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Purpose Genexol-PM is a Cremophor EL–free formulation of low-molecular-weight, non-toxic, and biodegradable polymeric micelle-bound paclitaxel. We conducted a phase III study comparing the clinical efficacy and toxicity of Genexol-PM with conventional paclitaxel (Genexol). Materials and Methods Patients were randomly assigned (1:1) to receive Genexol-PM 260 mg/m2 or Genexol 175 mg/m2 intravenously every 3 weeks. The primary outcome was the objective response rate (ORR). Results The study enrolled 212 patients, of whom 105 were allocated to receive Genexol-PM. The mean received dose intensity of Genexol-PM was 246.8±21.3 mg/m2 (95.0%), and that of Genexol was 168.3±10.6 mg/m2 (96.2%). After a median follow-up of 24.5 months (range, 0.0 to 48.7 months), the ORR of Genexol-PM was 39.1% (95% confidence interval [CI], 31.2 to 46.9) and the ORR of Genexol was 24.3% (95% CI, 17.5 to 31.1) (pnon-inferiority=0.021, psuperiority=0.016). The two groups did not differ significantly in overall survival (28.8 months for Genexol-PM vs. 23.8 months for Genexol; p=0.52) or progression-free survival (8.0 months for Genexol-PM vs. 6.7 months for Genexol; p=0.26). In both groups, the most common toxicities were neutropenia, with 68.6% occurrence in the Genexol-PM group versus 40.2% in the Genexol group (p < 0.01). The incidences of peripheral neuropathy of greater than grade 2 did not differ significantly between study treatments. Conclusion Compared with standard paclitaxel, Genexol-PM demonstrated non-inferior and even superior clinical efficacy with a manageable safety profile in patients with metastatic breast cancer.