Inhibition Of Kinesin Spindle Protein Research Articles

Kinesin spindle protein inhibitor exacerbates cisplatin-induced hair cell damage

There is emerging evidence indicating that Kinesin family, plays vital roles in influencing the growth of axons, interference with the progression of tumor. However, the function of Kinesin member in the auditory organs remains unknown. SB743921, a kinesin spindle protein (KSP) inhibitor, was applied in mouse organ of Corti and House Ear Institute-Organ of Corti 1 (HEI-OC1) cell line to examine the role of KSP in auditory system with and without cisplatin damage. Cell Counting Kit-8 (CCK-8) and Lactase dehydrogenase (LDH) release assay were conducted to evaluate cell activity and toxicity. Pretreatment with SB743921 increased the sensitivity of HEI-OC1 cells to cisplatin ototoxicity through promoting cell apoptosis and deteriorating superoxide generation mediated damage from cisplatin. SB743921 also enhanced cisplatin induced hair cell damage in explants of mouse cochleae in vitro. Furthermore, the combined N-acetylcysteine (NAC) treatment with cisplatin or with cisplatin and SB743921 both completely rescued the reduced number of hair cells impaired by cisplatin, confirming the strengthening function of superoxide accumulation by SB743921 after cisplatin treatment. Inhibition of kinesin spindle protein enhanced the susceptibility of hair cells to cisplatin induced damage in mouse cochlear explants and HEI-OC1 cells, indicating that kinesin spindle protein might be an unprecedented target to weaken the ototoxicity of platinum medicaments.

Abstract 3892: High-throughput combination screen for identifying novel therapies against high-risk neuroblastoma

Abstract Neuroblastoma is a childhood solid tumor commonly located in the adrenal gland. High-risk neuroblastoma patients undergo multidrug and multi-interventional treatment but despite these efforts many patients develop resistance to chemotherapy and eventually relapse. Neuroblastoma patients face an urgent need of novel therapeutic strategies that could effectively combat the disease. In a recent study we identified kinesin spindle protein (KSP) as a highly effective target in high-risk neuroblastoma. Pharmacological inhibition of KSP resulted in complete regression of a subset of neuroblastoma subcutaneous PDX tumors and tumor growth delay in orthotopic PDXs (Hansson, Radke et al, Sci. Transl. Med 2020). The purpose of the present study is to identify novel synergistic drug combinations effective against neuroblastoma and investigate their translational potential. We designed, optimized, and performed a high-throughput combination screen using 3D tumor organoids that represent three MYCN-amplified neuroblastoma tumors. KSP inhibition was tested in combination with the FIMM drug set of 527 approved and investigational drugs. Screening was performed stepwise: • Step 1: High-throughput drug screening 527 drug combinations and machine learning prediction (DECREASE tool) of drug combination synergy based on minimal required input. • Step 2: Screening of 6x6 dose-response matrices of top 26 predicted hits in high-risk neuroblastoma models. • Step 3: Screening of 6x6 dose-response matrices of top 26 hits in CD34+ cord blood controls to identify compounds that have low toxicity towards proliferating fraction of hematopoietic cells. Overall, we identified seven drug candidates (P1-P7). They classify as: kinase inhibitors, differentiating modifiers, and one hormone therapy. Screened combinations were chosen based on selective efficacy (difference between anti-neuroblastoma efficacy and efficacy towards CD34+ cord blood controls). Identified drug combinations are common among three high-risk neuroblastoma models and have most synergistic area score (MSA) over five. We further plan to validate seven hits in neuroblastoma models in vitro, investigate MOA of the most promising combination and test it in PDX models in vivo. Citation Format: Katarzyna Radke, Karin Hansson, Jani Saarela, Aleksandr Ianevski, Philipp Ianevski, Aurélie Baudet, Mattias Magnusson, Daniel Bexell. High-throughput combination screen for identifying novel therapies against high-risk neuroblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3892.

