495 5-ALA PDT and Targeting MEK/ERK Signaling Elicits Synergistic Antitumor Effects in Diffuse Midline Glioma

Abstract

INTRODUCTION: Diffuse midline gliomas are highly invasive, unresectable tumors in children. To date, there is no effective treatment for DMG. Concurrent radiotherapy (RT) and systemic therapies, the standard of care, has only been successful in providing limited disease control. The major obstacle to therapy is the selectively permeable blood brain barrier (BBB) that limits systemic drug delivery. Given the unmet need for penetrant and minimally invasive DMG treatments, photodynamic therapy (PDT), with the precursor photosensitizing agent 5-aminolevulinic acid (5-ALA), is an oncologic treatment that holds promise. 5-ALA PDT of tumors occurs by targeting tumor cells that accumulate the 5-ALA metabolite, protoporphyrin IX (PPIX), with 635 nm light to create deadly reactive oxygen species (ROS). METHODS: Growth delay assays were conducted for each individual treatment modality or in combination. FDA-approved MEK/ERK inhibitors were administered to mice with aggressive heterotopic allograft DMGs through intraperitoneal injection. Ex vivo immunofluorescence staining was performed on those mice that received 5-ALA PDT treatment. RESULTS: We demonstrated that sub-micromolar levels of 5-ALA PDT and nanomolar levels of MEK inhibitor successfully decrease cell proliferation and induce apoptosis in DMG cell lines. Cell viability assays revealed that drug response differs based on the histone mutation (H3.1 or H3.3) of the line. Mechanisms of decreased cell survival involves the generation of reactive oxygen species that induces programmed cell death. Through the use of a DMG genetically engineered mouse model, we also found 5-ALA PDT to induce apoptosis in vivo. CONCLUSION: The synergistic effects of MEK inhibition and 5-ALA PDT in vitro and apoptotic effects of 5-ALA PDT in vivo, highlights the potential therapeutic efficacy of this treatment modality.