Despite advances in treatment, multiple myeloma (MM) remains incurable. Here we propose the use of STK405759, a novel microtubule targeting agent (MTA) and member of the furan metotica family for MM therapy.STK405759 inhibited tubulin polymerization in a cell-free system and in myeloma cells. This molecule had potent cytotoxic activity against several MM cell lines and patient-derived MM cells. Moreover, STK405759 demonstrated cytotoxicity against drug-resistant myeloma cells that overexpressed the P-glycoprotein drug-efflux pump. STK405759 was not cytotoxic to peripheral blood mononuclear cells, including activated B and T lymphocytes. This compound caused mitotic arrest and apoptosis of myeloma cells characterized by cleavage of poly (ADP-ribose) polymerase-1 and caspase-8, as well as decreased protein expression of mcl-1. The combination of STK405759 with bortezomib, lenalidomide or dexamethasone had synergistic cytotoxic activity. In in vivo studies, STK405759-treated mice had significantly decreased MM tumor burden and prolonged survival compared to vehicle treated- mice.These results provide a rationale for further evaluation of STK405759 as monotherapy or part of combination therapy for treating patients with MM.

Multiple myeloma (MM) remains an incurable hematological malignancy. Combination regimens of conventional and novel drugs have improved patient’s survival. However, most patients inevitably relapse and become refractory to the current therapeutic armamentarium.We investigated the efficacy of combining the microtubule-targeting agent STK405759 with dexamethasone or bortezomib in vitro and in vivo.STK405759 combined with dexamethasone or bortezomib had synergistic cytotoxic activity in RPMIS, CAG and MM1.S human MM cell lines through activation of caspase 2, 3, 8, 9 and PARP. These treatments remained cytotoxic in the presence of bone marrow stroma cells. In other MM cells, including cells resistant to vincristine, melphalan, mitoxantrone or dexamethasone, these combinations decreased significantly survival as compared to single agents.In in vivo studies, STK405759 disrupted existing blood vessels in xenograft tumors, acting not only as a cytotoxic agent but also as an anti-angiogenic drug. Mice treated with STK405759 in combination with dexamethasone or bortezomib resulted in greater tumor growth inhibition, increased overall response and prolonged survival as compared to as compared to BTZ or DEXA alone. Their anticancer activity was mediated by activation of apoptosis and reduction of tumor microvessel density.These preclinical studies provide the rationale for future clinical trials of STK405759, dexamethasone and bortezomib combinations to improve the outcome of multiple myeloma patients.

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Background: Despite advances in treatment, multiple myeloma (MM) remains incurable due to development of drug resistance in the bone marrow microenvironment. Microtubules (MTs) are dynamic protein biopolymers formed through polymerization of heterodimers of α- and β-tubulins. Disruption of microtubules induces cell cycle arrest in G2-M phase and formation of abnormal mitotic spindles. MTs are also involved in many processes in interphase cells, including intracellular trafficking, cell motility and angiogenesis. The important functions of MT in the cells make them an attractive target for anti-myeloma drug discovery. Furan metotica is a novel class of anti-mitotic spindle drugs that inhibit kinetochore-microtubule binding and trigger a spindle checkpoint mediated arrest in mitosis, which frequently ends in cell death. We evaluated the activity of STK405759, a member of the furan metotica family, as a novel, potential antitubulin drug for MM treatment in preclinical models. Methods: Cytotoxic activity of STK405759 was evaluated by XTT assay. Apoptosis and cell cycle were analyzed by flow cytometry. Tubulin polymerization inhibition was evaluatedusing a biochemical cell free assay and by testingthe levels of soluble and polymerized tubulin in MM-treated cells using Western blot analysis. Efficacy and toxicity of the drug were checked in a murine MM xenograft model. Histochemistry was used to assay tumor apoptosis. Results: STK405759 had a potent cytotoxic activity against a wide variety of MM cell lines and patient-derived MM cells, regardless of their sensitivity to conventional therapy or novel agents. In contrast, the viability of normal peripheral blood mononuclear cells derived from healthy donors and MM patients was not affected. Importantly, STK405759 induced cell death of RPMI MM cells co-cultured with HS-5 bone marrow stromal cells. STK405759 inhibited tubulin polymerizationin a cell free system anddecreased the level of polymerized tubulin in MM treated cells.The STK405759 anti-tubulin activity was supported by demonstration of MM cell cycle arrest followed by activation of an apoptotic default pathway. Activation of pro-caspase-8 and poly (ADP-ribose) polymerase in the cleaved forms, as well as down-regulation of the Mcl-1 anti-apoptotic protein was detected in RPMI treated cells. Combination studies of STK405759 with bortezomib, lenalidomide or dexamethasone showed significant synergistic and additive cytotoxicity in MM cells. In vivo studies revealed decreased MM tumor burden and prolonged survival of STK405759-treated mice compared to controls. STK405759 induced apoptosis of tumors cells from treated mice. Summary/Conclusion: STK405759 is an active, microtubule-targeting agent with potent anti-myeloma activity. These results provide a rationale for further evaluation of STK405759 as monotherapy or part of combination therapy for treating patients with MM. Disclosures No relevant conflicts of interest to declare.