Direct determination of E2020 enantiomers in plasma by liquid chromatography-mass spectrometry and column-switching techniques

Influence of ligand geometry on cholinesterase enzyme - A comparison of 1-isoindolinone based structural analog with Donepezil

Donepezil (DNPZ) is one of the few FDA-approved widely used medication in the clinical care of Alzheimer's disease (AD) patients. To investigate the effect of geometry and to find the significance of an enol form if any in DNPZ on acetylcholinesterase (AChE) inhibition, we changed the tetrahedral geometry of DNPZ to planar trigonal pyramidal geometry by replacing the α-carbon atom next to ketone functionality with a nitrogen atom. To mimic 1-indanone in DNPZ, we selected 1-isoindolinone framework to synthesize 25 new DNPZ derivatives and characterized using ¹H NMR, ¹³C NMR and ESI-MS spectroscopy methods. Drug likeliness profile for each compound was predicted using Molinspiration online software following Lipinski's rule. Commercially available assay kits were used to measure AChE and butyrylcholinesterase (BuChE) inhibitory effects. NIH/3T3 mouse embryonic fibroblast cell line was used to measure cytotoxic and proliferation effects using LDH and MTT assay, respectively. Compound #20 was selected for comparative computational docking, modelling and physicochemical studies. Our results show that DNPZ with tetrahedral geometry has 3-fold higher AChE inhibition as compared to compound #20 with planar trigonal pyramidal geometry. Our approach may be useful as a novel indirect method to study the significance of the enol form in DNPZ (or similar compounds), since constant interconversion between the keto and enol forms does not permit a direct determination of the effect of the enol form of DNPZ in vivo. Overall, we conclude that the tetrahedral is a better fit and any change in geometry significantly drives down the cholinesterase inhibitory effect of DNPZ.

Synthetic Approaches for Piperidone-Based Templates as Scaffolds to Access Chirally Enriched Donepezil Analogues

A concise and high-yielding double aza-Michael reaction is presented as an atom-efficient method to access chiral 2-substituted 4-piperidone building blocks from divinyl ketones. The piperidones were further converted into analogues of donepezil, an acetylcholinesterase inhibiting drug used in the treatment of Alzheimer's disease. The donepezil analogues were obtained as inseparable diastereomeric mixtures with resolved stereochemistry in position 2 of the piperidine ring. Biological evaluation of the acetylcholinesterase inhibition by these analogues provides a new insight into structure-activity relationship studies with regard to donepezil's piperidine moiety toward stereochemical enhancement.

Classics in Chemical Neuroscience: Donepezil

The discovery of acetylcholine and acetylcholinesterase provided the first insight into the intricacies of chemical signal transduction and neuronal communication. Further elucidation of the underlying mechanisms led to an attendant leveraging of this knowledge via the synthesis of new therapeutics designed to control aberrant biochemical processes. The central role of the cholinergic system within human memory and learning, as well as its implication in Alzheimer's disease, has made it a point of focus within the neuropharmacology and medicinal chemistry communities. This review is focused on donepezil and covers the background, synthetic routes, structure-activity relationships, binding interactions with acetylcholinesterase, pharmacokinetics and metabolism, efficacy, adverse effects, and historical importance of this leading therapeutic in the treatment of Alzheimer's disease and true Classic in Chemical Neuroscience.