Ligand- and structure-based in silico studies to identify kinesin spindle protein (KSP) inhibitors as potential anticancer agents

Kinesin spindle protein (KSP) belongs to the kinesin superfamily of microtubule-based motor proteins. KSP is responsible for the establishment of the bipolar mitotic spindle which mediates cell division. Inhibition of KSP expedites the blockade of the normal cell cycle during mitosis through the generation of monoastral MT arrays that finally cause apoptotic cell death. As KSP is highly expressed in proliferating/cancer cells, it has gained considerable attention as a potential drug target for cancer chemotherapy. Therefore, this study envisaged to design novel KSP inhibitors by employing computational techniques/tools such as pharmacophore modelling, virtual database screening, molecular docking and molecular dynamics. Initially, the pharmacophore models were generated from the data-set of highly potent KSP inhibitors and the pharmacophore models were validated against in house test set ligands. The validated pharmacophore model was then taken for database screening (Maybridge and ChemBridge) to yield hits, which were further filtered for their drug-likeliness. The potential hits retrieved from virtual database screening were docked using CDOCKER to identify the ligand binding landscape. The top-ranked hits obtained from molecular docking were progressed to molecular dynamics (AMBER) simulations to deduce the ligand binding affinity. This study identified MB-41570 and CB-10358 as potential hits and evaluated these experimentally using in vitro KSP ATPase inhibition assays.

Abstract 3514: Inhibition of PI-3 kinase signaling potentiates the apoptotic response to mitotic arrest

Abstract Anti-mitotic drugs represent a common strategy for cancer chemotherapy by inducing a prolonged prometaphase delay and eventual cell death by apoptosis. Kinesin Spindle Protein (KSP) inhibitors arrest cells in mitosis by blocking bipolar spindle assembly, and these drugs have demonstrated limited side effects in clinical trials. However, they are not as effective as Taxol, whose efficacy is limited by the frequent development of resistance as well as the deleterious effects on non-dividing cells. Recent studies suggest that the ability of cells to maintain energy levels during mitosis could account for the differential responses of cancer cells to mitotic arrest. To determine if manipulating cellular metabolism could potentiate the efficacy of KSP inhibitors, we arrested cells in mitosis in the absence or presence of the PI3-kinase LY294002, and probed for the induction of apoptosis. Because tumor cells respond differently to mitotic arrest, we employed two cell lines that either survive mitotic arrest (MCF7) or undergo apoptosis shortly following mitotic slippage (HeLa). In both cases, simultaneous inhibition of KSP and PI3-kinase induced apoptosis more effectively than mitotic arrest or PI3-K inhibition alone. In the case of HeLa cells, there also appeared to be an acceleration of cell death such that arrested cells underwent apoptosis prior to mitotic slippage. Similar effects were observed using Akt inhibitors, providing additional evidence that PI3-kinase signaling was critical for surviving mitotic arrest. Lastly, we found that increased glucose had a protective effect, further supporting the notion that the ability to survive mitotic arrest depended on the ability to maintain energy levels. If confirmed, this may represent a novel approach for potentiating the anti-tumor activity of KSP inhibitors in the clinic. Citation Format: Roaa M. Kassim, Charles B. Shuster. Inhibition of PI-3 kinase signaling potentiates the apoptotic response to mitotic arrest. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3514.

KSP inhibitor SB743921 inhibits growth and induces apoptosis of breast cancer cells by regulating p53, Bcl-2, and DTL.

Kinesin spindle protein (KSP) is a microtubule-associated motor protein that is specifically expressed by mitosis cells. It is highly expressed in various types of tumors including hematomalignances and solid tumors. Chemical KSP inhibition has become a novel strategy in the development of anticancer drugs. SB743921 is a selective inhibitor for KSP, which is a mitotic protein essential for cell-cycle progression. Although SB743921 has shown antitumor activities for several types of cancers and entered into clinical trials, its therapeutic effects on breast cancer and mechanisms have not been explored. In this study, we tested the antitumor activity of SB743921 in breast cancer cell lines and partly elucidated its mechanisms. KSP and denticleless E3 ubiquitin–protein ligase homolog (DTL) are overexpressed in breast cancer cells compared with no-cancer tissues. Chemical inhibition of KSP by SB743921 not only reduces proliferation but also induces cell-cycle arrest and leads to apoptosis in breast cancer cells. Treatment of MCF-7 and MDA-MB-231 breast cancer cell lines with SB743921 results in decreased ability of colony formation in culture. SB743921 treatment also causes a KSP accumulation in protein level that is associated with cell arrest. Furthermore, we showed that SB743921 treatment significantly reduces the expression of bcl-2 and cell cycle-related protein DTL, and upregulates p53 and caspase-3 in breast cancer cells. Taken together, these data indicated that SB743921 can be expected to be a novel treatment agent for breast cancers.

