Utility of CSF in translational neuroscience

Human cerebrospinal fluid (CSF) sampling is of high value as the only general applicable methodology to obtain information on free drug concentrations in individual human brain. As the ultimate interest is in the free drug concentration at the CNS target site, the question is what CSF concentrations may tell us in that respect. Studies have been performed in rats and other animals for which concentrations in brain extracellular fluid (brain ECF) as a target site for many drugs, have been compared to (cisterna magna) CSF concentrations, at presumed steady state conditions,. The data indicated that CSF drug concentrations provided a rather good indication of, but not a reliable measure for predicting brain ECF concentrations. Furthermore, comparing rat with human CSF concentrations, human CSF concentrations tend to be higher and display much more variability. However, this comparison of CSF concentrations cannot be a direct one, as humans probably had a disease for which CSF was collected in the first place, while the rats were healthy. In order to be able to more accurately predict human brain ECF concentrations, understanding of the complexity of the CNS in terms of intrabrain pharmacokinetic relationships and the influence of CNS disorders on brain pharmacokinetics needs to be increased. This can be achieved by expanding a currently existing preclinically derived physiologically based pharmacokinetic model for brain distribution. This model has been shown to successfully predict data obtained for human lumbar CSF concentrations of acetaminophen which renders trust in the model prediction of human brain ECF concentrations. This model should further evolute by inclusion of influences of drug properties, fluid flows, transporter functionalities and different disease conditions. Finally the model should include measures of target site engagement and CNS effects, to ultimately learn about concentrations that best predict particular target site concentrations, via human CSF concentrations.     

The use of microdialysis in CNS drug delivery studies. Pharmacokinetic perspectives and results with analgesics and antiepileptics

Microdialysis can give simultaneous information on unbound drug concentration-time profiles in brain extracellular fluid (ECF) and blood, separating the information on blood-brain barrier (BBB) processes from confounding factors such as binding to brain tissue or proteins in blood. This makes microdialysis suitable for studies on CNS drug delivery. It is possible to quantify influx and efflux processes at the BBB in vivo, and to relate brain ECF concentrations to central drug action. The half-life in brain ECF vs. the half-life in blood gives information on rate-limiting steps in drug delivery and elimination from the CNS. Examples are given on microdialysis studies of analgesic and antiepileptic drugs.     

Physiologically Based Pharmacokinetic Modeling to Investigate Regional Brain Distribution Kinetics in Rats

One of the major challenges in the development of central nervous system (CNS)-targeted drugs is predicting CNS exposure in human from preclinical data. In this study, we present a methodology to investigate brain disposition in rats using a physiologically based modeling approach aiming at improving the prediction of human brain exposure. We specifically focused on quantifying regional diffusion and fluid flow processes within the brain. Acetaminophen was used as a test compound as it is not subjected to active transport processes. Microdialysis probes were implanted in striatum, for sampling brain extracellular fluid (ECF) concentrations, and in lateral ventricle (LV) and cisterna magna (CM), for sampling cerebrospinal fluid (CSF) concentrations. Serial blood samples were taken in parallel. These data, in addition to physiological parameters from literature, were used to develop a physiologically based model to describe the regional brain pharmacokinetics of acetaminophen. The concentration-time profiles of brain ECF, CSF(LV), and CSF(CM) indicate a rapid equilibrium with plasma. However, brain ECF concentrations are on average fourfold higher than CSF concentrations, with average brain-to-plasma AUC(0-240) ratios of 121%, 28%, and 35% for brain ECF, CSF(LV), and CSF(CM), respectively. It is concluded that for acetaminophen, a model compound for passive transport into, within, and out of the brain, differences exist between the brain ECF and the CSF pharmacokinetics. The physiologically based pharmacokinetic modeling approach is important, as it allowed the prediction of human brain ECF exposure on the basis of human CSF concentrations.     

The use of microdialysis for the study of drug kinetics: central nervous system pharmacokinetics of diphenhydramine in fetal, newborn, and adult sheep.

