Purine salvage promotes treatment resistance in H3K27M-mutant diffuse midline glioma

BACKGROUND

Diffuse midline gliomas (DMG), including diffuse intrinsic pontine gliomas (DIPGs), are a fatal form of brain cancer. These tumors often carry a driver mutation on histone H3 converting lysine 27 to methionine (H3K27M). DMG-H3K27M are characterized by altered metabolism and resistance to standard of care radiation (RT) but how the H3K27M mediates the metabolic response to radiation and consequent treatment resistance is uncertain.

METHODS

We performed metabolomics on irradiated and untreated H3K27M isogenic DMG cell lines and observed an H3K27M-specific enrichment for purine synthesis pathways. We profiled the expression of purine synthesis enzymes in publicly available patient data and our models, quantified purine synthesis using stable isotope tracing, and characterized the in vitro and in vivo response to de novo and salvage purine synthesis inhibition in combination with RT.

RESULTS

DMG-H3K27M cells activate purine metabolism in an H3K27M-specific fashion. In the absence of genotoxic treatment, H3K27M-expressing cells have higher relative activity of de novo synthesis and apparent lower activity of purine salvage demonstrated via stable isotope tracing of key metabolites in purine synthesis and by lower expression of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), the rate-limiting enzyme of purine salvage into IMP and GMP. Inhibition of de novo guanylate synthesis radiosensitized DMG-H3K27M cells in vitro and in vivo. Irradiated H3K27M cells upregulated HGPRT expression and hypoxanthine-derived guanylate salvage but maintained high levels of guanine-derived salvage. Exogenous guanine supplementation decreased radiosensitization in cells treated with combination RT and de novo purine synthesis inhibition. Silencing HGPRT combined with RT markedly suppressed DMG-H3K27M tumor growth in vivo.

CONCLUSIONS

Our results indicate that DMG-H3K27M cells rely on highly active purine synthesis, both from the de novo and salvage synthesis pathways. However, highly active salvage of free purine bases into mature guanylates can bypass inhibition of the de novo synthetic pathway. We conclude that inhibiting purine salvage may be a promising strategy to overcome treatment resistance in DMG-H3K27M tumors.

Potentiating the radiation-induced type I interferon antitumoral immune response by ATM inhibition in pancreatic cancer

Radiotherapy induces a type I interferon-mediated (T1IFN-mediated) antitumoral immune response that we hypothesized could be potentiated by a first-in-class ataxia telangiectasia mutated (ATM) inhibitor, leading to enhanced innate immune signaling, T1IFN expression, and sensitization to immunotherapy in pancreatic cancer. We evaluated the effects of AZD1390 or a structurally related compound, AZD0156, on innate immune signaling and found that both inhibitors enhanced radiation-induced T1IFN expression via the POLIII/RIG-I/MAVS pathway. In immunocompetent syngeneic mouse models of pancreatic cancer, ATM inhibitor enhanced radiation-induced antitumoral immune responses and sensitized tumors to anti-PD-L1, producing immunogenic memory and durable tumor control. Therapeutic responses were associated with increased intratumoral CD8+ T cell frequency and effector function. Tumor control was dependent on CD8+ T cells, as therapeutic efficacy was blunted in CD8+ T cell-depleted mice. Adaptive immune responses to combination therapy provided systemic control of contralateral tumors outside of the radiation field. Taken together, we show that a clinical candidate ATM inhibitor enhances radiation-induced T1IFN, leading to both innate and subsequent adaptive antitumoral immune responses and sensitization of otherwise resistant pancreatic cancer to immunotherapy.

GTP Signaling Links Metabolism, DNA Repair, and Responses to Genotoxic Stress

How cell metabolism regulates DNA repair is incompletely understood. Here, we define a GTP-mediated signaling cascade that links metabolism to DNA repair and has significant therapeutic implications. GTP, but not other nucleotides, regulates the activity of Rac1, a guanine nucleotide-binding protein, which promotes the dephosphorylation of serine 323 on Abl-interactor 1 (Abi-1) by protein phosphatase 5 (PP5). Dephosphorylated Abi-1, a protein previously not known to activate DNA repair, promotes nonhomologous end joining. In patients and mouse models of glioblastoma, Rac1 and dephosphorylated Abi-1 mediate DNA repair and resistance to standard-of-care genotoxic treatments. The GTP-Rac1-PP5-Abi-1 signaling axis is not limited to brain cancer, as GTP supplementation promotes DNA repair and Abi-1-S323 dephosphorylation in nonmalignant cells and protects mouse tissues from genotoxic insult. This unexpected ability of GTP to regulate DNA repair independently of deoxynucleotide pools has important implications for normal physiology and cancer treatment.

SIGNIFICANCE

A newly described GTP-dependent signaling axis is an unexpected link between nucleotide metabolism and DNA repair. Disrupting this pathway can overcome cancer resistance to genotoxic therapy while augmenting it can mitigate genotoxic injury of normal tissues. This article is featured in Selected Articles from This Issue, p. 5.

M2 isoform of pyruvate kinase rewires glucose metabolism during radiation therapy to promote an antioxidant response and glioblastoma radioresistance

BACKGROUND

Resistance to existing therapies is a significant challenge in improving outcomes for glioblastoma (GBM) patients. Metabolic plasticity has emerged as an important contributor to therapy resistance, including radiation therapy (RT). Here, we investigated how GBM cells reprogram their glucose metabolism in response to RT to promote radiation resistance.

METHODS

Effects of radiation on glucose metabolism of human GBM specimens were examined in vitro and in vivo with the use of metabolic and enzymatic assays, targeted metabolomics, and FDG-PET. Radiosensitization potential of interfering with M2 isoform of pyruvate kinase (PKM2) activity was tested via gliomasphere formation assays and in vivo human GBM models.

RESULTS

Here, we show that RT induces increased glucose utilization by GBM cells, and this is accompanied with translocation of GLUT3 transporters to the cell membrane. Irradiated GBM cells route glucose carbons through the pentose phosphate pathway (PPP) to harness the antioxidant power of the PPP and support survival after radiation. This response is regulated in part by the PKM2. Activators of PKM2 can antagonize the radiation-induced rewiring of glucose metabolism and radiosensitize GBM cells in vitro and in vivo.

CONCLUSIONS

These findings open the possibility that interventions designed to target cancer-specific regulators of metabolic plasticity, such as PKM2, rather than specific metabolic pathways, have the potential to improve the radiotherapeutic outcomes in GBM patients.

Metabolomic Profiles of Human Glioma Inform Patient Survival

Aims: Targeting tumor metabolism may improve the outcomes for patients with glioblastoma (GBM). To further preclinical efforts targeting metabolism in GBM, we tested the hypothesis that brain tumors can be stratified into distinct metabolic groups with different patient outcomes. Therefore, to determine if tumor metabolites relate to patient survival, we profiled the metabolomes of human gliomas and correlated metabolic information with clinical data. Results: We found that isocitrate dehydrogenase-wildtype (IDHwt) GBMs are metabolically distinguishable from IDH mutated (IDHmut) astrocytomas and oligodendrogliomas. Survival of patients with IDHmut gliomas was expectedly more favorable than those with IDHwt GBM, and metabolic signatures can stratify IDHwt GBMs subtypes with varying prognoses. Patients whose GBMs were enriched in amino acids had improved survival, while those whose tumors were enriched for nucleotides, redox molecules, and lipid metabolites fared more poorly. These findings were recapitulated in validation cohorts using both metabolomic and transcriptomic data. Innovation: Our results suggest the existence of metabolic subtypes of GBM with differing prognoses, and further support the concept that metabolism may drive the aggressiveness of human gliomas. Conclusions: Our data show that metabolic signatures of human gliomas can inform patient survival. These findings may be used clinically to tailor novel metabolically targeted agents for GBM patients with different metabolic phenotypes. Antioxid. Redox Signal. 39, 942-956.

Rewiring of cortical glucose metabolism fuels human brain cancer growth

The brain avidly consumes glucose to fuel neurophysiology. Cancers of the brain, such as glioblastoma (GBM), lose aspects of normal biology and gain the ability to proliferate and invade healthy tissue. How brain cancers rewire glucose utilization to fuel these processes is poorly understood. Here we perform infusions of 13 C-labeled glucose into patients and mice with brain cancer to define the metabolic fates of glucose-derived carbon in tumor and cortex. By combining these measurements with quantitative metabolic flux analysis, we find that human cortex funnels glucose-derived carbons towards physiologic processes including TCA cycle oxidation and neurotransmitter synthesis. In contrast, brain cancers downregulate these physiologic processes, scavenge alternative carbon sources from the environment, and instead use glucose-derived carbons to produce molecules needed for proliferation and invasion. Targeting this metabolic rewiring in mice through dietary modulation selectively alters GBM metabolism and slows tumor growth.

Significance

This study is the first to directly measure biosynthetic flux in both glioma and cortical tissue in human brain cancer patients. Brain tumors rewire glucose carbon utilization away from oxidation and neurotransmitter production towards biosynthesis to fuel growth. Blocking these metabolic adaptations with dietary interventions slows brain cancer growth with minimal effects on cortical metabolism.

Results of a Multi-institutional Randomized Phase 3 Trial of Parotid-Sparing Whole Brain Radiotherapy

PURPOSE/OBJECTIVE(S)

Observational studies have reported that xerostomia is common after conventional whole brain radiotherapy (WBRT) and associated with parotid dose. In this multi-institutional, single-blind randomized controlled trial, we hypothesized that patient-reported xerostomia is reduced in patients randomized to parotid-sparing vs. standard WBRT fields.

MATERIALS/METHODS

Between 2018 and 2021, patients receiving conventional WBRT (30-35 Gy in 10-15 fractions) for any diagnosis were enrolled at 3 academic institutions. Patients were randomized between standard WBRT fields covering the C1 vertebra with no prospective parotid delineation (control) vs. parotid-sparing fields without C1 coverage (experimental). Patients completed the University of Michigan Xerostomia Questionnaire (Scored 0-100, higher is worse) at baseline, EndRT, 2 weeks, 1 month, 3 months, and 6 months. Patients were excluded from toxicity analyses if baseline xerostomia score was >50 or if they did not complete any post-baseline questionnaires. The primary endpoint was proportion of patients with ≥15 point absolute increase in xerostomia score from baseline to 1 month; 108 patients were needed for an 80% power to detect a 22% absolute difference (1-sided significance of 0.05). The secondary endpoint was the rate of marginal failures.

RESULTS

The study closed early after 56 patients were randomized. Median survival was 4.6 months. 46 patients (23 in each arm) were eligible for analysis. Mean parotid dose was 17 vs. 10 Gy in the standard vs. parotid-sparing arms, respectively. The table below shows mean xerostomia score and proportion of patients with ≥15 increase in xerostomia score at each time point. There was no difference in the proportion of patients experiencing ≥15 increase in xerostomia score at 1 month, though there was a trend toward lower xerostomia score at 1 month in patients randomized to parotid-sparing fields (p = 0.07, Table). Xerostomia rates were also significantly improved in the parotid-sparing arm at EndRT (p = 0.03), but no longer-term difference was observed with greater attrition at 3 and 6 months. On linear regression, there was a trend toward association between mean parotid dose and xerostomia score at 1 month (p = 0.06). There were no reported marginal failures in either arm.

