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Vincristine is one of the most widely prescribed medications for treating solid tumors and acute lymphoblastic leukemia (ALL) in children and adults. However, its major dose-limiting toxicity is peripheral neuropathy that can disrupt curative therapy. Peripheral neuropathy can also persist into adulthood, compromising quality of life of childhood cancer survivors. Reducing vincristine-induced neurotoxicity without compromising its anticancer effects would be ideal. Here we show that low expression of NHP2L1 is associated with increased sensitivity of primary leukemia cells to vincristine, and that concomitant administration of vincristine with inhibitors of NHP2L1 increases vincristine cytotoxicity in leukemia cells, prolongs survival of ALL xenograft mice, but decreases vincristine effects on human induced pluripotent stem cell (hiPSC)-derived neurons and mitigates neurotoxicity in mice. These findings offer a strategy for increasing vincristine's antileukemic effects while reducing peripheral neuropathy in patients treated with this widely prescribed medication.

Acute erythroid leukemia (AEL) is characterized by a distinct morphology, mutational spectrum, lack of preclinical models, and poor prognosis. Here, using multiplexed genome editing of mouse hematopoietic stem and progenitor cells and transplant assays, we developed preclinical models of AEL and non-erythroid acute leukemia and describe the central role of mutational cooperativity in determining leukemia lineage. Different combination of mutations in Trp53, Bcor, Dnmt3a, Rb1, and Nfix resulted in the development of leukemia with an erythroid phenotype, accompanied by the acquisition of alterations in signaling and transcription factor genes that recapitulate human AEL by cross-species genomic analysis. Clonal expansion during tumor evolution was driven by mutational cooccurrence, with clones harboring a higher number of founder and secondary lesions (eg, mutations in signaling genes) showing greater evolutionary fitness. Mouse and human AEL exhibited deregulation of genes regulating erythroid development, notably Gata1, Klf1, and Nfe2, driven by the interaction of mutations of the epigenetic modifiers Dnmt3a and Tet2 that perturbed methylation and thus expression of lineage-specific transcription factors. The established mouse leukemias were used as a platform for drug screening. Drug sensitivity was associated with the leukemia genotype, with the poly (ADP-ribose) polymerase inhibitor talazoparib and the demethylating agent decitabine efficacious in Trp53/Bcor–mutant AEL, CDK7/9 inhibitors in Trp53/Bcor/Dnmt3a–mutant AEL, and gemcitabine and bromodomain inhibitors in NUP98-KDM5A leukemia. In conclusion, combinatorial genome editing has shown the interplay of founding and secondary genetic alterations in phenotype and clonal evolution, epigenetic regulation of lineage-specific transcription factors, and therapeutic tractability in erythroid leukemogenesis.

Survival in high-risk pediatric neuroblastoma has remained around 50% for the last 20 years, with immunotherapies and targeted therapies having had minimal impact. Here, we identify the small molecule CX-5461 as selectively cytotoxic to high-risk neuroblastoma and synergistic with low picomolar concentrations of topoisomerase I inhibitors in improving survival in vivo in orthotopic patient-derived xenograft neuroblastoma mouse models. CX-5461 recently progressed through phase I clinical trial as a first-in-human inhibitor of RNA-POL I. However, we also use a comprehensive panel of in vitro and in vivo assays to demonstrate that CX-5461 has been mischaracterized and that its primary target at pharmacologically relevant concentrations, is in fact topoisomerase II beta (TOP2B), not RNA-POL I. This is important because existing clinically approved chemotherapeutics have well-documented off-target interactions with TOP2B, which have previously been shown to cause both therapy-induced leukemia and cardiotoxicity—often-fatal adverse events, which can emerge several years after treatment. Thus, while we show that combination therapies involving CX-5461 have promising anti-tumor activity in vivo in neuroblastoma, our identification of TOP2B as the primary target of CX-5461 indicates unexpected safety concerns that should be examined in ongoing phase II clinical trials in adult patients before pursuing clinical studies in children., CX-5461 recently progressed through phase I clinical trial as a first-inhuman inhibitor of RNA-POL I. Here, the authors demonstrate that CX-5461 synergizes with topoisomerase I inhibitors to inhibit neuroblastoma cells and that its primary target in this disease is topoisomerase II beta and not RNA-POL I.

Pediatric high-grade glioma (pHGG) is a major contributor to cancer-related death in children. In vitro and in vivo disease models reflecting the intimate connection between developmental context and pathogenesis of pHGG are essential to advance understanding and identify therapeutic vulnerabilities. Here we report establishment of 21 patient-derived pHGG orthotopic xenograft (PDOX) models and eight matched cell lines from diverse groups of pHGG. These models recapitulate histopathology, DNA methylation signatures, mutations and gene expression patterns of the patient tumors from which they were derived, and include rare subgroups not well-represented by existing models. We deploy 16 new and existing cell lines for high-throughput screening (HTS). In vitro HTS results predict variable in vivo response to PI3K/mTOR and MEK pathway inhibitors. These unique new models and an online interactive data portal for exploration of associated detailed molecular characterization and HTS chemical sensitivity data provide a rich resource for pediatric brain tumor research., Patient-derived xenografts provide a resource for basic and translational cancer research. Here, the authors generate multiple pediatric high-grade glioma xenografts, use omics technologies to show that they are representative of primary tumours and use them to assess therapeutic response.

Obesity has been epidemiologically and empirically linked with more severe disease upon influenza infection. To ameliorate severe disease, treatment with antivirals, such as the neuraminidase inhibitor oseltamivir, are suggested to begin within days of infection, especially in hosts at higher risk for poor outcomes. However, this treatment is often poorly effective and can generate resistance variants within the treated host. Here, we hypothesized that oseltamivir treatment would not be effective in genetically obese mice and would generate a more diverse and drug resistant viral population. We demonstrated that oseltamivir treatment does not improve viral clearance in obese mice. While no traditional variants associated with oseltamivir resistance emerged, we did note that drug treatment failed to quench the viral population and did lead to phenotypic drug resistance in vitro. Mechanistically, we demonstrate the blunted interferon response in obese hosts may be contributing to treatment failure, as type I interferon receptor deficient mice also fail to clear influenza virus infection upon oseltamivir administration. Together, these studies suggest that the unique pathogenesis and immune responses in obese mice could have implications for pharmaceutical interventions and the within-host dynamics of the influenza virus population.IMPORTANCEInfluenza virus infections, while typically resolving within days to weeks, can turn critical especially in high-risk populations. Prompt antiviral administration is crucial to mitigating these severe sequalae, yet concerns remain if antiviral treatment is effective in hosts with obesity. Here, we show that oseltamivir does not improve viral clearance in genetically obese or type I IFN receptor-deficient mice and increases the genetic entropy of the within-host viral population. This suggests a blunted immune response may impair oseltamivir efficacy and render a host more susceptible to severe disease. This study furthers our understanding of oseltamivir treatment dynamics both systemically and in the lungs of obese mice, as well as the consequences of oseltamivir treatment for the within-host emergence of drug-resistant variants.

Pediatric high-grade glioma (pHGG) is a major contributor to cancer-related death in children.In vitroandin vivodisease models reflecting the intimate connection between developmental context and pathogenesis of pHGG are essential to advance understanding and identify therapeutic vulnerabilities. We established 21 patient-derived pHGG orthotopic xenograft (PDOX) models and eight matched cell lines from diverse groups of pHGG. These models recapitulated histopathology, DNA methylation signatures, mutations and gene expression patterns of the patient tumors from which they were derived, and included rare subgroups not well-represented by existing models. We deployed 16 new and existing cell lines for high-throughput screening (HTS).In vitroHTS results predicted variablein vivoresponse to inhibitors of PI3K/mTOR and MEK signaling pathways. These unique new models and an online interactive data portal to enable exploration of associated detailed molecular characterization and HTS chemical sensitivity data provide a rich resource for pediatric brain tumor research.

Purpose: Curing all children with brain tumors will require an understanding of how each subtype responds to conventional treatments and how best to combine existing and novel therapies. It is extremely challenging to acquire this knowledge in the clinic alone, especially among patients with rare tumors. Therefore, we developed a preclinical brain tumor platform to test combinations of conventional and novel therapies in a manner that closely recapitulates clinic trials. Experimental Design: A multidisciplinary team was established to design and conduct neurosurgical, fractionated radiotherapy and chemotherapy studies, alone or in combination, in accurate mouse models of supratentorial ependymoma (SEP) subtypes and choroid plexus carcinoma (CPC). Extensive drug repurposing screens, pharmacokinetic, pharmacodynamic, and efficacy studies were used to triage active compounds for combination preclinical trials with “standard-of-care” surgery and radiotherapy. Results: Mouse models displayed distinct patterns of response to surgery, irradiation, and chemotherapy that varied with tumor subtype. Repurposing screens identified 3-hour infusions of gemcitabine as a relatively nontoxic and efficacious treatment of SEP and CPC. Combination neurosurgery, fractionated irradiation, and gemcitabine proved significantly more effective than surgery and irradiation alone, curing one half of all animals with aggressive forms of SEP. Conclusions: We report a comprehensive preclinical trial platform to assess the therapeutic activity of conventional and novel treatments among rare brain tumor subtypes. It also enables the development of complex, combination treatment regimens that should deliver optimal trial designs for clinical testing. Postirradiation gemcitabine infusion should be tested as new treatments of SEP and CPC. Clin Cancer Res; 24(7); 1654–66. ©2018 AACR.

