Pharmacologic and clinicopharmacologic properties of SDZ ENA 713, a centrally selective acetylcholinesterase inhibitor

Drug Repurposing Analysis for Colorectal Cancer through Network Medicine Framework: Novel Candidate Drugs and Small Molecules

This study aimed to reveal the drug-repurposing candidates for colorectal cancer (CRC) via drug-repurposing methods and network biology approaches. A novel, differentially co-expressed, highly interconnected, and co-regulated prognostic gene module was identified for CRC. Based on the gene module, polyethylene glycol (PEG), gallic acid, pyrazole, cordycepin, phenothiazine, pantoprazole, cysteamine, indisulam, valinomycin, trametinib, BRD-K81473043, AZD8055, dovitinib, BRD-A17065207, and tyrphostin AG1478 presented as drugs and small molecule candidates previously studied in the CRC. Lornoxicam, suxamethonium, oprelvekin, sirukumab, levetiracetam, sulpiride, NVP-TAE684, AS605240, 480743.cdx, HDAC6 inhibitor ISOX, BRD-K03829970, and L-6307 are proposed as novel drugs and small molecule candidates for CRC.

Evaluation of rivastigmine in Alzheimer's disease

Dementia is the major cause of mortality and morbidity in older adults, with Alzheimer's disease (AD) being the most common cause. AD has a significant impact on economic and psychosocial status. Cholinesterase inhibitors (ChEIs) are currently the mainstay in the management of AD. Rivastigmine is the only ChEI that inhibits both acetylcholinesterase and butyrylcholinesterase enzymes in the brain. This dual inhibition makes it potentially more effective for AD patients. Its availability as both a transdermal formulation and oral capsule, may improve adherence rates and care giver satisfaction compared with other ChEIs. To date, the data from randomized clinical trials and post marketing observational studies have shown evidence for an impact on cognitive functions in AD with good safety and tolerability.

Decarbamoylation of acetylcholinesterases is markedly slowed as carbamoyl groups increase in size

Carbamates are esters of substituted carbamic acids that react with acetylcholinesterase (AChE) by initially transferring the carbamoyl group to a serine residue in the enzyme active site accompanied by loss of the carbamate leaving group followed by hydrolysis of the carbamoyl enzyme. This hydrolysis, or decarbamoylation, is relatively slow, and half-lives of carbamoylated AChEs range from 4 min to more than 30 days. Therefore, carbamates are effective AChE inhibitors that have been developed as insecticides and as therapeutic agents. We show here, in contrast to a previous report, that decarbamoylation rate constants are independent of the leaving group for a series of carbamates with the same carbamoyl group. When the alkyl substituents on the carbamoyl group increased in size from N-monomethyl- to N,N-dimethyl-, N-ethyl-N-methyl-, or N,N-diethyl-, the decarbamoylation rate constants decreased by 4-, 70-, and 800-fold, respectively. We suggest that this relationship arises as a result of active site distortion, particularly in the acyl pocket of the active site. Furthermore, solvent deuterium oxide isotope effects for decarbamoylation decreased from 2.8 for N-monomethylcarbamoyl AChE to 1.1 for N,N-diethylcarbamoyl AChE, indicating a shift in the rate-limiting step from general acid-base catalysis to a likely conformational change in the distorted active site.

Characterization of the interactions between coumarin-derivatives and acetylcholinesterase: Examination by NMR and docking simulations

Alzheimer's disease (AD) is one of the most common forms of dementia and a significant threat to the elderly populations, especially in the Western world. The rapid hydrolysis of the principal neurotransmitter into choline and acetate by acetylcholinesterase (AChE) at synapses causes the loss of cognitive response that becomes the real cause of AD. Therefore, inhibition of AChE is the most fundamental therapy among currently available treatments for AD. In this context, we designed and performed molecular recognitions studies of coumarin-based inhibitors towards AChE. STD NMR and Tr-NOESY applications were utilized to evaluate the binding epitope, the dissociation constant (K_D) and bound conformations of these inhibitors within this inhibitor-AChE complex. Compound 1, which has a similar inhibition activity to tacrine (a current drug) led in this study as a stronger binder with K_D = 30 μM ,even greater than tacrine (K_D = 140 μM). Moreover, docking simulations mimic NMR results and provided evidence of synchronizing binding of compound 1 with three sites; the peripheral anionic site, the bottom of the gorge, and the catalytic site. Therefore, we envisioned from our experimental and theoretical results that coumarin-based inhibitors containing a piperidinyl scaffold might be a potential drug candidates for AD in the future.

Cholinesterase inhibitors for the treatment of Alzheimer's disease:: getting on and staying on

BACKGROUND

Cholinesterase (ChE) inhibitors currently used in the treatment of Alzheimer's disease (AD) are the acetylcholinesterase (AChE)-selective inhibitors, donepezil and galantamine, and the dual AChE and butyrylcholinesterase (BuChE) inhibitor, rivastigmine. In addition to differences in selectivity for AChE and BuChE, ChE inhibitors also differ in pharmacokinetic and pharmacodynamic properties, and these differences could significantly impact on safety, tolerability, and efficacy.

OBJECTIVE

The aim of this article was to provide an overview of the ChE inhibitors widely used in AD, focusing on key pharmacologic differences among agents and how these may translate into important differences in safety, tolerability, and efficacy in clinical practice.

METHODS

Using published literature collected over time by the author, a review was conducted, focusing on the pharmacology and clinical data of donepezil, galantamine, and rivastigmine.

