Efflux of Zidovudine and 2′,3′-Dideoxyinosine Out of the Cerebrospinal Fluid When Administered Alone and in Combination to Macaca nemestrina

To determine if there is active efflux of zidovudine (ZDV) and 2,3-dideoxyinosine (ddl) out of the cerebrospinal fluid (CSF), and if this efflux is saturable, we investigated the steady-state CSF/plasma concentration ratio of the two drugs when administered alone or in combination. Constant-rate infusions of ZDV, ddl or both were administered to seven macaques (Macaca nemestrina) through a chronic venous catheter for a minimum of 28 hr. Antipyrine, a marker of passive diffusion, was coinfused in all experiments. Blood (5 mL) and CSF samples (0.5–1 mL) were collected by venous and lumbar/thoracic punctures, respectively, at 24 and 28 hr after beginning the infusion. When ZDV and ddl were administered alone, the steady-state CSF/plasma concentration ratios were significantly different from unity (ZDV, 0.20 ± 0.08; ddI, 0.09 ± 0.04) and were independent of the plasma concentration (P > 0.05). In contrast, the CSF/plasma concentration ratio of antipyrine (0.82 ± 0.19) was close but significantly smaller than unity (P > 0.05). The CSF/ plasma concentration ratios after simultaneous administration of ZDV and ddI were not significantly different (P > 0.05) from those obtained after administration of the drugs alone. These results suggest that ZDV and ddI are actively transported out of the CSF; however, within the concentration range studied, this efflux is neither saturable nor mutually competitive. Concomitant administration of ZDV and ddI did not produce a systemic interaction in the animals, indicating that the pharmacokinetics of either drug is unaffected by the presence of the other.     

Modeling the Enhanced Uptake of Zidovudine (AZT) into Cerebrospinal Fluid. 1. Effect of Probenecid

The kinetics of zidovudine (AZT) distribution into rabbit cerebrospinal fluid (CSF) were studied during continuous infusion of AZT and after iv bolus administration. The CSF/plasma steady-state AZT concentration ratio was 0.192 ± 0.003. That this ratio is less than unity, and the clearance from the CSF due to bulk flow is much smaller than the total CSF-to-plasma clearance, suggests active CSF-to-plasma transport of AZT. Probenecid coadministration significantly enhances AZT distribution into CSF when plasma and CSF concentrations of AZT are at steady state during continuous infusion of AZT or at a transient steady state after a single iv bolus dose administration. A linear pharmacokinetic model which describes the distribution of AZT into CSF and relates intercompartmental clearances between CSF and plasma was developed and was used to analyze the results. This analysis showed that probenecid enhances the distribution of AZT into the CSF by its effect on clearances between plasma and CSF. The CSF exit-rate constant for AZT estimated during probenecid coadministration was significantly different from controls. Probenecid coadministration resulted in a 36% reduction in the CSF-to-plasma transfer-rate constant. Reduction in the CSF to plasma clearance of AZT is probably due to the effect of probenecid on the active CSF-to-plasma transport of AZT. The model analysis also indicates that probenecid may have increased the plasma to CSF clearance of AZT. There was an increasing trend in the steady-state CSF/plasma AZT concentration ratio with increasing plasma probenecid concentrations. These results are consistent with probenecid competitively inhibiting the CSF-to-plasma transport of AZT.     

Transport of Desglycinamide-Arginine Vasopressin Across the Blood–Brain Barrier in Rats as Evaluated by the Unit Impulse Response Methodology

The pharmacokinetic characteristics of desglycinamide-arginine vasopressin (DGAVP) with respect to its transport across the blood–brain barrier (BBB) were studied with the use of serial CSF sampling in an individual animal and the unit impulse response methodology. Transport rate is determined as BBB clearance, the volume of plasma per unit time cleared of the peptide by BBB transport, and the extent of transport as the percentage of the administered dose transported into the central nervous system. Plasma kinetics of DGAVP were shown to be linear within the dose range studied (50–150 µg), plasma mean residence time (MRT) being 18 ± 4 min (mean ± SE; n = 9). Elimination of DGAVP from CSF after icv administration was linear, with an MRT of 10 ± 1 min (n = 9). After iv administration of 100 µg DGAVP, CSF concentrations were detectable for 90 min. Transport from plasma to the central nervous system was linear. The BBB transport clearance value was 1.0 ± 0.3 µl/min, and 0.026 ± 0.007% of the administered dose was transported into the central nervous system. Results demonstrate that, within the concentration range studied, DGAVP is transported across the BBB by passive diffusion, although to a very low extent.     

