3′-Fluoro substitution in the pyridine ring of epibatidine improves selectivity and efficacy for α4β2 versus α3β4 nAChRs

The analog of epibatidine having a fluoro substituent at the 3′ position of the pyridine ring has been recently developed and shown to possess binding affinity in the pM range to α4β2 nAChRs and in the nM range to α7 nAChRs and to exhibit potent agonist activity in nicotine-induced analgesia tests. Here we used patch-clamp technique in a whole-cell configuration to compare functional activity of 3′-fluoroepibatidine to that of epibatidine by itself on recombinant α4β2, α7 and α3β4 neuronal nAChRs. The agonist effect of (±)-epibatidine was partial and yielded comparable EC50s of 0.012 μM (72% efficacy) and 0.027 μM (81% efficacy) at α4β2 and α3β4 nAChRs, respectively, but was full at α7 nAChRs with an EC50 of 4.8 μM. Testing of the analog at different concentrations revealed that it acts as a full agonist with an EC50 of 0.36 μM at α4β2 nAChRs and induces partial agonist effect (66% efficacy) at α7 nAChRs with an EC50 of 9.8 μM and an IC50 corresponding to 225 μM. In contrast, the analog caused only 24% maximal activation at the range of concentrations from 0.1 to 100 μM and, in addition, induced an inhibition of α3β4 nAChR function with an IC50 of 8.3 μM. Our functional data, which are in agreement with previous binding and behavioral findings, demonstrate that 3′-fluoro substitution in the pyridine ring of epibatidine results in an improved pharmacological profile as observed by an increased efficacy and selectivity for α4β2 versus α3β4 nAChRs.