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Background: Therapy options for pts with HR-MDS or AML who are not candidates for intensive chemotherapy (IC) or hematopoietic stem cell transplantation (HSCT) are limited, and clinical outcomes are poor. Novel, effective and tolerable therapies are urgently needed. TIM-3 is an inhibitory receptor that regulates both adaptive and innate immune responses. It is expressed on immune cells and on leukemic stem cells (LSCs) and blasts, but not on normal hematopoietic stem cells, making it a promising target in MDS and AML. Sabatolimab is a high-affinity, humanized, anti-TIM-3 IgG4 (S228P) antibody that simultaneously targets TIM-3 on immune and myeloid cells; this may restore immune function while also directly targeting LSCs and blasts. In preclinical studies, sabatolimab enhanced immune cell-mediated killing of AML cells in vitro. In early results from a ph 1b study (NCT03066648), sabatolimab + hypomethylating agents (HMAs; decitabine [Dec] or azacitidine [Aza]) demonstrated encouraging overall response rates in pts with high-/very high-risk MDS (+ Dec, 61%; + Aza, 57%) and newly diagnosed AML (+ Dec, 47%; + Aza, 29%), and a favorable safety profile. Study Design and Methods: The STIMULUS clinical trial program currently includes 3 trials evaluating the efficacy and safety of sabatolimab combination therapy in pts with HR-MDS or AML who are ineligible for IC or HSCT: STIMULUS-MDS1 (NCT03946670; ph 2) in pts with HR-MDS, STIMULUS-MDS2 (NCT04266301; ph 3) in pts with HR-MDS or chronic myelomonocytic leukemia-2 (CMML-2), and STIMULUS-AML1 (NCT04150029; ph 2) in pts with AML. STIMULUS-MDS1 is a randomized, double-blind, placebo-controlled ph 2 study evaluating the addition of sabatolimab to HMA in pts with HR-MDS. The 2 primary endpoints are complete remission (CR) rate and/or progression-free survival. Secondary endpoints include overall survival (OS), event-free survival (EFS), leukemia-free survival, duration of CR, transfusion independence, safety, pharmacokinetics (PK), and immunogenicity. Approximately 120 pts will be randomized 1:1 to receive sabatolimab 400 mg or placebo IV Q2W (D8 and D22 of each 28-d cycle), + Dec 20 mg/m2 IV on D1-D5 or Aza 75 mg/m2 IV/SC on D1-D7, or D1-D5 + D8 and D9, of each 28-d cycle. STIMULUS-MDS2 is a randomized, double-blind, placebo-controlled ph 3 study of sabatolimab in combination with Aza in pts with HR-MDS or CMML-2; the primary endpoint is OS. Secondary endpoints include measures related to pt quality of life, such as time to definitive deterioration of fatigue, improvement of fatigue, transfusion-free intervals, and physical/emotional functioning, as well as additional efficacy and safety parameters. Approximately 500 pts will be randomized 1:1 to sabatolimab 800 mg or placebo IV Q4W at D8 of each 28-d cycle + Aza using the same Aza dosing schedule as in STIMULUS-MDS1. Eligible pts for STIMULUS-MDS1 or -MDS2 are treatment-naïve adults with HR-MDS (Revised International Prognostic Scoring System [IPSS-R] intermediate-/high-/very high-risk MDS). Pts with intermediate-risk MDS enrolling in STIMULUS-MDS1 are also required to have ≥5% bone marrow blasts at baseline. Pts with CMML-2 are eligible for STIMULUS-MDS2 only. STIMULUS-AML1 is a single-arm ph 2 study evaluating the safety and efficacy of sabatolimab in combination with Aza and venetoclax in pts with AML who are not candidates for IC or HSCT. Primary endpoints are incidence of dose-limiting toxicities (safety run-in) and CR rate. Secondary endpoints include safety and tolerability, CR/CRi rate, measurable residual disease-negative rate, durability of CR, relapse-free survival, EFS, OS, PK, transfusion independence, and immunogenicity. Eligible pts for STIMULUS-AML1 are adults with newly diagnosed