Methoxyflavones from Stachys glutinosa with binding affinity to opioid receptors: In silico, in vitro, and in vivo studies

Fractionation of the bioactive dichloromethane extract from the aerial parts ofStachys glutinosa led to the isolation of four flavones, xanthomicrol (1),sideritoflavone (2), 8-methoxycirsilineol (3), and eupatilin (4), along with two neo-clerodane diterpenes, roseostachenone (8) and a new compound,3?,4?-epoxyroseostachenol (7). In order to study structure-activityrelationships, two methoxyflavones [5-demethyltangeretin (5) and tangeretin (6)] were synthesized by the methoxylation of xanthomicrol. The isolated compounds(1-4, 7, and 8) as well as the xanthomicrol semisynthetic derivatives (5 and 6)were evaluated for their binding affinity to the ? and ? opioid receptors.Xanthomicrol was the most potent binder to both ? and ? receptors, with a Kivalue of 0.83 and 3.6 ?M, respectively. Xanthomicrol administeredintraperitoneally in mice at a dose of 80 mg/kg significantly reducedmorphine-induced antinociception in the tail flick test. Our results suggestedthat xanthomicrol is a ? opioid receptor antagonist. Docking experiments werecarried out to acquire a deeper understanding about important structural aspects of binding of xanthomicrol. In summary, these data suggest that xanthomicrol is avaluable structure for further development into a potential ? opioid receptorantagonist.