Antisense oligonucleotides as emerging drugs

Abstract

Virtually every major human disease can be characterised by the discordant over-expression of one or more genes whose protein products contribute to the underlying pathophysiology. Antisense oligonucleotides provide a direct means with which to attenuate such discordant gene expression and epigenetically modify disease. Antisense oligonucleotides have thus been called the pharmacology of the future and the next great wave of the biotechnology revolution'. Their attractiveness as pharmaceuticals derives from the fact that they can theoretically modify disease at a point more proximal to its cause - the messenger RNA (mRNA) that acts as the intermediary between the DNA code of the disease gene and its protein effector molecule - than do more traditional 'small molecule' drugs. Such 'small molecule' drugs target proteins already participating in the disease process. In contrast, antisense oligonucleotides prevent the very formation of the disease-associated protein. In addition, the potential specificity and binding avidity of antisense oligonucleotides are orders of magnitude greater than comparable traditional drugs. Their specificity derives from two facts: first, that a single base change is sufficient to destroy the hybridisation potential of an antisense oligonucleotide, making it possible to selectively target viral RNAs, mutant mRNAs (e.g., oncogenes) without affecting their wild-type isomers, or mRNAs of individual members of gene families without cross-reactivity (adenosine A1 receptors vs. A2A, A2B or A3 receptors, for example); and second, that, on a theoretical basis, an antisense oligonucleotide 14 nucleotides in length or longer will be specific for a single gene in human DNA. The avidity of binding of antisense oligonucleotides derives from the combinative strength of hydrogen bonding inherent in Watson-Crick base pairing. In terms of further comparison of antisense oligonucleotides to traditional pharmaceuticals, one might draw the following analogy. The disease-related protein can be considered to resemble a piece of malfunctioning machinery, the gears of which are turning too rapidly, out of control. Traditional 'small molecule' antagonists, then, can be compared to wrenches thrust into the machinery to stop the spinning of its gear mechanism. The antisense oligonucleotide approach is more akin to walking behind the machinery and switching it off. One might consider this a more physiological strategy - more the way nature itself might engineer a response to a disease process. It is at least, teleologically, a most elegant mechanism to intervene in the disease process, if such elegant theory can be reduced to practice. However, significant problems arose in the development of antisense therapeutics, which have only partially been answered to date. Among these, delivery of adequate amounts of material to the target tissue has been especially troublesome. When administered systemically, many applications of antisense oligonucleotides appear to require large doses. Such large doses permit a variety of toxicities that can occur with this class of molecules to become apparent, e.g., certain sequence-dependent, non-antisense effects related to specific sequence motifs within the oligonucleotide ('G strings' - CpG dinucleotide induction of immunoreactivity; [1,2]), as well as certain sequence-independent, non-antisense effects associated with chemical modifications employed to prevent nuclease degradation. Alternative routes of delivery, e.g., inhalation of respirable antisense oligonucleotides, or other means of local administration (topical, intravitreal, etc.) that deliver the drug directly to the target tissue, may permit the full potential of antisense oligonucleotides to be realised [3,4]. It was perhaps because of the fact that the antisense mechanism is so straightforward and understandable that an exaggerated significance was placed on problems that arose during initial attempts to reduce antisense theory to practice. Several observers wrongly inferred from initially confounding data that antisense oligonucleotides could not achieve selective ablation of gene function in vivo. However, with the passage of time, several well-controlled examples of powerful in vivo antisense effects have been published (e.g., [3,5-7]), and at least two antisense oligonucleotides have been proven to have clinical benefit in man, as discussed below. Therapeutic application of antisense oligonucleotides is a field still in its infancy. Yet very significant progress has been made, and at a rate in excess of that observed for most other therapeutic classes. Because there remain significant unmet medical needs that have not proven amenable to traditional drug design strategies, the future of therapeutic antisense oligonucleotides appears promising.