Abivax Shares Plunge 37% on Ulcerative Colitis Drug Data Amid Cancer Concerns

Shares of Abivax tumbled as much as 37% after it reported fresh data on its lead asset, a drug for ulcerative colitis, wiping out billions in market cap and raising questions over how a potential takeover of the French biotech could unfold.

The medicine met its endpoints of demonstrating clinically meaningful efficacy and a placebo-adjusted remission rate of about 40% for both doses tested, but also showed that there were cancer cases among patients taking the higher dose.

"[The] cancer signal complicates matters," a Jefferies analyst said. "Even if unrelated noise, we think the overhang will be real, especially considering absence of other value-inflecting data events over the next [year]."

In the study cohort, there was one reported case each of prostate cancer, breast cancer, and colonic dysplasia, which Abivax said were considered unrelated to treatment by investigators, with no organ-specific clustering observed.

There were also four cases of skin cancer on the higher 50 mg dose, of which two were deemed not or unlikely related to the drug, and the other two had a medical history of skin cancer, Abivax said.

The Jefferies analysts said that "a reasonable explanation" for the cancer cases was plausible but "doesn't seem like an easily dismissed overhang," and downgraded the stock to Hold from Buy.

The Paris-listed stock gained nearly 1,700% in 2025, but shares had fallen 7% coming into Tuesday trading.

It was last trading down 37.7%.

The trial results, published late Monday, were from a maintenance study in which Abivax assessed the effect of the drug, obefazimod, over 44 weeks. It comes after the results of a late-stage trial in mid-2025 beat even the most optimistic expectations.

Analysts have described the experimental drug as a potential best-in-class treatment for ulcerative colitis (UC). Abivax is also testing the medicine for Crohn's disease, opening it up to a multi-billion-dollar market for Irritable Bowel Syndrome (IBS).

"The novel mechanism, sustained clinical remission, and favorable long-term safety profile highlight its potential to address a significant unmet need in UC," said Abivax's David Rubin, also director of the inflammatory bowel disease center at the University of Chicago Medicine.

"While the malignancy signal cannot be ignored, we view it as a potential labelling overhang rather than evidence of a clear causal safety risk," said Stifel analyst Damien Choplain, noting that the efficacy data were strong.

AbbVie's Rinqvoc, another medication for ulcerative colitis and other inflammatory conditions, carries a U.S. Food and Drug Administration warning of malignancy but still generated $8.3 billion in sales in 2025, he added.

De Garidel told CNBC in March that he was confident the maintenance study would read out positively, and signaled that the company was in no rush to secure a deal or a partnership, as terms would be better after it.

It has been widely expected that Abivax would be acquired before launching obefazimod, and de Garidel told CNBC that it was planning to raise money after the June data readout. The company on Monday reiterated the expectation to submit obefazimod for FDA approval in the fourth quarter and a potential launch in 2027.

According to Choplain, the most likely scenario now is that Abivax will wait for the full data set, which is expected to be disclosed in October, to gain more granularity and confirm that the malignancies are not related to the treatment.

It also seems reasonable to assume that potential buyers will wait for the full data set before making a move, he told CNBC via email on Tuesday.