YN reports research funding from Gilead Sciences, Inc; and honoraria from AstraZeneca, Eisai, Ono, Guardant, Takeda, Eli Lilly, Novartis, Pfizer, Chugai, PDR Pharma, Nihon Kayaku, Taiho, Bristol, Bayer, and Daiichi Sankyo. SN, TI, TN, and TK report nothing to disclose. NKS reports research funding from Gilead Sciences, Inc; and honoraria from Chugai, Daiichi Sankyo, Kyowa Kirin, Nippon Kayaku, Pfizer, Taiho, Eisai, AstraZeneca, and Zena. YY reports honoraria from AstraZeneca, Chugai, Kyowa-Kirin, Novartis, Lilly, Pfizer, Daiichi Sankyo, Nippon-Kayaku, Taiho, Eisai, Takeda, MSD, Sysmex and Exact Science; and other financial or non-financial interests with the Japanese Breast Cancer Society and the Japan Breast Cancer Research Group. NM reports honoraria from AstraZeneca, Chugai, Daiichi Sankyo, Eli Lilly, and Pfizer; and uncompensated leadership roles for JBCRG, Japanese Breast Cancer Society (JBCS), and Japan Society of Clinical Oncology (JSCO). KM reports research funding from Gilead Sciences, Inc; and honoraria from MSD, Kyowa-Kirin, Eli Lilly, Daiichi-Sankyo, Eisai, and Chugai. KS reports research funding from Amgen, AstraZeneca, Daiichi Sankyo, PRA Health Sciences, NanoCarrier, Takeda, Gilead Sciences, Inc, and Merck; and honoraria from Astra Zeneca, Eisai, Bayer Yakuhin, Pfizer, Nihon Medi-Physics, and MSD. AS reports research funding from Gilead Sciences, Inc; and honoraria from Chugai, Eli Lilly, AstraZeneca, Gilead Sciences, Inc, Exact Science, Daiichi Sankyo, Kyowa-Kirin, MSD, Pfizer, and Nihon Medi-Physics. CL, DZ, YI, DG, and DH report employment and stock or other ownership interests in Gilead Sciences, Inc. HI reports research funding from Chugai, Daiichi Sankyo, and AstraZeneca; consulting fees from Gilead Sciences, Inc; and honoraria from Chugai, Daiichi Sankyo, and AstraZeneca.

The protocol and any amendments were approved by the institutional review board of each participating site. Each individual participating in the study provided written informed consent to participate. The study was conducted in accordance with the International Council for Harmonisation E6(R2) addendum to its guideline for Good Clinical Practice, and applicable laws and regulations. The safety and tolerability of each dose level in phase 1 was assessed by the Data Monitoring Committee.