Expert Testifies Viagra and Cialis May Cause Skin Cancer

SAN FRANCISCO (CN) – With billions of dollars on the line for two of the world’s largest pharmaceutical firms, a medical researcher testified Tuesday of a “plausible” link between erectile dysfunction drugs Viagra and Cialis and skin cancer.

Cialis tablet (Photo credit: JEAN-PATRICK DONZEY/Wikipedia)

“There is a biologically plausible mechanism by which sildenafil causes accelerated melanoma progression,” Dr. Rizwan Haq of the Dana Farber Cancer Institute testified in court Tuesday, basing his conclusion on an analysis of about 50 research studies.

Sildenafil is the erectile dysfunction drug sold under the brand names Viagra by Pfizer and Cialis by Eli Lilly.

Haq testified on the first day of a four-day causation hearing, after which U.S. District Judge Richard Seeborg must decide if the science underpinning claims against Pfizer and Eli Lilly is good enough to justify a jury trial. More than 800 lawsuits were filed against the pharmaceutical giants and consolidated in a multidistrict case in San Francisco.

Before Haq took the stand Tuesday, Pfizer attorney Joseph Petrosinelli of Williams and Connolly in Washington attempted to discredit the doctor as one of three lone medical researchers who believe erectile dysfunction drugs cause cancer. No one else in the medical community has accepted those claims, he said.

“They are the only three people in the world who have said that,” Petrosinelli told the judge.

Regulators, researchers and respected medical organizations, including the American Academy of Dermatology, have found no causal link between those medications and melanoma, he argued.

Tempering the Pfizer lawyer’s enthusiasm, Seeborg noted that no doctors have come forward to say conclusively that Viagra and Cialis do not cause cancer.

Petrosinelli responded that under the law, the plaintiffs bear the burden of establishing causation.

“Are you suggesting there’s a certain critical number I have to come to,” Seeborg asked, referring to the number of doctors who say the drugs may cause skin cancer.

Petrosinelli replied that no professionals who did original research on this subject concluded the erectile dysfunction drugs contribute to melanoma. The only reason a “weak association” has been found is because men who take Viagra and Cialis “for whatever reason are exposed to the sun more,” he insisted.

On the witness stand, Haq explained how phosphodiesterase type 5 (PDE5) inhibitors in Viagra and Cialis decrease degradation of cyclic guanosine monophosphate (cGMP), a nucleotide which relaxes smooth muscle tissue and increases blood flow to treat erectile dysfunction.

“It’s pretty clear that PDE5 is present in melanoma cells,” Haq said.

Addressing one study that found sildenafil did not increase tumor growth in the lungs of mice injected with melanoma cells, Haq said he suspects the mice were not given high enough doses of the drug to reach any valid conclusions in that study.

“I think they under-dosed the animal,” Haq said.

On another study that found PDE5 inhibitors help prevent the spread of cancer, Haq said the plausibility of a link between sildenafil and cancer “is not related to a patient who already has metastatic melanoma,” meaning cancer that has already spread.

Because melanoma requires multiple genetic changes before it shows up on skin and can be detected, Haq said it could take several years before symptoms appear.

Implying that his opinion is unbelievable, Petrosinelli asked Haq if a man who took one Viagra pill 20 years ago could have accelerated skin cancer in his body with that one act decades earlier.

“My opinion is not about a single patient,” Haq said. “Sildenafil can accelerate the progression of those melanoma cells – one effective dose.”

The causation hearing is set to continue through Oct. 22.

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