Introduction: Small cell transformation (SCT) is a typical mechanism of adaptive resistance to third generation epidermal growth factor receptor inhibitors (EGFRi) which have become the standard of care for EGFR-driven non-small cell lung cancer (EGFR+ NSCLC). Little is known about the optimal management of SCT patients. This study aimed to compare outcomes under platinum/etoposide chemotherapy alone (chemo) or in combination with EGFR inhibitors (EGFRi+chemo) or immune checkpoint inhibitors (ICI+chemo). In addition, DLL3 expression was explored as potential novel therapeutic target., Methods: We conducted a retrospective study on patients with EGFR+ NSCLC and SCT treated at 19 centers in Europe and the United States. A total of 47 patients were included of whom 17 received chemo, 20 ICI+chemo, and 10 EGFRi+chemo. We analyzed DLL3 expression by immunohistochemistry., Results: In the entire cohort, median overall survival (OS) from start of first SCT therapy was 11 months (95 % confidence interval [95 %CI] 9.1-12.9) and median progression-free survival (PFS) was 5 months (95 %CI 4.2-5.8). Median PFS was similar in all three groups (chemo and ICI+chemo 4 months, EGFRi+chemo 6 months), and 12-months PFS was 12 % (95 %CI 2 %-31 %), 13 % (95 %CI 0 %-43 %), and 0 % for ICI+chemo, EGFRi+chemo, and chemo, respectively. Median OS in the ICI+chemo group was 13 months (95 %CI 5.5-20.5) compared to 10 months (95 %CI 7.6-12.4) with chemo and EGFRi+chemo (95 %CI 8.1-11.9), respectively. Before and after SCT, 0 % and 93 % of tumors were DLL3-positive., Conclusions: Our results suggest that ICI+chemo and DLL3-targeting agents are worth further exploration in EGFR+ NSCLC undergoing SCT., Presented Elsewhere: Part of this work has been presented at ESMO annual meeting in Madrid, Spain in October 2023 (Poster 1336 P)., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Allgäuer, M received speaker fees from Boehringer Ingelheim. Alt, J received speaker fees and/or honoraria for advisory boards by Astra Zeneca, Pfizer, Daichii Sankyo, Roche, Amgen, MSD, Novartis, Janssen, Boehringer Ingelheim, Merck, BMS. Aust, DEA received honoraria from Roche, Astra Zeneca, MSD, Pfizer, Novartis. Christopoulos. P received research funding from Roche, Amgen, Boehringer Ingelheim, Takeda, Merck, AstraZeneca, and Novartis, honoraria from Roche, Takeda, Gilead, AstraZeneca, Merck, Thermo Fisher, Janssen, Pfizer, and Novartis, travel support from AstraZeneca, Pfizer, Janssen, Merck, Gilead, Daiichi Sankyo, Takeda, Novartis, Eli Lilly and compensation for advisory roles for Pfizer, Chugai, Boehringer Ingelheim, Takeda, Janssen, Novartis, astraZeneca, MSD, Roche. Griesinger, F received speaker fees and/or honoraria for advisory boards from AstraZeneca, Boehringer, BMS, Celgene, Lilly, MSD, Novartis, Pfizer, Roche, Takeda, Ariad, Abbvie, Tesaro/GSK, Siemens, Tesaro, Amgen, Sanofi, Daiichi-Sankyo, Beigene. Grohé, C received honoraria from AstraZeneca, Amgen, Bristol-Myers Squibb, Boehringer Ingelheim, Merck Sharp & Dohme, Pfizer, and Roche, and has had consulting or advisory roles with Boehringer Ingelheim, Merck Sharp & Dohme, Pfizer, Novartis, and Roche. Hochmair, M received honoraria from AstraZeneca, Amgen, Bristol-Myers Squibb, Boehringer Ingelheim, Merck Sharp & Dohme, Pfizer, and Roche, and has had consulting or advisory roles with Boehringer Ingelheim, Merck Sharp & Dohme, Pfizer, Novartis, and Roche. Illini, O received speaker fees and/or honoraria for advisory boards from Boehringer Ingelheim, Eli Lilly, Johnson & Johnson, Menarini, Merck Sharp & Dohme, Pfizer, and Roche and received research grants from Amgen and AstraZeneca outside of the submitted study. Jöhrens, K received compensation for advisory roles for BMS, GSK, Merck Sharp & Dohme, honoraria from AstraZeneca, Agilent, Boehringer Ingelheim, and DSO, and serves as medical advisor for QuIP. Kauffmann-Guerrero received speaker fees and/or honoraria for advisory boards from AstraZeneca, Boehringer Ingelheim, BMS, Lilly, MSD, Novartis, Pfizer, Roche, Takeda. Kirfel, J received speaker fees and/or honoraria for advisory boards, from AstraZeneca, Bristol-Myers Squibb, Novartis, Pfizer and Roche. König, D received a personal grant from Geistlich-Stucki-Stiftung, consulting fees from AstraZeneca, Merck, MSD, Novartis, PharmaMar, honoraria for presentations from Amgen, BMS, Mirati, Sanofi, Swiss Oncology in Motion, support for attending meetings from Amgen, Roche, Sanofi, honoraria for advisory boards from AstraZeneca, BMS, Merck, MSD, PharmaMar, Roche. Michels, S