To Conjugate or to Package? A Look at Targeted siRNA Delivery Through Folate Receptors

Advances in conjugate drug delivery System: Opportunities and challenges

Ideal drug delivery system is designed to accurately deliver the drug to its intended site. The development of conjugate drug delivery system introduces a novel pathway to precise drug delivery with advantages over traditional methods. The core of a conjugate drug delivery system comprises a molecule with two functional components, bounded by a linker structure. One component is responsible for delivering or stabilizing the conjugate, while the other is used to provide the therapeutic or diagnostic effects of the bioactivity. Conjugate drug delivery system improves patient health by maintaining the structural stability of drugs in molecular form, delivering therapeutics or diagnostic material to the target site, minimising off-target accumulation and promoting patient compliance. This system includes various types of drug conjugates that modulate drug pharmacokinetics, stability, absorption, and exposure in lesions and healthy tissues. In this review, we focus on the key characteristics and recent advances of various conjugate drug delivery systems and explore their mechanisms. We also point out the current challenges faced by conjugate drug delivery system and look forward to the future prospects.

Antitumor Effect of Oleoyl-siRNA against Pancreatic Cancer Using a Portal Vein Infusion Liver-Metastatic Mouse Model

In this study, we developed an oleoyl-siRNA conjugate in which oleic acid was conjugated at the 5'-end of the sense strand of the siRNA. Furthermore, we examined the effects of RNAi in a mouse model of pancreatic cancer with liver metastasis. The mouse model of pancreatic cancer with liver metastasis was developed by implanting Sui67Luc human pancreatic cancer cells into the portal veins of mice. Sui67Luc cells have high expression of tumor-related genes such as β-catenin, vascular endothelial growth factor, and programmed cell death ligand-1. All genes were knocked down using siRNA, among which siRNA targeting β-catenin exhibited the most suitable RNAi effect. Therefore, we investigated the in vitro RNAi effect of oleoyl-siRNA (Ole-siRNA) targeting the β-catenin gene in Sui67Luc cells and found that it was stronger than that of unmodified siRNA. For in vivo experiments, we investigated the biodistribution, antitumor effect, and change in life expectancy of mice upon systemic administration of Ole-siRNA complexed with Invivofectamine 3.0 (IVF). In terms of biodistribution, the Ole-siRNA/IVF complex likely accumulates in the liver of mice. The antitumor effect of Ole-siRNA in a portal vein infusion liver-metastatic Sui67Luc tumor mouse model was evaluated using an in vivo imaging system. Ole-siRNA had a significant antitumor effect compared with nonmodified siRNA. In addition, mice with metastatic liver Sui67Luc tumors treated with Ole-siRNA showed increased survival. These results suggest that Ole-siRNAs are useful novel RNAi molecules for treating pancreatic cancer and liver metastasis.

Mechanistic Pharmacokinetics and Pharmacodynamics of GalNAc-siRNA: Translational Model Involving Competitive Receptor-Mediated Disposition and RISC-Dependent Gene Silencing Applied to Givosiran

