Objective: Tamoxifen suppresses insulin-like growth factor-1 (IGF-1) plasma levels in early and advanced breast cancer patients. Relationships between tamoxifen (GH) and IGF-1 are complex and not completely described yet. The present investigation was performed to evaluate the effect of acute and chronic tamoxifen administration on GH response to growth hormone-releasing hormone (GHRH), as well as on IGF-1 serum levels.
Materials and methods: Evaluation of GH after administration of GHRH was performed (a) at baseline, (b) 3 hours after 20 mg oral administration of tamoxifen and (c) after 12 weeks of 20 mg a day oral tamoxifen treatment, in fifteen postmenopausal stage I-II breast cancer patients. IGF-I was measured at baseline and after chronic tamoxifen administration.
Results: The GH response to GHRH was significantly reduced after 12 weeks of tamoxifen 10 mg administered twice a day orally (mean peak 3.2 +/- 0.2 micrograms/l, mean AUC 261.3 +/- 18.2 micrograms/minute p < 0.01 versus basal AUC). A concomitant significant reduction of IGF-1 was observed after 3 months of tamoxifen treatment. Basal pretreatment levels of 113.2 +/- 15.5 micrograms/l were suppressed to 70 +/- 7.9 micrograms/l (p < 0.01).
Conclusion: Our study confirm the inhibitory effect of tamoxifen on IGF-I and suggested, as shown in previous in vitro data, that its suppression could be directly related to GH reduction in response to GHRH stimulation.