Peptidings Glossary

A

Absorption

How a compound gets from the injection site into your bloodstream.

For peptides administered subcutaneously, absorption occurs via the lymphatic system and capillary bed at the injection site. Absorption rate determines how quickly the compound reaches peak plasma concentration (Tmax) and is distinct from bioavailability — a compound can be absorbed slowly but still be fully bioavailable, or absorbed quickly but partially degraded in transit.

Acetylation

A chemical modification that adds a small molecular cap to a peptide, usually to make it more stable or to help it cross into skin cells.

An acetyl group is added, typically at the N-terminus of a peptide. Acetylation increases lipophilicity, improves membrane permeability, and extends half-life by blocking degradation at the peptide's exposed end. Commonly used in cosmetic peptides (e.g., acetyl hexapeptide-3 / argireline) to enhance transdermal penetration.

acromegaly

A condition caused by too much growth hormone in adults—leading to enlarged hands, feet, and facial features.

Acromegaly results from chronic GH hypersecretion, usually from a pituitary somatotroph adenoma. Excess GH and IGF-1 cause progressive somatic overgrowth, metabolic complications (insulin resistance, diabetes), cardiovascular disease, and increased mortality. First-line treatment after surgery is somatostatin analog therapy: octreotide LAR or lanreotide Autogel suppress GH secretion via SSTR2 activation on somatotroph tumor cells. Biochemical control (normalized IGF-1) is achieved in approximately 55–65% of patients on somatostatin analogs alone.

Active comparator trial

A clinical trial that tests a new compound against an existing real treatment, not just a sugar pill.

Considered more rigorous than placebo-only designs for conditions where withholding effective treatment would be unethical. When you see a trial compared only against placebo, ask whether an active comparator was available — its absence can mean the bar for efficacy was set lower than it needed to be.

Adaptive immune response

The branch of your immune system that learns, remembers specific threats, and improves with each encounter — your antibodies and T cells.

Generates antigen-specific responses via T lymphocytes and B lymphocytes, producing immunological memory. Several immune modulation peptides (thymosin alpha-1, LL-37) interact with adaptive immune pathways. Distinct from the innate immune response, which responds non-specifically and immediately to any threat.

Adenylyl cyclase

An enzyme inside cells that gets switched on when certain peptide hormones bind to the cell surface, triggering a chain reaction that changes what the cell does.

Catalyzes the conversion of ATP to cyclic AMP (cAMP) upon Gs-coupled receptor activation. The cAMP it produces acts as a second messenger, activating protein kinase A (PKA) and downstream targets. Central to melanocortin, GLP-1, VIP, and secretin receptor signaling — which is why so many diverse peptide effects share a similar intracellular mechanism.

Adipogenesis

The process by which stem cells become fat cells.

Precursor cells (pre-adipocytes) differentiate into mature adipocytes through a regulated program involving PPAR-γ and C/EBP transcription factors. Several metabolic peptides including AOD-9604 and GLP-1 analogs have proposed mechanisms involving inhibition of adipogenesis or promotion of adipocyte apoptosis — though the in vitro evidence for these effects often does not translate cleanly to human fat loss outcomes.

Adjuvant

In immunology, something added to a vaccine or immune treatment to make the immune response stronger.

Thymosin alpha-1 has been studied as a vaccine adjuvant for hepatitis B and influenza, particularly in immunocompromised patients who respond poorly to standard vaccination. Not to be confused with adjuvant therapy in oncology, which refers to treatment given after primary surgery to reduce recurrence risk — an entirely different use of the same word.

Adrenal insufficiency

A condition where the adrenal glands don't produce enough cortisol — resulting in fatigue, low blood pressure, and sometimes visible skin darkening.

Primary adrenal insufficiency (Addison's disease) is caused by adrenal gland failure. The pituitary compensates by massively increasing ACTH secretion. Because ACTH weakly activates MC1R on skin melanocytes, chronically elevated ACTH produces the characteristic bronze hyperpigmentation of Addison's disease. This is the endogenous human pharmacology that originally motivated the development of synthetic MC1R agonists for tanning — researchers asked whether a selective, stable MC1R agonist could replicate the pigmentation effect without also activating the adrenal stress axis.

adverse event

Any unwanted medical occurrence during a study—whether or not it was actually caused by the treatment.

An adverse event (AE) is any unfavorable medical occurrence in a patient administered a pharmaceutical product, regardless of causal relationship. Adverse events are graded by severity (mild, moderate, severe) and classified by seriousness (serious adverse events require hospitalization, are life-threatening, or cause death). The distinction between AE and adverse drug reaction (ADR) is causality: all ADRs are AEs, but not all AEs are drug-related. Clinical trial safety data reports AE incidence in both treatment and placebo groups to establish attribution.

Agonist

A molecule that binds to a receptor and turns it on.

A full agonist produces the maximum possible response at that receptor. A partial agonist produces a submaximal response even at full receptor occupancy — which can be useful therapeutically when you want activity but not maximum activation. Contrast with antagonist (blocks the receptor without activating it). Most therapeutic peptides are agonists.

Amino acid

The individual building blocks that peptides and proteins are assembled from — like letters in an alphabet that spell out different molecules.

Twenty standard amino acids are encoded by the human genome. Each contains an amino group (NH₂), a carboxyl group (COOH), and a variable side chain that determines its chemical properties — whether it's acidic, basic, hydrophobic, or polar. Peptide bonds form between the carboxyl group of one amino acid and the amino group of the next, releasing water. The sequence and chemistry of the side chains determine the three-dimensional structure and function of the finished peptide.

Amyloid

Sticky, misfolded protein aggregates that accumulate in tissue and are associated with diseases like Alzheimer's.

Some neuroprotective peptides (humanin, NAP/ADNP) have been studied in models of amyloid-related neurodegeneration. The connection between amyloid research and these peptides is relevant to understanding why they appear in cognitive and longevity cluster contexts.

Anagen

The active growth phase of a hair follicle — the phase during which your hair is actually getting longer.

Anagen duration determines hair length potential and varies by body region (scalp anagen lasts 2–6 years; eyebrow anagen lasts 3–5 months). Anagen is driven by Wnt/β-catenin signaling, IGF-1, and KGF in the dermal papilla. Several compounds in the hair cluster target anagen entry or extension as their proposed mechanism for reducing hair loss or increasing density.

Angiogenesis

The growth of new blood vessels from existing ones — essential for wound healing, but also the process tumors exploit to feed themselves.

BPC-157 and GHK-Cu have documented pro-angiogenic effects via VEGFR2 upregulation and VEGF stimulation respectively. Angiogenesis is therapeutically desirable in wound healing and ischemic tissue but pathological in tumor growth and retinal neovascularization. The same compound that promotes healing-appropriate angiogenesis in one context could theoretically worsen another condition — context and tissue matter.

Antagonist

A molecule that binds to a receptor and blocks it — preventing the receptor from being activated by anything else.

An antagonist occupies the receptor without activating it, effectively silencing that signal. GnRH antagonists (cetrorelix, ganirelix, degarelix) block GnRH receptors in the pituitary, immediately suppressing LH and FSH secretion and therefore testosterone or estrogen — used in fertility protocols and prostate cancer treatment. NK1 receptor antagonists block substance P signaling. Contrast with agonist.

Antibody

A large immune protein that recognizes and binds to a specific target with high precision — the basis for most biologic drugs.

Antibodies (~150 kDa) are structurally and mechanistically distinct from peptides (typically

antimicrobial resistance

When bacteria evolve to survive antibiotics that used to kill them—one of the most urgent threats in medicine today.

Antimicrobial resistance (AMR) occurs when microorganisms develop mechanisms to survive antimicrobial agents: enzymatic inactivation (beta-lactamases), target modification (ribosomal methylation), efflux pumps, and permeability changes. The WHO identifies AMR as a critical global health threat. Antimicrobial peptides represent a promising counter-strategy because their membrane-disrupting mechanism targets fundamental bacterial membrane composition, making resistance development difficult. However, resistance to AMPs can emerge through membrane charge modification, protease production, and efflux mechanisms.

Apoptosis

Programmed cell death — the body's controlled process for eliminating cells that are damaged, old, or no longer needed.

Distinct from necrosis, which is uncontrolled cell death that triggers inflammation. Apoptosis is tightly regulated and essential for normal development and tissue homeostasis. Several neuroprotective peptides (humanin, NAP) work by inhibiting apoptosis in neurons under metabolic stress. In oncology, inducing apoptosis in cancer cells is a core therapeutic goal.

AUC (area under the curve)

The total drug exposure your body receives over time—calculated from a graph of blood levels, it captures both how high levels get and how long they last.

Area under the concentration-time curve (AUC) is the fundamental pharmacokinetic measure of total systemic drug exposure. AUC₀₋∞ represents total exposure from administration to complete elimination; AUC₀₋τ represents exposure within a dosing interval. For antimicrobial peptides, the AUC/MIC ratio predicts efficacy. For peptide therapeutics generally, AUC determines whether sufficient receptor engagement occurs for therapeutic effect. AUC is influenced by dose, bioavailability, clearance, and volume of distribution.

Autocrine signaling

When a cell releases a signaling molecule that acts back on itself.

Contrast with paracrine (acts on neighboring cells in the same tissue) and endocrine (travels via bloodstream to act on distant cells). Many peptide hormones operate in multiple modes depending on concentration and tissue context — understanding which mode is relevant in a given study matters for interpreting whether local vs. systemic administration would replicate the observed effect.

B

bacteriocin

An antimicrobial peptide made by bacteria to kill competing bacteria—nisin is the most commercially important example.

Bacteriocins are ribosomally synthesized antimicrobial peptides produced by bacteria that inhibit closely related species. Unlike antibiotics (secondary metabolites), bacteriocins are gene-encoded and target specific bacterial populations. Nisin, produced by Lactococcus lactis, is the most studied bacteriocin—a 34-amino acid lantibiotic that binds lipid II (peptidoglycan precursor) and forms membrane pores. Bacteriocins are classified into classes based on structure and modification: Class I (lantibiotics like nisin), Class II (unmodified), Class III (large, heat-labile), and Class IV (complex).

Bacteriostatic water (BAC water)

The sterile water most research peptides get dissolved in before injection — it contains a small amount of preservative that keeps bacteria from growing in the vial.

0.9% benzyl alcohol is the preservative. It allows the same vial to be used multiple times over several weeks when refrigerated, unlike plain sterile water (single-use only). Do not use bacteriostatic water in neonates — benzyl alcohol is toxic in newborns at doses that are safe in adults. Distinct from normal saline (0.9% NaCl) and sterile water for injection, both of which lack preservative.

Beta-sheet

A flat, extended structural shape that some peptides and proteins fold into — associated with both stability and, in some cases, disease-causing aggregation.

Formed when backbone hydrogen bonds develop between adjacent peptide strands running parallel or antiparallel. Amyloid-forming peptides adopt beta-sheet conformations upon aggregation. Cyclic peptides are often designed to restrict conformational flexibility and prevent unwanted beta-sheet formation — one reason cyclization improves stability and receptor selectivity.

Bioavailability

The fraction of a dose that actually reaches your bloodstream in active form — the rest is lost to degradation or incomplete absorption.

IV administration gives 100% bioavailability by definition. Subcutaneous injection typically gives 70–95% for peptides. Oral bioavailability of most peptides is near zero — peptidases in the GI tract degrade the peptide bond before it can be absorbed, and the GI epithelium is poorly permeable to large polar molecules. This is why nearly all research peptides require injection. Oral "peptide" products that claim equivalent bioavailability to injectable forms are making a claim that requires extraordinary evidence.

biofilm

A slimy, protective community of bacteria that sticks to surfaces—much harder to kill with antibiotics than free-floating bacteria.

Biofilms are structured microbial communities embedded in a self-produced extracellular polymeric substance (EPS) matrix attached to surfaces. Biofilm bacteria exhibit 10–1,000× greater antibiotic resistance than planktonic cells due to restricted antibiotic penetration, metabolic dormancy, and persister cell formation. Several antimicrobial peptides (nisin, temporins, pexiganan) demonstrate anti-biofilm activity by disrupting biofilm matrix, penetrating established biofilms, or preventing initial biofilm attachment—advantages over conventional antibiotics.

Biomarker

A measurable signal in your blood, tissue, or elsewhere that tells you something is happening biologically — a proxy for what you're actually trying to track.

In peptide research, relevant biomarkers include IGF-1 levels (GH axis activity), inflammatory cytokines (IL-6, TNF-α, CRP), and tissue-specific enzymes. Biomarkers used as primary endpoints in clinical trials are surrogate endpoints — they substitute for harder-to-measure clinical outcomes. A drug that improves a biomarker does not necessarily improve the clinical outcome the biomarker is supposed to represent.

Blood-brain barrier (BBB)

A highly selective filter between your bloodstream and your brain that keeps most drugs — and most peptides — out of the CNS.

Formed by tight junctions between brain endothelial cells. Most peptides cannot cross the BBB due to size and polarity. Semax and selank are among the exceptions, attributed to specific structural features. Some neuroprotective peptides (NAP, humanin) have been studied with intranasal delivery as a way to bypass the BBB via olfactory nerve pathways, though the intranasal-to-brain transport efficiency for most peptides remains uncertain.

