A new study done in mice suggests that taking raloxifene after 5 years of tamoxifen therapy may not continue to prevent the growth of breast cancers. In fact, raloxifene may stimulate the growth of endometrial tumors, according to Ruth O'Regan, M.D., and V. Craig Jordan, Ph.D., of Northwestern University Medical School in Chicago and colleagues.

Writing in the Journal of the National Cancer Institute, they noted that adjuvant therapy with tamoxifen helps some women with early-stage breast cancer reduce their risk of recurrence. But some studies have suggested this benefit levels off after 5 years, and further treatment with tamoxifen not only doesn't help, it may increase a woman's risk of endometrial cancer.

To get around this, researchers have suggested following a 5-year tamoxifen treatment with the drug raloxifene. Results from an earlier clinical trial of raloxifene indicated that the drug may decrease a woman's risk of breast cancer without increasing her risk of endometrial cancer.

However, this new study suggests that raloxifene after tamoxifen may not be beneficial. O'Regan and her colleagues exposed breast and endometrial tumors in mice to combinations of estrogen, tamoxifen, and raloxifene or nothing at all. The authors found that raloxifene did not continue to prevent the growth of breast tumors previously exposed to more than 5 years of tamoxifen. Furthermore, raloxifene did not block the growth of endometrial tumors caused by low-dose estrogen.

These findings "do not support an optimistic future for the use of raloxifene as a replacement for tamoxifen after 5 years of adjuvant therapy," wrote Michael B. Sporn, M.D., of Dartmouth Medical School, in an accompanying editorial. However, he pointed out that the current study does not address whether raloxifene might be useful in preventing the development of a second primary breast cancer or a primary endometrial cancer.

Sporn noted that the ongoing Study of Tamoxifen and Raloxifene (STAR) Trial, a large randomized clinical trial, will go a long way toward determining the efficacy of raloxifene and tamoxifen for the primary prevention of breast cancer and endometrial cancer in high-risk women.

SOURCE:
Journal of the National Cancer Institute, February 20, 2002; 94:242-243, 274-283