The central nervous system (CNS) pharmacokinetics of the H(1) receptor antagonist diphenhydramine (DPHM) were studied in 100- and 120-day-old fetuses, 10- and 30-day-old newborn lambs, and adult sheep using in vivo microdialysis. DPHM was administered i.v. at five infusion rates, with each step lasting 7 h. In all ages, cerebrospinal fluid (CSF) and extracellular fluid (ECF) concentrations were very similar to each other, which suggests that DPHM between these two compartments is transferred by passive diffusion. In addition, the brain-to-plasma concentration ratios were >or=3 in all age groups, suggesting the existence of a transport process for DPHM into the brain. Both brain and plasma DPHM concentrations increased in a linear fashion over the dose range studied. However, the ECF/unbound plasma and CSF/unbound plasma DPHM concentration ratios were significantly higher in the fetus and lambs (approximately 5 to 6) than in the adult (approximately 3). The factors f(CSF) and f(ECF), the ratios of DPHM areas under the curves (AUCs) in CSF and ECF to the plasma DPHM AUC, respectively, decreased with age, indicating that DPHM is more efficiently removed from the brain with increasing age. The extent of plasma protein binding of the drug increased with age. This study provides evidence for a transporter-mediated mechanism for the influx of DPHM into the brain and also for an efflux transporter for the drug, whose activity increases with age. Moreover, the higher brain DPHM levels in the fetus and lamb compared with the adult may explain the greater CNS effects of the drug at these ages.     

A pharmacokinetic study of diphenhydramine transport across the blood-brain barrier in adult sheep: potential involvement of a carrier-mediated mechanism.

The purpose of this study was to examine the disposition of diphenhydramine (DPHM) across the ovine blood-brain barrier (BBB). In six adult sheep, we characterized the central nervous system (CNS) pharmacokinetics of DPHM in brain extracellular fluid (ECF) and cerebrospinal fluid (CSF) using microdialysis in two experiments. In the first experiment, DPHM was administered via a five-step i.v. infusion (1.5, 5.5, 9.5, 13.5, and 17.5 microg/kg/min; 7 h per step). Average steady-state CNS/total plasma concentration ratios (i.e., [CNS]/[total plasma]) for steps 1 to 5 ranged from 0.4 to 0.5. However, average steady-state [CNS]/[free plasma] ratios ranged from 2 to 3, suggesting active transport of DPHM into the CNS. Plasma protein binding averaged 86.1 +/- 2.3% (mean +/- S.D.) and was not altered with increasing drug dose. Plasma, CSF, and ECF demonstrated biexponential pharmacokinetics with terminal elimination half-lives (t1/2beta) of 10.8 +/- 5.4, 3.6 +/- 1.0, and 5.3 +/- 4.2 h, respectively. The bulk flow of CSF and transport-mediated efflux of DPHM may explain the observed higher CNS clearances. In the second experiment, DPHM was coadministered with propranolol (PRN) to examine its effect on blood-brain CSF and blood-brain ECF DPHM relationships. Plasma total DPHM concentration decreased by 12.8 +/- 6.3% during PRN, whereas ECF and CSF concentrations increased (88.1 +/- 45.4 and 91.6 +/- 34.3%, respectively). This increase may be due to the inhibitory effect of PRN on a transporter-mediated efflux mechanism for DPHM brain elimination.     

Toward the prediction of CNS drug-effect profiles in physiological and pathological conditions using microdialysis and mechanism-based pharmacokinetic-pharmacodynamic modeling

