Modeling Effective AML Therapies - Comparative Anti-Leukemic Synergy of BCL2 and MCL1 Inhibition Combined and with Chemotherapy

Background: Resistance to chemotherapy-induced apoptotic death is a major mechanism responsible for the failure of AML therapies. Levels of anti-apoptotic proteins BCL2 and MCL1 are increased in relapsed AML samples. Venetoclax, a BH3 mimetic that binds to BCL-2, was recently granted accelerated approval for use in AML in combination with cytotoxic chemotherapy. However, MCL-1 and BCL-XL are known pathways of resistance to BCL-2 directed therapy. AMG-176 is a first-in-class MCL-1 specific inhibitor that induces specific and significant apoptosis-mediated toxicity against leukemia cell lines, tumor xenograft models, and primary patient samples. We compared anti-leukemic synergy of Venetoclax and AMG-176 together and individually with cytotoxic chemotherapy used in AML. We sought to identify optimal combinations for efficacy against varying molecular signatures of AML.

Methods: To examine the potential of pairing Venetoclax with AMG-176 we performed a series of in vitro 8x8 dose response matrices on a panel of 11 leukemias spanning AML and Acute Lymphocytic Leukemia (ALL) (Table). This included 3 relapsed pediatric Patient Derived Xenograft (PDX) AML lines propagated in the presence of cytokines to better model inflammation, stem cell function, and niche derived support of leukemic blasts. An additional 8 established cell lines representing key AML & ALL molecular classes were employed. We used the Delta Bliss (DB) method for scoring matrices and report the DBsum, the summation of all of the synergistic(-) and antagonistic(+) well scores in an 8x8 matrix. Drugs tested in combinations with Venetoclax and/or AMG-176 included cytarabine and daunorubicin.

Results: The Figure shows average DBSum for the Venetoclax+AMG-176 combination was -3.41 in the 8 commercial lines, whereas combining the anthracycline Daunorubicin+AMG-176 combination yielded a DBsum score of -0.34, with the score being skewed by a marked antagonism observed with the t(8;21) AML line Kasumi-1 with a DBSum score of 2.13. The 3 relapsed pediatric PDX derived AML lines continued the trend of strong synergy with the Venetoclax+AMG-176 combination with an average score of -3.68 with no observed antagonistic interactions. For context, a synergistic DBSum score of -2.33 would be considered noteworthy from a combination dataset of over >4000 discreet combinations. The Venetoclax+AMG-176 combination exhibited the highest degree of synergy broadly across AML and ALL molecular subgroups. Different patterns of cytotoxicity of combination of chemotherapy with BCL2 or MCL1 inhibition were observed in leukemias with varying molecular backgrounds and histology, including patterns of strong synergy, additivity or antagonism. However, leukemias carrying KMT2A rearrangements exhibited strong synergy or additivity of BCL2 or MCL1 combinations with either daunorubicin or cytarabine, regardless of histology.

Conclusions: We identified broad synergy in high risk genetic subtypes of AML and ALL with the combination of BCL-2 and MCL-1 inhibition, and the effect was superior to either agent combined with chemotherapy individually. We identified potential genetic signatures associated with response to chemotherapy cytotoxicity with either Venetoclax or AMG-176. In vivo experiments are in progress to evaluate the efficacy of this combination and better determine genetic subtypes who would most benefit from this treatment approach.

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