Pharmacokinetics of ceftazidime and cefepime in burn patients: the importance of age and creatinine clearance

AIM: The standard dosage recommendations for beta-lactam antibiotics can result in very low drug levels in intensive care (IC) patients and burn patients in the absence of renal dysfunction. We studied the pharmacokinetic parameters and serum concentrations of ceftazidime (CF) and cefepime (CE) in burn patients and analyzed the modifications according to clinical and biological parameters and in particular age and creatinine clearance. MATERIAL AND METHODS: Two pharmacokinetic studies were carried out with daily doses of 1 g x 6 for CF (n = 17) and 2 g x 3 for CE (n = 13). Creatinine clearance (CL(CR)) was both estimated and measured. Blood was sampled at steady state after an initial and a subsequent antibiotic dose. C(max) (maximal) and C(min) (minimal) concentrations were measured by HPLC. The influence of clinical and biological data was analyzed using ANOVA, ANCOVA and stepwise multiple linear regression. RESULTS: The ratio of C(min) to the low MIC break point (4 mg/l) was lower than 4 in 52% of subjects receiving CF and in 80% of subjects receiving CE. The C(min) of CF was correlated with measured CL(CR) and was higher in mechanically ventilated patients than in non-ventilated patients. The clearance of CF was correlated with age. The C(min) of CE was correlated with age and drug clearance with measured CL(CR). Therefore dosage adjustment of these drugs in burn patients needs to take into account age, measured creatinine clearance and the danger of low concentrations occurring when the creatinine clearance is greater than 120 ml x min(-1). CONCLUSION: In burn patients, the pharmacokinetic disposition of CF and CE was much more variable than in healthy subjects. Age and CL(CR) were predictors of the disposition of these antibiotics. Shortening the dosage interval or using continuous infusions will prevent low serum levels and keep trough levels above the MIC for longer periods of time. In view of the lack of a bedside measurement technique for ceftazidime and cefepime levels, we suggest a more frequent use of measured CL(CR) in order to attain efficacious clinical concentrations.     

Assessment of renal function in clinical practice at the bedside of burn patients

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: * In burn patients it has been shown ([2]), that there is a correlation between the creatinine clearance (CL(CR)) and the clearance of inulin. * The CL(CR) has never been studied in burn patients who have normal serum creatinine. * The Robert, Kirkpatrick and sMDRD formulae have never been evaluated in burn patients. WHAT THIS STUDY ADDS: * Despite burn patients having normal serum creatinine concentrations, the study showed that there are large variations in CL(CR) which cannot be detected by single serum creatinine measurements, and which have important implications for drug therapy. * It showed that the formulae currently used to calculate creatinine clearance on the basis of serum creatinine are inadequate for use in burn patients, and they should be abandoned in favour of direct measurement from a 24 h urine collection. AIMS: The aim of this study was to evaluate whether the renal function of burn patients could be correctly assessed using a single serum creatinine measurement, within normal limits, and three prediction equations of glomerular filtration taking into account, serum creatinine, age, weight and sex. METHODS: This was a prospective study comprising 36 adult burn patients with a serum creatinine <120 micromol l(-1), within the second or third week following the burn injury. Renal function was assessed using serum creatinine, 24 h urinary CL(CR), and the Cockcroft-Gault, Robert, Kirkpatrick and simplified MDRD equations. RESULTS: Despite normal serum creatinine concentrations in all patients, a significant number had a decreased CL(CR). The urinary CL(CR) was <80 ml(-1) min(-1) 1.73 m(-2) in nine patients (25%), and <60 ml(-1) min(-1) 1.73 m(-2) in five patients (14%). Between the groups having a CL(CR) lower or greater than 80 ml(-1) min(-1) 1.73 m(-2) there were no differences in gender, burn indices, percentage of mechanically ventilated patients or length of hospital stay, but a difference in age. The highest CL(CR) (>140 ml(-1) min(-1) 1.73 m(-2)) was found in 13 patients younger than 40 years. Regression analysis, residual and Bland-Altman plots revealed that neither the Cockcroft-Gault, Robert, Kirkpatrick nor sMDRD equations were specific enough for the assessment of renal function. CONCLUSIONS: In burn patients with normal serum creatinine during the hypermetabolic phase, serum creatinine and creatine based predictive equations are imprecise in assessing renal function.     

