Modulating native GABAA receptors in medulloblastoma with positive allosteric benzodiazepine-derivatives induces cell death.

PURPOSE: Pediatric brain cancer medulloblastoma (MB) standard-of-care results in numerous comorbidities. MB is comprised of distinct molecular subgroups. Group 3 molecular subgroup patients have the highest relapse rates and after standard-of-care have a 20% survival. Group 3 tumors have high expression of GABRA5, which codes for the α5 subunit of the γ-aminobutyric acid type A receptor (GABAAR). We are advancing a therapeutic approach for group 3 based on GABAAR modulation using benzodiazepine-derivatives. METHODS: We performed analysis of GABR and MYC expression in MB tumors and used molecular, cell biological, and whole-cell electrophysiology approaches to establish presence of a functional 'druggable' GABAAR in group 3 cells. RESULTS: Analysis of expression of 763 MB tumors reveals that group 3 tumors share high subgroup-specific and correlative expression of GABR genes, which code for GABAAR subunits α5, ß3 and γ2 and 3. There are ~ 1000 functional α5-GABAARs per group 3 patient-derived cell that mediate a basal chloride-anion efflux of 2 × 109 ions/s. Benzodiazepines, designed to prefer α5-GABAAR, impair group 3 cell viability by enhancing chloride-anion efflux with subtle changes in their structure having significant impact on potency. A potent, non-toxic benzodiazepine ('KRM-II-08') binds to the α5-GABAAR (0.8 µM EC50) enhancing a chloride-anion efflux that induces mitochondrial membrane depolarization and in response, TP53 upregulation and p53, constitutively phosphorylated at S392, cytoplasmic localization. This correlates with pro-apoptotic Bcl-2-associated death promoter protein localization. CONCLUSION: GABRA5 expression can serve as a diagnostic biomarker for group 3 tumors, while α5-GABAAR is a therapeutic target for benzodiazepine binding, enhancing an ion imbalance that induces apoptosis.

First In Vivo Testing of Compounds Targeting Group 3 Medulloblastomas Using an Implantable Microdevice as a New Paradigm for Drug Development.

Medulloblastoma is the most common childhood malignant brain tumor. The most lethal medulloblastoma subtype exhibits a high expression of the GABAA receptor α5 subunit gene and MYC amplification. New benzodiazepines have been synthesized to function as α5-GABAA receptor ligands. To compare their efficacy with that of standard-of-care treatments, we have employed a newly developed microscale implantable device that allows for high-throughput localized intratumor drug delivery and efficacy testing. Microdoses of each drug were delivered into small distinct regions of tumors, as confirmed by tissue mass spectrometry, and the local drug effect was determined by immunohistochemistry. We have identified a benzodiazepine derivative, KRM-II-08, as a new potent inhibitor in several α5-GABAA receptor expressing tumor models. This is the first instance of in vivo testing of several benzodiazepine derivatives and standard chemotherapeutic drugs within the same tumor. Obtaining high-throughput drug efficacy data within a native tumor microenvironment as detailed herein, prior to pharmacological optimization for bioavailability or safety and without systemic exposure or toxicity, may allow for rapid prioritization of drug candidates for further pharmacological optimization.