Merck & Co has suffered a setback with the news that its investigational HIV drug vicriviroc has failed in two late-stage trials.

The company says that it will not submit a New Drug Application in the USA for vicriviroc in treatment-experienced HIV-infected patients “at this time” after two Phase III studies in this patient population did not meet their primary endpoints. The New Jersey-based drugs giant noted that these trials enrolled “a high percentage of patients who had three or more active drugs in their optimised background therapy regimen”.

This is not the end of the road for vicriviroc, however, as Merck said it will continue to evaluate the CCR5 inhibitor in treatment-naive HIV-infected patients. Ongoing Phase II studies in that set of patients have completed enrollment and will continue unchanged.

This is disappointing news because vicriviroc, which Merck got hold of through its recent $41 billion purchase of Schering-Plough, was touted as potentially the best drug in the CCR5 class, which is led by Pfizer’s Selzentry/Celsentri (maraviroc). The latter is on the market already but patients on the latter need to take a diagnostic test to see if they are suitable for treatment.

Peak sales of vicriviroc had been estimated at around $150-$200 million.