Multiparametric Longitudinal Profiling of RCAS-tva-Induced PDGFB-Driven Experimental Glioma

Glioblastomas are incurable primary brain tumors harboring a heterogeneous landscape of genetic and metabolic alterations. Longitudinal imaging by MRI and [18F]FET-PET measurements enable us to visualize the features of evolving tumors in a dynamic manner. Yet, close-meshed longitudinal imaging time points for characterizing temporal and spatial metabolic alterations during tumor evolution in patients is not feasible because patients usually present with already established tumors. The replication-competent avian sarcoma-leukosis virus (RCAS)/tumor virus receptor-A (tva) system is a powerful preclinical glioma model offering a high grade of spatial and temporal control of somatic gene delivery in vivo. Consequently, here, we aimed at using MRI and [18F]FET-PET to identify typical neuroimaging characteristics of the platelet-derived growth factor B (PDGFB)-driven glioma model using the RCAS-tva system. Our study showed that this preclinical glioma model displays MRI and [18F]FET-PET features that highly resemble the corresponding established human disease, emphasizing the high translational relevance of this experimental model. Furthermore, our investigations unravel exponential growth dynamics and a model-specific tumor microenvironment, as assessed by histology and immunochemistry. Taken together, our study provides further insights into this preclinical model and advocates for the imaging-stratified design of preclinical therapeutic interventions.

Imipridones affect tumor bioenergetics and promote cell lineage differentiation in diffuse midline gliomas

BACKGROUND

Pediatric diffuse midline gliomas (DMGs) are incurable childhood cancers. The imipridone ONC201 has shown early clinical efficacy in a subset of DMGs. However, the anticancer mechanisms of ONC201 and its derivative ONC206 have not been fully described in DMGs.

METHODS

DMG models including primary human in vitro (n = 18) and in vivo (murine and zebrafish) models, and patient (n = 20) frozen and FFPE specimens were used. Drug-target engagement was evaluated using in silico ChemPLP and in vitro thermal shift assay. Drug toxicity and neurotoxicity were assessed in zebrafish models. Seahorse XF Cell Mito Stress Test, MitoSOX and TMRM assays, and electron microscopy imaging were used to assess metabolic signatures. Cell lineage differentiation and drug-altered pathways were defined using bulk and single-cell RNA-seq.

RESULTS

ONC201 and ONC206 reduce viability of DMG cells in nM concentrations and extend survival of DMG PDX models (ONC201: 117 days, P = .01; ONC206: 113 days, P = .001). ONC206 is 10X more potent than ONC201 in vitro and combination treatment was the most efficacious at prolonging survival in vivo (125 days, P = .02). Thermal shift assay confirmed that both drugs bind to ClpP, with ONC206 exhibiting a higher binding affinity as assessed by in silico ChemPLP. ClpP activation by both drugs results in impaired tumor cell metabolism, mitochondrial damage, ROS production, activation of integrative stress response (ISR), and apoptosis in vitro and in vivo. Strikingly, imipridone treatment triggered a lineage shift from a proliferative, oligodendrocyte precursor-like state to a mature, astrocyte-like state.

CONCLUSION

Targeting mitochondrial metabolism and ISR activation effectively impairs DMG tumorigenicity. These results supported the initiation of two pediatric clinical trials (NCT05009992, NCT04732065).

