Background: The absence of infiltrating antigen-specific CD8+ T-cells at baseline is associated with low response rates to PD-1 blockade. SCCHN tumors often exclude effector T cells, and 2nd line response rates are low (13-18%). Highly immunogenic, antigen specific antitumor vaccines may expand intratumoral CD8+ T cells, potentially increasing durable response rates to PD-1 blockade. SNS-301 is a first-in-class, bacteriophage-based immune activating agent targeting human aspartate β-hydroxylase (ASPH), a tumor associated antigen overexpressed in multiple tumor types. SNS-301 is a self-adjuvanted vaccine consisting of λ-bacteriophage engineered to express an immunogenic fragment of ASPH fused to the phage gpD coat protein. The study objectives are to evaluate safety, immunogenicity and preliminary efficacy of SNS-301 added to pembrolizumab in patients (pts) not achieving tumor reductions on PD-1 blockade alone. Methods: Intradermal SNS-301 was combined with pembrolizumab in pts with locally advanced unresectable (LA) or metastatic/recurrent (met) SCCHN with a best response of stable disease (SD) or unconfirmed progressive disease (uPD) on ongoing PD-1 blockade > 12 weeks. Pts provided pre and on-treatment biopsies to characterize the tumor microenvironment using Nanostring and multiplex immunohistochemistry (mIHC). Blood samples were collected to evaluate B and T cell responses using ELISA/ELISPOT assays. Results: As of February 4, 2021, 13 pts were enrolled. Median duration of PD-1 blockade was 48 weeks (range 14-114) at study entry. There were no DLTs & mostly Grade 1-2 unrelated adverse events. Only two related Grade 3 events were reported: rash & dehydration (also a serious adverse event). Ten pts were evaluable for efficacy: 1 pt with PD-L1 negative (neg) disease & SD on pembrolizumab monotherapy achieved a partial response (PR; -52% at 8 months), 4 pts achieved SD & 5 pts had progressive disease. Two of the pts with SD had long-lasting duration (8 & 10 months) of which the latter had PD-L1 neg disease. One pt with uPD at enrollment achieved SD for 4 months. Analyses of pre- & on-treatment biopsies from the PR pt demonstrated an increase in infiltrating CD8+ T cells, PD-L1 expression & PD-1/PD-L1 proximity measures. Nanostring analysis demonstrated increased gene expression signatures for immune cells in the PR pt that was concordant with the mIHC & clinical outcome. Conclusions: The combination of SNS-301 and pembrolizumab was well-tolerated and resulted in encouraging clinical efficacy in pts not expected to respond to PD-1 blockade alone. Translational data suggest cellular response to SNS-301 and transformation of a poorly inflamed tumor to an immunologically active tumor in a responding pt (PR). Based on these data, an additional cohort will start enrolling PD-1 blockade naïve pts with LA/met SCHNN in the front-line setting. Clinical trial information: NCT04034225