Highly specific immunoproteasome inhibitor M3258 induces proteotoxic stress and apoptosis in MLL-AF4-driven acute lymphoblastic leukemia.

Proteasome inhibitors (PIs) Bortezomib, Carfilzomib and Ixazomib are approved for the treatment of multiple myeloma and mantle cell lymphoma and have clinical activity in acute lymphoblastic leukemia (ALL). The predominant form of proteasome in these hematologic malignancies is the lymphoid tissue specific immunoproteasome. FDA-approved PIs inhibit immunoproteasomes and ubiquitously expressed constitutive proteasomes causing on-target toxicities in non-hematological tissues. Replacing PIs with selective immunoproteasome inhibitors (IPIs) should reduce these toxicities. We have previously shown that IPI ONX-0914 causes apoptosis of ALL cells expressing the MLL-AF4 fusion protein but did not elucidate the mechanism. Here we show that a novel, highly specific IPI M3258 induces rapid apoptosis in ALL cells in vitro and is comparable to bortezomib in its ability to reduce tumor growth and enhance effect of chemotherapy in vivo. Treatment of ALL cells with M3258, ONX-0914 and bortezomib induced proteotoxic stress that was prevented by the protein synthesis inhibitor cycloheximide, which dramatically desensitized cells to PI-induced apoptosis. Thus, like MM, ALL cells are sensitive to PIs and IPIs because they are under proteotoxic stress caused by elevated rates of protein synthesis.