LAUSR

e20667 Background: The tissue microenvironment associated with specific organ metastases potentially influences the efficacy of checkpoint inhibitors. The presence of liver metastases is a predictor of poor response and survival in melanoma and is correlated with reduced CD8+ T cell infiltration. Our study examined clinicopathologic characteristics, focusing on sites of metastatic disease, that are associated with poor outcomes. Methods: Advanced NSCLC patients treated with ≥1 cycle of ICI were reviewed. Baseline age, sex, histology, stage, smoking status, ethnicity, PD-L1 expression and sites of metastases were recorded. Best overall response (BOR) was determined by clinical imaging response and categorized ordinally as shrinkage, stable, or progression, adapted from RECIST for CR/PR, SD, PD. A rapidly progressive phenotype (RPP) was defined as BOR of progression and ICI use of ≤2 months. The association between sites of metastases and clinical outcomes were investigated using logistic and cox regression models. Results: Among 219 eligible patients, bone was the most common metastatic site (34.7%), followed by brain (21.5%), adrenals (14.2%), and liver (13.7%). Bone metastases (OR 0.45, p = 0.004) were associated with a worse BOR, while only a trend was observed for liver metastases (OR 0.47, p = 0.06). Adrenal metastases were associated with a better BOR (OR 2.08, p = 0.04). But thorax limited disease did not associate with BOR (OR 1.08, p = 0.76). In a multivariate model, bone was the only metastatic site associated with a worse BOR (OR 0.50, p = 0.01). Further, bone metastases were associated with RPP (adjusted OR 1.91, p = 0.04). Both bone (adjusted hazard ratio/aHR 1.61, p = 0.01) and liver metastases (aHR 1.80, p = 0.02) were associated with a shorter time-to-treatment-failure. The presence of liver (aHR 2.63, p < 0.001) but not bone (aHR 1.04, p = 0.86) metastases was a significant predictor of poor OS. Conclusions: We report a novel finding that the presence of bone metastases was associated with a worse clinical overall response on ICI and a rapidly progressive phenotype. Further investigations into the mechanisms of RPP in the presence of bone metastases are needed.