RESULTS

All ChE inhibitors have the potential to induce centrally mediated cholinergic adverse events (AEs), such as nausea and vomiting, if the dose is increased too rapidly or in increments that are too large. These AEs, which are most likely to occur during the "getting on," or dose-escalation, phase of treatment, may result in patients discontinuing treatment early without achieving optimum therapeutic benefit. To reduce the incidence of these AEs, a slow dose-escalation schedule has been established in clinical practice, consisting of a "start low, go slow" procedure with a minimum of 4 weeks between dose increases. After "getting on" treatment, maintaining treatment in the long term, or "staying on," may be achieved with good safety, tolerability, and sustained symptomatic efficacy across the key symptom domains (activities of daily living, behavior, and cognition).

CONCLUSIONS

ChE inhibitors provide symptomatic benefit in AD across key symptom domains. Factors influencing the safety, tolerability, and efficacy of these agents in clinical practice include ChE enzymes inhibited, brain and brain-region ChE selectivity, and metabolism route. Class-specific cholinergic AEs can be minimized using slow, flexible dose escalation.

Determination of Bis(9)-(-)-Meptazinol, a bis-ligand for Alzheimer's disease, in rat plasma by liquid chromatography-tandem mass spectrometry: application to pharmacokinetics study

A rapid, simple and sensitive LC-MS/MS method was developed and validated for the determination of Bis(9)-(-)-Meptazinol (B9M) in rat plasma. Protein precipitation method was used for sample preparation, using five volumes of methanol as the precipitation agent. The analytes were separated by a Zorbax Extend-C18 column with the mobile phase of methanol-water (containing 5mM ammonium formate, pH 9.8) (95:5, v/v), and monitored by positive electrospray ionization in multiple reaction monitoring (MRM) mode. Retention time of IS (Bis(5)-(-)-Meptazinol) and B9M were 1.9 min and 3.3 min, respectively. The limit of detection was 0.1 ng/ml and the linear range was 1-500 ng/ml. The relative standard deviation (RSD) of intra-day and inter-day variation was 4.4-6.2% and 6.2-8.9%, respectively. The extraction recoveries of B9M in plasma were over 95%. The method proved to be applicable to the pharmacokinetic study of B9M in rat after intravenous and subcutaneous administration.

Effects of novel tacripyrines ITH12117 and ITH12118 on rat vas deferens contractions, calcium transients and cholinesterase activity

We have recently synthesized a new series of hybrid compounds having the moieties of tacrine, a potent inhibitor of brain and peripheral acetylcholinesterase (AChE), and nimodipine, a blocker of L-type voltage-dependent calcium channels (VDCCs). These compounds were designed to target AChE and L calcium channels in the brain, as potential therapeutic agents in Alzheimer's disease. We performed the present study to determine the main peripheral side effects of two of these compounds, ITH12117 and ITH12118. We have here shown that in rat vas deferens these compounds inhibited AChE with a potency about 1000-fold lower than that of physostigmine or tacrine. Furthermore, the hybrid compounds enhanced contractions evoked by acetylcholine, with a potency about 100-fold lower than that of physostigmine or tacrine. Additionally, contractions induced by Ca2+ on depolarized vas deferens were blocked by nimodipine with greater efficacy, compared with ITH12117 and ITH12118. Compound ITH12118 (1 μM) caused a pronounced inhibition of the tonic (but not phasic) contraction elicited by electrical field stimulation. Furthermore, the same dose of nimodipine and ITH12118 blocked by 75% cytosolic Ca2+ elevations produced by acetylcholine, noradrenaline, or ATP. As a matter of comparison, we showed that rat brain cortex AChE was inhibited by ITH12118 with a potency 10 to 20-fold higher than that for vas deferens. This study shows that ITH12118 could be a paradigmatic multitarget compound having selective brain effects with smaller peripheral side effects. This may help to orient the search of new neuroprotective compounds with potential therapeutic application in Alzheimer's disease.

Switching from oral cholinesterase inhibitors to the rivastigmine transdermal patch

Oral cholinesterase inhibitors (ChEIs) are associated with side effects such as nausea and vomiting. The use of transdermal patches for ChEI delivery may help to minimize these problems. The objective of this review was to consider available data from patients switching from oral ChEIs to transdermal rivastigmine treatment, and to suggest practical guidelines for patients wishing to do this. Literature database and reference list searches were performed to identify suitable publications. Data from two clinical trials and a series of open observational studies, in which patients were switched to the rivastigmine patch from oral rivastigmine, donepezil tablets, or galantamine, were evaluated. Adverse events were tabulated. In the studies reported here, nausea was reported in up to 3.2% and vomiting in up to 1.9% of patients switching to the rivastigmine patch from oral rivastigmine. Similar rates (up to 3.8% of patients for nausea and 0.8% of patients for vomiting) were reported when switching to the rivastigmine patch from donepezil tablets, and no nausea or vomiting was reported in a case study of patients switching to the rivastigmine patch from galantamine tablets. Switching regimes used in clinical trials appeared well tolerated. Data support recommendations for patients on high rivastigmine capsule doses to switch directly to the 9.5 mg/24 h rivastigmine patch, while those on lower oral rivastigmine doses should start on the 4.6 mg/24 h patch for 4 weeks before increasing to the 9.5 mg/24 h patch. This latter regimen is recommended for patients on other oral cholinesterase inhibitors if switching is medically indicated or requested by the patient or the caregiver.

