ARRY-520, a KSP inhibitor with efficacy and pharmacodynamic activity in animal models of solid tumors

1433 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-520 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) and a potent inducer of cellular phosphorylation of histone H3 (pHH3, a pharmacodynamic marker for accumulation of cells in mitosis), with in vitro antiproliferative IC50s ranging from subnanomolar to single digit nanomolar across a variety of human and rodent tumor cell lines. We report here the characterization of the in vivo efficacy of this compound. At its maximally tolerated dose (MTD) in mice of 25 - 30 mg/kg IP on a q4dx3 schedule, ARRY-520 caused tumor regressions, with some complete responses and cures, in several subcutaneous mouse xenograft models: HT-29 (colon), HCT-116 (colon), A2780 (ovarian), HL-60 (acute promyelocytic leukemia) and K-562 (chronic myelogenous leukemia). In the HT-29, HCT-116 and A2780 models, the antitumor efficacy of ARRY-520 was greater than that of paclitaxel. ARRY-520 had modest efficacy against subcutaneous HCT-15 xenografts (resistant to paclitaxel due to overexpression of p-glycoprotein), but the efficacy was greater than that of paclitaxel. Efficacy was greater than that of vincristine in the K562 model. ARRY-520 also had a potent pharmacodynamic effect in vivo. After a single IP dose of the compound at its MTD in nude mice bearing subcutaneous HT-29 xenografts, tumor pHH3 remained elevated for 4 days, indicating that the pharmacodynamic effect of the drug could be maintained for the duration of the dosing interval on the q4dx3 schedule. Quantitation of monopolar spindles in tumor sections from xenograft studies also demonstrated the duration of the pharmacodynamic effect. 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. Owing to this combination of excellent in vivo efficacy and good properties, ARRY-520 has been selected for clinical development.