No Effect of Soy on Breast Proliferation in Small Study


SAN ANTONIO — Consumption of soy isoflavones by postmenopausal breast cancer survivors doesn't appear to stimulate epithelial proliferative activity in the contralateral breast, according to a small pilot study.

This is a reassuring, albeit still preliminary, observation. The great majority of breast cancer patients are postmenopausal. They are discouraged from using hormone therapy to manage their menopausal symptoms, which can be quite severe. Soy supplements, which are rich in phytoestrogens, are growing in popularity as a nonpharmacologic alternative, Melanie R. Palomares, M.D., noted at a breast cancer symposium sponsored by the Cancer Therapy and Research Center.

Because preclinical work had shown conflicting stimulatory and inhibitory effects of soy isoflavones on breast tissue, Dr. Palomares and her coinvestigators launched the University of Washington/Seattle Cancer Care Alliance Phytoestrogen Trial. Participants were randomized to 100 mg/day of isoflavone tablets or placebo. Ultrasound-guided core biopsies of the contralateral breast were taken at baseline and after 6 and 12 months of therapy, explained Dr. Palomares of City of Hope National Medical Center in Duarte, Calif.

She reported on 23 postmenopausal disease-free women previously diagnosed with and treated for in situ or early-stage invasive breast cancer who have completed the year-long randomized trial.

The primary study end point was change in Ki-67 index, a widely used measure of epithelial proliferation. Ki-67 levels were elevated in both treatment and control groups at baseline, which was to be expected in light of the known elevated risk of contralateral breast cancer in women with a history of breast cancer. The Ki-67 index dropped steadily throughout the 12 months of follow-up, indicative of a decline in breast epithelial proliferation. The decline was greater in soy isoflavone-treated women, although not significantly so.

Hyperplasia was present in the contralateral breast tissue samples of 10 patients at baseline and 5 patients after a year. The treatment groups were too small to show significant differences in serial histology. There was a trend toward decreased estrogen receptor expression over time in both the soy isoflavone- and placebo-treated groups, but no significant differences between the two study arms.

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