Pharmacokinetic modelling of blood-brain barrier transport of escitalopram in rats

This study examined the pharmacokinetics and distribution of escitalopram in the brain extracellular fluid in rats by the concurrent use of intracerebral microdialysis and serial blood sampling. Following three constant intravenous infusions, drug concentrations in the hippocampus and plasma were monitored for 6 h. To estimate the integrated pharmacokinetics and intercompartmental transport parameters, including blood-brain barrier (BBB) transport over the entire dose range, unbound brain and plasma escitalopram concentration data from all doses were simultaneously analysed using compartmental modelling. The pharmacokinetic analysis revealed that systemic clearance decreased as a function of dose, which was incorporated in the integrated model. Escitalopram was rapidly and extensively transported across the BBB and distributed into the brain extracellular fluid. The modelling resulted in an estimated influx clearance into the brain of 535 microl/min/g brain, resulting in an unbound brain-to-plasma AUC ratio of 0.8 independent of escitalopram dose. The model may be applied for preclinical evaluations or predictions of escitalopram concentration-time courses in plasma as well as at the target site in the CNS for various dosing scenarios. In addition, this modelling approach may also be valuable for studying BBB transport characteristics for other psychotropic agents.     

Microdialysis in the study of drug transporters in the CNS

Quantitative microdialysis in the central nervous system (CNS) has recently provided evidence for the existence of transporters as they relate to the brain distribution of a variety of drugs. Support for the existence of drug transporters in the blood-brain barrier (or in the blood-CSF barrier) comes from investigations that have found: unbound drug concentrations in brain fluids that are lower than corresponding levels in plasma; saturability of transport clearances across the blood-brain barrier and; the regulation of transport by putative inhibitors. Additional confirmatory evidence for the existence of active transport or carrier-mediated processes has also been derived from models that relate observed drug levels in the CNS with those in plasma or blood. The conclusion that reduced drug levels in brain fluids generally indicate the existence of active efflux transport is questioned. In the case of relatively polar compounds with modest blood-brain barrier permeability, lower unbound concentrations in brain may be a consequence of dilution by turnover of brain fluids. This review summarizes recent reports (grouped by class of compounds) where investigators have used microdialysis to examine the distribution of therapeutic agents to the CNS, and have reached conclusions regarding the functional presence of drug transporters in the brain.     

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.     

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.     

Entry of two new asymmetric bispyridinium oximes (K-27 and K-48) into the rat brain: comparison with obidoxime

In the search for new oximes with higher reactivation potency and a broader spectrum, K-27 and K-48, have recently been synthesized. To test if their superior efficacy was related to better penetration across the blood-brain barrier, their brain entry was compared with that of obidoxime, when administered either alone or after the organophosphate paraoxon (POX). Rats received 50 micromol obidoxime, K-27 or K-48, either alone or in addition to 1 micromol POX. Oxime concentrations at various points in time in brain and plasma were measured using HPLC. The obidoxime C(max) in brain was 1.3% of the plasma C(max) when injected alone, and 1.5% when injected following POX. The ratio of the area under the curve (AUC) brain to plasma for obidoxime was around 6%, irrespective of whether it was administered alone or after POX. For K-27, C(max) (brain) was 0.6% of C(max) (plasma) when injected alone, and 0.7% when injected after POX (no significant difference). The AUC (brain) was 2% of AUC (plasma) for both K-27 groups. K-48, when injected alone reached 1.4% of C(max) (plasma) in the brain and 1.2% of C(max) (plasma), when injected following POX. The AUC (brain) was 5% of the AUC (plasma), both when K-48 was administered alone and in combination with POX. Entry of all three oximes into the brain is minimal and cannot explain the better therapeutic efficacy of K-27 and K-48. As already observed for pralidoxime, injection of POX before oxime administration had no influence upon penetration across the blood-brain barrier.     

Effects of a potent and specific P-glycoprotein inhibitor on the blood-brain barrier distribution and antinociceptive effect of morphine in the rat.

