Synergistic effect of acyclovir and 3,19-isopropylideneandrographolide on herpes simplex virus wild types and drug resistant strains

Background: An andrographolide analogue, 3,19-isopropylideneandrographolide (IPAD), exert an inhibitory effect on herpes simplex virus serotype 1 (HSV-1) wild types replication. In this study, we examined the anti-viral activity of IPAD on HSV-1 and HSV-2 wild types and HSV-1 drug-resistant strains (DRs). Synergistic effects of IPAD with ACV were also evaluated. Methods: MTT and plaque reduction assay were performed to determine the cytotoxicity and anti-viral activity, respectively. Combination assay was determined for synergistic effects. Viral DNA and protein were investigated by polymerase chain reaction and western blotting, respectively. Results: The results showed that, at the sub-cytotoxic concentration of 22.50 μM, IPAD completely inhibited the plaque formations of HSV wild types (HSV-1 kos and HSV-2) and HSV-1 DRs at post entry step. The activity was confirmed by complete inhibition of protein synthesis, but the low level viral DNA was still detected. The inhibitory concentration of ACV for HSV wild types and HSV-1 DRs was 22.20 and 2,220.00 μM, respectively. The combination of ACV and IPAD showed synergistic effect for complete inhibition of plaques, viral DNA and protein synthesis of HSV wild types and reduce ACV and IPAD concentration to 0.04 and 12.80 μM, respectively; whereas for the complete inhibition of HSV-1 DRs are at 22.20 and 12.8 μM, respectively. Conclusions: These results showed the inhibitory potential of IPAD on HSV wild type and HSV drug resistances and suggested that IPAD can be used as a synergistic drug for treatment of HSV-1 DRs.