Evaluation of pharmacokinetic methods used to estimate caffeine clearance and comparison with a Bayesian forecasting method

Serum Metabolite/Caffeine Ratios as a Test for Liver Function

The aim of this study was to test the usefulness of the metabolite/caffeine ratio for the evaluation of hepatic dysfunction. Subjects with liver cirrhosis and chronic hepatitis, as well as healthy volunteers, were given the oral dose of 300 mg caffeine. Blood samples were collected after 4, 8, and 12 hours. Concentrations of caffeine (CA) and its three metabolites-paraxanthine (PX), theobromine (TB), and theophylline (TP)-were determined by high-performance liquid chromatography. Pharmacokinetic parameters of caffeine and PX/CA, TB/CA, and TP/CA ratios were calculated. Elimination of caffeine was decreased in cirrhotics in comparison with healthy volunteers, as proved by the values of clearance (0.035 vs. 0.094 L/h/kg), elimination coefficient (0.061 vs. 0.153 h(-1)), and half-life (11.4 vs. 4.3 h). Serum metabolite/caffeine ratios were significantly reduced in cirrhotic patients: PX/CA by more than 80%, TB/CA by 50% to 70%, and TP/CA by 40% to 70%. The reduction of the ratios in chronic hepatitis patients was lower and did not occur at all time points. A high correlation was found between caffeine clearance and metabolite/caffeine ratios. Metabolite/caffeine ratios calculated in a single blood sample collected 8 or 12 hours after caffeine administration could provide a practical assessment of hepatic function in cirrhotic patients. The value of the test for the chronic hepatitis patients is limited.

Limitations in the Clinical Usefulness of Single-Dose Pharmacokinetic Studies of Drugs and a Bayesian Approach for the Estimation of Kinetic Parameters

This review presents several clinical examples indicating that physiological changes in the body dependent and/or independent of developmental age, genetic polymorphisms, different disease states, acute and/or chronic inflammations, and physicochemical properties of drugs as well as some environmental factors, such as viral infections, may exert a significant effect on the first-time assessment of kinetic parameters of drug absorption, disposition, metabolism, and excretion after a single-dose administration in children and adults. The available pharmacokinetic data in the literature suggest that one must be cautious in interpretation and practical use of pharmacokinetic variables derived from either single-dose studies or bayesian methods, especially in a pediatric population.

A Bayesian approach for the estimation of pharmacokinetic parameters in children

A Bayesian method is proposed for the first-time assessment of pharmacokinetic parameters (area under the curve [AUC], maximum concentration [C(max)], half-life [T(1/2)]) in children after a single oral dose. Pharmacokinetic parameters were compared using sparse sampling (three blood samples) with extensive sampling (nine blood samples). Two different therapeutic classes of drugs were evaluated in this study. The population pharmacokinetic parameters and the blood sampling time points were obtained from adult data. The results indicate that the Bayesian approach can be used to estimate pharmacokinetic parameters in children with fair degree of accuracy. The Bayesian method described here can be used to assess pharmacokinetic parameters in children, provided there is a prior knowledge of the pharmacokinetics of a drug in adult population.

Nonparametric Expectation Maximisation (NPEM) Population Pharmacokinetic Analysis of Caffeine Disposition from Sparse Data in Adult Caucasians

OBJECTIVE

To explore the ability of the nonparametric expectation maximisation (NPEM) method of population pharmacokinetic modelling to deal with sparse data in estimating systemic caffeine clearance for monitoring and evaluation of cytochrome P450 (CYP) 1A2 activity.

DESIGN AND PARTICIPANTS

Nonblind, single-dose clinical investigation in 34 non-related adult Bulgarian Caucasians (18 women and 16 men, aged between 18 and 62 years) with normal and reduced renal function.

METHODS

Each participant received oral caffeine 3 mg/kg. Two blood samples per individual were taken according to the protocol for measuring caffeine plasma concentrations. A total of 67 measured concentrations were used to obtain NPEM estimates of caffeine clearance. Paraxanthine/caffeine plasma ratios were calculated and correlated with clearance estimates. Graphical methods and tests for normality were applied and parametric and nonparametric statistical tests were used for comparison.

