In vivo oxidative cleavage of a pyridine-carboxylic acid ester metabolite of nifedipine

The pharmacokinetics of the primary pyridine metabolite of nifedipine (2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinecarboxylic acid dimethylester) (M-0) and its [2H6]dimethylester analog ([2H6]M-0) were studied in male rats. A large, 5.8-fold deuterium isotope effect for the formation clearance of the monomethylester (M-1) was observed, which is strongly indicative for an oxidative reaction mechanism involving the abstraction of a hydrogen atom, presumably by cytochrome P-450. M-0 exhibited a high systemic blood clearance (104 +/- 27 ml/min/kg) (mean +/- SD) which was not significantly influenced by deuterium substitution (125 +/- 13 ml/min/kg). Its systemic clearance is presumably flow limited, and extrahepatic metabolism can be anticipated. The major metabolic pathway for M-0 in male rats seems to be a direct oxidation at the 2-methyl position and subsequently a rapid conversion of the unstable 2-hydroxymethyl-dimethylester to the lactone of the monomethylester (M-2), as has been shown by others in vitro. Non-oxidative ester cleavage of M-0 in our rats was negligible. Deuterium substitution of M-0 at the ester methyl groups induced "metabolic switching" in favor of the direct oxidation of M-0 to M-2.

Prediction of response to antihypertensive therapy with enalapril and nifedipine

We evaluated the usefulness of an integrated concentration-effect modelling technique in predicting the long-term response to antihypertensive therapy with enalapril and nifedipine. Two groups of essential hypertensives were given monotherapy with 20 mg nifedipine twice a day (n = 14) or 20 mg enalapril once a day (n = 13), and were studied following the administration of the drugs and after at least 6 weeks' treatment. For both drugs the predicted responses (predose and 4 h postdose) were in close agreement with the observed responses. With enalapril the observed and predicted profiles over a 12-h study period were well correlated in all subjects. In contrast, with nifedipine, although there was generally good agreement, the model over-predicted the profile of response in one patient and under-predicted the profile in one patient. Thus, there is evidence that application of concentration-effect analysis is useful in predicting the steady-state antihypertensive effect from the first dose-response to the drug.

Nifedipine pharmacokinetics during preterm labor tocolysis

Nifedipine, a calcium entry blocker, has known relaxing effects on the myometrium. Thirteen women in preterm labor received nifedipine for tocolysis. Blood samples obtained serially during treatment and at the time of delivery were assayed for maternal and neonatal nifedipine concentrations. The peak concentration of nifedipine during sublingual therapy ranged from 23.4 to 197.9 ng/ml and reflected substantial interpatient variability. The mean (+/- SD) measurable trough value in patients who received 20 mg of nifedipine orally every 6 hours was 7.2 +/- 5.5 ng/ml. The maternal mean half-life of nifedipine was 81 minutes (range 49 to 137 minutes). At delivery, neonatal nifedipine levels were nondetectable in 6 of the 11 neonates available for study; in 5, values ranged from 29.5 to 1.8 ng/ml. From these results we conclude that both sublingual and oral nifedipine treatment results in variable but usually measurable maternal plasma concentrations and that placental transfer of nifedipine occurs.

Differences in the serum binding determinants of isradipine and darodipine--consequences for serum protein binding in various diseases

1. Serum protein binding of isradipine and darodipine, and serum concentrations of alpha 1-acid glycoprotein (AAG), albumin (HSA) and non-esterified fatty acids (NEFA) were measured in three groups of patients, I: healthy subjects (n = 20); II: patients with inflammatory disorders (n = 15) and III: patients with hepatic insufficiency (n = 17). 2. AAG was increased significantly in group II patients (P less than 0.001) and decreased in group III patients (P less than 0.001); HSA was decreased significantly in group II and group III patients (P less than 0.001). 3. The free percentage of isradipine was decreased significantly in group II patients (P less than 0.05) and increased in group III patients (P less than 0.05) and multivariate analysis showed that these variations were inversely related to changes in AAG concentration. 4. The free percentage of darodipine was increased significantly in group II and III patients (P less than 0.05) due to a decrease in HSA concentration, as shown by multivariate analysis. 5. The changes in free serum percentages of isradipine and darodipine were inversely related to concomitant changes in the concentration of the serum protein for which they showed the highest affinity, AAG for isradipine and HSA for darodipine, respectively. 6. The unexplained variability in the binding data was greater when AAG was the major determinant of binding (isradipine).

