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Phases of Clinical Studies

Clinical Studies Progress in Phases

Before a new cancer drug or treatment is tested in humans, scientists must have good reason to believe--from work in the laboratory and from pre-clinical trials in animals--that it will be at least as effective as the standard treatment.

Even then, testing with human subjects progresses slowly through Phase I, Phase II, and Phase III trials. A Phase I trial will have a small number of patients, perhaps as few as 10, while a Phase III trial may enroll hundreds or even thousands of people.

The risk to patients is higher in a Phase I trial--which is the first test of the drug or treatment in people--and to qualify you usually must have advanced cancer for which there is no effective treatment.

If you are considering a clinical trial, you will want to understand what is being tested in each type of trial.

More information in the Phases of Clinical Trials:

Phase I: Safety

Phase I trials look at how well a new drug is tolerated and are used to determine the highest dosage that can be safely given. Scientists will start by giving a low dose to a small group of people, a slightly larger dose to the next group, and so on, while closely monitoring the patients for side effects.

Generally, the study is only intended to determine the maximum tolerated dose and the toxicity, not to show whether or not the treatment is effective.


Some Phase I trials test new combinations or new dose schedules of drugs that are already shown to be effective.

Most cancer patients do not consider Phase I trials unless there are no Phase II or III trials for which they qualify.

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Phase II: Effectiveness

Phase II trials are done to determine if a treatment is effective. Phase II trials usually include a relatively small number of patients (20 to 100) who all have the same kind of cancer.

Researchers are looking at the response rate to the treatment. For most cancers, this means a reduction in the size of measurable tumors. Some patients may have a partial response, meaning tumors have shrunk but have not completely disappeared. Other patients may have a complete response, meaning no measurable tumor is left.

When considering a Phase II clinical trial, you should ask about the duration of the response in patients who have received the experimental treatment. The duration is typically reported in months, and indicates the period of time before the patients' cancers returned.

Typically, to qualify for a Phase II clinical trial you must have advanced cancer and you must have tried the standard therapy for your disease, if one if available.

One advantage of Phase II trials for patients with advanced disease is that these trials are not randomized: You will get a promising treatment at the best dose without the possibility of being randomly assigned to receive the standard treatment, as can happen in a Phase III trial.

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Phase III: Comparing Treatments

Once Phase I and II trials are completed, a treatment that continues to look promising will be tested in a Phase III trial with a large group of patients. The same Phase III trial may be available at a number of cancer centers throughout the country.

A Phase III trial compares two treatments, typically an experimental treatment and a standard treatment, to see if survival rates are better with the new treatment. Some Phase III trials compare the side effects of two treatments.

Typically, a Phase III trial will be randomized, which means that patients who enroll are assigned randomly to one of the two (or sometimes three or four) arms of the study. Randomization is important to the scientific validity of the research.

For a patient trying to choose among various treatment options, however, randomization means that you may receive the standard treatment rather than the promising new treatment. If you are considering participating in a randomized clinical trial, but you prefer one treatment to the other, you have several options:

  • If the treatment you prefer is available outside the trial as a standard treatment, don't enroll in the trial.
  • If the treatment you prefer is only available in clinical trials, look for another trial, perhaps a Phase II trial, where you can receive that treatment.
  • If the drug or treatment you prefer is FDA-approved for another purpose, you may be able to get that treatment outside of a clinical trial even if the treatment is not standard.

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Phase IV: Long-term Studies

New treatments receive government approval after successful Phase III clinical trials. Some drugs and treatments are also tested in Phase IV clinical trials. In these trials, researchers may monitor the effects of long-term use of the treatment. They may also monitor the use of the drug or treatment in different populations, or look at quality-of-life issues.

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