Cytolytic viruses as potential anti-cancer agents.

The resistance of cancers to conventional therapies has inspired the search for novel strategies. One such approach, namely gene therapy, is based upon the introduction of genes such as those encoding suicide proteins, tumour suppressor proteins or cytokines into tumour cells by means of a genetic vector. The efficiency with which viruses transfer their genes from one host cell to another has led to the widespread use of viruses as genetic vectors. For safety reasons, such virus vectors are generally replication-defective but, unfortunately, this has limited the efficacy of treatment by restricting the number of cells to which the therapeutic gene is delivered. For this reason, the use of replication-competent viruses has been proposed, since virus replication would be expected to lead to amplification and spread of the therapeutic genes in vivo. The replication of many viruses results in lysis of the host cells. This inherent cytotoxicity, together with the efficiency with which viruses can spread from one cell to another, has inspired the notion that replication-competent viruses could be exploited for cancer treatment. Some viruses have been shown to replicate more efficiently in transformed cells but it is unlikely that such examples will exhibit a high enough degree of tumour selectivity, and hence safety, for the treatment of patients. Our increasing knowledge of the pathogenesis of virus disease and the ability to manipulate specific regions of viral genomes have allowed the construction of viruses that are attenuated in normal cells but retain their ability to lyse tumour cells. Such manipulations have included modifying the ability of viruses to bind to, or replicate in, particular cell types, while others have involved the construction of replication-competent viruses encoding suicide proteins or cytokines. Naturally occurring or genetically engineered oncolytic viruses based upon adenovirus, herpes simplex virus, Newcastle disease virus, poliovirus, vesicular stomatitis virus, weasles virus and reovirus have been described. The results of animal studies are encouraging and a number of viruses are now being evaluated in clinical trials.