Acquired Venetoclax Resistance in an In Vivo Model of B-Cell Precursor Acute Lymphoblastic Leukemia Is Characterized By Altered Functions of Apoptosis Regulators

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Deregulation of cell death pathways such as apoptosis is a hallmark of cancer and contributes to leukemogenesis and treatment failure in B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Apoptosis is controlled by different pro- and anti-apoptotic molecules either inducing or counteracting cell death signaling. Inhibition of anti-apoptotic molecules of BCL-2 family cell death inhibitors has been developed as therapeutic strategy. Venetoclax (VEN) binds selectively to BCL-2 resulting in apoptosis induction and has shown clinical activity in hematologic malignancies, but primary and secondary resistance have also been described. In this study, we aimed to identify resistance mechanisms acquired during VEN treatment leading to therapy failure of BCL-2 inhibition. In order to mimic a clinical situation with consecutive treatment cycles, we employed a patient-derived human BCP-ALL xenograft mouse model with subsequent courses of VEN administration to ALL bearing recipients in vivo. First, we estimated VEN-sensitivities in a series of BCP-ALL PDX samples (N=30) by ex vivo titration of individual VEN-EC 50 values and identified a range of heterogeneous VEN-activities. A VEN-sensitive (low VEN-EC50) PDX-ALL sample with a pro-B ALL phenotype carrying a KMT2A:: MLL1 gene fusion was selected and transplanted onto immunodeficient NOD/SCID recipient mice. Upon leukemia manifestation (presence of 5% human ALL cells in peripheral blood), mice were either treated with VEN (100 mg/kg, two times five days) or vehicle control. Recipients were sacrificed upon onset of leukemia-related morbidity and ALL cells isolated from spleens were transplanted onto secondary recipients, which were again treated with VEN or vehicle. ALL cells isolated from secondary recipients were retransferred onto tertiary mice and again exposed to in vivo VEN-treatment providing a total of three subsequent transplantation and in vivo VEN-therapy cycles. Assessing VEN sensitivities of the ALL cells initially transplanted and isolated after VEN-treatment cycles by using an ex vivo cytochrome c release assay showed increasing VEN-resistance. Accordingly, comparison of VEN to control in vivo treated cells showed significantly decreased sensitivities over treatment cycles. Importantly, also the time periods between treatment and leukemia re-occurrence in mice indicating decreasing VEN-sensitivity became shorter with every additional in vivo therapy course. Paired RNA- seq analysis revealed that transcript levels of the pro-apoptotic molecules BAX and BIM were significantly downregulated in VEN-exposed/resistant ALL as compared to vehicle control treated cells. Basal BH3 profiling analysis showed impaired apoptotic priming (significantly lower cytochrome c release after BIM and PUMA peptide exposure) and decreased dependence on anti-apoptotic BCL-2 and MCL-1 indicating a shift from a pro-apoptotic to a pro-survival state. This shift was also reflected in an e x vivo drug treatment assay showing decreased sensitivity to the MCL-1 inhibitor S63845 and the BCL-XL inhibitor A-1331852 in ALL cells from VEN- treated mice compared to control- treated mice (S63845 EC 50 1.5 vs. 2.2 µM, A-1331852 EC 50 9.3 vs. 15.3 µM). In line with the low BAX levels in ALL cells with acquired VEN-resistance, PDX BCP-ALL samples with intrinsic VEN resistance also showed lower BAX protein expression. Taken together, acquired VEN-resistance was recapitulated in a co-clinical trial model of BCP-ALL with repeated in vivo treatment cycles showing lower drug sensitivities along with increasingly reduced in vivo anti-ALL activity of VEN. Characterization of acquired VEN-resistance revealed decreased functional dependency on anti-apoptotic proteins and downregulation of pro-apoptotic BIM and BAX, thus pointing to an imbalance of pro- and anti-apoptotic molecules, which can be potentially targeted by directed compounds bypassing resistance to specific BCL-2 inhibition.