MEK Inhibitor Research Articles

LGG-23. INHIBITORS IN THE TREATMENT OF LOW GRADE GLIOMAS- A SINGLE CENTER EXPERIENCE, BRATISLAVA, SLOVAKIA

Abstract BACKGROUND Pediatric LGGs are the most common CNS tumors in children. They are a very heterogeneous group of tumors of different histological types, according to the WHO classification of grade 1- 2. The major molecular pathway affected in these tumors is the mitogen-activated protein kinase (MAPK) signaling cascade. The frequency of MAPK pathway activation makes this pathway an attractive candidate for targeted therapy, and direct inhibition via BRAF and/or MEK inhibitors is emerging as a clinically relevant strategy. METHODS We performed a retrospective analysis of patients treated with inhibitors at the Department of Pediatric Hematology and Oncology in Bratislava since 03/2020. Currently 9 patients, 5 boys and 4 girls, are treated with inhibitors. They are pLGG patients, two of them with concomitant genetically confirmed neurofibromatosis type 1. Six patients are treated with the MEK inhibitor trametinib in monotherapy and three patients with BRAF V600-mutated LGG with a combination of the BRAF inhibitor dabrafenib and the MEK inhibitor trametinib. In our cohort with a small number of patients, with a median follow-up of 13 months, we did not observe disease progression in even one patient on inhibitor therapy. Tolerance to treatment is reasonable. CONCLUSIONS In recent years, our understanding of the molecular biology of pediatric brain tumors has improved significantly. Alterations in the mitogen-activated protein kinase (MAPK) pathway are generally the most common molecular alterations in cancer, and dysregulation of this pathway also dominates the development of pediatric LGGs. The introduction of targeted drugs not only allows to improve the disease prognosis and survival of children with CNS tumors, to improve their quality of life on treatment, but also to reduce toxicity caused by e.g. radiotherapy, and at the same time to reduce the incidence of late sequelae of treatment.

DIPG-50. TARGETING VULNERABILITIES IN PAEDIATRIC DIFFUSE MIDLINE GLIOMAS: LEVERAGING PLK1 INHIBITION TO EXPLOIT RAS/PI3K PATHWAY DEPENDENCY

Abstract BACKGROUND Diffuse Midline Gliomas (DMGs), characterised by prevalent H3K27M mutations, exhibit epigenetic modifications that drive a proliferative state resistant to conventional therapies. The interplay of the cell cycle with PI3K and RAS/MYC growth signalling pathways, pivotal in the context of the H3K27M mutation, is yet to be fully understood. The aim of this study was to assess the efficacy of the inhibition of Polo-like kinase 1 (PLK1), a critical regulator of mitosis and cell cycle progression, identify mechanisms of action, and to develop effective combination strategies against DMG. METHODS AND RESULTS PLK1 inhibition demonstrated significant anti-tumour activities both in vitro and within aggressive in vivo orthotopic DMG models, notably doubling the median survival of mice bearing tumours. High-throughput RNA-seq analysis of DMG samples particularly sensitive to PLK1 inhibition, revealed a distinct pattern of negative enrichment in ribosomal genes, indicating a marked reduction in ribosomal function-associated gene expression. This alteration suggests a potential for innovative drug combination strategies targeting the PI3K/mTOR and RAS/MYC pathways, crucial for ribosomal and RNA processing. Subsequently, our in vitro drug sensitivity assays demonstrated that PLK1 inhibition in combination with PI3K/mTOR and MEK inhibitors, exhibit strong drug synergies, evidenced by high Bliss synergy scores. CONCLUSIONS Our findings underscore the efficacy of simultaneous targeting of PI3K/mTOR and RAS/MYC pathways with PLK1 inhibition as an attractive strategy to mitigate DMG tumorigenesis. Furthermore this research not only sheds light on the crucial interplay between cell cycle and signalling pathways in DMG but also facilitates novel intervention strategies for a range of other paediatric CNS malignancies with similar characteristic molecular drivers.

ATRT-17. INHIBITING THE MAP KINASE PATHWAY WITH MEK INHIBITOR MIRDAMETINIB SUPPRESSES CANCER GROWTH IN ATYPICAL TERATOID/RHABDOID TUMORS