Dose dependent effect of diltiazem on the pharmacokinetics of nifedipine

The effect of diltiazem pretreatment on the pharmacokinetics of nifedipine were determined in six healthy male volunteers. Placebo or diltiazem (30 mg and 90 mg) was given orally three times daily for 3 days in a double-blind, Latin square method. On the fourth day, a 20-mg nifedipine was given orally 1 hour after the last dose of placebo or diltiazem. The mean elimination half-life of nifedipine prolonged significantly following diltiazem (2.54 hours on placebo vs 3.40 hours on 30 mg diltiazem and 3.47 on 90 mg diltiazem, both P less than .01). The mean AUC of nifedipine increased during diltiazem (1726.6 nmol X hr/ml on placebo vs 3838.0 on 30 mg diltiazem, and 5370.0 on 90 mg diltiazem, both P less than .05, 30 mg vs 90 mg, 0.1 less than P less than .05). The ratio of the AUC of primary metabolite (nitropyridine form) to the AUC of nifedipine was reduced by diltiazem pretreatment in a dose-dependent manner. ICG clearance was not influenced following diltiazem. These results indicate that diltiazem dose-dependently alters the pharmacokinetic profiles of nifedipine. The ICG clearance test showed that the liver blood flow did not decrease during diltiazem therapy, therefore, the reduction in the metabolic clearance of nifedipine might be caused by inhibiting effect of diltiazem on the activity of drug oxidizing enzymes.

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.

Slow release nifedipine plus atenolol in chronic stable angina pectoris

1. The effects of adding slow release nifedipine in doses of 20 mg and 40 mg twice daily to atenolol therapy (50 mg twice daily) were assessed in 18 patients with chronic stable angina. 2. The addition of the lower dose of nifedipine to atenolol did not significantly alter the weekly consumption of glyceryl trinitrate or the mean number of anginal attacks as assessed by diary cards. However, 2 h after dosing there was a significant prolongation during stress testing in the time to onset of both 1 mm ST depression on the ECG (by 28%) and to angina (by 37%) compared with atenolol alone, but no benefit was apparent by 12 h after dosing. 3. At a dose of 40 mg twice daily, nifedipine significantly reduced glyceryl trinitrate consumption by 25% and the number of anginal attacks by 36%. The times to onset of ST depression and angina were increased by 37% and 55% respectively at 2 h and by 24% and 26% respectively 12 h after dosing. 4. Analysis of the frequency distribution of anginal attacks showed decreasing efficacy with time after administration of nifedipine. The overall results also suggest a relationship between efficacy and the plasma nifedipine concentration, with a mean 20% improvement in time to development of angina occurring at a nifedipine plasma concentration of approximately 30-40 ng ml-1. 5. In conclusion, the reduction of effort-related angina by nifedipine is related to its plasma concentration and the effective duration of action of the 20 mg slow release formulation is less than 12 h.

