ALT-803 pharmacokinetics, serum cytokine levels, and vital sign changes following IV vs SQ administration. (A-B) Measurement of serum ALT-803 protein was performed by enzyme-linked immunosorbent assay with samples collected at various time points following the first ALT-803 injection administered on study. (A) IV ALT-803 administration by dose level, with a dose-dependent increase in ALT-803 concentrations, a high Cmax, and rapid clearance with return to baseline by 96 hours postinjection. (B) SQ ALT-803 administration by dose level, demonstrating a decreased ALT-803 Cmax that peaked after 4 hours, with sustained serum levels through 96 hours and clearance by 168 hours. Complete pharmacokinetic analysis is provided in supplemental Table 3. (C-D). IFN-γ and IL-6 were measured via cytokine bead array at various time points following the first ALT-803 administration. Depending on the route of administration, IFN-γ levels peaked early (IV) or late (SQ). IV ALT-803 resulted in an early IL-6 spike that is absent in SQ ALT-803 administration. Complete cytokine measurements are provided in supplemental Table 2. (E-G) Vital sign assessments immediately following the first dose ALT-803, including heart rate (HR) (E), temperature (F), and systolic blood pressure (G). IV ALT-803 administration resulted in fevers with increased heart rate 3 to 6 hours after administration, concurrent with elevated serum IL-6 levels and IFN-γ levels. SQ ALT-803 did not result in short-term alterations in vital signs.