CONCLUSION

Parotid-sparing without coverage of the C1 vertebra appears safe and may meaningfully reduce acute xerostomia in patients with limited life expectancy who are candidates for conventional WBRT, although the study was underpowered to detect a significant difference at 1 month.

A Phase 0 Study Assessing the Intracranial Activity of a Metabolic Radiosensitizer in Patients with Glioblastoma

PURPOSE/OBJECTIVE(S)

Efforts to overcome treatment resistance in glioblastoma (GBM) have been unsuccessful due to tumor heterogeneity and poor intracranial drug penetration. Targeting altered metabolism is a promising approach to improve GBM therapy despite this heterogeneity. Mycophenolate mofetil (MMF) is an inhibitor of purine synthesis that sensitizes GBM to radiation and temozolomide (TMZ) in vitro and in vivo, but its ability to cross the blood brain barrier and inhibit GBM metabolism in patients is unknown. NCT04477200 is a phase 0/1 dose escalation study of MMF combined with radiation and temozolomide in GBM. Here we report the phase 0 results of this study assessing the intracranial activity of MMF.

MATERIALS/METHODS

Purine (GTP and IMP) and mycophenolic acid (MPA, the active metabolite of MMF) concentrations were determined using mass spectrometry in flash-frozen tumor (enhancing and non-enhancing) and normal cortex obtained from 8 patients with recurrent GBM who received MMF (500, 1000, 1500 and 2000 mg BID, N = 2 patients each dose level) for 1 week prior to re-resection and 5 control patients who did not receive MMF prior to re-resection. Plasma MPA concentration was similarly quantified to calculate the enhancing tumor, non-enhancing tumor and normal cortex to plasma MPA ratios.

RESULTS

Patients who received MMF had a mean MPA concentration of 2.2 ± 0.7 µM in the enhancing tumor samples, 1.2 ± 0.5 µM in the non-enhancing tumor samples and 1.3 ± 0.5 µM in normal cortex. MPA concentration was negligible in control patients. This corresponded to tissue/plasma MPA ratios of 0.31, 0.17 and 0.10 for enhancing tumor, non-enhancing tumor and normal cortex, respectively. The GTP/IMP ratio was decreased by 75% in enhancing tumor in MMF-treated patients compared to untreated controls (p = 0.009), indicating effective target engagement and inhibition of purine synthesis. The GTP/IMP ratio was also decreased in cortex and non-enhancing tumor, though a paucity of control samples prevented statistical analysis.

CONCLUSION

Twice daily MMF treatment yields intracranial drug concentrations above 1 µM and lowers the GTP/IMP ratio in GBMs, consistent with target engagement. As we have previously observed radiosensitization in vitro with MPA concentrations of 1 µM, these data suggest that MMF may achieve adequate CNS penetration for therapeutic benefit. The Phase 1 component of this study to determine the dose limiting toxicity and maximally tolerated dose of MMF when combined with reirradiation in recurrent GBM and radiation and TMZ in newly diagnosed GBM is ongoing.

Identification of Excellent Prognosis IDH Wildtype Glioblastomas Using Genomic and Metabolic Profiling

PURPOSE/OBJECTIVE(S)

High grade gliomas (HGGs) are aggressive brain tumors with altered cellular metabolism. HGGs can carry mutations in the tricarboxylic acid (TCA) cycle enzyme isocitrate dehydrogenase 1 (IDH1), conferring distinct biology and improved patient prognosis compared to IDH wildtype (wt) tumors. Using metabolomic analyses of tumor tissue, we previously showed that IDH wt and IDH mutant (IDH mut) tumors have unique metabolomic signatures that correlate with different survival outcomes. Among this cohort of 69 HGG samples, we identified two unique patient tumors that metabolically clustered with IDH mut tumors, but lacked both the IDH mutation and its product 2-hydroxyglutarate. We aimed to discover unique mutations in these two tumors that may impart an IDH mutant-like phenotype in the absence of an IDH1 or IDH2 mutation.

MATERIALS/METHODS

Whole exome sequencing (WES) was performed on frozen tumor samples from two patients diagnosed as glioblastoma (GBM), IDH wt via Agilent v5 + IncRNA platform. Alignment to the hg38 genome and variant calling were completed using an accelerated implementation of GATK's BWA and MuTect2 algorithms from Sentieon. Variants were filtered based on supporting reads and variant allele thresholds, with synonymous variants and common SNPs removed. High-confidence variants were further filtered by membership in the four KEGG pathways associated with IDH1 and IDH2. Identified variants were corroborated with metabolomics data from the two unique IDH wt tumors compared with classical GBM IDH wt, oligodendrogliomas IDH mut and astrocytomas IDH mut to identify putative drivers of an IDH mutant-like metabolomic phenotype in these unique IDH wt tumors.

RESULTS

Despite the lack of an IDH mutation, one patient survived 45.6 months and the other patient remains alive at last follow up 64 months post diagnosis, much longer than the 16-18-month median survival typical of patients with GBM IDH wt. WES of outlier IDH wt tumor samples revealed 65 unique mutations in the queried KEGG pathways, of which 34 had a variant allele frequency > = 0.15. These variants were processed in Gprofiler, confirming expected enrichment of the carboxylic acid metabolic biologic process, a functional gene set consisting of TCA genes, among these variants (p = 0.002, 3.6-fold enrichment). Accordingly, metabolite levels of intermediates of the TCA cycle, including malate and isocitrate were decreased in the outlier tumor samples compared to classic GBMs IDH wt (p<0.001). Presence of genetic alterations in key variants of the carboxylic acid metabolic biologic process (including ME1, GYP4F3, PTGIS, PFKL, PSPH, AKR1A1, HK2, NOS1) correlated with improved overall survival among GBM patients in the TCGA (p = 0.04). Laboratory validation of these findings in preclinical GBM models is ongoing.

CONCLUSION

Disruption of the TCA cycle independent of an IDH mutation is associated with favorable survival in GBM. Pharmacologic inhibition of these pathways may be a promising strategy to improve GBM outcomes.

Adaptive rewiring of purine metabolism promotes treatment resistance in H3K27M-mutant diffuse midline glioma

Background

Diffuse midline gliomas (DMG), including diffuse intrinsic pontine gliomas (DIPGs), are a fatal form of brain cancer. These tumors often carry a driver mutation on histone H3 converting lysine 27 to methionine (H3K27M). DMG-H3K27M are characterized by altered metabolism and resistance to standard of care radiation (RT), but how the H3K27M mediates the metabolic response to radiation and consequent treatment resistance is uncertain.

Methods

We performed metabolomics on irradiated and untreated H3K27M isogenic DMG cell lines and observed an H3K27M-specific enrichment for purine synthesis pathways. We profiled the expression of purine synthesis enzymes in publicly available patient data and in our models, quantified purine synthetic flux using stable isotope tracing, and characterized the in vitro and in vivo response to de novo and salvage purine synthesis inhibition in combination with RT.

Results

DMG-H3K27M cells activate purine metabolism in an H3K27M-specific fashion. In the absence of genotoxic treatment, H3K27M-expressing cells have higher relative activity of de novosynthesis and lower activity of purine salvage due to decreased expression of the purine salvage enzymes. Inhibition of de novo synthesis radiosensitized DMG-H3K27M cells in vitro and in vivo. Irradiated H3K27M cells adaptively upregulate purine salvage enzyme expression and pathway activity. Silencing the rate limiting enzyme in purine salvage, hypoxanthine guanine phosphoribosyl transferase (HGPRT) when combined with radiation markedly suppressed DMG-H3K27M tumor growth in vivo.

Conclusions

H3K27M expressing cells rely on de novo purine synthesis but adaptively upregulate purine salvage in response to RT. Inhibiting purine salvage may help overcome treatment resistance in DMG-H3K27M tumors.

Combined cytotoxic and immune-stimulatory gene therapy for primary adult high-grade glioma: a phase 1, first-in-human trial

BACKGROUND

High-grade gliomas have a poor prognosis and do not respond well to treatment. Effective cancer immune responses depend on functional immune cells, which are typically absent from the brain. This study aimed to evaluate the safety and activity of two adenoviral vectors expressing HSV1-TK (Ad-hCMV-TK) and Flt3L (Ad-hCMV-Flt3L) in patients with high-grade glioma.

METHODS

In this dose-finding, first-in-human trial, treatment-naive adults aged 18-75 years with newly identified high-grade glioma that was evaluated per immunotherapy response assessment in neuro-oncology criteria, and a Karnofsky Performance Status score of 70 or more, underwent maximal safe resection followed by injections of adenoviral vectors expressing HSV1-TK and Flt3L into the tumour bed. The study was conducted at the University of Michigan Medical School, Michigan Medicine (Ann Arbor, MI, USA). The study included six escalating doses of viral particles with starting doses of 1×1010 Ad-hCMV-TK viral particles and 1×109 Ad-hCMV-Flt3L viral particles (cohort A), and then 1×1011 Ad-hCMV-TK viral particles and 1×109 Ad-hCMV-Flt3L viral particles (cohort B), 1×1010 Ad-hCMV-TK viral particles and 1×1010 Ad-hCMV-Flt3L viral particles (cohort C), 1×1011 Ad-hCMV-TK viral particles and 1×1010 Ad-hCMV-Flt3L viral particles (cohort D), 1×1010 Ad-hCMV-TK viral particles and 1×1011 Ad-hCMV-Flt3L viral particles (cohort E), and 1×1011 Ad-hCMV-TK viral particles and 1×1011 Ad-hCMV-Flt3L viral particles (cohort F) following a 3+3 design. Two 1 mL tuberculin syringes were used to deliver freehand a mix of Ad-hCMV-TK and Ad-hCMV-Flt3L vectors into the walls of the resection cavity with a total injection of 2 mL distributed as 0·1 mL per site across 20 locations. Subsequently, patients received two 14-day courses of valacyclovir (2 g orally, three times per day) at 1-3 days and 10-12 weeks after vector administration and standad upfront chemoradiotherapy. The primary endpoint was the maximum tolerated dose of Ad-hCMV-Flt3L and Ad-hCMV-TK. Overall survival was a secondary endpoint. Recruitment is complete and the trial is finished. The trial is registered with ClinicalTrials.gov, NCT01811992.

FINDINGS

Between April 8, 2014, and March 13, 2019, 21 patients were assessed for eligibility and 18 patients with high-grade glioma were enrolled and included in the analysis (three patients in each of the six dose cohorts); eight patients were female and ten were male. Neuropathological examination identified 14 (78%) patients with glioblastoma, three (17%) with gliosarcoma, and one (6%) with anaplastic ependymoma. The treatment was well-tolerated, and no dose-limiting toxicity was observed. The maximum tolerated dose was not reached. The most common serious grade 3-4 adverse events across all treatment groups were wound infection (four events in two patients) and thromboembolic events (five events in four patients). One death due to an adverse event (respiratory failure) occurred but was not related to study treatment. No treatment-related deaths occurred during the study. Median overall survival was 21·3 months (95% CI 11·1-26·1).

