151 Exceptional Responders to Base Excision Repair (BER) Inhibition for Recurrent Glioblastoma Display Enrichment for DNA Damage Response Pathways: RNA Sequencing Analysis From a Multicenter Trial

INTRODUCTION: Therapies for glioblastoma recurrence after surgery and chemoradiation are warranted. TRC102 (methoxyamine), a small molecule DNA base-excision repair (BER) inhibitor, reverses temozolomide (TMZ) resistance in preclinical glioma models. We had investigated efficacy of TRC102+TMZ for recurrent glioblastoma (rGBM) through multicenter trial. METHODS: A two-arm, two-stage, non-randomized, Phase II trial of TRC102+TMZ for adults with rGBM was conducted. Arm 1 included bevacizumab-naive patients at first recurrence, with primary aim of estimating response rates using RANO criteria. If sufficient activity was identified, arm 2 was planned in bevacizumab-refractory patients. Secondary aims were to determine overall survival (OS), progression-free survival (PFS), PFS at six months, and toxicity using CTCAE v4. 0. Exploratory aims were correlating clinical outcomes with MPG expression and MGMT methylation. Differential gene expression profiling was analyzed using DESeq2 GSVA (RRID: SCR₀15687) and transformed into enrichment scores for 'hallmark of DNA repair' pathways from Molecular Signatures Database (MSigDB, RRID: SCR₀16863) to assign mechanistic signatures of therapeutic vulnerability. RESULTS: Arm 1 enrolled 19 patients with median of two treatment cycles, but objective responses were not observed. Hence, arm 2 did not open. Median OS was 11. 1 months (95%CI 8. 2-17. 9). Median PFS was 1. 9 months (95%CI 1. 8-3. 7). MGMT promoter was unmethylated in 14/19 patients, and MPG expression was present in 8/14 cases with available tumor tissue. MGMT methylation and MPG non-expression were associated with better OS and PFS. However, two ‘exceptional responders’ had PFS = 11 months, with MGMT methylated and MPG expressed. RNA sequencing of their tumor tissue demonstrated significant enrichment for DNA damage response (DDR) pathways (MSigDB), chromosomal instability gene signatures (CIN70, CIN25), and proliferative gene signatures (PCNA25). CONCLUSIONS: rGBM patients with elevated levels of MPG and DDR molecular signature may have impaired BER and respond better to TRC102+TMZ.