Background: Ribavirin is a broad-spectrum antiviral agent that derives some of its activity from inhibition of cellular inosine monophosphate dehydrogenase (IMPDH), resulting in lower guanosine triphosphate (GTP) levels. Here we report the biological activities of three ribavirin analogues.
Methods: Antiviral activities of test compounds were performed by in vitro cytopathic effect inhibition assays against influenza A (H1N1, H3N2 and H5N1), influenza B, measles, parainfluenza type 3 (PIV-3) and respiratory syncytial viruses. Compounds were modelled into the ribavirin 5'-monophosphate binding site of the crystallographic structure of the human type II IMPDH (hIMPDH2) ternary complex. Effects of compounds on intracellular GTP levels were performed by strong anion exchange HPLC analysis.
Results: Of the three compounds evaluated, the 5-ethynyl nucleoside (ETCAR) exhibited virus-inhibitory activities (at 1.2-20 μM, depending upon the virus) against most of the viruses, except for weak activity against PIV-3 (62 μM). Antiviral activity of ETCAR was similar to ribavirin; however, cytotoxicity of ETCAR was greater than ribavirin. Replacing the 5-ethynyl group with a 5-propynyl or bromo substituent (BrCAR) considerably reduced antiviral activity. Computational studies of ternary complexes of hIMPDH2 enzyme with 5'-monophosphates of the compounds helped rationalize the observed differences in biological activity. All compounds suppressed GTP levels in cells; additionally, BrCAR suppressed adenosine triphosphate and elevated uridine triphosphate levels.
Conclusions: Three compounds related to ribavirin inhibited IMPDH and had weak to moderate antiviral activity. Cytotoxicity adversely affected the antiviral selectivity of ETCAR. As with ribavirin, reduction in intracellular GTP may play a role in virus inhibition.