Acute myeloid leukemia (AML) is a malignancy of aging endowed with poor prognosis. The combination of the hypomethylating agent azacitidine (AZA) with the BCL-2 inhibitor venetoclax (VEN) is the first-line treatment of older AML patients but is endowed with substantial resistance. The project leverages functional precision oncology, single-cell studies and mouse experiments to dissect the mechanisms of primary and adaptive resistance to AZA/VEN. The primary objective is to prospectively validate an ex vivo drug sensitivity testing (DST) assay as predictor of primary resistance to first-line AZA/VEN in 100 unfit AML patients. The study will also explore whether newer DST assays with enhanced niche mimicry can improve on the standard assay. By serially interrogating the short-term fate of both leukemic and immune cells upon AZA/VEN exposure in patients primed towards refractoriness, transient or prolonged remission, the aim is to dissect the cell-intrinsic and immune-mediated mechanisms of primary versus adaptive resistance. A parallel flow cytometry study will interrogate the role of senescence in AZA/VEN activity. These translational studies will be mirrored by experiments in a transplantable AML model derived from syngeneic mice harboring the age-related Tet2-/- leukemia-predisposing genotype. Lineage tracing single-cell experiments will backtrack AZA/VEN resistance to determine whether it is driven by selection or adaptation. The actionable stress sensor Pml will be invalidated in the same model to determine whether Pml-driven senescence contributes to AZA/VEN anti-leukemic activity in vivo. The project will pave the way to the clinical implementation of functional precision oncology in a high-risk malignancy. By simultaneously interrogating cell-intrinsic and immune-mediated drug resistance in vivo in a prospective patient cohort mirrored by controlled mice experiments, the project will provide a framework for the integrative analysis of drug resistance in cancers.