Frequency distribution of free warfarin and free phenytoin fraction values in serum of healthy human adults.

The protein binding of racemic warfarin was determined in serum from 57 normal adults (27 men and 30 women). The free fraction of warfarin (total concentration, 0.8 mug/ml) ranged from 0.0050 to 0.0186 and was log-normally distributed. The frequency distribution differs from that in rats in which the serum free fraction values of warfarin are trimodally distributed. The protein binding of phenytoin was determined in serum from 39 of the subjects. The free fraction of phenytoin (total concentration, 15 mug/ml) ranged from 0.111 to 0.155 and was unimodally distributed. There was no apparent correlation between the extent of protein binding of warfarin and phenytoin in individual serum samples. The pronounced intersubject variation in serum free fraction of warfarin observed in consistent with our previous finding that serum protein binding is an important determinant of interindividual differences in the total clearance of warfarin in man. On the other hand, the relatively narrow distribution of free phenytoin fraction values suggests that differences in serum protein binding of phenytoin are not an important cause of the prounounced interindividual differences in the total clearance of phenytoin by subjects with normal renal function.

Intrasubject variation of warfarin binding to protein in serum of patients with cardiovascular disease.

A study of 31 patients with cardiovascular disease who were taking warfarin regularly had shown pronounced intersubject differences in serum protein binding of warfarin and a highly significant correlation between the body clearance of warfarin and the free fraction of the drug in serum. Similar observations have been made in experimental animals and are consistent with predictions based on theoretical considerations. The purpose of this investigation was to determine the intrasubject variation in the free fraction of warfarin in serum. Samples of serum were obtained from 23 of the 31 patients previously studied. The time interval between the two studies was 3.4 to 5.7 mo. The daily dose of warfarin had been changed by 10.6% on the average. With two exceptions, there was no change in concurrent medications. The ratio of free fraction values of warfarin in serum, second/first study, was 0.948 +/- 0.297 (mean +/- S.D.), and there was a highly significant correlation (P less than 0.001) between the individual free fraction values in the first and second studies.

Serum protein binding as a determinant of warfarin body clearance and anticoagulant effect.

The serum protein binding and elimination kinetics of warfarin were determined in 31 patients with cardiovascular disease who were taking warfarin regularly. The free fraction of warfarin in the serum ranged from 0.00436 to 0.0189, indicating 98.11% to 99.56% protein binding. There was no apparent relationship between the extent of protein binding of warfarin and the concentration of albumin or total protein in the serum. The estimated total body clearance of warfarin in the patients ranged from 1.16 to 4.35ml/hr/kg of body weight and correlated significantly with the free fraction of warfarin in serum. This correlation has been predicted on theoretical grounds and shows that serum protein binding is a major determinant of the elimination kinetics of warfarin in man and an important cause of interindividual variations in its body clearance. The interindividual variation of free warfarin concentrations in the serum of patients with similar prothrombin times was somewhat smaller than the variations in total serum-warfarin concentrations and in the daily dose of warfarin. There was no correlation between prothrombin time and the concentration of free warfarin in serum, indicating that variables other than protein binding also affect the anticoagulant response of patients.

Bretylium kinetics in renal insufficiency.

Bretylium kinetics were examined in patients with varying degrees of renal impairment after a single intravenous dose of bretylium tosylate. Maximum plasma concentrations achieved at the end of the infusion, when normalized to the dose, correlated strongly with creatinine clearance. Drug disposition from plasma was biexponential, with a short distributive phase, but drug elimination was reduced, especially in patients with creatinine clearance below 30 ml/min X 1.73 m2. There was reduction in renal and total clearance and prolongation of t 1/2, with deteriorating renal function. In one patient who was reevaluated after a year, there was 76% reduction in the total clearance, corresponding to 43% deterioration of renal function. The difference of 33% between these values is due to a reduction of nearly 36% in volume of distribution, caused by the further deterioration of the renal function. Six-hour hemodialysis procedure on two anephric patients, resulted in an apparent one- to threefold increase in the computed bretylium clearance during dialysis, but the fraction of the total body load eliminated during the same period was not proportionally significant. The strong linear relationships between renal and total clearance, beta, and the creatinine clearance, may be helpful in adjusting dosage regimens for bretylium in patients with renal dysfunction.

