Strategies, design, and chemistry in siRNA delivery systems

Abstract

Emerging therapeutics that utilize RNA interference (RNAi) have the potential to treat broad classes of diseases due to their ability to reversibly silence target genes. In August 2018, the FDA approved the first siRNA therapeutic, called ONPATTRO™ (Patisiran), for the treatment of transthyretin-mediated amyloidosis. This was an important milestone for the field of siRNA delivery that opens the door for additional siRNA drugs. Currently, >20 small interfering RNA (siRNA)-based therapies are in clinical trials for a wide variety of diseases including cancers, genetic disorders, and viral infections. To maximize therapeutic benefits of siRNA-based drugs, a number of chemical strategies have been applied to address issues associated with efficacy, specificity, and safety. This review focuses on the chemical perspectives behind non-viral siRNA delivery systems, including siRNA synthesis, siRNA conjugates, and nanoparticle delivery using nucleotides, lipids, and polymers. Tracing and understanding the chemical development of strategies to make siRNAs into drugs is important to guide development of additional clinical candidates and enable prolonged success of siRNA therapeutics.

Keywords: Nanomaterials; Therapeutics; siRNA.

Fig. 1.. Mechanism of RNA interference and strategies for siRNA synthesis and delivery.

a. An illustration of RNA interference process. b. Strategies and procedures for siRNA development and delivery.

Fig. 2.. siRNA synthesis.

a. Chemical modification of nucleic acids including the 2’-position, phosphate linkage, ribose, and nucleobase. b. Solid-phase synthesis of RNA strands using automated RNA synthesizer.

Fig. 3.. Chemical strategies for synthesis of siRNA conjugates.

a. siRNA conjugates with ligands including small molecules, carbohydrates peptides, antibodies andaptamers. b. Parallel and linear synthesis siRNA-peptide and -cholesterol conjugates. c. An example of linear synthesis of GalNAc-siRNA conjugates.

Fig. 3.. Chemical strategies for synthesis of siRNA conjugates.

a. siRNA conjugates with ligands including small molecules, carbohydrates peptides, antibodies andaptamers. b. Parallel and linear synthesis siRNA-peptide and -cholesterol conjugates. c. An example of linear synthesis of GalNAc-siRNA conjugates.

Fig. 4.. Nucleotides derived nanoparticles.

a. Tetrahedron DNA origami. b. Spherical nucleic acid (SNA) conjugates with a gold core and siRNA shell. c. Three-way junction RNA nanoparticles. d. RNAi microsponges with multiple copies of shRNA.

Fig. 5.. Lipid analogs with cationic head groups, linker and hydrophobic tails and a representative synthetic route to DLin-MC3-DMA.

Cationic head groups can be single or multiple cationic centers. Linkers span from ester, amide to ketal. Hydrophobic tails can accommodate unsaturated bonds, cholesterol, and ester groups.

Fig. 6.. Combinatorial and high throughput strategies of lipid-like materials.

Thousands of lipid-like compounds were synthesized through Michael addition reactions, epoxide ring-opening reactions, reductive amination reactions and thiol-ene reactions. Materials were screened with high throughput bioassays both in vitro and in vivo.

Figure 7.

Selected examples of conjugate, linear, and branched type architectures for siRNA delivery.

Figure 8.

Selected examples of block copolymer, star / core-shell, and self-assembled designs for siRNA delivery.