Population pharmacokinetics of perhexiline from very sparse, routine monitoring data.

Using NONMEM, the population pharmacokinetics of perhexiline were studied in 88 patients (34 F, 54 M) who were being treated for refractory angina. Their mean +/- SD (range) age was 75 +/- 9.9 years (46-92), and the length of perhexiline treatment was 56 +/- 77 weeks (0.3-416). The sampling time after a dose was 14.1 +/- 21.4 hours (0.5-200), and the perhexiline plasma concentrations were 0.39 +/- 0.32 mg/L (0.03-1.56). A one-compartment model with first-order absorption was fitted to the data using the first-order (FO) approximation. The best model contained 2 subpopulations (obtained via the $MIXTURE subroutine) of 77 subjects (subgroup A) and 11 subjects (subgroup B) that had typical values for clearance (CL/F) of 21.8 L/h and 2.06 L/h, respectively. The volumes of distribution (V/F) were 1470 L and 260 L, respectively, which suggested a reduction in presystemic metabolism in subgroup B. The interindividual variability (CV%) was modeled logarithmically and for CL/F ranged from 69.1% (subgroup A) to 86.3% (subgroup B). The interindividual variability in V/F was 111%. The residual variability unexplained by the population model was 28.2%. These results confirm and extend the existing pharmacokinetic data on perhexiline, especially the bimodal distribution of CL/F manifested via an inherited deficiency in hepatic and extrahepatic CYP2D6 activity.     

Caffeine in apnoeic Asian neonates: a sparse data analysis

AIMS: To monitor plasma caffeine concentrations and adverse effects and to study the pharmacokinetics of caffeine in neonatal apnoea in the local Asian population after intravenous administration of caffeine. METHODS: Eighteen neonates with apnoea were treated with caffeine citrate at a loading dose of 10 mg caffeine base kg-1 and a maintenance dose of 2.5 mg kg-1 day-1. Blood samples, three after loading and two after the maintenance dose on day 2, 3, 7, 14 and 21 were taken and analysed for caffeine and its main metabolites using solid phase extraction and h.p.l.c. Adverse effects were monitored. Sparse data pharmacokinetic analysis was performed using P-Pharm. RESULTS: Mean caffeine concentrations varied from 10 to 20 mg l-1 throughout treatment (range 3.6-28.4 mg l-1). These concentrations were efficacious; less so in those with lower concentrations. Adverse effects included gastrointestinal disturbances, diuresis and hyperglycaemia. Pharmacokinetic parameter estimates [mean (coefficient of variation%)] were CL=0.00628 (17.5%) l h-1 and V=0.961 (20.3%) l. CL (l h-1)=0.004248 * wt(kg)+0.00154; r=0.8, P<0.01, explained 64% of the variation. V (l)=0.6299 * wt(kg)+0.259; r=0.67, P<0.01, explained 45% of variation. Model-predicted compared with observed plasma concentrations in a separate group of 10 neonates were unbiased and of good precision. CONCLUSIONS: The dosing regimen studied was suitable for our local Asian neonates as it resulted in therapeutic caffeine concentrations for adequate treatment of apnoea. Adverse effects were tolerable. Therefore, to avoid a higher incidence of adverse effects, this regimen should be retained and not increased as proposed by other workers. CL and V were within values of those reported for neonatal apnoea. Sparse data analysis showed that weight alone was adequate as the influential variable for the accurate prediction of individual pharmacokinetic parameters, plasma concentrations and for dosage adjustment if required.     

