Enantiomeric determination of amlodipine in human plasma by liquid chromatography coupled to tandem mass spectrometry

Determination of amlodipine, indapamide and perindopril in human plasma by a novel LC-MS/MS method: Application to a bioequivalence study

A robust and rapid UPLC-MS/MS method has been developed, optimized and validated for determination of amlodipine (AML), indapamide (IND) and perindopril (PRN) in human plasma. A positive electrospray ionization mode was used in a Xevo TQD LC-MS/MS instrument. A single sample preparation step using extraction technique was applied to extract the three analytes from plasma samples. There was no need to extract indapamide from blood samples in a further step. Extraction of the three drugs and internal standards was done using a solvent mixture composed of methyl tertiary butyl ether, dichloromethane and ethyl acetate. The prepared samples were analyzed using an Acquity UPLC HSS C₁₈ (100 × 2.1 mm, 1.7 μm) column. Ammonium acetate and methanol, pumped at a flow rate of 0.3 ml/min, were used as a mobile phase. Method validation was done as per the US Food and Drug Administration guidelines. Linearity was achieved in the range of 0.2-15 ng/ml for AML, 0.5-50 ng/ml for IND and 0.5-120 ng/ml for PRN. Accuracy and precision were estimated and found to be within the acceptable ranges. The rapid chromatography permits analysis of many samples per batch, making the method suitable for clinical and pharmacokinetic investigations. The developed and validated method was applied to estimate AML, IND, and PRN in a fasting bioequivalence study in healthy human volunteers.

A simple sonochemical assisted synthesis of NiMoO4/chitosan nanocomposite for electrochemical sensing of amlodipine in pharmaceutical and serum samples

In this investigation, a facile sonochemical route has been developed for the preparation of porous nickel molybdate nanosheets/chitosan nanocomposite (NiMoO₄/CHIT) by using ammonium molybdate and nickel(II) acetate tetrahydrate and as nickel and molybdate precursor, respectively (ultrasonic power 60 W/cm² and frequency 20 kHz). The ultrasonic based materials preparation as a fast, convenient and economical approach has been widely used to generate novel nanomaterials. Herein, we report an efficient voltammetric sensor for amlodipine drug by using porous nickel molybdate nanosheets/chitosan nanocomposite (NiMoO₄/CHIT). Its structure and properties were characterized by x-ray diffraction pattern, scanning electron microscope, transmission electron microscope, elemental analysis and mapping. The electrochemical studies are indicated the NiMoO₄/CHIT modified glassy carbon electrode (GCE) exhibited the good performance towards electrocatalytic sensing of amlodipine drug. Consequently, a linear correlation between the anodic peak current with sensor concentration 0.025-373.6 µM with a detection limit and sensitivity of 4.62 nM and 4.753 µA·µM^-1·cm^-2, respectively. A voltammetry based drug analysis was found to be high sensitive and reproducible, which able to detect nanomolar concentration. Furthermore, the fabricated electrochemical sensor was applied in drug and biological samples.

Validated LC-MS/MS method for the determination of amlodipine enantiomers in rat plasma and its application to a stereoselective pharmacokinetic study

A rapid and sensitive method was established to determine amlodipine enantiomers using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Stereoselective separation was performed on CHIRALCEL OZ-RH column (150 mm × 4.6 mm i.d., 5 μm) with acetonitrile-water (10 mM ammonium acetate, 0.5% ammonia solution) (95:5, v/v) at a flow rate of 0.5 mL/min. The substances were detected by mass spectrometer equipped with an electrospray ionization source interface in positive ion mode. Multiple reaction monitoring was selected with the transition of the m/z 409.1 → 238.0 for amlodipine enantiomers and m/z 237.0 → 194.1 for carbamazepine (IS) respectively. Calibration curves were linear at the range of 0.9375-120 ng/mL for both isomers with r > 0.99, while using a lower sample volume (50 μL) compared with previously reported enantiospecific methods The accuracy was at the range of 84.1-119.0% for R-amlodipine, and 87.4-118.2% for S-amlodipine, respectively. The within- and between-run precision (CV%) was within 10% in all cases for both enantiomers. Enantiomers were stable under different conditions, e.g. processed sample, short-term, residue, long-term and freeze/thaw. The LC-MS/MS method was successfully applied in pharmacokinetic study of amlodipine enantiomers in rats. It was observed the concentration of the S- amlodipine was significantly higher than that of the R-amlodipine in racemate-treated group. And there was no significant difference in the pharmacokinetic profiles of the S-amlodipine between the 10 mg/kg racemate- and 5 mg/kg S-amlodipine-treated groups. In addition, it was the first time to find that the main pharmacokinetic parameters (AUC_(0-t), AUC_(0-∞) and C_max) of R-amlodipine were significantly lower in the 5 mg/kg R-amlodipine-treated group compared with the racemate-treated group.