Coronary vasodilation with dihydropyridines--a pharmacokinetic study

In 26 patients with coronary artery disease, the mean diameters of angiographically 'normal' epicardial coronary arteries were assessed with the aid of a computer-assisted contour detection system (CAAS) before and up to 15 min after onset of a 4-min intravenous-infusion of 2 mg nifedipine (13 patients, group I) or 1 mg nisoldipine (13 patients, group II). Maximal coronary dilation amounted to 20 +/- 9% (4th min) in group I and to 18 +/- 9% (15th min) in group II. In addition, in group II changes of the minimal diameters of 9 coronary obstructions were measured; the maximum increase averaged 28 +/- 15% (7th min). In order to compare the pharmacokinetic properties of these compounds the dilation of the 'normal' coronary segments was correlated with the respective drug plasma levels; maximal plasma concentrations averaged 62 +/- 21 ng ml-1 (7th min) in group I and 17 +/- 7 ng ml-1 (4th min) in group II respectively. A positive, linear correlation between coronary dilation and plasma levels was only found with nifedipine (P less than 0.05); with nisoldipine, however, coronary dilation developed in form of a hysteresis curve, when plotted against plasma levels, probably due to the high receptor affinity of this substance. The prolonged efficacy of nisoldipine could be favourable in oral long-term treatment of patients with coronary artery disease.

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.

Usefulness of nifedipine for myocardial ischemia and the nifedipine gastrointestinal therapeutic system

Although conventional treatment of angina pectoris with nifedipine capsules involves frequent daily dosing, a new vehicle for the once-daily delivery of this potent but insoluble calcium channel blocking agent has been developed. With the gastrointestinal therapeutic system (GITS) formulation, constant effective drug levels of nifedipine are delivered for a full 24-hour period. When nifedipine GITS 30, 60 or 90 mg was administered once daily to patients with stable angina pectoris taking beta blockers, clinical effectiveness was demonstrated both by an increase in time to angina and by exercise time compared with placebo. Improvement was noted to be more significant with the higher doses of nifedipine administered once daily. This study indicates that nifedipine GITS provides additional antianginal protection in patients with exercise-induced angina secondary to coronary artery disease who are receiving a fixed dose of beta blocker.

Pharmacokinetics of nifedipine slow release tablet in Mexican subjects: further evidence for an oxidation polymorphism

Nifedipine kinetics after ingestion of 20 mg slow release tablets were studied in 12 young, healthy, Mexican subjects. Plasma levels were determined by a nifedipine-specific HPLC assay. Levels rose after drug administration reaching a maximum concentration of 48.7 +/- 7.3 ng/ml in 2.1 +/- 0.7 h (mean +/- SEM). Concentrations then decayed with a terminal half-life of 16.9 +/- 3.1 hours. AUC was 526 +/- 62 ng h/ml. Five individuals were fast and seven were slow nifedipine metabolizers, according to the AUC criterion proposed by Kleinbloesem and coworkers. Individual AUC/Dose values from this and from other two studies on oral nifedipine kinetics in Mexicans were cumulated and the frequency histogram and probit analyses were performed (N = 30). A bimodal distribution was clearly observed. Fast and slow metabolizers were distinguished as those subjects with AUC/Dose values either lower or higher than 22.5 ng h/ml mg. Unlike European populations, it appears that slow metabolization is more frequent in Mexicans. Data strongly support the hypothesis of the existence of a polymorphism concerning nifedipine disposition kinetics due to genetic basis.

Serum binding of nifedipine and verapamil in patients with ischaemic heart disease on monotherapy

Serum free fractions of nifedipine, verapamil and some of their metabolites were measured in patients with ischaemic heart disease receiving single oral dose and chronic monotherapy and were compared with those obtained in vitro. The percentages of unbound nifedipine and verapamil in vitro (concentration range 50-200 and 150-400 ng ml-1, respectively) were 2.51 and 7.23%, respectively, and did not differ from those found on monotherapy with these drugs (2.05 and 8.08%, respectively), and after single dosing. It is suggested that, during treatment with nifedipine or verapamil, their serum metabolites do not affect binding of the parent drugs to serum proteins.

