We have established 51 solid tumor and 10 ALL in vivo models. The models identify vincristine and cyclophosphamide as having broad-spectrum activity. The PPTP tumor panels appear to generally recapitulate the activity of these agents against specific childhood cancers and to have the potential for identifying novel agents having significant clinical activity.

Purpose:Vinca alkaloids, agents that cause depolymerization of microtubules, are highly active in treatment of many pediatric cancers. In contrast, taxanes, agents that stabilize microtubules, are far less effective against the same cancer types.The purpose of the current study was to evaluate the antitumor activity of ixabepilone, an epothilone B derivative representing a new class of microtubule-stabilizing antimitotic agent in a wide variety of pediatric solid tumor models. Experimental Design: Ixabepilone was administered i.v. every 4 days for three doses to scid mice bearing s.c. human rhabdomyosarcoma (three lines), neuroblastoma (four), Wilms' tumors (six), osteosarcoma (four), or brain tumors (seven). Tumor diameters were measured weekly, and tumor growth or regressions were determined. Pharmacokinetic studies were done following a single administration of drug at the maximum tolerated dose (MTD) level (10 mg/kg). Results: At the MTD (10 mg/kg), ixabepilone induced objective responses (all tumors in a group achieved z50% volume regression) in three of three rhabdomyosarcoma lines, three of five neuroblastomas, six of seven Wilms' tumor models, two of six osteosarcoma, and four of eight brain tumor models. However, the dose-response curve was steep with only 2 of 19 tumors models regressing (z50%) at 4.4 mg/kg. In comparison, paclitaxel administered at the MTD on the same schedule failed to induce objective regressions of three tumor lines that were highly sensitive to treatment with ixabepilone. Pharmacokinetics following single i.v. administration of ixabepilone at its MTD (10 mg/kg) were biexponential with C max of 12.5 Amol/L, elimination half-life of 19.2 hours, and total area under the curve of 5.8 Amol/L-h.The achieved drug exposure of ixabepilone at this efficacious MTD dose level in mice is similar to those achieved in patients given the recommended phase II dose of 40 mg/m 2 by either1-or 3-hour infusion every 3 weeks, a regimen that has shown significant anticancer activity in phase II clinical trials in adult patients. Conclusions: Administered at doses ranging from 66% to100% of its MTDinmice, the epothilone B derivativeixabepilone shows broad spectrum activity against apanelof pediatric tumor xenograft models.Pharmacokineticanalysisindicates that the systemicixabepiloneexposureachievedinmice at its MTD is similar to that achieved in patients at the recommended phase II dose of 40 mg/m 2 administered every 3 weeks. Importantly, the present results showed a clear distinction in sensitivity of pediatric solid tumors to this epothilone derivative compared with paclitaxel.

Background Tazemetostat (EPZ-6438) is a selective inhibitor of the histone methyltransferase EZH2, currently in clinical development for non-Hodgkin lymphoma and genetically defined tumors. Procedures Tazemetostat was tested against the PPTP solid tumor xenografts using a dose of 400 mg/kg administered twice-daily by oral gavage for 28 days. H3K27me3:H3 ratios were determined in control and treated tumors. Results Tazemetostat induced significant differences in event free survival (EFS) distribution compared to control in 9 of 30 (30%) of the xenografts studied. Significant differences in EFS distribution were observed in 5 of 7 (71%) rhabdoid tumor xenograft lines compared to 4 of 23 (17%) non-rhabdoid xenograft lines [chi square (χ2) test p=0.006]. Tazemetostat induced tumor growth inhibition meeting criteria for intermediate and high EFS treated to control (T/C) activity in 2 of 25 (8%) and 1 of 25 (4%) xenografts, respectively. Intermediate and high activity for the EFS T/C metric was observed exclusively among rhabdoid tumor xenografts (3 of 5 rhabdoid tumor versus 0 of 22 non-rhabdoid tumors (χ2 test p<0.001). One rhabdoid tumor xenograft (G401) showed stable disease. For one rhabdoid tumor (G401), delayed tumor regression to tazemetostat was noted following 1 week of tumor growth. Tazemetostat induced significant reduction of H3K27me3 levels in the majority of tumors compared to controls. Conclusions Tazemetostat demonstrated significant antitumor activity in rhabdoid tumor models, but showed no consistent activity against any other histology. Tazemetostat reduced H3K27me3 levels irrespective of tumor response. Further preclinical testing to evaluate tazemetostat in combination with other anticancer agents is warranted.