Targeting Alzheimer's disease by investigating previously unexplored chemical space surrounding the cholinesterase inhibitor donepezil

A series of twenty seven acetylcholinesterase inhibitors, as potential agents for the treatment of Alzheimer's disease, were designed and synthesised based upon previously unexplored chemical space surrounding the molecular skeleton of the drug donepezil, which is currently used for the management of mild to severe Alzheimer's disease. Two series of analogues were prepared, the first looking at the replacement of the piperidine ring in donepezil with different sized saturated N-containing ring systems and the second looking at the introduction of different linkers between the indanone and piperidine rings in donepezil. The most active analogue 5,6-dimethoxy-1-oxo-2,3-dihydro-1H-inden-2-yl 1-benzylpiperidine-4-carboxylate (67) afforded an in vitro IC₅₀ value of 0.03 ± 0.07 μM against acetylcholinesterase with no cytotoxicity observed (IC₅₀ of >100 μM, SH-SY5Y cell line). In comparison donepezil had an IC₅₀ of 0.05 ± 0.06 μM and an observed cytotoxicity IC₅₀ of 15.54 ± 1.12 μM. Molecular modelling showed a strong correlation between activity and in silico binding in the active site of acetylcholinesterase.

Synthesis of fluorinated donepezil by palladium-catalyzed decarboxylative allylation of α-fluoro-β-keto ester with tri-substituted heterocyclic alkene and the self-disproportionation of its enantiomers

Catalytic asymmetric synthesis of fluorinated donepezil, a promising new therapeutic agent for Alzheimer's disease, was achieved.

Review of the validated HPLC and LC-MS/MS methods for determination of drugs used in clinical practice for Alzheimer's disease

Alzheimer's disease (AD) is a neurological disorder and is the most frequent type of dementia among elderly people. Donepezil, rivastigmine, galantamine, tacrine and memantine are the US Food and Drug Administration approved oral drugs used in the treatment of AD. Quantitation of these drugs in various biological matrices and monitoring them in long-term treatment is essential to titer the dose of these drugs and ensure patient compliance. This review provides a comprehensive account of various HPLC and LC-MS/MS assays, which have been successfully employed to measure the drug levels in various biological matrices arising from preclinical and clinical studies. In addition, this review collates various considerations such as internal standard selection, extraction schemes, matrix effect, selectivity evaluation and optimization of mass spectrometric conditions to enable the development of sound bioanalytical methods for quantitation of Alzheimer's drugs. Overall LC-MS/MS methods have proven to be the choice of bioanalytical method for the quantification of Alzheimer's drugs in both preclinical and clinical studies. In conclusion, important features of LC-MS/MS methodology for Alzheimer's drugs include shortened analysis time, increased throughput, selectivity and lower cost of analysis.

Enantiomer separation of amino acids in immunoaffinity micro LC-MS

Chiral immunoaffinity microbore columns were directly interfaced with MS detection, and the effect of column length and temperature on the enantiomer separation of a number of underivatized aromatic and aliphatic amino acids was investigated utilizing an antibody chiral stationary phase that had been prepared by immobilizing a monoclonal anti-D-amino acid antibody onto silica. The stronger affinity of the antibody towards aromatic and bulky amino acids allowed separation of such analytes in a 0.75 x 150 mm column, while an increase in column length enabled separation of more weakly bound compounds. The strength of interaction between chiral selector and analytes could be modulated conveniently by lowering the temperature. For the first time, simultaneous enantiomer separation of mixtures of amino acids was achieved on antibody-based chiral stationary phases using extracted ion chromatograms.

Quantitative determination of donepezil in human plasma by liquid chromatography/tandem mass spectrometry employing an automated liquid–liquid extraction based on 96-well format plates

An automated high-throughput liquid chromatography/tandem mass spectrometry (LC-MS/MS) method was developed for quantitative determination of donepezil in human plasma. 150 MicroL of plasma samples were placed in 2.2 mL 96-deepwell plates and both donepezil and loratadine (IS) were extracted from human plasma by liquid-liquid extraction (LLE), using hexane as the organic solvent. Robotic liquid handling workstations were employed for all liquid transfer and solution preparation steps and resulted in a short sample preparation time. After vortexing, centrifugation and freezing, the supernatant organic solvent was evaporated and reconstituted in a small volume of reconstitution solution. The method developed, includes a sample analysis performed by reversed phase LC-MS/MS, with positive ion electrospray ionization, using multiple reaction monitoring (MRM). The chromatographic run time was set for 2.0 min with a flow rate of 0.7 mL/min in a C18 analytical column. The method was significantly sensitive, specific, accurate and precise for the determination of donepezil in human plasma and had the shortest run time. The curve was proven to be linear for the concentration range of 0.1-100 ng/mL. After validation, the method was applied to the rapid and reliable quantitative determination of donepezil in a bioequivalence study after per os administration of a 5mg donepezil tablet.

