Preferential inhibition of acetylcholinesterase molecular forms in rat brain

Recent Advances in the Treatment and Management of Alzheimer's Disease: A Precision Medicine Perspective

About 60% to 70% of people with dementia have Alzheimer's Disease (AD), a neurodegenerative illness. One reason for this disorder is the misfolding of naturally occurring proteins in the human brain, specifically β-amyloid (Aβ) and tau. Certain diagnostic imaging techniques, such as amyloid PET imaging, tau PET imaging, Magnetic Resonance Imaging (MRI), Computerized Tomography (CT), and others, can detect biomarkers in blood, plasma, and cerebral spinal fluids, like an increased level of β-amyloid, plaques, and tangles. In order to create new pharmacotherapeutics for Alzheimer's disease, researchers must have a thorough and detailed knowledge of amyloid beta misfolding and other related aspects. Donepezil, rivastigmine, galantamine, and other acetylcholinesterase inhibitors are among the medications now used to treat Alzheimer's disease. Another medication that can temporarily alleviate dementia symptoms is memantine, which blocks the N-methyl-D-aspartate (NMDA) receptor. However, it is not able to halt or reverse the progression of the disease. Medication now on the market can only halt its advancement, not reverse it. Interventions to alleviate behavioral and psychological symptoms, exhibit anti- neuroinflammation and anti-tau effects, induce neurotransmitter alteration and cognitive enhancement, and provide other targets have recently been developed. For some Alzheimer's patients, the FDA-approved monoclonal antibody, aducanumab, is an option; for others, phase 3 clinical studies are underway for drugs, like lecanemab and donanemab, which have demonstrated potential in eliminating amyloid protein. However, additional study is required to identify and address these limitations in order to reduce the likelihood of side effects and maximize the therapeutic efficacy.

AChE as a spark in the Alzheimer's blaze - Antagonizing effect of a cyclized variant

The amyloid precursor protein (APP), presenilin 1 (PS1), amyloid beta (Aβ), and GSK3 are the effectors, which are significantly associated with progression of Alzheimer's Disease (AD) and its symptoms. A significant protein, acetylcholinesterase (AChE) becomes dysfunctional as a result of cholinergic neuronal loss in AD pathology. However, certain associated peptides potentiate the release of primary neuropathological hallmarks, i.e., senile plaque and neurofibrillary tangles (NFTs), by modulating the alpha 7 acetylcholinesterase receptor (α7nAChR). The AChE variants, T30 and T14 have also been found to be elevated in AD patients and mimic the toxic actions of pathological events in patients. The manuscript discusses the significance of AChE inhibitors in AD therapeutics, by indicating the disastrous role of molecular alterations and elevation of AChE, accompanied with the downstream effects instigated by the peptide, supported by clinical evidence and investigations. The cyclized variant of AChE peptide, NBP14 has been identified as a novel candidate that reverses the harmful effects of T30, T14 and Aβ, mainly calcium influx, cell viability and AChE release. The review aims to grab the attention of neuro-researchers towards the significance of triggering effectors in propagating AD and role of AChE in regulating them, which can potentially ace the development of reliable therapeutic candidates, similar to NBP14, to mitigate neurodegeneration.

Differential Inhibition of the Brain Acetylcholinesterase Molecular Forms Following Sarin, Soman and VX Intoxication in Laboratory Rats

The female Wistar rats were intoxicated (i.m.) with sarin, soman and VX in doses equal to 1xLD50 and pontomedullar areas of the brain were prepared, homogenized, centrifuged and in these samples, acetylcholinesterase (AChE, EC 3.1.1.7) activities were determined. In the same samples, AChE was separated using polyacrylamide gel electrophoresis and AChE molecular forms were detected and densitometrically evaluated. In control animals, AChE was separated into four forms differing in their electrophoretic mobility and their quantitative content in the sample. The form with lowest electrophoretic mobility represent the main part of AChE activity constituting the whole enzymatic activity. Following intoxication with the nerve agents mentioned, the whole AChE activity in the pontomedullar area of the brain was decreasing in intervals of ten minutes (soman and sarin) or one hour (VX). The AChE activity at the time of death (or terminal stage) was represented 5-30 % of controls. Molecular forms of AChE were inhibited in different extent: the form with lowest electrophoretic mobility was diminished to zero level while the form with the highest mobility was practically unaffected, independently on the type of nerve agent. From quantitative expression of percentage content of the forms vs their activity we can imply that values of the total AChE activity represent the ãmeanÒ activity of the forms determined.

