Background: Metabolomic profiling of plasma from mUC pts has not been comprehensively examined. Plasma metabolomics may capture the effects of interactions between the malignancy, host and therapy. We hypothesized that Identifying metabolites in plasma from patients with mUC receiving an ICI or PBC may shed valuable insights regarding tumor biology and mechanisms of resistance. Methods: We obtained 0.2 ml plasma before and after starting therapy from pts with mUC receiving an ICI or PBC at the Dana-Farber Cancer Institute. Plasma metabolomic profiling was conducted at the Broad Institute using 3 complementary liquid chromatography tandem mass spectrometry (LC-MS)-based metabolomics platforms. We measured 648 metabolites at baseline prior to starting ICI/PBC and at a second time point in each subject following initiation of ICI/PBC. Metabolite levels were assumed to be normally distributed with log transformation to transform distributions to be approximately symmetric. We performed Wilcoxon-rank sum test to compare the levels of metabolites before and after initiation of the ICI or PBC (significance at p < 0.05). Results: Plasma was available at baseline and during therapy in 53 mUC pts (ICI: n = 43; PBC: n = 10). The median age was 68 (range: 39-86) years and 42 (82.3%) were male. The median time from baseline to the second time point was 4.7 months (range: 0.7-90.2). The ICIs administered were atezolizumab (n = 20), pembrolizumab (n = 16), nivolumab (n = 5), and durvalumab + tremelimumab (n = 1). We identified 20 metabolites that were significantly increased in post-PBC plasma samples (vs. pre-PBC) and 19 metabolites increased in post-ICI (vs. pre-ICI) samples (p < 0.05). All altered metabolites except one (Uracil) were exclusive for each treatment group. The most significant metabolites that increased following initiation of the ICI and PBC are shown in the Table. Evaluation of the association of plasma metabolomics with clinical outcomes and toxicities is ongoing. Conclusions: This is the first report, to our knowledge, of comprehensive metabolomic plasma profiling of pre- and post-ICI and PBC pts with mUC. The metabolomic changes after ICI appear distinct from those seen after PBC. Furthermore, our study sheds insights on potential mechanisms of resistance and new therapeutic targets in pts with mUC.