Metrifonate for Alzheimer's disease: is the next cholinesterase inhibitor better?

STRUCTURAL FEATURE OF AChE INHIBITOR HUPERZINE B IN NATURE AND IN THE BINDING SITE OF AChE: DENSITY FUNCTIONAL THEORY STUDY COMBINED WITH IR DETERMINATION

Quantum chemical DFT-B3LYP/6-31G* method and IR spectrometry have been used to investigate the natural and binding structures of Huperzine B (HupB) in order to better understand the interaction nature between acetylcholinesterase (AChE) and its inhibitor, with the view of designing new AChE inhibitors. The predicted and experimental results reveal that both the natural state and binding form of HupB adopt the chair conformation. Furthermore, the B3LYP/6-31G* results suggest that structure S1 should be the dominant form of the two possible chair structures (S1 and S2, Fig. 2). The calculated results also show that the condensed ring structure composing of rings A, B and C is very rigid. Therefore, its flexibility does not need to be considered when we try to dock this structure to its target. Indeed, this supposition is confirmed by the excellent alignment of the binding structure produced from our recent/break X-ray crystallographic structure of the HupB-AChE complex with the B3LYP/6-31G* predicted geometry. Among all the 111 predicted vibrational bands, the mode 110, which is resulted from the stretching of the bond N2–H and having the second highest frequency, is essential for the geometrical identification. The difference between our predicted strongest absorption band and experimental IR spectrum suggests that a strong intermolecular interaction, which could be a hydrogen bond, exists in HupB crystal. The electrostatic potential surface of HupB derived from our B3LYP/6-31G* CHelpG atomic charge suggests a mechanism of how HupB would interact with its target. In addition, the good agreement between predicted vibrational bands (scaled by a factor of 0.96) and experimental result shows that B3LYP/6-31G* is a good tool for studying such kind of natural compound.

Noncholinesterase effects induced by organophosphate pesticides and their relationship to cognitive processes: implication for the action of acylpeptide hydrolase

Organophosphate pesticides have been classically described as inhibitors of acetylcholinesterase (AChE) activity in insects and invertebrates. However, there is now more evidence supporting the hypothesis that these compounds also act through noncholinergic pathways, especially those related to cognitive processes. The enzyme acylpeptide hydrolase was identified as a new target for organophosphate pesticides. This enzyme is more sensitive than AChE to some organophosphates (OP), including dichlorvos, which is the parent compound for metrifonate, a therapeutic agent used in the treatment of cognitive impairment associated to Alzheimer's disease. Therefore, there is some doubt as to whether the mechanism of action of this drug is mediated by a potentiation of cholinergic transmission. However, the direct action of acylpeptide hydrolase in cognitive processes and the physiological and molecular mechanisms underlying subacute exposure to OP have yet to be demonstrated. This review deals with evidence demonstrating the existence of mechanisms of actions of OP, which are independent of cholinergic pathway potentiation and which have an effect on cognitive processes. In addition, the possible participation of the enzyme acylpeptide hydrolase in these processes is also discussed. Finally, the possibility of using this enzyme activity as a new biomarker for exposure to OP is considered.

Identification of acylpeptide hydrolase as a sensitive site for reaction with organophosphorus compounds and a potential target for cognitive enhancing drugs

We describe here the purification and identification of a previously unrecognized target for organophosphorus compounds. The target, acylpeptide hydrolase, was isolated as a tritiated-diisopropylfluorophosphate-reactive protein from porcine brain and purified to homogeneity using a combination of ion-exchange and gel-filtration chromatography. Biochemical characterization and internal sequence analysis confirmed identity. Acylpeptide hydrolase was found to be potently inhibited by the organophosphorus compounds chlorpyrifosmethyl oxon, dichlorvos, and diisopropylfluorophosphate (20-min IC(50) values of 18.3 +/- 2.0, 118.7 +/- 9.7, and 22.5 +/- 1.2 nM, respectively). The in vitro sensitivity of acylpeptide hydrolase toward these compounds is between six and ten times greater than that of acetylcholinesterase (AChE), making it a target of pharmacological and toxicological significance. We show that, in vivo, acylpeptide hydrolase is significantly more sensitive than AChE to inhibition by dichlorvos and that the inhibition is more prolonged after a single dose of inhibitor. Furthermore, using dichlorvos as a progressive inhibitor, it was possible to show that acylpeptide hydrolase is the only enzyme in the brain capable of hydrolyzing the substrate N-acetyl-alanyl-p-nitroanilide. A concentration of 154 +/- 27 pmol of acylpeptide hydrolase/gram of fresh rat brain was also deduced by specific labeling with tritiated-diisopropylfluorophosphate. We also suggest that, by comparison of structure-activity relationships, acylpeptide hydrolase may be the target for the cognitive-enhancing effects of certain organophosphorus compounds. Acylpeptide hydrolase cleaves N(alpha)-acylated amino acids from small peptides and may be involved in regulation of neuropeptide turnover, which provides a new and plausible mechanism for its proposed cognitive enhancement effect.

