updated 10/31/2003 5:30:21 PM ET 2003-10-31T22:30:21

Scientists have discovered the first drug that promises to prevent prostate cancer, but deciding who should use it won’t be easy: Sexual side effects aside, it may actually increase aggressive tumors in some men. The drug is finasteride, already sold as a treatment for enlarged prostates under the brand name Proscar and, in a much lower dose, as Propecia for baldness.

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Men who took Proscar daily for seven years cut their chances of getting prostate cancer by nearly 25 percent compared with men given a dummy pill, researchers reported Tuesday in the New England Journal of Medicine.

The results were strong enough that the study of 18,000 men age 55 and older, originally scheduled to run for another year, was stopped this month.

'A huge step forward'
“This trial proves prostate cancer, at least in part, is preventable,” said Dr. Peter Greenwald, cancer prevention chief at the National Cancer Institute, who participated in the study himself and so far is cancer-free. “It’s a huge step forward.”

Because 220,000 U.S. men are diagnosed annually with prostate cancer, Proscar has “extraordinary public health potential,” said lead researcher Dr. Ian Thompson of the University of Texas Health Sciences Center in San Antonio.

Proscar works by preventing testosterone from changing into another hormone that fuels prostate enlargement and cancer growth. In the study, it worked equally well for men at low risk of cancer, and those at high risk - black men and those whose fathers and brothers had the disease.

But some troubling findings have critics questioning just how often Proscar should be used:

Men who developed prostate cancer while taking Proscar were more likely to have tumors that appear aggressive, what doctors term “high grade.” Some 6.4 percent of Proscar patients were diagnosed with those aggressive tumors, compared with 5.1 percent of men given a dummy pill. No one knows if it was a fluke - or if Proscar, a hormonal treatment, alters the prostate in a way that favors growth of more aggressive tumors.

The medical importance of the overall cancer reduction isn’t clear because of another quirk - researchers diagnosed prostate cancer in four times more placebo patients than expected.

Many were small, early-stage tumors found only because every study participant received a prostate biopsy even if blood tests for cancer-signaling PSA were normal - biopsies that in the real world never would have occurred.

“It looks like Proscar prevented little tiny, insignificant cancers, but did nothing for high-grade cancers or maybe even allowed them to become more common,” said Dr. Peter Scardino of New York’s Memorial Sloan-Kettering Cancer Center, who wrote a cautionary editorial accompanying the research. “That doesn’t sound like a very good trade-off to me.”

Drug's usefulness debated
The study didn’t test whether taking Proscar helped men live longer, added Dr. Herman Kattlove of the American Cancer Society. He predicted a “huge debate” about its usefulness.

“If it were free of side effects, it would be another story,” he said, citing impotence and loss of libido that were more common among Proscar users. “If it’s not going to save your life and it’s just going to ruin your sex life, I won’t” take it.

How effective is Proscar? Track 1,000 men starting at age 63, and 60 will get prostate cancer by age 70. Eighteen of those cancers will be high-grade. Give those same 1,000 men Proscar every day, and only 45 would get prostate cancer - but 22 of them would be high-grade, the NCI estimated.

Other considerations: Impotence and loss of libido are common in older age, but more common in Proscar users. However, Proscar users suffered fewer of the urinary problems common in aging prostates.

“There are very few prevention strategies that are not trade-offs,” said Thompson.

Whether to use Proscar is an individual decision men must make based on their own risk of prostate cancer and tolerance of side effects, stressed NCI’s Dr. Leslie Ford, who monitored the study.

“Take the time to review this data and make informed choices,” she advised.

Prostate cancer kills almost 29,000 U.S. men each year. The risk of cancer increases with age. Treatment causes major side effects such as incontinence and impotence, and there’s no good way to predict who needs aggressive therapy and who has a slow-growing, unthreatening tumor.

Men already taking Proscar to treat enlarged prostates shouldn’t stop because of the aggressive-tumor concern - just get regular prostate exams, Scardino cautioned.

Researchers will track Proscar users diagnosed with high-grade tumors to see if they fare poorly - or if the drug falsely made it appear their cancer was more aggressive, which hormonal treatments sometimes do, Thompson said.

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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