Koshkin VS, et al. - In patients with metastatic bladder cancer who received treatment with anti-PD-1 or anti-PD-L1 agents, researchers aimed to describe circulating tumor cells (CTCs) and other single cell populations related to improved clinical outcomes. A total of 27 patients (median age 74 years, 70% male) were included who received pembrolizumab (n=15), atezolizumab (n=11), or nivolumab (n=1). Using RECIST v1.1, patient response to treatment was evaluated; prospectively obtained blood samples were analyzed. Comparisons were made between patients whose CTC and leukocyte values were above and below several pre-determined parameters to compare time to progression via Kaplan-Meier analysis. Findings revealed that a statistically non-significant tendency towards shorter time to progression (TTP) was observed among patients who exhibited an increased baseline CTC count. A link was observed between increased baseline CD4 cells and delayed TTP in this preliminary analysis.