Population pharmacokinetics of ceftazidime in burn patients

AIM: The aim of this study was to characterize, via a population pharmacokinetic approach, the pharmacokinetics of ceftazidime in burn patients who were not in the acute post-injury phase. METHODS: The development of the pharmacokinetic model was based on data from therapeutic drug monitoring (41 patients, 94 samples). The estimation of population pharmacokinetic parameters and the selection of covariates (age, gender, body weight, size of burn and creatinine plasma concentration) that could affect the pharmacokinetics were performed with a nonlinear mixed effect modelling method. RESULTS: No relationship between covariates and the pharmacokinetic parameters was established with the exception of an inverse-linear relationship between creatinine plasma concentration and ceftazidime total clearance. The total clearance of ceftazidime was 2.72 l h-1[coefficient variation (CV) = 56.3%] and the distribution volume of the central compartment was 0.28 l kg-1 (CV = 13.2%) The transfer rate constants (k12, k 21) between the central and peripheral compartments were 0.06718 h-1 (CV = 87.2%) and 0.001823 h-1 (CV = 82.7%), respectively. From these parameters, the total ceftazidime volume of distribution (10.64 l kg-1) was calculated. CONCLUSION: The population parameters were different from those obtained in a previous study performed in fewer patients and in the early period after burn injury. In our study, the lower ceftazidime clearance could be explained by the relative decrease in ceftazidime elimination in relation to the burn area, and the higher ceftazidime volume of distribution in the presence of interstitial oedema, which could act as a reservoir from which ceftazidime returns slowly to the circulation.     

Population pharmacokinetics of raltitrexed in patients with advanced solid tumours

AIMS: To investigate the population pharmacokinetics of raltitrexed in patients with advanced solid tumours and to identify patient covariates contributing to the interpatient variability in the pharmacokinetics of raltitrexed. METHODS: Patient covariate and concentration-time data were collected from patients receiving 0.1-4.5 mg m(-2) raltitrexed during the early clinical trials of raltitrexed. Data were fitted using nonlinear mixed effects modelling to generate population mean estimates for clearance (CL) and central volume of distribution (V). The relationship between individual estimates of the pharmacokinetic parameters and patient covariates was examined and the influence of significant covariates on the population parameter estimates and their variance was investigated using stepwise multiple linear regression. The performance of the developed model was tested using an independent validation dataset. All patient data were pooled in the total cohort to refine the population pharmacokinetic model for raltitrexed. RESULTS: three-compartment pharmacokinetic model was used to fit the concentration-time data of raltitrexed. Estimated creatinine clearance (CL(CR)) was found to influence significantly the CL of raltitrexed and explained 35% of variability in this parameter, whilst body weight (WT) and serum albumin concentrations (ALB) accounted for 56% of the variability in V. Satisfactory prediction (mean prediction error 0.17 micro g l(-1) and root mean square prediction error 4.99 micro g l(-1)) of the observed raltitrexed concentrations was obtained in the model validation step. The final population mean estimates were 2.17 l h(-1)[95% confidence interval (CI) 2.06, 2.28] and 6.36 l (95% CI 6.02, 6.70) for CL and V, respectively. Interpatient variability in the pharmacokinetic parameters was reduced (CL 28%, V 25%) when influential covariates were included in the final model. The following covariate relationships with raltitrexed parameters were described by the final population model: CL (l h(-1)) = 0.54 + 0.02 CL(CR) (ml min(-1)) and V (l) = 6.64 + 0.08 WT (kg) - 0.16 ALB (g l(-1)). CONCLUSIONS: A population pharmacokinetic model has been developed for raltitrexed in patients with advanced cancer. Pharmacokinetic parameters of raltitrexed are markedly influenced by the patient's renal function, body weight and serum albumin levels, which may be taken into account in dose individualization. The use of influential covariates to guide anticancer dosage selection may result in less variability in drug exposure and potentially a better clinical outcome.     

