Putranjivain A from Euphorbia jolkini inhibits both virus entry and late stage replication of herpes simplex virus type 2 in vitro

Aims: To investigate the in vitro antiviral properties of putranjivain A, isolated from the whole plant of Euphorbia jolkini Bioss (Euphorbiaceae). Methods and results: Herpes simplex virus (HSV)-2 strain 196-infected Vero cells were used. It was shown that putranjivain A exhibited antiviral activity with an IC ₅₀ of 7.9 ± 1.2 μM using the XTT assay, with the IC ₅₀ value increasing with increasing multiplicity of infection. Using the plaque reduction assay, the IC ₅₀ and IC ₉₀ were 6.3 ± 0.8 and 14.5 ± 3.1 μM, respectively. Putranjivain A showed no cytotoxic effect on cell multiplication at concentrations that achieved antiviral activity. The 50% cell cytotoxic concentration (CC ₅₀) was 80.3 ± 14.7 μM, and the selectivity index (SI) (ratio of CC ₅₀ to IC ₅₀) for the XTT and plaque reduction assays was 10.2 and 12.7, respectively. When tested for virucidal activity, putranjivain A significantly reduced viral infectivity at concentrations of 75 and 100 μM, but not at 50 μM or below. The results of the time-of-addition studies suggested that putranjivain A affected the late stage of HSV-2 replication at 25 μM. Interestingly, putranjivain A also showed inhibition of viral attachment and cell penetration. The combination of putranjivain A and aciclovir produced no interaction. Conclusions: Putranjivain A possesses antiviral activity, inhibiting viral attachment and penetration, and also interfering with the late stage of viral replication.