Our ultimate goal is to develop mechanism-based pharmacokinetic (PK)-pharmacodynamic (PD) models to characterize and to predict CNS drug responses in both physiologic and pathologic conditions. To this end, it is essential to have information on the biophase pharmacokinetics, because these may significantly differ from plasma pharmacokinetics. It is anticipated that biophase kinetics of CNS drugs are strongly influenced by transport across the blood-brain barrier (BBB). The special role of microdialysis in PK/PD modeling of CNS drugs lies in the fact that it enables the determination of free-drug concentrations as a function of time in plasma and in extracellular fluid of the brain, thereby providing important data to determine BBB transport characteristics of drugs. Also, the concentrations of (potential) extracellular biomarkers of drug effects or disease can be monitored with this technique. Here we describe our studies including microdialysis on the following: (1) the evaluation of the free drug hypothesis; (2) the role of BBB transport on the central effects of opioids; (3) changes in BBB transport and biophase equilibration of anti-epileptic drugs; and (4) the relation among neurodegeneration, BBB transport, and drug effects in Parkinson's disease progression.     

Brain microdialysis and PK/PD correlation of pregabalin in rats.

Pregabalin [PGB, (S)-3-isobutyl GABA, CI-1008] is a derivative of the inhibitory neurotransmitter g-aminobutyric acid (GABA). It has shown anticonvulsant, analgesia and anxiety activity in animal models. In this report, blood-brain barrier (BBB) influx and efflux of PGB were investigated with microdialysis at efficacious doses in rats. BBB influx (CLin) and efflux (CLout) permeability for pregabalin were 4.8 and 37.2 microL/min/g brain, respectively, following an intravenous infusion to rats. The results indicate that PGB is brain penetrable, supporting its anti-epilepsy and other CNS pharmacology. Significant anticonvulsant action of PGB was detected between 2 and 8 hr post oral dose, which is lag behind ECF drug concentrations lees. A PK/PD link model was used to describe the counter-clockwise hysteresis relationship between pregabalin brain ECF concentration and the anticonvulsant effect in rats. The resulting Ce (concentration in effect compartment) versus effect profile exhibits a sigmoidal curve and the calculated ECe50 and Keo values were 95.3 ng/mL and 0.0092 min-1, respectively. The small Keo value suggests that the effect is not directly proportional to the amount of pregabalin in the ECF compartment possibly due to inherent delay.     

Diurnal Variation in P-glycoprotein-Mediated Transport and Cerebrospinal Fluid Turnover in the Brain

Nearly all bodily processes exhibit circadian rhythmicity. As a consequence, the pharmacokinetic and pharmacodynamic properties of a drug may also vary with time of day. The objective of this study was to investigate diurnal variation in processes that regulate drug concentrations in the brain, focusing on P-glycoprotein (P-gp). This efflux transporter limits the distribution of many drugs in the brain. To this end, the exposure to the P-gp substrate quinidine was determined in the plasma and brain tissue after intravenous administration in rats at six different time points over the 24-h period. Our results indicate that time of administration significantly affects the exposure to quinidine in the brain. Upon inhibition of P-gp, exposure to quinidine in brain tissue is constant over the 24-h period. To gain more insight into processes regulating brain concentrations, we used intracerebral microdialysis to determine the concentration of quinidine in brain extracellular fluid (ECF) and cerebrospinal fluid (CSF) after intravenous administration at two different time points. The data were analyzed by physiologically based pharmacokinetic modeling using NONMEM. The model shows that the variation is due to higher activity of P-gp-mediated transport from the deep brain compartment to the plasma compartment during the active period. Furthermore, the analysis reveals that CSF flux is higher in the resting period compared to the active period. In conclusion, we show that the exposure to a P-gp substrate in the brain depends on time of administration, thereby providing a new strategy for drug targeting to the brain.     

