NEW YORK (AP) -- Drugmaker Bristol-Myers Squibb Co. is suspending a midstage study of a potential hepatitis C treatment because of a serious patient safety issue.

The New York company did not elaborate on the problem in a brief statement released Wednesday night after markets closed. It said the cause of the issue and potential relationship to the drug are unknown.

Bristol-Myers shares are down more than 5 percent, or $1.80, to $33.80 in premarket trading Thursday.

The drug company is making a push to become a player in the hepatitis C drug market, and it acquired the potential treatment, labeled BMS-986094, as part of a $2.5 billion acquisition of drug developer Inhibitex Inc. it completed earlier this year.

Hepatitis C is a virus that can lead to life-threatening liver damage and is the main cause of liver transplants in the United States. Analysts say the market for treatments is potentially lucrative for drugmakers. More people are expected to be diagnosed with the tough-to-treat disease as the baby boomer generation ages.

After a two-decade drought, the first two new hepatitis C drugs were approved last year: Victrelis from Merck & Co. and Incivek, marketed by partners Vertex Pharmaceuticals Inc. and Johnson & Johnson. Both significantly improve the cure rate over what has long been the standard of care - a mix of injections and pills with nasty, flu-like side effects that takes several months and still doesn't cure many patients.

Bristol-Myers is an important maker of medicines for viruses, including Baraclude for hepatitis B and several HIV drugs. It also sells the world's second-best-selling drug, the blood thinner Plavix, but revenue from that is expected to plunge due to competition from cheaper generic drugs in the U.S. market.

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