Identifying Mcl-1 protein dependencies using dimerization-specific antibody biomarker for predicting response to targeted apoptosis inducing therapies.

Abstract

7038 Background: The anti-apoptotic Bcl-2 family proteins facilitate pro-survival and resistance to anti-cancer therapies. Measuring the function of these proteins has shown utility in predicting response to therapy. A method that expands the principle of BH3 profiling to solid tumors is presented. Measuring the occurrence of heterodimers of Myeloid Leukemia Cell Differentiation Protein (Mcl-1), and pro-apoptotic binding protein Bim is an indicator of cancer cell apoptotic priming state. The readout of Mcl-1 containing complex-specific biomarkers can identify survival dependencies in cancer cells potentially providing clinical utility in guiding cancer treatments. Methods: Engineered immunogens that recapitulate conformation-specific epitopes induced during binding of the Mcl-1/Bim protein complex were used to generate. monoclonal antibodies. One Heterodimer Specific Mcl-1 Bim (HsMcB) was chosen. The selective binding was confirmed using ELISA, fluorescence polarization, immunofluorescence microscopy and flow cytometry in Bim or Mcl-1 knockdown cells, and by immunohistochemistry in formalin-fixed paraffin-embedded patient tissue. Correlation of HsMcB measurements to BH3 profiling readouts from the Mcl-1 restricted Noxa peptide was explored. Results: HsMcB signal depends on both Mcl-1 and Bim protein levels. The ratio of the HsMcB to unbound protein (Mcl-1) signal ([HsMcB]/[Mcl-1]) was measured. Disruption of the complexes by BH3 mimetics targeted to Mcl-1 and depletion of Mcl-1 level using CDK9 inhibitors diminished the [HsMcB]/[Mcl-1] readout. We see correlations between these readouts and BH3 mimetic peptides readouts from the Mcl-1 specific Noxa and MS1 BH3 mimetic peptides in AML patient samples that have been treated with Mcl-1 inhibitors. Conclusions: Mcl-1 dependence is a predictive biomarker for venetoclax resistance and for response to Mcl-1 targeted therapies. Flow cytometric and IHC based measurements of a heterodimer complex offer a direct and simpler approach that harbors potential for use in clinical settings. Additional antibodies targeting Mcl-1/Bak and Mcl-1 Noxa complexes are being tested.