G1TKSVNa.53

**G1TKSVNa.53** is an investigational, producer-cell-mediated retroviral gene therapy developed for recurrent malignant brain tumors. The product consists of a PA317 murine fibroblast producer-cell clone that releases replication-defective retroviral vectors encoding the **herpes simplex virus type 1 thymidine kinase** gene. Following intratumoral administration, tumor cells transduced with the HSV thymidine kinase gene become susceptible to intravenously administered ganciclovir; HSV thymidine kinase phosphorylates ganciclovir, enabling formation of cytotoxic metabolites that disrupt DNA synthesis and promote tumor-cell death. It was evaluated with systemic ganciclovir in a completed Phase 1 study in adults with recurrent malignant brain tumors.