Isolation and Pharmacological Characterization of Six Opioidergic Picralima nitida Alkaloids

The seeds of the akuamma tree (Picralima nitida) have been used as a traditional treatment for pain and fever. Previous studies have attributed these effects to a series of indole alkaloids found within the seed extracts; however, these pharmacological studies were significantly limited in scope. Herein, an isolation protocol employing pH-zone-refining countercurrent chromatography is developed to provide six of the akuamma alkaloids in high purity and quantities sufficient for more extensive biological evaluation. Five of these alkaloids, akuammine, pseudo-akuammigine, picraline, akuammicine, and akuammiline, were evaluated against a panel of >40 central nervous system receptors to identify that their primary targets are the opioid receptors. Detailed in vitro investigations revealed one alkaloid as a potent kappa opioid receptor agonist and three alkaloids with micromolar activity at the mu opioid receptor. The mu opioid receptor agonists were further evaluated for analgesic properties but demonstrated limited efficacy in assays of thermal nociception. These findings contradict previous reports of the antinociceptive properties of the akuamma alkaloids and the traditional use of akuamma seeds as analgesics. Nevertheless, their opioid preferring activity does suggest the akuamma alkaloids provide distinct scaffolds from which to develop novel opioids with unique pharmacologically properties and therapeutic utility.