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Wnt-activated medulloblastoma (MB) confers an excellent prognosis. However, specific treatment strategies for patients with relapsed Wnt-MB are unknown. We report two patients with recurrent beta-catenin nucleopositive Wnt-MB successfully treated by incorporating marrow-ablative chemotherapy and autologous hematopoietic progenitor cell rescue (HDCx/AuHPCR). We also present a review of the literature for previously reported cases of relapsed Wnt-MB. We propose that patients with recurrent Wnt-MB may be treated using a multi-disciplinary approach that includes HDCx/AuHPCR with or without re-irradiation.

INTRODUCTION: Medulloblastoma is the most common malignant brain tumor in children, with 5-year overall survival (OS) ranging from 60%-95% depending on subgroup and risk status. The POG 8631/CCG 923 trial (accrual 1986-1990) found a 63% 5 year OS for patients with local disease after gross total resection treated with radiation at a dose of 23.4 Gy craniospinal radiation and posterior fossa boost to 54 Gy, vs. 80.5% 5-year OS for patients treated as per full ACNS0331 therapy including maintenance chemotherapy. Herein, we describe a long-term survival with standard-risk medulloblastoma who only received surgical resection and radiation therapy. CASE: A 17-year-old male presented with acute onset of hypertension, bradycardia, headache, and blurry vision and was found to have a heterogeneously enhancing posterior fossa mass with mass effect on the fourth ventricle and hydrocephalus on brain MRI. He underwent gross total resection of the tumor and histopathology revealed medulloblastoma with classic and large cell features. Fluorescence in situ hybridization (FISH) was negative for MYC, MYCN amplification, or HER2 gain. Cerebrospinal fluid cytology was negative for neoplastic cells, and spinal MRI did not reveal any drop metastases. The patient initiated therapy per ACNS0331 with craniospinal irradiation posterior fossa boost. He also received weekly vincristine. After completion of radiation therapy, the family declined further chemotherapy despite medical advice. He had no evidence of relapse most recently at 51 months from completion of therapy. Next generation sequencing and methylation testing are currently pending. CONCLUSION: Current efforts aim at optimizing therapy based on molecular subgrouping, to minimize long-term adverse events associated with current therapies. We report a unique case of an adolescent male with an standard-risk medulloblastoma, who achieved remission with only radiotherapy. Further molecular tumor analysis may elucidate the response of the tumor.

BACKGROUND: Cutaneous side effects commonly occur with MEK inhibitor (MEKi) therapy and can be challenging to manage. METHODS: A retrospective chart review was performed on sixteen pediatric patients treated with MEKi therapy for at least three months. These patients were diagnosed with either a brain tumor, plexiform neurofibroma, Langerhans Cell Histiocytosis, or Parkes Weber Syndrome (PWS). OBJECTIVES: To describe cutaneous side effects from MEKi therapy, compare the side effect profiles of patients treated for different diagnoses, and compare the side effect profiles between different MEKi agents. RESULTS: The most prevalent cutaneous toxicities for all eligible patients were acneiform rash (81%, most common), paronychia (50%, second most common), hair changes and xerosis. Mean number of cutaneous skin toxicities were 3.2 (trametinib group: 3.4, selumetinib group: 2.8). Only one patient, treated with trametinib for PWS, did not experience a cutaneous side effect. The average number of interventions (new medication or over-the-counter product) recommended to prevent or treat cutaneous-related toxicities was 6.8 per person, the majority of those were for acne and paronychia. Over 75% of the interventions were topical. Of those who experienced an acneiform rash, the average number of recommended interventions were 3.1. Sixty two percent of these patients developed acne within the first cycle; average time to development was 2.6 months into therapy. Most exhibited improvement overtime following intervention. Those patients who experienced paronychia were treated with a mean of 3.1 interventions – 37.5% developed paronychia within the first cycle of therapy; average time to development was 3.9 months into therapy. CONCLUSIONS: Cutaneous side effects are common, occur early in therapy, and require multiple interventions. The number and complexity of these interventions may further complicate the overall management of cutaneous side effects given that the responsibility of the administration falls on the patient. This differs from traditional chemotherapy regimens.

BACKGROUND: Primary intracranial germ cell tumors (GCTs) are rare heterogeneous tumors, with germinoma accounting for two-thirds of cases. Neoadjuvant chemotherapy with response-based reduced radiotherapy dose and field has become the standard management of localized CNS germinomas, however, treatment of primary metastatic disease has remained controversial. Furthermore, there is limited published research on the use of neoadjuvant chemotherapy in primary metastatic germinoma. METHODS: We performed a retrospective multi-institutional data collection and analysis of patients diagnosed with metastatic germinoma since 2000 to assess the overall survival (OS) and event-free survival (EFS) of the different treatment modalities administered. RESULTS: We identified 78 patients with germinoma, in two tertiary care centers, of which nine patients (11.5%; eight males) had metastatic disease. The median age at presentation was 13.3 years. All patients had a biopsy at presentation confirming the diagnosis. Three patients had positive CSF cytology (M1). Six patients received craniospinal irradiation (CSI) with boost to primary and metastatic sites, of which five patients received total CSI dose of 24 Gy, while the dose was unknown for one patient. One patient required one cycle of chemotherapy prior to CSI due to worsening visual changes, which subsequently resolved. One patient received two cycles followed by whole ventricular irradiation (WVI) of 23.4 Gy with a boost to the primary bed. One patient received WVI without neoadjuvant chemotherapy. One patient developed anoxic brain injury and only received chemotherapy. He died of recurrent progressive disease 15 months post-diagnosis. The median follow-up time was 77.5 months (range 15-130.5 months), with an OS of 88.9%. Further multi-institutional data collection and analysis is underway and will be presented at the meeting. CONCLUSION: We anticipate our results may elucidate the role of neoadjuvant chemotherapy in the treatment of metastatic germinoma, and whether when combined with lower CSI doses did not compromise EFS/OS.

