Kinesins are eukaryotic microtubule-associated motor proteins. One of these, kinesin spindle protein (KSP) is a mitotic-specific kinesin that plays a key role in spindle pole separation and production of the bipolar spindle, as well as centrosome separation and maturation. As KSP is expressed predominately in proliferating cells and is absent from postmitotic neurons, inhibition of KSP has the potential to provide the antitumor activity of a mitotic inhibitor while avoiding peripheral neuropathy. ARRY-429520 is a member of a series of KSP inhibitors discovered and optimized by structure-based design. It is a potent inhibitor of human KSP (IC50 6 nM), with an EC50 of 1.5 nM for cellular phosphorylation of histone H3 (a pharmacodynamic marker for accumulation of cells in mitosis), and in vitro antiproliferative IC50s ranging from subnanomolar to low single digit nanomolar across a varity of human and murine tumor cell lines. We report here the in vivo characterization of this compound. ARRY-429520 inhibits histone (H3) phosphorylation in tumor xenografts , showing that the compound has pharmacodynamic activity in vivo. As a result, it is a highly efficacious inhibitor of the growth of tumor xenografts. At its maximally tolerated dose in mice of 25 – 30 mg/kg i.p. on a Q4Dx3 schedule, ARRY-429520 caused tumor regressions, with some complete responses, in subcutaneous mouse xenograft models of leukemia: HL-60 (acute promyelocytic leukemia) and K-562 (chronic myelogenous leukemia). The compound was also highly efficacious against other tumor types, including HT-29 (colon), HCT-116 (colon), and A2780 (ovarian). ARRY-429520 also showed activity in intravenous leukemia models. The molecule possess desirable drug-like properties, including high aqueous solubility (> 4 mg/ml at physiological pH), low CYP inhibition (> 25uM for 3A4, 2C9, 1A2, 2D6, and 2C19), and pharmacokinetics favorable for an IV-infused targeted chemotherapeutic.

Disclosures: Array BioPharma.; Array BioPharma.

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