A review of butyrylcholinesterase as a therapeutic target in the treatment of Alzheimer's disease

OBJECTIVE

To examine the role of butyrylcholinesterase (BuChE) in cholinergic signaling and neurologic conditions, such as Alzheimer's disease (AD). The rationale for inhibiting cholinesterases in the management of AD, including clinical evidence supporting use of the dual acetylcholinesterase (AChE) and BuChE inhibitor rivastigmine, is discussed.

DATA SOURCES

PubMed searches were performed using butyrylcholinesterase as a keyword. English-language articles referenced in PubMed as of September 2011 were included. Study Selection and Data Synthesis: English-language articles related to BuChE considered to be of clinical relevance to physicians were included. English-language articles specifically related to AChE were not included, as the role of AChE in cholinergic signaling and the underlying pathology of AD is well documented. Reference lists of included publications were used to supplement the search.

RESULTS

AChE and BuChE play a role in cholinergic signaling; BuChE can hydrolyze acetylcholine and compensate for AChE when levels are depleted. In the AD brain, AChE levels decrease, while BuChE levels are reportedly increased or unchanged, with changes becoming more pronounced during the disease course. Furthermore, BuChE genotype may influence AD risk and rate of disease progression. Strategies that increase acetylcholine levels (eg, cholinesterase inhibitors) demonstrate symptomatic efficacy in AD. Rivastigmine has proven cognitive efficacy in clinical trials, and data suggest that its action is mediated, in part, by inhibition of BuChE. Retrospective analyses of clinical trials provide evidence that BuChE genotype may also influence treatment response.

CONCLUSIONS

AChE-selective inhibitors and a dual AChE and BuChE inhibitor demonstrate symptomatic efficacy in AD. Mounting preclinical and clinical evidence for a role of BuChE in maintaining normal cholinergic function and the pathology of AD provides a rationale for further studies investigating use of rivastigmine in AD and the influence of BuChE genotype on observed efficacy.

Revisiting the Role of Acetylcholinesterase in Alzheimer's Disease: Cross-Talk with P-tau and β-Amyloid

A common feature in the Alzheimer’s disease (AD) brain is the presence of acetylcholinesterase (AChE) which is commonly associated with β-amyloid plaques and neurofibrillary tangles (NFT). Although our understanding of the relationship between AChE and the pathological features of AD is incomplete, increasing evidence suggests that both β-amyloid protein (Aβ) and abnormally hyperphosphorylated tau (P-tau) can influence AChE expression. We also review recent findings which suggest the possible role of AChE in the development of a vicious cycle of Aβ and P-tau dysregulation and discuss the limited and temporary effect of therapeutic intervention with AChE inhibitors.

