The ability of short, synthetic, single- stranded DNA or RNA oligonu- cleotides to interdict individual gene expression in a sequence-specific manner is the basis for antisense oligonucleotide-based therapies (for recent reviews see Refs 1,2). Unlike traditional drugs which typically interact with proteins, these oligonu- cleotide molecules are designed to selectively bind, by Watson-Crick base pairing, to complementary sequences in the target messenger RNA to interfere with and/or pre- vent formation of the gene product. In principle, antisense oligonu- cleotides can bind to and inhibit the translation of a given messenger RNA by several putative mecha- nisms including simply blocking ribosomal reading or by activating RNaseH, an endogenous enzyme which selectively degrades the message. The fact that very specific Watson-Crick binding of oligo- nucleotides to target sequences may be possible suggests that antisense- based therapies have the potential to be orders of magnitude more specific than conventional therapies. Antisense therapeutics, therefore, represents an exciting new tech- nology for manipulating gene expression in the treatment of human viral diseases such as AIDS, inflammatory disorders and cancers. The purpose of this article is to review the current literature on in vioo pre-clinical and clinical studies with a view to assessing the extent to which antisense oligonucleotides have ‘lived up’ to their initial thera- peutic potential and expectations. Pre-clinical efficacy studies The fact that antisense oligonu- cleotides may be useful therapeutic agents, particularly as antiviral agents, was demonstrated almost a decade ag+. However, it was quickly realized that unmodified phosphodiester oligonucleotides were rapidly degraded in biological fluid&-7 and in viao following intra- venous (i.v.) administration”. Hence, for therapeutic purposes biologically stable analogues would be desirable. The most widely studied of these analogues at present are the phosphorothioate oligonucleotides (S-oligos) where one of the non- bridging oxygens in the phospho- diester backbone is replaced with a sulphur. The initial ‘proof-of concept’ dis- coveries were made with such first- generation oligonucleotides in cell culture (for review see Ref. 9) but rapid development to in viva models followed. Table 1 (see Refs 10-23) highlights some of the in viva studies that have formed the basis for evalu- ating oligonucleotide-based thera- pies in clinical trials (see below). These and the pharmacokinetic dis- tribution studies (Table 2; see Refs 24-36) suggest that the organs of the reticuloendothelial system (RES) such as liver, spleen, kidney and lungs may be good sites for oligonucleotide efficacy as much of the oligonucleotide administered, by almost any route, appears to end up in these tissues (see below). For example, the study of Dean and McKay17 reported a dose-dependent, antisense-mediated downregulation (up to 90%) of pro- tein kinase C-a mRNA in the livers of hairless mice following intraperi- toneal (i.p.) administration. In another study, an oligonucleotide complementary to angiotensinogen that was administered to the liver via the portal vein showed a measurable reduction in angiotensinogen levels in plasma and a lowering of mean blood pressure for three to five days20. However, preferential accu- mulation of oligonucleotides within RES organs may also precipitate adverse effects (see below).