Distribution,elimination, metabolism and bioavailability of hexamethylenebisacetamide in rats

Summary Hexamethylenebisacetamide (HMBA), an in vitro differentiating agent, was studied for its pharmaco-dynamic actions in animals. Plasma stability, organ distribution, excretion, oral bioavailability, and estimates of pharmacokinetic parameters and acute lethality were determined in rats. The single dose intraperitoneal LD₅₀ was greater than 3000 mg/kg in both mice and rats. The drug was stable in plasma from several different species during an 8 h in vitro incubation at 37°C. Following a single intravenous (iv) bolus injection (1000 mg/kg) to rats, HMBA was removed from the plasma with a half time of 2.2 ± 0.5 h, and 65 ± 8% of the dose was excreted unchanged in the urine during the first 24 h after dosing. During an 8 h iv infusion, plasma concentrations of 4 mM were easily maintained with no apparent adverse effects. Drug was uniformly distributed, with highest concentrations found in thymus, kidney, liver, and lymph node throughout the first 24 h after a single iv bolus dose. In vivo metabolism was very small, and the presence of apparent metabolites was undetectable until 48 h after iv administration. Oral bioavailability was good (32%), with peak plasma concentrations of 2 mM achieved one hour after oral administration. After oral dosing urinary excretion and plasma decay were comparable to similar data obtained after iv dosing.     

In Vitro and in Vivo Transport of Zidovudine (AZT) Across the Blood–Brain Barrier and the Effect of Transport Inhibitors

The transport of the antiviral nucleoside analogue zidovudine (3-azido-3-deoxythymidine; AZT) into the central nervous system (CNS) was characterized in vitro and in vivo. The in vitro model consisted of primary cultures of isolated bovine capillary endothelial cells. The transport rate of AZT across the monolayer, expressed as endothelial permeability P, was determined following luminal and abluminal administration. P did not differ between the two administration sites (luminal, 1.65 ± 0.44 cm/min/10³; abluminal, 1.63 ± 0.28 cm/min/10³). The transport of AZT across the endothelial cell monolayer was found to be concentration independent in the range between 0.4 and 50 µg/mL. AZT transport was not affected by pre-treatment of the cells with either metabolic inhibitors (DODG and DODG/NaN₃) or probenecid. This suggests that AZT passes the monolayer mainly by passive diffusion. The in vivo transport of AZT across the blood–brain barrier and the blood–CSF barrier was studied in male Wistar rats after coadministration of potential inhibitors of active transport of AZT: probenecid (organic anion transport) and thymidine (nucleoside transport). Intracerebroventricular and intravenous coadministration of probenecid caused a significant (P < 0.001) increase in the CSF/plasma concentration ratio compared to the control phase, indicating that the organic anion carrier is involved in AZT transport from CSF to blood. Since there was no effect of probenecid on the transport of AZT in vitro, it is suggested that this carrier is located at the choroid plexus. Coadministration of thymidine did not affect the CSF/plasma concentration ratio, suggesting that a nucleoside carrier system is not involved in AZT transport into or out of the CNS.     

