Diurnal oscillations of CSF valproate in monkey

Probenecid-inhibitable efflux transport of valproic acid in the brain parenchymal cells of rabbits: a microdialysis study

Delivery of valproic acid (VPA) to the human brain is relatively inefficient as reflected by a low brain-to-unbound plasma concentration ratio (< or =0.5) at steady state. Previous pharmacokinetic studies suggested that the unfavorable brain-to-plasma gradient is maintained by coupled efflux transport processes at both the brain parenchymal cells and blood-brain barrier (BBB); one or both of the efflux transporters are inhibitable by probenecid. The present study in rabbits utilized microdialysis to measure drug concentration in the brain extracellular fluid (ECF) of the cerebral cortex during steady-state i.v. infusion with VPA alone or with VPA plus probenecid. Probenecid co-infusion elevated VPA concentration in the brain tissue surrounding the tip of the microdialysis probe to a greater extent than in the ECF (230% versus 47%). Brain intracellular compartment (ICC) concentration was estimated. In control rabbits, the ICC concentration was 2.8+/-0.28 times higher than the ECF concentration. Probenecid co-infusion elevated the ICC-to-ECF concentration ratio to 4.2+/-0.44, which confirms the existence of an efflux transport system in brain parenchymal cells. The ECF-to-unbound plasma concentration ratio was well below unity (0.029), indicating an uphill efflux transport of VPA across the BBB. Co-infusion of probenecid did not have a significant effect on VPA efflux at the BBB as evidenced by a minimal change in the ECF-to-unbound plasma concentration ratio. This study suggests the presence of distinctly different organic anion transporters for the efflux of VPA at the parenchymal cells and capillary endothelium in the brain.

Effect of para-aminohippurate on the efflux of valproic acid from the central nervous system of the rabbit

Recently we investigated the mechanisms mediating the transport of valproic acid (VPA) between blood and brain. In one study efflux of valproic acid (VPA) from rabbit brain was inhibited by probenecid. Efflux of VPA decreased when probenecid was given intravenously but not when probenecid was given by ventriculocisternal (VC) perfusion indicating that the major site of probenecid-sensitive transport was at the brain capillary endothelium and not at the choroid plexus. In another study VPA transport into rat brain was inhibited by para-aminohippurate (PAH). The purpose of the present study were to determine (a) if the efflux of VPA from rabbit brain was also inhibited by PAH, and (b) whether efflux of VPA could occur at the choroid plexus via an PAH-selective transport system. Six control rabbits received VPA by intravenous infusion and tracer concentrations of [3H]VPA and [14C]PAH by VC perfusion. Rabbits in the PAH group (n = 6) received identical treatment with VPA, tracer concentrations of [3H]VPA and [14C]PAH and, in addition, received 20 mM PAH by VC perfusion. PAH had no effect on the VC extraction ratio of [3H]VPA or the steady-state brain concentration of intravenously administered VPA. It is concluded that the efflux of VPA at the rabbit blood-brain barrier is mediated by a transporter different from the PAH-like transporter responsible for the uptake of VPA into rat brain. In addition, the finding that VC perfusion with PAH had no effect on the VC extraction of [3H]VPA provides further evidence that the choroid plexus plays a negligible role in removal of VPA from the CNS.

Assessment of valproic acid serum-cerebrospinal fluid transport by microdialysis

The systemic disposition and serum-cerebrospinal fluid (CSF) translocation of valproic acid (VPA) were examined in rats after administration of VPA as a bolus, as a continuous infusion, or with probenecid. VPA in CSF was monitored continuously by in vivo microdialysis. Both prolonged VPA infusion and probenecid pretreatment increased the Km for saturable VPA elimination and decreased intrinsic hepatic clearance, perhaps due to competition of probenecid or accumulated VPA metabolites for glucuronidation or depletion of hepatic UDP-glucuronic acid. The CSF/serum VPA ratio increased rapidly initially, then decreased with time throughout the remainder of the experiment in all three groups. This time- and/or concentration-dependent behavior suggested that the rate of CSF penetration increased disproportionately with increasing serum VPA and could be described by a kinetic model incorporating a concentration-dependent first-order rate constant for VPA influx into CSF. Under all experimental conditions, the VPA efflux from CSF appeared to be saturable; an increase in the Michaelis constant for efflux was observed following probenecid pretreatment and during VPA infusion, suggesting competitive inhibition of transport by probenecid and derived metabolites of VPA, respectively. The mechanisms responsible for asymmetric VPA transport between serum and CSF, in particular the apparent concentration-dependent change in the rate constant governing CSF penetration, remain to be elucidated.

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.

Biological rhythms in the absorption, distribution, metabolism and excretion of drugs

Studies carried out in the last 20 years indicated that biological rhythms can be detected in the pharmacokinetics of most classes of drugs. These time-dependent variations could be due to parallel changes in the physiological functions and variables involved in the absorption, distribution, metabolism and excretion of drugs. A review of the data available suggests that the peak and trough values of these functions and variables do not occur at the same hour of the day in every factor involved in drug disposition. This information could be used to predict the time-dependent changes in the pharmacokinetics. The presence of circadian variations in the kinetics of drugs raise the rather old question: "When to administer drug?"