Purpose: Combination of antiprogrammed cell death protein-1 (PD-1) plus anti-cytotoxic T-cell lymphocyte-4 (anti-CTLA-4) immunotherapy shows greater response rates (RRs) than anti-PD-1 antibody alone in melanoma, but RR after initial anti-PD-1 and programmed death ligand-1 (PD-L1) antibody progression awaits robust investigation. Anti-CTLA-4 antibody alone after anti-PD-1/L1 antibody progression has a historical RR of 13%. We report the results of the first prospective clinical trial evaluating ipilimumab 1 mg/kg plus pembrolizumab following progression on anti-PD-1 immunotherapy., Methods: Patients with advanced melanoma who had progressed on anti-PD-1/L1 antibody as immediate prior therapy (including non-anti-CTLA-4 antibody combinations) were eligible. Patients received pembrolizumab 200 mg plus ipilimumab 1 mg/kg once every 3 weeks for four doses, followed by pembrolizumab monotherapy. The primary end point was RR by irRECIST. After 35 patients, the trial met the primary end point and was expanded to enroll a total of 70 patients to better estimate the RR., Results: Prior treatments included 60 on anti-PD-1 antibody alone and 10 on anti-PD-1/L1 antibody-based combinations. Thirteen patients had progressed in the adjuvant setting. The median length of prior treatment with anti-PD-1/L1 antibody was 4.8 months. Response assessments included five complete and 15 partial responses, making the irRECIST RR 29% among the entire trial population. The median progression-free survival was 5.0 months, and the median overall survival was 24.7 months. The median duration of response was 16.6 months. There was no difference in median time on prior anti-PD1/L1 or time to PD1 + CTLA4 initiation between responders and nonresponders. Grade 3-4 drug-related adverse events occurred in 27% of patients. Responses occurred in PD-L1-negative, non-T-cell-inflamed, and intermediate tumor phenotypes., Conclusion: To our knowledge, this is the first prospective study in melanoma of pembrolizumab plus low-dose ipilimumab after anti-PD-1/L1 immunotherapy failure, demonstrating significant antitumor activity and tolerability., Competing Interests: Zeynep ErogluConsulting or Advisory Role: Regeneron, Sun Pharma, Genentech/Roche, Novartis, OncoSec, Natera, ElsevierResearch Funding: Novartis, Pfizer Bruce BrocksteinConsulting or Advisory Role: Exelixis/IpsenResearch Funding: AstraZenecaPatents, Royalties, Other Intellectual Property: UpToDate. Author and Editorial duties Andrew S. PoklepovicConsulting or Advisory Role: NovartisSpeakers' Bureau: Bristol Myers Squibb Sunil BabuEmployment: Fort Wayne Medical Oncology & HematologyStock and Other Ownership Interests: Fort Wayne Medical Oncology & Hematology, Lutheran HospitalHonoraria: Bristol Myers Squibb, Alexion Pharmaceuticals, Lilly, Bayer, AstraZenecaConsulting or Advisory Role: Bristol Myers Squibb, Alexion Pharmaceuticals, AstraZeneca, argenx, Boehringer Ingelheim, Bayer, Kite, a Gilead company, Janssen Oncology, AmgenSpeakers' Bureau: Alexion PharmaceuticalsResearch Funding: Bristol Myers Squibb, Novartis, Genentech/Roche, AstraZeneca/MedImmune, Janssen Oncology, Amgen, TG Therapeutics, Abbvie, Lilly, Alexion Pharmaceuticals, Merck, Novartis, Syndax, Nektar, Sanofi, argenxTravel, Accommodations, Expenses: Bristol Myers Squibb, Alexion Pharmaceuticals, Lilly, Janssen Oncology, Genentech/Roche Sigrun HallmeyerLeadership: Association of Community Cancer Centers (ACCC)Honoraria: Cardinal HealthConsulting or Advisory Role: Bristol Myers Squibb, Cardinal Health, Array PharamceuticalSpeakers' Bureau: Bristol Myers