Previous data suggest that the analgesic effect of morphine may be modulated by P-glycoprotein (P-gp) inhibition. The effects of the P-gp inhibitor GF120918 on brain distribution and antinociceptive effects of morphine were examined in a rat cerebral microdialysis model. Pretreatment with GF120918 increased both the area under the concentration-time curve of unbound morphine in brain extracellular fluid (ECF) and morphine-associated antinociception. The area under the concentration-time curve ratio for unbound morphine in brain ECF versus unbound morphine in blood was significantly higher in GF120918-treated rats compared with control rats (1.21 +/- 0.34 versus 0.47 +/- 0.05, respectively; p <.05). Modulation of morphine brain-blood distribution was confirmed by quantitating brain tissue morphine in a separate group of rats; GF120918 increased the brain tissue:serum concentration ratio approximately 3-fold. The half-life of unbound morphine in brain ECF was approximately 3-fold longer in GF120918-treated rats compared with controls (p <.05). The fraction unbound of morphine in whole blood was not altered significantly in the presence of GF120918 (0.651 +/- 0.039) as compared with controls (0.662 +/- 0.035). Concentrations of unbound morphine-3-glucuronide in blood and brain ECF were increased in GF120918-treated rats versus controls. An integrated pharmacokinetic/pharmacodynamic model was developed to characterize the unbound blood and brain ECF morphine concentration profiles and concentration-effect relationships. The results of this study indicate that alteration of morphine antinociception by a potent P-gp inhibitor appears to be mediated at the level of the blood-brain barrier.     

Effect of probenecid and quinidine on the transport of alovudine (3'-fluorothymidine) to the rat brain studied by microdialysis

Microdialysis was used to sample extracellular unbound concentrations of alovudine in order to study the influence of well-known transport inhibitors (probenecid and quinidine) on the transport of alovudine between the blood and the brain extracellular fluid or whole brain tissue. The AUC (area under the time versus concentration curve) ratio brain extracellular fluid/serum was 0.17+/-0.036 after a subcutaneous injection of alovudine 25 mg/kg in rats treated with probenecid 25 mg/kg subcutaneous (n=5), which was not significantly different from the control group (AUC ratio 0.24+/-0.039). Perfusion through the microdialysis probe with probenecid 100 microM (n=4) also had no effect on the brain extracellular fluid/serum AUC ratio after alovudine 25 mg/kg subcutaneous. The AUC ratio brain extracellular fluid/serum was 0.085+/-0.009 after subcutaneous injection of alovudine 25 mg/kg in rats treated with quinidine 25 mg/kg intraperitoneally (n=8), which was significantly lower than the control group. However, the whole brain tissue concentration was not significantly different between control rats (n=5) and rats treated with quinidine (n=4) 1 hr after subcutaneous injection of alovudine 25 mg/kg (brain to serum ratios being 0.11+/-0.006 and 0.10+/-0.005 respectively). Finally, the microdialysis recovery of alovudine increased with increasing concentrations (10, 50, 250, 1250 microM) of alovudine in the perfusion fluid. The recovery of alovudine was increased in quinidine-treated rats but not in those given probenecid. Thus, probenecid does not significantly influence the concentration gradient of alovudine over the blood-brain barrier in the rat after systemic or after local administration, while quinidine lowered brain extracellular fluid concentration of alovudine, but not total brain tissue concentration. The mechanism behind this phenomenon is not yet known.     

Blood-Brain Barrier Equilibration of Codeine in Rats Studied with Microdialysis

Purpose. The purpose of the study was to investigate the distribution of codeine across the blood-brain barrier (BBB) in rats by micro-dialysis (MD). Methods. Rats were administered intravenous infusion of codeine in doses of (1) 10 mg/kg, (2) 20 mg/kg for 10 min, and (3) an exponential infusion for 2 h aiming at a plasma concentration of 2500 ng/ml, in a crossover design (n = 6). Microdialysis was used to determine codeine unbound concentrations in blood and brain extracellular fluid (ECF). Total brain tissue and plasma concentrations were also determined. Nalorphine was used as a calibrator for measurement of in vivo recovery. Results. Relative recovery and retrodialysis loss of codeine and nalorphine were similar both in vitro and in vivo. Codeine was rapidly transported into the brain ECF with identical influx and efflux clearance across the BBB. The AUC ratios of brain to blood were 0.99 ± 0.25 and 0.95 ± 0.16 for Dose 1 and 2, respectively. The C_ss ratio of brain to blood was 1.06 ± 0.12 for the exponential infusion. The half-lives were 25 ± 4 min, 22 ± 2 min in blood and 27 ± 5 min, 25 ± 5 min in brain for Dose 1 and Dose 2, respectively. Total brain tissue concentrations were 3.6 ± 1.2-fold higher than the unbound concentrations in brain. Codeine was demethylated to morphine with an unbound AUC_{bIood,morphine}/AUC_{blood,codeine} ratio of 7.7 ± 5.1% in blood. No morphine was detected in brain MD, but total concentrations were possible to measure. Conclusions. Codeine rapidly reached a distributional equilibrium with equal unbound concentrations in blood and brain. The brain transport of codeine did not show any dose-dependency.     