INTERPRETATION

The combination of two adenoviral vectors demonstrated safety and feasibility in patients with high-grade glioma and warrants further investigation in a phase 1b/2 clinical trial.

FUNDING

Funded in part by Phase One Foundation, Los Angeles, CA, The Board of Governors at Cedars-Sinai Medical Center, Los Angeles, CA, and The Rogel Cancer Center at The University of Michigan.

Pancreatic tumors exhibit myeloid-driven amino acid stress and upregulate arginine biosynthesis

Nutrient stress in the tumor microenvironment requires cancer cells to adopt adaptive metabolic programs for survival and proliferation. Therefore, knowledge of microenvironmental nutrient levels and how cancer cells cope with such nutrition is critical to understand the metabolism underpinning cancer cell biology. Previously, we performed quantitative metabolomics of the interstitial fluid (the local perfusate) of murine pancreatic ductal adenocarcinoma (PDAC) tumors to comprehensively characterize nutrient availability in the microenvironment of these tumors. Here, we develop Tumor Interstitial Fluid Medium (TIFM), a cell culture medium that contains nutrient levels representative of the PDAC microenvironment, enabling us to study PDAC metabolism ex vivo under physiological nutrient conditions. We show that PDAC cells cultured in TIFM adopt a cellular state closer to that of PDAC cells present in tumors compared to standard culture models. Further, using the TIFM model, we found arginine biosynthesis is active in PDAC and allows PDAC cells to maintain levels of this amino acid despite microenvironmental arginine depletion. We also show that myeloid derived arginase activity is largely responsible for the low levels of arginine in PDAC tumors. Altogether, these data indicate that nutrient availability in tumors is an important determinant of cancer cell metabolism and behavior, and cell culture models that incorporate physiological nutrient availability have improved fidelity to in vivo systems and enable the discovery of novel cancer metabolic phenotypes.

GAP43-dependent mitochondria transfer from astrocytes enhances glioblastoma tumorigenicity

The transfer of intact mitochondria between heterogeneous cell types has been confirmed in various settings, including cancer. However, the functional implications of mitochondria transfer on tumor biology are poorly understood. Here we show that mitochondria transfer is a prevalent phenomenon in glioblastoma (GBM), the most frequent and malignant primary brain tumor. We identified horizontal mitochondria transfer from astrocytes as a mechanism that enhances tumorigenesis in GBM. This transfer is dependent on network-forming intercellular connections between GBM cells and astrocytes, which are facilitated by growth-associated protein 43 (GAP43), a protein involved in neuron axon regeneration and astrocyte reactivity. The acquisition of astrocyte mitochondria drives an increase in mitochondrial respiration and upregulation of metabolic pathways linked to proliferation and tumorigenicity. Functionally, uptake of astrocyte mitochondria promotes cell cycle progression to proliferative G2/M phases and enhances self-renewal and tumorigenicity of GBM. Collectively, our findings reveal a host-tumor interaction that drives proliferation and self-renewal of cancer cells, providing opportunities for therapeutic development.

GTP signaling links metabolism, DNA repair, and responses to genotoxic stress

How cell metabolism regulates DNA repair is incompletely understood. Here, we define a GTP-mediated signaling cascade that links metabolism to DNA repair and has significant therapeutic implications. GTP, but not other nucleotides, regulates the activity of Rac1, a G protein, that promotes the dephosphorylation of serine 323 on Abl-interactor 1 (Abi-1) by protein phosphatase 5 (PP5). Dephosphorylated Abi-1, a protein previously not known to activate DNA repair, promotes non-homologous end joining. In patients and mouse models of glioblastoma, Rac1 and dephosphorylated Abi-1 mediate DNA repair and resistance to standard of care genotoxic treatments. The GTP-Rac1-PP5-Abi-1 signaling axis is not limited to brain cancer, as GTP supplementation promotes DNA repair and Abi-1-S323 dephosphorylation in non-malignant cells and protects mouse tissues from genotoxic insult. This unexpected ability of GTP to regulate DNA repair independently of deoxynucleotide pools has important implications for normal physiology and cancer treatment.

Abstract 2820: Defining the role of purine metabolism in radiation resistance in H3K27M–mutant diffuse midline glioma

Diffuse midline gliomas (DMG) are an aggressive and treatment resistant form of brain cancer in urgent need of new and effective treatment strategies. Radiation therapy (RT) is the standard of care for DMGs, but these tumors often recur locally within the high-dose radiation field. Therefore, it is imperative to discover methods of overcoming RT resistance. DMGs are often characterized by the presence of driving mutation in the tail domain of histone H3 that converts the 27th residue from a lysine to a methionine (termed H3K27M). Using liquid chromatography/mass spectrometry “snapshot” metabolomics, we determined that human H3K27M-expressing cells (referred to as K27M cells going forward) possess altered metabolic profiles distinct from their H3-wildtype (H3WT) counterparts characterized by enrichment in metabolites associated with purine metabolism. RT-treated K27M isogenic-paired cells displayed K27M-specific increases in the abundance of both hypoxanthine and guanine and decreased abundance of guanosine, which can be degraded into guanine from mature guanylate species (GMP/GDP/GTP). Hypoxanthine and guanine are the substrates for HGPRT (encoded by HPRT1), the rate-limiting enzyme in purine salvage. Using publicly available data, pediatric high-grade gliomas (pHGG) expressing K27M were found to have decreased HPRT1 transcript compared to H3WT tumors, and we found decreased HGPRT protein expression in K27M cell lines compared to their isogenic H3WT counterparts. The decreased abundance of guanosine, increased abundance of both purine salvage metabolites following RT, and decreased expression of HGPRT suggest impaired guanylate purine salvage in K27M cells that is exacerbated by RT. We hypothesized that K27M cells are deficient in purine salvage and may rely on de novo purine synthesis (DNPS) for purine production following RT. Consistent with this hypothesis, we found that a clinically available, blood-brain penetrant inhibitor of DNPS increased RT-mediated killing of K27M neurospheres in vitro. Combination RT+DNPS inhibition (DNPSi) increased survival (38d post-implantation) over RT alone (31.5d post-implantation) in Rag1-KO mice bearing orthotopically implanted K27M tumors, but did not cure tumors. To understand potential mechanisms of treatment resistance, we interrogated metabolic pathway utilization with stable isotope tracing. Using 2,8-deuterium-hypoxanthine to interrogate purine salvage and 15N-glutamine to interrogate de novo purine synthesis, we found an RT-mediated increase in purine salvage that could mediate resistance to DNPSi inhibition. Future experiments will determine whether inhibition of purine salvage has efficacy for those tumors that recur or progress following treatment with RT and DNPSi inhibition.

Citation Format: Erik R. Peterson, Peter Sajjakulnukit, Andrew Scott, Caleb Heaslip, Costas Lyssiotis, Maria G. Castro, Daniel R. Wahl. Defining the role of purine metabolism in radiation resistance in H3K27M–mutant diffuse midline glioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2820.

Abstract 331: Mitochondrial transfer from astrocytes drives glioblastoma tumorigenicity

Mitochondrial transfer in the central nervous system occurs from astrocytes to neurons in stroke. Mitochondrial exchange has also been reported among tumor cells in glioblastoma (GBM), the most common primary brain tumor. However, the role of mitochondrial transfer from non-neoplastic cells in the surrounding microenvironment to GBM remains poorly understood. We hypothesized that mitochondrial transfer from these non-neoplastic to GBM cells supports tumor metabolism and growth. Using transgenic mice expressing fluorophore-tagged mitochondria, we found that ~50% of orthotopically-implanted mouse GBM cells acquire host mitochondria. Brain-resident cells, mainly astrocytes, but not infiltrating immune cells were the primary mitochondrial donors in vivo and in vitro. Mitochondrial transfer also occurred from immortalized human astrocytes to a broad array of patient-derived xenograft (PDX) models of GBM in vitro at rates of 15-35%. GBM cells that acquired mitochondria expressed higher levels of the ATP-synthase subunit ATP5A and produced more ATP, while metabolomics revealed upregulated amino acid metabolism in recipient cells. In vivo, mouse GBM cells that acquired mitochondria were more likely to be in G2/M proliferative cell cycle phases. We observed a similar effect in PDX that acquired astrocyte mitochondria from co-cultures in vitro. To mechanistically link increased proliferation specifically to mitochondrial transfer, we isolated astrocyte mitochondria by differential centrifugation and found that addition and uptake of cell-free mitochondria in human GBM cells recapitulated the increased proliferation. Using sorted mouse and human GBM cells with/without astrocyte mitochondrial acquisition, we further found that mitochondrial transfer promoted in vitro self-renewal and in vivo tumorigenicity, leading to significant reduction in survival and increased penetrance in orthotopic GBM models. Transfer in mouse and human systems was contact-dependent and was abrogated by physical separation of donor and recipient cells by transwell inserts. We visualized contact-dependent transfer across actin-based intercellular connections consistent with previously reported microtubes. We confirmed the critical role of actin and the actin-associated protein, growth-associated protein 43 (GAP43) in facilitating mitochondrial transfer by showing that pharmacologic inhibition and genetic knockdown (respectively) significantly decreased the rate of mitochondrial transfer. Taken together, mitochondrial transfer comprises a fundamental, protumorigenic mechanism of GBM, enhancing metabolic activity and driving tumor cell proliferation. Further elucidating the molecular machinery regulating astrocyte mitochondrial transfer and its downstream protumorigenic effects will lead to therapeutic opportunities targeting this understudied tumor microenvironment interaction.

Citation Format: Dionysios C. Watson, Defne Bayik, Simon Storevik, Shannon S. Moreino, Samuel S. Sprowls, Gauravi Deshpande, Palavalasa Sravya, Costas A. Lyssiotis, Daniel R. Wahl, Hrvoje Miletic, Justin D. Lathia. Mitochondrial transfer from astrocytes drives glioblastoma tumorigenicity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 331.

Abstract 1095: Exploiting altered methionine metabolism to overcome treatment resistance in glioblastoma

Glioblastoma (GBM) is the most aggressive adult brain tumor and is uniformly fatal due to resistance to standard therapies such as radiation (RT) and chemotherapy. Our group and others have identified altered metabolism as a key mediator of GBM RT resistance. Methionine is an essential sulfur-containing amino acid that cells use to synthesize antioxidants, polyamines and S-adenosyl methionine (SAM), which drives intracellular methylation reactions. Methionine uptake is dramatically elevated in GBM compared to normal brain, but what GBMs use this methionine for, and whether it governs GBM treatment resistance, is unknown. Here, we find that RT acutely increases the levels of numerous methionine-related metabolites in multiple RT-resistant GBM models. To interrogate metabolic pathway activity, we used 13C5 methionine stable isotope tracing to show that GBMs respond to RT by activating the conversion of methionine to SAM, which is dependent on signaling through the DNA damage response. We developed in vivo methionine stable isotope tracing techniques to confirm these findings in orthotopic PDX models of GBM. Blocking the conversion of methionine to SAM, through pharmacologic inhibition of methionine adenosyltransferase 2A (MAT2A), slowed the repair of RT-induced DNA damage and increased cell death in GBM models following RT. These effects were especially pronounced in GBM models lacking the methionine salvage enzyme methylthioadenosine phosphorylase (MTAP). Pharmacologic inhibition of MAT2A in flank and orthotopic in vivo GBM models depleted SAM levels and slowed tumor growth when combined with RT. Combining MAT2A inhibition with dietary methionine restriction and RT slowed GBM tumor growth even further. Together, our work demonstrates a new signaling link between DNA damage and methionine-driven SAM synthesis in GBM. Inhibiting SAM synthesis slows the repair of RT-induced DNA damage and augments RT efficacy. This therapeutic strategy may be especially effective in GBMs defective in methionine salvage and spare normal cortex in which methionine salvage is active.