Stereoselective Transport of Baclofen Across the Blood–Brain Barrier in Rats as Determined by the Unit Impulse Response Methodology

The blood–brain barrier transport characteristics of racemic baclofen and the separate R- and S-enantiomers have been determined in vivo in rats by using the unit impulse response methodology. Transport rate was determined as blood–brain barrier clearance, the volume of plasma per unit time cleared of baclofen by transport across the blood–brain barrier. Plasma elimination kinetics and CSF elimination kinetics did not differ among racemic baclofen and the R-and S-enantiomers. Transport of each compound could be described by a linear V(t] curve, suggesting the absence of saturable transport processes in the concentration range studied. However, for R-baclofen the blood–brain barrier clearance (4.7 ± 1.0 µl/min, mean ± SE; n = 6) and cumulative transported amount (0.085 ± 0.007%; n = 6) were significantly higher than these values for the S-enantiomer (1.1 ± 0.3 µl/min, 0.031 ± 0.005%; n = 6) and racemic baclofen (1.0 ± 0.1 µl/min, 0.036 ± 0.003%; n = 6). These findings indicate that there is stereoselective transport of baclofen across the blood–brain barrier.     

Central nervous system kinetics of atenolol and metoprolol in the dog during long term treatment

Beagle dogs with catheters chronically implanted into the lateral cerebral ventricle were used to study the distribution of atenolol and metoprolol between the cerebrospinal fluid (CSF) and blood plasma over a 24-hr period during long term treatment. The concentration of atenolol declined more slowly in CSF than in blood plasma and the CSF/plasma ratio of atenolol (after iv administration for 7 days) increased from 0.08 +/- 0.02 (2 hr after dose) to 0.83 +/- 0.14 (24 hr after dose) (mean +/- SD). Furthermore, the CSF concentration of atenolol, relative to the plasma concentration, increased during repeated drug administration. The CSF/plasma ratio 24 hr after an iv dose was 0.48 +/- 0.12 on day 1 and 0.83 +/- 0.14 on day 7. The CSF concentration of the more lipophilic beta 1-adrenoceptor antagonist metoprolol was almost the same as the concentration of the drug in blood plasma. After 7 days of oral treatment, the CSF/plasma ratio of metoprolol 24 hr after dosing was 0.81 +/- 0.10. The regional CSF concentration of atenolol along the neuraxis was determined in anaesthetized dogs after acute iv administration of the drug. The atenolol concentration in CSF from the lateral cerebral ventricle was similar to that in the cisterna magna but lower than the concentration in CSF sampled from the lumbar region. It is concluded that the CSF concentration of the moderately lipophilic beta 1-adrenoceptor antagonist metoprolol equilibrates with the plasma concentration of the drug more rapidly compared with the hydrophilic drug atenolol.     

Pharmacokinetics of epidural morphine in man

Summary Cerebrospinal fluid (CSF) and plasma morphine concentrations were determined in 5 patients after epidural administration of 6 mg morphine; plasma samples were collected frequently during the initial 6 h and 6–7 CSF samples were obtained from each patient over a 24 h period. Morphine was analysed using gas chromatography and electron capture detection. Individual morphine concentration-time curves were plotted for plasma and CSF and various pharmacokinetic variables were calculated. Plasma morphine concentrations after epidural injection were similar to those found after intramuscular administration; C_max (66±8 mg/ml: mean±SEM) appeared within 12±3 min, and the terminal elimination half-life in plasma was 213±24 min. In CSF, morphine reached a peak (1575±359 ng/ml) after 135±40 min. The terminal elimination half-life for morphine in CSF was 239±10 min. The CSF bioavailability of morphine after epidural administration was calculated to be 1.9±0.5%.The study showed that epidural administration of morphine resulted in CSF concentrations many times higher than those in plasma, but still only 2% of the dose administered was available to the CSF compartment. Morphine was eliminated with similar speed from CSF and plasma.     