Study of the percutaneous penetration of flurbiprofen by cutaneous and subcutaneous microdialysis after iontophoretic delivery in rat

The percutaneous penetration of flurbiprofen delivered by iontophoresis was investigated in the hairless rat. Unbound concentrations of flurbiprofen in dermis and subcutaneous tissue were continuously measured by on-line microdialysis. Simultaneously, a conventional blood sampling was performed. Linear microdialysis probes were implanted in dermis and in subcutaneous tissue at a depth of 398.3 +/- 15.3 and 1878 +/- 35.8 microm, respectively. Commercial patches were used to deliver flurbiprofen for 15 min at a current density of 0.4 mA/cm(2). In vivo recoveries of both probes, determined by using naproxen as retrodialysis calibrator, were 26.0 +/- 0.3 and 72.9 +/- 0.7% for dermal and subcutaneous probe, respectively. After iontophoretic delivery, a gradient in mean tissue unbound concentrations was observed, with a C(max) in dermis of 8.7 +/- 0.4 microg/mL as compared with subcutaneous C(max) of 0.5 +/- 0.1 microg/mL. The area under the unbound concentration curve in dermis was 13-fold higher than that in the subcutaneous tissue. Total plasma concentration curves showed a rapid absorption phase with a T(max) of 30 min and C(max) of 1.8 +/- 0.1 microg/mL. In conclusion, iontophoresis delivery was demonstrated to be efficient to deliver a high amount of flurbiprofen in dermis and underlying tissue with a fast input rate whereas maintaining a low plasma exposure.     

Neuropharmacokinetics of a new alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) modulator, S18986 [(S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide], in the rat.

The aim of our study was to determine the neuropharmacokinetics of S18986 [(S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide], a new positive allosteric modulator of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-type receptors, in the rat. We focused on its blood-brain barrier (BBB) uptake and on its brain intra- and extracellular fluid (bICF-bECF) partitioning. BBB transport of S18986 was measured using the in situ brain perfusion technique. bECF concentrations were determined by microdialysis in the two effector areas, i.e., frontal cortex (FC) and dorsal hippocampus (DH), and blood samples were collected simultaneously through a femoral catheter. Cerebrospinal fluid and brain tissue concentrations were determined using a conventional pharmacokinetic approach. Using all the experimental data, pharmacokinetic modeling was applied to describe the S18986 blood-brain disposition. The brain uptake clearance of S18986 was found to be high, about 20 mul s(-1) g(-1). Terminal half-lives were similar in plasma and brain, at around 1 h. Experimental and predicted blood and brain concentrations were a good fit with the pharmacokinetic model, which assumed first-order rate constants at each interface. Ratios of bECF to the unbound plasma area under the curve (AUC) were 0.24 in FC and 0.25 in DH, whereas ratios of bICF/plasma AUC were 1 in FC and 1.5 in DH. We conclude that despite the ratio of bECF/plasma AUC below 1, there is nevertheless an elevated BBB uptake of S18986. This can be explained by the S18986 nonhomogenous bECF/bICF partitioning, since S18986 mainly distributes into hippocampal bICF. This illustrates the importance of taking bECF/bICF partitioning into account when interpreting the neuropharmacokinetics of a drug.     

