The protein coded by the open-reading-frame 3a of SARS coronavirus has been demonstrated to form a cation-selective channel that may become expressed in the infected cell. The activity of the channel is involved in the mechanism of virus release. Drugs that inhibit the ion channel can, therefore, inhibit virus release, and they could be a source for development of novel therapeutic antiviral agents. Various drugs found in Chinese herbs that are well known as anticancer agents also have an antiviral potency. Here we tested the flavonols kaempferol, kaempferol glycosides, and acylated kaempferol glucoside derivatives with respect to their potency to block the 3a channel. We used the <i>Xenopus</i> oocyte with a heterologously expressed 3a protein as a model system to test the efficacy of the flavonols. Some of these drugs turned out to be potent inhibitors of the 3a channel. The most effective one was the glycoside juglanin (carrying an arabinose residue) with an IC<sub>50</sub> value of 2.3 µM for inhibition of the 3a-mediated current. Kaempferol derivatives with rhamnose residue also seem to be quite effective. We suggest that viral ion channels, in general, may be a good target for the development of antiviral agents, and that, in particular, kaempferol glycosides are good candidates for 3a channel proteins of coronaviruses.

Kaempferol Derivatives as Antiviral Drugs against the 3a Channel Protein of Coronavirus