Discovery of novel spiro 1,3,4-thiadiazolines as potent, orally bioavailable and brain penetrant KSP inhibitors

Kinesin spindle protein (KSP) 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, as well as centrosome separation and maturation. Inhibition of KSP has the potential to provide anti-tumor activity while avoiding peripheral neuropathy associated with some microtubule-targeted drugs. Based on MK-0731 and related heterocyclic compounds targeting the KSP monastrol binding site, structurally constrained spiro-cyclic KSP inhibitors were designed. In particular, rapid evaluation and optimization of the novel spiro 1,3,4-thiadiazolines resulted in a series of potent KSP inhibitors demonstrating mechanism based activities in cells, including induction of the mitotic marker phospho-histone H3 and induction of monaster spindle formation. Further optimization of the pharmacokinetic (PK) properties afforded MK-8267 as a potent, orally bioavailable and brain penetrant KSP inhibitor which showed anti-tumor activity in preclinical xenograft models.

ARRY-520 Combined With Pomalidomide Displays Enhanced Anti-Tumor Activity In Preclinical Models Of Multiple Myeloma

Inhibitors of Kinesin Spindle Protein (KSP), a mitosis-specific motor protein, represent a novel class of targeted anti-cancer therapies that have demonstrated clinical activity in hematological cancers. Inhibition of KSP results in the formation of aberrant monopolar spindles, mitotic arrest, and rapid apoptosis through degradation of the survival protein Mcl-1. The majority of KSP-inhibitor sensitive cells are proliferating hematopoietic cells which are dependent on Mcl-1 for survival. ARRY-520, a highly selective KSP inhibitor, is currently in Phase 2 clinical studies in patients with relapsed and refractory multiple myeloma. To date, ARRY-520 has demonstrated a well-tolerated safety profile and clinical activity both alone and in combination with bortezomib, dexamethasone, or carfilzomib in heavily pretreated patients. Updated clinical data will be disclosed at this meeting in separate abstracts. While prior preclinical studies have shown that ARRY-520 is additive or synergistic when combined with bortezomib in several in vivo models of multiple myeloma, the combinability of ARRY-520 with other myeloma standards of care, such as immunomodulatory drugs (IMiD), has not been thoroughly investigated. In the present studies, we have evaluated the combination utility of ARRY-520 and pomalidomide, an IMiD recently approved by the FDA, in preclinical models of multiple myeloma. Our findings show that the combination of ARRY-520 (12.5 mg/kg, IP, D1, 2) with pomalidomide (10 mg/kg, IP, QD) is significantly more active than either monotherapy alone in several in vivo models of multiple myeloma. In particular, complete responses and cures were observed in two models. In the RPMI-8226 model, a 100% cure rate was observed with the combination regimen as compared to a 0% cure rate for either monotherapy (cure is defined as no palpable tumor 100 days from study initiation). Likewise, in the JJN3 model a 100% complete response rate was observed with the combination regimen versus a 0% complete response rate for either monotherapy (complete response is defined as no palpable tumor for 2 consecutive tumor measurements). Notably, similar combination efficacy was also observed in the RPMI-8226 model at full and significantly reduced doses of both ARRY-520 and pomalidomide, supporting a synergistic interaction between these drugs. Characterization of the mechanism underlying the anti-tumor activity of this combination is currently being explored. The combination regimen was well tolerated causing no significant enhancement of body weight loss (not exceeding 10% for any group). Additionally, hematology analysis showed that the combination regimen caused no significant increase in thrombocytopenia or leukopenia. Consistent with clinical findings, both single agent ARRY-520 and pomalidomide caused neutropenia while the combination of these drugs moderately enhanced these effects. Pharmacokinetic studies confirmed that administration of pomalidomide with ARRY-520 does not alter the exposure of either compound, suggesting that the mechanism of enhanced anti-tumor activity is dependent upon pharmacodynamic rather than pharmacokinetic effects. Taken together, these findings show that the novel combination of ARRY-520 and pomalidomide is a well-tolerated and synergistic preclinical regimen that warrants investigation in future clinical trials in patients with multiple myeloma. Disclosures:Humphries:Array BioPharma: Employment. Anderson:Array BioPharma: Employment. Williams:Array BioPharma: Employment. Rieger:Array BioPharma: Employment. Tunquist:Array BioPharma: Employment. Walker:Array BioPharma: Employment.

