Shares in SciClone Pharmaceuticals are on the slide after the US and Italian partner Sigma-Tau revealed that their hepatitis C drug candidate thymalfasin has failed in a late-stage trial.

The firms presented disappointing results from a 552-patient Phase III trial evaluating thymalfasin in combination with pegylated interferon alpha-2a (peg-IFN-2a) and ribavirin (RBV) as a treatment for patients with HCV who have not responded to prior therapy of peg-IFN and RBV alone, the current standard of care.

The thymalfasin treatment group did not achieve statistical significance for the primary endpoint of sustained virological response. However SciClone noted that in the secondary population of patients, involving 182 patients who completed the full 48-week treatment course, the drug did meet its goal.

The treatment is currently approved in more than 30 countries worldwide to treat a variety of indications and is marketed as Zadaxin. In clinical studies, more than 4,000 patients with viral hepatitis B and C, primary immunodeficiency diseases and numerous cancers have been treated with thymalfasin, the firms noted.

SciClone chief executive Friedhelm Blobel said the firm is disappointed that the study did not reach its primary efficacy endpoint, but the subset suggests “a potential benefit of using thymalfasin in patients who completed the full course of treatment". He added that the firm plans to analyse the data further in coming weeks and present the full results at an upcoming conference.

Chief medical officer Israel Rios noted that although the safety analyses are still ongoing, thymalfasin appeared to be well tolerated. He added that “during the course of the trial, the side effects reported were those usually associated with the use of interferon and ribavirin."

Investors seem to have made their mind up about thymalfasin, however, and SciClone shares ended the day down 12.8% at $0.81.