Research and development of donepezil hydrochloride,a new type of acetylcholinesterase inhibitor

A wide range of evidence shows that cholinesterase (ChE) inhibitors can interfere with the progression of Alzheimer's disease (AD). The earliest known ChE inhibitors, namely, physostigmine and tacrine, showed modest improvement in the cognitive function of AD patients. However, clinical studies show that physostigmine has poor oral activity, brain penetration and pharmacokinetic parameters, while tacrine has hepatotoxic liability. Studies were then focused on finding a new type of acetylcholinesterase (AChE) inhibitor that would overcome the disadvantages of these two compounds. During the study, by chance we found a seed compound. We then conducted a structure-activity relationship study of this compound. After four years of exploratory research, we found donepezil hydrochloride (donepezil). Donepezil showed several positive characteristics including the following: 1) It has a novel structure compared to other conventional ChE inhibitors; 2) It shows strong anti-AChE activity and has long lasting efficacy; 3) The inhibitory characteristic of donepezil shows that it is highly selective for AChE as compared to butyrylcholinesterase (BuChE) and showed reversibility; 4) The results of clinical studies on donepezil show a very high significant difference on ADAS cog and CIBIC plus scores of AD patients. Donepezil is currently marketed in 56 countries all over the world.     

Cerebral acetylcholinesterase imaging: development of the radioprobes

Cerebral acetylcholinesterase (AChE) imaging is not only useful for diagnosis of dementia disorders but also for therapeutic monitoring of the effects of cholinesterase (ChE) inhibitors and for decision of the appropriate clinical dosage of newly developed ChE inhibitors. Several ChE inhibitors or the derivatives such as 1,2,3,4-tetrahydro-9-methylaminoacridine (MTHA), donepezil, physostigmine, CP126,998 and 2-fluoro-CP118,954 have been labeled with positron emitters for mapping cerebral AChE by positron emission tomography (PET). When [(11)C]MTHA or [(11)C]donepezil was injected in animals, the uptake poorly reflect the regional distribution of AChE in the brain because of high non-specific binding and/or less specific to AChE in vivo in the brain tissue. [(11)C]physostigmine, [(11)C]CP126,998 and 2-[(18)F]fluoro-CP118,954 were distributed corresponding well to the regional AChE activity in animals, and also former two probes were successfully applied to clinical PET trial. The other approach is measuring cerebral AChE activity with radiolabeled acetylcholine analogue substrates. We have developed the principle for measuring cerebral enzyme activity by PET and radiolabeled N-methylpiperidinyl esters for quantitative measurement of cerebral AChE activity. N-[(11)C]methylipiperidin-4-yl acetate (MP4A) and N-[(11)C]methylpiperidin-4-yl propionate (MP4P) have been used for clinical studies of other demented disorders including Alzheimer's disease (AD), and the probes have demonstrated not only the reduction of AChE activity in the cerebral cortex of patients with AD but also the inhibitory effects of donepezil and rivastigmine on AChE activity in the brain of AD patients. Following this succession, widely available [(18)F]-labeled derivatives of MP4A and MP4P have been developed based on the structure-activity relationships between AChE and piperidinol esters.     

A New Therapeutic Target in Alzheimer's Disease Treatment: Attention to Butyrylcholinesterase

