Determination of nifedipine and its three principal metabolites in plasma and urine by automated electron-capture capillary gas chromatography

Comparison of short-acting versus extended-release nifedipine: Effects on hemodynamics and sympathetic activity in patients with stable coronary artery disease

We investigated the impact of short-acting and extended release nifedipine on sympathetic activity using radiotracer methodology in patients with stable coronary artery disease in order to more accurately document the response of the sympathetic nervous system to different formulations of this dihydropyridine calcium channel antagonist. Participants were randomized to placebo, short-acting or extended release nifedipine for 7–10 days. On the final day, systemic blood pressure, cardiac filling pressures, cardiac output, plasma norepinephrine (NE) and total body NE spillover were measured at baseline (time 0) and repeated at intervals for 6 hours. There were no differences in baseline measures between groups. Following the morning dose of study medication there were no changes in hemodynamics or sympathetic activity in the placebo group. However, there was a significant fall in blood pressure and a significant increase in total body NE spillover in both nifedipine groups. Importantly, the increase in sympathetic activity in response to short-acting nifedipine began earlier (30 minutes) and was much greater than that observed in the extended release group, which occurred later (270 minutes). These findings confirm that sustained therapy with nifedipine is associated with activation of the sympathetic nervous system which is dependent on the pharmacokinetics of the formulation.

Analysis of nifedipine in human plasma and amniotic fluid by liquid chromatography-tandem mass spectrometry and its application to clinical pharmacokinetics in hypertensive pregnant women

Nifedipine is a dihydropyridine calcium channel blocker used for the treatment of hypertension in pregnant women. A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for analysis of nifedipine in human plasma and amniotic fluid. Separation of nifedipine and nitrendipine (IS) was performed using a LiChroCART(®) RP-Select B column and a mixture of water:acetonitrile:glacial acetic acid (30:70:0.5 v/v) as the mobile phase. Aliquots of 500μL of biological samples were extracted at pH 13 using dichloromethane:n-pentane (3:7 v/v). The validated method was applied to a study of the pharmacokinetics of nifedipine in human plasma and amniotic fluid samples collected up to 12h after administration of the last slow-release nifedipine (20mg/12h) dose to 12 hypertensive pregnant women. The estimated pharmacokinetic parameters of nifedipine showed a mean AUC(0-12) of 250.2ngh/mL, ClT/F of 89.2L/h, Vd/F of 600.0L and t1/2 5.1h. The mean amniotic fluid/plasma concentration ratio was 0.05. The methods proved to be highly sensitive by showing a lower quantification limit of 0.1ng/mL for both matrices. And this study reports for the first time the complete development and validation of the method to quantify nifedipine in amniotic fluid using LC-MS-MS.

Simultaneous determination of nitrendipine and one of its metabolites in plasma samples by gas chromatography with electron-capture detection

A sensitive method for GC-ECD simultaneous determination of nitrendipine and its pyridine metabolite M1 in human plasma is described. Felodipine was used as the internal standard. The plasma samples were extracted with toluene. One microlitre of the extract was injected onto the capillary column (polymethylsiloxane) and measured with electron-capture detector. The developed method showed to be linear over the range 0.25-70 for nitrendipine and 0.3-61 ng/ml for its metabolite M1 with an inter-day and intra-day precision in terms of R.S.D. lower than 8% except the concentrations near lowest limit of quantification (LLOQ) (<11% R.S.D.). The LLOQ for nitrendipine was 0.25 and 0.3 ng/ml for its metabolite, respectively. The analytical recovery was 94% for nitrendipine and 89% for its pyridine metabolite M1. This GC-ECD method was developed for being used in clinical pharmacokinetic studies.

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.

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.