Rational design and synthesis of highly potent anti-acetylcholinesterase activity huperzine A derivatives

By targeting multi-active sites of acetylcholinesterase (AChE), a series of huperzine A (Hup A) derivatives with various aromatic ring groups were designed and synthesized by Schiff reaction. They were evaluated as AChE and butyrylcholinesterase (BChE) inhibitors. Results showed very significant specificity that the group of imine derivatives could inhibit TcAChE and hAChE, but no inhibitory effect on hBChE was detected. The experiment was explained by a docking study. In the docking model, we confirmed that aromatic ring of Hup A derivatives played the pi-pi stacking against aminophenol residues of AChE, and the structure-activity relationship (SAR) was discussed.

Complete atrioventricular block associated with rivastigmine therapy

PURPOSE

A case of complete atrioventricular block associated with rivastigmine use is presented.

SUMMARY

A 67-year-old Turkish woman with Alzheimer's disease was admitted to the hospital because of dizziness and syncope. Her medical history included diagnoses of hypertension (treated with amlodipine 5 mg daily) and diabetes mellitus (treated with nateglinide 120 mg daily). She had been taking both drugs for over five years. She had also been taking rivastigmine 6 mg p.o. daily for five months for the treatment of Alzheimer's disease. She had experienced dizziness since the onset of rivastigmine therapy but had not reported it to any health care provider. On admission, she had a blood pressure measurement of 90/60 mm Hg and a pulse rate of 34 beats/min. A 12-lead electrocardiogram revealed complete atrioventricular block. Echocardiography results, blood electrolyte levels, and cardiac biochemical markers were normal. After initial evaluation, a temporary transvenous pacemaker was implanted via the right femoral vein. Amlodipine and rivastigmine were discontinued. On the first day of hospitalization, a coronary angiogram revealed normal coronary anatomy. Two days later, the complete atrioventricular block resolved spontaneously to sinus rhythm. Rivastigmine 6 mg p.o. daily was reinitiated, and complete atrioventricular block recurred on the fourth day of therapy. A VVI permanent pacemaker was implanted on the fifth day of hospitalization. Amlodipine and rivastigmine were reinitiated. The patient continued rivastigmine 6 mg p.o. daily after permanent pacemaker implantation. A three-month follow-up appointment revealed that no further syncope episodes or dizziness had occurred.

CONCLUSION

A 67-year-old woman developed complete atrioventricular block after receiving rivastigmine for the treatment of Alzheimer's disease.

Rivastigmine for the treatment of dementia associated with Parkinson's disease

Parkinson's disease (PD) afflicts millions of people worldwide and leads to cognitive impairment or dementia in the majority of patients over time. Parkinson's disease dementia (PDD) is characterized by deficits in attention, executive and visuospatial function, and memory. The clinical diagnostic criteria and neuropathology surrounding PDD remain controversial with evidence of overlap among PDD, dementia with Lewy bodies (DLB) and Alzheimer's disease (AD). Cortical cholinergic deficits are greater in PDD than in AD, and are well-correlated with the cognitive and neuropsychiatric dysfunction that occurs in PDD. Inhibition of acetylcholine metabolism is therefore a practical therapeutic strategy in PDD.This review examines current evidence for rivastigmine (a cholinesterase/butyrylcholinesterase inhibitor) treatment in PDD. In addition to its efficacy, we examine the safety profile, side effects, and cost effectiveness of rivastigmine in PDD. Rivastigmine provides modest benefit in PDD and further long-term studies are needed to determine the effectiveness and safety of rivastigmine over time. Tolerability is a problem for many PDD patients treated with rivastigmine. Future studies of rivastigmine in PDD should focus on pragmatic outcomes such as time to need for nursing home placement, pharmacoeconomic outcomes and simultaneous patient/caregiver quality of life assessments.

Cardiac safety of rivastigmine in Lewy body and Parkinson's disease dementias

Cholinesterase inhibition in patients with Alzheimer's disease (AD) may affect heart rate, sometimes inducing bradycardia. Additional cardiac safety considerations apply in patients with dementia with Lewy bodies (DLB) and Parkinson's disease (PDD), in whom cardiovascular autonomic nervous system dysfunction is common. We conducted a review of the safety data available for rivastigmine in these two conditions. A modest reduction in the mean heart rate of 1.5-2 bpm was seen. No clinically meaningful treatment differences in bradycardia or ECG abnormalities were apparent. Compared with placebo, rivastigmine appeared to be associated with fewer vascular disorder adverse events (AEs) (p = 0.002) and fewer AEs of syncope (p = 0.018) in PDD patients (n = 541). A smaller randomised, placebo-controlled study of rivastigmine in DLB (n = 120) showed similar findings. Rivastigmine appears to have a favourable cardiac safety profile in PDD and DLB patients.

Case of the month: rivastigmine (Exelon) toxicity with evidence of respiratory depression

Rivastigmine, which has been approved by the US Food and Drugs Administration for the treatment of Alzheimer's disease, is a non-competitive reversible inhibitor of acetylcholinesterase. We present a case of rivastigmine toxicity at a dose of 90 mg, with evidence of respiratory depression. To our knowledge, this case report provides evidence of the highest rivastigmine ingestion recorded (90 mg) that caused respiratory depression but requiring only supportive intervention without the need for ralidoxime. Emergency physicians should strongly consider cholinesterase inhibitor (rivastigmine, galantamine, and tacrine) ingestion in patients who present with short and temporary organophosphate-like toxidromes.