received research grants from Novartis and Pfizer, personal fees from Eli Lilly, Janssen, and Astra Zeneca as well as support for attending meetings and/or travel from Eli Lilly, and Janssen. Overbeck, T received speaker fees and/or honoraria for advisory boards by AstraZeneca, BMS, Boehringer-Ingelheim, Eli Lilly, Janssen-Cilag, Merck, MSD, Novartis, Roche, Takeda, Tesaro/GSK, Lilly, Roche, travel reimbursement by AstraZeneca, Boehringer-Ingelheim, Janssen-Cilag, Lilly, and Roche. Rohde, G received personal fees from Astra Zeneca, Atriva, Boehringer Ingelheim, GSK, Insmed, MSD, Sanofi, Novartis and Pfizer for consultancy during advisory board meetings and personal fees from Astra Zeneca, Berlin Chemie, BMS, Boehringer Ingelheim, Chiesi, Essex Pharma, Grifols, GSK, Insmed, MSD, Roche, Sanofi, Solvay, Takeda, Novartis, Pfizer and Vertex for lectures. Rothschild, S received honoraria (institutional) from, Roche, AstraZeneca, BMS, Boheringer Ingelheim, MSD Oncology, Novartis, Amgen, Lilly, Eisai, Merck serono, Pfizer, Takeda, Bayer, Janssen Oncology, Otsuka, PharmaMar, and Sanofi, compensation for advisory roles for AstraZeneca, Boerhinger Ingelheim, BMS, Pfizer, Eisai, Eli Lilly, Merck Serono, MSD Oncology, Novartis, Roche Pharma AG, Takeda, Amgen, Otsuka, PharmaMar, serves for the speakers bureau of Roche Pharma AG, Sanodi/Aventis, Amgen, AstraZeneca, Takeda, received research funding from Abbvie, BMS, AstraZeneca, Boerhinger Ingelheim, Merck Serono, Roche Pharma AG, and travel support, accomodation, expenses from Sanofi, Roche Pharma AG, BMS, MSD Oncology, AstraZeneca, Takeda, Boehringer Ingelheim, Amgen. He serves at the Federal Drug Comission of the Federal Office of Public Health and the Swiss Group for Clinical Cancer Research (SAKK). Saalfeld, F received research funding from Roche, consultancy fees from Boehringer Ingelheim, honoraria for lectures from AstraZeneca, Janssen, Takeda, Pfizer, Novartis, Thieme, and GWT-TUD, travel support from Janssen and Eli Lilly, and compensation for advisory meetings with BMS, Pfizer, AstraZeneca, Janssen, MSD, and Roche. Stenzinger, A received research grants from Bayer, BMS, Chugai, Incyte, and MSD, and compensation for advisory boards/speaker’s bureau: Agilent, Aignostics, Amgen, Astellas, Astra Zeneca, Bayer, BMS, Eli Lilly, Illumina, Incyte, Janssen, MSD, Novartis, Pfizer, Qlucore, QuiP, Roche, Sanofi, Seagen, Servier, Takeda, Thermo Fisher. Syrigos, K received honoraria for advisory boards from Merck Sharp & Dohme, AstraZeneca, BMS Amgen. Thomas, M received research funding (institution) from AstraZeneca, BMS, Merck, Roche, Takeda, speaker fees and/or honoraria for advisory boards by Amgen, AstraZeneca, Beigene, BMS, Boehringer Ingelheim, Daiichi Sankyo, GlaxoSmithKline, Janssen Oncology, Lilly, Merck, MSD, Novartis, Pfizer, Pierre Fabre, Roche, Sanofi, Takeda. Veluswamy, R served on advisory boards for Bristol-Myers Squibb, Astrazeneca, Merck, Boehringer Ingelheim, Merus, Novocure, Regeneron, BerGenBio, and Serna Bio, on unbranded speaker’s bureau of Astrazeneca and EMD Serono, received consulting honorarium from Beigene, received research funding from Bristol-Myers Squibb, Onconova, Astrazeneca, Boehringer Ingelheim, Lung Cancer Research Foundation, Stand Up 2 Cancer. Wenzel, C received speaker fees from Astra Zeneca. Wermke, M received honorary from Lilly, Boehringer-Ingelheim, SYNLAB, Janssen, Merck Serono, GWT, Amgen, Novartis, Pfizer, BMS and has/had paid consulting or advisory positions for BMS, Novartis, Lilly, Boehringer-Ingelheim, ISA Pharmaceuticals, Amgen, Immatics, Bayer, ImCheck, AstraZeneca, Tacalyx, Regeneron, Daiichi, Zymeworks, PharmaMar. He receive research funding (instution) from Roche and travel support from Pfizer, BMS, AstraZeneca, Amgen, Gemoab, Sanofi-Aventis, Merck Serono, Immatics, Janssen, Iovance, Daiichi-Sankyo. Wesseler, C received speaker fees and/or honoraria for advisory boards from Boehringer Ingelheim, Eli Lilly, Merck Sharp & Dohme, Pfizer, Roche, MSD, Novartis, Takeda, AstraZeneca, Chugai, BMS. Wiesweg, M received honoraria and Advisory Role: Amgen, AstraZeneca, Bristol-Myers Squibb, Daiichi Sankyo, GlaxoSmithKline, Janssen, Novartis, Pfizer, Roche, Takeda, and travel costs from Amgen, Janssen, Daiichi Sankyo, and research funding from Bristol-Myers Squibb, Takeda. Klimova, A, Kulhvay, J, Möller, J, Pelusi, N, Rasokat, A, Schubart, C, Schütz, M, Wang, A, and Vathiotis, J have nothing to disclose., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)