The influence of body posture on lithium clearance

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.

Clinical relevance of drug interactions with lithium

Although lithium continues to be regarded as the treatment of choice for bipolar disorders, the clinical use of this mood stabiliser is associated with an extremely narrow therapeutic range. Relatively minor increases in serum concentrations may induce serious adverse sequelae, and concentrations within the therapeutic range may result in toxic reactions. The safety of combining lithium with other medications, therefore, is a major concern, and extensive clinical experience has served to identify several significant drug interactions. Lithium removal from the body is achieved almost exclusively via renal means. As a result, any medication that alters glomerular filtration rates or affects electrolyte exchange in the nephron may influence the pharmacokinetic disposition of lithium. Concomitant use of diuretics has long been associated with the development of lithium toxicity, but the risk of significant interactions varies with the site of pharmacological action of the diuretic in the renal tubule. Thiazide diuretics have demonstrated the greatest potential to increase lithium concentrations, with a 25 to 40% increase in concentrations often evident after initiation of therapy. Osmotic diuretics and methyl xanthines appear to have the opposite effect on lithium clearance and have been advocated historically as antidotes for lithium toxicity. Loop diuretics and potassium-sparing agents have minor variable effects. Nonsteroidal anti-inflammatory drugs (NSAIDs) have also been associated with lithium toxicity, although the relative interactive potential of specific NSAIDs is difficult to determine. Small prospective studies have demonstrated large interindividual differences in lithium clearance values associated with different NSAIDs. A growing body of evidence also suggests that ACE inhibitors may impair lithium elimination, but further investigations are needed to identify patients at risk. Anecdotal reports have linked numerous medications with the development of neurotoxicity without an apparent effect on the pharmacokinetic disposition of lithium. Antipsychotics, anticonvulsants and calcium antagonists have all be implicated in a sufficient number of case reports to warrant concern. As these medications have all been commonly coadministered with lithium, the relative risk of serious interactions appears to be quite low, but caution is advised.

A comparison between endogenous and exogenous lithium clearance in the anaesthetized rat

The present study tests whether the endogenous renal lithium clearance (CLi) may be used as an alternative to the conventional method in order to avoid the adverse effects of exogenous lithium on renal function. Endogenous CLi) was measured by flameless atomic absorption spectrometry (AAS) in halothane anaesthetized, laparotomized rats and compared with values obtained after subsequent intravenous (i.v.) administration of LiCl. Endogenous plasma lithium concentration (PLi) was 0.30 +/- 0.06 mumol l-1. When measured by both conventional flame emission AAS and flameless AAS on the same samples, values for PLi did not differ significantly. Endogenous CL1/GFR was 0.195 +/- 0.01 (mean +/- SEM). Amiloride caused on significant change in either the endogenous CLi or 51Cr-EDTA clearance, a measure of glomerular filtration rate (GFR), but it caused a significant increase in CNa. Administration of exogenous Li+ had no significant effect on CLi (230 +/- 33 vs. 234 +/- 35 microliters min-1 gKW-1); both urine flow and CNa increased significantly (P < 0.01) (5.4 +/- 0.4 vs. 9.6 +/- 1.3, and 2.4 +/- 0.5 vs. 10.7 +/- 2.4 microliters min-1 gKW-1, respectively). GFR and calculated proximal tubular reabsorption rate (APR) both decreased by approximately 100 microliters min-1 gKW-1 (both P < or = 0.025), suggesting a moderate inhibition of APR after the acute elevation of plasma Li+ concentration. It is concluded that the endogenous CLi may be used as an alternative to the conventional method.