Investigation of the binding mode of (−)-meptazinol and bis-meptazinol derivatives on acetylcholinesterase using a molecular docking method

Molecular docking has been performed to investigate the binding mode of (-)-meptazinol (MEP) with acetylcholinesterase (AChE) and to screen bis-meptazinol (bis-MEP) derivatives for preferable synthetic candidates virtually. A reliable and practical docking method for investigation of AChE ligands was established by the comparison of two widely used docking programs, FlexX and GOLD. In our hands, we had more luck using GOLD than FlexX in reproducing the experimental poses of known ligands (RMSD<1.5 A). GOLD fitness values of known ligands were also in good agreement with their activities. In the present GOLD docking protocol, (-)-MEP seemed to bind with the enzyme catalytic site in an open-gate conformation through strong hydrophobic interactions and a hydrogen bond. Virtual screening of a potential candidate compound library suggested that the most promising 15 bis-MEP derivatives on the list were mainly derived from (-)-MEP with conformations of (S,S) and (SR,RS) and with a 2- to 7-carbon linkage. Although there are still no biological results to confirm the predictive power of this method, the current study could provide an alternate tool for structural optimization of (-)-MEP as new AChE inhibitors. [Figure: see text].

A study paradigm allowing comparison of multiple high-resolution rCBV-maps for the examination of drug effects

Owing to the neuro-vascular coupling, measurement of changes in regional cerebral blood flow and blood volume (rCBV) can be used as surrogates reflecting the effects of central nervous system active drugs on neural transmission. As most such drugs are administered orally or intramuscularly and, in many cases, beneficial effects due to drug treatment can be observed only after chronic administration for days or weeks, the evaluation of drug efficacy requires the development of acquisition and analysis tools that allow for comparison of imaging data sets obtained in multiple sessions and for multiple subjects. In the present study, high-resolution susceptibility contrast MR perfusion imaging using a super-paramagnetic contrast agent (CA) was applied to study the effect of a single oral administration of the acetylcholine-esterase inhibitor rivastigmine (Exelon) on rCBV in rats. rCBV maps were calculated from two T2-weighted three-dimensional fast-spin-echo scans recorded before and after the injection of the CA, respectively. All MRI data sets were mapped to a reference data set obtained from a normal male Sprague-Dawley rat using an automated co-registration procedure prior to the analysis for drug effects. Rivastigmine was orally administered at doses of 2, 4 or 8 mg/kg 1 h prior to the rCBV measurement. Rivastigmine increased rCBV in several brain areas including cortex, caudate putamen and hippocampus. The observed effects were dose-dependent and the changes reached the order of 5-12% as compared with baseline levels. Vehicle-treated animals showed no significant alterations of blood volume, demonstrating the reproducibility and stability of rCBV measurements.

Treatments for Alzheimer Disease

As our population ages, the incidence and prevalence of Alzheimer disease (AD) will increase dramatically. A number of therapies have been investigated for the treatment and prevention of AD. Clinicians should be prepared to provide evidence-based answers to inquiries regarding AD treatment. There is insufficient evidence to recommend ginkgo biloba, estrogen, statins, or nonsteroidal anti-inflammatory drugs for the prevention or treatment of AD. The use of vitamin E is supported by a single randomized controlled trial, while data on other antioxidants is mixed. There is good evidence that cholinesterase inhibitors and memantine are modestly effective in the treatment of AD. Cholinesterase inhibitors appear to be effective throughout the spectrum of AD, while memantine, alone or in combination with cholinesterase inhibitors, is effective in late stage disease. There is insufficient evidence to suggest superiority of one cholinesterase over another.

Enhancement of purinergic neurotransmission by galantamine and other acetylcholinesterase inhibitors in the rat vas deferens

Galantamine, a mild acetylcholinesterase inhibitor and an allosteric ligand of nicotinic receptors, enhanced in a concentration-dependent manner the amplitude of purinergic twitch contractions of the electrically stimulated rat vas deferens (0.2 Hz, 1 ms, 60 V). Other acetylcholinesterase inhibitors also increased the twitches, showing a hierarchy of potencies of galantamine>physostigmine>tacrine>rivastigmine=donepezil. The potentiations seem to be unrelated to the ability to inhibit acetylcholinesterase, since the hierarchy of potencies to block the enzyme in vas deferens was tacrine>physostigmine>rivastigmine>donepezil>galantamine. Acetylcholine also increased the twitches; such effect was produced by a low range of concentrations of acetylcholine (10(-10)-10(-7) M). This facilitatory effect of acetylcholine on twitches was significantly potentiated by galantamine (10(-7)-10(-6) M), but not by rivastigmine or donepezil. A striking enhancement of twitches was also caused by charybdotoxin, a blocker of high-conductance Ca2+-activated K+ channels, and by 4-aminopyridine, a non-specific blocker of K+ channels; in addition, apamin, a blocker of small-conductance Ca2+-activated K+ channels, induced a lower potentiation. The antagonist mecamylamine (10(-7)-10(-6) M) reduced by 80% the potentiation by galantamine, indicating the involvement of nicotinic receptors. Therefore, it is suggested that, besides an inhibition of acetylcholinesterase, some additional mechanisms, such as blockade of Ca2+-dependent K+ channels, or activation of nicotinic receptors of nerve terminals, might be involved in twitch potentiation. These results are relevant in the context of the clinical use of galantamine to improve cognition and behaviour in patients with Alzheimer's disease.

