Association of HER2 and NECTIN4 expression with clinical outcomes in patients (pts) with advanced urothelial carcinoma (aUC) treated with enfortumab vedotin +/- pembrolizumab (EVP).

796 Background: NECTIN4 and HER2 immunohistochemistry (IHC) expression are emerging biomarkers and targets of approved therapies in aUC. There are limited data on outcomes with EVP in combination biomarker subsets. Methods: Pts with aUC treated with EVP who had tissue samples available for IHC staining were separated based on expression of NECTIN4 (H-score < 200 vs ≥200) and HER2 (IHC 0/1+ vs 2+/3+). Clinical outcomes were compared in 4 biomarker subsets (NECTIN4 High /HER2 High , NECTIN4 High /HER2 Low , NECTIN4 Low /HER2 High , NECTIN4 Low /HER2 Low ). For IHC staining, Abcam clone EPR15613-68 and Ventana clone 4B5 were used for NECTIN4 and HER2, respectively. Observed response rate (ORR) was evaluated using χ² test; progression-free survival (PFS) and overall survival (OS) were analyzed via Kaplan-Meier and Cox proportional hazards models (NECTIN4 Low / HER2 Low used as reference). Results: Among 69 pts with aUC and available biopsies between 9/2017 – 10/2024, 58 had both NECTIN4 and HER2 IHC data. Breakdown into 4 biomarker subsets and outcomes with EV+/-P are shown in the Table. Baseline pt characteristics including age, sex, primary tumor location, tumor histology, and prior lines of therapy were similar across groups. HER2 and NECTIN4 expression were significantly correlated (p = 0.01). For the overall cohort (n = 58), 28 pts had EVP and 30 had EV treatment. With a median follow up of 19.7 months (mos), ORR was 44%, mPFS 6.4 mos, and mOS 19.7 mos. No statistically significant differences in ORR, PFS, and OS were observed among the 4 subsets. Analyzing NECTIN4 IHC alone, pts with very high expression (H-score > 250, n = 19) had prolonged mPFS relative to the rest (16.9 vs 5.4 mos, p = 0.02), but no mOS difference (45 vs 15.9 mos, p = 0.1). Conclusions: In this single-institution retrospective study of pts with aUC treated with EV-based regimens, there were no significant differences in clinical outcomes between groups separated by combination of NECTIN4 and HER2 tumor expression. Analysis was limited by modest sample size across the 4 biomarker subsets. In an exploratory analysis, superior mPFS was observed in pts with very high NECTIN4 H-score (> 250). These findings should be validated in larger cohorts. NECTIN4 High / HER2 High (n=26) NECTIN4 High / HER2 Low (n=9) NECTIN4 Low / HER2 High (n=9) NECTIN4 Low / HER2 Low (n=14) ORR - % 48% (12/25) 38% (3/8) 57% (4/7) 33% (4/12) ORR Comparison* OR (95% CI), p 1.9 (0.5 - 10), p=0.4 1.2 (0.2 - 8.0), p=0.8 2.6 (0.4 - 20), p=0.3 NA mPFS - months (95% CI) 9.7 (6.4 - NR) 4.6 (1.8 - NR) 37.8 (4.2 - NR) 4.3 (1.9 - NR) mPFS Comparison*HR (95% CI), p 0.5 (0.2 - 1.1), p=0.08 0.8 (0.3 – 2.5), p=0.7 0.4 (0.1–1.3), p=0.1 NA mOS - months (95% CI) 22.4 (10.4 - NR) 9.6 (7.4 - NR) NR (6.5 - NR) 8.6 (4.2 - NR) mOS Comparison*HR (95% CI), p 0.5 (0.2–1.3), p=0.1 1.0 (0.3–2.5), p=0.8 0.4 (0.1–1.3), p=0.1 NA *NECTIN4 Low / HER2 Low group used as reference.