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Intro. Glioblastoma is the most common and aggressive adult brain cancer, with a 5-year survival of less than 15%. Treatment decisions are guided by imaging, which at early timepoints cannot distinguish tumor pseudoprogression from non-response. Our work seeks to identify circulating biomarkers that can classify response as an alternative or adjunct to imaging. Here we present preliminary data exploring the potential of circulating cytokines and tumor cells collected before, during, and after treatment with the goal to identify potential makers for treatment adaptation. Methods. After consent, patient blood was collected the week before initiating chemoradiotherapy, weekly during treatment, and three weeks after treatment. Serum was tested by multiplex bead-based immunoassay (Legendplex Human Inflammation Panel 1, Biolegend inc.) targeting 13 cytokines and ELISA targeting soluble Human GFAP (Glial fibrillary acidic protein) (Biovendor R&D). The Circulogix filtration system was also used to capture cells of size consistent with circulating tumor cells (CTCs). Initial efforts focused on cells co-staining for GFAP and Olig2 and negative for CD45 (hematopoietic marker). All patient tumor specimens and cell lines were verified for GFAP and Olig2 expression by IHC. We selected 13 patients for the current analysis: 5 with growth due to non-response (NR), 6 stable disease (S) and two pseudo progression (PP). These classifications were evaluated radiologically at the first follow up after chemoradiotherapy (S or no) and based on whether changes observed stabilized or resolved spontaneously within six months or continued to progress (NR or PP). Results. The immunoassays detected approximately 70% of the analytes in our samples. Among those, IL10 and IL33 were higher before treatment on patients later determined to have stable disease (IL10 p=0.09; IL33 p=0.043 KW test; IL10 median S=22.93 ng/mL, NR=13.415ng/mL; IL33 median S= 164.10ng/mL, NR=55.57ng/mL). Interestingly, IL33 is reduced on stable patients during week 5 of treatment (PRE vs. W5 p=0.033; median PRE=164.1ng/mL, W5=57.57ng/mL). GFAP presented a slightly lower level after treatment in stable patients (S vs. NR p=0.014; median S=0.441 ng/ml, NR=0.473 ng/ml). As proof of concept, 50 GBM cells from each of U87 adherent cells, or 0913 and 0821 neurosphere cells (CD45 negative) glioblastoma cell lines were mixed in separate vials with 106 hematopoietic cells (RAJI; CD45 positive) and filtered per the procedure to identify CTCs in patient blood. We separated GBM cells from hematopoietic cells on the filtration system. Conclusion. Our preliminary data suggest that blood obtained during chemoradiotherapy can identify biomarkers associated with response. For example, IL10, IL33, GFAP, and CTCs might be used to classify treatment response at very early timepoints. We continue to recruit additional patients for additional analyses. Citation Format: Paulo Roberto Del Valle, Jonathan B. Bell, Scott M. Welford, Kaylie Kullison, Alessandro Valderrama, Gregory A. Azzam, Jessica J. Meshman, Macarena I. De La Fuente, Eric A. Mellon. Preliminary analysis of circulating biomarkers of glioblastoma response during chemoradiotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2162.

Patients with head and neck malignancies commonly develop metastatic disease, yet rarely do these carcinomas metastasize to the brain. Stereotactic radiosurgery (SRS) is routinely employed to treat brain metastases (BM). This study was undertaken to examine the efficacy of SRS for BM from primary head and neck carcinomas. From 2000 to 2016, a total of 19 patients with 38 lesions were retrospectively identified. All patients presented with a primary head and neck malignancy and subsequently developed metastatic disease to the brain treated with SRS at our institution. Actuarial rates for overall survival (OS), local control (LC) and distant brain metastases (DBM) were calculated using Kaplan-Meier estimates. Median follow up was 6.8 months and median survival was 15.8 months. Eleven lesions received post-operative SRS to a surgical cavity and 27 lesions received definitive SRS to a metastasis. The median dose prescribed was 18 Gy. One-year actuarial rate for LC was 77.3% (95% confidence interval [CI] 44-92%) while 1 year and 2 year rates of OS were 52.9% (CI 28-73%) and 31.7% (CI 11-55%) respectively. The median time to develop DBM was 8.4 months. Three patients (16%) underwent repeat SRS following development of new BM and three patients (16%) underwent salvage whole brain radiotherapy (WBRT). SRS may be utilized in the treatment of patients with primary head and neck malignancies metastasized to the brain with high efficacy. Patients with well-controlled systemic disease and good performance status may benefit the most from definitive SRS while avoiding WBRT.