BACKGROUND: The management of childhood central nervous system (CNS) tumors is complex and often faces numerous challenges in low- and middle- income countries (LMICs), including delayed diagnosis and limited treatment resources. Twinning initiatives between LMICs with high- income countries are feasible and proven to be highly effective at exchanging skills and expertise to improve diagnosis, treatment and care for children with brain tumors. METHODS: A monthly multidisciplinary international pediatric neuro-oncology tumor board via zoom videoconferencing was established in January 2021. This effort is a collaboration between Washington University School of Medicine, in St. Louis, Missouri, USA and nine international sites. Given the significant contributions of this international effort, it has since grown to include 20 institutions and cancer centers from 12 countries in the Middle East, Europe, Australia and South America. RESULTS: As of January 2022, we have held 11 tumor boards, 35 cases were reviewed, and have had 320 experts attend from several specialties – neuro-oncology, neurology, neurosurgery, neuroradiology and neuropathology. A multidisciplinary team of physicians reviewed each case and recommendations were given accordingly. We also started a quarterly neurofibromatosis (NF) meeting focused to leverage the expertise of dedicated specialists in the NF center. Two NF-focused meetings took place since establishing the program, and total of five cases were discussed. CONCLUSION: Virtual videoconferencing promotes a multi-disciplinary approach for the management of pediatric CNS tumors, and it allows access for medical expertise. We anticipate the current initiative will also provide a platform for future international research collaborations and deliver the optimal medical care for neuro-oncology patients globally. Multiple potential collaborative projects are currently underway.

BACKGROUND: Low-grade gliomas (LGG) are the most common solid tumor of childhood and can result in neurologic complications, including seizures, focal neurologic deficits, and learning difficulties. Molecularly targeted agents are increasingly being utilized to treat LGG, but the effect of these agents on accompanying neurologic complications are poorly understood. CASE: An 8-years old male with Neurofibromatosis Type 1 (NF1), medically refractory epilepsy and deep extensive glioma (extending from the optic pathway and involving the basal ganglia and corpus collosum) began selumetinib therapy due to radiographic and symptomatic tumor progression. Radiographic response (resolution of enhancement) was observed at 12 weeks of therapy, accompanied by improvement in seizure frequency, hemiparesis, and academic performance. Due to cardiotoxicity observed at that time (asymptomatic decreased ejection fraction and shortening fraction on echocardiogram), selumetinib was reduced to 50% dosing. On this reduced dose of selumetinib, seizures increased in frequency with subsequent worsening hemiparesis and recurrence of learning difficulties. One month later, dosing was escalated back to 100% due to interval resolution of cardiotoxicity, resulting in resolution of seizures and improvement in focal neurologic deficits and cognition. DISCUSSION: Dose-dependent response to MEK inhibition was observed without concurrent changes in anti-epileptic medications. The tumor was stable in size despite improved enhancement with treatment, suggesting that objective response by RANO criteria is not necessary for improved seizure control in LGG. Recent work has implicated the RAS/MEK/ERK pathway in neuronal precursor cells as a cause for epilepsy, suggesting that MEK inhibition of NF1-heterozygous neurons could be contributing to treatment response in this patient. Improvements in weakness and academic performance may have been due to improved seizure control or a direct effect of MEK inhibition on NF1-heterozygous neurons. CONCLUSION: MEK inhibition may have a clinically relevant anti-seizure effect for patients with pediatric LGG or NF1.

Fetoplacental neuroblastoma metastasis has been postulated as a mechanism accounting for concordant cases where one twin develops a primary tumour and the second twin manifests the disease without an identifiable primary site. These tumours may originate and spread concomitantly due to the same genetic background shared by monozygotic twins. This study investigated the molecular profile of stage MS neuroblastoma presenting concomitantly in monozygotic twins. Comparative genomic hybridisation (aCGH) was done for each of the twin liver tumour and peripheral blood samples at diagnosis. Comparison of copy-number variation (CNV) regions revealed a set of CNVs that were common to both tumour specimens and not apparent in the blood. The CNV signature in both twins’ tumours was highly similar, suggesting a common clonal origin. Additional findings included large deletion of chromosome 10 and amplification of chromosome 17. Notably, both liver samples had amplification of a short region involving DEIN (chromosome 4q34.1). Similar CNVs strongly support a common clonal origin and metastatic spread from one twin to the other. DEIN is a long-coding RNA (IncRNA) that has been found highly expressed in stage MS neuroblastoma and is likely involved in biological processes such as cell migration and metastasis.

BACKGROUND No consensus exists on the optimal irradiation volume/dose for patients with metastatic CNS germinoma. We reviewed the literature for a potential association between different treatment modalities and survival. METHODS We searched PubMed for studies on patients with primary metastatic CNS germinoma including detailed survival outcome data. We used the Kaplan-Meier method to estimate event-free survival (EFS) rates and log-rank test for comparison among treatment modalities. RESULTS We identified 124 patients from 38 studies published between 1990 and 2021. The median age of presentation was 15 years (range: 1.5–47 years). Treatment modalities included: chemotherapy only (n = 5), radiotherapy only (n = 31), and radio-chemotherapy (n = 88). Radiotherapy modalities included: craniospinal irradiation (CSI) (n = 69), whole-brain irradiation (WBI) (n = 19), whole ventricular irradiation (WVI) (n=16), focal and spine irradiation (n = 5), focal irradiation only (n = 9), and not specified (n = 1). The median focal irradiation dose was 36.6 Gy (range:19.5-59.8 Gy). The median doses for CSI, WBI, and WVI were 24 Gy (range: 10.8-49 Gy). Eighteen patients relapsed, 111 were alive without disease, 2 were alive with disease, and 12 died of disease. The median follow-up time was 5 years in survivors. Four patients who received only chemotherapy relapsed. Five-year EFS rates were similar in patients who received chemo-radiotherapy and radiotherapy alone (89%, 95%CI 81-97%, vs 88%, 95%CI 74-100%, p = 0.6). Patients who were treated with CSI had a longer 5-year EFS compared to patients treated with other modalities (92%, 95%CI 84-100% vs 80%, 95%CI 64-94% p = 0.03). EFS was similar in patients treated with CSI whether they received ≥ 24 Gy or less. CONCLUSION CSI is required to achieve prolonged relapse-free survival (RFS). CSI dose of < 24 Gy may be sufficient to achieve sustained RFS. Future trials are needed to evaluate the potential role of neoadjuvant chemotherapy in reducing the dose/volume of irradiation in patients with metastatic CNS germinoma.

Retinoblastoma is a childhood cancer of the retina involving germline or somatic alterations of the RB Transcriptional Corepressor 1 gene, RB1. Rare cases of sellar-suprasellar region retinoblastoma without evidence of ocular or pineal tumors have been described. A nine-month-old male presented with a sellar-suprasellar region mass. Histopathology showed an embryonal tumor with focal Flexner-Wintersteiner-like rosettes and loss of retinoblastoma protein (RB1) expression by immunohistochemistry. DNA array-based methylation profiling confidently classified the tumor as pineoblastoma group A/intracranial retinoblastoma. The patient was subsequently enrolled on an institutional translational cancer research protocol and underwent comprehensive molecular profiling, including paired tumor/normal exome and genome sequencing and RNA-sequencing of the tumor. Additionally, Pacific Biosciences (PacBio) Single Molecule Real Time (SMRT) sequencing was performed from comparator normal and disease-involved tissue to resolve complex structural variations. RNA-sequencing revealed multiple fusions clustered within 13q14.1-q21.3, including a novel in-frame fusion of RB1-SIAH3 predicted to prematurely truncate the RB1 protein. SMRT sequencing revealed a complex structural rearrangement spanning 13q14.11-q31.3, including two somatic structural variants within intron 17 of RB1. These events corresponded to the RB1-SIAH3 fusion and a novel RB1 rearrangement expected to correlate with the complete absence of RB1 protein expression. Comprehensive molecular analysis, including DNA array-based methylation profiling and sequencing-based methodologies, were critical for classification and understanding the complex mechanism of RB1 inactivation in this diagnostically challenging tumor.