HGG-32. ONC201 AND ONC206 TARGET TUMOR CELL METABOLISM IN PEDIATRIC DIFFUSE MIDLINE GLIOMA PRECLINICAL MODELS

Diffuse midline gliomas (DMGs) remain incurable cancers and new treatments are urgently needed. One promising new therapeutic avenue for these cancers is targeting of metabolic vulnerabilities including a heightened dependence on mitochondrial metabolism. We and others have shown that the oral, brain-penetrant imipridone drugs ONC201 and ONC206 target mitochondrial metabolism in cancer cells. In particular, ONC201 and ONC206 hyper-activate the mitochondrial protease ClpP, impair mitochondrial oxidative phosphorylation (OXPHOS), activate the integrated stress response (ISR) signaling pathway, and induce apoptosis in DMG preclinical models. We validated ClpP as a key target of ONC206 by showing that CRISPR/Cas9-mediated CLPP knockout significantly decreased ONC206 sensitivity in DMG cells. We further showed that imipridone-mediated ClpP activation resulted in significant degradation of the chaperone protein ClpX. Moreover, ONC201 and ONC206 treatment inhibited mitochondrial respiration, decreased mitochondrial membrane potential and triggered extensive mitochondrial structural damage, including disintegration of mitochondrial cristae. Time-course RNA sequencing of five DMG cell lines treated with ONC201 and ONC206, alone or in combination, revealed robust ATF4 and CHOP upregulation, indicating potent activation of ISR signaling. Notably, ATF4/CHOP upregulation was strongest in ONC201/6 combination-treated cells, indicating synergy between the two drugs. We further explored drug combinations by testing ONC201 together with ONC206, Panobinostat, JQ1, and Osimertinib to identify synergistic combination treatments. The strongest synergistic effect was found over a broad IC50 range for ONC201 and ONC206. Finally, we showed that ONC201 and ONC206 significantly prolonged survival of mice bearing brainstem DIPG xenografts. Ongoing studies include assessment of the in vivo efficacy of ONC201 and ONC206 across different CNS tumor models, as well as investigation and validation of clinically relevant biomarkers of response to treatment. In summary, our preclinical data strongly support the utility of the mitochondrial targeting agents ONC201 and ONC206 for the treatment of DMG and other malignant brain tumors.

HGG-23. IN VITRO AND IN VIVO PRECLINICAL DRUG SCREENING OF PROMISING THERAPEUTICS FOR DIFFUSE MIDLINE GLIOMA (DMG)

Introduction

Diffuse midline gliomas (DMGs) are amongst the most unforgiving pediatric brain tumors, characterized by an intrinsic resistance to therapy. Despite major advances in understanding of tumor biology, the prognosis remains exceedingly poor, and treatment options are limited. New therapeutics are being evaluated at a fast rate by different laboratories. In order to prioritize effective drug candidates for DMG treatment, we comprehensively characterized a panel of promising therapeutic agents in in vitro and in different vivo systems.

Methods

We determined the sensitivity of primary DMG cell lines to a panel of small molecule inhibitors targeting known DMG targets and pathways. Dose response curves were generated for more than 20 different compounds and possible synergistic effects were investigated by SynergieFinder. In an effort to highlight potential toxicities and associated mechanisms at a large scale, we performed a preclinical toxicity evaluation in zebrafish larvae, with a slightly modified version of the official Fish Embryo Acute Toxicity (FET) test. Drug toxicity was tested by continuous exposure of zebrafish larvae to increasing concentrations of the different compounds. Survival curves, morphological analyses and behavioral tests were performed at a maximum tolerated dose (MTD). To confirm the findings obtained in zebrafish, we further performed in vivo studies in mice for promising candidates.

Results

Among the tested drugs in vitro we found 10 drugs showing promising dose- dependent reduction in cell viability with IC₅₀ in nM to µM range. These were further evaluated for toxicity in zebrafish. The zebrafish larvae toxicities observations strongly correlated with the findings in murine in vivo studies, reinforcing the importance of zebrafish as an accurate investigative toxicology model to assess acute toxicity of molecules in preclinical studies.

Conclusions

By testing a wide range of drugs, targeting different pathways on DMG cells and in different in vivo systems we identified promising drug candidates for clinical management of children diagnosed with DMG.

Targeting CSF1R Alone or in Combination with PD1 in Experimental Glioma

Glioblastoma is an aggressive primary tumor of the central nervous system. Targeting the immunosuppressive glioblastoma-associated microenvironment is an interesting therapeutic approach. Tumor-associated macrophages represent an abundant population of tumor-infiltrating host cells with tumor-promoting features. The colony stimulating factor-1/ colony stimulating factor-1 receptor (CSF-1/CSF1R) axis plays an important role for macrophage differentiation and survival. We thus aimed at investigating the antiglioma activity of CSF1R inhibition alone or in combination with blockade of programmed death (PD) 1. We investigated combination treatments of anti-CSF1R alone or in combination with anti-PD1 antibodies in an orthotopic syngeneic glioma mouse model, evaluated post-treatment effects and assessed treatment-induced cytotoxicity in a coculture model of patient-derived microtumors (PDM) and autologous tumor-infiltrating lymphocytes (TILs) ex vivo. Anti-CSF1R monotherapy increased the latency until the onset of neurological symptoms. Combinations of anti-CSF1R and anti-PD1 antibodies led to longterm survivors in vivo. Furthermore, we observed treatment-induced cytotoxicity of combined anti-CSF1R and anti-PD1 treatment in the PDM/TILs cocultures ex vivo. Our results identify CSF1R as a promising therapeutic target for glioblastoma, potentially in combination with PD1 inhibition.