Brain cholinergic impairment in liver failure

The cholinergic system is involved in specific behavioural responses and cognitive processes. Here, we examined potential alterations in the brain levels of key cholinergic enzymes in cirrhotic patients and animal models with liver failure. An increase (∼30%) in the activity of the acetylcholine-hydrolyzing enzyme, acetylcholinesterase (AChE) is observed in the brain cortex from patients deceased from hepatic coma, while the activity of the acetylcholine-synthesizing enzyme, choline acetyltransferase, remains unaffected. In agreement with the human data, AChE activity in brain cortical extracts of bile duct ligated (BDL) rats was increased (∼20%) compared to controls. A hyperammonemic diet did not result in any further increase of AChE levels in the BDL model, and no change was observed in hyperammonemic diet rats without liver disease. Portacaval shunted rats which display increased levels of cerebral ammonia did not show any brain cholinergic abnormalities, confirming that high ammonia levels do not play a role in brain AChE changes. A selective increase of tetrameric AChE, the major AChE species involved in hydrolysis of acetylcholine in the brain, was detected in both cirrhotic humans and BDL rats. Histological examination of BDL and non-ligated rat brains shows that the subcellular localization of both AChE and choline acetyltransferase, and thus the accessibility to their substrates, appears unaltered by the pathological condition. The BDL-induced increase in AChE activity was not parallelled by an increase in mRNA levels. Increased AChE in BDL cirrhotic rats leads to a pronounced decrease (∼50–60%) in the levels of acetylcholine. Finally, we demonstrate that the AChE inhibitor rivastigmine is able to improve memory deficits in BDL rats. One week treatment with rivastigmine (0.6 mg/kg; once a day, orally, for a week) resulted in a 25% of inhibition in the enzymatic activity of AChE with no change in protein composition, as assessed by sucrose density gradient fractionation and western blotting analysis. In conclusion, this study is the first direct evidence of a cholinergic imbalance in the brain as a consequence of liver failure and points to the possible role of the cholinergic system in the pathogenesis of hepatic encephalopathy.

N1phenethyl-norcymserine, a selective butyrylcholinesterase inhibitor, increases acetylcholine release in rat cerebral cortex: a comparison with donepezil and rivastigmine

The effects of (-)-N(1)phenethyl-norcymserine (PEC, 5 mk/kg, i.p.) on acetylcholine release and cholinesterase activity in the rat cerebral cortex were compared with those of donepezil (1 mg/kg, i.p.), a selective acetylcholinesterase inhibitor, and rivastigmine (0.6 mg/kg, i.p.), an inhibitor of acetylcholinesterase and butyrylcholinesterase. Acetylcholine extracellular levels were measured by microdialysis coupled with HPLC; acetylcholinesterase and butyrylcholinesterase activity were measured with colorimetric and radiometric methods. It was found that comparable 2-3 fold increases in cortical extracellular acetylcholine level, calculated as areas under the curve, followed the administration of the three drugs at the doses used. At the peak of acetylcholine increase, a 27% acetylcholinesterase inhibition and no butyrylcholinesterase inhibition was found after donepezil (1 mg/kg, i.p) administration. At the same time point, rivastigmine (0.6 mg/kg, i.p.) inhibited acetylcholinesterase by 40% and butyrylcholinesterase by 25%. After PEC (5 mg/kg, i.p.) administration, there was a 39% butyrylcholinesterase inhibition and no effect on acetylcholinesterase. Since in the present study it was also confirmed that in the brain butyrylcholinesterase activity is only about 10% of acetylcholinesterase activity, it is surprising that its partial inhibition is sufficient to increase extracellular acetylcholine levels. The importance of butyrylcholinesterase as a "co-regulator" of synaptic acetylcholine levels should thus be reconsidered.

Metrifonate for Alzheimer's disease

BACKGROUND

Metrifonate is a long-acting irreversible cholinesterase inhibitor, originally used to treat schistosomiasis. Its potential to enhance central nervous system cholinergic neurotransmission led to clinical trials for the treatment of people with Alzheimer's disease (AD). Although low incidence of serious side effects occurred during short-term use as an antihelmintic, in studies of the treatment of AD extending over 6 months, 20 patients experienced respiratory paralysis and problems with neuromuscular transmission. These findings have led to a halt to trials of metrifonate for AD and Bayer, the pharmaceutical company, has withdrawn its FDA application.

OBJECTIVES

1) To establish the efficacy of metrifonate for patients with Alzheimer's disease, in terms of cognition, global impression, functional activity, non cognitive symptoms, rate of institutionalization and mortality.2) Assess the safety and tolerability of metrifonate.

SEARCH STRATEGY

The Cochrane Dementia and Cognitive Improvement Group's Specialized Register was searched on 5 December 2005 using the term metrifonat*. This Register is regularly updated with records from all major health care databases (MEDLINE, EMBASE, CINAHL, PsycINFO) and many trials databases. One of the authors (LS), as member of the Metrifonate Study Group has had the opportunity to contact other metrifonate trialists to obtain data from potentially non published data of metrifonate clinical trials.

