Population pharmacokinetics of enterally administered cisapride in young infants with gastro-oesophageal reflux disease.

AIMS: To investigate the pharmacokinetics of enterally administered cisapride suspension in young infants being treated for gastro-oesophageal reflux disease. METHODS: Plasma cisapride concentrations in 49 subjects (weight: 825-5010 g; n=108 samples, median two per patient; concentration: 14.8-170 ng ml-1 ) were fitted to a one-compartment model with first-order absorption and elimination in the NONMEM program using a logarithmic transformation of the observed and predicted concentrations. Fitting was achieved using the first order conditional estimation (FOCE) method with interaction between the interpatient and intrapatient variabilities. The interpatient variance of clearance (CL/F ) and volume of distribution (V /F ) and their covariance were estimated using an exponential error model. Intrapatient (residual) variance was estimated using an additive model. RESULTS: The clearance of cisapride was shown to be linearly related to current body weight, slope: 0.538. The typical population values of CL/F, V /F and Ka (absorption rate constant) were 0.538 l h-1 kg-1, 21.9 l, and 2.58 h-1, respectively. The population coefficients of variation (CV%) for CL/F and V/F were 34.4% and 84.3%, respectively. The squared coefficient of correlation between random effects for CL/F and V /F was 0.45. The intrapatient variance was 0.15. V /F and Ka were not influenced significantly by any patient characteristic. CONCLUSIONS: Cisapride pharmacokinetics in infants with reflux disease were satisfactorily described by a one-compartment model. Current weight should be taken into account when calculating maintenance cisapride doses in these infants.

Population analysis of the non linear red blood cell partitioning and the concentration-effect relationship of draflazine following various infusion rates.

AIMS: To investigate the impact of the specific red blood cell binding on the pharmacokinetics and pharmacodynamics of the nucleoside transport inhibitor draflazine after i.v. administration at various infusion rates. It was also aimed to relate the red blood cell (RBC) occupancy of draflazine to the ex vivo measured adenosine breakdown inhibition (ABI). METHODS: Draflazine was administered to healthy volunteers as a 15-min i.v. infusion of 0.25, 0.5, 1, 1.5 and 2.5 mg immediately followed by an infusion of the same dose over 1 h. Plasma and whole blood concentrations were measured up to 120 h post dose, and were related to the ex vivo measured ABI, serving as a pharmacodynamic endpoint. The capacity-limited specific binding of draflazine to the nucleoside transporter located on the erythrocytes was evaluated by a population approach. RESULTS: The estimate of the population parameter typical value (%CV) of the binding constant Kd and the maximal specific binding capacity (Bmax) was 0.385 (3.5) ng ml-1 plasma and 158 (2.1) ng ml-1 RBC, respectively. The non-specific binding was low. The specific binding to the erythrocytes was a source of non-linearity in the pharmacokinetics of draflazine. The total plasma clearance of draflazine slightly decreased with increasing doses, whereas the total clearance in whole blood increased with increasing doses. The sigmoidal Emax equation was used to relate the plasma and whole blood concentration of draflazine to the ex vivo determined ABI. In plasma, typical values (%CV) of Emax, IC50 and Hill factor were 81.4 (1.9)%, 3.76 (9.3) ng ml-1 and 1.06 (3.4), respectively. The relationship in whole blood was much steeper with population parameter typical values (%CV) of Emax, IC50 and Hill factor of 88.2 (2.0)%, 65.7 (2.8) ng ml-1 and 4.47 (5.5), respectively. The RBC occupancy of draflazine did not coincide with the ex vivo measured ABI. The observed relationship between RBC occupancy and ABI was not directly proportional but similar for all studied infusion schemes. CONCLUSIONS: The findings of this study show that the occupancy of the nucleoside transporter by draflazine should be at least 90% in order to inhibit substantially adenosine breakdown in vivo. On the basis of these findings it is suggested that a 15 min infusion of 1 mg draflazine followed by an infusion of 1 mg h-1 could be appropriate in patients undergoing a coronary artery bypass grafting.

A model with separate hepato-portal compartment ("first-pass" model): fitting to plasma concentration-time profiles in humans.

