Pre-clinical properties of the alpha 4 beta 2 nicotinic acetylcholine receptor partial agonists varenicline, cytisine and dianicline translate to clinical efficacy for nicotine dependence

Background and purpose:

Smoking cessation trials with three high-affinity partial agonists of alpha 4 beta 2 neuronal nicotinic acetylcholine receptors (nAChRs) have demonstrated differences in their clinical efficacy. This work examines the origin of the differences by taking into account brain exposure and pharmacological effects at human alpha 4 beta 2 nAChRs.

Experimental approach:

Rat plasma and brain pharmacokinetics were characterized and used to predict human steady-state plasma and brain concentrations following recommended doses of each of the three compounds. The pharmacological characterization included in vitro affinities at different nAChR subtypes, functional efficacies and potencies at the human alpha 4 beta 2 nAChR, as well as in vivo effects on rat mesolimbic dopamine turn-over.

Key results:

A comparison of predicted human brain concentrations following therapeutic doses demonstrated that varenicline and nicotine, but not dianicline and cytisine, can extensively desensitize and, to a lesser extent, activate alpha 4 beta 2 nAChRs. The limited clinical efficacy of dianicline may be accounted for by a combination of weak functional potency at alpha 4 beta 2 nAChRs and moderate brain penetration, while recommended doses of cytisine, despite its high in vitro potency, are predicted to result in brain concentrations that are insufficient to affect alpha 4 beta 2 nAChRs.

Conclusions and implications:

The data provide a plausible explanation for the higher abstinence rate in smoking cessation trials following treatment with varenicline than with the two other alpha 4 beta 2 nAChR partial agonists. In addition, this retrospective analysis demonstrates the usefulness of combining in vitro and in vivo parameters with estimated therapeutic human brain concentrations for translation to clinical efficacy.