Background: The role of immunotherapy in curative viral-mediated oropharyngeal cancer (OPC) remains undefined. Human papillomavirus (HPV) 16 positive (+) OPC constitutively expresses viral epitopes that drive antigen-specific anti-tumor immunity, supporting rationale for evaluating neoadjuvant HPV16 directed therapeutics in non-metastatic OPC. HB-200 is an arenavirus-based vector therapy derived from LCMV (HB-201) and Pichinde virus (HB-202) each expressing a non-oncogenic HPV16 E7E6 fusion protein to induce HPV16 specific CD8+ T-cell responses. Building on our OPTIMA II results demonstrating that neoadjuvant chemotherapy/nivolumab led to deep responses in 71% of patients who went on to receive reduced radiation (RT), we hypothesized that adding HB-200 to chemotherapy to drive HPV16-specific anti-tumor immunity would be safe and effective to further deepen responses and facilitate more de-escalation. Methods: In this Phase 1 dose escalation investigator-initiated study, patients with non-metastatic HPV16+ OPC were treated with escalating doses of HB-201 single vector (Arm A) or HB-202/201 alternating vector (Arm B) x3 with neoadjuvant carboplatin AUC5 on day 1 and paclitaxel 100mg/m2 on days 1/8/15 of a 21-day cycle for 3 cycles. Deep responders (≥50% tumor shrinkage) received transoral robotic surgery (TORS) alone or RT to 50Gy +/- cisplatin, while non-responders (<50% shrinkage) received 50 or 70Gy of RT with cisplatin. Primary objective was safety/tolerability and recommended phase 2 dose of HB-200 with chemotherapy. Additional objectives included deep response rate, circulating tumor (ct) HPV-DNA dynamics, and HPV16-specific T-cell response. Results: Twenty-one patients with HPV16+ OPC were enrolled and treated (Arm A, n=9, 43%; Arm B, n=12, 57%). Median age 57, 91% male, 75% base of tongue, 52% non-smokers, and 33% stage II/III (AJCC 8^th edition). ≥Grade 3 treatment-emergent adverse events (AEs) occurred in 13 (62%) of patients overall and 3 (14%) non-hematologic during neoadjuvant. There were no Grade 4 AEs reported. All patients completed neoadjuvant HB-200/chemotherapy and response-stratified locoregional treatment. Deep responses following HB-200/chemotherapy were observed in 17/21 (81%) of patients, and in 14/15 (93%) treated on higher dose levels 1 or 2 (p=0.05). All three patients who underwent TORS had no viable tumor at time of surgery. Two patients (9%) had persistent disease following CRT and underwent salvage surgery with no evidence of disease at last follow-up. ctHPV-DNA and HPV16-specific T-cell response data will be presented at the meeting. Conclusions: Neoadjuvant HB-200/chemotherapy is safe and feasible with early efficacy signal in this setting warranting further study. Enrollment to the subsequent randomized phase II part is ongoing (NCT05108870). Clinical trial information: NCT05108870.