By Ransdell Pierson

NEW YORK, May 01 (Reuters) - Merck & Co.'s arthritis drug Vioxx, taken by more than 6 million Americans since its 1999 debut, is likely to face stepped up scrutiny from US regulators because of the number of patients who suffered strokes and heart attacks after taking the medicine in new clinical trials, analysts said.

"The heart attacks and strokes could be a serious concern for the [US] Food and Drug Administration, and could delay or derail Merck's plans to market Vioxx for rheumatoid arthritis," said David Saks, manager of Gruntal & Co's Medical Sciences Fund.

The FDA in recent months has shown far more concern over safety of drugs already on the market and those awaiting approval, following the recall of several medicines linked to deaths and serious side effects.

Vioxx is one of the fastest selling new drugs in the world. It was approved last May by the FDA for the treatment of acute pain and osteoarthritis  the most common form of arthritis, caused by normal wear and tear of joints and afflicting 21 million Americans. Some analysts expect sales of Vioxx, which were worth $472 million last year, to swell to $2 billion or more by 2002.

The nation's largest US drug maker has said that it plans soon to ask the FDA to widen the approved use of Vioxx to include rheumatoid arthritis, the crippling hereditary form of arthritis afflicting 2.1 million Americans.

In late March, Merck said in a statement that preliminary data from a study of 8,000 rheumatoid arthritis patients showed that those taking Vioxx suffered from far fewer ulcers and other gastrointestinal side effects than those who took a popular older arthritis drug, naproxen.

But Merck said that patients using naproxen  a generic painkiller made by many companies  had significantly fewer heart attacks and strokes than the Vioxx group. Merck has attributed that negative finding for Vioxx to naproxen's ability to prevent platelets from sticking together. That study did not include a placebo comparison.

Some analysts, though, are less than convinced by Merck's explanation.

"This whole situation about heart attacks and strokes adds an element of risk to Vioxx that most people are not looking at. It's not on most people's radar screens, but it should be," said Mario Corso, a Boston-based drug analyst for Dutch bank ABN-Amro.

Gruntal's Saks, who had previously forecast Vioxx could achieve annual sales of $2 billion by 2002, including between $200 million and $300 million for rheumatoid arthritis if it wins that approval, said FDA scrutiny could threaten those sales prospects.

"The rheumatoid arthritis indication would be icing on the cake. The larger question is how this issue (of heart attacks and strokes) might affect the whole cake," Saks said, referring to the drug's total sales.

FDA spokesperson Susan Cruzan told Reuters that she could not comment on how closely her agency might scrutinize Merck's rheumatoid arthritis marketing application and the related heart attack and stroke data. "It's too early to comment about that. We will review the study and the results for both drugs," Cruzan said, referring to comparisons between Vioxx and naproxen.

Meanwhile, Cruzan said that the FDA continues to believe that Vioxx is "effective and adequately safe" for its labeled uses.

The FDA has overseen a spate of drug recalls in the past 3 years, nudging companies to voluntarily withdraw their medicines after side effects rarely seen in clinical trials became apparent once the drugs hit the market and were taken by millions of people.

On Wednesday, Merck spokeswomen Jan Weiner told Reuters that the lower rate of heart attacks and strokes in the naproxen group, compared to Vioxx, was a "statistically significant" finding  a scientific term meaning it was a clear trend. But Weiner said that there was no evidence that Vioxx had actually put patients at higher risk of such adverse events. Instead, she said that it was likely that naproxen had conferred protection to patients taking that drug.

"Naproxen blocks platelet aggregation, it keeps them from clumping together. That's in the literature," Weiner said, referring to previous medical studies.

ABN Amro's Corso said he was not reassured by Merck's suggestion that naproxen conferred protection against heart attacks and strokes. "Medical authorities I've spoken to don't see any special reduction of such cardiovascular events in people taking naproxen," Corso said. "This could conceivably delay approval of Vioxx for rheumatoid arthritis. The FDA could ask for another trial to see if the results were a fluke or to add more validity to the findings," he said.