* Cites increase in nonfatal side effects
* Says it will not seek U.S. approval of high-profile drug
* Failure is latest setback for HDL-raising drugs
* Merck shares fall 2.7 percent (Adds details on findings, side effects, byline)
By Ransdell Pierson
Dec 20 (Reuters) - A major trial of Merck & Co Inc's Tredaptive medicine to increase "good" HDL cholesterol has raised safety concerns and shown it was no better at preventing heart attacks, deaths or strokes than traditional statin drugs that lower "bad" LDL cholesterol, the company said on Thursday.
Merck said the experimental medicine significantly raised the incidence of some types of nonfatal but serious side effects in the study, which followed more than 25,000 patients for almost four years.
Merck shares fell 2.7 percent to $42.46 in morning trading on the failure of one of its most closely watched experimental drugs.
Tredaptive combines an extended release form of niacin, a nutrient that has been used for decades to raise HDL cholesterol, with a drug called laropiprant, which is meant to reduce the incidence of facial flushing that is a side effect of niacin therapy.
Merck said it no longer planned to seek regulatory approval for the drug in the United States and is advising doctors in other countries against starting new patients on it. The medicine was approved in the European Union in 2008, but U.S. regulators were unwilling to approve it until Merck conducted the costly long-term study to better assess its safety and effectiveness.
The U.S. Food and Drug Administration has been especially concerned about potential heart risks from laropiprant.
The failed study, called HPS2-THRIVE, is the latest high-profile trial that has called into question the heart-protective value of niacin.
Merck said Tredaptive had sales of about $13 million for the first three quarters of 2012 in the 40 countries where it is already sold. Had the drug been successful in the HPS2-THRIVE study, brokerage Cowen and Co predicted its annual global sales would have jumped to $300 million by 2016.
(Reporting by Caroline Humer and Ransdell Pierson; Editing by Gerald E. McCormick and Lisa Von Ahn)