Overview of Clinical Trial Process

PhaseNo. of subjectsScope
0/IFew dozen (therapeutic); 10–30 (diagnostic)New drug is first given to small number of people; studies might be exploratory, involving several candidate molecules to select lead for development (46), and are intended to demonstrate safety of new agent and dose-limiting toxicity
IIFew hundred (therapeutic); 20–40 (diagnostic)Study is undertaken to demonstrate proof of efficacy and to reveal less common side effects; if enough patients benefit from new drug, and side effects are acceptable, then phase III trials can be initiated; additional safety data are obtained
IIISeveral hundred (therapeutic); 50–100 (diagnostic)Trials are intended to compare safety and efficacy of new diagnostic or therapeutic agent against standard of care and typically are large and multicenter; research subjects are picked at random to receive standard of care or new treatment; when neither doctor nor subject knows which treatment subject is getting, trial is considered double-masked; additional safety data are obtained, and if new drug is found to be as effective as or more effective than existing drugs, or is safer, NDA can be submitted to FDA
IVThousandsPostapproval studies are intended to monitor FDA-approved drugs over long period (e.g., several years)