Men on "watchful waiting" for prostate cancer may be eligible to participate in a UC Davis Cancer Center study of a food extract that could help prevent disease progression.

The extract is genistein concentrated polysaccharide, or GCP. Derived from soybeans and shiitake mushrooms, GCP is used as a complementary prostate cancer therapy in Japan, Korea and other Asian countries. A preliminary study of GCP at UC Davis Cancer Center last year found the supplement reduced prostate-specific antigen (PSA) levels in a small number of "watchful waiting" patients. Rising PSA levels in prostate cancer patients can be a signal of disease progression, while falling levels can signal remission.

Watchful waiting is recommended for some small prostate cancers that cause no symptoms, are expected to grow very slowly, and are contained within one area of the prostate.

"If we can find a chemopreventive agent capable of slowing or stopping the progression of early, localized prostate cancer, we'll have something to offer men besides watchful waiting," said Ralph deVere White, professor and chair of urology at UC Davis School of Medicine and Medical Center, director of the UC Davis Cancer Center and a principal investigator of the GCP trial. "It would be an important development in prostate cancer."

To join the study, volunteers must have biopsy-proven prostate cancer and have undergone no previous treatment for the cancer. Patients also must have decided, with their doctors, to continue receiving no treatment for the next six months. Participants also must have a PSA level of 2 nanograms per milliliter of blood or higher; those with a PSA above 10 must have been on watchful waiting at least one year.

Volunteers will be randomized to receive either GCP or placebo capsules for six months. The study is "double-blinded" — neither volunteers nor study investigators will know who is taking which capsule until the end of the six months. At that point, volunteers who received the placebo, along with volunteers who received GCP but had minimal change in their PSAs, will be offered a six-month supply of GCP. All patients who continue to have decreasing or stable PSAs then may continue receiving GCP at no cost, as long as they have their PSAs checked every six months.

Volunteers will visit UC Davis Cancer Center three times during the study to have their blood drawn for PSA testing and other analyses, and to be monitored for such side effects as diarrhea.

Men who have an allergy to soy or soy products, who are already taking more than 2 grams of genistein daily in nutritional supplements or who have participated in a previous GCP trial at UC Davis are not eligible for the study.

The study is sponsored by Amino Up Chemical Co., Ltd., of Sapporo, Japan, a GCP manufacturer.