Exploratory translational modeling approach in drug development to predict human brain pharmacokinetics and pharmacologically relevant clinical doses

The central nervous system (CNS) pharmacokinetics (PK) of drugs that have pharmacological targets in the brain are not often understood during drug development, and this gap in knowledge is a limitation in providing a quantitative framework for translating nonclinical pharmacologic data to the clinical patient population. A focus of translational sciences is to improve the efficiency of clinical trial design via a more judicious selection of clinical doses on the basis of nonclinical data. We hypothesize that this can be achieved for CNS-acting drugs based on knowledge of CNS PK and brain target engagement obtained in nonclinical studies. Translating CNS PK models from rat to human can allow for the prediction of human brain PK and the human dose-brain exposure relationship, which can provide insight on the clinical dose(s) having potential brain activity and target engagement. In this study, we explored the potential utility of this translational approach using rat brain microdialysis and PK modeling techniques to predict human brain extracellular fluid PK of atomoxetine and duloxetine. The results show that this translational approach merits consideration as a means to support the clinical development of CNS-mediated drug candidates by enhancing the ability to predict pharmacologically relevant doses in humans in the absence of or in association with other biomarker approaches.     

Simultaneous central nervous system distribution and pharmacokinetic-pharmacodynamic modelling of the electroencephalogram effect of norfloxacin administered at a convulsant dose in rats

The objective of this study was to investigate the contribution of norfloxacin blood-brain barrier (BBB) transport to its delayed electroencephalogram (EEG) effect in rats. Norfloxacin was injected as a bolus dose of 150 mg kg(-1). Blood samples were collected for total norfloxacin plasma concentration measurements. The corresponding unbound levels were determined in brain extracellular fluid (ECF) using microdialysis. Quantitative EEG recording was conducted during 9 h post-dose. Brain ECF norfloxacin concentrations were much lower than plasma levels (AUC ratio=9.7+/-2.8%) but peaked very early, and concentration versus time profiles were parallel in both biological fluids. The best pharmacokinetic (PK) modelling was obtained by considering that ECF concentrations were part of the central compartment, with a proportionality factor. The peak of EEG effect was delayed and the effect versus plasma concentration curves exhibited a dramatic hysteresis. A PK-pharmacodynamic (PD) effect compartment model with a spline function to describe the relationship between effect and concentration at the effect site successfully described the data. Comparisons of PK-PD parameters estimated from plasma and ECF concentrations show that most of the delayed norfloxacin EEG effect is not due to BBB transport, but also that PD parameters derived from plasma data must be carefully interpreted when drug distribution at the effect site is restricted, as may often be the case for centrally acting drugs.     

Microdialysis in peripheral tissues

The objective of this review is to survey the recent literature regarding the applications of microdialysis in pharmacokinetic studies and facilitating many other studies in peripheral tissues such as muscle, subcutaneous adipose tissue, heart, lung, etc. It has been reported extensively that microdialysis is a useful technique for monitoring free concentrations of compounds in extracellular fluid (ECF), and it is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses the use of microdialysis technique for ECF sampling in peripheral tissues in animal studies. The second part of the review describes the use of microdialysis for ECF sampling in peripheral tissues in human studies. Microdialysis has been applied extensively to measure both endogenous and exogenous compounds in ECF. Of particular benefit is the fact that microdialysis measures the unbound concentrations in the peripheral tissue fluid which have been shown to be responsible for the pharmacological effects. With the increasing number of applications of microdialysis, it is obvious that this method will have an important place in studying drug pharmacokinetics and pharmacodynamics.     

A modified surgical procedure for microdialysis probe implantation in the lateral ventricle of a FVB mouse

A modified surgical procedure is described to implant a microdialysis probe to sample ventricular cerebrospinal fluid (vCSF) in FVB mice. Microdialysis sampling of drugs in vCSF provides insight into drug penetration into the brain across the blood brain barrier (BBB) and the blood CSF barrier (BCB); however, this method has been reported primarily in larger animal species. Implanting a microdialysis probe in the lateral ventricle of a mouse is technically very challenging. The modification consisted of changes in the stereotaxic coordinates and insertion of the cannula and ultimately the probe at a 20 degrees angle. Exact placement of the probe was confirmed using ultrasound (US), micro-computed tomography (CT), and histologic review of serial paraffin sections. Additionally, studies of topotecan CSF penetration in the FVB mouse were conducted. With this modified procedure, the ventricular CSF to plasma AUC ratio of unbound topotecan lactone was greater than that previously reported using conventional methods. We speculate this is due to changes incorporated by the modified procedure that places the probe directly into the lateral ventricle allowing sampling of that discrete compartment. Thus, we propose that this modified procedure for placement of the microdialysis probe is superior to the conventional perpendicular method previously reported.     