Optimal therapeutic targeting by HDAC inhibition in biopsy-derived treatment-naïve diffuse midline glioma models

BACKGROUND

Diffuse midline gliomas (DMGs), including diffuse intrinsic pontine gliomas (DIPGs), have a dismal prognosis, with less than 2% surviving 5 years postdiagnosis. The majority of DIPGs and all DMGs harbor mutations altering the epigenetic regulatory histone tail (H3 K27M). Investigations addressing DMG epigenetics have identified a few promising drugs, including the HDAC inhibitor (HDACi) panobinostat. Here, we use clinically relevant DMG models to identify and validate other effective HDACi and their biomarkers of response.

METHODS

HDAC inhibitors were tested across biopsy-derived treatment-naïve in vitro and in vivo DMG models with biologically relevant radiation resistance. RNA sequencing was performed to define and compare drug efficacy and to map predictive biomarkers of response.

RESULTS

Quisinostat and romidepsin showed efficacy with low nanomolar half-maximal inhibitory concentration (IC50) values (~50 and ~5 nM, respectively). Comparative transcriptome analyses across quisinostat, romidepsin, and panobinostat showed a greater degree of shared biological effects between quisinostat and panobinostat, and less overlap with romidepsin. However, some transcriptional changes were consistent across all 3 drugs at similar biologically effective doses, such as overexpression of troponin T1 slow skeletal type (TNNT1) and downregulation of collagen type 20 alpha 1 chain (COL20A1), identifying these as potential vulnerabilities or on-target biomarkers in DMG. Quisinostat and romidepsin significantly (P < 0.0001) inhibited in vivo tumor growth.

CONCLUSIONS

Our data highlight the utility of treatment-naïve biopsy-derived models; establishes quisinostat and romidepsin as effective in vivo; illuminates potential mechanisms and/or biomarkers of DMG cell lethality due to HDAC inhibition; and emphasizes the need for brain tumor-penetrant versions of potentially efficacious agents.

MODL-22. DEVELOPING A REAL-TIME PERSONALIZED DRUG TESTING PLATFORM FOR PEDIATRIC CNS CANCERS

INTRODUCTION

The relatively small size of biopsied CNS tumors has presented a historical challenge for real-time drug screens. Moreover, in vivo assessment of drug response does not often benefit patients with aggressive gliomas given the relatively long time (&gt;8 months) of tumor engraftment in the classic mouse PDX models. Here, we aimed to develop an innovative real-time in vivo and in vitro drug screening platform capable of analyzing a minimal number (&lt;1E6) of cells obtained at biopsy.

METHODS

Existing primary cells were used to test 6 different culture platforms. The top platform was selected and used to expand tumor cells obtained of DMG biopsy. Tumor cells were validated using the minION sequencing platform. Single and combination drug (n=7) screens were performed. Effective drugs were further evaluated in zebrafish PDX and non-tumor bearing models to assess efficacy and toxicity, respectively.

RESULTS

A total of 8 biopsies were obtained. Successful cell expansion was achieved in 6/8 (75%) and a limited drug screen in 3/6 (50%) of cases. Single and combination drug (n=7) assays identified responder and non-responders to candidate drugs. Systemic toxicity of effective drugs was tested in non-tumor bearing zebrafish. Tumor cells were engrafted in zebrafish providing the opportunity for an in vivo screen. The entire process was completed within 21 days on average.

CONCLUSIONS

A novel platform was developed for rapid in vitro and in vivo drug screens of tumor cells obtained at biopsy. This platform will provide the opportunity to establish personalized therapy for heterogeneous cancers including DMGs.

