Background: TVEC is a modified oncolytic herpes simplex 1 (HSV1) virus currently FDA approved for the treatment of unresectable cutaneous and nodal melanoma. TVEC is designed to preferentially lyse tumor cells over normal tissue to release tumor associated antigens, produces GM-CSF to activate dendritic cells, and stimulates T cells to infiltrate the tumor (TILs). TILs in breast cancer are associated with better response to neoadjuvant chemotherapy (NAC), so we hypothesized that intratumoral TVEC may enhance response to NAC. We report results of a phase 2 trial combining NAC with TVEC in stage 2-3 TNBC. Methods: Stage II-III TNBC pts (N = 37) were to be enrolled into a single arm, optimal Simon 2 stage phase 2 trial with TVEC (10^6 PFU 1^st dose then 10^8 PFU x 4 doses) weeks 1,4,6,8,10 + weekly paclitaxel (80mg/m2) IV x 12, followed by dose dense AC (doxorubicin/cyclophosphamide 60/600 mg/m2) IV q2weeks x 4 alone given preoperatively. Primary endpoint was residual cancer burden 0 rate (RCB0). Trial meets primary endpoint with ≥15 RCB0 responses out of 37 evaluable pts, assuming p1 = 45% vs. p0 = 30% with one sided type I error rate at 0.10 and power at 70%. Results: Forty pts were enrolled at Moffitt (5/2018 – 4/2020) and evaluable for safety with 3 pts non-evaluable for efficacy due to incomplete treatment. Study demographics: median age 49 (27-66), 67.5% White, 10% Black, 15% Hispanic, clinical stage II 83% and III 17%, node + 42%. The RCB0 rate = 16/37 (43%, 95% CI 27-61%) and additional 9 pts with RCB-1 (RCB0/1 rate 68%, 95% CI 50-82%). Toxicities did not differ significantly from expected NAC toxicities except for increased brief G1-2 fevers, chills, injection site pains. Four pts had G2-3 thromboembolic events (10%) slightly greater than expected 6% rate on NAC. Conclusions: Addition of TVEC to NAC increased RCB0 rates with manageable toxicities and warrants additional investigation in TNBC. Immune correlates and updated survival data will be presented at the meeting. Clinical trial information: NCT02779855