RESULTS

NPEM median estimates of caffeine absorption and elimination rate constants, k(a) = 4.54 h(-1) and k(el) = 0.139 h(-1), as well as of fractional volume of distribution and plasma clearance, V(S1) = 0.58 L/kg and CL(S1) = 0.057 L/h/kg, agreed well with reported values from more 'data rich' studies. Significant correlations were observed between paraxanthine/caffeine ratios at 3, 8 and 10 hours and clearance (Spearman rank correlation coefficients, r(s), >0.74, p </= 0.04). Sex or renal function caused no significant differences in clearance. Heavy smokers and drinkers showed 2-fold higher CYP1A2 activity. Normality tests and graphical methods of analysing caffeine clearance supported a non-Gaussian and multicomponent distribution of CYP1A2 activity.

CONCLUSIONS

Collectively, the results show that the NPEM method is suitable and relevant for large-scale epidemiological studies of population phenotyping for cancer susceptibility and for abnormal liver function by monitoring CYP1A2 activity based on sparse caffeine data.

Population pharmacokinetics of fentanyl in healthy volunteers

The authors compared the population pharmacokinetics of fentanyl using a standard individualized modeling (SIM) approach versus that of a nonparametric expectation maximization (NPEM) approach. The pharmacokinetic properties of fentanyl administered as a single 5 ug/kg intravenous infusion were evaluated in 18 healthy volunteers by use of SIM as well as with NPEM. NPEM-derived parameters were a total body clearance of 2.12 +/- 0.28 L/kg/h, distributional clearance of 8.43 +/- 4.58 L/kg/h, central volume of distribution of 1.22 +/- 0.21 L/kg, and peripheral volume of distribution of 1.81 +/- 1.47 L/kg. Identified parameter values from the modeling methods resulted in virtually identical simulated profiles; this finding was confirmed when median values noted were not statistically significantly different between modeling methods (SIM or NPEM). However, the NPEM algorithm uniquely identified a greater distributional clearance in the elderly population and also illustrated a profile with at least 10% of the study population having a very high clearance of fentanyl. This finding may affect the therapeutic use of fentanyl. NPEM allows for a more informative global representation of a drug's pharmacokinetics.

Limited sampling model for the estimation of pharmacokinetic parameters in children

A limited sampling model (LSM) is proposed for the first-time assessment of pharmacokinetic parameters (area under the concentration-time curve (AUC), Cmax, and T1/2) in children after a single oral dose of drug. Three drugs were evaluated in this study. The LSM was developed for each drug from the data of 10 healthy adult volunteers. The relationship at selected time points between plasma concentration and the AUC or Cmax was evaluated by multiple linear regression. The multiple linear regression that gave the best correlation coefficient (r) for 3 sampling times versus AUC or Cmax was chosen as the LSM. Pharmacokinetic parameters generated using sparse sampling (3 blood samples) were compared with pharmacokinetic parameters generated using extensive sampling (>7 blood samples). The results indicated that a limited sampling model can be developed from adult data to estimate pharmacokinetic parameters in children with fair degree of accuracy.

Validation of a high-performance liquid chromatography assay for quantification of caffeine and paraxanthine in human serum in the context of CYP1A2 phenotyping

In this study the validation of a reversed-phase high-performance liquid chromatography (HPLC) method, with UV-detection, for both caffeine and paraxanthine in human serum is described. This method is feasible for cytochrome P450 1A2 (CYP1A2) phenotyping, according to the results of a pilot study. With this HPLC method caffeine and paraxanthine can be determined selectively and specifically. In the expected concentration range, caffeine recoveries were 98-108% (within-run variation 4.0-6.4%, between-run variation 6.4-8.8%), paraxanthine recoveries were 96.6-97.5% (within-run variation 5.0-7.2%, between-run variation 7.2-10.8%). The limits of detection for caffeine and paraxanthine using this HPLC system were 0.3 and 0.1 mg/L, respectively. Linear calibration curves for both caffeine and paraxanthine were obtained in the concentration range 0.5-30 mg/L (r > 0.9999. Serum samples were stable for a week, when stored at -20 and +4 degrees C.