Summary

Alzheimer's disease (AD) is a progressive neurodegenerative disorder of the elderly, characterised by widespread loss central cholinergic function. The only symptomatic treatment proven effective, to date is the use of cholinesterase (ChE) inhibitors to augment surviving cholinergic activity. ChE inhibitors act on the enzymes that hydrolyse acetylcholine (ACh) following synaptic release. In the healthy brain, acetylcholinesterase (AChE) predominates (80%) and butyrylcholinesterase (BuChE) is considered to play a minor role in regulating brain ACh levels. In the AD brain, BuChE activity rises while AChE activity remains unchanged or declines. Therefore both enzymes are likely to have involvement in regulating ACh levels and represent legitimate therapeutic targets to ameliorate the cholinergic deficit. The two enzymes differ in location, substrate specificity and kinetics. Recent evidence suggests that BuChE may also have a role in the aetiology and progression of AD beyond regulation of synaptic ACh levels. Experimental evidence from the use of agents with enhanced selectivity for BuChE (cymserine, MF-8622) and ChE inhibitors such as rivastigmine, which have a dual inhibitory action on both AChE and BuChE, indicate potential therapeutic benefits of inhibiting both AChE and BuChE in AD and related dementias. The development of specific BuChE inhibitors and the continued use of ChE inhibitors with the ability to inhibit BuChE in addition to AChE should lead to improved clinical outcomes.     

A new therapeutic target in Alzheimer's disease treatment: attention to butyrylcholinesterase

Alzheimer's disease (AD) is a progressive neurodegenerative disorder of the elderly, characterised by widespread loss of central cholinergic function. The only symptomatic treatment proven effective to date is the use of cholinesterase (ChE) inhibitors to augment surviving cholinergic activity. ChE inhibitors act on the enzymes that hydrolyse acetylcholine (ACh) following synaptic release. In the healthy brain, acetylcholinesterase (AChE) predominates (80%) and butyrylcholinesterase (BuChE) is considered to play a minor role in regulating brain ACh levels. In the AD brain, BuChE activity rises while AChE activity remains unchanged or declines. Therefore both enzymes are likely to have involvement in regulating ACh levels and represent legitimate therapeutic targets to ameliorate the cholinergic deficit. The two enzymes differ in location, substrate specificity and kinetics. Recent evidence suggests that BuChE may also have a role in the aetiology and progression of AD beyond regulation of synaptic ACh levels. Experimental evidence from the use of agents with enhanced selectivity for BuChE (cymserine, MF-8622) and ChE inhibitors such as rivastigmine, which have a dual inhibitory action on both AChE and BuChE, indicate potential therapeutic benefits of inhibiting both AChE and BuChE in AD and related dementias. The development of specific BuChE inhibitors and the continued use of ChE inhibitors with the ability to inhibit BuChE in addition to AChE should lead to improved clinical outcomes.     

New classes of AChE inhibitors with additional pharmacological effects of interest for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is associated to a gradual loss of attention and memory that have been associated to impairment of brain cholinergic neurotransmission, particularly a deficit of cholinergic neurons in the nucleus basalis of Meynert. Thus, it is not surprising that the first therapeutic target that has demonstrated therapeutic efficacy on cognition, behaviour and functional daily activities has been the inhibitors of acetylcholinesterase (AChE), i.e. tacrine, donepezil, rivastigmine and galanthamine. But not all inhibitors of AChE have the same potency to block the enzyme and have a different pharmacological profile. For instance, rivastigmine is a dual inhibitor of AChE and butyrylcholinesterase (BuChE), and galanthamine is a mild inhibitor of AChE and an allosteric potentiating ligand of neuronal nicotinic receptors for acetylcholine (nAChRs). In addition, we have recently found that galanthamine has neuroprotective effects by inducing calcium signals and the induction of the antiapoptotic protein Bcl-2. In this frame, we have been synthesizing new tacrine derivatives that keep their ability to inhibit AChE but that interfere with neuronal calcium overloading and prevent apoptosis. Some of these compounds exhibit neuroprotecting properties and thus, could be useful in the treatment of neurodegenerative and ischaemic brain diseases.     

