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Comprehensive genomic profiling (CGP) of histologic subtypes of urethral carcinomas (UrthCa).

5087 Background: UrthCa is an uncommon GU malignancy that can progress to advanced metastatic disease. Methods: 127 metastatic UrthCa underwent hybrid-capture based CGP to evaluate all classes of genomic alterations (GA). Tumor mutational burden (TMB) was determined on up to 1.1 Mbp of sequenced DNA and microsatellite instability (MSI) was determined on 114 loci. PD-L1 expression was determined by IHC (Dako 22C3). Results: 49 (39%) urothelial (UrthUC), 31 (24%) squamous (UrthSCC), 34 (19%) adenocarcinomas (UrthAC) and 13 (9%) clear cell (UrthCC) were evaluated along with a control cohort of 2,130 bladderUC cases. UrthUC and UrthSCC were more common in men; UrthAC and UrthCC more common in women. Age was similar in all 4 groups. GA in PIK3CA were the most frequent potentially targetable GA; MTOR pathway GA in PTEN also identified. GA in other potentially targetable genes were also identified including ERBB2(6% in UrthUC, 3% in UrthSCC and 12% in UrthAC), FGFR1-3 (3% in UrthSCC), BRAF (3% in UrthAC), PTCH1 (8% in UrthCC) and MET (8% in UrthCC). Higher TMB was seen in UrthUC and UrthCC compared to UrthAC and UrthSCC, possibly reflecting their higher GA/tumor status and suggesting potential for immunotherapy benefit. MSI high status was absent throughout. The bladderUC cases had similar genomic pattern as UrthUC with significantly lower frequency of HPV16/18 positive cases. Conclusions: CGP reveals GA that may be predictive of both targeted and immunotherapy benefit in patients with advanced UrthCa and that could potentially be used in future adjuvant, neoadjuvant and metastatic disease trials. [Table: see text]