BACKGROUND: The optimal management of pediatric papillary tumors of the pineal region (PTPR) is not yet established. We report three cases, as an addition to the existing literature. PATIENT 1: A 22-month-old female presented with progressive ataxia and incoordination, found to have an enhancing solid and cystic mass centered within the pineal region, with associated obstructive hydrocephalus. Tumor biopsy revealed PTPR, prompting a subsequent subtotal resection. No adjuvant therapy was recommended. Patient has remained without evidence of disease progression for 92 months, since resection. PATIENT 2: An 8-year-old female presented with progressive headaches and new-onset generalized seizures, found to have a heterogeneously enhancing pineal lesion, with associated obstructive hydrocephalus. Following a biopsy demonstrating PTPR, patient underwent subtotal resection of the tumor. A second resection was completed 6 months later followed by focal irradiation. Targeted next-generation sequencing (NGS) demonstrated two non-targetable genomic alterations (CREBBP and MLL2). The patient remains without tumor recurrence, now 37 months, since irradiation. PATIENT 3: A 7-year-old male with Autism spectrum disorder presented with new-onset focal complex seizures. A brain MRI showed a heterogeneously enhancing lesion in the region of the pineal gland, with associated obstructive hydrocephalus. A tumor biopsy revealed PTPR, prompting a subsequent gross total resection (GTR). NGS demonstrated no reportable genomic alterations. Methylome profiling classified the tumor as PTPR, group B. Post-operative imaging with no residual tumor and he has remained without tumor progression, now eight months, since resection. CONCLUSION: PTPR are extremely rare in the pediatric setting, and were initially introduced in the WHO 2007 classification as grade II-III pineal gland tumors with distinct morphologic and immunohistochemical features. GTR is the mainstay of treatment but post-surgical management remains controversial. The clinical course is characterized by frequent local recurrence, hence, adjuvant chemotherapy and/or irradiation may be necessary upon disease progression.

INTRODUCTION: Ganglioglioma is a low grade neoplasm consisting of dysplastic neuronal and neoplastic glial cells and accounts for 5% of pediatric CNS tumors. Management often includes CNS diversion. There have been case reports in which platinum containing chemotherapy has been thought to contribute to CSF malabsorption leading to ascites. CASE: A 13 month old male developed progressive macrocephaly, developmental delay, chronic emesis, and intermittent bilateral cranial nerve VI palsy over the 5 months prior to presentation. MRI brain/spine was significant for an enhancing nodule in the left posterior lateral medulla, nodular thickening and enhancement along the brainstem down to the conus medullaris and in the tentorium, with associated hydrocephalus. Biopsy of the medullary nodule and of the enhancement were consistent with ganglioglioma with BRAF-KIAA1549 fusion, equivocal MYCN amplification, and no BRAF V600E mutation. A ventriculo-peritoneal shunt was placed at the time of biopsy. Therapy was initiated with vincristine (1.5 mg/m2) and carboplatin (175 mg/m2.) Following the 12 week induction phase of therapy, he developed increasing diarrhea, emesis, and abdominal ascites. Peritoneal fluid analysis had no malignant cells and low protein compared to CSF. Ascites was responsive to drainage but would rapidly re-accumulate. Ultimately the patient’s chemotherapy was discontinued after 2 maintenance cycles due to continued symptoms. Acetazolamide was trialed but discontinued due to side effects, so its efficacy could not be determined. He underwent shunt externalization followed by venticulo-atrial (VA) shunt re-internalization. He has not had ascites since that time, at 4 months from surgery. His CNS disease burden has been stable at 6 months off therapy. DISCUSSION: Ascites was most likely due to CSF malabsorption in the abdomen with a possible contribution from platinum containing chemotherapy and less likely secondary to malignant peritoneal cells. Resolution since VA shunt internalization makes alternate explanations less likely.

BACKGROUND: Outcomes of patients with primary metastatic CNS Germinoma are excellent. There is no global consensus over the optimal irradiation doses and fields to reduce long-term radiotherapy side effects. Our literature review explores the different treatment modalities utilized for patients with metastatic CNS germinoma. METHODS: A literature review of studies that included patients with primary metastatic CNS germinoma via PubMed database. RESULTS: Sixty-eight patients (male: 48, female: 13, not specified: 7) were identified. At presentation, the median age was 14.4 years (range: 1.5–47 years). Primary tumor location included the following: pineal (n=15), suprasellar (n=11), bifocal (suprasellar and pineal) (n=21), other (n=9), not specified (n=12). Metastasis sites included the following: multifocal intracranial with/without ventricular metastasis (n=18), ventricular metastasis only (n=21), spinal metastasis only (n=8), CSF cytology dissemination only (n=3), and not specified (n=18). Seventeen patients received radiotherapy only, and 51 received radio-chemotherapy. Radiotherapy modalities included the following: craniospinal irradiation (n=36) with a mean whole-brain dose of 23.3 Gy (range:10.8-40.5 Gy) and spinal dose of 21.6 Gy (range: 10.8 – 30.6 Gy), whole-brain irradiation (n=13) with a mean dose of 24.2 Gy (range: 18 - 40 Gy), whole ventricular irradiation (n=16) with a mean dose of 24.7 Gy (range: 23.4 – 30 Gy), Focal irradiation only (n=1) with a dose of 40 Gy, and not specified (n=2). Four patients died of disease progression or relapse. Two patients were alive with relapse/residual disease. Sixty-two patients were alive with no evidence of disease with a mean follow of 77 months (range: 10.3 – 256 months). Further analysis is underway and will be presented at the meeting. CONCLUSION: The role of neoadjuvant chemotherapy in reducing the dose and field of radiation therapy remains unclear. Further analysis is pending which may highlight the role of neoadjuvant chemotherapy in managing patients with metastatic germinoma.

