Dihydroquinoline Carbamate Derivatives as "Bio-oxidizable" Prodrugs for Brain Delivery of Acetylcholinesterase Inhibitors: [¹¹C] Radiosynthesis and Biological Evaluation
Résumé
With the aim of improving the efficiency of marketed acetylcholinesterase (AChE) inhibitors in the symptomatic treatment of Alzheimer’s disease, plagued by adverse effects arising from peripheral cholinergic activation, this work reports a biological evaluation of new central AChE inhibitors based on an original “bio-oxidizable” prodrug strategy. After peripheral injection of the prodrug (1a) [IC50 > 1 mM (hAChE)] in mice, monitoring markers of central and peripheral cholinergic activation provided in vivo proof-of-concept for brain delivery of the drug (2a) [IC50 = 20 nM (hAChE)] through central redox activation of (1a). Interestingly, peripheral cholinergic activation has been shown to be limited in time, likely due to the presence of a permanent positive charge in (2a) promoting rapid elimination of the AChE inhibitor from the circulation of mice. To support these assumptions, the radiosynthesis with carbon-11 of prodrug 1a was developed for additional ex vivo studies in rats. Whole-body biodistribution of radioactivity revealed high accumulation in excretory organs along with moderate but rapid brain uptake. Radio-HPLC analyses of brain samples confirm rapid CNS penetration of [11C] (1a), while identification of [11C] (2a) and [11C] (3a) both accounts for central redox activation of (1a) and pseudoirreversible inhibition of AChE, respectively. Finally, Caco-2 permeability assays predicted metabolite (3a) as a substrate for efflux transporters (P-gp inter alia), suggesting that metabolite (3a) might possibly be actively transported out of the brain. Overall, a large body of evidence from in vivo and ex vivo studies on small animals has been collected to validate this “bio-oxidizable” prodrug approach, emerging as a very promising strategy in the rational design of selective central AChE inhibitors.