Structure of acetylcholinesterase complexed with (-)-galanthamine at 2.3 A resolution

(-)-Galanthamine (GAL), an alkaloid from the flower, the common snowdrop (Galanthus nivalis), shows anticholinesterase activity. This property has made GAL the target of research as to its effectiveness in the treatment of Alzheimer's disease. We have solved the X-ray crystal structure of GAL bound in the active site of Torpedo californica acetylcholinesterase (TcAChE) to 2.3 A resolution. The inhibitor binds at the base of the active site gorge of TcAChE, interacting with both the choline-binding site (Trp-84) and the acyl-binding pocket (Phe-288, Phe-290). The tertiary amine group of GAL does not interact closely with Trp-84; rather, the double bond of its cyclohexene ring stacks against the indole ring. The tertiary amine appears to make a non-conventional hydrogen bond, via its N-methyl group, to Asp-72, near the top of the gorge. The hydroxyl group of the inhibitor makes a strong hydrogen bond (2.7 A) with Glu-199. The relatively tight binding of GAL to TcAChE appears to arise from a number of moderate to weak interactions with the protein, coupled to a low entropy cost for binding due to the rigid nature of the inhibitor.

Pharmacokinetics and pharmacodynamics of the acetylcholinesterase inhibitor metrifonate in patients with renal impairment

The aim of this study was to assess the influence of renal function on the pharmacokinetics, pharmacodynamics, safety, and tolerability of the acetylcholinesterase inhibitor metrifonate. Four groups of six age- and gender-matched subjects with varying degrees of renal function (creatinine clearances more than 90, 60-90, 30-60, and less than 30 mL/min/ 1.73 m2, respectively) were administered a single 50-mg oral dose of metrifonate. Blood and urine samples were collected for 24 hours and concentrations of metrifonate and its metabolites dichlorvos, dichloroacetic acid, and M3 were determined. Inhibition of acetylcholinesterase activity in erythrocytes and butyrylcholinesterase in plasma were also measured. Metrifonate was well tolerated in all treatment groups. The urinary excretion of metrifonate and dichlorvos decreased with decreasing renal function but accounted for less than 2% of the elimination. There were no statistically significant differences in primary pharmacokinetic parameters--Cmax, t(max), area under the concentration-time curve (AUC), and t1/2--of metrifonate and dichlorvos among the different groups. The excretion of dichloroacetic acid and M3 was not influenced by renal impairment. Acetylcholinesterase was not inhibited, whereas butyrylcholinesterase was inhibited markedly but independently of renal function. No metrifonate dose adjustments are needed when treating subjects with renal impairment.

Cholinesterase inhibitors: a therapeutic strategy for Alzheimer disease

OBJECTIVE

To provide a review of acetylcholinesterase inhibitors (AChEIs) tested as therapeutic agents for Alzheimer disease (AD).

DATA SOURCES

MEDLINE searches (January 1986-July 1998) identified pertinent literature. Selected references from these articles, as well as abstracts from recent meetings and package insert literature from approved compounds, were also used as source material.

DATA EXTRACTION

AChEIs were reviewed with regard to chemical structure, mechanism of inhibition, substrate specificity, pharmacokinetics/pharmacodynamics, safety/tolerability, and efficacy.

