Shares of Achillion Pharmaceuticals Inc. have more than doubled in value this week, with the latest gain coming Tuesday after the drugmaker said regulators have removed a clinical hold on a study of one of its most advanced potential hepatitis C treatments.

The New Haven, Connecticut, company said the Food and Drug Administration's decision allows researchers to continue testing of the drug, sovaprevir, in patients with hepatitis C. The agency placed a hold on the research last summer and asked for reports on sovaprevir's interaction with another drug and a safety analysis of the trials.

Achillion also said Tuesday that it has started treating hepatitis C patients in an early stage study of another drug labeled ACH-3422.

The pharmaceutical company has no products on the market but it is testing treatments in one of the hottest categories of drug research. Companies are racing to develop a combination hepatitis C therapy without injections and debilitating side effects.

One drugmaker, Gilead Sciences, is already raking in billions of dollars from its groundbreaking new drug Sovaldi, which regulators approved late last year.

On Monday, giant drugmaker Merck & Co. said it will spend about $3.85 billion to buy Idenix Pharmaceuticals Inc., which also is developing hepatitis C treatments. Merck said it will spend $24.50 in cash for each Idenix share, or more than triple the value of the stock the last trading day before Merck announced its offer.

Achillion's ACH-3422 is in the same class of drugs as Sovaldi and potential treatments being developed by Idenix. Achillion shares jumped about 48 percent to close at $4.25 Monday after the Merck deal was announced.

The stock then climbed another 63 percent, or $2.66, to $6.91 Tuesday afternoon, while broader trading indexes fell slightly.