Pharmacokinetics and effects of propofol 6% for short-term sedation in paediatric patients following cardiac surgery

AIMS: This paper describes the pharmacokinetics and effects of propofol in short-term sedated paediatric patients. METHODS: Six mechanically ventilated children aged 1-5 years received a 6 h continuous infusion of propofol 6% at the rate of 2 or 3 mg kg-1 h-1 for sedation following cardiac surgery. A total of seven arterial blood samples was collected at various time points during and after the infusion in each patient. Pharmacokinetic modelling was performed using NONMEM. Effects were assessed on the basis of the Ramsay sedation score as well as a subjective sedation scale. RESULTS: The data were best described by a two-compartment pharmacokinetic model. In the model, body weight was a significant covariate for clearance. Pharmacokinetic parameters in the weight-proportional model were clearance (CL) = 35 ml kg-1 min-1, volume of central compartment (V1) = 12 l, intercompartmental clearance (Q) = 0.35 l min-1 and volume of peripheral compartment (V2) = 24 l. The interindividual variabilities for these parameters were 8%, < 1%, 11% and 35%, respectively. Compared with the population pharmacokinetics in adults following cardiac surgery and when normalized for body weight, statistically significant differences were observed the parameters CL and V1 (35 vs 29 ml kg-1 min-1 and 0.78 vs 0.26 l kg-1P < 0.05), whereas the values for Q and V2 were similar (23 vs 18 ml kg-1 min-1 and 1.6 vs 1.8 l kg-1, P > 0.05). In children, the percentage of adequately sedated patients was similar compared with adults (50% vs 67%) despite considerably higher propofol concentrations (1.3 +/- 0.10 vs 0.51 +/- 0.035 mg l-1, mean +/- s.e. mean), suggesting a lower pharmacodynamic sensitivity to propofol in children. CONCLUSIONS: In children aged 1-5 years, a pharmacokinetic model for propofol was described using sparse data. In contrast to adults, body weight was a significant covariate for clearance in children. The model may serve as a useful basis to study the role of covariates in the pharmacokinetics and pharmacodynamics of propofol in paediatric patients of different ages.     

Population pharmacokinetics of tobramycin in hospitalized patients receiving once-daily dosing regimen

The population pharmacokinetics of tobramycin was investigated in a group of 327 adult hospitalized patients receiving once-daily administration of tobramycin at a dose of 7 mg kg(-1). The patients had an average age of 57+/-18 y and an average weight of 65+/-14 kg; 153 of the patients were female. Data, comprised of 575 serum concentrations, were analyzed using a nonlinear mixed-effect model (NONMEM) with a first-order conditional estimation method and were best described with a one-compartment model. The patient covariates including body weight, gender, age and creatinine clearance (CL(CR)) were added in a stepwise fashion to identify their potential influences on tobramycin pharmacokinetics. Results showed that tobramycin clearance (CL) was linearly correlated with CL(CR) (proportionality constant: 0.066+/-0.002 x CL(CR) (ml min(-1))) and the volume of distribution (Vd) was linearly related to body weight (proportionality constant: 0.40+/-0.024 x body weight (1 kg(-1))). The mean population estimates for CL and Vd were 4.53 l h(-1) and 27.3 l, respectively. The half-life of tobramycin was estimated to be 4.2 h. The inter-individual variability in CL and Vd were 37.0 and 28.5%, respectively. The residual error was 1.2 mg l(-1). Based on the results, optimal dosing intervals for renal impaired patients were calculated and were comparable with the intervals derived from the previous established nomogram.     

Population pharmacokinetics of gentamicin in hospitalized patients receiving once-daily dosing