Despite recent advances in the treatment of medulloblastoma, patients in high-risk categories still face very poor outcomes. Evidence indicates that a subpopulation of cancer stem cells contributes to therapy resistance and tumour relapse in these patients. To prevent resistance and relapse, the development of treatment strategies tailored to target subgroup specific signalling circuits in high-risk medulloblastomas might be similarly important as targeting the cancer stem cell population. We have previously demonstrated potent antineoplastic effects for the PI3Kα selective inhibitor alpelisib in medulloblastoma. Here, we performed studies aimed to enhance the anti-medulloblastoma effects of alpelisib by simultaneous catalytic targeting of the mTOR kinase. Pharmacological mTOR inhibition potently enhanced the suppressive effects of alpelisib on cancer cell proliferation, colony formation and apoptosis and additionally blocked sphere-forming ability of medulloblastoma stem-like cancer cells in vitro. We identified the HH effector GLI1 as a target for dual PI3Kα and mTOR inhibition in SHH-type medulloblastoma and confirmed these results in HH-driven Ewing sarcoma cells. Importantly, pharmacologic mTOR inhibition greatly enhanced the inhibitory effects of alpelisib on medulloblastoma tumour growth in vivo. In summary, these findings highlight a key role for PI3K/mTOR signalling in GLI1 regulation in HH-driven cancers and suggest that combined PI3Kα/mTOR inhibition may be particularly interesting for the development of effective treatment strategies in high-risk medulloblastomas.

Medulloblastoma is a highly malignant pediatric brain tumor associated with poor outcome. Developing treatments that target the cancer stem cell (CSC) population in medulloblastoma are important to prevent tumor relapse and induce long-lasting clinical responses. We utilized medulloblastoma neurospheres that display CSC characteristics and found activation of the PI3K/AKT pathway in sphere-forming cells. Of all class IA PI3Ks, only the PI3Kα isoform was required for sphere formation by medulloblastoma cells. Knockdown of p110α, but not p110β or p110δ, significantly disrupted cancer stem cell frequencies as determined by extreme limiting dilution analysis (ELDA), indicating an essential role for the PI3Kα catalytic isoform in medulloblastoma CSCs. Importantly, pharmacologic inhibition of the MAPK-interacting kinase (MNK) enhanced the antineoplastic effects of targeted PI3Kα inhibition in medulloblastoma. This indicates that MNK signaling promotes survival in medulloblastoma, suggesting dual PI3Kα and MNK inhibition may provide a novel approach to target and eliminate medulloblastoma CSCs. We also observed a significant reduction in tumor formation in subcutaneous and intracranial mouse xenograft models, which further suggests that this combinatorial approach may represent an efficient therapeutic strategy for medulloblastoma. Implications: These findings raise the possibility of a unique therapeutic approach for medulloblastoma, involving MNK targeting to sensitize medulloblastoma CSCs to PI3Kα inhibition.

Glioblastoma multiforme remains the deadliest malignant brain tumor, with glioma stem cells (GSC) contributing to treatment resistance and tumor recurrence. We have identified MAPK-interacting kinases (MNK) as potential targets for the GSC population in glioblastoma multiforme. Isoform-level subtyping using The Cancer Genome Atlas revealed that both MNK genes (MKNK1 and MKNK2) are upregulated in mesenchymal glioblastoma multiforme as compared with other subtypes. Expression of MKNK1 is associated with increased glioma grade and correlated with the mesenchymal GSC marker, CD44, and coexpression of MKNK1 and CD44 predicts poor survival in glioblastoma multiforme. In established and patient-derived cell lines, pharmacologic MNK inhibition using LY2801653 (merestinib) inhibited phosphorylation of the eukaryotic translation initiation factor 4E, a crucial effector for MNK-induced mRNA translation in cancer cells and a marker of transformation. Importantly, merestinib inhibited growth of GSCs grown as neurospheres as determined by extreme limiting dilution analysis. When the effects of merestinib were assessed in vivo using an intracranial xenograft mouse model, improved overall survival was observed in merestinib-treated mice. Taken together, these data provide strong preclinical evidence that pharmacologic MNK inhibition targets mesenchymal glioblastoma multiforme and its GSC population. Implications: These findings raise the possibility of MNK inhibition as a viable therapeutic approach to target the mesenchymal subtype of glioblastoma multiforme. Mol Cancer Res; 14(10); 984–93. ©2016 AACR.

