Absolute Configuration and Pharmacology of the Poison Frog Alkaloid Phantasmidine

Abstract

Phantasmidine, a rigid congener of the well-known nicotinic acetylcholine receptor agonist epibatidine, is found in the same species of poison frog ( Epipedobates anthonyi). Natural phantasmidine was found to be a 4:1 scalemic mixture, enriched in the (2a R,4a S,9a S) enantiomer by chiral-phase LC-MS comparison to the synthetic enantiomers whose absolute configurations were previously established by Mosher's amide analysis. The major enantiomer has the opposite S configuration at the benzylic carbon to natural epibatidine, whose benzylic carbon is R. Pharmacological characterization of the synthetic racemate and separated enantiomers established that phantasmidine is ∼10-fold less potent than epibatidine, but ∼100-fold more potent than nicotine in most receptors tested. Unlike epibatidine, phantasmidine is sharply enantioselective in its activity and the major natural enantiomer whose benzylic carbon has the 4a S configuration is more active. The stereoselective pharmacology of phantasmidine is ascribed to its rigid and asymmetric shape as compared to the nearly symmetric conformations previously suggested for epibatidine enantiomers. While phantasmidine itself is too toxic for direct therapeutic use, we believe it is a useful platform for the development of potent and selective nicotinic agonists, which may have value as pharmacological tools.

Figure 1.

Chiral-phase LC-MS chromatograms showing extracted ions for phantasmidine (1), epibatidine (2a) and N-methylepibatidine (2b) in E. anthonyi extract. All ions are shown in the top trace. Ions in the mass range indicated on the right are shown in subsequent traces.

Figure 2.

Functional profiles of phantasmidine as the racemate (top panel) and individual enantiomers (bottom panels) at rat nicotinic and serotonin receptor-expressing Xenopus oocytes.

Figure 3.

Molecular Electrostatic Potential Maps of Epibatidine, Phantasmidine, Varenicline and Cytisine.