Background: Despite improvement in outcomes with checkpoint (PD-1) inhibitors in some patients (pts) with recurrent and metastatic head and neck squamous cell carcinoma (RM HNSCC), overall survival (OS) remains poor, necessitating novel therapy combinations. Fanconi Anemia (FA) and FA-related DNA repair pathways alterations have been associated with cumulative tumor mutational burden (TMB) due to genomic instability. High TMB is associated with improved response to PD-1 inhibitors. DNA pathway repair mutations have been reported in 17% of sporadic HNSCC. Ataxia-Telangiesctasia Mutated (ATM) loss has been reported in 60% of human HNSCC biopsies and FA-related defects were reported in 15-21% of human HNSCC biopsies and cell lines. Poly (ADP-ribose) polymerase (PARP) inhibition has already demonstrated efficacy as a single agent in multiple cancers which harbor a DNA repair defect. Preclinical data also showed PARP inhibitor (PARPi) upregulates PD-L1 expression in cancer lines and animal models via inactivation of glycogen synthase kinase 3 (GSK3β). Importantly, blockade of PD-1 re-sensitized PARPi treated cells to induce T-cell mediated cytotoxicity. Combination of PARPi and anti-PD-1 therapy significantly increased therapeutic efficacy in vivo compared to single agent. Therefore, we hypothesized that the combination of a PARPi (niraparib) and PD-1 blockade (dostarlimab) in our ongoing phase II study would enhance overall response (OR) and survival in R/M HNSCC pts (NCT04313504). Methods: Eligible pts must have a confirmed histopathology of non-cutaneous RM HNSCC not amenable to local therapy, progressive disease (PD) after at least one line of platinum-based chemotherapy and/or immunotherapy, age>18, ECOG <2, no prior therapy with PARPi and adequate bone marrow, liver, and renal function. Seven of 14 pts have been enrolled in the first stage of the study. If 8 or more pts develop either stable disease (SD), partial (PR) or complete response (CR), 9 additional patients will be accrued for a total of 23 patients. Pts will receive 200 mg PO niraparib daily in 28-day cycles along with dostarlimab 500mg (Q3W) for 4 doses followed by 1000mg (Q6W) until PD or unacceptable toxicity. An interim analysis will be performed to assess efficacy and safety after 14 patients become evaluable. Tumor response will be assessed per RECIST v1.1. Primary endpoint is best OR including SD, PR, and CR; Secondary endpoints include adverse events determined by CTCAE v5.0, progression free survival and OS. Integrated tissue analysis includes DNA pathway repair defects and TMB. Additional exploratory analyses including peripheral and immune cell activation will be performed on tissue and plasma samples. Clinical trial information: NCT04313504.