PEG-RLI: A Long-Acting IL-15 Agonist That Produces Massive Levels of CD8+ and CD44hiCD8+ Cells for Cancer Immunotherapy.

Background/Objectives: Interleukin-15 (IL-15) is a promising immunotherapeutic cytokine, but its short half-life limits clinical utility. We developed a stable PEGylated receptor-linker IL-15 agonist (PEG-RLI) to improve pharmacokinetic properties while preserving biological activity and antitumor efficacy. Methods: PEG-RLI was synthesized by site-specific N-terminal conjugation of a 40 kDa methoxy polyethylene glycol (MeOPEG) to RLI. The conjugate was characterized for purity and receptor agonism. Pharmacokinetics were evaluated in mice, and pharmacodynamic effects on NK cells and CD8+ T cell subsets were assessed. Antitumor activity was tested in CT26 tumor-bearing mice alone and in combination with anti-PD-1 therapy. Results: PEGylation preserved RLI agonistic activity and extended its mouse half-life from approximately 3 h to approximately 15 h. PEG-RLI induced robust expansion of NK cells and CD8+ T cells, with preferential enrichment of CD8+ T cells and CD44hiCD8+ T cells. In CT26 solid tumors, PEG-RLI enhanced the antitumor efficacy of anti-PD-1 treatment. Conclusions: Stable PEGylation improved the pharmacokinetic profile of RLI while maintaining potent immunostimulatory activity. PEG-RLI preferentially expanded CD8+ T cells and showed promising efficacy with immune checkpoint blockade, supporting further development as a differentiated IL-15-based immunotherapeutic.