Radiation Therapy and Talimogene Laherparepvec (TVEC) Synergize in the Treatment of Melanoma

Abstract Talimogene laherparepvec (TVEC), an oncolytic virus, is an FDA-approved therapy that increases the overall survival of locally advanced and metastatic melanoma patients. Radiation therapy (RT) in combination with immunotherapies improves response rates (compared to either modality alone). Therefore, we hypothesized that combination RT and TVEC results in a synergistic therapeutic effect on melanoma. We treated 8 human melanoma cell lines with radiation (0, 4, or 8 Gy) and/or T-VEC (0, 0.01, 0.1, or 1 MOI) and determined its therapeutic effects using the AlamarBlue cell viability assay. Treatment of UACC257 melanoma cells with combination RT (8 Gy) and TVEC (1 MOI) resulted in a significant decrease in tumor cell viability (a decrease of 44.9%) compared to no treatment, RT alone (33.2%, P=0.0004), or T-VEC alone (24.2%, P=0.0088), with similar findings in other cell lines. We then injected a B16 mouse melanoma cell line, permissive to T-VEC infection, into B6 mice to assess in vivo synergy. Mice were treated when tumors were ~7mm × 7mm. RT resulted in an early 20% decrease in tumor size within 4 days, followed by an increase thereafter. Intra-tumoral TVEC resulted in no clear benefit until 9 days post-treatment, when it showed a slight decrease in size sustained for ~10 days. Combination treatment, however, led to an early response that was sustained until 18 days after the initial treatment. When tumors were assessed 18 days after initial treatment, a 3-fold or more increase in tumor size was lowest in the combination T-VEC + RT group (17%) compared to the control (100%), RT (60%), or TVEC (50%) groups. These data propose that combination T-VEC + RT may be beneficial for cancer patients, and studies establishing the mechanism of synergy are ongoing.