Pharmacokinetic and Pharmacodynamic Study of NKTR-255, a Polymer-Conjugated Human IL-15, in Cynomolgus Monkey

Introduction

IL-15 is a common gamma chain cytokine that activates and provides a survival benefit to T-cells and NK cells and has long been recognized as having potential as an immunotherapeutic agent for the treatment of cancer. Therapeutic use of native IL-15 has been challenging due to, for example, its unfavorable pharmacokinetic and safety properties. NKTR-255 is a polymer-conjugated human IL-15 that retains binding affinity to the alpha subunit of IL-15 receptor and exhibits reduced clearance to thereby provide a sustained pharmacodynamics response. Here we investigate the biological effects of NKTR-255 in naïve cynomolgus monkey.

Methods

In vitro monkey whole blood was treated with NKTR255 and the percentage of pSTAT5 positive populations in each NK, CD4 T and CD8 T cells was determined by flow cytometry. In an PK/PD study, monkeys received single IV doses of 0.001, 0.003, 0.01, 0.03, or 0.1 mg/kg NKTR-255. Blood samples were collected to determine the plasma concentrations of NKTR-255 and to assess the effects of NKTR-255 on NK and CD8 T cells at multiple time points; flow cytometry was used to measure STAT5 phosphorylation, Ki-67 expression and frequency of cell populations. Granzyme B expression was assessed in NK and CD8 T cells by flow cytometry.

Results

NKTR-255 induced dose-dependent phosphorylation of STAT5 in monkey whole blood (EC50 values NK cells: 6.9 ng/ml, CD8 T cells: 39 ng/ml, CD4 T cells: 53 ng/ml). The half-life and clearance of NKTR-255 were 26x longer and 38x lower, respectively, than IL-15. NKTR-255 engaged the IL-15 signaling pathway, in vivo, demonstrating both robust and sustained STAT5 phosphorylation in lymphocytes. NKTR-255 drove the proliferation of total CD8 T cells and NK cells in a dose-dependent manner, with dramatic and durable increases observed in Ki67 positive population and absolute cell numbers (NK cells: 6.1 fold; CD8 T cells: 7.8 fold from baseline on day 5 at 0.1 mg/kg). These effects were strongly biased towards CD8 T cells and NK cells, with substantially less induction of CD4 T cells. The Ki67 response analyses of the T cell subpopulation revealed a higher response of memory populations than for naive T cells. Among memory T cells, effector memory T cells showed the highest response over stem cell memory T cells and central memory T cells. Finally, NKTR-255 also increased the expression of Granzyme B in both NK and CD8 T cells, concomitant with an enhancement in target cell lysis.

Conclusions

Nektar has generated a novel and potent molecule in NKTR-255 that not only preserves the relevant biology of IL-15, but additionally provides enhanced PK and PD properties relative to the native IL-15 cytokine. NKTR-255 is being developed as an immune-stimulatory agent to target NK and CD8 T cell biology for the treatment of cancer.