Pharmacokinetics of repinotan in healthy and brain injured animals

Repinotan hydrochloride (repinotan) is a highly potent and selective 5-HT(1A) full receptor agonist. The ability of repinotan to cross the blood-brain barrier (BBB) and penetrate into rat brain tissue was investigated, because rapid penetration into brain tissue is thought to be essential for neuroprotective efficacy. Intravenous (i.v.) repinotan was rapidly distributed into brain, and the distribution equilibrium between blood and brain was reached immediately after the start of infusion. Free concentrations of repinotan were identical in brain and plasma, indicating that repinotan crosses the BBB freely in both directions with diffusion as a driving force. The brain concentration of repinotan was determined by the free plasma concentration. Thus, the total plasma concentration of repinotan (sum of bound and unbound compound) is only indicative for the brain concentration as long as the unbound fraction remains constant. Metabolites of repinotan do not penetrate the BBB and are retained in the perfusing blood due to their increased polarity. The penetration of [14C] repinotan into ischemic areas of the brain was dependent on time. In studies using injured animals (pMCAO), high levels of [14C] repinotan could be detected in ischemic areas when the compound was administered up to 5 h post injury. [14C] repinotan radioactivity could no longer be detected in ischemic areas when administered 18 h after pMCA-O. After the end of infusion, repinotan was rapidly and completely eliminated from rat brains. Elimination occurred in parallel from plasma and brain with half-lives of about 1 h. In conclusion, repinotan rapidly and to a considerable extent penetrates into brain tissue of healthy and injured animals.     

The role of P-glycoprotein in blood-brain barrier transport of morphine: transcortical microdialysis studies in mdr1a (-/-) and mdr1a (+/+) mice.

1. The aim of this study was to investigate whether blood-brain barrier transport of morphine was affected by the absence of mdr1a-encoded P-glycoprotein (Pgp), by comparing mdr1a (-/-) mice with mdr1a (+/+) mice. 2. Mdr1a (-/-) and (+/+) mice received a constant infusion of morphine for 1, 2 or 4 h (9 nmol/min/mouse). Microdialysis was used to estimate morphine unbound concentrations in brain extracellular fluid during the 4 h infusion. Two methods of estimating in vivo recovery were used: retrodialysis with nalorphine as a calibrator, and the dynamic-no-net-flux method. 3. Retrodialysis loss of morphine and nalorphine was similar in vivo. Unbound brain extracellular fluid concentration ratios of (-/-)/(+/+) were 2.7 for retrodialysis and 3.6 for the dynamic-no-net-flux at 4 h, with corresponding total brain concentration ratios of (-/-)/(+/+) being 2.3 for retrodialysis and 2.6 for the dynamic-no-net-flux. The total concentration ratios of brain/plasma were 1.1 and 0.5 for mdr1a (-/-) and (+/+) mice, respectively. 4. No significant differences in the pharmacokinetics of the metabolite morphine-3-glucoronide were observed between (-/-) and (+/+) mice. 5. In conclusion, comparison between mdr1a (-/-) and (+/+) mice indicates that Pgp participates in regulating the amount of morphine transport across the blood-brain barrier.     

Skin and soft tissue concentrations of tedizolid (formerly torezolid), a novel oxazolidinone, following a single oral dose in healthy volunteers

Plasma concentrations of antimicrobial drugs have long been used to correlate exposure with effect, yet one cannot always assume that unbound plasma and tissue concentrations are similar. Knowledge about unbound tissue concentrations is important in the development of antimicrobial drugs, since most infections are localised in tissues. Therefore, a clinical microdialysis study was conducted to evaluate the distribution of tedizolid (TR-700), the active moiety of the antimicrobial prodrug tedizolid phosphate (TR-701), into interstitial fluid (ISF) of subcutaneous adipose and skeletal muscle tissues following a single oral 600 mg dose of tedizolid phosphate in fasting conditions. Twelve healthy adult subjects were enrolled. Two microdialysis probes were implanted into the thigh of each subject, one into the vastus medialis muscle and one into subcutaneous adipose tissue. Probes were calibrated using retrodialysis. Dialysate samples were collected every 20 min for 12h following a single oral dose of 600 mg tedizolid phosphate, and blood samples were drawn over 24h. Unbound tedizolid levels in plasma were similar to those in muscle and adipose tissue. The ratios of unbound (free) AUC in tissues over unbound AUC in plasma (fAUC(tissue)/fAUC(plasma)) were 1.1 ± 0.2 and 1.2 ± 0.2 for adipose and muscle tissue, respectively. The median half-life was 8.1, 9.2 and 9.6h for plasma, adipose tissue and muscle tissue, respectively. Mean protein binding was 87.2 ± 1.8%. The study drug was very well tolerated. The results of this study show that tedizolid distributes well into ISF of adipose and muscle tissues. Unbound levels of tedizolid in plasma, adipose tissue and muscle tissue were well correlated. Free plasma levels are indicative of unbound levels in the ISF of muscle and adipose tissues.     