Reduction of carry-over in column-switching HPLC/MS system with automated system washing procedure for highly sensitive direct analysis of donepezil in dog plasma

To solve the problem of carry-over--a persistent chromatographic challenge for bioanalytical assays with highly sensitive detector such as a mass spectrometry (MS)--a new on-line sample pretreatment HPLC/MS system using a column-switching technique was established. This system was designed to reduce carry-over based on a hydrophobic interaction mechanism using a washing function (multi-mobile phase flow system), as well as to remove impurities on-line in a mobile phase for the pretreatment. As a result, a washing function in this system was enabled to reduce carry-over and to remove impurities in a mobile phase by automatic operation. Therefore, concentration levels of donepezil (DH) in dog plasma as low as 10 pg/mL could be determined using this on-line sample pretreatment HPLC/MS system. In addition, method validation results of specificity, linearity, accuracy, precision, lower limit of quantitation (LLOQ), and carry-over demonstrated that this on-line sample pretreatment HPLC/MS system was robust and valid as a practical assay of drugs and metabolites in the biological samples.

Determination of donepezil hydrochloride in human and rat plasma, blood and brain microdialysates by HPLC with a short C30 column

A simple and sensitive HPLC method with fluorescence (FL) detection for determination of donepezil (DP) in plasma and microdialysate samples was developed. A rapid isocratic separation of DP could be achieved by a short C30 column using mobile phases of 25 mM citric acid/50 mM Na2HPO4 (pH 6.0)-CH3CN (73:27%, v/v) containing 3.5 mM sodium 1-octanesulfonate for plasma and H2O-CH3CN-CH3OH (80:17:3%, v/v/v) containing 0.01% acetic acid for microdialysate. The eluate was monitored at 390 nm with an excitation at 325 nm. The detection limits (S/N = 3) of DP for human plasma, rat plasma and rat brain or blood microdialysates were 0.2, 1.0 and 2.1 ng/ml, respectively. Reproducible results could be obtained by using (+/-)-2-[(1-benzyl-piperidine-4-yl)ethyl]-5,6-dimethoxyindan-1-one hydrochloride as an internal standard (IS). The method was successfully applied for monitoring of DP levels in rat plasma, blood and brain microdialysates and patient plasma.

Rapid and sensitive determination of donepezil in human plasma by liquid chromatography/tandem mass spectrometry: application to a pharmacokinetic study

A liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed and validated for the determination of donepezil in human plasma samples. Diphenhydramine was used as the internal standard. The collision-induced transition m/z 380 --> 91 was used to analyze donepezil in selected reaction monitoring mode. The signal intensity of the m/z 380 --> 91 transition was found to relate linearly with donepezil concentrations in plasma from 0.1-20.0 ng/mL. The lower limit of quantification of the LC/MS/MS method was 0.1 ng/mL. The intra- and inter-day precisions were below 10.2% and the accuracy was between -2.3% and +2.8%. The validated LC/MS/MS method was applied to a pharmacokinetic study in which healthy Chinese volunteers each received a single oral dose of 5 mg donepezil hydrochloride. The non-compartmental pharmacokinetic model was used to fit the donepezil plasma concentration-time curve. Maximum plasma concentration was 12.3 +/- 2.73 ng/mL which occurred at 3.50 +/- 1.61 h post-dosing. The apparent elimination half-life and the area under the curve were, respectively, 60.86 +/- 12.05 h and 609.3 +/- 122.2 ng . h/mL. LC/MS/MS is a rapid, sensitive and specific method for determining donepezil in human plasma samples.

