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

Abstract

Akuammine (1) and pseudoakuammigine (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 semisynthetic 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 μOR. 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.

Discovery of Potent Kappa Opioid Receptor Agonists Derived from Akuammicine

“…Because our previous studies indicated the akuamma alkaloids are moderately potent κOR and µOR agonists with negligible activity at the dOR, all compounds were first evaluated at the κOR and µOR using a cell-based system that measured inhibition of forskolin-induced production of cAMP as a measure of G-protein pathway activation (Table 1). 35,36 Notably, consistent with our initial characterization of 1 as a selective κOR agonists, except in instances where the modifications removed all agonist activity, the derivatives were full κOR agonists with considerably greater potency at the κOR than the µOR. And while most of these derivatives possess similar efficacies, it was noted that several modifications had considerable effects on the compounds' potency at the κOR.…”