What are the phases of a clinical trial?

Development of new anticancer agents and treatment strategies occurs in four phases, each of which is extremely important to advancing cancer treatments. The goal of a clinical trial is to develop a new therapy that is more effective than previous standard therapies.

  • Phase I: This is probably the most important step in the development of a new drug or therapy. Generally trials involve a small number of patients for whom other standard therapies have failed or who have no known alternative treatments available. These treatments may produce some positive effects and a small number may benefit from them. The main goal of this phase is to determine anticancer activity in humans. Secondary goals include finding the maximum tolerated dose of a treatment as well as the manner in which the drug works best in the body. Additional goals may be to determine the toxic side effects related to different doses and whether those side effects are reversible. When a drug completes Phase I studies and meets the required criteria it moves to Phase II.
  • Phase II: The information from Phase I studies is used to design Phase II studies. This phase determines the effectiveness of a treatment in a specific population at the doses and schedules determined in Phase I. These trials usually require a slightly higher number of patients than Phase I. The number may also increase depending on the number of responses as the Phase II trial progresses. Drugs or therapies shown to be active in Phase II trials may become standard treatment or may be further evaluated for effectiveness in phase III trials. 
  • Phase III: Phase III trials compare a new drug or therapy with standard therapy in a randomized and controlled manner in order to determine proof of effectiveness. This phase requires a large number of patients – generally thousands – to measure the statistical validity of the results, since a patient’s age, sex, race and other unknown factors can affect the results. Phase III trials typically involve many physicians from different institutions.
  • Phase IV: Once a drug or treatment becomes a standard of care, the manufacturer of the drug may elect to initiate a Phase IV trial. This means there will be continued evaluation of the treatment's effectiveness and monitoring of side effects as well as implementation of studies to evaluate its usefulness for different types of cancers.