Antiviral Action of Tryptanthrin Isolated from Leaf against Human Coronavirus NL63.

(Nees) Kuntze is a Chinese herbal medicine used in the treatment of respiratory virus infections. The methanol extract of leaf contains chemical components such as β-sitosterol, indirubin, tryptanthrin, betulin, indigodole A, and indigodole B that have diverse biological activities. However, the antiviral action of leaf and its components against human coronavirus remains to be elucidated. Human coronavirus NL63 infection is frequent among immunocompromised individuals, young children, and in the elderly. This study investigated the anti-Human coronavirus NL63 (HCoV-NL63) activity of the methanol extract of leaf and its major components. The methanol extract of leaf effectively inhibited the cytopathic effect (CPE) and virus yield (IC = 0.64 μg/mL) in HCoV-NL63-infected cells. Moreover, this extract potently inhibited the HCoV-NL63 infection in a concentration-dependent manner. Among the six components identified in the methanol extract of leaf, tryptanthrin and indigodole B (5a-ethyltryptanthrin) exhibited potent antiviral activity in reducing the CPE and progeny virus production. The IC values against virus yield were 1.52 μM and 2.60 μM for tryptanthrin and indigodole B, respectively. Different modes of time-of-addition/removal assay indicated that tryptanthrin prevented the early and late stages of HCoV-NL63 replication, particularly by blocking viral RNA genome synthesis and papain-like protease 2 activity. Notably, tryptanthrin (IC = 0.06 μM) and indigodole B (IC = 2.09 μM) exhibited strong virucidal activity as well. This study identified tryptanthrin as the key active component of leaf methanol extract that acted against HCoV-NL63 in a cell-type independent manner. against HCoV-NL63 infection.