Population approaches in drug development

An expert meeting to discuss population pharmacokinetic/pharmacodynamic software was held in Brussels in November 1993 under the auspices of the European Co-operation in Science and Technology (COST), Medicine (B1) programme.Recently developed statistical methods offer the possibility of gaining integrated information on pharmacokinetics and response from relatively sparse observational data obtained directly in patients who are being treated with the drug under development. These methods can minimize the need to exclude patient groups and also allow analysis of a variety of unbalanced designs that frequently arise in the evaluation of the relationships between dose or concentration on the one hand and efficacy or safety on the other relationships that do not readily lend themselves to other forms of statistical analysis.The purpose of the Brussels meeting was to evaluate the state of both existing software and software under development, and to specify the needs and wishes of potential users of such software. It was apparent from the meeting that software development for population data analysis is currently a very active area of investigation and that several very good packages are already available, with more in development.The general consensus of the meeting was that well validated, easy to use software was essential to the implementation of the population approach to drug development. Participants: L.Aarons (UK), L.Balant (Switzerland), P.Bechtel (France), A.Boobis (UK), R.Bruno (France), H.Fluhler (Switzerland), R.Gomeni (France), U.Gundert-Remy (Germany), A.Iliadis (France), M.Karlsson (Sweden), P.Kremers (Belgium), L.Lacey (UK), P.Maire (France), A.Mallet (France), G.Pacifici (Italy), K.Pithan (CEC), J.Rodriguez (Spain), M.Rowland (UK), J-L.Steimer (Switzerland), A.Thomson (UK) J. van Bree (Switzerland), S.Vozeh (Switzerland), J.Wakefield (UK), B.Whiting (UK), A.Zipfel (France)All authors were members of the COST-B1 Working Party on Population Approaches     

Allosteric kinetics of human carboxylesterase 1: species differences and interindividual variability

Esterified drugs such as imidapril, derapril, and oxybutynin hydrolyzed by carboxylesterase 1 (CES1) are extensively used in clinical practice. The kinetics using the CES1 substrates have not fully clarified, especially concerning species and tissue differences. In the present study, we performed the kinetic analyses in humans and rats in order to clarify these differences. The imidaprilat formation from imidapril exhibited sigmoidal kinetics in human liver microsomes (HLM) and cytosol (HLC) but Michaelis-Menten kinetics in rat liver microsomes and cytosol. The 2-cyclohexyl-2-phenylglycolic acid (CPGA) formation from oxybutynin were not detected in enzyme sources from rats, although HLM showed high activity. The kinetics were clarified to be different among species, tissues, and preparations. In individual HLM and HLC, there was large interindividual variability in imidaprilat (31- and 24-fold) and CPGA formations (15- and 9-fold). Imidaprilat formations exhibited Michaelis-Menten kinetics in HLM and HLC with high activity but sigmoidal kinetics in those with low activity. CPGA formations showed sigmoidal kinetics in high activity HLM but Michaelis-Menten kinetics in HLM with low activity. We revealed that the kinetics were different between individuals. These results could be useful for understanding interindividual variability and for the development of oral prodrugs.     

Approaches for modeling within subject variability in pharmacometric count data analysis: dynamic inter-occasion variability and stochastic differential equations

Parameter variation in pharmacometric analysis studies can be characterized as within subject parameter variability (WSV) in pharmacometric models. WSV has previously been successfully modeled using inter-occasion variability (IOV), but also stochastic differential equations (SDEs). In this study, two approaches, dynamic inter-occasion variability (dIOV) and adapted stochastic differential equations, were proposed to investigate WSV in pharmacometric count data analysis. These approaches were applied to published count models for seizure counts and Likert pain scores. Both approaches improved the model fits significantly. In addition, stochastic simulation and estimation were used to explore further the capability of the two approaches to diagnose and improve models where existing WSV is not recognized. The results of simulations confirmed the gain in introducing WSV as dIOV and SDEs when parameters vary randomly over time. Further, the approaches were also informative as diagnostics of model misspecification, when parameters changed systematically over time but this was not recognized in the structural model. The proposed approaches in this study offer strategies to characterize WSV and are not restricted to count data.     

Multiscale and Bayesian approaches to data analysis in genomics high-throughput screening

Tremendous amounts of data are produced by high-throughput screening methods currently employed in drug discovery and product development. A typical cDNA microarray or oligonucleotide-based gene chip experiment easily generates over 10,000 data points for each array or chip. The challenge of inferring meaningful information is formidable given the size and number of these datasets. This paper reviews the current status of statistical tools available for gene expression analysis, with emphasis on Bayesian approaches and multiscale wavelet filtering. Fundamental concepts of Bayesian and multiscale modeling are discussed from the perspective of their potential to address important issues related to the analysis of gene expression data, such as the fact that genomic data often have non-Gaussian distributions and feature localization and multiple scales in both frequency and measurement dimension. Recent publications in these areas are reviewed. Wavelet filtering and the advantages of multiscale methods are demonstrated by application to publicly available gene expression data from the National Cancer Institute (NCI). Multiscale methods, including multiscale principal component analysis (MSPCA), are applied to extract gene subsets and to visualize data in multidimensions for comparisons. Similarity in cell lines and gene selection are effectively visualized and quantitatively compared.     