[Determination of nifedipine and its metabolites in plasma using gas chromatography]

A method was worked out to determine nifedipine and its metabolite in plasma with the use of the packed chromatographic column 3% OV-101 and the detector of electron capture. The adjustment od plasma is simple. It consists in denaturation of plasma proteins and subsequent simple extraction with benzene. Denaturation of proteins present in canine plasma is carried out with methanol, where as denaturation of human plasma with ammonium phosphomolybdate. The minimal detectable amount of nifedipine is 0.5 ng.ml-1 of plasma and that of its metabolite, 0.1 ng.ml-1 of plasma. As nifedipine is unstable in daylight, the whole extraction of plasma including the withdrawal of blood must be performed in yellow light. At -18 degrees C and in darkness, nifedipine samples in plasma can be kept for 30 days.

Clinical utility of nifedipine and diltiazem plasma levels in patients with angina pectoris receiving monotherapy and combination treatment

The clinical utility of nifedipine and diltiazem blood levels in patients with angina pectoris receiving monotherapy (N = 14) and combination treatment (N = 9) were assessed in a placebo run-in, double blind, randomized, crossover study. Compared to placebo, diltiazem (mean daily dose 360 mg), nifedipine (mean daily dose 90 mg) and combination diltiazem-nifedipine therapy (mean daily dose 55 mg of nifedipine, 360 mg of diltiazem) were associated with reductions in weekly angina attacks and nitroglycerin consumption. Although both drugs used as monotherapy and in combination were also associated with significant increments in exercise tolerance and other improved angina parameters, these changes were not related to the plasma levels of either drug. Nifedipine plasma levels were measured by gas chromatography and diltiazem plasma levels measured by reverse high-pressure liquid chromatography from specimens obtained 2-5 hours after the last previous dose, after 1, 2 and 3 weeks of treatment, and during baseline placebo and placebo washout periods. With combination therapy, there was no effect on the diltiazem plasma level compared to monotherapy. The significant decrease in the nifedipine dose in patients on combination therapy did not significantly change nifedipine plasma levels. Determinations of plasma levels of diltiazem and nifedipine in the management of patients is of no value in the management of patients with angina pectoris except for monitoring treatment compliance and overdosage.

Absorption of amlodipine unaffected by food. Solid dose equivalent to solution dose

The oral bioavailability of amlodipine in healthy volunteers was compared in two separate studies after solution and capsule doses, and after capsule doses in fed and fasting states. The bioavailability of amlodipine was equivalent both in terms of rate and in extent of absorption between solution and capsule doses and in the fed and fasting states.

The effect of sublingual nifedipine on coronary venous graft resistance immediately following cardiopulmonary bypass

The purpose of this study was to determine the effects of sublingual nifedipine administered immediately after discontinuation of cardiopulmonary bypass on coronary graft resistance and systemic hemodynamics. Twenty patients were prospectively randomized into two groups; one given 10 mg sublingual nifedipine after weaning from bypass, the other given a placebo. Coronary graft blood flow was measured under blinded conditions and graft resistance calculated from measurements obtained with an electromagnetic flow probe applied directly to the graft prior to and 15 minutes after drug administration. Serum nifedipine levels were determined immediately before and 15, 30, and 60 minutes after sublingual administration. All patients receiving nifedipine had therapeutic serum levels. Graft resistance in patients given nifedipine decreased a statistically significant average of 27% and increased slightly, but not statistically significantly so, in patients given sublingual placebos. There were no differences between the two groups in cardiac index or pulmonary capillary wedge pressures. We conclude that the administration of sublingual nifedipine to patients in the immediate postbypass period results in therapeutic serum nifedipine levels and decreases coronary graft resistance without affecting cardiac performance.

