Endocrine Biomarker Changes in a Randomised Low-Dose Tamoxifen Trial for Breast Cancer Prevention.

BACKGROUND: Tamoxifen reduces breast cancer incidence and recurrence, but uptake for primary prevention remains limited, largely because of concerns regarding adverse effects at the standard 20 mg dose. Understanding systemic endocrine effects of lower tamoxifen doses may help improve future prevention strategies. METHODS: We analysed data from 1,055 healthy women enrolled in the randomised, double-blind, placebo-controlled KARISMA trial, assigned to placebo or tamoxifen 1, 2.5, 5, 10, or 20 mg daily for 6 months. Plasma concentrations of endocrine biomarkers and tamoxifen metabolites were measured at study end. Associations between randomised tamoxifen dose, circulating metabolite concentrations, and endocrine biomarker plasma concentration (estrogens, androgens, progestogens, cortisol, prolactin, and sex hormone-binding globulin (SHBG)), were evaluated. RESULTS: Tamoxifen dose was associated with measurable endocrine changes after six months, most consistently increased SHBG levels, with additional associations observed for cortisol and hydroxyprogesterone. SHBG demonstrated the clearest dose-response relationship, with increasing levels across tamoxifen dose groups and evidence of attenuated increase at intermediate doses. Large relative differences between placebo and 20 mg tamoxifen were additionally observed for estrone, estrone sulphate and estradiol. Unadjusted analyses of circulating endoxifen showed broadly similar endocrine patterns; however, these associations were substantially attenuated after adjustment for randomised tamoxifen dose. Circulating tamoxifen metabolites were strongly correlated with administered dose and with one another. CONCLUSIONS: Low-dose tamoxifen was associated with measurable endocrine changes, particularly in SHBG, cortisol, and hydroxyprogesterone. These findings show endocrine pharmacodynamic responses during low-dose tamoxifen therapy warrants further investigation as a potential component of future individualised prevention and adjuvant endocrine therapy strategies. TRIAL REGISTRATION: ClinicalTrials.gov ID: NCT03346200.