Population kinetics of tobramycin in neonates

The population kinetics of tobramycin were studied in 140 neonates (100/40 patients for the index/validation groups, respectively) of 30 to 42 weeks' gestational age and 0.8 to 4.25 kg current body weight in their first 2 weeks of life, undergoing routine therapeutic drug monitoring of their tobramycin serum levels. The 365 tobramycin concentration measurements obtained were analyzed by use of NONMEM according to a one-compartment open model with zero-order absorption and first-order elimination. The effect of a variety of demographic, developmental, and clinical factors (gender, height, birth weight, current weight, gestational age, postnatal age, postconceptional age, and serum creatinine concentration) on clearance and volume of distribution was investigated. Forward selection and backward elimination regression identified significant covariates. The final pharmacostatistical model with influential covariates was as follows (full population): clearance (L/h) = 0.0508 x current weight (kg), multiplied by 0.843 if birth weight was 2.5 kg or less (low-birthweight infants), and volume of distribution (L) = 0.533 x current weight (kg). Using the proportional error model for the random-effects parameters, interindividual variability for clearance and for volume of distribution was determined to be 25.8% and 21.9%, respectively, and the residual variability was 19.2%. In this study, the use of the NONMEM gave significant and consistent information on the pharmacokinetics and the determinants of the pharmacokinetic variability of tobramycin in neonates when compared with available bibliographic information. Moreover, the final population pharmacokinetic model may be used to design a priori recommendations for tobramycin and to improve the dosing readjustments through Bayesian estimation.     

Methodological approaches to the population analysis of toxicity data

Toxicokinetics is the assessment of systemic exposure in toxicity studies, in which pharmacokinetic data are generated, either as an integral component in the conduct of the nonclinical toxicity studies or in specially designed supportive studies, in order to assess systemic exposure. The data may be used in the interpretation of toxicity findings and contribute to the assessment of the relevance of these findings to clinical safety. Data may be obtained from all animals in a toxicity study, in representative subgroups, in satellite groups or in separate studies. Applying a mixed effects modelling approach in toxicokinetics offers many advantages over the current approach of having satellite groups. Sparse samples for measuring drug/metabolite concentration are collected in all main animals in the majority of studies where toxicological findings are obtained. Such sampling is unlikely to distress the animals, disturb the conduct of a toxicological study or affect the outcome of the study. Many of the outcome measures in toxicological studies are categorical in nature. For example, lesions may be scored on a one to four scale, from none to severe. The analysis of such data is usually carried out using a general mixed modelling approach. We have implemented such models in a nonlinear mixed effects modelling framework which allows us to relate pharmacokinetic response to outcome. A case study is used to illustrate the principles of general mixed effects modelling in toxicokinetics.     

Population kinetics of gentamicin in neonates

A population kinetic analysis was carried out on sparse plasma gentamicin (GE) concentration data from 469 neonates obtained as part of a routine therapeutic drug monitoring (TDM) programme in the hospital neonatology unit.The best predictors of the kinetic parameters of the monoexponential model, volume of distribution (Vd) and clearance (CL), were the weight (WT) and gestational age (GA). Vd of the neonates was only related to WT, whereas the half-life was only related to the GA.     

Pharmacokinetic analysis of sparse in vivo NMR spectroscopy data using relative parameters and the population approach