Extractive spectrophotometric methods for the determination of nifedipine in pharmaceutical formulations using bromocresol green, bromophenol blue, bromothymol blue and eriochrome black T

Four simple, sensitive and accurate spectrophotometric methods have been developed for the determination of nifedipine in pharmaceutical formulations. These methods are based on the formation of ion-pair complexes of amino derivative of the nifedipine with bromocresol green (BCG), bromophenol blue (BPB), bromothymol blue (BTB) and eriochrome black T (EBT) in acidic medium. The coloured products are extracted with chloroform and measured spectrophotometrically at 415 nm (BCG, BPB and BTB) and 520 nm (EBT). Beer's law was obeyed in the concentration range of 5.0-32.5, 4.0-37.5, 6.5-33.0 and 4.5-22.5 microg ml(-1) with molar absorptivity of 6.41 x 10(3), 4.85 x 10(3), 5.26 x 10(3) and 7.69 x 10(3) l mol(-1) cm(-1) and relative standard deviation of 0.82%, 0.72%, 0.66% and 0.68% for BCG, BPB, BTB and EBT methods, respectively. These methods have been successfully applied for the assay of drug in pharmaceutical formulations. No interference was observed from common pharmaceutical adjuvants. Statistical comparison of the results with the reference method shows excellent agreement and indicates no significant difference in accuracy and precision.

Determination of nifedipine in human plasma by solid-phase extraction and high-performance liquid chromatography: validation and application to pharmacokinetic studies

Nifedipine, a dihydropyridine calcium channel antagonist, is widely used in the treatment of hypertension and other cardiovascular disorders. A simple, rapid, sensitive, precise and accurate HPLC method, using solid-phase extraction, for the quantitation of nifedipine in human plasma was developed and validated. The calibration graphs were linear in the 5-400 ng/ml concentration range (r>0.999). Recovery for nifedipine was greater than 93.9% and for internal standard nitrendipine was 96.1%. Intra-day and inter-day precision ranged from 1.4 to 4.2 and 3.9 to 5.6%, respectively. Intra-day and inter-day accuracy was ranged from 94.5 to 98.0 and 93.1 to 98.0%, respectively. The method was not interfered with by other plasma components and was applied for the determination of nifedipine in pharmacokinetic study after single oral administration of 10 mg nifedipine to 18 healthy male subjects.

Determination of nifedipine in human plasma by square wave adsorptive stripping voltammetry

A simple, sensitive and selective square-wave adsorptive stripping voltammetric method has been developed and validated for the determination of nifedipine (NIF) in plasma. The assay was performed after single extraction of NIF from alkalinised plasma into organic phase. The adsorption behaviour of NIF on a hanging mercury drop electrode (HMDE) was explored by square-wave and cyclic voltammetry. The drug was accumulated at HMDE and a well-defined peak was obtained at -730 mV versus Ag/AgCl in borate buffer of pH 9.0 including 0.01 M KCl. The linear concentration range was 2.89 x 10(-9) M-3.61 x 10(-7) M (1.00-125.01 ng ml(-1)) when using 30 s accumulation time at -300 mV. Limit of detection and limit of quantification were 1.21 x 10(-9) M (0.42 ng ml(-1)) and 2.89 x 10(-9) M (1.00 ng ml(-1)) respectively. The intra-day relative standard deviation (RSD) ranged from 1.93 to 4.12% at three concentrations and the inter-day RSDs varied from 2.53 to 6.68%. The method was applied, to the plasma of pregnant women suffering from pregnancy induced hypertension, for the determination of NIF. The percentage recoveries varied from 96.26 to 99.49%. It has been shown that NIF could be determined in the presence of its main metabolite (dehydronifedipine) by the developed method.

Photodegradation monitoring of amlodipine by derivative spectrophotometry

A derivative spectrophotometric method for the simultaneous determination of amlodipine and its pyridine photodegradation product has been developed. The analytes concentrations were linearly correlated with spectral measurements in the 3rd order UV derivative spectrum through equations obtained by simple and multiple regression analysis. The recovery values were estimated to range from 95 to 99% and the quantitation limit of the photoproduct was found to be equivalent to an impurity level of 1%, with respect to the content of amlodipine. The method could usefully be applied to routine quality control of pharmaceutical formulations containing amlodipine.

