8001 Background: Atezolizumab combined with carboplatin and etoposide is approved as first-line treatment for patients with ES-SCLC, based on results from the Phase I/III IMpower133 trial (NCT02763579). The VEGF inhibitor bevacizumab (bev) is approved for the treatment of many tumor types and has synergistic effects when combined with atezolizumab, as demonstrated by the Phase III IMpower150 study (NCT02366143) in non-small cell lung cancer. BEAT-SC (jRCT2080224946) is evaluating the efficacy and safety of bev combined with atezolizumab and platinum-based chemotherapy in patients with ES-SCLC from Japan and China. Here, we report the primary analysis for progression-free survival (PFS) and the first interim analysis for overall survival (OS) from BEAT-SC. Methods: Eligible patients had measurable ES-SCLC, were aged ≥20 y (≥18 y for patients from China), had an ECOG performance status of 0 or 1 and had no prior systemic treatment for ES-SCLC. Patients were randomized 1:1 to receive 4 cycles (21 days/cycle) of induction therapy with bev combined with atezolizumab + cisplatin or carboplatin + etoposide (ACE) or placebo combined with ACE, followed by maintenance therapy with bev + atezolizumab or placebo + atezolizumab, respectively. The primary endpoint was investigator-assessed PFS (INV-PFS). Key secondary endpoints included OS and safety. Results: The ITT population comprised 333 patients with a median age of 65.0 y; 82.6% of the patients were male, 57.7% were from China, 91.8% received carboplatin and 87.6% were current or former smokers. At data cutoff (June 30, 2023; median follow-up, 10.2 mo), median INV-PFS was 5.7 mo for bev + ACE vs 4.4 mo for placebo + ACE (HR, 0.70; 95% CI: 0.54, 0.90; P=0.0060; 2-sided α boundary=0.05). Median OS was 13.0 mo for bev + ACE vs 16.6 mo for placebo + ACE (HR, 1.22; 95% CI: 0.89, 1.67; P=0.2212; 2-sided α boundary=0.0079). Among the safety analysis population (n=330), bev + ACE was well-tolerated and treatment-related adverse events (TRAEs) were generally similar between treatment arms (Table). Conclusions: BEAT-SC met its primary endpoint, demonstrating that the addition of bev to ACE significantly increased PFS vs placebo + ACE. OS data were immature at the first interim OS analysis, and the numerical OS improvement of bev + ACE was not shown vs placebo + ACE; OS follow-up will continue. No new safety signals were observed. Clinical trial information: jRCT2080224946 . [Table: see text]