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.     

Comparison of in vitro BBMEC permeability and in vivo CNS uptake by microdialysis sampling

The studies presented in this report were designed to assess the correlation of the bovine brain microvessel endothelial cell (BBMEC) apparent permeability coefficient (P_app) and in vivo BBB penetration using microdialysis sampling. A mathematical model was developed to describe the relationship of brain extracellular fluid (ECF) concentration to free drug in plasma. The compounds studied have a broad range of physico-chemical characteristics and have widely varying in vitro and in vivo permeability across the blood–brain barrier (BBB). BBMEC permeability coefficients vary in magnitude from a low of 0.9×10⁻⁵ cm/s to a high value of 7.5×10⁻⁵ cm/s. Corresponding in vivo measurements of BBB permeability are represented by clearance (CL_in) into the brain ECF and range from a low of 0.023 μl/min/g to a high of 12.9 μl/min/g. While it is apparent that in vitro data from the BBMEC model can be predictive of the in vivo permeability of a compound across the BBB, there are numerous factors both prior to and following entry into the brain which impact the ultimate uptake of a compound. Even in the presence of high BBB permeability, factors such as high plasma protein binding, active efflux across the BBB, and metabolism within the CNS can greatly limit the ultimate concentrations achieved. In addition, concentrations in the intracellular space may not be the same as concentrations in the extracellular space. While these data show that the BBMEC permeability is predictive of the in vivo BBB permeability, the complexity of the living system makes prediction of brain concentrations difficult, based solely on the in vitro measurement.     

The Simultaneous Estimation of the Influx and Efflux Blood-Brain Barrier Permeabilities of Gabapentin Using a Microdialysis-Pharmacokinetic Approach

Purpose. To determine the apparent bidirectional permeabilities of gabapentin (GBP) across the blood-brain barrier (BBB) using a novel microdialysis-pharmacokinetic approach. Methods. Rats were administered intravenous infusions of [¹⁴C]GBP to achieve clinically relevant steady-state plasma concentrations. Microdialysis was used to monitor GBP concentration in brain extracellular fluid (ECF) in conscious animals. Brain tissue GBP concentration was measured at termination. The BBB influx (CL₁) and efflux (CL₂) permeabilities of GBP were estimated with a hybrid pharmacokinetic model assuming that transport between intra-and extracellular space was more rapid than transport across the BBB. The time course of GBP concentration in brain tissue was determined independently to validate the model assumption. Results and Conclusions. Simulations of the concentration-time course of GBP in brain tissue based on this modeling correlated well with the time-course of brain tissue concentrations determined after intravenous bolus administration and validated this pharmacokinetic-microdialysis approach for estimation of BBB permeabilities. The values for CL₁ and CL₂ were 0.042 (0.017) and 0.36 (0.16) ml/min·g-brain, respectively, indicating that GBP was more efficiently transported from brain ECF to plasma. The total brain tissue concentration of GBP was significantly higher than the ECF concentration at steady-state due to intracellular accumulation and tissue binding, that if not considered, will lead to underestimated efflux BBB permeability using the tissue homogenate-pharmacokinetic approach.     

Acetyl-L-carnitine permeability across the blood-brain barrier and involvement of carnitine transporter OCTN2

OCTN2 (SLC22A5), an organic cation/carnitine transporter, is widely distributed throughout the body, including the brain. In the present study, the involvement of OCTN2 in acetyl-L-carnitine (ALCAR) permeation across the blood-brain barrier (BBB) was examined using a microdialysis method in mouse. OCTN2 function was examined by comparison of wild-type mice with jvs mice, which express defective OCTN2 and are considered a model for primary systemic carnitine deficiency. Zero-net-flux method analysis indicated higher in vivo recovery of ALCAR and lower physiological ALCAR concentration in thalamus extracellular fluid (ECF) in jvs mice compared with wild-type mice. Externally added ALCAR showed significantly slower initial uptake across the BBB in jvs mouse. These results indicated that OCTN2 is functionally involved in ALCAR transfer across the BBB. Total radioactivity in ECF after i.v. administration of radiolabelled ALCAR remained constant for the rest of the experimental period. Accordingly, our results indicate that ALCAR is transported from blood to brain ECF by OCTN2 at least in part, and its concentration in brain ECF is regulated by other events such as protein binding and anabolic reactions in the brain, as well as by transport across the BBB.     

