Purpose: Patients with relapsed pediatric solid malignancies have few therapeutic options, and many of these patients die of their disease. B7-H3 is an immune checkpoint protein encoded by the CD276 gene that is overexpressed in many pediatric cancers. Here, we investigate the activity of the B7-H3–targeting antibody–drug conjugate (ADC) m276-SL-PBD in pediatric solid malignancy patient-derived (PDX) and cell line–derived xenograft (CDX) models. Experimental Design: B7-H3 expression was quantified by RNA sequencing and by IHC on pediatric PDX microarrays. We tested the safety and efficacy of m276-SL-PBD in two stages. Randomized trials of m276-SL-PBD of 0.5 mg/kg on days 1, 8, and 15 compared with vehicle were performed in PDX or CDX models of Ewing sarcoma (N = 3), rhabdomyosarcoma (N = 4), Wilms tumors (N = 2), osteosarcoma (N = 5), and neuroblastoma (N = 12). We then performed a single mouse trial in 47 PDX or CDX models using a single 0.5 m/kg dose of m276-SL-PBD. Results: The vast majority of PDX and CDX samples studied showed intense membranous B7-H3 expression (median H-score 177, SD 52). In the randomized trials, m276-SL-PBD showed a 92.3% response rate, with 61.5% of models showing a maintained complete response (MCR). These data were confirmed in the single mouse trial with an overall response rate of 91.5% and MCR rate of 64.4%. Treatment-related mortality rate was 5.5% with late weight loss observed in a subset of models dosed once a week for 3 weeks. Conclusions: m276-SL-PBD has significant antitumor activity across a broad panel of pediatric solid tumor PDX models.

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.

Purpose-Defining robust criteria for drug activity in preclinical studies allows for fewer animals per treatment group, and potentially allows for inclusion of additional cancer models that more accurately represent genetic diversity and, potentially, allows for tumor sensitivity biomarker identification.Methods-Using a single mouse design, 32 pediatric xenograft tumor models representing diverse pediatric cancer types (Ewing sarcoma [9], brain [4], rhabdomyosarcoma [10], Wilms tumor [4], and non-CNS rhabdoid tumors [5]) were evaluated for response to a single administration of pegylated-SN38 (PLX038A), a controlled-release PEGylated formulation of SN-38. Endpoints measured were percent tumor regression, and Event Free Survival (EFS). The correlation between response to PLX038A was compared to that for 10 models treated with irinotecan (2.5 mg/kg × 5 days × 2 cycles), using a traditional design (10 mice/group). Correlations between tumor sensitivity, genetic mutations and gene expression were sought. Models showing no disease at week 20 were categorized as 'extreme responders' to PLX038A, whereas those with EFS less than 5 weeks were categorized as 'resistant'.Results-The activity of PLX038A was evaluable in 31/32 models. PLX038A induced >50% volume regressions in 25 models (78%). Initial tumor volume regression correlated only modestly Terms of use and reuse: academic research for non-commercial purposes, see here for full terms. https://www.springer.com/aamterms-v1

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.

Developing new therapeutics for the treatment of childhood cancer has challenges not usually associated with adult malignancies. Firstly, childhood cancer is rare, with approximately 12,500 new diagnoses annually in the U.S. in children 18 years or younger. With current multimodality treatments, the 5-year event-free survival exceeds 80%, and 70% of patients achieve long-term “cure”, hence the overall number of patients eligible for experimental drugs is small. Childhood cancer comprises many disease entities, the most frequent being acute lymphoblastic leukemias (25% of cancers) and brain tumors (21%), and each of these comprises multiple molecular subtypes. Hence, the numbers of diagnoses even for the more frequently occurring cancers of childhood are small, and undertaking clinical trials remains a significant challenge. Consequently, development of preclinical models that accurately represent each molecular entity can be valuable in identifying those agents or combinations that warrant clinical evaluation. Further, new regulations under the Research to Accelerate Cures and Equity for Children Act (RACE For Children Act) will change the way in which drugs are developed. Here, we will consider some of the limitations of preclinical models and consider approaches that may improve their ability to translate therapy to clinical trial more accurately.