Spinal bulbar muscular atrophy (SBMA) is a motor neuron disease caused by toxic gain of function of the androgen receptor (AR). Previously, we found that co-regulator binding through the activation function-2 (AF2) domain of AR is essential for pathogenesis, suggesting that AF2 may be a potential drug target for selective modulation of toxic AR activity. We screened previously identified AF2 modulators for their ability to rescue toxicity in a Drosophila model of SBMA. We identified two compounds, tolfenamic acid (TA) and 1-[2-(4-methylphenoxy)ethyl]-2-[(2-phenoxyethyl)sulfanyl]-1H-benzimidazole (MEPB), as top candidates for rescuing lethality, locomotor function and neuromuscular junction defects in SBMA flies. Pharmacokinetic analyses in mice revealed a more favorable bioavailability and tissue retention of MEPB compared with TA in muscle, brain and spinal cord. In a preclinical trial in a new mouse model of SBMA, MEPB treatment yielded a dose-dependent rescue from loss of body weight, rotarod activity and grip strength. In addition, MEPB ameliorated neuronal loss, neurogenic atrophy and testicular atrophy, validating AF2 modulation as a potent androgen-sparing strategy for SBMA therapy.

Paediatric solid tumours arise from endodermal, ectodermal, or mesodermal lineages. Although the overall survival of children with solid tumours is 75%, that of children with recurrent disease is below 30%. To capture the complexity and diversity of paediatric solid tumours and establish new models of recurrent disease, here we develop a protocol to produce orthotopic patient-derived xenografts at diagnosis, recurrence, and autopsy. Tumour specimens were received from 168 patients, and 67 orthotopic patient-derived xenografts were established for 12 types of cancer. The origins of the patient-derived xenograft tumours were reflected in their gene-expression profiles and epigenomes. Genomic profiling of the tumours, including detailed clonal analysis, was performed to determine whether the clonal population in the xenograft recapitulated the patient's tumour. We identified several drug vulnerabilities and showed that the combination of a WEE1 inhibitor (AZD1775), irinotecan, and vincristine can lead to complete response in multiple rhabdomyosarcoma orthotopic patient-derived xenografts tumours in vivo.

Purpose: Rhabdoid tumors are devastating pediatric cancers in need of improved therapies. We sought to identify small molecules that exhibit in vitro and in vivo efficacy against preclinical models of rhabdoid tumor. Experimental Design: We screened eight rhabdoid tumor cell lines with 481 small molecules and compared their sensitivity with that of 879 other cancer cell lines. Genome-scale CRISPR–Cas9 inactivation screens in rhabdoid tumors were analyzed to confirm target vulnerabilities. Gene expression and CRISPR–Cas9 data were queried across cell lines and primary rhabdoid tumors to discover biomarkers of small-molecule sensitivity. Molecular correlates were validated by manipulating gene expression. Subcutaneous rhabdoid tumor xenografts were treated with the most effective drug to confirm in vitro results. Results: Small-molecule screening identified the protein-translation inhibitor homoharringtonine (HHT), an FDA-approved treatment for chronic myelogenous leukemia (CML), as the sole drug to which all rhabdoid tumor cell lines were selectively sensitive. Validation studies confirmed the sensitivity of rhabdoid tumor to HHT was comparable with that of CML cell lines. Low expression of the antiapoptotic gene BCL2L1, which encodes Bcl-XL, was the strongest predictor of HHT sensitivity, and HHT treatment consistently depleted Mcl-1, the synthetic-lethal antiapoptotic partner of Bcl-XL. Rhabdoid tumor cell lines and primary-tumor samples expressed low BCL2L1, and overexpression of BCL2L1 induced resistance to HHT in rhabdoid tumor cells. Furthermore, HHT treatment inhibited rhabdoid tumor cell line and patient-derived xenograft growth in vivo. Conclusions: Rhabdoid tumor cell lines and xenografts are highly sensitive to HHT, at least partially due to their low expression of BCL2L1. HHT may have therapeutic potential against rhabdoid tumors.

Summary Background (+)-SJ000557733 (SJ733) is a novel, orally bioavailable inhibitor of Plasmodium falciparum ATP4. In this first-in-human and induced blood-stage malaria phase 1a/b trial, we investigated the safety, tolerability, pharmacokinetics, and antimalarial activity of SJ733 in humans. Methods The phase 1a was a single-centre, dose-escalation, first-in-human study of SJ733 allowing modifications to dose increments and dose-cohort size on the basis of safety and pharmacokinetic results. The phase 1a took place at St Jude Children's Research Hospital and at the University of Tennessee Clinical Research Center (Memphis, TN, USA). Enrolment in more than one non-consecutive dose cohort was allowed with at least 14 days required between doses. Participants were fasted in seven dose cohorts and fed in one 600 mg dose cohort. Single ascending doses of SJ733 (75, 150, 300, 600, 900, or 1200 mg) were administered to participants, who were followed up for 14 days after SJ733 dosing. Phase 1a primary endpoints were safety, tolerability, and pharmacokinetics of SJ733, and identification of an SJ733 dose to test in the induced blood-stage malaria model. The phase 1b was a single-centre, open-label, volunteer infection study using the induced blood-stage malaria model in which fasted participants were intravenously infected with blood-stage P falciparum and subsequently treated with a single dose of SJ733. Phase 1b took place at Q-Pharm (Herston, QLD, Australia) and was initiated only after phase 1a showed that exposure exceeding the threshold minimum exposure could be safely achieved in humans. Participants were inoculated on day 0 with P falciparum-infected human erythrocytes (around 2800 parasites in the 150 mg dose cohort and around 2300 parasites in the 600 mg dose cohort), and parasitaemia was monitored before malaria inoculation, after inoculation, immediately before SJ733 dosing, and then post-dose. Participants were treated with SJ733 within 24 h of reaching 5000 parasites per mL or at a clinical score higher than 6. Phase 1b primary endpoints were calculation of a parasite reduction ratio (PRR48) and parasite clearance half-life, and safety and tolerability of SJ733 (incidence, severity, and drug-relatedness of adverse events). In both phases of the trial, SJ733 hydrochloride salt was formulated as a powder blend in capsules containing 75 mg or 300 mg for oral administration. Healthy men and women (of non-childbearing potential) aged 18–55 years were eligible for both studies. Both studies are registered with ClinicalTrials.gov ( NCT02661373 for the phase 1a and NCT02867059 for the phase 1b). Findings In the phase 1a, 23 healthy participants were enrolled and received one to three non-consecutive doses of SJ733 between March 14 and Dec 7, 2016. SJ733 was safe and well tolerated at all doses and in fasted and fed conditions. 119 adverse events were recorded: 54 (45%) were unrelated, 63 (53%) unlikely to be related, and two (2%) possibly related to SJ733. In the phase 1b, 17 malaria-naive, healthy participants were enrolled. Seven participants in the 150 mg dose cohort were inoculated and dosed with SJ733. Eight participants in the 600 mg dose cohort were inoculated, but two participants could not be dosed with SJ733. Two additional participants were subsequently inoculated and dosed with SJ733. SJ733 exposure increased proportional to the dose through to the 600 mg dose, then was saturable at higher doses. Fasted participants receiving 600 mg exceeded the target area under the concentration curve extrapolated to infinity (AUC0–∞) of 13 000 μg × h/L (median AUC0–∞ 24 283 [IQR 16 135–31 311] μg × h/L, median terminal half-life 17·4 h [IQR 16·1–24·0], and median timepoint at which peak plasma concentration is reached 1·0 h [0·6–1·3]), and this dose was tested in the phase 1b. All 15 participants dosed with SJ733 had at least one adverse event. Of the 172 adverse events recorded, 128 (74%) were mild. The only adverse event attributed to SJ733 was mild bilateral foot paraesthesia that lasted 3·75 h and resolved spontaneously. The most common adverse events were related to malaria. Based on parasite clearance half-life, the derived log10 PRR48 and corresponding parasite clearance half-lives were 2·2 (95% CI 2·0–2·5) and 6·47 h (95% CI 5·88–7·18) for 150 mg, and 4·1 (3·7–4·4) and 3·56 h (3·29–3·88) for 600 mg. Interpretation The favourable pharmacokinetic, tolerability, and safety profile of SJ733, and rapid antiparasitic effect support its development as a fast-acting component of combination antimalarial therapy. Funding Global Health Innovative Technology Fund, Medicines for Malaria Venture, and the American Lebanese Syrian Associated Charities.

Developing drugs for CNS related diseases continues to be one of the most challenging endeavors in drug discovery. This is at least in part related to the existence of the Blood Brain Barrier (BBB), a complex multicellular organization that provides selective access to required nutrients and hormones, while removing waste and restricting exposure to potential harmful toxins, pathogens, and xenobiotics. Consequently, designing and selecting molecules that can overcame this protection system are unique and critical aspects of the CNS drug discovery. Here we review modern CNS pharmacokinetic concepts and methods suitable for early drug discovery, and medicinal chemistry strategies towards molecules with optimal CNS exposure.

