By Anna Prior 

Achillion Pharmaceuticals Inc. said Tuesday the U.S. Food and Drug Administration has removed the clinical hold on sovaprevir, the company's experimental hepatitis C treatment.

Shares surged 47% to $6.24 in early trading.

The news comes amid heightened interest in makers of hepatitis C drugs, as the pharmaceutical industry bets that improved medicines for the liver disease represent a large, long-term market opportunity. Illustrating that demand, Merck & Co. on Monday agreed to acquire Idenix Pharmaceuticals Inc., a developer of hepatitis C drugs, for $24.50 a share in cash, more than triple the stock's Friday closing price of $7.23.

Achillion, for its part, has also been seen a takeover candidate.

In a note to clients, JMP Securities said that Merck's planned acquisition of Idenix leaves Achillion as "the only unencumbered, clinical-stage, HCV nucleotide-analog company," with several potential acquirers in the wings.

The firm went so far as to put a $13 per-share, or $1.5 billion, price tag on Achillion, assuming a premium for intellectual property and expertise in nucleotide-analog development.

The FDA's move Tuesday allows Achillion to conduct trials in patients with hepatitis C, the company said, adding that the previously evaluated dose of sovaprevir that was well-tolerated with clinical activity in two completed Phase 2 studies may be used in additional therapeutic clinical trials.

The company is "very pleased that the effort by the Achillion team, working in collaboration with the FDA, has resulted in this response for the sovaprevir program," said Achillion executive vice president and chief medical officer David Apelian.

The company also said it has begun dosing ACH-3422, a uridine-analog nucleotide polymerase inhibitor, for patients with a certain type of hepatitis C in an ongoing Phase 1 clinical trial.

Achillion said it expects to report proof-of-concept results from this trial in the fall.

In July, Achillion said the FDA put sovaprevir on clinical hold after the company's early-stage study of drug interactions in healthy patients resulted in elevated liver enzymes tied to higher-than-expected exposures to sovaprevir and another drug, atazanavir.

The company had voluntarily stopped dosing in the study after finding high levels of the liver enzymes in several people enrolled in the study. Its preliminary investigation found that dosing with both of the drugs caused a metabolic interaction that substantially increased the plasma concentration of both drugs.

No serious adverse events were reported at that time.

In September, Achillion said the FDA had decided against lifting its clinical hold on sovaprevir, news that sent the company's stock plunging at the time.

The FDA had continued to allow the enrollment of patients in a separate midstage study evaluating sovaprevir in combination with other drugs in patients with hepatitis C.

Achillion is competing with Gilead Sciences Inc. and other drug companies to bring an all-oral hepatitis C regimen to market, hoping to capitalize on what is expected to be a multibillion-dollar market for the therapy. The treatment from Gilead called Sovaldi recorded nearly $2.3 billion in first-quarter sales.

Write to Anna Prior at anna.prior@wsj.com

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