Abstract B54: Treatment of breast cancer cells with histone deacetylase inhibitors increases the replication of an oncolytic herpes simplex virus

The purpose of this study was to improve the efficacy of oncolytic herpes simplex virus (HSV) against metastatic breast cancer by combining treatment with histone deacetylase (HDAC) inhibitors. Metastatic breast cancer continues to be a significant clinical challenge as the survival rates remain below 25%, illustrating the dire need for newly developed therapeutic strategies. Oncolytic viruses selectively replicate in cancer cells but not in normal cells and are an exciting potential therapy being evaluated in patients with breast cancer. HSV is a well-studied oncolytic virus that has been employed in clinical trials for several cancers. These trials have demonstrated safety, but HSV has generally lacked sufficient potency to yield durable tumor responses. In part, this is due to low viral replication and persistence. For tumor-selective replication, oncolytic HSV (oHSV) typically lack the diploid viral γ134.5 gene, which encodes a multifunctional protein that also contributes to viral evasion of host interferon response. Because γ134.5-deleted HSVs are particularly sensitive to interferon, reducing this response may be an effective means of enhancing oncolytic HSV therapy while maintaining deficiency of replication in normal cells. HDAC inhibitors have been shown to reduce the expression of interferon-stimulated genes. Therefore, we hypothesized that oHSV replication and efficacy could be improved by treatment with particular HDAC inhibitors. We tested a panel of HDAC inhibitors with distinct inhibition profiles and selectivity towards different classes of HDACs. Cell viability assays were used to establish lethal dose 50 (LD50) values for each compound and each was tested for its ability to modulate the replication of a γ134.5-deleted oncolytic HSV. The inhibitors were tested at three different dose levels: low (non-lethal), middle (partially lethal), and high (lethal), and two different treatment schedules: prior to viral infection (pre-treatment) or immediately following viral infection (co-treatment). The cell lines selected for this study included the metastatic breast cancer line MDA-MB-231 that was previously shown to be oHSV replication-competent and the oHSV replication-resistant cell lines: immortalized but otherwise normal breast epithelial cell line MCF10A and metastatic murine mammary carcinoma 4T1. Although pre-treatment did not affect viral replication and co-treatment actually reduced it at a high multiplicity of infection (MOI), at a low MOI, significantly enhanced viral replication was observed with both pre- and co-treatment in MDA-MB-231 cells (magnitude of increase dependent on particular HDAC inhibitor), but not in MCF10A. Interestingly, the oHSV-resistant 4T1 cells were rendered permissive by HDAC-inhibitor treatment as significant increases in oHSV replication were noted. These studies demonstrate that HDAC inhibitors can be used to induce viral replication depending on the particular compound and MOI of the virus. Furthermore, these results suggest that HDAC inhibitors could prove useful in combination with oHSV as a novel treatment strategy for metastatic breast cancer. Citation Format: James J. Cody, James M. Markert, Douglas R. Hurst. Treatment of breast cancer cells with histone deacetylase inhibitors increases the replication of an oncolytic herpes simplex virus. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Jun 19-22, 2013; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2013;73(13 Suppl):Abstract nr B54.