Purpose: To evaluate the safety and efficacy of gemcitabine and cisplatin in combination with the immune checkpoint inhibitor pembrolizumab as neoadjuvant therapy before radical cystectomy (RC) in muscle-invasive bladder cancer., Methods: Patients with clinical T2-4aN0/XM0 muscle-invasive bladder cancer eligible for RC were enrolled. The initial six patients received lead-in pembrolizumab 200 mg once 2 weeks prior to pembrolizumab 200 mg once on day 1, cisplatin 70 mg/m 2 once on day 1, and gemcitabine 1,000 mg/m 2 once on days 1 and 8 every 21 days for four cycles. This schedule was discontinued for toxicity and subsequent patients received cisplatin 35 mg/m 2 once on days 1 and 8 without lead-in pembrolizumab. The primary end point was pathologic downstaging (< pT2N0) with null and alternative hypothesis rates of 35% and 55%, respectively. Secondary end points were toxicity including patient-reported outcomes, complete pathologic response (pT0N0), event-free survival, and overall survival. Association of pathologic downstaging with programmed cell death ligand 1 staining was explored., Results: Thirty-nine patients were enrolled between June 2016 and March 2020 (72% cT2, 23% cT3, and 5% cT4a). Patients received a median of four cycles of therapy. All patients underwent RC except one who declined. Twenty-two of 39 patients (56% [95% CI, 40 to 72]) achieved < pT2N0 and 14 of 39 (36% [95% CI, 21 to 53]) achieved pT0N0. Most common adverse events (AEs) of any grade were thrombocytopenia (74%), anemia (69%), neutropenia (67%), and hypomagnesemia (67%). One patient had new-onset type 1 diabetes mellitus with ketoacidosis related to pembrolizumab and no patients required steroids for immune-related AEs. Clinicians consistently under-reported AEs when compared with patients., Conclusion: Neoadjuvant gemcitabine and cisplatin plus pembrolizumab met its primary end point for improved pathologic downstaging and was generally safe. A global study of perioperative chemotherapy plus pembrolizumab or placebo is ongoing., Competing Interests: Tracy L. RoseResearch Funding: Roche/Genentech, GeneCentric, Bristol Myers Squibb, Merck, AstraZeneca Michael R. HarrisonConsulting or Advisory Role: Bayer, Exelixis, Genentech, Fujifilm, Janssen Oncology, AstraZeneca, Pfizer, Bristol Myers SquibbSpeakers' Bureau: Genentech, ExelixisResearch Funding: Bristol Myers Squibb, Genentech, Pfizer, Merck, Clovis Oncology, Acerta Pharma, AstraZeneca, Astellas Pharma, Bayer, Exelixis, Seattle Genetics Sundhar RamalingamHonoraria: Bayer Young E. WhangResearch Funding: Astellas Pharma, Clovis Oncology, Constellation Pharmaceuticals, Amgen, Regeneron, Arvinas Blaine BrowerConsulting or Advisory Role: Seattle Genetics/AstellasTravel, Accommodations, Expenses: Seattle Genetics/Astellas Marc A. BjurlinHonoraria: Intuitive Surgical Angela B. SmithConsulting or Advisory Role: Merck, Photocure, Urogen Pharma, Fergene, Ambu Matthew E. NielsenStock and Other Ownership Interests: Grand Rounds Health Eric WallenStock and Other Ownership Interests: MDxHealth Daniel GeorgeLeadership: Capio BioSciencesHonoraria: Sanofi, Bayer, Exelixis, EMD Serono, OncLive, Pfizer, UroToday, Acceleron Pharma, American Association for Cancer Research, Axess Oncology, Janssen Oncology, Millennium Medical PublishingConsulting or Advisory Role: Bayer, Exelixis, Pfizer, Sanofi, Astellas Pharma, Innocrin Pharma, Bristol Myers Squibb, Genentech, Janssen, Merck Sharp & Dohme, Myovant Sciences, AstraZeneca, Michael J. Hennessy Associates, Constellation Pharmaceuticals, Physicans' Education Resource, Propella Therapeutics, RevHealth LLCSpeakers' Bureau: Sanofi, Bayer, ExelixisResearch Funding: Exelixis, Janssen Oncology, Novartis, Pfizer, Astellas Pharma, Bristol Myers Squibb, Acerta Pharma, Bayer, Dendreon, Innocrin Pharma, Calithera Biosciences, Sanofi/AventisTravel, Accommodations, Expenses: Bayer, Exelixis, Merck, Pfizer, Sanofi, Janssen Oncology, UroToday Tian ZhangLeadership: Capio BioSciences, Archimmune TherapeuticsStock and Other Ownership Interests: Capio Biosciences, Archimmune Therapeutics, NanoroboticsHonoraria: Exelixis, Genentech/Roche, MJH Life Sciences, Pacific GenuityConsulting or Advisory Role: Janssen, Genentech/Roche, Sanofi, Exelixis, AstraZeneca, Pfizer, Bristol Myers Squibb, Foundation Medicine, Pharmacyclics, Amgen, Merck, Seattle Genetics, Dendreon, Calithera BiosciencesSpeakers' Bureau: Exelixis, Genentech/Roche, Genomic Health, Sanofi/AventisResearch Funding: Janssen, Acerta Pharma, Pfizer, Merrimack, Stem CentRx, Novartis, OmniSeq, Personal Genome Diagnostics, Regeneron, Merck, Mirati Therapeutics, Astellas PharmaPatents, Royalties, Other Intellectual Property: Circulating tumor cell novel capture by c-MET technology, Prochelators as Targeted Prodrugs for Prostate CancerTravel, Accommodations, Expenses: Acerta Pharma, Genomic Health, AstraZeneca Anthony DrierEmployment: Docs Global William Y. KimLeadership: Advanced Chemotherapy TechnologiesStock and Other Ownership Interests: Johnson & Johnson, Bristol Myers Squibb, Kura Oncology, BeiGene, FibroGen, Amgen, AbbVie, G1 Therapeutics, Spectrum Pharmaceuticals, Illumina, Arvinas, Fibrogen, NantWorks, Oramed, Natera, ZentalisHonoraria: Takeda, H3 BiomedicineConsulting or Advisory Role: GeneCentric, Foundation MedicineResearch Funding: Merck, GeneCentric, Foundation MedicinePatents, Royalties, Other Intellectual Property: BASE47 bladder cancer subtype classifier, PurIST (Purity Independent Subtyping of Tumors)Travel, Accommodations, Expenses: Takeda, H3 Biomedicine Matthew I. MilowskyResearch Funding: Merck, Roche/Genentech, Bristol Myers Squibb, Astellas Pharma, Clovis Oncology, Inovio Pharmaceuticals, Mirati Therapeutics, Constellation Pharmaceuticals, Syndax, Incyte, Amgen, Regeneron, Arvinas, Seagen, Pfizer, Johnson & Johnson/JanssenNo other potential conflicts of interest were reported.