PURPOSE: To demonstrate the value of "first-pass" pharmacokinetic models (FPMs) in which the hepato-portal (HP) system is kinetically separated from the central compartment in fitting pharmacokinetic data obtained after intravenous (IV) and oral administration. METHODS: Plasma concentration-time profiles of an investigational drug obtained in six healthy subjects each received 4 mg as an intravenous (IV) bolus dose and 10 mg as an oral solution served as a real data example. The common three- and four-compartment models with the first-order absorption and lag time (3CM and 4CM, respectively) in which HP system is assumed to be part of the central compartment were used as alternative models. We tested also: (i) the sensitivity of the output of FPM to variations in its parameters assuming IV and oral administration; (ii) practical estimability of the FPM parameters by fitting it to 20 simulated noisy data sets; (iii) distinguishability of FPM, 3CM and 4CM by fitting them to the simulated data sets. RESULTS: FPM was shown to give the best fit as compared to 3CM or 4CM in 5 subjects of 6. The sensitivity of FPM was sufficient for the sake of parameter estimation. The "individual" means of parameter estimates obtained after fitting simulated data did not differ significantly from the preselected values. The variance in "individual" estimates was dependent on the sampling frequency. FPM was demonstrated to be distinguishable among relevant models. CONCLUSIONS: FPM is preferable as compared to standard compartmental models for drugs extensively taken up by the intestine and/or the liver, and may have a broad spectrum of applications.

Population analysis of the non-linear red blood cell partitioning of draflazine following various infusion durations.

OBJECTIVE: The pharmacokinetics and non-linear red blood cell partitioning of the nucleoside transport inhibitor draflazine were investigated in 19 healthy male and female subjects (age range 22-55 years) after a 15-min i.v. infusion of 1 mg, immediately followed by infusions of variable rates (0.25, 0.5 and 1 mg.h-1) and variable duration (2-24 h). METHODS: The parameters describing the capacity-limited specific binding of draflazine to the nucleoside transporters located on erythrocytes were determined by NONMEM analysis. The red blood cell nucleoside transporter occupancy of draflazine (RBC occupancy) was evaluated as a pharmacodynamic endpoint. RESULTS: The population typical value for the dissociation constant Kd (%CV) was 0.648 (12) ng.ml-1 plasma, expressing the very high affinity of draflazine for the erythrocytes. The typical value of the specific maximal binding capacity Bmax (%CV) was 155 (2) ng.ml-1 RBC. The interindividual variability (%CV) was moderate for Kd (38.9%) and low for Bmax (7.8%). As a consequence, the variability in RBC occupancy of draflazine was relatively low, allowing the justification of only one infusion scheme for all subjects. The specific binding of draflazine to the red blood cells was a source of non-linearity in draflazine pharmacokinetics. Steady-state plasma concentrations of draflazine virtually increased dose-proportionally and steady state was reached at about 18 h after the start of the continuous infusion. The t1/2 beta averaged 11.0-30.5 h and the mean CL from the plasma was 327 to 465 ml.min-1. The disposition of draflazine in whole blood was different from that in plasma. The mean t1/2 beta was 30.2 to 42.2 h and the blood CL averaged 17.4-35.6 ml.min-1. CONCLUSION: Although the pharmacokinetics of draflazine were non-linear, the data of the present study demonstrate that draflazine might be administered as a continuous infusion over a longer time period (e.g., 24 h). During a 15-min i.v. infusion of 1 mg, followed by an infusion of 1 mg.h-1, the RBC occupancy of draflazine was 96% or more. As the favored RBC occupancy should be almost complete, this dose regimen could be justified in patients.

Pharmacokinetics of acetylsalicylic acid and its metabolites at low doses: a compartmental modeling.

The pharmacokinetics of acetylsalicylic acid (ASA) and its metabolites salicylic acid (SA) and salicyluric acid (SUA) were studied in 12 healthy young volunteers after oral administration of low (30 and 100 mg) and moderate (400 mg) doses. Plasma and urine were assayed for the above drugs by high-performance liquid chromatographic method. Individual pharmacokinetic parameters were estimated by compartmental modeling (ASA and SA) and by model-independent methods (SUA). ASA parameter values estimated in this study were in agreement with those reported by other authors after administration of higher doses, which confirms the linearity of ASA pharmacokinetics in a broad dose range. On the contrary, both metabolic and renal elimination routes for SA were found to be saturable. The relative changes in SA renal clearance with the dose were more pronounced than those in metabolic clearance. Particularly, there was no statistically significant difference in SA metabolic clearance between 30 and 100 mg doses, indicating the linear kinetics in this dose range. Further increase in the dose resulted in significant decrease in SA metabolic clearance. At the same time, both SA excretion rate constant and fraction excreted significantly diminished across the entire dose range studied. The dependence of SUA renal clearance upon the dose was shown to be complex, reflecting possible saturability of its excretion.

Modeling of the saturable time-constrained amoxicillin absorption in humans.