NMR spectroscopy in vivo when applied to studying drugs and their metabolites usually measures relative concentration in a tissue over time. Only ratios of clearance and volume parameters can be estimated from these data. Low drug dosages (relative to the sensitivity of in vivo NMR) or rapid drug elimination create the additional problem of data sparsity where a pharmacokinetic model cannot be fitted individually. We have investigated whether relative and absolute pharmacokinetic parameters can be estimated from such data by applying a population model.The data analysed were relative concentractions of 5-fluorouracil (FU) and of the sum of its catabolits -fluoro--ureido-propanoic acid (FUPA) and -fluoro--alanine (FBAL) in te liver, as monitored in 16 cancer patients by [¹⁹F]-NMP spectroscopy during and after a 10-min intravenous infusion of 650 mg FU·m^–2. The structural part of the population model was a non-linear, two-compartment model featuring one FU compartment with volume V _FU , a saturable clearance of FU by conversion into the catabolites where CL=v _max /(k _M +C _FU ), a catabolite compartment with volume V _cat , and a concentration-independent clearance of the catabolites, CL _cat . The parameters actually fitted were: , v _max , k _M ·V _FU , V _cat /V _FU , and CL _cat /V _cat where is a proportionality factor relating the NMR signal intensity of FU to the amount of FU in the body and, therefore, has no purely pharmacokinetic interpretation. All parameters were checked for random interindividual variation; and v _max were also tested for inter-occasion variation. The program system NONMEM was used for model fitting.The estimated mean population parameters were: v _max =121 mol·min^–1, k _M ·V _FU =2590 mol, V _cat /V _FU =0.0648, CL _cat /V _cat =0.0555·min^–1. The proportionality factor was found to depend on body weight and, in addition, to have an inter-occasion random variation (within patients, between examinations). No other random variation of a kinetic parameter could be identified. The estimated v _max is similar to a reported estimate of 2.02 mol·min^–1·kg^–1 derived from FU plasma kinetics.This study shows that sparse relative concentration data can be analysed by using relative parameters in a population model. Only one parameter has no unequivocal pharmacokinetic meaning due to the lack of absolute concentration information. Any contribution of the measuring procedure to the inter-occasion variation of in vivo NMP spectroscopy measurements should be minimized in order to allow the detection of possible inter-individual variances of the pharmacokinetic parameters.Dedicated to Prof. Dr. Rudolf Preussmann on the occasion of his 65th birthday     

Polymorphic CYP2C19 and N-acetylation: human variability in kinetics and pathway-related uncertainty factors.

CYP2C19-mediated oxidation and N-acetylation constitute major phase I and phase II polymorphic pathways of xenobiotic metabolism in humans. Analysis of human variability in kinetics for these pathways has been carried out for compounds metabolised extensively (>60%) by these routes. Data for minor substrates for CYP2C19 metabolism (10-60%) have also been analysed. Published pharmacokinetic studies (after oral and intravenous dosing) in CYP2C19 non-phenotyped healthy adults (NPs), and phenotyped extensive (EMs), slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve) and primarily to acute exposure (peak concentration). Similar analyses were performed for the N-acetylation pathway using data for fast acetylators (FA) and slow acetylators (SA). Interindividual variability in the kinetics of CYP2C19 substrates after oral dosage was greater in EMs than in NPs (60 vs 43% for clearances and 54 vs 45% for Cmax). Lower variability was found for N-acetylation for both phenotypes (32 and 22% for FA and SA, respectively). The internal dose of CYP2C19 substrates in PM subjects would be 31-fold higher than in EMs, while for N-acetylated substrates there was a three-fold difference between SA and FA subjects. Pathway-related uncertainty factors were above the default safety factor of 3.16 for most subgroups and values of 52 and 5.2 would be necessary to cover to the 99th centile of the poor metaboliser phenotype for CYP2C19 and N-acetylation, respectively. An exponential relationship (R(2)=0.86) was found between the extent of CYP2C19 metabolism and the difference in internal dose between EMs and PMs. The kinetic default factor (3.16) would cover PMs for substrates for which CYP2C19 was responsible for up to 20-30% of the metabolism in EMs.     

