Pharmacokinetics of oral valganciclovir solution and intravenous ganciclovir in pediatric renal and liver transplant recipients.

In an open-label, prospective, pharmacokinetic assessment, we evaluated total drug exposure (area under the curve [AUC]) of intravenous (IV) ganciclovir (GCV) and oral (p.o.) valganciclovir when normalized for body surface area (BSA) in pediatric liver (n=20) and renal (n=26) transplant patients Reference doses for IV GCV (200 mg/m(2)) and p.o. valganciclovir (520 mg/m(2)) were based on adult doses, and adjusted for BSA initially, and BSA and renal function (estimated via creatinine clearance [CrCL]) thereafter. Renal transplant patients received GCV on days 1-2, valganciclovir 260 mg/m(2) on day 3, and valganciclovir 520 mg/m(2) on day 4. Liver transplant patients received twice daily GCV from enrollment to day 12, and then valganciclovir twice daily on days 13-14. GCV pharmacokinetics were described using a population pharmacokinetic approach. Type of solid organ transplant (kidney or liver) had no effect on GCV pharmacokinetics. Median GCV exposure following valganciclovir 520 mg/m(2) was similar to that with IV GCV, and to that reported in adults. Patients <5 years of age had AUC values approximately 50% of those compared with older age ranges; dosing based on both BSA and CrCL increased drug exposure in younger patients. A dosing algorithm based on BSA and CrCL should be tested in future studies.

Valganciclovir dosing according to body surface area and renal function in pediatric solid organ transplant recipients.

Oral valganciclovir is effective prophylaxis for cytomegalovirus (CMV) disease in adults receiving solid organ transplantation (SOT). However, data in pediatrics are limited. This study evaluated the pharmacokinetics and safety of valganciclovir oral solution or tablets in 63 pediatric SOT recipients at risk of CMV disease, including 17 recipients < or =2 years old. Patients received up to 100 days' valganciclovir prophylaxis; dosage was calculated using the algorithm: dose (mg) = 7 x body surface area x creatinine clearance (Schwartz method; CrCLS). Ganciclovir pharmacokinetics were described using a population pharmacokinetic approach. Safety endpoints were measured up to week 26. Mean estimated ganciclovir exposures showed no clear relationship to either body size or renal function, indicating that the dosing algorithm adequately accounted for both these variables. Mean ganciclovir exposures, across age groups and organ recipient groups were: kidney 51.8 +/- 11.9 microg * h/mL; liver 61.7 +/- 29.5 microg * h/mL; heart 58.0 +/- 21.8 microg * h/mL. Treatment was well tolerated, with a safety profile similar to that in adults. Seven serious treatment-related adverse events (AEs) occurred in five patients. Two patients had CMV viremia during treatment but none experienced CMV disease. In conclusion, a valganciclovir-dosing algorithm that adjusted for body surface area and renal function provides ganciclovir exposures similar to those established as safe and effective in adults.

Establishing pharmacokinetic bioequivalence of valganciclovir oral solution versus the tablet formulation.

Dosing with valganciclovir tablets may not be appropriate in some patients, such as those on hemodialysis or children. A "tutti-frutti" flavored oral valganciclovir solution has been developed to provide flexibility in dosage needed to accommodate these patients. An adult, multicenter, open-label randomized trial was conducted to establish bioequivalence between valganciclovir oral solution and valganciclovir tablets. Pharmacokinetic profiles and safety of the oral solution versus the tablet formulation were determined in 23 renal transplant recipients with estimated creatinine clearance>or=60 mL/min who had been receiving cytomegalovirus prophylaxis with valganciclovir for >or=4 days prior to the administration of the study drug. Patients received two doses of 900 mg valganciclovir either by tablet or oral solution in random order once daily over 6 days. Plasma concentrations of ganciclovir were assessed on days 2, 4, and 6 predose and at 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, and 12 hours after the dose. Maximum mean plasma concentrations (Cmax) were 6.73 microg/mL and 6.39 microg/mL for the valganciclovir tablet and oral solution, respectively, with identical mean AUC0-24 of 51.2 microg.h/mL. For both the AUC0-24 and Cmax ratio, the 90% Cl of the mean ratios of the oral solution relative to the tablet formulation lies within the acceptance region (80% to 125%) required by the US Food and Drug Administration and European Agency for the Evaluation of Medicinal Products. With the demonstration of bioequivalence and no differences in the incidence of adverse effects, it will be possible to interchangeably use the oral formulation.

Influence of biophase distribution and P-glycoprotein interaction on pharmacokinetic-pharmacodynamic modelling of the effects of morphine on the EEG.

