High-performance liquid chromatographic determination of the polar metabolites of nifedipine in plasma, blood and urine

Rapid and simultaneous determination of nifedipine and dehydronifedipine in human plasma by liquid chromatography-tandem mass spectrometry: Application to a clinical herb-drug interaction study

Nifedipine (NIF), a calcium channel antagonist, is metabolized primarily by cytochrome P450 (CYP3A4) to dehydronifedipine (DNIF). As such, NIF is often used as a probe drug for determining CYP3A4 activity in human studies. A rapid and sensitive liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed and validated to simultaneously determine NIF and DNIF in human plasma using nitrendipine as the internal standard (IS). After extraction of the plasma samples by ether-n-hexane (3:1, v/v), NIF, DNIF and the IS were subjected to LC/MS/MS analysis using electro-spray ionization (ESI). Chromatographic separation was performed on a Hypersil BDS C(18) column (50 mm x 2.1 mm, i.d., 3 microm). The method had a chromatographic running time of approximately 2.5 min and linear calibration curves over the concentrations of 0.5-100 ng/mL for NIF and DNIF. The recoveries of the one-step liquid extraction method were 81.3-89.1% for NIF and 71.6-80.4% for DNIF. The lower limit of quantification (LLOQ) of the analytical method was 0.5 ng/mL for both analytes. The intra- and inter-day precision was less than 15% for all quality control samples at concentrations of 2, 10, and 50 ng/mL. The validated LC/MS/MS method has been successfully used to study pharmacokinetic interactions of NIF with the herbal antidepressant St. John's wort in healthy volunteers. These results indicated that the developed LC/MS/MS method was efficient with a significantly shorter running time (2.5 min) for NIF and DNIF compared to those methods previously reported in the literature. The presented LC/MS/MS method had acceptable accuracy, precision and sensitivity and was used in a clinical pharmacokinetic interaction study of NIF with St. John's wort, a known herbal inducer of CYP3A4. St. John's wort was shown to induce NIF metabolism with increased plasma concentrations of DNIF.

Liquid chromatographic assay of nifedipine in human plasma and its application to pharmacokinetic studies

A highly sensitive, selective and reproducible reversed-phase high-performance liquid chromatographic method has been developed for the determination of nifedipine in human plasma with minimum sample preparation. The method is sensitive to 3 ng/ml in plasma, with acceptable within- and between-day reproducibilities and linearity (r2 > 0.99) over a concentration range from 10-200 ng/ml. Acidified plasma samples were extracted using diethyether containing diazepam as internal standard and chromatographic separation was accomplished on C18 column using a mobile phase consisting of acetonitrile, methanol and water (35:17:48, v/v). The within-day precision ranged from 2.22 to 4.64% and accuracy ranged from 102.4-106.4%. The day-to-day precision ranged from 2.34-7.07% and accuracy from 95.1-100.1%. The relative recoveries of nifedipine from plasma ranged from 91.0-107.3% whereas extraction recoveries were 88.6-93.3%. Following eight 6-week freeze-thaw cycles, nifedipine in plasma samples proved to be stable with accuracy ranging from 0.64 to 3.0% and precision ranging from 3.6 to 4.15%. Nifedipine was also found to be photostable for at least 120 min in plasma, 30 min in blood and for 60 min in aqueous solutions after exposure to light. The method is sensitive and reliable for pharmacokinetic studies and therapeutic drug monitoring of nifedipine in humans after the oral administration of immediate-release capsules and sustained-release tablets to five healthy subjects.

Inhibition of nifedipine metabolism in dogs by erythromycin: difference between the gut wall and the liver

The purpose of this study was to evaluate possible interaction of nifedipine with erythromycin or rokitamycin in the intestinal mucosa. Male beagle dogs were orally administered nifedipine (10 mg), with or without oral pre-medication with erythromycin (300 mg), and 300 mg erythromycin or rokitamycin twice a day for 3 days. The experiments were of randomized cross-over design with a two-week wash-out period between dosing regimens. Erythromycin pre-medication for 3 days resulted in a significant increase in the area under the serum nifedipine concentration-time curve (AUC), whereas the curve for one nifedipine metabolite (M-2) decreased significantly. When the effects of erythromycin on the metabolism of nifedipine were studied using dog liver microsomes it was found that erythromycin significantly inhibited formation of M-2 but not of the metabolite M-1. These results indicate that formation of M-2 from M-1 in the liver might be reduced by erythromycin pre-medication. To avoid possible metabolism in the gut, the dogs were then administered 8 mg nifedipine into the peritoneal cavity, with or without multiple dose pre-treatment with erythromycin for 3 days. After intraperitoneal administration of nifedipine, the maximum concentration (Cmax) of nifedipine increased significantly. After pre-administration of erythromycin the relative bioavailability of nifedipine after oral administration was increased compared with injection into the peritoneal cavity. In-vitro study using rat intestinal microsomes and the in-vivo rat intestinal loop technique also showed that pre-administration of erythromycin inhibits nifedipine metabolism in the small intestine.

Liquid chromatographic determination of manidipine in serum with electrochemical detection

A highly sensitive and selective method for determining a dihydropyridine calcium antagonist, manidipine, by liquid chromatography using column switching with electrochemical detection was developed. Manidipine in serum was extracted by a rapid and simple procedure based on C8 bonded-phase extraction and then silica extraction. Manidipine and nilvadipine as an internal standard were separated on a C8 bonded-phase HPLC column and detected by high conversion efficiency amperometric detection at +0.7 V. Manidipine and nilvadipine (I.S.) were separated from an endogenous interference peak in serum and concentrated on a pre-column (C18) by column switching using an isocratic mobile phase, and then the corresponding fractions were introduced to an analytical column with a C8 stationary phase. Determination of manidipine was possible over the concentration range 0.5-10 ng/ml: the limit of detection was 0.3 ng/ml. The recovery of manidipine added to serum was 93.1-98.4% with coefficients of variation of less than 7.1%. The method is applicable to drug level monitoring in the serum of healthy volunteers treated with manidipine and to the analysis of pharmacokinetics.