Population pharmacokinetic analysis of pegylated human erythropoietin in rats

The purpose of this study was to model the pharmacokinetics of the pegylated human erythropoietin (PEG-EPO) after single-dose administration in rats, and to evaluate the influence of weight, sex, and pregnancy status on the pharmacokinetic parameters. A total of 436 serum concentrations from 193 Sprague-Dawley rats were obtained from four pharmacokinetic/toxicokinetic studies, in which a single dose of PEG-EPO was administered by the intravenous (i.v.; dose range: 2.5 to 500 microg/kg) and subcutaneous (s.c.; dose range: 12.5 to 500 microg/kg) route. Pharmacokinetic analysis was performed using nonlinear mixed effect modeling (NONMEM V software) to determine the population mean of pharmacokinetic parameters and the variances of the interindividual random effects. The effect of weight, sex, and pregnancy status on the pharmacokinetic parameters was evaluated by forward inclusion and backward elimination process, using the likelihood ratio test. Nonparametric bootstrap analysis was employed as an internal model evaluation technique to qualify the model developed. An open two-compartment model with linear elimination from the central compartment, a first-order absorption with lag time characterized the serum concentration-time profiles of PEG-EPO after i.v. and s.c. administration. For a male rat of 0.24 kg, the average CL, Vc, Q, Vp, Ka, Tlag, and F was estimated to be 0.728 mL/h, 15.8 mL, 0.373 mL/h, 6.99 mL, 0.0618 h(-1), 3.13 h, and 48.8%, respectively. A twofold increase in weight corresponded with a 170 and 238% increase in CL and Vc, respectively. In female rats, Vp was reduced by 11%, whereas F was increased by 15%. No effect of pregnancy status on any of the parameters could be identified. The interindividual variability in CL, Vc, Vp, Ka, and F was estimated at 10.7, 14.7, 16.6, 11.0, and 13.6%, respectively. Nonparametric bootstrap analysis confirmed the accuracy and the precision of the NONMEM parameter estimates. A population pharmacokinetic approach was used to integrate the knowledge gathered from several pharmacokinetic/toxicokinetic studies in rats. The pharmacokinetics of PEG-EPO in the rat was successfully modeled using a two-compartmental model with a linear elimination from the central compartment and a first-order absorption process with lag time. Weight and sex, but not pregnancy status, were identified as covariates of interest during preclinical development. The population pharmacokinetic model developed will be further used for the purpose of interspecies scaling and PK/PD modeling.     

Methodology for pharmacokinetic/pharmacodynamic data analysis

This review will highlight the role of exploratory data analysis and nonlinear regression software in pharmacokinetic–pharmacodynamic (PK/PD) data analysis. Kinetic and dynamic modelling situations typical to the pharmacokineticist in the drug industry will be addressed, and two case studies, including single-dose intravenous bolus plasma data and multiple-dose response-time data will be analysed. Approaches to assessing the suitability of model fit to the observed data will be discussed, and a summary of the key features of available commercial PK software will be given. Specific emphasis will be placed on the application of WinNonlin.     

Catecholamines and the kinetics of lipolysis in isolated rat adipocytes: Statistical analysis and handling of day-to-day variability in dose-response curves: A general procedure for assessing and manipulating dose-response data

(1) As a first step in studying the kinetics of the lipolytic system of rat adipocytes, the day-to-day variation between dose-response curves has been analysed. (2) Methods are described for the evaluation of large quantities of data relating noradrenaline to lipolysis. (3) A ‘clustering’ technique is presented which can be used both to estimate and minimise differences between curves along the response axis. Criteria are outlined to determine whether clustering is appropriate. (4) Our findings indicate that the basic form of the relationship between lipolysis and noradrenaline concentration is relatively stable and consequently that the data can be utilised to study the relationship in greater detail. (5) The techniques described should be applicable to all hormone-mediated responses and may therefore provide the first step towards a meaningful analysis of the relationship between hormone concentrations and response in many systems.     

Statistical evaluation of stability data: criteria for change-over-time and data variability

In a recently issued ICH Q1E guidance on evaluation of stability data of drug substances and products, the need to perform a statistical extrapolation of a shelf-life of a drug product or a retest period for a drug substance is based heavily on whether data exhibit a change-over-time and/or variability. However, this document suggests neither measures nor acceptance criteria of these two parameters. This paper demonstrates a useful application of simple statistical parameters for determining whether sets of stability data from either accelerated or long-term storage programs exhibit a change-over-time and/or variability. These parameters are all derived from a simple linear regression analysis first performed on the stability data. The p-value of the slope of the regression line is taken as a measure for change-over-time, and a value of 0.25 is suggested as a limit to insignificant change of the quantitative stability attributes monitored. The minimal process capability index, Cpk, calculated from the standard deviation of the regression line, is suggested as a measure for variability with a value of 2.5 as a limit for an insignificant variability. The usefulness of the above two parameters, p-value and Cpk, was demonstrated on stability data of a refrigerated drug product and on pooled data of three batches of a drug substance. In both cases, the determined parameters allowed characterization of the data in terms of change-over-time and variability. Consequently, complete evaluation of the stability data could be pursued according to the ICH guidance. It is believed that the application of the above two parameters with their acceptance criteria will allow a more unified evaluation of stability data.     

