The chemistry and biology of oligonucleotide conjugates

Short DNA or RNA oligonucleotides have tremendous potential as therapeutic agents. Because of their ability to engage in Watson-Crick base pairing, they can interact with mRNA or pre-mRNA targets with high selectivity. As a result, they could precisely manipulate gene expression. This possibility has engendered extensive efforts to develop oligonucleotides as drugs, and many candidates are already in clinical trials. However, a major impediment to the maturation of this field of oligonucleotide-based therapeutics remains: these relatively large and often highly charged molecules don't easily cross cellular membranes, making it difficult for them to reach their sites of action in the cytosol or nucleus. In this Account, we summarize some basic features of the biology of antisense and siRNA oligonucleotides. We then discuss chemical conjugation as an approach to improving the intracellular delivery and therapeutic potential of these agents. Instead of focusing on the details of conjugation chemistry, we emphasize the pharmacological ramifications of oligonucleotide conjugates. In one important approach to improving delivery and efficacy, researchers have conjugated oligonucleotides with ligands designed to bind to particular receptors and thus provide specific interactions with cells. In another strategy, researchers have coupled antisense or siRNA with agents such as cell penetrating peptides that are designed to provoke escape of the conjugate from intracellular vesicular compartments. Although both of these strategies have had some success, further research is needed before oligonucleotide conjugates can find an important place in human therapeutics.