Bjelic-Radisic, Vesna

Abstract Purpose :Tamoxifen undergoes bioactivation to its active metabolite (Z)-endoxifen, which blocks estrogen-dependent breast tumor growth at high potency. We tested the feasibility and safety of supplementing standard tamoxifen therapy with low-dose (Z)-endoxifen in patients with breast cancer with compromised tamoxifen bioactivation. Patients and Methods :We conducted a prospective, interventional, three group randomized trial including 235 patients with hormone receptor–positive breast cancer who received standard tamoxifen therapy (20 mg/day). Patients were stratified by CYP2D6 genotype (n = 78), defining poor, intermediate, and normal metabolizers, or by baseline (Z)-endoxifen plasma concentration (n = 78), defining ≤15, 15 to 25, and ≥25 nmol/L. Co-treatment with (Z)-endoxifen 3 and 1.5 mg/day or placebo was performed, respectively. A control group (n = 79) received placebo regardless of metabolizer phenotype. The primary endpoint was the number of patients with (Z)-endoxifen levels >32 nmol/L after 6 weeks of treatment. Adverse events were continuously monitored. Results: A higher proportion of patients in both intervention groups achieved target concentrations >32 nmol/L compared with control (P < 0.0001). At 3 mg (Z)-endoxifen supplementation, 92.3% of CYP2D6 poor metabolizer patients and all patients with baseline (Z)-endoxifen ≤15 nmol/L achieved the target concentration. At 1.5 mg (Z)-endoxifen supplementation, 88% of CYP2D6 intermediate metabolizer patients and 95% of patients with 15 to 25 nmol/L baseline (Z)-endoxifen levels achieved the target concentration. Similar proportions of patients receiving (Z)-endoxifen (6/80, 7.5%) or placebo (8/155, 5.2%) experienced grade 3 adverse events. Conclusions: Adding low-dose (Z)-endoxifen to standard tamoxifen is safe and provides a new approach to personalized antiestrogen treatment for patients with low endoxifen plasma levels.