3-Hydroxy-3-methylglutaryl coenzyme A reductase inhibitors were potent hits against a mouse ependymoma cell line, but their effectiveness against central nervous system tumors will depend on their ability to cross the blood–brain barrier and attain a sufficient exposure at the tumor. Among 3-hydroxy-3-methylglutaryl coenzyme A inhibitors that had activity in vitro, we prioritized simvastatin (SV) as the lead compound for preclinical pharmacokinetic studies based on its potential for central nervous system penetration as determined from in silico models. Furthermore, we performed systemic plasma disposition and cerebral microdialysis studies of SV (100 mg/kg, p.o.) in a murine model of ependymoma to characterize plasma and tumor extracellular fluid (tECF) pharmacokinetic properties. The murine dosage of SV (100 mg/kg, p.o.) was equivalent to the maximum tolerated dose in patients (7.5 mg/kg, p.o.) based on equivalent plasma exposure of simvastatin acid (SVA) between the two species. SV is rapidly metabolized in murine plasma with 15 times lower exposure compared with human plasma. SVA exposure in tECF was

Using a high-throughput small molecule screen, Teitz et al. identify kenpaullone, a cyclin-dependent kinase 2 inhibitor, which when delivered locally confers protection against cisplatin- and noise-induced hearing loss in zebrafish, mice, and rats and reduces mitochondrial ROS production and cochlear cell death., Hearing loss caused by aging, noise, cisplatin toxicity, or other insults affects 360 million people worldwide, but there are no Food and Drug Administration–approved drugs to prevent or treat it. We screened 4,385 small molecules in a cochlear cell line and identified 10 compounds that protected against cisplatin toxicity in mouse cochlear explants. Among them, kenpaullone, an inhibitor of multiple kinases, including cyclin-dependent kinase 2 (CDK2), protected zebrafish lateral-line neuromasts from cisplatin toxicity and, when delivered locally, protected adult mice and rats against cisplatin- and noise-induced hearing loss. CDK2-deficient mice displayed enhanced resistance to cisplatin toxicity in cochlear explants and to cisplatin- and noise-induced hearing loss in vivo. Mechanistically, we showed that kenpaullone directly inhibits CDK2 kinase activity and reduces cisplatin-induced mitochondrial production of reactive oxygen species, thereby enhancing cell survival. Our experiments have revealed the proapoptotic function of CDK2 in postmitotic cochlear cells and have identified promising therapeutics for preventing hearing loss.

Purpose Retinoblastoma is a childhood cancer of the retina. Clinical trials have shown that local delivery of broad spectrum chemotherapeutic agents is efficacious. Recent studies characterizing the genomic and epigenomic landscape of retinoblastoma identified spleen tyrosine kinase (SYK) as a promising candidate for targeted therapy. The purpose of this study was to conduct preclinical testing of the SYK antagonist R406 to evaluate it as a candidate for retinoblastoma treatment. Methods The efficacy of the SYK antagonist R406 delivered locally in a human orthotopic xenograft mouse model of retinoblastoma was tested. Intraocular exposure of R406 was determined for various routes and formulations. Results There was no evidence of efficacy for subconjunctival. R406. Maximal vitreal concentration was 10-fold lower than the minimal concentration required to kill retinoblastoma cells in vitro. Dosage of R406 subconjunctivally from emulsion or suspension formulations, direct intravitreal injection of the soluble prodrug of R406 (R788), and repeated topical administration of R406 all increased vitreal exposure, but failed to reach the exposure required for retinoblastoma cell death in culture. Conclusion Taken together, these data suggest that R406 is not a viable clinical candidate for the treatment of retinoblastoma. This study highlights the importance of pharmacokinetic testing of molecular targeted retinoblastoma therapeutics. Electronic supplementary material The online version of this article (doi:10.1007/s11095-014-1399-y) contains supplementary material, which is available to authorized users.

The treatment of children with primary central nervous system (CNS) tumors continues to be a challenge despite recent advances in technology and diagnostics. In this overview, we describe our approach for identifying and evaluating active anticancer drugs through a process that enables rational translation from the lab to the clinic. The preclinical approach we discuss uses tumor subgroup-specific models of pediatric CNS tumors, cerebral microdialysis sampling of tumor extracellular fluid (tECF), and pharmacokinetic modeling and simulation to overcome challenges that currently hinder researchers in this field. This approach involves performing extensive systemic (plasma) and target site (CNS tumor) pharmacokinetic studies. Pharmacokinetic modeling and simulation of the data derived from these studies are then used to inform future decisions regarding drug administration, including dosage and schedule. Here, we also present how our approach was used to examine two FDA approved drugs, simvastatin and pemetrexed, as candidates for new therapies for pediatric CNS tumors. We determined that due to unfavorable pharmacokinetic characteristics and insufficient concentrations in tumor tissue in a mouse model of ependymoma, simvastatin would not be efficacious in further preclinical trials. In contrast to simvastatin, pemetrexed was advanced to preclinical efficacy studies after our studies determined that plasma exposures were similar to those in humans treated at similar tolerable dosages and adequate unbound concentrations were found in tumor tissue of medulloblastoma-bearing mice. Generally speaking, the high clinical failure rates for CNS drug candidates can be partially explained by the fact that therapies are often moved into clinical trials without extensive and rational preclinical studies to optimize the transition. Our approach addresses this limitation by using pharmacokinetic and pharmacodynamic modeling of data generated from appropriate in vivo models to support the rational testing and usage of innovative therapies in children with CNS tumors.

Purpose: To describe sources of interindividual variability in bevacizumab disposition in pediatric patients and explore associations among bevacizumab pharmacokinetics and clinical wound healing outcomes. Experimental Design: Before tumor resection, three doses of bevacizumab (15 mg/kg) were administered to patients (median age, 12.2 years) enrolled in a multi-institutional osteosarcoma trial. Serial sampling for bevacizumab pharmacokinetics was obtained from 27 patients. A population pharmacokinetic model was fit to the data, and patient demographics and clinical chemistry values were systematically tested as predictive covariates on model parameters. Associations between bevacizumab exposure and wound healing status were evaluated by logistic regression. Results: Bevacizumab concentration–time data were adequately described by a two-compartment model. Pharmacokinetic parameter estimates were similar to those previously reported in adults, with a long median (range) terminal half-life of 12.2 days (8.6 to 32.4 days) and a volume of distribution indicating confinement primarily to the vascular space, 49.1 mL/kg (27.1 to 68.3 mL/kg). Body composition was a key determinant of bevacizumab exposure, as body mass index percentile was significantly (P < 0.05) correlated to body-weight normalized clearance and volume of distribution. Furthermore, bevacizumab exposure before primary tumor resection was associated with increased risk of major wound healing complications after surgery (P < 0.05). Conclusion: A population pharmacokinetic model for bevacizumab was developed, which demonstrated that variability in bevacizumab exposure using weight-based dosing is related to body composition. Bevacizumab dosage scaling using ideal body weight would provide an improved dosing approach in children by minimizing pharmacokinetic variability and reducing likelihood of major wound healing complications. Clin Cancer Res; 20(10); 2783–92. ©2014 AACR.

We devised a high-throughput, cell-based assay to identify compounds to treat Group3 medulloblastoma (G3 MB). Mouse G3 MBs neurospheres were screened against a library of approximately 7,000 compounds including U.S. Food and Drug Administration (FDA)-approved drugs. We identified pemetrexed and gemcitabine that preferentially inhibited G3 MB proliferation in vitro compared to control neurospheres and substantially inhibited G3 MB proliferation in vivo. When combined, these two drugs significantly increased survival of mice bearing cortical implants of mouse and human G3 MBs that overexpress MYC compared to each agent alone, while having little effect on mouse MBs of the Sonic Hedgehog (SHH) subgroup. Our findings strongly suggest that combination therapy of pemetrexed and gemcitabine is a promising treatment for G3 MBs.

Medulloblastoma (MB), the most common malignant pediatric brain tumor, is classified into four major molecularly and histopathologically distinct subgroups, among which MYC-driven Group 3 MBs confer a poor prognosis. The Cyclin D/CDK4/CDK6/RB pathway is frequently deregulated in MB leading to uncontrolled cell proliferation, but tumors express an intact RB protein (Northcott et al., Nature, 2017). Therefore, CDK4/6 inhibitor drugs offer a possible therapeutic approach to treat MBs. Because single agent therapy ultimately leads to drug resistance, we initiated in vitro combination drug screens to identify drug classes potentiating CDK4/6 inhibitors. We used Group 3 MB patient-derived orthotopic xenografts (PDOXs), a human cell line (HDMB03), and freshly dissociated tumor cells propagated only in the mouse brain. The drug screen included 90 compounds comprising targeted and cytotoxic drugs that are FDA approved or under active clinical investigation. Using a bioluminescence-based assay that measures ATP consumption (CellTiter-Glo) to evaluate the number of viable cells, these 90 compounds were screened in combination with a fixed concentration of ribociclib, one of the three FDA approved CDK4/6 inhibitors. The primary screen, carried out in HDMB03 cells, revealed several drugs with additive or synergistic potential when combined with ribociclib, including BET inhibitors, MEK inhibitors, PI3K/mTOR inhibitors and gemcitabine. We are currently evaluating the combination of brain penetrant compounds in Group 3 MB PDOXs. The identification of potent drug combinations should provide new therapeutic options for the treatment of Group 3 MB, one of the most difficult to treat.

