A Study to Evaluate S241656 Alone or in Combination in Participants With Selected Myeloid Malignancies

Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2.
• Investigator-assessed life expectancy of ≥ 3 months.
• Able and willing to comply with requirements of the study protocol.
• Documented genetic characterization of the disease as per local practice.
• Clinical and laboratory thresholds:
• Cytoreduction: white blood cell (WBC) < 25 × 10⁹/L (hydroxyurea/cytarabine/leukapheresis allowed).
• Renal: creatinine clearance (CrCl) ≥ 60 mL/min (Cockcroft-Gault).
• Hepatic: aspartate aminotransferase (AST)/alanine aminotransferase (ALT) ≤ 3 × upper limit of normal (ULN) (5 × if leukemic); Total bilirubin ≤ 1.5 × ULN (≤ 3 × ULN for Gilbert's syndrome).
• Part 1 Only: Relapsed/Refractory (R/R), pathologically confirmed AML, MDS/AML, or CMML.
• Part 1 Only: Must have failed ≥ 1 approved standard therapy and have no other approved standard options.

Exclusion Criteria:

• Known hypersensitivity to S241656, or Posaconazole (Part 1B).
• Pregnant or breastfeeding; positive serum pregnancy test for WOCBP.
• Diagnosis of acute promyelocytic leukemia (French-American-British [FAB] M3 classification), MPN, mixed/ambiguous lineage, histiocytic/dendritic cell neoplasms, or AML with isolated extramedullary disease (no marrow/blood involvement).
• Active Central Nervous System (CNS) disease (by cytologic or radiographic evidence).
• Failure to recover to ≤ Grade 1 from previous toxicities (except Grade 2 neuropathy/alopecia).
• Major surgery within 4 weeks.
• Any anticancer therapy within 2 weeks or 5 half-lives (28 days for biologics). Cytoreduction with hydroxyurea or cytarabine is permitted.
• Prior use of experimental KRAS/BRAF/MEK/ERK inhibitors (prior FLT3 inhibitors are permitted).
• Uncontrolled infections (human immunodeficiency virus (HIV)/hepatitis B virus (HBV)/hepatitis C virus (HCV) permitted only if viral load is undetectable/controlled and specific cluster of differentiation 4 (CD4)+ criteria are met).
• Malabsorption, Crohn's, or chronic vomiting that impacts oral drug absorption.
• History/risk of retinal vein occlusion (RVO), glaucoma, or hyper-viscosity syndromes.
• Other active malignancy requiring systemic therapy within 2 years (except non-melanoma skin cancer or localized/cured tumors).
• Stroke, myocardial infarction (MI), unstable angina, or acute coronary syndrome within 6 months.
• Congestive heart failure (CHF), clinically significant cardiac arrhythmias according to the investigator's judgement (e.g., ventricular tachycardia), complete left bundle branch block and high-grade atrioventricular (AV) block (e.g., bifascicular block, Mobitz type II- and third degree AV block).
• Fridericia-corrected QT interval (QTcF) > 470 msec or history of Torsades de pointes.
• Disseminated intravascular coagulation (DIC), significant coagulopathy according to the investigator's judgement, or uncontrolled bleeding.
• Proton Pump Inhibitors (PPIs) and potassium-competitive acid blockers ≥ 7 days prior to Cycle 1 Day 1.
• Breast cancer resistance protein (BCRP) sensitive substrate or with a narrow therapeutic index (NTI)
• All herbal preparations/supplements are prohibited.