CSF as a surrogate for assessing CNS exposure: an industrial perspective

For drugs that directly act on targets in the central nervous system (CNS), sufficient drug delivery into the brain is a prerequisite for drug action. Systemically administered drugs can reach CNS by passage across the endothelium of capillary vasculatures, the so-called blood-brain barrier (BBB). Literature data suggest that most marketed CNS drugs have good membrane permeability and relatively high plasma unbound fraction, but are not good P-glycoprotein (P-gp) substrates. Therefore, it is important to use the in vitro parameters of P-gp function activity, membrane permeability and plasma unbound fraction as key criteria for lead optimization during the early stage of drug discovery. Evidence from preclinical and clinical studies suggests that drug concentration in cerebrospinal fluid (CSF) appears to be reasonably accurate in predicting unbound drug concentration in the brain. Therefore, CSF can be used as a useful surrogate for in vivo assessment of CNS exposure and provides an important basis for the selection of drug candidates for entry into development. However, it is important to point out that CSF drug concentration is not always an accurate surrogate for predicting unbound drug concentration in the brain. Depending on the physicochemical properties of drugs and the site/timing of CSF sampling, the unbound drug concentration at the biophase within the brain could differ significantly from the corresponding CSF drug concentration.     

Dermal Drug Levels of Antibiotic (Cephalexin) Determined by Electroporation and Transcutaneous Sampling (ETS) Technique

The purpose of this project was to assess the validity of a novel Electroporation and transcutaneous sampling (ETS) technique for sampling cephalexin from the dermal extracellular fluid (ECF). This work also investigated the plausibility of using cephalexin levels in the dermal ECF as a surrogate for the drug levels in the synovial fluid. In vitro and in vivo studies were carried out using hairless rats to assess the workability of ETS. Cephalexin (20 mg/kg) was administered (i.v.) through tail vein and the time course of drug concentration in the plasma was determined. In the same rats, cephalexin concentration in the dermal ECF was determined by ETS and microdialysis techniques. In a separate set of rats, only intraarticular microdialysis was carried out to determine the time course of cephalexin concentration in synovial fluid. The drug concentration in the dermal ECF determined by ETS and microdialysis did not differ significantly from each other and so as were the pharmacokinetic parameters. The results provide validity to the ETS technique. Further, there was a good correlation (~ 0.9) between synovial fluid and dermal ECF levels of cephalexin indicating that dermal ECF levels could be used as a potential surrogate for cephalexin concentration in the synovial fluid.     

An integrated microdialysis rat model for multiple pharmacokinetic/pharmacodynamic investigations of serotonergic agents

INTRODUCTION: Integrated in vivo models applying intracerebral microdialysis in conjunction with automated serial blood sampling in conscious, freely moving rodents are an attractive approach for pharmacokinetic (PK) and simultaneous pharmacokinetic/pharmacodynamic (PK/PD) investigations of CNS active drugs within the same animal. In this work, the ability to obtain and correlate data in this manner was evaluated for the selective serotonin (5-HT) reuptake inhibitor (SSRI) escitalopram. METHODS: An instrumented rat model equipped with an intracerebral hippocampal microdialysis probe and indwelling arterial and venous catheters was applied in the studies. Concomitant with brain microdialysis, serial blood sampling was conducted by means of an automated blood sampling device. The feasibility of the rat model for simultaneous PK/PD investigations was examined by monitoring plasma and brain extracellular concentrations of escitalopram along with SSRI-associated pharmacological activity, monitored as changes in brain 5-HT levels and plasma corticosterone levels. RESULTS: Combining intracerebral microdialysis and automated blood sampling did not cause any detectable physiological changes with respect to basal levels of plasma corticosterone or brain 5-HT levels. Furthermore, the PK of escitalopram in hippocampus following intravenous injection was not influenced by the presence of vascular catheters. Conversion of escitalopram dialysate concentrations into absolute extracellular levels by means of in vivo retrodialysis was verified by the no-net-flux method, which gave similar recovery estimates. The PK of escitalopram could be characterized simultaneously in plasma and the hippocampus of conscious, freely moving rats. Concomitantly, the modulatory and functional effects of escitalopram could be monitored as increases in brain 5-HT and plasma corticosterone levels following drug administration. DISCUSSION: The applicability of intracerebral microdialysis combined with arterial blood sampling was demonstrated for simultaneous PK/PD investigations of escitalopram in individual rats under non-stressful conditions. Together, these temporal relationships provide multiple PK/PD information in individual animals, hence minimizing inter-animal variation using a reduced number of animals.     