Pharmacokinetics of pefloxacin and its interaction with cyclosporin A, a P-glycoprotein modulator, in rat blood, brain and bile, using simultaneous microdialysis

1. In vivo microdialysis with HPLC was used to investigate the pharmacokinetics of pefloxacin and its interaction with cyclosporin A. Microdialysis probes were inserted into the jugular vein/right atrium, the striatum and the bile duct of male Sprague-Dawley rats. Biological fluid sampling thereby allowed the simultaneous determination of pefloxacin levels in blood, brain and bile. 2. Following pefloxacin administration, the brain-to-blood coefficient of distribution was 0.036. This was calculated by dividing the area under the concentration curve (AUC) of pefloxacin in brain by its AUC in blood (k=AUC(brain)/AUC(blood)). 3. When the P-glycoprotein cyclosporin A (10 mg kg(-1)) was co-administered with pefloxacin (10 mg kg(-1)), the AUC and the mean residence time in rat blood did not differ significantly (P>0.05). Similarly, the pharmacokinetics of pefloxacin in rat brain was not affected by the presence of cyclosporin A. 4. The AUC of unbound pefloxacin in bile was significantly greater than that in blood. The disposition of pefloxacin in rat bile shows a slow elimination phase following a peak concentration 30 min after pefloxacin administration (10 mg kg(-1), i.v.). The bile-to-blood coefficient of distribution (k=AUC(bile)/AUC(blood)) was 1.53. 5. The results indicated that pefloxacin was able to penetrate the blood-brain barrier and that the concentration in bile was greater than that in the blood, suggesting active biliary excretion of pefloxacin. Current data obtained from rats show no significant impact of cyclosporin A on the pharmacokinetics of pefloxacin in rat blood and brain when administered by concomitant i.v. bolus.     

Microdialysis combined blood sampling technique for the determination of rosiglitazone and glucose in brain and blood of gerbils subjected to cerebral ischemia

Rosiglitazone is a potent synthetic peroxisome proliferator-activated receptor-gamma (PPAR-γ) agonist which improves glucose control in the plasma and reduces ischemic brain injury. However, the pharmacokinetics of rosiglitazone in the brain is still unclear. In this study, a method using liquid chromatography-mass spectrometry coupled with microdialysis and an auto-blood sampling system was developed to determine rosiglitazone and glucose concentration in the brain and blood of gerbils subjected to treatment with rosiglitazone (3.0 mg kg(-1), i.p.). The results showed the limit of detection was 0.04 μg L(-1) and the correlation coefficient was 0.9997 for the determination of rosiglitazone in the brain. The mean parameters, maximum drug concentration (C(max)) and the area under the concentration-time curve from time zero to time infinity (AUC(inf)), following rosiglitazone administration were 1.06±0.28 μg L(-1) and 296.82±44.67 μg min L(-1), respectively. The time to peak concentration (C(max) or T(max)) of rosiglitazone occurred at 105±17.10 min, and the mean elimination half-life (t(1/2)) from brain was 190.81±85.18 min after administration of rosiglitazone. The brain glucose levels decreased to 71% of the basal levels in the rosiglitazone-treated group when compared with those in the control (p<0.01). Treatment with rosiglitazone decreased blood glucose levels to 80% at 1h after pretreatment of rosiglitazone (p<0.05). In addition, pretreatment with rosiglitazone significantly reduced the cerebral infarct volume compared with that of the control group. These findings suggest that this method may be useful for simultaneous and continuous determination of rosiglitazone and glucose concentrations in brain and plasma. Rosiglitazone was effective at penetrating the blood-brain barrier as evidenced by the rapid appearance of rosiglitazone in the brain, and rosiglitazone may contribute to a reduction in the extent of injuries related to cerebral ischemic stroke via its hypoglycemic effect.     

