Pharmacokinetics of a commercially available product and a compounded formulation of extended-release levetiracetam after oral administration of a single dose in cats.

OBJECTIVE: To compare pharmacokinetics of levetiracetam in serum and CSF of cats after oral administration of extended-release (ER) levetiracetam. ANIMALS: 9 healthy cats. PROCEDURES: Cats received 1 dose of a commercially available ER levetiracetam product (500 mg, PO). Thirteen blood and 10 CSF samples were collected over a 24-hour period for pharmacokinetic analysis. After 1 week, cats received 1 dose of a compounded ER levetiracetam formulation (500 mg, PO), and samples were obtained at the same times for analysis. RESULTS: CSF concentrations of levetiracetam closely paralleled serum concentrations. There were significant differences between the commercially available product and the compounded formulation for mean ± SD serum maximum concentration (Cmax; 126 ± 33 µg/mL and 169 ± 51 µg/mL, respectively), Cmax corrected for dose (0.83 ± 0.10 µg/mL/mg and 1.10 ± 0.28 µg/mL/mg, respectively), and time to Cmax (5.1 ± 1.6 hours and 3.1 ± 1.5 hours, respectively). Half-life for the commercially available product and compounded formulation of ER levetiracetam was 4.3 ± 2.0 hours and 5.0 ± 1.6 hours, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: The commercially available product and compounded formulation of ER levetiracetam both maintained concentrations in healthy cats 12 hours after oral administration that have been found to be therapeutic in humans (ie, 5 µg/mL). Results of this study supported dosing intervals of 12 hours, and potentially 24 hours, for oral administration of ER levetiracetam to cats. Monitoring of serum concentrations of levetiracetam can be used as an accurate representation of levetiracetam concentrations in CSF of cats.

Modification of polydopamine-coated Fe3O4 nanoparticles with multi-walled carbon nanotubes for magnetic-µ-dispersive solid-phase extraction of antiepileptic drugs in biological matrices.

In this study, multi-walled carbon nanotubes were coated on the surface of magnetic nanoparticles modified by polydopamine. The synthesized composite was characterized and applied to magnetic-µ-dispersive solid-phase extraction of oxcarbazepine (OXC), phenytoin (PHT), and carbamazepine (CBZ) from human plasma, urine, and cerebrospinal fluid samples prior to analysis by a high-performance liquid chromatography-photodiode array detector. The extraction parameters were investigated and the optimum condition was obtained when the variables were set to the following: sorbent type, Fe3O4@polyDA-MWCNTs (length < 2 µm); sample pH, 6; amount of sorbent, 15 mg; sorption time, 1.5 min at room temperature; type and volume of the eluent, 2.5 mL methanol; and salt content, none added. Under the optimized conditions, the calibration curves are linear in the concentration range 2-2000 ng/mL, the limits of detection are in the range 0.4-3.1 ng/mL, and the relative standard deviations and relative recoveries of plasma (spiked at 200 ng/mL) and CSF (spiked at 50 ng/mL) are in the ranges 1.4-8.2% and 92.8-96.5%, respectively. The applicability of the method was successfully confirmed by extraction and determination of OXC, PHT, and CBZ in biological matrices. Graphical abstract Magnetic multi-walled carbon nanotube core-shell composites were applied as magnetic-µ-dispersive solid-phase extraction adsorbents for determination of antiepileptic drugs in biological matrices.

Acute or chronic use of lacosamide does not alter its distribution between serum and cerebrospinal fluid.

OBJECTIVE: The site of action for antiepileptic drugs (AEDs) is within the brain; however, cerebrospinal fluid (CSF) concentration is highly variable. Lacosamide (LCM) is approved by the U.S. Food and Drug Administration (FDA) for treatment of partial-onset seizures in adults, and has linear pharmacokinetics in serum. Penetration across the blood-brain barrier (BBB) is unknown. This study aims to provide additional insights into the pharmacokinetics of LCM. METHODS: Thirty adults undergoing craniotomy for treatment of intractable epilepsy or brain tumor were recruited and were either taking LCM long term (group 1, n = 15), or were LCM naive, receiving LCM as prophylaxis for surgery (group 2, n = 15). All patients received one intravenous (IV) dose (15 min infusion) immediately prior to craniotomy. CSF and arterial blood were collected simultaneously following craniotomy. LCM concentrations were measured in serum and CSF. RESULTS: LCM concentration differences between groups 1 and 2 for both CSF and serum were statistically significant (p ≤ 0.0005), but there was no statistically significant difference in CSF/serum ratios (group 1 = 0.726 ± 0.231; group 2 = 0.556 ±0.241; p = 0.0585). LCM concentration in serum correlated positively with CSF concentration in group 1 (Pearson r = 0.8527, p < 0.0001). The time interval between the end of dose delivery and sample collection correlated positively with the CSF/serum ratio for the drug-naive group (Pearson r = 0.6525; p = 0.0084). Treatment with other AEDs did not affect LCM distribution between serum and CSF. SIGNIFICANCE: Although chronic dosing resulted in higher LCM concentrations in serum and CSF compared to drug-naive patients, the CSF/serum ratio was not affected by LCM pretreatment. These data suggest that LCM serum concentration may reliably predict CSF concentration.

