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Clinical Study Phases

Clinical studies, which test interventions in people, occur in several steps, known as phases. While each phase is designed to answer different research questions, studies in all phases evaluate the safety of the intervention.

Typically, there are four phases of clinical studies:

If a new treatment (also called an investigational or experimental treatment) is successful in one phase, it may move on to the next phase. The information gained in each phase helps researchers design subsequent studies. As a treatment progresses through the phases, the number of participants in the studies typically increases. For example, a Phase I study will have a small number of participants, perhaps as few as 10, while a Phase III study may enroll hundreds or even thousands of people.

Sometimes two phases may be combined (as in Phase I/II or Phase II/III studies) to provide a smooth transition between phases and allow the research questions to be answered more quickly or with fewer participants.

Although treatment studies are typically assigned a phase, other types of clinical studies, such as prevention studies, may not be labeled this way.

Phase I: Safety

Phase I is typically the first time the treatment is tested in humans, the first time it is studied for a particular condition, or the first time it is being administered in a particular way (for example, in a new combination with other drugs).

Even though the investigational treatment has been studied in the laboratory (nonclinical studies) and in animals (pre-clinical studies), the risk to participants can be significant. For this reason, Phase I studies often enroll small numbers of participants (about 15 to 30) who typically don’t have any other treatment options—patients who cannot be treated with the standard (most widely accepted) treatment or for whom there is no standard treatment. In cancer research, these tend to be patients with advanced disease.

Phase I studies evaluate the safety of a new treatment, including side effects, dose, and impact on the body. For a new drug, Phase I studies are designed to find the safe dose amount and schedule as well as the best way to give the drug, such as orally or by injection. To determine the highest dose that can be safely given (maximum tolerated dose), researchers may start by giving a low dose to a small group of participants, a slightly larger dose to the next group, and so on, while closely monitoring the participants for side effects.

Generally, the study is only intended to determine the safety of the new treatment, not to show whether it is effective; however, results may provide an indication of whether it is effective.

Learn about the Seattle Cancer Care Alliance Phase I program.

Phase II: Efficacy

Phase II studies are done to determine if a treatment is effective for a certain condition. In the case of cancer, researchers look at the response to the treatment in participants who have the same cancer or similar types of cancer. In Phase II studies, participants are usually given the treatment in whatever way (dose, schedule, method of administration, and so on) researchers determined was best during Phase I studies; sometimes Phase II studies test variations in dose or schedule.

Phase II studies often include more participants than Phase I studies but are usually limited to fewer than 100 people.

Phase III: Comparison

Phase III studies compare the effectiveness and side effects of treatments, typically a new treatment and the standard treatment, in patients with a particular type of cancer. The goal is to determine whether the investigational treatment is more effective than the standard.

To make this comparison, patients may be randomized—that is, randomly assigned to one of the two (or sometimes more) arms of the study. Neither you nor your doctor can choose or influence the arm (or group) you will be in.

  • If you are assigned to the investigational group, you will get the new treatment being tested.
  • If you are assigned to the control group, you will get the standard treatment for your cancer.

Randomization, used in some Phase II and nearly all Phase III studies, is important to the scientific validity of the research. It eliminates bias in selecting which patients get which treatment. This helps ensure that differences between the groups’ results are due to the different treatments they received and not other factors. Often, randomized studies are single blind, meaning you don’t know which treatment you are getting, or double blind, meaning neither you nor your doctor knows.

Phase III studies enroll a large number of participants—typically between 100 and several thousand—and often take place at a number of cancer centers throughout the country (which is why they may be called multicenter studies).

In the United States, after successful Phase III clinical studies, the developer of the treatment (such as a pharmaceutical company in the case of a drug) may apply for U.S. Food and Drug Administration (FDA) approval of the treatment. If the FDA approves the treatment, it may become a standard therapy.

Phase IV: Post-Marketing

Some treatments that are approved by the FDA are tested in Phase IV clinical studies. In these studies, also known as post-marketing studies, researchers may monitor the effectiveness and safety of the treatment over a long period of time and in a larger, more diverse, or different population than in previous studies.

How Are Placebos Used in Clinical Studies?

In cancer research, placebos, which are inactive treatments, may be given to the control group in randomized studies to test interventions that might prevent cancer in people who do not have the disease. Placebos are almost never used in cancer treatment studies. They might be used in these situations:

  • If no standard treatment exists for the study participants, researchers might compare the effects of a new treatment with the effects of a placebo. This helps them distinguish between the actual benefits of the investigational treatment and the benefits of believing you’re getting effective treatment.
  • More often a placebo is given along with a standard treatment so researchers can compare this with an investigational treatment plus the same standard treatment.

You will be told in advance if the study you’re considering uses a placebo and you could be assigned to the placebo group.