Introduction. Acute erythroid leukemia (AEL) is a high-risk leukemia subtype of poorly understood genetic basis. In integrated comparative genomic analysis of 159 AEL and 1509 non AEL myeloid tumors, we identified 5 age-related AEL subtypes with distinct genomic features and outcome: adult AEL with bi-allelic alterations in TP53 (31%), frequently co-occurring with alterations in DNMT3A,BCOR, EZH2, RB1, or NFIX; NPM1-mutated (12%); KMT2A-mutated/rearranged (11%), mostly co-mutated with STAG2; pediatric, NUP98-rearranged (5%) and adult, DDX41-mutated (3%). Thirty-eight percent of cases lacked an identifiable exclusive recurrent founding alteration but were enriched in mutations in ASXL1, TET2 and splicing factors. Methods. To explore the roles and cooperativity of the identified alterations in leukemogenesis we used CRISPR-Cas9 genome editing to induce combinations of loss-of-function mutations in 9 recurrently mutated genes in AEL (Tp53, Tet2, Dnmt3a, Asxl1, Ezh2, Stag2, Bcor, Ppm1d, Rb1 and Nfix). Based on patterns of mutation association, we generated 6 pools of lentiviral vectors (Table 1) with different combinations of single guide RNA (sgRNA) to induce multiplex genome editing in Cas9-eGFP-mouse lineage-negative hematopoietic stem cells (HSCs). Transduced cells were transplanted into lethally irradiated congenic mice. Tumors were characterized by morphology, immunophenotyping, and genomic analysis (sequencing of sites of editing, and exome, methylation and transcriptome sequencing). Ex vivo drug screening was performed to test sensitivity to 192 therapeutic agents, including conventional chemotherapeutic agents, and compounds targeting epigenetic regulators, protein kinases and cell cycle checkpoints. Results. In contrast to the uniform representation of guide RNAs observed in HSCs pre-transplant, tumors exhibited enrichment of specific sgRNAs with tumors of specific phenotype. We frequently observed bi-allelic editing of Tp53, Bcor, Tet2, Dnmt3a, Rb1 and Nfix in agreement with the presence of bi-allelic loss in patients. Concomitant editing and inactivation of Tp53/Bcor/Dnmt3a, or Tp53/Bcor/Rb1/Nfix promoted the development of acute erythroid leukemia (GATA1+/RUNX1+/GlyA+/-Ter119+/- and B220/CD19/PAX5/CD3/Mac1/Gr1-). Concomitant editing of Tp53/Bcor/Tet2 promoted the development of B-lineage ALL, and editing of Dnmt3a and Tet2 without Tp53 promoted T-cell ALL. Leukemic clones from primary tumors were serially transplantable across multiple different genetic backgrounds, with the same dominant clone present in all transplanted mice. Notably, mice that did not develop leukemia showed enrichment of different combinations of sgRNAs for Tet2, Dnmt3a and/or Asxl1, genes commonly mutated in clonal hematopoiesis of indeterminate potential, confirming that these mutations are alone not sufficient to induce leukemogenesis. Additional somatic mutations were acquired during clonal expansion of leukemic cells such as alterations of Notch1 and Ikzf1 in T-ALL; Setd2 at the serial passaging of T-ALL; Ptpn11, Kit (D816V), Kras (Q61L) and Lmo7 in the AEL models; and Sf3b3 in the B-ALL model (Fig 1A). Tumors with mutated Tp53 acquired aneuploidy whereas Tet2-mutated cells were genomically stable (Fig. 1B). Unsupervised hierarchical clustering of gene expression profiling identified 3 subgroups (Fig. 1C) associated with distinct genotypes and methylation profiles (Fig. 1D). Tet2-mutated tumors showed increase of hypermethylated sites and co-mutated Bcor/Dnmt3a leukemic cells showed loss of methylation. Eleven tumors representative of key AEL genotypes from the established models were selected to explore drug sensitivity. Response to individual drugs was associated with genotype with co-mutated Tet2/Dnmt3a T-ALL cells sensitive to bromodomain and histone methyltransferase inhibitors; co-mutated Bcor/Tp53/Dnmt3a or Bcor/Rb1 AEL cells to CDK9 inhibitor (LY2857785); Tp53 mutations alone were exclusively sensitive only to PARP inhibition (Talazoparib). In vivo mouse studies are ongoing to confirm ex vivo results. Conclusions: We successfully generated genetically defined models of AEL, demonstrated the role of Tp53 and Bcor mutations in driving the erythroid phenotype, and showed that sensitivity to different classes of compounds is genotype-dependent. These results provide the rational for testing new targeted agents in AEL. Disclosures Mullighan: Abbvie: Research Funding; Amgen: Honoraria, Speakers Bureau; Loxo Oncology: Research Funding; Cancer Prevention and Research Institute of Texas: Consultancy; Pfizer: Honoraria, Research Funding, Speakers Bureau.

Summary Personalized cancer therapy targeting somatic mutations in patient tumors is increasingly being incorporated into practice. Other therapeutic vulnerabilities resulting from changes in gene expression due to tumor specific epigenetic perturbations are progressively being recognized. These genomic and epigenomic changes are ultimately manifest in the tumor proteome and phosphoproteome. We integrated transcriptomic, epigenomic, and proteomic/phosphoproteomic data to elucidate the cellular origins and therapeutic vulnerabilities of rhabdomyosarcoma (RMS). We discovered that alveolar RMS occurs further along the developmental program than embryonal RMS. We also identified deregulation of the RAS/MEK/ERK/CDK4/6, G2/M, and unfolded protein response pathways through our integrated analysis. Comprehensive preclinical testing revealed that targeting the WEE1 kinase in the G2/M pathway is the most effective approach in vivo for high-risk RMS.

Reopening a critical period Young brains, compared with adult brains, are plastic. This phenomenon has given rise to the concept of critical periods, during which acquisition of certain skills is optimal. In mice, an auditory critical period is only open in early postnatal days. The youthful brain tunes circuits to sounds in its environment in a way that the adult brain does not. This facility may form the basis for childhood language acquisition in humans. Blundon et al. show that by manipulating adenosine signaling in mice, some plasticity of the adult auditory cortex can be regained (see the Perspective by Kehayas and Holmaat). Disruption of adenosine production or adenosine receptor signaling in adult mice leads to improved tone discrimination abilities. Science , this issue p. 1352 ; see also p. 1335

Purpose To determine the maximum-tolerated dose (MTD), dose-limiting toxicities (DLT), pharmacokinetic profile, and pharmacodynamics of the histone deacetylase inhibitor, depsipeptide, in children with refractory or recurrent solid tumors. Patients and Methods Depsipeptide was administered as a 4-hour infusion weekly for 3 consecutive weeks every 28 days at dose levels of 10 mg/m2, 13 mg/m2, 17 mg/m2, and 22 mg/m2. Pharmacokinetics and histone acetylation studies were performed in the first course. The levels of H3 histone and acetyl-H3 histone were evaluated in peripheral blood mononuclear cells (PBMC) using immunofluorescence techniques. Results There were 24 patients, and 18 who were assessable were enrolled. DLTs included reversible, asymptomatic T-wave inversions, without any associated changes in troponin levels or evidence of ventricular dysfunction, in the inferior leads in two patients at 22 mg/m2 and in the lateral leads in one patient at 13 mg/m2 (n = 1), and transient asymptomatic sick sinus syndrome and hypocalcemia in one patient at 17 mg/m2. At the MTD (17 mg/m2), the median depsipeptide clearance was 6.8 L/h/m2 with an area under the plasma depsipeptide concentration-time curve from 0 to infinity of 2,414 ng/mL/h, similar to adults. Accumulation of acetylated H3 histones was seen in all patients in a dose independent manner, with maximal accumulation at a median of 4 hours, (range, 0 hours to 20 hours) after the end of the infusion. No objective tumor responses were observed. Conclusion Depsipeptide is well tolerated in children with recurrent or refractory solid tumors when administered weekly for 3 consecutive weeks every 28 days and inhibits histone deacetylase activity in PBMC in a dose-independent manner. The recommended phase II dose in children with solid tumors is 17 mg/m2.

The purpose of this study was to estimate ventricular cerebrospinal fluid (vCSF) topotecan lactone (TPT) exposures in pediatric medulloblastoma patients from plasma concentration–time data by using a pharmaco‐kinetic (PK) model. We studied children with high‐risk medulloblastoma who received pharmacokinetically guided TPT (target plasma area under the concentration–time curve [AUC], 120–160 ng/ml‐h) and obtained serial vCSF samples to assess TPT exposure. Population pharmacokinetic parameters were determined by using linear mixed‐effects modeling via the two‐stage approach. We simulated TPT vCSF exposure duration at plasma TPT AUC values of 120 to 200 ng/ml‐h and determined percentages of studies meeting or exceeding the vCSF exposure duration threshold (EDT) of 1 ng/ ml for 8 h. We then used bootstrap methods to estimate variability in vCSF EDT. Eighteen PK studies were conducted in six patients (median age, 4.8 years). In these patients, seven of nine studies attaining target plasma TPT AUC achieved the vCSF EDT. Given a plasma TPT AUC of 120 ng/ml‐h, the median percentage of results meeting or exceeding EDT was 78% (95% CI, 61%–100%). One patient (four studies) with tumor blockage of CSF flow, which can alter CSF pharmacokinetics, was removed, and the bootstrap analysis was repeated. In this subset, a median 93% (95% CI, 79%–100%) of studies achieved vCSF EDT. Increasing plasma TPT AUC values resulted in increased ability to achieve vCSF EDT. We demonstrated that a plasma PK model could estimate vCSF TPT concentrations. Further, our results indicate that the TPT vCSF EDT can be achieved in more than 80% of studies targeted to a plasma TPT AUC of 120 ng/ml‐h.

Gefitinib (ZD1839, Iressa), a member of the 4-anilinoquinazoline class of compounds, has the chemical name 4-quinazolinamine, N-(3-chloro-4-flurophenyl)-7-methoxy-6-[3-(4-morpholinyl)propoxy]. Gefitinib often is referred to as a "specific" or "selective" inhibitor of epidermal growth factor receptor (EGFR). EGFR expression has been noted in neuroblastoma and rhabdomyosarcoma cell lines and in tumor specimens from children with Wilms tumor, osteosarcoma, and glioma. Thus, gefitinib, the first marketed EGFR tyrosine kinase inhibitor, was chosen for study in children with refractory solid tumors and central nervous system (CNS) malignancies. This review discusses findings from 3 clinical trials of gefitinib in children with refractory solid tumors and CNS malignancies, focusing on the clinical pharmacology of the compound. To date, gefitinib has been studied in children as a single agent and in combination with irinotecan. Overall, the compound has been well tolerated in children and has a safety profile similar to that observed in adults. The clinical pharmacokinetics of gefitinib in children are similar to those observed in adults. Finally, the future for the use of gefitinib in pediatrics is similar to that of other molecularly targeted agents and awaits definition of tumors and patient populations in which it will be most advantageous.