Donepezil (E2020): a new acetylcholinesterase inhibitor. Review of its pharmacology, pharmacokinetics, and utility in the treatment of Alzheimer’s disease

Donepezil is an acetylcholinesterase inhibitor under development for the treatment of mild-moderately Alzheimer's disease. In vitro, donepezil is about 10 times more potent than tacrine as an inhibitor of acetylcholinesterase. Donepezil is 500 - 1000 fold selective for acetylcholinesterase over butyrylcholinesterase. In animal models, donepezil produces positive effects on both working memory and long term memory. In man, donepezil is slowly absorbed from the gastrointestinal (GI) tract. The compound has a terminal elimination half-life of 50 - 70 h in young volunteers; in elderly volunteers, the half-life of the compound is extended to over 100 h. Donepezil is extensively metabolised after oral administration. The parent compound is 93% bound to plasma proteins. Results from two clinical trials with donepezil were published. The largest of these trials was a 12 week 161 patient Phase II investigation in the USA. Results from this investigation showed that donepezil produced dose-related improvements, with statistically significant effects occurring at doses of 3 and 5 mg/day. The results published to date suggest that donepezil will be a useful agent in the symptomatic treatment of Alzheimer's disease.

Brain levels and acetylcholinesterase inhibition with galantamine and donepezil in rats, mice, and rabbits

Galantamine is a rather weak acetylcholinesterase (AChE) inhibitor, currently approved for the symptomatic treatment of Alzheimer's disease, with possible additional allosteric potentiating effects at the nicotinic ACh receptor (nAChR). Earlier data from in vitro biochemical tests suggest that donepezil is 40- to 500-fold more potent than galantamine in inhibiting AChE. In this study, both brain levels and Ki values for AChE inhibition for donepezil and galantamine in rat, mouse, and rabbit after subcutaneous application were determined. Clearance of galantamine from the brain is in general faster that donepezil and is faster in rabbits compared to rats and mice. The brain-to-plasma ratio for galantamine and donepezil, respectively, ranges from 1.2 to 1.5 in the rabbit, 3.3 to 5.2 in the mouse, and 6.6 to 13 in the rat. Galantamine doses between 1.5 and 5 mg/kg are appropriate to reach brain levels within the documented optimal allosteric potentiating ligand dose-response. Ki values of brain AChE inhibition for galantamine and donepezil, respectively, are 7.1 and 2.3 microg/g in rats, 8.3 and 0.65 microg/g for mice, and 19.1 and 1.3 microg/g in rabbits. The data also suggest that for a similar degree of brain AChE inhibition, 3-15 times higher galantamine than donepezil doses are needed.

Aromatic amino-acid residues at the active and peripheral anionic sites control the binding of E2020 (AriceptR) to cholinesterases

E2020 (R,S)-1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl)piperidine hydrochloride is a piperidine-based acetylcholinesterase (AChE) inhibitor that was approved for the treatment of Alzheimer's disease in the United States. Structure-activity studies of this class of inhibitors have indicated that both the benzoyl containing functionality and the N-benzylpiperidine moiety are the key features for binding and inhibition of AChE. In the present study, the interaction of E2020 with cholinesterases (ChEs) with known sequence differences, was examined in more detail by measuring the inhibition constants with Torpedo AChE, fetal bovine serum AChE, human butyrylcholinesterase (BChE), and equine BChE. The basis for particular residues conferring selectivity was then confirmed by using site-specific mutants of the implicated residue in two template enzymes. Differences in the reactivity of E2020 toward AChE and BChE (200- to 400-fold) show that residues at the peripheral anionic site such as Asp74(72), Tyr72(70), Tyr124(121), and Trp286(279) in mammalian AChE may be important in the binding of E2020 to AChE. Site-directed mutagenesis studies using mouse AChE showed that these residues contribute to the stabilization energy for the AChE-E2020 complex. However, replacement of Ala277(Trp279) with Trp in human BChE does not affect the binding of E2020 to BChE. Molecular modeling studies suggest that E2020 interacts with the active-site and the peripheral anionic site in AChE, but in the case of BChE, as the gorge is larger, E2020 cannot simultaneously interact at both sites. The observation that the KI value for mutant AChE in which Ala replaced Trp286 is similar to that for wild-type BChE, further confirms our hypothesis.