Kinetics of esmolol, an ultra-short-acting beta blocker, and of its major metabolite.

Esmolol is an ultra-short-acting beta blocker. Its kinetics was studied in eight healthy subjects after continuous intravenous infusion of 400 micrograms/kg/min over 2 hr. The concentrations of esmolol and its major metabolite, 3-[4-(2-hydroxy-3-[isopropylamino]propoxy)phenyl]propionic acid, in blood and urine were determined by gas chromatographic-mass spectrometric assay and HPLC. The distribution and elimination t1/2s of esmolol averaged 2.03 and 9.19 min. The apparent volume of distribution of esmolol averaged 3.43 l/kg and was four times the volume of the central compartment. The total clearance of esmolol averaged 285 ml/min/kg, indicating that nonhepatic routes play a predominant role in its clearance. The t1/2s of formation and elimination of the metabolite averaged 2.82 min and 3.72 hr. The ratio of the metabolite formation and elimination rate constants of the parent drug (kf/k10) averaged 0.829, suggesting that 82.9% of esmolol was converted to the metabolite (which is consistent with the urinary recovery of 71% of the dose as unconjugated metabolite). The volume of distribution and total clearance of the metabolite averaged 0.411 l/kg and 1.28 ml/min/kg. Esmolol was followed by a significant reduction of isoproterenol-induced increase in heart rate and systolic blood pressure at doses of 50, 150, and 400 micrograms/kg/min.(ABSTRACT TRUNCATED AT 250 WORDS)

A multiple-dose safety and bioequivalence study of a narrow therapeutic index drug: a case for carbamazepine.

BACKGROUND AND OBJECTIVES: Carbamazepine is among those drugs that have been considered to have a narrow therapeutic plasma concentration range, that is, a narrow therapeutic index. Although the US Food and Drug Administration has approved new generic products based on standard single-dose bioequivalence studies, several state formularies, including the New Jersey Drug Utilization Review Council, have recently established additional criteria for acceptance of bioequivalence of narrow therapeutic index drugs, limiting the use of some approved generic drugs in specific states. To further validate the adequacy of single-dose studies for the determination of bioequivalence of narrow therapeutic index drugs, a multiple-dose study was conducted that more closely reflected therapeutic use. METHODS: A single-center, multiple-dose, randomized, open-label, 2-way crossover bioequivalence study was conducted in 32 fasting volunteers at steady state. Subjects received the test and reference products as a 200 mg carbamazepine tablet 3 times a day in a crossover fashion. Concentrations of carbamazepine and carbamazepine-10,11-epoxide in plasma were measured by a validated specific HPLC method. RESULTS: A total of 28 subjects completed the study. Pharmacokinetic parameters and measures of fluctuation for both products at steady state were similar, with 90% and 95% confidence intervals falling within 90% and 110%. CONCLUSION: The multiple-dose study provided reliable safety and bioequivalence data under rigorous statistical conditions and confirmed bioequivalence of test and reference products determined by a single-dose study.

Bretylium: relations between plasma concentrations and pharmacologic actions in high-frequency ventricular arrhythmias.