Long-lasting acetylcholinesterase splice variations in anticholinesterase-treated Alzheimer's disease patients

Protein levels of different acetylcholinesterase (AChE) splice variants were explored by a combination of immunoblot techniques, using two different antibodies, directed against the C-terminus of the AChE-R splice variant or the core domain common to all variants. Both AChE-R and AChE-S splice variants as well as several heavier AChE complexes were detected in brain homogenates from the parietal cortex of patients with or without Alzheimer's disease (AD) as well as the cerebrospinal fluid (CSF) of AD patients, compatible with the assumption that CSF AChEs might originate from CNS neurons. Long-term changes in the composition of CSF AChE variants were further pursued in AD patients treated with rivastigmine (n = 11) or tacrine (n = 17) in comparison to untreated AD patients (n = 5). In untreated patients, AChE-R was markedly reduced as compared with the baseline level (37%), whereas the medium size AChE-S complex was increased by 32%. Intriguingly, tacrine produced a general and profound up-regulation of all detected AChE variants (up to 117%), whereas rivastigmine treatment caused a mild and selective up-regulation of AChE-R ( approximately 10%, p < 0.05). Moreover, the change in the ratio of AChE-R to AChE-S (R/S-ratio) strongly and positively correlated with sustained cognition at 12 months (p < 0.0001). Thus, evaluation of changes in the composition of CSF AChE variants may yield important information referring to the therapeutic efficacy and/or development of drug tolerance in AD patients treated with anti-cholinesterases.

Novel cognitive improving and neuroprotective activities of Polygala tenuifolia Willdenow extract, BT-11

We carried out this study to search a new active constituent that had cognitive enhancing activity and low side effects from natural source. We found that the extract of dried root of Polygala tenuifolia Willdenow (BT-11, 10 mg/kg, i.p.) could significantly reverse scopolamine-induced cognitive impairments in rat, using a passive avoidance and a water maze test. We also investigated the effects of BT-11 on neurotoxicity induced by glutamate (Glu) and toxic metabolites of amyloid precursor protein (APP) such as amyloid beta protein (A beta) and C-terminal fragment of APP (CT) in primary cultured neurons of rat. The pretreatment of BT-11 (0.5, 3, and 5 micro g/ml) significantly reduced cell death induced by Glu (1 mM), A beta (10 micro M) and CT105 (10 micro M) in a dose-dependent manner. In addition, BT-11 inhibited acetylcholinesterase (AChE) activity in a dose-dependent and non-competitive manner (IC(50) value; 263.7 micro g/ml). Our novel findings suggest the possibility that this extract may have some protective effects against neuronal death and cognitive impairments in Alzheimer's disease (AD), or other neurodegenerative diseases related to excitotoxicity and central cholinergic dysfunction.

Pharmacologic treatments of dementia

The management of dementia patients encompasses pharmacologic, behavioral, and psychosocial intervention strategies. Before pharmacologic intervention is instituted, it is important that sources of excess disability and comorbidity be eliminated or reduced. Identification of comorbid medical and psychiatric conditions, such as depression and delirium, should be identified and appropriately treated. Providing caregivers with education, support, and practical advice is a critical component of the management of the demented patient. The current standard of care for pharmacologic management of the cognitive and functional disabilities of AD consists of the combination of a cholinesterase inhibitor and high-dose vitamin E. This standard is based on the results of large-scale, double-blind, placebo-controlled trials. Cholinesterase inhibitors are the only FDA-approved pharmacologic treatments for AD. Cholinesterase inhibitors have been shown to be effective in the treatment of the cognitive, behavioral, and functional deficits of AD. Large-scale placebo-controlled trials of tacrine, donepezil, rivastigmine, and galantamine have demonstrated moderate benefits in patients with mild to moderate AD. Donepezil, rivastigmine, and galantamine are the first-line choices in the treatment of AD because of their lack of hepatotoxicity, ease of administration, few significant drug-drug interactions, and mild to moderate side effects. There are few contraindications to the use of cholinesterase inhibitors. Known hypersensitivity to a specific drug or its derivatives is the only true contraindication. Cautious administration of cholinesterase inhibitors is advised in patients who have a previous history of allergy or adverse reactions to prior cholinesterase inhibitors, severe liver disease, preexisting bradycardia, peptic ulcer disease, current alcoholism, asthma, or chronic obstructive pulmonary disease. Nausea, vomiting, diarrhea, and anorexia are the most common side effects of cholinesterase inhibitors. These gastrointestinal side effects can be minimized by gradual dose increases, administration with food, adequate hydration, and judicious use of an antiemetic. Vitamin E has been demonstrated to slow the progression of AD in several small and one large placebo-controlled trials. Because of its low cost and safety, it is recommended in addition to a cholinesterase inhibitor for the treatment of AD. There are no FDA-approved treatments for DLB and VaD. One small placebo-controlled trial demonstrated that rivastigmine may be effective in the treatment of DLB. More large-scale placebo-controlled trials are needed to confirm the results of this study. Treatment of VaD focuses on the control, identification, and management of cerebrovascular disease and vascular risk factors. Although there are no peer-reviewed reports on the efficacy of cholinesterase inhibitors for VaD or mixed AD/VaD, early reports suggest that these agents may also be effective for mixed AD/VaD. The indications for the use of cholinesterase inhibitor drugs are eventually likely to broaden to include DLB, mixed AD/VaD, and AD in its more advanced stages.