Abstract BACKGROUND Atypical teratoid/rhabdoid tumors (ATRT) are poor-prognosis infantile brain tumors. Our prior studies examining ATRT primary tissues have shown an increased expression of phospho-ERK, a downstream effector of MEK in the mitogen-activated protein kinase (MAPK) pathway. The elevated MAPK pathway potentially contributes to drug resistance, aggressive cancer growth, and poor treatment outcomes. Mirdametinib is a highly specific MEK1/2 inhibitor reported to have superior blood-brain-barrier penetration compared to other MEK inhibitors. We hypothesized that mirdametinib can suppress ATRT tumor growth and improve overall survival. METHODS Mirdametinib effects were assessed through western blot for MAPK markers. Western blot analysis, Muse flow cytometer assays, and immunofluorescence were used for proliferation and apoptosis. Mirdametinib impact on survival was determined in orthotopic xenograft models of ATRT. RESULTS Mirdametinib decreased MAPK pathway activity in five ATRT cell lines (western blot: phospho-ERK). Cell lines treated with mirdametinib exhibited lower rates of proliferation (western blot: phospho-Rb; BrdU immunofluorescence staining: CHLA05 p=0.015, CHLA06 p<0.0001, BT37 p<0.0001 compared to DMSO control by t-test). Mirdametinib induced apoptosis in ATRT cells (western blot: cPARP; Muse Annexin V Assay: CHLA04, CHLA05, CHLA06, and BT37, p<0.0001, CHLA266, p<0.01 compared to DMSO control by t-test; cleaved caspase-3 immunofluorescence staining: CHLA05, CHLA06, and BT37, p<0.0001 compared to DMSO control by t-test). Daily oral delivery of mirdametinib treatment significantly extended the survival of mice bearing the very aggressive orthotopic CHLA06 tumors (17 to 24 days, log-rank test p=0.038 compared to vehicle control) and decreased the expression of phospho-ERK (western blot). Testing against additional ATRT orthotopic xenografts is currently underway. CONCLUSIONS Mirdametinib inhibits the MAPK pathway both in vitro and in vivo, suppressing proliferation and inducing apoptosis in ATRT. Our studies suggest that pharmacologically inhibiting the MAPK pathway with mirdametinib may be an effective strategy to target ATRT and help extend survival in this deadly disease.

LGG-49. EVALUATING THE EFFECTS OF MIRDAMETINIB IN NF1-MUTANT LOW-GRADE GLIOMAS USING BOTH CULTURED CELLS AND A NOVEL ZEBRAFISH XENOGRAFT MODEL

Abstract BACKGROUND Pediatric low-grade gliomas (LGGs) with NF1 mutations represent a significant portion of childhood brain tumors. NF1 mutations lead to the dysregulation of the MAPK kinase and mTOR pathways, contributing to tumor growth. However, obtaining cell lines and xenograft animal models for NF1-mutated LGGs remains challenging due to their slow growth and less aggressive nature. Methods and RESULTS We examined the therapeutic efficacy of a novel MEK inhibitor mirdametinib in targeting the MAPK pathway within JHH-NF1-PA1 cells, a cell line derived from a low-grade optic pathway glioma originating from a patient with type 1 neurofibromatosis. Mirdametinib decreased p-ERK protein expression levels on western blot within four hours. We confirmed that mirdametinib significantly suppressed tumor cell proliferation by approximately 60% (BrdU staining (P<0.05), CellTiter Blue assay). β-galactosidase staining revealed approximately a 50% induction of cellular senescence in JHH-NF1-PA1 cells following mirdametinib treatment (P<0.05). We confirmed our findings using murine Nf1-null low grade glioma cell lines 1861 and 1491. To establish an in vivo model for drug testing, we utilized zebrafish due to their transparency, rapid reproduction, and short development time. Injecting RFP-labeled JHH-NF1-PA1 cells into the zebrafish at the 1000-cell stage, we monitored tumor growth using TECAN fluorescence detection from the third to eighth days post-injection. The fluorescence signals obtained from TECAN were validated by fluorescence microscopy. Western blot analysis confirmed a decrease in p-ERK levels within the zebrafish following mirdametinib treatment. We are evaluating the effect of mirdametinib on NF1 low grade glioma cell survival, migration and senescence in the zebrafish brain. CONCLUSION Our findings show that mirdametinib inhibits cell proliferation and induces senescence in NF1-mutated LGGs. Moreover, our zebrafish model provides a valuable tool for evaluating the efficacy of MEK inhibitors in vivo, particularly in cases where traditional xenograft models are difficult to establish.

LGG-59. DIFFUSE LEPTOMENINGEAL GLIONEURONAL TUMOR; MOLECULAR IMPLICATIONS ON PROGNOSIS

Abstract BACKGROUND Diffuse leptomeningeal glioneuronal tumors (DLGNTs) are extremely rare tumors that were first described in the WHO Classification in 2016. While usually considered low-grade tumors, DLGNTs currently lack a standard of care as well as literature on molecular profiling and treatment responses. METHODS Retrospective review of pediatric DLGNTs seen at MD Anderson Cancer Center with Institutional Board Review approval. RESULTS Twelve patients with a histopathological and neuroradiological diagnosis of DLGNT were identified with a median age at diagnosis of 7.5 years (range 4 months – 21 years). Histologically, 10/12 (83%) tumors were low-grade while 2 were reported as high-grade. BRAF gene alterations were reported in 50% (6/12; 4 fusions and 2 mutations). Other reported genetic alterations were mutations [PTPN11 (n=2), PIK3CA (n=1), FGFR1 (n=1), ALK (n=1)], amplifications [NOTCH1 (n=1) and STK11(n=1)], and fusion [NTRK (n=1)]. Upfront, 9 patients received chemotherapy including temozolomide (n=5), carboplatin (n=4), and vincristine (n=4) or vinblastine (n=1). Nine patients (88%) received proton radiation, including 5 upfront and 4 upon progression. Targeted therapy [NTRK (n=1), BRAF (n=1) or MEK inhibitors (n=6)] was initiated upon progression in 8 patients, and at diagnosis in one. The median progression-free survival (PFS) in the group receiving upfront chemoradiation (n=3) is 57 months versus chemotherapy alone (n=6) of 30.5 months, and 26.7 months for radiotherapy alone (n=3). The PFS at 2 years and 5 years were 58.3% and 17% respectively; OS was 100% at both time intervals. BRAF genetic alterations and younger age trended toward a worse outcome, hence upfront targeted therapy may be warranted when radiation needs to be avoided. CONCLUSION In our experience, DLGNTs are more commonly low-grade with frequent BRAF gene alterations; upfront chemoradiation provided the longest PFS compared to chemotherapy or radiation alone. In the absence of established standard of care, our case series emphasizes the potential role of targeted therapy upfront for younger patients and progressive disease post-chemoradiation. Larger prospective, collaborative studies are greatly needed for DLGNTs.