Background: VTP-50469 is a potent inhibitor of the menin-MLL1 interaction and is implicated in signaling downstream of EWSR1-FLI1.Procedure: VTP-50469 was evaluated against seven Ewing sarcoma (EwS) xenograft models and in vitro against EwS cell lines.Results: VTP-50469 showed limited antitumor activity, statistically significantly slowing tumor progression in four tumor models but with no evidence of tumor regression. In vitro, the IC 50 concentration was 10 nM for the mixed lineage leukemia (MLL)-rearranged leukemia cell line MV4;11, but > 3 M for EwS cell lines. Conclusions:In contrast to its high level of activity against MLL1-rearranged leukemia xenografts, VTP-50469 shows little activity against EwS models. K E Y W O R D SEwing sarcoma, pediatric oncology, xenograft 1 of 4 wileyonlinelibrary.com/journal/pbc https://doi.

Purpose: We investigated why three patient derived xenograft (PDX) childhood BRAFV600E-mutant brain tumor models are highly sensitive to trametinib. Mechanisms of acquired resistance selected in situ, and approaches to prevent resistance were also examined, which may translate to both LGG molecular subtypes. Experimental Design: Sensitivity to trametinib (MEKi) alone or in combination with rapamycin (TORC1 inhibitor), was evaluated in pediatric PDX models. The effect of combined treatment of trametinib with rapamycin on development of trametinib resistance in vivo was examined. PDX tissue and tumor cells from trametinib-resistant xenografts were characterized. Results: In pediatric models TORC1 is activated through ERK-mediated inactivation of the tuberous sclerosis complex TSC: consequently inhibition of MEK also suppressed TORC1 signaling. Trametinib-induced tumor regression correlated with dual inhibition of MAPK/TORC1 signaling, and decoupling TORC1 regulation from BRAF/MAPK control conferred trametinib resistance. In mice, acquired resistance to trametinib developed within 3 cycles of therapy in all three PDX models. Resistance to trametinib developed in situ is tumor cell intrinsic and the mechanism was tumor line specific. Rapamycin, retarded or blocked development of resistance. Conclusions: In these three pediatric BRAF-mutant brain tumors, TORC1 signaling is controlled by the MAPK cascade. Trametinib suppressed both MAPK/TORC1 pathways leading to tumor regression. While low-dose intermittent rapamycin to enhance inhibition of TORC1 only modestly enhanced the antitumor activity of trametinib, it prevented or retarded development of trametinib resistance, suggesting future therapeutic approaches using rapamycin analogs in combination with MEK inhibitors that may be therapeutically beneficial in both KIAA1549::BRAF and BRAFV600Edriven gliomas.

Aim: Despite aggressive multiagent protocols, patients with metastatic rhabdomyosarcoma (RMS) have poor prognosis. In a recent high-risk trial (ARST0431), 25% of patients failed within the first year, while on therapy and 80% had tumor progression within 24 months. However, the mechanisms for tumor resistance are essentially unknown. Here we explore the use of preclinical models to develop resistance to complex chemotherapy regimens used in ARST0431. Methods: A Single Mouse Testing (SMT) protocol was used to evaluate the sensitivity of 34 RMS xenograft models to one cycle of vincristine, actinomycin D, cyclophosphamide (VAC) treatment. Tumor response was determined by caliper measurement, and tumor regression and event-free survival (EFS) were used as endpoints for evaluation. Treated tumors at regrowth were transplanted into recipient mice, and the treatment was repeated until tumors progressed during the treatment period (i.e., became resistant). At transplant, tumor tissue was stored for biochemical and omics analysis. Results: The sensitivity to VAC of 34 RMS models was determined. EFS varied from 3 weeks to > 20 weeks. Tumor models were classified as having intrinsic resistance, intermediate sensitivity, or high sensitivity to VAC therapy. Resistance to VAC was developed in multiple models after 2-5 cycles of therapy; however, there were examples where sensitivity remained unchanged after 3 cycles of treatment. Conclusion: The SMT approach allows for in vivo assessment of drug sensitivity and development of drug resistance in a large number of RMS models. As such, it provides a platform for assessing in vivo drug resistance mechanisms at a “population” level, simulating conditions in vivo that lead to clinical resistance. These VAC-resistant models represent “high-risk” tumors that mimic a preclinical phase 2 population and will be valuable for identifying novel agents active against VAC-resistant disease.