Phase 1, 2 and 3 trials

Phase 1 escalates the dose in small groups until side effects limit it, defining the maximum tolerated dose and the dose for later study; in oncology, phase 1 patients have usually exhausted standard options and modern phase 1 trials often expand into hundreds of patients with early efficacy readouts. Phase 2 tests the chosen dose for response in a specific cancer, sometimes randomised and sometimes single-arm, and phase 3 is the large randomised, often blinded, comparison against the standard of care that regulators normally require for full approval. Phase 4 studies follow a marketed drug. Only around 5% of oncology drugs entering phase 1 reach approval, and around a third of phase 3 trials fail, which is a major driver of drug cost.