<i>CYP2D6</i> Genotype-Guided Tamoxifen Dosing in Hormone Receptor-Positive Metastatic Breast Cancer (TARGET-1): A Randomized, Open-Label, Phase 2 Study

Abstract

Background: In patients taking tamoxifen, the CYP2D6 genotype causes different exposure of active metabolite endoxifen. There is a high frequency of CYP2D6 decreased-function variant in Asian populations. Therefore, we prospectively evaluated whetherCYP2D6 genotype-guided tamoxifen dosing in patients with hormone receptor-positive metastatic breast cancer could have an impact in the clinical outcome. Methods: In this randomized, open-label, multicenter, phase 2 study, patients who needed first-line tamoxifen therapy were enrolled in Japan. Based on individual CYP2D6 genotype, patients heterozygous (wt/V) or homozygous (V/V) for variant alleles of decreased or no-function were randomly assigned (1:1) to tamoxifen at increased dose (40 mg daily, ID arm) or regular dose (20 mg daily, RD arm), and patients homozygous for wild-type alleles (wt/wt) received tamoxifen at 20 mg daily. The primary endpoint was progression-free survival (PFS) rate at 6-month. The secondary endpoints included PFS and correlation of (Z)-endoxifen concentration with clinical outcomes. Findings: Between December 2012 and July 2016, 186 patients were enrolled. 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-month were not significantly different between ID and RD arms (67·6% vs. 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 vs. 51·1 nM; P < 0.0001) and were also higher compared with wt/wt patients (72·0 nM; P = 0·0452). No significant difference in (Z)-endoxifen concentrations was observed between patients with and without progression at 6-month (p = 0·43). Interpretation: In patients with CYP2D6 variant alleles, increasing tamoxifen dosing did not achieve higher PFS rate at 6-month. The CYP2D6 genotype solely cannot explain individual variability in the efficacy of tamoxifen. Clinical Trial Number: This study is registered with UMIN Clinical Trials Registry, number UMI09155. Funding Statement: This study was supported by grants from the Project for Development of Innovative Research on Cancer Therapeutics and from the Tailor-made Medical Treatment Program (BioBank Japan Project) funded by the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and the AMED under Grant Number 17ck0106191h0003). Declaration of Interests: YF reports grants from the Japan Agency for Medical Research and Development (AMED). MK reports grants from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT). KT reports grants from AstraZeneca and Daiichi-Sankyo outside the submitted work. CKI, MT, and YT report personal fees from AstraZeneca outside the submitted work. TT and JT report grants and personal fees from Daiichi-Sankyo outside the submitted work. SS reports grants and personal fees from AstraZeneca, and personal fees from Daiichi-Sankyo outside the submitted work. TY and YF report personal fees from AstraZeneca and Daiichi-Sankyo outside the submitted work. NTU reports grants from Daiichi-Sankyo outside the submitted work. YT reports personal fees from Meiji Seika Pharma outside the submitted work. All other authors declare no competing interests. Ethics Approval Statement: We obtained written informed consent from each patient before participation. The trial followed the ethical principles of the Declaration of Helsinki and Japanese Ethical Guidelines and Clinical Research. This study protocol and any amendments thereof were approved by an independent Ethics Committee or Institutional Review Board at each institution.