The Bioavailability and Nonlinear Clearance of (–)-Carbovir in the Rat

The pharmacokinetics and bioavailability of (±)-carbovir, a carbocyclic nucleoside active against human immunodeficiency virus, have been described previously. To determine the bioavailability of (–)-carbovir, the biologically active enantiomer, four male Sprague–Dawley rats received 18 mg/kg of (–)-carbovir through the jugular vein and 54 mg/kg orally. Following the pilot studies, five rats were randomly assigned to receive (–)-carbovir in a three-way crossover design as either a single 18-mg/kg iv bolus, a single 54-mg/kg oral dose, or a single iv infusion of 18 mg/kg to achieve a target steady-state concentration (C _ss) of 1 µg/ml, the peak concentration after an oral dose. Blood and urine samples were analyzed by an improved ion-paired reversed-phase HPLC method with fluorescence detection. Blood concentrations of (–)-carbovir declined in a biphasic manner after the iv bolus dose. The terminal half-life was 116 and 106 min after the iv bolus and oral dose, respectively. The blood/plasma distribution ratio was approximately 1.0 in the range of 1 to 10 µg/ml of (–)-carbovir in blood. The free fraction in serum was concentration dependent. Significant differences in the renal, nonrenal, and total-body clearances after the iv bolus and iv infusion suggested nonlinear elimination of (–)-carbovir. The oral bioavailabilities derived from blood data were significantly different when the iv bolus was used as a reference rather than the iv infusion. However, the bioavailabilities were not significantly different when the total urinary excretion of unchanged (–)-carbovir after iv bolus or infusion was used as a reference. Concomitant saturation of renal and nonrenal clearances might explain these findings. The oral bioavailability was about 20% at concentrations approximating 1 µg/ml in blood.     

The pharmacokinetics of digoxin in newborn and adult sheep

The pharmacokinetics of digoxin were determined in 12 ewes and 13 newborn sheep after bolus drug administration and under steady state drug conditions. After death, tissue distribution of digoxin was determined and normalized to plasma drug concentrations at steady state. Volume of distribution and total drug clearance were lower at steady state than the comparable variables calculated from bolus drug administration. No significant difference between ewes and newborns was shown for drug distribution half-life (0.72 vs. 0.76 hr), drug elimination halflife (15.2 vs. 13.7), or renal drug clearance (0.86 vs. 0.89 liters/kg/hr). Total drug clearance as well as the area derived and steady state volumes of distribution were higher in newborns than in ewes. Digoxin secretion into the urine was limited in newborns, as evidenced by a lower renal digoxin clearance to creatinine clearance ratio in newborns than in ewes (371 vs. 600%). The plasma concentration of digoxin at steady state correlated well with myocardial drug concentrations. Drug distribution was similar in both age groups; however, the tissue to plasma digoxin ratio in kidney was higher in newborns than in ewes (mean 469 vs. 263, respectively). Although agerelated differences in drug clearance and distribution volume existed, intersubject variation was substantial, and the demonstrated variations were not large enough to account for the high doses of digoxin used to treat congestive heart failure in immature subjects.This work was supported in part by United States Public Health Service Heart, Lung, and Blood Institute Grant #1-R01-HL-20830.     

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.     

The Pharmacokinetics and Absorption of Recombinant Human Relaxin in Nonpregnant Rabbits and Rhesus Monkeys After Intravenous and Intravaginal Administration

Recombinant human relaxin (rhRlx) is being developed as a potential cervical ripening agent to be applied intravaginally or intracervically prior to parturition. The pharmacokinetics and absorption of rhRlx were determined in nonpregnant female rabbits and rhesus monkeys after intravenous bolus (iv) and intravaginal administration of 0.1 mg/kg; additionally, rabbits were dosed with 0.5 mg/kg intravaginally. In rabbits (n = 6), mean (±SD) peak concentrations following iv bolus administration were 1554 ± 296 ng/mL. The weight-normalized clearance (CL/W) was 5.9 ± 0.4 mL/min/kg, initial volume of distribution (V ₁/W) was 57 ± 9 mL/kg, and volume of distribution at steady state (V _SS/W), assuming central compartment elimination, was 240 ± 20 mL/kg. V _ss/W could be as large as 2000 ± 400 mL/kg without this assumption. The estimated amounts of rhRlx absorbed in rabbits following intravaginal administration of 0.1 and 0.5 mg/kg (n = 5/dose) were 3.1 ± 1.4 and 0.7 ± 0.3%, respectively; peak concentrations were 600 ± 297 and 1066 ± 584 pg/mL, respectively. In rhesus monkeys (n = 5) after iv administration, peak concentrations were 971 ± 277 ng/mL; CL/W was 4.1 ± 0.6 mL/ min/kg, V ₁/W was 78 ± 25 mL/kg, and V _ss/W, assuming central compartment elimination, was 690 ± 220 mL/kg. The upper limit for V _ss/W was 1600 ± 200 mL/kg when no assumptions were made regarding site (compartment) of elimination. After intravaginal administration (n = 6), two monkeys had undetectable rhRlx concentrations throughout the 48-hr sampling interval; one monkey had only one sample containing measurable rhRlx (51 pg/mL) at 24 hr; and three monkeys absorbed <2% of the 0.1 mg/kg dose. Peak concentrations in these three animals ranged from 64 to 1475 pg/mL. The absorption of rhRlx was low and variable in both species, and similar results have been observed in women.     