OTHR-26. MINIMALLY-INVASIVE MOLECULAR DIAGNOSIS OF DIFFUSE LEPTOMENINGEAL GLIONEURONAL TUMOR (DLGNT) USING CEREBROSPINAL FLUID CELL-FREE DNA SEQUENCING IN PEDIATRIC AND YOUNG ADULT PATIENTS

Abstract BACKGROUND Diffuse leptomeningeal glioneuronal tumor (DLGNT) presents with nodular leptomeningeal disease throughout the neuroaxis and is characterized by the presence of KIAA1549::BRAF fusion and chromosome arm 1p deletion. Concomitant gain of chromosome arm 1q is associated with poor prognosis. Diagnosis by meningeal biopsy is often non-diagnostic due to sampling error or yields insufficient material for molecular testing. We hypothesized that analysis of cell free DNA (cfDNA) from cerebrospinal fluid (CSF) could represent a superior diagnostic modality. METHODS We analyzed CSF samples from 9 suspected DLGNT patients, median age of 19.5 years with a 5:4 male:female ratio using MSK-IMPACT, a hybridization capture-based next-generation DNA sequencing panel, including matched germline. RESULTS Prior tissue biopsy was done in 7 patients; 6 were negative for the KIAA1549:BRAF fusion and 1 was positive. Of the 6 biopsies negative for the fusion, analysis of CSF cfDNA detected KIAA1549::BRAF fusion in 3/6 (50%) of patients, who were all subsequently started on MEK inhibitor therapy. The other 3 patients were negative for KIAA1549:BRAF fusion on both biopsy and CSF cfDNA. One patient received biopsy confirmation of KIAA1549::BRAF fusion after CSF cfDNA analysis detected the mutation. In addition, in one patient with prior tissue-confirmed KIAA1549::BRAF fusion, the fusion was not detected in CSF cfDNA during/after treatment with a MEK inhibitor, but additional somatic alterations and DNA copy number profiles were similar to those found in prior biopsies. Additional somatic alterations were identified in ATRX, BCOR, RARA, STAT5A, SPOP, PPM1D, MED12, KMT2C, RECQL andSETD2, including likely oncogenic secondary driver alterations that have not been previously reported in DLGNT. CONCLUSIONS Given the high molecular diagnostic yield and concordance with open biopsy, cfDNA CSF sequencing should be considered as a first line diagnostic maneuver for patients with suspected DLGNT, followed by surgical biopsy only if CSF testing is non-diagnostic.

LGG-27. LOWER GRADE GLIOMA (LGG) IN INFANTS. ANALYSIS OF 25 NON-NF1 PATIENTS

Abstract INTRODUCTION Infants constitute approximately 5% of all pediatric LGG. This group is identified as having aggressive course and worse prognosis as compared to older children. No specific therapeutic guidelines are yet available for this specific age group. AIM: Retrospective analysis of infants diagnosed and treated for LGG. METHODOLOGY: 25 patients treated between 2001-2023 were analyzed for: sex, age, tumor location, pathology, treatment and outcome. NF1 patients were excluded. RESULTS There were 13 boys and 12 girls, aged 3 months-11 months, median 6 months. 12 had optic chiasm tumors, 6 of visual pathway, 3 had tumors of posterior fossa, 2 of pineal region and 1 spinal tumor. Pathology was available for 16 children revealing Pilocytic astrocytoma in 9, Pilomyxoid astrocytoma in 6, ganglioglioma in 1. Resection as first line treatment was performed in posterior fossa and spinal locations, in 4 cases of visual tracts tumors. All patients were treated with chemotherapy with a maximum of 6 subsequent episodes in a patient with visual pathway tumor. MEK inhibitors were given to 3 pts (in 2 discontinued due to cardiotoxicity). Radiotherapy was applied in 5 (older than 4 years). Twenty two patients are alive, from 4 months to 22 years 4 months (median 8 years 11 months) from diagnosis. Three patients died (midline tumors) 2 from electrolyte disorders and 1 from radiation related cerebral bleeding with observation time of 3, 10, 9 years. Among children with visual tract tumors 5 are blind, 3 have impaired vision. Five patients with very good response to first line treatment have excellent quality of life. Ten patients are disabled, requiring help in everyday life. CONCLUSIONS The risk of disability or death is highest in infants of all LGG patients, those who respond the first line treatment have very good quality of life.

