Aim: Previous in vitro studies have shown that tamoxifen down-regulates prolactin receptors in breast cancer cells. The aim of this study was to determine whether similar changes might provide the basis for a predictive test in patients.
Methods: Biopsy specimens were obtained from 28 post-menopausal women immediately before initiation of treatment with tamoxifen (20 mg daily) and after treatment for 7 days. Prolactin receptor mRNA, determined by reverse-transcription polymerase chain reaction, was then expressed relative to 18S ribosomal RNA.
Results: There was good evidence for a decline in receptor expression in response to treatment with tamoxifen in the whole group (p = 0.036) but with a particularly marked decrease (>60%) in a sub-group of 11 patients. No clear correlation with tumour type or grade, or with several other markers (progesterone receptor, c-erb B-2, pS2, or Bcl-2) was apparent.
Conclusion: Tamoxifen reduces expression of mRNA encoding the prolactin receptor in a sub-group of breast tumours and might provide the basis for a predictive test for tamoxifen therapy.