CDK8/19 Inhibition Attenuates the Tolerance of Leukemia Cells to BCR-ABL1 Blockers in the Medium Conditioned by Bone Marrow Fibroblasts

Tyrosine kinase inhibitors (TKIs) targeting BCR-ABL1 have transformed the treatment of chronic myelogenous leukemia (CML) and other malignancies, including FLT3-ITD-positive acute myelogenous leukemia (AML). However, soluble factors produced by the bone marrow microenvironment can attenuate the cytotoxic response to TKIs. CDK8 and CDK19 are mediator-associated kinases involved in stimulus-induced transcriptional programs, thereby representing plausible targets for limiting the adaptive drug tolerance. Here, we used HS-5 bone marrow fibroblast-conditioned medium as a model of soluble niche support and examined its effects in K562 CML and MV4.11 AML-cells treated with catalytic BCR-ABL inhibitors (imatinib, dasatinib, vamotinib, and olverembatinib) or with the allosteric blocker asciminib. In K562 cells, the conditioned medium reproducibly diminished TKI-induced accumulation of subG1 nuclei, indicating an attenuated DNA fragmentation. YO-PRO-1/propidium iodide staining further showed a redistribution from terminal cell damage by a short-term (72 h) treatment. Protective effects remained sustained for at least 9 days of intermittent exposure to TKI. Two structurally distinct, minimally cytotoxic CDK8/19 inhibitors senexin B (SenB) and SNX631, enhanced TKI-induced cell death in both regular and conditioned media. In MV4.11 cells, SenB and SNX631 increased the cytotoxicity of olverembatinib preferentially in the conditioned medium. Together, these data support a model in which soluble fibroblast-derived factor(s) delay the execution of TKI-induced cell death, while CDK8/19 inhibition antagonizes this adaptive response and restores TKI efficacy under bone marrow-mimicking conditions.