Bioassay-Guided Isolation of Epiquinamide, a Novel Quinolizidine Alkaloid and Nicotinic Agonist from an Ecuadoran Poison Frog, Epipedobates tricolor

Analytical HPLC fractionation, combined with an off-line 96-well fluorescent bioassay screen, has been developed and used for the separation and screening of a natural product extract. This method was used to guide the isolation of a novel quinolizidine alkaloid from the methanolic skin extracts of an Ecuadoran frog, Epipedobates tricolor. The structure was determined on the basis of MS, IR, and NMR analysis as (1R,10R)-1-acetamidoquinolizidine (alkaloid 196). We have named this compound epiquinamide, reflecting its origin and structure. The activity of the isolated compound was determined in five cell lines expressing various nicotinic acetylcholine receptor subtypes. The bioactivity of epiquinamide was evaluated on the basis of membrane potential fluorescence and was found to be beta2 selective. This compound represents a new structural class of nicotinic agonists and a potential lead compound for the development of new therapeutics and pharmacological probes for nicotinic receptors. The off-line screening technique was found to be very sensitive for the detection of compounds active at nicotinic receptors.