Blood-retinal barrier

The protective filter around the retina, similar to the blood-brain barrier, that prevents most drugs from reaching the back of the eye via the bloodstream.

This is why intravitreal injection — directly into the vitreous of the eye — is the standard delivery route for retinal therapeutics. Systemic dosing of anti-VEGF biologics or somatostatin analogs for retinal disease would require concentrations that could produce unacceptable systemic side effects. Local delivery solves the access problem but requires a clinical procedure. See also: intravitreal injection.

BNP / NT-proBNP

Proteins the heart releases when it's under stress — used as blood tests to diagnose heart failure and track disease severity.

B-type natriuretic peptide (BNP) and its cleavage product NT-proBNP are among the most important cardiac biomarkers in clinical medicine. Nesiritide (Natrecor) is recombinant BNP — the same molecule used as a biomarker repurposed as a therapeutic to dilate blood vessels in acute decompensated heart failure. The ASCEND-HF trial showed nesiritide improved dyspnea but not mortality or re-hospitalization — a reminder that biomarker improvement and clinical benefit are not the same thing.

bone mineral density

A measurement of how much mineral (mostly calcium) is packed into your bones—the standard test for osteoporosis.

Bone mineral density (BMD) is measured by dual-energy X-ray absorptiometry (DXA) at the lumbar spine, femoral neck, and total hip, expressed as T-scores (standard deviations from young adult mean). T-score ≤ −2.5 defines osteoporosis; −1.0 to −2.5 defines osteopenia. BMD is a surrogate endpoint—fracture reduction is the clinically meaningful outcome. Anabolic peptides (teriparatide, abaloparatide) increase BMD more rapidly than antiresorptives, but BMD gains do not always proportionally predict fracture risk reduction.

bone remodeling

The continuous process where old bone is broken down by osteoclasts and replaced with new bone by osteoblasts—the balance between these determines bone strength.

Bone remodeling is the coupled process of osteoclastic bone resorption followed by osteoblastic bone formation, occurring continuously in basic multicellular units (BMUs). Normal remodeling maintains skeletal integrity; imbalance causes disease. In osteoporosis, resorption exceeds formation. Antiresorptive agents (calcitonin, bisphosphonates) reduce resorption; anabolic agents (teriparatide, abaloparatide) stimulate formation. The anabolic window—the period where formation exceeds resorption—is time-limited with PTH analogs, explaining the 24-month treatment duration limits.

C

cAMP (cyclic adenosine monophosphate)

A molecular "messenger" inside cells that relays instructions from surface receptors to the machinery inside the cell — a pivotal relay in many peptide signaling pathways.

Produced by adenylyl cyclase when Gs-coupled receptors are activated. cAMP activates protein kinase A (PKA), which phosphorylates downstream targets including CREB (altering gene expression), MLCK (relaxing smooth muscle), and MITF (driving melanogenesis). This single second messenger is central to melanocortin, GLP-1, VIP, secretin, and many other peptide receptor pathways — explaining why selective receptor agonism matters so much when you want a specific effect without activating the whole cascade.

carcinoid syndrome

The cluster of symptoms—flushing, diarrhea, wheezing—caused by a neuroendocrine tumor releasing too much serotonin into the bloodstream.

Carcinoid syndrome occurs when neuroendocrine tumors (typically with hepatic metastases) secrete serotonin, histamine, tachykinins, and prostaglandins into systemic circulation, bypassing hepatic first-pass metabolism. Symptoms include episodic flushing, secretory diarrhea, bronchospasm, and carcinoid heart disease (endocardial fibrosis). Somatostatin analogs (octreotide, lanreotide) are first-line treatment, suppressing hormone secretion via SSTR2 activation on tumor cells. Rescue octreotide injections manage acute carcinoid crises.

case series

A report describing outcomes in a small group of patients—useful for generating hypotheses but too weak to prove a treatment works.

A case series is an uncontrolled, observational report of clinical outcomes in a group of patients receiving a treatment or sharing a diagnosis. Case series lack comparison groups, randomization, and blinding, placing them low in the evidence hierarchy. They are valuable for identifying rare adverse events, generating hypotheses, and reporting preliminary clinical observations. In peptide research, case series may represent the only human data available for compounds not yet in formal clinical trials—their limitations must be stated explicitly.

Catabolic

Metabolic processes that break things down — the opposite of the anabolic (building) state that GH secretagogues and IGF-1 are often used to promote.

Excessive catabolic signaling — from cortisol, myostatin, inflammatory cytokines — drives muscle wasting in disease states. Several GH secretagogues and IGF-1 analogs are studied in genuine catabolic disease contexts (HIV wasting, cachexia, sarcopenia). The performance community uses "anti-catabolic" language to describe any compound that might preserve muscle, but catabolic signaling exists for physiological reasons — chronic suppression carries its own consequences.

Catagen

The brief transitional phase of hair follicle cycling — the growth phase switches off, the follicle shrinks, and the hair prepares to shed.

Lasts approximately 2–3 weeks. Matrix cell proliferation stops; the follicle involutes. Neurogenic inflammation via substance P is proposed to prematurely push follicles into catagen in stress-related hair shedding (telogen effluvium). Understanding catagen is relevant to interpreting any compound claimed to prevent hair loss — if the mechanism doesn't address premature catagen entry, it may not address the most common cause of shedding.

cathelicidin

A family of antimicrobial peptides stored in immune cells and released during infection—LL-37 is the only human cathelicidin.

Cathelicidins are a family of antimicrobial peptides characterized by a conserved cathelin pro-domain and a variable C-terminal antimicrobial domain. Humans express a single cathelicidin gene (CAMP) producing hCAP-18, which is proteolytically cleaved to release the active peptide LL-37. LL-37 exhibits broad-spectrum antimicrobial activity, immunomodulatory functions, and wound healing promotion. Cathelicidins are stored in neutrophil specific granules and epithelial cells, released upon infection or injury.

Cell-permeable peptide

A peptide engineered to pass through cell membranes so it can reach targets inside the cell, not just on the surface.

Standard peptides can only bind receptors on the cell surface. Cell-permeable peptides contain a protein transduction domain (PTD) or cell-penetrating peptide (CPP) sequence that enables membrane crossing. PTD-DBM uses this property to deliver its Dvl-binding domain into dermal papilla cells where the CXXC5-Dvl interaction occurs. Intracellular targets inaccessible to conventional receptor-binding peptides become tractable with this approach.

Circadian rhythm

Your internal biological clock — the ~24-hour cycle that regulates when you sleep, when hormones peak, when your immune system is most active, and dozens of other processes.

The suprachiasmatic nucleus (SCN) in the hypothalamus serves as the master circadian pacemaker. VIP and PACAP are critical SCN synchronization signals — neuropeptides that help the internal clock stay aligned with the external light-dark cycle. Disruption of circadian rhythm is associated with metabolic dysfunction, immune dysregulation, and mood disorders, which is why the sleep and circadian peptide cluster is pharmacologically broader than just sleep aids.

clearance

How fast your body eliminates a drug from your bloodstream—measured in volume per time.

Clearance (CL) represents the volume of plasma completely cleared of drug per unit time, typically expressed in mL/min or L/hr. For peptides, clearance occurs primarily through renal filtration, hepatic metabolism, and enzymatic degradation by tissue peptidases. High clearance necessitates frequent dosing or formulation modifications (PEGylation, fatty acid conjugation, albumin binding) to extend exposure. Clearance determines the maintenance dose required to achieve target steady-state concentrations.

Clinical trial phase

The standardized stages of human drug testing — each phase answers different questions and involves more people than the last.

Phase I: Is this safe? Conducted in 20–100 healthy volunteers or patients. Establishes safety, tolerability, and dosing. Phase II: Does it work, and at what dose? 100–300 patients in the target population. Generates efficacy signals. Phase III: Does it work definitively, at scale? 300–3,000+ patients in randomized controlled trials against placebo or standard of care. Required for regulatory approval. Phase IV: Post-marketing surveillance after approval. A compound that has only completed Phase I has established basic safety — not efficacy.

Cmax

The highest blood level a drug reaches after a dose—the peak concentration on the drug level curve.

Cmax (maximum plasma concentration) is the peak drug concentration achieved after administration. It is determined by the rate and extent of absorption, volume of distribution, and early elimination. For peptide therapeutics, Cmax is clinically relevant in two contexts: efficacy (whether peak concentrations achieve therapeutic receptor occupancy) and safety (whether peak concentrations approach toxic thresholds). The Cmax/MIC ratio is a key efficacy predictor for concentration-dependent antimicrobial peptides.

cohort study

A study that follows a defined group of people over time to see what happens to them—observational, not experimental.

A cohort study follows a group of individuals sharing a common characteristic (exposure, treatment, condition) over time, measuring outcomes prospectively or retrospectively. Unlike RCTs, cohort studies are observational and cannot establish causation, but they can identify associations and generate hypotheses. In peptide research, cohort data from self-experimentation communities provides real-world exposure information but lacks the controls, blinding, and standardization of clinical trials.

Collagen

The main structural protein in skin, tendons, ligaments, bone, and cartilage — what gives these tissues their tensile strength.

The most abundant protein in the human body. Major collagen subtypes: Type I (skin, tendon, bone); Type II (cartilage); Type III (blood vessels, early wound healing). GHK-Cu, matrixyl (palmitoyl pentapeptide-4), and several injury recovery peptides have proposed mechanisms involving collagen synthesis stimulation or MMP regulation. The claim that a topical peptide "boosts collagen" requires specifying whether the evidence is from cell culture, animal models, or human skin biopsy — the difference matters enormously.

compassionate use

Access to an unapproved drug for a seriously ill patient who has no other options—granted on a case-by-case basis outside of clinical trials.

Compassionate use (expanded access) is a regulatory pathway allowing patients with serious or life-threatening conditions to access investigational drugs outside clinical trials when no comparable alternative exists. In the US, FDA grants expanded access through individual patient INDs, intermediate-size programs, or treatment INDs. For peptide therapeutics, compassionate use data occasionally provides human exposure data for compounds not yet in formal Phase I trials, though such data lacks the controls needed for efficacy assessment.

Competitive binding assay

A lab test that measures how strongly a compound grabs onto a receptor by seeing how well it displaces a reference molecule from that receptor.

Produces Ki or IC50 values that quantify receptor binding affinity. In vitro receptor binding data characterizes the pharmacological interaction but does not predict in vivo efficacy or safety — a compound that binds tightly to a receptor in a test tube may still be metabolized before it reaches that receptor in vivo, or may produce different effects in a complex physiological system than it does in an isolated assay.

Confidence interval (CI)

A range around a study's result that tells you how precise the estimate is — the wider the range, the less certain the finding.

A 95% CI means: if the study were repeated 100 times, the true value would fall within this range 95 times. A 95% CI for relative risk that does not include 1.0 (or a difference that does not include 0) indicates statistical significance at p

Crossover trial

A trial design where each participant receives both the real treatment and the comparison treatment in sequence — so every person acts as their own control.

Eliminates between-person variability because the same individual experiences both treatments. Requires a washout period between treatments to allow the first compound to clear. Appropriate for stable, reversible conditions. Some SS-31/elamipretide heart failure trials used crossover designs. Not appropriate when treatments have long-lasting or irreversible effects, or when the condition is expected to change over time.

Cyclic peptide

A peptide whose ends are joined together in a ring — making it more resistant to degradation and often more selective for its target receptor.

The N- and C-termini are linked by a peptide bond, or a side chain forms an intramolecular bond creating a ring. Cyclization constrains the peptide's three-dimensional shape, improving receptor selectivity by limiting the range of conformations it can adopt. Also significantly increases resistance to proteolytic degradation — enzymes that break down linear peptides often cannot process the ring structure. Melanotan II, bremelanotide, and setmelanotide are all cyclic peptides.

Cytokine

Small signaling proteins secreted by immune and other cells that coordinate inflammation and immune responses — the "messenger chemicals" of the immune system.

Includes interleukins (IL-1β, IL-6, IL-10, IL-17), tumor necrosis factors (TNF-α), interferons, and chemokines. Pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) amplify inflammatory responses. Anti-inflammatory cytokines (IL-10, TGF-β) dampen them. Many peptides modulate cytokine production — typically by reducing pro-inflammatory and increasing anti-inflammatory cytokines via NF-κB inhibition. The clinical significance of cytokine changes observed in in vitro or animal studies is not always reflected in human outcomes.

D

D-amino acid

A mirror-image version of the normal amino acid — enzymes that break down standard amino acids typically cannot break down the mirror version, making D-amino acid-containing peptides much harder to degrade.

All naturally occurring amino acids in human proteins are L-configured. Peptidases are stereospecific — they cleave L-peptide bonds efficiently but cannot process D-configured ones. Inserting D-amino acids at specific positions in synthetic peptides (D-Phe7 in Melanotan II and afamelanotide; D-amino acids in selank and semax) significantly extends plasma half-life by blocking proteolytic degradation at those positions. A key medicinal chemistry tool for improving peptide stability without changing receptor binding properties.

DAC (Drug Affinity Complex)

A molecular add-on that causes a peptide to bind to a blood protein, dramatically slowing how quickly it's cleared from the body.