Optimization of the chiral separation of some 2-arylpropionic acids on an avidin column by modeling a combined response

The enantiomeric separation of some nonsteroidal antiinflammatory drugs was investigated on an avidin column. An experimental design approach (central composite design) was used to evaluate the effects of three method parameters (pH, concentration of organic modifier, and buffer concentration) on the analysis time and the resolution, as well as to model these responses. This revealed that the organic modifier concentration and sometimes the pH are significant parameters to control because of their influence on both analysis time and resolution. Furthermore, the central composite design results were combined in a multicriteria decision-making approach in order to obtain a set of optimal experimental conditions leading to the most desirable compromise between resolution and analysis time.

A new HPLC method to determine Donepezil hydrochloride in tablets

A HPLC stability-indicating assay for Donepezil hydrochloride in tablets was developed and validated. Donepezil hydrochloride is a reversible inhibitor of acetylcholinesterase, indicated for the treatment of mild to moderate dementia of the Alzheimer's type. The HPLC method was performed with a reversed phase C18 column, detection at 268 nm and a mixture of methanol, phosphate buffer 0.02 M and triethylamine (50:50:0.5) as mobile phase. Typical retention time for Donepezil was 9 min. The method was statistically validated for linearity, accuracy, precision and selectivity following ICH recommendations. Due to its simplicity and accuracy, the method can be used for routine quality control analysis.

Analysis and enantioresolution of donepezil by capillary electrophoresis

The analysis of donepezil, a centrally acting acetylcholine esterase inhibitor, is described by a CZE method suitable for applications in pharmaceutical field. A rapid migration of the analyte was obtained under acidic conditions (pH 3.0); with detection wavelength of 320 nm a LOD of 0.8 x 10(-3) mg/ml was provided. Applications on real sample (pharmaceuticals) were carried out using two different instruments with comparable results in terms of reproducibility and accuracy. The use of chiral selectors in the running buffer allowed the enantioseparation of donepezil; charged cyclodextrins (carboxymethyl-beta-cyclodextrin and sulfated-beta-cyclodextrin) were suitable for the chiral resolution of the analyte. Interesting results were also obtained using human serum albumin. The protein-based CE enantioseparation was carried out at pH 7.4 avoiding the partial filling technique due to the good absorptivity of donepezil at 320 nm. Interestingly, the use of bicine as BGE provided a significative improvement in the enantioresolution compared to that obtained by phosphate buffer.

Fully automated determination of eserine N-oxide in human plasma using on-line solid-phase extraction with liquid chromatography coupled with electrospray ionization tandem mass spectrometry

A sensitive and entirely automated solid-phase extraction/liquid chromatography/electrospray ionization tandem mass spectrometric (SPE/LC/ESI-MS/MS) method was developed and validated for the determination of eserine N-oxide (ENO), a cholinesterase inhibitor-like physostigmine in human plasma, for use in pharmacokinetic studies. ENO is light-sensitive and the use of a fully on-line process increased the reliability of the assay. Plasma samples previously mixed with neostigmine bromide to prevent in vitro degradation, and tacrine as internal standard (IS), were directly injected into the SPE/LC/ESI-MS/MS system. MS software piloted the overall system. MS/MS detection of ENO and the IS was performed in the positive ion ESI mode using multiple reaction monitoring. The linear calibration curve for ENO ranged from 25 pg ml(-1) to 12.5 ng ml(-1). The limit of quantitation was 25 pg ml(-1) with 250 microl of plasma injected. Precision, accuracy and stability tests were within the acceptable range and just one analyst is required to analyze 50 unknown samples a day five days per week, from the preparation of the samples (i.e. thawing and centrifugation) to data processing. A pilot pharmacokinetic study in three healthy volunteers treated with 4.5 mg of ENO (Génésérine3((R))) showed that the method was suitable for pharmacokinetic studies in humans.

