R. A. A. Mathôt

Population pharmacokinetics of mycophenolic acid in renal transplant recipients

AbstractBackground: Mycophenolate mofetil is the prodrug of mycophenolic acid (MPA) and is used as an immunosuppressant following renal, heart, lung and liver transplantation. Although MPA plasma concentrations have been shown to correlate with clinical outcome, there is considerable inter- and intrapatient pharmacokinetic variability. Consequently, it is important to study demographic and pathophysiological factors that may explain this variability in pharmacokinetics.n Objective: The aim of the study was to develop a population pharmacokinetic model for MPA following oral administration of mycophenolate mofetil, and evaluate relationships between patient factors and pharmacokinetic parameters.n Patients and methods: Pharmacokinetic data were obtained from a randomised concentration-controlled trial involving 140 renal transplant patients. Pharmacokinetic profiles were assessed on nine occasions during a 24-week period. Plasma samples for description of full 12-hour concentration-time profiles on the first three sampling days were taken predose and at 0.33, 0.66, 1.25, 2, 6, 8 and 12 hours after oral intake of mycophenolate mofetil. For the remaining six occasions, serial plasma samples were taken according to a limited sampling strategy predose and at 0.33, 0.66, 1.25 and 2 hours after mycophenolate mofetil administration. The resulting 6523 plasma concentration-time data were analysed using nonlinear mixed-effects modelling.n Results: The pharmacokinetics of MPA were best described by a two-compartment model with time-lagged first-order absorption. The following population parameters were estimated: absorption rate constant (ka) 4.1h−1, central volume of distribution (V1) 91L, peripheral volume of distribution (V2) 237L, clearance (CL) 33 L/h, intercompartment clearance (Q) 35 L/h and absorption lag time 0.21h. The interpatient variability for ka, V1, V2 and CL was 111%, 91%, 102% and 31%, respectively; estimates of the intrapatient variability for ka, V1 and CL were 116%, 53% and 20%, respectively. For MPA clearance, statistically significant correlations were found with creatinine clearance, plasma albumin concentration, sex and ciclosporin daily dose (p < 0.001). For V1, significant correlations were identified with creatinine clearance and plasma albumin concentration (p < 0.001).n Conclusion: The developed population pharmacokinetic model adequately describes the pharmacokinetics of MPA in renal transplant recipients. The identified correlations appear to explain part of the observed inter- and intrapatient pharmacokinetic variability. The clinical consequences of the observed correlations remain to be investigated.

Effect of CYP2C19*2 and *17 mutations on pharmacodynamics and kinetics of proton pump inhibitors in Caucasians

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECTnThe influence of CYP2C19 on the kinetics and dynamics of omeprazole, lansoprazole and rabeprazole has been studied in Japanese subjects. * It has been suggested that subjects with *1/*1 genotype might need stronger acid suppression than *1/*2 and *2/*2 subjects. This suggestion comes from data in Japanese subjects and has not been confirmed in Caucasians. * Furthermore, a novel CYP2C19 mutation, *17, which mainly occurs in Caucasians has been discovered. This mutation has been associated with clinical failure, but its relevance for therapy with PPIs has not been studied yet.nnnWHAT THIS STUDY ADDSnIn this study, the influence of CYP2C19 on both the pharmacokinetics and dynamics in Caucasian subjects after single and repeated dosing has been investigated. * This is the first study showing that Caucasian subjects with *1/*1 and *1/*17 mutations need stronger acid-inhibition. In this study three proton pump inhibitors (omeprazole, lansoprazole and pantoprazole, in different doses) were studied of which pantoprazole had not been studied before in this setting, not even in Japanese.nnnAIMSnTo investigate the impact of CYP2C19 mutations *2-*6 and *17 on acid-inhibition and pharmacokinetics of lansoprazole (L15), omeprazole (O10, O20) and pantoprazole (P40) in Caucasians.nnnMETHODSnCYP2C19 genotyping for *2-*6 and *17 mutations was assessed in subjects who were H. pylori negative in two randomized crossover trials. The influence of CYP2C19 mutations on single and repeated administration of L15 and O10 (study A) and O20 and P40 (study B) was investigated. Pharmacokinetics and the cumulative percentage of time with intragastric pH above 4 (% > pH 4) were assessed on day 1 and 6.nnnRESULTSnFor study A CYP2C19 genotyping found five *1/*1, four *1/*2, one *1/*17 and one *2/*17. For study B the results were six *1/*1, two *1/*2, six *1/*17, one *2/*2 and one *2/*17. For all PPIs AUC was highest in *2/*2 and lowest in *1/*17. On day 1, all PPIs significantly increased percentage >pH 4 compared with baseline. *1/*1 genotype showed no significant acid-inhibition after L15, O10 and O20. *1/*17 genotype showed no significant acid-inhibition after O20 and P40. *1/*2 genotype showed significant acid-inhibition after L15 and O10. On day 6, all four PPIs showed significantly increased acid-inhibition. *1/*1 and *1/*17 showed a significantly increased percentage > pH 4 after treatment with O20 and P40. However, in *1/*1 subjects percentage > pH 4 was not significantly increased after L15 and O10. *1/*2 genotype showed a significant acid-inhibitory effect after repeated dosing with L15 and O10.nnnCONCLUSIONSnCaucasian subjects with *1/*1 and *1/*17 genotype need stronger acid-suppression therapy, especially during the first days of treatment or with on-demand therapy.