Comparison of lacosamide concentrations in cerebrospinal fluid and serum in patients with epilepsy.

OBJECTIVE: This study was carried out to estimate the exposure of the central nervous system (CNS) to the antiepileptic drug (AED) lacosamide, under steady state conditions, in patients with epilepsy who take oral lacosamide alongside up to three other AEDs. METHODS: Twenty-seven serum and cerebral spinal fluid (CSF) samples were collected from 21 patients receiving lacosamide for the treatment of epilepsy (50-600 mg/day over two or three doses). This included 23 time-matched pairs of serum and CSF samples from 19 patients. The concentration of lacosamide in each sample was determined using high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). Linear regression was used to characterize the relationship between the CSF-to-serum ratio of lacosamide concentration and the time since dosing, the daily lacosamide dose, or the daily dose normalized by volume of distribution (Vd , approximated to total body water), and between the drug concentrations in each compartment (CSF vs. serum). RESULTS: Concentrations of lacosamide in CSF (mean ± standard deviation [SD] 7.37 ± 3.73 µg/ml, range 1.24-14.95, n = 27) and serum (mean ± SD 8.16 ± 3.82 µg/ml, range 2.29-15.45, n = 27) samples showed a good correlation over the dose range investigated. The mean CSF-to-serum ratio of lacosamide concentrations was 0.897 ± 0.193 (range 0.492-1.254, n = 23 time-matched pairs) and was independent of lacosamide dose. SIGNIFICANCE: Drug concentrations in the CSF are often used to indicate those in the brain interstitial fluid. In patients with epilepsy who follow a stable oral AED dosing regimen, lacosamide concentration in CSF is approximately 85% of that found in serum, suggesting that serum may be a valuable indicator of lacosamide concentration in the CNS.

Steady-state plasma and cerebrospinal fluid pharmacokinetics and tolerability of eslicarbazepine acetate and oxcarbazepine in healthy volunteers.

PURPOSE: To evaluate the pharmacokinetics and tolerability of once-daily eslicarbazepine acetate (ESL) and twice-daily oxcarbazepine (OXC) and their metabolites in cerebrospinal fluid (CSF) and plasma following repeated oral administration. METHODS: Single-center, open-label, randomized, parallel-group study in healthy volunteers. Volunteers in ESL group (n = 7) received 600 mg on days 1-3 and 1,200 mg on days 4-9, once daily. Volunteers in the OXC group (n = 7) received 300 mg on days 1-3 and 600 mg on days 4-9, twice daily. Plasma and CSF sampling was performed following the last dose. KEY FINDINGS: Eslicarbazepine was the major drug entity in plasma and CSF, accounting for, respectively, 93.84% and 91.96% of total exposure in the ESL group and 78.06% and 76.42% in the OXC group. The extent of exposure to drug entities R-licarbazepine and oxcarbazepine was approximately four-fold higher with OXC as compared with ESL. There was relatively little fluctuation from peak-to-trough (ratio) in the CSF for both eslicarbazepine (ESL = 1.5; OXC = 1.2) and R-licarbazepine (ESL = 1.2; OXC = 1.2). In contrast, oxcarbazepine showed larger differences between peak and trough (ESL = 3.1; OXC = 6.4). A total of 84 and 24 treatment-emergent adverse events (TEAEs) were reported with OXC and ESL, respectively. SIGNIFICANCE: In comparison to OXC, administration of ESL resulted in more eslicarbazepine, less R-licarbazepine, and less oxcarbazepine in plasma and CSF, which may correlate with the tolerability profile reported with ESL. The smaller peak-to-trough fluctuation of eslicarbazepine in CSF (a measure of sustained delivery to the brain) than in plasma supports once-daily dosing of ESL.

Investigating paradoxical hysteresis effects in the mouse neocortical slice model.