A Phase I study to assess the safety, tolerability, and pharmacokinetics of AZD4877, an intravenous Eg5 inhibitor in patients with advanced solid tumors

Inhibition of kinesin spindle protein or Eg5 causes the formation of monoastral mitotic spindles, which leads to cell death. AZD4877 is a specific, potent inhibitor of Eg5. This was a Phase I, open-label, two-part study to evaluate the maximum tolerated dose (MTD) and safety and tolerability of AZD4877 in patients with advanced solid malignancies. In part A, the MTD of AZD4877, administered as three weekly 1-h intravenous (iv) infusions in a 28-day schedule, was determined by evaluating dose-limiting toxicity (DLT). In part B, the safety, tolerability, and pharmacokinetic profile of AZD4877 at the MTD were evaluated. In part A, 29 patients received at least one dose of AZD4877 (5mg, n=4; 7.5mg, n=4; 10mg, n=3; 15mg, n=3; 20mg, n=3; 30mg, n=6; 36mg, n=3; 45mg, n=3). The MTD was defined as 30mg, with the primary DLT being neutropenia. Although exposures appeared to be similar at the AZD4877 20 and 30mg doses, dose reductions and omissions were higher in the 30-mg cohort; therefore, an intermediate dose, 25mg, was evaluated in part B (n=14). In part B, neutropenia remained the most commonly reported causally related adverse event. Exposure to AZD4877 was approximately dose proportional. Severity of neutropenia was related to exposure. The MTD of AZD4877 given as a 1-h iv infusion on days 1, 8, and 15 of a 28-day cycle was 30mg. At the selected 25mg dose, AZD4877 had an acceptable safety profile.

A Phase I/II Trial of the KSP Inhibitor ARRY-520 In Relapsed/Refractory Multiple Myeloma

A Phase I/II Trial of the KSP Inhibitor ARRY-520 In Relapsed/Refractory Multiple Myeloma

Discovery of Allosteric Inhibitors of Kinesin Spindle Protein (KSP) for the Treatment of Taxane-Refractory Cancer: MK-0731 and Analogs

Current cancer chemotherapy relies heavily on cytotoxic agents, such as the taxanes and Vinca alkaloids, that interfere with the cellular machinery required for cell division and divert the cell down a pathway of programmed cell death. These antimitotic agents, or spindle poisons, target the mitotic spindle by binding to tubulin, a protein required not only for mitosis but also for structural integrity and proper function of healthy, terminally differentiated cells. To avoid side effects attributed to this nonselective mechanism of action, new targets in the mitotic pathway that act only in dividing cells were sought and a leading candidate to emerge from these efforts was kinesin spindle protein (KSP or HsEg5). KSP is a molecular motor protein that is expressed only during mitosis and controls the formation of a functional mitotic spindle. Inhibition of KSP causes mitotic arrest followed by cell death in malignant cells and thus has the potential to become a novel chemotherapeutic strategy with the potential for reduced toxicity. This article summarizes efforts carried out at Merck to discover potent, selective and water soluble KSP inhibitors that culminated in the discovery of MK-0731, the second KSP inhibitor to enter clinical trials. Of special focus in this article is how an HTS lead was optimized in apparently divergent directions, but these disparate leads converged in the design of compounds that overcame P-glycoprotein efflux and hERG channel activity, two issues that required considerable optimization within our program.

Kinesin Spindle Protein (KSP) Inhibitors with 2,3-Fused Indole Scaffolds

Mitotic kinesin spindle protein (KSP) is involved in the assembly of the bipolar spindle during cell division. On the basis of a common 2,3-fused indole substructure within the complex frameworks of terpendole E and other KSP inhibitors, the carbazoles with a bulky alkyl group were identified as a novel KSP inhibitory scaffold. Additionally, among several naturally occurring cell growth inhibitors with 2,3-fused indole structures, beta-carboline alkaloids, harman and harmine, showed moderate inhibition of KSP.

