PD-L1 expression patterns in the metastatic tumors to the lung: a comparative study with the primary non-small cell lung cancer Zoran Gatalica1*, Jude Senarathne1, Semir Vranic2,3  1 Caris Life Sciences, Phoenix, AZ, USA; 2 Department of Pathology, Clinical Center, University of Sarajevo, Sarajevo, Bosnia and Herzegovina; 3 School of Medicine, University of Sarajevo, Sarajevo, Bosnia and Herzegovina Results Table 1. Abstract Introduction: Immune check point inhibitors (anti-PD-1/PD-L1) therapy has revolutionized cancer treatment of several, advanced and chemotherapy resistant malignancies. PD-L1 expression on tumor (TC) and/or inflammatory cells (IC) has been associated with a more favorable therapy response. We compared PD-L1 distribution in a large cohort of advanced tumors metastatic to the lungs and compared it with the primary lung non-small cell carcinomas (NSCLC). Materials and Methods: The study groups included 176 metastatic cancers and 81 NSCLC. Expression of PD-L1 was assessed using immunohistochemistry (SP142, Ventana). PD-L1 positivity was defined as 2+ intensity at ≥5% in TC or IC cells. All cases were further stratified into 4 categories based on the expression presence or absence of PD-L1 expression on tumor or IC cells. PD-L1 expression was correlated with total mutational load (TML) measured in tumors using Next-Generation Sequencing (NGS). Results: Overall TC PD-L1 positivity was significantly higher in NSCLC compared with metastatic tumors (28% vs. 14%, p=0.009) although some metastatic cancers (e.g. triple-negative breast and head/neck carcinomas, melanoma) exhibited higher TC PD-L1 expression. In contrast, overall IC PD-L1 expression was predominantly observed in metastatic tumors (28% vs. 0%, p<0.001). The IC PD-L1 expression ranged from 0% for metastatic renal cell carcinomas to 36-38% in the metastatic breast and colon carcinomas and melanoma (Table 1). Consequently, the stratification based on PD-L1 distribution (TC vs. IC), resulted in significantly different patterns between the primary and metastatic tumors (p<0.001, Table 2). Mean TML (±SD) for NSCLC (10±5.6) differed significantly from metastatic carcinomas from other sites (6.6±2.7) (p=0.013) (Table 3). Conclusion: Our study indicate that a substantial proportion of metastatic tumors to the lung exhibit PD-L1 expression on either tumor or inflammatory (immune) cells and are potentially amenable for the treatment with immune check point inhibitors. Histotype PD-L1 expression in tumor cells Total [<5%] [≥5%] NSCLC Adenocarcinoma Squamous cell carcinoma Other NSCLC 58 (72%) 44 (72%) 11 (73%) 3 (60%) 23 (28%) 17 (28%) 4 (27%) 2 (40%) 81 61 15 5 Metastatic carcinomas Colorectal carcinoma Gynecologic carcinomas Breast carcinoma Head and neck carcinomas Pancreatic carcinoma Renal cell carcinoma 113 (90%) 49 (96%) 21 (95%) 18 (86%) 9 (60%) 10 (100%) 6 (86%) 13 (10%) 2 (4%) 1 (5%) 3 (14%) 6 (40%) 0 (0%) 1 (14%) 126 51 22 21 10 7 Other metastatic tumors Soft tissue tumors Malignant melanoma Other cancers 39 (78%) 12 (80%) 7 (64%) 20 (83%) 11 (22%) 3 (20%) 4 (36%) 4 (17%) 50 11 24 210 47 257   PD-L1 expression in inflammatory cells [<1%] [≥1%] 81 (100%) 61 (100%) 15 (100%) 5 (100%) Breast carcinomas 87 (69%) 32 (63%) 16 (73%) 13 (62%) 7 (70%) 7 (100%) 39 (31%) 19 (37%) 6 (27%) 8 (38%) 3 (30%) 40 (81%) 13 (87%) 20 (80%) 10 (19%) 2 (13%) 4 (20%) 208 49 Histotype TML category Total Low (≤6) Intermediate (7-16) High (≥17) NSCLC (mean: 10.07, SD 5.608) Metastatic carcinomas (mean: 6.60, SD 2.785) Other metastatic tumors (mean 11.76, SD 21.405) 22 (29%) 49 (64%)* 5 (7%) 76 64 (60%)* 41 (39%) 1 (1%) 106 27 (57%)* 12 (26%) 8 (17%)** 47 113 102 14 229 Histotype TME categories (PD-L1 expression) Total TC+/IC+ TC-/IC- TC+/IC-* TC-/IC+* NSCLC Metastatic carcinomas 0 (0%) 58 (72%) 23 (28%) 81 4 (3%) 78 (62%) 9 (7%) 35 (28%) 126 4 136 32 35 207 TC = tumor cells; IC = inflammatory (immune cells); TME = tumor microenvironment; NSCLC = non-small cell lung cancer.   Table 2. Significantly different TME categories* between NSCLC and metastatic carcinomas to the lung (p<0.001). TML = tumor mutational load; SD = standard deviation Table 3. Tumor mutational load differences* between the primary (NSCLC) and metastatic tumors to the lung (p<0.001); Metastatic melanomas** exhibited particularly high TML (mean: 32.70, SD: 43.721). Conclusions Our study revealed significant differences in the presence of PD-L1 expressing cells in the microenvironments of primary NSCLC and metastatic to lungs carcinomas. It also showed that the tumor mutational burdens differ between the primary and metastatic tumors to the lung. These observations should be further explored for the refinement of the immune therapy treatment strategies, as the evidence emerges on the value of distinction between various cellular components in the tumor microenvironments for the purpose of selection of patients for the immune check point inhibition. *Correspondence: zgatalica@carisls.com