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Background Platinum chemotherapy (carboplatin or cisplatin) + etoposide is the current 1L standard of care treatment (tx) for ES-SCLC. Despite high response rates on this regimen, patient (pt) outcomes remain poor. IMpower133 is a Ph1/3, double-blind, randomized, placebo (PBO)-controlled trial that is evaluating the efficacy and safety of adding PD-L1-inhibitor atezo or PBO to 1L tx for ES-SCLC (NCT02763579). Methods IMpower133 enrolled pts without prior tx for ES-SCLC; PD-L1 testing was not required. Pts were randomized 1:1 to four 21-day cycles of carboplatin (AUC 5 mg/mL/min IV, Day 1) + etoposide (100 mg/m2 IV, Days 1-3) with either atezo (1200 mg IV, Day 1) or PBO, then maintenance atezo or PBO until PD, unacceptable toxicity or loss of clinical benefit. Maintenance prophylactic cranial irradiation (PCI) was allowed per protocol, while definitive thoracic radiation (TR) was not. Coprimary endpoints were OS and investigator-assessed PFS. A key exploratory endpoint was outcomes by blood tumor mutation burden (bTMB) analyzed at prespecified cutoffs (≥ 16 vs < 16 and ≥ 10 vs < 10 mutations/Mb). Results Adding atezo to carboplatin + etoposide demonstrated significant improvement in efficacy (OS and PFS) in 1L ES-SCLC (Table). Survival benefits were consistent across key subgroups and prespecified bTMB cutoffs. 44 pts (22 in each arm [per ITT]) received PCI, and 7 (3 in atezo arm, 4 in PBO arm) received TR. Gr 3-4 tx-related adverse events (AEs) occurred in 57% of atezo pts vs 56% of PBO pts; serious tx-related AEs occurred in 23% vs 19%. Occurrence of CNS-related AEs in pts who had PCI are shown in the Table. Conclusion Adding atezo to carboplatin + etoposide resulted in a significant improvement in survival in this 1L ES-SCLC all-comers population. There were no new safety signals, including in pts who received PCI or TR. Atezo + carboplatin + etoposide may represent a new standard regimen for untreated ES-SCLC. Atezo + carboplatin + etoposiden = 201PBO + carboplatin + etoposiden = 202Co-primary endpointsMedian OS, mo12.310.3HR 0.70 (95% CI: 0.54, 0.91); P = 0.0069Median PFS, mo5.24.3HR 0.77 (95% CI: 0.62, 0.96) P = 0.017Secondary efficacy endpointsInvestigator-assessed confirmed ORR, %60.264.4Median DoR, mo4.23.9CNS-related AEs in pts who had PCI, n (%)a,bn = 23n = 21Headache8 (34.8)4 (19.0)Asthenia5 (21.7)2 (9.5)Dizziness2 (8.7)0Insomnia2 (8.7)1 (4.8)Fall2 (8.7)1 (4.8)aSafety population; safety analyses conducted according to tx received (1 pt randomized to the control arm received a dose of atezo).bIncludes AEs occurring in ≥ 2 patients. Citation Format: Aaron S. Mansfield, Stephen V. Liu, Aleksandra Szczęsna, Libor Havel, Maciej Kzrakowski, Maximilian J. Hochmair, Florian Huemer, György Losonczy, Melissa L. Johnson, Makoto Nishio, Martin Reck, Tony S. Mok, Sivuonthanh Lam, David S. Shames, Juan Liu, Fairooz Kabbinavar, Alan Sandler, Leora Horn. IMpower133: Primary efficacy and safety + CNS-related adverse events in a Ph1/3 study of first-line (1L) atezolizumab (atezo) + carboplatin + etoposide in extensive-stage SCLC (ES-SCLC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT199.