Simultaneous determination of donepezil (aricept) enantiomers in human plasma by liquid chromatography-electrospray tandem mass spectrometry

A rapid, sensitive and enantioselective LC-MS-MS method using deuterium-labeled internal standard was developed and evaluated for the simultaneous quantitative determination of donepezil enantiomers in human plasma without interconversion during clean-up process and measurement. The use of an avidin column allowed the separation of donepezil enantiomers, which were specifically detected by MS-MS without interference from its metabolites and plasma constituents. Evaluation of this assay method shows that samples can be assayed with acceptable accuracy and precision within the range from 0.0206 ng/ml to 51.6 ng/ml for both R-donepezil and S-donepezil. This analytical method was applied to the simultaneous quantitation of donepezil enantiomers in human plasma.

Three-dimensional structure of a complex of E2020 with acetylcholinesterase from Torpedo californica

The 3D structure of a complex of the anti-Alzheimer drug, E2020, also known as Aricept, with Torpedo californica acetylcholinesterase is reported. The X-ray structure, at 2.5 A resolution, shows that the elongated E2020 molecule spans the entire length of the active-site gorge of the enzyme. It thus interacts with both the 'anionic' subsite, at the bottom of the gorge, and with the peripheral anionic site, near its entrance, via aromatic stacking interactions with conserved aromatic residues. It does not interact directly with either the catalytic triad or with the 'oxyanion hole'. Although E2020 is a chiral molecule, and both the S and R enantiomers have similar affinity for the enzyme, only the R enantiomer is bound within the active-site gorge when the racemate is soaked into the crystal. The selectivity of E2020 for acetylcholinesterase, relative to butyrylcholinesterase, can be ascribed primarily to its interactions with Trp279 and Phe330, which are absent in the latter.

Mapping of peptides and protein fragments in human urine using liquid chromatography-mass spectrometry

A method for the mapping of peptide mixtures, heterogeneous with respect to the concentration and the size of individual peptides, was established with the aim of obtaining a comprehensive analysis of human urine peptides. Peptide extraction and fractionation were optimized to achieve a two-step analysis, using reversed-phase and ion-exchange chromatography. Highly sensitive detection of peptides was performed by coupling microbore HPLC with electrospray mass spectrometry (ESI-MS). Peptides such as urodilatin, angiotensin and fragments of psoriasin, granulin and uromodulin were isolated and sequenced. The procedure presented here is a tool for the analysis of complex peptide mixtures from human urine.

Quantitative determination of a nonpeptide antithrombotic in dog plasma by microbore high-performance liquid chromatography-tandem mass spectrometry utilizing pneumatically assisted electrospray ionization

A method has been developed and is described for the quantitative determination of a nonpeptide antithrombotic in dog plasma. The assay employs reversed phase microbore high-performance liquid chromatography in conjunction with tandem mass spectrometry utilizing pneumatically assisted electrospray ionization. The analyte and internal standard are isolated from the plasma matrix by solid-phase extraction. The mass spectrometer is operated in the positive ion multiple reaction monitoring mode and is set to detect the presence of a precursor-product ion pair for both the analyte and internal standard to generate product ion chromatograms for both species. The analyte is quantified by using weighted least-squares regression of the peak height ratio of drug:internal standard. The method provides linear response for plasma concentrations ranging from 5 ng/mL (25 pg on-column) to 2500 ng/mL. Statistical evaluation and examples of authentic sample assays are also presented.