Most clinical trials are carried out in steps called phases. Each phase is designed to find different information. Patients may be eligible for studies in different phases depending on their general condition, the type and stage of their condition, and what therapy, if any, they have already had. Patients are seen regularly to determine the effect of the treatment, and treatment is always stopped if side effects become too severe. (See the phase descriptions below)
Phase I studies are primarily concerned with assessing the drug's safety. This initial phase of testing in humans is done in a small number of healthy volunteers (20 to 100), who are usually paid for participating in the study. The study is designed to determine what happens to the drug in the human body--how it is absorbed, metabolized, and excreted. A phase I study will investigate side effects that occur as dosage levels are increased. This initial phase of testing typically takes several months. About 70 percent of experimental drugs pass this initial phase of testing.
Once a drug has been shown to be safe, it must be tested for effectiveness. This second phase of testing may last from several months to two years and involve up to several hundred patients. Most phase II studies are randomized trials. One group of patients will receive the experimental drug while a second "control" group will receive a standard treatment or placebo. Often these studies are "blinded"--neither the patients nor the researchers know who is getting the experimental drug. In this manner, the study can provide the pharmaceutical company and the FDA comparative information about the relative safety of the new drug and its effectiveness. Only about one-third of experimental drugs successfully complete both phase I and phase II studies. Safety continues to be evaluated.
In a phase III study, a drug is tested in several hundred to several thousand patients. This large-scale testing provides the pharmaceutical company and the FDA with a more thorough understanding of the drug's safety, effectiveness, benefits, and the range of possible adverse reactions. Most phase III studies are randomized and blinded trials. Phase III studies typically last several years. Seventy to Ninety percent of drugs that enter phase III studies successfully complete this phase of testing. Once a phase III study is successfully completed, a pharmaceutical company can request FDA approval for marketing the drug.
In late phase III/phase IV studies, pharmaceutical companies have several objectives: (1) studies often compare a drug with other drugs already in the market; (2) studies are often designed to monitor a drug's long-term safety, effectiveness and impact on a patient's quality of life; and (3) many studies are designed to determine the cost-effectiveness of a drug therapy relative to other traditional and new therapies.