Simultaneous determination of rosuvastatin and amlodipine in human plasma using tandem mass spectrometry: Application to disposition kinetics

The liquid chromatography–tandem mass spectrometric assay method for the simultaneous determination of rosuvastatin and amlodipine in human plasma using deuterated analogs as internal standards has been developed and validated. The analytes were extracted from 100 μL aliquots of human plasma via liquid–liquid extraction using a mixture of ethyl acetate and n-hexane (80:20, v/v) as an extraction solvent. The optimized mobile phase was composed of 0.1% formic acid in 5 mM ammonium acetate, methanol, and acetonitrile (20:20:60, v/v/v) and delivered at a flow rate of 0.75 mL/min. The calibration curve obtained was linear (R² ⩾ 0.999) over the concentration range of 0.52–51.77 ng/mL for rosuvastatin and 0.10–10.07 ng/mL for amlodipine. A sample turnover rate of less than 2.5 min makes it an attractive procedure in high-throughput bioanalysis of rosuvastatin and amlodipine. The present method was found to be applicable to clinical studies and the results were authenticated by incurred sample reanalysis.

Simultaneous Quantitative Analysis of Olmesartan Medoxomil and Amlodipine Besylate in Plasma by High-performance Liquid Chromatography Technique

A rapid, simple and sensitive high-performance liquid chromatography (HPLC) method has been developed for quantification of olmesartan medoxomil (OLM) and amlodipine besylate (AM) in plasma. The assay enables the measurement of OLM and AM for therapeutic drug monitoring with a minimum detectable limit of 2 ng mL. The method involves a simple, one-step extraction procedure and analytical recovery was above 50%. The separation was performed on an analytical 250 × 4.6 mm Eurospher 100^-5 C18 column. The wavelength was set at 239 nm. The mobile phase was a mixture of acetonitrile:0.05 M ammonium acetate buffer: 0.1 mL triethylamine at pH 6.8 was selected at a flow rate of 1.0 mL min. The calibration curve for the determination of OLM and AM in plasma was linear over the range 2–2500 and 8–10,000 ng mL AM and OLM. The coefficients of variation for interday and intraday assay were found to be <15%. The method can be applied to a pharmacokinetic and pharmacodynamic study of OLM and AM in a combined dosage form.

Simultaneous determination of losartan, losartan acid and amlodipine in human plasma by LC-MS/MS and its application to a human pharmacokinetic study

INTRODUCTION

A simple, rapid and sensitive liquid chromatography-tandem mass spectrometric assay method has been developed and fully validated for simultaneous quantification of losartan and its active metabolite, losartan carboxylic acid, and amlodipine in human plasma. Irbesartan was used as an internal standard.

MATERIALS AND METHODS

The analytes were extracted from human plasma samples by solid-phase extraction technique using Oasis HLB cartridges, (Waters Corporation, Mumbai, India). The reconstituted samples were chromatographed on a C18 column by using an 85:15, v/v mixture of methanol and 0.1% v/v formic acid as the mobile phase at a flow rate of 1.0 mL/min. A detailed validation of the method was performed as per the FDA guidelines.

RESULTS

The calibration curves obtained were linear (r ≥ 0.99) over the concentration range of 0.5-1000 ng/mL for losartan and for its active metabolite losartan acid and 0.05-10.1 ng/mL for amlodipine. The results of the intra- and inter-day precision and accuracy studies were well within the acceptable limits.

CONCLUSIONS

A run time of 2.5 min for each sample made it possible to analyze more than 300 plasma samples per day. The proposed method was found to be applicable to clinical studies.