Design, synthesis and biological evaluation of new 2-benzoxazolinone derivatives as potential cholinesterase inhibitors for therapy of alzheimer's disease

Currently acetylcholinesterase inhibitor (AChEI) therapy is one of the most frequently used methods in the treatment of Alzheimer's disease; tacrine, donepezil, rivastygmine and galantamine are applied in different stages of AD. In the present study, we propose a new series of 2-benzoxazolinone derivatives as potential cholinesterase inhibitors. These compounds were synthesized by condensation of 6-chloro acetyl-2-benzoxa zolinone with the corresponding amine and evaluated as acetylcholinesterase inhibitors using the colorimetric Ellman's method. Selectivity and the IC50 values were determined for the received derivatives. All tested compounds exhibited the inhibitory activity towards acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Compound 3e showed stronger activity than the standard tacrine, and compound 3a showed activity similar to that of tacrine for AChE. Compounds 3a, 3b, 3c, and 3e showed stronger activity than the standard donepezil towards the inhibition of BChE, and the compound 3e showed stronger activity than donepezil towards AChE.     

Inhibitory effect of orally administered donepezil hydrochloride (E2020), a novel treatment for Alzheimer's disease, on cholinesterase activity in rats

Donepezil hydrochloride ((+/-)-2-[(1-benzylpiperidin-4-yl)methyl]-5, 6-dimethoxy-indan-1-one monohydrochloride: E2020: donepezil) is a potent and selective acetylcholinesterase inhibitor developed for the treatment of Alzheimer's disease. The present experiments were designed to compare the inhibitory effects of orally administered donepezil and other cholinesterase inhibitors, tacrine (9-amino-1,2, 3,4-tetrahydroacridine hydrochloride), (S)-N-ethyl-3-[(1-dimethyl-amino)ethyl]-N-methyl-phenylcarbamate hydrogentartrate (ENA-713, rivastigmine) and 3-[1-(phenylmethyl)-4-piperidinyl]-1-(2,3,4, 5-tetrahydro-1H-1-benzazepin-8-yl)-1-propanone fumarate (TAK-147), on the cholinesterase activity in the brain and plasma of rats. Moreover, in order to validate the cholinesterase inhibition data, we measured the brain and plasma concentrations of these drugs. Oral administration of donepezil, tacrine, ENA-713 or TAK-147, caused a dose-dependent inhibition of brain and plasma cholinesterase activities. The ID(50) values of these compounds for brain cholinesterase activity were 6.3, 40.5, 7.2 and 26.8 micromol/kg, respectively. On the other hand, the ID(50)170, 9.7 and 51.2 micromol/kg, respectively. Thus, the ratios of the ID(50)4.2, 1.3 and 1.9, respectively. Brain and plasma concentrations of donepezil, tacrine and TAK-147 increased dose-dependently. The ratios of the concentrations (brain/plasma) of these compounds were 6.1-8.4 for donepezil, 14.5-54.6 for tacrine and 7.0-20.6 for TAK-147. The values of 50% inhibitory concentration of these drugs in the brain were 0.42, 3.5 and 1.1 nmol/g, respectively. In contrast, the brain and plasma concentrations of ENA-713 at all doses, except the two highest doses, were below the quantification limit. These results suggest that orally administered donepezil satisfactorily penetrates into the brain and inhibits cholinesterase there, and that donepezil is a potent and selective inhibitor of brain cholinesterase in comparison with plasma cholinesterase in vivo.     

Effects of huperzine A on acetylcholinesterase isoforms in vitro: comparison with tacrine,donepezil, rivastigmine and physostigmine

