Overcoming the challenges of tissue delivery for oligonucleotide therapeutics

Synthetic therapeutic oligonucleotides (STOs) target genes and gene products by utilizing endogenous endonucleases for the degradation of cellular RNA, RNA maturation pathways by modulating splicing, and protein translation by sterically blocking the access of ribosomal machinery to mRNA. STOs cross at least two phospholipid bilayers, plasma, and endosomal membranes, to reach their biological targets, mRNA and DNA inside cells. They enter the cell through receptor-mediated endocytic uptake and escape endosomal compartments to access the cytoplasm and/or nucleus, where they exert pharmacological effects through different antisense mechanisms. Backbone and sugar modifications improve the drug properties of STOs by increasing their metabolic stability and affinity for plasma proteins to reduce glomerular filtration and urinary excretion, thereby facilitating distribution to peripheral tissues. Subsequent interactions with cell surface proteins promotes cellular internalization by endocytic pathways. Next-generation oligonucleotide drugs are targeted therapeutics conjugated to endogenous or exogenous ligands for cell-surface receptors expressed in specific cell types or tissues. Givosiran (Givlaari®) is the first ligand conjugated STO developed by Alnylam for the treatment of adults with acute hepatic porphyria. Synthetic therapeutic oligonucleotides (STO) represent the third bonafide platform for drug discovery in the pharmaceutical industry after small molecule and protein therapeutics. So far, thirteen STOs have been approved by regulatory agencies and over one hundred of them are in different stages of clinical trials. STOs hybridize to their target RNA or DNA in cells via Watson-Crick base pairing to exert their pharmacological effects. This unique class of therapeutic agents has the potential to target genes and gene products that are considered undruggable by other therapeutic platforms. However, STOs must overcome several extracellular and intracellular obstacles to interact with their biological RNA targets inside cells. These obstacles include degradation by extracellular nucleases, scavenging by the reticuloendothelial system, filtration by the kidney, traversing the capillary endothelium to access the tissue interstitium, cell-surface receptor-mediated endocytic uptake, and escape from endolysosomal compartments to access the nuclear and/or cytoplasmic compartments where their targets reside. In this review, we present the recent advances in this field with a specific focus on antisense oligonucleotides (ASOs) and siRNA therapeutics. Synthetic therapeutic oligonucleotides (STO) represent the third bonafide platform for drug discovery in the pharmaceutical industry after small molecule and protein therapeutics. So far, thirteen STOs have been approved by regulatory agencies and over one hundred of them are in different stages of clinical trials. STOs hybridize to their target RNA or DNA in cells via Watson-Crick base pairing to exert their pharmacological effects. This unique class of therapeutic agents has the potential to target genes and gene products that are considered undruggable by other therapeutic platforms. However, STOs must overcome several extracellular and intracellular obstacles to interact with their biological RNA targets inside cells. These obstacles include degradation by extracellular nucleases, scavenging by the reticuloendothelial system, filtration by the kidney, traversing the capillary endothelium to access the tissue interstitium, cell-surface receptor-mediated endocytic uptake, and escape from endolysosomal compartments to access the nuclear and/or cytoplasmic compartments where their targets reside. In this review, we present the recent advances in this field with a specific focus on antisense oligonucleotides (ASOs) and siRNA therapeutics. chemically modified, short, single-stranded oligonucleotides (typically 12–20 mer) that bind to their target RNA by Watson-Crick base-pairing principles and modulate expression of their target mRNA by multiple antisense mechanisms. routes of drug administration where the compound is directly delivered into the tissue of interest. These methods of administration bypass tissue barriers such as the blood–brain barrier (BBB) and blood–retinal barrier and reduce exposure to systemic tissues. Examples of local delivery methods include intrathecal or intraocular injections for delivery to the CNS or the eye, and aerosol delivery to the lungs. an endonuclease that specifically cleaves the RNA strand of a DNA–RNA hybrid. ASOs bind to target mRNA through complementary base pairing, recruiting RNaseH1 that leads to degradation of the target mRNA, thus preventing the translation of mRNA into protein. an siRNA or miRNA-directed endonuclease that is composed of multiprotein complex, which incorporates one strand of a siRNA or miRNA duplex for interaction with complementary mRNA sequence. Upon activation, one of the proteins in the complex (Argonaute 2), cleaves the target mRNA. double-stranded short RNA or modified duplexes that mediate targeted mRNA degradation through RISC complex formation. routes of drug administration where the compound is directly delivered into the blood stream to access tissues. Systemic delivery methods for synthetic therapeutic oligonucleotides (STO) include intravenous, intramuscular, and subcutaneous injections.