Pharmacokinetics, pharmacodynamics, and toxicity of a PD-1-targeted IL-15 in cynomolgus monkeys.

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e14568 Background: Interleukin (IL)-15 is a potent cytokine that enhances the proliferation and effector functions of IL-2Rβ/γ-expressing lymphocytes, such as NK cells and T cell subtypes (CD8+, CD4+, gd, and NKT). PF-07209960 is a novel bispecific fusion protein composed of an anti-PD-1 antibody and engineered IL-15 cytokine mutein with reduced binding affinity to its receptors. PD-1 is expressed on tumor-infiltrating T cells at relatively high levels. PF-07209960 has reduced activity on PD-1-negative NK cells and enhanced activity on PD-1-expressing intratumoral CD8+ T cells, therefore is expected to have improved therapeutic window. Methods: The pharmacokinetics (PK), pharmacodynamics (PD), and toxicity of PF-07209960 were evaluated following once every other week subcutaneous (SC) or intravenous (IV) administration to cynomolgus monkeys in a two-dose PKPD (0.01 – 0.3 mg/kg/dose) study and in a three-dose (SC route) GLP toxicity study (0.1 – 3 mg/kg/dose). Results: PF-07209960 showed dose-dependent pharmacokinetics with a terminal T1/2 of 8 and 13 hours following IV administration at 0.03 and 0.1 mg/kg, respectively. The clearance is faster than a typical IgG1 antibody. Slightly faster clearance was also observed following the second dose, likely due to increased target pool and formation of anti-drug antibodies (ADA). Despite a high incidence rate of ADA (92%) observed in GLP toxicity study, PD-1 receptor occupancy, IL-15 signaling (STAT5 phosphorylation) and T cell expansion were comparable following the first and second doses. Activation and proliferation of T cells were observed with largest increase in cell numbers found in gamma delta T cells, followed by CD4+ and CD8+ T cells, and then NK cells. The expansion of T cells and NK cells in peripheral blood peaked around Day 8 and waned to near baseline levels by Day 15. Release of cytokines IL-6, IFNɣ, and IL-10 were detected, which peaked at 72 hours postdose. All animals at 3 mg/kg/dose were euthanized in a moribund condition on Day 4 of the dosing phase, coincident with the peak of cytokine release. Four out of six monkeys administered 1 mg/kg were euthanized between Day 6 and Day 16 due to thrombocytopenia, bleeding, and poor physical condition. Microscopic evaluations revealed generalized mononuclear infiltration in various tissues. Both the no observed adverse effect level (NOAEL) and the highest non severely toxic dose (HNSTD) were determined to be 0.3 mg/kg/dose, which corresponded to mean Cmax and AUC48 values of 1.15 µg/mL and 37.9 µg*h/mL, respectively. Conclusions: PF-07209960 demonstrated dose-dependent exposure and robust PD (T cell and NK cell subsets expansion and cytokine release) in cynomolgus monkeys, with well-characterized safety profile. In summary, the results of the present study support Phase I clinical evaluation of PF-07209960 in cancer patients.