Glucose-β-CD interaction assisted ACN field-amplified sample stacking in CZE for determination of trace amlodipine in beagle dog plasma

A simple, sensitive and low-cost method using CE coupled with glucose-β-CD interaction assisted ACN stacking technique has been developed for quantification of trace amlodipine in dog plasma. The plasma samples were extracted with methyl tert-butyl ether. The separation was performed at 25°C in a 31.2 cm × 75 μm fused-silica capillary with an applied voltage of 15 kV. The BGE was composed of 6.25 mM borate/25 mM phosphate (pH 2.5) and 5 mg/mL glucose-β-CD. The detection wavelength was 200 nm. Because CD could diminish the interaction between drugs and matrix, and derivation groups of CD play an important role in separation performance, the effects of β-CD, and its derivatives on the separation were studied at several concentrations (0, 2.5, 5.0, 10.0 mg/mL). In this study, organic solvent field-amplified sample stacking technique in combination with glucose-β-CD enhanced the sensitivity about 60-70 folds and glucose-β-CD could effectively improve the peak shape. All the validation data, such as accuracy, precision extraction recovery, and stability, were within the required limits. The calibration curve was linear for amlodipine from 1 to 200 ng/mL. The method developed was successfully applied to the pharmacokinetic studies of amlodipine besylate in beagle dogs.

Pharmacokinetic interaction of telmisartan with s-amlodipine: an open-label, two-period crossover study in healthy Korean male volunteers

BACKGROUND

Telmisartan belongs to a class of orally active angiotensin II receptor blockers (ARBs), and S-amlodipine is an enantiomer of amlodipine. Amlodipine is a racemic mixture and the calcium channel blocking (CCB) effect is confined to S-amlodipine, whereas R-amlodipine has a 1000-fold lower activity and no racemization occurs in vivo in human plasma. Combination therapy of ARBs with CCBs provides advantages for blood pressure control and vascular protection over monotherapy.

OBJECTIVE

To investigate the effects of coadministration of telmisartan and S-amlodipine on the steady-state pharmacokinetic properties of each drug as a drug-drug interaction study required before developing the fixed-dose combination agent.

METHODS

This study comprised 2 separate parts, A and B; each was a multiple-dose, open-label, 2-sequence, 2-period, crossover study in healthy male Korean volunteers. In part A, volunteers were administered 80 mg of telmisartan, either alone or with 5 mg of S-amlodipine. In part B, volunteers were administered 5 mg of S-amlodipine, either alone or with 80 mg of telmisartan. Blood samples were taken on days 9 and 37, following the final dose of each treatment, and at 0 (predose), 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, and 24 hours after administration in part A, and were taken at 0 (predose), 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, and 24 hours after administration in part B. Plasma concentrations were determined using LC-MS/MS. The pharmacokinetic properties of each drug after coadministration of telmisartan and S-amlodipine were compared with those of each drug administered alone. Tolerability was assessed using measurements of vital signs, clinical chemistry tests, and interviews.

RESULTS

Fifty-six volunteers were enrolled (32 in part A and 24 in part B), and all completed except 4 volunteers (3 withdrawn in part A and 1 withdrawn in part B). The geometric mean ratios (GMRs) (90% CI) for the C(max,ss) and AUC(τ,ss) of telmisartan (with or without S-amlodipine) were 1.039 (0.881-1.226) and 1.003 (0.926-1.087), respectively. The GMRs (90% CI) for C(max,ss) and AUC(τ,ss) of S-amlodipine (with or without telmisartan) were 0.973 (0.880-1.076) and 0.987 (0.897-1.085). Total 11 adverse events (AEs) were reported in 7 volunteers (21.9%) in part A. A total of 9 AEs were reported in 6 volunteers (25.0%) in part B. Statistical analysis confirmed that the 90% CIs for these pharmacokinetic parameters were within the commonly accepted bioequivalence range of 0.8 to 1.25, indicating that the extent of bioavailability of S-amlodipine was not affected by telmisartan. The intensity of all AEs was considered to be mild, and there were no significant differences in the prevalences of AEs between the 2 formulations.

CONCLUSIONS

Following multiple-dose coadministration of high doses of telmisartan and S-amlodipine, the steady-state pharmacokinetic properties of telmisartan were not significantly affected, and telmisartan had no significant effect on the pharmacokinetic properties of S-amlodipine at steady state in these selected groups of healthy volunteers. Both formulations were generally well-tolerated.

