Ribavirin, viramidine and adenosine-deaminase-catalysed drug activation: implication for nucleoside prodrug design (2003)

The epidemics caused by pathogenic viruses, such as HIV, hepatitis C virus (HCV) and most recently the severe acute respiratory syn- drome (SARS) by a new coronavirus, represent major global health hazards, which demand novel broad-spectrum antiviral intervention. Although discovered 40 years ago, ribavirin (1-β-D-ribofuranosyl- 1,2,4-triazole-3-carboxamide) remains the only approved small mol- ecule drug that is active against both DNA and RNA viruses. 1 The unique antiviral profile of ribavirin deserves a brief review of its dis- covery. Back in the 1960s, two five-membered ring nucleoside anti- biotics, showdomycin and pyrazomycin, were isolated from bacterial culture, and both (Figure 1) showed significant activity against a broad series of viruses in cell culture. The novel properties of these nucleoside antibiotics prompted synthesis and testing of a number of imidazole- and triazole-based ribonucleosides. 2 Among them, riba- virin (originally termed Virazole) stood out as the most promising broad-spectrum antiviral agent. Structurally, ribavirin is a ribo- nucleoside with an unconventional triazole base (Figure 1). X-ray crystallographic studies indicated that the five-membered triazole ribonucleoside is capable of mimicking the conformation of several canonical purine ribonucleosides with its 3-carboxamide oxygen and nitrogen spatially eclipsing the O6 and N1 atoms of inosine and guanosine. By rotating the C3-C6 bond by 180°, ribavirin also resembles the conformation of adenosine. The multiple conforma- tions adopted by ribavirin make it possible to target both viral and host enzymes that utilize purine ribonucleosides/nucleotides as a substrate or a cofactor, and may hold the key for its broad-spectrum antiviral activity. Current data support the notion that ribavirin acts by pleiotropic modes of action. 3 Ribavirin itself is a prodrug that is metabolized to the active 5'-monophosphate, -diphosphate and -tri- phosphate in vivo. Ribavirin 5'-monophosphate (RMP) inhibits cel- lular inosine monophosphate dehydrogenase (IMPDH), thereby depleting intracellular pools of GTP for viral replication. Ribavirin 5'-triphosphate (RTP) is the major metabolite accumulated in vivo. It is an effective inhibitor of viral RNA guanylyl transferase and (gua- nine-7N-)-methyl transferase, contributing to the defective 5'-cap structure of viral transcripts and insufficient translation. All the 5'-phosphate forms of ribavirin have been implicated as direct inhibitors of viral RNA polymerases. As a nucleotide analogue, RTP can also be incorporated into a viral genome and induces virus error catastrophe by mutations. 4 In a recent study of poliovirus replication in cell culture, a single mutation in the poliovirus RNA-dependent RNA polymerase was shown to confer resistance to ribavirin. 5 This is the first report of viral resistance to ribavirin. This study reinforces the notion that the transition mutations caused by incorporation of ribavirin as a mutagen are responsible for antiviral activity, at least in the case of poliovirus. On the other hand, consistent with the broad-spectrum antiviral property, ribavirin is also a small molecule immunomodulator and is capable of eliciting immuno-based anti- viral activities by shifting the host immune response to 'type 1', thereby enhancing T cell mediated immunity that favours elimin- ation of intracellular viral pathogens. The fact that no ribavirin resist- ance has been convincingly identified in the clinical setting supports the notion that ribavirin exerts its effect through non-specific or pleiotropic mechanisms. Despite its success as an effective antiviral in cell culture and in animal infection models, the clinical application of ribavirin has so far met with mixed results. One factor that may limit its clinical potential is the dose-limiting haemolytic anaemia when high doses are administered. Ribavirin formulated as small particle aerosol is currently approved for treatment of the respiratory syncytial virus (RSV) infection. In capsule or tablet form it is used as a part of combination therapy for chronic HCV infection. Notably, the com- bination therapy of ribavirin with interferon-α (both native and pegylated forms) has become the 'gold' standard for anti-HCV chemotherapy.