Concentration-effect relationship of the positive chronotropic and hypokalaemic effects of fenoterol in healthy women of childbearing age

Interpreting population pharmacokinetic-pharmacodynamic analyses - a clinical viewpoint

The population analysis approach is an important tool for clinical pharmacology in aiding the dose individualization of medicines. However, due to their statistical complexity the clinical utility of population analyses is often overlooked. One of the key reasons to conduct a population analysis is to investigate the potential benefits of individualization of drug dosing based on patient characteristics (termed covariate identification). The purpose of this review is to provide a tool to interpret and extract information from publications that describe population analysis. The target audience is those readers who are aware of population analyses but have not conducted the technical aspects of an analysis themselves. Initially we introduce the general framework of population analysis and work through a simple example with visual plots. We then follow-up with specific details on how to interpret population analyses for the purpose of identifying covariates and how to interpret their likely importance for dose individualization.

Pharmacokinetic-pharmacodynamic modeling of the effect of fluvoxamine on p-chloroamphetamine-induced behavior

The pharmacokinetic-pharmacodynamic (PK-PD) correlation of the effect of fluvoxamine on para-chloroamphetamine (PCA)-induced behavior was determined in the rat. Rats (n=66) with permanent arterial and venous cannulas received a 30-min intravenous infusion of 1.0, 3.7 or 7.3 mg kg(-1) fluvoxamine. At various time points after the start of fluvoxamine administration, a single dose of PCA (2.5 mg kg(-1)) was injected in the tail vein and resulting behavioral effects, excitation (EXC), flat body posture (FBP) and forepaw trampling (FT), were immediately scored (scores: 0, 1, 2 or 3) over a period of 5 min. In each individual animal the time course of the fluvoxamine plasma concentration was determined up to the time of PCA administration. Observed behavioral effects were related to fluvoxamine plasma concentrations. Fluvoxamine pharmacokinetics was described by a population three-compartment pharmacokinetic model. The effects of fluvoxamine on PCA-induced behavior (probability of EXC, FBP and FT) were directly related to fluvoxamine plasma concentration on the basis of the proportional odds model. For EXC, EC(50) values for the cumulative probabilities P(Y<1), P(Y<2), P(Y<3) were 237+/-39, 174+/-28 and 100+/-20 ng ml(-1), respectively. Slightly higher EC(50) values were obtained for the corresponding effects on FBP and FT. This investigation demonstrates the feasibility of PK-PD modeling of categorical drug effects in animal behavioral pharmacology. This constitutes a basis for the future development of a mechanism-based PK-PD model for fluvoxamine in this paradigm.

Determination of fenoterol in human plasma by HPLC with fluorescence detection after derivatization

A new method for the determination of fenoterol is described, which uses HPLC separation with fluorescence detection. Dobutamine is employed as an internal standard. The separation was achieved on a short reversed phase column with a mobile phase consisting of water, acetonitrile and methanol. Prior to chromatography both analytes are derivatized with 9-chloroformyl-carbazole. Isolation of the analytes from plasma is carried out by liquid-liquid extraction into 2-butanol after protein precipitation with acetonitrile. The method is capable of estimating fenoterol concentrations in the sub-nanogram per ml range with sufficient accuracy and precision. The determination of fenoterol can now be carried out in the average laboratory without radiolabelled material.

Pharmacokinetic-pharmacodynamic modelling of insulin: comparison of indirect pharmacodynamic response with effect-compartment link models

