Definition

An orally bioavailable, potent, selective, reversible inhibitor of the large multifunctional peptidase 7 (LMP7, Beta5i, PSMB8), a chymotrypsin-like, proteolytic subunit of the immunoproteasome, with potential antineoplastic activity. Upon oral administration, LMP7 inhibitor M3258 targets and inhibits the proteolytic activity of the LMP7 subunit of immunoproteasome, thereby blocking its deubiquitylating activity. This blocks the ubiquitin proteasome degradation pathway, prevents the degradation of defective proteins, and leads to an accumulation of poly-ubiquitylated proteins. This induces the unfolded protein response (UPR) and results in both the induction of tumor cell apoptosis and the inhibition of tumor cell growth. Proteasomes are large multi-subunit protease complexes that degrade unneeded or damaged proteins that have been ubiquitinated, thereby restoring protein homeostasis. Unlike the constitutive proteasome, which is expressed in most tissues, immunoproteasome is specifically present in normal and malignant hematopoietic cells, including multiple myeloma. Immunoproteasome degrades ubiquitinated proteins, generates peptides for presentation on MHC class I, and plays a key role in the adaptive immune response and inflammatory diseases.