Background: NK105 is a paclitaxel (PTX)-incorporating "core-shell-type" polymeric micellar nanoparticle formulation composed of block copolymers (polyethylene glycol and a polyamino acid). The efficacy and safety of NK105 and paclitaxel in advanced or recurrent breast cancer have never been compared at equivalent dose levels., Patients and Methods: Patients were randomly assigned to either NK105 or PTX in a 1:1 ratio. The study drug was administered on Day 1, 8, and 15 of a 28-day cycle with 80 mg/m 2 . The primary endpoint was overall response rate (ORR), secondary endpoints were progression-free survival (PFS), overall survival (OS), and adverse events., Results: A total of 123 patients (NK105, n=62; PTX, n=61) received one of the two drugs. There was no significant difference in ORR, the median PFS, or OS (NK105 group: 41.9%, 9.1, and 27.5 months, respectively; PTX group: 45.9%, 7.8, and 32.4 months, respectively). Neutropenia occurred more frequently in the NK105 group, but most patients did not require granulocyte-colony stimulating factor or dose-reduction. The median time to onset of peripheral sensory neuropathy (PSN) in the NK105 group was significantly longer than that in the PTX group (p=0.001), and PSN (≥ grade 3) was not observed in the NK105 group., Conclusion: Weekly NK105 administration was well-tolerated. Efficacy was similar in both groups. The PSN profile was better in the NK105 group., Competing Interests: Y.K. reports having received honoraria from Chugai, AstraZeneca, Eisai, Pfizer, Kyowa Kirin, Novartis, Taiho, and Nippon Kayaku; and financial support from Nippon Kayaku for attending meetings. T.S. reports having received grants from Eisai, Kyowa Kirin, Taiho, Chugai, Nippon Kayaku, AstraZeneca, NRG Oncology Japan, Sawai, JBCRG, Daiichi Sankyo, Novartis, and West Japan Oncology Group; honoraria from Aska, AstraZeneca, Eisai, Ono, Taiho, Takeda, Chugai, Eli Lilly, Nippon Kayaku, Novartis, Pfizer, MiRTeL, and Meiji; and financial support from Nippon Kayaku for attending meetings. T.T. reports having received grants from Chugai, Daiichi Sankyo, Ono, MSD, and Eisai, and honoraria from Chugai, Daiichi Sankyo, Eisai, Eli Lilly, and Celltrion Healthcare. T.A. reports having received honoraria and support for attending meetings from Pfizer, Chugai, AstraZeneca, Eli Lilly, Eisai, Daiichi Sankyo, MSD, Taiho, Celltrion, Nihon Medi-Physics, Kirin, Konica Minolta REALM, and Nippon Kayaku. T.Y. reports having received grants from Chugai, Taiho, Kyowa Kirin, and Nippon Kayaku; honoraria from Chugai, Daiichi Sankyo, Eisai, Eli Lilly, Novartis Pharma, Taiho, AstraZeneca, Pfizer Japan, and Nippon Kayaku; and support for attending meetings from Nippon Kayaku. N.M. reports having received grants from AstraZeneca, Daiichi Sankyo, Eli Lilly, MSD, Sanofi, Chugai, Eisai, Kyowa Kirin, Novartis, Pfizer, and Nippon Kayaku; honoraria from AstraZeneca, Chugai, Eisai, Eli Lilly, Pfizer, and Nippon Kayaku; and board memberships from the Japanese Breast Cancer Society and the Japan Breast Cancer Research Group Association. Y.K. reports having received honoraria and support for attending meetings from Nippon Kayaku. A. O. reports having received grants from AstraZeneca, Eisai, Kyowa Kirin, Taiho, Chugai, Nippon Kayaku, Covance Japan, Maruho, Bayer, Sanofi, Eli Lilly, MSD, Takeda, Sawai, Daiichi Sankyo, Pfizer, and West Japan Oncology Group; honoraria from AstraZeneca, Eisai, Daiichi Sankyo, Chugai, Nippon Kayaku, Pfizer, Shionogi, and Eli Lilly; and financial support from Nippon Kayaku for attending meetings. J.W. reports having received grants and honoraria from AstraZeneca, Eisai, MSD, Gilead, Daiichi Sankyo, and Eli Lilly; honoraria from Chugai, Pfizer, Taiho, Novartis, Takeda, and Nippon Kayaku; and support for attending meetings from Nippon Kayaku. R.S. is an employee of Nippon Kayaku Co. Ltd. The authors report no other conflicts of interest in this work., (© 2022 Kosaka et al.)