The DAC modification on CJC-1295 covalently binds to albumin after injection via a maleimide-thiol reaction. Albumin's long half-life (~19 days) effectively extends CJC-1295 with DAC's plasma half-life from minutes to 6–8 days, producing sustained GH elevation. CJC-1295 without DAC (Modified GRF 1-29) has a half-life of ~30 minutes and produces a discrete GH pulse that more closely mimics physiological secretion. The two compounds have the same binding target (GHRH receptor) but very different pharmacokinetic profiles — the choice between them is a decision about what pattern of GH stimulation you're trying to produce.

defensin

A small antimicrobial peptide made by your immune cells and epithelial surfaces—your body’s natural antibiotic.

Defensins are cationic, cysteine-rich antimicrobial peptides of the innate immune system. Alpha-defensins (HNP-1–4 from neutrophils; HD-5, HD-6 from Paneth cells) and beta-defensins (hBD-1–4 from epithelial surfaces) kill pathogens through membrane disruption. Beyond direct antimicrobial activity, defensins function as chemoattractants, promote wound healing, and bridge innate and adaptive immunity. Defensins represent both a natural defense system and a template for antimicrobial drug development.

Dermal papilla

A cluster of specialized cells at the base of the hair follicle that act as the follicle's "command center" — directing hair growth, cycling, and what the hair shaft looks like.

Dermal papilla cells (DPCs) are mesenchymal in origin and produce the paracrine signals (KGF, IGF-1, Wnt ligands) that drive follicle keratinocyte proliferation and anagen entry. DPCs are the primary target of PTD-DBM — the CXXC5-Dvl interaction that PTD-DBM disrupts occurs specifically in dermal papilla cells. The DPC niche is also where androgenetic alopecia acts, via DHT-mediated miniaturization signals from DPCs to the follicle matrix.

diabetic retinopathy

Damage to the blood vessels in the back of the eye caused by diabetes—a leading cause of blindness in working-age adults.

Diabetic retinopathy (DR) is a microvascular complication of diabetes characterized by progressive damage to retinal capillaries. Non-proliferative DR involves microaneurysms, hemorrhages, and macular edema; proliferative DR involves pathological neovascularization driven by VEGF upregulation. Current treatments include anti-VEGF injections, laser photocoagulation, and vitrectomy. Peptide-based approaches under investigation include intravitreal octreotide (somatostatin receptor-mediated neuroprotection), anti-VEGF peptides, and SP-IGF-1 (retinal neuroprotection).

Disulfide bond

A chemical link between two specific amino acids (cysteines) that acts like a molecular staple, holding a peptide in a stable three-dimensional shape.

Formed by oxidation of cysteine thiol groups. Disulfide bonds stabilize peptide structure and reduce susceptibility to proteolytic degradation. Many endogenous peptide hormones contain disulfide bonds — insulin has two disulfide bonds that are essential to its activity. Breaking disulfide bonds (by reducing agents) typically abolishes activity. Relevant when considering storage conditions and reconstitution — oxidizing or reducing environments can disrupt disulfide-bonded peptides.

Dose-response relationship

The connection between how much of a compound you take and how strong the effect is — ideally a smooth curve, but sometimes surprisingly non-linear.

A simple dose-response curve increases with dose until a plateau (Emax). Some peptides exhibit non-monotonic or hormetic dose-response curves, where low doses and high doses produce opposite effects — low-dose angiogenesis vs. high-dose anti-angiogenesis in some VEGF-related compounds, for example. This non-linearity is one reason extrapolating from animal dosing to human dosing is unreliable, and one reason community "more is better" assumptions about peptide dosing are sometimes specifically wrong.

DPP-IV (dipeptidyl peptidase-4)

An enzyme that rapidly inactivates GLP-1 and other incretin hormones—the reason native GLP-1 only lasts minutes in your blood.

Dipeptidyl peptidase-4 (DPP-IV/CD26) is a serine protease that cleaves N-terminal dipeptides from substrates with proline or alanine at position 2. DPP-IV rapidly inactivates GLP-1 (t½ ~2 min), GIP, GLP-2, PYY, and neuropeptide Y. DPP-IV inhibitors (gliptins) extend incretin half-life for diabetes treatment. Long-acting GLP-1 receptor agonists (semaglutide, tirzepatide) incorporate structural modifications (fatty acid conjugation, amino acid substitutions) that resist DPP-IV cleavage, extending half-life from minutes to days or weeks.

E

EC50

The dose that produces exactly half the maximum possible effect — a standard way of comparing how potent different compounds are.

Lower EC50 = higher potency (less compound needed to achieve the same effect). Derived from in vitro functional assays. Distinct from IC50 (concentration producing 50% inhibition) and Ki (binding affinity constant). EC50 values from cell culture experiments provide comparative pharmacology data but do not directly translate to human dosing — in vivo PK, receptor distribution, and downstream signaling complexity all mediate the relationship between plasma concentration and biological effect.

ECM (Extracellular Matrix)

The structural scaffolding that surrounds cells in tissues — a complex mesh of proteins and sugars that gives tissues their architecture and mechanical properties.

Major ECM components: collagen (tensile strength), elastin (elasticity), fibronectin and laminin (cell adhesion), hyaluronic acid (hydration and volume). ECM remodeling — breakdown and replacement of old matrix — is central to wound healing, follicle cycling, and tissue repair. GHK-Cu, matrixyl, and several injury recovery peptides target ECM remodeling via matrix metalloproteinase regulation and collagen synthesis stimulation. Excessive ECM breakdown impairs structure; insufficient turnover impairs healing.

Effect size

How large the treatment effect actually is — separate from whether it's statistically significant.

A large enough trial can make a trivially small effect statistically significant (p

EMA (European Medicines Agency)

The European Union’s drug regulator—the EU equivalent of the FDA.

The EMA evaluates and supervises medicines across the European Economic Area. Marketing authorization through the EMA’s centralized procedure grants approval in all EU member states simultaneously. For peptide therapeutics, EMA and FDA approval timelines often diverge—compounds may be approved in one jurisdiction years before the other, creating access disparities that affect clinical practice and research availability.

Endogenous

Produced naturally inside the body — as opposed to something you'd administer from outside.

Endogenous peptides (GLP-1, insulin, oxytocin, alpha-MSH) have defined physiological roles produced by specific cells in response to specific signals. The fact that a peptide is endogenous does not make exogenous administration of it safe, effective, or physiologically equivalent. Exogenous doses, timing, receptor context, and feedback regulation all differ substantially from endogenous physiology. "It's natural" is not a safety or efficacy argument.

endopeptidase

An enzyme that cuts a peptide chain in the middle rather than nibbling from the ends—often the first step in peptide degradation.

Endopeptidases cleave internal peptide bonds, distinguishing them from exopeptidases that remove terminal residues. Major endopeptidases relevant to peptide therapeutics include neprilysin (NEP/CD10), endothelin-converting enzyme (ECE), insulin-degrading enzyme (IDE), and matrix metalloproteinases (MMPs). Endopeptidase-mediated degradation is the primary mechanism limiting the plasma half-life of most linear peptides. Drug design strategies to resist endopeptidase cleavage include cyclization, D-amino acid substitution at cleavage sites, and backbone modifications.

Endophthalmitis

A serious infection inside the eye that can cause permanent blindness — a known risk of intravitreal injection even when performed correctly.

Infection of the vitreous and aqueous humor by bacteria or fungi. Occurs as a complication of intravitreal injection even under sterile surgical conditions in clinical settings. Incidence with anti-VEGF intravitreal injections is approximately 0.03–0.05% per injection in clinical practice — low, but not zero, and the consequence is potentially catastrophic. This is one key reason intravitreal injection is not an appropriate self-experimentation route under any circumstances.

ERK1/2

A key relay in the cell’s growth signaling chain—when activated, it tells the cell to grow, divide, or survive.

Extracellular signal-regulated kinases 1 and 2 (ERK1/2) are serine/threonine kinases in the Ras-Raf-MEK-ERK (MAPK) cascade. Activated by receptor tyrosine kinases and GPCRs, ERK1/2 phosphorylates transcription factors controlling cell proliferation, differentiation, survival, and migration. ERK1/2 activation is a common downstream effect of peptide receptor signaling—including GHS-R1a (ghrelin, ipamorelin), melanocortin receptors, and growth factor receptors. Sustained versus transient ERK activation can produce opposing cellular outcomes.

Erythropoietic protoporphyria (EPP)

A rare genetic disorder that makes sunlight excruciatingly painful — caused by a deficiency in a blood enzyme that leaves a toxic compound accumulating in skin.

Deficiency of ferrochelatase leads to accumulation of protoporphyrin IX in red blood cells, plasma, and skin. Protoporphyrin IX absorbs visible light (not just UV) and releases the energy as reactive oxygen species, causing severe burning pain within minutes of sun exposure. Afamelanotide (Scenesse) is FDA-approved specifically for EPP — by stimulating melanogenesis, the induced pigmentation absorbs light before it reaches protoporphyrin IX. This is a rare disease approval, not a cosmetic tanning approval.

Eumelanin

The brown and black form of melanin — the protective pigment that gives skin its darker tones and shields DNA from UV damage.

Produced in melanocytes via the MC1R–cAMP–PKA–MITF–tyrosinase pathway. Eumelanin absorbs UV radiation and quenches reactive oxygen species generated by UV exposure, providing genuine photoprotection. Contrast with pheomelanin (red-yellow pigment, produced when MC1R signaling is low), which provides less UV protection and may actually generate reactive oxygen species under UV exposure. MC1R agonists specifically promote eumelanin synthesis — the pharmacologically relevant distinction for both EPP therapy and tanning applications.

Evidence hierarchy

A ranking of research study types by how reliably they establish cause and effect — not all research is equally convincing.

From highest to lowest: systematic reviews of RCTs > randomized controlled trials > cohort studies > case-control studies > case series > in vitro/animal studies. This hierarchy exists because each step down introduces more potential for confounding and bias. A compound supported only by in vitro data has not cleared the bar for human efficacy evidence, regardless of how compelling the mechanism. Peptidings applies this hierarchy explicitly — the evidence tier on every compound is an honest assignment within this framework.

Exogenous

Administered from outside the body — whether injected, swallowed, or applied to skin.

The distinction between endogenous and exogenous matters because the body regulates its own peptide production through feedback loops that exogenous administration bypasses or disrupts. Exogenous GH secretagogues cause GH release in a pattern determined by injection timing rather than physiological demand. Exogenous oxytocin in pharmacological doses activates receptors in tissues that endogenous oxytocin never reaches in those concentrations. "Natural" is not the same as "safe at pharmacological doses."

F

FDA Category 1

The compound has been fully approved by the FDA for human use — meaning it passed Phase III clinical trials and was reviewed by the agency.

Approval is indication-specific: an FDA-approved drug for condition X has not been evaluated by the FDA for conditions Y or Z. Prescribing an approved drug off-label is legal in the US when a physician judges it appropriate, but off-label use does not carry the same evidence weight as on-label use. Category 1 on this site means there is at least one human indication with formal regulatory approval.

FDA Category 2

The compound is not yet approved but is under active investigation — it has been tested in humans in clinical trials.

IND (Investigational New Drug) status means a sponsor has provided sufficient preclinical data to justify human trials. Being in clinical trials means the FDA has permitted human research, not that the compound is safe or effective. Most compounds that enter Phase I trials never reach approval. Category 2 is meaningfully different from Category 3 (no human trials) but equally meaningfully different from Category 1 (approved).

FDA Category 3

The compound has no FDA approval and no active clinical development in the US — it has not been formally evaluated for human safety or efficacy.

Most research peptides used in self-experimentation communities are Category 3. Category 3 status means the FDA has not evaluated the compound at all. This is not the same as "proven unsafe" — it means there is no systematically collected human safety data. The absence of evidence of harm is not evidence of absence of harm, particularly for long-term or high-dose use.

fibrin

The protein that forms the structural mesh of blood clots—it polymerizes into long strands that trap blood cells and seal wounds.

Fibrin is an insoluble protein polymerized from fibrinogen by thrombin-mediated cleavage of fibrinopeptides A and B. Fibrin monomers spontaneously polymerize into protofibrils, then are cross-linked by Factor XIIIa into a stable clot matrix. Fibrin clots are eventually dissolved by plasmin (fibrinolysis). In peptide cardiovascular therapeutics, controlling fibrin formation is the target: bivalirudin inhibits thrombin to reduce fibrin generation; eptifibatide blocks platelet aggregation that contributes to clot formation.

fibroblast

The cell that builds and maintains connective tissue—it produces collagen, the structural protein in skin, tendons, and organs.

Fibroblasts are the primary cell type of connective tissue, responsible for synthesizing extracellular matrix components including collagen, elastin, fibronectin, and glycosaminoglycans. They play central roles in wound healing (migration, contraction, matrix remodeling) and fibrosis (excessive matrix deposition). Several peptides on Peptidings interact with fibroblast biology: GHK-Cu stimulates fibroblast collagen synthesis, BPC-157 promotes fibroblast migration to injury sites, and antimicrobial peptides modulate fibroblast inflammatory responses.

First-pass metabolism

The degradation of a compound in the gut and liver before it reaches the bloodstream — the main reason you can't just swallow most peptides.