Electro membrane extraction combined with capillary electrophoresis for the determination of amlodipine enantiomers in biological samples

Electro membrane extraction as a new microextraction method was applied for the extraction of amlodipine (AM) enantiomers from biological samples. During the extraction time of 15  min, AM enantiomers migrated from a 3  mL sample solution, through a supported liquid membrane into a 20  μL acceptor solution presented inside the lumen of the hollow fiber. The driving force of the extraction was 200  V potential, with the negative electrode in the acceptor solution and the positive electrode in the sample solution. 2-Nitro phenyl octylether was used as the supported liquid membrane. Using 10  mM HCl as background electrolyte in the sample and acceptor solution, enrichment up to 124 times was achieved. Then, the extract was analyzed using CD modified CE method for separation of AM enantiomers. Best results were achieved using a phosphate running buffer (100  mM, pH 2.0) containing 5  mM hydroxypropyl-α-CD. The range of quantitation for both enantiomers was 10-500  ng/mL. Intra- and interday RSD (n=6) were less than 14%. The limits of quantitation and detection for both enantiomers were 10 and 3  ng/mL respectively. Finally, this procedure was applied to determine the concentration of AM enantiomers in plasma and urine samples.

Enantioselective quantification of chiral drugs in human plasma with LC–MS/MS

Today, approximately 60% of synthetic drugs are chiral and 88% of these chiral synthetic drugs are used therapeutically as racemates. However, for many racemic drugs, their stereospecific plasma pharmacokinetics in humans are not known due to the limitations of the analytical methods. Nowadays, liquid chromatography (LC)–tandem mass spectrometry (MS/MS) methods based on various chiral stationary phases (CSPs), with a high degree of specificity and sensitivity, have been widely used in enantioselective determination of chiral drugs and/or their metabolites in human plasma. The technologies and issues when coupling chiral chromatography with MS/MS detection in bioanalytical methods will be reviewed herein. The introduction and applications of various CPSs, including polysaccharide-, macrocyclic glycopeptide-, protein- and cyclodextrin-based phases, are described here. This review also includes a discussion of interface and matrix effects in enantioselective LC–MS/MS methods.

Enantioselective determination of azelnidipine in human plasma using liquid chromatography–tandem mass spectrometry

A sensitive and simple method was developed for determination of the enantiomers of azelnidipine, (R)-(-)-azelnidipine and (S)-(+)-azelnidipine, in human plasma using chiral liquid chromatography with positive ion atmospheric pressure chemical ionization tandem mass spectrometry. Plasma samples spiked with stable isotope-labeled azelnidipine, [(2)H(6)]-azelnidipine, as an internal standard, were processed for analysis using a solid-phase extraction in a 96-well plate format. The azelnidipine enantiomers were separated on a chiral column containing alpha(1)-acid glycoprotein as a chiral selector under isocratic mobile phase conditions. Acquisition of mass spectrometric data was performed in multiple reaction monitoring mode, monitoring the transitions from m/z 583-->167 for (R)-(-)-azelnidipine and (S)-(+)-azelnidipine, and from m/z 589-->167 for [(2)H(6)]-azelnidipine. The standard curve was linear over the studied range (0.05-20 ng/mL), with r(2)>0.997 using weighted (1/x(2)) quadratic regression, and the chromatographic run time was 5.0 min/injection. The intra- and inter-assay precision (coefficient of variation), calculated from the assay data of the quality control samples, was 1.2-8.2% and 2.4-5.8% for (R)-(-)-azelnidipine and (S)-(+)-azelnidipine, respectively. The accuracy was 101.2-117.0% for (R)-(-)-azelnidipine and 100.0-107.0% for (S)-(+)-azelnidipine. The overall recoveries for (R)-(-)-azelnidipine and (S)-(+)-azelnidipine were 71.4-79.7% and 71.7-84.2%, respectively. The lower limit of quantification for both enantiomers was 0.05 ng/mL using 1.0 mL of plasma. All the analytes showed acceptable short-term, long-term, auto-sampler and stock solution stability. Furthermore, the method described above was used to separately measure the concentrations of the azelnidipine enantiomers in plasma samples collected from healthy subjects who had received a single oral dose of 16 mg of azelnidipine.

