Retifanlimab (INCMGA00012) is a humanized, hinge-stabilized immunoglobulin G4 kappa (IgG4κ), anti-programmed cell death protein (PD)-1 monoclonal antibody with safety and clinical pharmacology that are characteristic for the class. Evaluation of retifanlimab in solid tumors is under investigation in phase 2 and 3 studies. POD1UM-201 is an open-label, single-arm, multicenter, phase 2 study evaluating the efficacy and safety of retifanlimab in patients with chemotherapy-naïve or chemotherapy-refractory advanced/metastatic Merkel cell carcinoma (MCC). Updated results from the chemotherapy-naïve cohort are reported here.Eligible patients were ≥18 years of age, had metastatic or recurrent unresectable loco-regional MCC, Eastern Cooperative Oncology Group performance status ≤1, measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1, and had not received prior systemic treatment for MCC. Retifanlimab 500 mg IV every 4 weeks (Q4W) was administered for up to 2 years. The primary endpoint was overall response rate (ORR) assessed by independent central review per RECIST v1.1. Secondary endpoints included duration of response, disease control rate (DCR; defined as proportion of patients with either an objective response or stable disease lasting at least 6 months), progression-free survival, overall survival, safety, and pharmacokinetics.As of April 16, 2021, 87 patients with chemotherapy-naïve advanced/metastatic MCC had received retifanlimab. Per protocol, the primary efficacy analyses are based on the first 65 patients assessed. At the data cutoff, 34 of these 65 patients (52.3%) were on treatment; 4 (6.2%) had completed treatment; and 27 (41.5%) had discontinued treatment for reasons including disease progression (18 [27.7%]), adverse event (AE; 7 [10.8%]), death (1 [1.5%]), and physician decision (1 [1.5%]). The ORR in these patients was 46.2% (n=30: complete response, 8 [12.3%]; partial response, 22 [33.8%]). The DCR was 53.8% (n=35). Other secondary efficacy results are not yet mature. Among all treated patients (n=87), 66 (75.9%) had a treatment-emergent AE (TEAE), 25 (28.7%) had a grade ≥3 TEAE, and 12 (13.8%) had a grade ≥3 treatment-related AE. Twenty-three patients (26.4%) had an immune-related AE (irAE), and 8 (9.2%) had a grade ≥3 irAE. Four patients (4.6%) discontinued treatment due to irAEs (peripheral sensorimotor neuropathy, pancreatitis, eosinophilic fasciitis, and polyarthritis [each n=1]). One patient (1.1%) had a grade 3 infusion reaction.These data from the POD1UM-201 trial show that retifanlimab monotherapy at 500 mg Q4W continues to demonstrate promising clinical activity and safety in patients with advanced/metastatic chemotherapy-naïve MCC. Updated results will be presented at the meeting.The study is sponsored by Incyte Corporation (Wilmington, DE). Statistical support was provided by Xiaohan Xu of Incyte Corporation. Editorial assistance was provided by Matthew Bidgood of Envision Pharma Group (Philadelphia, PA, USA).Clinicaltrials.gov NCT03599713; EudraCT 2018-001627-39The study was approved by institutional review boards or independent ethics committees in Canada (McGill University Health Center-Research Ethics Board [MP-37-2019-5103, MEO-37-2019-1616]; Ontario Cancer Research Ethics Board [1728]; Health Research Ethics Board of Alberta – Cancer Committee [HREBA.CC-19-0004, HREBA.CC-19-0020]); Czech Republic (Eticka komise Fakultni nemocnice Kralovske Vinohrady, Eticka komise IKEM a FTNsP, Eticka komise Nemocnice Na Bulovce, Statni ustav pro kontrolu leciv, Eticka komise FN a LF UP Olomouc [169/18MEK24, LEK/04/07/2018, (L-18-85) 8522/23.3.2021, 22.3.2021/9965/EK-Z]); France (Comité de Protection des Personnes Ile de France X [CNRIPH : 18.11.19.49212/Id. 2043]; Agence Nationale de Sécurité du Médicament et des Produits de Santé); Germany (Ethik-Kommission der Medizinischen Fakultaet der Universitaet Duisburg-Essen [18-8371-AF]; Bundesamt fuer Strahlenschutz; Paul-Ehrlich Institute); Hungary (Egeszsegugyi Tudomanyos Tanacs Klinikai Farmakologiai Etikai Bizottsaga [IV/2407-0/2021-EKL, OGYÉI/11697-2/2021]; Orszagos Gyogyszereszeti es Elelmezes-egeszsegugyi Intezet); Italy (Comitato Etico IRCCS Pascale Napoli [116/21 E - 87/18]; Comitato Etico IRCCS di Candiolo [232/2021]; Istituto Tumori Giovanni Paolo II IRCCS Ospedale Oncologico Bari [736/CE]; Comitato Etico Locale per la Sperim. Clin. dei Medicinali dell’Az. Osp.ra Univ.ria Senese di Siena [14107]; Comitato Etico dell’IRCCS Istituto Nazionale per la Ricerca sul Cancro di Genova [389/2018 - 24/05/2021]; Comitato etico degli IRCCS Istituto Europeo di Oncologia e Centro Cardiologico Monzino [IEO 948 - RE3065/IB Edition 7 dated 10Nov2020 (SA7)]; Comitato Etico, Fondazione IRCCS Istituto Nazionale dei Tumori, .c. Medicina Oncologica 1 – Fondazio [INT 01/19]; Comitato Etico IRCCS Istituto Oncologico Veneto di Padova [EM 109/2021]; Comitato Etico dell’IRCCS Istituto Dermopatico dell’Immacolata Ospedale Generale S. Carlo di Roma [550/7]; AIFA – Agenzia Italiana del Farmaco [0040152-01/04/2021-AIFA-AIFA_USC-P]; Comitatao Etico Policlinico di Modena [1017/2018/FARM/AOUMO - EMENDAMENTO SOSTANZIALE IB EDIZIONE 7 DEL 10/11/20 (201800162739-010) (p. 9869/21)]); Poland (Komisja Bioetyczna przy Centrum Onkologii [no. 55/2019]; Office for Registration of Medicinal Products, Medical Devices and Biocidal Products [UR/DBL/D/328/2019]); Spain (CEIC Hospital General Universitario Gregorio Marañon [280/18]; Agencia Española del Medicamento y Productos Sanitarios); Switzerland (Kantonale Ethikkommission Zürich (KEK-Zürich) [2019-00200]; Swissmedic [2019DR2035]); United Kingdom (North East – York Research Ethics Committee [248465]; Medicines and Healthcare products Regulatory Agency; Health Research Authority); United States (Copernicus Group IRB; Western Institutional Review Board [20181738, Work order number -– IQV1-18-309]; Roswell Park Cancer Institute IRB [STUDY00000802/P 75918]; Inova Institutional Review Board, Human Research Protection Program; Stanford IRB Research Compliance Office [48198]; Rush University Medical Center [18072304-IRB01]; University of Miami IRB; Mayo Clinic IRB – Rochester).