After oral absorption, compounds travel through the portal vein to the liver before entering systemic circulation. Peptidases in the GI epithelium and hepatic enzymes degrade most peptides before they can be absorbed or distributed. The result is near-zero oral bioavailability for most therapeutic peptides. This is why injectable administration is standard for peptide therapeutics. Companies selling oral peptide supplements are claiming a delivery mechanism that basic biochemistry says should not work for most peptides — claims that require extraordinary evidence.

Follicle immune privilege

The hair follicle maintains a partial "immune-free zone" around its growing cells, protecting the genetically distinct hair shaft from immune attack.

The hair shaft is antigenically distinct from the host and would normally trigger immune responses. The follicle suppresses local immune activity via IL-10, TGF-β, and MHC class I downregulation to prevent this. In alopecia areata, this immune privilege collapses — for reasons not fully understood — allowing cytotoxic T cells to infiltrate the follicle matrix, producing the characteristic patchy hair loss. Thymulin's proposed mechanism for hair loss partly involves restoration of follicle immune privilege via immunomodulatory signaling.

Formyl peptide receptor (FPR)

A receptor on immune cells originally designed to detect bacterial peptides — but also activated by some endogenous peptides involved in resolving inflammation.

Humanin activates FPRL1 (FPR2) as part of its cytoprotective mechanism. The FPR family was originally characterized as a detector for N-formylmethionine-containing bacterial peptides, triggering neutrophil chemotaxis toward infection sites. Some endogenous peptides that resolve inflammation (annexin A1, lipoxins, humanin) also signal through this pathway — an example of the immune system repurposing a pathogen-detection receptor for tissue-protective signaling.

G

G protein-coupled receptor (GPCR)

The most common type of receptor on cell surfaces — activated by peptide hormones, neurotransmitters, and many drugs, triggering changes inside the cell via a relay protein.

The largest receptor superfamily in the human genome (~800 members), characterized by seven transmembrane domains. G protein coupling determines what happens inside the cell after activation: Gs activates adenylyl cyclase, producing cAMP; Gi inhibits adenylyl cyclase; Gq activates phospholipase C, producing IP3 and DAG. The vast majority of peptide hormone receptors are GPCRs. Understanding which G protein a receptor couples to predicts the downstream cellular response.

GHRH (Growth Hormone-Releasing Hormone)

The brain's signal to the pituitary to release growth hormone — the endogenous molecule that sermorelin and CJC-1295 mimic.

Produced in the hypothalamus; acts on GHRH receptors (GHRHRs) on pituitary somatotrophs to stimulate GH synthesis and secretion. Sermorelin (29-amino acid synthetic GHRH fragment) and Modified GRF(1-29)/CJC-1295 are synthetic GHRH analogs acting via the same receptor. Distinct from ghrelin and the GHS-R1a pathway — a second independent GH secretagogue mechanism. The two-pathway system explains the pharmacological rationale for GHRH analog + GHRP combination protocols.

GHS-R1a (Growth Hormone Secretagogue Receptor)

The ghrelin receptor — the second pathway through which growth hormone can be stimulated, separate from the GHRH pathway.

A G protein-coupled receptor expressed in the pituitary, hypothalamus, and peripheral tissues. Activated by ghrelin (the endogenous ligand) and synthetic GH secretagogues: ipamorelin, hexarelin, GHRP-2, GHRP-6, and MK-677. GHS-R1a agonism stimulates GH secretion and also stimulates appetite — ghrelin is the "hunger hormone." The dual GHRH+GHS-R1a stimulation approach amplifies GH release because both pathways converge on the same pituitary cells via different receptors.

GLP-1 receptor agonist

A class of drugs that mimic GLP-1, a gut hormone released after eating — they stimulate insulin, suppress appetite, and slow digestion, making them the most effective pharmaceutical weight loss tools currently available.

Mechanism: glucose-dependent insulin secretion (safe — no hypoglycemia at normal glucose levels), glucagon suppression, gastric emptying delay, and hypothalamic satiety signaling. The glucose-dependence is the key safety feature that differentiates GLP-1 agonists from insulin. Includes semaglutide, liraglutide, and tirzepatide (GLP-1/GIP dual agonist). The most clinically successful class of metabolic peptide therapeutics to date, with Phase III trial data far exceeding any other compound in the weight loss space.

gut-brain axis

The two-way communication highway between your gut and your brain—including nerves, hormones, and immune signals.

The gut-brain axis encompasses bidirectional communication between the gastrointestinal tract and central nervous system via the vagus nerve, enteric nervous system, circulating hormones (ghrelin, CCK, GLP-1, PYY), immune mediators, and microbial metabolites. Gut peptide hormones including cholecystokinin, ghrelin, and neuropeptide Y are key mediators of appetite regulation, satiety signaling, and stress responses. Disruption of gut-brain axis signaling is implicated in functional gastrointestinal disorders, obesity, and mood disorders.

H

Half-life (plasma)

How long it takes for half the compound to clear from your blood — a measure of how long it stays active.

Most endogenous peptides have half-lives of minutes due to proteolytic degradation. Medicinal chemistry modifications extend half-life: D-amino acid substitutions block proteolysis, PEGylation reduces renal clearance, albumin binding via DAC leverages albumin's 19-day half-life. Important: plasma half-life is not the same as duration of biological effect. A compound cleared from the blood may have already activated signaling cascades that persist for hours or days. Conversely, a long plasma half-life does not guarantee prolonged receptor activation.

HPA axis (Hypothalamic-Pituitary-Adrenal axis)

The brain-to-adrenal gland stress response system — the chain of command that produces cortisol when you're under stress.

CRH (hypothalamus) → ACTH (pituitary) → cortisol (adrenal cortex). Cortisol feeds back to suppress both CRH and ACTH production (negative feedback), creating the regulatory loop. Several peptides in the sleep and stress cluster interact with HPA axis components: CRH amplifies the stress response; NPY dampens it; cortisol's circadian rhythm is the most prominent example of HPA axis cycling. Chronic HPA axis dysregulation is implicated in PTSD, major depression, and metabolic syndrome.

Human studies gap

The disconnect between what we know from animal and lab studies and what we actually know from human clinical trials — for many research peptides, this gap is the most important fact about them.

Animal model results predict human outcomes imperfectly and sometimes not at all. This is not a theoretical concern — most compounds that show dramatic results in rodent models fail to replicate in humans. The human studies gap is the central limiting factor in evaluating the majority of research peptides. When community discourse cites rat studies as evidence of human efficacy, or in vitro cell culture data as evidence of clinical effect, the human studies gap is being ignored. This site documents the gap explicitly for every compound where it exists.

I

IC50

The concentration that cuts a biological process in half — used to measure how potent an inhibitor is.

Lower IC50 = more potent inhibitor. Used to characterize inhibitors of enzymes, receptors, or signaling pathways. Complement to EC50, which characterizes agonist potency. IC50 values are assay-condition-dependent — the same compound can show different IC50 values in different assay systems, which is why comparing IC50 values across different studies requires caution.

IGF-1 (Insulin-like Growth Factor 1)

A growth hormone produced mainly by the liver that drives most of GH's tissue-building effects — the downstream mediator of the GH axis.

Produced primarily in the liver in response to GH stimulation, but also locally in muscle, skin, bone, and scalp as a paracrine signal. Mediates GH's anabolic effects via the IGF-1 receptor (IGF1R) through PI3K/Akt and MAPK signaling. Serum IGF-1 is the standard clinical biomarker for assessing GH axis activity and monitoring GH secretagogue effects. IGF-1-LR3 and IGF-1-DES are synthetic analogs with modified IGFBP binding (LR3) or truncation (DES) that extend activity duration or increase local potency respectively.

immunomodulation

Adjusting the immune system’s activity—not just suppressing it, but tuning it up or down depending on what’s needed.

Immunomodulation encompasses interventions that modify immune function without complete suppression—including upregulation of deficient responses, downregulation of overactive responses, and redirection of immune polarization. Key immunomodulatory peptides on Peptidings include thymosin alpha-1 (T cell maturation, dendritic cell activation), cortistatin (anti-inflammatory via M2 macrophage polarization), VIP (VPAC-mediated immune suppression), and alpha-MSH/MC1R agonists (anti-inflammatory melanocortin signaling). Immunomodulation is distinct from immunosuppression.

In vitro

Research done outside a living organism — in cell cultures, test tubes, or biochemical assays. Useful for understanding mechanism; insufficient on its own to prove clinical benefit.

Latin: "in glass." In vitro data characterizes mechanism, identifies receptor interactions, and generates hypotheses. It cannot account for metabolism, distribution, protein binding, off-target effects in complex tissues, or compensatory physiological responses. On this site, in vitro data alone does not qualify a compound for any clinical evidence tier. "Works in cell culture" is the beginning of a research program, not the end.

In vivo

Research conducted in a living organism — a necessary step between cell culture and human trials.

Latin: "in life." Animal (rodent, primate) in vivo data accounts for absorption, metabolism, distribution, and whole-organism responses that in vitro data cannot capture. Rodent models have produced many false positives for human efficacy — particularly in CNS, oncology, and metabolic disease research. The translation failure rate from rodent to human is high enough that even compelling in vivo data requires cautious interpretation as a predictor of human outcomes. See: human studies gap.

IND (Investigational New Drug)

An FDA application that permits a company to start human clinical trials for a new compound.

IND submission requires sufficient preclinical data (safety pharmacology, toxicology, manufacturing) to justify exposing humans to the compound. IND approval is not drug approval — it is permission to conduct research in humans. A compound with IND status is in Phase I or later clinical trials. IND status means regulatory-standard preclinical data exists; it does not mean the compound is safe or effective.

inflammatory bowel disease

Chronic inflammation of the digestive tract—either Crohn’s disease (anywhere in the GI tract) or ulcerative colitis (colon only).

Inflammatory bowel disease (IBD) encompasses Crohn’s disease and ulcerative colitis, characterized by chronic relapsing intestinal inflammation driven by dysregulated mucosal immunity. Pathogenesis involves genetic susceptibility, environmental triggers, microbiome disruption, and barrier dysfunction. Several peptides are being investigated for IBD: PL-8177 (MC1R agonist, anti-inflammatory), VIP (VPAC1/2-mediated immunosuppression), and KPV (NF-κB suppression in preclinical models). Tight junction restoration (larazotide approach) represents an alternative mechanism.

Innate immune response

The immune system's immediate, non-specific first response to infection or injury — the rapid reaction that buys time while the adaptive system prepares a targeted attack.

Mediated by neutrophils, macrophages, dendritic cells, and NK cells via pattern recognition receptors (Toll-like receptors, NOD receptors) that detect conserved molecular patterns of pathogens. Antimicrobial peptides (LL-37, defensins) are innate immune effectors that directly kill bacteria and recruit immune cells. Distinct from the adaptive immune response, which requires days to develop but produces highly specific and long-lasting immunity.

integrin

A protein on cell surfaces that anchors cells to their surroundings and transmits signals between the outside and inside of the cell.

Integrins are heterodimeric transmembrane receptors (α and β subunits) that mediate cell-extracellular matrix adhesion and bidirectional signaling. They regulate cell migration, proliferation, survival, and differentiation. The αvβ3 and αvβ5 integrins are overexpressed on tumor vasculature and promote angiogenesis. Cilengitide (cyclic RGD pentapeptide) was designed to inhibit αvβ3/αvβ5 integrins to block tumor angiogenesis. Eptifibatide targets the αIIbβ3 integrin on platelets to prevent platelet aggregation in acute coronary syndromes.

Intravitreal injection

A needle injection directly into the gel-filled interior of the eye, performed by an ophthalmologist in a clinical setting — the only effective way to deliver drugs to the retina.

Bypasses the blood-retinal barrier to deliver therapeutic concentrations directly to retinal tissue. Used for anti-VEGF biologics (ranibizumab, bevacizumab, aflibercept) and being studied for somatostatin analogs in diabetic retinopathy. The procedure carries a risk of endophthalmitis (intraocular infection causing potential permanent blindness) even under sterile surgical conditions. This is categorically not a route for self-experimentation. No research peptide context, no community discussion of any compound in this cluster changes that fact.

K

Ki (inhibition constant)

A precise measure of how strongly a molecule grips a receptor — a lower number means tighter binding.

Thermodynamic measure of binding affinity, derived from equilibrium binding experiments. Unlike IC50 (which varies with assay conditions), Ki is a fundamental constant that describes the receptor-ligand interaction. Tight binding (low Ki) is necessary but not sufficient for therapeutic activity — a compound must bind, activate or block the receptor appropriately, and survive long enough in the body to do so.

KPV

A tiny three-amino-acid fragment from the tail end of alpha-MSH that carries the anti-inflammatory activity of the parent peptide — studied for gut and skin inflammation.

Lys-Pro-Val: the C-terminal tripeptide of α-MSH. Retains MC1R-mediated anti-inflammatory activity via NF-κB inhibition with improved stability vs. full-length α-MSH. Studied in colitis and other gut inflammation models. Appears in the Injury Recovery, Gut Health, and Immune Modulation clusters because its anti-inflammatory mechanism crosses all three therapeutic areas. Also has potential for oral delivery because its small size and stability may allow it to survive GI transit — an unusual feature in peptide therapeutics.

L

Ligand

Any molecule that binds to a receptor — includes both the body's own hormones and drugs.