Intermittent MEK Inhibition with GITR Costimulation Rescues T-cell Function for Increased Efficacy with CTLA-4 Blockade in Solid Tumor Models.

MEK inhibitors (MEKi) have shown limited success as a treatment for MAPK/ERK pathway-dependent cancers due to various resistance mechanisms tumor cells can employ. CH5126766 (CKI27) is an inhibitor that binds to MEK and prevents release of RAF, reducing the relief of negative feedback commonly observed with other MEKis. We observed that CKI27 increased MHC expression in tumor cells and improved T cell-mediated killing. Yet, CKI27 also decreased T-cell proliferation, activation, and cytolytic activity by inhibiting the MAPK/ERK pathway that is activated downstream of T-cell receptor signaling. Therefore, we aimed to balance the positive and negative immunomodulatory effects of MEKis for optimal combination with immunotherapy. Intermittent administration of CKI27 allowed T cells to partially recover and costimulation via GITR and OX-40 agonist antibodies completely alleviated inhibition of function. In Kras mutant lung and colon tumor mouse models, intermittent CKI27 and anti-GITR significantly decreased tumor growth and prolonged survival when further combined with CTLA-4 immune checkpoint blockade. Moreover, this triple combination increased CD8+ and CD4+ T-cell proliferation, activation, and effector/memory subsets in the tumor-draining lymph nodes and tumors and led to intratumoral regulatory T-cell destabilization. These data, collectively, will allow for more informed decisions when optimizing combination regimens by overcoming resistance, reducing toxicity, and generating long-term immune responses.

LGG-55. DIFFERENTIAL TOXICITY PROFILE OF INDIVIDUAL MEK INHIBITORS: A MULTI-INSTITUTION RETROSPECTIVE REVIEW

Abstract BACKGROUND Most pediatric low-grade gliomas (pLGGs) are driven by molecular aberrations in the mitogen-activated protein kinase pathway (MAPK), which can be targeted by novel agents such as MEK inhibitors (MEKi.) However, MEKi use is accompanied by a varied side effect profile, with the most severe dose-limiting effects involving dermatologic, cardiac, and ophthalmic toxicities. METHODS Multi-institutional retrospective chart review of patients less than 25 years of age who underwent MEKi monotherapy between 2015-2022 was conducted. Data was collected from 4 sites, including Connecticut Children’s, St. Louis Children’s Hospital, Cardinal Glennon Children’s Hospital, and Children’s Hospital of Philadelphia. RESULTS Ninety-six patients were included in the analysis; 54 (56.4%) with brain tumors and 56 (58.3%) with germline mutation in NF-1. Forty-nine (51%) received trametinib, 42 (43.8%) selumetinib, 5 (5%) binimetinib. Of 42 patients with skin toxicity data available, 35 (83%) had cutaneous toxicity, with the most prevalent being acneiform rash (40.0%), followed by maculopapular rash (28.6%) and eczematous dermatitis (11.4%). The mean number of cutaneous skin toxicities was 2.4 (trametinib group = 2.2, selumetinib group = 2.6). Eighteen of 96 (18.8%) patients experienced cardiac toxicity, of which 11 (61.1%), experienced reduction in left ventricular ejection fraction (6 trametinib, 5 selumetinib), 4 (22.2%) developed pericardial effusions. 10 of 96 (10.4%) patients experienced ophthalmic toxicities, most commonly blurred vision (30.0%). Other toxicities included elevated CK (36, 37.5%), anemia (32, 33.3%), and eosinophilia (17, 17.7%). CONCLUSION There was not an observable clinically relevant difference in toxicities between MEKi, with skin toxicity the most prevalent. The prevalence of cardiac toxicity in the patient cohort reinforces the need for appropriate monitoring via echocardiography. MEKi remain an excellent option for therapy for LGG but with the need for continued supportive care and monitoring.

HGG-30. TARGETED THERAPY OF PEDIATRIC HIGH GRADE GLIOMAS

Abstract BACKGROUND The prognosis for patients with pediatric high-grade glioma (pHGG) is poor despite aggressive multimodal therapy. Recent advances in understanding tumor biology facilitate utilization of targeted therapies (TT) in pediatric neuro-oncology. METHODS We analyzed the efficacy of targeted therapy in patients with progressive pHGG treated with TT according to molecularly-identified actionable events. Molecular characterization of the tumor included DNA panel and transcriptomic RNA sequencing. RESULTS Eighteen patients were included (median age 3.7 years, range 0.1-17.7) with anaplastic pleomorphic xanthoastrocytoma (n=10), infant-type hemispheric glioma (n=4), and diffuse high-grade glioma (n=4). Metastatic disease was present in 3/18 patients. Molecular genetic alterations included BRAF V600E mutations (n=8), NTRK1/2/3 fusions (n=4), ROS1 fusions (n=4), MET fusion (n=1), KIT P627L mutation (n=1). Eight patients received an NTRK/ALK/ROS1 inhibitor (entrectinib), seven patients received combination therapy with BRAF and MEK inhibitors (dabrafenib and trametinib), and one patient anti-BRAF monotherapy (dabrafenib). Two patients were treated with a multi-tyrosine kinase inhibitor (cabozantinib). The median duration of targeted therapy was 10.6 months (1-42). Most patients tolerated treatment well with no grade 4-5 toxicities. The overall response rate was 66% (12/18), including 4 complete (22%) and 8 partial responses (44%). At 12 months, PFS was 86% with dabrafenib/trametinib and 42% with entrectinib; at 18 months PFS was 44 % with dabrafenib plus trametinib and 42% with entrectinib. Three out of four patients with ROS1 rearranged glioma experienced disease progression on entrectinib after an initial good response in all cases; two of them subsequently received lorlatinib with clinical benefit. All four patients with NTRK1/2/3 rearranged gliomas remain on entrectinib therapy without signs of progression. Cabozantinib therapy resulted in prolonged complete and partial responses in both cases. CONCLUSIONS TT for patients with progressive pHGG is feasible and provides unexpectedly good clinical efficacy.

