What Is a Phase 1 vs. Phase 2 vs. Phase 3 Clinical Trial?

Phase 1 Trials: Establishing Safety

Phase 1 clinical trials are the first stage of testing a new treatment in humans. These trials enroll a small number of patients — typically 15 to 30 — and focus primarily on determining the safe dosage range, identifying side effects, and understanding how the body absorbs, distributes, and eliminates the drug. According to the FDA, Phase 1 trials are essential for establishing baseline safety data.

In Phase 1 trials, participants often receive increasing doses of the treatment (dose-escalation design) to find the maximum tolerated dose. Because these are the earliest human tests, the risk of unknown side effects is higher than in later phases. However, for mesothelioma patients who have exhausted standard treatments, Phase 1 trials may offer access to entirely new therapeutic approaches.

Phase 2 Trials: Testing Effectiveness

Phase 2 trials expand enrollment to 25 to 100 or more patients and focus on whether the treatment actually works against the specific cancer. Researchers use the safe dose established in Phase 1 and measure tumor response rates, progression-free survival, and other indicators of effectiveness. Phase 2 trials also continue to monitor for side effects in the larger patient group.

Phase 2 results determine whether a treatment is promising enough to advance to Phase 3. Many treatments that show safety in Phase 1 do not demonstrate sufficient effectiveness in Phase 2 to continue development. For mesothelioma, Phase 2 trials have been critical in identifying treatments like immunotherapy combinations that later proved successful in Phase 3.

Phase 3 Trials: Definitive Comparison

Phase 3 trials are large, randomized studies that compare the new treatment to the current standard of care. These trials typically enroll hundreds to thousands of patients across multiple institutions. Participants are randomly assigned to receive either the experimental treatment or the current standard, and outcomes are compared to determine whether the new treatment offers a meaningful improvement.

Successful Phase 3 results are generally required for FDA approval. The landmark CheckMate 743 trial, for example, was a Phase 3 study that demonstrated the nivolumab-ipilimumab immunotherapy combination improved survival compared to chemotherapy in unresectable mesothelioma, leading to FDA approval of that treatment in 2020.

Choosing the Right Phase

The appropriate phase depends on your treatment history and goals. Phase 1 trials are typically considered when standard treatments have been exhausted. Phase 2 and 3 trials may be options at various points in your treatment journey, including as first-line therapy. Discuss with your oncologist which phase aligns with your medical situation.

Regardless of which phase you participate in, you retain all rights to pursue legal compensation for asbestos exposure. Clinical trial participation and legal claims are entirely separate processes.