Researchers have shown that giving men the drug finasteride will lower their chance of developing prostate cancer by 25%. The study, which appears in the New England Journal of Medicine online, reports on the results of treating 9,060 men.

Prostate cancer is the most common non-skin cancer found in men, and is the second leading cause of cancer deaths in men. The American Cancer Society estimates that nearly 221,000 men in the United States will get prostate cancer this year, and nearly 29,000 will die from it.

Early detection through blood PSA (prostate-specific antigen) testing may be lowering the chances of a man dying from this disease. But treatment of early prostate cancer with either surgery or radiation carries major side effects, including impotence and incontinence, prompting the National Cancer Institute to look for ways to prevent the disease.

In this study, the Prostate Cancer Prevention Trial, the researchers tested whether the drug finasteride can prevent cancer development, thus avoiding the need for any treatment.

RESOURCES: Risk Factors for Prostate Cancer ACS Prostate Cancer Detection Guidelines

Finasteride blocks prostate growth by preventing the gland from responding to stimulation by male hormones. The drug is currently used to treat men with enlarged prostates to help them urinate freely (Proscar). In lower dosages, it is also used to treat male-pattern baldness (Propecia).

Nearly 25,000 men 55 and older were recruited at 221 study sites nationwide. They were screened for early prostate cancer by PSA testing and rectal exams. Many were eliminated because they had abnormal PSA levels, or an abnormal prostate exam. One defect of the study was that fewer than 4% of the men recruited were African American, though African Americans have a 60% higher risk of getting prostate cancer than whites and are more than twice as likely to die of the disease.

Half the men who made the final cut were given finasteride, a single 5-milligram pill taken once a day. The other half received a placebo. The men were followed for seven years with yearly PSA tests and rectal exams. If they showed an abnormality in either of these tests, a biopsy of the prostate was done.

Those men who showed no abnormality were offered a biopsy at the end of the study. Many refused the biopsy. Also, the study was closed early when the outcome became clear. This limited the final analysis to 9,060 men whose prostate cancer status was known by biopsy.

Finasteride reduced the prostate cancer rate by 25%. More than 24% of the men in the placebo group were found to have prostate cancer on biopsy compared to only 18.4% of the men who took finasteride. Only 10 men died of prostate cancer, five in each group.

Side effects differed in the two groups. Men taking the finasteride had more erectile dysfunction and loss of interest in sex. But they also had fewer urinary problems.

More disturbing were the results of the biopsies. Men taking finasteride who were found to have prostate cancer were more likely to have a fast growing kind, which would be expected to be more deadly.

So should men start taking finasteride to prevent prostate cancer? Not according to Peter Scardino, MD, chief of urology and head of the prostate cancer program at Memorial Sloan Kettering Cancer Center. In an accompanying editorial he urged caution because of the drug’s side effects and the higher rate of potentially deadlier cancers in the finasteride group.

He also questioned whether the cancers found in the placebo group were dangerous. Would they have eventually killed the men? If the cancers in the placebo group weren’t the dangerous kind, the benefits of finasteride would be less than what the study found. Scardino thinks the men in both groups will need to be followed to learn their ultimate outcome before concluding that finasteride is a worthwhile drug. “On balance, finasteride does not seem to be an attractive agent for the chemoprevention of prostate cancer,” he writes.

Harmon J. Eyre, MD, the Society's chief medical officer and executive vice president for research and cancer control, is more optimistic. "This study is a major step forward, providing the first clear evidence that chemoprevention of prostate cancer can work,” he says.

But he also cautions that there are no easy answers.

“The detection and treatment of prostate cancer already involves some fairly complex decision making, and this groundbreaking trial adds a significant element to the mix. The study will no doubt prompt a lot of men to start asking their doctors whether they should be on the drug, and we would encourage men to carefully weigh their options as this information is very new. There are still some important unanswered questions, especially regarding side effects, whether it can benefit men at increased risk, especially African Americans, who are twice as likely as white men to die of prostate cancer, and the mechanism by which men taking the drug develop higher grade tumors."

The study will be published in the July 17, 2003, issue of NEJM.