BACKGROUND: MRI-guided laser interstitial thermal therapy (LITT) is a minimally invasive, cytoreductive surgery useful for managing unresectable brain tumors. LITT disrupts the blood brain barrier (BBB) and facilitates chemotherapy delivery. We report the toxicity and outcome for pediatric brain tumors treated on a pilot trial of LITT and chemotherapy. The primary objectives were to quantify peritumoral BBB disruption following LITT and evaluate toxicity and efficacy. METHODS: The trial had two arms, A: patients with newly diagnosed gliomas underwent LITT followed by standard of care management, and B: patients with relapsed malignant brain tumors received 6 weeks of weekly doxorubicin post-LITT followed by maintenance etoposide. RESULTS: Between 2015 – 2018, six patients were enrolled: five on arm A (four with low-grade gliomas, one with high-grade glioma), one on Arm B with progressive anaplastic astrocytoma. All patients tolerated the procedure well; four experienced a transient hemiparesis post-LITT. The Arm B patient progressed and died of disease 2 months and 22 months post-LITT, respectively. The HGG patient received standard therapy and remains without disease progression 44 months post-LITT. One patient with LGG required additional treatment for disease progression 14 months post-LITT. Two patients with LGGs did not require additional therapy, now 51 and 41 months post-LITT. One patient was alive 24 weeks post-LITT and subsequently lost to follow-up. Peritumoral BBB disruption was analyzed in two ways: serum abundance of brain-derived proteins and MRI Dynamic contrast enhancement (DCE). Neuron-specific enolase were measurable in the serum of all patients, using ELISA up to 84 days post-LITT. DCE 2 weeks post-LITT demonstrated increased enhancement and FLAIR signal, consistent with BBB disruption and vasogenic edema. This effect was evident up to 4 months post-procedure. CONCLUSION: LITT is safe in children with brain tumors and can be combined with chemotherapy. DCE and serum brain-derived proteins can measure BBB disruption.

BACKGROUND: MEK inhibitor therapy is increasingly being utilized for the treatment of pediatric tumors, including low-grade glioma, plexiform neurofibroma and Langerhans cell histiocytosis. These drugs are well-tolerated but do have risk of toxicity, including cardiac toxicity. The purpose of this study is to better characterize MEK inhibitor-induced cardiac toxicity in pediatric patients. METHODS: Retrospective review of all patients who underwent MEK inhibitor mono-therapy for at least 3 months, 2015- 2021, age 25 years or less, at St. Louis Children’s hospital and Cardinal Glennon Children's hospital. RESULTS: We evaluated 31 patients, 19 (61%) with brain tumors and 12 (39%) without. Of the thirty-one, fifteen (48%) had NF1, 1 had Tuberous sclerosis. Cardiac toxicity consisted of asymptomatic sinus tachycardia, bradycardia, or decreased ejection fraction (EF). Thirteen patients (42%) experienced an asymptomatic decrease in left-ventricular ejection fraction (EF), Grade I-III. Time on therapy before decreased EF was 5 days to 21 months, median 2.8 months. Decreased EF developed in 5 of 13 patients receiving selumetinib and 8 of 18 receiving trametinib. Of the patients who developed decreased EF, 11 (85%) had brain tumors, 6 (46%) had NF1, and 89% had received prior systemic therapy. Out of the patients who had received no prior systemic therapy (6), 2 (33%) had decreased EF, while 11/25 (44%) of those who had received prior systemic therapy did. Drug was held temporarily for 6 patients, with dose limiting toxicity for 5 patients. Drug was discontinued for 1 patient after EF continued to decline despite dose reduction. Patients showed improvement in EF as early as 2 weeks after holding therapy. CONCLUSIONS: Cardiac toxicity in our patients was limited to asymptomatic reduction in ejection fraction, sinus bradycardia and tachycardia, reinforcing the need for appropriate monitoring via echocardiography. Prior systemic therapy was associated with decreased EF.

BACKGROUND: CNS Germinomas are highly radio-sensitive tumors with an excellent survival rate of more than 90%. The current standard of care combines chemotherapy with reduced-dose radiotherapy to minimize the adverse effects and long-term effects associated with radiotherapy. In the latest Children’s Oncology Group clinical trial (ACNS1123 stratum 2), patients with residual or progressive disease following chemotherapy can be considered for a “second-look” surgery to assess tumor viability. Patients with residual disease who do not undergo second-look surgery receive 24 Gy of whole ventricular radiation with a 12 Gy boost compared to 18 Gy plus boost given to patients in complete remission or without viable tumor on second-look. Conversely, the International Society of Paediatric Oncology (SIOP) protocol does not stratify based on response to chemotherapy, and the Korean SMC GCT trial uses 18 Gy CSI plus boost for all patients regardless of chemotherapy response. METHODS: Single center retrospective chart review of germinoma patients treated at St Louis Children’s Hospital between 2011 and 2021. RESULTS: We analyzed data for all 15 germinoma patients treated between 2011 and 2021. Five patients had residual disease following chemotherapy. Of these five, one had complete remission at the end of radiotherapy, one had partial response, and three had stable disease. All patients remain relapse-free with time of follow-up ranging between 6.3-109.3 months from the end of therapy (median 24 months). CONCLUSION: None of the five patients with residual disease following chemotherapy demonstrated disease progression following chemotherapy and radiotherapy. Future prospective clinical trials are needed in order to test the possibility of treating germinomas patients who have residual disease after chemotherapy with a low dose of radiotherapy similar to that used in patients with complete remission.

INTRODUCTION: Congenital intracranial teratomas (CITs) are rare tumors occurring in the first 60 days of life, often associated with dismal prognosis, posing challenges in both oncologic and surgical management. METHODS: We report a patient with a congenital mature teratoma diagnosed prenatally, who underwent successful surgical resection following neoadjuvant chemotherapy. We also performed an updated literature review on CIT outcomes. RESULTS: A newborn female, diagnosed prenatally with a complex midline intracranial echogenic mass, underwent postnatal brain MRI which demonstrated a supratentorial large complex enhancing mass with solid and cystic components, with associated obstructed hydrocephalus. Serum alpha-fetoprotein (AFP) and beta-human chorionic gonadotropin were normal for age. Tumor biopsy revealed mature teratoma. AFP obtained from cyst fluid during fenestration was significantly elevated at 8 weeks of life. Due to concerns for possible malignant transformation and in an attempt to alter tumor vascularity facilitating surgical resection, the patient received two cycles of neoadjuvant carboplatin and etoposide, followed by tumor resection without significant intraoperative bleeding. There was no evidence of immature components on final pathology. The patient remains without tumor recurrence, now 9 months, since surgical resection. Review of literature yielded 76 cases of CITs: eight patients with mature teratoma, of which six remained alive following tumor resection. Five patients with immature teratoma received chemotherapy prior to resection; three remained alive post-resection (follow-up range 3-20 years). DISCUSSION: CITs are typically associated with poor prognosis, with reported one-year survival of 7.2-12%. Surgical excision is the mainstay of treatment, often limited by intraoperative hemorrhage. Neoadjuvant chemotherapy has been employed as a successful strategy to reduce tumor volume and vascularity in a variety of pediatric brain tumors; more specifically, carboplatin and etoposide are utilized as neoadjuvant chemotherapy for non-germinomatous germ cell tumors to facilitate excision in older children. Their role in CITs requires further evaluation.

