Inhibitory effect of mizoribine and ribavirin on the replication of severe acute respiratory syndrome (SARS)-associated coronavirus

Abstract

The activity of inosine-5'-monophosphate dehydrogenase (IMPDH) inhibitors, mizoribine and ribavirin, against severe acute respiratory syndrome (SARS)-associated coronavirus (SARS-CoV) was determined by plaque reduction and yield reduction assays. Mizoribine and ribavirin selectively inhibited replication of SARS-CoV. The 50% inhibitory concentration (IC50) of mizoribine for SARS-CoV Frankfurt-1 and SARS-CoV HKU39849, as determined by plaque reduction was 3.5 microg/ml and 16 microg/ml, respectively, and the IC50 of ribavirin for SARS-CoV Frankfurt-1 and SARS-CoV HKU39849 was 20 microg/ml and 80 microg/ml, while the 50% cytotoxic concentration of mizoribine and ribavirin for Vero E6 cells exceeded 200 microg/ml. In a yield reduction assay, mizoribine (10 microg/ml) and ribavirin (40 microg/ml) inhibited the replication of SARS-CoV and reduced the infectious SARS-CoV titers to one-tenth or less. Mizoribine inhibited replication of SARS-CoV more strongly than ribavirin. However, neither drug could completely inhibit replication of SARS-CoV even at concentrations up to 100 microg/ml.

Fig. 1

Cytotoxicity (A), inhibitory effect on plaque formation of SARS-CoV Frankfurt-1 (B) and HKU39849 (D) in a plaque reduction assay and inhibitory effect on replication of SARS-CoV Frankfurt-1 (C) and HKU39849 (E) in yield reduction assay. The symbols, “●” and “■” represent mizoribine and ribavirin, respectively. The vertical bar indicates 1 S.D. One hundred percent of plaque numbers in (B) and (D) correspond to the control (no compound). The SARS-CoV titers at 0 μg/ml of drugs are not shown in these figures, as these titers were not significantly different from those at a concentration of 0.1 μg/ml.

Fig. 2

Inhibitory effect of mizoribine (●) and ribavirin (■) on vaccinia virus (Lister strain) (A) and HSV-1 VR-3 (B) determined by plaque reduction assay. The vertical bar indicates 1 S.D. calculated from the data obtained by independent three experiments. One hundred percent of plaque numbers correspond to the control (no compound).