AML who are not candidates for IC or HSCT, based on comorbidities (including renal and hepatic impairments, cardiac and pulmonary comorbidities), age (≥75 years old), or Eastern Cooperative Oncology Group performance status of 2 or 3. STIMULUS-AML1 will enroll approximately 86 pts. Part 1 consists of a safety run-in (≈18 pts) across 2 escalating sabatolimab dose levels (400/800 mg IV Q4W on D8 of each 28-d cycle) in combination with Aza (75 mg/m2 IV/SC on D1-D7, or D1-D5 + D8 and D9) and venetoclax 400 mg PO QD. If this triple combination is assessed to be safe, part 2 (expansion) will open and enroll approximately 68 additional pts who will receive sabatolimab 800 mg Q4W in combination with Aza and venetoclax. Disclosures Zeidan: Incyte: Consultancy, Honoraria, Research Funding; Trovagene: Consultancy, Honoraria, Research Funding; Astex: Research Funding; Celgene / BMS: Consultancy, Honoraria, Research Funding; Otsuka: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria, Research Funding; Pfizer: Consultancy, Honoraria, Research Funding; CCITLA: Other; Taiho: Consultancy, Honoraria; Cardinal Health: Consultancy, Honoraria; Daiichi Sankyo: Consultancy, Honoraria; Astellas: Consultancy, Honoraria; Acceleron: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Boehringer-Ingelheim: Consultancy, Honoraria, Research Funding; Agios: Consultancy, Honoraria; MedImmune/Astrazeneca: Research Funding; Leukemia and Lymphoma Society: Other; Epizyme: Consultancy, Honoraria; Ionis: Consultancy, Honoraria; Takeda: Consultancy, Honoraria, Research Funding; Cardiff Oncology: Consultancy, Honoraria, Other; BeyondSpring: Consultancy, Honoraria; Seattle Genetics: Consultancy, Honoraria; Jazz: Consultancy, Honoraria; Aprea: Research Funding; ADC Therapeutics: Research Funding. Giagounidis:AMGEN: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Kim:SL VaxiGen: Consultancy, Honoraria; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Daiichi Sankyo: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; AML Global Portal: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Hanmi: Consultancy, Honoraria; Astellas: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Yuhan: Consultancy, Honoraria; Chugai: Consultancy, Honoraria; BL&H: Research Funding; Sanofi Genzyme: Honoraria; Abbvie: Honoraria. Miyazaki:Novartis Pharma KK: Honoraria; NIPPON SHINYAKU CO.,LTD.: Honoraria; Astellas Pharma Inc.: Honoraria; Otsuka Pharmaceutical: Honoraria; Chugai Pharmaceutical Co., Ltd.: Honoraria; Kyowa Kirin Co., Ltd.: Honoraria; Celgene: Honoraria; Sumitomo Dainippon Pharma Co., Ltd.: Honoraria. Platzbecker:Amgen: Honoraria, Research Funding; Takeda: Consultancy, Honoraria; Geron: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; BMS: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding. Sekeres:BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees; Takeda/Millenium: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees. Malek:Novartis: Current Employment. Scott:Novartis: Current Employment. Niolat:Novartis: Current Employment. Peyrard:Novartis: Current Employment. Ma:Novartis: Current Employment. Kiertsman:Novartis: Current Employment. Stegert:Novartis AG: Current Employment, Current equity holder in publicly-traded company. Hertle:Novartis Pharma AG: Current Employment, Current equity holder in publicly-traded company. Fenaux:BMS: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Jazz: Honoraria, Research Funding; Abbvie: Honoraria, Research Funding. Santini:Takeda: Membership on an entity's Board of Directors or advisory committees; Johnson & Johnson: Honoraria; Novartis: Consultancy, Honoraria; Acceleron: Consultancy; Takeda: Consultancy, Honoraria; Menarini: Consultancy, Honoraria; Pfizer: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Honoraria.