LGG-12. INTEGRATED CLINICAL AND MOLECULAR CHARACTERIZATION OF 200 DISSEMINATED PEDIATRIC LOW-GRADE GLIOMAS

Abstract Pediatric low-grade gliomas (PLGG) have excellent outcomes overall but are a major clinical challenge when disseminated. Diffuse leptomeningeal glioneuronal tumor (DLGNT) is a recognised entity both clinically and pathologically, however many disseminated LGG (DLGG) fall outside this diagnosis. To better understand the clinical and molecular features of DLGG as well as risk factors for dissemination, we assembled an international consortium of 30 sites, contributing over 200 patients with clinical annotation, along with genomic and methylation profiling. DLGG have worse progression-free (PFS) and overall survival (OS) than PLGG overall (p<0.0001). Seventy (35%) presented with a localized mass and secondary dissemination, including some with initial gross-total resection. The most common growth pattern observed (n=77, 40%) is a dominant suprasellar/optic pathway tumor with leptomeningeal drop-metastases involving the brainstem and spinal cord. Only 27 (14%) patients had diffuse tumors (without an identifiable dominant mass), and these had significantly worse OS (p=0.03). The most common pathologic diagnosis was pilocytic/pilomyxoid astrocytoma (n=92; 53%). DLGNT comprised a minority of cases (n=23, 14%), with a trend (p=0.056) towards worse survival compared to other diagnoses. The most frequent molecular alteration was BRAF fusion (78/151 with molecular testing; 52%), followed by FGFR mutations or fusions (18/151; 12%). BRAF V600E was underrepresented (14/151; 9%). 1p deletion was rare (17 patients) but highly associated with DLGNT. Methylation classification (n=70) was highly concordant with histologic diagnoses rather than clinical behavior. Importantly, patients who received upfront targeted therapies (TT; BRAF and/or MEK inhibition, n=9) had better PFS compared to those receiving chemotherapy (n=146, p=0.05). Furthermore, patients who were treated sequentially with chemo then TT had longer PFS on TT (p=0.011). This study presents the largest cohort of DLGG to date, expanding our understanding of the clinical, pathologic, and molecular features of this disease and supports the use of upfront targeted therapy.

HGG-46. PEDIATRIC RADIATION-INDUCED GLIOMA PATIENT SAMPLES AND NOVEL PATIENT-DERIVED MODELS REVEAL TRANSLATIONALLY RELEVANT TREATMENT SUSCEPTIBILITIES AND POTENTIAL GERMLINE RISK FACTORS

Abstract BACKGROUND Pediatric radiation-induced glioma (RIG) is an incurable secondary high-grade glioma (HGG) afflicting up to 4% of children who previously received cranial radiotherapy (RT) for other malignancies. RIG accounts for up to 10% of pediatric brain tumor deaths. It has no known genetic risk factors nor effective treatment and is uniformly fatal, partly due to extremely limited available models. Our recent molecular characterization of patient samples showed that while RIG and primary pediatric HGG share histological similarities, their genetic alterations and gene expression profiles differ. METHODS We obtained germline DNA sequencing from 28 RIG patients and 112 matched controls who received RT but did not develop RIG. In addition, we developed cell culture and murine orthotopic patient-derived xenograft (PDX) models from four RIGs. We conducted single-cell RNA-seq (scRNA-seq), drug screening/validation, and CRISPR functional screening on our novel models. RESULTS Germline mutations in TP53, PARP10, POLI, DCLRE1A, and MLH3 were more common in RIG patients than controls (p<0.002). Our matched cell culture and PDX models of RIG represent the genetic diversity of the disease. The PDX models form tumors in 30-120 days and closely recapitulate human RIG by histology and scRNA-seq. They show susceptibility across models to DNA-damaging treatments (etoposide, irinotecan, radiation), as well as MEK, PARP, and proteasome inhibition, with orthogonal supporting findings via CRISPR screening. Several two-treatment combinations also show efficacy and synergy across models, including radiation and trametinib in vivo (logrank p=0.029). Discussion: RIG is an understudied but major cause of pediatric CNS tumor mortality that differs genetically from primary pediatric HGG. Our findings suggest specific germline alterations in DNA repair pathways may put some patients at higher risk of developing RIG after cranial RT. We also introduce the first set of patient-derived RIG models in order to advance translational RIG research, with characterization of translationally relevant treatment susceptibilities.

