A state in which a cell permanently exits the cell division cycle but remains metabolically active and resistant to apoptosis (programmed cell death). Senescent cells accumulate with age and secrete a complex mixture of inflammatory cytokines, growth factors, and proteases collectively called the senescence-associated secretory phenotype (SASP).

The SASP is what makes senescent cells harmful: it drives chronic low-grade inflammation, degrades surrounding tissue, and can promote cancer in neighboring cells. Clearing senescent cells (senolysis) is a major target in longevity research. FOXO4-DRI is a peptide specifically designed to kill senescent cells by disrupting the FOXO4-p53 interaction that keeps them alive. The compound showed dramatic results in aged mice but has never been tested in humans—a gap that epitomizes the field’s distance between preclinical promise and clinical proof.