NCP26 is a potent prolyl-tRNA synthetase inhibitor and induces pro- apoptotic responses with in vivo potency in multiple myeloma. Bulk RNA-seq data.

Glutamyl-prolyl-tRNA synthetase (EPRS) is a unique bifunctional aminoacyl-tRNA synthetase which catalyzes the ATP-dependent aminoacylation of glutamyl and prolyl residues to cognate tRNAs, constituting fundamental building blocks in protein synthesis. Genomic data suggest that EPRS promotes disease progression and is associated with poor prognosis in multiple myeloma (MM), while downregulation of EPRS triggers MM cell apoptosis implying an amino acid starvation response as a central mechanism. We developed a novel ATP competitive inhibitor, NCP26, targeting the prolyl-tRNA synthetase (PRS) component. The inhibitor demonstrates significant anti-tumor activity against human MM cell lines, regardless of their sensitivity to other therapeutic agents, against patient MM cells and is active in vivo using a MM xenograft mouse model. In contrast to the proline competitive PRS inhibitor halofuginone, NCP26 effects are not altered by exogenous proline levels. NCP26 treatment induces G0/G1 cell cycle arrest followed by apoptotic MM cell death, evidenced by depletion of mitochondrial membrane potential as well as caspase and PARP cleavage. Using combined transcriptomic and proteomic approaches we demonstrate a complex phenotypic response involving multiple pathways in protein quality control which center around the ribosome as integrating hub. Furthermore, we identified multiple proline rich motif containing protein targets of NCP26 as downstream effectors. These include myeloma survival factors such as syndecan 1 (SDC1, CD138), or basic helix-loop-helix transcription factors such as MYC, and transcription factor 3 (TCF3) suggesting that acute blockade of prolyl- aminoacylation evokes a complex pro-apoptotic response beyond the canonical amino acid starvation and integrated stress response. Taken together, our pre-clinical studies validate EPRS as a possible therapeutic target and provide the framework for evaluation of PRS inhibitors in a clinical setting. Overall design: 22 unique conditions, in triplicate = 6 samples total. 2 cell types (WT AMO-1 and CFZ-resistant L363) and two time points. CFZr treated with DMSO (control), NCP22, NCP26, MAZ1392 (Halofuginone) and MAZ1805 (Halofuginol). WT AMO-1 treated with DMSO (control), NCP22, NCP26, MAZ1392 (Halofuginone) and MAZ1805 (Halofuginol) and carfilzomib.