Background: ISA101b (peltopepimut-S), a therapeutic vaccine against the HPV-specific E6/E7 oncoproteins, induces specific expansion of HPV16 targeted T-cells when given together with cemiplimab, an anti-PD-1 agent. Previously we showed that ISA101 with nivolumab achieved a higher response rate compared to immune checkpoint inhibitor (ICI) alone in trials for patients with recurrent/metastatic (R/M) HPV16+ oropharyngeal cancer (OPC)^1,2. Methods: In this single arm phase 2 study, patients with R/M HPV16+ OPC with progressive disease (PD) within 6 months after prior 1^st or 2^nd line anti-PD-1 ICI were treated with ISA101b (subcutaneously 100µg/peptide on days 1, 29, and 50) with cemiplimab (intravenously 350mg as a 3-weekly regimen) until PD. Patients without PD at 6 months were offered a booster ISA101b injection. HPV16 positive tumor status was confirmed by a central reference laboratory with an established PCR assay. The primary efficacy endpoint was defined as overall response rate (ORR) as per RECIST1.1. An analysis was planned at the end of Stage 1, when 26 patients who had received at least one dose of ISA101b had been followed for 6 months. The cut-off date for this analysis, approved by the DSMB, was 31 January 2023. Results: Twenty-six patients (mean age 60.7 ±12.9 years; 22 (84.6%) male and 4 (15.4%) female) enrolled in Stage 1. Median follow-up was 16.2 weeks (range 1.6-78.8). Three patients were confirmed with partial response (PR) (11.5%); two of the 3 patients with PR never responded to previous ICI treatment. In 13 patients (50.0%) best overall response (BOR) was stable disease (SD); in 8 patients (30.8%) BOR was PD. Two (7.7%) patients did not have a tumor assessment after baseline: one patient died on day 8 due to OPC and one patient withdrew consent shortly after enrolment. The clinical benefit ratio (CBR) was 61.5%. Nine (34.6%) patients received a booster injection. At the time of the booster, 2 patients had PR and 5 patients had meaningful (≥6 months) SD. Time to response ranged between 127 and 307 days. Median duration of study treatment was 13.1 weeks (range 1.1-75.9), with 4 patients still on treatment. There were two grade 3 adverse events (AEs) related to ISA101b: erythema at the injection site and diarrhea. Two patients (7.7%) had grade 3 ICI-related auto-immune events. Grade 4-5 AEs related to study treatment did not occur. Median overall survival was 8.1 months (range 0.3-17.5). Conclusions: This treatment appears promising for HPV16+ OPC patients who failed prior anti-PD-1 ICI therapy, with an ORR of 11.5% and a CBR of 61.5%. In this small, ongoing study the combination of cemiplimab and ISA101b was well tolerated with a safety profile resembling anti-PD-1 monotherapy. This is the first study of the combination of ISA101b and cemiplimab in 2^nd/3^rd line R/M anti-PD-1 refractory OPC patients. ¹Massarelli E. JAMA Oncol 2019 ²de Sousa LG. J Immunother Cancer 2022. Clinical trial information: NCT04398524.