In a letter to the Lancet, doctors at the Royal Bolton Hospital have reported a possible fatal interaction between protease inhibitors and the anti-impotence drug Viagra™ (sildenafil).
The case involved a 47 year old man treated with ritonavir and saquinavir for over a year, who had been prescribed Viagra™ for erectile dsyfunction. Having experienced no untoward effects on eight prior occasions, the man was admitted to hospital suffering severe chest pain soon after taking a 25mg dose of Viagra™. He had a heart attack and died the next day.
Just two months ago, Pfizer issued new information concerning potential interactions between Viagra™ and other drugs. They advised a dose reduction to 25mg if taken alongside protease inhibitor treatment, and that a single 25mg dose should not be exceeded in any 48 hour period. Similar advice applies when the drug is taken with ketoconazole, itraconazole and erythromycin.
Protease inhibitors and Viagra™ are metabolised through a common enzyme in the liver, P450 3A4. Consequently, PIs can cause blood levels of Viagra™ to rise, increasing the risk of potentially serious side-effects.
The letter appeared in the June 12th issue of the Lancet 353:2071-2072.