In the United States, approval of a new drug for clinical use follows a series of developmental and evaluative steps under an Investigational New Drug (IND) Application, which precedes submission of a New Drug Application (NDA) for marketing approval from the Food and Drug Administration (FDA). The process is divided into preclinical and three clinical, investigational phases followed by NDA review by the FDA.
Preclinical studies are usually performed in animals to address biological activity, toxicity, and safety data, and collection of medication concentration and effect data to assist in developing initial human dosing regimens.
Phase I studies are performed in healthy volunteers and patients to define initial toxicity parameters, the tolerated dose range, how the medication appears and distributes through the body, how it is removed from the body, and the effects of the medication on humans. Special patient target populations may be evaluated, and different methods of drug formulation or delivery systems may be tested (usual, n = 20-80).
Phase II studies test clinical efficacy or therapeutic effectiveness, and are designed to provide dose/concentration versus response relationships in closely monitored patient groups of limited numbers (usual, n = 100-200). Results aid in the design of studies in phase III.
Phase III studies are patient studies that document clinical safety and efficacy, further refine the dose/concentration versus response relationships, and assess adverse drug reaction rates. Larger numbers of patients are usually studied in phase III (usual, n = a few hundred to thousands) than in phase II; however, phase II and phase III studies often go on simultaneously.
Phase IV trials are postmarketing approval surveillance trials, and are variable in design and objective.
From: Schwartz, JB. Pharmacy Focus. The evaluation of pharmacologic therapy in humans: a brief summary of the drug evaluation process and guidelines for clinical trials as they relate to women. J Gender-specific Medicine 2001; 4(4), 13-17