Background: The discovery of the driving oncogenic alterations in pediatric low-grade gliomas (pLGGs) has shifted our focus towards management with targeted therapies, especially in relapsing or progressive disease. Limited data is available on the durability of response to targeted therapy in pLGGs once the therapy has ceased. Methods: Multi-institutional retrospective chart review of patients with pLGGs younger than 25 years, between 2010-2021, was undertaken to evaluate the durability of response to targeted therapy and determine risk factors associated with disease progression after cessation of therapy. Results: Current analysis included 18 patients from two centers. Seven (39%) had neurofibromatosis type-1 (NF-1). Diagnoses included: optic pathway glioma (OPG) (6/18, 33%), pilocytic astrocytoma (8/18, 44%), diffuse fibrillary astrocytoma (1/18), ganglioglioma (1/18), glioneural neoplasm (2/18). Sixteen patients received at least one prior line of chemotherapy (range 1-5). Targeted agents included trametinib (50%), selumetinib (5%), binimetinib (22%), vemurafenib (11%) and everolimus (11%). Median time on therapy was 351 days (range 29-979 days). All, but one patient had residual intracranial findings at the end of therapy: eight patients (44%) had stable disease, while ten required additional therapy; 50% were NF-1 patients with OPG. Median time to progression was 203 days (range 29-615 days). Of those who did not require any additional therapies, 50% had suprasellar tumors. Genomic data was available for twelve patients; BRAF-KIAA1549 fusion was the most common genomic alteration. Others included mutations in KRAS, BRAF (V600E), PTPN11, SOX6-RAF1 fusion, NF-1, and a patient with FGFR1, KMT2C, and PTPN11 alterations. Conclusion: Preliminary analysis demonstrates that despite initial response, the majority of patients required additional line of therapy. Patients with NF-1 and OPGs tend to progress after discontinuing therapy, while suprasellar non-NF1 pLGGs tend to develop sustained response to targeted therapies. Additional multi-institutional analysis is underway and will be presented at the meeting.

BACKGROUND: Biologically targeted agents such as the BRAF inhibitor dabrafenib are now used to treat progressive low-grade glioma (LGG), but the effect of these agents on the neurologic symptoms that accompany LGGs is poorly understood. CASE: A 21-year-old male was diagnosed with medullary ganglioglioma (GG) after presenting with persistent vertigo and positional headaches. Immunohistochemistry was consistent with BRAF V600E mutation and dabrafenib was started. The patient had resolution of his neurologic symptoms within 3 weeks of treatment initiation; surveillance MRI several months later demonstrated interval decrease in tumor size. The patient remained on therapy for 2 years. Several days after planned drug discontinuation the patient had re-emergence of persistent vertigo that impaired his ability to work. Repeat imaging two months later demonstrated an increase in tumor size and solid enhancement along tumor margins. He was restarted on dabrafenib. Within one week, he again had complete resolution of his vertigo. DISCUSSION: Gangliogliomas are comprised of mixed neuronal and glial components and associated with epilepsy, headache and other localizing neurologic symptoms. In this report, we describe clinical and radiographic response from single-therapy dabrafenib in a patient with BRAF V600E+ GG, along with a very rapid resolution of neurologic symptoms upon initiation of the drug and recrudescence shortly following cessation of therapy. The symptoms subsided within a week of restarting dabrafenib, suggesting a separate, more rapid mechanism of symptom reversal than decline in tumor size. BRAF mutations have been identified in both the neuronal and glial components of ganglioglioma, suggesting that dabrafenib may provide symptomatic relief via inhibition of abberant neuronal processes. CONCLUSION: BRAF inhibitors can rapidly improve neurological symptoms via a rapid, not yet fully elucidated mechanism. Kinetics of this response suggest it is independent of effects on tumor volume and the degree of compression to tissue surrounding the tumor.

BACKGROUND: Atypical choroid plexus papillomas (aCPP) are rare central nervous system (CNS) tumors often occurring in very young children. While surgical resection has been a mainstay of therapy, there is no consensus and limited data on the treatment of relapsed or metastatic tumors. METHODS: Retrospective review of the treatment and outcome of patients diagnosed with aCPP since 2011 was performed. RESULTS: Of the seven patients, 4 were male and 3 were female with a median age of 3 years at diagnosis (range: antenatal to 18 years old). All non-metastatic patients (six) were treated with surgery and all achieved gross total resection. Two patients had diffuse leptomeningeal contrast enhancement on diagnosis MRI that resolved after resection of primary tumor alone. One patient developed local relapse underwent re-resection with a GTR then was treated with 4 cycles of chemotherapy based on CPT-SIOP-2000 protocol (carboplatin, etoposide) and has not had further relapse in 24 months. One patient had metastatic disease at the time of diagnosis. They were treated with adjuvant chemotherapy, which stabilized disease for 36 months until they had progression. Additional four cycles were given and has again stabilized disease now 8 months from completion of that therapy. One non-metastatic patient died of unknown causes 28 months from diagnosis. CONCLUSIONS: Surgical resection remains the standard of care for patients with aCPP. However, chemotherapy based on the SIOP backbone may be useful to reduce the need for or to delay radiation therapy in select patients in the relapsed or metastatic setting.

BACKGROUND The optimal management for children with recurrent choroid plexus carcinoma (CPC), is not established. We report two children with germline TP53 mutations, whose CPC relapses were managed with marrow-ablative chemotherapy and oral biologically-targeted therapies. PATIENTS: Patient A: A 17 months old male presented with non-metastatic bilateral CPC. A de novo mosaic germline TP53 mutation was identified. After near-total resections, 16 months of standard chemotherapy were administered; 18 months later, localized tumor growth developed, again near-totally resected. Two cycles of re-induction chemotherapy were administered followed by three cycles of thiotepa/carboplatin with autologous hematopoietic cell rescue (AuHCR) and subsequently 21 months of sirolimus and thalidomide, continuing without residual or recurrent disease. Patient B: A 30 months old male presented with left lateral ventricular non-metastatic CPC. A de novo TP53 germline mutation was identified. Following sub-total resection, craniospinal irradiation with boost was administered followed by eight cycles of standard chemotherapy; 18 months later, localized recurrence developed; gross total resection was followed by 15 months of standard dose chemotherapies; four months thereafter, a second local recurrence developed, again gross totally resected. He then received one cycle of high-dose cyclophosphamide followed by three cycles of thiotepa/carboplatin with AuHCR. Subsequently he received sirolimus and thalidomide for 12 months, complicated by progressive pancytopenia. A small localized CPC recurrence was noted, gross totally resected, concomitant with myelodysplastic syndrome; he underwent an allogeneic matched unrelated donor marrow transplantation. CONCLUSIONS Marrow-ablative chemotherapy with post-transplant targeted biological therapy may afford durable survival for select children with recurrent CPC.

