Development of Specific Covalent MALT1 Inhibitors and Biomarkers for B-cell Lymphomas

The MALT1 paracaspase plays an essential role in Activated B-cell like Diffuse Large B-cell Lymphoma (ABC DLBCL) downstream of B-cell and Toll-like receptor pathway genes mutated in these tumors. Although MALT1 is considered to be a compelling therapeutic target, development of tractable and specific MALT1 protease inhibitors has thus far been elusive. Herein, we developed a target engagement assay that provides a quantitative readout for specific MALT1 inhibitory effects in living cells. This enabled a structure-guided medicinal chemistry effort culminating in a novel class of substrate-mimetic compounds that bind MALT1 active site and, constitute the first pharmacologically tractable irreversible MALT1 inhibitor. We confirmed MALT1 targeting is effective at suppressing ABC DLBCL cells in vitro and in vivo. We show that reduction in serum IL10 levels exquisitely correlates with drug PK and degree of MALT1 inhibition in vitro and in vivo and constitutes a useful pharmacodynamic biomarker to evaluate these compounds in clinical trials. Our new inhibitor revealed insights into the biology of MALT1 in ABC DLBCL, such as driving JAK-STAT signaling and suppressing type I interferon (IFN) response and MHC class II expression, suggesting that MALT1 inhibition could prime lymphomas for immune recognition by cytotoxic immune cells. ABC-DLBCL cell lines treated with MALT1 inhibitor compound #3