XmAb5574 but not anti-CD19 IgG1 depletes B cells in the blood and tissue. XmAb5574, anti-CD19 IgG1, or vehicle was administered as a single 3-mg/kg infusion (4 cynomolgus monkeys per treatment group). B cells were measured in the blood throughout the study (A) and in the blood, bone marrow, spleen, and lymph node on study conclusion, 29 days later (B). B-cell levels were assessed using flow cytometry with a CD20⁺CD3⁻ gate and expressed as the percentage of baseline (mean ± SE). XmAb5574 caused a substantial reduction in blood and tissue B cells, whereas anti-CD19 IgG1 was indistinguishable from vehicle. The rank order of depletion was blood > spleen > bone marrow > lymph node. (C) Serum concentrations of XmAb5574 and anti-CD19 IgG1 were measured using an immunoassay and are shown on a log-linear plot (mean ± SD of 4 animals). The half-lives were 11 (± 1) and 13 (± 2) days for XmAb5574 and anti-CD19 IgG1, respectively, demonstrating that these antibodies displayed similar exposure and pharmacokinetic properties with the given dosing regimen.