Total and free steady-state plasma levels and pharmacokinetics of nifedipine in patients with terminal renal failure

The total and free steady-state plasma levels of nifedipine in patients with renal failure have been compared with those in subjects with normal renal function. Studies were done after administration of nifedipine 10 mg t.d.s. p.o. for 5 days, after i.v. infusion of 4.4 mg, and after a single 10 mg oral dose. The systemic clearance of nifedipine after a single i.v.-dose was higher in subjects with renal insufficiency (854 ml/min) than in those with normal renal function (468 ml/min). After the single oral dose the AUC (6100 ng.min.ml-1) and maximum plasma concentration (75.0 ng.ml-1) were lower than in subjects with normal renal function (19300 ng.ml-1; 122 ng.ml-1). The plasma protein binding of nifedipine averaged 95.5% in normal subjects and 94.8% in patients with renal failure. Although free and total steady-state plasma levels of nifedipine tended to be somewhat lower than normal in renal failure, the changes in pharmacokinetics and decreased protein binding of nifedipine did not result in a significantly different steady-state plasma level of the drug. The blood pressure response to a given plasma nifedipine level appeared to be enhanced in renal failure.

Pharmacokinetics of oral nifedipine: relevance of the distribution phase

Pharmacokinetics of oral Nifedipine was studied in 12 Mexican young healthy volunteers, six men and six women, who received a 10 mg capsule. Plasma levels were determined by a nifedipine specific HPLC assay. Experimental data were fitted and pharmacokinetic parameters were calculated using an open two compartment model. No statistically significant difference was detected between men and women, thus both sexes were considered as a single population. Nifedipine plasma levels rose rapidly (ka = 8.46 +/- 1.96 h-1) reaching a maximum concentration of 145 +/- 23 ng/ml in 0.61 +/- 0.07 h. Plasma levels then decayed with a distribution phase (alpha = 1.98 +/- 0.40 h-1, t1/2 alpha = 0.46 +/- 0.06 h) and a terminal elimination phase (beta = 0.17 +/- 0.03 h-1, t1/2 beta = 4.98 +/- 0.55 h). AUC was 384 +/- 41 ng h/ml. Values of AUC and t1/2 beta were higher than those reported by other authors. Differences in the AUC could be due to ethnic origin, environmental factors or nutritional habits. Ten subjects presented plasma concentration-time curves in which the distribution phase was clearly distinguishable, having a ka/alpha relationship higher than 1.5. For the other two subjects, the distribution phase was not apparent and ka/alpha was lower than 1.5. The results show that an adequate characterization of the distribution phase is required if one pretends to use pharmacokinetic data for dosage regimen design.

Cardiovascular effects of a nifedipine infusion during fentanyl-nitrous oxide anesthesia in dogs

The hemodynamic effects of a nifedipine infusion were investigated in eight dogs given fentanyl/pancuronium/nitrous oxide/oxygen anesthesia. Nifedipine (20 micrograms/kg) was given intravenously over two minutes immediately prior to each 30-minute infusion at 2 micrograms/kg/min, 4 micrograms/kg/min, and 6 micrograms/kg/min. The range of plasma nifedipine levels obtained was 52.1 to 113.7 ng/mL. The predominant hemodynamic effects were significant reductions in systemic vascular resistance (SVR) and mean aortic pressure (MAP), accompanied by a rise in cardiac index and heart rate (HR). Administration of calcium chloride (20 mg/kg) after the nifedipine infusion had no effect on SVR or MAP, but HR was significantly reduced. Serum epinephrine and norepinephrine levels increased after the infusion of nifedipine and suggested that fentanyl did not completely overcome the sympathetic response to the profound vasodilatation. The resulting tachycardia in combination with diastolic hypotension from nifedipine could have a detrimental effect on the myocardial oxygen balance.

Analysis of amlodipine in serum by a sensitive high-performance liquid chromatographic method with amperometric detection

An analytical method for a new calcium channel blocking agent, amlodipine, has been developed using high-performance liquid chromatography with electrochemical detection. No compound modification is required for detection and the calibration curve in spiked sera is linear and reproducible over the range 0.2-2.0 ng ml-1. The method has been applied successfully to pharmacokinetics studies in rats and also can be used for other dihydropyridine compounds such as nifedipine and nicardipine.