Amoxicillin pharmacokinetics was modeled using a two-compartment disposition model and a saturable time-constrained absorption model with a storage compartment. The absorption model parameters estimated by the nonlinear regression are: a rate constant of the systemic input, ksys, (median: 1.31 h-1, range: 0.79-7.01 h-1), a maximal absorption rate, Vmax (median: 1407 mg/h, range: 703-4181 mg/h), an account corresponding to the half maximal rate, Kma, (median: 1077 mg, range: 235-4376 mg), time of the absorption cessation, Tabs, (median: 1.72 h, range: 0.82-4.53 h) and absorption lag time. Tlag, (median: 0.085 h, range: 0-0.123 h). It was shown, that the first-order absorption parallel to the saturable process is negligible in the dose range studied. The model described well the dependence of areas under concentration-time curves on the dose determined in several earlier studies. It was used also to predict the fraction of the amoxicillin dose absorbed for different doses. Simulations performed over a wide dose range (50-10000 mg) demonstrated that the fraction absorbed decreases nonlinearly from 90% at 50 mg to 22% at 10000 mg and strongly depends on the duration of the absorption period.

Dose-ranging study of NG-nitro-L-arginine pharmacokinetics in rats after bolus intravenous administration.

1. NG-nitro-L-arginine (10, 30 and 100 mg/kg) was administered intravenously to the male Wistar rat. Plasma was collected over 48, 72 and 120 h and was analysed for the drug by hplc. Pharmacokinetic parameters were calculated using a non-compartmental method. 2. Drug concentration-time profiles of individual rats after all doses studied exhibited secondary peaks, while geometric mean concentration-time curves showed plateaus. 3. NG-nitro-L-arginine plasma concentrations divided by dose almost coincided. Pharmacokinetic parameters were not dose-dependent in the range of 10-30 mg/kg, but changed after 100 mg/kg of NG-nitro-L-arginine indicating some decline from linearity. 4. NG-nitro-L-arginine is a low-extracted drug in rat as the total clearance was low (0.05-0.07 l/h/kg). Half-life and mean residence time were found to be long (17-30 and 23-40 h, respectively). Despite its low lipophilicity, NG-nitro-L-arginine exhibited large steady-state and terminal volumes of distribution (1.4-2.2 l/kg and 1.4-2.4 l/kg, respectively). Together with the double peak phenomenon, these results may be explained by assuming NG-nitro-L-arginine is involved in a recirculation process in the body.

The use of physiologically based models to simulate enantioselective differences in pharmacokinetics.

The majority of synthetic drugs are chiral and are usually administered as racemates. However, body proteins can recognize the steric configuration, and the pharmacological activity and pharmacokinetics of enantiomers usually differ. The key parameters that determine the overall enantioselectivity in disposition are those quantifying protein binding and biotransformation of drugs. Physiologically based models (PBMs) are effectively applied to predict pharmacokinetics of drugs using those parameters. In this work we used PBM to evaluate a range of changes in the model-independent pharmacokinetic parameters (total clearance, mean residence time, volume of distribution, half-life) with changing relative fraction of enantiomers unbound in the blood (range 1-8) and relative intrinsic hepatic clearance (range 1-10). A pharmacokinetic interaction between enantiomers was also simulated and it was shown that experiments with separate administration of pure enantiomers and of a racemate are equally important to avoid biases in pharmacokinetic parameter estimates due to interactions between enantiomers. Concentration-effect relationships were analyzed for the case of one enantiomer being active, and it was demonstrated that hysteresis as well as proteresis may appear depending on the differences in hepatic intrinsic clearances of enantiomers or fractions unbound in blood. The great predictive potential of physiologically based models in stereopharmacokinetics was thereby demonstrated.

The use of a nonlinear absorption model in the study of ascorbic acid bioavailability in man.

A two-compartment disposition model of ascorbic acid (AA) pharmacokinetics with saturable and time-constrained intestinal absorption was developed. The model was fitted to pharmacokinetic data obtained after oral administration to nine healthy volunteers of two effervescent dosage forms differing in AA content: Celaskon 60 mg (CK60) and Celaskon 500 mg (CK500). It was demonstrated that in the case of CK500 less than 30% of the dose was absorbed as compared with CK60. Parameters of the AA nonlinear absorption kinetics were assessed by simultaneous fitting of mean concentration-time data for both doses and placebo. The relatively short duration of absorption found (3.2 h) can explain the failure of past attempts to increase the AA bioavailability using sustained-release dosage forms. Model simulation showed that the ingestion of 60 mg with 3-4 h intervals is optimal for maximal bioavailability of AA.