Pharmacokinetic role of protein binding of mycophenolic acid and its glucuronide metabolite in renal transplant recipients

Mycophenolic acid (MPA), the active compound of mycophenolate mofetil (MMF), is used to prevent graft rejection in renal transplant recipients. MPA is glucuronidated to the metabolite MPAG, which exhibits enterohepatic recirculation (EHC). MPA binds for 97% and MPAG binds for 82% to plasma proteins. Low plasma albumin concentrations, impaired renal function and coadministration of cyclosporine have been reported to be associated with increased clearance of MPA. The aim of the study was to develop a population pharmacokinetic model describing the relationship between MMF dose and total MPA (tMPA), unbound MPA (fMPA), total MPAG (tMPAG) and unbound MPAG (fMPAG). In this model the correlation between pharmacokinetic parameters and renal function, plasma albumin concentrations and cotreatment with cyclosporine was quantified. tMPA, fMPA, tMPAG and fMPAG concentration–time profiles of renal transplant recipients cotreated with cyclosporine (nxa0=xa048) and tacrolimus (nxa0=xa045) were analyzed using NONMEM. A 2- and 1-compartment model were used to describe the pharmacokinetics of fMPA and fMPAG. The central compartments of fMPA and fMPAG were connected with an albumin compartment allowing competitive binding (bMPA and bMPAG). tMPA and tMPAG were modeled as the sum of the bound and unbound concentrations. EHC was modeled by transport of fMPAG to a separate gallbladder compartment. This transport was decreased in case of cyclosporine cotreatment (Pxa0<xa00.001). In the model, clearance of fMPAG decreased when creatinine clearance (CrCL) was reduced (Pxa0<xa00.001), and albumin concentration was correlated with the maximum number of binding sites available for MPA and MPAG (Pxa0<xa00.001). In patients with impaired renal function cotreated with cyclosporine the model adequately described that increasing fMPAG concentrations decreased tMPA AUC due to displacement of MPA from its binding sites. The accumulated MPAG could also be reconverted to MPA by the EHC, which caused increased tMPA AUC in patients cotreated with tacrolimus. Changes in CrCL had hardly any effect on fMPA exposure. A decrease in plasma albumin concentration from 0.6 to 0.4xa0mmol/l resulted in ca. 38% reduction of tMPA AUC, whereas no reduction in fMPA AUC was seen. In conclusion, a pharmacokinetic model has been developed which describes the relationship between dose and both total and free MPA exposure. The model adequately describes the influence of renal function, plasma albumin and cyclosporine co-medication on MPA exposure. Changes in protein binding due to altered renal function or plasma albumin concentrations influence tMPA exposure, whereas fMPA exposure is hardly affected.

Population pharmacokinetics of mycophenolic acid : a comparison between enteric-coated mycophenolate sodium and mycophenolate mofetil in renal transplant recipients.

