IDENTIFICATION OF POTENT BH3-MIMETIC COMBINATIONS TARGETING PRO-SURVIVAL PATHWAYS IN HUMAN B-CELL ACUTE LYMPHOBLASTIC LEUKEMIA

Precursor-B acute lymphoblastic leukemia (B-ALL) is an aggressive hematological malignancy. Relapsed disease has a poor prognosis despite improved outcomes with tyrosine kinase inhibitors (TKIs) and immunotherapeutic approaches such as CAR-T cells. Targeting cell survival with small molecule BH3 mimetic inhibitors of BCL-2, BCL-XL or MCL1 is an emerging therapeutic option. We report that dual BH3 mimetic targeting of BCL-2/MCL1 is strongly synergistic in SUPB15, BV173, MUTZ5 and MHHCALL4 B-ALL cell lines and was more effective than single BH3 mimetic combinations with dexamethasone (DXM) or TKIs (dasatinib/ruxolitinib). In patient samples, combined BCL-2/MCL1 targeting lowered the LC50 by 10-1000 fold (LC50< 10nM) in 4/4 Ph+ ALL cases and 8/10 Ph- cases. Similarly, combined MCL1 and BCL-XL targeting demonstrated synergy in 3/4 Ph+ cases and 7/10 Ph- cases (to LC50< 10nM) confirming strong anti-leukemic activity compared to BH3-mimetics alone or combined with chemotherapy. BH3 mimetic combination therapy (S55746/S63845) compared favourably in Ph+ ALL cases to S55746 or S63845 used in combination with dasatinib. Preliminary data from patient-derived xenografts in NSG mice revealed in vivo efficacy of combined S55746 and S63845 therapy in 3 adult B-ALL cases (1 Ph+ and 2 Ph-) with reduction of established ALL in the bone marrow of transplanted recipients after one week of treatment (p < 0.05). In conclusion, dual BH3-mimetic targeting of BCL-2/MCL1 induces synergistic killing of human B-ALL cell lines and primary B-ALL samples in vitro and rapid cytoreduction in vivo. Simultaneous inhibition of BCL-2 and MCL1 represents a novel and effective approach for targeting Ph+, Ph- and Ph-like B-ALL without need for additional chemotherapy or kinase inhibition. Our results support the translational investigation of BH3-mimetic targeting of BCL-2 and MCL1 in the clinic.