## 4 Discussion  
Nivolumab and pembrolizumab are two anti-PD-1 antibodies that are indicated for R/M HNSCC. FDA approved these two immunotherapeutic agents in 2016 for R/M HNSCC patients in the second-line setting following progression from platinum-containing regimens. More recently, pembrolizumab, based on phase III clinical trial evidence, was approved as first-line therapy R/M HNSCC in combination with platinum-based chemotherapy or as a single agent in patients with PD-L1-positive (CPS \(\geq\) 1) tumor expression [8]. To the best of our knowledge, pivotal clinical trials with these two immunotherapies did not ascertain if specific head and neck anatomical sites are associated with distinct clinical responses and outcomes. Our results provide an initial signal that anatomical site may matter in immunotherapy response; oral cavity SCC patients are less likely to have durable responses to anti-PD-1 therapies compared to oropharyngeal and laryngeal SCC patients. HPV+, based on HPV in situ hybridization or p16 IHC, is a favorable prognostic biomarker in OPSCC in response to various treatment approaches, surgical and chemotherapeutics, in the locally advanced and R/M settings [11; 12; 13; 14]. In light of these clinical studies and the notion that HPV+ tumors have an intrinsic and persistent source of viral antigens to illicit an anti-tumor immunity response, it was anticipated that anti-PD-1 immunotherapies may be more active in p16+ than in p16- R/M OPSCC patients. The results from phase I and II trials with pembrolizumab were mixed in R/M OPSCC patients; some studies showed that p16+ status was associated with an improved response rate, while another study reported similar clinical benefit between p16+ and p16- patients [5; 15; 16]. Unfortunately, results from the two multi-institutional phase III trials with pembrolizumab (KEYNOTE-048) or nivolumab (CheckMate 141) did not provide sufficient clarity to ascertain the impact of p16 status on the OS of R/M OPSCC patients treated with these PD-1 blocking agents [7; 8]. Our single institutional study, with a heterogeneous patient population, showed that p16+ status is an independent prognostic biomarker in immunotherapy-treated R/M OPSCC patients; p16- patients had a 7.67-fold increase in risk of death compared to p16+ patients. In light of our data, post-hoc analyses of KEYNOTE-048 and CheckMate 141 to compare OS of p16+ and p16- OPSCC patients treated with anti-PD-1 antibodies should be performed to determine if our real-world cohort finding will be validated in a well-defined, homogenous clinical trial population. HPV/p16 status, tobacco smoking, N-stage, and T-stage are the major determinants of OS in OPSCC and these four variables can be used as risk classifiers in this population [11]. HPV+OPSCC patients are classified as low-risk and intermediate-risk categories in the definitive setting based on tobacco smoking history and N-stage; low-risk patients are non-smokers (\(\leq\)10 pack-years) or smokers (>10 pack-years) with N0-N2a, whereas intermediate-risk patients are smokers with N2b-N3 [11]. HPV-OPSCC patients are at high risk, with the exception of a small subset of non-smoker individuals with T2-3 tumors, which are considered intermediate risk [11]. In our OPSCC study cohort, 76% (16/21) were self-reported smokers (current or former) and this proportion was similar regardless of p16 status, which was 75% (12/16) and 80% (4/5) for p16+ and p16- patients, respectively. The predominance of smokers in our OPSCC subset was anticipated, since HPV+ smokers are at intermediate-risk, and thus have a higher probability of progressing with R/M disease. Tobacco smoking history was not an independent prognostic biomarker in this OPSCC subset, suggesting that tumor mutational load, recognized to be higher in smokers than non-smokers, may not be a robust biomarker of immunotherapy response in the R/M OPSCC setting. Pembrolizumab and nivolumab are designed to work against the same target, PD-1, suggesting that these therapeutics may have equivalent efficacies.