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UC Davis Health System

UC Davis Health System

Soy vs. prostate cancer?

Ralph deVere White is testing a soy-shiitake supplement in men with early prostate cancer. 

Many men with early, low-grade prostate cancer choose to postpone surgery or radiation, treatments that can result in impotence or incontinence. But the conservative alternative, often called "active surveillance" or "watchful waiting," can be tough psychologically: Some patients call the approach “worry and pray.”

UC Davis Cancer Center researchers are testing what they hope will be a third option for such men: A daily handful of soy-mushroom capsules.

Known as genistein combined polysaccharide, or GCP, the supplement has been studied in two clinical trials at UC Davis Cancer Center. Results are promising. In the first trial, the compound slowed or halted prostate cancer progression in men with early, untreated disease. Data from the second are still being analyzed.

Watchful waiting

GCP is a proprietary extract cultured from soybeans and shiitake mushrooms and marketed by Amino-Up of Japan. It's not a cure, and it's unknown how long its therapeutic effects might last. But GCP could enable men to postpone the day they'll need aggressive medical treatment.

"It's a promising approach," said Ralph deVere White, professor of urology, director of the UC Davis Cancer Center, and principal investigator of the GCP studies. "If we can find a chemopreventive agent capable of slowing or stopping the progression of early, localized prostate cancer, it would be an important development in our treatment of the disease."

Long used in Asia

GCP is used as a complementary therapy for prostate cancer in Japan, Korea and other parts of Asia. Before bringing it to clinical trials, UC Davis studied the extract in the laboratory for many years. Robert Hackman, a researcher in the UC Davis Department of Nutrition, has collaborated with deVere White on the studies.

The first clinical study, a small Phase I trial involving 13 prostate cancer patients, took place at the Cancer Center four years ago. Results were published in the Journal of Urology.

In that study, deVere White found that the supplement reduced PSA levels by as much as 61 percent in the prostate cancer patients who had received no previous treatment for their disease. PSA, for prostate-specific antigen, is a marker for prostate cancer. For unknown reasons, GCP didn't benefit men who had received radiation, surgery or hormone therapy. The only side effect was diarrhea.

Phase II trial under way

A second, phase II study enrolled 64 men. The volunteers received either GCP or a placebo for six months. Neither the patients nor the study investigators knew who took the placebo until the six months were up. At that point, patients who received the placebo, along with patients who received GCP but had minimal change in their PSAs, were offered a six-month supply of the supplement at no cost. At the end of that second six-month period, all patients who continued to show a decreased or stable PSA could opt to continue receiving GCP for free, as long as they agreed to have their PSAs checked every six months.

If the results of this study are as promising as the phase I findings, deVere White will seek funding for a much larger, Phase III study.