An opiate alkaloid isolated from the plant Papaver somniferum and produced synthetically. As a direct-acting smooth muscle relaxant, papaverine is not closely related to the other opium alkaloids in structure or pharmacological actions; its mechanism of action may involve the non-selective inhibition of phosphodiesterases and direct inhibition of calcium channels. This agent also exhibits antiviral activity against respiratory syncytial virus, cytomegalovirus, and HIV. Check for active clinical trials using this agent. (NCI Thesaurus)