Characterization of benidipine and its enantiomers' metabolism by human liver cytochrome P450 enzymes

Benidipine is a dihydropyridine calcium antagonist that has been used clinically as an antihypertensive and antianginal agent. It is used clinically as a racemate, containing the (-)-alpha and (+)-alpha isomers of benidipine. This study was performed to elucidate the metabolism of benidipine and its enantiomers in human liver microsomes (HLMs) and to characterize the cytochrome P450 (P450) enzymes that are involved in the metabolism of benidipine. Human liver microsomal incubation of benidipine in the presence of NADPH resulted in the formation of two metabolites, N-desbenzylbenidipine and dehydrobenidipine. The intrinsic clearance (CL(int)) of the formation of N-desbenzylbenidipine and dehydrobenidipine metabolites from (-)-alpha isomer was similar to those from the (+)-alpha isomer (1.9 +/- 0.1 versus 2.3 +/- 2.3 microl/min/pmol P450 and 0.5 +/- 0.2 versus 0.6 +/- 0.6 microl/min/pmol P450, respectively). Correlation analysis between the known P450 enzyme activities and the rate of the formation of benidipine metabolites in the 15 HLMs showed that benidipine metabolism is correlated with CYP3A activity. The P450 isoform-selective inhibition study in liver microsomes and the incubation study of cDNA-expressed enzymes also showed that theN-debenzylation and dehydrogenation of benidipine are mainly mediated by CYP3A4 and CYP3A5. The total CL(int) values of CYP3A4-mediated metabolite formation from (-)-alpha isomer were similar to those from (+)-alpha isomer (17.7 versus 14.4 microl/min/pmol P450, respectively). The total CL(int) values of CYP3A5-mediated metabolite formation from (-)-alpha isomer were also similar to those from (+)-alpha isomer (8.3 versus 11.0 microl/min/pmol P450, respectively). These findings suggest that CYP3A4 and CYP3A5 isoforms are major enzymes contributing to the disposition of benidipine, but stereoselective disposition of benidipine in vivo may be influenced not by stereoselective metabolism but by other factors.

A stability-indicating high performance liquid chromatographic (HPLC) assay for the simultaneous determination of atorvastatin and amlodipine in commercial tablets

A simple, rapid, precise and accurate isocratic reversed-phase stability-indicating HPLC method was developed and validated for the simultaneous determination of atorvastatin (AT) and amlodipine (AM) in commercial tablets. The method has shown adequate separation for AM, AT from their associated main impurities and their degradation products. Separation was achieved on a Perfectsil Target ODS-3, 5 microm, 250 mm x 4.6 mm i.d. column using a mobile phase consisting of acetonitrile-0.025 M NaH(2)PO(4) buffer (pH 4.5) (55:45, v/v) at a flow rate of 1 ml/min and UV detection at 237 nm. The drugs were subjected to oxidation, hydrolysis, photolysis and heat to apply stress conditions. The linearity of the proposed method was investigated in the range of 2-30 microg/ml (r=0.9994) for AT and 1-20 microg/ml (r=0.9993) for AM. The limits of detection were 0.65 microg/ml and 0.35 microg/ml for AT and AM, respectively. The limits of quantitation were 2 microg/ml and 1 microg/ml for AT and AM, respectively. Degradation products produced as a result of stress studies did not interfere with the detection of AT and AM and the assay can thus be considered stability-indicating.

Stability Indicating RP-HPLC Method for Simultaneous Determination of Atorvastatin and Amlodipine from Their Combination Drug Products

The study describes development and subsequent validation of a stability indicating reverse-phase HPLC method for the simultaneous estimation of atorvastatin (ATV), and amlodipine (AML) from their combination drug product. The proposed RP-HPLC method utilizes a Lichrospher 100 C18, 5 microm, 250 mm x 4.0 mm i.d. column, at ambient temperature, optimum mobile phase consisted of acetonitrile and 50 mM potassium dihydrogen phosphate buffer (60 : 40, v/v), apparent pH adjusted to 3+/-0.1 with 10% phosphoric acid solution, effluent flow rate monitored at 1.0 ml/min, and UV detection at 254 nm. ATV, AML, and their combination drug product were exposed to thermal, photolytic, hydrolytic, and oxidative stress conditions, and the stressed samples were analyzed by proposed method. The method was applied for the in vitro dissolution of marketed combination drug products. The described method was linear over the range of 1-90 microg/ml and 1-80 microg/ml for ATV and AML, respectively. The mean recoveries were 99.76 and 98.12% for ATV and AML, respectively. The intermediate precision data obtained under different experimental setup, the calculated value of coefficient of variation (CV, %) was found to be less than critical value. The limit of detection for ATV and AML were found to be 0.4 and 0.6 mug/ml, respectively and the limit of quantification was 1.0 microg/ml for both drugs. The average percentage drug release was found to be more than 70% within 30 min for both drugs. Chromatographic peak purity data of ATV and AML indicated no co-eluting peaks with the main peaks of drugs which demonstrated the specificity of assay method for their estimation in presence of degradation products. The proposed method can be useful in the quality control and in vitro dissolution of combination drug products.