Citation Format: Navyateja Korimerla, Kari-Wilder Romans, Peter Kalev, Ayesha Kothari, Nathan Qi, Charles Evans, Maureen Kachman, Marc L Hyer, Katya Marjon, Taryn Sleger, Daniel R Wahl. Exploiting altered methionine metabolism to overcome treatment resistance in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1095.

Abstract 3677: DNA damage signaling activates de novo GTP synthesis to promote chemoradiation resistance in glioblastoma

Glioblastoma (GBM) is uniformly fatal due to inherent radiation (RT) and chemotherapy resistance. We have found this therapeutic resistance is mediated by alterations in tumor cellular metabolic activity. Our group and others have found that metabolites can regulate DNA repair and RT resistance in brain tumors, but little is known about how DNA damage regulates metabolic pathway activity in cancer. Here, we show that DNA damage acutely increases guanine-containing purine metabolites in multiple in vitro and intracranial GBM models. By interrogating metabolic fluxes in vitro using a variety of stable isotope tracers, we confirmed RT-induced elevation in guanylates was due to increased de novo purine synthesis (DNPS) rather than activation of purine salvage. By developing and using novel stable isotope tracing methods to directly measure DNPS in awake, unrestrained mice, we confirmed that orthotopic GBMs have higher DNPS rates than adjacent cortical tissue that further increase after treatment with RT. Neither salvage synthesis of purines nor pyrimidine synthesis were impacted by RT in any intracranial tissues. With these findings, we opened a clinical study to directly measure purine synthesis in patients, and we found that human GBMs have similarly high purine synthesis rates compared to normal brain tissue. Because DNA damage activated DNPS without affecting purine salvage or pyrimidine synthesis, we reasoned that active signaling may be involved. Indeed, therapy-induced DNPS increases are lost in vitro and in vivo upon pharmacological or genomic inhibition of the DNA-damage sensing kinase DNA-PK. Moreover, RT and DNA-PK have direct influence over the spatial organization of DNPS enzymes, including IMPDH, the rate-limiting step in guanylate synthesis. Because purines can promote DNA repair, these findings suggest that DNA-PK signaling helps promote DNA repair in part by causing the spatial reorganization of DNPS enzymes, thereby activating purine synthesis. To determine if disrupting this regulation can augment GBM treatment efficacy, we combined an FDA-approved inhibitor of purine synthesis with chemoradiation in a variety of mouse models of GBM. Critically, targeting GTP synthesis improved the efficacy of both RT alone and chemoradiation in multiple patient-derived and syngeneic intracranial models, suggesting a potential therapeutic targeting opportunity in patients. In this study, we have developed novel methodology to directly measure purine synthesis in brain tumors in mice and humans. With these tools, we discovered that after DNA damage, DNA-PK mediates a novel pathway controlling the spatial reorganization of purine synthesis enzymes and subsequent DNPS increases. The resulting elevation of GTP levels promotes therapy resistance in tumors, and we are now directly measuring and inhibiting this molecular activity in patients with GBM in an effort to improve standard therapy.

Citation Format: Andrew J. Scott, Alexandra M. O'Brien, Weihua Zhou, Vidhi Pareek, Zhou Sha, Sravya Palavalasa, Ayesha U. Kothari, Kari Wilder-Romans, Li Zhang, Anthony C. Andren, Sriram Chandrasekaran, Jason Heth, Yoshie Umemura, Nathan Qi, John Woulfe, Sriram Venneti, Meredith A. Morgan, Theodore S. Lawrence, Wajd N. Al-Holou, Costas A. Lyssiotis, Daniel R. Wahl. DNA damage signaling activates de novo GTP synthesis to promote chemoradiation resistance in glioblastoma. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3677.

Validation of the Combination Gleason Score as an Independent Favorable Prognostic Factor in Prostate Cancer Treated With Dose-Escalated Radiation Therapy

PURPOSE

Prognostic factors for prostate cancer include tumor, node, metastases stage, pretreatment prostate-specific antigen, and pathology (via Gleason score [GS] or grade group). Of these, GS yields the largest effect on prostate cancer specific mortality. It was previously determined that those with cores with a mix of higher and lower GS at biopsy (which was termed a "ComboGS") had decreased risk for prostate cancer specific mortality after either surgical or radiation treatment. We validate the effect of ComboGS in an independent cohort of patients with prostate cancer treated with definitive dose-escalated radiation therapy (DE-RT) at 2 institutions.

METHODS AND MATERIALS

DE-RT was administered to 2539 men, of which 687 men had a ComboGS. To further ascertain the ComboGS effect we employed the modified Cancer of the Prostate Risk Assessment (mCAPRA) score. Rates of biochemical event-free survival and distant metastasis-free survival were compared across CAPRA scores, with and without modification, and the prognostic value of the CAPRA scores was compared using Harrel's concordance index.

RESULTS

On univariate analysis in Gleason 7 to 10 patients the presence of ComboGS improved 10-year biochemical event-free survival from 76.6% to 82.4% (hazard ratio [HR], 0.75; confidence interval [CI], 0.59-0.96; P = .021), 10-year distant metastasis-free survival from 89.3% to 93.2% (HR, 0.57; CI, 0.39-0.85; P = .005), 10-year prostate cancer specific survival from 93.9% to 97.4% (HR, 0.39; CI, 0.21-0.7; P = .001), and 10-year overall survival from 65.7% to 75.6% (HR, 0.69; CI, 0.57-0.83; P < .001). Multivariable analysis also supported that ComboGS is protective for biochemical failure (HR, 0.64; CI, 0.50-0.83; P < .001), distant metastasis (HR, 0.42; CI, 0.28-0.63; P < .001), death from prostate cancer (HR, 0.32; CI, 0.17-0.58; P < .001), and overall mortality (HR, 0.65; CI, 0.54-0.79; P < .001). Additionally, adjusting the mCAPRA score for ComboGS decreased the risk of biochemical failure by nearly 30% (HR, 0.70; 95% CI, 0.55-0.88; P = .003) and by 50% (HR, 0.54; 95% CI, 0.37-0.80; P = .002) for distant metastasis.

CONCLUSIONS

ComboGS is a useful and readily available independent prognostic factor for all clinical endpoints evaluated. Moreover, the ComboGS can be used in conjunction with the extensively validated CAPRA scoring to better risk stratify patients being treated with definitive DE-RT for GS 7 to 10 disease.

Developing H3K27M mutant selective radiosensitization strategies in diffuse intrinsic pontine glioma

Diffuse intrinsic pontine glioma (DIPG) is a rare but highly lethal pediatric and adolescent tumor located in the pons of the brainstem. DIPGs harbor unique and specific pathological and molecular alterations, such as the hallmark lysine 27-to-methionine (H3K27M) mutation in histone H3, which lead to global changes in the epigenetic landscape and drive tumorigenesis. While fractionated radiotherapy, the current standard of care, improves symptoms and delays tumor progression, DIPGs inevitably recur, and despite extensive efforts chemotherapy-driven radiosensitization strategies have failed to improve survival. Advances in our understanding of the role of epigenetics in the cellular response to radiation-induced DNA damage, however, offer new opportunities to develop combinational therapeutic strategies selective for DIPGs expressing H3K27M. In this review, we provide an overview of preclinical studies that explore potential radiosensitization strategies targeting the unique epigenetic landscape of H3K27M mutant DIPG. We further discuss opportunities to selectively radiosensitize DIPG through strategic inhibition of the radiation-induced DNA damage response. Finally, we discuss the potential for using radiation to induce anti-tumor immune responses that may be potentiated in DIPG by radiosensitizing-therapeutic strategies.

Subclonal evolution and expansion of spatially distinct THY1-positive cells is associated with recurrence in glioblastoma

PURPOSE

Glioblastoma(GBM) is a lethal disease characterized by inevitable recurrence. Here we investigate the molecular pathways mediating resistance, with the goal of identifying novel therapeutic opportunities.

EXPERIMENTAL DESIGN

We developed a longitudinal in vivo recurrence model utilizing patient-derived explants to produce paired specimens(pre- and post-recurrence) following temozolomide(TMZ) and radiation(IR). These specimens were evaluated for treatment response and to identify gene expression pathways driving treatment resistance. Findings were clinically validated using spatial transcriptomics of human GBMs.

RESULTS

These studies reveal in replicate cohorts, a gene expression profile characterized by upregulation of mesenchymal and stem-like genes at recurrence. Analyses of clinical databases revealed significant association of this transcriptional profile with worse overall survival and upregulation at recurrence. Notably, gene expression analyses identified upregulation of TGFβ signaling, and more than one-hundred-fold increase in THY1 levels at recurrence. Furthermore, THY1-positive cells represented <10% of cells in treatment-naïve tumors, compared to 75-96% in recurrent tumors. We then isolated THY1-positive cells from treatment-naïve patient samples and determined that they were inherently resistant to chemoradiation in orthotopic models. Additionally, using image-guided biopsies from treatment-naïve human GBM, we conducted spatial transcriptomic analyses. This revealed rare THY1+ regions characterized by mesenchymal/stem-like gene expression, analogous to our recurrent mouse model, which co-localized with macrophages within the perivascular niche. We then inhibited TGFBRI activity in vivo which decreased mesenchymal/stem-like protein levels, including THY1, and restored sensitivity to TMZ/IR in recurrent tumors.

CONCLUSIONS

These findings reveal that GBM recurrence may result from tumor repopulation by pre-existing, therapy-resistant, THY1-positive, mesenchymal cells within the perivascular niche.

Microenvironmental ammonia enhances T cell exhaustion in colorectal cancer

Effective therapies are lacking for patients with advanced colorectal cancer (CRC). The CRC tumor microenvironment has elevated metabolic waste products due to altered metabolism and proximity to the microbiota. The role of metabolite waste in tumor development, progression, and treatment resistance is unclear. We generated an autochthonous metastatic mouse model of CRC and used unbiased multi-omic analyses to reveal a robust accumulation of tumoral ammonia. The high ammonia levels induce T cell metabolic reprogramming, increase exhaustion, and decrease proliferation. CRC patients have increased serum ammonia, and the ammonia-related gene signature correlates with altered T cell response, adverse patient outcomes, and lack of response to immune checkpoint blockade. We demonstrate that enhancing ammonia clearance reactivates T cells, decreases tumor growth, and extends survival. Moreover, decreasing tumor-associated ammonia enhances anti-PD-L1 efficacy. These findings indicate that enhancing ammonia detoxification can reactivate T cells, highlighting a new approach to enhance the efficacy of immunotherapies.