Clinically, anesthetic drugs show hysteresis in the plasma drug concentrations at induction versus emergence from anesthesia induced unconsciousness. This is assumed to be the result of pharmacokinetic lag between the plasma and brain effect-site and vice versa. However, recent mathematical and experimental studies demonstrate that anesthetic hysteresis might be due in part to lag in the brain physiology, independent of drug transport delay - so-called "neural inertia". The aim of this study was to investigate neural inertia in the reduced neocortical mouse slice model. Seizure-like event (SLE) activity was generated by exposing cortical slices to no-magnesium artificial cerebrospinal fluid (aCSF). Concentration-effect loops were generated by manipulating SLE frequency, using the general anesthetic drug etomidate and by altering the aCSF magnesium concentration. The etomidate (24 µM) concentration-effect relationship showed a clear hysteresis, consistent with the slow diffusion of etomidate into slice tissue. Manipulation of tissue excitability, using either carbachol (50 µM) or elevated potassium (5mM vs 2.5mM) did not significantly alter the size of etomidate hysteresis loops. Hysteresis in the magnesium concentration-effect relationship was evident, but only when the starting condition was magnesium-containing "normal" aCSF. The in vitro cortical slice manifests pathway-dependent "neural inertia" and may be a valuable model for future investigations into the mechanisms of neural inertia in the cerebral cortex.

Fluconazole-induced intoxication with phenytoin in a patient with ultra-high activity of CYP2C9.

PURPOSE: The cytochrome P450 enzyme CYP2C9 metabolizes several important drugs, such as warfarin and oral antidiabetic drugs. The enzyme is polymorphic, and all known alleles, for example, CYP2C9*2 and*3, give decreased activity. Ultra-high activity of the enzyme has not yet been reported. METHODS: We present a patient with Behçet's disease who required treatment with high doses of phenytoin. When fluconazole, a potent inhibitor of CYP2C9, was added to the treatment regimen, the patient developed ataxia, tremor, fatigue, slurred speech and somnolence, indicating phenytoin intoxication. On suspicion of ultra-high activity of CYP2C9, a phenotyping test for CYP2C9 with losartan was performed. RESULTS: The patient was shown to have a higher activity of CYP2C9 than any of the 190 healthy Swedish Caucasians used as controls. CONCLUSIONS: Our finding of an ultrarapid metabolism of losartan and phenytoin may apply to other CYP2C9 substrates, where inhibition of CYP2C9 may cause severe adverse drug reactions.

Antiepileptic drugs abolish ictal but not interictal epileptiform discharges in vitro.

PURPOSE: We established the effects of the antiepileptic drugs (AEDs) carbamazepine (CBZ), topiramate (TPM), and valproic acid (VPA) on the epileptiform activity induced by 4-aminopyridine (4AP) in the rat entorhinal cortex (EC) in an in vitro brain slice preparation. METHODS: Brain slices were obtained from Sprague-Dawley rats (200-250 g). Field and intracellular recordings were made from the EC during bath application of 4AP (50 microm). AEDs, and in some experiments, picrotoxin were bath applied concomitantly. RESULTS: Prolonged (>3 s), ictal-like epileptiform events were abolished by CBZ (50 microm), TPM (50 microm), and VPA (1 mm), whereas shorter (<3 s) interictal-like discharges continued to occur, even at concentrations up to 4-fold as high. gamma-Aminobutyric acid (GABA)(A)-receptor antagonism changed the 4AP-induced activity into recurrent interictal-like events that were not affected by CBZ or TPM, even at the highest concentrations. To establish whether these findings reflected the temporal features of the epileptiform discharges, we tested CBZ and TPM on 4AP-induced epileptiform activity driven by stimuli delivered at 100-, 10-, and 5-s intervals; these AEDs reduced ictal-like responses to stimuli at 100-s intervals at nearly therapeutic concentrations, but did not influence shorter interictal-like events elicited by stimuli delivered every 10 or 5 s. CONCLUSIONS: We conclude that the AED ability to control epileptiform synchronization in vitro depends mainly on activity-dependent characteristics such as discharge duration. Our data are in keeping with clinical evidence indicating that interictal activity is unaffected by AED levels that are effective to stop seizures.

Status epilepticus in encephalitis: a study of clinical findings, magnetic resonance imaging, and response to antiepileptic drugs.

This study evaluates clinical findings, magnetic resonance imaging (MRI), and response to antiepileptic drugs (AEDs) in encephalitis patients with status epilepticus (SE). Encephalitis patients with SE were included and they were grouped into herpes (HSE), Japanese (JE), dengue, and nonspecific encephalitis on the basis of virological studies. The demographic and clinical details, including SE type and duration, were noted. Cranial MRI and cerebrospinal fluid (CSF) were carried out. Response to first, second, and third AEDs were noted and the patients not responding to the second AED were considered refractory SE. The relationships of the mortality and the refractoriness of SE with various clinical findings, MRI, CSF, and the type of encephalitis were evaluated. Thirty SE patients with encephalitis aged 1 to 64 years were included. Nine patients had JE, 4 HSE, 1 dengue, and 16 nonspecific encephalitis. Generalized convulsive SE was present in 26 and nonconvulsive SE in 4 patients. The mean duration of SE was 21 (0.83 to 72) h. MRI was abnormal in 20 patients. A 46.7% of patients responded to the first AED and 36.7% remained refractory to the second AED. In 26.7% patients, the seizure continued even after the third AED. The response to AED was not related to the clinical, MRI, and laboratory variables. Nine patients died and the mortality was related to gender and Glasgow Coma Scale (GCS) score. In encephalitis with SE, 46.7% patients responded to the fist AED and 36.7% remained refractory to the second AED. One third of patients of died, which was related to the depth of coma.