The pharmacokinetic and pharmacodynamic modelling of insulin has been reported using a combined pharmacokinetic/pharmacodynamic (PK/PD) model, in which a hypothetical effect compartment is linked to a pharmacokinetic compartment. Review of the literature, however, indicated that the recently developed PK/PD models have consisted of an indirect pharmacodynamic response component, but none of them has been applied to the modelling of insulin. To study the relative relevance of the indirect pharmacodynamic response model and the effect-compartment link model in modelling the pharmacokinetics and pharmacodynamics of insulin, regular human insulin was administered intravenously at a dose of 0.1 IU kg(-1) to healthy Yucatan minipigs (after an overnight fasting). The plasma concentrations of insulin were measured by radioimmunoassay at predetermined time intervals, while blood glucose levels were monitored continuously using a glucose monitor. Analysis of the plasma insulin and the blood glucose profiles was performed by fitting with various PK/PD models and the results indicated that all of the 12 sets of plasma insulin data (after normalizing by the basal levels) have been adequately fitted to the two-compartment open pharmacokinetic model (a mean+/-s.e. correlation coefficient of 0.996+/-0.001 was obtained). The mean+/-s.e. correlation coefficient, the weighted residuals sum of squares (WRSS), and the Akaike's information criterion (AIC) were found, respectively, to be 0.935+/-0.008, 624+/-67, and 522+/-9 for the inhibitory indirect pharmacodynamic response model and 0.941+/-0.010, 547+/-63 and 513+/-9 for the stimulatory indirect pharmacodynamic response model, as compared with 0.725+/-0.041, 2309+/-276 and 628+/-10 for the effect-compartment link model. Based on these results, one may conclude that the indirect pharmacodynamic response model is a more appropriate approach for modelling the PK/PD of insulin than the effect-compartment link model.

High-performance liquid chromatographic determination of the beta2-selective adrenergic agonist fenoterol in human plasma after fluorescence derivatization

A sensitive high-performance liquid chromatographic method has been developed for the determination of the beta2-selective adrenergic agonist fenoterol in human plasma. To improve the sensitivity of the method, fenoterol was derivatized with N-(chloroformyl)-carbazole prior to HPLC analysis yielding highly fluorescent derivatives. The assay involves protein precipitation with acetonitrile, liquid-liquid-extraction of fenoterol from plasma with isobutanol under alkaline conditions followed by derivatization with N-(chloroformyl)-carbazole. Reversed-phase liquid chromatographic determination of the fenoterol derivative was performed using a column-switching system consisting of a LiChrospher 100 RP 18 and a LiChrospher RP-Select B column with acetonitrile, methanol and water as mobile phase. The limit of quantitation in human plasma was 376 pg fenoterol/ml. The method was successfully applied for the assay of fenoterol in patient plasma.

Expanding clinical applications of population pharmacodynamic modelling

Population pharmacokinetics or pharmacodynamics is the study of the variability in drug concentration or pharmacological effect between individuals when standard dosage regimens are administered. We provide an overview of pharmacokinetic models, pharmacodynamic models, population models and residual error models. We outline how population modelling approaches seek to explain interpatient variability with covariate analysis, and, in some approaches, to characterize the unexplained interindividual variability. The interpretation of the results of population modelling approaches is facilitated by shifting the emphasis from the perspective of the modeller to the perspective of the clinician. Both the explained and unexplained interpatient variability should be presented in terms of their impact on the dose-response relationship. Clinically relevant questions relating to the explained and unexplained variability in the population can be posed to the model, and confidence intervals can be obtained for the fraction of the population that is estimated to fall within a specific therapeutic range given a certain dosing regimen. Such forecasting can be used to develop optimal initial dosing guidelines. The development of population models (with random effects) permits the application of Bayes's formula to obtain improved estimates of an individual's pharmacokinetic and pharmacodynamic parameters in the light of observed responses. An important challenge to clinical pharmacology is to identify the drugs that might benefit from such adaptive-control-with-feedback dosing strategies. Drugs used for life threatening diseases with a proven pharmacokinetic-pharmacodynamic relationship, a small therapeutic range, large interindividual variability, small interoccasion variability and severe adverse effects are likely to be good candidates. Rapidly evolving changes in health care economics and consumer expectations make it unlikely that traditional drug development approaches will succeed in the future. A shift away from the narrow focus on rejecting the null hypothesis towards a broader focus on seeking to understand the factors that influence the dose-response relationship--together with the development of the next generation of software based on population models--should permit a more efficient and rational drug development programme.