ET-27 * REPLICATION AND SPREAD OF ONCOLYTIC HERPES VIRUS IN GLIOMA STEM CELLS CAN BE ENHANCED BY SPECIFIC INHIBITION OF HISTONE DEACETYLASE 6

Abstract

High-grade gliomas remain clinically challenging and under current therapies the prognosis is generally dismal. Intratumoral heterogeneity, therapy resistance, and tumor recurrence likely caused by glioma stem cells (GSCs) still represent major challenges, which need to be addresses by novel, multimodal therapies. We attempted to combine rQnestin34.5, an oncolytic herpes simplex virus expressing its ICP34.5 virulence gene under the glioma-specific nestin promoter, with specific inhibition of histone deacetylase 6 (HDAC6). We have previously reported that pan-HDAC inhibition using valproic acid enhances rQnestin34.5's antitumor activity and our data obtained in established glioma cell lines indicate that sole inhibition of HDAC6 can at least in part be accounted for this effect. We now analyzed the influence of the specific HDAC6 inhibitor Tubacin on rQnestin34.5 replication in GSC lines originating from proneural, classic, and mesenchymal glioma subtypes. Time-lapse microscopy revealed that plaque formation and spread were accelerated within the first 48 hours after infection when cells were pre-treated with Tubacin, translating into significantly increased cytotoxicity in the respective time frame. Viral DNA replication was enhanced after combination treatment as increased viral copy numbers per cell were detected by qPCR. Furthermore, HDAC6 inhibition increased mRNA expression levels of immediate early and late genes. Importantly, the positive effect of Tubacin on rQnestin34.5 replication and spread in GSCs was critically dependent on drug pre-treatment regimens and very low infectious titers. We also noted that the benefit of rQnestin34.5 oncolysis from HDAC6 inhibition varied quantitatively between GSCs. While responsiveness was not determined by glioma subtype or HDAC6 expression levels, we are currently investigating whether the choice of the exact viral entry mechanism can be used as a predictive marker. Future studies will also aim at establishing a therapeutic regimen demonstrating the potential benefit of HDAC6 inhibition in addition to herpes simplex oncolysis in a GSC-derived orthotopic glioma model.