// Asneha Iqbal 1, 2 , Frank Eckerdt 1 , Jonathan Bell 1, 3 , Ichiro Nakano 4 , Francis J. Giles 1, 3 , Shi-Yuan Cheng 1 , Rishi R. Lulla 1, 2 , Stewart Goldman 1, 2 , Leonidas C. Platanias 1, 3, 5 1 Robert H. Lurie Comprehensive Cancer Center of Northwestern University, Chicago, IL, USA 2 Division of Hematology/Oncology/Stem Cell Transplantation, Ann and Robert H. Lurie Children’s Hospital of Chicago, Department of Pediatrics, Northwestern University Feinberg School of Medicine, Chicago, IL, USA 3 Division of Hematology/Oncology, Northwestern University, Feinberg School of Medicine, Chicago, IL, USA 4 Department of Neurological Surgery and James Comprehensive Cancer Center, The Ohio State University, Columbus, OH, USA 5 Department of Medicine, Jesse Brown VA Medical Center, Chicago, IL, USA Correspondence to: Leonidas C. Platanias, e-mail: l-platanias@northwestern.edu Keywords: glioblastoma, PIM kinase, mTOR signaling, p110α Received: February 04, 2016 Accepted: April 05, 2016 Published: April 21, 2016 ABSTRACT The PIM family of proteins encodes serine/threonine kinases with important roles in protein synthesis and cancer cell metabolism. In glioblastoma (GBM) cell lines, siRNA-mediated knockdown of PIM kinases or pharmacological inhibition of PIM kinases by SGI-1776 or AZD-1208 results in reduced phosphorylation of classic PIM effectors and also elements of the PI3K/mTOR pathway, suggesting interplay between PIM and mTOR signals in GBM cells. Combination of PIM kinase inhibitors with BYL-719, an inhibitor specific for the PI3K catalytic isoform p110α, results in enhanced antineoplastic effects in GBM cells. Additionally, pharmacologic inhibition of PIM kinases impairs growth of patient-derived glioma sphere cells, suggesting an important role for PIM kinases in cancer stem cell (CSC) function and survival. Such effects are further enhanced by concomitant inhibition of PIM kinase and p110α activities. Altogether these findings suggest that pharmacological PIM targeting in combination with PI3K inhibition may provide a unique therapeutic approach for the treatment of heterogeneous tumors containing populations of therapy-resistant CSCs in GBM.

Medulloblastoma is the most common paediatric malignant brain cancer and there is a need for new targeted therapeutic approaches to more effectively treat these malignant tumours, which can be divided into four molecular subtypes. Here, we focus on targeting sonic hedgehog (SHH) subtype medulloblastoma, which accounts for approximately 25% of all cases. The SHH subtype relies upon cholesterol signalling for tumour growth and maintenance of tumour-initiating cancer stem cells (CSCs). To target cholesterol signalling, we employed biomimetic high-density lipoprotein nanoparticles (HDL NPs) which bind to the HDL receptor, scavenger receptor type B-1 (SCARB1), depriving cells of natural HDL and their cholesterol cargo. We demonstrate uptake of HDL NPs in SCARB1 expressing medulloblastoma cells and depletion of cholesterol levels in cancer cells. HDL NPs potently blocked proliferation of medulloblastoma cells, as well as hedgehog-driven Ewing sarcoma cells. Furthermore, HDL NPs disrupted colony formation in medulloblastoma and depleted CSC populations in medulloblastoma and Ewing sarcoma. Altogether, our findings provide proof of principle for the development of a novel targeted approach for the treatment of medulloblastoma using HDL NPs. These findings present HDL-mimetic nanoparticles as a promising therapy for sonic hedgehog (SHH) subtype medulloblastoma and possibly other hedgehog-driven cancers.

