Background: SCLC rapidly recurs after first-line platinum therapy, and while several agents are approved in the relapsed/refractory setting, there is no approved agent or existing standard of care for third-line in Japan. Rovalpituzumab tesirine (Rova-T™) is an antibody-drug conjugate targeting Delta-like 3 protein (DLL3), an atypical Notch ligand that is highly expressed in SCLC but not in normal tissue. This was the first study evaluating safety, PK, and preliminary anti-tumor activity of Rova-T in Japanese pts. Methods: This was an open label Phase 1, 3+3 dose-escalation study of Rova-T in Japanese pts with advanced recurrent SCLC (NCT03086239). Eligibility: progressive disease after ≥2 prior systemic regimens incl. ≥1 platinum-based regimen; ECOG 0-1. Pts received 0.2 or 0.3 mg/kg Rova-T IV on Day 1 of a 6-week cycle for 2 cycles. Objective was to evaluate safety, tolerability, PK, and preliminary efficacy and expression of DLL3. Antitumor activity was measured by RECISTv1.1, and DOR, PFS, OS were evaluated. Results: 29 pts were treated with Rova-T (6 at 0.2mg/kg, 23 at 0.3 mg/kg). Median age 68 yrs; 76% male; 64% DLL3 high (≥75% expression); 86% DLL3 positive (≥25%). 20 pts (69%) had received ≥3 prior lines of therapy. Similar PK and AEs were seen compared to previous studies in non-Japanese pts. The most frequently reported study drug-related AEs were platelet count decreased, pleural effusion, oedema peripheral, and aspartate aminotransferase increased, the majority Grade 1/2. No DLTs occurred, and both dose levels were tolerated. Three pts previously treated with ≥3 prior lines of therapy had confirmed partial response by investigator (10% of all pts; 17% of DLL3 high pts). For DLL3 high pts, mDOR was 3.0 mos (95% CI: 2.9, 4.1), mPFS was 2.9 mos (1.2-3.6), and mOS was 7.4 mos (4.1-11.9). Individual responses were analyzed in detail and radiographic data with tumor shrinkage will be shown. Conclusions: Rova-T demonstrated a manageable safety profile with promising preliminary efficacy in Japanese SCLC pts, in particular pts with DLL3 high expression. These data support further exploration of Rova-T treatment in Japanese pts with SCLC in global Phase 3 studies. Clinical trial information: NCT03086239