Background: Therapies for relapsed/refractory acute myeloid leukemia remain limited and outcomes poor, especially amongst patients who are ineligible for cytotoxic chemotherapy or targeted therapies., Patients and Methods: This phase 1b trial evaluated venetoclax, a B-cell lymphoma-2 (BCL-2) inhibitor, plus cobimetinib, a MEK1/2 inhibitor, in patients with relapsed/refractory acute myeloid leukemia, ineligible for cytotoxic chemotherapy. Two-dimensional dose-escalation was performed for venetoclax dosed daily, and for cobimetinib dosed on days 1-21 of each 28-day cycle., Results: Thirty patients (median [range] age: 71.5 years [60-84]) received venetoclax-cobimetinib. The most common adverse events (AEs; in ≥40.0% of patients) were diarrhea (80.0%), nausea (60.0%), vomiting (40.0%), febrile neutropenia (40.0%), and fatigue (40.0%). Overall, 66.7% and 23.3% of patients experienced AEs leading to dose modification/interruption or treatment withdrawal, respectively. The composite complete remission (CRc) rate (complete remission [CR] + CR with incomplete blood count recovery + CR with incomplete platelet recovery) was 15.6%; antileukemic response rate (CRc + morphologic leukemia-free state/partial remission) was 18.8%. For the recommended phase 2 dose (venetoclax: 600 mg; cobimetinib: 40 mg), CRc and antileukemic response rates were both 12.5%. Failure to achieve an antileukemic response was associated with elevated baseline phosphorylated ERK and MCL-1 levels, but not BCL-xL. Baseline mutations in ≥1 signaling gene or TP53 were noted in nonresponders and emerged on treatment. Pharmacodynamic biomarkers revealed inconsistent, transient inhibition of the mitogen-activated protein kinase (MAPK) pathway., Conclusion: Venetoclax-cobimetinib showed limited preliminary efficacy similar to single-agent venetoclax, but with added toxicity. Our findings will inform future trials of BCL-2/MAPK pathway inhibitor combinations., Competing Interests: Disclosure MYK is a consultant for AbbVie, Inc., Genentech, Inc., and F. Hoffmann La-Roche; is an advisory board member for F. Hoffmann La-Roche; holds shares from Reata Pharmaceuticals; has received honoraria from Amgen, AbbVie, Inc., and Genentech, Inc.; and has received research funding from AbbVie, Inc., Genentech, Inc., Eli Lilly, Cellectis, Calithera, Stemline, Threshold, Flexus Biosciences, Novartis, Ablynx, and Agios. MD, MO, JW, WH, DD, and HH are employees of Genentech, Inc. and may hold Roche stock or stock options. NGD has received research funding from Daiichi Sankyo, Bristol-Myers Squibb, Pfizer, Gilead, Sevier, Genentech, Inc., Astellas, Daiichi Sankyo, AbbVie, Inc., Hanmi, Trovagene, Fate Therapeutics, Amgen, Novimmune, GlycoMimetics, Trillium, Kite Pharma, Aptose, Shattuck Labs, KAHR, ArcellX, Sanofi, Sumitomo, and ImmunoGen; and has served in a consulting or advisory role for Daiichi Sankyo, Bristol-Myers Squibb, Arog Pharmaceuticals, Pfizer, Novartis, Jazz Pharmaceuticals, Celgene, AbbVie, Inc., Astellas, Genentech, Inc., Immunogen, Servier, Syndax, Trillium, Gilead, Amgen, Shattuck Labs, ArcellX, Kite Pharma, Sumitomo, Caribou Biosciences, Sanofi, Rigel, Aptose, KURA, GlycoMimetics, and Agios. JSG has received research funding (for trials) from AbbVie, Inc., Genentech, Inc., Pfizer, Prelude, and AstraZeneca; and has served on advisory boards for AbbVie, Inc., Astellas, Bristol-Myers Squibb and Servier. BAJ is a consultant/advisor for AbbVie, Inc., Bristol-Myers Squibb, Daiichi Sankyo, Genentech, Inc., Gilead, GlycoMimetics, Jazz Pharmaceuticals, Kymera, Pfizer, Rigel, Servier, and Takeda; protocol steering committee for GlycoMimetics; data monitoring committee for Gilead; travel reimbursement/support from Rigel; and research funding to his institution from AbbVie, Inc., Amgen, Arog Pharmaceuticals, Aptose, BMS, Celgene, Daiichi Sankyo, F. Hoffmann-La Roche, Forma Therapeutics, Forty-Seven, Genentech, Inc./Roche, Gilead, GlycoMimetics, Hanmi, Immune-Onc, Incyte, Jazz Pharmaceuticals, Loxo Oncology, Pfizer, Pharmacyclics, Sigma Tau, and Treadwell. KWLY is a consultant for Astex, Bristol-Myers Squibb/Celgene, F. Hoffmann-La Roche, Novartis, Otsuka, Paladin, Pfizer, Shattuck Labs, Taiho, and Takeda; has received honorarium from AbbVie, Inc. and Novartis; and has received research funding from Astex, Forma Therapeutics, F. Hoffmann-La Roche, Genentech, Inc., Geron, Janssen, Jazz Pharmaceuticals, MedImmune, Novartis, Onconova, and Tolero. KRK, NV, SP, AT, AP, PF, and GV declare no competing financial interests. SA reports non-financial support from Roche/Genentech, Inc. during the conduct of the study; and has received personal fees from Roche Canada, Pfizer, Bristol-Myers Squibb, Palladin, and Lundbeck outside of the submitted work. GJR consulted for AbbVie, Inc., Amgen, Argenx, AstraZeneca, Bluebird Bio, Blueprint Medicines, Bristol-Myers Squibb, Caribou Biosciences, Celgene, Daiichi Sankyo, Ellipses Pharma, GlaxoSmithKline, Janssen, Jasper Pharmaceuticals, Jazz Pharmaceuticals, Molecular Partners, Novartis, Pfizer, Roche, Syndax, Takeda (IRC Chair), and Telix Pharma; and has received research funding from Janssen. DAP has received research funding from AbbVie, Inc. and has served as a consultant or advisory board member for AbbVie, Inc. and Genentech, Inc. JMB consulted for AbbVie, Inc., Bristol-Myers Squibb, Astex, Pfizer, Astellas, Taiho, and Jazz Pharmaceuticals. BLP has received research funding from Ambit Biosciences, Genentech, Inc., F. Hoffmann-La Roche, Jazz Pharmaceuticals, Novartis, Pfizer, and Cornerstone Pharmaceuticals; and is a consultant for Cornerstone Pharmaceuticals. RLO has received research support for trials from Astellas, Pfizer, Genentech, Inc., Daiichi Sankyo, and Cellectis; and consulted for AbbVie, Inc., Astellas, and Actinium. GM is a consultant for AbbVie, Inc., Celgene, Roche, Janssen, Astellas, Pfizer, and Incyte. W-JH is a former employee of Genentech, Inc. and may hold Roche stock; and is a current employee of Prelude Therapeutics. MGO is an employee of F. Hoffmann-La Roche and may hold stock or stock options. MA consulted for Amgen, AstraZeneca, Daiichi Sankyo, Syndax, GlycoMimetics, Oncoceutics, and Aptose; has received research funding from F. Hoffmann-La Roche, AstraZeneca, Amgen, Daiichi Sankyo, Jazz Pharmaceuticals, GlycoMimetics; and holds stock from Reata, Oncoceutics/Chimerix and Aptose., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)