Longitudinal tumor-informed ctDNA monitoring and clinical outcomes in advanced urothelial carcinoma treated with enfortumab vedotin ± pembrolizumab.

4569 Background: Enfortumab vedotin plus pembrolizumab (EV±P) is standard-of-care for patients (pts) with locally advanced/metastatic urothelial carcinoma (la/mUC), although not all pts experience durable benefit. Tumor-informed circulating tumor DNA (ctDNA) assays enable a personalized, real-time assessment of disease kinetics, and early ctDNA dynamics (<12 weeks) have been linked to EV±P radiographic response and survival outcomes. However, the prognostic relevance of ctDNA dynamics relative to durable clinical benefit is not well defined. Methods: We conducted a multicenter, retrospective real-world analysis of pts with la/mUC who received ≥1 cycle of EV±P and ≥1 commercial plasma ctDNA test using a clinically validated, personalized, tumor-informed assay (Signatera, Natera, Inc.). Associations between ctDNA features and progression-free survival (PFS) and overall survival (OS) were evaluated using Cox proportional hazards models. Results: A total of 110 pts with 545 plasma samples were analyzed. All pts had ≥1 ctDNA test (median: 4 tests/pt) immediately prior to and/or after EV±P initiation, with median time between serial ctDNA collections of 7.3 (IQR: 5.4-12.0) weeks. In this cohort, 69% (76/110) of pts had ≥1 ctDNA test (355 total) beyond 12 weeks post-EV±P initiation. Among these 76 pts, 31.5% (24/76) were persistently ctDNA negative (≥2 consecutively negative samples with no positives), while 65.8% (50/76) were anytime ctDNA-positive. Time-varying covariate analysis incorporating all serial results after 12-weeks reaffirmed that ctDNA-positivity was associated with worse PFS and OS (PFS: HR: 36.8, p<0.005; OS: HR: 17.7, p=0.01). Serial ctDNA negativity within 4-24 weeks (i.e. ≥2 consecutively negative samples with no positives) was strongly associated with durable OS/PFS beyond 24 weeks (PFS: HR=0.05, 95% CI: 0.01-0.42, p=0.005; OS: HR=0.046, 95% CI: 0.0004-0.33 [Firth], p=0.0002). Furthermore, single-timepoint ctDNA positivity evaluated at 12–24 weeks (HR: 11.8; 95% CI: 2.72–51.60; p=0.001) and >24 weeks (HR: 6.8; 95% CI: 2.5–18.6; p=0.0002) was strongly associated with inferior PFS. Findings were similar for OS (12-24wk: HR: 7.2, 95% CI: 1.7-31.3; p=0.008; >24wk: HR=3.8, CI: 1.2-12.3; p=0.03). Conclusions: Longitudinal ctDNA dynamics are strongly prognostic in pts with la/mUC treated with EV±P, even when assessed beyond 12 weeks from treatment start. Sustained ctDNA negativity further identifies pts with durable clinical benefit. These findings support ctDNA as a potentially complementary biomarker to standard imaging in this clinical context.