Marizomib eradicates primary and metastatic triple-negative breast cancer via proteasome and oxidative phosphorylation inhibition

Lacking effective targeted therapies, triple-negative breast cancer (TNBCs) is highly aggressive, metastatic, and clinically challenging breast cancer subtype with worst prognosis. Despite survival dependency on the proteasome pathway genes, the FDA-approved proteasome inhibitors induced minimal clinical response in TNBC patients due to weaker proteasome inhibition. Here, we show that a novel proteasome inhibitor Marizomib (Mzb), inhibited multiple proteasome catalytic activities and induced better anti-tumor response in TNBC cell line and patient-derived xenografts alone and in combination with a standard-of-care chemotherapy, doxorubicin. Mechanistically, Mzb inhibits oxidative phosphorylation (OXPHOS) via PGC-1α suppression in conjunction with proteasome inhibition in TNBC cells. Development of metastatic disease, especially brain metastasis, remains a reason for a greater mortality rate amongst TNBC patients. Mzb reduces lung and brain metastasis in vivo by reducing circulating tumor cells and the expression of multiple epithelial-to-mesenchymal genes. We also demonstrate that Mzb-induced OXPHOS inhibition upregulates glycolysis to fulfill the metabolic demand of TNBC cells and hence, combined inhibition of glycolysis with Mzb leads to a synergistic anti-cancer activity in vivo. Collectively, our data provide a strong rationale for the clinical evaluation of Mzb in primary and metastatic TNBC patients.