Triantennary N-acetyl-D galactosamine (GalNAc)3-conjugated small interfering RNA (siRNA) have majorly advanced the development of RNA-based therapeutics. Chemically stabilized GalNAc-siRNAs exhibit extensive albeit capacity-limited (nonlinear) distribution into hepatocytes with additional complexities in intracellular liver disposition and pharmacology. A mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) model of GalNAc-siRNA was developed to i) quantitate ASGPR-mediated disposition and downstream RNA-induced silencing complex (RISC)-dependent pharmacology following intravenous (IV) and subcutaneous (SC) dosing, ii) assess the kinetics of formed active metabolite, iii) leverage, as an example, published experimental data for givosiran, and iv) demonstrate PK translation across two preclinical species (rat and monkey) with subsequent prediction of human plasma PK. The structural model is based on competition between parent and formed active metabolite for occupancy and uptake via ASGPR into hepatocytes, intracellular sequestration and degradation, and downstream engagement of RNA-induced silencing complex (RISC) governing target mRNA degradation. The model jointly and accurately captured available concentration-time profiles of givosiran and/or AS(N-1)3' givosiran in rat and/or monkey plasma, liver, and/or kidney following givosiran administered both IV and SC. RISC-dependent gene silencing of ALAS1 mRNA was well-characterized. The model estimated an in vivo affinity (KD) value of 27.7 nM for GalNAc-ASGPR and weight-based allometric exponents of -0.27 and -0.24 for SC absorption and intracellular (endolysosomal) degradation rate constants. The model well-predicted reported givosiran plasma PK profiles in humans. PK simulations revealed net-shifts in liver-to-kidney distribution ratios with increasing IV and SC dose. Importantly, decreases in the relative liver uptake efficiency were demonstrated following IV and, to a lesser extent, following SC dosing explained by differential ASGPR occupancy profiles over time.

Nucleic acid nanoassembly-enhanced RNA therapeutics and diagnosis

RNAs are involved in the crucial processes of disease progression and have emerged as powerful therapeutic targets and diagnostic biomarkers. However, efficient delivery of therapeutic RNA to the targeted location and precise detection of RNA markers remains challenging. Recently, more and more attention has been paid to applying nucleic acid nanoassemblies in diagnosing and treating. Due to the flexibility and deformability of nucleic acids, the nanoassemblies could be fabricated with different shapes and structures. With hybridization, nucleic acid nanoassemblies, including DNA and RNA nanostructures, can be applied to enhance RNA therapeutics and diagnosis. This review briefly introduces the construction and properties of different nucleic acid nanoassemblies and their applications for RNA therapy and diagnosis and makes further prospects for their development.

Synthesis of folate-labeled siRNAs from a folate derivative phosphoramidite

With the recent success of GalNAc and the need for extra-hepatic RNAi delivery systems, other receptor-targeting ligands, like folate, have gained increased attention. The folate receptor is an important molecular target in cancer research, as it is overexpressed on numerous tumours while having limited expression in non-malignant tissues. Despite the promise of folate conjugation as a delivery platform in cancer therapeutics, its application in RNAi has been limited by sophisticated, and often expensive, chemistry. Here, we report a straightforward and cost-effective strategy to synthesize a novel folate derivative phosphoramidite for siRNA incorporation. In the absence of a transfection carrier, these siRNAs were selectively taken up by folate receptor-expressing cancer cell lines and displayed potent gene-silencing activity.

The therapeutic prospects of N-acetylgalactosamine-siRNA conjugates

RNA interference has become increasingly used for genetic therapy following the rapid development of oligonucleotide drugs. Significant progress has been made in its delivery system and implementation in the treatment of target organs. After a brief introduction of RNA interference technology and siRNA, the efficiency and stability of GalNAc-siRNA conjugates are highlighted since several oligonucleotide drugs of GalNAc have been approved for clinical use in recent years. The structure and features of GalNAc-siRNA conjugates are studied and the clinical efficiency and limitations of oligonucleotide-based drugs are summarized and investigated. Furthermore, another delivery system, lipid nanoparticles, that confer many advantages, is concluded, includ-ing stability and mass production, compared with GalNAc-siRNA conjugates. Importantly, developing new approaches for the use of oligonucleotide drugs brings hope to genetic therapy.

Synthesis of 5’-fluorophosphate-modified short-interfering RNAs

We have developed an improved scheme for the synthesis of a mono-fluorinated phosphoramidite for the 5'-modification of nucleic acids using standard phosphoramidite chemistry. We describe the first report of a phosphofluoridate modified siRNA strand and have evaluated C18 HPLC for purification of modified strands from unreacted siRNA strands. Lastly, the biological activity of the high purity siRNA strands, when placed on the sense and/or antisense strand, was evaluated to assess the impact of 5' phosphofluoridate modifications on siRNA activity.