Development Of Selective Small Molecule Inhibitors Of Cellular Malt1 Protease Activity

Background: The aim of this study was to develop potent, selective and cell-permeable inhibitors of MALT1 protease activity. MALT1 is an Arg-specific protease that cleaves multiple substrates, acting as a key component in T- and B-cell receptor signaling in lymphocytes. MALT1 protease activity is reported to mediate normal and disease-associated lymphocyte proliferation and survival as part of the CARMA-1/BCL-10/MALT1 (CBM) signaling complex, making MALT1 an attractive therapeutic target. Constitutive MALT1 activity is a hallmark of Diffuse Large B Cell Lymphoma of the activated B cell type (ABC-DLBCL). This aberrant activation is observed as a consequence of mutations in CARMA-1/CARD11, and in multiple regulatory proteins upstream of the CBM complex. Methods: MALT1 enzyme assay with Ac-Leu-Arg-Ser-Arg-AMC as substrate was used to measure Ki. Selectivity towards other Arg-specific proteases was measured by inhibition of Trypsin and Thrombin. For determination of cellular potency the ABC-DLBCL cell line OCI-LY3, carrying the L251P mutation of CARMA-1/CARD11, was used. A MALT1 cell assay to measure A20 substrate cleavage was adapted to capillary electrophoresis and quantification. MALT1 down-stream signaling was measured by inhibition of IL-6 expression. Results: Published inhibitors of MALT1 were profiled for potency, selectivity, DMPK properties and activity in cell-based assays, and showed several drawbacks such as low MALT1 activity, low cell permeability and/or selectivity. We developed several novel reversible small molecule inhibitors of MALT1 with Ki 100μM, Thrombin Kiu003e100μM, and Caco-2 Papp = 1×10 −6 cm/s shows intracellular A20 cleavage IC50 = 1100nM, whereas compound B with MALT1 Ki = 47nM and Caco-2 Papp = 5.6×10 −6 cm/s, displays intracellular A20 cleavage IC50 = 600nM. We observe a clear dissociation between intracellular inhibition of MALT1 and inhibition of cell proliferation of DLBCL cell lines of ABC-type with constitutive MALT1 activity, e.g. compound C (MALT1 Ki = 35nM, Trypsin Kiu003e100μM, Thrombin Kiu003e100μM and intracellular A20 cleavage IC50 = 260nM) inhibits the proliferation of OCI-LY3 cells and TMD8 cells with CC50 values of 73μM and 96μM respectively, after 5 days exposure. Conclusion: Inhibition of MALT1 protease activity did not translate into significant anti-proliferative effects in DLBCL cell lines. These new MALT1 inhibitors provide the opportunity to test other therapeutic hypotheses involving the targeting of MALT1 protease activity. Citation Format: Fredrik G. Oberg, Sofia Karlstrom, Ian Henderson, Ellen Hewitt, Anders Kallin, Susanne Sedig, Jimmy Lindberg, Anna-Karin Sohlenius-Sternbeck, Richard Bethell, Mark Albertella. Development of selective small-molecule inhibitors of cellular MALT1 protease activity. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3011.