Background: Hypomethylating agents are approved in higher-riskmyelodysplastic syndromes. The combination of a hypomethylating agent with venetoclax is standard of care in acute myeloid leukaemia. We investigated the safety and activity of the first totally oral combination of decitabine plus cedazuridine and venetoclax in patients with higher-risk-myelodysplastic syndromes and chronic myelomonocytic leukaemia., Methods: We did a single-centre, dose-escalation and dose-expansion, phase 1/2, clinical trial. Patients with treatment-naive higher-risk-myelodysplastic syndromes or chronic myelomonocytic leukaemia (risk level categorised as intermediate-2 or higher by the International Prognostic Scoring System) with excess blasts (>5%). Treatment consisted of oral decitabine 35 mg plus cedazuridine 100 mg on days 1-5 and venetoclax (variable doses of 100-400 mg, day 1 to 14, 28-day cycle). The primary outcomes were safety for the phase 1 part and the overall response for the phase 2 part of the study. The trial is ongoing and this analysis was not prespecified. This study is registered with ClinicalTrials.gov, NCT04655755, and is currently enrolling participants., Findings: Between Jan 21, 2021, and Jan 20, 2023, we enrolled 39 patients (nine in phase 1 and 30 in phase 2). The median age was 71 years (range 27-94), 28 (72%) patients were male, and 11 (28%) were female. The maximum tolerated dose was not reached, and the recommended phase 2 dose was established as oral decitabine 35 mg plus cedazuridine 100 mg for 5 days and venetoclax (400 mg) for 14 days. The most common grade 3-4 adverse events were thrombocytopenia (33 [85%] of 39), neutropenia (29 [74%]), and febrile neutropenia (eight [21%]). Four non-treatment-related deaths occurred on the study drugs due to sepsis (n=2), lung infection (n=1), and undetermined cause (n=1). The median follow-up time was 10·8 months (IQR 5·6-16·4). The overall response rate was 95% (95% CI 83-99; 37/39). 19 (49%) patients proceeded to hematopoietic stem-cell transplantation., Interpretation: This early analysis suggests that the combination of oral decitabine plus cedazuridine with venetoclax for higher-risk-myelodysplastic syndromes and chronic myelomonocytic leukaemia is safe in most patients, with encouraging activity. Longer follow-up will be needed to confirm these data., Funding: MD Anderson Cancer Center, MDS/AML Moon Shot, Genentech/AbbVie, and Astex Pharmaceuticals., Competing Interests: Declaration of interests GM-B declares research support from Takeda, Rigel, and IFN Therapeutics. KSC declares research funding from Amgen and consultancy fees from AbbVie. JI declares funding from the US National Institutes of Health, the Leukemia Research Foundation, and the US Department of Defense, and honoraria from AstraZeneca. KS declares consultancy fees from Novartis and honoraria from Otsuka Pharmaceuticals and Amgen. NS declares grants from Takeda Oncology, Astellas Pharma, Xencor, Stemline Therapeutics, and Nexcure; consultancy fees from Pfizer, GSK, NKARTA, and Sanofi; and honoraria from Adaptive Biotechologies, Novartis, Amgen, Takeda, Pfizer, Astellas Pharma, Sanofi, and BeiGene. GCI declares consultancy fees from Novartis, Kura Oncology, and Nuprobe; and research funding from Celgene, Kura Oncology, Syndax, Merck, Cullinan, and Novartis. ND declares grants from Daiichi-Sankyo, Bristol Myers Squibb (BMS), Pfizer, Gilead, Servier, Genentech, Astellas, AbbVie, ImmunoGen, Amgen, Trillium, Hanmi, Trovagene, FATE Therapeutics, Novimmune, Glycomimetics, and KITE; and consultancy fees from Daiichi-Sankyo, BMS, Pfizer, Gilead, Servier, Genentech, Astellas, AbbVie, ImmunoGen, Amgen, Trillium, Arog, Novartis, Jazz Pharmaceuticals, Celgene, Syndax, Shattuck Labs, Agios, KITE, and Stemline/Menarini. TMK declares grants from BMS, Abbvie, Amgen, Ascentage Pharma Group, Astellas Pharma, DrenBio, Astex, AstraZeneca, Celgene, Incyte, Cellenkos, Cyclacel, Delta-Fly Pharma, Genentech, Genfleet, Glycomimetics, Iterion, Janssen, Jazz Pharmaceuticals, Pfizer, Pulmotect, Regeneron, and SELLAS; consultancy fees from AbbVie, Agios, Daiichi Sankyo, Genentech, Genzyme, Jazz Pharmaceuticals, Liberum, Novartis, Pfizer, PinotBio, Pukmotect, Sanofi-Aventis, Servier; and honoraria from AbbVie, Agios, Daiichi Sankyo, DAVA Oncology, Delta-Fly, DrenBio, Genentech, Genfleet, Genzyme, Jazz Pharmaceuticals, Liberum, Novartis, Pfizer, Rigel, Sanofi-Aventis, SELLAS, and Servier. FR declares research funding from ASTEX and TAIHO. EJ received research grants from AbbVie, Adaptive Biotechnologies, Amgen, Pfizer, and Takeda; and consultancy fees from AbbVie, Adaptive Biotechnologies, Amgen, BMS, Genentech, Incyte, Novartis, Pfizer, and Takeda. HMK declares research funding from Abbvie, Amgen, Ascentage, BMS, Daiichi Sankyo, Immunogen, Jazz Pharmaceuticals, and Novartis; and honoraria from Abbvie, Amgen, Amphista, Ascentage, Astellas, Biologix, Curis, Ipsen Biopharmaceuticals, KAHR Medical, Labcorp, Novartis, Pfizer, Precision Biosciences, Shenzhen Target Rx, Stemline, and Takeda. GG-M declares research funding from Astex Pharmaceuticals, Novartis, Abbvie, BMS, Genentech, Aprea Therapeutics, Curis, and Gilead Sciences; consultancy fees from Astex Pharmaceuticals, Acceleron Pharma, and BMS; and honoraria from Astex Pharmaceuticals, Acceleron Pharma, Abbvie, Novartis, Gilead Sciences, Curis, Genentech, and BMS. All other authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved.)