CSF and plasma pharmacokinetics of pethidine and norpethidine in man after epidural and intrathecal administration of pethidine

Summary The disposition of pethidine and its main metabolite, norpethidine, in cerebrospinal fluid (CSF) and plasma was studied in 11 thoracic surgery patients after lumbar epidural (100 mg;n=6) or lumbar intrathecal (25 mg;n=5) administration of pethidine. Pethidine appeared more slowly in plasma after intrathecal than after epidural administration (t_max 2.3 h and 14 min, respectively), but systemic bioavailability was similar. The CSF concentrations of pethidine were higher than those in plasma after both routes of administration. The maximal CSF/plasma concentration ratio was 6000 to 45000 after intrathecal administration but was only 26 to 97 after the epidural route. Pethidine was rapidly distributed in CSF; nine to ten h after the intrathecal and epidural injections the CSF/plasma concentration ratios were 12 to 89 and 2 to 33, respectively. The calculated bioavailability in CSF of epidural pethidine was 10.3%. The terminal elimination half-life of pethidine was 6.0 h (CSF) and 5.4 h (plasma) after intrathecal administration and 8.6 h (CSF) and 8.8 h (plasma) after epidural injection. The volume of distribution of unchanged pethidine in the subarachnoid space was 13 ml·kg^–1 and clearance from the CSF was 15 µl·kg^–1·min^–1. In all patients receiving intrathecal pethidine and in some patients after epidural pethidine, CSF norpethidine concentrations were higher than those in plasma; the maximum CSF norpethidine was 102 to 1211 ng·ml^–1 and 14 to 210 ng·ml^–1 and the maximum CSF/plasma norpethidine concentration ratios were 21 to 652 and 0.6 to 14 times after intrathecal and epidural administration, respectively. Norpethidine was rapidly distributed and its level in CSF was about the same or lower than in plasma during the terminal elimination phase. The maximum CSF norpethidine level was 1.2±1.0% of that of pethidine after intrathecal injection. Thus, epidural pethidine enters the CSF more rapidly and to a greater extent than has been previously shown for epidural morphine, but pethidine is more rapidly redistributed from CSF. The terminal elimination half-life of pethidine was found to be long in relation to the reported duration of analgesia after a single spinal dose of pethidine, which suggests a potential risk of accumulation within the CSF on multiple spinal injections of pethidine. Pethidine is partly metabolised within the subarachnoid space by N-dealkylating enzymes in the CNS. After intrathecal injection of more than 25 mg pethidine, the concentration of the principle metabolite, norpethidine, in CSF may be higher than that associated with CNS toxicity in man.Presented at the III World Conference on Clinical Pharmacology and Therapeutics, Stockholm, Sweden 1986     

