Effect of posture on total phenytoin plasma concentration

Influence of posture on pharmacokinetics

INTRODUCTION

Body position may influence physiological characteristics, such as perfusion, gastrointestinal function and plasma volume. These characteristics may interact with key factors determining the pharmacokinetics of drugs (dissolution, absorption, distribution, metabolism, excretion).

OBJECTIVES

Based on a systematic literature search, current data on the effect of posture on physiological characteristics and/or pharmacokinetics are summarized, and the relevance of possible effects, such as those presenting in clinical practice and clinical pharmacokinetic studies, is assessed.

RESULTS

Postures which favour rapid gastric emptying (sitting, standing, recumbent right) accelerate the absorption of orally administered drugs. Consequently, these postures favour a shorter time to reach peak plasma drug concentration (t(max)) and a higher maximum plasma drug concentration (C(max)) and--in the case of transient saturation of first-pass metabolism--total exposure (area under the concentration-time curve, AUC) in comparison to recumbent left and supine positions (e.g. nifedipine: AUC 30 and 38% higher in standing and right lateral position vs. left lateral position; C(max) 149 and 80% higher, respectively). The magnitude of these postural effects depends strongly on the nature and amount of liquids and food ingested before drug administration and is most pronounced in the fasting state and after administration with a nonnutrient liquid. Changes in splanchnic-hepatic blood flow (e.g. reduction of estimated hepatic perfusion by 37% in standing vs. supine position) may substantially affect the metabolism of orally administered drugs, especially of those with a high/saturable first-pass metabolism. For highly protein-bound drugs (e.g. phenytoin, imipramine), the total plasma concentration has been found to be approximately 10% higher in standing than lying subjects due to changes in plasma volume.

CONCLUSIONS

Positioning of a patient may be an effective method of enhancing or retarding absorption of some drugs in appropriate clinical situations (e.g. toxic ingestions, bedridden patients). In clinical pharmacokinetic trials, such as bioequivalence studies, defining and maintaining posture precisely is a useful approach for reducing within- and between-subject variability.

Recent advances in chronopharmacokinetics: methodological problems

Chronopharmacokinetics deals with the study of the temporal changes in absorption, distribution, metabolism and elimination and thus takes into account the influence of time of administration on these different steps. In the last decade, numerous studies have been devoted to chronokinetics: recent advances will be reviewed in the first part. As representative examples, the main chronokinetic changes of anaesthetics, cardiovascular active drugs and antiinflammatory drugs in men are listed. Temporal changes can be involved at each step of the sequence of pharmacokinetic processes: temporal variations in drug absorption from the gastro-intestinal tract, in plasma protein binding and drug distribution, in drug metabolism (temporal variations in enzyme activity, hepatic blood flow) and in renal drug excretion may play a role. Thus, the time of administration of a drug is an important source of variation which must be taken into account in kinetic studies and particular methodological aspects of chronokinetics are needed. In a chronopharmacokinetic study many factors of variation must be controlled: factors related to the drug itself (influence of food, galenic formulation, drug interactions), subject related-factors (age, gender, pathology, posture, exercise, synchronization) and factors related to the conditions of the administration (single or repeated dosing, constant rate delivery, route of administration). In conclusion, there are some instances in which a chronokinetic study is needed: 1) when possible daily variations in pharmacokinetics may be responsible for time dependent variations in drug effects, 2) when drugs have a narrow therapeutic range, 3) when symptoms of a disease are clearly circadian phase-dependent (e.g. nocturnal asthma, angina pectoris, myocardial infarction, ulcer disease) 4) when drug plasma concentrations are well correlated to the therapeutic effect in case the latter is circadian phase-dependent. Variables influencing pharmacokinetics such as fasting, meals and meal times, galenic formulation, posture, activity-rest, have to be controlled according to the aim of the investigation. The main aim of chronokinetic studies is to control the time of administration which among others, can be responsible for variations of drug kinetics but also may explain chronopharmacological effects observed with certain drugs.