A receptor may have multiple ligands with different selectivity profiles and different effects. The same receptor can be activated (agonist), blocked (antagonist), partially activated (partial agonist), or even constitutively suppressed (inverse agonist) depending on the ligand and its binding characteristics. Understanding ligand-receptor relationships is foundational to understanding why different compounds in the same receptor family (e.g., different melanocortin agonists) produce different physiological effects.

lipid bilayer

The two-layer fat membrane that surrounds every cell—it controls what gets in and out.

The lipid bilayer is the fundamental structural component of all biological membranes, composed of amphipathic phospholipids arranged with hydrophilic heads facing aqueous environments and hydrophobic tails forming the membrane interior. Bacterial and mammalian membranes differ in lipid composition—bacterial membranes are enriched in negatively charged phospholipids, which is why cationic antimicrobial peptides preferentially target bacteria over host cells. This selectivity is the basis for the therapeutic window of antimicrobial peptides like magainins, temporins, and defensins.

Lyophilization

Freeze-drying: the process that turns a liquid peptide into the dry powder you receive in a research vial, making it stable for storage and shipping.

Water is removed by freezing the solution and then reducing pressure, allowing ice to sublimate directly to vapor, leaving a dry powder (lyophilizate). Lyophilized peptides are stable at room temperature for shipping and at refrigerator temperature for months to years, depending on the compound. Solutions are not. Reconstitution with bacteriostatic water restores the peptide to injectable form. See also: bacteriostatic water; reconstitution.

M

macrophage

A large immune cell that engulfs pathogens, cleans up dead cells, and signals other immune cells about what it found.

Macrophages are phagocytic cells of the innate immune system derived from monocytes. They exist on a polarization spectrum: M1 (classically activated, pro-inflammatory, antimicrobial) and M2 (alternatively activated, anti-inflammatory, tissue repair). Macrophage polarization is a key therapeutic target—several peptides modulate this balance. VIP and cortistatin promote M2 polarization; antimicrobial peptides like LL-37 and defensins activate M1 responses. Tumor-associated macrophages (TAMs) in the M2 phenotype promote cancer progression.

macular edema

Swelling in the central part of the retina—the area responsible for sharp, detailed vision—usually caused by leaky blood vessels.

Macular edema is the accumulation of fluid within the macula due to breakdown of the blood-retinal barrier, causing visual acuity loss. It occurs in diabetic retinopathy (diabetic macular edema, DME), retinal vein occlusion, uveitis, and post-surgical inflammation. VEGF-mediated vascular hyperpermeability is the primary driver. Anti-VEGF intravitreal injections are first-line therapy. Peptide-based approaches under investigation include somatostatin analogs (octreotide intravitreal), anti-VEGF peptides, and neurotrophic peptides targeting retinal neurodegeneration.

mast cell

An immune cell packed with granules of histamine and other chemicals—it triggers allergic reactions and plays a role in wound healing.

Mast cells are tissue-resident immune cells containing granules of histamine, heparin, proteases (tryptase, chymase), and cytokines. Degranulation is triggered by IgE crosslinking, complement activation, and neuropeptides (substance P, VIP). Mast cells are central to allergic and anaphylactic responses but also contribute to wound healing, angiogenesis, and host defense. MrgX2 receptor activation by peptides (including cortistatin and some antimicrobial peptides) can trigger non-IgE-mediated mast cell degranulation—a consideration for peptide safety assessment.

Matrix metalloproteinase (MMP)

Enzymes that remodel the structural scaffolding of tissue — essential for wound healing and follicle cycling, but potentially harmful if overactivated.

A family of zinc-dependent endoproteases that degrade ECM components: collagen, gelatin, fibronectin, laminin. MMPs are required for tissue remodeling in wound healing (clearing damaged matrix) and in hair follicle cycling (enabling stem cell migration from the bulge). GHK-Cu upregulates MMP-2, facilitating ECM remodeling in wound contexts. Excessive MMP activity contributes to cartilage destruction in arthritis and tissue invasion in cancer. MMP regulation — not simple activation or suppression — is the therapeutic goal in most healing contexts.

Mechanism of action (MOA)

The specific biological process a compound uses to produce its effects — which receptor it targets, what signal it triggers, what happens downstream.

A well-characterized MOA provides a mechanistic basis for predicting efficacy, selectivity, and potential off-target effects. MOA alone does not establish clinical efficacy — it is the hypothesis that clinical trials test. Many compounds with compelling MOA data in cell culture and animal models fail Phase II/III trials because the mechanism does not translate cleanly to the complex physiology of human disease. On this site, MOA is documented carefully as part of each compound's evidence profile, with explicit distinction between MOA evidence and clinical efficacy evidence.

Melanocortin receptor (MC1R–MC5R)

A family of five related receptors activated by the same group of peptides but located in different tissues — controlling everything from skin color to appetite to stress response.

MC1R: skin melanocytes (pigmentation) and immune cells (anti-inflammatory). MC2R: adrenal cortex (cortisol production; exclusive ACTH receptor). MC3R: hypothalamus (energy sensing, homeostasis). MC4R: hypothalamus (appetite suppression, sexual function, energy expenditure). MC5R: exocrine glands (sebaceous, sweat). Receptor subtype selectivity explains why the same peptide family produces apparently unrelated effects — and why selective agonists (afamelanotide for MC1R; setmelanotide for MC4R) have been developed from the non-selective parent compounds.

melanocyte

The cell in your skin that produces melanin pigment—it determines your skin color and protects against UV radiation.

Melanocytes are neural crest-derived cells located in the basal layer of the epidermis, hair follicles, and uveal tract. They synthesize melanin within melanosomes and transfer pigment to surrounding keratinocytes. Melanocyte activity is regulated primarily through MC1R signaling: alpha-MSH binding activates cAMP/MITF/tyrosinase cascade, increasing eumelanin production. Afamelanotide, melanotan I, and alpha-MSH analogs all target this pathway. Melanocyte dysfunction underlies vitiligo; melanocyte malignant transformation produces melanoma.

Melanogenesis

The biological process that makes melanin — the pigment that determines skin color and protects skin from UV radiation.

Initiated by MC1R activation, which increases cAMP, activating PKA, which activates MITF (the master melanocyte transcription factor). MITF upregulates tyrosinase, TRP-1, and TRP-2 — the three enzymes that convert tyrosine into eumelanin. The pathway is specific to melanocytes and is the molecular basis for both normal tanning and the therapeutic mechanism of afamelanotide in EPP.

membrane disruption

How most antimicrobial peptides kill bacteria—they punch holes in or dissolve bacterial cell membranes.

Membrane disruption is the primary bactericidal mechanism of most antimicrobial peptides (AMPs). Cationic AMPs are electrostatically attracted to negatively charged bacterial membranes (phosphatidylglycerol, lipopolysaccharide), then insert and disrupt membrane integrity through several models: barrel-stave (transmembrane pores), toroidal pore (lipid-peptide pores), or carpet model (surface dissolution). This mechanism is difficult for bacteria to develop resistance against because it targets fundamental membrane composition rather than specific protein targets.

Meta-analysis

A statistical method that combines results from many individual studies into one overall estimate — the most powerful form of clinical evidence when done well.

Requires that pooled studies address the same research question with comparable designs, populations, and outcomes. When studies are methodologically heterogeneous, pooling them produces a statistically precise but potentially misleading estimate (garbage in, garbage out). The Cochrane meta-analysis of thymosin alpha-1 for HBV found insufficient evidence for overall survival benefit — a meaningful negative finding from a systematic synthesis of available data, not just an absence of studies.

metastasis

The spread of cancer from its original site to distant organs—the primary reason cancer becomes life-threatening.

Metastasis is the multistep process by which cancer cells disseminate from the primary tumor to establish secondary tumors at distant sites. Steps include local invasion, intravasation, circulation, extravasation, and colonization. Integrins, matrix metalloproteinases, and VEGF-mediated angiogenesis are key mediators. Anti-metastatic peptide strategies include integrin inhibition (cilengitide targeting αvβ3), CXCR4 antagonism (motixafortide disrupting CXCL12-mediated homing), and anti-angiogenic approaches.

MIC (minimum inhibitory concentration)

The lowest concentration of an antimicrobial compound that stops visible bacterial growth overnight.

MIC is the standard laboratory measure of antimicrobial potency, determined by serial twofold dilutions in broth or agar. Lower MIC values indicate greater potency. MIC alone does not predict clinical efficacy—it must be interpreted alongside pharmacokinetic parameters (Cmax/MIC ratio, AUC/MIC ratio, time above MIC) that determine whether achievable tissue concentrations can sustain bactericidal or bacteriostatic activity in vivo.

MITF (Microphthalmia-Associated Transcription Factor)

The master switch for melanocyte identity — turns on the entire melanin-production machinery.

MITF expression is induced by MC1R–cAMP–PKA signaling. Once activated, MITF binds to the promoters of tyrosinase, TRP-1, and TRP-2 — the key enzymes of eumelanin biosynthesis — and also regulates genes involved in melanocyte survival and differentiation. Loss-of-function MITF mutations produce Waardenburg syndrome (depigmentation, hearing loss). This centrality makes MITF a useful pharmacological endpoint for measuring MC1R pathway activation.

mTOR

A master switch inside cells that controls growth and metabolism—when nutrients are abundant it says ‘grow,’ when they’re scarce it says ‘conserve.’

The mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that integrates nutrient, energy, and growth factor signals to regulate cell growth, proliferation, autophagy, and metabolism. mTOR complex 1 (mTORC1) promotes protein synthesis and inhibits autophagy; mTOR complex 2 (mTORC2) regulates cytoskeletal organization and Akt activation. mTOR signaling intersects with multiple peptide pathways: IGF-1/insulin signaling activates mTOR; caloric restriction and autophagy-promoting peptides inhibit it. mTOR is a central node in aging biology research.

N

NDA (New Drug Application)

The formal submission to the FDA asking for permission to sell a drug — requires years of clinical trial data proving safety and efficacy.

Comprehensive submission including Phase I–III clinical data, manufacturing specifications, proposed labeling, and risk management plans. FDA review typically takes 6–12 months for standard applications. Approval grants the right to market for the specific approved indication only. A successful NDA means the FDA concluded the benefit outweighs the risk for a defined patient population — not that the compound is safe for any use or population.

NEP (neutral endopeptidase)

An enzyme on cell surfaces that chops up many signaling peptides—including VIP and natriuretic peptides—limiting how long they can act.

Neutral endopeptidase (NEP/CD10, neprilysin, EC 3.4.24.11) is a zinc-dependent metalloprotease expressed on cell surfaces throughout the body. It cleaves numerous bioactive peptides: natriuretic peptides (ANP, BNP), VIP, substance P, enkephalins, amyloid-beta, and bradykinin. NEP inhibition (sacubitril in Entresto) is an established cardiovascular therapy. For peptide therapeutics, NEP represents a major degradation pathway that limits half-life and bioavailability of subcutaneously or intravenously administered peptides.

neuroendocrine tumor

A tumor arising from hormone-producing cells—often slow-growing but capable of secreting excess hormones that cause symptoms.

Neuroendocrine tumors (NETs) arise from enterochromaffin and other hormone-secreting cells, most commonly in the gastrointestinal tract and pancreas (GEP-NETs). They are classified by grade (Ki-67 proliferation index) and differentiation. Functional NETs secrete bioactive substances causing clinical syndromes (carcinoid syndrome from serotonin, insulinoma, gastrinoma). Somatostatin analogs (octreotide, lanreotide) are first-line therapy for symptom control and tumor growth inhibition via SSTR2 activation. Lutathera (¹⁷⁷Lu-DOTATATE) uses somatostatin receptor targeting for peptide receptor radionuclide therapy.

neuroinflammation

Inflammation inside the brain and nervous system—a common driver of neurodegenerative diseases from Alzheimer’s to Parkinson’s.

Neuroinflammation involves activation of microglia and astrocytes within the central nervous system, producing pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), reactive oxygen species, and complement factors. Chronic neuroinflammation contributes to neurodegeneration in Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis. Several peptides demonstrate anti-neuroinflammatory properties: cortistatin (suppresses microglial activation), PACAP (neuroprotective via PAC1), orexin (modulates neuroinflammatory cascades), and galanin (regulates hippocampal inflammation).

neuropeptide

A small protein used by neurons to communicate—unlike fast electrical signals, neuropeptides provide slower, longer-lasting modulation of brain circuits.

Neuropeptides are small proteins (3–100 amino acids) synthesized in neuronal cell bodies, packaged in dense-core vesicles, and released from axon terminals to modulate neural circuits. Unlike classical neurotransmitters, neuropeptides act at nanomolar concentrations, have longer durations of action, and signal through GPCRs. Key neuropeptides on Peptidings include orexin (wakefulness), galanin (sleep/cognition), NPY (appetite/stress), cortistatin (sleep), PACAP (neuroprotection), ghrelin (appetite/GH), and cholecystokinin (satiety). Neuropeptide signaling is volumetric—not confined to synapses.

neuroprotection

Protecting nerve cells from damage or death—a therapeutic goal in conditions from stroke to Alzheimer’s to traumatic brain injury.

