PK and PD of NKTR-255 in NHPs. (A) Cynomolgus monkeys (n = 2 per group) received a single IV dose of rhIL-15 at 0.05 mg/kg or NKTR-255 at 0.1 mg/kg. Blood samples were collected to determine plasma concentrations of IL-15 or NKTR-255 (IL-15 protein content, IL-15 equivalent). (B) Whole blood samples from cynomolgus monkeys (n = 3 per group) were incubated with the indicated concentrations of rhIL-15 or NKTR-255 for 20 minutes. Phosphorylation of STAT5 was measured by flow cytometry as the percentage of positive CD3⁻CD56⁺ NK cells (left), CD3⁺CD8⁺ T cells (middle), and CD3⁺CD4⁺ T cells (right). Mean ± standard error of the mean (SEM) is shown. (C) Cynomolgus monkeys (n = 3-4 per group) received a single IV dose of 0.001, 0.01, or 0.1 mg/kg of NKTR-255 or vehicle. Blood samples were collected before infusion and at the indicated time points to assess the absolute numbers of CD3⁻CD56⁺ NK cells (left), CD3⁺CD8⁺ T cells (middle), CD3⁺CD4⁺ T cells (right), percent of pSTAT5 positive cells, and percent of Ki-67⁺ cells using flow cytometry. Figures show mean values.