Effect Modifiers of Low-Dose Tamoxifen in a Randomized Trial in Breast Noninvasive Disease

Purpose: Low-dose tamoxifen halved recurrence after surgery in a phase III trial in breast noninvasive disease without increasing adverse events. We explored the effect of low-dose tamoxifen in clinically relevant subgroups, including menopausal status, estradiol levels, smoking, body mass index, and proliferation of baseline lesion. Patients and Methods: Incidence of invasive breast cancer or ductal carcinoma in situ was the primary endpoint. HRs and interaction terms were estimated using Cox models. Results: A favorable HR and 95% confidence interval (CI) could be demonstrated for postmenopausal status (HR = 0.30; 95% CI, 0.11-0.82 vs. HR = 0.73; 95% CI, 0.30-1.76 in premenopausal women; P-interaction = 0.13), women with estradiol less than 15.8 pg/mL, presence of menopausal symptoms at baseline, and never smoking (P-interaction = 0.07), although the interaction P value was >0.05 for all characteristics. Efficacy was similar in all body mass index categories. Tumors with Ki-67 above the median level of 10% had a greater benefit (HR = 0.27; 95% CI, 0.09-0.81) than those with Ki-67 10%. Our results by menopausal status provide important insight into low-dose tamoxifen personalized treatment, although caution is necessary given their exploratory nature. Observation of an improved response in tumors with Ki-67 >10% is consistent but the use of the marker in this setting is investigational.