JAK1/2 and BCL2 Inhibitors Synergize to Counter-Act Bone Marrow Stromal Cell-Induced Protection of AML

BACKGROUND AND OBJECTIVES: The bone marrow (BM) microenvironment supports the survival of leukemic cells and influences their response to therapeutic agents by promoting drug tolerance and resistance. Novel therapeutic strategies are therefore needed that can override the BM mediated protection of AML cells in patients undergoing drug treatment. To address this we used a high-throughput drug screening method to identify novel drug combinations to reverse stromal-induced cytoprotection against the BCL2 antagonist venetoclax in primary AML samples.

METHODS: Sensitivity of mononuclear cells collected from 18 AML BM aspirates or peripheral blood samples to a range of BCL2 inhibitors and tyrosine kinase inhibitors (TKIs) was assessed either in mononuclear cell medium (MCM, Promocell) or in a 25% HS-5 stromal cell-conditioned medium plus 75% RPMI medium mix (CM) to mimic cytoprotective bone marrow conditions. Cell viability was measured after 72 h and dose response curves generated for each tested drug. Drug sensitivity scores were calculated based on the area under the dose response curve. For the drug combination studies single agents (venetoclax, WEHI-539, ruxolitinib) were added simultaneously at fixed concentrations to AML cells and incubated for 72 h either in the MCM or CM medium. Cell viability was measured using the CellTiter-Glo assay. The expression of BCL2 genes was measured by qPCR after incubating the AML patient cells in either MCM or CM for 48 h.

RESULTS: Incubation of primary AML cells in the CM culture condition led to reduced sensitivity to BCL2 family inhibitors, suggesting that stromal-derived factors in the CM promote cytoprotection. This effect was particularly pronounced for the selective BCL2 inhibitor venetoclax, where the CM-induced loss of sensitivity coincided with decreased BCL2 expression and increased BCL2L1 expression. In contrast, JAK inhibitors showed improved efficacy in CM compared to MCM culture conditions. To determine if the protective effects of CM stromal-like conditions against venetoclax could be diminished, the drug was tested in combination with the JAK1/2 inhibitor ruxolitinib using AML cells cultured in MCM or CM. When tested on AML cells from 4 patients with the FLT3-ITD alteration, we found that ruxolitinib rescued the sensitivity of venetoclax in leukemic cells in the presence of CM and the combination of two drugs exhibited synergistic effects in this setting. The combinatorial activity, however, was not recapitulated in the MCM condition. Since CM was found to induce BCL2L1 expression, venetoclax was also tested in combination with a BCLX_Lspecific inhibitor WEHI-539. Analogously to the ruxolitinib-venetoclax combination, synergistic activity between venetoclax and WEHI-539 was observed towards leukemic cells in the presence of CM.

CONCLUSIONS: By applying a functional, drug-based approach to understand microenvironment-induced mechanisms of drug resistance in AML, we found that the activity of the selective BCL2 inhibitor venetoclax towards AML cells is adversely affected in stromal-based conditions, while JAK inhibitors, in contrast, exhibit increased efficacy in these conditions. Our results suggest stroma-derived cytokines induce JAK-STAT signaling in AML cells, which results in increased BCL2L1 expression and drives resistance to venetoclax. However, blocking JAK1/2 with ruxolitinib restores the sensitivity of AML cells to venetoclax.  We found that JAK1/2 inhibitors such as ruxolitinib can act synergistically with BCL2/BCLX_L inhibitors, suggesting clinically useful combination treatments.