Neuroprotection encompasses mechanisms that preserve neuronal structure and function against injury, degeneration, or toxic insult. Neuroprotective strategies target excitotoxicity (glutamate), oxidative stress, mitochondrial dysfunction, neuroinflammation, and apoptotic cascades. Several peptides on Peptidings demonstrate neuroprotective properties in preclinical models: PACAP (anti-apoptotic via PAC1/VPAC), NAD+ precursors, cortistatin (anti-neuroinflammatory), and orexin (neuronal survival signaling). Translation from preclinical neuroprotection to clinical benefit has historically been challenging.

neurotrophic keratitis

A degenerative eye condition where damaged corneal nerves can’t maintain the cornea’s surface—leading to persistent ulcers that won’t heal.

Neurotrophic keratitis (NK) is a degenerative corneal disease caused by impaired trigeminal innervation, resulting in reduced corneal sensitivity, epithelial breakdown, and impaired healing. Classified in three stages: Stage 1 (punctate keratopathy), Stage 2 (persistent epithelial defect), Stage 3 (corneal ulcer with stromal melting). Cenegermin (recombinant human NGF) is the first FDA-approved treatment, directly addressing the neurotrophic deficit. RGN-259 (thymosin beta-4) and other peptide-based eye drops are in development for corneal nerve regeneration.

neutrophil

The most common white blood cell and your immune system’s first responder—it arrives fast, kills pathogens, and dies quickly.

Neutrophils are the most abundant circulating leukocytes and primary effectors of acute innate immunity. They kill pathogens through phagocytosis, degranulation (releasing antimicrobial peptides including alpha-defensins HNP-1–4), reactive oxygen species production, and neutrophil extracellular trap (NET) formation. Neutrophil alpha-defensins constitute 5–7% of total neutrophil protein. Excessive neutrophil activation contributes to inflammatory tissue damage; insufficient neutrophil function increases infection susceptibility.

NF-κB (Nuclear Factor kappa-light-chain-enhancer of activated B cells)

A key "inflammation switch" inside cells — when it's activated, cells produce inflammatory signals; when it's blocked, inflammation is reduced.

A transcription factor family that controls expression of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-8), adhesion molecules, and immune mediators. Many anti-inflammatory peptides (KPV, α-MSH, PL-8177, BPC-157) work partly through NF-κB inhibition. NF-κB is also activated by cancer-promoting signals and by viral infections — which is why compounds that modulate it have research relevance across immune, oncology, and infectious disease contexts.

Nociception

The nervous system's detection and processing of painful stimuli — what turns tissue damage into the experience of pain.

Substance P is a key nociceptive neurotransmitter via NK1 receptor activation on pain-processing neurons in the dorsal horn of the spinal cord. Ziconotide (Prialt), derived from cone snail venom, blocks N-type calcium channels in spinal pain-transmission neurons — the most potent analgesic peptide drug approved for intrathecal use in refractory chronic pain (including cancer pain and failed back surgery syndrome). Its approval is specifically for intrathecal (spinal) delivery, not systemic.

Non-inferiority trial

A trial designed to show a new treatment is "not much worse" than an existing standard — sometimes a legitimate scientific goal, sometimes a way to get approval without proving superiority.

The margin for "not much worse" (the non-inferiority margin) is set in advance and needs clinical justification. Pexiganan's Phase III failure for diabetic foot infections was a regulatory decision about whether the non-inferiority design met the evidentiary standard — the FDA advisory committee actually voted to approve it, but the FDA rejected the NDA because it felt the non-inferiority design wasn't the right test. This is a regulatory standard dispute, not a safety failure or evidence of inefficacy.

O

Off-label use

Using an approved drug for a purpose not listed in its official FDA approval — legal when prescribed by a physician, but not evidence of efficacy for the off-label use.

Off-label prescribing is common and sometimes evidence-based, sometimes not. Key distinctions: (1) Off-label use of an approved drug is not the same as community use of a research peptide with no approval at all. (2) FDA approval for indication X does not establish efficacy for indication Y, even if the mechanism seems related. (3) The compound being off-patent and widely prescribed off-label does not mean Phase III evidence exists for the off-label indication. These distinctions are frequently blurred in community discourse.

open-label study

A study where both the researchers and participants know who is getting the treatment—less rigorous than blinded trials, but still useful.

In open-label studies, treatment allocation is known to investigators and participants. This design is susceptible to placebo effects, observer bias, and expectation bias. Open-label studies are appropriate for Phase I safety evaluation, dose-finding, and conditions where blinding is impractical. However, open-label efficacy data must be interpreted cautiously—particularly for subjective endpoints like pain, fatigue, or well-being. Many peptide compounds have only open-label human data, making honest assessment of evidence quality critical.

Orphan drug designation

An FDA status for drugs targeting rare diseases — comes with development incentives and market exclusivity, but is not itself an approval.

For diseases affecting fewer than 200,000 people in the US. Incentives include 50% tax credit on clinical trial costs, waived FDA fees, and 7 years of market exclusivity after approval. Afamelanotide (Scenesse for EPP) and setmelanotide (Imcivree for POMC/PCSK1/LEPR-deficiency obesity) both received orphan drug designation before approval. Designation is granted based on disease prevalence and unmet need — it does not indicate the drug works.

osteoblast

The cell that builds new bone—it lays down the mineral matrix that makes bone hard and strong.

Osteoblasts are bone-forming cells derived from mesenchymal stem cells. They synthesize and secrete osteoid (unmineralized bone matrix composed primarily of type I collagen), which subsequently mineralizes with hydroxyapatite crystals. Osteoblast activity is stimulated by PTH (intermittent exposure), PTHrP, and mechanical loading. Teriparatide and abaloparatide exert their anabolic bone-building effects primarily by stimulating osteoblast differentiation and activity, increasing bone formation rate above resorption rate.

osteoclast

The cell that breaks down bone—it dissolves old or damaged bone so osteoblasts can replace it with new tissue.

Osteoclasts are large, multinucleated cells derived from monocyte/macrophage lineage that resorb bone through acidification and proteolytic enzyme secretion (cathepsin K, TRAP, MMPs). Osteoclast activity is regulated by RANK/RANKL/OPG signaling. Excessive osteoclast activity relative to osteoblast formation drives osteoporosis and Paget’s disease. Calcitonin directly inhibits osteoclast function via calcitonin receptors on the osteoclast surface—the basis for its therapeutic use in bone diseases.

osteoporosis

A condition where bones become so thin and fragile that they break easily—especially at the spine, hip, and wrist.

Osteoporosis is characterized by reduced bone mineral density (T-score ≤ −2.5) and deteriorated bone microarchitecture, resulting in increased fracture risk. Bone remodeling imbalance—where osteoclast resorption exceeds osteoblast formation—drives progressive bone loss. Peptide therapeutics for osteoporosis include anabolic agents (teriparatide, abaloparatide) that stimulate bone formation, and calcitonin that inhibits osteoclast resorption. The distinction between antiresorptive and anabolic mechanisms is clinically significant.

P

p-value

The probability that the study's result could have occurred by chance alone — conventionally, below 5% (p

A p-value does not tell you the size of the effect, whether the result is clinically important, or whether the finding will replicate. A study with n = 10,000 can produce p

Paneth cell

A specialized cell at the base of intestinal crypts that secretes antimicrobial peptides—your gut’s built-in defense system.

Paneth cells are secretory epithelial cells located at the base of small intestinal crypts. They produce and secrete alpha-defensins (HD-5, HD-6), lysozyme, phospholipase A2, and REG3 lectins, maintaining antimicrobial defense at the crypt-villus interface. Paneth cell dysfunction is implicated in Crohn’s disease pathogenesis. Alpha-defensins from Paneth cells regulate intestinal microbiome composition and provide innate immune defense against enteric pathogens.

Paracrine signaling

When a cell releases a signal that acts on neighboring cells in the same tissue — local communication, not systemic circulation.

Distinct from endocrine signaling (travels via bloodstream to act on distant cells) and autocrine signaling (acts on the same cell). IGF-1 produced locally in scalp dermis acts on adjacent follicle keratinocytes and dermal papilla cells parabolically — this local production, not circulating IGF-1 levels, is the therapeutically relevant signal for the follicle. This is why intramuscular IGF-1 injection to raise systemic levels is a different pharmacological intervention from local scalp IGF-1 delivery.

PEGylation

Attaching chains of PEG (a inert polymer) to a peptide to make it last longer in the body and be less likely to cause an immune reaction.

Polyethylene glycol attachment reduces renal clearance (by increasing molecular size), decreases immunogenicity (by shielding the peptide from immune recognition), and extends plasma half-life. PEG-MGF is pegylated mechano growth factor, developed to extend MGF's plasma half-life for potential systemic delivery. Limitations: PEGylation can reduce receptor binding affinity by sterically hindering the active binding site, and theoretical concerns about PEG accumulation with chronic dosing exist. PEG-related hypersensitivity reactions have been documented with pegylated biologics.

peptidase

An enzyme that breaks down peptides by cutting their amino acid chains—the main reason most peptides have short half-lives.

Peptidases (proteases) cleave peptide bonds through hydrolysis. They are classified as endopeptidases (cleave internal bonds) or exopeptidases (remove terminal residues). Key peptidases affecting therapeutic peptides include DPP-IV (cleaves N-terminal dipeptides from GLP-1, GIP), NEP/CD10 (degrades natriuretic peptides, VIP, substance P), ACE (cleaves angiotensin I, bradykinin), and aminopeptidases (degrade N-terminal residues). Peptide drug design frequently incorporates modifications—D-amino acids, cyclization, PEGylation—specifically to resist peptidase degradation.

Peptide

A small protein — a chain of amino acids, shorter than a full protein, that acts as a biological signal or structural molecule.

By convention, molecules with fewer than ~50 amino acids are called peptides; larger molecules are proteins — though this boundary is approximate. Peptides can be linear or cyclic, naturally occurring or synthetic, and range from 2 amino acids (dipeptides) to 49 amino acids. Most peptide hormones are 3–40 amino acids. Shorter peptides are more susceptible to proteolytic degradation; modifications like cyclization and D-amino acid substitution are used to improve stability. Not all molecules marketed as "peptides" meet this definition — verify structure when evaluating claims.

Peptide bond

The chemical link that joins amino acids together to form a peptide — and the bond that digestive enzymes break down when you swallow a peptide.

Formed between the carboxyl group of one amino acid and the amino group of the next, releasing a water molecule. The backbone of all peptides and proteins. Peptide bonds are hydrolyzed by peptidases (proteases) — which is why oral bioavailability of peptides is near zero. The peptide bond's vulnerability to hydrolysis is the fundamental pharmacological challenge of peptide drug delivery.

Pharmacodynamics (PD)

What the compound does to the body — how it works, what it activates, what changes as a result.

Covers receptor binding, mechanism of action, downstream biological effects, and dose-response relationships. A complete pharmacological characterization requires both pharmacodynamics (what the drug does) and pharmacokinetics (what the body does to the drug). PD data from in vitro assays tells you the compound can activate a receptor; PD data from human trials tells you what happens when it does so in a complex physiological system.

Pharmacokinetics (PK)

What the body does to the compound — how it's absorbed, where it goes, how it's broken down, and how it's eliminated.

Encompasses absorption, distribution, metabolism, and excretion (ADME). Key PK parameters: bioavailability, Tmax (time to peak plasma concentration), Cmax (peak concentration), half-life, volume of distribution, and clearance. Animal PK does not reliably predict human PK — metabolic enzymes, protein binding, and renal/hepatic clearance rates all differ between species. Human PK data is essential for dosing design; community dosing protocols based on animal PK are working with incomplete information.

pharmacovigilance

The ongoing monitoring of drug safety after approval—watching for rare or delayed side effects that clinical trials were too small or short to detect.

Pharmacovigilance is the science and practice of detecting, assessing, understanding, and preventing adverse effects of pharmaceutical products throughout their lifecycle. Post-marketing surveillance (Phase IV) captures rare adverse events that pre-approval trials (typically thousands of patients, months to years of follow-up) cannot detect. For peptide therapeutics, pharmacovigilance is particularly important for identifying long-term effects of chronic receptor activation, immunogenicity (anti-drug antibody formation), and off-target effects at receptors with broad physiological roles.

Phase I trial

The first time a drug is tested in humans—focused on safety and dosing, not whether it works.

Phase I clinical trials evaluate safety, tolerability, pharmacokinetics, and pharmacodynamics in a small number of subjects (typically 20–80), usually healthy volunteers. Primary objectives are determining maximum tolerated dose (MTD), dose-limiting toxicities (DLTs), and pharmacokinetic parameters. Phase I does not assess efficacy. For peptide therapeutics, Phase I trials establish injection site tolerance, immunogenicity potential, and metabolic stability in humans. A compound reaching Phase I has completed preclinical toxicology and obtained an IND.

Phase II trial

The stage where a drug is first tested for whether it actually works—in a few hundred patients with the target condition.

Phase II trials evaluate preliminary efficacy and optimal dosing in 100–300 patients with the target condition. Phase IIa is exploratory (proof-of-concept); Phase IIb is dose-ranging. These trials typically use randomized, controlled designs with clinical endpoints. Approximately 33% of compounds that enter Phase II progress to Phase III. For peptide therapeutics, Phase II establishes whether the pharmacological mechanism observed preclinically translates to clinical benefit at tolerable doses.

Phase III trial

The final and largest stage of testing—hundreds to thousands of patients, designed to prove a drug works well enough for regulatory approval.