Mesenchymal (MES) and proneural (PN) are two distinct glioma stem cell (GSC) populations that drive therapeutic resistance in glioblastoma (GBM). We screened a panel of 650 small molecules against patient-derived GBM cells to discover compounds targeting specific GBM subtypes. Arsenic trioxide (ATO), an FDA-approved drug that crosses the blood–brain barrier, was identified as a potent PN-specific compound in the initial screen and follow-up validation studies. Furthermore, MES and PN GSCs exhibited differential sensitivity to ATO. As ATO has been shown to activate the MAPK-interacting kinase 1 (MNK1)-eukaryotic translation initiation factor 4E (eIF4E) pathway and subsequent mRNA translation in a negative regulatory feedback manner, the mechanistic role of ATO resistance in MES GBM was explored. In GBM cells, ATO-activated translation initiation cellular events via the MNK1–eIF4E signaling axis. Furthermore, resistance to ATO in intracranial PDX tumors correlated with high eIF4E phosphorylation. Polysomal fractionation and microarray analysis of GBM cells were performed to identify ATO's effect on mRNA translation and enrichment of anti-apoptotic mRNAs in the ATO-induced translatome was found. Additionally, it was determined that MNK inhibition sensitized MES GSCs to ATO in neurosphere and apoptosis assays. Finally, examination of the effect of ATO on patients from a phase I/II clinical trial of ATO revealed that PN GBM patients responded better to ATO than other subtypes as demonstrated by longer overall and progression-free survival. Implications: These findings raise the possibility of a unique therapeutic approach for GBM, involving MNK1 targeting to sensitize MES GSCs to drugs like arsenic trioxide. Mol Cancer Res; 16(1); 32–46. ©2017 AACR.

BACKGROUND: The phosphoinositide 3-kinase (PI3K) and sonic hedgehog (SHH) pathways play important roles in medulloblastoma (MB) and other pediatric cancers. Aberrant activation of the PI3K pathway has been shown to be an important regulator of cancer cell proliferation, metabolism, protein synthesis and apoptosis. Additionally, mTOR activation contributes to therapy resistance likely in part through SMO-independent activation of the transcription factor GLI. Thus we sought to evaluate the effects of combined PI3K/mTOR inhibition in medulloblastoma and Ewing sarcoma. METHODS: Medulloblastoma (DAOY and D556) and Ewing Sarcoma cell lines (TC71, RDES) were grown in 2-D culture to investigate the effects of pharmacologic inhibition of PI3K and mTOR (using BYL-719, a p110α isoform specific PI3K inhibitor and OSI-027, a catalytic inhibitor of mTOR1/2, respectively) on protein kinase signaling, cell proliferation, colony formation, intracellular localization of GLI, and GLI target gene expression. RESULTS: Of all four class I PI3Ks only the p110α isoform is required for MB cell proliferation and colony formation. Pharmacologic targeting of the p110α isoform of PI3K with BYL-719 synergized with the catalytic mTORC1/2 inhibitor OSI-027, resulting in inhibition of effector pathways, and blocked cell proliferation and colony formation in both medulloblastoma and Ewing Sarcoma. Moreover, dual PI3K/mTOR inhibition resulted in decreased nuclear localization of GLI and reduced GLI target gene expression in both pediatric tumor cell lines. CONCLUSIONS: Inhibition of p110α and mTOR exerts potent antineoplastic effects on cancer cell proliferation and transformation. This effect may be due in part to inhibition of GLI nuclear localization. Thus, besides established roles in inhibition of cancer cell proliferation and protein synthesis, dual inhibition of p110α and mTOR is particularly promising for targeting SHH-driven cancers such as medulloblastoma and Ewing sarcoma.

