266 Combination Drug Therapy in H3 K27-Altered Diffuse Midline Glioma Enhances Tumor Retention and Treatment Efficacy

INTRODUCTION: Diffuse midline gliomas (DMG) with H3 K27 alterations are uniformly lethal tumors for which no therapy has been found to be effective. One reason for our therapy failure is the blood-brain barrier (BBB), which severely restricts uptake and accumulation of systemically administered drugs in the brain. Convection-enhanced delivery (CED) has emerged as a technique that circumvents the BBB by directly infusing therapeutics into the tumor parenchyma through generation of a mechanical pressure gradient, allowing for homogenous drug distribution at high local concentrations with minimal systemic toxicity. However, efflux transport proteins expressed by the BBB and the tumor, including P-gp and BCRP, rapidly clear the majority of drugs given by CED, severely limiting its therapeutic potential. METHODS: A panel of FDA-approved drugs (ponatinib, topotecan, tretinoin) were evaluated alone and in combination with the known efflux transporter inhibitor everolimus. RESULTS: A high degree of formal synergy was observed in multiple DMG cell lines in vitro, which was attributed to everolimus’ ability to inhibit mTOR signaling and P-gp/BCRP. In vivo, drugs that were expected to penetrate the BBB and be less likely to undergo active efflux persisted at the site of infusion the longest following CED. Concurrent dosing with everolimus further enhanced this effect, which was particularly apparent with the dual P-gp/BCRP substrate ponatinib. CONCLUSIONS: The proclivity of many drugs to be rapidly cleared from the brain by efflux transporters generates a vulnerability that can be exploited for the development of novel therapeutic regimens in DMG, such as with concomitant dosing of P-gp/BCRP-inhibiting drugs when using CED.