Phenotyping of drug-metabolizing enzymes in adults: a review of in-vivo cytochrome P450 phenotyping probes

Cytochrome P450 phenotyping provides valuable information about real-time activity of these important drug-metabolizing enzymes through the use of specific probe drugs. Despite more than 20 years of research, few conclusions regarding optimal phenotyping methods have been reached. Caffeine offers many advantages for CYP1A2 phenotyping, but the widely used caffeine urinary metabolic ratios may not be the optimal method of measuring CYP1A2 activity. Several probes of CYP2C9 activity have been suggested, but little information exists regarding their use, largely due to the narrow therapeutic index of most CYP2C9 probes. Mephenytoin has long been considered the standard CYP2C19 phenotyping probe, but problems such as sample stability and adverse effects have prompted the investigation of potential alternatives, such as omeprazole. Several well-validated CYP2D6 probes are available, including dextromethorphan, debrisoquin and sparteine, but, in most cases, dextromethorphan may be preferred due to its wide safety margin and availability. Chlorzoxazone remains the only CYP2E1 probe that has received much study. However, questions concerning phenotyping method and involvement of other enzymes have impaired its acceptance as a suitable CYP2E1 phenotyping probe. CYP3A phenotyping has been the subject of numerous investigations, reviews and commentaries. Nevertheless, much controversy regarding the selection of an ideal CYP3A probe remains. Of all the proposed methods, midazolam plasma clearance and the erythromycin breath test have been the most rigorously studied and appear to be the most reliable of the available methods. Despite the limitations of many currently available probes, with continued research, phenotyping will become an even more valuable research and clinical resource.

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.

The role of metabolism in determining susceptibility to parathion toxicity in man

Human liver microsomes (n = 16) activated parathion (O, O, diethyl O-p-nitrophenyl phosphorothioate, 20 and 200 microM) to paraoxon at a rate of 23.3-199.3 and 18.7-310.3 pmol/min per mg protein, respectively. p-Nitrophenol, was also formed, at 321.1-769.2 and 406.2-778.3 pmol/min per mg protein. This represented a 16-fold and 2-fold range in capacity to activate and detoxify parathion, respectively. Parathion was activated with an apparent Km of 9-16 microM (n = 3). The activation of parathion (200 microM) was positively correlated with nifedipine oxidation, indicating the involvement of CYP3A. Correlations were not significant with ethoxyresorufin-O-dealkylation (CYP1A1/2), pentoxyresorufin-O-dealkylation (CYP2B6), p-nitrophenol hydroxylation (CYP2E1), paraoxon hydrolysis (A-esterase) or phenylvalerate hydrolysis (B-esterase). Paraoxon formation from parathion was markedly reduced by CYP3A inhibitors. Experiments with EDTA indicated that A-esterase was not functionally important at low levels of paraoxon. Human P450s 3A4 and 3A5 expressed microsomes were the most efficient at biotransforming parathion to paraoxon, although P450s 1A1, 2B6 and 2C8 also catalysed the reaction. This study has determined wide interindividual variations in capacity to metabolise parathion, mainly by CYP3A, which may influence its manifest toxicity.

Determination of nifedipine in human plasma by flow-injection tandem mass spectrometry

For use in clinical studies, a fast and sensitive assay method was developed for the determination of nifedipine in human plasma samples. The assay method is based on tandem mass spectrometry detection (HPLC-MS-MS). The effect of flow injection as well as HPLC separation on the results of the nifedipine determination were evaluated. The limit of quantification is 0.5 ng/ml and the accuracy (as determined by spiking recovery) was found to be good.

Differential activation of cardiac and peripheral sympathetic nervous system by nifedipine: role of pharmacokinetics

OBJECTIVES

We sought to study the effects of short-acting and long-acting nifedipine on the sympathetic nervous system (SNS), heart rate (HR) and blood pressure (BP) of normotensive subjects under baseline conditions and during SNS stimulation.

BACKGROUND

Calcium channel antagonists in different pharmacokinetic formulations are widely used in patients with coronary artery disease or hypertension. Short-acting formulations activate the SNS, an action that may be disadvantageous in patients with coronary disease, especially if left ventricular function is impaired. The effects of slow-release formulations on the SNS are unknown.

