Jeffrey R. Koup

Pharmacokinetics of digoxin in normal subjects after intravenous bolus and infusion doses

Normal subjects were given 0.75 mg of intravenous digoxin as a bolus and a 1-hr infusion, Radio-immunoassayed serum concentrations obtained over 48 hr and urinary excretion rates over 6 days were simultaneously fitted to a two- compartment open model by computer nonlinear least-squares regression. Serum concentration data alone were also fitted by this program. There was good agreement in calculated parameters between the two routes of administration in five of eight subjects, but the infusion mode of administration produced less variability in the apparent pharmacokinetic constants. The β half-life values obtained from serum concentration data alone (24.2 hr) underestimated the half-lives obtained by the simultaneous fit (44.1 hr). The steady-state volume of distribution of digoxin averaged 590±164 liters (±1sd).The renal clearance of digoxin (140±41 ml/min/1.73 m2)was significantly higher than creatinine clearance (101±13 ml/min/ 1.73 m2),indicating tubular secretion of the drug. Digoxin body clearances were 188±44 ml/min/ 1.73 m2,indicating elimination of 25% of the dose by nonrenal mechanisms. Urinary excretion data are essential for proper pharmacokinetic analysis of digoxin disposition and reveal a slower rate of elimination than that suggested by earlier studies which determined only serum concentrations.

Digoxin pharmacokinetics: Role of renal failure in dosage regimen design

Radioimmunoassayed serum concentration and urinary excretion data for digoxin from azotemic patients were characterized using a 2‐compartment open model. Urinary excretion rates of digoxin as well as serum concentration data are needed to accurately characterize the disposition of the drug. Seven patients with renal failure showed highly variable steady‐state volumes of distribution (VSSD = 195 to 489 liters/1.73 m2) and tÛβ values (1.5 to 5.2 days). This variability is a major limiting factor in the use of dosage regimen nomograms that assume a constant VSSD and a rigorous relationship between tÛβ and creatinine clearance (ClCR). Body clearance (ClB) is a parameter that is affected by both elimination and distribution of drugs. A linear relationship between ClB and renal clearance of digoxin or ClCR was found and was used to develop a model‐independent approach to calculation of maintenance doses of digoxin. Several methods for calculating steady‐state serum concentrations of digoxin (CSSD) were compared with actual measurements obtained in 16 chronically medicated patients. Optimum computation of CSSD is obtained by use of digoxin renal and body clearances. Variability in the digoxin: creatinine renal clearance ratio is the major limiting factor in prediction of digoxin dosage regimens.

Intravenous Theophylline Therapy: Nomogram Guidelines

We evolved a nomogram for guiding and standardizing intravenous theophylline therapy in hospitalized patients. It provides rapid calculation of a loading dose based on body weight and previous therapy and a maintenance infusion rate related to three categories of expected metabolic activity. The guidelines were prospectively used in the treatment of 72 patients, mainly in a respiratory care unit. The nomogram was successfully used to attain near-steady-state serum concentrations in the therapeutic range of 8 to 20 mg/litre in 72% of patients, with only two patients outside of the range of 5 to 25 mg/litre. These guidelines facilitate initial theophylline dosage in older patients with liver and cardiac disease and provide a rational basis for interpreting serum concentration measurements and adjustment of drug therapy.

Theophylline pharmacokinetics in premature infants with apnea.

The pharmacokinetics of theophylline were examined in eight low-birth-weight infants (gestation: 26-32 weeks: birth-weight: 887-1,480 gm), who received the drug for treatment of primary apnea. The drug was assayed by high pressure liquid chromatography. The final dosage was 1 to 3 mg/kg/6 hour at 25 to 37 days of age. At the time, theophylline had a prolonged half-life ranging from 13 to 29 hours, a relatively large volume of distribution of 0.65 to 2.86 1/kg, and a small body clearance of 23 to 68 ml/hr/kg. The extremely slow and variable elimination of theophylline must be considered in treatment of apneic infants. The initial dosage regimen suggested is a loading dose of 6 mg/kg and a maintenance dose of 2 mg/kg/ 12 hours, with adjustments made based on monitoring of the serum concentration and on an increased biotransformation capability as maturation occurs.

Nonlinear assessment of phenytoin bioavailability

The bioavailability of phenytoin, a drug subject to capacity-limited disposition, was examined using linear and nonlinear pharmacokinetic techniques. The linear method (comparative areas) underestimates the essentially complete bioavailability of this drug from capsules (Epanutin, Parke-Davis). The error incurred in using area ratios is inversely related to the rate of absorption of the drug. The time course of absorption of phenytoin capsules is irregular and prolonged over nearly 2 days.

Pharmacokinetics of ciprofloxacin in cystic fibrosis.