Simultaneous determination of a new anticancer drug galocitabine and its metabolites in blood by high-performance liquid chromatography

A relatively simple and sensitive high-performance liquid chromatographic (HPLC) method is described for measuring galocitabine (Ro 09-1390) and its meatbolites, i.e. 5'-deoxy-fluorocytidine (5'-DFCR), 5'-deoxy-fluorouridine (5'-DFUR) and 5-fluorouracil (5-FU), in blood for the purpose of studying pharmacokinetics and toxicokinetics in small animals. The procedure for blood includes deproteinization with acetonitrile. Blood components were separated on a reversed-phase C18 column with a linear gradient of acetonitrile and water and detected at a wavelength of 270 nm. The between-day relative standard deviation (RSD) was less than 10% for all compounds at concentrations of 10-100 micrograms ml-1. The calibration curves obtained from the analysis of blood samples were linear and the correlation coefficients ranged from 0.997 to 0.999. The calculated determination limits were 6.9 micrograms ml-1 for galocitabine, 3.0 micrograms ml-1 for 5'-DFCR, 4.0 micrograms ml-1 for 5'-DFUR and 3.7 micrograms ml-1 for 5-FU.

Intraindividual variability in nifedipine pharmacokinetics and effects in healthy subjects

The intraindividual variability in pharmacokinetics and effects of oral nifedipine (10 mg), administered with 1 week intervals, was investigated in twelve young healthy subjects. The population estimate of the coefficient of intraindividual variability (CVw) in AUC of nifedipine (13%) was much smaller than the pure between-subject variability (CVb 54%). The long-term (1 1/2 year) intraindividual variability was much larger than the short-term variability. Maximum changes from baseline-values of mean blood pressure (SBP -5%, DBP -4%) and mean heart rate (HR +21%) were small. Individual maximum changes in systolic blood pressure, diastolic blood pressure, and heart rate (SBP, DBP, and HR) and areas under effect curves were highly variable (CVw 34-250%, CVb 8-88%). For most subjects a significant positive linear relation was observed between nifedipine plasma concentration and the change in HR (mean r = 0.63). The CVw in slope (106%) and intercept (685%) were even larger than the high CVb in these parameters (38% and 252%). Changes in blood pressure were not significantly related to nifedipine plasma concentrations within these healthy subjects. The small intraindividual variability in nifedipine pharmacokinetics allows crossover studies to detect pharmacokinetic relationships between nifedipine and other dihydropyridine calcium entry blockers.

Influence of single- and multiple-dose omeprazole treatment on nifedipine pharmacokinetics and effects in healthy subjects

The effects of single dose (20 mg) and short-term (20 mg/day for 8 days) oral treatment with omeprazole on the pharmacokinetics and effects of oral nifedipine (10 mg capsule) and on gastric pH have been investigated in a randomized, double-blind, placebo-controlled cross-over study in 10 non-smoking healthy male subjects. The single dose of omeprazole had no significant effect on any pharmacokinetic parameter of nifedipine, nor on gastric pH, or blood pressure or heart rate. Short-term omeprazole treatment increased the AUC of nifedipine by 26% (95% confidence interval 9-46%), but all other pharmacokinetic parameters of nifedipine, including elimination half-life, Cmax, tmax, and recovery of the main urinary metabolite, were not significantly changed. The median gastric pH during the absorption phase of nifedipine was increased by short-term omeprazole (pH 4.2) compared to placebo treatment (pH 1.4). Blood pressure and heart rate did not differ between treatments. The interaction between nifedipine and omeprazole is not likely to be of major clinical relevance.

Determination of four carboxylic acid metabolites of felodipine in plasma by high-performance liquid chromatography

A reversed-phase high-performance liquid chromatography method with ultraviolet detection at 220 nm was developed to determine four carboxylic acid metabolites in plasma following therapeutic doses of the calcium antagonist felodipine. After the addition of an internal standard the analytes were isolated by liquid-liquid and solid-phase extraction. The metabolites were applied to a C2 cartridge in their free acid form, but they were transformed and retained as ion pairs with tetrabutylammonium during a wash with phosphate buffer (pH 7), prior to automated elution and injection by the Varian AASP system onto the analytical C18 column. Using a sample volume of 1 ml of plasma, the lower limit of determination for the metabolites was about 20 nmol/l. The influence of the pH of the mobile phase on the retention time of the metabolites and the structural requirements for the internal standard were studied. The method was applied to plasma samples from four dogs collected after an oral dose of felodipine. The plasma concentration-time profiles of the metabolites gave useful information about the mechanisms by which they were formed and eliminated.

Analysis of nifedipine and its pyridine metabolite dehydronifedipine in blood and plasma: review and improved high-performance liquid chromatographic methodology

A reversed-phase HPLC method is described for the simultaneous determination of nifedipine and its primary pyridine metabolite dehydronifedipine in blood and plasma, that involves UV detection and neutral (blood) or alkaline (plasma) extraction. The limit of reliable determination is found to be 3 ng ml-1 with an inter-assay RSD of below 11%. In the presence of haemoglobin, nifedipine is unstable at pH greater than 10, necessitating neutral extraction for the measurement of nifedipine in haemolysed blood. Published methods for analysis of nifedipine are reviewed, emphasizing the lack of specificity and sensitivity which render many of them unsuitable for the investigation of nifedipine disposition in man.