Tamoxifen-to-Exemestane Switch of Benefit in Early Breast Cancer


ATLANTA — Postmenopausal women with early-stage hormone-receptive breast cancer who have done well on 2–3 years of tamoxifen therapy have significantly improved disease-free and overall survival if they are switched to exemestane midway through their hormonal treatment, researchers reported at the annual meeting of the American Society of Clinical Oncology.

The beneficial effects of tamoxifen tend to wane the longer women are treated, said Dr. Judith Bliss, director of the Clinical Trials and Statistics Unit at the Institute for Cancer Research in London. “Five years of tamoxifen has been the gold standard of treatment for hormone-sensitive postmenopausal breast cancer patients. But we know that the amount of benefit for tamoxifen is greater in the early years, compared with the later years of its use. We wanted to determine what we could add to this treatment to improve on the good results we already have,” she said.

Accordingly, Dr. Bliss and her colleagues initiated the Intergroup Exemestane Study of more than 4,700 patients from 37 countries and 366 centers worldwide. In the study, patients who remained disease free after 2–3 years of tamoxifen therapy were randomized to continue on tamoxifen, or to start treatment with exemestane, an aromatase inhibitor approved for this purpose by the Food and Drug Administration and marketed as Aromasin by Pfizer. Disease-free survival, the trial's primary end point, was increased by approximately 25% in the women who were switched to exemestane, compared with women who remained on tamoxifen, Dr. Bliss reported.

At the time Dr. Bliss and her colleagues started the trial, routine testing of estrogen-receptor status was not being done in several countries. When they went back to determine estrogen-receptor status in their population, they were able to identify 122 women as hormone-receptor negative. After omitting them and reanalyzing the data, the results remained virtually the same, Dr. Bliss said in an interview.

Serious side effects overall were very low. Cardiovascular side effects were equal for both treatments. There was a slight increase in musculoskeletal side effects in women switched to exemestane. However, it is difficult to determine whether this increase was due to cessation of tamoxifen, which is known to be protective of bone, or due to exemestane, Dr. Bliss noted. “I think what we are seeing here is very exciting. By incorporating this switch, we add to the early benefit from treatment with tamoxifen,” she said.

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