SELECTION CRITERIA

All unconfounded, randomized double-blind clinical controlled trials comparing metrifonate to placebo in people with AD.

DATA COLLECTION AND ANALYSIS

Data were extracted by the two reviewers, cross-checked, and pooled when appropriate and possible.

MAIN RESULTS

Most studies assessed changes in cognitive function, global function, activities of daily living, behavioural problems, severity of disease and adverse events. Occasionally the results were not reported in sufficient detail to allow extraction of data for the meta-analyses. The treatment regimens were varied: loading doses were used in some trials. The range of maintenance doses and studies were not pooled unless the treatment regimens were considered comparable. The lengths of treatment varied from 6 to 26 weeks and studies were not pooled unless the treatment duration was similar. The results are derived from the ITT populations. Metrifonate at various doses, fixed and loading doses, was associated with significant cognitive improvement compared to placebo, except for weekly doses where there was no difference from placebo: MMSE (metrifonate 60-80 mg/day with initial loading at 26 weeks; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD 1.85, 95% CI 1.06 to 2.64, p<0.00001); ADAS-Cog (metrifonate 60-80 mg/day with initial loading at 26 weeks MD -3.24, 95% CI -4.40 to -2.08, p<0.00001)In most trials, there was improvement in clinical global impression: CIBIC-Plus (metrifonate 30-55 mg/day, approximately 0.65 mg/kg body weight, with initial loading at 26 weeks MD -0.25, 95% CI -0.41 to -0.09 p=0.002; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD -0.20, 95% CI -0.39 to -0.01, p=0.04). There were generally-significant drug-placebo differences in activities of daily living but this often depended on sample size and the characteristics of the instrument used: DAD (metrifonate 30-55 mg/day, 0.65 mg/kg body weight, with initial loading at 26 weeks MD 2.72, 95% CI 0.66 to 4.77, p=0.01; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD 4.07, 95% CI 0.29 to 7.85, p=0.03)Also there were differences associated with metrifonate compared with placebo for different doses of metrifonate in scores on a behavioural symptom scale, caregiver burden scale, and severity of disease scale. Adverse events occurring more often with metrifonate included abdominal pain, bloating, bradycardia, diarrhoea, leg cramps, nausea and rhinitis and were described as mostly mild and transient, but occasionally moderately severe, and infrequently severe and serious. Analysis of the number of patients suffering at least one mild, moderate, severe or serious adverse event before the end of treatment showed that there was usually no difference between placebo and metrifonate.

AUTHORS' CONCLUSIONS

Metrifonate given once per day appears to be related to clinical response in cognition, global improvement, and activities of daily living in patients with mild to moderate Alzheimer's disease. Tolerability is good with adverse events as expected from a cholinesterase inhibitor, but with a low incidence of neuromuscular dysfunction and respiratory failure, too low to be detected in this review. It has been withdrawn from further development.

Effects per se of Organic Solvents in the Cerebral Acetylcholinesterase of Rats

Acetylcholinesterase (AChE) was studied in different rat brain regions (cerebellum, hypothalamus, striatum, hippocampus and cortex) in the presence of different organic solvents normally used in the in vitro assay. The organic solvents used were acetone (C3H6O), acetonitrile (C2H3N), ethyl alcohol (C2H6O), isopropyl alcohol (C3H8O), methyl alcohol (CH4O), tert-butyl alcohol (C4H10O) and dimethyl sulfoxide (DMSO, C2H6OS) ranging from 0.6 to 10%. Ethyl and methyl alcohol presented no effect on AChE activity at any of the concentrations and brain structures tested. In the hippocampus, isopropyl alcohol did not demonstrate a significant inhibitory effect, even at high concentrations. Tert-butyl alcohol presented an interesting result, increased AChE activity (P < .05) in the hypothalamus (1.8%), cortex (1.8 and 2.5) and striatum (1.2, 1.8 and 2.5%) and decreased activity at a concentration of 10% in the cortex (P < .05) and striatum (P < .01). Acetone and acetonitrile presented similar results, both significantly inhibiting AChE in all structures (5%, P < .05 and 10%, P < .01). DMSO exhibited a highly inhibitory effect at practically all concentrations tested (P < .01). In conclusion, for testing new compounds on AChE activity in vitro, methyl and ethyl alcohol may be the best organic solvent choice.

