Schering-Plough has decided to press on with a Phase II trial of its investigational HIV treatment vicriviroc, despite the discovery of five cases of cancer among patients taking the drug.

The company said the AIDS Clinical Trial Group (ACTG) tasked with carrying out the trial believes there is no evidence that vicriviroc is directly responsible for the malignancies, which included four cases of lymphoma and one of stomach cancer.

The 118-patient study included individuals who had already received a number of treatment regimens for HIV. As vicriviroc was showing benefits in reducing levels of the virus and boosting immune cells even in these hard-to-treat patients, the ACTG said the trial should continue.

As a precaution, the ACTG is introducing long term follow-up (three to five years) of patients and is unblinding the study, so that patients and their physicians may make a more informed decision about their treatment options.

Vicriviroc, which inhibits the entry of HIV into cells by blocking a cell surface receptor called CCR5, was one of the pipeline products highlighted by S-P at its investors’ presentation last November.

The company has been trying to show that it has new products in development that will help bring it out of a slump caused by the loss of patent protection on its blockbuster antihistamine Claritin (loratadine) in 2002, as well as government fines for manufacturing problems and investigations into both financial disclosures and its sales and marketing.

But there was a further disappointment for S-P last year after a Phase II trial of vicriviroc in treatment-naïve patients showed that although use of the drug in combination with other antiretrovirals causedblood levels of HIV to become undetectable, the virus staged a comeback a few months later.

“Schering-Plough intends to continue to work closely with the ACTG as well as the FDA and other health authorities, to fully determine the potential role of vicriviroc in HIV therapy,” said the company in a statement.