Antisense Technology: Selection and Delivery of Optimally Acting Antisense Oligonucleotides

Abstract

Antisense technology has increasingly become a reality. It's objective is simple: to inhibit gene expression in a highly sequence-specific and selective manner. The technology has come to encompass three distinct strategies: antigene and antisense oligonucleotides (ODNs) and ribozymes. The antigene strategy relies on short, synthetic ODNs to form triple helical structures with duplex DNA to interfere with gene expression at the level of transcription, whereas antisense ODNs and ribozymes are designed to interfere with gene expression at the level of single stranded RNA (pre-mRNA or mRNA). Antisense ODNs simply rely on the fidelity of Watson:Crick base pairing interactions to hybridize (bind) to the target RNA and inhibit translation by several putative mechanisms including steric hindrance of ribosomal read-through or, depending on the ODN chemistry, by activation of RNaseH to selectively destroy the RNA component of the duplex. In contrast, ribozymes are RNA-based “antisense” oligonucleotides which exhibit the intrinsic capability of catalyzing the hydrolytic destruction of the target RNA within the RNA:ribozyme duplex. The catalytic activity of ribozymes is achieved by having a defined motif of core ribonucleotides which are not antisense (i.e. complementary) to the targeted mRNA molecules. Several RNA motifs are catalytic but the most commonly studied is the hammerhead motif (Symons, 1992; Akhtar and Rossi, 1996). Hammerhead Ribozymes and antisense ODNs are currently undergoing clinical evaluation as potential therapeutic agents in the treatment of HIV-infection in AIDS patients and also for a variety of cancers and inflammatory disorders (Akhtar and Agrawal, 1997; Narayanan and Akhtar, 1996). Although ribozymes and antisense molecules can be delivered as genes (gene therapy approach), in this article I will largely discuss the exogenous delivery of synthetic antisense ODNs and ribozymes.