Electrocardiographic effects of rivastigmine

The electrocardiographic (ECG) effects of rivastigmine treatment were assessed in mild to moderately severe Alzheimer's disease (AD) by analysis of four 26-week, double-blind, multicenter, placebo-controlled, phase III clinical trials. Of an initial 2791 patients, 77% completed treatment. Seventy-one percent required at least one concomitant medication for conditions other than AD, with 34% requiring cardiovascular medications. Safety assessments included ECGs, adverse events, vital signs, and clinical laboratory parameters. Pooled 12-lead ECG data were analyzed by an independent cardiologist blinded to treatment group and clinical information. Heart rate, PR, QRS, and QTc intervals did not differ significantly between treatment and placebo groups. Percentage change from baseline for PR, QRS, and QTc intervals was also no different. In conclusion, rivastigmine appears not to produce adverse effects on cardiac function assessed by ECG.

Structure-based 3D QSAR and design of novel acetylcholinesterase inhibitors

The paper describes the construction, validation and application of a structure-based 3D QSAR model of novel acetylcholinesterase (AChE) inhibitors. Initial use was made of four X-ray structures of AChE complexed with small, non-specific inhibitors to create a model of the binding of recently developed aminopyridazine derivatives. Combined automated and manual docking methods were applied to dock the co-crystallized inhibitors into the binding pocket. Validation of the modelling process was achieved by comparing the predicted enzyme-bound conformation with the known conformation in the X-ray structure. The successful prediction of the binding conformation of the known inhibitors gave confidence that we could use our model to evaluate the binding conformation of the aminopyridazine compounds. The alignment of 42 aminopyridazine compounds derived by the docking procedure was taken as the basis for a 3D QSAR analysis applying the GRID/GOLPE method. A model of high quality was obtained using the GRID water probe, as confirmed by the cross-validation method (q2LOO = 0.937, q2L50%O = 0.910). The validated model, together with the information obtained from the calculated AChE-inhibitor complexes, were considered for the design of novel compounds. Seven designed inhibitors which were synthesized and tested were shown to be highly active. After performing our modelling study the X-ray structure of AChE complexed with donepezil, an inhibitor structurally related to the developed aminopyirdazines, has been made available. The good agreement found between the predicted binding conformation of the aminopyridazines and the one observed for donepezil in the crystal structure further supports our developed model.

Review of rivastigmine and its clinical applications in Alzheimer's disease and related disorders

Rivastigmine (Exelontrade mark, Novartis) is a novel intermediate-acting reversible and non-competitive carbamate acetylcholinesterase (AChE) that was recently introduced for the treatment of Alzheimer's disease (AD). Preclinical studies have shown that rivastigmine has many similarities to other currently available cholinesterase inhibitors (ChEIs) and some important differences. The drug has been evaluated for this use in two well designed, published, adequately powered, Phase III, 26 week clinical trials that included a total of more than 1500 rivastigmine and 700 placebo recipients. Most of these patients had concomitant disorders that were being treated with numerous other drugs. These studies indicate that rivastigmine (6 - 12 mg/day) usually produces cognitive, global and functional changes that indicate significantly less deterioration than was observed with placebo in patients with mild-to-moderate AD, with higher doses producing more benefits. Rivastigmine is beneficial in all three domains (namely cognition, daily activities and behaviour) of AD. Data on long-term efficacy support continued benefits of rivastigmine beyond 6 months. Rivastigmine is generally well-tolerated with no requirement for routine electrocardiogram (ECG) or blood monitoring. Rivastigmine causes adverse events that are generally those expected from a ChEI and mainly involve gastrointestinal system. They are usually mild-to-moderate, of short duration and responsive to dosage reduction. They occur mostly during the dosage titration phase and decrease during the maintenance phase. Clinically significant drug-drug interactions are unlikely. The consistent efficacy in treating all three domains (cognition, daily functioning and behaviour) and good tolerability, particularly with slow titration, makes rivastigmine a good first-line ChEI therapy for the treatment of AD. Therapeutic dose range is 6 - 12 mg/day. Rivastigmine should be started at 1.5 mg b.i.d. with meals and increased at 2 - 4 week intervals to achieve the highest tolerated dose.

Review of the next generation of Alzheimer's disease therapeutics: challenges for drug development

1. AD is believed to stem from dysfunctional cholinergic signaling in the regions of the brain associated with memory and cognition. 2. The occurrence of AD in afflicted individuals correlates with an increase in the accumulation of A beta-rich senile plaques and neurofibrillary tangles in the brain. 3. Currently, the only FDA-approved AD therapies are a group of acetylcholinesterase inhibitors which slow the turnover of the neurotransmitter acetylcholine in the synapse. 4. Many other compounds which target other aspects of the disease, such as reducing neuronal damage and limiting oxidation, are in clinical trials. These include monoamine oxidase (MAO-B) inhibitors, NSAIDs, antioxidants and estrogen, among others. 5. Recent research discoveries have more completely defined the molecular nature of AD, and are generating new approaches for treatment. One idea is to limit the ability of the protein tau to become phosphorylated in hopes that this will limit the formation of neurofibrillary tangles in the brain. 6. A separate approach that is being pursued is to prevent formation and accumulation of A beta plaques. This may be accomplished by either regulating gamma-secretase activity, or using anti-beta-amyloid antibodies to reduce the size of existing plaques. 7. Employing improved procedural and technological approaches during clinical trials, such as bridging studies, dynabridge studies and PET analysis, promises to streamline the drug development process. 8. The use of biomarkers and MRI analysis may be an effective means by which to identify the disease early. Consequently, early intervention treatment therapies may be an effective way of delaying onset of the disease. 9. Long term AD studies, particularly those focusing on the MCI population, are likely to provide statistically valid results using a smaller study population.

