Modified Akuamma Alkaloids with Increased Potency at the Mu-opioid Receptor

Akuammine (1) and pseudo-akuammigine (2) are indole alkaloids found in the seeds of the akuamma tree (Picralima nitida). Both alkaloids are weak agonists of the mu opioid receptor (µOR); however, they produce minimal effects in animal models of antinociception. To probe the interactions of 1 and 2 at the opioid receptors, we have prepared a collection of 22 semi-synthetic derivatives. Evaluation of this collection at the µOR and kappa opioid receptor (κOR) revealed structural-activity relationship trends and derivatives with improved potency at the OR. Most notably, the introduction of a phenethyl moiety to the N1 of 2 produces a 70-fold increase in potency and a 7-fold increase in selectivity for the µOR. The in vitro potency of this compound resulted in increased efficacy in the tail-flick and hot-plate assays of antinociception. The improved potency of these derivatives highlights the promise of exploring natural product scaffolds to probe the opioid receptors.