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.