Five inhibitors of acetylcholinesterase, huperzine A, donepezil, tacrine, rivastigmine and physostigmine, were compared with regard to their effects on different molecular forms of acetylcholinesterase in cerebral cortex, hippocampus, and striatum from the rat brain. In general, huperzine A preferentially inhibited tetrameric acetylcholinesterase (G4 form), while tacrine and rivastigmine preferentially inhibited monomeric acetylcholinesterase (G1 form). Donepezil showed pronounced selectivity for G1 acetylcholinesterase in striatum and hippocampus, but not in cortex. Physostigmine showed no form-selectivity in any brain region. In cortex, the most potent inhibitors of G4 acetylcholinesterase were huperzine A (K(i) 7 x 10(-9) M) and donepezil (K(i) 4 x 10(-9) M). The potent inhibitors of cortical G1 acetylcholinesterase were donepezil (K(i) 3.5 x 10(-9) M) and tacrine (K(i) 2.3 x 10(-8) M). In hippocampus, huperzine A and physostigmine were the most potent inhibitors of G4 acetylcholinesterase, while donepezil and tacrine were most potent against G1 acetylcholinesterase. In striatum, huperzine A and donepezil were the most potent against G4 acetylcholinesterase, while again donepezil was the most potent against G1. Although the inhibition constants (K(i)) of these acetylcholinesterase inhibitors differed significantly from region to region, the nature of the inhibition did not vary. These results suggest that the use of acetylcholinesterase inhibitors in treatment of Alzheimer's disease must consider both form-specific and region-specific characteristics of acetylcholinesterase inhibition.     

Design, synthesis, and structure-activity relationships of a series of 3-[2-(1-benzylpiperidin-4-yl)ethylamino]pyridazine derivatives as acetylcholinesterase inhibitors

Starting from the 3-[2-(1-benzylpiperidin-4-yl)ethylamino]-6-phenylpyridazine 1, we performed the design, the synthesis, and the structure-activity relationships of a series of pyridazine analogues acting as AChE inhibitors. Structural modifications were achieved on four different parts of compound 1 and led to the following observations: (i) introduction of a lipophilic environment in the C-5 position of the pyridazine ring is favorable for the AChE-inhibitory activity and the AChE/BuChE selectivity; (ii) substitution and various replacements of the C-6 phenyl group are possible and led to equivalent or slightly more active derivatives; (iii) isosteric replacements or modifications of the benzylpiperidine moiety are detrimental to the activity. Among all derivatives prepared, the indenopyridazine derivative 4g was found to be the more potent inhibitor with an IC(50) of 10 nM on electric eel AChE. Compared to compound 1, this represents a 12-fold increase in potency. Moreover, 3-[2-(1-benzylpiperidin-4-yl)ethylamino]-5-methyl-6-phenylpyridazine 4c, which showed an IC(50) of 21 nM, is 100-times more selective for human AChE (human BuChE/AChE ratio of 24) than the reference compound tacrine.     

TAK—147逆转东莨菪碱诱发的大鼠空间记忆障碍

目的:研究和阐明TAK-147对东莨菪碱诱发的大鼠空间记忆损伤的作用.方法:采用Morris水迷宫的程序研究大鼠的空间记忆,利用开场实验方法测定动物自发活动量.结果:在水迷宫的学习过程中,腹腔内注射东莨菪碱(0.4mg/kg,ip)明显延长大鼠上台的潜伏期,而腹腔内注射TAK-147或donepezil(多奈哌齐)能剂量依赖性地改善东莨菪碱诱发的记忆损伤,两药在0.1-1.0mg/kg的剂量时具有显著性差异.在记忆的再生过程中,腹腔内注射东莨菪碱(1.5mg/kg,ip)引起空间记忆再生过程的障碍分别被TAK-147(0.1,0.3和1.0mg/kg)、多奈哌齐(0.3和1.0mg/kg)以及他克林(3和5mg/kg)显著性改善.TAK-147的作用比多奈哌齐略强却明显强于他克林.此外,在开场实验中,TAK-147和多奈哌齐与生理盐水和东莨菪碱相比,对大鼠运动量未产生明显改变.结论:TAK-147在空间认知功能上起重要的作用,进一步证明TAK-147能够成为一个治疗阿尔采默病的理想的胆碱酯酶抑制药.     

