Population pharmacokinetics of mycophenolic acid in kidney transplant pediatric and adolescent patients.

Current data on mycophenolate mofetil (MMF) suggest that there is a pharmacokinetic/pharmacodynamic relationship between the mycophenolic acid (MPA) area under the curve (AUC) during treatment and both the risk of acute rejection and the occurrence of side effects. The aim of this study was to characterize the population pharmacokinetics of MPA in kidney transplant patients between the ages of 2 and 21 years and to propose a limited sampling strategy to estimate individual MPA AUCs. Forty-one patients received long-term oral MMF continuous therapy as part of a triple immunosuppressive regimen, which also included cyclosporine or tacrolimus (n=3) and corticosteroids. Therapy was initiated at a dose of 600 mg/m twice daily. The population parameters were calculated from an initial group of 32 patients. The data were analyzed by nonlinear mixed-effect modeling using a 2-compartment structural model with first-order absorption and a lag time. The interindividual variability in the initial volume of distribution was partially explained by the fact that this parameter was weight-dependent. Fifteen concentration-time profiles from 13 patients were used to evaluate the predictive performance of the Bayesian approach and to devise a limited sampling strategy. The protocol, involving two sampling times, 1 and 4 hours after oral administration, allows the precise and accurate determination of MPA AUCs (bias -0.9 microg.h/mL; precision 6.02 microg.h/mL). The results of this study combine the relationships between the pharmacokinetic parameters of MPA and patient covariates, which may be useful for dose adjustment, with a convenient sampling procedure that may aid in optimizing pediatric patient care.

Intravenous omeprazole in children: pharmacokinetics and effect on 24-hour intragastric pH.

BACKGROUND: Omeprazole is a proton pump inhibitor, acting selectively on the gastric parietal cell H+K+-adenosine triphosphatase. Data on the intravenous route are limited in children and not available in infants. OBJECTIVE: This study was designed to determine the pharmacokinetics and the optimal dosage of intravenous omeprazole in patients younger than 30 months of age. METHODS: Nine children (three girls), aged 4.5 to 27 months, with normal liver and renal functions requiring intravenous omeprazole were studied. After enrollment in the study and randomization, omeprazole was administered once daily, at 8 am, as a 1-hour infusion. Group 1, consisting of the first four patients, received 20 mg/1.73 m2, and group 2, consisting of the following five patients, received 40 mg/1.73 m2. At day 3, a 24-hour intragastric pH and a pharmacokinetic study of omeprazole were performed. Plasma concentrations were measured by high-performance liquid chromatography. RESULTS: Patients in group 2 had a significantly higher median pH (6.99 vs. 3.35; P = 0.01) and percent of monitored time with gastric pH >4 than children given 20 mg/1.73 m2 (90.6% vs. 44.8%; P < 0.01). Four had a pH more than 4 during more than 90% of the time versus none of the patients of group 1. The plasma concentration versus time curves showed rapid elimination of the drug. The median area under the curve of omeprazole was 0.78 microg. mL-1. h-1 (range, 0.55-1.64 microg. mL-1. h-1) and 3.95 microg. mL-1. h-1 (range, 1.9-4.9 microg. mL-1. h-1), respectively, in groups 1 and 2 (P < 0.05). Systemic clearance was not different between the two groups: median values were 0.68 and 0.42 L. kg-1. h-1 (P = 0.22). CONCLUSIONS: In critical situations, intravenous administration of omeprazole may be required in infants. The authors demonstrate that the dose of 20 mg/1.73 m2 is not effective in maintaining 24-hour gastric pH of more than 4 and that a dose of 40 mg/1.73 m2 is required.

Lansoprazole in children: pharmacokinetics and efficacy in reflux oesophagitis.

