Combination therapy for treating drug-resistant multiple myeloma

Abstract

Abstract Proteasome inhibitors (PIs) have remarkably improved the survival of multiple myeloma (MM) patients but dose-limiting toxicities and the development of drug resistance limit their long-term utility. Elevated TGF-β levels in MM patient sera correlate with chemoresistance, disease progression, metastasis and poor prognosis. Therefore, we evaluated the anti-MM therapeutic potential of a TGF-β type I receptor kinase inhibitor, Vactosertib. In vitro treatment of Vactosertib synergistically inhibited the growth of PI-resistant MM cells in combination with the ixazomib by suppressing TGF-β activation of Smad2/3 and expression of PSMB5, which encodes the proteasome 5 catalytic subunit targeted by PIs. Oral administration of Vactosertib as a single agent inhibited MM progression and induced a decrease in mortality and an increase in body weight of the mice bearing MM. Vactosertib alone also attenuated TGF-β activation of Smad2/3, reduced the expansion of CD11b+Gr-1+ myeloid derived suppressor cells (MDSCs) in bone marrow tumor microenvironment, and diminished the population of Foxp3+ regulatory T cells (Treg) in the spleen. Combination therapy of Vactosertib plus ixazomib was exhibited a synergistic anti-tumor effect when compared to either Vactosertib or ixazomib alone by greatly prolonged survival and significant a reduction in both MDSCs and Tregs. Taken together, our data provide the rationale for clinical evaluation of Vactosertib in MM and demonstrate proof-of-concept that combination of Vactosertib and PIs may overcome chemoresistance and enhance durable patient responses. Supported by 1R03CA259901-01A1