Effect of donepezil hydrochloride (E2020) on extracellular acetylcholine concentration in the cerebral cortex of rats

Donepezil hydrochloride (donepezil), a potent and selective acetylcholinesterase inhibitor, has been developed for the treatment of Alzheimer's disease. We studied the effect of oral administration of this drug on the extracellular acetylcholine (ACh) concentration in the cerebral cortex of rats using microdialysis. We also observed fasciculation, a peripheral cholinergic sign induced by activation of neuromuscular transmission, after oral administration of the drug as an index of peripheral cholinergic activation. Other cholinesterase inhibitors, tacrine, ENA-713 and TAK-147, were used as reference drugs. Donepezil significantly and dose-dependently increased the extracellular ACh concentration in the rat cerebral cortex within the dose range of 2.5-10 mg/kg. Tacrine, ENA-713 and TAK-147 also elevated the extracellular concentration of ACh. The minimum effective doses of donepezil, tacrine, ENA-713 and TAK-147 were (< or = 2.5, 10, 10 and < or = 10 mg/kg, respectively. Donepezil produced fasciculation at doses of 2.5 mg/kg and above, with a dose-dependent increase in incidence and intensity. The reference compounds also induced fasciculation in a dose-dependent manner. The threshold doses of tacrine, ENA-713 and TAK-147 for fasciculation were 5, 2.5 and 2.5 mg/kg, respectively. The values of the ratio of the minimum effective dose for the ACh-increasing action to that for the fasciculation-producing action were: donepezil, < or = 1; tacrine, 2; ENA-713, 4; TAK-147, < or = 4. These results indicate that orally administered donepezil has a potent and selective activity on the central cholinergic system.     

石杉碱乙的抗胆碱酯酶作用

目的：测试石杉碱乙的抗胆碱酯酶作用并与他克林进行比较,方法：比色法用于测定胆碱酶活性。结果：石杉碱乙和他克林对ＢｕＣｈＥ和ＡＣｈＥ抑制的ＩＣ５０值的比率分别为６５．８和０．５４,石杉碱乙对乙酰胆碱酯酶有更强的选择性抑制作用。石杉碱乙灌胃对脑内ＡＣｈＥ的抑制作用明业强于他克林,而他克林对ＢｕＣｈＥ的抑制作用强于石杉碱乙,并有严重副反应,单次灌胃石杉碱乙在４小时内对脑内ＡＣｈＥ产生较对稳定的抑制作用,     

Effects of switching from an AChE inhibitor to a dual AChE-BuChE inhibitor in patients with Alzheimer's disease

OBJECTIVE: Cholinesterase (ChE) inhibitors are the only medications approved for the treatment of Alzheimer's disease (AD). The features of ChE inhibitors differ considerably. In addition to acetylcholinesterase (AChE) inhibition, rivastigmine also inhibits butrylcholinesterase (BuChE), providing dual AChE and BuChE inhibition. An observational study was performed to determine the response in routine clinical practice to switching AD patients to rivastigmine from a selective AChE inhibitor when that treatment no longer delivered a satisfactory clinical response. RESEARCH DESIGN AND METHODS: A prospective, multicentre, 3-month observational trial in patients with mild to moderately severe AD (adjusted Mini Mental State Examination [MMSE] score 10-26) deteriorating (at least 2 adjusted MMSE points in last 6 months) on selective AChE inhibitor treatment. Adjusted MMSE, activities of daily living (ADL) and instrumental activities of daily living (IADL), the Zarit caregiver burden and global function (short Clinical Global Impression of Change, CGIC) scores were noted before the switch and 3 months after the switch. RESULTS: 225 patients entered the study. The switches made were from donepezil to rivastigmine in (D-R) in 188 patients, galantamine to rivastigmine (G-R) in 33 patients and donepezil to galantamine (D-G) in four patients. Ten patients discontinued due to adverse events and eight for other reasons. More than half of the switches were within 36 hours of a patient's first treatment visit. In the D-R and G-R groups, 67.7% and 66.7% of patients responded (CGIC score < or = 4), respectively. In non-responders, worsening (CGIC score 5-7) was mild in approximately 80% or more of patients. Adjusted MMSE improved after the switch from both donepezil and galantamine to rivastigmine (+0.69 +/- 3.2, p = 0.008 and +0.6 +/- 1.6, p = 0.05, respectively). Mean ADL, IADL, and Zarit scores remained stable. The proportion of patients on concomitant antipsychotic therapy diminished by 30.5% and benzodiazepines were discontinued in all patients, except one. CONCLUSIONS: AD patients deteriorating on selective AChE inhibitor treatment can benefit from switching to a dual AChE-BuChE inhibitor, such as rivastigmine, in terms of stabilization of disease, improvement in cognitive function and reduction in the burden of concomitant psychoactive treatment. The switch was well tolerated. Confirmation of these results is required in a controlled study.     