METHODS

We used microneurography to investigate the influence of nifedipine (5 mg; 10 mg; and slow-release [GITS], 60 mg) on muscle sympathetic nerve activity (MSA) and skin sympathetic nerve activity (SSA) in healthy volunteers.

RESULTS

Peak plasma levels after short-acting and slow-release nifedipine were achieved within 60 min and 330 min, respectively. Short-acting (10 mg, n = 10) and slow-release (n = 10) nifedipine, but not placebo, markedly activated MSA and increased plasma norepinephrine; plasma endothelin increased only with slow-release nifedipine. HR increased after short-acting nifedipine, but not after nifedipine GITS. Nifedipine had no effect on SSA (n = 6). Blockade of cardiac sympathetic activity (with esmolol) led to similar decreases in HR with or without nifedipine, whereas parasympatholysis (with atropine) led to similar increases in HR with or without nifedipine. The cold pressor test markedly increased MSA in all treatment groups and further increased MSA beyond the increase induced by nifedipine.

CONCLUSIONS

Nifedipine markedly increased MSA, but not SSA, independently of drug release formulation. In contrast, HR increased with short-acting, but not with slow-release, nifedipine. Therefore, nifedipine activates cardiac and peripheral sympathetic nerves differently depending on pharmacokinetics. These effects of nifedipine may be disadvantageous in cardiac patients with increased sympathetic activity or congestive heart failure, or both.

Screening for cytochrome P450 3A in man: studies with midazolam and nifedipine

This report describes work directed towards the development of a screening technique for cytochrome P450 3A activity which should be valid for a variety of drugs metabolized by this enzyme. A significant correlation (P < 0.01) was found between the ratio of the plasma concentration of nifedipine to that of its oxidized metabolite and the area under the time curve for the plasma concentration of midazolam. It is suggested that the nifedipine: metabolite ratio might have general predictive value for the metabolism of orally administered cytochrome P450 3A substrates.

Periodontal variables affecting nifedipine sequestration in gingival crevicular fluid

We previously demonstrated the sequestration of nifedipine in gingival crevicular fluid (GCF), especially in patients exhibiting significant gingival overgrowth. The aim of the present study is to determine the role of site specific periodontal factors in this phenomenon. 10 adult patients exhibiting nifedipine induced gingival overgrowth were studied. In each patient GCF was harvested from two sites that demonstrated inflammation and increased probing depth as well as from two clinically healthy sites. The concentration of nifedipine was determined using gas chromatography. Drug concentrations were significantly increased in the presence of inflammation (p = 0.004) and plaque (p = 0.029) whilst increased probing depths and gingival overgrowth were not significantly related to drug sequestration. We can conclude that inflammatory changes in gingival tissues appear to be a significant determinant for the sequestration of nifedipine in the GCF.

Determination of nifedipine in human serum by gas chromatography-mass spectrometry: validation of the method and its use in bioavailability studies

A procedure for the determination of nifedipine in human serum is described. The light-sensitive substance is isolated from serum by liquid-liquid extraction and analyzed using capillary gas chromatography with a mass-selective detector. The validation of the method shows that the extraction recovery is ca. 85%, the limit of detection is 2 ng/ml and the standard deviations of the intra-day precision test range from 5.8 to 7.4% with respect to the concentration. The procedure is highly selective and sensitive. It is especially suited for bioavailability studies because of its stability and high sampling rate.

Identification of the major human hepatic cytochrome P450 involved in activation and N-dechloroethylation of ifosfamide