AbstractObjective: The pharmacokinetics of mycophenolic acid (MPA) were compared in renal transplant patients receiving either mycophenolate mofetil (MMF) or enteric-coated mycophenolate sodium (EC-MPS).n Methods: MPA concentration-time profiles were included from EC-MPS- (n = 208) and MMF-treated (n = 184) patients 4–257 months after renal transplantation. Population pharmacokinetic analysis was performed using nonlinear mixed-effects modelling (NONMEM®). A two-compartment model with first-order absorption and elimination was used to describe the data.n Results: No differences were detected in MPA clearance, intercompartmental clearance, or the central or peripheral volume of distribution. Respective values and interindividual variability (IIV) were 16 L/h (39%), 22 L/h (78%), 40 L (100%) and 518 L (490%). EC-MPS was absorbed more slowly than MMF with respective absorption rate constant values of 3.0 h−1 and 4.1 h−1 (p < 0.001) [IIV 187%]. A mixture model was used for the change-point parameter lag-time (tlag) in order to describe IIV in this parameter adequately for EC-MPS. Following the morning dose of EC-MPS, the tlag values were 0.95, 1.88 and 4.83 h for 51%, 32% and 17% of the population (IIV 8%), respectively. The morning tlag following EC-MPS administration was significantly different from both the tlag following MMF administration (0.30 h; p < 0.001 [IIV 11%]) and the tlag following the evening dose of EC-MPS (9.04 h; p < 0.001 [IIV 40%]). Post hoc analysis showed that the tlag was longer and more variable following EC-MPS administration (morning median 2.0 h [0.9–5.5 h], evening median 8.9 h [5.4–12.3 h]) than following MMF administration (median 0.30 h [0.26–0.34 h]; p < 0.001). The morning MPA predose concentrations were higher and more variable following EC-MPS administration than following MMF administration, with respective values of 2.6 mg/L (0.4–24.4 mg/L) and 1.6 mg/L (0.2–7.6 mg/L). The correlation between predose concentrations and the area under the plasma concentration-time curve (AUC) was lower in EC-MPS-treated patients (r2 = 0.02) than in MMF-treated patients (r2 = 0.48).n Conclusion: Absorption of MPA was delayed and also slower following EC-MPS administration than following MMF administration. Furthermore, the tlag varied more in EC-MPS-treated patients. MPA predose concentrations were poorly correlated with the MPA AUC in both MMF- and EC-MPS-treated patients.

Population Pharmacokinetics of Midazolam and Its Metabolites during Venoarterial Extracorporeal Membrane Oxygenation in Neonates

Background and ObjectiveMidazolam is used to sedate children during extracorporeal membrane oxygenation (ECMO). Pharmacokinetic changes are expected because of extracorporeal circulation and maturation. We present a population pharmacokinetic model for midazolam and its major metabolites in neonates during venoarterial ECMO.MethodsWe studied 20 neonates on venoarterial ECMO, with a median postnatal age of 0.79 (range 0.17–5.8) days and a bodyweight of 3.0 (range 2.7–3.9) kg at the onset of ECMO. The median ECMO duration was 124 (range 70–275) hours. Serum concentrations were measured at the initiation and discontinuation of the midazolam infusion (100–300 μg/kg/h). Analysis of concentrations of midazolam, 1-hydroxymidazolam and its glucuronide were performed using nonlinear mixed-effects modelling. A two-compartment model for midazolam and a one-compartment model for the metabolites 1-hydroxymidazolam and hydroxymidazolam glucuronide adequately described the data, with allometric scaling of all parameters.ResultsFollowing the start of ECMO, the volume of distribution of midazolam increased from 4.29 to 14.6 L/3kg, with an elimination half-life of 1.85 hours. The median midazolam and 1-hydroxymidazolam clearance values increased 3-fold within the first 5 days (up to 1.38 and 5.31 L/h/3 kg, respectively), whereas hydroxymidazolam glucuronide clearance remained constant at 0.18 L/h/3 kg. Interpatient variability estimates of midazolam, 1-hydroxymidazolam and hydroxymidazolam glucuronide clearance and midazolam and hydroxymidazolam glucuronide volumes of distribution varied between 87% and 129%. Concomitant inotropic infusion increased hydroxymidazolam glucuronide clearance by 23%.ConclusionAfter allometric scaling, clearance of midazolam and 1-hydroxymidazolam increases as a result of maturation or recovery from critical illness. In ECMO patients weighing 2.7–3.9 kg, continuously infused midazolam doses of 300 μg/kg/h for 6 hours and 150 μg/kg/h thereafter provide adequate serum concentrations for sedation. The dose must be increased substantially after 5–7 days. Hydroxymidazolam glucuronide accumulates during ECMO, providing an increased proportion of the overall effect, up to 34% after 7 days. Large unexplained interpatient variability warrants careful titration of sedation and adverse effects.

