Merck & Co. has halted a study of an experimental drug it had touted as key to energizing its sagging cholesterol franchise, barely three weeks after U.S. regulators refused to allow sales of the drug.

The drugmaker said Wednesday an advisory committee recommended ending the study of cholesterol drug MK-0524A, two months after the same scientists urged enrollment of more patients be put on hold. MK-0524A combines the B vitamin niacin and a drug called laropiprant to limit the facial flushing caused by niacin.

The halted study, called ACHIEVE, used ultrasound imaging to monitor plaque buildup in arteries of people whose genes give them sky-high cholesterol levels — the same kind of patients and technology in a study of Merck's popular Vytorin cholesterol drug that backfired earlier this year.

Whitehouse Station, N.J.-based Merck said the FDA rejection was unrelated to its ending the ACHIEVE study, in which half the patients got MK-0524A and half got dummy pills; both groups also got standard cholesterol-lowering drugs.

Dr. Yale Mitchel, vice president of cardiovascular disease at Merck Research Labs and a member of the steering committee, said in an interview the decision to stop the study was driven by data from three recently released studies that tested various cholesterol drugs on people with ultra-high cholesterol.

Mitchel said analysis of those studies and a similar, earlier one unexpectedly found that because all the patients already took cholesterol drugs, plaque didn't build up in their arteries during the studies enough to see much difference between patients getting the new treatment being tested and those getting dummy pills.

Mitchel said the steering committee members, who met May 14, recommended ending the study, expecting a similar result.

Ultimately, the company decided to end it, a Merck spokesman said.

Dr. Harlan Krumholz, a heart specialist at Yale University School of Medicine, said, "What needs to be clear is exactly what's different now from when they started the study."

"When things like this occur, it's even more important (than usual) to provide assurance that the integrity of the scientific endeavors is maintained," he said.

Dr. Steve Nissen, chairman of cardiovascular medicine at the Cleveland Clinic, said Mitchel's explanation is inadequate and that it's not appropriate to end medical experiments on human volunteers "without proper cause."

"I am concerned whether the reasons for terminating this trial are commercial or scientific," said Nissen.

The studies the steering committee analyzed included a controversial one called ENHANCE, which found Vytorin, the pricey combination cholesterol drug Merck markets jointly with partner Schering-Plough Corp., is no better at limiting plaque buildup than a much-cheaper generic. The long delay in releasing that data led to suspicions the companies withheld it to protect sales. Several government investigations are ongoing.

Merck spokesman Ron Rogers said Merck won't release results of the analysis now but will submit them and the data from ACHIEVE to regulatory agencies and release the findings at a medical meeting or in a journal.

Rogers said the ACHIEVE data have not been seen by Merck executives or steering committee members.

Merck has not disclosed why the Food and Drug Administration rejected MK-0524A but said Wednesday it is continuing other studies of the drug.

John Kastelein, chair of the ACHIEVE steering committee, said it recommended a new imaging trial "in a more appropriate patient population."

Only 433 of the 900 patients planned for the study had been enrolled, and none finished the intended two years of testing.