LGG-46. MEKI WITH MIRDAMETINIB: EXPLORING ADAPTIVE SIGNALING PATHWAY DYNAMICS AND IMMUNE MODULATION IN PEDIATRIC GLIOMAS

Abstract INTRODUCTION Glial neoplasms are among the most common types of solid tumors in children. The mitogen-activated protein kinase (MAPK) pathway is implicated in glioma tumorigenesis. Drugs targeting this pathway hold promise as novel therapy for this disease. Vertical inhibition of the MAPK pathway has been linked to increased inflammation in several other cancers but similar exploration in pediatric brain tumors is limited. Within the MAPK pathway, MEK plays a significant role as a modulator of ERK phosphorylation. This study investigates the therapeutic potential of the MEK inhibitor, Mirdametinib, and evaluates the drugs’ capacity to induce tumor-intrinsic inflammation by characterizing changes in PD-L1 expression and proliferation in a panel of pediatric glial neoplasms. METHODS Three cell lines were included in the panel (Res186, Res259, SF188) and were treated with Mirdametinib for 3, 5, or 7 days. P-ERK expression, analyzed by western blot, verified successful inhibition of MEK by Mirdametinib. The CCK8 cytotoxic assay was utilized to determine the drug’s 50% inhibitory concentration as well as its direct effects on cell viability. Changes in Ki67 expression assessed by flow cytometry further characterized Mirdametinib’s effects on cell proliferation. Flow cytometry was also utilized to measure changes in PD-L1 expression following treatment. RESULTS Despite minimal effects on proliferation as measured by Ki67 and CCK8, expression of P-ERK verifies successful inhibition of MEK. Western blot data also reveals increased Pi3K/AKT pathway activity in SF188 groups treated with Mirdametinib. FACS data for SF188 also demonstrates increased PD-L1 expression in response to treatment. CONCLUSIONS Data suggests that, in response to MEKi, gliomas utilize compensatory growth mechanisms through Pi3K/AKT signaling. Findings also point to immune-modulatory effects of MEKi. Supporting results in ongoing experiments are expected to further our understanding of these mechanisms and may help inform effective combination treatment strategies for pediatric gliomas.

NFS-20. DECIPHERING CELLULAR CROSSTALK IN PERIPHERAL NERVOUS SYSTEM TUMORS ASSOCIATED WITH NEUROFIBROMATOSIS TYPE 1 THROUGH SINGLE-CELL RNA SEQUENCING

Abstract BACKGROUND Neurofibromatosis Type 1 (NF1) is a genetic disorder characterized by mutations and dysfunctional expression of neurofibromin 1 gene, a critical tumor-suppressor gene. NF1’s complex pathophysiology involves dysregulation of the RAS-MAPK signaling pathway, resulting in sustained MAPK signaling. Among others, the spectrum of clinical features includes low-grade tumors (e.g. plexiform neurofibromas (pNFs)) and malignant peripheral nerve sheath tumors (MPNST). While surgical removal of peripheral nerve tumors is common, total resection is often unattainable. Additionally, approved therapies comprising MEK inhibitors fail to prevent malignization, highlighting the urgent need for novel targets for effective therapies. METHODS To explore novel potential targets across a broad spectrum, we analyzed published scRNA-seq datasets (Kershner et al. 2021, Wu et al. 2022) from human patients, including pNFs and MPNSTs. We further investigated key pathways in vitro with patient-derived cell culture models. RESULTS ScRNA-seq provides insight into complex networks of cell-to-cell interactions that drive the malignant phenotype of cancerous tissues. Due to the rich presence of ECM in NF1 tumors we aimed to investigate cell types contributing to key extracellular matrix (ECM) components and associated signaling pathways. Analysis of 19 NF1 tumor samples revealed consistent presence of Schwann cells (SCs), fibroblasts, endothelial cells, and immune cells. Proliferating cells were found in MPNSTs, while pericytes were exclusive to pNFs. We compared secreted and ECM signaling between the cell types of both tumors. Our analysis discovered, among other findings, collagen signaling, highlighting fibroblasts as emitters and SCs as receivers. We further confirmed these findings through culturing of NF1-derived cells in vitro with and without collagen, observing increased cell proliferation in the presence of collagen. Further experiments and analyses are ongoing. CONCLUSIONS The emphasis on ECM and signaling adds depth to our understanding of NF1 pathogenesis and suggests potential therapeutic targets.