Association of graded allele-specific changes in CYP2D6 function with imipramine dose requirement in a large group of depressed patients.

The inactivation and clearance of the tricyclic antidepressant imipramine is dependent on CYP2D6 activity. First, CYP2C19 converts imipramine into the active metabolite desipramine, which is then inactivated by CYP2D6. This retrospective single center study aimed to prove whether CYP2C19 and ample CYP2D6 genotyping (taking into consideration four null alleles and three decreased-activity alleles) could be used to predict imipramine and desipramine plasma concentrations in depressed patients, and whether genotype-based drug dose recommendations might assist in the early management of imipramine pharmacotherapy. In 181 subjects with major depressive disorder, drug doses were recorded, imipramine and desipramine plasma concentrations were monitored and CYP2C19 (*2) and CYP2D6 genotype (*3, *4, *5, *6, *9, *10, *41 and gene duplication) were obtained, yielding graded allele-specific CYP2D6 patient groups. Desipramine and imipramine+desipramine plasma concentration per drug dose unit, imipramine dose at steady state, and imipramine dose requirement significantly depended on CYP2D6 genotype (Kruskal–Wallis test, P<0.0001). Mean (±s.d.) drug dose requirements were 131 (±109), 155 (±70), 217 (±95), 245 (±125), 326 (±213), and 509 (±292) mg imipramine/day in carriers of 0, 0.5, 1, 1.5, 2, and >2 active CYP2D6 genes, respectively. Our protocol for CYP2D6 genotyping will thus importantly aid in the prediction of imipramine metabolism, allowing for the use of an adjusted starting dose and faster achievement of predefined imipramine+desipramine plasma levels in all genetic patient subgroups. Therefore, therapeutic efficacy and efficiency may be improved, the number of adverse drug reactions decreased, and hospital stay reduced.

Sildenafil exposure in neonates with pulmonary hypertension after administration via a nasogastric tube

Objective To describe the pharmacokinetics and exposure of oral sildenafil (SIL) in neonates (2–5 kg) with pulmonary hypertension (PH). Design We included 11 neonates (body weight 2–5 kg, postnatal age 2–121 days) who received SIL and extracorporeal membrane oxygenation (ECMO) treatment for PH. SIL capsules were given via a nasogastric tube. Blood samples were collected via a pre-existing arterial line to quantify SIL and metabolite plasma levels (219 samples). Non-linear mixed effects modelling was used to describe SIL and desmethylsildenafil (DMS) pharmacokinetics. Results A one-compartment model was suitable for SIL and DMS. Interpatient and intrapatient variability for clearance at 100% bioavailability were 87% and 27% (SIL) and 62% and 26% (DMS). Patient weight, postnatal age and post-ECMO time did not explain variability. Concomitant fluconazole use was associated with a 47% reduction in SIL clearance. The exposure expressed as average plasma concentration area under the curve over 24 h (AUC24 (SIL+DMS)) ranged from 625 to 13 579 ng/h/ml. An oral dose of 4.2 mg/kg/24 h would lead to a median AUC24 (SIL+DMS) of 2650 ng/h/ml equivalent to 20 mg three times a day in adults. Interpatient variability was large, with a simulated AUC24 (SIL+DMS) range (10th and 90th percentiles) of 1000–8000 ng/h/ml. Conclusions SIL pharmacokinetics are highly variable in post-ECMO neonates and infants. In a median patient, the current dose regimen of 0.5–2.0 mg/kg four times a day leads to an exposure comparable to the recommended adult dose of 20 mg four times a day. Careful dose titration, based on efficacy and the occurrence of hypotension, remains necessary. Follow-up research should include appropriate pharmacodynamic endpoints, with a population pharmacokinetic/pharmacodynamic analysis to assign a suitable exposure window or target concentration.