The purpose of this study was to describe the population pharmacokinetics of gentamicin in a group of 939 adult hospitalized patients receiving once-daily administration of gentamicin and to evaluate the potential influence of patient covariates on gentamicin disposition. Data comprising 1294 serum concentrations from 939 patients, were analyzed using a nonlinear mixed-effect model (NONMEM). The patients had an average age of 55 and an average weight of 70 kg, 431 of the patients were female. The patient covariates including body weight, gender, age, and creatinine clearance (CL(CR)) were analyzed in a stepwise fashion to identify their potential influences on gentamicin pharmacokinetics. The data were best described with a two-compartment model. NONMEM analyses showed that gentamicin clearance (CL, l/h) was linearly correlated with CLcR with proportionality constant: 0.047 (S.E.: 0.0035) x CL(CR) (ml/min). Volume of the central compartment (V1, 1) was linearly related to body weight with proportionality constant: 0.28 (S.E.: 0.021) x body weight (kg). The mean population estimates of CL and V1 were 4.32 l/h and 19.61. respectively. The inter-individual variability in CL and V1 were 29.6 and 5.8%, respectively. Residual errors were 0.23 mg/l and 23.7%. The mean population values of CL and V1 of gentamicin dosed once daily are in agreement with those described by others. This analysis indicates that once-daily dosing (7 mg/kg) of gentamicin should achieve satisfactory concentration in patients with normal renal function although serum concentration monitoring is required to confirm the optimal dosing interval in patients with impaired renal function.     

Population pharmacokinetics of total and unbound plasma cisplatin in adult patients

AIMS: To investigate the pharmacokinetics of unbound (ultrafilterable) and total plasma platinum using a population approach and to identify patient characteristics that may influence the disposition of the drug. METHODS: Pharmacokinetic and demographic data were collected from adult patients treated with 30-min daily infusions of cisplatin for various malignancies. Unbound and total platinum concentration-time data were analysed using a nonlinear mixed effects model. RESULTS: Data from 43 patients were available for analysis. A linear two-compartment model best described total and unbound platinum plasma concentration-time data. The mean population estimates for total and unbound drug were, respectively, 0.68 and 35.5 l h(-1) for clearance and 21.1 and 23.4 l for central distribution volume (V(1)). Unbound clearance (CL) was dependent on body surface area (BSA) and creatinine clearance, and V(1) was dependent on BSA. The elimination rate constant for plasma-bound platinum (modelled as metabolite formation) was 0.014 h(-1). The pharmacokinetic parameter, f(m)/V(m), a measure of the clearance of unbound platinum due to irreversible plasma binding, was related to serum protein concentration and to the inverse of dose per m(2). The covariate modelling of CL, V(1) and f(m)/V(m) improved the intersubject variabilities associated with these parameters. The final pharmacokinetic models were validated using 200 bootstrap samples from the original datasets. CONCLUSIONS: The results support the conventional dose adjustment of cisplatin based on BSA. They also support the need for a dose reduction in case of renal insufficiency.     

Population pharmacokinetics of digoxin in Japanese patients: a 2-compartment pharmacokinetic model.

OBJECTIVE: To clarify the observed variability of digoxin disposition by performing a population pharmacokinetic analysis in a Japanese population. DESIGN: Retrospective analysis of clinical pharmacokinetic data. PATIENTS AND PARTICIPANTS: Data were obtained from 106 patients with heart failure and atrial fibrillation (43 males and 63 females). METHODS: Digoxin concentrations in serum were measured by fluorescence polarisation immunoassay. Population pharmacokinetic analysis was performed using a 2-compartment open pharmacokinetic model with the computer program NONMEM. RESULTS: 246 serum concentrations were obtained. Final pharmacokinetic parameters were: CL (L/h) = (0.036 x TBW + 0.112 x CL(CR)) x 0.77SPI x 0.784CCB, V1 = 1.83 L/kg, V2 = 22.6 L/kg and Q = 0.629 L/h/kg, where CL is total body clearance, V1 and V2 are the apparent volumes of distribution in the central and peripheral compartments, Q is intercompartmental clearance, TBW is total bodyweight (in kg), CL(CR) is creatinine clearance (in ml/min), SPI = 1 for concomitant administration of spironolactone (and zero otherwise) and CCB = 1 for concomitant administration of calcium antagonists (and zero otherwise). Concomitant administration of digoxin and spironolactone resulted in a 23% decrease in digoxin clearance. Concomitant administration of digoxin and calcium antagonists (diltiazem, nicardipine, nifedipine or verapamil) resulted in a 21.6% decrease in digoxin clearance. CONCLUSIONS: The estimated population parameter values may assist clinicians in the individualisation of digoxin dosage regimens.     

