Characterizing the immune effects of enfortumab vedotin (EV) on peripheral blood mononuclear cells (PBMCs) in metastatic urothelial cancer patients (mUC).

833 Background: EV has high antitumor activity and improved survival in advanced urothelial cancer both as a single agent and in combination with pembrolizumab. We hypothesized that EV primes a systemic immune response for effective T cell mediated killing. Methods: We conducted a retrospective analysis of mUC patients treated with EV at Memorial Sloan Kettering Cancer Center to evaluate its impact on peripheral blood immune cells. PBMCs were isolated from 32 patients with mUC on C1D1 and C2D1 of EV monotherapy. A 32-color spectral flow cytometry panel was used to identify T-cell subsets, and their correlation with treatment response using Wilcoxon rank sum test. Response was determined by an independent radiologist’s assessment of the first on-treatment scan according to RECIST v1.1 criteria. Results: Of 32 patients, 75% were men with median age 73. 81.3% received prior platinum-based chemotherapy and 93.8% received prior PD-1/PD-L1 immunotherapy. Partial response (PR) was observed in 16 patients, while 16 had stable disease (SD) or progressive disease (PD). At C1D1, CD8+ T cell frequencies did not correlate with response; however, a reduction in circulating CD8+ T cells at C2D1 correlated with poor response (SD + PD, p < 0.05). At baseline, high frequency of CD8+PD-1+ T cells correlated with poor response (p < 0.05). Baseline high frequencies of CD8+PD-1+LAG3+ and CD8+PD-1-LAG3+ T cells were also associated with poor response (p < 0.05). At C2D1, high frequency of CD8+TIM3+ (p < 0.05) and CD8+LAG3+ (p < 0.05) T cells correlated with non-response. Additional activation markers on CD8+ T cells at C1D1 were analyzed (Table). Within the memory CD8+ and CD4+ T cell compartments, patients with higher circulating naïve memory cells (CD45RA+CCR7+) at baseline were less likely to respond to EV (p < 0.05), while those with higher frequencies of T effector memory (CD45RA-CCR7-) and effector T cells (CD45RA+CCR7-) were more likely to respond to EV (p < 0.05). No significant correlations were found between memory T cells and response at C2D1. Conclusions: At baseline and on treatment, distinct immunophenotypes are detected in the peripheral blood of EV treated patients, potentially correlating with treatment response. Ongoing studies aim to further elucidate the influence of prior anti-PD-1/PD-L1 therapy and the combination of EV with pembrolizumab on these immunophenotypic profiles. Peripheral T cell phenotypes at C1D1 from mUC patients treated with EV. Peripheral Immune Cell at C1D1 Responders (PR, %CD45+) Non-Responders (SD + PD, %CD45+) p-value CD8+ 11.1 13.7 0.10 CD8+ Fold Change (C1D1/ C2D1) 1.07 0.74 0.04 Responders (PR, %CD8+) Non-Responders (SD + PD, %CD8+) p-value CD8+ ICOS+ 37.6 56.5 0.01 CD8+ PD1+ 40.9 64 0.04 CD8+ TIGIT+ 39.9 54.6 0.01 CD8+ LAG3+ 1.16 1.23 0.67 CD8+ TIM3+ 23.6 18.5 0.04