Brain tumor eradication and prolonged survival from intratumoral conversion of 5-fluorocytosine to 5-fluorouracil using a nonlytic retroviral replicating vector

The histone deacetylase inhibitor valproic acid lessens NK cell action against oncolytic virus-infected glioblastoma cells by inhibition of STAT5/T-BET signaling and generation of gamma interferon

Tumor virotherapy has been and continues to be used in clinical trials. One barrier to effective viral oncolysis, consisting of the interferon (IFN) response induced by viral infection, is inhibited by valproic acid (VPA) and other histone deacetylase inhibitors (HDACi). Innate immune cell recruitment and activation have been shown to be deleterious to the efficacy of oncolytic herpes simplex virus (oHSV) infection, and in this report we demonstrate that VPA limits this deleterious response. VPA, administered prior to oHSV inoculation in an orthotopic glioblastoma mouse model, resulted in a decline in NK and macrophage recruitment into tumor-bearing brains at 6 and 24 h post-oHSV infection. Interestingly, there was a robust rebound of recruitment of these cells at 72 h post-oHSV infection. The observed initial decline in immune cell recruitment was accompanied by a reduction in their activation status. VPA was also found to have a profound immunosuppressive effect on human NK cells in vitro. NK cytotoxicity was abrogated following exposure to VPA, consistent with downmodulation of cytotoxic gene expression of granzyme B and perforin at the mRNA and protein levels. In addition, suppression of gamma IFN (IFN-γ) production by VPA was associated with decreased STAT5 phosphorylation and dampened T-BET expression. Despite VPA-mediated immune suppression, mice were not at significantly increased risk for HSV encephalitis. These findings indicate that one of the avenues by which VPA enhances oHSV efficacy is through initial suppression of immune cell recruitment and inhibition of inflammatory cell pathways within NK cells.

Retroviral replicating vectors in cancer

The use of replication-competent viruses for the treatment of cancer is an emerging technology that shows significant promise. Among the various different types of viruses currently being developed as oncolytic agents, retroviral replicating vectors (RRVs) possess unique characteristics that allow highly efficient, non-lytic, and tumor-selective gene transfer. By retaining all of the elements necessary for viral replication, RRVs are capable of transmitting genes via exponential in situ amplification. Their replication-competence also provides a powerful means whereby novel and useful RRV variants can be generated using natural selection. Their stringent requirement for cell division in order to achieve productive infection, and their preferential replication in cells with defective innate immunity, confer a considerable degree of natural specificity for tumors. Furthermore, their ability to integrate stably into the genome of cancer cells, without immediate cytolysis, contributes to long-lasting therapeutic efficacy. Thus, RRVs show much promise as therapeutic agents for cancer and are currently being tested in the clinic. Here we describe experimental methods for their production and quantitation, for adaptive evolution and natural selection to develop novel or improved RRV, and for in vitro and in vivo assessment of the therapeutic efficacy of RRVs carrying prodrug activator genes for treatment of cancer.