Multiple myeloma cells have greater phosphorylated Eg5 than hematopoietic cells. Eg5 phosphorylation increases microtubule binding and uniquely enhances BRD9876 activity. BRD9876 acts as an ATP- and ADP-competitive inhibitor with a Ki of 4 nM.

BRD9876 is a distinctive kinesin-5 (Eg5) inhibitor that specifically targets microtubule-bound Eg5 allowing greater selectivity over hematopoietic cells. In particular, BRD9876 slows gliding speeds, consistent with locking Eg5 in a rigor state. Especially, BRD9876 may paradoxically have the capacity to stabilize microtubule arrays in cells.

The rigor inhibitor BRD9876 enhances the braking ability of Eg5. Moreover, BRD9876 stabilizes microtubules against depolymerization in vitro and stabilizes metaphase spindles against collapse. Furthermore, BRD9876 enhances the “braking” ability of Eg5, consistent with BRD9876-inhibited motors being in a strong-binding state. Finally, BRD9876 has a very slow unloaded off-rate and a similar Fc as the control.

BRD9876 causes Eg5 to bind tightly to microtubules in either ATP or ADP in marked contrast. BRD9876 also inhibits the diffusion of Eg5 in 5 mM ADP. Therefore, BRD9876 may also be a competitive inhibitor of ADP. In addition, BRD9876 displays selectivity over normal hematopoietic progenitors and is an unusual ATP non-competitive Eg5 inhibitor.

BRD9876 may be specific for cycling multiple myeloma cells and reveals rapid arrest of cells at the G2/M phase. BRD9876 inhibits diverse multiple myeloma cell lines, overcome stromal resistance, and displays selectivity over hematopoietic progenitors. In contrast, CD34 hematopoietic cells treated with BRD9876 show markedly less G2/M arrest albeit these cultures contained fewer cycling cells.

All in all, BRD9876 acts as an Eg5 inhibitor that binds to the α4-α6 interface. BRD9876 specifically inhibits microtubule-bound kinesin-5.

Geng-Yuan Chen, et al. Eg5 Inhibitors Have Contrasting Effects on Microtubule Stability and Metaphase Spindle Integrity. ACS Chem Biol. 2017 Apr 21;12(4):1038-1046.