Phase III trials are pivotal, large-scale (300–3,000+ patients), randomized controlled studies designed to confirm efficacy and monitor adverse reactions. Results form the primary basis for regulatory approval (NDA/BLA submission). Phase III trials require pre-specified primary endpoints, statistical powering, and often include active comparator arms. Approximately 50–60% of Phase III trials succeed. For the peptide compounds on Peptidings, Phase III completion distinguishes Tier 1 (Approved Drug) and advanced Tier 2 compounds from earlier-stage research.

phosphorylation

The cell’s on/off switch—adding a phosphate group to a protein changes its shape and activates (or deactivates) it.

Phosphorylation is the enzymatic addition of a phosphate group (PO₄³⁻) to serine, threonine, or tyrosine residues on proteins, catalyzed by kinases. This post-translational modification alters protein conformation, activity, localization, or interaction partners. Dephosphorylation by phosphatases reverses the signal. Most peptide receptor signaling cascades—GPCR/cAMP/PKA, receptor tyrosine kinase/MAPK, JAK-STAT—propagate through sequential phosphorylation events. Understanding which phosphorylation cascades a peptide activates is essential to predicting both therapeutic effects and off-target risks.

PI3K/Akt pathway

A major cell survival and growth signaling relay — activated by IGF-1, KGF, and other growth factors to tell cells to grow, divide, and not die.

Phosphoinositide 3-kinase (PI3K) is activated by growth factor receptors including IGF1R and KGFR. PI3K produces PIP3, which recruits and activates Akt (protein kinase B). Akt phosphorylates numerous targets including mTOR (protein synthesis), FOXO transcription factors (apoptosis suppression), and glycogen synthase kinase-3 (glycogen metabolism). This pathway drives cell survival, proliferation, and metabolic activity — and is constitutively activated in many cancers, which is why growth factors that stimulate it require careful consideration in oncology contexts.

placebo-controlled

A study where some participants receive an inactive treatment—the gold standard for proving a drug actually works beyond the placebo effect.

Placebo-controlled trials compare an active treatment against an inert control, isolating the treatment’s pharmacological effect from placebo response, natural disease course, and regression to the mean. For peptide research, placebo control is especially important because injection-based delivery can produce substantial placebo responses in subjective endpoints. A compound with only open-label data has not demonstrated efficacy above placebo—this distinction is critical for honest evidence evaluation.

POMC (Pro-opiomelanocortin)

A large protein precursor that gets cut up into multiple smaller active peptides — including ACTH, alpha-MSH, and beta-endorphin — depending on which tissue is doing the cutting.

241-amino acid precursor cleaved by tissue-specific prohormone convertases (PC1, PC2). In the pituitary anterior lobe: PC1 produces ACTH and β-endorphin. In the intermediate lobe, hypothalamus, and skin: PC2 further cleaves ACTH to produce α-MSH. In the gut: produces β-endorphin and other fragments. Genetic defects in POMC, PCSK1 (encodes PC1), or LEPR (leptin receptor, which regulates POMC neuron activity) impair MC4R signaling downstream and produce severe early-onset obesity — the specific genetic context for setmelanotide's approval.

Preclinical

Research done before human trials — in cells and animals. A necessary step, but not proof that something works or is safe in people.

Regulatory agencies require preclinical evidence (safety pharmacology, toxicology, mechanism characterization) before permitting Phase I human trials. Preclinical evidence establishes that a compound has a plausible mechanism, is not immediately lethal in animals at relevant doses, and has sufficient rationale to justify human exposure. It does not establish human efficacy or safety. The failure rate from promising preclinical results to successful human trials is high — estimates range from 85–95% of compounds that enter Phase I never reaching approval.

Primary endpoint

The main thing a clinical trial was designed to measure — the result that actually determines whether the trial succeeded or failed.

A trial that fails its primary endpoint has not demonstrated efficacy for that indication, regardless of secondary endpoint results. Secondary endpoints in a failed trial are hypothesis-generating, not confirmatory — they can justify designing a new trial, but they do not rescue the primary failure. This distinction is frequently ignored in community reporting of trial results: "the trial failed but secondary endpoints were promising" is often presented as evidence of efficacy. It is not.

prodrug

A medication that is inactive when administered but gets converted into the active drug inside your body.

A prodrug is a pharmacologically inactive compound that undergoes biotransformation in vivo to release the active drug. Prodrug strategies for peptides address bioavailability, stability, or targeting challenges. Palopegteriparatide (TransCon PTH) is a peptide prodrug: PTH(1-34) is conjugated to a PEG carrier via a transient linker that slowly hydrolyzes, providing sustained PTH release from a weekly injection. Melflufen uses a peptide-conjugated prodrug approach where aminopeptidase cleavage releases the cytotoxic alkylating agent preferentially inside tumor cells.

prospective study

A study that follows people forward in time from enrollment—stronger evidence than looking back at old records.

Prospective studies recruit subjects and follow them forward through time, collecting data on exposures and outcomes as they occur. This design minimizes recall bias and allows controlled data collection. Prospective studies include clinical trials (interventional) and cohort studies (observational). When evaluating peptide research, prospective human data is substantially more reliable than retrospective analyses, case reports, or anecdotal community reports.

proteolysis

The breaking down of proteins and peptides into smaller pieces—the reason injectable peptides don’t last long in your body.

Proteolysis is the hydrolytic cleavage of peptide bonds, catalyzed by proteases/peptidases. For therapeutic peptides, proteolysis is the dominant elimination pathway determining plasma half-life. Proteolytic degradation occurs in plasma (by circulating proteases), at injection sites (by tissue peptidases), in the liver (first-pass metabolism for oral peptides), and at target tissues. Resistance to proteolysis is the central pharmacokinetic challenge in peptide drug design—addressed through chemical modifications including D-amino acid substitution, cyclization, PEGylation, and fatty acid conjugation.

R

radionuclide therapy

Cancer treatment that delivers radioactive atoms directly to tumor cells using a targeting molecule—like a guided missile with a radioactive warhead.

Peptide receptor radionuclide therapy (PRRT) uses radiolabeled peptides to deliver targeted radiation to tumors expressing specific receptors. Lutathera (¹⁷⁷Lu-DOTATATE) targets SSTR2-positive neuroendocrine tumors, delivering beta radiation selectively to tumor cells while sparing normal tissue. The NETTER-1 trial demonstrated significant progression-free survival benefit. PRRT represents a paradigm where peptide receptor specificity enables precision oncology—the peptide provides targeting, the radionuclide provides cytotoxicity.

RCT (Randomized Controlled Trial)

The gold standard study design for establishing whether a treatment actually works — participants are randomly assigned to treatment or control, and the results are compared.

Randomization minimizes selection bias and allows causal inference — the gold standard distinction from observational studies, which can only show association. Double-blind RCTs (neither participants nor assessors know group assignment) further reduce measurement bias. Sample size must be adequate to detect the expected effect — underpowered RCTs produce unreliable positive and negative results. For most compounds on this site, the absence of an RCT is the most important single fact about the evidence record.

Receptor occupancy

The proportion of a receptor type that is bound by a compound at a given concentration — not the same as biological effect.

A compound may produce near-maximum biological response at 10–20% receptor occupancy, because signal amplification downstream multiplies the initial receptor activation. Conversely, a compound with high receptor occupancy may produce little effect if the receptor-effector coupling is inefficient. Receptor reserve, signal amplification, and desensitization all mediate the relationship between receptor occupancy and observed effect.

Receptor selectivity

How specifically a compound targets one receptor type versus related receptors — higher selectivity generally means fewer unintended effects.

Receptor selectivity is pharmacologically critical in families with multiple subtypes. Afamelanotide (MC1R-selective) has a different profile than Melanotan II (non-selective MC1R/MC3R/MC4R/MC5R agonist) — the non-selectivity explains why Melanotan II produces both tanning and spontaneous erections simultaneously. Higher selectivity generally means fewer off-target effects; in some cases, intentional multi-receptor engagement is the therapeutic strategy (tirzepatide as GLP-1/GIP dual agonist).

Reconstitution

Dissolving a freeze-dried peptide powder in sterile water to produce an injectable solution — the step that converts what arrived in the vial into what goes in the syringe.

Inject bacteriostatic water slowly down the side of the vial — not directly onto the powder — and allow to dissolve without agitation. Do not shake; swirl gently if needed. The volume of bacteriostatic water you add determines the resulting concentration, which determines the injection volume per dose. Calculate this carefully before drawing up a dose. Reconstituted peptides should be stored refrigerated (2–8°C / 35–46°F) and used within the timeframe recommended for that compound (typically 2–4 weeks for bacteriostatic water reconstitutions).

RGD motif

A three-amino-acid sequence (Arg-Gly-Asp) that acts as a universal ‘docking code’ recognized by integrins on cell surfaces.

The RGD (arginine-glycine-aspartate) tripeptide sequence is the minimal recognition motif for several integrin receptors, including αvβ3, αvβ5, and αIIbβ3. Found in extracellular matrix proteins (fibronectin, vitronectin, fibrinogen), RGD mediates cell adhesion, migration, and survival signaling. Cilengitide is a cyclic RGD pentapeptide designed to competitively inhibit αvβ3/αvβ5 integrin binding. Eptifibatide contains a KGD (lysine-glycine-aspartate) motif that selectively blocks αIIbβ3 on platelets. RGD-based targeting is also used in radiolabeled peptide imaging.

S

Second messenger

A molecular relay inside cells — when a peptide binds a surface receptor, the second messenger carries the signal from the membrane into the interior of the cell.

Common second messengers: cAMP (produced by adenylyl cyclase; used by melanocortin, GLP-1, VIP, and secretin receptors), IP3/DAG (produced by phospholipase C; used by Gq-coupled receptors), and Ca²⁺ (calcium ion). Second messengers amplify the original signal — one receptor-ligand binding event can produce thousands of cAMP molecules, which activate thousands of PKA enzyme molecules. This amplification explains why peptide hormones are biologically potent at nanomolar concentrations.

slow-wave sleep

The deepest stage of sleep—when your body does most of its physical repair work and your brain consolidates memories.

Slow-wave sleep (SWS, N3 stage) is characterized by high-amplitude, low-frequency (0.5–4 Hz) delta EEG activity. SWS is the period of maximal growth hormone secretion, tissue repair, immune function enhancement, and memory consolidation. Cortistatin is the most potent known endogenous promoter of slow-wave sleep. Galanin promotes sleep onset via hypothalamic circuits. GH secretagogue timing is typically recommended before bedtime to coincide with the natural nocturnal GH pulse during SWS.

Somatostatin

A broad inhibitory peptide produced in the brain, pancreas, and gut that suppresses growth hormone, insulin, glucagon, and multiple GI hormones.

Endogenous 14-amino acid peptide (also a 28-amino acid form) produced in the hypothalamus, pancreatic delta cells, and GI tract. Acts on five receptor subtypes (SSTR1–5). Octreotide and lanreotide are synthetic somatostatin analogs with preferential SSTR2/SSTR5 selectivity and extended half-life. Their therapeutic utility in neuroendocrine tumors derives from SSTR2 overexpression on NET cells. Their investigational use in diabetic retinopathy derives from SSTR2 expression in retinal tissue and somatostatin's ability to suppress local IGF-1 and VEGF production.

somatotroph

The specific cell in your pituitary gland that makes and releases growth hormone.

Somatotrophs are acidophilic cells constituting approximately 50% of anterior pituitary hormone-producing cells. They synthesize, store, and secrete growth hormone (GH) in a pulsatile pattern regulated by opposing hypothalamic signals: GHRH (stimulatory via GHRH-R/cAMP) and somatostatin (inhibitory via SSTR2/SSTR5). GH secretagogues (ipamorelin, GHRP-2, GHRP-6, MK-677) stimulate somatotrophs via GHS-R1a. Somatotroph adenomas cause acromegaly—the primary indication for somatostatin analog therapy (octreotide, lanreotide).

SSTR2 (Somatostatin Receptor 2)

The somatostatin receptor subtype that octreotide and lanreotide bind most strongly — and the reason those drugs work in neuroendocrine tumors.

SSTR2 is overexpressed on neuroendocrine tumor cells at levels far exceeding normal tissue — which is both the therapeutic mechanism (SSTR2 activation inhibits tumor cell proliferation and hormone secretion) and the diagnostic mechanism (SSTR2 is the target for somatostatin receptor scintigraphy and DOTATATE PET/CT). The same SSTR2 scaffold in octreotide was repurposed as a targeting vector for Lutathera (lutetium-177 DOTATATE), linking tumor-targeting SSTR2 binding to radioisotope-mediated cell killing. SSTR2 is also expressed in retinal tissue, the basis for the diabetic retinopathy application.

stem cell mobilization

Pushing stem cells out of the bone marrow into the bloodstream so they can be collected for transplantation.

Stem cell mobilization is the pharmacological release of hematopoietic stem and progenitor cells (HSPCs) from bone marrow into peripheral blood for collection via apheresis. The CXCR4/CXCL12 axis is the primary retention mechanism—CXCR4 on HSPCs binds CXCL12 (SDF-1) on marrow stromal cells. Motixafortide (BL-8040) is a CXCR4 antagonist that disrupts this interaction, mobilizing stem cells for autologous transplantation. G-CSF is the standard mobilization agent; CXCR4 antagonists provide rapid, complementary mobilization.