BACKGROUND: Data on the proton pump inhibitor lansoprazole in paediatric patients are limited. AIM: To investigate the pharmacokinetics, optimal dosage and efficacy of lansoprazole in paediatric patients. METHODS: A 24-h gastric pH recording and a pharmacokinetic study were performed after 7 days of lansoprazole, 17 mg/m2, in 23 patients with reflux oesophagitis (median age, 3.5 years). Response was defined as pH > 3 for > 65% of the recording. The dosage was doubled in non-responders. Patients with no response on day 14 were excluded. Responders underwent endoscopy after 4 weeks on the response-inducing dosage; abnormal findings led to a repeat endoscopy after four additional weeks. RESULTS: Nine patients responded to 17 mg/m2 and six to 30.3 mg/m2. On day 7, time with pH > 3 was significantly correlated with the area under the plasma concentration-time curve (P=0.003). The area under the plasma concentration-time curve was significantly greater in the nine responders to 17 mg/m2 than in the 14 other patients. Pharmacokinetic parameters were similar in responders and non-responders to the higher dose. After 4 weeks, oesophagitis was healed in 80% of responders. Adverse events occurred in three patients and required treatment discontinuation in one. CONCLUSIONS: Lansoprazole is effective and safe in children. The optimal starting dosage is 30 mg/m2 or 1.4 mg/kg.

Pharmacokinetics and tolerance of mycophenolate mofetil in renal transplant children.

Mycophenolate mofetil (MMF) is a prodrug that is hydrolyzed to the active immunosuppressant mycophenolic acid (MPA). The drug is now widely prescribed for adult renal transplant recipients and its use has been extended to pediatric patients, although pharmacological data in this age group are limited. Nine pediatric renal transplant recipients received MMF with corticosteroids and either cyclosporine or tacrolimus a median of 55 months (range 7.5-124 months) months after transplantation. The pharmacokinetic parameters of MPA and MPA glucuronide (MPAG) were determined at steady state by high-performance liquid chromatography after administration of MMF at the oral dose of 494+/-142 mg/m(2) twice daily. MPA was rapidly absorbed, with a peak concentration at 1.4 h. The mean plasma concentration of MPA at steady state was 4.7+/-1.3 microg/ml. The areas under the plasma concentration-time curves (AUCs) over 12 h (between two administrations) were 57.0+/-15.3 microg.h/ml for MPA and 1,515+/-722 microg.h/ml for MPAG, and the apparent oral clearance was 11.7+/-7.0 and 0.5+/-0.4 l/h for MPA and MPAG, respectively. Assuming that the pharmacokinetics of MPA was dose dependent, the mean concentration at steady state and the AUC for MPA were calculated for the recommended dosage schedule of 600 mg/m(2) every 12 h and were 6.3+/-2.7 microg/ml and 75.2+/-32.9 microg.h/ml, respectively. The tolerance of MMF was studied prospectively with a follow-up of 1.1+/-0.2 years. Gastrointestinal disorders requiring dosage reduction or discontinuation of therapy, observed in five of nine patients, occurred at an incidence higher than expected from adult data. Our results suggest that the dose of 600 mg/m(2) every 12 h extrapolated from adult data for use in pediatric patients would be associated with plasma levels and AUCs higher than expected and may be associated with a higher incidence of side-effects, primarily gastrointestinal.

In-vitro cytochrome P450 dependent metabolism of oxybutynin to N-deethyloxybutynin in humans.

Oxybutynin (Ditropan), a direct antagonist of acetylcholine on muscarinic receptors, is administered in children for the treatment of vesical immaturity and is responsible for atropinic adverse effects, which are more frequent in paediatric than in adult patients. Oxybutynin is metabolized by oxidation to N-desethyloxybutynin, a stable and toxic metabolite, and previous results in vivo have suggested that cytochrome P450 2D6 (CYP2D6) may be involved in this pathway of metabolism. We used human liver microsomes genotyped as extensive metabolizers for CYP2D6 to determine the kinetic parameters of N-desethyloxybutynin formation: Km was 16.5+/-5.2 microM and Vmax was 76.8+/-3.7 mmol/mg/h. Quinidine and anti-liver kidney microsome antibody type I had no inhibitory effects on N-desethyloxybutynin formation, demonstrating that CYP2D6 has no role in oxybutynin metabolism. The effects of specific inhibitors of other cytochromes P450 were also investigated. Recombinant human CYP 3A4 but not 2B6, 2D6, 2C8 and 2E1, displayed significant N-desethylation activity. Our results demonstrate that the CYP3A subfamily, and not CYP2D6, is involved in N-desethyloxybutynin formation.