Next-generation of targeted AAVP vectors for systemic transgene delivery against cancer

Bacteriophage (phage) have attractive advantages as delivery systems compared with mammalian viruses, but have been considered poor vectors because they lack evolved strategies to confront and overcome mammalian cell barriers to infective agents. We reasoned that improved efficacy of delivery might be achieved through structural modification of the viral capsid to avoid pre- and postinternalization barriers to mammalian cell transduction. We generated multifunctional hybrid adeno-associated virus/phage (AAVP) particles to enable simultaneous display of targeting ligands on the phage's minor pIII proteins and also degradation-resistance motifs on the very numerous pVIII coat proteins. This genetic strategy of directed evolution bestows a next-generation of AAVP particles that feature resistance to fibrinogen adsorption or neutralizing antibodies and ability to escape endolysosomal degradation. This results in superior gene transfer efficacy in vitro and also in preclinical mouse models of rodent and human solid tumors. Thus, the unique functions of our next-generation AAVP particles enable improved targeted gene delivery to tumor cells.

Proteasome inhibition in cancer is associated with enhanced tumor targeting by the adeno-associated virus/phage

Bacteriophage (phage), which are viruses that infect bacteria only, have shown promise as vehicles for targeted cancer gene therapy, albeit with poor efficiency. Recently, we generated an improved version of phage vectors by incorporating cis genetic elements of adeno-associated virus (AAV). This novel AAV/phage hybrid (AAVP) efficiently delivered systemically administered therapeutic genes to various tumor targets by displaying an integrin tumor-targeting ligand on the phage capsid. However, inherent limitations in bacteriophage mean that these AAVP vectors still need to be improved. One of the limitations of AAVP in mammalian cells may be its susceptibility to proteasomal degradation. The proteasome is upregulated in cancer and it is known that it constitutes a barrier to gene delivery by certain eukaryotic viruses. We report here that inhibition of proteasome improved targeted reporter gene delivery by AAVP in cancer cells in vitro and in tumors in vivo after intravenous vector administration to tumor-bearing mice. We also show enhanced targeted tumor cell killing by AAVP upon proteasome inhibition. The AAVP particles persisted significantly in cancer cells in vitro and in tumors in vivo after systemic administration, and accumulated polyubiquitinated coat proteins. Our results suggest that the proteasome is indeed a barrier to tumor targeting by AAVP and indicate that a combination of proteasome-inhibiting drugs and AAVP should be considered for clinical anticancer therapy.

Dual systemic tumor targeting with ligand-directed phage and Grp78 promoter induces tumor regression

The tumor-specific Grp78 promoter is overexpressed in aggressive tumors. Cancer patients would benefit greatly from application of this promoter in gene therapy and molecular imaging; however, clinical benefit is limited by lack of strategies to target the systemic delivery of Grp78-driven transgenes to tumors. This study aims to assess the systemic efficacy of Grp78-guided expression of therapeutic and imaging transgenes relative to the standard cytomegalovirus (CMV) promoter. Combination of ligand and Grp78 transcriptional targeting into a single vector would facilitate systemic applications of the Grp78 promoter. We generated a dual tumor-targeted phage containing the arginine-glycine-aspartic acid tumor homing ligand and Grp78 promoter. Next, we combined flow cytometry, Western blot analysis, bioluminescence imaging of luciferase, and HSVtk/ganciclovir gene therapy and compared efficacy to conventional phage carrying the CMV promoter in vitro and in vivo in subcutaneous models of rat and human glioblastoma. We show that double-targeted phage provides persistent transgene expression in vitro and in tumors in vivo after systemic administration compared with conventional phage. Next, we showed significant tumor killing in vivo using the HSVtk/ganciclovir gene therapy and found a systemic antitumor effect of Grp78-driven HSVtk against therapy-resistant tumors. Finally, we uncovered a novel mechanism of Grp78 promoter activation whereby HSVtk/ganciclovir therapy upregulates Grp78 and transgene expression via the conserved unfolded protein response signaling cascade. These data validate the potential of Grp78 promoter in systemic cancer gene therapy and report the efficacy of a dual tumor targeting phage that may prove useful for translation into gene therapy and molecular imaging applications.