Two NADPH-dependent metabolic routes for the anticancer drug ifosfamide, 4-hydroxylation (activation) and N-dechloroethylation (a detoxication pathway), were studied in human liver microsomes to identify the cytochrome P450 enzymes involved. Naringenin, a grapefruit aglycone and an inhibitor of cytochrome P450 3A4 (CYP3A4)-catalysed reactions, was found to inhibit ifosfamide activation and N-dechloroethylation by human liver microsomes. IC50 for both reactions was of the order of 70 microM. The CYP3A4-specific inhibitor triacetyloleandomycin inhibited ifosfamide N-dechloroethylation by human liver microsomes with an IC50 of approximately 10 microM. Furthermore, anti-human CYP3A4 antiserum inhibited by about 80% N-dechloroethylation of ifosfamide by human liver microsomes. The relative levels of cytochromes P450 1A, 2C, 2E and 3A4 in 12 human livers were determined by western blotting analysis. A strong correlation (P < 0.001) was observed between CYP3A4 expression and both activation and N-dechloroethylation of ifosfamide. A role for human CYP3A4 in both pathways of ifosfamide metabolism was thus demonstrated. This was substantiated by the observation that the nifedipine oxidase activities of the 12 samples of human liver microsomes correlated with ifosfamide activation (P < 0.009) and N-dechloroethylation (P < 0.001). These findings have important clinical implications. The involvement of the same key cytochrome P450 enzyme in both reactions prohibits selective inhibition of the N-dechloroethylation pathway, as might be desirable to reduce toxic side effects. They also demonstrate the need to consider interaction with co-administered drugs that are CYP3A4 substrates.

Sensitive high-performance liquid chromatographic assay for nifedipine in human plasma utilizing ultraviolet detection

A rapid, simple, sensitive and selective reversed-phase high-performance liquid chromatographic (HPLC) technique is reported for the determination of nifedipine in human plasma. The procedure involves extraction of nifedipine from plasma under alkaline conditions (pH 12), separation via reversed-phase HPLC and ultraviolet detection (350 nm). The peak corresponding to nifedipine was free of interference from its photodegradation products or metabolites. The method was validated over the range 5-250 ng/ml nifedipine using weighted least-squares linear regression analysis. Accuracy and precision were within approximately 10% or less over the concentration range, except for the lowest concentration point which, nonetheless, was acceptable and approached 15%. The minimum quantifiable concentration of nifedipine was determined to be 5 ng/ml. The minimum detectable concentration was in the order of 1 ng/ml. Analysis of plasma samples collected from healthy volunteers demonstrate that this assay is applicable to clinical and pharmacokinetic studies.

Quantitative analysis of nifedipine in plasma by high-performance liquid chromatography

A rapid, sensitive and specific high-performance liquid chromatographic assay for the quantification of nifedipine in human plasma was developed, satisfactorily validated and applied to samples of plasma from healthy volunteers. The sample pre-treatment incorporating protein denaturation by urea and ethyl acetate extraction compared favourably in terms of selectivity with previously published methods. The limit of quantitation of this reversed-phase LC method was 7.0 ng ml-1 for the analysis of 0.5 ml samples.

Determination of nifedipine in gingival crevicular fluid: a capillary gas chromatographic method for nifedipine in microlitre volumes of biological fluid

This paper describes a sensitive capillary gas chromatographic (GC) method for the determination of nifedipine in sub-microliter samples of gingival crevicular fluid (GCF) in order to assess if nifedipine is present in the GCF and if so, whether the local tissue concentrations of this drug are an important determinant in the development of gingival overgrowth. Liquid-liquid and solid-phase extraction were combined to give adequate sample clean-up and concentration for measurement by automated capillary GC with electron capture detection. Nifedipine and its principal metabolite, M-I, were analysed in both plasma and GCF in 9 adult male patients who had been taking nifedipine for over six months. M-I could not be measured in GCF. Plasma nifedipine and M-I levels were normal, but the nifedipine levels found in the GCF of 7 patients (including all those with overgrowth) were remarkably elevated, 15 to 316-fold greater. This massive concentration of nifedipine into the GCF is therefore linked with gingival overgrowth. This is the first time that a GC method has been developed which permits determination of GCF pharmacokinetics of a drug which causes gingival overgrowth, and further investigation will lead to a better understanding of the tissue mechanisms involved.

