US drug major Merck & Co has come out in support of its embattled painkiller Vioxx, saying that it believes the findings of two observational studies published by the Journal of the American Medical Association are “not supported by the current weight of the clinical trial data.”

One study, which was conducted by researchers at the Brigham and Women’s Hospital in Boston and Harvard Medical School and looked at 114 clinical trials involving COX-2 inhibitors, threw up an increased risk of kidney problems and arrhythmias with use of Vioxx (rofecoxib), but concluded that other drugs in the class, such as Pfizer’s Celebrex (celecoxib), did not pose the same risk to patients.

And a second analysis of studies delivered a further punch to Merck’s one-time blockbuster. Researchers at the University of Newcastle in Australia found that the higher heart risks associated with Vioxx can be seen as early as in the first month after treatment, which goes against Merck’s claim that it is only related to longer-term use.

In addition, the findings indicate that the NSAID diclofenac, which is sold by Novartis as Voltaren and Cataflam, induced the same cardiovascular risks as Vioxx. This may have implications for Merck’s Vioxx successor Arcoxia (etoricoxib), the safety of which was recently touted as being the same as diclofenac in a clinical trial, casting a shadow of doubt over the future of the drug, which is still awaiting approval in the US.

But Merck argues that observational data on COX-2 inhibitors and NSAIDs have “resulted in a range of different findings with a cardiovascular risk observed in some studies but not others,” and notes that studies of this nature are “associated with inherent limitations, including factors that cannot be adequately controlled for and ability to fully account for differences in risk factors between groups.” Therefore, the company says it believes it is “not appropriate to draw conclusions from these types of analyses when they contradict findings from randomised, controlled clinical trials, which are the gold standard in evaluating the safety and efficacy of medicines.”

The findings are, however, bound to make a mark in the 14,000 or so liability lawsuits still outstanding against Merck, spurred by Vioxx’ withdrawal in 2004 when its increased cardiovascular risk first came to light. “What the studies are going to do is provide substantial support to the opinions being expressed by the plaintiff’s experts, and that will serve to increase their credibility and persuasiveness to juries,” Reuters quotes Frank McClellan, Professor at Temple University’s Beasley School of Law, as saying.