Radiation Therapy for IDH-Mutant Grade 2 and Grade 3 Diffuse Glioma: An ASTRO Clinical Practice Guideline

PURPOSE

This guideline provides evidence-based recommendations for adults with isocitrate dehydrogenase (IDH)-mutant grade 2 and grade 3 diffuse glioma, as classified in the 2021 World Health Organization (WHO) Classification of Tumours. It includes indications for radiation therapy (RT), advanced RT techniques, and clinical management of adverse effects.

METHODS

The American Society for Radiation Oncology convened a multidisciplinary task force to address 4 key questions focused on the RT management of patients with IDH-mutant grade 2 and grade 3 diffuse glioma. Recommendations were based on a systematic literature review and created using a predefined consensus-building methodology and system for grading evidence quality and recommendation strength.

RESULTS

A strong recommendation for close surveillance alone was made for patients with oligodendroglioma, IDH-mutant, 1p/19q codeleted, WHO grade 2 after gross total resection without high-risk features. For oligodendroglioma, WHO grade 2 with any high-risk features, adjuvant RT was conditionally recommended. However, adjuvant RT was strongly recommended for oligodendroglioma, WHO grade 3. A conditional recommendation for close surveillance alone was made for astrocytoma, IDH-mutant, WHO grade 2 after gross total resection without high-risk features. Adjuvant RT was conditionally recommended for astrocytoma, WHO grade 2, with any high-risk features and strongly recommended for astrocytoma, WHO grade 3. Dose recommendations varied based on histology and grade. Given known adverse long-term effects of RT, consideration for advanced techniques such as intensity modulated radiation therapy/volumetric modulated arc therapy or proton therapy were given as strong and conditional recommendations, respectively. Finally, based on expert opinion, the guideline recommends assessment, surveillance, and management for toxicity management.

CONCLUSIONS

Based on published data, the American Society for Radiation Oncology task force has proposed recommendations to inform the management of adults with IDH-mutant grade 2 and grade 3 diffuse glioma as defined by WHO 2021 classification, based on the highest quality published data, and best translated by our task force of subject matter experts.

Efficacy and toxicity with radiation field designs and concurrent temozolomide for CNS lymphoma

Background

There is no consensus on the treatment of central nervous system (CNS) lymphoma refractory to first-line methotrexate-based chemotherapy. Whole brain radiotherapy (WBRT) is sometimes used but may result in unacceptable neurocognitive dysfunction. We examined the efficacy and toxicities of WBRT with or without concurrent temozolomide in CNS lymphoma treatment.

Methods

This single-institution IRB-approved retrospective study included adults with CNS lymphoma who received WBRT, either consolidative low-dose WBRT alone or low-dose WBRT with a focal boost to residual disease and were previously treated with high-dose methotrexate. The relationships between the WBRT regimen, concurrent temozolomide, and clinical outcomes and toxicities were assessed using proportional hazards and logistic regression models.

Results

A total of 45 patients with a median age of 64 years (range 24–74) treated from 2004 to 2019 were included. In total, 20 patients received concurrent temozolomide. In the WBRT + Boost cohort (n = 32), concurrent temozolomide resulted in better 2-year overall survival (OS) and progression free survival (PFS) (73% OS and 66% PFS) compared to patients treated without concurrent temozolomide (44% OS and 24% PFS). On multivariate analysis, concurrent temozolomide was associated with significantly better PFS (HR 0.28, P = .02). There were no significant differences between the two radiation groups or between those treated with or without concurrent temozolomide, with respect to significant acute hematologic, non-hematologic, and long-term neurocognitive toxicities (P &gt; .05).

Conclusions

In this study, concurrent temozolomide with radiotherapy in CNS lymphoma was associated with better PFS and was well tolerated. Low-dose WBRT with a boost is a safe and reasonable treatment approach for focal refractory disease. Prospective research that includes rigorous neurocognitive assessments is now warranted.

Epigenetically defined therapeutic targeting in H3.3G34R/V high-grade gliomas

High-grade gliomas with arginine or valine substitutions of the histone H3.3 glycine-34 residue (H3.3G34R/V) carry a dismal prognosis, and current treatments, including radiotherapy and chemotherapy, are not curative. Because H3.3G34R/V mutations reprogram epigenetic modifications, we undertook a comprehensive epigenetic approach using ChIP sequencing and ChromHMM computational analysis to define therapeutic dependencies in H3.3G34R/V gliomas. Our analyses revealed a convergence of epigenetic alterations, including (i) activating epigenetic modifications on histone H3 lysine (K) residues such as H3K36 trimethylation (H3K36me3), H3K27 acetylation (H3K27ac), and H3K4 trimethylation (H3K4me3); (ii) DNA promoter hypomethylation; and (iii) redistribution of repressive histone H3K27 trimethylation (H3K27me3) to intergenic regions at the leukemia inhibitory factor (LIF) locus to drive increased LIF abundance and secretion by H3.3G34R/V cells. LIF activated signal transducer and activator of transcription 3 (STAT3) signaling in an autocrine/paracrine manner to promote survival of H3.3G34R/V glioma cells. Moreover, immunohistochemistry and single-cell RNA sequencing from H3.3G34R/V patient tumors revealed high STAT3 protein and RNA expression, respectively, in tumor cells with both inter- and intratumor heterogeneity. We targeted STAT3 using a blood-brain barrier–penetrable small-molecule inhibitor, WP1066, currently in clinical trials for adult gliomas. WP1066 treatment resulted in H3.3G34R/V tumor cell toxicity in vitro and tumor suppression in preclinical mouse models established with KNS42 cells, SJ-HGGx42-c cells, or in utero electroporation techniques. Our studies identify the LIF/STAT3 pathway as a key epigenetically driven and druggable vulnerability in H3.3G34R/V gliomas. This finding could inform development of targeted, combination therapies for these lethal brain tumors.

Predicting cancer drug TARGETS - TreAtment Response Generalized Elastic-neT Signatures

We are now in an era of molecular medicine, where specific DNA alterations can be used to identify patients who will respond to specific drugs. However, there are only a handful of clinically used predictive biomarkers in oncology. Herein, we describe an approach utilizing in vitro DNA and RNA sequencing and drug response data to create TreAtment Response Generalized Elastic-neT Signatures (TARGETS). We trained TARGETS drug response models using Elastic-Net regression in the publicly available Genomics of Drug Sensitivity in Cancer (GDSC) database. Models were then validated on additional in-vitro data from the Cancer Cell Line Encyclopedia (CCLE), and on clinical samples from The Cancer Genome Atlas (TCGA) and Stand Up to Cancer/Prostate Cancer Foundation West Coast Prostate Cancer Dream Team (WCDT). First, we demonstrated that all TARGETS models successfully predicted treatment response in the separate in-vitro CCLE treatment response dataset. Next, we evaluated all FDA-approved biomarker-based cancer drug indications in TCGA and demonstrated that TARGETS predictions were concordant with established clinical indications. Finally, we performed independent clinical validation in the WCDT and found that the TARGETS AR signaling inhibitors (ARSI) signature successfully predicted clinical treatment response in metastatic castration-resistant prostate cancer with a statistically significant interaction between the TARGETS score and PSA response (p = 0.0252). TARGETS represents a pan-cancer, platform-independent approach to predict response to oncologic therapies and could be used as a tool to better select patients for existing therapies as well as identify new indications for testing in prospective clinical trials.

Clinical Targeting of Altered Metabolism in High-Grade Glioma

ABSTRACT

High-grade gliomas are among the deadliest of all cancers despite standard treatments, and new therapeutic strategies are needed to improve patient outcome. Targeting the altered metabolic state of tumors with traditional chemotherapeutic agents has a history of success, and our increased understanding of cellular metabolism in the past 2 decades has reinvigorated the concept of novel metabolic therapies in brain tumors. Here we highlight metabolic alterations in advanced gliomas and their translation into clinical trials using both novel agents and already established drugs repurposed for cancer treatment in an effort to improve outcome for these deadly diseases.

A Complementary Strategy to Mitigate Radiation-Induced Cognitive Decline

Cranial radiation activates an upstream complement cascade component, C1q, leading to brain injury. Microglia-specific deletion of C1q prevents astrocyte and microglial activation, synaptic loss, neuroinflammation, and cognitive impairment. Therapeutically inhibiting complement activation may help mitigate radiation-induced cognitive decline.See related article by Markarian et al., p. 1732.

Survival Prediction Analysis in Glioblastoma With Diffusion Kurtosis Imaging

SIMPLE SUMMARY

Glioblastoma (GBM) is the most common and aggressive primary brain tumor. Diffusion kurtosis imaging (DKI) has characterized non-Gaussian diffusion behaviors in brain normal tissue and gliomas, but there are very limited efforts in investigating treatment responses of kurtosis in GBM. This study aimed to investigate whether any parameter derived from the DKI is a significant predictor of overall survival (OS). We found that the large mean, 80 and 90 percentile kurtosis values in the contrast enhanced gross tumor volume (Gd-GTV) on post-Gd T1-weighted images pre-RT were significantly associated with reduced OS. In the multivariate Cox model, the mean kurtosis Gd-GTV pre-RT after considering effects of age, extent of surgery, and methylation were significant predictors of OS. In addition, the 80 and 90 percentile kurtosis values in Gd-GTV post RT were significantly associated with progression free survival (PFS). The DKI model demonstrates the potential to predict outcomes in the patients with GBM.

PURPOSE

Non-Gaussian diffusion behaviors in gliomas have been characterized by diffusion kurtosis imaging (DKI). But there are very limited efforts in investigating the kurtosis in glioblastoma (GBM) and its prognostic and predictive values. This study aimed to investigate whether any of the diffusion kurtosis parameters derived from DKI is a significant predictor of overall survival.

METHODS AND MATERIALS

Thirty-three patients with GBM had pre-radiation therapy (RT) and mid-RT diffusion weighted (DW) images. Kurtosis and diffusion coefficient (DC) values in the contrast enhanced gross tumor volume (Gd-GTV) on post-Gd T1 weighted images pre-RT and mid-RT were calculated. Univariate and multivariate Cox models were used to evaluate the DKI parameters and clinical factors for prediction of OS and PFS.

RESULTS

The large mean kurtosis values in the Gd-GTV pre-RT were significantly associated with reduced OS (p = 0.02), but the values at mid-RT were not (p > 0.8). In the multivariate Cox model, the mean kurtosis in the Gd-GTV pre-RT (p = 0.009) was still a significant predictor of OS after adjusting effects of age, O6-Methylguanine-DNA Methyl transferase (MGMT) methylation and extent of resection. In Gd-GTV post-RT, 80 and 90 percentile kurtosis values were significant predictors (p ≤ 0.05) for progression free survival (PFS).

CONCLUSION

The DKI model demonstrates the potential to predict OS and PFS in the patients with GBM. Further development and histopathological validation of the DKI model will warrant its role in clinical management of GBM.

