PO-360 Vaccinia virus delivered by isolated limb perfusion combines with PD-1 blockade to prevent local and distant relapse in soft-tissue sarcoma

Introduction Systemic relapse is the major cause of mortality in extremity soft tissue sarcoma (ESTS). Traditional systemic therapies for metastatic sarcoma have limited efficacy, as do novel agents such as immune checkpoint inhibitors (ICI). Oncolytic virotherapy is capable of improving the efficacy of ICI in some non-sarcomatous pathology. In a rat model of ESTS, we have previously found isolated limb perfusion (ILP) to be ideally suited to delivering a vaccinia virus (GLV-1h68), alongside melphalan and tumour necrosis factor-α (TNFα). Using this model, we sought to investigate the effects of GLV-1h68 delivered by ILP on the tumour microenvironment and determine whether viral ILP can sensitise sarcomas to subsequent immune checkpoint blockade. Material and methods In vivo experiments were performed in Brown Norway rats bearing BN175 sarcoma in accordance with a Home Office animal license. Therapeutics included GLV-1h68, a PD-1 inhibitor (J43), melphalan and TNFα. Results and discussions PD-1 inhibition had limited in vivo monotherapy efficacy in BN175 sarcoma. Pre-treatment with GLV-1h68 delivered by ILP prior to PD-1 blockade markedly improved therapy, with complete tumour regression in a third of animals. Without further treatment, resistant disease rapidly evolved leading to both local and distant relapse. However, when performed as a neoadjuvant treatment prior to surgery and radiotherapy, viral ILP and PD-1 blockade prevented local and distant relapse in all animals. Treatment with both GLV-1h68 and PD-1 blockade were found to be necessary for durable cure. In vitro , GLV-1h68 induced ATP, calreticulin and HMGB1, markers of immunogenic cell death. In vivo , viral ILP and PD-1 blockade significantly increased the number of intra-tumoural CD4 + and CD8 + effector cells, with an increased proportion of these cells expressing activation markers. Viral ILP and PD-1 blockade also altered the topography of intra-tumoural immune invasion; significantly increasing CD8 + cells within the tumour parenchyma relative to the invasive margin. The accumulation of effector cells within regional lymph nodes, located outside of the perfusion field, was also noted indicative of at least a locoregional, and perhaps, systemic immune-priming. Conclusion Viral ILP and PD-1 blockade combine to prevent local and distant relapse in an animal model of high-grade ESTS. These data provide a strong rationale for clinical translation of this neoadjuvant combination immunotherapy.