Population pharmacokinetics of weekly docetaxel in patients with advanced cancer

AIMS: Previous pharmacokinetic studies of the 3-weekly regimen (100 mg m(-2) every 3 weeks) of docetaxel have shown that docetaxel clearance is affected by liver function, body surface area, age, serum alpha1-acid glycoprotein and cytochrome P450 3A4 (CYP3A4) activity. However, the pharmacokinetics of a weekly docetaxel (40 mg m(-2) week(-1)) schedule are not well characterized. The aims of this study were (a) to investigate the pharmacokinetics of docetaxel (40 mg m(-2) week(-1)) using sparse concentration-time data collected from patients with advanced cancer and (b) to utilize a population pharmacokinetic approach to identify patient covariates that significantly influence the clearance of docetaxel when administered according to this regimen. METHODS: A two-compartment pharmacokinetic model was used to describe the docetaxel concentration-time data from 54 patients with advanced cancer. The mean population and individual posterior Bayesian estimates of docetaxel clearance were estimated using P-PHARM. The relationships between docetaxel clearance and 21 covariates were investigated. This included estimates of CYP3A4 function in each patient using the erythromycin breath test (1/tmax). Significant covariates were included into the final population pharmacokinetic model. Pharmacokinetic models were validated using a data splitting approach with a dataset consisting of 16 patients. RESULTS: Significant relationships were found between docetaxel clearance and 1/tmax (erythromycin breath test parameter) and several of the liver function enzymes and CL was best described by the equation; CL = 21.51 + 217 (1/tmax) - 0.13 (ALT). This final population pharmacokinetic model provided both precise and unbiased predictions of docetaxel concentrations in a validation group of patients and an estimate of the population mean (95% confidence interval) clearance of docetaxel was 30.13 l h(-1) (12.54, 46.04 l h(-1)) with an intersubject variability 30%. CONCLUSIONS: A population pharmacokinetic model has been developed and validated for weekly docetaxel (40 mg m(-2)) in patients with advanced cancer. These results indicate that CYP3A4 activity and hepatic function have an impact on the pharmacokinetics of docetaxel when administered weekly.     

Weight related differences in the pharmacokinetics of abacavir in HIV-infected patients

AIM: To study the possible influence of patient characteristics on abacavir pharmacokinetics. METHODS: A population pharmacokinetic model for abacavir was developed using data from 188 adult patients by the use of a nonlinear mixed effects modelling method performed with NONMEM. RESULTS: Abacavir pharmacokinetics was well described by a two-compartment open model with linear absorption and elimination. Typical population estimates for the absorption rate constant (Ka), the apparent central distribution volume (Vc/F), the apparent peripheral distribution volume (Vp/F), the apparent intercompartmental clearance (Q/F) and the apparent plasma clearance (CL/F) were 1.8 h(-1), 75 l, 23.6 l, 10 l h(-1) and 47.5 l h(-1), respectively. Apparent plasma clearance was positively related to bodyweight. Individual Bayesian estimates of CL/F were used to calculate abacavir AUC. The latter decreased from 10.7 +/- 5.0 to 5.7 +/- 1.6 mgh l(-1) when bodyweight increased from 36 to 102 kg. This drop in abacavir exposure could lead to suboptimal treatment for the heaviest patients, as antiviral efficacy of abacavir is known to be related to its AUC. A 400 mg abacavir dose would be necessary to achieve adequate exposure to abacavir in patients weighing more than 60 kg. CONCLUSIONS: The apparent plasma clearance of abacavir was positively related to bodyweight. The efficacy of the current recommended abacavir dosage for patients with high bodyweight should be evaluated in further studies.     

