Peptide Coacervates Promote Cytosolic Delivery of STING Agonists for Cancer Immunotherapy

Background/Objectives: Cyclic dinucleotide stimulator of interferon genes (STING) agonists have emerged as potential agents in cancer immunotherapy, but their clinical applications are limited by relatively poor pharmacokinetic properties. Methods: A luciferase reporter assay was employed to screen delivery peptides capable of promoting cellular activating effect of cyclic dinucleotide STING agonists. The potent candidates were further confirmed by enzyme-linked immunosorbent assay (ELISA), real-time quantitative PCR (qPCR) and Western blotting analysis. Colon and melanoma cancer mouse models were used to examine the antitumor efficacy of the delivery peptides with cyclic GMP–AMP (cGAMP) as a therapeutic agents or vaccine adjuvant. Results: We identify a class of STING agonist delivery peptides that efficiently facilitate cytosolic delivery of cyclic dinucleotide STING agonists and promote STING activation by forming peptide coacervates. Intratumoral administration of Sti3-4A and cGAMP effectively suppressed tumor growth and promoted antitumor immune response. Furthermore, the conjugation of tumor-specific antigen peptides with Sti3-4A promoted cytosolic co-delivery of antigen peptides and cGAMP, thus significantly boosting APC maturation, antigen cross-presentation, and T cell responses to peptide antigens. Prophylactic and therapeutic immunization with the conjugated peptides and cGAMP inhibited tumor growth in multiple murine tumor models. Conclusion: These findings establish STING agonist delivery peptides as a versatile platform for cancer immunotherapy.