Shares of Achillion Pharmaceuticals sank Tuesday before markets opened and a day after the drug developer said federal regulators placed a hold on an early-stage study of a drug combination involving its potential hepatitis C treatment sovaprevir.

Hepatitis C has become a major area of drug research because it is a chronic illness and more cases are expected to be diagnosed as Baby Boomers age. Companies are racing to develop new and more convenient treatments.

The New Haven, Conn., company said Monday shortly after markets closed that the Food and Drug Administration placed a clinical hold on the study after researchers saw an unanticipated elevation in liver enzymes in several subjects.

Achillion was studying the effects of administering sovaprevir with ritonavir-boosted atazanivir when researchers detected the unexpected elevations, stopped administering the combination and contacted the FDA.

The FDA has asked for reports from some drug interaction studies and a safety analysis of on-going sovaprevir studies. Achillion said it expects to provide that information in about six weeks.

Meanwhile, regulators will allow the continued enrollment and treatment of patients in a mid-stage study of sovaprevir.

Achillion currently is studying three potential treatments for chronic hepatitis C, and sovaprevir is one of its most advanced candidates. The company expects to report some results from that study in the third quarter.

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. The disease is spread through the blood, and that can happen through sharing intravenous drug needles or having sex with an infected person. It can go undetected for many years until the liver is severely damaged.

Achillion shares sank more than 23 percent, or $1.98, to $6.38 in premarket trading Tuesday about 90 minutes ahead of the market opening. Its shares have traded in a 52-week range of $5.42 to $11.36.