Liquid separation techniques coupled with mass spectrometry for chiral analysis of pharmaceuticals compounds and their metabolites in biological fluids

Determination of the chiral composition of drugs is nowadays a key step in order to determine purity, activity, bioavailability, biodegradation, etc., of pharmaceuticals. In this article, works published for the last 5 years on the analysis of chiral drugs by liquid separation techniques coupled with mass spectrometry are reviewed. Namely, chiral analysis of pharmaceuticals including, e.g., antiinflammatories, antihypertensives, relaxants, etc., by liquid chromatography-mass spectrometry and capillary electrophoresis-mass spectrometry are included. The importance and interest of the analysis of the enantiomers of the active compound and its metabolites in different biological fluids (plasma, urine, cerebrospinal fluid, etc.) are also discussed.

Sensitive and rapid liquid chromatography/tandem mass spectrometry assay for the quantification of amlodipine in human plasma

A simple, sensitive and rapid high-performance liquid chromatography/electrospray ionization tandem mass spectrometry method was developed and validated for the assay of amlodipine in human plasma. Following liquid-liquid extraction, the analytes were separated using an isocratic mobile phase on a reverse-phase C(18) column and analyzed by MS in the multiple reaction monitoring mode using the respective [M+H]+ ions, m/z 409/238 for amlodipine and m/z 409/228 for the IS. The assay exhibited a linear dynamic range of 50-10,000 pg/mL for amlodipine in human plasma. The lower limit of quantification was 50 pg/mL with a relative standard deviation of less than 8%. Acceptable precision and accuracy were obtained for concentrations over the standard curve range. The average absolute recoveries of amlodipine and the IS from spiked plasma samples were 74.7 +/- 4.6 and 72.1 +/- 2.0%, respectively. A run time of 1.5 min for each sample made it possible to analyze more than 400 human plasma samples per day. The validated method has been successfully used to analyze human plasma samples for application in pharmacokinetic, bioavailability or bioequivalence studies. The observed maximum plasma concentration (Cmax) of amlodipine (2.5 mg oral dose) was 1425 pg/mL, time to observed maximum plasma concentration (Tmax) was 8.1 h and elimination half-life (T(1/2)) was 50.1 h.

Validated HPLC method for determination of amlodipine in human plasma and its application to pharmacokinetic studies

A rapid, simple and sensitive high-performance liquid chromatography (HPLC) method has been developed for quantification of amlodipine in plasma. The assay enables the measurement of amlodipine for therapeutic drug monitoring with a minimum detectable limit of 0.2 ng ml(-1). The method involves simple, one-step extraction procedure and analytical recovery was about 97%. The separation was performed on an analytical 125 x 4.6 mm i.d. Nucleosil C8 column. The wavelength was set at 239 nm. The mobile phase was a mixture of 0.01 M sodium dihydrogen phosphate buffer and acetonitrile (63:37, v/v) adjusted to pH 3.5 at a flow rate of 1.5 ml min(-1). The calibration curve was linear over the concentration range 0.5-16 ng ml(-1). The coefficients of variation for inter-day and intra-day assay were found to be less than 10%.

Development and validation of a selective and robust LC-MS/MS method for quantifying amlodipine in human plasma

A liquid chromatographic-tandem mass spectrometric method (LC-MS/MS) for quantifying amlodipine in human plasma was developed and validated. Sample preparation was based on liquid-liquid extraction using NaOH and a mixture of ethyl acetate/hexane (80/20; v/v). Chromatography was performed on a C-18 analytical column and the retention times were 1.9 and 3.0 min for amlodipine and nimodipine (internal standard), respectively. The ionization was optimized using ESI(+) and enhanced selectivity was achieved using tandem mass spectrometric analysis via two MRM functions, 409 --> 238 and 418 --> 343 for amlodipine and nimodipine. The calibration curve ranged from 0.2 to 20.0 ng/mL. The inter-day precision and accuracy and the relative standard deviation (RSD) were <15%. The analyte was shown to be stable over the time-scale of the whole procedure. The robustness of the method was demonstrated by the good reproducibility of the results obtained during the analysis of clinical samples.