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,11kidney 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 oneWilms' tumor]. Depsipeptide inhibited growth of many tumor lines but achieved stable disease (<25% increase in volume during treatment cycle 1) in only two tumor models (anaplastic astrocytoma, two rhabdomyosarcomas, and a Wilms' tumor). Pharmacokinetic analysis showed that the population estimated AUC 0-24 was 1,123 ng h/mL, similar to the exposure following 13 mg/m 2 in ongoing phase I trials. Pharmacodynamic changes in histone acetylation (H2A, H2B, H3, and H4) in three depsipeptide-sensitive and three intrinsically resistant tumors followed a similar pattern; maximal increases in histone acetylation occurred at 8 hours and were elevated for up to 96 hours. In two sensitive tumor lines, IRS56 and BT27 (both wild-type p53) p53 increased in treated tumors being maximal at 8 hours and associated with induction of p21 cip1 , whereas p53 was stable in tumors with mutant p53. Sensitivity to depsipeptide did not correlate with p53 genotype, p53 acetylation, cleaved poly(ADP-ribose) polymerase, or phosphorylation of Akt (Ser 473 ). 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.

ABT-751 demonstrated intermediate activity against this tumor panel. Neuroblastoma models appear somewhat more sensitive to this agent, with objective regressions also in rhabdomyosarcoma and Wilms tumor. ABT-751 was also active in several tumor lines intrinsically refractory to vincristine or paclitaxel.

Background: Entinostat, a selective class I histone deacetylase inhibitor, has been reported to enhance the activity of cytotoxic agents and suppress expression of PAX3-FOXO1 in alveolar rhabdomyosarcoma (ARMS). Procedures: Entinostat was tested against three rhabdomyosarcoma cell lines using 96-hour drug exposure. Entinostat alone or in binary combination with vincristine, actinomycin D or cyclophosphamide was tested in ARMS and two embryonal rhabdomyosarcoma (ERMS) xenograft models. Tumor growth was measured at weekly intervals. Drug-induced changes in acetylated histone H3(K9) and entinostat pharmacokinetics were determined. Results: In vitro, the IC50 concentration of entinostat ranged from 280 to 1300 nM. In vivo, entinostat significantly inhibited the growth of only Rh10 xenografts. For most studies, entinostat did not potentiate the activity of the cytotoxic agent. Exceptions included the vincristine and entinostat combination for Rh10 and the entinostat and actinomycin D combination for Rh10 and Rh18, although the effects were modest. For Rh18, the combination of entinostat with vincristine showed evidence of an antagonistic interaction compared with single-agent vincristine. Pharmacokinetic studies showed the average Cmax was 569.4 ng/mL (1.51 μM) with Tmax at 15 minutes, and total exposure (AUC0–12 h) was 435.6 h × ng/mL. Entinostat treatment increased acetylated histone H3. Conclusions: Entinostat demonstrated modest antitumor activity in only one of four models at dose and shedule that gave drug exposures relevant to human treatment. The addition of entinostat to standard-of-care cytotoxic agents was in most instances no more effective than the cytotoxic agents used alone. Entinostat demonstrated target inhibition with increased histone 2A acetylation.

P had no single-agent activity, although it marginally potentiated the activity of vincristine and cisplatin in one of three models studied. However, the addition of E necessitated dose reduction of each cytotoxic agent, abrogating the enhancement observed with P alone.

Treatment with the nucleoside analog cytarabine has been shown to mimic changes in gene expression associated with down-regulation of the EWS-FLI1 oncogene in Ewing sarcoma cell lines, selectively inhibit their growth in vitro, and cause tumor regression in athymic nude mice. For this report cytarabine was studied in vitro against a panel of 23 pediatric cancer cell lines and in vivo against 6 Ewing sarcoma xenografts. Acute lymphoblastic leukemia cell lines were the most sensitive to cytarabine in vitro (median IC50 9 nM), while Ewing sarcoma cell lines showed intermediate sensitivity (median IC50 232 nM). Cytarabine at a dose of 150 mg/kg administered daily 5× failed to significantly inhibit growth of five xenograft models, but reduced growth rate of the A673 xenograft by 50%. Cytarabine shows no differential in vitro activity against Ewing sarcoma cell lines and is ineffective in vivo against Ewing sarcoma xenografts at the dose and schedule studied.