Background Epoetin alfa (EPO, PROCRIT®) pharmacokinetics and pharmacodynamics were evaluated in children with malignant solid tumors receiving chemotherapy. Procedure Children initially received IV EPO 600 IU/kg (max dose 40,000 IU) or placebo once weekly for 16 weeks. Dose was increased to 900 IU/kg (max dose 60,000 IU) for patients not achieving a 1 g/dl increase in hemoglobin by study week 3 or 4. Serial PK samples were collected for 24 hr after the first study dose, and after the 10th or 11th dose. Serum EPO concentrations were analyzed using an ELISA assay, and pharmacokinetics were evaluated using compartmental methods. Results Twelve children participated; six (median age 15.2 years; range 9.3–18.6 years) were randomized to receive EPO. All children required dosage increases to 900 IU/kg due to no response. The median (range) apparent EPO AUC0–24 and clearance (CL) were 67.1 IU/ml·hr (13.8–102.6) and 0.26 L/hr/m2 (0.19–1.08), respectively. After the 10th or 11th EPO dose in four of these six EPO patients, the median (range) apparent AUC0–24 and CL of EPO was 126.5 IU/ml·hr (107.3–161.1) and 0.21 L/hr/m2 (0.15–0.25), respectively. No significant correlations were observed between pharmacokinetic parameters and pharmacodynamic effects. Conclusions EPO disposition in our patients was similar to other pediatric patient populations or adults receiving IV EPO. Interesting but insignificant trends were noted in pharmacodynamic effects. Pediatr Blood Cancer 2006; 47:572–579. © 2005 Wiley-Liss, Inc.

BACKGROUND. An open-label Phase II study of oxaliplatin was conducted to evaluate its safety and efficacy in children with recurrent or refractory medulloblastoma (MB), supratentorial primitive neuroectodermal tumors (SPNET), and atypical teratoid rhabdoid tumor (ATRT). METHODS. Patients were stratified as follows: stratum IA, first recurrence MB with measurable disease; IB, recurrent MB with only cerebral spinal fluid (CSF) positivity or linear leptomeningeal disease (LLD); IC, MB ≥second recurrence; stratum II, recurrent SPNET; stratum III, recurrent ATRT. Patients received oxaliplatin, 130 mg/m2 intravenously over 2 hours every 3 weeks. The primary objective was to estimate the sustained response rate in stratum 1A. Plasma ultrafiltrate platinum pharmacokinetics were evaluated. RESULTS. A total of 43 patients with a median age of 8.5 years (range, 0.6–18.9 years) were enrolled. In stratum 1A, 2 of 15 had partial responses (PRs, 1 sustained PR). No responses were observed in other strata. The most frequent Grade 3 and 4 toxicities included thrombocytopenia (25.6%), neutropenia (16.3%), leukopenia (12%), increase in serum alanine transaminase (ALT) (7%), vomiting (4.7%), and sensory neuropathy (4.7%). No severe ototoxicity or nephrotoxicity was reported. Plasma ultrafiltrate platinum pharmacokinetic parameters were similar to adults, with a median clearance of 12.2 L/hr (range, 4.4–30 L/hr) and median area under the curve (AUC0-∞) of 9.4 μg/mL/hr (range, 6.2–13.9 μg/mL/hr). CONCLUSIONS. Oxaliplatin was well tolerated in children but has limited activity in children with recurrent CNS embryonal tumors previously treated with platinum compounds. Cancer 2006. © 2006 American Cancer Society.

To develop a pharmacokinetic limited sampling model (LSM) for temozolomide and its metabolite MTIC in infants and children.LSMs consisting of either two or four samples were determined using a modification of the D-optimality algorithm. This accounted for prior distribution of temozolomide and MTIC pharmacokinetic parameters based on full pharmacokinetic sampling from 38 patients with 120 pharmacokinetic studies (dosage range 145-200 mg/m(2) per day orally). Accuracy and bias of each LSM were determined relative to the full sampling method. We also assessed the predictive performance of the LSMs using Monte-Carlo simulations.The four strategies generated from the D-optimality algorithm were as follows: LSM 1=0.25, 1.25, and 3 h; LSM 2=0.25, 1.25, and 6 h; LSM 3=0.25, 0.5, 1.25, and 3 h; LSM 4=0.25, 0.5, 1.25, and 6 h. LSM 2 demonstrated the best combination of low bias [0.1% (-8.9%, 11%) and 11% (4.3%, 15%)] and high accuracy [-1.0% (-12%, 24%) and 14% (7.9%, 37%)] for temozolomide clearance and MTIC AUC, respectively. Furthermore, adding a fourth sample (e.g., LSM 4) did not substantially decrease the bias or increase the accuracy for temozolomide clearance or MTIC AUC. Results from Monte-Carlo simulations also revealed that LSM 2 had the best combination of lowest bias (0.1+/-6.1% and -0.8+/-6.5%), and the highest accuracy (4.5+/-4.1% and 5.0+/-4.3%) for temozolomide clearance and MTIC apparent clearance, respectively.Using data derived from our population analysis, the sampling times for a limited sample pharmacokinetic model for temozolomide and MTIC in children are prior to the temozolomide dose, and 15 min, 1.25 h and 6 h after the dose.

Clofarabine, a deoxyadenosine analog, was an active anticancer drug in our in vitro high-throughput screening against mouse ependymoma neurospheres. To characterize the clofarabine disposition in mice for further preclinical efficacy studies, we evaluated the plasma and central nervous system disposition in a mouse model of ependymoma. A plasma pharmacokinetic study of clofarabine (45 mg/kg, IP) was performed in CD1 nude mice bearing ependymoma to obtain initial plasma pharmacokinetic parameters. These estimates were used to derive D-optimal plasma sampling time points for cerebral microdialysis studies. A simulation of clofarabine pharmacokinetics in mice and pediatric patients suggested that a dosage of 30 mg/kg IP in mice would give exposures comparable to that in children at a dosage of 148 mg/m2. Cerebral microdialysis was performed to study the tumor extracellular fluid (ECF) disposition of clofarabine (30 mg/kg, IP) in the ependymoma cortical allografts. Plasma and tumor ECF concentration–time data were analyzed using a nonlinear mixed effects modeling approach. The median unbound fraction of clofarabine in mouse plasma was 0.79. The unbound tumor to plasma partition coefficient (K pt,uu: ratio of tumor to plasma AUCu,0–inf) of clofarabine was 0.12 ± 0.05. The model-predicted mean tumor ECF clofarabine concentrations were below the in vitro 1-h IC50 (407 ng/mL) for ependymoma neurospheres. Thus, our results show the clofarabine exposure reached in the tumor ECF was below that associated with an antitumor effect in our in vitro washout study. Therefore, clofarabine was de-prioritized as an agent to treat ependymoma, and further preclinical studies were not pursued.

10535 Background: Rhabdomyosarcoma (RMS) is an aggressive malignancy of childhood with a poor prognosis in patients with metastatic or recurrent disease. Inhibitors of Wee1 kinase and heat shock protein 90 (HSP90) have in vitro activity in RMS and have emerged as potential novel treatment strategies. We performed a comprehensive preclinical phase III study to compare the Wee1 inhibitor AZD1775 and HSP90 inhibitor ganetespib (GANET) in combination with irinotecan (IRN) and vincristine (VCR). Methods: Orthotopic xenografts (O-PDXs) were created by injecting luciferase labeled RMS cells into the hind-leg muscle of CD-1 nude mice. Pharmacokinetic studies on RMS O-PDXs were performed to determine matched human AUC-guided dosing. A total of 540 O-PDXs derived from 4 high risk RMS patients, 2 alveolar and 2 embryonal, were randomly enrolled into 14 treatment groups. Six courses of blinded placebo-controlled therapy were given on a clinically relevant schedule. Mice were classified as having progressive disease if tumor approached 20% body weight at any time in the study. For mice completing all 6 courses, bioluminescence was used to determine complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD). Results: The addition of AZD1775 to IRN and VCR demonstrated the most significant response for all 4 O-PDX lines tested; 70% of mice achieved a CR or PR. GANET combined with IRN and VCR had a 54% response (CR + PR) which was not significantly better than IRN plus VCR for most O-PDXs tested. Overall response data for select treatment groups is shown in the table below. Conclusions: Comprehensive preclinical testing using multiple O-PDX models of RMS that represent the clinical spectrum of disease is feasible. Comparison of novel treatment regimens to standard of care at clinically relevant doses is warranted as justification for future clinical trials. Observation to assess durable response following completion of therapy is ongoing to determine if AZD1775 remains a promising treatment strategy. [Table: see text]

SummaryAlterations of IKZF1, encoding the lymphoid transcription factor IKAROS, are a hallmark of high-risk acute lymphoblastic leukemia (ALL), however the role of IKZF1 alterations in ALL pathogenesis is poorly understood. Here, we show that in mouse models of BCR-ABL1 leukemia, Ikzf1 and Arf alterations synergistically promote the development of an aggressive lymphoid leukemia. Ikzf1 alterations result in acquisition of stem cell-like features, including self-renewal and increased bone marrow stromal adhesion. Retinoid receptor agonists reversed this phenotype, partly by inducing expression of IKZF1, resulting in abrogation of adhesion and self-renewal, cell cycle arrest, and attenuation of proliferation without direct cytotoxicity. Retinoids potentiated the activity of dasatinib in mouse and human BCR-ABL1 ALL, providing an additional therapeutic option in IKZF1-mutated ALL.

