Study of Efficacy and Safety of Myl1401O + Taxane vs Herceptin©+ Taxane for 1st Line, Met. Br. Ca.

Part 1: A multicenter, double-blind, randomized, parallel-group, Phase III study to compare the efficacy and safety of MYL-1401O (Mylan Trastuzumab biosimiliar) plus taxane versus Herceptin® plus taxane in patients with HER2+ MBC. Either docetaxel or paclitaxel (Investigator site level choice) is planned for at least 24 weeks until documented response to therapy, disease progression, or discontinuation. Disease response and progression will be assessed locally by the Investigator on the basis of clinical and radiographic evidence using RECIST 1.1 criteria. The primary and secondary efficacy analyses will be performed using independently assessed radiographic evidence for the Intent-to-Treat (ITT) population. The primary analysis population for best ORR will be those patients who had measureable disease at baseline and for the secondary endpoint, DR, only those who are responders will be included in the analysis. Part 2: A multicenter, double-blind, parallel-group study to continue to compare the safety and immunogenicity and efficacy of MYL-1401O versus Herceptin® in patients with HER2+ MBC who have clinical benefit to first-line combination therapy with a taxane. All patients who have at least stable disease (SD), will continue with either single agent MYL-1401O or Herceptin® alone until death, unacceptable toxicity or disease progression. Population pharmacokinetics: During Part 1 of the study, for both MYL-1401O, and Herceptin® treatment arms, PK sampling for Cmin and Cmax (end of infusion) will be collected for all patients. A PopPK subset, with sufficient samples available to perform the necessary analysis of PopPK modeling will be used to assess AUC, Cmax, Cmin, clearance, Vd, and T1/2 at various time points in the PopPK. Patients randomized into the main study will sign an additional consent form for the PopPK subset. We anticipate that approximately 80 patients will need to be enrolled in this subset collection in order to obtain sufficient samples for analysis. Long term survival follow-up: OS for this treated population will be determined with every 3 month follow-up until either 240 deaths or 36 months, whichever occurs first, as observed from the time of randomization. The results for OS are expected to be reported March 2019. Exploratory evaluations: In addition, during Part 1 of the study, blood samples will be collected in all patients to assess the impact of soluble shed HER2/ECD on PK and efficacy at pre-dose on Cycles 1, 3, 5, 7, and end of treatment (EOT). Additional samples (ECD) will be obtained on Cycles 9, 13 and every 4 cycles thereafter, EOT and end of study (EOS) for continued evaluation of immunogenicity in patients continuing to receive therapy. A blood sample will be obtained on Cycle 1, Day 1, to be used for assay development and validation