Phase-Separated Peptide Coacervates as Delivery Vehicles for mRNA Vaccines

Nucleic acid therapeutics, such as mRNA vaccines, require effective delivery vehicles; yet widely used systems like lipid nanoparticles or cationic polymers are often associated with adverse responses. Developing low-molecular-weight compounds as delivery vehicles for nucleic acids is promising but remains challenging. Here, we identify a disulfide-bond-linked, six-residue peptide, WWKssKWW (WWK), which forms coacervates through liquid-liquid phase separation (LLPS) in aqueous solutions. The WWK coacervates encapsulate diverse nucleic acids—including single-stranded DNA, siRNA, mRNA, and plasmids—and deliver them into cells with efficiencies equivalent to or exceeding conventional agents (lipofectamine, InstantFECT, PEI, and lipid nanoparticles). Subcutaneous and intramuscular injection of mRNA-loaded WWK coacervates in mice results in robust protein expression and a favorable safety profile. Importantly, when human PBMCs were transfected with various mRNA formulations, WWK/mRNA induced significantly lower levels of the inflammatory cytokine IL-1β compared to liposomes and lipid nanoparticles, similar to naked mRNA, suggesting reduced toxicity risk. WWK coacervates also enabled efficient intramuscular delivery of SARS-CoV-2 spike mRNA, producing spike-specific cellular responses comparable to the commercial Comirnaty vaccine. Furthermore, in a B16-OVA cancer model, WWK/OVA mRNA coacervates exhibited potent anti-tumor effects and significantly enhanced tumor-free survival. Overall, our findings demonstrate that peptide coacervates are promising vehicles for mRNA delivery, offering improved safety and efficacy for gene therapy and vaccine development.