Although it is widely assumed that the early arrhythmogenic and pressor responses to bretylium are caused by catecholamine release from the adrenergic neuron, this assumption has not been systematically studied in humans. Pharmacologic responses to a placebo infusion and 3 separate bretylium infusions (2.5, 5.0, 10 mg/kg over 60 minutes) were assessed in 6 patients with recurrent, nonsustained ventricular tachycardia. Plasma bretylium concentration, blood pressure (BP), plasma norepinephrine (NE) concentration, arrhythmia frequency and adrenergic neuronal blockade (assessed by the presence or absence of reflex venoconstriction) were measured. Adrenergic blockade was seen with every bretylium infusion and at a time when relatively small amounts of bretylium had been administered (range 160 to 750 mg, median 252). Temporal relations (p less than 0.03) were noted among the time of onset of adrenergic neuronal blockade, onset of the pressor response, increase in NE plasma concentration and increase in ventricular arrhythmia frequency. BP responses during the infusions were linearly related to change in plasma NE at the time of development of adrenergic neuronal blockade. Bretylium plasma concentrations higher than 3 micrograms/ml were frequently associated with a short-lived pressor response. There was a significant relation (p less than 0.06) between the increase in plasma NE during the infusion and an increase in ventricular arrhythmia frequency. Reduction in arrhythmia frequency was seen in only 1 patient, beginning 6 hours after the development of adrenergic neuronal blockade.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic profile of cefixime in man.

Cefixime is an orally absorbed third generation cephalosporin with a broad spectrum of activity against Gram-positive and Gram-negative bacteria and is highly resistant to beta-lactamase degradation. In general serum and urinary concentrations of cefixime achieved after recommended daily doses are well above the minimal inhibitory concentrations at 90% for indicated microorganisms. The pharmacokinetics of cefixime were determined in adult and pediatric subjects. In general the half-life of the drug is about 3 to 4 hours and is not dependent on dose. The drug is not extensively bound to serum proteins; the free fraction is about 31% and is concentration-independent. The absolute bioavailability, based on comparisons of area under the serum concentration-time curve values after 200-mg intravenous, 200-mg oral solution, and 200- and 400-mg capsule doses, ranged from 40 to 52%, showing a comparable bioavailability for cefixime at single 200- and 400-mg oral doses. In a dose proportionality study, over an oral dose range of 200 to 2000 mg, peak serum concentration (Cmax) and area under the concentration-time values increased linearly but were not directly proportional with dose. Upon multiple dosing for 2 weeks on a 400-mg daily or 200-mg twice a day regimen, serum concentrations and urinary recovery of unchanged drug were similar for each group, and there was no drug accumulation. Peak serum concentration and area under the concentration-time curve values after a 400-mg dose were almost double those after a 200-mg dose. All formulations of cefixime were bioequivalent to an oral reference solution at doses of 200 and 400 mg.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of aspirin absorption rate from urinary excretion rate measurements.

It is shown on the basis of pharmacokinetic simulations and experimental data that adequate evaluation of aspirin dosage forms with different absorption rates by urinary excretion measurements in man requires that such measurements be made during the first hour after drug administration. Measurements made only after 3 hours are totally inadequate for that purpose. An assessment of the completeness of absorption obviously does not require serial measurements but needs only a determination of the total amount excreted.

Dose proportionality of bisoprolol enantiomers in humans after oral administration of the racemate.

Dose proportionality of racemic bisoprolol and the stereoselectivity of its enantiomers were studied after single oral dosing of 5 to 40 mg of bisoprolol hemifumarate in eight healthy male volunteers in an open-label, randomized, four-way cross-over trial. There were dose-proportional increases in mean peak plasma concentration (Cmax) and area under the plasma concentration versus time curve (AUC) values for the racemate and the individual enantiomers. No statistically significant differences were detected between the mean half life (t 1/2), Cmax, and time to reach Cmax (tmax) of the R- and S-isomers at each of the four dose levels studied. These findings support dose proportionality and absence of stereoselective pharmacokinetics for bisoprolol in the dose range studied.

Absolute bioavailability of cefixime in man.