The CYP2C19*17 genotype is associated with lower imipramine plasma concentrations in a large group of depressed patients

CYP2C19 converts the tricyclic antidepressant imipramine to its active metabolite desipramine, which is subsequently inactivated by CYP2D6. The novel CYP2C19*17 allele causes ultrarapid metabolism of CYP2C19 substrates. We genotyped 178 depressed patients on imipramine for CYP2C19*17, and measured steady-state imipramine and desipramine plasma concentrations. Mean dose-corrected imipramine plasma concentration was significantly dependent on CYP2C19 genotype (Kruskal–Wallis test, P=0.01), with circa 30% lower levels in CYP2C19*17/*17 individuals compared with CYP2C19*1/*1 (wild-type) patients. The mean dose-corrected imipramine+desipramine plasma concentrations and imipramine/desipramine ratios were not significantly different between genotype subgroups (Kruskal–Wallis tests, P⩾0.12). In a multivariate analysis, we found a significant, but limited effect (P=0.035, η2=0.031) of the CYP2C19*17 genotype on imipramine+desipramine concentrations. Although the CYP2C19*17 allele is associated with a significantly increased metabolism of imipramine, CYP2C19*17 genotyping will, in our view, not importantly contribute to dose management of patients on imipramine therapy guided by imipramine+desipramine plasma concentrations.

Quantification of levetiracetam in plasma of neonates by ultra performance liquid chromatography–tandem mass spectrometry

A sensitive and specific method using ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was developed for the determination of levetiracetam (LEV) in plasma of neonates. A plasma aliquot of 50 microl was deproteinized by addition of 500 microl methanol which contained 5 microg/ml UCB 17025 as an internal standard. After centrifugation, 50 microl of supernatant was diluted with 1000 microl of 0.1% formic acid-10 mM ammonium formate in water (pH 3.5) (mobile phase solution A) and 2 microl was injected onto the UPLC-system. Compounds were separated on a Acquity UPLC BEH C(18) 2.1 mm x 100 mm column using gradient elution with mobile phase solution A and 0.1% formic acid in methanol (mobile phase solution B) with a flow rate of 0.4 ml/min and a total runtime of 4.0 min. LEV and the internal standard were detected using positive ion electrospray ionization followed by tandem mass spectrometry (ESI-MS/MS). The assay allowed quantification of LEV plasma concentrations in the range from 0.5 microg/ml to 150 microg/ml. Inter-assay inaccuracy was within +/-2.7% and inter-assay precision was less than 4.5%. Matrix effects were minor: the recovery of LEV was between 97.7% and 100%. The developed method required minimal sample preparation and less plasma sample volume compared to earlier published LC-MS/MS methods. The method was successfully applied in a clinical pharmacokinetic study in which neonates received intravenous administrations of LEV for the treatment of neonatal seizures.

Quantification of midazolam, morphine and metabolites in plasma using 96‐well solid‐phase extraction and ultra‐performance liquid chromatography–tandem mass spectrometry

Currently, pharmacokinetic-pharmacodynamic studies of sedatives and analgesics are performed in neonates and children to find suitable dose regimens. As a result, sensitive assays using only small volumes of blood are necessary to determine drug and metabolite concentrations. We developed an ultra-performance liquid chromatographic method with tandem mass spectrometry detection for quantification of midazolam, 1-hydroxymidazolam, hydroxymidazolamglucuronide, morphine, morphine-3-glucuronide and morphine-6-glucuronide in 100 microL of plasma. Cleanup consisted of 96 wells micro-solid phase extraction, before reversed-phase chromatographic separation (ultra-performance liquid chromatography) and selective detection using electrospray ionization tandem mass spectrometry. Separate solid-phase extraction methods were necessary to quantify morphine, midazolam and their metabolites because of each groups physicochemical properties. Standard curves were linear over a large dynamic range with adequate limits of quantitation. Intra- and interrun accuracy and precision were within 85-115% (of nominal concentration using a fresh calibration curve) and 15% (coefficient of variation, CV) respectively. Recoveries were >80% for all analytes, with interbatch CVs (as a measure of matrix effects) of less than 15% over six batches of plasma. Stability in plasma and extracts was sufficient, allowing large autosampler loads. Runtime was 3.00 min per sample for each method. The combination of 96-well micro-SPE and UPLC-MS/MS allows reliable quantification of morphine, midazolam and their major metabolites in 100 microL of plasma.