Abivax shares fell 32% after the company disclosed several cancer cases among patients treated with obefazimod, overshadowing strong Phase 3 maintenance-trial results in ulcerative colitis.
In the 580-patient study, 50.8% on the 25-mg dose and 51.3% on the 50-mg dose reached clinical remission after 44 weeks, versus 10.4% on placebo.
The cancer findings were concentrated on the higher dose, including prostate and breast cancer, colonic dysplasia and four skin cancers; Abivax said the cases were unrelated to the drug.
Analysts said the efficacy looked best-in-class for a long-term ulcerative colitis trial, but the safety signal now clouds a program that had helped drive a roughly 1,700% stock surge in 2025.