Trial Design
Phase 1, 2, 3, and 4 Clinical Trials
Clinical trials are conducted in four sequential phases: Phase 1 evaluates safety and pharmacokinetics in a small cohort; Phase 2 evaluates efficacy and dose; Phase 3 confirms efficacy and safety in large randomized populations to support marketing approval; Phase 4 monitors real-world safety and effectiveness post-approval.
Phase 1 trials enroll 20; 100 healthy volunteers (or patients in oncology) and answer: is the compound safe and what dose can be tolerated? Designs include single ascending dose (SAD), multiple ascending dose (MAD), and food-effect studies. Phase 1 oncology trials use specialized escalation designs (3+3, BOIN, mTPI).
Phase 2 trials enroll 100; 300 patients and answer: does it work, and at what dose? Phase 2a is proof-of-concept; Phase 2b is dose-finding for Phase 3.
Phase 3 trials enroll 300; 3,000+ patients across many sites and many countries, generating the pivotal evidence regulators use to grant marketing approval. Phase 3 site selection and operational execution are where most clinical-development budget is spent.
Phase 4 (post-marketing) trials are conducted after approval to detect rare adverse events, study additional populations, or support label expansion.
Related terms
Clinical Trial Protocol
A clinical trial protocol is the master document that defines a clinical study's objectives, design, methodology, statistical considerations, and operational conduct, and that all investigators must follow to ensure scientific validity and participant safety.
Endpoint (Clinical Trial)
A clinical trial endpoint is a defined, measurable outcome that determines whether the intervention being studied has the intended effect; it is the variable on which the trial's success or failure is statistically evaluated.
Site Selection (Clinical Trials)
Site selection is the process by which sponsors and CROs identify, evaluate, and choose the clinical trial sites best suited to enroll a given study, based on patient population access, investigator experience, operational capacity, regulatory standing, and historical performance.
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