β-Carboline derivatives as novel antivirals for herpes simplex virus

Several commercial and novel synthetic β-carbolines (βCs) were evaluated for their antiviral activity against herpes simplex virus type 1 (HSV-1) using an adapted MTS assay. Of 21 drugs tested, although 11 exerted antiviral activity at non-cytotoxic concentrations, only 3 of them [9-methyl-norharmane (9-Me-nHo), 9-methyl-harmane (9-Me-Ho) and 6-methoxy-harmane (6-MeO-Ho)] completely avoided virus-driven cytopathic effects. Half-maximal effective concentrations (EC₅₀ values) (4.9 ± 0.4, 5.9 ± 0.8 and 19.5 ± 0.3 µM, respectively) and selectivity indexes (88.8, 40.2 and 7.0, respectively) of the latter three βCs against HSV-1 were determined by MTS, flow cytometry and plaque reduction assays. The mode of action of these drugs was also evaluated. According to time-of-addition assays, the selected compounds were not virucidal and did not interfere with attachment or penetration of HSV-1, but interfered with later events of virus infection. Western blot studies showed that early and late protein expression was significantly delayed or even suppressed. Herpes simplex virus type 2 (HSV-2) was also inhibited by the selected substances in a similar manner. Interestingly, 6-MeO-Ho, 9-Me-Ho and 9-Me-nHo restricted HSV-1 ICP0 localisation to the nucleus during later stages of infection, possibly affecting its functionality in the cytoplasm where ICP0 normally inhibits antiviral signalling and promotes viral replication. In silico prediction of ADME (Absorption, Distribution, Metabolism and Excretion) properties showed that all compounds fulfilled Lipinski's rule and their calculated absorptions were >95%.