Abstract PO-008: Mechanistic and therapeutic links between purine biosynthesis and DNA damage in glioblastoma

Glioblastoma (GBM) is the most common and aggressive adult brain cancer. Radiation therapy (RT) is a critical treatment modality, and development of RT resistance is the predominant cause of recurrence and mortality in GBM patients. Using cell line models as well as patient-derived xenografts and neurospheres in orthotopic brain tumor models, we have identified increased rates and dependence upon de novo purine biosynthesis as a hallmark of GBM RT resistance. More recently, we have discovered that radiation treatment acutely stimulates flux through de novo purine synthesis in cell line and neurosphere models of GBM. This RT-induced increase in de novo purine synthesis is dependent on signaling through the DNA damage response and thus appears to be an adaptive mechanism to supply purines to repair radiation-induced DNA damage. To determine whether this regulatory mechanism also exists in vivo, we have used advanced metabolomic and metabolic tracing techniques with 13C-labeled glucose and 15N-labeled glutamine in mice bearing RT-resistant GBM patient-derived orthotopic brain tumors. We found that that orthotopic GBM PDXs had elevated activity of de novo purine synthesis that increased further after RT, while normal cortex had little activity even after RT. These observations have therapeutic relevance, as targeting this metabolic pathway with the FDA-approved purine biosynthesis inhibitor mycophenolate mofetil (MMF) overcomes GBM radiation resistance in mouse models in vivo. The lack of de novo purine synthesis in normal cortex suggests that targeting this pathway may be tumor specific. Collectively our data suggest that de novo synthesis of purines mediates RT resistance in GBM and that treatment of brain tumors with MMF in combination with RT may be a promising therapeutic strategy in patients.

Citation Format: Andrew J. Scott, Weihua Zhou, Kari Wilder-Romans, Anthony C. Andren, Li Zhang, Yoshie Umemura, Nathan Qi, Theodore S. Lawrence, Costas A. Lyssiotis, Daniel R Wahl. Mechanistic and therapeutic links between purine biosynthesis and DNA damage in glioblastoma [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-008.

Response assessment during chemoradiation using a hypercellular/hyperperfused imaging phenotype predicts survival in patients with newly diagnosed glioblastoma

BACKGROUND

Adversely prognostic hypercellular and hyperperfused regions of glioblastoma (GBM) predict progression-free survival, and are a novel target for dose-intensified chemoradiation (chemoRT) recently implemented in a phase II clinical trial. As a secondary aim, we hypothesized that dose-intensified chemoRT would induce greater mid-treatment response of hypercellular/hyperperfused tumor regions vs standard chemoradiation, and that early response would improve overall survival (OS).

METHODS

Forty-nine patients with newly diagnosed GBM underwent prospective, multiparametric high b value diffusion-weighted MRI (DW-MRI) and perfusion dynamic contrast-enhanced MRI (DCE-MRI) pre-RT and 3-4 weeks into RT. The hypercellular tumor volume (TVHCV, mean contralateral normal brain + 2SD) and hyperperfused tumor volume (TVCBV, contralateral normal frontal gray matter + 1SD) were generated using automated thresholding. Twenty-six patients were enrolled on a dose-escalation trial targeting TVHCV/TVCBV with 75 Gy in 30 fractions, and 23 non-trial patients comprised the control group. OS was estimated using the Kaplan-Meier method and compared using the log-rank test. The effect of TVHCV/TVCBV and Gd-enhanced tumor volume on OS was assessed using multivariable Cox proportional-hazard regression.

RESULTS

Most patients had gross total (47%) or subtotal resection (37%), 25% were MGMT-methylated. Patients treated on the dose-escalation trial had significantly greater reduction in TVHCV/TVCBV (41% reduction, IQR 17%-75%) vs non-trial patients (6% reduction, IQR 6%-22%, P = .002). An increase in TVHCV/TVCBV during chemoRT was associated with worse OS (adjusted hazard ratio [aHR] 1.2, 95%CI 1.0-1.4, P = .02), while pre-treatment tumor volumes (P > .5) and changes in Gd-enhanced volume (P = .9) were not.

CONCLUSIONS

Multiparametric MRI permits identification of therapeutic resistance during chemoRT and supports adaptive strategies in future trials.

Combinatorial Efficacy of Olaparib with Radiation and ATR Inhibitor Requires PARP1 Protein in Homologous Recombination-Proficient Pancreatic Cancer

PARP inhibitor monotherapy (olaparib) was recently FDA approved for the treatment of BRCA1/2-mutant, homologous recombination (HR) repair-deficient pancreatic cancer. Most pancreatic cancers, however, are HR proficient and thus resistant to PARP inhibitor monotherapy. We tested the hypothesis that combined therapy with radiation and ataxia telangiectasia and Rad3-related (ATR) inhibitor (AZD6738) would extend the therapeutic indication of olaparib to HR-proficient pancreatic cancers. We show that olaparib combined with AZD6738 significantly reduced radiation survival relative to either agent alone, regardless of HR status. Whereas catalytic inhibition of PARP with low concentrations of olaparib radiosensitized HR-deficient models, maximal sensitization in HR-proficient models required concentrations of olaparib that induce formation of PARP1-DNA complexes. Furthermore, CRISPR-Cas9-mediated PARP1 deletion failed to recapitulate the effects of olaparib on radiosensitivity and negated the combinatorial efficacy of olaparib and AZD6738 on radiosensitization, suggesting that PARP1-DNA complexes, rather than PARP catalytic inhibition, were responsible for radiosensitization. Mechanistically, therapeutic concentrations of olaparib in combination with radiation and AZD6738 increased DNA double-strand breaks. DNA fiber combing revealed that high concentrations of olaparib did not stall replication forks but instead accelerated replication fork progression in association with an ATR-mediated replication stress response that was antagonized by AZD6738. Finally, in HR-proficient tumor xenografts, the combination of olaparib, radiation, and AZD6738 significantly delayed tumor growth compared with all other treatments. These findings suggest that PARP1-DNA complexes are required for the therapeutic activity of olaparib combined with radiation and ATR inhibitor in HR-proficient pancreatic cancer and support the clinical development of this combination for tumors intrinsically resistant to PARP inhibitors.

Radiation-Induced Imaging Changes and Cerebral Edema following Stereotactic Radiosurgery for Brain AVMs

BACKGROUND AND PURPOSE

T2 signal and FLAIR changes in patients undergoing stereotactic radiosurgery for brain AVMs may occur posttreatment and could result in adverse radiation effects. We aimed to evaluate outcomes in patients with these imaging changes, the frequency and degree of this response, and factors associated with it.

MATERIALS AND METHODS

Through this retrospective cohort study, consecutive patients treated with stereotactic radiosurgery for brain AVMs who had at least 1 year of follow-up MR imaging were identified. Logistic regression analysis was used to evaluate predictors of outcomes.

RESULTS

One-hundred-sixty AVMs were treated in 148 patients (mean, 35.6 years of age), including 42 (26.2%) pediatric AVMs. The mean MR imaging follow-up was 56.5 months. The median Spetzler-Martin grade was III. The mean maximal AVM diameter was 2.8 cm, and the mean AVM target volume was 7.4 mL. The median radiation dose was 16.5 Gy. New T2 signal and FLAIR hyperintensity were noted in 40% of AVMs. T2 FLAIR volumes at 3, 6, 12, 18, and 24 months were, respectively, 4.04, 55.47, 56.42, 48.06, and 29.38 mL Radiation-induced neurologic symptoms were encountered in 34.4%. In patients with radiation-induced imaging changes, 69.2% had new neurologic symptoms versus 9.5% of patients with no imaging changes (P = .0001). Imaging changes were significantly associated with new neurologic findings (P < .001). Larger AVM maximal diameter (P = .04) and the presence of multiple feeding arteries (P = .01) were associated with radiation-induced imaging changes.

CONCLUSIONS

Radiation-induced imaging changes are common following linear particle accelerator-based stereotactic radiosurgery for brain AVMs, appear to peak at 12 months, and are significantly associated with new neurologic findings.

Xenograft-based, platform-independent gene signatures to predict response to alkylating chemotherapy, radiation, and combination therapy for glioblastoma

BACKGROUND

Predictive molecular biomarkers to select optimal treatment for patients with glioblastoma and other cancers are lacking. New strategies are needed when large randomized trials with correlative molecular data are not feasible.

METHODS

Gene signatures (GS) were developed from 31 orthotopic glioblastoma patient-derived xenografts (PDXs), treated with standard therapies, to predict benefit from radiotherapy (RT-GS), temozolomide (Chemo-GS), or the combination (ChemoRT-GS). Independent validation was performed in a heterogeneously treated clinical cohort of 502 glioblastoma patients with overall survival as the primary endpoint. Multivariate Cox analysis was used to adjust for confounding variables and evaluate interactions between signatures and treatment.

RESULTS

PDX models recapitulated the clinical heterogeneity of glioblastoma patients. RT-GS, Chemo-GS, and ChemoRT-GS were correlated with benefit from treatment in the PDX models. In independent clinical validation, higher RT-GS scores were associated with increased survival only in patients receiving RT (P = 0.0031, hazard ratio [HR] = 0.78 [0.66-0.92]), higher Chemo-GS scores were associated with increased survival only in patients receiving chemotherapy (P < 0.0001, HR = 0.66 [0.55-0.8]), and higher ChemoRT-GS scores were associated with increased survival only in patients receiving ChemoRT (P = 0.0001, HR = 0.54 [0.4-0.74]). RT-GS and ChemoRT-GS had significant interactions with treatment on multivariate analysis (P = 0.0009 and 0.02, respectively), indicating that they are bona fide predictive biomarkers.

CONCLUSIONS

Using a novel PDX-driven methodology, we developed and validated 3 platform-independent molecular signatures that predict benefit from standard of care therapies for glioblastoma. These signatures may be useful to personalize glioblastoma treatment in the clinic and this approach may be a generalizable method to identify predictive biomarkers without resource-intensive randomized trials.

Purine metabolism promotes radioresistance and is a therapeutic target in glioblastoma

Profound intratumoral genomic heterogeneity has limited the ability of targeted therapies to overcome therapy resistance in glioblastoma. We have defined purine metabolism as a key mediator of DNA repair and radiation resistance in glioblastoma. Because many glioblastoma oncogenic drivers activate purine metabolism, its inhibition may overcome therapy resistance despite intratumoral genomic heterogeneity.

Expression of the Androgen Receptor Governs Radiation Resistance in a Subset of Glioblastomas Vulnerable to Antiandrogen Therapy

New approaches are needed to overcome intrinsic therapy resistance in glioblastoma (GBM). Because GBMs exhibit sexual dimorphism and are reported to express steroid hormone receptors, we reasoned that signaling through the androgen receptor (AR) could mediate therapy resistance in GBM, much as it does in AR-positive prostate and breast cancers. We found that nearly half of GBM cell lines, patient-derived xenografts (PDX), and human tumors expressed AR at the transcript and protein level-with expression levels overlapping those of primary prostate cancer. Analysis of gene expression datasets also revealed that AR expression is higher in GBM patient samples than normal brain tissue. Multiple clinical-grade antiandrogens slowed the growth of and radiosensitized AR-positive GBM cell lines and PDXs in vitro and in vivo Antiandrogens blocked the ability of AR-positive GBM PDXs to engage adaptive transcriptional programs following radiation and slowed the repair of radiation-induced DNA damage. These results suggest that combining blood-brain barrier permeable antiandrogens with radiation may have promise for patients with AR-positive GBMs.

