Small interfering RNA (siRNA) has emerged as a promising approach for targeted gene silencing and therapeutic development. However, effective delivery remains one of the major challenges in siRNA pharmacology. Current delivery strategies, including lipid nanoparticles, viral vectors, and molecular conjugates, each have limitations related to stability, tissue distribution, cellular uptake, specificity, toxicity, or immunogenicity. Improving delivery remains essential for achieving sufficient exposure at the intended site of action while maintaining an acceptable safety profile.
Nonhuman primates (NHPs) can provide valuable translational information during the preclinical development of siRNA therapeutics. Their similarities to humans in anatomy, physiology, genetics, and immune biology make them useful for evaluating pharmacokinetics, pharmacodynamics, tissue distribution, and safety. Depending on the disease model and study design, NHP studies can also help assess target engagement and the relationship between tissue exposure and pharmacological effects.
This article reviews selected approaches for improving siRNA delivery in NHP studies, with a focus on emerging delivery technologies and their potential applications in preclinical research. It also considers the value and limitations of NHP models in the development of siRNA therapeutics.
New Delivery Approaches for siRNA Pharmacology in NHPs
One approach under investigation is the use of exosomes as siRNA delivery vehicles. Exosomes are extracellular vesicles released by cells that can carry nucleic acids and other biomolecules. Their ability to interact with recipient cells and participate in intercellular transport has led to interest in their potential use as carriers for RNA therapeutics. However, questions regarding loading efficiency, tissue targeting, manufacturing consistency, pharmacokinetics, and safety remain important considerations for further development.
A study by Wang et al. (2021) investigated exosome-mediated siRNA delivery in NHPs. In the study, exosomes derived from human embryonic kidney cells were loaded with siRNA targeting hepatitis B virus (HBV) surface antigen (HBsAg) and administered to HBV-infected cynomolgus monkeys. The investigators assessed serum HBsAg and HBV DNA levels following treatment. The reported findings indicated reductions in these viral markers following exosome-mediated siRNA delivery, supporting the potential of exosome-based systems for in vivo RNA delivery. Further studies are required to establish the reproducibility, pharmacological mechanism, biodistribution, and safety of this approach.
Ultrasound-mediated microbubble destruction (UMMD) represents another approach being investigated for localized delivery of nucleic acid therapeutics. In this strategy, therapeutic cargo is associated with ultrasound-responsive microbubbles. Following administration, focused ultrasound can induce microbubble disruption and may facilitate local release and cellular uptake of the associated cargo. The potential advantage of this approach is the possibility of achieving greater spatial control over delivery, although its effectiveness depends on factors such as microbubble formulation, cargo characteristics, ultrasound parameters, tissue accessibility, and treatment conditions.
Zhang et al. (2020) reported an NHP study investigating ultrasound-assisted siRNA delivery targeting angiotensin-converting enzyme 2 (ACE2). The study evaluated whether ultrasound-mediated delivery could reduce ACE2 expression in lung tissue. The reported results indicated reduced ACE2 expression following treatment and provided preliminary evidence supporting the feasibility of ultrasound-assisted nucleic acid delivery in the lung. Such approaches remain at an experimental stage, and further work is needed to establish their delivery efficiency, tissue specificity, durability of gene silencing, and safety.
The Role of NHP Models in siRNA Development
The development of siRNA therapeutics requires evaluation across multiple stages, from molecular design and delivery optimization to pharmacokinetic, pharmacodynamic, biodistribution, and safety assessment. NHP studies can provide an important translational bridge between rodent studies and clinical development, particularly when species differences in target biology, tissue distribution, or immune responses may affect the interpretation of preclinical findings.
At the same time, NHP studies should be designed around specific development questions rather than used as a direct substitute for earlier-stage models. The appropriate species, disease model, administration route, endpoints, and sampling strategy depend on the mechanism of action, delivery platform, target tissue, and intended clinical application.
Continued development of delivery technologies, together with well-designed NHP pharmacology studies, may help address some of the key challenges in siRNA development. Integrating PK/PD, biodistribution, molecular biomarkers, and other translational endpoints can provide a more comprehensive assessment of the relationship between siRNA exposure, tissue delivery, target engagement, and pharmacological response.
