Inhibition of Kinesin Spindle Protein Induces Apoptosis and Overcomes Drug Resistance in Models of Multiple Myeloma.

Introduction: Kinesin spindle protein is a mitotic kinesin that is expressed only in proliferating cells and plays a key role in spindle pole separation, formation of a bipolar mitotic spindle, and thus in satisfaction of the mitotic checkpoint. Ispinesib (SB-715992) is a potent and selective inhibitor of kinesin spindle protein with a K_i of 0.6 nM, has cytotoxic activity at less than 10 nM in a spectrum of tumor cell lines, and disrupts the assembly of functional bipolar mitotic spindles.

Methods: This study sought to examine whether spindle disruption by inhibition of kinesin spindle protein with ispinesib may have therapeutic potential in the treatment of multiple myeloma.

Results: Ispinesib reduced cell viability in both interleukin-6-independent (RPMI 8226 and U266) and interleukin-6-dependent (ANBL-6 and KAS-6/1) models of multiple myeloma in a time- and concentration-dependent fashion. The average IC₅₀ for ispinesib against these cell lines was 3.0 nM, 1.7 nM, 1.8 nM, and 1.8 nM, respectively. Cell cycle analysis showed that ispinesib induced growth arrest of myeloma cells with 4N DNA content (in M phase) within 24-hours. Two days after treatment at a 1 nM concentration, cells were able to recover from M phase arrest and resume normal cycling but, after exposure to 10 nM, treated cells could not escape M phase arrest, and instead entered apoptosis as determined by an increased sub-G₁ population. Ispinesib was able to overcome resistance to melphalan in that the IC₅₀ in melphalan-resistant RPMI 8226/LR5 cells (1.6 ± 0.2 nM) was comparable to that in parental RPMI 8226 controls (3.0 ± 0.9 nM). Similarly, ispinesib was also able to overcome dexamethasone resistance and bortezomib resistance. In regard to the latter, KAS-6/VR5 bortezomib-resistant cells (IC₅₀ of 12.5 nM for bortezomib) retained sensitivity to ispinesib (IC₅₀ 1.6 ± 0.2 nM). Combination therapy with ispinesib and bortezomib in these cells resulted in enhanced levels of specific apoptosis (24%) that were greater than the sum of either agents alone (7% for ispinesib and 1% for bortezomib), suggesting synergy. Importantly, ispinesib was also active against freshly isolated CD138⁺ patient-derived multiple myeloma cells, while relatively sparing CD138⁻ cells.

Conclusions: Taken together, these studies demonstrate that kinesin spindle protein inhibition with ispinesib was able to induce growth arrest and apoptosis in myeloma cells, and overcome resistance to both conventional drugs and novel agents such as bortezomib. Moreover, the preferential activity against transformed plasma cells with sparing of normal bone marrow cells provides a strong rationale for translation of this agent into the clinic to combat relapsed/refractory multiple myeloma.