SquibbTravel, Accommodations, Expenses: Cardinal Health, Bristol Myers SquibbUncompensated Relationships: Society for Immunotherapy of Cancer Mario VelascoEmployment: Cancer Care Specialists of IL Jose LutzkyConsulting or Advisory Role: Castle Biosciences, Iovance Biotherapeutics, Replimune, RegeneronResearch Funding: Bristol Myers Squibb, Novartis, Iovance Biotherapeutics, Immunocore, Regeneron, Replimune, Vyriad Emily HiggsEmployment: LillyStock and Other Ownership Interests: LillyResearch Funding: LillyPatents, Royalties, Other Intellectual Property: Jeffrey D. Helterbrand, Richard E. Higgs, Phillip W. Iversen, Automatic Contextual Segmentation for Imaging Bones for Osteoporosis Therapies (Granted February 1, 2000, US Patent 6,021,213). Raymond E. Kaiser, Richard E. Higgs, Randall K. Julian Jr, System and Methods for Quantitatively Comparing Complex Mixtures Using Single Ion Chromatograms Derived From Spectroscopic Analysis of Such Admixtures (Granted March 23, 1999, US Patent 5,885,841)Travel, Accommodations, Expenses: Lilly Vernon K. SondakConsulting or Advisory Role: Merck/Schering Plough, Novartis, Bristol Myers Squibb, Regeneron, Replimune, EisaiTravel, Accommodations, Expenses: Polynoma, Merck, Bristol Myers Squibb, Replimune Thomas F. GajewskiStock and Other Ownership Interests: Jounce Therapeutics, Evelo Therapeutics, Five Prime Therapeutics, PyxisConsulting or Advisory Role: Merck, Jounce Therapeutics, Adaptimmune, FOGPharma, Allogene Therapeutics, Pyxis, Trillium Therapeutics, Janssen Oncology, Gilead SciencesResearch Funding: Bristol Myers Squibb, Merck, Roche/Genentech, Incyte, Seattle Genetics, Ono Pharmaceutical, Aduro Biotech, Pyxis, BayerPatents, Royalties, Other Intellectual Property: Licensing to Evelo, Licensing to Aduro, Licensing to BMS, Licensing to Pyxis Nikhil I. KhushalaniStock and Other Ownership Interests: Bellicum Pharmaceuticals, Mazor Robotics, Amarin Corporation, Asensus SurgicalHonoraria: SanofiConsulting or Advisory Role: Bristol Myers Squibb, AstraZeneca, Regeneron, Array BioPharma, Immunocore, Merck, Incyte, Jounce Therapeutics, Iovance Biotherapeutics, NCCN/PfizerResearch Funding: Bristol Myers Squibb, Merck, Novartis, GlaxoSmithKline, HUYA Bioscience International, Amgen, Regeneron, Celgene, Replimune Jason J. LukeStock and Other Ownership Interests: Actym Therapeutics, Mavu Pharmaceutical, Pyxis, Alphamab, Tempest Therapeutics, Kanaph Therapeutics, Onc.AI, Arch OncologyConsulting or Advisory Role: Array BioPharma, Bristol Myers Squibb, Merck, EMD Serono, Novartis, 7 Hills Pharma, Janssen, Reflexion Medical, Tempest Therapeutics, TTC Oncology, Alphamab, Compugen, Spring Bank, Abbvie, Astellas Pharma, Bayer, Incyte, Mersana, Partner Therapeutics, Synlogic, Eisai, Werewolf Therapeutics, Ribon Therapeutics, Checkmate Pharmaceuticals, CStone Pharmaceuticals, Nektar, Regeneron, Rubius Therapeutics, Tesaro, Xilio Therapeutics, XencorResearch Funding: Merck, Bristol Myers Squibb, Incyte, Corvus Pharmaceuticals, Abbvie, Checkmate Pharmaceuticals, Macrogenics, Xencor, Array BioPharma, Agios, Astellas Pharma, EMD Serono, Immatics, Kadmon, Moderna Therapeutics, Nektar, Spring Bank, Trishula TherapeuticsPatents, Royalties, Other Intellectual Property: Serial #15/612,657 (Cancer Immunotherapy), Serial #PCT/US18/36052 (Microbiome Biomarkers for Anti-PD-1/PD-L1 Responsiveness: Diagnostic, Prognostic and Therapeutic Uses Thereof)Travel, Accommodations, Expenses: Bristol Myers Squibb, Array BioPharma, EMD Serono, Janssen, Merck, Novartis, Reflexion Medical, Mersana, Pyxis, Xilio TherapeuticsNo other potential conflicts of interest were reported.