LGG-60. DISTRIBUTION AND CLINICAL SIGNIFICANCE OF FGFR ALTERATIONS IN PEDIATRIC AND AYA GLIOMAS

Abstract Fibroblast growth factor receptor (FGFR) alterations are prevalent in human cancers. In a large Canadian population-based cohort, encompassing over 1,300 pediatric and Adolescent and young adult (AYA) gliomas, FGFR mutations occurred in 10% of pediatric low-grade gliomas (LGG) and 20% of IDH-WT AYA LGGs. Here we assembled a large multi-institutional cohort including more than 300 mutated gliomas, aged 6 months to 87 years, 53% of which were LGG. High-grade gliomas were more common with increasing age; over 90% of pediatric cases were LGGs, compared to 30% in the AYA group. Fusions, predominantly seen in the pediatric group (70% of LGGs), contrast with FGFR3-TACC3 fusions frequent found in adult high-grade gliomas. Interestingly, FGFR mutations sometimes existed as sole drivers or co-occurred with other alterations (~30%), it is unclear how this alters tumor behavior, although gain of additional alterations could be implied in malignant transformation and poor prognosis. Despite higher incidence of high-grade gliomas in AYAs, their progression-free survival was significantly better than that of pediatric LGGs (p=0.007), largely due to the prevalence of hemispheric tumors amenable to gross total resection. Approximately 30% of our LGG cohort underwent adjuvant therapy; with available data for 28 patients, objective responses were noted in 2/12 to chemotherapy, 2/9 to MEK inhibitors, and 3/7 to FGFR inhibitors, respectively. With the increasing promise of precision targeted therapy, molecular profiling and assessment for FGFR alterations is essential. Although encouraging, further studies are needed to assess the benefit of FGFR inhibition in these patients.

DIPG-33. COMBINING CLONAL TRACKING WITH SINGLE CELL RNA SEQUENCING REVEALS TEMPORAL SHIFTS IN CELL STATES ASSOCIATED WITH TRAMETINIB RESISTANCE IN MAPK ALTERED DIFFUSE MIDLINE GLIOMA

Abstract BACKGROUND H3K27-altered diffuse midline glioma (DMG) is an aggressive and incurable paediatric high-grade glioma with median overall survival of 9-12 months. A distinct subset of DMG harbour alterations in the MAPK pathway, and we have previously shown that DMG-MAPK may exhibit sensitivity to MEK inhibitors but develop resistance under long-term exposure to drug. Elucidating these resistance mechanisms could spearhead better treatment strategies for these patients. METHODS To study clonal evolution under trametinib, we used CloneSifter lineage tracking technology to uniquely barcode 300,000 DMG-MAPK cells which were then treated with a GI80 dose of trametinib for 11 weeks. We extracted DNA to quantify barcode enrichment at endpoint across six technical replicate experiments and leveraged CloneSifter’s compatibility with single cell RNA sequencing (scRNA-seq) to couple clonal identity with transcriptional states in ~90,000 trametinib treated cells at varying timepoints. RESULTS DNA sequencing revealed 0.5% (1500/300,000) of the initial barcodes were enriched across trametinib replicates and were also detectable by scRNA-seq. Conducting cNMF analysis on the single cell dataset showed populations of trametinib treated cells associated with each other in a temporal manner, whereby early and late-stage resistant populations were defined by neuronal and stem cell like markers, respectively. A time-dependent enrichment of CloneSifter barcodes was also observed, suggesting dynamic expansion of clones over the treatment window. Notably, week 11 trametinib treated cells showed higher expression of resistance driver genes related to EMT, stemness and p38/MAPK upregulation. CONCLUSIONS Utilizing the sequence-specific reporter construct built in to the CloneSifter system allows for retrieval of resistant clones marked by enriched barcodes under treatment prior to drug exposure, enabling further characterization of monoclonal populations of trametinib-resistant cells. In summary, we have employed novel tools to study resistance development in DMG-MAPK that could be exploited to devise rational treatment strategies for these patients in an evolutionary-guided manner.

BIOM-03. IMMUNO-ONCOLOGIC PROFILING OF PEDIATRIC BRAIN TUMORS REVEALS MAJOR CLINICAL SIGNIFICANCE OF THE TUMOR IMMUNE MICROENVIRONMENT

Abstract With the success of immunotherapy in cancer, understanding the tumor immune microenvironment (TIME) has become increasingly important; however in pediatric brain tumors this remains poorly characterized. Accordingly, we developed a clinical immune-oncology gene expression assay, including the 18-gene tumor inflammation signature (TIS) as a marker of overall immune activation, and validated this with immunohistochemistry for key cell-type markers. We used this assay, in conjunction with public data from the Pediatric Brain Tumor Atlas (PBTA), to profile a diverse range of 1382 samples in total with detailed clinical and molecular annotation. Overall among the main types of pediatric brain tumors, low-grade gliomas (LGG) had the highest inflammation levels (regardless of genetic driver alteration) and medulloblastomas the lowest. In LGG we identified three distinct patterns of immune activation with prognostic significance in BRAF V600E-mutant tumors, specifically that higher inflammation predicted worse outcomes. In high-grade gliomas (HGG), we observed immune activation and T-cell infiltrates in tumors that have historically been considered exclusively immune cold. There were genomic correlates of inflammation levels, with BRAF-V600E-mutant HGG having the highest inflammation levels and suggesting these may be candidates for combination MEK inhibition and immunotherapy. In mismatch repair deficient HGG, we found that high TIS was a significant predictor of response to immune checkpoint inhibition, and demonstrated the potential for multimodal biomarkers (TIS plus tumor mutation burden) to improve treatment stratification. Importantly, while overall patterns of immune activation were observed for histologically and genetically defined tumor types, there was significant variability within each entity, indicating that the TIME must be evaluated as an independent feature from diagnosis. In sum, in addition to the histology and molecular profile, this work underscores the importance of reporting on the TIME as an essential axis of cancer diagnosis in the era of personalized medicine.

