Clinical outcomes of muscle-invasive and metastatic, bladder, and urethral pure adenocarcinoma (BUAdenoCa): An international study from the Global Society of Rare Genitourinary Tumors (GSRGT).

e16565 Background: BUAdenoCa are rare and their management options are limited. The GSRGT assembled an international cohort of pts with BUAdenoCa to evaluate the natural history and clinical outcomes of this rare cancer in the perioperative and metastatic settings. Methods: We retrospectively collected data on pts with locally advanced or metastatic BUAdenoCa receiving systemic therapy between 2000-2026 at 25 medical centers in the US, Europe, South America, and Asia. Median overall survival (mOS), disease-free survival (mDFS), and progression free survival (mPFS) were estimated by the Kaplan-Meier method for perioperative and metastatic pts. Observed objective response rate (ORR) was determined for metastatic pts receiving systemic therapy by the clinical investigator per RECIST v1.1 criteria when feasible. Results: Among 328 pts with BUAdenoCa, including urachal (47%), urethral (11%), and non- urachal-nonurethral bladder AdenoCa /other (42%). Median age was 61 years; 61% were male; 68% were White, 17% were Black, 12% were Asian; 87% were non-Hispanic. At time of study, most patients were metastatic (65%), followed by muscle-invasive/locally advanced (16%), and non-muscle-invasive/NED (19%). 80% of stage IV pts had visceral metastases, including lung (44%), bone (36%), and liver (17%), while 10% had LN-only disease. Other metastatic sites included peritoneal/mesentery, gynecologic organs, soft tissue, muscle, CNS, and other abdominal or retroperitoneal locations. Among pts with molecular data, 98% were microsatellite stable. Common alterations included TP53 (18%), KRAS (9%), and SMAD4 (5%). Clinical outcomes by type and treatment setting are summarized in the Table. Conclusions: To date, this represents the largest retrospective analysis of BUAdenoCa. These data suggest outcomes differ by type of BUAdenoCa and there is an association between 5-FU based therapy and improved outcomes in both the perioperative and metastatic settings. Analysis Cohort Variable mDFS / mPFS (yrs) mOS (yrs) ORR (% [n/N]) Median DoR (months) Surgery ± perioperative systemic therapy (n=220) Urachal 1.5 6.3 — — Urethral 1.0 4.2 — — Other BUAdenoCa 1.8 3.2 — — 5-FU–based 2.7 NR — — Non–5-FU 2.0 2.7 — — Metastatic 1L systemic therapy (n=138) Urachal 0.68 2.3 — — Urethral 0.37 2.9 — — Other BUAdenoCa 0.54 1.6 — — 5-FU–based 0.64 2.3 29.3% (24/82) 6.8 Non–5-FU 0.57 1.5 22.5% (11/49) 8.7