Background: Although first-line treatment with the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor osimertinib improved survival in patients (pts) with advanced/metastatic EGFR-mutated (EGFRm) non-small cell lung cancer (NSCLC), therapy after acquired resistance to osimertinib remains an unmet need. HER3 is expressed in the majority of EGFRm NSCLCs. HER3-DXd is a novel, investigational, HER3-directed antibody-drug conjugate that demonstrated preliminary single-agent clinical activity in EGFRm NSCLC. In preclinical models of EGFRm NSCLC, osimertinib increased membrane HER3 expression, improved the internalization rate of HER3-DXd, and enhanced tumor growth inhibition by HER3-DXd. Therefore, HER3-DXd with osimertinib might improve outcomes in pts with disease that progressed with osimertinib therapy. This phase 1 study (NCT04676477; U31402-A-U103) evaluates HER3-DXd in combination with osimertinib in pts with advanced EGFRm NSCLC that has progressed with first-line osimertinib therapy. Methods: This is an open-label, 2-part study of HER3-DXd in combination with osimertinib. Pts are enrolling in North America, Europe, and Asia. Dose escalation enrolls pts with locally advanced/metastatic NSCLC with an EGFR-activating mutation (exon 19 del or L858R) and progression during or after osimertinib therapy. Pts receive HER3-DXd 1.6, 3.2, 4.8, or 5.6 mg/kg intravenously (IV) every 3 weeks (Q3W) in combination with osimertinib 40 or 80 mg orally (PO) once daily (QD). Pts are enrolled in each cohort guided by safety (dose-limiting toxicities) using a Bayesian logistic regression model. Primary objectives of dose escalation are to assess safety and tolerability of the combination and identify a recommended combination dose (RCD). In dose expansion, pts will be randomized 1:1 to receive either HER3-DXd and osimertinib at the RCD (arm 1, [≈60 pts]) or HER3-DXd 5.6 mg/kg IV Q3W (arm 2, [≈60 pts]). A third arm (arm 1b, [≈60 pts]) may be added to evaluate 2 provisional RCDs of HER3-DXd + osimertinib. The primary objective of dose expansion (arms 1, 2, and 1b) is to evaluate efficacy of the combination vs that of monotherapy. The primary endpoint is objective response rate (ORR) as assessed by blinded independent central review. Other efficacy endpoints include ORR by investigator and duration of response, disease control rate, time to response, progression-free survival, and overall survival. If the RCD includes an osimertinib dose of 80 mg PO QD, ≈30 pts with advanced EGFRm NSCLC without prior treatment will be enrolled and treated at the RCD in a separate cohort; the primary objective is to assess safety and tolerability. A tumor sample after progression with osimertinib (or prior to entry) is required for pts enrolled in dose expansion for retrospective evaluation of HER3 and biomarker analyses. Clinical trial information: NCT04676477.