Chemosensitization Effect of SP1017 on Multiple Myeloma
Multiple myeloma (MM) is a hematological malignancy of plasma cells that are predominantly located in bone marrow (BM). Despite recent improvements in MM treatment by introduction of several novel agents includingimmunomodulatory drugs, proteasome inhibitors and the use of the drug combinations, MM remains incurable and almost all patients eventually relapse or become refractory to the current treatment regimens. So the current challenge of MM treatments is to maintain treatment response, prevent relapse and eventually prolong survival.
Here we demonstrated that Pluronic block copolymers ((Pluronic L61: Pluronic F127 = 1:8 w/w, SP1017) significantly increase cytotoxicity of proteasome inhibitors (Bortezomib, BTZ or Carfilzomib, CFZ) in a panel of MM cells. Specifically, SP1017 (0.005%) co-treatment triggered 2-fold increase in drug cytotoxicity to MM cells. Lower concentrations of SP1017 (0.002%) showed lower, but still significant cytotoxicity when combined with proteasome inhibitors. The mechanistic basis for sensitization effect of SP1017 was multifactorial and included: 1) augmented inhibition of chymotrypsin-like proteolytic activity, concomitant with increased accumulation of poly-ubiquitinated proteins and proteotoxic stress; 2) translocation of Pluronic L61 into the endoplasmic reticulum (ER) and increase of ER stress response; 3) translocation of Pluronic L61 into the Golgi apparatus and increase of rate of Golgi fragmentation, concomitant with reduction of secretion of paraprotein; 4) enhanced depletion of reduced glutathione, that is essential for mitigation of drug-induced oxidative stress; 5) enhanced pro-apoptotic activity of the proteasome inhibitors and; 6) decreased anti-apoptotic defense in MM cells. Importantly, SP1017 co-treatments restore drug sensitivity in BTZ/CFZ-resistant MM cells. Further studies have revealed that SP1017 co-treatment could also sensitize MM cells in co-culture models of the BM microenvironment, which triggers cytokine- and adhesion-mediated MM drug resistance. These results provide support for the design of therapeutic strategies aimed to counteract the drug resistance mechanisms in MM. We also demonstrated that combination of BTZ and SP1017 exerted enhanced antitumor efficacy in human MM/SCID mice model compared to BTZ alone, delaying the disease progression without causing systemic toxicity or hematological toxicity. Moreover, we observed the accumulation of the Cy5-L61 in the BM, which was proved by both fluorescence imaging and flow cytometry, indicating that Pluronic L61 could target and accumulate within the BM, thus playing an important role in sensitizing MM cells in the bone microenvironment to proteasome inhibitors.