Everolimus improves the efficacy of dasatinib in PDGFRα-driven glioma

Pediatric and adult high-grade gliomas (HGGs) frequently harbor PDGFRA alterations. We hypothesized that cotreatment with everolimus may improve the efficacy of dasatinib in PDGFRα-driven glioma through combinatorial synergism and increased tumor accumulation of dasatinib. We performed dose-response, synergism, P-glycoprotein inhibition, and pharmacokinetic studies in in vitro and in vivo human and mouse models of HGG. Six patients with recurrent PDGFRα-driven glioma were treated with dasatinib and everolimus. We found that dasatinib effectively inhibited the proliferation of mouse and human primary HGG cells with a variety of PDGFRA alterations. Dasatinib exhibited synergy with everolimus in the treatment of HGG cells at low nanomolar concentrations of both agents, with a reduction in mTOR signaling that persisted after dasatinib treatment alone. Prolonged exposure to everolimus significantly improved the CNS retention of dasatinib and extended the survival of PPK tumor-bearing mice (mutant TP53, mutant PDGFRA, H3K27M). Six pediatric patients with glioma tolerated this combination without significant adverse events, and 4 patients with recurrent disease (n = 4) had a median overall survival of 8.5 months. Our results show that the efficacy of dasatinib treatment of PDGFRα-driven HGG was enhanced with everolimus and suggest a promising route for improving targeted therapy for this patient population.

Integrated Metabolic and Epigenomic Reprograming by H3K27M Mutations in Diffuse Intrinsic Pontine Gliomas

H3K27M diffuse intrinsic pontine gliomas (DIPGs) are fatal and lack treatments. They mainly harbor H3.3K27M mutations resulting in H3K27me3 reduction. Integrated analysis in H3.3K27M cells, tumors, and in vivo imaging in patients showed enhanced glycolysis, glutaminolysis, and tricarboxylic acid cycle metabolism with high alpha-ketoglutarate (α-KG) production. Glucose and/or glutamine-derived α-KG maintained low H3K27me3 in H3.3K27M cells, and inhibition of key enzymes in glycolysis or glutaminolysis increased H3K27me3, altered chromatin accessibility, and prolonged survival in animal models. Previous studies have shown that mutant isocitrate-dehydrogenase (mIDH)1/2 glioma cells convert α-KG to D-2-hydroxyglutarate (D-2HG) to increase H3K27me3. Here, we show that H3K27M and IDH1 mutations are mutually exclusive and experimentally synthetic lethal. Overall, we demonstrate that H3.3K27M and mIDH1 hijack a conserved and critical metabolic pathway in opposing ways to maintain their preferred epigenetic state. Consequently, interruption of this metabolic/epigenetic pathway showed potent efficacy in preclinical models, suggesting key therapeutic targets for much needed treatments.

Abstract 6271: Hormone receptor inhibition as a strategy for radiosensitization of breast cancer

Purpose: Expression of the androgen receptor (AR) has been identified as a driver of tumor growth in triple negative breast cancers (TNBC), and previous work has nominated AR as a target for radiosensitization. In addition, 70-95% of all estrogen receptor (ER) positive (ER+) breast cancers also have coexpression of AR, suggesting extended utility of AR inhibition in the radiosensitization of these AR+, ER+ tumors. Here we assessed the efficacy of AR inhibition in ER+, AR+ breast cancers to better understand the role of AR signaling across breast cancer models. Further, we also investigated the effect of ER inhibition on radiosensitization of ER+ breast cancer models.

Methods: IC50 values were determined for MDV3100 (enzalutamide), ARN-509 (apalutamide), and ODM-201 (darolutamide) in TNBC cell lines (AR+ TNBC: MDA-MB-453, ACC-422, and SUM-185PE, and AR- TNBC: MDA-MB-231) and ER+ breast cancer cell lines (AR+, ER+: ZR-75-1, BT-474, CAMA-1, and AR-, ER+: MCF-7). IC50 values for tamoxifen were determined for ER+ breast cancer cell lines (MCF-7, T47D, ZR-75-1), and ER- (SUM-159) cells. Clonogenic survival assays were performed to assess radiosensitization with ER or AR inhibition with tamoxifen or second generation anti-androgens, respectively, in TNBC and ER+ breast cancer models.

Results: AR inhibition with enzalutamide, apalutamide, and darolutamide showed limited single agent growth inhibition efficacy in AR+ TNBC and AR+, ER+ breast cancer cell lines (IC50 &gt; 10 μM). AR inhibition with enzalutamide did not induce radiosensitivity in vitro. In AR+, ER+ CAMA-1 cells, AR blockade with enzalutamide had a radioprotective effect with enhancement ratios (enhR) of 0.76-0.83. No radiosensitization was observed in BT-474 (enhR: 0.92-1.01) or ZR-75-1 cells (enhR: 0.94-1.00). Radiosensitization was also assessed with anti-androgens apalutamide and darolutamide in AR+ breast cancer models. Inhibition of ER with tamoxifen, however, induced radiosensitization in MCF-7 (enhR: 1.14-1.50) and T47D (enhR: 1.33-1.60) cells. No radiosensitization was observed with tamoxifen in ER- SUM-159 cells.

Conclusion: Although AR is a mediator of radioresistance in AR+ TNBC, AR inhibition does not provide comparable radiosensitization in AR+, ER+ models and may actually confer a radioprotective effect. In contrast, our results demonstrate ER inhibition is an effective radiosensitizing strategy in ER+ breast cancers, independent of AR status. This work highlights the complexities of androgen and estrogen receptor signaling in AR+, ER+ breast tumors and underscores the necessity for understanding context dependent effects when translating into patients with AR+ breast cancer.

Citation Format: Anna R. Michmerhuizen, Amanda Zhang, Rachel Schwartz, Andrea M. Pesch, Benjamin C. Chandler, Cassandra L. Ritter, Meilan Liu, Kari Wilder-Romans, Daniel E. Spratt, Daniel R. Wahl, Shyam Nyati, Lori J. Pierce, Corey Speers. Hormone receptor inhibition as a strategy for radiosensitization of breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6271.

Abstract 6267: Repurposing antiandrogens to overcome therapy resistance in androgen receptor-positive glioblastoma

New approaches are needed to overcome intrinsic therapy resistance in glioblastoma (GBM). Because GBMs exhibit sexual dimorphism and are reported to express steroid hormone receptors, we reasoned that signaling through the androgen receptor (AR) could mediate therapy resistance in GBM, as it does in AR-positive prostate and breast cancers. Using RNAseq, immunoblot and immunohistochemistry, we found that nearly half of GBM cell lines, patient-derived xenografts and human tumors express AR transcript and protein with levels that overlap those of primary prostate cancer. AR expression in GBM did not vary by sex, age or common molecular alterations. We identified two cell line models of GBM that expressed AR protein (LN18 and T98G: termed “AR positive”) and two that did not (8MGBA and AM38: termed “AR negative”). Seviteronel, a blood-brain barrier permeable CYP17 lyase inhibitor and antiandrogen slowed growth in AR positive GBM cell lines (GI50 3-4 µM) but not AR negative lines (GI50 &gt; 500 µM) as measured by the colony formation assay. The antiandrogen enzalutamide, which also penetrates the blood brain barrier, similarly preferentially slowed growth in AR positive GBM cell lines. Seviteronel and enzalutamide sensitized AR positive GBM cell lines to radiation with enhancement ratios of 1.3-1.6 as measured by the clonogenic survival assay. Antiandrogens had no effect on the radiosensitivity of AR negative GBM cell lines. Seviteronel treatment did not affect the growth of AR positive T98G xenografts grown in vivo, but did sensitize these tumors to radiation (median time to tripling: 15 d with radiation alone and not reached with radiation combined with seviteronel). Enzalutamide similarly had modest single agent effects on an AR positive GBM patient-derived xenograft (GBM26 from the Mayo Clinic GBM PDX national resource) grown in vivo but sensitized these tumors to radiation (median time to tripling: 25.5 d with radiation alone and 39 d with radiation combined with enzalutamide). RNAseq performed on GBM26 tumors grown in vivo revealed that enzalutamide treatment caused minimal transcriptional changes when given as monotherapy but, when given in combination with radiation, blocked the ability of AR-positive GBMs to engage adaptive transcriptional programs related to multiple DNA repair pathways. We confirmed these mechanistic findings in vitro, as antiandrogens selectively impaired the repair of radiation-induced double strand DNA breaks in AR positive GBM cell lines. These results suggest that AR signaling may mediate therapy resistance in AR positive GBMs, and patients with these tumors could derive clinical benefit from combination therapies involving radiation and blood-brain-barrier permeable antiandrogens.

Citation Format: Christian K. Werner, Uchechi Nna, Hanshi Sun, Kari Wilder-Romans, Joseph Dresser, Ayesha Kothari, Weihua Zhou, Yangyang Yao, Arvind Rao, Stefanie Stallard, Carl Koschmann, Tarik Bor, Waldemar Debinski, Alexander Hegedus, Meredith Morgan, Sriram Venneti, Edwina Baskin-Bey, Daniel Spratt, Howard Colman, Jann Sarkaria, Arul Chinnayain, Joel Eisner, Corey Speers, Theodore S. Lawrence, Roy Strowd, Daniel R. Wahl. Repurposing antiandrogens to overcome therapy resistance in androgen receptor-positive glioblastoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6267.

Purine metabolism regulates DNA repair and therapy resistance in glioblastoma

Intratumoral genomic heterogeneity in glioblastoma (GBM) is a barrier to overcoming therapy resistance. Treatments that are effective independent of genotype are urgently needed. By correlating intracellular metabolite levels with radiation resistance across dozens of genomically-distinct models of GBM, we find that purine metabolites, especially guanylates, strongly correlate with radiation resistance. Inhibiting GTP synthesis radiosensitizes GBM cells and patient-derived neurospheres by impairing DNA repair. Likewise, administration of exogenous purine nucleosides protects sensitive GBM models from radiation by promoting DNA repair. Neither modulating pyrimidine metabolism nor purine salvage has similar effects. An FDA-approved inhibitor of GTP synthesis potentiates the effects of radiation in flank and orthotopic patient-derived xenograft models of GBM. High expression of the rate-limiting enzyme of de novo GTP synthesis is associated with shorter survival in GBM patients. These findings indicate that inhibiting purine synthesis may be a promising strategy to overcome therapy resistance in this genomically heterogeneous disease.

Targeting genotype-independent abnormalities may overcome therapy resistance in glioblastoma despite intratumoral genomic heterogeneity. Here, the authors show that glioblastoma radiation resistance is promoted by purine metabolism and can be overcome by inhibitors of purine synthesis.

Stereotactic Radiosurgery for Brain Arteriovenous Malformations: Evaluation of Obliteration and Review of Associated Predictors

BACKGROUND

High arteriovenous malformation (AVM) obliteration rates have been reported with stereotactic radiosurgery (SRS), and multiple factors have been found to be associated with AVM obliteration. These predictors have been inconsistent throughout studies. We aimed to analyze our experience with linear accelerator (LINAC)-based SRS for brain AVMs, evaluate outcomes, assess factors associated with AVM obliteration and review the various reported predictors of AVM obliteration.

