CTIM-28. FIRST-IN-HUMAN TRANSGENE DETECTION 17 MONTHS POST-ADENOVIRAL THERAPY

Abstract

Abstract Gene therapy was developed as a promising approach to treat high-grade glioma (HGG). One of the main strategies is suicide gene therapy. This method introduces the expression of an enzyme that converts a non-toxic prodrug to an active toxic compound, thereby causing the death of transduced cells. Specifically, herpes simplex virus thymidine kinase (HSV1-TK) converts the pro-drug valacyclovir into phosphorylated valacyclovir. When cells initiate DNA replication, phosphorylated valacyclovir is incorporated into de novo DNA strands, causing DNA replication to arrest, followed by apoptosis. This approach is lethal only for replicating cells, (i.e., tumor cells), without harming non-dividing brain cells. An infected cell, expressing HSV1-TK, requires two further simultaneous events to induce cell death: (1) to be exposed to valacyclovir and (2) enter into cell division. Without either of these events, the cell will remain alive and will still express HSV1-TK. Previous work from our lab in murine models demonstrated the long-term expression of transgenes β-galactosidase and HSV1-TK up to 12 months after viral therapy administration (Zermansky et al 2001; Barcia et al 2007). We recently completed and published our Phase I clinical trial (NCT01811992), where dual vector viral therapy, Ad-TK/Ad-Flt3L, and valacyclovir was administered to patients suffering from HGG (Umemura et al, 2023). In paraffin embedded tumor samples from recurrent tumors to our surprise we detected HSV1-TK immunoreactivity up to 17 months after initial treatment. HSV1-TK immunoreactive cells were found in five out of eight patients’ samples. We analyzed HSV1-TK expression to further characterize HSV1-TK positive cells using combination immunohistochemistry, allowing us to identify them as tumor cells, neurons, or astrocytes. The presence of HSV-TK positive cells in the recurrent tumors strongly supports the administration of further cycles of pro drug (valacyclovir), following Ad-TK administration.