CHICAGO, Jan. 9 (UPI) -- The drug Viagra appears to stimulate the formation of blood clots, which could explain the heart attacks and strokes experienced by some men taking the erectile dysfunction drug, scientists said Thursday.
"We found that Viagra actually has a stimulatory role in ... clumping platelets, which is important in causing heart attacks and strokes," Dr. Xiaoping Du, the study's lead author and an associate professor of pharmacology at the University of Illinois at Chicago's College of Medicine, told United Press International. Platelets are blood components that help form blood clots and repair damaged blood vessels.
This was surprising, Du said, because "originally this drug was designed to ... prevent heart attack."
In addition, Du's research revealed a compound in the body called cyclic guanosine monophosphate, or cGMP, that was long thought to inhibit platelet clumping and blood clots actually does the opposite.
"We found cGMP actually ... promotes platelet aggregation," Du said. This could explain some cases of heart attack and stroke in men taking Viagra because the drug raises cGMP levels, Du said. Heart attacks and strokes have occurred in men taking the drug who had a history of heart problems, and more than 500 men have died while taking it.
"For years people argued about why Viagra causes this but nobody really provided a clear explanation but I think we have quite a good explanation for this," Du said.
"The study has not progressed to the point when it can be claimed to offer a valid explanation of heart attacks or stroke reported with Viagra use," Pankaj Ganguly, of the National Heart, Lung and Blood Institute in Bethesda, Md., told UPI. "The laboratory findings of this study need to be verified in a clinical setting to establish that point," he added.
In the study, which appears in the Jan. 10 issue of the journal Cell, Du's team obtained platelets from normal, healthy individuals and exposed them to Viagra. By itself, Viagra did not induce platelet aggregation. But the drug did cause clumping when other chemicals, which are released by the body when a blood vessel is damaged, were added. The researchers noted clumping was seen with Viagra concentrations much lower than normally used for treating erectile dysfunction.
The findings indicate Viagra is safe for healthy men, Du said. "This drug by itself cannot cause (clots in blood vessels and the heart) but it promotes (clots) when the patient already has a precondition" such as blocked or damaged arteries, he said. "And that explains why for most people it's OK but some people die," he added.
The research "goes against a lot of the prevailing thinking in this area" and could "open up our minds to other ways of thinking about" blood clot formation and hardening of arteries, Ed Plow, chairman of the department of molecular cardiology at the Cleveland Clinic, told UPI. The research also could lead to new treatments or methods to prevent these conditions, he said.
Plow noted, however, that because the findings conflict with long-held beliefs about the role of cGMP "independent verification is going to be called for" before the medical community fully accepts it.
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(Reported by Steve Mitchell, UPI Medical Correspondent, in Washington.)