BACKGROUND Wnt-MB infers an excellent prognosis, and metastatic disease is rare. However, specific treatment strategies and patterns of failure for patients with recurrent Wnt-MB are unknown. We report two cases of recurrent beta-catenin nucleopositive Wnt-MBs treated with an irradiation-sparing strategy, incorporating HDCx/AuHPCR. PATIENT 1: A nine-year-old female experienced local recurrence of a non-metastatic Wnt-MB nine months after gross total resection (GTR) followed by 18Gy craniospinal irradiation (CSI) with primary site boost to 54Gy, accompanied by weekly vincristine, followed by a maintenance regimen of nine cycles of cisplatin/lomustine/vincristine alternating with cyclophosphamide/vincristine every third cycle. GTR of the relapsed tumor was followed by three cycles of HDCx/AuHPCR. She is disease-free for over three years following relapse treatment. PATIENT 2: A 17-year-old male initially underwent GTR, followed by 23.4Gy CSI with 54Gy posterior fossa boost with concomitant weekly vincristine, followed by a maintenance regimen that included nine alternating cycles of vincristine/lomustine/cisplatin and cyclophosphamide/vincristine. Isolated right frontal horn metastatic recurrence developed 19 months post-treatment; three cycles of irinotecan/temozolomide/bevacizumab and gamma-knife radiosurgery produced complete response. A second isolated metastatic recurrence within the left frontal horn occurred 13 months post-treatment, which was treated with two cycles of cyclophosphamide/etoposide followed by two cycles of HDCx/AuHPCR. MRI of the brain showed no residual tumor one month post-treatment. He currently awaits follow-up stereotactic radiosurgery. CONCLUSION Patients with recurrent Wnt-MB may be treated with curative intent using a multi-disciplinary approach that includes HDCx/AuHPCR, and minimization or avoidance of re-irradiation.

BACKGROUND We report the responses, toxicities and long-term outcomes of gemcitabine, paclitaxel and oxaliplatin (GemPOx) regimen administered, in responsive patients, prior to single cycle marrow-ablative chemotherapy (thiotepa, etoposide and carboplatin) with autologous hematopoietic progenitor cell rescue (HDCx+AuHPCR). METHODS Since December 2009, 11 recurrent/refractory patients (10 MMGCT, 1 germinoma; 10 males; mean age 16.5 years, range 7–46 years) have been treated with up to four cycles of gemcitabine (800mg/M2), paclitaxel (170mg/M2) and oxaliplatin (100mg/M2) administered on one day at 14 days intervals. RESULTS All 11 patients were enrolled on a prospective multi-center trial, which was closed in October 2019. Three patients achieved complete remissions (tumor marker and/or imaging studies), five achieved partial remissions, two developed disease progression (PD), and one was withdrawn after one cycle for severe paclitaxel neurotoxicity followed by rapid tumor progression and death. One patient with PD after one cycle had pathologically-confirmed metastatic transformation to pure embryonal rhabdomyosarcoma, and rapidly expired. A second patient, with pure pineal choriocarcinoma, progressed after the second GemPOx cycle, ultimately died of tumor progression. Eight of the 11 responsive patients subsequently underwent HDCx+AuHPCR; five of these received some form of radiotherapy. Seven patients (six MMGCT, one germinoma) are alive and disease-free without recurrence for a mean of 94 months (range 74–118 months) since completion of therapy. CONCLUSION GemPOx is an effective re-induction regimen for patient with recurrent CNS germ cell tumors, with acceptable toxicities; when followed by marrow-ablative chemotherapy and subsequent irradiation/re-irradiation, the regimen produces encouraging long-term disease-free survival.

BACKGROUND We report a unique case of spinal ependymoma with classic histology and aggressive clinical behavior which harbored a focal MYC (8q24) amplification. CASE REPORT: A-12-year old male presented with a three months history of back pain and acute onset weakness with ataxia. A spine MRI revealed an avidly enhancing intradural, extramedullary mass occupying the dorsal spinal canal from C6 through T2. The tumor demonstrated mild diffusion restriction and was associated with severe cord compression and mild edema. He underwent gross total resection. Pathological diagnosis was classic grade II ependymoma. Eleven months later, he re-presented with acute onset lower extremity paresthesia and left-handed weakness. Spine MRI demonstrated tumor recurrence extending from C2 through T1-T2 with resultant severe cord compression, again demonstrating avid enhancement and restricted diffusion. He underwent subtotal resection of the mass and focal proton beam irradiation. MOLECULAR CHARACTERISTICS: The patient was enrolled on an institutional comprehensive genomic profiling protocol. The tumor’s copy number profile was complex, including homozygous loss of 17p and notably, amplification of the MYC oncogene. Using fluorescence in situ hybridization, we identified >20 copies of MYC in interphase cells, confirming the gene amplification, while two copies of MYCN (2p24) were seen. DNA methylation further classified this tumor as clustering near posterior fossa group A (score=0.6073) tumors. CONCLUSION We report a unique case of an adolescent male with aggressive spinal ependymoma harboring focal MYC amplification. Testing for MYC amplification may be reasonable in newly-diagnosed spinal ependymomas to aid in characterization.