BACKGROUND AND PURPOSE: The aim was to investigate the influence of biophase distribution including P-glycoprotein (Pgp) function on the pharmacokinetic-pharmacodynamic correlations of morphine's actions in rat brain. EXPERIMENTAL APPROACH: Male rats received a 10-min infusion of morphine as 4 mg kg(-1), combined with a continuous infusion of the Pgp inhibitor GF120918 or vehicle, 10 or 40 mg kg(-1). EEG signals were recorded continuously and blood samples were collected. KEY RESULTS: Profound hysteresis was observed between morphine blood concentrations and effects on the EEG. Only the termination of the EEG effect was influenced by GF120918. Biophase distribution was best described with an extended catenary biophase distribution model, with a sequential transfer and effect compartment. The rate constant for transport through the transfer compartment (k(1e)) was 0.038 min(-1), being unaffected by GF120918. In contrast, the rate constant for the loss from the effect compartment (k(eo)) decreased 60% after GF120918. The EEG effect was directly related to concentrations in the effect compartment using the sigmoidal E(max) model. The values of the pharmacodynamic parameters E(0), E(max), EC(50) and Hill factor were 45.0 microV, 44.5 microV, 451 ng ml(-1) and 2.3, respectively. CONCLUSIONS AND IMPLICATIONS: The effects of GF120918 on the distribution kinetics of morphine in the effect compartment were consistent with the distribution in brain extracellular fluid (ECF) as estimated by intracerebral microdialysis. However, the time-course of morphine concentrations at the site of action in the brain, as deduced from the biophase model, is distinctly different from the brain ECF concentrations.

Population pharmacokinetic modelling of non-linear brain distribution of morphine: influence of active saturable influx and P-glycoprotein mediated efflux.

BACKGROUND AND PURPOSE: Biophase equilibration must be considered to gain insight into the mechanisms underlying the pharmacokinetic-pharmacodynamic (PK-PD) correlations of opioids. The objective was to characterise in a quantitative manner the non-linear distribution kinetics of morphine in brain. EXPERIMENTAL APPROACH: Male rats received a 10-min infusion of 4 mg kg(-1) of morphine, combined with a continuous infusion of the P-glycoprotein (Pgp) inhibitor GF120918 or vehicle, or 40 mg kg(-1) morphine alone. Unbound extracellular fluid (ECF) concentrations obtained by intracerebral microdialysis and total blood concentrations were analysed using a population modelling approach. KEY RESULTS: Blood pharmacokinetics of morphine was best described with a three-compartment model and was not influenced by GF120918. Non-linear distribution kinetics in brain ECF was observed with increasing dose. A one compartment distribution model was developed, with separate expressions for passive diffusion, active saturable influx and active efflux by Pgp. The passive diffusion rate constant was 0.0014 min(-1). The active efflux rate constant decreased from 0.0195 min(-1) to 0.0113 min(-1) in the presence of GF120918. The active influx was insensitive to GF120918 and had a maximum transport (N(max)/V(ecf)) of 0.66 ng min(-1) ml(-1) and was saturated at low concentrations of morphine (C(50)=9.9 ng ml(-1)). CONCLUSIONS AND IMPLICATIONS: Brain distribution of morphine is determined by three factors: limited passive diffusion; active efflux, reduced by 42% by Pgp inhibition; low capacity active uptake. This implies blood concentration-dependency and sensitivity to drug-drug interactions. These factors should be taken into account in further investigations on PK-PD correlations of morphine.

Blood-brain barrier transport and brain distribution of morphine-6-glucuronide in relation to the antinociceptive effect in rats--pharmacokinetic/pharmacodynamic modelling.

1. The objective of this study was to investigate the contribution of the blood-brain barrier (BBB) transport to the delay in antinociceptive effect of morphine-6-glucuronide (M6G), and to study the equilibration of M6G in vivo across the BBB with microdialysis measuring unbound concentrations. 2. On two consecutive days, rats received an exponential infusion of M6G for 4 h aiming at a target concentration of 3000 ng ml(-1) (6.5 microM) in blood. Concentrations of unbound M6G were determined in brain extracellular fluid (ECF) and venous blood using microdialysis and in arterial blood by regular sampling. MD probes were calibrated in vivo using retrodialysis by drug prior to drug administration. 3. The half-life of M6G was 23+/-5 min in arterial blood, 26+/-10 min in venous blood and 58+/-17 min in brain ECF (P<0.05; brain vs blood). The BBB equilibration, expressed as the unbound steady-state concentration ratio, was 0.22+/-0.09, indicating active efflux in the BBB transport of M6G. A two-compartment model best described the brain distribution of M6G. The unbound volume of distribution was 0.20+/-0.02 ml g brain(-1). The concentration-antinociceptive effect relationships exhibited a clear hysteresis, resulting in an effect delay half-life of 103 min in relation to blood concentrations and a remaining effect delay half-life of 53 min in relation to brain ECF concentrations. 4. Half the effect delay of M6G can be explained by transport across the BBB, suggesting that the remaining effect delay of 53 min is a result of drug distribution within the brain tissue or rate-limiting mechanisms at the receptor level.

Pharmacokinetic-pharmacodynamic modelling of morphine transport across the blood-brain barrier as a cause of the antinociceptive effect delay in rats--a microdialysis study.