Disposition of morphine in tissues of the pregnant rat and foetus following single and continuous intraperitoneal administration to the mother

Foetal exposure to maternally administered opiates such as morphine represent a serious human health problem but disposition studies in man are difficult to perform. Morphine disposition was therefore investigated in pregnant rats and their foetuses near term as a model. Disposition was examined either following intraperitoneal dosing as a single dose or continuous infusion. A high-pressure liquid chromatography assay for morphine in plasma and tissue was developed and validated. Following the single morphine dose, foetal distribution was rapid and concentrations in foetal and placental tissue were from 2.6 (whole foetus) to 27.6 (placenta) times higher compared with maternal plasma. The rank order of the area under the concentration vs time curve (AUC) of morphine in tissues was: placenta > or = foetal liver > foetal brain > whole foetus > maternal brain. The foetal brain to maternal brain AUC ratio for morphine was 9.5, suggesting large differences in their blood-brain barrier permeability. Following continuous administration of morphine there were significant linear relationships between maternal plasma and tissue concentrations with the same rank order as the single dose study. However, following continuous administration the relative amount of morphine in placenta and foetal liver was reduced by half and one-third, respectively, compared with the single dose study. These results document why the rat foetus is particularly susceptible to the pharmacodynamic effects of morphine following maternal administration.     

Species comparison of in vivo P-glycoprotein-mediated brain efflux using mdr1a-deficient rats and mice

The experiments described herein compared the extent of in vivo P-glycoprotein (P-gp)-mediated brain efflux between rats and mice for a set of known central nervous system compounds. With use of newly introduced genetically modified mdr1a-deficient rats and their gene-competent counterparts, the brain to plasma distribution was assessed and compared with the distribution pattern in mdr1a-deficient and wild-type mice. Four compounds (aripiprazole, citalopram, risperidone, and venlafaxine) were administered using a continuous subcutaneous osmotic minipump infusion paradigm. Steady-state brain and plasma concentrations of the compounds, including selected metabolites (9-hydroxyrisperidone, O-desmethyl-venlafaxine and N-desmethyl-venlafaxine) were measured in mdr1a-deficient rats and mice and their wild-type counterparts along with their free fractions to determine total and unbound brain to plasma distribution between genotypes within and between species. The results revealed qualitative as well as quantitative similarities between P-gp functionality in vivo at the blood-brain barrier level in rats and mice. All compounds tested were shown to have a significantly higher brain to plasma distribution in both mdr1a-deficient rats and mice compared with that in their wild-type counterparts. Moreover, the relative enhancement in extent of brain penetration between mdr1a-deficient and wild-type rats could be directly correlated to the enhancement ratios obtained in mice. From the unbound brain to unbound plasma distributions, the impact of P-gp on the overall brain penetration capabilities showed minor differences between rats and mice for the compounds tested. In conclusion, a comparable functional role of P-gp between rats and mice with respect to brain efflux mediated by this transporter is suggested.     

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.     

Paraoxon has only a minimal effect on pralidoxime brain concentration in rats

Clinical experience with oximes, cholinesterase reactivators used in organophosphorus poisoning, has been disappointing. Their major anatomic site of therapeutic action and their ability to pass the blood-brain barrier (BBB) are controversial. Although their physico-chemical properties do not favour BBB penetration, access of oximes to the brain may be facilitated by organophosphates. The effect of the organophosphate paraoxon (POX) on pralidoxime (2-PAM) brain entry was therefore determined. Rats either received 50 micromol 2-PAM only (G(1)) or additionally 1 micromol POX ( approximately LD(75)) (G(2)). Three animals each were killed after 5, 15, 30, 60, 90, 120, 180, 240, 360, 480 min, and 2-PAM concentrations in the brain and plasma were measured using HPLC. Moreover, the effect of brain perfusion with isotonic saline on subsequent 2-PAM measurements was assessed. The maximal 2-PAM concentration (C(max)) in G(1) brain was 6% of plasma C(max), while in G(2) brains it was 8%. Similarly, the ratio of the area under the curve (AUC) brain to plasma was 8% in G(1) and 12% in G(2). Brain t(max) (15 min) was slightly higher than plasma t(max) (5 min). The AUC of plasma 2-PAM did not differ between G(1) and G(2). However, in G(1), AUC brain was significantly lower than in G(2), the differences probably being clinically irrelevant. In perfused brains, 2-PAM concentrations were very close to those of non-perfused brains. The results indicate that brain penetration of 2-PAM is poor and that organophosphates only have a modest effect on 2-PAM BBB penetration. Brain perfusion does not significantly alter 2-PAM measurements and is therefore considered unnecessary.     

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.