Cyclopiazonic acid, an inhibitor of calcium-dependent ATPases with antiviral activity against human respiratory syncytial virus

- We identified indole alkaloid cyclopiazonic acid as an inhibitor of RSV by screening of natural product library.
- Cyclopiazonic acid was efficacious against strains of both RSV subgroups A and B, and PIV-3, but not EV-A71.
- Cyclopiazonic acid acts at the step of virus genome replication and/or transcription.
- Pharmacological inhibitors capable of elevating intracellular calcium concentration could also inhibit RSV replication.

Human respiratory syncytial virus (RSV) is a common cause of lower respiratory tract infections in infants and young children worldwide, yet no vaccine or effective antiviral treatment is available. To search for new anti-RSV agents, we developed a cell-based assay that measures inhibition of RSV-induced cytopathic effect (CPE) and identified cyclopiazonic acid (CPA), an intracellular calcium ATPase inhibitor as a RSV inhibitor (EC50 values 4.13 μM) by screening of natural product library. CPA inhibited the replication of RSV strains belonging to both A and B subgroups and human parainfluenza virus type 3, but not Enterovirus 71. Mechanism of action study by time-of-addition assay and minigenome assay revealed that CPA acts at the step of virus genome replication and/or transcription. Moreover, two other calcium ATPase inhibitors (Thapsigargin and BHQ) and calcium ionophores (A23187 and ionomycin), but not calcium channel blockers (nifedipine, nimodipine, and tetrandrine), also had similar effect. These results indicate that an increase in intracellular calcium concentration is detrimental to RSV replication. Thus, our findings provide a new strategy for anti-RSV therapy via increasing intracellular calcium concentration.