Metrifonate (Trichlorfon): a review of the pharmacology, pharmacokinetics and clinical experience with a new acetylcholinesterase inhibitor for Alzheimer’s disease

Metrifonate is a cholinesterase inhibitor, effective in the treatment of both the cognitive and behavioural symptoms of Alzheimer's disease (AD). Previously used as an antihelminthic and insecticide, clinical experience with metrifonate in AD patients is large and growing. The parent compound is relatively inactive; it is metabolised non-enzymatically to 2,2-dimethyl dichlorovinyl phosphate (DDVP), which irreversibly inhibits the acetylcholinesterase enzyme. The elimination half-life of DDVP is 2.1 h; cholinesterase inhibition by DDVP is stable and may persist for up to 55 days. Metrifonate can be administered once daily. In vitro and animal data regarding possible carcinogenesis of metrifonate and DDVP are conflicting; experience in the treatment of humans with schistosomiasis or AD support its safety. Animal studies demonstrate its efficacy in enhancing memory in animals with cholinergic deficits. Double-blind, placebo-controlled studies have shown the benefit of metrifonate compared to placebo in improving scores on the Clinical Global Impression of Change, Alzheimer's Disease Assessment Scale-Cognitive Subscale and the Neuropsychiatric Inventory.

Organophosphates/nerve agent poisoning: mechanism of action, diagnosis, prophylaxis, and treatment

OP/nerve agents are still considered as important chemicals acting on living organisms and are widely used. They are characterized according to their action as compounds influencing cholinergic nerve transmission via inhibition of AChE. Modeling of this action and extrapolation of experimental data from animals to humans is more possible for highly toxic agents than for the OP. The symptoms of intoxication comprise nicotinic, muscarinic, and central symptoms; for some OP/nerve agents, a delayed neurotoxicity is observed. Cholinesterases (AChE and BuChE) are characterized as the main enzymes involved in the toxic effect of these compounds, including molecular forms. The activity of both enzymes (and molecular forms) is influenced by inhibitors (reversible, irreversible, and allosteric) and other factors, such as pathological states. There are different methods for cholinesterase determination; however, the most frequent is the method based on the hydrolysis of thiocholine esters and subsequent detection of free SH-group of the released thiocholine. The diagnosis of OP/nerve agent poisoning is based on anamnesis, the clinical status of the intoxicated organism, and on cholinesterase determination in the blood. For nerve agent intoxication, AChE in the red blood cell is more diagnostically important than BuChE activity in the plasma. This enzyme is a good diagnostic marker for intoxication with OP pesticides. Some other biochemical examinations are recommended, especially arterial blood gas, blood pH, minerals, and some other specialized parameters usually not available in all clinical laboratories. These special examinations are important for prognosis of the intoxication, for effective treatment, and for retrospective analysis of the agent used for exposure. Some principles of prophylaxis against OP/nerve agent poisoning comprising the administration of reversible cholinesterase inhibitors such as pyridostigmine (alone or in combination with other drugs), scavengers such as preparations of cholinesterases, some therapeutic drugs, and possible combinations are given. Basic principles of the treatment of nerve agent OP poisoning are described. They are based on the administration of anticholinergics (mostly atropine but some other anticholinergics can be recommended) as a symptomatic treatment, cholinesterase reactivators as a causal treatment (different types but without a universal reactivator against all OP/nerve agents) as the first aid and medical treatment, and anticonvulsants, preferably diazepam though some other effective benzodiazepines are available. New drugs for the treatment are under experimental study based on new approaches to the mechanism of action. Future trends in the complex research of these compounds, which is important not only for the treatment of intoxication but also for the quantitative and qualitative increase of our knowledge of toxicology, neurochemistry, neuropharmacology, clinical biochemistry, and analytical chemistry in general, are characterized.