PURPOSE: To quantify the contribution of distributional processes across the blood-brain barrier (BBB) to the delay in antinociceptive effect of morphine in rats. METHODS: Unbound morphine concentrations were monitored in venous blood and in brain extracellular fluid (ECF) using microdialysis (MD) and in arterial blood by regular sampling. Retrodialysis by drug was used for in vivo calibration of the MD probes. Morphine was infused (10 or 40 mg/kg) over 10 min intravenously. Nociception, measured by the electrical stimulation vocalisation method, and blood gas status were determined. RESULTS: The half-life of unbound morphine in striatum was 44 min compared to 30 min in venous and arterial blood (p<0.05). The BBB equilibration of morphine, expressed as the ratio of areas under the curve between striatum and venous blood, was less than unity (0.28+/-0.09 and 0.22+/-0.17 for 10 and 40 mg/kg), respectively, indicating active efflux of morphine across the BBB. The concentration-effect relationship exhibited a clear hysterisis with an effect delay half-life of 32 and 5 min based on arterial blood and brain ECF concentrations, respectively. CONCLUSIONS: Eighty five percent of the effect delay was caused by morphine transport across the BBB, indicating possible involvement of rate limiting mechanisms at the receptor level or distributional phenomena for the remaining effect delay of 5 min.

Modelling of the blood-brain barrier transport of morphine-3-glucuronide studied using microdialysis in the rat: involvement of probenecid-sensitive transport.

The objective of this study was to investigate the impact of probenecid on the blood-brain barrier (BBB) transport of morphine-3-glucuronide (M3G). Two groups of rats received an exponential infusion of M3G over 4 h to reach a target plasma concentration of 65 microM on two consecutive days. Probenecid was co-administered in the treatment group on day 2. Microdialysis was used to estimate unbound M3G concentrations in brain extracellular fluid (ECF) and blood. In vivo recovery of M3G was calculated with retrodialysis by drug, preceding the drug administration. The BBB transport was modelled using NONMEM. In the probenecid group, the ratio of the steady-state concentration of unbound M3G in brain ECF to that in blood was 0.08+/-0.02 in the absence and 0.16+/-0.05 in the presence of probenecid (P=0.001). In the control group, no significant difference was found in this ratio between the 2 days (0.11+/-0.05 and 0.10+/-0.02, respectively). The process that appears to be mainly influenced by probenecid is influx clearance into the brain (0.11 microl min(-1) g-brain(-1) vs 0.17 microl min(-1) g-brain(-1), in the absence vs presence of probenecid, P:<0.001). The efflux clearance was 1.15 microl min(-1) g-brain(-1). The half-life of M3G was 81+/-25 min in brain ECF vs 22+/-2 min in blood (P<0.0001). Blood pharmacokinetics was not influenced by probenecid. In conclusion, a probenecid-sensitive transport system is involved in the transport of M3G across the BBB.

Methodological aspects of the use of a calibrator in in vivo microdialysis-further development of the retrodialysis method.

PURPOSE: To investigate the performance of two alternative retrodialysis recovery methods and to describe the influence of different recoveries on the reliability in estimating unbound extracellular concentrations of morphine. METHODS: Unbound concentrations of morphine in striatum and in blood were determined by microdialysis after a 10 min i.v. infusion in freely moving rats. In vivo recovery of morphine was determined by morphine itself, retrodialysis by drug, and by the calibrator nalorphine, retrodialysis by calibrator. RESULTS: The low calibrator recovery in striatum (8.6%) resulted in large variability in the estimation of unbound extracellular concentrations when retrodialysis by calibrator was used. In blood, where the recovery was higher (36%), the variability was smaller. Also, when retrodialysis by drug was used, the variability remained low. This difference is caused by the propagation of errors in the way retrodialysis recovery is determined. Therefore, calibrator recovery values > or =20% are preferable in concentration estimations using retrodialysis by calibrator. CONCLUSIONS: When no time-dependent change in recovery is observed, retrodialysis by drug determined before the systemic administration is the best method. The calibrator is valuable as a quality control during the experiment.

Application of intracerebral microdialysis to study regional distribution kinetics of drugs in rat brain.

1. The purpose of the present study was to determine whether intracerebral microdialysis can be used for the assessment of local differences in drug concentrations within the brain. 2. Two transversal microdialysis probes were implanted in parallel into the frontal cortex of male Wistar rats, and used as a local infusion and detection device respectively. Within one rat, three different concentrations of atenolol or acetaminophen were infused in randomized order. By means of the detection probe, concentration-time profiles of the drug in the brain were measured at interprobe distances between 1 and 2 mm. 3. Drug concentrations were found to be dependent on the drug as well as on the interprobe distance. It was found that the outflow concentration from the detection probe decreased with increasing lateral spacing between the probes and this decay was much steeper for acetaminophen than for atenolol. A model was developed which allows estimation of kbp/Deff (transfer coefficient from brain to blood/effective diffusion coefficient in brain extracellular fluid), which was considerably larger for the more lipohilic drug, acetaminophen. In addition, in vivo recovery values for both drugs were determined. 4. The results show that intracerebral microdialysis is able to detect local differences in drug concentrations following infusion into the brain. Furthermore, the potential use of intracerebral microdialysis to obtain pharmacokinetic parameters of drug distribution in brain by means of monitoring local concentrations of drugs in time is demonstrated.