Background: SNS-301 is a first-in-class immune activating agent targeting human aspartyl (asparaginyl) β-hydroxylase (ASPH). ASPH is overexpressed in +20 tumor types. SNS-301 is an inactivated λ-bacteriophage, engineered to express a highly immunogenic fragment of ASPH fused to the phage gpD coat protein, previously shown to safely and effectively generate CD4, CD8 and B cell responses in a Phase I study (NCT04034225). Anti-PD-1 antibodies such as pembrolizumab and nivolumab can induce durable clinical responses in 13-16% SCCHN patients. While ASPH is frequently expressed in SCCHN, effective de novo responses to this tumor-specific antigen may be limited due to immune tolerance. Our hypothesis is that SNS-301 will break tolerance and trigger a significant anti-ASPH T-cell immune response, which in combination with anti-PD-1 blockade, will result in improved anti-tumor responses in this difficult-to-treat patient population. Methods: This is a Phase I/II study of SNS-301 delivered intradermally in combination with pembrolizumab. The population consists of 30 patients with ASPH+ locally advanced unresectable or metastatic/recurrent SCCHN who are actively receiving pembrolizumab or nivolumab for ≥12 weeks and have a best response of stable disease (SD) or first evidence of progressive disease (PD). A safety run-in will be performed for safety and any dose limiting toxicities (DLT) using a modified rolling six design prior to proceeding to full enrollment. Patients will provide a pre-SNS-301 tumor sample, an on-treatment tumor biopsy and an optional biopsy at PD to characterize the tumor microenvironment and correlate with clinical outcomes. SNS-301 in combination with pembrolizumab is given until confirmed PD or unacceptable toxicity. Study Objectives: Safety, tolerability and anti-tumor activity such as objective response rate, duration of response, progression-free and overall survival, immune responses, tumor/immune biomarkers and their associated treatment outcome. Results: Three patients have been enrolled. There have been no DLTs. Emerging safety data includes G1 adverse events with one G1 injection site pain. Patient status includes 1 PD and 2 patients with SD of at least six weeks duration. Immunological analyses will be presented. Conclusions: Early safety/efficacy warrant further evaluation of this combination therapy. Trial will be proceeding to full enrollment after the safety run-in is completed. Clinical trial information: NCT04034225