Background: CT-P6 (C) is a proposed biosimilar to trastuzumab. This trial (NCT02162667) evaluated the similarity of C and trastuzumab in efficacy and safety for HER2+ EBC. Methods: 549 patients with HER2+ EBC were randomized to receive C (n=271) or trastuzumab (n=278) in combination with docetaxel (Cycles 1-4) and 5-fluorouracil, epirubicin, and cyclophosphamide (Cycles 5-8). C or trastuzumab was administered at 8 mg/kg (Cycle 1 only) followed by 6 mg/kg every 3 weeks. The primary endpoint was pathological complete response (pCR) rate at surgery. Secondary endpoints were overall response rate (ORR), PK, PD and safety. After surgery, patients received adjuvant C or trastuzumab to complete a total of 1-year treatment. Results: The pCR rate was 46.8% for C and 50.4% for trastuzumab. The 95% CIs for the risk ratio estimate were within the equivalence margin (0.74, 1.35) in PPS and ITT analyses. Other efficacy endpoints were similar between C and trastuzumab. The proportion of patients with at least 1 treatment-emergent SAE was 6.6% for C and 7.6% for trastuzumab. Only 1 patient in each group withdrew treatment due to significant LVEF decrease. Infusion-related reaction was reported for 8.5% of patients in C and 9.0% of patients in trastuzumab. Conclusions: This study demonstrated the similarity of efficacy in terms of pCR between CT-P6 and trastuzumab in EBC patients. Secondary efficacy endpoints also supported the similarity between CT-P6 and trastuzumab. CT-P6 was well tolerated with a similar safety profile to that of trastuzumab during the neoadjuvant period. Clinical trial information: NCT02162667