Development of an oncolytic Herpes Simplex Virus using a tumor-specific HIF-responsive promoter

We exploited the differential activation of hypoxia-inducible factor (HIF)-dependent gene expression in tumors versus normal tissue for the design of a targeted oncolytic Herpes simplex virus type-1 (HSV-1). A gene that is essential for viral replication, ICP4, was placed under the regulation of a HIF-responsive promoter and then introduced into the thymidine kinase locus (U(L)23) of HSV d120 which contains partial deletions in the two endogenous ICP4 genes. Recombinant HIF-HSV were isolated and their derivation from d120 was verified by expression of a truncated, nonfunctional form of ICP4 protein. Disruption of the U(L)23 locus was confirmed by loss of thymidine kinase expression and resistance to acyclovir. Unexpectedly, HIF-HSV expressed ICP4 and induced tumor cell lysis at similar levels under normoxia and hypoxia. The lack of HIF-dependent ICP4 transgene expression by HIF-HSV was due to two factors that have not previously been reported- reversion of the ICP4 gene region to its wild-type configuration and increased HIF-transcriptional activity under normoxia when cells were infected with any strain of HSV-1. The findings that an oncolytic HSV-1 is genetically unstable and can activate a tumor-related promoter in a non-specific manner have important implications for any proposed use of this virus in cancer therapy.