What Are the Different Stages (Phases) of Clinical Trials?

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Clinical trials are conducted in a series of four steps or phases. Each step or phase builds on the results of the phase before it.

Phase I trials: Is it safe?

Phase I trials are the first trials to involve people. (Preclinical studies in petri dishes and animals already have been done.) Phase I trials are small, with usually between 15 to 50 people participating. In phase I trials, researchers are figuring out:

  • the best way to give a new treatment (as an injection or as a pill)
  • the highest dose that can be given safely, without serious side effects

In phase I trials, researchers closely monitor the participants and adjust the dose a little at time until they find the amount that works the best with acceptable side effects. This dose is usually the one used for further testing.

Phase II trials: Does it work?

Phase II trials look at how effective the new treatment is. Phase II trials are slightly larger than phase I trials and usually involve 25 to 100 people. Researchers start with the dose and method of giving the new treatment that were found to be best in phase I. The phase II participants are given the new treatment and the researchers watch to see if the treatment has some benefit. The benefits the researchers look for can be different, depending on the goals of the research:

  • cancer tumor gets smaller
  • cancer stops growing
  • longer time before the cancer comes back
  • longer survival time
  • better quality of life

If a certain percentage of participants benefit from the treatment and the side effects are still acceptable, the new treatment will probably go on to a phase III trial.

Phase III trials: Is it better than what we have now?

Phase III trials compare the safety and effectiveness of the new treatment to the current standard of care. Phase III trials are usually large (some involve tens of thousands of participants) and are done at many places in the United States and sometimes around the world. A phase III trial is the last step a new treatment goes through before the U.S. Food and Drug Administration considers approving it for general use.

Participants are usually randomly assigned to receive the current standard treatment or the new treatment. If possible, the trial is double-blinded, which means neither the researchers nor the participants know who's getting which treatment. Double-blinded clinical trials help researchers see the actual benefits and side effects of a treatment without bias or outside influence. Results from a randomized, double-blinded trial are considered more credible than results from a trial that isn't randomized or double-blinded.

Just like phase I and phase II trials, phase III trial participants are watched closely to see if any serious side effects develop. Treatment is stopped if side effects appear to be dangerous.

Phase IV trials: Are there any other uses or benefits?

Phase IV trials usually look at whether the treatment offers benefits or produces long-term side effects that weren't studied or seen in the phase II or phase III trials. Phase IV trials usually are done after a treatment has been approved for use by the U.S. Food and Drug Administration. Phase IV trials are less common than phase I, II, or III trials and may involve hundreds of thousands of people.

For example, one phase IV trial is interested in participation from women diagnosed with breast cancer who are taking an aromatase inhibitor. Aromatase inhibitors are known to cause joint pain and bone loss. This phase IV trial is looking at whether the joint pain some women have while taking an aromatase inhibitor is linked to more defects in their cartilage compared to women not taking an aromatase inhibitor.

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