A drug used to treat enlarged prostate or hair loss in men has been shown to have a long-term protective effect against prostate cancer.

The study, led by Fred Hutchinson Cancer Research Center biostatistician Dr. Joe Unger, and recently published in Journal of the National Cancer Institute, linked data from a large prostate cancer clinical trial conducted by the clinical trial network SWOG with Medicare claims data to determine that the steroid tablet finasteride could protect men from developing the cancer for up to 16 years.

The original NCI-funded study, known as the Prostate Cancer Prevention Trial, set out to determine if finasteride (also known as Proscar or Propecia) could prevent prostate cancer in men ages 55 and older. That study, which came to an end in 2003, found finasteride use for 7 years reduced a man’s risk of developing prostate cancer by 25% and that the protective benefit lasted through that 7-year period, which is how long the study followed participants.

Unger and his colleagues went back to the PCPT cohort of nearly 19,000 healthy men (half of whom were given finasteride and half of whom received a placebo) to see if finasteride offered a longer-term benefit.

Linking patient data to gain new insights

They matched participants with Medicare claims data and found 75% of the PCPT participants were in the Medicare system. The researchers, who included Dr. Ian Thompson Jr of CHRISTUS Santa Rosa Hospital, Dr. Dawn Hershman of Columbia University Medical Center, and Drs. Scott Ramsey and Catherine Tangen of Fred Hutch, then created an algorithm to flag PCPT participants on Medicare who’d been diagnosed with and/or treated for prostate cancer.

A total of 3,244 PCPT participants developed the disease in the 16 years since the initiation of the original trial: 1,805 of those received placebo and 1,439 had taken finasteride. Overall, researchers found men who’d taken finasteride had a 21% decreased risk of getting prostate cancer over the course of 16 years compared to those who took placebo.

“These findings raise the intriguing possibility that 7 years of finasteride can reduce prostate cancer diagnoses over a much longer period than was previously shown,” Unger said. “It’s a low-cost generic drug, with minimal side effects, that can have a benefit that lasts long after men stop taking it.”

Side effects of finasteride include decreased libido, impotence, and decreased ejaculate volume. Potential side effects of prostate cancer treatment, according to Fred Hutch’s clinical care partner Seattle Cancer Care Alliance, include infertility, incontinence, reduced libido, decreased ejaculate volume, and impotence.

The innovative design of this newly published, NCI-funded study was also notable, Unger said. By tapping Medicare claims data, the researchers could examine outcomes for trial participants many years after the original clinical trial had ended to answer new questions regarding reduction of cancer risk.

“These secondary data sources are emerging as a new paradigm for long-term follow up for cancer clinical trials,” he said. “It’s an exciting new avenue of research.”