MicroRNA-focused CRISPR/Cas9 screen identifies miR-142 as a key regulator of Epstein-Barr virus reactivation.

Reactivation from latency plays a significant role in maintaining persistent lifelong Epstein-Barr virus (EBV) infection. Mechanisms governing successful activation and progression of the EBV lytic phase are not fully understood. EBV expresses multiple viral microRNAs (miRNAs) and manipulates several cellular miRNAs to support viral infection. To gain insight into the host miRNAs regulating transitions from EBV latency into the lytic stage, we conducted a CRISPR/Cas9-based screen in EBV+ Burkitt lymphoma (BL) cells using anti-Ig antibodies to crosslink the B cell receptor (BCR) and induce reactivation. Using a gRNA library against >1500 annotated human miRNAs, we identified miR-142 as a key regulator of EBV reactivation. Genetic ablation of miR-142 enhanced levels of immediate early and early lytic gene products in infected BL cells. Ago2-PAR-CLIP experiments with reactivated cells revealed miR-142 targets related to Erk/MAPK signaling, including components directly downstream of the B cell receptor (BCR). Consistent with these findings, disruption of miR-142 enhanced SOS1 levels and Mek phosphorylation in response to surface Ig cross-linking. Effects could be rescued by inhibitors of Mek (cobimetinib) or Raf (dabrafenib). Taken together, these results show that miR-142 functionally regulates SOS1/Ras/Raf/Mek/Erk signaling initiated through the BCR and consequently, restricts EBV entry into the lytic cycle.

Molecular-targeted therapy for childhood low-grade glial and glioneuronal tumors.

Since the discovery of the association between BRAF mutations and fusions in the development of childhood low-grade gliomas and the subsequent recognition that most childhood low-grade glial and glioneuronal tumors have aberrant signaling through the RAS/RAF/MAP kinase pathway, there has been a dramatic change in how these tumors are conceptualized. Many of the fusions and mutations present in these tumors are associated with molecular targets, which have agents in development or already in clinical use. Various agents, including MEK inhibitors, BRAF inhibitors, MTOR inhibitors and, in small subsets of patients NTRK inhibitors, have been used successfully to treat children with recurrent disease, after failure of conventional approaches such as surgery or chemotherapy. The relative benefits of chemotherapy as compared to molecular-targeted therapy for children with newly diagnosed gliomas and neuroglial tumors are under study. Already the combination of an MEK inhibitor and a BRAF inhibitor has been shown superior to conventional chemotherapy (carboplatin and vincristine) in newly diagnosed children with BRAF-V600E mutated low-grade gliomas and neuroglial tumors. However, the long-term effects of such molecular-targeted treatment are unknown. The potential use of molecular-targeted therapy in early treatment has made it mandatory that the molecular make-up of the majority of low-grade glial and glioneuronal tumors is known before initiation of therapy. The primary exception to this rule is in children with neurofibromatosis type 1 who, by definition, have NF1 loss; however, even in this population, gliomas arising in late childhood and adolescence or those not responding to conventional treatment may be candidates for biopsy, especially before entry on molecular-targeted therapy trials.

MEK inhibitors and DA-Raf, a dominant-negative antagonist of the Ras-ERK pathway, prevent the migration and invasion of KRAS-mutant cancer cells.

The Ras-induced ERK pathway (Raf-MEK-ERK signaling cascade) regulates a variety of cellular responses including cell proliferation, survival, and migration. Activating mutations in RAS genes, particularly in the KRAS gene, constitutively activate the ERK pathway, resulting in tumorigenesis, cancer cell invasion, and metastasis. DA-Raf1 (DA-Raf) is a splicing isoform of A-Raf and contains the Ras-binding domain but lacks the kinase domain. Consequently, DA-Raf antagonizes the Ras-ERK pathway in a dominant-negative manner and can serve as a tumor suppressor that targets mutant Ras protein-induced tumorigenesis. We show here that MEK inhibitors and DA-Raf interfere with the in vitro collective cell migration and invasion of human KRAS-mutant carcinoma cell lines, the lung adenocarcinoma A549, colorectal carcinoma HCT116, and pancreatic carcinoma MIA PaCa-2 cells. DA-Raf expression was silenced in these cancer cell lines. All these cell lines had high collective migration abilities and invasion properties in Matrigel, compared with nontumor cells. Their migration and invasion abilities were impaired by suppressing the ERK pathway with the MEK inhibitors U0126 and trametinib, an approved anticancer drug. Expression of DA-Raf in MIA PaCa-2 cells reduced the ERK activity and hindered the migration and invasion abilities. Therefore, DA-Raf may function as an invasion suppressor protein in the KRAS-mutant cancer cells by blocking the Ras-ERK pathway when DA-Raf expression is induced in invasive cancer cells.