Differential expression of melanoma-associated antigen A3/6 and associated immune molecules prior to and post treatment with immune checkpoint inhibitors (ICI) in patients with mUC

433 Background: MAGE is an attractive target for IO given its expression restricted to carcinomas and the testes. Engineered T-cell targeting MAGE A3/6 has shown promise. However, successful treatment is dependent on circumventing the potentially hostile immune microenvironment in metastatic tumors. In this study, we aim to assess the effects of ICI on the immune microenvironment in patients who progressed on ICI. Methods: We obtained FFPE tissue from 16 patients with mUC across 3 institutions. Samples from the primary or metastatic tumor prior to treatment with PD1/PD-L1 inhibitors (ICI) were paired with samples from metastatic sites post-treatment. Differential mRNA expression was assessed using NanoString PanCancerIO360 panel for the main genes of interest ( MAGEA3/6, CD274, HLADP, HLAA, B2M and PDCD1) in addition to immune profiling of the tumor samples. Also, IHC was done on the paired samples to assess DE for the genes of interest using an H-score analyzed via the paired T-test. p-values