An adenoviral vector engineered to express the herpes simplex virus thymidine kinase (HSV-tk) gene, which, when administered in conjunction with a synthetic acyclic guanosine analogue, possesses potential antineoplastic activity. Aglatimagene besadenovec is transduced into tumor cells, sensitizing tumor cells that overexpress HSV-tk to synthetic acyclic guanosine analogues. Subsequently, a low dose of a synthetic acyclic guanosine analogue such as valacyclovir (VCV) or ganciclovir (GCV) is given, which may preferentially kill tumor cells containing the adenoviral vector and overexpressing HSV-tk. Release of tumor-associated antigens (TAAs) by dying tumor cells may then stimulate an antitumor cytotoxic T lymphocyte (CTL) response.