DATA SYNTHESIS

Cholinergic deficits, leading to cognitive impairment, are a significant aspect of neurodegeneration in AD. AChEIs reduce the degradation of acetylcholine, thus enhancing cholinergic transmission. In addition to the two agents approved by the Food and Drug Administration, tacrine and donepezil, six other compounds of diverse chemical structure and mechanism of inhibition including physostigmine, metrifonate, rivastigmine, and galantamine are under investigation as potential therapy for AD. These compounds are structurally diverse, possess unique patterns of specificities for the various forms of cholinesterase enzymes, use distinct mechanisms of enzyme inhibition, present unique adverse event profiles, and offer relatively similar mean gains in cognitive abilities to patients with AD in controlled clinical trials.

CONCLUSIONS

Relative to placebo, new AChEIs in development provide modest improvements in cognition for patients with mild to moderate AD, with improved tolerability profiles and more convenient dosing relative to tacrine. The availability of a wide array of AChEIs soon to be accessible to patients with AD will provide additional options to those who cannot tolerate or do not respond to drugs currently used for AD.

Structure of acetylcholinesterase complexed with E2020 (Aricept): implications for the design of new anti-Alzheimer drugs

BACKGROUND

Several cholinesterase inhibitors are either being utilized for symptomatic treatment of Alzheimer's disease or are in advanced clinical trials. E2020, marketed as Aricept, is a member of a large family of N-benzylpiperidine-based acetylcholinesterase (AChE) inhibitors developed, synthesized and evaluated by the Eisai Company in Japan. These inhibitors were designed on the basis of QSAR studies, prior to elucidation of the three-dimensional structure of Torpedo californica AChE (TcAChE). It significantly enhances performance in animal models of cholinergic hypofunction and has a high affinity for AChE, binding to both electric eel and mouse AChE in the nanomolar range.

RESULTS

Our experimental structure of the E2020-TcAChE complex pinpoints specific interactions responsible for the high affinity and selectivity demonstrated previously. It shows that E2020 has a unique orientation along the active-site gorge, extending from the anionic subsite of the active site, at the bottom, to the peripheral anionic site, at the top, via aromatic stacking interactions with conserved aromatic acid residues. E2020 does not, however, interact directly with either the catalytic triad or the 'oxyanion hole', but only indirectly via solvent molecules.

CONCLUSIONS

Our study shows, a posteriori, that the design of E2020 took advantage of several important features of the active-site gorge of AChE to produce a drug with both high affinity for AChE and a high degree of selectivity for AChE versus butyrylcholinesterase (BChE). It also delineates voids within the gorge that are not occupied by E2020 and could provide sites for potential modification of E2020 to produce drugs with improved pharmacological profiles.

The cholinergic hypothesis of Alzheimer's disease: a review of progress

Alzheimer's disease is one of the most common causes of mental deterioration in elderly people, accounting for around 50%-60% of the overall cases of dementia among persons over 65 years of age. The past two decades have witnessed a considerable research effort directed towards discovering the cause of Alzheimer's disease with the ultimate hope of developing safe and effective pharmacological treatments. This article examines the existing scientific applicability of the original cholinergic hypothesis of Alzheimer's disease by describing the biochemical and histopathological changes of neurotransmitter markers that occur in the brains of patients with Alzheimer's disease both at postmortem and neurosurgical cerebral biopsy and the behavioural consequences of cholinomimetic drugs and cholinergic lesions. Such studies have resulted in the discovery of an association between a decline in learning and memory, and a deficit in excitatory amino acid (EAA) neurotransmission, together with important roles for the cholinergic system in attentional processing and as a modulator of EAA neurotransmission. Accordingly, although there is presently no "cure" for Alzheimer's disease, a large number of potential therapeutic interventions have emerged that are designed to correct loss of presynaptic cholinergic function. A few of these compounds have confirmed efficacy in delaying the deterioration of symptoms of Alzheimer's disease, a valuable treatment target considering the progressive nature of the disease. Indeed, three compounds have received European approval for the treatment of the cognitive symptoms of Alzheimer's disease, first tacrine and more recently, donepezil and rivastigmine, all of which are cholinesterase inhibitors.

Combination therapy for early Alzheimer's disease: what are we waiting for?

The practical pharmacological approaches currently available to palliate the cognitive and functional losses in early Alzheimer's disease (AD) include cholinesterase inhibitors (ChEI), antioxidants (e.g., vitamin E), anti-inflammatory agents, estrogen, seligiline, vasoactive agents, and ginkgo biloba. Reviewing available data on these therapies and using models from medical illnesses such as cancer and hypertension, we highlight the urgent need for evaluating combination therapies in early AD.