Background: While outcomes for patients with EGFR-mutant NSCLC have significantly improved with the use of EGFR tyrosine kinase inhibitors, there remain limited treatment options for many patients once they develop resistance to these agents. The HER3/ERBB3 oncogene is overexpressed in many cancers, including NSCLC, and higher expression is correlated with poorer outcomes. U3-1402 is a HER3-targeting antibody-drug conjugate (ADC) of high drug-to-antibody-ratio (DAR = 7 to 8) with a novel linker and topoisomerase I inhibitor payload. Methods: This is a multicenter Phase 1, Dose Escalation, and Dose Expansion study of U3-1402 in metastatic or unresectable adenocarcinoma NSCLC subjects harboring EGFR-activating mutation who (a) are T790M mutation-negative after disease progression during treatment with erlotinib, gefitinib, or afatinib or (b) develop disease progression while on osimertinib. Eligible subjects are at least 18 years of age, have ECOG PS 0 or 1, have radiological documentation of disease progression while receiving continuous treatment with erlotinib, gefitinib, afatinib, or osimertinib, have at least one measurable lesion per RECIST v1.1, have adequate bone marrow and organ function, do not have mean QTc prolongation to > 470 ms for females and > 450 ms for males, and do not have spinal cord compression or clinically active brain metastases. In Dose Escalation, subjects receive U3-1402 via intravenous infusion in 21-day cycles. In Dose Escalation, escalation of U3-1402 dosing is based on dose-limiting toxicity data in subjects, guided by the modified Continuous Reassessment Method (mCRM). In Dose Expansion, subjects receive U3-1402 at the recommended dose for expansion (RDE) determined in Dose Escalation. Primary objectives are to determine the safety, tolerability, and RDE of U3-1402. Secondary objectives are to assess the pharmacokinetic parameters of U3-1402 and its components, and to assess antitumor activity of U3-1402 (RECIST v1.1). Enrollment to cohort 1 began in January 2018. Clinical trial information: NCT03260491