Microdialysis Studies of the Distribution of Stavudine into the Central Nervous System in the Freely-Moving Rat

Purpose. To study the extent and time course of distribution of stavudine (d4T) into the central nervous system (CNS) and to investigate the transport mechanisms of antiviral nucleosides in the CNS. Methods. Microdialysis with on-line HPLC analysis was used to measure drug concentrations in the brain extracellular fluid (ECF) and cerebrospinal fluid (CSF) in the freely-moving rat. The in vivo recovery of d4T and zidovudine (AZT) was estimated by retrodialysis, which was validated by the zero-net flux method. The CNS distribution of d4T was investigated during iv and intracerebroventricular (icv) infusion. In the subsequent studies, the effect of AZT on CNS distribution of d4T was examined. Results. During iv infusion, d4T distributed rapidly into the CNS. Its brain ECF/plasma and CSF/plasma steady-state concentration ratios were 0.33 ± 0.06 and 0.49 ± 0.12, respectively (n = 15). During icv infusion, the steady-state d4T concentrations in the brain ECF were 23-fold higher than those during iv infusion, whereas its steady-state plasma levels were about the same for these two routes. Coadministration of AZT with d4T did not alter their respective brain distribution and systemic clearance at the concentrations examined. More importantly, the steady-state brain ECF/plasma and CSF/plasma concentration ratios of d4T were about 2-fold higher than those of AZT (0.15 ± 0.04 and 0.25 ± 0.08) determined in the same animals. Conclusions. d4T readily crosses the blood-brain barrier (BBB) and blood-CSF barrier. An active efflux transport system in the BBB and blood-CSF barrier may be involved in transporting d4T out of the CNS. Direct icv administration of d4T can be used to enhance its brain delivery. Moreover, d4T exhibits a more favorable penetration into the CNS than AZT and therefore may be useful in the treatment of AIDS dementia complex.     

Blood-brain barrier transport and brain distribution of morphine-6-glucuronide in relation to the antinociceptive effect in rats--pharmacokinetic/pharmacodynamic modelling.

1. The objective of this study was to investigate the contribution of the blood-brain barrier (BBB) transport to the delay in antinociceptive effect of morphine-6-glucuronide (M6G), and to study the equilibration of M6G in vivo across the BBB with microdialysis measuring unbound concentrations. 2. On two consecutive days, rats received an exponential infusion of M6G for 4 h aiming at a target concentration of 3000 ng ml(-1) (6.5 microM) in blood. Concentrations of unbound M6G were determined in brain extracellular fluid (ECF) and venous blood using microdialysis and in arterial blood by regular sampling. MD probes were calibrated in vivo using retrodialysis by drug prior to drug administration. 3. The half-life of M6G was 23+/-5 min in arterial blood, 26+/-10 min in venous blood and 58+/-17 min in brain ECF (P<0.05; brain vs blood). The BBB equilibration, expressed as the unbound steady-state concentration ratio, was 0.22+/-0.09, indicating active efflux in the BBB transport of M6G. A two-compartment model best described the brain distribution of M6G. The unbound volume of distribution was 0.20+/-0.02 ml g brain(-1). The concentration-antinociceptive effect relationships exhibited a clear hysteresis, resulting in an effect delay half-life of 103 min in relation to blood concentrations and a remaining effect delay half-life of 53 min in relation to brain ECF concentrations. 4. Half the effect delay of M6G can be explained by transport across the BBB, suggesting that the remaining effect delay of 53 min is a result of drug distribution within the brain tissue or rate-limiting mechanisms at the receptor level.     

CSF,blood-brain barrier,and brain drug delivery

ABSTRACT

Introduction: There are 2 misconceptions about the cerebrospinal fluid (CSF), the blood-brain barrier (BBB), and brain drug delivery, which date back to the discovery of a barrier between blood and brain over 100 years ago. Misconception 1 is that drug distribution into CSF is a measure of BBB transport. Misconception 2 is that drug injected into the CSF compartment distributes to the inner parenchyma of brain.

Areas Covered: Drug distribution into the CSF is a function of drug transport across the choroid plexus, which forms the blood-CSF barrier, and not drug transport across the BBB, which is situated at the microvascular endothelium of brain. Drug injected into CSF undergoes rapid efflux to the blood compartment via bulk flow. Drug penetration into brain parenchyma from the CSF is limited by diffusion and drug concentrations in brain decrease exponentially relative to the CSF concentration.