Disposition of nifedipine in plasma and gingival crevicular fluid in relation to drug-induced gingival overgrowth

The present study investigates the relationship between the pharmacokinetic variables of nifedipine with the incidence and severity of gingival overgrowth in 9 adult male patients medicated with the drug for at least 6 months. Five of the patients had experienced significant gingival changes and were thus designated "responders". The remaining four patients exhibited no gingival overgrowth, and thus acted as a control. A baseline periodontal examination (plaque scores, bleeding index and gingival overgrowth assessment) was carried out on each patient, and confined to the upper and lower anterior teeth. Serial blood and gingival crevicular fluid samples were collected over an eight-hour investigation period. Samples were analyzed for nifedipine by gas chromatography. No significant difference (p > 0.05) was seen between responders and non-responders with regard to drug therapy, periodontal parameters or plasma pharmacokinetics of nifedipine. Nifedipine was detected in the gingival crevicular fluid of seven subjects (all responders, and two non-responders). The peak concentration of nifedipine in crevicular fluid was 15-90 fold greater than levels observed in plasma.

Evaluation of the pharmacokinetic and pharmacodynamic interaction between quinidine and nifedipine

Quinidine and nifedipine appear to be subject to metabolism by the same isozyme of cytochrome P-450. In addition, both drugs have been reported to alter the pharmacokinetics of other compounds. To investigate a potential interaction, 10 healthy subjects (five male, five female) received quinidine sulfate (200 mg orally), nifedipine (20 mg orally), or the combination of both drugs every 8 hours for 4 doses using a randomized, cross-over study design with a 2-week washout period between treatments. Drug concentration, heart rate, and mean arterial pressure were measured at frequent intervals after the final dose. Quinidine concentrations were unchanged by the co-administration of nifedipine. Nifedipine area under the curve (AUC0-8) increased 36.6% from 333 to 455 micrograms.hr/L (P < .05) after quinidine administration. Heart rate was significantly higher in the nifedipine-quinidine treatment at 0.5, 1.0, 1.5, and 2.0 hours when compared with either drug alone. The maximum increase in heart rate (17.9 beats/minute) occurred at 0.5 hours after nifedipine administration and was significantly correlated with serum concentrations at that time (r = .78). These results suggest that quinidine inhibits nifedipine metabolism, and this pharmacokinetic interaction results in enhanced pharmacologic response.

Gingival sequestration of nifedipine in nifedipine-induced gingival overgrowth

The mechanism of gingival overgrowth associated with long-term use of nifedipine and of other drugs that affect calcium homoeostasis, such as cyclosporin and phenytoin, is unknown. With an ultrasensitive assay, we measured the pharmacokinetics of nifedipine in plasma and gingival crevicular fluid (GCF) of nine patients receiving this drug for angina and hypertension. In seven patients, the maximum nifedipine concentration was in the range 15-316 (mean 84 [SD 105]) times greater in GCF than in plasma. The two patients with low (undetectable) GCF nifedipine did not have overgrowth. We propose that gingival tissues sequester nifedipine and that the very high nifedipine concentrations predispose the tissues to overgrowth.

A phase I study on the reversal of multidrug resistance (MDR) in vivo: nifedipine plus etoposide

Multidrug resistance (MDR) is one of the mechanisms of resistance to multiple cytotoxic drugs and is mediated by the expression of a membrane pump called the P-glycoprotein. Nifedipine is one of the calcium channel blocking agents which reverses MDR in vitro. Fifteen patients with various malignancies received nifedipine at three dose levels: 40 mg, 60 mg and 80 mg orally twice daily for 6 days. Etoposide was administered intravenously on day 2 in a dose of 150-250 mg m-2 and orally 150-300 mg twice daily on days 3 and 4. Cardiovascular effects of nifedipine were dose limiting and the maximum tolerated dose was 60 mg bid. Mean area under the plasma concentration curve (AUC0-00) and plasma half-life (beta) of nifedipine and its major metabolite MI at the highest dose level were 7.87 microM.h, 7.97 h and 4.97 microM.h, 14.0 h respectively. Nifedipine did not interfere with the pharmacokinetics of etoposide.

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.