Lidocaine distribution into the CNS following nasal and arterial delivery: a comparison of local sampling and microdialysis techniques

The disposition of lidocaine within the CNS of the rat following nasal and intra-arterial delivery was characterized using a microdialysis technique. Lidocaine concentrations in the cisterna magna were determined using microdialysis and compared to those previously determined using a direct CSF sampling method. The disposition profiles for lidocaine into the cisternal CSF obtained using microdialysis were found to be similar to those obtained by direct CSF sampling techniques over an initial 120-min interval. In other experiments, lidocaine disposition in the right (dosed side) and left olfactory bulb following nasal (i.n.) and intra-arterial (i.a.) administration was studied using microdialysis. The lidocaine concentrations in the ipsilateral olfactory bulb were slightly higher after drug administration into the nasal cavity than those in the contralateral olfactory bulb over the initial 20-min sampling interval. Drug concentrations found in the right olfactory bulb were not significantly different from those found in the left olfactory bulb following intra-arterial administration. Comparisons of lidocaine disposition in the right olfactory bulb and cerebellum, two CNS sites with the same regional vascular supply, showed that the disposition patterns were nearly identical for the two sites following i.a. administration. There was a significant lengthening in the t_max at both sites following i.n. delivery compared to i.a. delivery, and the relative concentrations at each site were no longer equivalent. From these results, it appears that the microdialysis technique is a useful tool for studying drug distribution into the CNS. The changes in disposition patterns between i.a. and i.n. administration indicate that other factors or pathways, in addition to the systemic circulation, play a role in the transport of lidocaine into the brain following nasal administration.     

Elevated concentrations of morphine 6-beta-D-glucuronide in brain extracellular fluid despite low blood-brain barrier permeability

1 This study was done to find out how morphine 6-beta-D-glucuronide (M6G) induces more potent central analgesia than morphine, despite its poor blood-brain barrier (BBB) permeability. The brain uptake and disposition of these compounds were investigated in plasma and in various brain compartments: extracellular fluid (ECF), intracellular space (ICS) and cerebrospinal fluid (CSF). 2 Morphine or M6G was given to rats at 10 mg kg(-1) s.c. Transcortical microdialysis was used to assess their distributions in the brain ECF. Conventional tissue homogenization was used to determine the distribution in the cortex and whole brain. These two procedures were combined to estimate drug distribution in the brain ICS. The blood and CSF pharmacokinetics were also determined. 3 Plasma concentration data for M6G were much higher than those of morphine, with Cmax and AUC 4-5 times more higher, Tmax shorter, and VZf-1 (volume of distribution) and CL f(-1) (clearance) 4-6 times lower. The concentrations of the compounds in various brain compartments also differed: AUC values for M6G were lower than those of morphine in tissue and CSF and higher in brain ECF. AUC values in brain show that morphine levels were four times higher in ICS than in ECF, whereas M6G levels were 125 higher in ECF than in ICS. 4 Morphine entered brain cells, whereas M6G was almost exclusively extracellular. This high extracellular concentration, coupled with extremely slow diffusion into the CSF, indicates that M6G was predominantly trapped in the extracellular fluid and therefore durably available to bind at opioid receptors.     

