Structure-Based Optimization and Discovery of M3258, a Specific Inhibitor of the Immunoproteasome Subunit LMP7 (β5i)

Proteasomes are broadly expressed key components of the ubiquitin-dependent protein degradation pathway containing catalytically active subunits (beta 1, beta 2, and beta 5). LMP7 (beta 5i) is a subunit of the immunoproteasome, an inducible isoform that is predominantly expressed in hematopoietic cells. Clinically effective pan-proteasome inhibitors for the treatment of multiple myeloma (MM) nonselectively target LMP7 and other subunits of the constitutive proteasome and immunoproteasome with comparable potency, which can limit the therapeutic applicability of these drugs. Here, we describe the discovery and structure-based hit optimization of novel amido boronic acids, which selectively inhibit LMP7 while sparing all other subunits. The exploitation of structural differences between the proteasome subunits culminated in the identification of the highly potent, exquisitely selective, and orally available LMP7 inhibitor 50 (M3258). Based on the strong antitumor activity observed with M3258 in MM models and a favorable preclinical data package, a phase I clinical trial was initiated in relapsed/refractory MM patients.

Automated summary

Proteasomes are essential components of the ubiquitin-dependent protein degradation pathway, with LMP7 being a subunit of the immunoproteasome. Current pan-proteasome inhibitors nonselectively target LMP7 and other subunits, limiting their therapeutic applicability. Novel amido boronic acids were discovered to selectively inhibit LMP7, leading to the identification of the highly potent and orally available inhibitor M3258, which showed strong antitumor activity in MM models.