Abstract LB-054: Discovery and profiling of M3258, a potent and selective LMP7 inhibitor demonstrating high efficacy in multiple myeloma models

Proteasomes are large multi-subunit proteolytic complexes which are key components of the ubiquitin-dependent protein degradation pathway. The constitutive proteasome is an isoform which is expressed in most tissues, where it regulates protein homeostasis. In contrast, the immunoproteasome is predominantly expressed in hematopoietic cells and can be induced in non-hematopoietic cells by inflammatory stimuli such as IFNγ. The constitutive proteasome contains the catalytically different subunits β1c, β2c and β5c, which possess caspase-like, trypsin-like and chymotrypsin-like activity, respectively. These subunits are replaced in the immunoproteasome by β1i, β2i and β5i (LMP7, PSMB8) which display chymotrypsin-like, trypsin-like and chymotrypsin-like activity, respectively. The enhanced proteolytic capacity of the LMP7 subunit underpins the key function of the immunoproteasome in restoring homeostasis under conditions of elevated proteolytic or oxidative stress. Furthermore, the peptides generated by LMP7 are efficiently loaded onto MHC I and as such, the immunoproteasome plays an essential role in the adaptive immune response and inflammatory diseases. The clinically approved proteasome inhibitors Bortezomib, Ixazomib, or Carfilzomib indiscriminately inhibit multiple subunits of the constitutive proteasome and immunoproteasome, including LMP7, in a comparable potency range. The therapeutic applicability of these drugs is limited by the diverse toxicities that arise as a result of the inhibition of the constitutive proteasome in healthy tissues. To reduce toxicities associated with pan-proteasome inhibition, a drug discovery program was initiated to identify selective inhibitors of the LMP7 subunit of the immunoproteasome. A hit series containing the well-established α-aminoboronic acid scaffold, which interacts covalently and reversibly with the hydroxyl group of Thr-1 in the active site of LMP7, served as starting point. Co-crystallization of series representatives in complex with the 20S core particle of the human immunoproteasome guided rational optimization of both potency and selectivity. Based on the underlying structure activity relationship differences in the P1 and P3 pockets of the proteasome subunits were exploited to gain high specificity for LMP7. Further optimization of ADME and physicochemical properties led to the discovery of M3258, a potent, orally-bioavailable inhibitor of the LMP7 subunit demonstrating >500-fold selectivity against β1c, β2c, β5c, β1i and β2i. M3258 displayed favorable pharmacokinetics and demonstrated strong LMP7 inhibition and anti-tumor activity in several multiple myeloma xenografts, including models that were refractory to Bortezomib. Supported by favorable preclinical data, phase I assessment of M3258 is planned in 2019. Citation Format: Markus Klein, Michael Busch, Christina Esdar, Manja Friese-Hamim, Mireille Krier, Djordje Musil, Felix Rohdich, Michael Sanderson, Gina Walter, Oliver Schadt, Ugo Zanelli, Jianguo Ma. Discovery and profiling of M3258, a potent and selective LMP7 inhibitor demonstrating high efficacy in multiple myeloma models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-054.