Achillion Pharmaceuticals Inc. said U.S. regulators put a hold on a clinical trial of one of the company’s experimental drugs for hepatitis C after some patients experienced elevated liver enzymes. The company’s shares tumbled in extended trading after the announcement Monday.

Liver enzyme elevations were observed in healthy patients when Achillion’s hepatitis C drug sovaprevir was combined with two medicines used to treat HIV infections in order to test for possible drug interactions. The combination may have resulted in a drug interaction that produced higher than expected blood levels of the therapies, New Haven, Connecticut-based Achillion said Monday in a statement.

Achillion has three drugs in clinical testing for hepatitis C, a liver disease estimated to affect 170 million people worldwide. The company announced April 23 that one of the treatments, ACH-3102, appeared effective in five out of eight patients, reducing the amount of the liver-destroying virus in their bloodstream to undetectable levels after 12 weeks of treatment. Achillion is competing with drugmakers including AbbVie Inc. and Gilead Sciences Inc. to develop new treatments for the disease, a market analysts’ estimate may be $20 billion.

“Achillion voluntarily stopped further dosing” and “promptly notified” the Food and Drug Administration of the finding in the early-stage trial involving sovaprevir, the company said. The hold doesn’t affect a current second-stage study of the drug in hepatitis C patients, Achillion said.

To resolve the hold, the regulatory agency has asked for data from two drug interaction studies as well as a safety analysis of continuing sovaprevir studies, which the company said it plans to provide in about six weeks.

Achillion shares dropped 21 percent to $6.60 in extended trading at 5:16 p.m. New York time after closing at $8.36. The company had gained 35 percent in the past 12 months.