Determination of nifedipine in plasma by a rapid capillary gas chromatographic method

A rapid, specific and reliable gas chromatographic assay procedure for Nifedipine in plasma has been developed. With a single-step solvent extraction, and electron capture detection, the method is sensitive to 0.5 ng/mL of plasma and the standard curve is linear from 0.5 to 500 ng/mL. Samples are protected from light to prevent formation of photodecomposition products. The method has been used to monitor drug concentrations in patients receiving therapeutic doses.

Transcranial Doppler to evaluate the effects of antihypertensive medication on cerebral blood flow velocity

The effect of oral nifedipine on cerebral blood flow velocity was studied in six elderly hypertensive patients using transcranial Doppler. Serial measurements of blood pressure (BP), middle cerebral artery (MCA) flow velocity and nifedipine serum concentrations were obtained over an 8-hour period. The authors found a significant inverse relationship between MCA velocities and nifedipine concentrations, independent of BP changes. These results are consistent with a direct vasodilatory effect of nifedipine on cerebral vessels and support the use of TCD in pharmacodynamic investigations of the cerebral vasculature.

Two deaths from intravenous nifedipine abuse

An incident is reported in which 2 intravenous drug abusers died as the result of uncontrolled experimentation with intravenous injection of the common anti-hypertensive and anti-anginal drug Nifedipine (Adalat t.m. Bayer), probably in mistake for the commonly abused short-acting benzodiazepine drug Temazepam. Large quantities of Nifedipine were identified in the blood of both decreased men by gas chromatography. Apart from intense gastric mucosal congestion, pulmonary oedema and general visceral congestion, the autopsy findings were entirely nonspecific. The similarity in colour, shape and texture between capsules of Nifedipine and those of Temazepam is likely to have prompted the mistake.

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

A relatively simple reversed-phase high-performance liquid chromatographic method for the determination of the polar metabolites of nifedipine in biological fluids is described. After conversion of 2-hydroxymethyl-6-methyl-4-(2-nitrophenyl)pyridine-3,5-dicarboxylic acid 5-methyl ester (IV) into 5,7-dihydro-2-methyl-4-(2-nitrophenyl)-5-oxofuro[3,4-b] pyridine-3-carboxylic acid methyl ester (V) by heating under acidic conditions, V was extracted with n-pentane-dichloromethane (7:3) and analysed on a C18 column with ultraviolet detection. Subsequently, 2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinedicarboxylic acid monomethyl ester (III) was extracted with chloroform and analysed on the same system. Limits of determination in blood were 0.1 microgram/ml for III and 0.05 microgram/ml for IV and V; these limits were two to ten times higher for urine. This inter-assay variability was always less than 7.5%.

Chronopharmacokinetics and cardiovascular effects of nifedipine

Circadian phase dependency in pharmacokinetics and hemodynamic effects on blood pressure and heart rate of different galenic formulations of nifedipine (immediate-release, sustained-release, and i.v. solution) were studied in healthy subjects or in hypertensive patients. Pharmacokinetics of immediate-release but not sustained-release and i.v. nifedipine were dependent on time of day: immediate-release nifedipine had higher Cmax (peak concentration) and shorter tmax (time-to-peak concentration) after morning than evening application, and bioavailability in the evening was reduced by about 40%. Circadian rhythm in estimated hepatic blood flow as determined by indocyanine green kinetics may contribute to these chronokinetics. A circadian time dependency was also found in nifedipine-induced effects on blood pressure and heart rate as monitored by 24-h ambulatory blood pressure measurements. In conclusion, the dose response relationship of oral nifedipine is influenced by the circadian organization of the cardiovascular system as well as by the galenic drug formulation.

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.

Lack of effect of co-trimoxazole on the pharmacokinetics and pharmacodynamics of nifedipine

The pharmacokinetics of nifedipine and its primary oxidised metabolite, M-I were studied in nine healthy volunteers following a single oral dose of 20 mg nifedipine alone or after pretreatment with oral co-trimoxazole. Following pretreatment with co-trimoxazole, no significant effect was detected on maximum plasma concentration, elimination half-life, or area under the plasma concentration-time curve of either nifedipine or M-I, nor on the blood pressure response to nifedipine.