Pharmacokinetics and Anticonvulsant Effect of a New Hypnotic,CL 284,846, in Rats

Purpose. CL 284,846 (CL846) is an investigational non-benzodiazepine agent with hypnotic, anxiolytic, myorelaxant and anticonvulsant properties. This study assessed the pharmacokinetics and anticonvulsant action of CL846 in female Sprague-Dawley rats. Methods. CL846 pharmacokinetics were examined after either an iv bolus dose (2.5 mg/kg) or a 6-hr infusion (0.4 mg/kg/hr). CL846 pharmacodynamics were evaluated with a pentylenetetrazol (PTZ) infusion 5 min after a CL846 in bolus dose (0 to 10 mg/kg). CL846 and the derived metabolite CL 284,859 (CL859) concentrations in serum and brain tissue were determined by HPLC with fluorescence detection. Results. Both the steady-state volume of distribution (1636 ± 162 and 1804 ± 293 ml/kg, after bolus and infusion administration, respectively) and systemic clearance (19.1 ± 7.1 and 22.2 ± 4.3 ml/min/kg for bolus and infusion administration, respectively) were high. No differences in pharmacokinetic parameters were noted between the two modes of administration. The relationship between anticonvulsant effect and brain/serum concentrations was well described by an E_max model. CL846 was as effective as triazolam in antagonizing PTZ-induced seizures. Conclusions. Under the conditions of the present study, CL846 pharmacokinetics were linear and stationary. Further evaluation of the anticonvulsant properties of CL846 is warranted, including the potential development of tolerance, which is well known for benzodiazepines.     

Modeling of Subcutaneous Absorption Kinetics of Infusion Solutions in the Elderly Using Technetium

Absorption kinetics of solutes given with the subcutaneous administration of fluids is ill-defined. The gamma emitter, technitium pertechnetate, enabled estimates of absorption rate to be estimated independently using two approaches. In the first approach, the counts remaining at the site were estimated by imaging above the subcutaneous administration site, whereas in the second approach, the plasma technetium concentration-time profiles were monitored up to 8 hr after technetium administration. Boluses of technetium pertechnetate were given both intravenously and subcutaneously on separate occasions with a multiple dosing regimen using three doses on each occasion. The disposition of technetium after iv administration was best described by biexponential kinetics with a V_ss of 0.30±0.11 L/kg and a clearance of 30.0±13.1 ml/min. The subcutaneous absorption kinetics was best described as a single exponential process with a half-life of 18.l6±3.97min by image analysis and a half-life of 11.58±2.48 min using plasma technetium time data. The bioavailability of technetium by the subcutaneous route was estimated to be 0.96±0.12. The absorption half-life showed no consistent change with the duration of the subcutaneous infusion. The amount remaining at the absorption site with time was similar when analyzed using image analysis, and plasma concentrations assuming multiexponential disposition kinetics and a first-order absorption process. Profiles of fraction remaining at the absorption site generated by deconvolution analysis, image analysis, and assumption of a constant first-order absorption process were similar. Slowing of absorption from the subcutaneous administration site is apparent after the last bolus dose in three of the subjects and can be associated with the stopping of the infusion. In a fourth subject, the retention of technetium at the subcutaneous site is more consistent with accumulation of technetium near the absorption site as a result of systemic recirculation.     

Absorption and Disposition of a New Antiarrhythmic Agent Bidisomide in Man

Absorption and disposition of bidisomide were studied in 12 healthy male subjects after a 20-min iv (1 mg/kg; N = 6) infusion and oral (2 mg/kg; N = 6) administration of the ¹⁴C-labeled drug. The oral absorption profile of unlabeled bidisomide was also studied after administration of a solution by a nasoenteric tube to different sites of the gastrointestinal tract (stomach, duodenum, jejunum, and ileum). The systemic availability was 61%. Absorption was slow initially and then rapid, achieving peak plasma concentrations between 2 and 4 hr. Less than complete systemic availability was attributed to incomplete absorption rather than first-pass metabolism. When the drug solution was delivered directly to the stomach, two distinct peak plasma levels were found. This was attributed to the more rapid absorption of bidisomide in the duodenum and ileum (and/or possibly colon). Following an iv dose, plasma levels of the drug declined with mean half-lives of 0.11, 2.0, and 12 hr for , , and phases, respectively, and a plasma clearance of 380 mL/min. The percentages of the dose recovered as bidisomide in urine and feces were 19 ± 1 and 29 ± 4 for the iv dose and 9.1 ± 0.9 and 48 ± 5 for the oral dose. Bidisomide did not exhibit substantial enantioselective pharmacokinetics in plasma regardless of the route of administration. The mean urinary excretion of the (–) enantiomer was, however, slightly higher than that of the (+) enantiomer, with (–)/(+) enantiomeric ratios of 1.2 and 1.3 after iv and oral administration, respectively. The enantiomeric ratio of bidisomide recovered in the feces was approximately 1.     

