Background: The role of the HER3 receptor and its ligand heregulin (HRG) in the progression of multiple cancers has been well established. Seribantumab (MM-121) is a fully human, monoclonal IgG2 antibody that binds to the HRG domain of HER3, blocking HER3 activity. In retrospective analyses of prior seribantumab Phase 2 studies, high levels of HRG mRNA appeared to predict poor outcome when patients received standard of care (SOC) treatment. Addition of seribantumab to SOC improved progression-free survival (PFS) in patients with HRG positive (HRG+) tumors, consistent with the hypothesis that blockade of HRG-induced HER3 signaling by seribantumab can restore sensitivity to SOC impacted by HRG. Methods: In the current randomized, open-label, international, Phase 2 study, NSCLC patients will be screened for HRG using an RNA in situ hybridization assay on a recent biopsy tissue sample. Approximately 560 patients will be screened to support enrollment of 280 HRG+ patients, who will be randomized in a 2:1 ratio to receive seribantumab plus investigator’s choice of docetaxel or pemetrexed, or docetaxel or pemetrexed alone. Patients will be wild-type for EGFR and ALK and will have progressed following one to three systemic therapies for locally advanced and/or metastatic disease, including one platinum-containing regimen and anti-PD-1/PD-L1 where available and clinically indicated. The primary endpoint is overall survival (OS). Secondary endpoints include PFS, objective response rate and time to progression. The study has > 80% power to detect a 33% risk improvement in median OS (hazard ratio ≤ 0.67), using a one-sided, stratified log-rank test at a significance level of 0.025. An interim analysis is planned when 50% of final OS events have been reported. Enrollment was initiated in June 2015. Approximately 80 sites worldwide will be open to enrollment by the end of 2016. Clinical trial information: NCT02387216