Simple and rapid HPLC method for determination of amlodipine in human serum with fluorescence detection and its use in pharmacokinetic studies

A fast, sensitive and specific high performance liquid chromatographic method using fluorescence detection is described for analysis of amlodipine in human serum. Amlodipine is extracted from serum by ethyl acetate and involves precolumn derivatization with 4-chloro-7-nitrobenzofurazan (NBD-Cl) and reverse-phase chromatography on C18 column. The mobile phase was sodium phosphate buffer (pH 2.5) containing 1 ml/l triethylamine and methanol at flow rate of 2.8 ml/min. Propranolol was used as internal standard. The standard curve was linear over the range 0.25-16 ng/ml of amlodipine in human serum. The within-day and between-day precision studies showed good reproducibility with coefficients of variation less than 12% for all the analytes. The limit of quantification was 0.25 ng/ml of serum. The method has been applied to a bioequivalence study after administration of 10 mg amlodipine in 12 normal subjects.

Simultaneous quantification of the enantiomers of verapamil and its N-demethylated metabolite in human plasma using liquid chromatography-tandem mass spectrometry

A stereoselective bioanalytical method for the simultaneous quantification of the enantiomers of verapamil and its active main metabolite norverapamil in human plasma has been developed and validated. The samples were analysed by liquid chromatography-electrospray-tandem mass spectrometry (LC-ESI-MS/MS) in the Selected Reaction Monitoring (SRM) mode using a deuterated internal standard. The stationary phase used for the chiral separation was a Chiral-AGP. The enantiomers of verapamil were selectively detected from those of norverapamil by the mass spectrometer due to different molecular masses, although there was a chromatographic co-elution. Thus, time-consuming procedures like achiral preseparation or chemical derivatisation could be avoided. Higher detection sensitivity than earlier published methods based on fluorescence detection was obtained, although a mobile phase of high water-content and high flow-rate was introduced into the electrospray interface (85% aqueous ammonium acetate pH 7.4 +15% acetonitrile at 0.6 ml/min). The enantiomers of verapamil and norverapamil could be quantified at levels down to 50 pg and 60 pg/500 microl plasma sample, respectively, with R.S.D. in the range of 3.6-7.8%. The presented method was successfully applied to an in vivo intestinal absorption and bioavailability study in humans, using the Loc-I-Gut method.

Determination of benidipine in human plasma using liquid chromatography–tandem mass spectrometry

We developed a method for determining benidipine, a dihydropyridine analogue calcium-channel blocker, in plasma using liquid chromatography-tandem mass spectrometry (LC-MS-MS). Benidipine and benidipine-d5, an internal standard, were extracted from plasma using diethyl ether in the presence of 5M NaOH. After drying the organic layer, the residue was reconstituted in acetonitrile and injected onto a reversed-phase C18 column. The isocratic mobile phase (acetonitrile-5mM ammonium acetate, 90:10, v/v) was eluted at 0.2 ml/min. The ion transitions monitored in multiple reaction-monitoring mode were m/z 506-174 for benidipine and m/z 511-179 for the internal standard. The coefficient of variation of the assay precision was less than 13%, and the accuracy exceeded 92%, except at the limit of quantification, 0.05 ng/ml with 1ml of plasma, when it was 85%. This method was used to measure the benidipine concentration in plasma from healthy subjects after a single 4-mg oral dose of benidipine. This method is a very simple, sensitive, and accurate way to determine the plasma benidipine concentration.

Selective and rapid liquid chromatography-mass spectrometry method for the determination of lercanidipine in human plasma

A specific liquid chromatography-mass spectrometric (LC-MS/MS) assay was developed and validated for the determination of lercanidipine, a dihydropyridine calcium channel blocker, in human plasma. Lercanidipine R-D3 was used as internal standard (IS). The drug was extracted from plasma using liquid-liquid extraction technique utilizing hexane: ethyl acetate as extraction solvent. The samples were analyzed using a prepacked Thermo Hypersil C(8) column and a mobile phase composed of a mixture of aqueous acetic acid and triethylamine in methanol. An ion trap mass spectrometer equipped with electrospray ionization (ESI) source operating in the positive ion mode was used to develop and validate the method. The method was proved to be sensitive and specific by testing six different human plasma batches. Linearity was established for the concentration ranges of 0.1-16 ng/ml with a regression factor of 0.9996. The lower limit of quantitation was identifiable and reproducible at 0.1 ng/ml with a precision of 7.2%.