Circulating tumor DNA dynamics as a predictive biomarker of response and survival in metastatic urothelial carcinoma treated with enfortumab vedotin and pembrolizumab.

860 Background: On treatment biomarkers that connect molecular clearance to clinical outcomes are needed in metastatic urothelial carcinoma (mUC). We tested whether circulating tumor DNA (ctDNA) kinetics, especially ctDNA negativity, track response and survival during enfortumab vedotin plus pembrolizumab (EV+pembro) treatment. Methods: This single-center retrospective study included 25 mUC patients treated with EV+pembro at The Ohio State University Comprehensive Cancer Center between August 2023 and August 2025. A tumor informed assay (Signatera) measured ctDNA longitudinally. Metrics included baseline detectability, early decrease (≤60 days), ctDNA negativity (0 MTM/mL) at any time, and rise after nadir. PFS and OS were measured from treatment start using Kaplan Meier and log rank methods. Hazard ratios were derived from univariate Cox regression analysis. Fisher’s exact test was used to assess associations with the ORR. Results: Median age 69 yrs; 68% male; ECOG 0–1 = 80%. Primary tumor site was bladder (80%) or upper tract (20%). Median follow up was 15.4 months. Baseline ctDNA was available for 18/25 patients and was detectable in all 18 (100%). Among those with baseline ctDNA, an on-treatment decrease occurred in 15/18 (83%) patients, including 13/18 (72%) within ≤60 days. Overall, ctDNA negativity was achieved in 15/25 (60%) patients. The best overall response was 5 CR, 14 PR, 4 SD, and 2 PD, for an ORR of 76%. ctDNA negativity aligned with response: the ORR was 100% (15/15) in patients who achieved ctDNA negativity versus 40% (4/10) in those who did not (p = 0.0012). ctDNA negativity was also associated with significantly improved PFS (HR 0.16, 95% CI 0.05–0.54; p = 0.003) and OS (HR 0.08, 95% CI 0.01–0.68; p = 0.021). Early decrease (≤60 days) showed non-significant trends toward improved PFS (HR 0.88, 95% CI 0.22–3.47; p = 0.86) and OS (HR 0.46, 95% CI 0.05–4.12; p = 0.49). The median PFS for the entire cohort was 9.59 months; median OS was not reached. Among 13 patients with a ctDNA rise after nadir, the median time from nadir to rise was 126 days, and the median time from rise to radiographic progression was 115 days, indicating potential lead time. In exploratory analyses, TP53 mutations (n = 11) trended toward shorter PFS (HR 4.44; p = 0.061) and lower ctDNA negativity (45.5% vs 88.9% without mutation; p = 0.07). TSC1 mutations (n = 3) trended toward shorter PFS (HR 4.51; p = 0.095) but showed no association with ctDNA negativity (p = 1.0). Conclusions: In EV+pembro treated mUC, achieving ctDNA negativity is a powerful on treatment biomarker, coinciding with a near universal radiographic response and substantially longer PFS and OS. An early decrease in ctDNA similarly indicated treatment activity and trended toward improved outcomes. These hypothesis generating findings support the prospective validation of ctDNA guided management strategies.