In a four-way cross-over study, the absolute bioavailability of cefixime was determined in 16 healthy volunteers. Each subject received a single 200-mg dose as an intravenous (IV) and oral solution, and 200-mg and 400-mg capsule doses of the drug. Blood and urine samples were collected for 24 hours after each dose. Cefixime was well tolerated after IV and oral doses of the drug and no serious drug-related adverse effects were observed. The maximal serum concentration (Cmax) of cefixime following the 200-mg oral solution and 200-mg and 400-mg capsule doses were 3.22, 2.92, and 4.84 micrograms/mL, respectively. Mean area under the serum concentration time curves (AUC) following the IV, 200-mg oral solution, and 200-mg and 400-mg capsule doses were 47.0, 26.0, 23.6, and 39.4 micrograms.hr/mL, respectively. Mean elimination half-life values of the drug were comparable after oral and IV doses, ranging from 3.2 to 3.5 hours. Based on serum AUC values, the absolute bioavailability of cefixime was 52.3%, 47.9%, and 40.2% after the 200-mg oral solution, 200-mg capsule and 400-mg capsule doses, respectively. Respective ratios based on 24-hour urinary recovery data were 44.7%, 41.7%, and 40.5%. Therefore, the results show that the percent of cefixime adsorbed after 200-mg and 400-mg oral doses was similar.

Importance of oral dosing rate on the hemodynamic and pharmacokinetic profile on nilvadipine.

Nilvadipine was administered as an oral solution formulation to 12 normotensive subjects in a three-way randomized crossover study at a dose of 16 mg as three different dosing regimens: 1) as a single 16 mg dose, 2) as a 1.6 mg dose given hourly for 10 doses, and 3) as an initial dose of 4.8 mg, followed by 1.6 mg doses given every hour for seven additional doses. After each dose, clinical effects, hemodynamic changes and the pharmacokinetic profile of the drug were determined. The mean maximum changes in diastolic (DBP) and systolic (SBP) blood pressure and heart rate (HR) after dosing regimens 1, 2, and 3 were: -33, -13 and +46%; -17, -14 and +38%; and -24, -14 and +36%, respectively. There was a relationship between the changes in DBP and HR and plasma concentrations of nilvadipine only after dosing regimen 1. The effect-concentration relationships were fit to a modified Emax model. There was no relationship between the change in SBP and plasma concentration after any of the dosing regimens. While there were no significant differences in the mean area under the plasma concentration-time curve (AUC0----infinity) between dosing regimens 2 (38.7 ng.hr/mL) and 3 (42.1 ng.hr/mL) (P greater than 0.05), the mean AUC0----infinity after regimen 1 (76.3 ng.hr/mL) was significantly greater than after dosing regimens 2 or 3 (P less than 0.05). The mean maximal plasma concentrations (Cmax) were 31.6, 1.3 and 6.3 ng/mL after dosing regimens 1, 2 and 3, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Who needs individual bioequivalence studies for narrow therapeutic index drugs? A case for warfarin.

Warfarin is, among drugs, considered to have a narrow therapeutic index for which individual bioequivalence has been suggested. To establish the propriety of "switching," an individual bioequivalence study involving a replicate-design study and three "switchings" in healthy subjects was undertaken using the U.S.-brand warfarin sodium tablet and a generic product. A randomized, single-center, open-label, single-dose, four-way crossover replicate bioequivalence study was performed in 24 healthy male volunteers in which each subject received the same 5 mg warfarin test and reference tablets twice on different occasions under fasting conditions. Concentrations of warfarin in plasma were measured by a validated specific HPLC method. The individual pharmacokinetic parameters obtained with test and reference products were compared using pooled data and Liu's method. Bioequivalence was shown with both average and individual bioequivalence methods. The individual bioequivalence assessment did not show a subject-by-formulation interaction, nor did it add value to the bioequivalence assessment of warfarin.

The assessment of the intrasubject variability in digoxin absorption in man from two oral dosage forms.

