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Annotated Bibliography of NSABP Publications Effect of Tamoxifen on Serum Insulinlike Growth Factor - I Levels in Stage I Breast Cancer Patients Pollak M, Costantino J, Constantin P, Blauer S-A, Guyda H, Redmond C, Fisher B, and Margolese R Journal of the National Cancer Institute 82:1693-1697, 1990 Abstract Insulinlike growth factor I (IGF-I) has been shown to be a potent mitogen for breast cancer cells in vitro, and IGF-I receptors have been demonstrated on human primary breast neoplasms. In a randomized, placebo-controlled study, we document that administration of the antiestrogen tamoxifen to patients with breast cancer was associated with a statistically significant (P = .002) reduction in the serum level of IGF-I. The mean IGF-I level was 1.4 U/mL in the placebo-treated group and 0.9 U/mL in the tamoxifen-treated group. Because serum IGF-I level is growth hormone (GH) dependent and because data suggest that the pubertal surge in GH and IGF-I levels is sex steroid dependent, we speculate that the mechanism underlying our observation may involve blockade by tamoxifen of estrogen action in the hypothalamic-pituitary axis. We conclude that tamoxifen treatment reduces IGF-I levels and that this reduction may contribute to the therapeutic effect of the drug. Department of Oncology, Jewish General Hospital, Montreal, Canada