EXTH-68. RECURRENT XENOGRAFT TUMORS UPREGULATE EGFR AFTER LENTIVIRAL VECTOR MEDIATED SUICIDE GENE THERAPY FOR GLIOBLASTOMA, BUT ARE RESISTANT TO COMBINATORIAL TREATMENT WITH ERLOTINIB

Lentiviral vector mediated herpes simplex virus thymidine kinase (HSV-Tk)/ ganciclovir (GCV) therapy is a very promising therapeutic option for the treatment of glioblastoma (GBM). Although this therapy leads to complete remission of GBM in an orthotopic PDX model, recurrent tumors are observed, which contain a fraction of surviving Tk-GFP+ cells. Here, we hypothesize that prolonged prodrug treatment could provide a significant survival benefit through killing of these remaining Tk-GFP+ cells. After intracranial implantation of a patient derived biopsy, tumor growth was monitored by MRI and when the tumors reached a measurable size, lentiviral vectors encoding Tk-GFP were injected intratumorally followed by intraperitoneal GCV administration for 3 weeks or oral Valganciclovir (ValGCV) administration for 3 months. After euthanasia, brains were fixed and paraffin embedded for immunohistochemical analysis and/or snap-frozen for RNA-sequencing. Initially, we sorted the remaining Tk-GFP+ glioma cells from recurrent tumors after GCV treatment and showed that these cells retained sensitivity to GCV in vitro. Therefore we decided to use prolonged administration (3 months) of valganciclovir (valGCV), which is similar to GCV but tailored for oral administration, and compared it to the 3-week period of GCV administration in vivo. Prolonged ValGCV therapy resulted in a significant survival advantage compared to short-term (3 weeks) GCV application. Nonetheless, the majority of animals treated with valGCV also developed recurrent tumors. These were more invasive compared to the primary tumors and showed significant upregulation of the epidermal growth factor receptor (EGFR) warranting for a combinatorial treatment of HSV-Tk/valGCV with an anti-EGFR therapy. However, the combination of suicide gene therapy with erlotinib, a widely used EGFR small molecule inhibitor, failed to provide survival benefit. Currently we are initiating RNA sequencing experiments on recurrent compared to primary tumors to unravel potential changes in glioma cells refractory to suicide gene therapy and/or erlotinib.