Acetylcholinesterase imaging: its use in therapy evaluation and drug design

Several cholinesterase (ChE) inhibitors have been labeled with carbon-11 for visualizing binding sites on acetylcholinesterase (AChE) by positron emission tomography (PET). Following intravenous injection of 1,2,3,4-tetrahydro-9-[(11)C]methylaminoacridine or [(11)C]donepezil, however, the radioactivity distribution does not reflect the regional distribution of AChE in the brain of animals, probably because these compounds have high non-specific binding and/or other specific binding sites in vivo in the brain. PET studies with [(11)C]physostigmine and [(11)C]CP-126,998 in the brain of healthy subjects have shown a radioactivity distribution corresponding to the regional distribution of AChE activity measured in postmortem human brains. These radiotracers may be useful for measuring the occupancy of binding sites on AChE by AChE inhibitors, and for investigating the cerebral pharmacokinetics of such therapeutic drugs. An alternative approach to map AChE is the use of acetylcholine analogue substrates. We have developed N-methylpiperidinyl esters labeled with carbon-11 for quantitative measurement of AChE activity. Currently, two N-[(11)C]methylpiperidine esters, N-[(11)C]methylipiperidin-4-ylacetate (MP4A) and N-[(11)C]methylpiperidin-4-yl propionate (MP4P or PMP), have been used for clinical studies of Alzheimer's disease and other neurodegenerative diseases. Both [(11)C]MP4A- and [(11)C]MP4P-PET have demonstrated not only the reduction of AChE activity in the cerebral cortex of patients with Alzheimer's disease (AD) but also the inhibitory effects of donepezil and rivastigmine on AChE activity in the brain of AD patients. AChE imaging should prove useful for therapeutic monitoring of the effects of ChE inhibitors, including determination of the appropriate clinical doses of newly developed compounds, and can thus prompt the development of novel drugs targeting AChE.     

Central selective acetylcholinesterase inhibitor with neurotrophic activity: structure-activity relationships of TAK-147 and related compounds

A series of benzylamino inhibitors of acetylcholinesterase (AChE) have been designed based on a working hypothesis of the enzyme s active site. These compounds were tested for their inhibitory activities on AChE and potent inhibitors were further evaluated in terms of central selectivity. These studies led to a discovery of 3-[1-(phenylmethyl)-4-piperidinyl]-1-(2,3,4, 5-tetrahydro-1H-1-benzazepin-8-yl)-1-propanone fumarate (TAK-147). Pharmacokinetic study has shown that the compound has high central selectivity, as demonstrated by rapid elimination from plasma and long-term existence in the brain. As a consequence, TAK-147 ameliorates impairments of learning and memory in various animal models without producing peripheral side effects. TAK-147 also activates the monoaminergic systems and energy metabolism. Furthermore, TAK-147 was revealed to have NGF-like neurotrophic activity on central cholinergic neurons at concentrations where it inhibits AChE activity. Therefore, TAK-147 is expected not only to ameliorate the clinical symptoms in Alzheimer s disease via AChE inhibition but to prevent or slow the progression of the disease via its neurotrophic action. TAK-147 is now under clinical trial as a therapeutic drug for Alzheimer s disease. This article reviews design and structure-activity relationships of TAK-147 and related compounds. Preclinical pharmacology of TAK-147 is also summarized.     

