Synthesis of Monovalent N-Acetylgalactosamine Phosphoramidite for Liver-Targeting Oligonucleotides

Drug discovery and development scheme for liver-targeting bridged nucleic acid antisense oligonucleotides

Antisense oligonucleotides (ASOs) containing bridged nucleic acids (BNAs) have been proven to be very powerful. However, ensuring a reliable discovery and translational development scheme for this class of ASOs with wider therapeutic windows remains a fundamental challenge. We here demonstrate the robustness of our scheme in the context of the selection of ASOs having two different BNA chemistries (2,′4′-BNA/locked nucleic acid [LNA] and amido-bridged nucleic acid [AmNA]) targeting human proprotein convertase subtilisin/kexin type 9 (PCSK9). The scheme features a two-step process, including (1) a unique and sensitive in vitro screening approach, called Ca²⁺ enrichment of medium (CEM) transfection, and (2) a ligand-targeted drug delivery approach to better reach target tissues, averting unintended accumulation of ASOs. Using CEM screening, we identified a candidate ASO that shows >70% cholesterol-lowering action in monkeys. An N-acetylgalactosamine (GalNAc) ligand then was appended to the candidate ASO to further broaden the therapeutic margin by altering the molecule’s pharmacokinetics. The GalNAc conjugate, HsPCSK9-1811-LNA, was found to be at least ten times more potent in non-human primates (compared with the unconjugated counterpart), with reduced nephrotoxicity in rats. Overall, we successfully showed that our drug development scheme is better suited for selecting clinically relevant BNA-based ASOs, especially for the treatment of liver-associated diseases.

Programmed Instability of Ligand Conjugation Manifold for Efficient Hepatocyte Delivery of Therapeutic Oligonucleotides

Ligand-targeted drug delivery (LTDD) has gained more attention in the field of nucleic acid therapeutics. To further elicit the potential of therapeutic oligonucleotides by means of LTDD, we newly developed (R)- and (S)-3-amino-1,2-propanediol (APD) manifold for ligand conjugation. N-acetylgalactosamine (GalNAc)/asialoglycoprotein receptor (ASGPr) system has been shown to be a powerful and robust paradigm of LTDD. Our novel APD-based GalNAc (GalNAc_APD) was shown to have intrinsic chemical instability that could play a role in better manipulation of active drug release. The APD manifold also enables facile production of conjugates through an on-support ligand cluster synthesis. We showed in a series of in vivo studies that while the knockdown activity of antisense oligonucleotides (ASOs) bearing 5'-GalNAc_APD was comparable to the conventional hydroxy-L-prolinol-linked GalNAc (GalNAc_HP), 3'-GalNAc_APD elicited ASO activity by more than twice as much as the conventional 3'-GalNAc_HP. This was ascribed partly to the GalNAc_APD's ideal susceptibility to nucleolytic digestion, which is expected to facilitate cytosolic internalization of ASO drugs. Moreover, an in vivo/ex vivo imaging study visualized the enhancement effect of monoantennary GalNAc_APD on liver localization of ASOs. This versatile manifold with chemical and biological instability would benefit therapeutic oligonucleotides that target both the liver and extrahepatic tissues.

Highly Potent GalNAc-Conjugated Tiny LNA Anti-miRNA-122 Antisense Oligonucleotides

The development of clinically relevant anti-microRNA antisense oligonucleotides (anti-miRNA ASOs) remains a major challenge. One promising configuration of anti-miRNA ASOs called "tiny LNA (tiny Locked Nucleic Acid)" is an unusually small (~8-mer), highly chemically modified anti-miRNA ASO with high activity and specificity. Within this platform, we achieved a great enhancement of the in vivo activity of miRNA-122-targeting tiny LNA by developing a series of N-acetylgalactosamine (GalNAc)-conjugated tiny LNAs. Specifically, the median effective dose (ED50) of the most potent construct, tL-5G3, was estimated to be ~12 nmol/kg, which is ~300-500 times more potent than the original unconjugated tiny LNA. Through in vivo/ex vivo imaging studies, we have confirmed that the major advantage of GalNAc over tiny LNAs can be ascribed to the improvement of their originally poor pharmacokinetics. We also showed that the GalNAc ligand should be introduced into its 5' terminus rather than its 3' end via a biolabile phosphodiester bond. This result suggests that tiny LNA can unexpectedly be recognized by endogenous nucleases and is required to be digested to liberate the parent tiny LNA at an appropriate time in the body. We believe that our strategy will pave the way for the clinical application of miRNA-targeting small ASO therapy.

Effect of modular conjugation strategy for N-acetylgalactosamine-targeted antisense oligonucleotides

The asialoglycoprotein receptor (ASGPr) and N-acetylgalactosamine (GalNAc) is one of the most reliable receptor-ligand combinations for delivering antisense oligonucleotides (ASOs) to the liver. Here, we show that a modular GalNAc conjugation strategy allows us to reinforce the activity of the parent, naked 2',4'-BNA/LNA gapmer targeting apolipoprotein B. The conjugation partly reduced a possible hepatotoxicity of the parent ASO. The structure-activity study revealed the significance of the metabolic susceptibility of the GalNAc moiety to nucleolytic cleavage that results in exposure of the parent gapmer. The broad usefulness of our delivery strategy of ASOs to the liver has been demonstrated.