Syrbactin-class dual constitutive- and immuno-proteasome inhibitor TIR-199 impedes myeloma-mediated bone degenerationin vivo

Abstract

Proteasome-addicted neoplastic malignancies present a considerable refractory and relapsed phenotype with patients exhibiting drug-resistance and high mortality rates. To counter this global problem, novel proteasome-based therapies are being developed. In the current study we extensively characterize TIR-199, a syrbactin-class proteasome inhibitor derived from a plant virulence factor of bacterium Pseudomonas syringae pv syringae. We report that TIR-199 is a potent constitutive and immunoproteasome inhibitor, capable of inducing cell death in multiple myeloma, triple-negative breast cancer and non-small cell lung cancer lines, effectively inhibit proteasome in primary myeloma cells of refractory patients, and bypass the PSMB5 A49T+A50V bortezomib-resistant mutant. TIR-199 treatment leads to accumulation of canonical proteasome substrates in cells, it is specific, and does not inhibit 50 other enzymes tested in vitro. The drug exhibits synergistic cytotoxicity in combination with proteasome-activating-kinase DYRK2 inhibitor LDN192960. Furthermore, low dose TIR-199 exhibits in vivo activity in delaying myeloma-mediated bone degeneration in a mouse xenograft model. Together, our data indicates that proteasome inhibitor TIR-199 could indeed be a next generation drug within the repertoire of proteasome-based therapeutics with a potential to target relapsed and refractory proteasome-addicted neoplasia.