Effect of methylmercury administration on choroid plexus function in rats

Methylmercury (MeHg) is a well-known environmental neurotoxin. The choroid plexus (CP), the main component of the blood–cerebrospinal fluid (CSF) barrier (BCSFB), protects the brain from xenobiotics, similar to the blood–brain barrier. Because CP is considered a critical target site of MeHg-induced neurotoxic damage, functional alterations in CP may be caused in relation to the extent of MeHg-induced brain injury. To test this hypothesis, we examined time-dependent pathological alterations in rats administered subtoxic (asymptomatic group) or toxic (symptomatic group) MeHg doses for 3 weeks after the cessation of MeHg administration. We primarily assessed (1) mercury concentrations in the brain, CSF, and plasma; (2) histopathological changes in the brain; (3) albumin CSF/plasma concentration quotient (Q_alb), an index of BCSFB dysfunction; and (4) concentration of CSF transthyretin (TTR), which is primarily produced in CP. Mercury concentrations in the brain, CSF, and plasma decreased, and Q_alb and CSF TTR concentrations did not change significantly in the asymptomatic group. In the symptomatic group, brain and CSF mercury concentrations did not decrease for 2 weeks after the cessation of MeHg administration, but no pathological alteration occurred in the brain during this period. Pathological changes in the cerebellum became evident 3 weeks after the cessation of MeHg administration. Furthermore, Q_alb continued to increase after the cessation of MeHg administration, whereas no decrease in CSF TTR concentration was observed, indicating selective impairment of CP function. These findings suggest that MeHg at toxic doses causes selective functional alteration of CP before leading to pathological alterations in the brain.     

Study on Brain Interstitial Fluid Distribution and Blood-Brain Barrier Transport of Baclofen in Rats by Microdialysis

Purpose. This study was performed to examine the distribution in the brain interstitial fluid (ISF) and the blood-brain barrier (BBB) transport of baclofen in rats by a microdialysis technique. Methods. Following an i.v. bolus administration and/or the constant i.v. infusion of baclofen to the microdialysis cannula-bearing anesthetized rats, the concentrations of baclofen in the hippocampal ISF, whole brain tissue, cerebrospinal fluid (CSF), and plasma were determined by high-performance liquid chromatography (HPLC). Data were kinetically analyzed to estimate the transport parameters, i.e., the influx clearance (CL_in) from plasma to brain and the efflux rate constant (k_eff) from brain to plasma, and the steady-state volume of distribution in the brain (V_d). Results. The concentrations of baclofen in ISF, whole brain tissue, and CSF at the pseudo-steady state were almost 30-fold lower than the plasma unbound concentration, suggesting the restricted distribution of baclofen in the brain. The estimated values of CL_in and k_eff were 0.00157 ± 0.00076 ml/min/g of brain and 0.0872 ± 0.0252 min^–1, respectively. The efflux clearance (CL_out) calculated by multiplying k_eff by V_d (0.816 ± 0.559 ml/g of brain) was 0.0712 ± 0.0529 ml/min/g of brain, and it was significantly 40-fold greater than the CL_in value and fully greater than the convective flow in ISF. Furthermore, no significant concentration gradient was observed between ISF and CSF. These results suggest that the CL_out value mainly reflects the efflux clearance through the BBB. Additionally, the hippocampal ISF/plasma concentration ratio of baclofen was markedly increased by both systemic administration of probenecid and its direct instillation into ISF. Conclusions. The restricted distribution of baclofen in the brain ISF may be ascribed to the efficient efflux from the brain through the BBB which is regulated possibly by a probenecid-sensitive organic anion transport system.     