Chromatography of calcium channel blockers

Numerous publications during the past ten years have described the determination of various calcium channel blockers in biological fluids, using gas and liquid chromatographic techniques. Diltiazem, verapamil, flunarizine and a growing number of dihydropyridines belong to this group of drugs, which in most instances are active at low plasma concentrations. From a bioanalytical point of view these compounds have many features in common, such as high lipophilicity and favourable detection properties.

Pharmacokinetics and toxic effects of nifedipine in massive overdose

A 57-year-old man took 30 x 20 mg nifedipine retarded release tablets. He developed hypotension, tachycardia and flushing, but remained in sinus rhythm. The concentration of nifedipine 10 h after overdose was 604 micrograms 1-1, and of the M-I metabolite 110 micrograms 1-1. Log concentration time curves were linear from 10-72 h for nifedipine, with a half-life of 7.5 h; and for M-I with a half-life of 8.2 h. On this evidence, oral absorption of nifedipine retarded release is complete by 10 h. There was no evidence of saturation of nifedipine or M-I metabolism, even at concentrations ten times above the therapeutic concentration.

The influence of food on the pharmacokinetics of 'biphasic' nifedipine at steady state in normal subjects

Previous studies following single dose administration have suggested that the pharmacokinetics of various nifedipine formulations could be influenced by the timing of associated food consumption. In order more closely to reflect the clinical situation we have carried out a study at steady state using a 'biphasic' formulation comprising 'rapid' and 'retarded' drug release components. Fifteen normal subjects took 20 mg 'biphasic' nifedipine 12 hourly for 10 days. Studies were carried out on days 4, 7 and 10. On these days the nifedipine was taken 2 h or 1 h before or immediately following a light breakfast. A light breakfast influenced neither the rate nor the extent of nifedipine absorption nor the rate or extent of major metabolite appearance. We conclude that at steady state the timing of a light meal is unlikely to alter in any clinically important manner the pharmacokinetics of nifedipine released from 'biphasic' tablets.

Gas chromatographic-mass spectrometric analysis of plasma nifedipine

A gas chromatographic-mass spectrometric assay is described for the determination of plasma nifedipine using nitrendipine as an internal standard. Chromatographic separation was performed on a dimethylsilicone capillary column, and detection was by selected ion monitoring of electron impact-generated base peak ions. The lower limit of quantifiable detection of nifedipine was 2 ng/ml. The method was applied to plasma samples obtained from a human subject who had been dosed with a 10-mg nifedipine capsule every 8 h for eight doses.

Potential of wide bore open tubular columns in gas chromatographic analysis of drugs

The recent introduction of wide bore flexible fused silica columns and the development of chemically bonded stationary phases for gas chromatography (GC) has opened up new possibilities for carrying out medium resolution capillary GC on packed column instruments. The favourable chromatographic properties of wide bore capillary columns make them ideal for applications in many areas of GC. This review discusses the characteristics of more popular 0.53 mm I.D. wide bore columns, experimental conditions necessary for their use and their applications in the pharmaceutical field with an emphasis on the analysis of drugs in biological fluids.

Gas chromatographic-negative-ion chemical ionization mass spectrometric determination of a new dihydropyridine calcium antagonist (MPC-1304) and its metabolites in human plasma and urine

A gas chromatographic-negative-ion chemical ionization mass spectrometric method was developed for the determination of a new calcium antagonist, (+/-)-methyl 2-oxopropyl 1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinedicarboxylate, and its metabolites in plasma and urine. The sample was extracted with n-hexane-diethyl ether. The dried organic layer was subjected to acetylation: the aqueous layer was acidified and extracted with ethyl acetate, and after the ethyl acetate extract was dried the resulting residue was subjected to methylation. Aliquots of each reactant solution were injected into the gas chromatograph-mass spectrometer, equipped with a chemical ionization source and negative-ion monitoring mode, and analysed by the selected-ion monitoring method using deuterium-labelled internal standards. Detection was limited to 0.02-0.05 ng/ml of plasma and urine for each metabolite. A precise and sensitive assay for the determination of a new dihydropyridine calcium antagonist and its metabolites in plasma and urine was thus established.