Potencies and selectivities of inhibitors of acetylcholinesterase and its molecular forms in normal and Alzheimer's disease brain

Eight inhibitors of acetylcholinesterase (AChE), tacrine, bis-tacrine, donepezil, rivastigmine, galantamine, heptyl-physostigmine, TAK-147 and metrifonate, were compared with regard to their effects on AChE and butyrylcholinesterase (BuChE) in normal human brain cortex. Additionally, the IC50 values of different molecular forms of AChE (monomeric, G1, and tetrameric, G4) were determined in the cerebral cortex in both normal and Alzheimer's human brains. The most selective AChE inhibitors, in decreasing sequence, were in order: TAK-147, donepezil and galantamine. For BuChE, the most specific was rivastigmine. However, none of these inhibitors was absolutely specific for AChE or BuChE. Among these inhibitors, tacrine, bis-tacrine, TAK-147, metrifonate and galantamine inhibited both the G1 and G4 AChE forms equally well. Interestingly, the AChE molecular forms in Alzheimer samples were more sensitive to some of the inhibitors as compared with the normal samples. Only one inhibitor, rivastigmine, displayed preferential inhibition for the G1 form of AChE. We conclude that a molecular form-specific inhibitor may have therapeutic applications in inhibiting the G1 form, which is relatively unchanged in Alzheimer's brain.

Selective inhibitors of butyrylcholinesterase: a valid alternative for therapy of Alzheimer's disease?

The brain of mammals contains two major forms of cholinesterases (ChEs): acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). The two forms differ genetically, structurally and in their kinetics. Butyrylcholine is not a physiological substrate in mammalian brains which makes the function of BuChE difficult to interpret. In human brains, BuChE is found in neurons and glial cells as well as in neuritic plaques and tangles in patients with Alzheimer's disease (AD). While AChE activity decreases progressively in the brain of patients with AD, BuChE activity shows some increase. In order to study the function of BuChE, we perfused intracortically the rat brain with a selective BuChE inhibitor. We found that extracellular acetylcholine levels increased 15-fold from 5 nmol/L to 75 nmol/L concentrations, with little cholinergic adverse effect in the animal. Based on these data, we postulated that two pools of ChEs may be present in the brain: one mainly neuronal and AChE dependent; and one mainly glial and BuChE dependent. The two pools show different kinetic properties with regard to regulation of acetylcholine concentration in the brain and can be separated with selective inhibitors. The recent development of highly selective BuChE inhibitors will allow us to test these new agents in patients with AD in order to find out whether or not they represent an advantage for the treatment of patients with AD as compared with selective (donepezil) or relatively non-selective (rivastigmine, galantamine) ChE inhibitors presently in use. The association between a BuChE-K variant and AD has not been confirmed in several studies. In conclusion, additional experimental and clinical work is necessary in order to elucidate the role of BuChE in normal brain function and in the brains of patients with AD. In the future, it may be possible that selective BuChE inhibitors will have a role in treatment of patients with advanced AD.

Eptastigmine: ten years of pharmacology, toxicology, pharmacokinetic, and clinical studies

