A box of Viagra, typically used to treat erectile dysfunction, is seen in a pharmacy in Toronto January 31, 2008. REUTERS/Mark Blinch

WASHINGTON (Reuters) - Impotence drugs may help carry cancer-fighting drugs through the brain to treat malignant tumors, U.S. researchers reported on Monday.

Tests in rats showed two erectile dysfunction drugs -- Schering-Plough’s Levitra and Pfizer’s Viagra -- helped carry a chemotherapy drug past the blood-brain barrier, the team at Cedars-Sinai Medical Center in Los Angeles said.

Rats with brain tumors lived 42 days when injected with the cancer drug adriamycin. But when they also got Levitra, known generically as vardenafil, the rats survived an average of 53 days. Levitra appeared to be more effective, the researchers reported in the journal Brain Research.

Levitra and Viagra, known generically as sildenafil, are in a class of drugs known as PDE5 inhibitors. They were originally tested as heart drugs because they increase blood flow in small vessels.

“We chose adriamycin for this study because it is one of the most effective drugs against brain tumor cell lines in the laboratory but it has very little effect in animals and humans because it is unable to cross the blood-brain tumor barrier,” neurosurgeon Dr. Keith Black, who led the study, said in a statement.

“The combination of vardenafil and adriamycin resulted in longer survival and smaller tumor size,” Black said.

The blood-brain barrier is a molecular mechanism that keeps harmful agents out of the brain. Brain tumors grow little blood vessels to supply themselves with nutrients and these also have a barrier, called the blood-brain tumor barrier.

Black said the impotence drugs appear to affect the tumor blood-brain barrier but not the larger blood-brain barrier, which may help doctors use chemotherapy drugs to kill off brain tumors without damaging healthy brain tissue, he said.