EFFECT OF TAMOXIFEN ON SERUM INSULIN-LIKE GROWTH FACTOR-I LEVELS IN STAGE-I BREAST-CANCER PATIENTS

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 arandomized, 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 sepeculate 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. [J Natl Cancer Inst 82:1693-1697,1990) © 1990 Oxford University Press.