Rivastigmine, a new-generation cholinesterase inhibitor for the treatment of Alzheimer's disease

Rivastigmine is a cholinesterase inhibitor (ChEI) with a structural formula different from that of currently available ChEIs. Tacrine and donepezil are classified as short-acting or reversible agents since binding to acetylcholinesterase enzyme (AChE) is hydrolyzed within minutes. Rivastigmine is classified as an intermediate-acting or pseudo-irreversible agent due to its long inhibition on AChE of up to 10 hours. Preclinical biochemical studies indicated that rivastigmine has central nervous system selectivity over peripheral inhibition. It ameliorated memory impairment in rats with forebrain lesions. The drug is rapidly absorbed orally, with a bioavailability of 0.355 and low protein binding (40%). Its elimination half-life is less than 2 hours, and it is converted to an inactive metabolite at the site of action, bypassing hepatic metabolic pathways. Its disposition essentially is unaltered in patients with renal or hepatic impairment. It also has dose-dependent effects on AChE inhibition. In the two large multicenter clinical trials (total 1324 patients) that used a forced-dosage titration scheme, rivastigmine 6-12 mg/day was superior to placebo on three cognitive and functioning scales (p<0.001). Gastrointestinal symptoms are the most frequently reported adverse events. They occurred mostly during the dosage titration phase and decreased during the maintenance phase. Rivastigmine offers clinicians another therapeutic agent to treat Alzheimer's disease.

Rivastigmine, a brain-region selective acetylcholinesterase inhibitor for treating Alzheimer's disease: review and current status

Alzheimer's disease is, in part, characterised by the loss of neurones in the basal forebrain cholinergic cells that project to the cerebral cortex and hippocampus. These impairments have correlated with the memory loss noted in dementia of the Alzheimer's type. This 'cholinergic hypothesis' has led to the rational design of drugs to enhance or stimulate acetylcholine-mediated neurotransmission. Early acetylcholinesterase inhibitors, such as tacrine and physostigmine, are poorly tolerated and have a short duration of action. Rivastigmine is a centrally-selective acetylcholinesterase inhibitor with a relatively long duration of action and is a 'pseudo-irreversible' cholinesterase inhibitor due to slow dissociation of a carbamoyl derivative from the esteratic site of acetylcholinesterase. Preclinical studies confirmed the central selectivity of the drug and its distribution into the cerebrospinal fluid (CSF). Early studies demonstrated that rivastigmine improved cognition and was relatively well-tolerated at moderate doses. Clinical investigations of rivastigmine administered at doses of 6 - 12 mg/day significantly improved cognition, as measured by the ADAS-Cog score, and activities of daily living, as measured by the Progressive Deterioration Scale. Significant global improvements were also noted as measured by the Clinician's Interview Based Impression of Change that required the use of caregiver information. The most frequent adverse effects noted in clinical trials were consistent with peripheral cholinergic stimulation and included nausea, vomiting, abdominal pain, dizziness and diarrhoea. These effects were dose-related and minimised by slow dose-escalation upon initiation of therapy. Rivastigmine undergoes minimal metabolism by the cytochrome P450 system. As a result, it has few drug interactions. The drug is currently marketed widely in over 60 countries worldwide. In the United States, the drug received 'approvable' status subsequent to the NDA filing, and should be available later this year.

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.

Rivastigmine. A review of its use in Alzheimer's disease

Rivastigmine (SDZ ENA 713) is a carbamylating, long-acting reversible and noncompetitive carbamate acetylcholinesterase inhibitor that is indicated as an oral treatment for patients with mild to moderately severe Alzheimer's disease. The drug has been evaluated for this use in 3 well designed, adequately powered, phase II/III, 26-week clinical trials that included a total of 1479 rivastigmine and 647 placebo recipients. Most of these patients had concomitant disorders that were being treated with numerous other drugs. Individual and pooled results of these trials indicate that rivastigmine 6 to 12 mg/day usually produces cognitive, global and functional changes that indicate significantly less deterioration than was observed with placebo in patients with mild to moderately severe Alzheimer's disease. Individual results of the 2 pivotal trials and pooled analysis also show that, compared with placebo recipients, significantly more rivastigmine 6 to 12 mg/day recipients respond to therapy. Indeed, after 26 weeks of therapy in the 2 pivotal trials, significantly more rivastigmine 6 to 12 mg/day than placebo recipients achieved clinically meaningful improvements as defined by 3 separate response criteria. The lower dosage range of 1 to 4 mg/day was not as effective as 6 to 12 mg/day, as measured using these criteria and other efficacy parameters. Rivastigmine causes adverse events that are generally those expected from an acetylcholinesterase inhibitor. They are usually mild to moderate, of short duration and responsive to dosage reduction. Unpublished data from 3989 patients indicate that rivastigmine and placebo were associated with similar incidences of serious adverse events and changes in laboratory parameters, ECG and cardiorespiratory vital signs. The most common events were gastrointestinal, central and peripheral nervous system and whole body adverse events. However, compared with placebo, rivastigmine more commonly caused adverse events resulting in treatment withdrawal. These events were most frequently gastrointestinal and were more common in women.

CONCLUSION

Rivastigmine is a useful option for the treatment of patients with mild to moderately severe Alzheimer's disease. Although only short term (6- month) comparisons with placebo are available, given the lack of established treatment options it should be considered for first-line use in this population.