© 2014 The Authors. Ewing sarcoma (EWS) is a tumor of the bone and soft tissue that primarily affects adolescents and young adults. With current therapies, 70% of patients with localized disease survive, but patients with metastatic or recurrent disease have a poor outcome. We found that EWS cell lines are defective in DNA break repair and are sensitive to PARP inhibitors (PARPis). PARPi-induced cytotoxicity in EWS cells was 10- to 1,000-fold higher after administration of the DNAdamaging agents irinotecan or temozolomide. We developed an orthotopic EWS mouse model and performed pharmacokinetic and pharmacodynamic studies using three different PARPis that are in clinical development for pediatric cancer. Irinotecan administered on a low-dose, protracted schedule previously optimized for pediatric patients was an effective DNA-damaging agent when combined with PARPis; it was also better tolerated than combinations with temozolomide. Combining PARPis with irinotecan and temozolomide gave complete and durable responses in more than 80% of the mice.

The spliceosome regulates pre-mRNA splicing, which is a critical process in normal mammalian cells. Recently, recurrent mutations in numerous spliceosomal proteins have been associated with a number of cancers. Previously, natural product antitumor agents have been shown to interact with one of the proteins that is subject to recurrent mutations (SF3B1). We report the optimization of a class of tumor-selective spliceosome modulators that demonstrate significant in vivo antitumor activity. This optimization culminated in the discovery of sudemycin D6, which shows potent cytotoxic activity in the melanoma line SK-MEL-2 (IC50 = 39 nM) and other tumor cell lines, including JeKo-1 (IC50 = 22 nM), HeLa (IC50 = 50 nM), and SK-N-AS (IC50 = 81 nM). We also report improved processes for the synthesis of these compounds. Our work supports the idea that sudemycin D6 is worthy of further investigation as a novel preclinical anticancer agent with application in the treatment of numerous human cancers.

Introduction: Hypodiploid acute lymphoblastic leukemia (ALL) is a rare, high-risk subtype of B-ALL associated with poor outcome. Genomic analysis of over 130 Hypodiploid ALL patients defined the distinct genomic landscape of this disease, defining two principal subtypes including Ras-activating alterations and IKZF3 loss in near haploid tumors (24-31 chromosomes) and mutation of TP53 coupled with homozygous IKZF2 loss in low hypodiploid tumors (32-39 chromosomes). New therapies are needed to improve outcome of this disease, necessitating preclinical studies in hypodiploid tumors in vitro, ex vivo and in vivo. Hypothesis: The observation of frequent Ras pathway activating mutations in hypodiploid ALL tumors, as well as increased phosphorylation of downstream Ras signaling targets, suggests that these tumors may be susceptible to PI3K/MEK inhibition. Ras signaling target activation was observed in hypodiploid tumors irrespective of mutational status. Here we used cell lines and representative xenograft models to explore the therapeutic efficacy of inhibiting PI3K/MAPK signaling in hypodiploid ALL. Methods: The near haploid cell lines NALM-16 and MHH-CALL-2 and eight patient-derived xenograft (PDX) models (four near haploid and four low hypodiploid) were selected for preclinical studies. Xenografts were representative of the most common genomic lesions observed in hypodiploid ALL patients, including deletion of NF1, PAG1 and IKZF3 in near haploid and mutation of TP53 and IKZF2 deletion in low hypodiploid (Holmfeldt et al. Nat Genet 2013; Mullighan et al. Blood 2015). Next-gen sequencing of xenograft and diagnosis tumor DNA confirmed the presence of key genetic alterations in the xenografts. A lentiviral vector was used for stable expression of luciferase in xenograft tumors for in vivo disease burden monitoring. Cell lines were treated in vitro with PI3K, mTOR, MEK, CDK4/6 and BET inhibitors either as single agents, inhibitor combinations or combined with chemotherapeutic agents. Xenografts were treated ex vivo with single agents and in vivo with PI3K and MEK inhibitors with or without chemotherapy. Results: NALM-16 and MHH-CALL-2 cell lines were most sensitive to PI3K, mTOR and BET inhibitors with IC50 values in the sub-micromolar range (Fig. 1).Treatment of xenografts ex vivo showed near haploid and low hypodiploid tumors to be four-fold more sensitive to pan-PI3K inhibitors and PI3K/mTOR dual inhibitors compared to PI3K α and d isoform-specific inhibitors. Near haploid and low hypodiploid xenografts were sensitive to MEK and BET inhibition ex vivo, but MEK inhibitors showed weak activity in the near haploid cell lines. Treatment with CDK4/6 inhibitors either as single agents or in combination with MEK inhibitors revealed minimal efficacy. Changes in cell signaling upon treatment was assayed by phosphoflow (Fig. 2). Akt and Erk1/2 phosphorylation was variable between tumors, whereas 4E-BP1 and S6 phosphorylation was consistently elevated, suggesting strong signaling through mTOR pathways. Pan-PI3K and PI3K/mTOR dual inhibitors showed potent activity against hypodiploid ALL tumors in vitro and ex vivo, however PI3K inhibition alone was insufficient to eliminate tumors in vivo. Substantial weight loss in mice treated with PI3K/mTOR dual inhibitors limited prolonged study in vivo. MAPK inhibition showed activity in both near haploid and low hypodiploid tumors ex vivo and displayed a modest reduction in tumor growth as a single agent in vivo. Combining the MEK inhibitor GDC-0973 with dexamethasone treatment increased efficacy, but induced significantly more weight loss than either monotherapy. Conclusion: Treatment of hypodiploid cell lines and xenografts in vitro and ex vivo, respectively, with PI3K and MEK inhibitors showed promising results, however the anti-tumor effect in vivo appeared mostly cytostatic and inhibitors were not sufficient as single agents to kill the tumor cells. MEK inhibitors showed stronger activity than PI3K inhibitors on xenografts in vivo and ex vivo, but this was not significantly increased when combined with dexamethasone treatment. The notable resistance of hypodiploid cells to dexamethasone may underlie this limited efficacy in addition to the increase in GDC-0973 clearance observed upon combination. Our findings also highlight the need to consider the PK properties of agents in the preclinical setting, particularly in combinations. Figure 1 Figure 1. Figure 2 Figure 2. Disclosures No relevant conflicts of interest to declare.

Myelodysplastic syndromes (MDSs) represent a group of incurable stem-cell malignancies which are predominantly treated by supportive care. Epigenetic silencing through promoter methylation of a number of genes is present in poor-risk subtypes of MDS and often predicts transformation to acute myelogenous leukemia (AML). Azacitidine and decitabine, two FDA-approved DNA methyltransferase (DNMT) inhibitors, are able to improve overall response although their oral bioavailability complicates their clinical use. This study evaluated -triacetyl-5-azacitidine (TAC) as a potential prodrug for azacitidine. The prodrug demonstrated significant pharmacokinetic improvements in bioavailability, solubility, and stability over the parent compound. In vivo analyses indicated a lack of general toxicity coupled with significantly improved survival. Pharmacodynamic analyses confirmed its ability to suppress global methylation in vivo. These data indicate that esterified nucleoside derivatives may be effective prodrugs for azacitidine and encourages further investigation of TAC into its metabolism, activity, and possible clinical evaluation.

Purpose: To characterize the population pharmacokinetics of topotecan lactone in children with cancer and identify covariates related to topotecan disposition. Patients and Methods: The study population consisted of 162 children in seven clinical trials receiving single agent topotecan as a 30-min infusion. A population approach via nonlinear mixed effects modeling was used to conduct the analysis. Results: A two-compartment model was fit to topotecan lactone plasma concentrations (n = 1874), and large pharmacokinetic variability was observed among studies, among individuals, and within individuals. We conducted a covariate analysis using demographics, biochemical data, trial effects, and concomitant drugs. The most significant covariate was body surface area, which explained 54% of the interindividual variability for topotecan systemic clearance. Interoccasion variability was considerable in both clearance and volume (20% and 22%, respectively), but was less than interindividual variability in both variables. Other covariates related to clearance were concomitant phenytoin, calculated glomerular filtration rate, and age ( Conclusions: We developed a descriptive and robust population pharmacokinetic model which identified patient covariates that account for topotecan disposition in pediatric patients. Additionally, dosing topotecan based on the covariate model led to a more accurate and precise estimation topotecan systemic exposure compared with a fixed dosing approach, and could be a tool to assist clinicians to individualize topotecan dosing.

Purpose A phase II study was conducted to evaluate the activity and safety of topotecan in pediatric patients with recurrent Wilms' tumor. Patients and Methods Patients with favorable histology Wilms' tumor (FHWT) and recurrence after at least one salvage chemotherapy regimen or with anaplastic histology Wilms' tumor (AHWT) in first or subsequent recurrence were eligible. Patients were stratified according to histology, with statistical considerations based on the FHWT stratum. Topotecan was administered intravenously over 30 minutes for 5 days on 2 consecutive weeks. Treatment dosages were adjusted to achieve a target area under the curve (AUC) of 80 ± 10 ng/mL*h. Tumor responses were measured after two cycles of treatment. Results Thirty-seven patients (26 patients with FHWT) were enrolled and received a total of 94 cycles of topotecan (range, one to six cycles). The median topotecan dosage required to achieve the target AUC was 1.8 mg/m2 (range, 0.7 to 3.2 mg/m2). Of 25 assessable patients with FHWT, 12 had partial response (PR), six had stable disease (SD), and seven had progressive disease (PD), for an overall response rate of 48% (95% CI, 27.8% to 68.7%). Of 11 assessable patients with AHWT, two had PR, one had SD, and eight had PD. The main toxicities were grade 3 and 4 neutropenia (median duration, 13 days) and thrombocytopenia (median duration, 7.5 days). Conclusion Topotecan administered on a protracted schedule is active against recurrent FHWT. Inclusion of topotecan in front-line clinical trials for patients with recurrent Wilms' tumor should be considered.

