(P 140) TRABECTEDIN ENHANCES ONCOLYTIC HERPES SIMPLEX VIROIMMUNOTHERAPY BY REDUCING BARRIERS TO VIRUS SPREAD AND IMMUNE EFFECTOR FUNCTION IN PRECLINICAL BONE SARCOMA MODELS

Objective: In pursuit of rational therapeutic combinations to treat pediatric sarcomas, we previously reported that trabectedin enhances the anti-tumor effects of oncolytic herpes simplex viroimmunotherapy in a preclinical model of Ewing sarcoma. We tested the generalizability of this efficacy in multiple models of Ewing sarcoma and osteosarcoma and sought to determine the mechanism(s) of synergy.

Methods: We combined the oncolytic herpes simplex virus HSV1716 with trabectedin in multiple preclinical murine models of Ewing sarcoma and osteosarcoma to determine the antitumor efficacy and survival benefits of the combination therapy. To probe the mechanism of synergy, we leveraged both fluorescence-activated cell sorting (FACS) and single-cell RNA sequencing in both types of models. We utilized these approaches to achieve an in-depth understanding of the treatment-induced changes in tumor and immune cell phenotypes, viral infection, and infiltrating immune populations.

Results: Multiple models of osteosarcoma and Ewing sarcoma treated with the combination of trabectedin and oncolytic herpes simplex virus displayed total and partial regressions, both in immunocompetent and immunodeficient mice, with significant survival benefits (p < 0.05). For Ewing sarcoma models, the best response rates (partial + total regressions) for animals bearing tumors treated with combination therapy were 5/5 (100%) for CHLA-258 and 4/5 (80%) for EW-5. In both models, only 1/11 (9%) of single-treated tumors showed a response. For osteosarcoma models, the best response rates for tumors treated with combination therapy were 5/5 (100%) for K7M2, with 2/11 (18%) for single-treated tumors, and 5/5 (100%) for combination-treated F420. FACS analysis showed increased T (syngeneic models) and NK cell (both in xenogeneic and syngeneic models) infiltration and activation in virus-treated tumors. Single-cell RNA sequencing data analysis revealed that trabectedin markedly increased the number of tumor cells containing oncolytic herpes simplex virus transcripts, likely resulting from a decrease in the expression of genes involved in antiviral intrinsic cellular responses. Both FACS and single-cell RNA sequencing analyses revealed compositional differences between those tumors treated with the combination therapy compared to single agents, particularly in the reduction of a complex myeloid population expressing immunosuppressive signatures alongside the activation of immune effector populations, namely, NK (in both immunodeficient and syngeneic hosts) and T (in syngeneic hosts) cells.

Conclusion: Our results indicate that trabectedin displays profound synergy with oncolytic herpes simplex viroimmunotherapy via multiple mechanisms including enhancement of virus infectivity, reduction of intratumoral immune-suppressive myeloid populations, and promotion of anti-tumor innate and adaptive immunity. Given that trabectedin is FDA-approved for human use and HSV1716 has shown clinical safety in pediatric patients, our findings should be readily translatable to the clinical setting where this combination therapy offers the potential to improve patient outcomes for those suffering from pediatric, adolescent and young adult bone sarcomas.