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.     

Characterization of the in vitro kinetic interaction of chlorpyrifos-oxon with rat salivary cholinesterase: a potential biomonitoring matrix

The primary mechanism of action for organophosphorus (OP) insecticides such as chlorpyrifos (CPF) involves the inhibition of acetylcholinesterase (AChE) by their active oxon metabolites resulting in a wide range of neurotoxic effects. These oxons also inhibit other cholinesterases (ChE) such as butyrylcholinesterase (BuChE), which represents a detoxification mechanism and a potential biomarker for OP insecticide exposure/response. Salivary biomonitoring has recently been explored as a practical method for examination of chemical exposure, however, there are few studies exploring the use of saliva for OP insecticides. To evaluate the use of salivary ChE as a biological monitor for OP insecticide exposure, a modified Ellman assay in conjunction with a pharmacodynamic model was used to characterize salivary ChE in adult male Sprague-Dawley rats. Comparison of rat saliva, brain, and plasma ChE activity in the presence of selective inhibitors of AChE and BuChE (BW284C51 and iso-OMPA, respectively) with different ChE substrates indicated that rat salivary ChE activity is primarily associated with BuChE (>95%). Further characterization of rat salivary BuChE kinetics yielded an average total BuChE active site concentration of 1.20+/-0.13 fmol ml(-1) saliva, an average reactivation rate constant (Kr) of 0.070+/-0.008 h(-1), and an inhibitory rate constant (Ki) of approximately 9 nM(-1) h(-1). The pharmacodynamic model successfully described the in vitro BuChE activity profile as well as the kinetic parameters. These results support the potential utility of saliva as a biomonitoring matrix for evaluating occupational and environmental exposure to CPF and other OP insecticides.     

Effects of switching from an AChE inhibitor to a dual AChE-BuChE inhibitor in patients with Alzheimer's disease

ABSTRACT

Objective: Cholinesterase (ChE) inhibitors are the only medications approved for the treatment of Alzheimer's disease (AD). The features of ChE inhibitors differ considerably. In addition to acetylcholinesterase (AChE) inhibition, rivastigmine also inhibits butyrylcholinesterase (BuChE), providing dual AChE and BuChE inhibition. An observational study was performed to determine the response in routine clinical practice to switching AD patients to rivastigmine from a selective AChE inhibitor when that treatment no longer delivered a satisfactory clinical response.

Research design and methods: A prospective, multicentre, 3‐month observational trial in patients with mild to moderately severe AD (adjusted Mini Mental State Examination [MMSE] score 10–26) deteriorating (at least 2 adjusted MMSE points in last 6 months) on selective AChE inhibitor treatment. Adjusted MMSE, activities of daily living (ADL) and instrumental activities of daily living (IADL), the Zarit caregiver burden and global function (short Clinical Global Impression of Change, CGIC) scores were noted before the switch and 3 months after the switch.

Results: 225 patients entered the study. The switches made were from donepezil to rivastigmine (D‐R) in 188 patients, galantamine to rivastigmine (G‐R) in 33 patients and donepezil to galantamine (D‐G) in four patients. Ten patients discontinued due to adverse events and eight for other reasons. More than half of the switches were within 36 hours of a patient's first treatment visit. In the D‐R and G‐R groups, 67.7% and 66.7% of patients responded (CGIC score ≤ 4), respectively. In non-responders, worsening (CGIC score 5–7) was mild in approximately 80% or more of patients. Adjusted MMSE improved after the switch from both donepezil and galantamine to rivastigmine (+0.69 ± 3.2, p = 0.008 and +0.6 ± 1.6, p = 0.05, respectively). Mean ADL, IADL and Zarit scores remained stable. The proportion of patients on concomitant antipsychotic therapy diminished by 30.5% and benzodiazepines were discontinued in all patients, except one.