The reproducibility of drug absorption within a given subject as well as the evaluation of bioavailability of two digoxin dosage forms were studied. The data showed (a) a higher initial plasma digoxin concentration after the soft elastic gelatin (SEG) capsule; (b) a more irregular absorption after the tablet; (c) on the average, the coefficients of variation of individual plasma concentrations were lower after the capsule; and (d) for the capsule, the intrasubject variations of the peak plasma concentrations, time of peak, area under plasma concentrations-versus-time curve (AUC), and amount digoxin excreted in urine (Ae) were on the average 60 per cent of the variations in the tablet parameters. The ratios of AUC and Ae for capsule/tablet were essentially unity, indicating that the amount digoxin absorbed from the 0.4-mg digoxin SEG capsule is identical to that from a 0.5-mg standard reference tablet.

Pharmacokinetics of hydroxyethyl starch in normal subjects.

To determine the elimination of high-molecular-weight hydroxyethyl starch (HES, Mw 450,000) in normal subjects, ten volunteers were given 500 ml 6% HES solution by intravenous infusion, and serial blood and urine samples were collected for nonglucose total carbohydrate determination. On the average, 46 and 64 per cent of the dose was excreted in the urine within two and eight days, respectively. The plasma concentration declined rapidly during the first week after infusion. The average terminal half-life was 17 days during the first 42 days, which accounted for elimination of about 90 per cent of the dose. The remainder was eliminated with a terminal half-life of 48 days determined between days 42 and 83 of the study. As expected, the infusion of HES resulted in plasma volume expansion over a 48-hour period during which time levels of nonglucose carbohydrates were above 3.5 mg/ml. HES is metabolized by alpha-amylase in the body. During the first 48 hours after infusion of HES, plasma alpha-amylase activity was significantly increased over control. Concomitantly, alpha-amylase activity in urine was also elevated but not significantly so.

Existing and new criteria for bioequivalence evaluation of new controlled release (CR) products of carbamazepine.

While the three classical pharmacokinetic (PK) parameters, AUC, Cmax and tmax are adequate to assess bioequivalence of immediate release (IR) formulations, they are not designed to fully characterize the pharmacokinetic (PK) performance of controlled release (CR) formulations and provide only limited insight into the function of carbamazepine (CBZ) CR products. Thus, for reliable assessment of bioequivalence in CR formulations, there is a role for the use of additional criteria (parameters). The following are the proposed new parameters: MRT (mean residence time), Cmax/AUC, plateau time or POT (the time span associated with the concentrations within 25% of Cmax), tapical (the arithmetic mean of the times associated with POT) and Capical (the arithmetic mean of the concentrations within 25% of Cmax). The above proposed parameters, were utilized in a recent PK study of new CR products of CBZ (600 mg) designed for once daily dosing. The comparative PK analysis was conducted in a three-way crossover single dose studies of three CBZ CR formulations (Teril 600 CR tablet, CBZ 600 granulate and Timonil 600 Retard tablet). Teril 600 CR was found to be bioequivalent to Timonil 600 Retard while CBZ 600 granulate was not. This conclusion was reached utilizing both the classical and the proposed new parameters. The new parameters showed that CBZ 600 granulate has similar rate of absorption as the two 600 mg CR tablets, but its extent of absorption was lower. The new parameters examined in this paper are more attractive than the single point parameters, Cmax and tmax, for assessment of rate of absorption and the flatness of the plasma concentration versus time curve. Their potential benefit and practical utility was confirmed in this study, which demonstrated bioequivalence between a new CR and an innovator CBZ (600 mg) tablet. Absorption rate assessment is important in light of concentration-related side effects associated with CBZ therapy and the impact of fluctuations and the flatness of the CBZ plasma concentration curve on the drug efficacy and tolerability.

Chemical characterization of the persistent fraction of hydroxyethyl starch in rat serum and spleen.

