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6051 Background: In platinum-resistant recurrent-metastatic (R/M) head and neck squamous cell carcinoma (HNSCC) patients (pts), survival improvements have been achieved with immune checkpoint inhibitors (ICI), however this benefit is limited to a relatively small subgroup of pts. Predictive biomarkers are still under investigation and their contribution to the clinic has been limited. An immune-related cluster (Cl6) has been defined by means of a metanalysis in locally advanced HNSCC. Methods: Gene expression profiling was performed on two cohorts of platinum resistant R/M HNSCC pts treated with ICI alone: i) 20 patients as training set, divided in long-term (OS >18 months; n=12) and short-term (OS < 6 months; n=8) survivors matched according to site of recurrence (distant metastases only, or locoregional recurrence with/out metastases); ii) 80 patients enrolled in a phase II trial (EudraCT number: 2017-000562-30; NIVACTOR study) as testing set. The molecular subtyping stratification (De Cecco, Oncotarget 2015) was applied. Subsequently, a score was determined between each sample and the centroids for the 6 clusters previously identified. The threshold point was imputed in the training set as the closest value to the observed prevalence. A Cox multivariable analysis including TMB, CPS and TPS PD-L1 status, age, tumoral subsite and performance status (PS) was performed. Results: Among all the clusters, Cl6 turned out to be significant and applied to training set it provided evidence to discriminate pts’ survivals (AUC=0.86, 95%CI=0.7-1.0; p=2E-05). The stratification based on Cl-6 along with the cut-off point defined in the training set were challenged in the testing set: samples were divided in 28 (35%) having high Cl-6 scores and 52 (65%) with low scores, reaching HR=0.46 (95% CI= 0.27-0.76; p=0.0024). Multivariate analysis showed that only Cl6 (high vs low, HR=0.44; p=0.00443) and PS (1,2 vs 0, HR 1.85; p=0.02686) resulted to be significant on pts’ OS. Conclusions: By analysing two series of pts receiving immunotherapy alone, we identified an immune-related gene expression signature, able to discriminate the prognosis of platinum-resistant R/M HNSCC pts. The multivariate analysis confirmed the immune-related Cl6 as factor linked to outcome in pts treated with immunotherapy, as well as it confirmed the importance of PS. An analysis of the signature on pts receiving first line treatment with immunotherapy is currently ongoing. [Table: see text]