Purpose To evaluate the maximum-tolerated dose (MTD), dose-limiting toxicity (DLT), pharmacokinetics (PK), and adverse effect profile of oxaliplatin in pediatric patients with refractory solid tumors and to determine whether carbamazepine reduces oxaliplatin-induced neurotoxicity. Patients and Methods Three regimens of oxaliplatin (given intravenously over 2 hours) were tested: regimen A (100 mg/m2, 130 mg/m2, or 160 mg/m2 every 3 weeks to determine the MTD of oxaliplatin); regimen B (to determine whether carbamazepine starting 24 hours before and ending 48 hours after oxaliplatin reduced the dose-limiting neurotoxicity and increased the MTD of regimen A); and regimen C (to evaluate the safety of a fixed dose two-thirds the MTD of regimen A given every 2 weeks [more frequent administration but comparable dose intensity]). Results Twenty-six patients were enrolled on regimens A (n = 11), B (n = 6), and C (n = 9). The DLT was grade 3 pharyngolaryngeal dysesthesia, sensory neuropathy, and ataxia at 160 mg/m2. The MTD was 130 mg/m2 every 3 weeks. At the MTD, the median clearance rate of ultrafiltrable platinum was 9.7 L/h/m2 (range, 6.5 to 15.5 L/h/m2). Addition of carbamazepine permitted dose escalation to 160 mg/m2 without DLT. DLT was not observed with a fixed dose of 85 mg/m2 given every 2 weeks. On all regimens, hematologic toxicity was mild. No significant nephrotoxicity, ototoxicity, or cumulative neurologic toxicity was observed. Conclusion The DLT, MTD, PK, and adverse effect profile of oxaliplatin in pediatric patients with refractory solid tumors are similar to those observed in adults. Carbamazepine may reduce the dose-limiting neurotoxicity of oxaliplatin.

Purpose: Histone acetyltransferases and histone deacetylases (HDAC) control the acetylation state of histones and other proteins regulating transcription and protein function. Several structurally diverse HDAC inhibitors have been developed as cancer therapeutic agents and in vitro have been shown to cause differentiation, cell cycle arrest, or apoptosis. Here, we have evaluated depsipeptide, a natural tetrapeptide HDAC inhibitor, against a panel of pediatric solid tumor models in vivo and evaluated pharmacokinetic and pharmacodynamic variables with tumor sensitivity. Experimental Design: Depsipeptide was administered at the maximum tolerated dose (4.4 mg/kg administered every 7 days × 3 i.v. repeated q21d for a total of two cycles) to scid mice bearing 39 independently derived childhood tumors (9 brain tumors, 11 kidney cancers, 9 rhabdomyosarcomas, 3 neuroblastomas, and 7 osteosarcomas). Pharmacokinetic variables were determined, as were changes in histone and p53 acetylation, induction of p53 and p53 genotype, and alterations in Akt phosphorylation. Results: Of 39 tumors evaluated, three showed objective tumor regressions [two brain tumors (primitive neuroectodermal tumor and atypical teratoid malignant rhabdoid tumor) and one Wilms' tumor]. Depsipeptide inhibited growth of many tumor lines but achieved stable disease ( Conclusions: Our results show that depsipeptide inhibits its target in vivo causing increased histone acetylation; however, this does not correlate with drug sensitivity. The relatively low objective response rate [3 of 39 (8%) tumor lines showing greater than or equal to partial response and 4 (10%) stable disease] administered at dose levels that give clinically relevant drug exposures suggests that as a single agent depsipeptide may have limited clinical utility against pediatric solid tumors in a first-line setting.

OBJECTIVE To review the resistance mechanisms of Enterococcus and Staphylococcus spp. and summarize quinupristin/dalfopristin's (QD's) effects on these resistant organisms when combined with other antibiotics via review of the literature and unpublished data. DATA SOURCES Data were identified by a PubMed search (1996—May 2003) using the search terms quinupristin/dalfopristin, synergy, in vitro, in vivo, vancomycin-resistant Enterococcus faecium (VREF), methicillin-resistant Staphylococcus aureus (MRSA), and individual antibiotic names. Bibliographies of the resultant PubMed searches were reviewed and included if applicable. STUDY SELECTION AND DATA EXTRACTION All studies reviewed were analyzed; specific drug data were included only if clinically pertinent. In vitro data from studies with adequate design were discussed, whereas all case reports and clinical trials were utilized. DATA SYNTHESIS In the treatment of VREF, available information seems conflicting, although some clear differences have become apparent. QD—ampicillin and QD—doxycycline combinations have demonstrated beneficial activity, usually displaying synergistic or additive effects even in macrolide-, lincosamine-, and streptogramin-resistant (MLSB) isolates. Vancomycin and chloramphenicol have shown some efficacy, but antagonistic or null results also have been observed. Regarding MRSA, results from many studies of QD combinations have been ambiguous. More common combinations displayed synergy or additive effects against MRSA, but only QD—rifampin showed consistent beneficial activity against MRSA and MLSB isolates. Most other combinations displayed antagonism when tested in vitro. CONCLUSIONS Data supporting the use of various QD—antibiotic combinations against VREF and MRSA are increasing, but further in vitro and in vivo data are needed to confirm the findings.

Clofarabine, a deoxyadenosine analog, was a potent hit in our in vitro high-throughput screening against murine ependymoma neurospheres. To prioritize clofarabine for further preclinical efficacy studies, we evaluated the plasma pharmacokinetic (PK) disposition and central nervous system penetration in a murine model of ependymoma. A plasma PK study of clofarabine (45 mg/kg IP) was performed using CD1 nude mice bearing ependymoma cortical allographs (Ink4a/Arf-null + RTBDN) to obtain initial plasma PK parameters. These estimates were used to derive D-optimal plasma sampling time-points (e.g., 0.25, 2.5, and 5 hr) for cerebral microdialysis studies. Comparison of the clofarabine systemic exposure obtained from the plasma PK study and that simulated from pediatric patients using a published population PK model (Bonate, Cancer Chemotherap Pharmacol, 2011) suggested a dosage of 30 mg/kg in mice would be equivalent to a pediatric dosage of 180 mg/m2 given as a 2 hr infusion. Cerebral microdialysis was applied in CD1 nude mice bearing ependymoma cortical allographs (Ink4a/Arf-null + RTBDN), which permitted repeated in situ sampling of clofarabine tumor extracellular fluid (tECF). A microdialysis probe (BASi; 1 mm membrane) was introduced into the tumor through a cannula inserted during tumor cell implantation. After microdialysis probe equilibration, 7 mice were dosed with 30 mg/kg of clofarabine IP. In each mouse, serial plasma samples were collected at 0.25, 2.5, and 5 h post-dose, and tECF dialysate fractions were collected over 60 min intervals for up to 5 h post-dose. To measure clofarabine in both plasma and tECF, a robust, sensitive LC-MS/MS method was developed and validated. Both within-day and between-day precision (%CV) were ≤ 5.1% and accuracy ranged from 86% to 109%. A two-compartment model with absorption and tumor compartments linked to a central compartment was fitted to plasma and tECF concentration-time data using a nonlinear mixed effects modeling approach (NONMEM 7.2.0). For modeling purposes, the volume of the tECF compartment was fixed to published values. Unbound fraction of clofarabine in murine plasma was 0.82 ± 0.14. The model derived area under unbound concentration-time curve (AUCu,0-8) values for 30 and 45 mg/kg dosages were 5185 ± 550 µg/L*hr and 7677 ± 699 µg/L*hr, respectively. Clofarabine was absorbed rapidly from the peritoneal cavity with Tmax (time to reach maximum concentration) value of 0.33 ± 0.17 hr. Tumor to plasma partition coefficient (Kpt,uu: ratio of tumor to plasma AUCu,0-inf) of clofarabine was 0.12 ± 0.05. The model predicted mean tECF clofarabine concentrations were below the in vitro 1-hr IC50 (1.34 µM) for ependymoma neurospheres. In summary, we have shown the tECF clofarabine concentrations were below that required for antitumor effect in our in vitro washout studies, thus we have not pursued clofarabine for detailed efficacy studies in our preclinical pipeline. Citation Format: Yogesh T. Patel, Megan O. Jacus, Abigail D. Davis, Pradeep Vuppala, Jason D. Dapper, Burgess B. Freeman, Nidal Boulos, Stacy L. Throm, Richard J. Gilbertson, Clinton F. Stewart. Clofarabine, a potent anticancer compound with limited penetration in an orthotopic murine model of ependymoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4645. doi:10.1158/1538-7445.AM2014-4645

