Merck & Co gave its investors the jitters on Friday after withdrawing the forecast filing date for one of the great hopes in its product pipeline, the cholesterol-lowerer MK-524B, and refusing to give a new timeline for the project.

MK-524B combines Merck’s big-selling statin drug Zocor (simvastatin), now facing generic competition, with MK-524A, a novel compound that is currently in Phase III testing. The main hurdle for the project is getting the fixed-dose combination of the two drugs to function well.

The company also said that the HSP2-THRIVE study, organised with Oxford University in the UK and testing the co-administration of MK-524A and simvastatin as separate pills in around 20,000 patients, is gearing up to start recruitment as planned.

MK-524A is a DP-1 inhibitor coupled with extended-release niacin that is designed to raise protective HDL cholesterol in the blood, and Merck stressed that clinical development of this project remains on track, with regulatory filing still scheduled for 2007.

News of the delay to MK-524B benefited rival company Kos Pharmaceuticals, which is developing Simcor, a rival HDL-boosting combination drug that combines simvastatin with Kos’ Niaspan (niacin) product. Kos’ shares rose nearly 7% on Merck’s announcement.

Gustav Ando, an analyst with Global Insight, said the news “creates significant uncertainty around a company that has already been suffering from numerous problems in its research department, which was once the envy of the global pharmaceutical industry.”