Correlation of the pharmacokinetic parameters of amikacin and ceftazidime

BACKGROUND AND OBJECTIVES: Ceftazidime and amikacin are often prescribed concomitantly to treat infections caused by Gram-negative bacteria. Their physicochemical properties are quite similar. Both drugs are highly soluble in water, have low plasma protein binding and are >95% excreted unchanged by the kidney via glomerular filtration. Their pharmacokinetic parameters are therefore expected to correlate. This study was performed to explore the correlation between the pharmacokinetic parameters of these two drugs. PATIENTS AND METHODS: Patients at Phramongkutklao Hospital, Bangkok, Thailand, who met the inclusion criteria participated in the study. They all received ceftazidime and amikacin concomitantly to treat their infections. After steady-state conditions had been reached, two blood samples were collected during the elimination phase of both drugs. Plasma drug concentrations were analysed and the pharmacokinetic parameters of each drug were calculated. The pharmacokinetic parameters that were examined included total drug clearance (CL), the elimination rate constant (k(e)), the elimination half life (t(1/2)) and the volume of distribution (V(d)). The correlations of the pharmacokinetic parameters of amikacin and ceftazidime were determined using regression analysis. RESULTS: Regression analysis showed that the pharmacokinetic parameters of ceftazidime and amikacin were highly correlated. The correlation coefficients (r) of CL, k(e), t(1/2) and V(d) of the two drugs were 0.966, 0.943, 0.888 and 0.671, respectively. The correlation between amikacin clearance and ceftazidime clearance was higher than the correlation between either amikacin or ceftazidime clearance and creatinine clearance, for which the r values were 0.647 and 0.661, respectively. CONCLUSIONS: The pharmacokinetic parameters of ceftazidime and amikacin were highly correlated. Knowledge of the pharmacokinetic parameters of one of these drugs can be used to predict the pharmacokinetic parameters of the other drug.     

Cefepime in intensive care unit patients: validation of a population pharmacokinetic approach and influence of covariables

AIM: The purpose of our study was to define and validate a population-pharmacokinetic model including the influence of patients' characteristics on the pharmacokinetics of cefepime. PATIENTS AND METHODS: A total of 55 patients were randomized in Group 1 (34 patients, 320 cefepime concentrations) for the model building and Group 2 (21 patients, 196 cefepime concentrations) for the validation group. They received cefepime as 2 g A 2 or as 4 g continuously. The population pharmacokinetic analysis was carried out using NONMEM and a baseline model was constructed for studying the influence of demographic and biological variables. The model was then validated by a comparison of the predicted and observed concentrations in Group 2. A final model was elaborated from the whole population. RESULTS: Total clearance (CL) was significantly correlated with the serum creatinine (CREA) and the central volume of distribution (V1) was correlated with the body weight (WT). The final model was: CL = 7.14 + (-0.0133 A CREA). V1 = (-16.8) + (0.475 A WT). Q (intercompartmental clearance) = 10.5. V2 = 18.1. The mean pharmacokinetic parameters and their individual variability were: CL (8.24 l/h, 45%), V1 (20.89 l, 60%), V2 (17.95 l, 49%), total volume (38.85 l, 42%) and Q (10.56 l/h, 9%). The bias (1.07 mg/l, IC 95% = -40.46 -+42.60), precision (21.19%) and AFE (1.15) demonstrated the performance of the model. CONCLUSION: We have developed and validated a pharmacokinetic model to estimate cefepime concentrations. We showed that serum creatinine and body weight are factors that may influence the standard dose of cefepime. Our model enabled us to predict cefepime concentrations in other patients.     

The disposition of alfentanil in neonates with respiratory distress

The disposition of continuous infusion alfentanil was evaluated in 13 mechanically ventilated neonates (gestational age 37.6 +/- 2.4 wks) with hyaline membrane disease (n = 7) or persistent pulmonary hypertension of the newborn (n = 6). Alfentanil was administered as a loading dose 8 micrograms/kg, followed by a variable-rate continuous infusion (maximum 10 micrograms/kg/hr; minimum 2.5 micrograms/kg/hr) for 27 hours. Serial plasma samples were obtained for pharmacokinetic analysis. Noncompartmental pharmacokinetic analysis of the data revealed the following estimates (mean +/- SD): total-body clearance 3.24 +/- 2.23 ml/kg/minute, volume of distribution 0.54 +/- 0.21 L/kg, and elimination half-life 4.14 +/- 2.58 hours. A significant effect of alfentanil plasma concentration on total-body clearance was found (r = -0.75; p = 0.02), suggesting nonlinear pharmacokinetics. No correlation was seen between total-body clearance and alfentanil dose (r = -0.37; p = 0.32). The results suggest that a larger dose-proportionality study is required to determine the linearity or nonlinearity of alfentanil pharmacokinetics in neonates.     

