Characterization of genomic alterations as biomarkers of immune checkpoint inhibitor (ICI) response in metastatic urothelial carcinoma (mUC).

Abstract

400 Background: ICIs have expanded therapeutic options for mUC patients (pts); however, biomarkers such as PD-L1 have not served as reliable predictors of treatment efficacy. High tumor mutation burden (TMB) has been previously described as a potential biomarker for predicting ICI response in several indications, but its utility is still being explored in mUC. Here, we compare the genomic landscapes and clinical outcomes of mUC pts following ICI therapy using an investigational solid tissue-based next-generation sequencing assay to assess TMB and identify genetic correlates of ICI response. Methods: 20 pts with mUC treated with ICIs at Duke Cancer Institute were identified. Tumor samples were retrospectively evaluated with a Personal Genome Diagnostics Assay for somatic variants across > 500 genes, as well as TMB and microsatellite status. Tumor samples were also stained for PD-L1 status using the Dako 22C3 IHC assay. Deidentified clinical information was extracted from the medical record and tumor response was evaluated using RECIST 1.1 criteria. Results: Pts were grouped by overall response following ICI therapy as either responders (“R”, n = 6) or non-responders (“NR”, n = 13). Pts exhibited a wide range of TMB scores (0.7 to 30.4 mutations/Mb), with a mean TMB score of 9.60 vs. 3.87 mut/Mb in R vs NR groups, respectively; however, this difference was not statistically significant ( p = 0.284). 18 pts were evaluated for PD-L1 status, with only 2 positive samples (one in each group). Rs had significantly more mutations in histone methylation genes (KDM6A, KMT2C, and KMT2D), (67% vs 15% in NRs, p = 0.0039). FGFR3 mutations were also more frequent in R vs NR (67% vs 5%, p = 0.0339). Finally, there was a higher frequency of mutations in TP53 and BRCA1 in the NRs. Conclusions: In this mUC cohort, neither TMB nor PD-L1 correlated with response to ICI therapy. Histone modifying genes and FGFR3 mutations were more frequent in responders, whereas BRCA1 and TP53 mutations were enriched in non-responders, warranting future prospective studies to understand underlying mechanisms of ICI response and resistance in mUC.