Hydroxyethyl starch (HES) found in rat serum and spleen after single and daily administrations of 0.9 g/kg for 1 week was characterized by gas-liquid chromatography. There was very little difference in the degree of substitution (D.S.) and molar substitution (M.S.) of HES in serum samples obtained at 1 hour and 57 days after multiple doses and of HES in spleen samples obtained at 1 hour and 168 days after a single dose of HES. The small increase in D.S. and M.S. was due to a decrease in the glucose content and not due to a change in the ratio of mono- to poly-substituted glucoses.

Comparative pharmacokinetics of coumarin anticoagulants. XIV: relationship between protein binding, distribution, and elimination kinetics of warfarin in rats.

The relationships between the protein binding, distribution in the body, and kinetics of elimination of warfarin were studied. Individual rats eliminated warfarin by apparent first-order kinetics, with a biological half-life of 5.9-41 hr and a total plasma clearance of 2.4-22 ml kg(-1) hr(-1). There is a strong positive correlation between the apparent volume of distribution (Vd) and the elimination rate constant (kel). There was no apparent concentration dependance of warfarin binding to serum proteins over a wide concentration range, but there were pronounced intersubject variations in protein binding, with the free fraction of drug (f) in serum ranging from 0.172 x 10(-2) to 1.53 x 10(-2). There are strong positive correlations between f and kel, f and Vd, and f and the kidney-serum concentration ratio of warfarin. Consistent with theory, there is an excellent positive linear correlation between f and total plasma clearance of the drug. The intersubject variation in f is not related to variations in serum albumin or total protein concentration. There is a strong correlation between values of f for serum and liver homogenate in individual animals, consistent with the lack of correlation between f in serum and the liver-serum concentration ratio of warfarin. These results show that the pronounced intersubject variation in the elimination of warfarin observed in this investigation was related to interindividual differences in plasma protein binding of the drug. The differences in protein binding cannot be ascribed to differences in plasma protein concentrations and may reflect configurational differences of proteins or the presence of an endogenous displacing agent at different concentrations.

Effect of serum protein binding on sulfisoxazole distribution, metabolism, and excretion in rats.

This investigation determined the effect of serum protein binding on the kinetics of sulfisoxazole distribution, metabolism, and excretion. Adult rats, whose serum free fraction of sulfisoxazole (at a total concentration of 81 +/- 6 micrograms/ml) was 0.05-0.24, received a rapid intravenous injection of 20 mg/kg. Sulfisoxazole concentrations in plasma declined biexponentially with time. There were pronounced and reproducible interindividual differences in the total, metabolic, and renal sulfisoxazole clearances, each positively correlated with the serum free fraction of sulfisoxazole. The renal sulfisoxazole clearance had a component unaffected by serum protein binding. The apparent central compartment volume increased with an increasing serum free sulfisoxazole fraction, but the latter had not apparent effect on the first exponential term of the biexponential equation describing sulfisoxazole elimination kinetics in rats. Serum protein binding was a major determinant of intersubject differences in sulfisoxazole excretion and biotransformation kinetics.

Intraindividual relationships between serum protein binding of drugs in normal human subjects, patients with impaired renal function, and rats.

The serum protein binding of phenytoin, salicylic acid, sulfisoxazole, and warfarin was determined in normal human adults, in patients with impaired renal function (kidney donor and recipient), and in adult male Sprague--Dawley rats. The free fraction values for salicylate and sulfisoxazole were significantly correlated in all three groups. The other correlations were statistically significant in only one or two of these groups. There was a statistically significant negative correlation between albumin concentration and the free fraction values of salicylic acid and sulfisoxazole (but not of phenytoin and only under special circumstances with warfarin) in normal subjects and of phenytoin, salicylic acid, and sulfisoxazole (but not warfarin) in rats. No such correlation was observed for any of the drugs in patients with impaired renal function. These observations show that no single weakly acidic drug can serve as an index for quantitatively determining the effect of disease or species differences on the serum protein binding of other weakly acidic drugs.