[Clinical and pharmacokinetic comparison of 2 oral preparations of nifedipine: 10 mg capsules and 20 mg delayed-release tablets]

The effects and pharmacokinetic parameters of two oral formulations of nifedipine, 10 mg capsule (Adaltat) and 20 mg slow release tablet (Adalat a.p.). With the 10 mg capsule nifedipine was rapidly absorbed, reaching a maximum concentration of 120 +/- 39 ng/ml in 0.52 +/- 0.07 h, and also rapidly eliminated with an apparent halflife of 5.51 +/- 0.64 h. A fall in blood pressure and a raise in heart rate, that significantly correlated with plasma levels, were observed. 83% of the subjects reported headache, that was probably due to the sudden increase in plasma levels. With the 20 mg slow release tablet nifedipine absorption was slower, reaching a maximum concentration of 39 +/- 7 ng/ml in 1.82 +/- 0.43 h, and the apparent half-life (16.89 +/- 3.14 h) was longer than with the capsule. A fall in blood pressure was observed that significantly correlated with plasma levels; however, there was no significant correlation between these and changes in heart rate. Only 17% of the subjects reported headache. Pharmacokinetic data indicate that, in most subjects, nifedipine therapeutics plasma levels (over 15 ng/ml) can be maintained with the administration of a 20 mg slow release tablet every 12 hours. This, joined to the reduction in side effects, suggest that this formulation is the adequate alternative in chronic treatments with nifedipine, such as arterial hypertension.

Determination of the calcium antagonist benidipine hydrochloride in plasma by sensitive radioimmunoassay

A sensitive radioimmunoassay of the 1,4-dihydropyridine calcium channel blocker (+/-)-(R*)-2,6-dimethyl-4-(m-nitrophenyl)-1,4-dihydropyridine-3,5-dicarb oxylic acid (R*)-1-benzyl-3-piperidinyl ester, methyl ester hydrochloride (benidipine hydrochloride, KW-3049) has been developed. Antiserum against KW-3049 was produced in rabbits by immunization with an immunogen prepared by conjugating a derivative of KW-3049 to bovine serum albumin. This antiserum was found to specifically bind to [3H]-KW 3049, while the recognition to [3H]-nitrendipine, another well-known dihydropyridine calcium channel blocker, was less pronounced. With the antiserum, [3H]-KW-3049 and dextran coated charcoal, this radioimmunoassay could detect 39 approximately equal to 500 pg/tube of KW-3049 in a buffer system, and 156 to 5000 pg/ml of KW-3049 in plasma by using 0.5 ml of the plasma which was pretreated with MeOH for deproteinization and extracted with diethyl ether under alkaline condition. To assess the specificity of the radioimmunoassay, the inhibition of [3H]-KW-3049 binding to the antiserum by the presumable metabolites was examined. Though three of these presumable metabolites could slightly inhibit the binding of [3H]-KW-3049, they were not detected in rat and dog plasma at 0.5 h after oral administration of KW-3049. Plasma levels of KW-3049 in rats receiving a single oral dose (1 mg/kg) determined by the radioimmunoassay show good agreement with those obtained by gas chromotography.

Gas chromatographic method for the quantification of the new calcium antagonist benidipine hydrochloride in plasma using electron capture detection

A gas chromatographic assay procedure was developed for measuring subnanogram order concentrations of (+/-)-(R*)-2,6-dimethyl-4-(m-nitrophenyl)-1,4-dihydropyridine-3,5- dicarboxylic acid (R*)-1-benzyl-3-piperidinyl ester, methyl ester hydrochloride (benidipine hydrochloride, KW-3049) in plasma. KW-3049 yielded the oxidation product partially during gas chromatography. To avoid decomposition, KW-3049 was oxidized in advance by nitrogen dioxide, extracted by diethylether under alkaline condition, chromatographed on the OV-1 column and measured using electron capture detector.