Eptastigmine (heptyl-physostigmine tartrate) is a carbamate derivative of physostigmine in which the carbamoylmethyl group in position 5 of the side chain has been substituted with a carbamoylheptyl group. In vitro and ex vivo results suggest that eptastigmine has a long-lasting reversible brain cholinesterase (i.e., acetylcholinesterase and butyryl-cholinesterase) inhibitory effect. When administered in vivo to rodents by various routes, eptastigmine inhibits cerebral acetylcholinesterases (AChE) and increases acetylcholine (Ach) brain levels by 2500-3000%, depending on the dose. This effect leads to an improvement in the cerebral blood flow in the ischemic brain, excitatory and inhibitory effects on the gastrointestinal tract and to a protection from acute soman and diisopropylfluorophosphate intoxication. Eptastigmine, by either acute or chronic administration, has been found to have memory enhancing effects in different species of normal, aged and lesioned animals. It also restored to normal the age-related increase of EEG power without affecting spontaneous motor activity. Clinical investigations on more than 1500 patients with Alzheimer's disease demonstrated that eptastigmine significantly improved cognitive performance (as assessed by the cognitive subscale of the Alzheimer's Disease Assessment Scale) as compared with placebo. This improvement was most evident in patients with more severe cognitive impairment at the baseline. The relationship between patient performance and average steady-state AChE inhibition was described by an inverted U-shaped dose-response curve. Pharmacokinetic studies have revealed that after oral administration eptastigmine is rapidly distributed to the tissues and readily enters the CNS, where it can be expected to inhibit AChE for a prolonged period. Eptastigmine is generally well tolerated and the majority of adverse events (cholinergic) were mild to moderate in intensity. However, the adverse hematologic (granulocytopenia) effects reported in two studies have resulted in the suspension of further clinical trials.

Is there a rationale for the use of acetylcholinesterase inhibitors in the therapy of Alzheimer's disease?

Since the 1980s, the cholinergic hypothesis of the pathogenesis of Alzheimer's disease has proven to be a strong stimulus to pharmacological strategies aimed at correcting the cognitive deficit by manipulating cholinergic neurotransmission. Among these strategies, the one based on acetylcholinesterase inhibition is currently the most extensively developed for the therapy of Alzheimer's disease. The inhibitors' mechanisms of action are complex, including changes in the release of acetylcholine, and modulation of acetylcholine receptors. Various clinical trials of various inhibitors have shown that, on the whole, their effects were modest and, in the case of some drugs, were associated with frequent adverse reactions. Among the conceivable reasons for the limited efficacy of these drugs, those related to the pharmacological target deserve particular attention. This review, therefore, focuses on the complex nature of the acetylcholine system, the alterations of acetylcholinesterase and muscarinic receptor signal transduction in Alzheimer's disease, and the involvement of other neurotransmitters.

Differential recovery of acetylcholinesterase in guinea pig muscle and brain regions after soman treatment

1. In brain areas of untreated guinea-pigs the highest activity of acetylcholinesterase was seen in the striatum and cerebellum, followed by the midbrain, medulla-pons and cortex, and the lowest in the hippocampus. The activity in diaphragm was seven-fold lower than in the hippocampus. 2. At 1 h after soman (27 micrograms/kg) administration the activity of the enzyme was dramatically reduced in all tissues studied. In muscle the three major molecular forms (A12, G4 and G1) showed a similar degree of inhibition and a similar rate of recovery and the activity had returned to normal by 7 days. 3. In the brain soman inhibited the G4 form more than the G1 form. The hippocampus, cortex and midbrain showed the greatest reductions in enzyme activity. At 7 days the activity in the cortex, medulla pons and striatum had recovered but in the hippocampus, midbrain and cerebellum it was still inhibited. 4. Thus the effects of soman administration varied in severity and time course in the different tissues studied. However the enzyme activity was still reduced in all tissues at 24 h when the overt signs of poisoning had disappeared.

The effects of multiple low doses of organophosphates on target enzymes in brain and diaphragm in the mouse

1. Multiple low doses of the direct acting organophosphates, ecothiopate, paraoxon and mipafox produced persistent and additive inhibition of diaphragm acetylcholinesterase. Paraoxon and mipafox had similar effects on brain acetylcholinesterase. There was greater recovery from inhibition between doses for paraoxon and ecothiopate than for mipafox. 2. Ecothiopate did not inhibit brain acetylcholinesterase but there was a small increase in activity. 3. Mipafox also had a cumulative inhibitory effect on brain neuropathy target esterase. 4. These results have particular implication for the use of multiple low doses of organophosphates occupationally by man.