Investigation of the high partition of YM992, a novel antidepressant,in rat brain - in vitro and in vivo evidence for the high binding in brain and the high permeability at the BBB

Brain extracellular fluid (ECF) concentration of YM992, a novel antidepressant, was determined using brain microdialysis to investigate the high partition of this drug to the brain after systemic administration to rats. Plasma, cerebrospinal fluid (CSF), ECF and brain concentrations were determined at the steady-state after intravenous infusion to rats. The concentration ratio of brain to plasma at the total concentration base was 71.3, while those of ECF to plasma and CSF to plasma at the free concentration base were comparable. The distribution volume in brain was 375 ml/g brain and in vitro binding of YM992 to rat brain was 98.1-98.5%, suggesting a high binding in the brain. The carotid artery injection study showed that the brain uptake index of YM992 was 141%, furthermore, the uptake clearance into brain after i.v. dosing to rats was 0.6 ml/min/g brain, indicating a high permeability at the blood-brain barrier (BBB). These findings suggest that the high partition of YM992 to rat brain is attributed to its high level of binding in the brain as well as its high permeability at the BBB.     

Intracerebral microdialysis technique and its application on brain pharmacokinetic-pharmacodynamic study

Intracerebral microdialysis (IC-MD) has been developed as a well-validated and powerful technique for decades. As a practical sampling tool, it can gain the continuous dialysates of endogenous and exogenous substances in extracellular fluid (ECF) of awake freely moving animals. Also, variform IC-MD probes (IC-MDPs) have grown more exquisite. The implantation of the IC-MDP in certain tissue of brain allows monitor drug distribution and measure drug and corresponding neurotransmitters levels in brain ECF after administration for brain pharmacokinetic-pharmacodynamic (B-PK-PD) study. So it is suitable for IC-MD to B-PK-PD study (IC-MD/B-PK-PD). The performance of IC-MD/B-PK-PD can not only elevate the degree of precision and accuracy of experimental data, minimize the individual difference by reduced number of animals, but also give important information for the prediction and optimization of drug effective dose in preclinical study. In this review, we have discussed various IC-MD/B-PK-PD studies of analgesic, antiepileptic and antidepressant drug. The role of IC-MD/B-PK-PD in confirming and assessing the drug effect before clinic trials is highlighted.     

Comparison of serum, cerebrospinal fluid and brain extracellular fluid pharmacokinetics of lamotrigine

We investigated the rate of penetration into and the intra-relationship between the serum, cerebrospinal fluid (CSF) and regional brain extracellular fluid (bECF) compartments following systemic administration of lamotrigine in rat. The serum pharmacokinetics were biphasic with an initial distribution phase, (half-life approximately 3 h), and then a prolonged elimination phase of over 30 h. The serum pharmacokinetics were linear over the range 10 - 40 mg kg(-1). Using direct sampling of CSF with concomitant serum sampling, the calculated penetration half-time into CSF was 0.42+/-0.15 h. At equilibrium, the CSF to total serum concentration ratio (0.61+/-0.02) was greater than the free to total serum concentration (0.39+/-0.01). Using in vivo recovery corrected microdialysis sampling in frontal cortex and hippocampus with concomitant serum sampling, the calculated penetration half-time of lamotrigine into bECF, 0.51+/-0.11 h, was similar to that for CSF and was not area or dose dependent. At equilibrium, the bECF to total serum concentration ratio (0.40+/-0.04) was similar to the free to total serum concentration (0.39+/-0.01), and did not differ between hippocampus and frontal cortex. The species specific serum kinetics can explain the prolonged action of lamotrigine in rat seizure models. Lamotrigine has a relatively slow penetration into both CSF and bECF compartments compared with antiepileptic drugs used in acute seizures. Furthermore, the free serum drug concentration is not the sole contributor to the CSF compartment, and the CSF concentration is an overestimate of the bECF concentration of lamotrigine.