Clinical trials to test new cancer treatments involve a series of steps called phases. If a new treatment is successful in one phase, it will proceed to further testing in the next phase. During the early phases (phases 1 and 2), researchers figure out whether a new treatment is safe, its side effects, and its best dose. They also ensure that the therapy has some benefits, such as slowing tumor growth. In the later phase (phase 3), researchers study whether the treatment works better than the current standard therapy. They also compare the safety of the new treatment with that of current treatments. Phase 3 trials include many people to ensure the result is valid.
There are also very early (phase 0) and later (phase 4) clinical trials. These trials are rare. Phase 0 trials are tiny trials that help researchers decide if a new agent should be tested in a phase 1 trial. Phase 4 trials look at long-term safety and effectiveness. They occur after a new treatment has been approved and is on the market.
The following shows the number of patients participating and the purpose of the most common phases. Although the trial phases are explained in the context of drug treatment trials, the same concepts apply to most types of clinical trials.
Phase 1
Purpose:
- To find a safe dose
- To decide how the new treatment should be given (by mouth, in a vein, etc.)
- To see how the new treatment affects the human body and fights cancer
Number of people taking part: 15–30
Phase 2
Purpose:
- To determine if the new treatment has an effect on a certain cancer
- To see how the new treatment affects the body and fights cancer
Number of people taking part: Less than 100
Phase 3
Purpose:
- To compare the new treatment (or new use of a treatment) with the current standard treatment
Number of people taking part: From 100 to several thousand
Some researchers design trials that combine two phases (phase 1/2 or phase 2/3 trials) in a single study. In this combined design, there is a seamless transition between trial phases, which may allow research questions to be answered more quickly or with fewer patients.
Phase 4
Purpose - To find out more information about the long-term benefits and side effects of an approved drug.
Number of People: Thousands
Text adapted from the National Cancer Institute; images provided by Genentech. Reviewed on November 15, 2023.