Design, Synthesis, and Biological Evaluation of Conformationally Restricted Rivastigmine Analogues

Abstract

Rivastigmine (1), an acetylcholinesterase (AChE) inhibitor approved in 2000 for the treatment of Alzheimer disease, bears a carbamate moiety in its structure, which is able to react covalently with the active site of the enzyme. Kinetic and structural studies on the interaction of 1 with different cholinesterases have been published, giving deeper, but not definitive, insights on the catalysis mechanism. On the basis of these findings and in connection with our previous studies on a series of benzopyrano[4,3-b]pyrrole carbamates as AChE inhibitors, we designed a series of conformationally restricted analogues of 1 by including the dimethylamino-alpha-methylbenzyl moiety in different tricyclic systems. A superimposition between the conformation of 1 and the carbon derivative 4, as obtained from Monte Carlo simulations, supported the idea that the tricyclic derivatives might act as rigid analogues of 1. The biological profile of 4-9, assessed in vitro against human AChE and BChE, validated our rational design. Compound 5, bearing a sulfur-containing system, showed the highest inhibitory activity, being 192-fold more potent than 1. In the present study, the most potent inhibitors were always methyl derivatives 3-5, endowed with a nanomolar range potency, whereas the ethyl ones were 40 times less potent. A reasonable explanation for this finding might be a steric hindrance effect between the ethyl group of 1 and His440 in the active site, as already suggested by the crystal structure of the complex AChE/1. The unfavorable influence of the carbamic N-alkyl chain on AChE inhibition is less striking when considering BChE inhibition, since BChE is characterized by a bigger acyl binding pocket than AChE. In fact, methyl carbamates 3-5 did not show AChE/BChE selectivity, whereas compounds 6-9 were significantly more potent in inhibiting BChE than AChE activity. At 100 microM, 5 was found to inhibit the AChE-induced aggregation only by 19% likely because it is not able to strongly interact with the peripheral anionic site of AChE, which plays an essential role in the Abeta aggregation mediated by the enzyme but is lacking in BChE structure.

The Kinetics of Inhibition of Human Acetylcholinesterase and Butyrylcholinesterase by Two Series of Novel Carbamates

“…Most published analyses of structure-activity relationships of carbamates designed as potential drugs for the treatment of AD are based on measurements of IC 50 (Yu et al, 2001;Sterling et al, 2002;Bolognesi et al, 2004;Luo et al, 2005Luo et al, , 2006Bartolucci et al, 2006). However, this parameter lacks information on the individual rate constants that govern the approach to, and the steady-state level of, enzyme activity and the rate of release of a leaving group designed to exert independent biological activity.…”

Bifunctional phenolic-choline conjugates as anti-oxidants and acetylcholinesterase inhibitors

“…The AChE inhibitory activities of the new phenoliccholine esters, and their parent compounds (Figures 2 and 3), were evaluated and the results are summarized 37 or a choline ester derivative of α,β-dehydrophenylalanine (37 μM). 22 Within this series of caffeic analogues (1-3), compound 3 displayed the lowest activity, with IC 50 value of 91 μM.…”

Neuroprotective Tri- and Tetracyclic BChE Inhibitors Releasing Reversible Inhibitors upon Carbamate Transfer

“…The K C value calculated for physostigmine as positive control is in agreement with the value reported in the literature. 29 For compound 8a at AChE, the actual K C value is even smaller than determined as the IC 50 values with K C = 1.1 μM and at BChE with K C = 274 nM, resulting in 4-fold selectivity. The rate constants of 0.31 min −1 (AChE) and 0.18 min −1 (BChE) show rapid carbamoylation, albeit slower than for physostigmine (Figure 1).…”

“…A heptyl group was identified that led to high acitivity, in the case of conformationally restricted rivastigmine analogues with 16-fold selectivity [heptylphysostigmine (eptastigmine) shows 4-fold selectivity]. 29 The synthesis of phenylcarbamates has been described, again connected to a physostigmine-related heterocyclic core. 29 In this case, modulation of selectivity could be achieved by fairly minor structural modifications, for example, a methyl group in the o-position led to AChE selectivity, whereas an isobutyl substituent in the p-position led to BChE selectivity.…”

“…29 The synthesis of phenylcarbamates has been described, again connected to a physostigmine-related heterocyclic core. 29 In this case, modulation of selectivity could be achieved by fairly minor structural modifications, for example, a methyl group in the o-position led to AChE selectivity, whereas an isobutyl substituent in the p-position led to BChE selectivity. 28 We therefore also applied these carbamate structures to our heterocyclic templates to investigate SARs.…”