We studied the pharmacokinetics of ciprofloxacin in 12 adult males with and 12 adult males without cystic fibrosis (CF). In a randomized crossover sequence, the subjects received 200 mg intravenously or 750 mg orally. With intravenous dosing, subjects also received 651 mg of iothalamate, a marker of glomerular filtration, and 700 mg of antipyrine, an indicator of hepatic oxidative drug metabolism. Pharmacokinetic parameters were determined by model independent methods. In the CF subjects, the ciprofloxacin concentration in serum during the first hour after intravenous administration was higher, and the oral absorption rate was slower. Other parameters did not differ between the groups. Mean concentrations in serum 5 min postinfusion were 3.08 and 2.14 micrograms/ml, and mean peak concentrations after oral dosing were 3.24 and 3.34 micrograms/ml in subjects with and without CF, respectively. Mean values for elimination half-life in all subjects were 4.8 and 5.0 h after intravenous and oral administration, respectively. The mean renal clearances in all subjects after intravenous and oral administration were 19.4 and 14.5 liters/h and accounted for 64 and 47% of the total clearance, respectively. These values were significantly greater than renal iothalamate clearance, indicating that tubular secretion contributed to the renal clearance of ciprofloxacin. A total of 69 and 35.4% of the administered ciprofloxacin was recovered from the urine within 48 h after intravenous and oral administration, respectively. The mean bioavailability was 71.2% and did not differ between the groups. We conclude that similar dosing regimens can be used to treat patients with CF and their normal counterparts.

Pharmacokinetics of three oral formulations of ciprofloxacin.

We compared the absorption of three formulations of ciprofloxacin after oral administration in 18 normal adult male volunteers. Each subject received 500 mg of ciprofloxacin as two 250-mg tablets, one 500-mg tablet, or a solution in a randomized crossover sequence. Pharmacokinetic parameters were determined by model independent methods. Because a solution is considered to be the ideal oral dosage form, the results determined for the tablets were compared to those for the solution. Mean values for the maximum concentration of drug in serum, the time to maximum concentration of drug in serum, and the elimination half-life were 3.23 micrograms/ml, 1.00 h, and 5.04 h, respectively, for the solution. The mean renal clearance of ciprofloxacin was 372 ml/min and accounted for at least 50% of the total clearance. We recovered 44.4, 48.6, and 55.8% of the administered ciprofloxacin from the urine as unchanged drug within 24 h after dosing with the 250-mg tablets, 500-mg tablets, or solution, respectively. The 500-mg tablets were found to be bioequivalent to the solution with regard to all pharmacokinetic parameters. The 250-mg tablet was not bioequivalent to either of the other formulations; the relative bioavailability values were 78.7 and 74.1%, respectively, for the 500-mg tablet and the solution. The clinical significance of this difference in bioavailability is yet to be determined.

Ethosuximide pharmacokinetics in a pregnant patient and her newborn.

Ethosuximide concentration in serum was monitored during the last trimester of pregnancy in a patient. After delivery, the decline in serum concentration of ethosuximide was observed in the nonnursing neonate. The half‐life of elimination of transplacentally acquired ethosuximide in this neonate was 41.3 hr. The ratio of breast milk to maternal serum concentration of ethosuximide was approximately 1. A total daily exposure to ethosuximide of 12.8 to 38.4 mg (3.6 to 11.0 mg/kg) as a result of nursing was predicted.

Chloramphenicol Pharmacokinetics in Hospitalized Patients

The apparent body clearance of chloramphenicol was investigated in 21 hospitalized adult patients on 27 occasions. Apparent body clearance was found to be significantly lower (1.99 ± 1.49 ml/min per kg) in patients with total serum bilirubin concentrations of >1.5 mg/100 ml than in patients with serum bilirubin concentrations of ≤1.5 mg/100 ml (3.57 ± 1.72 ml/min per kg; P < 0.001). Serum protein binding of chloramphenicol was lower in cirrhotic patients (42.2 ± 6.8% bound) than in normal adults (53.1 ± 5.2% bound; P < 0.001). Low binding of chloramphenicol was also found in the serum of premature neonates (32.4 ± 8.2% bound; P < 0.001). Reduced binding in neonates implies the need for a lower therapeutic range of total chloramphenicol concentration (3.5 to 13.9 μg/ml) compared with the usual adult range (5 to 20 μg/ml). Finally, three case reports are presented which demonstrate marked abnormalities and intrasubject variation in chloramphenicol clearance.

Pharmacokinetics of rifampin in children. II. Oral bioavailability.

The absolute bioavailability of oral rifampin was determined in 20 pediatric patients. Intravenous doses of rifampin (mean 287 mg/m2) were compared with p.o. doses (mean 324 mg/m2). Serum concentrations of rifampin, 25-O-desacetylrifampicin, and 3-formylrifamycin SV were determined by high performance liquid chromatography. Following a 1/2-h intravenous infusion, serum rifampin concentrations declined in a monoexponential fashion. Pharmacokinetic analysis of the rifampin serum concentration data indicated that only 50 +/- 22% of a freshly prepared p.o. suspension was absorbed. The rifampin elimination half-life following i.v. administration (2.25 +/- 0.64 h) was not different from that observed following p.o. dose administration (2.61 +/- 1.35 h). Peak rifampin concentrations were significantly higher following i.v. administration when corrected to a 300 mg/m2 dose (27.4 vs. 9.1 micrograms/ml, respectively, p less than 0.0001) than after p.o. administration. The peak concentration following a p.o. dose occurred at 2.0 +/- 0.9 h. The ratio of desacetylrifampicin to rifampin areas under the curves were similar for i.v. and p.o. routes of administration (0.23 vs. 0.19), suggesting linear metabolism of rifampin to this metabolite. 3-formylrifamycin SV concentrations were lower than those of desacetylrifampicin and were detectable in less than half of the patients. The results of this study indicate the need for larger p.o. doses when serum concentrations similar to those obtained following intravenous doses are desired.