Investigation of Natural Product Scaffolds for the Development of Opioid Receptor Ligands

Abstract

Kappa opioid (KOP) receptors have been suggested as an alternative target to the mu opioid (MOP) receptor for the treatment of pain because KOP activation is associated with fewer negative side-effects (respiratory depression, constipation, tolerance, and dependence). The KOP receptor has also been implicated in several abuse-related effects in the central nervous system (CNS). KOP ligands have been investigated as pharmacotherapies for drug abuse; KOP agonists have been shown to modulate dopamine concentrations in the CNS as well as attenuate the self-administration of cocaine in a variety of species, and KOP antagonists have potential in the treatment of relapse. One drawback of current opioid ligand investigation is that many compounds are based on the morphine scaffold and thus have similar properties, both positive and negative, to the parent molecule. Thus there is increasing need to discover new chemical scaffolds with opioid receptor activity. The flavonoid class of natural products has been identified as a potential source of novel opioid ligands. In particular, dioclein (86) and dioflorin (87) have been reported to have an antinociceptive effect in rodent models of pain, although there has been no in vitro pharmacological evaluation to date. Dioclein and several simplified analogs of dioflorin were synthesized in order to develop structure activity relationships (SAR) for the flavonoid scaffold at opioid receptors. The analogs were pharmacologically evaluated in several cell-based assays (radioligand binding, fluorescent calcium mobilization, and luminescent PathHunterTM beta-arrestin) and found to be inactive at both opioid and cannabinoid receptors. The novel KOP receptor agonist and neoclerodane diterpene salvinorin A was also investigated. Salvinorin A is the main active component of the hallucinogenic plant Salvia divinorum and is the first opioid ligand reported that lacks a basic nitrogen atom in the structure. Ether analogs at the C-2 position of salvinorin A have been reported to have improved affinity and potency over the parent molecule. As alkyl chain ethers have a high degree of flexibility and the oxygen atom may allow for extra hydrogen bonding interactions in the receptor, modifications were made at the C-2 position in order to develop analogs to elucidate the molecular basis for this improved affinity and potency. Tetrahydropyranyl ether 220, ether 223a, and methyltetrahydropyranyl ether 231 were found to have similar KOP affinity and potency to salvinorin A in radioligand binding, [35S]GTP-gamma-S functional, and fluorescent calcium mobilization assays. Tetrahydropyranyl ether 220 was further evaluated for its effects on the cocaine-primed reinstatement of extinguished cocaine self-administration in rats; 220 (1 mg/kg) was found to attenuate cocaine self-administration comparably to salvinorin A (0.3 mg/kg), previously reported to be effective in this animal model. This represents the first report of a salvinorin A derivative with demonstrated anti-addictive capability.