Expert Opinion: The barrier between blood and brain was discovered in 1913, when it was believed that the BBB was localized to the choroid plexus, and that nutrient flow from blood passed through the CSF en route to brain. These misconceptions are still widely held, and hinder progress in the development of technology for BBB drug delivery.     

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.     

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.     

Transport of a new sleep-inducer, 1H-1,2,4-triazolyl benzophenone derivative (450191-S), and its active metabolites to the brain in rats and mice

The transport of a new sleep-inducer, 450191-S, and its metabolites, M-1, M-2, M-A, M-3 and M-4, to the brain was examined by the BUI (Brain Uptake Index) method in rats and by comparing the plasma and the brain concentration of the metabolites in mice. 450191-S was not passed to the brain and M-A was hardly passed, but the permeability of M-1, M-2 and M-3 was as high as that of diazepam. The blood-brain barrier (BBB) permeability of these benzodiazepines correlated with the lipid solubility expressed by the Rm value. After oral administration of 450191-S or M-1 to mice, the common major metabolites, M-1, M-2, M-A, M-3 and M-4, were detected in the plasma. At the dose level of 5.5 mumol/kg, M-A showed the highest plasma concentration among the metabolites, but only a low level in the brain. At an increased dose of 55 mumol/kg, M-2 showed the highest concentration in both the plasma and the brain. These results with mice correlated well with the results of BBB permeability in rats. The brain level of M-4 was almost at the background level in spite of a considerable plasma level. The total brain concentration of pharmacologically active metabolites immediately after administration of M-1 rose much faster and to a higher level than after 450191-S administration. These results may explain the pharmacological character of 450191-S.     

Dose dependence in the plasma pharmacokinetics and uptake kinetics of 2',3'-dideoxyinosine into brain and cerebrospinal fluid of rats.

Dose dependence in the plasma pharmacokinetics of 2',3'-dideoxyinosine (ddI) was examined during and after 2-hr iv infusions in rats at infusion rates of 12.4, 32.7, and 125 mg/kg/hr. After termination of the infusions, the disappearance of ddI from plasma was distinctly biphasic, suggesting that the majority of ddI is eliminated before distribution equilibrium is achieved. The mean alpha t1/2 following the infusions was 2.7 min and was independent of dose. The mean terminal half-life (beta t1/2) was approximately 24 min and also independent of dose. Nonlinear pharmacokinetic behavior in plasma after infusions was manifested in a decreased clearance with increasing dose, as determined from steady state plasma concentrations of ddI during infusions. In parallel with the decreased clearance, the apparent volume of distribution of the central compartment, Vcapp, decreased with increasing dose. Nonlinearity in clearance with increasing dose could be accounted for using a model which includes rapid, saturable tissue binding. Dose dependence in the kinetics of uptake of ddI into brain tissue and cerebrospinal fluid (CSF) was also examined during and after iv infusions. Steady state concentrations of ddI in brain tissue and CSF varied linearly with steady state plasma concentrations over a plasma concentration range of greater than 30-fold. Mean tissue to plasma concentration ratios, expressed as percentages, were 2% in CSF, 5% in brain tissue, and 1-2% in brain parenchymal tissue (corrected for the contribution of the cerebral vascular space).     

全颅常规外照射对大鼠血脑屏障通透性的影响及相关机制研究

目的探讨全颅常规外照射增加大鼠血脑屏障药物通透性的产生机制,为临床对颅内肿瘤寻找合适的化疗时机提供理论依据。方法 60只正常成年Wistar大鼠随机分为5组,分别采用0、10、20、30、40Gy,^60Coγ射线进行全脑常规分割外照射,2Gy/次、1野/次,5次/周。各组完成照射计划后16h经尾静脉注射氨甲喋呤（MTX）25mg/kg,2h后采集脑脊液,用RP-HPLC法检测其中的MTX浓度。之后所有大鼠均断头取脑,40g/L中性甲醛固定24h,石蜡包埋,采用免疫组化法检测各组大鼠血脑屏障上P-gp的表达情况。结果对照组P-gp阳性表达最强,10Gy组与对照组,20Gy与30Gy组,40Gy组分别与20Gy、30Gy组比较无统计学意义（P〉0.05）,其余各组比较均有统计学意义（P〈0.05）。各组大鼠脑脊液中的MTX平均浓度分别为：0.06、0.08、0.19、0.24、0.23mg/L。经方差分析及秩和检验,10Gy组与对照组,20Gy与30Gy组,40Gy组分别与20Gy、30Gy组比较无统计学意义（P〉0.05）,其余各组比较均有统计学意义（P〈0.05）。结论一定剂量的放射线可能通过降低血脑屏障上P-gp的表达,增加其对药物的通透性,并且在放射剂量达到20Gy的时候化疗效果最好。     

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.     

