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The development of biomaterials for efficient intracellular delivery of the nucleic acid-based immune-stimulating molecule, CpG oligodeoxynucleotides (CpG), is crucial for their biological activity. In this study, we successfully fabricated polydopamine-coated porous poly(lactic-co-glycolic acid) microparticles (PPM) for the delivery of CpGs. After conjugation of CpGs to PPMs via cleavable disulfide linkages, CpGs were readily released from CpG-conjugated PPM (PPM-s–s-CpGs) in reductive conditions. Released CpGs exhibited significantly enhanced induction of two cytokines, TNF-α and IL-6, in RAW264.7 cells. GpC-conjugated PPMs showed negligible cytokine induction, whereas CpG-conjugated PPMs exhibited strong induction of the two cytokines in RAW264.7 cells. The PPM-s–s-CpGs can serve as immune-stimulating adjuvants to enhance the immune responses of vaccines.

Recent promising clinical results of RNA therapeutics have drawn big attention of academia and industries to RNA therapeutics and their carrier systems. To improve their feasibility in clinics, systemic evaluations of currently available carrier systems under clinical trials and preclinical studies are needed. In this review, we focus on recent noticeable preclinical studies and clinical results regarding siRNA-based conjugates for clinical translations. Advantages and drawbacks of siRNA-based conjugates are discussed, compared to particle-based delivery systems. Then, representative siRNA-based conjugates with aptamers, peptides, carbohydrates, lipids, polymers, and nanostructured materials are introduced. To improve feasibility of siRNA conjugates in preclinical studies, several considerations for the rational design of siRNA conjugates in terms of cleavability, immune responses, multivalent conjugations, and mechanism of action are also presented. Lastly, we discuss lessons from previous preclinical and clinical studies related to siRNA conjugates and perspectives of their clinical applications.

Polydopamine-coated porous microsphere (PPM) is investigated as a simple and versatile immobilization strategy for immune-stimulating biomolecules to enhance delivery efficiency and immune-stimulating effects such as cytokine induction in macrophages. The PPMs, with diameters of about 2 μm, exhibit simultaneous and efficient incorporation of biomolecules (nucleotides and proteins), which is comparable to that achieved using microspheres carrying biomolecules internally by virtue of their porous structure. Ovalbumin-conjugated PPMs are internalized into macrophages efficiently and selectively via the phagocytic pathway, without any noticeable toxicity. Internalized CpG oligodeoxynucleotide (ODN)-conjugated PPMs (PPM-CpG) greatly enhance the induction of selected cytokines (TNF-α and IL-6) in RAW 264.7 cells compared to that by the soluble CpG ODN and ionic complexes. Therefore, PPMs generated in this study may serve as effective carriers of immune-stimulating biomolecules such as diverse toll-like receptor agonists.