Acetaminophen in cerebrospinal fluid in children

Background There are few studies describing acetaminophen (APAP) cerebrospinal fluid (CSF) concentrations in children. This current study was undertaken in children—from neonates to adolescents—in order to investigate age-related changes in the plasma to CSF equilibration half-time (Teq) of APAP.Methods Children (n=41) 1 week to 18 years of age undergoing (semi) elective surgery for placement or revision of a ventriculo-peritoneal shunt or insertion of a temporary external ventricular drain received a loading dose of 30–40 mg/kg APAP 1 h before scheduled surgery. Blood and CSF samples for APAP concentration analysis were collected during surgery. In those children with a temporary external drain, blood and CSF sampling were extended into the postoperative period. APAP and CSF pharmacokinetics were estimated using non-linear mixed-effects models. Size was standardized to a 70-kg person using allometric "1/4 power models".Results Median (25–75th percentile) age and weight of the patients included in this study were 12 months (3–62 months) and 10.0 kg (5.8–20.0 kg). Median (25–75th percentile) time between APAP loading dose administration and collection of blood samples and median time (25–75th percentile) between APAP loading dose and collection of CSF were, respectively, 125 min (95–210 min) and 133 min (33–202 min). The population mean Teq, standardized to a 70-kg person, was 1.93 h (CV 43%), an estimate similar to that described in adults (2.1 h). There was no relationship between age and Teq other than that predicted by size. APAP plasma concentrations ranged from 0.0 mg/l to 33.0 mg/l, APAP CSF concentrations ranged from 0.0 mg/l to 21.0 mg/l.Conclusion Size rather than blood–brain-barrier maturation determines Teq changes with age in children. We predict a neonate (3.5 kg), 1-year-old child (10 kg), 5-year-old child (20 kg), 10-year-old child (30 kg) and adult (70 kg) to have Teq values of 0.9, 1, 1.4, 1.6, and 1.93 h, respectively.     

Estimation of amobarbital plasma-effect site equilibration kinetics. Relevance of polyexponential conductance functions

The time delay between drug plasma concentrations and effect has been modeled most commonly by the effect compartment approach, assuming first-order monoexponential equilibrium kinetics between plasma and effect site. So far this assumption has not been rigorously probed. The purpose of the present investigation was to model the delay between amobarbital plasma concentrations and EEG effect using a new approach based on system analysis principles. This approach models the equilibrium between plasma and effect site without assuming a specific kinetic structure. Assuming linear distribution kinetics between plasma and effect site, the relationship between the two variables may be described by a convolution type of linear operation, involving a conductance function (t),which is approximated by a sum of exponentials. Six male Wistarderived rats received an iv infusion of amobarbital at a rate of 10mg/kg per min until isoelectric periods of 5sec or longer appeared on the EEG. Frequent arterial blood samples were obtained and EEG was continuously quantified using aperiodic analysis. The amplitudes in the 2.5–30Hz frequency band were used as EEG effect measure. The delay between plasma concentrations and EEG effect was best modeled by a biexponential conductance function. The use of a biexponential conductance function resulted in a significant further reduction (41 ± 10%)in hysteresis when compared to a monoexponential function, indicating that the assumption of simple first-order monoexponential equilibration kinetics is inadequate. The use of a biexponential conductance function also resulted in a significantly different shape of the effect site concenration- EEG effect relationship and hence the estimated pharmacodynamic parameters, when compared with a monoexponential function. This relationship showed a biphasic behavior, with EEG effects being maximal at amobarbital concentrations of 29.6± 1.3mg/L. At 80.2±2.0mg/L the EEG effect was reduced 50%below baseline values. A comparison was made with the equilibration between amobarbital plasma and cerebrospinal fluid (CSF) concentrations. Six male Wistarderived rats received an iv infusion of amobarbital, 10mg per min for 15min. Arterial blood and CSF samples were taken simultaneously at regular intervals. The equilibration between plasma and CSF concentrations was best fitted by a monoexponential conductance function. Significant differences in equilibration profiles of CSF and effect site with the plasma site were observed. To reach 50%equilibrium the effect site requires 2.5±0.3min and the CSF 3.5±0.2min, to reach 95%the values were, respectively, 90± 27and 15± 1min. This suggests that CSF is kinetically distinguishable from the effect site.Supported by MEDIGON grant 900-521-106.