Background: Intratumoral injection of Talimogene Laherparepvec (TVEC) has a local oncolytic effect and evokes a cytotoxic immune response. The combination of Trabectedin (T) and Nivolumab (N) is a safe and effective therapy in soft tissue sarcoma (STS). This study aims to determine the safety and efficacy of adding TVEC to the combination of T and N in advanced leiomyosarcoma (LMS). Methods: Objectives: Primary: To assess progression-free survival (PFS). Secondary: (1) To evaluate the best overall response, (2) PFS rate at 6 and 9 months, (3) Overall survival (OS) rate at 6, 9, and 12 months, (4) Incidence of conversion from unresectable to the resectable tumor, and (5) Incidence of treatment-related adverse events (TRAEs). Patients and Methods: Eligible patients included patients ≥ 18 years of age with locally advanced unresectable or metastatic LMS, measurable disease by RECIST v1.1, and at least one accessible tumor for TVEC intratumoral injection. N (3 mg/kg q2 weeks), T (1.2 mg/m2 q3 weeks), and TVEC (1x10e8 PFU/ml q 2 weeks depending on tumor size) were administered. A starting dose of TVEC (1x10e6 PFU/ml) was initially given, followed by a total dose of 1x10e8 PFU/ml q 2 weeks depending on tumor size) three weeks later. Results: Efficacy: Per protocol, there were 11 evaluable subjects (Modified Intention to Treat [mITT] patients who had completed at least one treatment cycle and had a follow-up CT scan). The median number of prior lines of therapy was 4 (range 1-8). Confirmed Best Overall Response (BOR)by RECIST v1.1 = 2 PR, 9 SD (BOR Rate 18.2%). The disease control rate (PR+SD) at week 6 was 100%. The median PFS was 7 months (range: 3- 18); 6-month PFS rate, 55%; median OS 18.2 months (range: 4- 32); 6-month OS rate, 91%. There was no conversion from unresectable to resectable tumor. Response was not related to PD-L1 positivity but both patients with PR were ER+/PR+ and had uterine LMS. There were 15 evaluable subjects for OS analysis under the Intention-to-Treat (ITT) population who received at least one dose of study drug. The median OS was 12 (range 0-32) months; 6-month OS rate, 60%. Safety: Eight of 15 (53.3%) patients experienced at least one > Grade 3 treatment-related adverse event (TRAE). Grade 3 TRAEs include anemia (n=3), thrombocytopenia (n=2), neutropenia (n=1), increased ALT (n=1), increased GGT (n=1), decreased LVEF (n=1), myalgia (n=1) . Grade 4 TRAE include thrombocytopenia (n=1). The TRAEs were related to Trabectedin use. No new safety signals noted in this study. Conclusions: These results suggest that (1) By indirect comparison, the combination regimen using Talimogene laherparepvec, Nivolumab & Trabectedin may be more effective as second/third-line/fourth therapy for advanced leiomyosarcoma with manageable toxicity (Trabectedin alone for LMS= 4.3 mos; Dacarbazine alone = 1.6 mos; Demetri 2015), and (2) The best responders are patients with HR+ uterine LMS. Clinical trial information: NCT03886311.