METHODS

Electronic medical records were retrospectively reviewed to identify consecutive patients with brain AVMs treated with SRS over a 27-year period with at least 2 years of follow-up. Logistic regression analysis was performed to identify factors associated with AVM obliteration.

RESULTS

One hundred twenty-eight patients with 142 brain AVMs treated with SRS were included. Mean age was 34.4 years. Fifty-two percent of AVMs were associated with a hemorrhage before SRS, and 14.8% were previously embolized. Mean clinical and angiographic follow-up times were 67.8 months and 58.6 months, respectively. The median Spetzler-Martin grade was 3. Mean maximal AVM diameter was 2.8 cm and mean AVM target volume was 7.4 cm³ with a median radiation dose of 16 Gy. Complete AVM obliteration was achieved in 80.3%. Radiation-related signs and symptoms were encountered in 32.4%, only 4.9% of which consisted of a permanent deficit. Post-SRS AVM-related hemorrhage occurred in 6.3% of cases. In multivariate analysis, factors associated with AVM obliteration included younger patient age (P = .019), male gender (P = .008), smaller AVM diameter (P = .04), smaller AVM target volume (P = .009), smaller isodose surface volume (P = .005), a higher delivered radiation dose (P = .013), and having only one major draining vein (P = .04).

CONCLUSIONS

AVM obliteration with LINAC-based radiosurgery was safe and effective and achieved complete AVM obliteration in about 80% of cases. The most prominent predictors of AVM success included AVM size, AVM volume, radiation dose, number of draining veins and patient age.

Phase 2 Study of a Temozolomide-Based Chemoradiation Therapy Regimen for High-Risk, Low-Grade Gliomas: Long-Term Results of Radiation Therapy Oncology Group 0424

PURPOSE

To report the long-term outcomes of the RTOG 0424 study of a high-risk, low-grade glioma population treated with concurrent and adjuvant temozolomide (TMZ) and radiation therapy (RT).

METHODS AND MATERIALS

For this single-arm, phase 2 study, patients with low-grade gliomas with ≥3 risk factors (age ≥40 years, astrocytoma, bihemispheric tumor, size ≥6 cm, or preoperative neurologic function status >1) received RT (54 Gy in 30 fractions) with TMZ and up to 12 cycles of post-RT TMZ. The initial primary endpoint P was overall survival (OS) at 3 years after registration. Secondary endpoints included progression-free survival (PFS) and the association of survival outcomes with methylation status. The initial 3-year report of this study was published in 2015.

RESULTS

The study accrued 136 patients, of whom 129 were analyzable. The median follow-up for surviving patients was 9.0 years. The 3-year OS was 73.5% (95% confidence interval, 65.8%-81.1%), numerically superior to the 3-year OS historical control of 54% (P < .001). The median survival time was 8.2 years (95% confidence interval, 5.6-9.1). Five- and 10-year OS rates were 60.9% and 34.6%, respectively, and 5- and 10-year PFS rates were 46.8% and 25.5%, respectively.

CONCLUSIONS

The long-term results confirmed the findings from the initial report for efficacy, suggesting OS and PFS outcomes with the RT-TMZ regimen exceeded historical control groups treated with radiation alone. Toxicity was acceptable.

Tissue of origin dictates GOT1 dependence and confers synthetic lethality to radiotherapy

BACKGROUND

Metabolic programs in cancer cells are influenced by genotype and the tissue of origin. We have previously shown that central carbon metabolism is rewired in pancreatic ductal adenocarcinoma (PDA) to support proliferation through a glutamate oxaloacetate transaminase 1 (GOT1)-dependent pathway.

METHODS

We utilized a doxycycline-inducible shRNA-mediated strategy to knockdown GOT1 in PDA and colorectal cancer (CRC) cell lines and tumor models of similar genotype. These cells were analyzed for the ability to form colonies and tumors to test if tissue type impacted GOT1 dependence. Additionally, the ability of GOT1 to impact the response to chemo- and radiotherapy was assessed. Mechanistically, the associated specimens were examined using a combination of steady-state and stable isotope tracing metabolomics strategies and computational modeling. Statistics were calculated using GraphPad Prism 7. One-way ANOVA was performed for experiments comparing multiple groups with one changing variable. Student's t test (unpaired, two-tailed) was performed when comparing two groups to each other. Metabolomics data comparing three PDA and three CRC cell lines were analyzed by performing Student's t test (unpaired, two-tailed) between all PDA metabolites and CRC metabolites.

RESULTS

While PDA exhibits profound growth inhibition upon GOT1 knockdown, we found CRC to be insensitive. In PDA, but not CRC, GOT1 inhibition disrupted glycolysis, nucleotide metabolism, and redox homeostasis. These insights were leveraged in PDA, where we demonstrate that radiotherapy potently enhanced the effect of GOT1 inhibition on tumor growth.

CONCLUSIONS

Taken together, these results illustrate the role of tissue type in dictating metabolic dependencies and provide new insights for targeting metabolism to treat PDA.

Running the Light: Nucleotide Metabolism Drives Bypass of Senescence in Cancer

Senescence is engaged in response to oncogenes to suppress proliferation. Cancers rewire metabolism to facilitate proliferation; however, it is not well appreciated how this enables senescence bypass. Recent work by Buj et al. demonstrates that loss of the tumor suppressor p16 engages a mTORC1-dependent increase in nucleotide pools to override senescence.

Radiotherapy and Immunotherapy Promote Tumoral Lipid Oxidation and Ferroptosis via Synergistic Repression of SLC7A11

A challenge in oncology is to rationally and effectively integrate immunotherapy with traditional modalities, including radiotherapy. Here, we demonstrate that radiotherapy induces tumor-cell ferroptosis. Ferroptosis agonists augment and ferroptosis antagonists limit radiotherapy efficacy in tumor models. Immunotherapy sensitizes tumors to radiotherapy by promoting tumor-cell ferroptosis. Mechanistically, IFNγ derived from immunotherapy-activated CD8⁺ T cells and radiotherapy-activated ATM independently, yet synergistically, suppresses SLC7A11, a unit of the glutamate-cystine antiporter xc^-, resulting in reduced cystine uptake, enhanced tumor lipid oxidation and ferroptosis, and improved tumor control. Thus, ferroptosis is an unappreciated mechanism and focus for the development of effective combinatorial cancer therapy. SIGNIFICANCE: This article describes ferroptosis as a previously unappreciated mechanism of action for radiotherapy. Further, it shows that ferroptosis is a novel point of synergy between immunotherapy and radiotherapy. Finally, it nominates SLC7A11, a critical regulator of ferroptosis, as a mechanistic determinant of synergy between radiotherapy and immunotherapy.This article is highlighted in the In This Issue feature, p. 1631.

Metabolic Abnormalities in Glioblastoma and Metabolic Strategies to Overcome Treatment Resistance

Glioblastoma (GBM) is the most common and aggressive primary brain tumor and is nearly universally fatal. Targeted therapy and immunotherapy have had limited success in GBM, leaving surgery, alkylating chemotherapy and ionizing radiation as the standards of care. Like most cancers, GBMs rewire metabolism to fuel survival, proliferation, and invasion. Emerging evidence suggests that this metabolic reprogramming also mediates resistance to the standard-of-care therapies used to treat GBM. In this review, we discuss the noteworthy metabolic features of GBM, the key pathways that reshape tumor metabolism, and how inhibiting abnormal metabolism may be able to overcome the inherent resistance of GBM to radiation and chemotherapy.

Inhibition of ATM Increases Interferon Signaling and Sensitizes Pancreatic Cancer to Immune Checkpoint Blockade Therapy

Combinatorial strategies are needed to overcome the resistance of pancreatic cancer to immune checkpoint blockade (ICB). DNA damage activates the innate immune response and improves ICB efficacy. Because ATM is an apical kinase in the radiation-induced DNA damage response, we investigated the effects of ATM inhibition and radiation on pancreatic tumor immunogenicity. ATM was inhibited through pharmacologic and genetic strategies in human and murine pancreatic cancer models both in vitro and in vivo. Tumor immunogenicity was evaluated after ATM inhibition alone and in combination with radiation by assessing TBK1 and Type I interferon (T1IFN) signaling as well as tumor growth following PD-L1/PD-1 checkpoint inhibition. Inhibition of ATM increased tumoral T1IFN expression in a cGAS/STING-independent, but TBK1- and SRC-dependent, manner. The combination of ATM inhibition with radiation further enhanced TBK1 activity, T1IFN production, and antigen presentation. Furthermore, ATM silencing increased PD-L1 expression and increased the sensitivity of pancreatic tumors to PD-L1-blocking antibody in association with increased tumoral CD8+ T cells and established immune memory. In patient pancreatic tumors, low ATM expression inversely correlated with PD-L1 expression. Taken together, these results demonstrate that the efficacy of ICB in pancreatic cancer is enhanced by ATM inhibition and further potentiated by radiation as a function of increased tumoral immunogenicity, underscoring the potential of ATM inhibition in combination with ICB and radiation as an efficacious treatment strategy for pancreatic cancer. SIGNIFICANCE: This study demonstrates that ATM inhibition induces a T1IFN-mediated innate immune response in pancreatic cancer that is further enhanced by radiation and leads to increased sensitivity to anti-PD-L1 therapy.See related commentary by Gutiontov and Weichselbaum, p. 3815.

Dose-intensified chemoradiation is associated with altered patterns of failure and favorable survival in patients with newly diagnosed glioblastoma

BACKGROUND AND PURPOSE

We evaluated whether dose-intensified chemoradiation alters patterns of failure and is associated with favorable survival in the temozolomide era.

MATERIALS AND METHODS

Between 2003 and 2015, 82 patients with newly diagnosed glioblastoma were treated with 66-81 Gy in 30 fractions using conventional magnetic resonance imaging. Progression-free (PFS) and overall survival (OS) were calculated using Kaplan-Meier methods. Factors associated with improved PFS, OS, and time to progression were assessed using multivariate Cox model and linear regression.

RESULTS

Median follow-up was 23 months (95% CI 4-124 months). Sixty-one percent of patients underwent subtotal resection or biopsy, and 38% (10/26) of patients with available data had MGMT promoter methylation. Median PFS was 8.4 months (95% CI 7.3-11.0) and OS was 18.7 months (95% CI 13.1-25.3). Only 30 patients (44%) experienced central recurrence, 6 (9%) in-field, 16 (23.5%) marginal and 16 (23.5%) distant. On multivariate analysis, younger age (HR 0.95, 95% CI 0.93-0.97, p = 0.0001), higher performance status (HR 0.39, 95% CI 0.16-0.95, p = 0.04), gross total resection (GTR) versus biopsy (HR 0.37, 95% CI 0.16-0.85, p = 0.02) and MGMT methylation (HR 0.25, 95% CI 0.09-0.71, p = 0.009) were associated with improved OS. Only distant versus central recurrence (p = 0.03) and GTR (p = 0.02) were associated with longer time to progression. Late grade 3 neurologic toxicity was rare (6%) in patients experiencing long-term survival.

CONCLUSION

Dose-escalated chemoRT resulted in lower rates of central recurrence and prolonged time to progression compared to historical controls, although a significant number of central recurrences were still observed. Advanced imaging and correlative molecular studies may enable targeted treatment advances that reduce rates of in- and out-of-field progression.