PURPOSE In patients taking tamoxifen, the CYP2D6 genotype causes different exposure of active metabolite endoxifen. The objective of this randomized, open-label, multicenter, phase II study was to prospectively evaluate whether CYP2D6 genotype–guided tamoxifen dosing in patients with hormone receptor–positive metastatic breast cancer could have an impact on the clinical outcome. METHODS Patients who needed first-line tamoxifen therapy were enrolled. Based on individual CYP2D6 genotype, patients heterozygous (wild type [wt]/variant [V]) or homozygous (V/V) for variant alleles of decreased or no function were randomly assigned to receive tamoxifen at an increased dose (ID arm; 40 mg daily) or regular dose (RD arm; 20 mg daily), and patients homozygous for wild-type alleles (wt/wt) received tamoxifen at 20 mg daily. The primary endpoint was the progression-free survival (PFS) rate at 6 months. The secondary endpoints included PFS and correlation of Z-endoxifen concentration with clinical outcomes. RESULTS Between December 2012 and July 2016, 186 patients were enrolled in Japan. Of 184 evaluable patients, 136 carried wt/V or V/V (ID arm, 70; RD arm, 66), and 48 carried wt/wt. PFS rates at 6 months were not significantly different between the ID and RD arms (67.6% v 66.7%). The serum trough concentrations of Z-endoxifen in the ID arm were significantly higher than those in the RD arm (median, 89.2 nM v 51.1 nM; P < .0001) and were also higher compared with wt/wt patients (72.0 nM; P = .045). No significant difference in Z-endoxifen concentrations was observed between patients with disease progression and those who were progression free at 6 months ( P = .43). CONCLUSION In patients with CYP2D6-variant alleles, increasing tamoxifen dosing did not achieve a higher PFS rate at 6 months. The CYP2D6 genotype solely cannot explain individual variability in the efficacy of tamoxifen.
CDK4/6 Inhibitors in Hormone Receptor-Positive Metastatic Breast Cancer: Current Practice and Knowledge