HSV-1 Vectors for Gene Therapy of Experimental CNS Tumors

Gliomas account for about 60% of all primary CNS tumors; two-thirds of all gliomas comprise the most malignant form, glioblastoma multiforme, or glioma grade IV. Although much progress has been achieved in the treatment of other solid tumors over the last few decades, the median survival of patients with glioblastoma remains at around 12 mo after standard treatment, which includes bulk resection and irradiation, as well as chemotherapy in some cases (1). Essentially, no patient can expect to survive 5 yr. New treatment modalities like immunotherapy have been applied so far with only limited success (2). With the improvement of methods for in vivo and ex vivo gene delivery, gene therapy became a new, promising approach to glioma therapy. Gliomas appear to be a particularly good target for a gene therapy approach using locally applied vectors, as the growth of gliomas is restricted to the brain. Clinical trials are under way using retrovirus and adenovirus vectors which carry the herpes simplex virus type-1 (HSV-1) thymidine kinase gene (HSV-tk). This gene encodes a prodrug-activating enzyme, which in infected cells converts the nontoxic prodrug, ganciclovir (GCV), to its cytotoxic phosphorylated form (3-5). There is an ever-increasing list of other prodrug-activation systems that showed efficacy in culture and in preclinical studies using rodent glioma models. These include, for example, cytosine deaminase converting 5-fluorocytosine to 5-fluoro-uracil (6), cytochrome P450-2B1 converting cyclophosphamide to phosphoramide mustard (7), deoxycytidine kinase phosphorylating cytosine arabinoside (8), and the Escherichia coli guanine phosphoribosyl transferase (gpt) metabolizing 6-thioxanthine and 6-thioguanine to toxic nucleoside analogs (9). Moreover, gene therapy approaches to brain tumors include the viral transfer of immune-enhancing cytokines, particularly granulocyte/macrophage colony-stimulating factor (10), or antisense to TGF-β to glioma cells (11) used for vaccination purposes. Other approaches use the transfer of genes that modulate angiogenesis (12,13) or are involved in apoptosis like p53 (14). All aforementioned gene-transfer methods use nonreplicative viral vectors.