Inhibition of KSP by ARRY-520 induces cell cycle block and cell death via the mitochondrial pathway in AML cells

Kinesin spindle protein (KSP), a microtubule-associated motor protein essential for cell cycle progression, is overexpressed in many cancers and is a potential anti-tumor target. We found that inhibition of KSP by a selective inhibitor, ARRY-520, blocked cell cycle progression, leading to apoptosis in acute myeloid leukemia cell lines that express high levels of KSP. Knockdown of p53, overexpression of XIAP and mutation in caspase-8 did not significantly affect sensitivity to ARRY-520, suggesting that the response is independent of p53, XIAP and the extrinsic apoptotic pathway. Although ARRY-520 induced mitotic arrest in both HL-60 and Bcl-2-overexpressing HL-60Bcl-2 cells, cell death was blunted in HL-60Bcl-2 cells, suggesting that the apoptotic program is executed through the mitochondrial pathway. Accordingly, inhibition of Bcl-2 by ABT-737 was synergistic with ARRY-520 in HL-60Bcl-2 cells. Furthermore, ARRY-520 increased Bim protein levels prior to caspase activation in HL-60 cells. ARRY-520 significantly inhibited tumor growth of xenografts in SCID mice and inhibited AML blast but not normal colony formation, supporting a critical role for KSP in proliferation of leukemic progenitor cells. These results demonstrate that ARRY-520 potently induces cell cycle block and subsequent death in leukemic cells via the mitochondrial pathway and has the potential to eradicate AML progenitor cells.

Kinesin Spindle Protein (KSP) Inhibitors. 9. Discovery of (2S)-4-(2,5-Difluorophenyl)-N-[(3R,4S)-3-fluoro-1-methylpiperidin-4-yl]-2-(hydroxymethyl)-N-methyl-2-phenyl-2,5-dihydro-1H-pyrrole-1-carboxamide (MK-0731) for the Treatment of Taxane-Refractory Cancer

Inhibition of kinesin spindle protein (KSP) is a novel mechanism for treatment of cancer with the potential to overcome limitations associated with currently employed cytotoxic agents. Herein, we describe a C2-hydroxymethyl dihydropyrrole KSP inhibitor ( 11) that circumvents hERG channel binding and poor in vivo potency, issues that limited earlier compounds from our program. However, introduction of the C2-hydroxymethyl group caused 11 to be a substrate for cellular efflux by P-glycoprotein (Pgp). Utilizing knowledge garnered from previous KSP inhibitors, we found that beta-fluorination modulated the p K a of the piperidine nitrogen and reduced Pgp efflux, but the resulting compound ( 14) generated a toxic metabolite in vivo. Incorporation of fluorine in a strategic, metabolically benign position by synthesis of an N-methyl-3-fluoro-4-(aminomethyl)piperidine urea led to compound 30 that has an optimal in vitro and metabolic profile. Compound 30 (MK-0731) was recently studied in a phase I clinical trial in patients with taxane-refractory solid tumors.

An inhibitor of the kinesin spindle protein activates the intrinsic apoptotic pathway independently of p53 and de novo protein synthesis.

The kinesin spindle protein (KSP), a microtubule motor protein, is essential for the formation of bipolar spindles during mitosis. Inhibition of KSP activates the spindle checkpoint and causes apoptosis. It was shown that prolonged inhibition of KSP activates Bax and caspase-3, which requires a competent spindle checkpoint and couples with mitotic slippage. Here we investigated how Bax is activated by KSP inhibition and the roles of Bax and p53 in KSP inhibitor-induced apoptosis. We demonstrate that small interfering RNA-mediated knockdown of Bax greatly attenuates KSP inhibitor-induced apoptosis and that Bax activation is upstream of caspase activation. This indicates that Bax mediates the lethality of KSP inhibitors and that KSP inhibition provokes apoptosis via the intrinsic apoptotic pathway where Bax activation is prior to caspase activation. Although the BH3-only protein Puma is induced after mitotic slippage, suppression of de novo protein synthesis that abrogates Puma induction does not block activation of Bax or caspase-3, indicating that Bax activation is triggered by a posttranslational event. Comparison of KSP inhibitor-induced apoptosis between matched cell lines containing either functional or deficient p53 reveals that inhibition of KSP induces apoptosis independently of p53 and that p53 is dispensable for spindle checkpoint function. Thus, KSP inhibitors should be active in p53-deficient tumors.

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 Ki 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.

ARRY-429520, a KSP Inhibitor with Efficacy in Animal Models of Solid and Liquid Tumors.

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.