Clinical pharmacology of rivastigmine: a new-generation acetylcholinesterase inhibitor for the treatment of Alzheimer's disease

Rivastigmine (ENA 713, or carbamoylatine) is an acetylcholinesterase (AChE) inhibitor with brain-region selectivity and a long duration of action. Both preclinical studies and studies in human volunteers have shown that rivastigmine induces substantially greater inhibition of AChE in the central nervous system (CNS) compartment than in the periphery (40% inhibition of central AChE compared with 10% inhibition of plasma butylcholinesterase in healthy volunteers). Moreover, rivastigmine preferentially inhibits the G1 enzymatic form of AChE, which predominates in the brains of patients with Alzheimer's disease (AD). Evidence from animal studies also suggests that rivastigmine is a more potent inhibitor of AChE in the cortex and hippocampus, the brain regions most affected by AD. Absorption of rivastigmine is rapid and almost complete (>96% of the administered dose). Extensive, saturable first-pass metabolism, however, leads to bioavailability of approximately 35% of the administered dose and nonlinear pharmacokinetics. The principal metabolite of rivastigmine has at least 10-fold lower activity against AChE compared with the parent drug. Rivastigmine is completely metabolized; the major route of elimination of the metabolites is renal. Although patients with AD demonstrate 30% to 50% higher plasma concentrations of rivastigmine and its principal metabolite than do healthy elderly patients, there is no evidence of drug accumulation, which is consistent with rivastigmine's short pharmacokinetic half-life. Distribution of rivastigmine into the CNS is extensive, and inhibition of AChE in the cerebrospinal fluid is detectable 1.2 hours after oral dosing in both healthy volunteers and patients with AD. Peak activity is reached somewhat more slowly in AD patients than in healthy subjects, and the inhibitory effects have a longer duration (6.0 vs 2.4 hours and 12.0 vs 8.5 hours, respectively). Rivastigmine is inactivated during the process of interacting with and inhibiting AChE, and, in contrast to other AChE inhibitors, the hepatic cytochrome P-450 (CYP-450) system is not involved in the metabolism of rivastigmine. This reduces its propensity to interact with drugs metabolized by specific CYP-450 isoenzymes. Consistent with rivastigmine's pharmacokinetic and pharmacodynamic profiles, Phase II and III trials have demonstrated that the drug is a well-tolerated and effective treatment for AD.

Possible role of the cholinergic system and disease models

Memory impairment associated with the loss of cortical cholinergic neurons in AD has stimulated the development of animal models based on blockade or destruction of these systems. Strategies include mechanical lesions, local injection of excitotoxic amino acids or ethylcholine aziridinium (AF 64A), which disrupt reference and working memory in rats, but lack specificity for cholinergic systems. Other models involving, reduction in cerebral blood flow and interference with oxidative metabolism of glucose, mimic those found in AD, and also interfere with working and long-term memory in the rat. Memory impairments can be reversed by acetylcholinesterase inhibitors and cholinergic agonists but beneficial effects of these agents in AD patients are small and inconsistent. This may be partly due to unfavorable pharmacokinetics and dose-limiting side effects of existing drugs. Newer, brain specific acetylcholinesterase inhibitors and M1 muscarinic agonists with a lower incidence of unwanted effects are currently being evaluated.

Engineering of human cholinesterases explains and predicts diverse consequences of administration of various drugs and poisons

The acetylcholine hydrolyzing enzyme, acetylcholinesterase, primarily functions in nerve conduction, yet it appears in several guises, due to tissue-specific expression, alternative mRNA splicing and variable aggregation modes. The closely related enzyme, butyrylcholinesterase, most likely serves as a scavenger of toxins to protect acetylcholine binding proteins. One or both of the cholinesterases probably also plays a non-catalytic role(s) as a surface element on cells to direct intercellular interactions. The two enzymes are subject to inhibition by a wide variety of synthetic (e.g., organophosphorus and carbamate insecticides) and natural (e.g., glycoalkaloids) anticholinesterases that can compromise these functions. Butyrylcholinesterase may function, as well, to degrade several drugs of interest, notably aspirin, cocaine and cocaine-like local anesthetics. The widespread occurrence of butyrylcholinesterase mutants with modified activity further complicates this picture, in ways that are only now being dissected through the use of site-directed mutagenesis and heterologous expression of recombinant cholinesterases.

Brain selective inhibition of acetylcholinesterase: a novel approach to therapy for Alzheimer's disease

It could be argued that clinical experience with cholinergic drugs in the therapy of AD has not yet shown relevant symptomatic improvements. The main reasons for this might be attributed to peripheral cholinergic effects and the liver toxicity of some of these drugs, which limit their use and prevent confirmation of the cholinergic hypothesis (Gray et al., 1989). The main disadvantages of the cholinesterase inhibitors used in clinical trials are the short duration of action in the case of physostigmine and the potential for liver toxicity seen with the aminoacridine derivatives. The results presented with SDZ ENA 713 indicate that the disadvantages of AChE inhibitors might be overcome by improving CNS selectivity and thereby decreasing the peripheral cholinergic effects and toxicity. Clinico-pharmacological studies with SDZ ENA 713 have been performed in healthy volunteers; while central activity was clearly demonstrated in an EEG-sleep study (Holsboer et al., 1992), no prohibitive peripheral side effects were seen, confirming in humans the results obtained in experimental animals (Enz et al., 1991). A multicentre clinical investigation in AD patients has been performed in Europe and is currently being evaluated.