LAUSR

1024Background: Recently the immune checkpoint inhibitors (ICIs) have demonstrated efficacy across a wide variety of cancers, but have been less effective in breast cancer. PD-L1 (B7-H1, CD274) is a ligand produced by many tumor cells and some immune cells, and suppresses the T cell immune response. This allows tumor cells to escape immune detection. PD-L1 is used as a companion tumor tissue IHC biomarker for patient selection for some of the FDA-approved ICIs (pembrolizumab), but not for others (nivolumab, atezolizumab). Circulating PD-L1 has been detected in multiple myeloma, renal, lung, and gastric cancer, but not in breast cancer. Here we correlated serum PD-L1 levels with outcome in a HER2-positive metastatic breast cancer cohort treated with trastuzumab. Methods: Pretreatment serum was obtained from 63 metastatic breast cancer patients before starting first-line trastuzumab-containing therapy. A novel ELLA microfluidic channel immunoassay platform (ProteinSimple, San Jose, CA) was employed to quant...