Merck & Co’s bid to fill the gap in its painkiller portfolio left by the withdrawal of Vioxx has suffered a setback as the firm and partner Neuromed Pharmaceuticals decided to halt development of NMED-160.

NMED-160, which was also known as MK-6721, is an oral N-type calcium channel blocker that was in Phase II studies as a treatment of chronic pain but the firms say they have jointly decided that it “does not demonstrate the ideal pharmaceutical characteristics considered necessary to advance the compound further in development”. No other details were given but it was noted that no serious adverse events with the drug were observed in clinical trials in which up to a 1,600mg single dose was administered.

Christopher Gallen was not especially downhearted about the compound’s failure and said that “we are encouraged by what we have learned from MK-6721 and are continuing our productive collaboration with Merck with a focus on improving the pharmaceutical properties of our compounds to produce a best-in-class pain treatment". He added that the N-type calcium channel “is an important target for the development of new pain treatments and our studies to date support that,” saying that the alliance with Merck has resulted in the discovery of additional N-type calcium channel blockers “that we continue to advance towards clinical trials”.

Darryle Schoepp, head of neuroscience at Merck, was also upbeat, saying that the firms are committed to the further R&D of oral N-type calcium channel blockers for pain," adding that “we are pleased with the results of our ongoing collaboration". When the deal was first announced, in March last year, it was valued at up to $475 million but it is now almost back to the drawing board as other compounds in the collaboration have yet to make it into trials.

The search for a successor to the COX-2 inhibitor Vioxx (rofecoxib) – once a $2.5 billion product but withdrawn from the market in 2004 after being linked to an increased risk of heart attacks and stroke – goes on.