BACKGROUND Rhabdoid predisposition syndrome is characterized by germline alterations in SMARCB1 or SMARCA4, leading to synchronous or metachronous central nervous system (CNS) and extra-CNS rhabdoid tumors. Rare survivors have been reported to date. METHODS We describe the molecular profiling and treatment regimen of three patients with synchronous atypical teratoid/rhabdoid tumor (ATRT) and malignant rhabdoid tumor of the kidney (MRT-K). All patients underwent radical nephrectomy of the kidney, and gross total resection of the primary CNS tumor was achieved for two patients. An intensive chemotherapy regimen was administered; an induction phase based on the modified Third Intergroup Rhabdomyosarcoma Study (IRS-III) for ATRT followed by a consolidation phase with three cycles of high-dose chemotherapy and autologous hematopoietic progenitor cell rescue, without irradiation. All three patients were enrolled on an institutional comprehensive genomic profiling protocol. RESULTS A germline focal 22q deletion, including SMARCB1, was detected in two patients, while the third patient had a maternally-inherited heterozygous frameshift variant in SMARCB1. Somatic loss of heterozygosity of 22q was identified in all patients, resulting in biallelic inactivation of SMARCB1. Divergent tumor subgroups were described using DNA methylation. The three MRT-K samples were classified as MYC subtype. One ATRT was classified as SHH while the other as TYR. One patient is currently three years off-therapy without evidence of disease, while the other two patients have completed the consolidation phase without recurrent disease. CONCLUSION Molecular profiling of CNS and extra-CNS rhabdoid tumors revealed different epigenetic subgroups. An intensive multimodal therapeutic approach without irradiation may achieve prolonged survival.

BACKGROUND About one-third of children with primary CNS MMGCT experience incomplete responses to initial induction chemotherapy prior to irradiation, many of whom will subsequently relapse. Such high-risk patients are variably defined as having initial alpha-fetoprotein (AFP) elevations exceeding 1,000ng/mL, predominant histopathologies of malignant non-germinomatous GCT and incomplete responses to induction chemotherapy. Drugs targeting GCT-specific molecular markers have been identified for non-germinomatous GCT elements but have yet to be incorporated into prospective clinical trials. Four children with clearly identified HR-MMGCT characteristics have been treated on an innovative pilot regimen incorporating intensified chemotherapy and molecularly targeted agents, with avoidance or minimization of irradiation. METHODS Four children (two with pure suprasellar embryonal carcinoma (EC) - one with Down syndrome and the other with pre-diagnosis cognitive dysfunction; one with initial serum AFP exceeding 7,000ng/mL and yolk sac tumor (YST)+EC+Teratoma pathology; one with initial serum AFP exceeding 1,000ng/mL) were treated with 3 cycles of “standard” induction chemotherapy (ACNS1123), followed by 1–3 transplant cycles (thiotepa/carboplatin) each with complete radiographic and tumor marker responses. Two children with pure EC subsequently received six cycles of brentuximab-vedotin without irradiation and remain disease-free off therapy for 2–4 years. One child with YST+EC+Teratoma has subsequently received reduced dose craniospinal irradiation and pineal region boost, and will receive oral everolimus, erlotinib, palbocyclib and intravenous brentuximab-vedotin. The fourth child with YST+MT will commence everolimus, erlotinib and palbocyclib without irradiation. CONCLUSION This treatment strategy for HR-MMGCT patients provides preliminary tolerance and response data justifying extension to a multi-center trial.

BACKGROUND Noonan syndrome (NS) is associated with germline Ras signaling pathway mutations, RAS overactivation and increased tumorigenesis risk. Rosette-forming glioneuronal tumors (RFGT) are rare indolent tumors. We report the molecular profiling of two patients with NS and RFGT. PATIENT 1: A 22-year-old male with NS was diagnosed with RFGT after partial tumor resection followed by focal irradiation. He was enrolled on a comprehensive genomic profiling study involving paired tumor-normal whole exome sequencing and RNA sequencing of the disease-involved tissue, revealing a germline PTPN11 alteration (p.Gly60Ala) consistent with NS, and a somatic deletion (p.Ile442_Thr454del) in PIK3R1 and a somatic variant (p.Lys656Glu) in FGFR1 with concomitant increased expression of PIK3R1 and FGFR1 by RNA-sequencing. The patient remains without tumor progression now nine months since irradiation. PATIENT 2: A 19-year-old male with persistent headaches, underwent a brain MRI demonstrated multiple abnormal signals in the pineal region and midbrain. He had a stereotactic biopsy revealing RFGT. He was enrolled on the genomic study revealing a germline PTPN11 alteration (p.Asn308Asp) resulting in a new diagnosis of NS. Several family members were subsequently identified with clinical features of NS, including his mother and two siblings, enabling appropriate counseling. Two somatic variants were found in trans in PIK3R1 (p.Thr454_Phe456del and p.Glu451_Asn453delinsAsp), and a somatic variant (p.Val695Met) in FGFR1, with resultant overexpression of PIK3R1. The patient is monitored with surveillance imaging. CONCLUSION We report the molecular profiling of two patients with NS and RFGT; strongly suggesting their connection to RASopathies through the overactivation of the MAPK and PI3K/AKT/mTOR signaling pathways.

Diffuse Intrinsic Pontine Glioma (DIPG) is an extensively invasive tumor of brainstem. Tumor biopsy was rarely performed until recently, because of the possible complications associated with the procedure, therefore, the data on the DIPG histology is relatively limited. We identified 29 consecutive cases of DIPG diagnosed from 2002 to 2017 at Children’s Hospital of Michigan, and tumor biopsy was performed in 22 cases. The histology was consistent with low grade glioma (LGG) in 4 of the 22 cases (18%). We retrospectively reviewed these 4 cases, including the histology, MRI findings, H3 mutation and BRAF alteration status, treatment and survival. There was one Grade I astrocytoma, one Grade I ganglioglioma, and two grade II astrocytomas. Radiologically, 3 were consistent with typical DIPG, one with atypical DIPG. The testing for H3 mutation and BRAF alteration was performed in two recent cases. One case had H3F3A K27 mutation, but no BRAF alteration was identified in either case. All patients received involved-field external beam radiation as part of their initial treatment. Three patients succumbed to their disease with overall survival of 9–17 months, and one patient is alive with no clinical or radiological sign of disease at 6 months following diagnosis. In summary, histology was consistent with LGG in 4 out of 22(18%) of our biopsied DIPG series. One patient with ganglioglioma was found to have histone H3 mutation, a rare finding in LGG but common in midline high grade glioma, who progressed at 11 months, and died at 17 months following initial diagnosis.

