Measurements of Mcl-1 protein dependencies using dimerization-specific antibodies as predictive biomarkers for BH-3 mimetic class therapies in AML.

Abstract

e19505 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 treatment. A biomarker platform that assesses the functionality of these proteins by measuring BH3 mediated signaling potential in individual patients’ cancers, could provide a potential guiding platform for treating AML. In AML venetoclax is becoming widely prescribed and highly effective in combination with other drugs. Recent data indicate that Mcl-1 dependence is a resistance factor to venetoclax and that methods for identifying this could provide guidance for combination treatments. We are developing a technology to directly measure the occurrence of heterodimers of Mcl-1, or Bcl-xL, bound to pro-apoptotic BH3-only protein Bim. These measurements are combined in algorithms developed to indicate the cancer cell apoptotic priming state and offer great potential for identifying best AML treatment options. Methods: Monoclonal antibodies against conformation-specific epitopes induced in Mcl-1/Bim and Bcl-xL/Bim protein complexes were made. Selective bonding of Heterodimer Specific Mcl-1 Bim (HSMCB) and Heterodimer Specific Bcl-xL Bim (HSBXB) mabs were confirmed using ELISA, fluorescence polarization, immunofluorescence microscopy and flow, and by immunohistochemistry in genetically defined cell lines and in AML patient biopsied samples. Results: We show that HSMCB signal depends on both Mcl-1 and Bim protein levels while HSBXB requires Bc-xL and Bim levels. The relative signals of HSMCB to unbound Mcl-1 signal ([HsMcB]/[Mcl-1]) provide a biomarker for Mcl-1 dependence. We carefully established binding metrics that correlate to drug response in cell lines. The pharmacological disruption of these complexes is monitored by the ratiometric readouts that serve as predictive biomarkers for BH-3 mimetic drugs in AML cells. 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 Bcl-2/Bim complexes are being tested.