The preparation and kinetic analysis of multiple forms of human erythrocyte acetylcholinesterase

A method for preparing various forms of acetylcholinesterase (AChE) from human erythrocyte has been established and they have been characterized in terms of kinetic parameters such as K(m), rate constant (k), turnover number (kcat), specificity constant (ksp), Vmax, half-life (t1/2), IC50 and Ki for tetrabutylammonium hydroxide, procaine and physostigmine in the present study. The solubility experiments show that, there is one major form of AChE i.e. membrane bound AChE (MBAChE) and one minor form i.e. water soluble form (WSAChE). The MBAChE shows several subforms, and on the basis of percentage activity only three MBAChE forms have been selected for complete characterization by various kinetic parameters and found that these three forms of MBAChE demonstrate significant differences in their kinetic properties except IC50. This study supports the recommendation of the use of these kinetic parameters as a tool for the analysis of the multiple forms of the various enzymes in the biological systems.

Antischistosomal drugs: past, present ... and future?

The major antischistosomal drugs that have been or still are in use against infections with schistosomes are considered here together with some compounds that have not been in clinical use, but show interesting characteristics. Each individual compound presents aspects that may be enlightening about parasite biochemistry, parasite biology, and host-parasite relationships. Special attention is given to the mechanisms of action, an understanding of which is seen here as a major factor of progress in chemotherapy. Three compounds are currently in use, i.e., metrifonate, oxamniquine, and praziquantel, and all three are included in the World Health Organization list of essential drugs. They are analyzed in some detail, as each one presents advantages and disadvantages in antischistosomal therapy. The reported occurrence of drug-resistant schistosomes after treatment with oxamniquine and praziquantel suggests strict monitoring of such phenomena and encourages renewed efforts toward the development of multiple drugs against this human parasite.

Differential inhibition of soluble and membrane-bound acetylcholinesterase forms from mouse brain by choline esters with an acyl moiety of an intermediate size

Differential inhibitions of soluble and membrane-bound acetylcholinesterase forms purified from mouse brain were examined by the comparison of kinetic constants such as a Km value, a Kss value (substrate inhibition constant), and IC50 values of active site-selective ligands including choline esters. Membrane-bound acetylcholinesterase form (solubilized only in the presence of detergent) showed lower Km and Kss values than soluble acetylcholinesterase form (easily solubilized without detergent). Edrophonium expressed a slightly but significantly (p < 0.01) higher inhibition of detergent-soluble acetylcholinesterase form than aqueous-soluble acetylcholinesterase form, while physostigmine inhibited both forms with a similar potency. A remarkable difference in inhibition was observed using choline esters; although choline esters with acyl chain of a short size (acetyl- to butyrylcholine) or a long size (heptanoyl- to decanoylcholine) showed a similar inhibitory potency for two forms of acetylcholinesterase, pentanoylcholine and hexanoylcholine inhibited more strongly aqueous-soluble acetylcholinesterase than detergent-soluble acetylcholinesterase. Thus, it is suggested that the two forms of AChE may be distinguished kinetically by pentanoyl- or hexanoylcholine.

Differential inhibition of acetylcholinesterase molecular forms in normal and Alzheimer disease brain

Molecular forms of acetylcholinesterase were studied in three brain regions from Alzheimer disease patients and non-demented, age-matched controls. In Alzheimer disease patients, the membrane-bound G4 form was decreased in frontal (-71%) and parietal cortex (-45%) and in the caudate-putamen (-47%) from control levels. We also found a decrease of aqueous-soluble acetylcholinesterase molecular forms in the aqueous-soluble acetylcholinesterase molecular forms in the caudate-putamen region. The effect of three clinically significant acetylcholinesterase inhibitors, heptyl-physostigmine, physostigmine and edrophonium, on aqueous-soluble acetylcholinesterase molecular forms of the caudate-putamen was investigated. Heptyl-physostigmine, a physostigmine analogue, showed preferential inhibition for the G1 form. On the contrary, edrophonium inhibited the G4 form more potently than the G1 form. Physostigmine inhibited both forms with similar potency. The clinical implications of selective acetylcholinesterase inhibitors are discussed.