孕酮对缺氧缺血性脑损伤新生大鼠血脑屏障通透性及脑水肿的影响

目的研究孕酮(PROG)对缺氧缺血性脑损伤(HIBD)新生大鼠血脑屏障(BBB)通透性和脑水肿的作用,并进一步探讨其潜在的机制。方法 7 d龄SD大鼠96只,随机分成4组:正常组、假手术组、缺氧缺血(HI)组和PROG组。建立HIBD动物模型,HI后24 h,伊文思蓝示踪剂检测BBB;干/湿法测定脑含水量;免疫组织化学法观察大脑皮质水孔通道蛋白4(AQP4)表达。结果正常组和假手术组BBB通透性、脑含水量和脑皮质AQP4的表达差异无显著性(P>0.05);HI组BBB通透性、脑含水量和脑皮质AQP4的表达明显高于假手术组(P<0.01);PROG组BBB通透性、脑含水量和脑皮质AQP4的表达明显低于HI组(P<0.05)。结论 PROG通过减轻BBB破坏和脑水肿对HIBD新生大鼠起脑保护作用,PROG的脑保护作用可能与下调新生大鼠脑皮质AQP4的表达有关。     

A critical evaluation of the brain efflux index method as applied to the nitric oxide synthase inhibitor, aminoguanidine

The Brain Efflux Index (BEI) method is an in vivo procedure designed to quantitate saturable efflux mechanisms resident at the blood--brain barrier (BBB). The present work utilized the BEI method to assess the BBB efflux mechanisms of [(14)C]aminoguanidine, a nitric oxide synthase inhibitor. The BEI for [(14)C]aminoguanidine was >100% (relative to [(3)H]inulin diffusion) over a range of 41-184 pmol after 40 min. The unusually high retention (>100%) of [(14)C]aminoguanidine suggested brain parenchymal sequestration, either by neuronal uptake or tissue protein binding. The uptake of [(14)C]aminoguanidine in dendritic neuronal endings (synaptosomes) showed a saturable concentration dependency, consistent with a carrier-mediated process. Nonlinear least-squares regression yielded the following Michaelis--Menten and diffusional (k(ns)) parameters for synaptosomal [(14)C]aminoguanidine uptake: V(max)=118.50 +/- 28.77 pmol x mg protein(-1)/3 min; K(m)=58.34 +/- 8.33 muM; k(ns)=0.15 +/- 0.029 pmol x mg protein(-1)/3 min/muM; mean +/- SEM; n=3 concentration profiles). Protein binding studies using brain tissue showed negligible binding. In summary, this work identified three principle findings: (1) An apparent lack of quantifiable aminoguanidine BBB efflux; (2) a previously undescribed synaptosomal accumulation process for aminoguanidine; and (3) an interesting limitation of the BEI technique where unusual brain parenchymal sequestration yields values >100%.     

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.     

Pharmacokinetics of YM466, a new factor Xa inhibitor, in rats and dogs.

The pharmacokinetics of YM466, a selective inhibitor for factor Xa, was investigated after single intravenous and oral dosing to rats and dogs. After i.v. dosing, plasma YM466 concentration declined in a bi-phasic manner with a terminal elimination half-life of 1.4 h in rats and 0.8 h in dogs. Total plasma clearance values were 884 and 1212 ml/h/kg in rats and dogs, respectively. After oral dosing, plasma YM466 concentrations reached maximum within 2 h and increased in a dose-proportional manner in rats while increase was nonlinear in dogs. The absolute bioavailability of YM466 was 2.7-4.5% in rats, almost constant regardless of the dose levels investigated, while it was 6.9-24.6% in dogs, indicating nonlinear pharmacokinetics. The plasma protein binding of YM466 was 54.7-56.5% in rats and 45.2-49.0% in dogs and almost constant regardless of the concentration. No metabolism of YM466 was observed in an in vitro liver microsome study. These findings suggest that the low bioavailability of YM466 is attributable to the poor absorption not to the extensive metabolism.     