Leukemia is the most common type of childhood cancer. It accounts for about 30% of all malignancies in children and adolescents less than 14 years of age. Acute myeloid leukemia (AML) accounts for one-fourth of acute childhood leukemias and is considered a leading cause of death in the pediatric population with an overall 5-year survival of 65% and relapse rates approaching 40-50%. AML stem cells are believed to be the source of AML relapse, thus new molecular therapies targeting AML stem cells are urgently needed to lower risk of relapse and improve clinical outcomes. The rate of ribosome production controls cell growth and proliferation. RNA polymerase (Pol I), which is upregulated in acute myeloid leukemia (AML), transcribes ribosomal DNA, regulating ribosome production. CX-5461 is a novel inhibitor of Pol I transcription and has shown therapeutic efficacy in hematological cancers with minimal effect on normal cells. Recent evidence suggests that CX-5461 exerts its antitumor activity by blocking DNA replication forks and inducing DNA single strand breaks, leading to ATR activation and G2/M cell cycle arrest (Xu H, et al. Nat Commun. 2017;8:14432), representing a novel mechanism of resistance to this promising antitumor agent. Therefore, we hypothesized that targeting ATR using its selective inhibitor AZD-6738 would abolish CX-5461-induced G2 cell cycle checkpoint activation, preventing repair of damaged DNA, leading to more DNA damage and cell death. To begin to determine if AZD-6738 enhances CX-5461-induced AML cell death, we treated a panel of AML cell lines with variable concentrations of CX-5461 alone or in combination with AZD-6738 for 48 hours. CX-5461 treatment induced cell death independent of the status of TP53gene, as determined by Annexin V/Propidium Iodide (PI) staining and flow cytometry analysis, and was accompanied by increased cleavage of caspase 3 and PARP, demonstrating that CX-5461 induces apoptosis. Lentiviral shRNA knockdown revealed that CX-5461-induced cell death was partly dependent on both Bax and Bak, demonstrating that CX-5461 induces apoptosis, at least partially through the intrinsic apoptotic pathway. When combined, CX-5461 and AZD-6738 synergistically induced apoptosis in AML cell lines (n=6) and primary patient samples (n=3), determined by Annexin V/PI staining, flow cytometry analyses, and calculation of combination index values using the CalcuSyn software. Synergy was also confirmed in an additional 10 primary AML patient samples using MTT assays and standard isobologram analyses. Colony formation assays using primary AML patient samples showed that both CX-5461 and AZD-6738 individual treatments significantly reduced colony formation capacity compared to vehicle control, and was further reduced by combination treatment. Treatment of AML cells with CX-5461 induced G2/M cell cycle arrest, which was abolished by AZD-6738. This was accompanied by significantly increased cell death. CHK-1-selective inhibitor LY2603618 also abolished CX-5461-induced G2/M cell cycle arrest and synergistically induced cell death when combined with CX-5461 in AML cell lines. Protein levels of γH2AX increased with single drug treatment, which was enhanced in combination treated AML cells, indicating that the two drugs cooperatively induced DNA damage, which was confirmed by comet assays. Further, chromatin fractionation results showed that AZD-6738 enhanced binding of RPA32 to chromatin induced by CX-5461 treatment. Moreover, CX-5461 treatment substantially increased protein levels for ribonucleotide reductase subunit M2, which was abolished by the addition of AZD-6738. Taken together, inhibition of ATR, using AZD-6738, can synergistically enhance cell death induced by the Pol I transcription inhibitor CX-5461 in both AML cell lines and primary patient samples in vitro. Additional studies in both AML cell line- and patient-derived xenograft mouse models are warranted to confirm the efficacy of the combinationin vivo. Leukemia is the most common type of childhood cancer. It accounts for about 30% of all malignancies in children and adolescents less than 14 years of age. Acute myeloid leukemia (AML) accounts for one-fourth of acute childhood leukemias and is considered a leading cause of death in the pediatric population with an overall 5-year survival of 65% and relapse rates approaching 40-50%. AML stem cells are believed to be the source of AML relapse, thus new molecular therapies targeting AML stem cells are urgently needed to lower risk of relapse and improve clinical outcomes. The rate of ribosome production controls cell growth and proliferation. RNA polymerase (Pol I), which is upregulated in acute myeloid leukemia (AML), transcribes ribosomal DNA, regulating ribosome production. CX-5461 is a novel inhibitor of Pol I transcription and has shown therapeutic efficacy in hematological cancers with minimal effect on normal cells. Recent evidence suggests that CX-5461 exerts its antitumor activity by blocking DNA replication forks and inducing DNA single strand breaks, leading to ATR activation and G2/M cell cycle arrest (Xu H, et al. Nat Commun. 2017;8:14432), representing a novel mechanism of resistance to this promising antitumor agent. Therefore, we hypothesized that targeting ATR using its selective inhibitor AZD-6738 would abolish CX-5461-induced G2 cell cycle checkpoint activation, preventing repair of damaged DNA, leading to more DNA damage and cell death. To begin to determine if AZD-6738 enhances CX-5461-induced AML cell death, we treated a panel of AML cell lines with variable concentrations of CX-5461 alone or in combination with AZD-6738 for 48 hours. CX-5461 treatment induced cell death independent of the status of TP53gene, as determined by Annexin V/Propidium Iodide (PI) staining and flow cytometry analysis, and was accompanied by increased cleavage of caspase 3 and PARP, demonstrating that CX-5461 induces apoptosis. Lentiviral shRNA knockdown revealed that CX-5461-induced cell death was partly dependent on both Bax and Bak, demonstrating that CX-5461 induces apoptosis, at least partially through the intrinsic apoptotic pathway. When combined, CX-5461 and AZD-6738 synergistically induced apoptosis in AML cell lines (n=6) and primary patient samples (n=3), determined by Annexin V/PI staining, flow cytometry analyses, and calculation of combination index values using the CalcuSyn software. Synergy was also confirmed in an additional 10 primary AML patient samples using MTT assays and standard isobologram analyses. Colony formation assays using primary AML patient samples showed that both CX-5461 and AZD-6738 individual treatments significantly reduced colony formation capacity compared to vehicle control, and was further reduced by combination treatment. Treatment of AML cells with CX-5461 induced G2/M cell cycle arrest, which was abolished by AZD-6738. This was accompanied by significantly increased cell death. CHK-1-selective inhibitor LY2603618 also abolished CX-5461-induced G2/M cell cycle arrest and synergistically induced cell death when combined with CX-5461 in AML cell lines. Protein levels of γH2AX increased with single drug treatment, which was enhanced in combination treated AML cells, indicating that the two drugs cooperatively induced DNA damage, which was confirmed by comet assays. Further, chromatin fractionation results showed that AZD-6738 enhanced binding of RPA32 to chromatin induced by CX-5461 treatment. Moreover, CX-5461 treatment substantially increased protein levels for ribonucleotide reductase subunit M2, which was abolished by the addition of AZD-6738. Taken together, inhibition of ATR, using AZD-6738, can synergistically enhance cell death induced by the Pol I transcription inhibitor CX-5461 in both AML cell lines and primary patient samples in vitro. Additional studies in both AML cell line- and patient-derived xenograft mouse models are warranted to confirm the efficacy of the combinationin vivo. Keywords:CX-5461; AZD-6738; ATR; Pol I; acute myeloid leukemia Disclosures Ge: MEI Pharma: Research Funding.