Fluvoxamine affects sildenafil kinetics and dynamics

Sildenafil used as oral drug treatment for erectile dysfunction is predominantly metabolized by the cytochrome P450 isozyme 3A4. The antidepressant fluvoxamine is an inhibitor of cytochrome P450 3A4. In a randomized, double-blind, placebo-controlled, crossover study, we evaluated the effects of fluvoxamine dosed to steady state on the pharmacokinetics and pharmacodynamics of sildenafil. Twelve healthy men received oral fluvoxamine or placebo for 10 days (50 mg every day on days 1-3; 100 mg every day on days 4-10). On day 11, all participants received a single, oral, open-label dose of 50 mg sildenafil, and blood samples were collected for analysis of sildenafil plasma concentrations by liquid chromatography/mass spectrometry. Concurrently, the effect of sildenafil on venodilation induced by a constant dose of sodium nitroprusside was assessed using the dorsal hand vein compliance technique. Sildenafil was well tolerated in the presence of fluvoxamine. During fluvoxamine, sildenafil exposure (area under the curve) significantly increased by 40% (P < 0.001), and its half-life increased by 19% (P = 0.034). Concurrently, sodium nitroprusside-induced venodilation was significantly augmented by 59% during fluvoxamine compared to placebo (P = 0.012). In conclusion, sildenafil kinetics are mildly affected by fluvoxamine which translates into an increase in vascular sildenafil effects. Whereas the pharmacokinetic changes do not suggest a large clinically relevant interaction, it may be prudent to consider a starting dose of 25 mg in patients concurrently treated with fluvoxamine.     

Local kinetics and dynamics of xenobiotics

The linkage point between kinetics and dynamics of drugs and other foreign chemicals is at the active site of the target. The response is determined within certain limits by the concentration of the substance at the target and its mechanism of action. However, the concentration of the substance at the active site under in vivo conditions cannot be easily measured, and surrogates such as plasma concentration are used instead. The emergence of in vitro approaches to study some primary pharmacokinetic (PK) and pharmacodynamic (PD) processes in simpler experimental setups could provide ways to estimate the concentration at the active site, which then can be used in a more reliable extrapolation and prediction of in vivo behavior and action of the substance. Imaging and other novel analytical techniques are providing direct or indirect measurements of in vivo concentrations in tissues. In this review, processes such as transport, metabolism, etc. affecting the concentration of the substance at the active site are discussed, analytical possibilities to measure directly or estimate indirectly the concentration at the active site are illustrated, the overall design and performance of physiologically based PK-PD models are described, and some examples concerning blood-brain barrier, liver, and lungs are presented. In conclusion, several important recommendations are made: (1) In vitro systems should be characterized in a much more detailed way than has been done so far, especially with respect to unbound concentrations at the sites of disposition and action; (2) kinetics of the compound in the in vitro system have to be more thoroughly characterized; (3) local models encompassing the in vitro system should be developed and used; and (4) PBPK/PD models should incorporate these local models.     

Population balance modeling of aggregation kinetics of recombinant human interleukin-1 receptor antagonist

The kinetics of benzyl alcohol-induced nonnative aggregation of recombinant human interleukin-1 receptor antagonist (rhIL-1ra) were investigated using a population balance model. Steady-state size distributions of rhIL-1ra aggregates formed in a continuous mixed suspension, mixed product removal (MSMPR) reactor were measured and used to extrapolate aggregate nucleation and growth rates parameters. Aggregate growth rate was size-dependent and a linear growth rate model was used to derive a population density function. Addition of 0.9 wt/v% benzyl alcohol increased the nucleation rate by approximately four orders of magnitude. The growth rate for aggregates, however, changed little as a function of benzyl alcohol concentration in the range of 0-0.9%. The addition of sucrose to buffer containing 0.9% benzyl alcohol decreased rhIL1-ra nucleation rate by orders of magnitude and had little impact on growth rate kinetics. The simplicity of the population balance model and the physical relevance of the information obtained from this model render it a useful tool to study protein aggregation kinetics and the effects of excipients on this process.