Subcutaneous (SC)

The tissue layer just beneath the skin — where most research peptides are injected, using a short needle at approximately a 45-degree angle.

Subcutaneous injection delivers the compound into the fat and connective tissue between skin and muscle, where it is absorbed via local capillaries and lymphatic vessels. Absorption is slower and more sustained than intramuscular injection. Common SC injection sites: abdomen (preferred — consistent absorption), lateral thigh, outer upper arm. Rotate sites to prevent lipodystrophy. SC needles are typically 25–31 gauge, 8–12mm length.

Surrogate endpoint

A measurable proxy used in a clinical trial instead of the actual health outcome you care about — useful for speed, but not always a reliable stand-in.

Examples: using IGF-1 levels as a surrogate for GH axis activity; using HbA1c as a surrogate for diabetes complications; using tumor size reduction as a surrogate for overall survival. Surrogate endpoints can accelerate drug development, but a drug that improves the surrogate does not necessarily improve the clinical outcome if the surrogate is not causally linked to that outcome. The FDA has approved drugs based on surrogate endpoints that later failed to improve survival (accelerated approval pathway), which is why surrogate endpoint trials are viewed as preliminary evidence pending confirmatory outcomes data.

Systematic review

A comprehensive, structured review of all available evidence on a specific question — more rigorous and less biased than a traditional review article.

Uses pre-specified, transparent criteria for identifying studies (search strategy), assessing their quality (risk of bias tools), and synthesizing their results. When combined with meta-analysis, a systematic review of RCTs represents the highest tier in the evidence hierarchy. Systematic reviews can also conclude that evidence is insufficient to make a determination — which is itself a meaningful scientific finding, not a null result.

T

T cell

An immune cell that learns to recognize specific threats and either kills infected cells directly or coordinates the broader immune response.

T lymphocytes mature in the thymus and mediate adaptive cellular immunity. CD4+ helper T cells coordinate immune responses by activating B cells and macrophages; CD8+ cytotoxic T cells directly kill virus-infected or tumor cells. Regulatory T cells (Tregs) suppress excessive immune activation. Several peptide therapeutics (thymosin alpha-1, thymalin) target T cell maturation and function. T cell exhaustion in chronic disease and cancer is a key target of immunotherapy research.

Telogen

The resting phase of the hair follicle — the growth phase is over, the follicle is dormant, and the hair is preparing to shed.

Typically lasts 3–5 months. At any given time, roughly 10–15% of scalp follicles are in telogen. Telogen effluvium is diffuse hair shedding triggered by a stressor (physical illness, surgery, severe psychological stress, rapid weight loss, childbirth) that prematurely drives a large cohort of follicles into telogen simultaneously. The shedding appears 2–3 months after the triggering event — the lag time is the remaining telogen phase before those follicles shed. Most telogen effluvium resolves spontaneously once the stressor is removed.

TGF-beta

A signaling protein with dual personality—it suppresses inflammation and promotes tissue repair, but can also drive fibrosis and help tumors evade the immune system.

Transforming growth factor beta (TGF-β) is a pleiotropic cytokine superfamily member that regulates cell growth, differentiation, apoptosis, and immune function. TGF-β signals through SMAD-dependent and SMAD-independent pathways. In early-stage disease it acts as a tumor suppressor; in advanced cancer it promotes epithelial-mesenchymal transition, metastasis, and immune evasion. In tissue repair, TGF-β drives fibroblast activation and collagen deposition—beneficial in wound healing but pathological in fibrosis.

Therapeutic index

The gap between the dose that helps and the dose that harms — a wide gap means more room for error; a narrow gap means precision dosing is critical.

Formally: the ratio between the toxic dose (TD50) and the effective dose (ED50). Acetaminophen has a relatively narrow therapeutic index; penicillin has a wide one. For most research peptides, the therapeutic index in humans is genuinely unknown because systematic dose-ranging and toxicology studies in humans do not exist. Community dosing protocols that use animal-derived doses assume a therapeutic index that has not been characterized in human subjects.

thrombin

The enzyme at the center of blood clotting—it converts fibrinogen into fibrin strands that form the structural mesh of a blood clot.

Thrombin (Factor IIa) is a serine protease generated from prothrombin by the prothrombinase complex during coagulation. It converts fibrinogen to fibrin, activates platelets via PAR-1, amplifies coagulation through positive feedback (activating Factors V, VIII, XI), and regulates anticoagulation via thrombomodulin/protein C. Bivalirudin is a synthetic peptide (20 amino acids, derived from hirudin) that directly and reversibly inhibits thrombin—both clot-bound and circulating—providing anticoagulation during percutaneous coronary intervention.

tight junction

The seal between cells lining your gut, blood vessels, and other barriers—when tight junctions leak, things get through that shouldn’t.

Tight junctions are multiprotein complexes (claudins, occludins, ZO proteins) that form paracellular barriers between epithelial and endothelial cells. They regulate permeability across mucosal surfaces, the blood-brain barrier, and blood-retinal barrier. Tight junction dysfunction (increased intestinal permeability, ‘leaky gut’) is implicated in celiac disease, IBD, and autoimmune conditions. Larazotide (AT-1001) specifically targets tight junction regulation by antagonizing zonulin-mediated tight junction opening in the intestinal epithelium.

Tmax

The time it takes to reach peak blood levels after a dose—how fast the drug kicks in.

Tmax (time to maximum concentration) is the time point at which Cmax is achieved following drug administration. For subcutaneous peptide injections, Tmax typically ranges from 30 minutes to several hours depending on molecular weight, formulation, and injection site perfusion. Tmax influences dosing timing recommendations—for example, GH secretagogues are typically administered before bedtime so that peak peptide levels coincide with the natural nocturnal GH secretory window.

TNF-alpha

A major inflammation signal—one of the first alarm molecules your immune system releases when it detects a threat.

Tumor necrosis factor alpha (TNF-α) is a pro-inflammatory cytokine produced primarily by activated macrophages. It signals through TNFR1 (ubiquitous) and TNFR2 (immune cells), activating NF-κB and MAPK pathways that amplify inflammation, induce apoptosis, and recruit immune cells. Chronically elevated TNF-α drives pathology in rheumatoid arthritis, IBD, and psoriasis. Several peptides studied on Peptidings (cortistatin, VIP, thymosin alpha-1) modulate TNF-α signaling as part of their anti-inflammatory mechanism.

TrkA (Tropomyosin Receptor Kinase A)

The primary receptor for nerve growth factor (NGF) — when activated, it tells neurons to survive, grow, and maintain their connections.

TrkA is a receptor tyrosine kinase that autophosphorylates upon NGF binding, activating downstream PI3K/Akt (survival) and MAPK/ERK (growth and differentiation) signaling. Expressed on sensory neurons, sympathetic neurons, and corneal epithelial cells. Cenegermin (recombinant human NGF/Oxervate) acts via TrkA on corneal epithelial cells and trigeminal sensory nerve terminals to support corneal innervation and epithelial healing in neurotrophic keratitis.

U

uveitis

Inflammation of the middle layer of the eye (the uvea)—it can cause pain, redness, and vision loss if untreated.

Uveitis is inflammation of the uveal tract (iris, ciliary body, choroid), classified by anatomical location: anterior (iritis), intermediate (pars planitis), posterior (choroiditis), and panuveitis. Etiologies include autoimmune disease, infection, and idiopathic causes. Treatment relies on corticosteroids and immunosuppressive agents. Melanocortin receptor agonists (PL-8177 targeting MC1R) represent a peptide-based anti-inflammatory approach under investigation for ocular inflammation, potentially offering targeted immunomodulation with fewer corticosteroid side effects.

V

vasoconstriction

The narrowing of blood vessels—it raises blood pressure and reduces blood flow to tissues.

Vasoconstriction is the contraction of vascular smooth muscle, decreasing vessel diameter and increasing peripheral resistance and blood pressure. Endogenous vasoconstrictors include angiotensin II, endothelin-1, norepinephrine, and vasopressin (ADH). Desmopressin (DDAVP) is a vasopressin analog modified to retain V2 receptor (renal) selectivity while minimizing V1a receptor (vascular smooth muscle) vasoconstrictor effects—this selective design is why desmopressin treats diabetes insipidus without causing significant hypertension.

vasodilation

The widening of blood vessels—it lowers blood pressure and increases blood flow to tissues.

Vasodilation is the relaxation of vascular smooth muscle, increasing vessel diameter, blood flow, and reducing peripheral resistance. Peptide-mediated vasodilation occurs through multiple mechanisms: VIP activates VPAC1/2 receptors on vascular smooth muscle (cAMP-mediated relaxation); CGRP is the most potent known endogenous vasodilator (acting through CLR/RAMP1); BNP/nesiritide activates NPR-A/cGMP signaling. Vasodilation is both a therapeutic mechanism (nesiritide in heart failure) and a dose-limiting adverse effect (hypotension with VIP, CGRP).

VEGF (Vascular Endothelial Growth Factor)

A protein that stimulates new blood vessel growth — essential for wound healing, but also the molecule that enables tumors to grow their own blood supply and causes blindness in diabetic retinopathy.

VEGF-A is the primary pro-angiogenic signal. In wound healing, VEGF promotes healing-appropriate vascularization. In wet AMD and proliferative diabetic retinopathy, pathological VEGF-driven neovascularization in the retina causes bleeding and vision loss — the target for anti-VEGF biologics. BPC-157 and GHK-Cu upregulate VEGF/VEGFR2 as part of their wound healing mechanisms. The same pathway that is therapeutic in one tissue context is pathological in another.

VPAC1/VPAC2

The two receptors that VIP binds to—found throughout the body, they mediate VIP’s effects on blood flow, inflammation, and gut function.

VPAC1 (vasoactive intestinal peptide receptor 1) and VPAC2 are class B GPCRs that signal primarily through Gs/cAMP/PKA. VPAC1 is widely expressed in the CNS, liver, lung, and immune cells; VPAC2 predominates in smooth muscle, pancreatic beta cells, and circadian pacemaker neurons. VIP and PACAP are the primary endogenous ligands—VIP binds both receptors with similar affinity, while PACAP preferentially activates PAC1 with higher affinity. The pleiotropic effects of VIP (vasodilation, anti-inflammation, circadian regulation) reflect this broad receptor distribution.

W

WADA (World Anti-Doping Agency)

The international body that sets anti-doping rules in competitive sport — their Prohibited List determines which substances athletes cannot use.

WADA publishes the Prohibited List annually. The S2 category (Peptide Hormones, Growth Factors, and Related Substances) covers most research peptides including GH secretagogues, IGF-1 analogs, and performance compounds. WADA prohibition means athletes subject to anti-doping testing rules cannot use the compound in or out of competition (depending on category). It does not make possession or personal use illegal in most jurisdictions — the prohibition applies to sport participation, not criminal law. WADA status is documented for every compound on this site.

Washout period

The time between treatment periods in a crossover trial — needed to allow the first compound to clear the body completely before the second treatment starts.

Duration is determined by the compound's half-life (typically 5 half-lives to reduce plasma concentration to

Wnt/β-catenin signaling

A fundamental cell signaling pathway that drives cell growth and stem cell activation — in hair follicles, it's the primary switch that starts the growth phase.

When Wnt ligands bind Frizzled receptors, they stabilize β-catenin (normally targeted for degradation), allowing it to translocate to the nucleus and activate pro-proliferative target genes. In hair follicles, dermal papilla Wnt signaling activates follicle stem cells in the bulge to enter anagen. CXXC5 provides negative feedback by binding Dishevelled (Dvl), preventing β-catenin stabilization. PTD-DBM disrupts this brake specifically, releasing Wnt-driven follicle regeneration. The Wnt pathway is also critical in colorectal cancer (where CTNNB1/β-catenin is constitutively activated by mutations) — a context where the pathway's growth-promoting role is pathological.

Z

zonulin

A protein your body makes that opens the gaps between intestinal cells—too much zonulin may contribute to ‘leaky gut’ conditions.

Zonulin (pre-haptoglobin 2) is an endogenous regulator of intestinal tight junction permeability. It binds to EGF receptor and PAR2 on enterocytes, triggering cytoskeletal rearrangement that opens tight junctions and increases paracellular permeability. Elevated zonulin levels are associated with celiac disease, type 1 diabetes, and other autoimmune conditions. Larazotide (AT-1001) is a synthetic octapeptide designed to antagonize zonulin-mediated tight junction opening, representing the only therapeutic specifically targeting this permeability pathway.

Zymogen

An enzyme that ships in a switched-off state and gets activated later by a chemical cut that removes an inhibitory piece — like an enzyme with a safety pin that needs to be pulled.

Zymogens (also called proenzymes) prevent uncontrolled enzymatic activity by keeping the enzyme inactive until activation is needed. POMC processing is an example of zymogen-like precursor activation — prohormone convertases PC1 and PC2 cleave POMC at specific sites to release ACTH, α-MSH, and other bioactive peptides at the right time and place. The coagulation cascade involves multiple zymogens activated in sequence. Peptide anticoagulants like bivalirudin and eptifibatide act on already-activated clotting enzymes (thrombin and Factor Xa respectively).