Our recent work suggested that inhibition of mTORC1 activates Mnk in a PI3K-dependent manner, thereby providing a survival mechanism for medulloblastoma cells. The PI3K-Akt axis represents an important survival pathway that also confers therapy resistance to medulloblastoma stem cells, resulting in tumor recurrence. Here, we investigated a role for p110 isoforms of PI3K in medulloblastoma stem-like cancer cell biology and studied the potential of Mnk inhibition for sensitizing medulloblastoma stem-like cancer cells and orthotopic xenograft tumors to PI3K inhibition. We used medulloblastoma cell lines Daoy and D556 grown as conventional 2-D cultures or under stem-cell conditions as 3-D neurospheres to elucidate the roles of PI3K-Akt signaling in medulloblastoma proliferation, colony formation and stem cell function. We employed extreme limiting dilution analysis (ELDA) to ask whether concomitant Mnk inhibition enhances antineoplastic effects of PI3K inhibition on cancer stem cell growth. Additionally, in a preliminary intracereballar xenograft mouse study, we investigated the effects of pharmacologic PI3K and Mnk inhibition. We found that Akt activity greatly increased when 2-D cultures were converted into 3-D neurospheres. This Akt activation coincided with increased expression of CD133 and nestin, suggesting an important role for PI3K-Akt signaling in medulloblastoma stem cells. The p110a specific inhibitor BYL-719 blocked this Akt activation in neurospheres indicating this Akt activation is mediated by p110a. Consistently, of all class I PI3K catalytic isoforms (p110a, p110b, p110d and p110g) only knockdown of p110a disrupted stem cell frequencies in ELDA. We previously reported that mTORC1 inhibition engages Mnk signaling in a negative feedback manner to promote survival. Here we show that Mnk inhibition by CGP57380 sensitized medulloblastoma cells for pharmacologic inhibition and siRNA-mediated knockdown of p110a both in 2-D cancer cells and 3-D stem-like cancer cell cultures. After intracerebellar injection of medulloblastoma cells in nude mice, we found that combined targeting of PI3K-p110a and Mnk results in inhibition of tumor growth and increased survival. In summary, pharmacologic inhibition of PI3K-p110a by BYL-719 showed potent activity against medulloblastoma cells and stem-like cancer cells. Knockdown of p110a disrupted cancer stem cell frequency in ELDA and this effect was greatly enhanced by pharmacologic inhibition of Mnk. Finally, in an orthotopic mouse model we found that concomitant inhibition of p110a and Mnk prolonged survival and reduced tumor size. The striking effects of concomitant Mnk inhibition on stem-like cancer cells, neurospheres and tumors is particularly interesting as it suggests enhanced vulnerability of the therapy-resistant, tumor-initiating cancer stem cell population to p110a inhibition in medulloblastoma. Citation Format: Jonathan B. Bell, Frank Eckerdt, Quanhong Ma, Jessica R. Clymer, Stewart Goldman, Rintaro Hashizume, Leonidas C. Platanias. Mnk targeting enhances vulnerability of medulloblastoma stem-like cancer cells to PI3K-p110alpha inhibition. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-009.