Time-dependent clearance of mycophenolic acid in renal transplant recipients

AIMS: Pharmacokinetic studies of the immunosuppressive compound mycophenolic acid (MPA) have shown a structural decrease in clearance (CL) over time after renal transplantation. The aim of this study was to characterize the time-dependent CL of MPA by means of a population pharmacokinetic meta-analysis, and to test whether it can be described by covariate effects. METHODS: One thousand eight hundred and ninety-four MPA concentration-time profiles from 468 renal transplant patients (range 1-9 profiles per patient) were analyzed retrospectively by nonlinear mixed effect modelling. Sampling occasions ranged from day 1-10 years after transplantation. RESULTS: The pharmacokinetics of MPA were described by a two-compartment model with time-lagged first order absorption, and a first-order term for time-dependent CL. The model predicted the mean CL to decrease from 35 l h(-1) (CV = 44%) in the first week after transplantation to 17 l h(-1) (CV = 38%) after 6 months. In a covariate model without a term for time-dependent CL, changes during the first 6 months after transplantation in creatinine clearance from 19 to 71 ml min(-1), in albumin concentration from 35 to 40 g l(-1), in haemoglobin from 9.7 to 12 g dl(-1) and in cyclosporin predose concentration from 225 to 100 ng ml(-1) corresponded with a decrease of CL from 32 to 19 l h(-1). Creatinine clearance, albumin concentration, haemoglobin and cyclosporin predose concentration explained, respectively, 19%, 12%, 4% and 3% of the within-patient variability in MPA CL. CONCLUSIONS: By monitoring creatinine clearance, albumin concentration, haemoglobin and cyclosporin predose concentration, changes in MPA exposure over time can be predicted. Such information can be used to optimize therapy with mycophenolate mofetil.     

Population pharmacokinetics of digoxin in Korean patients

AIM: Digoxin possesses a narrow therapeutic index and shows a large inter-patient pharmacokinetic variability. The purpose of this study was to develop a population model for the pharmacokinetics of digoxin in Korean patients. METHODS: Plasma concentrations of digoxin after multiple administration at varying dosing schedules in Korean patients were used for population modeling. Data analysis was performed with the P-Pharm software. The data were best fitted by a one-compartment model. The effect of demographic and clinical factors like sex, age, weight, disease state, and renal function on the pharmacokinetic parameters of digoxin was investigated. RESULTS: The study indicated that the clearance of digoxin was influenced by creatinine clearance, while body weight and creatinine clearance were the covariates for its volume of distribution. The population mean estimates for CL and V were 4.4 l/h and 535 l, respectively. Absorption rate constant was lower in females and in the presence of concomitant drug treatment. CONCLUSION: A population pharmacokinetic model for the digoxin pharmacokinetics in a section of Korean patients was developed. The relationships between the pharmacokinetic parameters and the demographic data and the patient-specific covariates were established.     

Population pharmacokinetics of meropenem in critically ill patients undergoing continuous renal replacement therapy

