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 <0.05 were considered significant and FDR corrected. Results: MAGEA3/A6 mRNA expression was unchanged in the post-treatment samples. In addition, time to progression was not influenced by the mRNA DE. Similarly, IHC expression was not decreased post-treatment. The other genes of interest were also similarly expressed pre and post treatment. Immune cell localization genes CPA3,TPSAB1/B2, HDC, MS4A2 were significantly downregulated post treatment. Genes implicated in cancer cell killing ( MX1,KIT) were similarly downegulated whereas CD276, TLR1 were upregulated. Expression in the post treatment tissue of PTPN1, CES3, ACVR1C, and BAD were associated with increased progression. LY9, PTPRC, CTLA4, HLA-DMA, IL10RA, HLA-DMB, ITGAL, CD48 were associated with decreased progression. Conclusions: Prior ICI treatment does not decrease expression of MAGE A3/6, suggesting that salvage treatments targeting this antigen remain a viable strategy. Suggestion of immune microenvironment alterations were also noted. More work is necessary to understanding the tumor microenvironment in order to design rational treatment combinations and sequences.