Conclusions: AD patients deteriorating on selective AChE inhibitor treatment can benefit from switching to a dual AChE-BuChE inhibitor, such as rivastigmine, in terms of stabilization of disease, improvement in cognitive function and reduction in the burden of concomitant psychoactive treatment. The switch was well tolerated. Confirmation of these results is required in a controlled study.     

Comparison of donepezil-, tacrine-, rivastigmine- and metrifonate-induced central and peripheral cholinergically mediated responses in the rat

There are now several acetylcholinesterase inhibitors in clinical use for the treatment of Alzheimer's disease, however, no systematic comparative studies of their central and peripheral cholinergic mediated effects in rats appear to have been reported. The present study investigated the dose-response characteristics of donepezil, tacrine, rivastigmine and metrifonate in inducing tremor, lacrimation, salivation and hypothermia and the duration of action of these compounds in Lister hooded rats. Data obtained were compared with the clinical observations on these drugs. Three doses of each compound were given orally to establish a dose-response curve for each behaviour, Tremor and lacrimation were scored, salivation was measured by weighing swabs applied to the mouth area and hypothermia was measured with a rectal probe. ED50 values were calculated for tremor. Using a just sub-maximal tremorigenic dose, the duration of response was examined. All four compounds produced dose-dependent increases in tremor and hypothermia. Only tacrine also produced marked salivation and lacrimation. The order of potency (ED50 value in micromol/kg) was rivastigmine (3.7), donepezil (18.0), tacrine (37.5), metrifonate (470). Tremor following tacrine (150 micromol/kg) and donepezil (20 micromol/kg) was prolonged (> 6 h) with a similar hypothermic response. The duration of these responses following metrifonate (777 micromol/kg) and rivastigmine (12.5 micromol/kg) did not exceed 3 h. Tacrine had poor selectivity for central (tremor) versus peripheral (salivation/lacrimation) effects compared to the other compounds. Donepezil also had a sustained duration of action. The data are consistent with clinical results and indicate that simple in-vivo models may assist in the selection of acetylcholinesterase inhibitors with a suitable response profile for use in the symptomatic treatment of Alzheimer's disease.     

Reversible and persistent decreases in cocaine self-administration after cholinesterase inhibition: different effects of donepezil and rivastigmine

We recently observed that pretreatment with the cholinesterase inhibitor, tacrine can produce long-lasting reductions in cocaine-reinforced behavior, described as persistent attenuation. In addition to inhibiting both acetylcholinesterase (AChE) and butyrylcholinesterase, tacrine can potentiate actions of dopamine. This study was carried out to evaluate the effects of donepezil (which selectively inhibits AChE) and rivastigmine (which inhibits both AChE and butyrylcholinesterase) on cocaine self-administration. High self-administration rats self-administered different doses of cocaine under a fixed ratio-5 schedule. Over a 4-day period, vehicle, donepezil, or rivastigmine was infused as animals were maintained in home cages (21 h per day), with signs of cholinergic stimulation (fasciculation, vacuous jaw movements, yawning, and diarrhea) scored by a blinded observer. Both compounds dose-dependently decreased cocaine self-administration, but differed in the potency and temporal pattern of their effects. Self-administration of low-dose cocaine was decreased to a greater degree by rivastigmine than donepezil (50% effective doses of 2.33 and 6.21 mg/kg/day, respectively), but this early effect did not continue beyond sessions immediately after treatment with rivastigmine. Group means for cocaine self-administration were decreased at some time points occurring between 1 and 3 days after the treatment with 10 mg/kg/day of donepezil (late effects), with decreases of more than 80% observed in some individual rats that persisted for 1 week or longer. Early, but not late, effects were correlated with signs of cholinergic stimulation. In summary, pretreatment with donepezil, but not rivastigmine produced persistent reductions in cocaine-reinforced behavior, which were not associated with signs of cholinergic stimulation.