Medulloblastoma is the most common malignant tumor that arises from the cerebellum of the central nervous system. Clinically, medulloblastomas are treated by surgery, radiation, and chemotherapy, all of which result in toxicity and morbidity. Recent reports have identified that DDX3, a member of the RNA helicase family, is mutated in medulloblastoma. In this study, we demonstrate the role of DDX3 in driving medulloblastoma. With the use of a small molecule inhibitor of DDX3, RK-33, we could inhibit growth and promote cell death in two medulloblastoma cell lines, DAOY and UW228, with IC50 values of 2.5 μM and 3.5 μM, respectively. Treatment of DAOY and UW228 cells with RK-33 caused a G1 arrest, resulted in reduced TCF reporter activity, and reduced mRNA expression levels of downstream target genes of the WNT pathway, such as Axin2, CCND1, MYC, and Survivin. In addition, treatment of DAOY and UW228 cells with a combination of RK-33 and radiation exhibited a synergistic effect. Importantly, the combination of RK-33 and 5 Gy radiation caused tumor regression in a mouse xenograft model of medulloblastoma. Using immunohistochemistry, we observed DDX3 expression in both pediatric (55%) and adult (66%) medulloblastoma patients. Based on these results, we conclude that RK-33 is a promising radiosensitizing agent that inhibits DDX3 activity and down-regulates WNT/β-catenin signaling and could be used as a frontline therapeutic strategy for DDX3-expressing medulloblastomas in combination with radiation.

A subset of poor-prognosis medulloblastoma has genomic amplification of MYC. MYC regulates glutamine metabolism in multiple cellular contexts. We modified the glutamine analog 6-diazo-5-oxo-l-norleucine (DON) to mask its carboxylate and amine functionalities, creating a prodrug termed JHU-083 with increased oral bioavailability. We hypothesized that this prodrug would kill MYC-expressing medulloblastoma. JHU-083 treatment caused decreased growth and increased apoptosis in human MYC-expressing medulloblastoma cell lines. We generated a mouse MYC-driven medulloblastoma model by transforming C57BL/6 mouse cerebellar stem and progenitor cells. When implanted into the brains of C57BL/6 mice, these cells formed large cell/anaplastic tumors that resembled aggressive medulloblastoma. A cell line derived from this model was sensitive to JHU-083 in vitro. Oral administration of JHU-038 led to the accumulation of micromolar concentrations of DON in the mouse brain. JHU-083 treatment significantly increased the survival of immune-competent animals bearing orthotopic tumors formed by the mouse cerebellar stem cell model as well as immune-deficient animals bearing orthotopic tumors formed by a human MYC-amplified medulloblastoma cell line. These data provide pre-clinical justification for the ongoing development and testing of orally bioavailable DON prodrugs for use in medulloblastoma patients.