Local distribution into brain tumor and pharmacokinetics of 4-pyridoxate diammine hydroxy platinum, a novel cisplatin derivative, after intracarotid administration in rats with 9L malignant glioma: simultaneous brain microdialysis study

Local distribution into brain tumor and the pharmacokinetics of 4-pyridoxate diammine hydroxy platinum (PyPt), a novel cisplatin derivative, were examined using rats implanted with 9L glioma and compared with cisplatin. PyPt (5.0 mg/kg) and cisplatin (3.5 mg/kg) were administered as selective intracarotid infusions for 30 min to the rats. Dialysates from extracellular fluid (ECF) in tumor and non-tumor brain tissues were collected by simultaneous microdialysis. The amount of platinum was determined by atomic absorption spectrophotometry, as representative of the drug administered. Plasma concentration of total and protein unbound platinum, and urinary excretion amount and tissue distribution of total platinum were also determined. Unbound platinum was accumulated preferentially in the brain tumor tissue ECF after drug administration, while there was little distribution into normal tissue ECF of the brain. In the brain tumor, the values of the unbound platinum AUC and MRT, where AUC is the area under the concentration-time curve and MRT is the mean residence time, for PyPt were 1.7 and 1.3 times larger than with cisplatin, respectively. The brain tumor distribution coefficient (the ratio of brain tumor ECF platinum AUC to plasma protein unbound platinum AUC) for PyPt (0.85) was higher than that for cisplatin (0.69), indicating that the local amount of platinum distributed into the glioma is enhanced by PyPt rather than by cisplatin. The binding to plasma proteins of PyPt (23%) was lower than that of cisplatin (65%). The total platinum concentration in tissues after administration of PyPt was significantly lower than that of cisplatin in the kidney, liver and spleen. In addition, the urinary excretion amount of total platinum after the administration of PyPt was significantly larger than that of cisplatin. These results suggested that PyPt is easily eliminated by rapid urinary excretion because of its reduced interaction with plasma proteins and poor distribution to the kidney or reticuloendothelial tissues such as the liver and spleen. It is concluded that PyPt is an effective cisplatin derivative for the treatment of gliomas with the added advantage of enhancing local distribution of drug into the brain tumor and reducing its accumulation in the kidney, which has previously caused severe nephrotoxicity.     

Regulation of brain copper homeostasis by the brain barrier systems: effects of Fe-overload and Fe-deficiency

Maintaining brain Cu homeostasis is vital for normal brain function. The role of systemic Fe deficiency (FeD) or overload (FeO) due to metabolic diseases or environmental insults in Cu homeostasis in the cerebrospinal fluid (CSF) and brain tissues remains unknown. This study was designed to investigate how blood-brain barrier (BBB) and blood-SCF barrier (BCB) regulated Cu transport and how FeO or FeD altered brain Cu homeostasis. Rats received an Fe-enriched or Fe-depleted diet for 4 weeks. FeD and FeO treatment resulted in a significant increase (+ 55%) and decrease (− 56%) in CSF Cu levels (p < 0.05), respectively; however, neither treatment had any effect on CSF Fe levels. The FeD, but not FeO, led to significant increases in Cu levels in brain parenchyma and the choroid plexus. In situ brain perfusion studies demonstrated that the rate of Cu transport into the brain parenchyma was significantly faster in FeD rats (+ 92%) and significantly slower (− 53%) in FeO rats than in controls. In vitro two chamber Transwell transepithelial transport studies using primary choroidal epithelial cells revealed a predominant efflux of Cu from the CSF to blood compartment by the BCB. Further ventriculo-cisternal perfusion studies showed that Cu clearance by the choroid plexus in FeD animals was significantly greater than control (p < 0.05). Taken together, our results demonstrate that both the BBB and BCB contribute to maintain a stable Cu homeostasis in the brain and CSF. Cu appears to enter the brain primarily via the BBB and is subsequently removed from the CSF by the BCB. FeD has a more profound effect on brain Cu levels than FeO. FeD increases Cu transport at the brain barriers and prompts Cu overload in the CNS. The BCB plays a key role in removing the excess Cu from the CSF.     

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.