Market reaction
Amgen Inc. (NASDAQ:AMGN) stock declined 2.1% on Friday after Kissei Pharmaceutical Co., Ltd. issued a safety advisory concerning Tavneos, the oral treatment for rare autoimmune kidney conditions Amgen gained via its $16.6 billion acquisition of Horizon Therapeutics in 2023.
Kissei advisory and reported outcomes
Kissei, which holds exclusive rights to develop and commercialize Tavneos in Japan, told healthcare professionals on Friday to avoid initiating prescriptions for new patients and to carefully consider whether existing patients should remain on therapy. The company cited concerns over severe liver injury in patients taking the drug.
In its announcement, Kissei reported 20 fatal cases linked to Tavneos in Japan, including 13 deaths attributed to Vanishing Bile Duct Syndrome. As of April 27, 2026, there have been 22 severe instances of Vanishing Bile Duct Syndrome among an estimated 8,503 patients treated in Japan during the post-marketing period.
Regulatory activity in major markets
Regulatory scrutiny has been escalating. The U.S. Food and Drug Administration issued a notice on March 31, 2026, warning about severe drug-induced liver injury in patients taking Tavneos. On April 27, 2026, the FDA's Center for Drug Evaluation and Research proposed withdrawing U.S. approval, stating there is new information indicating a lack of substantial evidence of effectiveness and that the original application contained untrue statements of material facts. The European Medicines Agency opened a review on January 30, 2026, citing concerns about data integrity from the Phase III clinical trial.
Commercials and next steps
Kissei reported sales revenue of 11.524 billion yen for Tavneos in the fiscal year ending March 2026. The company said it is collecting information from regulators in the United States and Europe and is in discussions with Japan's Ministry of Health, Labour and Welfare. At present, Tavneos' approval in Japan remains in effect.
Clinical pattern of liver events
The drug, generically known as avacopan and indicated for microscopic polyangiitis and granulomatosis with polyangiitis, has shown a concentration of serious liver dysfunction cases early in treatment. Most severe events occurred within three months of treatment initiation, with particularly high incidence between 29 and 56 days after starting therapy.
This report presents the current facts released by Kissei and regulatory agencies and reflects market response; it does not introduce new data beyond those statements.