BACKGROUND AND OBJECTIVE: Meropenem is a carbapenem antibacterial frequently prescribed for the treatment of severe infections in critically ill patients, including those receiving continuous renal replacement therapy (CRRT). The objective of this study was to develop a population pharmacokinetic model of meropenem in critically ill patients undergoing CRRT. PATIENTS AND METHODS: A prospective, open-label study was conducted in 20 patients undergoing CRRT. Blood and dialysate-ultrafiltrate samples were obtained after administration of 500 mg, 1000 mg or 2000 mg of meropenem every 6 or 8 hours by intravenous infusion. The data were analysed under the population approach using NONMEM version V software. Age, bodyweight, dialysate plus ultrafiltrate flow, creatinine clearance (CL(CR)), the unbound drug fraction in plasma, the type of membrane, CRRT and the patient type (whether septic or severely polytraumatized) were the covariates studied. RESULTS: The pharmacokinetics of meropenem in plasma were best described by a two-compartment model. CL(CR) was found to have a significant correlation with the apparent total clearance (CL) of the drug during the development of the covariate model. However, the influence of CL(CR) on CL differed between septic and polytraumatized patients (CL = 6.63 + 0.064 x CL(CR) for septic patients and CL = 6.63 + 0.72 x CL(CR) for polytraumatized patients). The volume of distribution of the central compartment (V(1)) was also dependent on the patient type, with values of 15.7 L for septic patients and 69.5 L for polytraumatized patients. The population clearance was 15 L/h, and the population apparent volume of distribution of the peripheral compartment was 19.8 L. From the base to the final model, the interindividual variabilities in CL and the V(1) were significantly reduced. When computer simulations were carried out and efficacy indexes were calculated, it was shown that polytraumatized patients and septic patients with conserved renal function may not achieve adequate efficacy indexes to deal with specific infections. Continuous infusion of meropenem is recommended for critically septic patients and polytraumatized patients when pathogens with a minimum inhibitory concentration (MIC) of > or =4 mg/L are isolated. Infections caused by pathogens with an MIC of > or =8 mg/L should not be treated with meropenem in polytraumatized patients without or with moderate renal failure because excessive doses of meropenem would be necessary. CONCLUSION: A population pharmacokinetic model of meropenem in intensive care patients undergoing CRRT was developed and validated. CL(CR) and the patient type (whether septic or polytraumatized) were identified as significant covariates. The population pharmacokinetic model developed in the present study has been employed to recommend continuous infusion protocols in patients treated with CRRT.     

Pharmacokinetics of a glycine site antagonist (gavestinel) following multiple dosing in patients with acute stroke

OBJECTIVE: The objective of this study was to characterize the pharmacokinetics of gavestinel in patients with acute stroke. METHODS: Gavestinel was administered as an 800-mg loading dose and followed by either 100-, 200-, or 400-mg maintenance doses given every 12 h for five doses. Blood and urine samples were collected for pharmacokinetic evaluation. The pharmacokinetics of gavestinel were determined using compartmental analysis. RESULTS: The mean clearance (CL) and central (Vc) and steady-state (Vss) volumes of distribution across the dose groups were 0.31-0.40 l x h(-1), 3.3-3.9 l, and 9.8-17 l, respectively. The mean terminal half-life ranged from 29 h to 56 h. Gavestinel was extensively bound to plasma protein (median percentage free <0.01). During gavestinel administration, some patients exhibited elevated levels of bilirubin, which may be the result of shared mechanisms of elimination (glucuronide conjugation and excretion in bile). CONCLUSIONS: This study characterized the pharmacokinetics of gavestinel following multiple doses in acute stroke patients and showed that the pharmacokinetics are similar for increasing maintenance doses. The high protein binding of gavestinel was confirmed in acute stroke patients. A pharmacokinetic interaction between gavestinel and bilirubin may contribute to the increase in bilirubin.     

Influence of clinical diagnosis in the population pharmacokinetics of amikacin in intensive care unit patients

The aim of the present study was to analyse the pharmacokinetic behaviour of amikacin in intensive care unit (ICU) patients using a mixed-effect model and sparse data collected during routine clinical care. The patient population comprised 158 medical ICU patients divided into two groups: one for computing the population model (n = 120) and the other for validation (n = 38). A 1-compartment model was used and the following covariates were tested for their influence on clearance (CL) and volume of distribution (Vd): age, gender, weight, parenteral nutrition, creatinine clearance, duration of therapy and clinical diagnosis. The nonlinear mixed-effect model (NONMEM) was used to assess the population pharmacokinetic model of amikacin in this patient population. In this study, the final population model accounting for amikacin pharmacokinetics in ICU patients was: CL = 0.93 CL(CR) (1 + 0.22 Trauma), Vd = 0.39 TBW (1 + 0.24 Sepsis), where CL(CR) and TBW corresponded to the patients' creatinine clearance and total bodyweight, respectively. The 'Trauma' and 'Sepsis' variables referred to the clinical diagnosis of the patients. This model was subsequently used to predict amikacin serum levels obtained in the validation population by a priori and Bayesian methods. The predictive performance was adequate for clinical purposes, pointing to the feasibility of our population model to provide reference values for a priori prediction as well as the Bayesian approach for individualisation of amikacin therapy in ICU patients.