GEM, a fluorinated nucleoside analog, was identified by in vitro high-throughput screening as a potent inhibitor of murine tumor cell lines derived from a preclinical model of a highly-aggressive subgroup of pediatric MB (MYC-driven Group 3). To support the potential preclinical-to-clinical transition of GEM for this indication, we determined the pharmacokinetic (PK) disposition and central nervous system (CNS) penetration of GEM in a murine Group 3 MB model. A serial sacrifice plasma PK study (single sample per mouse) was performed in tumor-bearing mice to obtain initial GEM plasma PK parameter estimates. These estimates were used to inform a D-optimal, limited sampling strategy for microdialysis experiments. Cerebral microdialysis was applied in CD-1 nude mice bearing orthotopic allograft Group 3 mouse MB tumor lines (SJCRH 2416/2417), which permitted repeated in situ measurements of GEM tumor extracellular fluid (tECF) concentrations. Microdialysis probes (BASi; 1 mm membrane) were introduced into the tumor through cannulae inserted during tumor cell implantation. After microdialysis probe equilibration, 5 mice were each dosed with 60 mg/kg of GEM via tail vein injection. In each mouse, serial plasma samples were collected at 0.083, 1.5, and 6 hrs after dosing, and tECF dialysate fractions were collected over 60 min intervals for up to 6 hrs post-dose. To measure GEM in both plasma and tECF, a robust, sensitive LC-MS/MS method was developed and validated. Both within-day and between-day precision (%CV) were ≤ 7% and accuracy ranged from 95.3% to 103%. GEM concentration-time plasma and tECF data were accurately described using a unified population PK model, consisting of a central plasma compartment, a peripheral compartment, and a perfusion-limited “well-stirred” tumor compartment. For modeling purposes, both the plasma flow to the brain and the volume of the brain ECF were fixed to published values. The PK analysis showed that plasma elimination of GEM followed bi-exponential kinetics, with an initial rapid decay (mean ± SD) t1/2,alpha of 8.2 ± 3.3 min followed by a relatively slower beta phase t1/2,beta of 1.6 ± 0.3 hrs. The (mean ± SD) area under the concentration-time curve (AUC0-∞) for GEM in plasma was 544 ± 199 μg/ml*min, consistent with previously published AUC values in pediatric patients at tolerable dosages (1,200 mg/m2). On average, GEM CNS penetration (calculated as the tECF/plasma GEM AUC0-8hrs ratio from the simulated concentration-time curves) was 0.21 ± 0.16. The peak tECF concentrations ranged from 2.7 to 76 μM, and simulated tECF concentrations exceeded in vitro cell line IC50 values for approximately 6 hours. Overall, these data provide convincing rationale to prioritize GEM efficacy evaluations in additional MB models within our preclinical brain tumor drug development program. Citation Format: David C. Turner, Burgess B. Freeman, Megan O. Jacus, Marie Morfouace, Stacy L. Throm, Pradeep K. Vuppala, Martine Roussel, Amar Gajjar, Clinton F. Stewart. Gemcitabine (GEM), a drug with clinical activity against pediatric solid tumors, demonstrates satisfactory CNS penetration in a preclinical murine medulloblastoma (MB) model. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2749. doi:10.1158/1538-7445.AM2013-2749

e13584 Background: Neuroblastoma is an aggressive malignancy that accounts for 15% of pediatric cancer deaths. There are limited well characterized neuroblastoma models for use in preclinical trials. We characterized a human neuroblastoma orthotopic xenograft, performed comprehensive preclinical studies, and evaluated the pharmacokinetics of drugs targeting the PI3K pathway. Methods: Tumor tissue (MAST3) was obtained from a 2 year old with stage IV metastatic MYCN amplified neuroblastoma. Using ultrasound guidance, MAST3 cells were injected into the para-adrenal space of nod-scid mice. Tumors were monitored for growth with routine ultrasound and passaged. The initial patient tumor (MAST3) and tumor from each passage were extensively analyzed including: histology, electron microscopy (EM), gene expression profiling, single nucleotide polymorphism (SNP ) microarrays, whole genome sequencing (WGS), and spectral karyotyping (SKY). Using this orthotopic xenograft model, a randomized preclinical trial was conducted to evaluate the response to standard chemotherapeutic agents. Pharmacokinetic studies of oral BEZ-235, BKM-120, OSI-906 and everolimus were conducted. Results: Analysis of the histology, EM, gene expression profiling, SNP microarrays, WGS, and SKY showed minimal variability between the MAST3 tumor and the passaged tumors in the orthotopic xenografts. Notably, MAST3 metastasized to the liver, lung and spleen in vivo. Administering standard chemotherapeutic agents every 3 weeks for a total of 6 courses, we observed a significant response in 40% of treated mice. Further ongoing preclinical trials will evaluate the comparative efficacy of IGFR/PI3K/mTOR molecular targeted therapies. Conclusions: We developed the first well characterized neuroblastoma orthotopic xenograft. Our comprehensive characterization of this xenograft indicates that it retains many of the molecular, cellular and genetic properties of the primary lesion. A preclinical trial using standard chemotherapeutic agents has provided a valuable baseline for comparison with novel therapeutics and will inform our selection of the most promising agents to move into clinical trials.

Abstract 1959 Background: Kinesin Spindle Protein (KSP) is required for cell cycle progression through mitosis. Inhibition of KSP induces mitotic arrest and cell death. ARRY-520 is a potent, selective KSP inhibitor. Cancers such as multiple myeloma (MM) which depend on the short-lived survival protein MCL-1 are highly sensitive to treatment with ARRY-520. ARRY-520 shows potent activity in preclinical MM models, providing a strong rationale for its clinical investigation in this disease. Methods: This Phase 1 study was designed to evaluate the safety and pharmacokinetics (PK) of ARRY-520 administered intravenously (IV) on Day 1 and Day 2 q 2 weeks without/with granulocyte-colony stimulating factor (G-CSF). Patients (pts) with relapsed/refractory (RR) MM with 2 prior lines of therapy (including both bortezomib and an immunomodulatory agent, unless ineligible for or refusing to receive this therapy) were eligible. Cohorts of at least 3 pts were enrolled in a classical 3 + 3 dose escalation design. Pts were treated for 2 cycles (4 weeks) to evaluate safety prior to dose escalation. Results: Twenty five pts have been treated to date, with a median age of 60 years (range 44–79) and a median of 5 prior regimens (range 2–16). All pts received prior bortezomib or carfilzomib, 21 pts received prior lenalidomide, 17 pts prior thalidomide, and 18 pts had a prior stem cell transplant. Pts received ARRY-520 without G-CSF at 1 mg/m2/day (n = 3), and at 1.25 mg/m2/day (n = 7, 6 evaluable). A dose-limiting toxicity (DLT) of Grade 4 neutropenia was observed at 1.25 mg/m2/day, and this was considered the maximum tolerated dose (MTD) without G-CSF. As neutropenia was the DLT, dose escalation with prophylactic G-CSF support was initiated, at doses of 1.5 mg/m2/day (n = 7, 6 evaluable), 2.0 mg/m2/day (n = 6) and 2.25 mg/m2/day (n = 2) with G-CSF. Both the 2.0 mg/m2/day and 2.25 mg/m2/day dose levels were determined to be non-tolerated, with DLTs of febrile neutropenia (FN) (2 pts at 2.0 mg/m2/day and both pts at 2.25 mg/m2/day) and Grade 3 mucositis (both pts at 2.25 mg/m2/day). One out of 6 evaluable pts at 1.5 mg/m2/day also developed a DLT of FN. In an attempt to optimize the Phase 2 dose, an intermediate dose level of 1.75 mg/m2/day with G-CSF is currently being evaluated. The most commonly reported treatment-related adverse events (AEs) include those observed with other KSP inhibitors, such as hematological AEs (thrombocytopenia, neutropenia, anemia, leukopenia), fatigue, mucositis and other gastro-intestinal AEs. Pts displayed linear PK, a low clearance and a moderate volume of distribution, with moderate-to-high inter-individual variability in PK parameters. The median terminal elimination half life is 65 hours. The preliminary efficacy signal as a single agent is encouraging with 2 partial responses (PR) observed to date per IMWG and EBMT criteria in a heavily pretreated population (23 evaluable pts). A bortezomib-refractory pt with 8 prior lines of therapy, including a tandem transplant, treated at 1 mg/m2/day of ARRY-520 obtained a PR after Cycle 6, with urine protein and kappa light chain levels continuing to decline over time. He remains on-study after 15 months of ARRY-520 treatment. A pt with 2 prior lines of therapy, including prior carfilzomib, has obtained a PR after Cycle 8 at 2 mg/m2/day of ARRY-520, and she is currently ongoing after 4.5 months on therapy. Fifteen pts had a best response of stable disease (SD), including 1 pt with a thus far unconfirmed minimal response, and 6 had progressive disease. A total of 10 pts (43%) achieved a PR or SD lasting > 12 weeks. Several additional pts have shown other evidence of clinical activity, with decrease in paraproteins, increase in hemoglobin levels and regression of plasmacytomas. The median number of cycles is 4 (range 1–28+). Treatment activity has not correlated with any baseline characteristics or disease parameters to date. Conclusions: : The selective KSP inhibitor ARRY-520 has been well tolerated, and shows promising signs of single agent clinical activity in heavily pretreated pts with RR MM. Prophylactic G-CSF has enabled higher doses to be tolerated. No cardiovascular or liver enzyme toxicity has been reported. Enrollment is ongoing at 1.75 mg/m2/day with G-CSF support, and a planned Phase 2 part of the study will be initiated as soon as the MTD is determined. Complete Phase 1 data will be disclosed at the time of the meeting. Disclosures: Shah: Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Millennium: Research Funding. Off Label Use: Revlimid (lenalidomide) in combination with dexamethasone is indicated for the treatment of multiple myeloma patients who have received at least one prior therapy. Zonder:Millennium: Consultancy, Myeloma and Amyloidosis Patient Day Symposium – Corporate support from multiple sponsors for a one-day educational event, Research Funding; Celgene:; Novartis:; Proteolix: . Weber:novartis-unpaid consultant: Consultancy; Merck- unpaid consultant: Consultancy; celgene- none for at least 2 years: Honoraria; millenium-none for 2 years: Honoraria; celgene, Millenium, Merck: Research Funding. Wang:Celgene: Research Funding; Onyx: Research Funding; Millenium: Research Funding; Novartis: Research Funding. Kaufman:Celgene: Consultancy, Honoraria, Research Funding; Millenium: Consultancy, Honoraria; Merck: Research Funding; Genzyme: Consultancy. Walker:Array Biopharma: Employment, Equity Ownership. Freeman:Array Biopharma: Employment, Equity Ownership. Rush:Array Biopharma: Employment, Equity Ownership. Ptaszynski:Array Biopharma: Consultancy. Lonial:Millennium, Celgene, Bristol-Myers Squibb, Novartis, Onyx: Advisory Board, Consultancy; Millennium, Celgene, Novartis, Onyx, Bristol-Myers Squibb: Research Funding.

2570 Background: Antimitotic therapy stabilizes microtubule polymers and inhibits tubulin polymerization, but is nonselective and causes neurotoxicity. Kinesin Spindle Protein (KSP) is responsible ...