The MORE trial: multiple outcomes for raloxifene evaluation--breast cancer as a secondary end point: implications for...

Breast cancer is a common disease in the United States and Europe and is therefore a major target for prevention strategies. Estrogen plays a central role in its pathogenesis, and treatment with estrogen deprivation has long been recognized to be an effective therapy. Tamoxifen is the first selective estrogen receptor modulator (SERM) to be widely used for the treatment of breast cancer and has been demonstrated to reduce the risk of breast cancer in high-risk women. Raloxifene is a second-generation SERM that has estrogenic effects on bone and lipid metabolism, and antiestrogenic effects on breast tissue. Unlike tamoxifen, raloxifene displays antiestrogenic effects on the endometrium and may serve as a safer alternative to tamoxifen in the prevention setting. The MORE trial is a multicenter randomized placebo-controlled trial designed to determine whether 3 years of raloxifene reduces the risk of fracture in postmenopausal women with osteoporosis. As a secondary end point of the trial, raloxifene was shown to reduce the risk of both in situ and invasive breast cancer by 65% (RR = 0.35; 95% CI = 0.21-0.58; P < 0.001). The benefits were most significant in women who developed estrogen receptor (ER)-positive cancers, with a relative risk of 0.10 (95% CI = 0.04-0.24). This reduced incidence of breast cancer may be due to an anticarcinogenic effect or to a slowing of growth of occult ER-positive cancer, with a shift to the right in the time-to-cancer curve. A second large-scale prevention trial in breast cancer comparing tamoxifen to raloxifene is presently enrolling cancer-free, but high-risk postmenopausal women (the STAR trial). Future directions include combined estrogen blockade of the breast by the addition of an aromatase inhibitor to a SERM. New trial designs, including those based on biochemical changes at the tissue level, will be required to allow future progress in this field with adequate rapidity.