MB-67. DUAL TARGETING OF PI3K AND mTOR SIGNALING IN MEDULLOBLASTOMA Jessica Clymer1,2, Frank Eckerdt2, Jonathan Bell2, Rishi Lulla1,2, Stewart Goldman1,2, and Leonidas Platanias2,3; Division of Hematology/ Oncology/Stem Cell Transplantation, Department of Pediatrics, Ann & Robert H. Lurie Children’s Hospital, Chicago, IL, USA; Robert H. Lurie Comprehensive Cancer Center, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA; Division of Hematology/Oncology, Department of Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA BACKGROUND: Aberrant activation of the PI3K pathway has been shown to play a role in medulloblastoma cell proliferation, while mTOR activation contributes to therapy resistance. Here we sought to evaluate the effects of combined targeting of the PI3K and mTOR pathways in medulloblastoma. METHODS: Medulloblastoma cell lines Daoy and D556 were grown in 2-D cultures to investigate the effects of pharmacologic inhibition of PI3K and mTORon kinase signaling, medulloblastomacell proliferation, colony formation, and apoptosis. After knockdown of p110 isoforms, cells were also grown under stem cell conditions as 3-D neurospheres and subjected to extreme limiting dilution analysis (ELDA) for assessment of stem-like cancer cell growth. RESULTS: Of all class I PI3K isoforms (p110a, p110b, p110d, and p110g), only p110a was essential for medulloblastoma cell proliferation, colony formation and neurosphere growth. Accordingly, pharmacologic targeting of the p110a isoform of PI3K with BYL-719 synergizedwith the mTOR inhibitor OSI-027, as it resulted in inhibition of effector pathways, blocked cell proliferation and colony formation, and increased malignant cell death by apoptosis. Finally,BYL-719and OSI-027 greatly impairedgrowthof stem-likecancer cells as 3-D neurospheres. CONCLUSIONS: The p110a isoform of PI3K exerts crucial roles in medulloblastoma. Inhibition of p110a and mTOR exerts potent antineoplastic effects on medulloblastoma cells and also blocks neurosphere growth. Dual inhibition of p110a and mTOR might be promising for targeting both malignant medulloblastoma cells and the therapy-resistant cancer stem cell population. Neuro-Oncology 18:iii97–iii122, 2016. doi:10.1093/neuonc/now076.63 #The Author(s) 2016. Published by Oxford University Press on behalf of the Society for Neuro-Oncology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Glioblastoma (GBM) is the deadliest primary brain tumor with a median survival of around one year. Arsenic trioxide (ATO) is an emerging therapy for the treatment of GBM and other malignant brain tumors. The cytotoxic effects of ATO are mainly mediated by the production of reactive oxygen species and induction of cell death pathways. However, glioma stem cells in heterogeneous GBM tumors impart resistance by activation of survival pathways, thereby preventing therapeutic responses to cytotoxic agents such as ATO. We have previously shown that ATO responses in leukemia are antagonized by the MAPK-interacting kinases (MNKs), which activate protein translation and survival pathways including the eukaryotic translation initiation factor 4E (eIF4E) in response to ATO treatment. Yet, the role of MNK signaling in GBM and glioma stem cells and the potential of using MNK inhibitors to sensitize GBM to ATO has not been explored. In this study, we sought to determine the mechanisms by which MNK signaling regulates arsenic trioxide responses in GBM and glioma stem cells. GBM cell lines were treated with ATO in the presence or absence of MNK inhibitors or siRNA against MNK isoforms. Western blots of treated samples were analyzed with antibodies against phosphorylated eIF4E, the key downstream effector of the MNKs. Following treatment with ATO and MNK inhibitors, proliferation rate and apoptosis were determined by WST-1 assay and Annexin V-FITC/PI staining. GBM cell lines were grown under stem cell conditions and subjected to qPCR and flow cytometry to monitor CD44 expression and aldehyde dehydrogenase (ALDH) activity, both markers of stemness. Patient-derived glioma stem cell lines displaying mesenchymal-like phenotype were treated with ATO and MNK inhibitors and analyzed by neurosphere formation assay. Treatment of GBM cell lines with ATO resulted in MNK activation and induction of eIF4E phosphorylation in a MNK1-depedent manner. Furthermore, MNK inhibition sensitized GBM cells to the anti-proliferative and pro-apoptotic effects of ATO. Knockdown of MNK1 in GBM cell lines grown under stem cell conditions decreased neurosphere formation. Finally, pharmacological MNK inhibition sensitized mesenchymal-like glioma stem cells to ATO. Our results suggest ATO in combination with MNK inhibition might represent a new approach for the treatment of GBM, in particular the therapy-resistant glioma stem cell subpopulation. Citation Format: Jonathan B. Bell, Frank Eckerdt, Ahmet Dirim Arslan, Asneha Iqbal, Angel A. Alvarez, Shi-Yuan Cheng, Ichiro Nakano, Leonidas C. Platanias. MAPK-interacting kinase inhibition sensitizes glioblastoma and glioma stem cells to arsenic trioxide. [abstract]. In: Proceedings of the AACR Special Conference: Advances in Brain Cancer Research; May 27-30, 2015; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2015;75(23 Suppl):Abstract nr B26.

The mTOR pathway controls mRNA translation of mitogenic proteins and is a central regulator of metabolism in malignant cells. Development of malignant cell resistance is a limiting factor to the effects of mTOR inhibitors, but the mechanisms accounting for such resistance are not well understood. We provide evidence that mTORC1 inhibition by rapamycin results in engagement of a negative feedback regulatory loop in malignant medulloblastoma cells, involving phosphorylation of the eukaryotic translation-initiation factor eIF4E. This eIF4E phosphorylation is Mnk2- mediated, but Mnk1-independent, and acts as a survival mechanism for medulloblastoma cells. Pharmacological targeting of Mnk1/2 or siRNA-mediated knockdown of Mnk2 sensitizes medulloblastoma cells to mTOR inhibition and promotes suppression of malignant cell proliferation and anchorage-independent growth. Altogether, these findings provide evidence for the existence of a Mnk2-controlled feedback loop in medulloblastoma cells that accounts for resistance to mTOR inhibitors, and raise the potential for combination treatments of mTOR and Mnk inhibitors for the treatment of medulloblastoma.