Comparison of drug concentrations in postmortem cerebrospinal fluid and blood specimens.

The concentration of drugs and metabolites in cerebrospinal fluid (CSF) and blood were determined in 282 autopsied cases using liquid-liquid extraction techniques and gas chromatographic analyses. All drugs were confirmed in one matrix by gas chromatography-mass spectrometry. CSF/blood ratios were used to compare the two biological fluids. Classes of drugs evaluated in this study included: benzodiazepines, anticonvulsants, sedatives, opioids, antidepressants, anesthetics, and antihistamines. The majority of the drugs tested were readily detected in CSF specimens. The average CSF/blood ratio for most drugs was in the range of 0.05-0.50. Interpretation of these results is difficult because protein binding, half-life, hydrophobic properties, and pKa of a drug, in addition to survival time after drug use, influence the CSF/blood ratio. While CSF specimens do provide a viable alternative testing matrix when blood specimens are not available, they should not be used to estimate blood drug concentrations.

The pharmacokinetics of vigabatrin in rat blood and cerebrospinal fluid.

PURPOSE: Data on the blood pharmacokinetics of vigabatrin, an antiepileptic drug with a unique and novel mechanism of action, in the rat are sparse. Additionally, little is known of the kinetics of vigabatrin in the central cerebrospinal fluid (CSF) compartment. We therefore investigated the rate of penetration into and the inter-relationship between serum and CSF compartments following systemic administration of vigabatrin in the rat. METHODS: Sprague-Dawley rats were implanted with a jugular vein catheter and a cisterna magna catheter for blood and CSF sampling, respectively. Vigabatrin was administered by intraperitonial injection at three different doses (250, 500 and 1000mg/kg) and blood and CSF collected at timed intervals up to 8h. Vigabatrin concentrations in sera and CSF were determined by high performance liquid chromatography. RESULTS: Vigabatrin concentrations in blood and CSF rose linearly and dose-dependently and the time to maximum concentration (Tmax) was 0.4 and 1.0h, respectively. Vigabatrin is not protein bound in serum and its elimination from serum (mean t1/2 values, 1.1-1.4 h) is rapid and dose-independent. The efflux of vigabatrin from CSF was significantly slower than that seen for serum (mean t1/2 values, 2.2-3.3h). CONCLUSIONS: The kinetics of vigabatrin are linear with rapid entry into CSF. However, although vigabatrin CSF kinetics parallel that seen in serum, CSF vigabatrin concentrations represent only 2% of concentrations seen in serum and do not reflect free drug concentrations in serum.

Development and validation of a high performance liquid chromatographic method for the determination of oxcarbazepine and its main metabolites in human plasma and cerebrospinal fluid and its application to pharmacokinetic study.

An isocratic reversed-phase HPLC-UV procedure for the determination of oxcarbazepine and its main metabolites 10-hydroxy-10,11-dihydrocarbamazepine and 10,11-dihydroxy-trans-10,11-dihydrocarbamazepine in human plasma and cerebrospinal fluid has been developed and validated. After addition of bromazepam as internal standard, the analytes were isolated from plasma and cerebrospinal fluid by liquid-liquid extraction. Separation was achieved on a X-TERRA C18 column using a mobile phase composed of 20 mM KH(2)PO(4), acetonitrile, and n-octylamine (76:24:0.05, v/v/v) at 40 degrees C and detected at 237 nm. The described assay was validated in terms of linearity, accuracy, precision, recovery and lower limit of quantification according to the FDA validation guidelines. Calibration curves were linear with a coefficient of variation (r) greater than 0.998. Accuracy ranged from 92.3% to 106.0% and precision was between 2.3% and 8.2%. The method has been applied to plasma and cerebrospinal fluid samples obtained from patients treated with oxcarbazepine, both in monotherapy and adjunctive therapy.

Comparison of brain extracellular fluid, brain tissue, cerebrospinal fluid, and serum concentrations of antiepileptic drugs measured intraoperatively in patients with intractable epilepsy.

PURPOSE: The mechanisms of drug resistance in epilepsy are only incompletely understood. According to a current concept, overexpression of drug efflux transporters at the blood-brain barrier may reduce levels of antiepileptic drugs (AEDs) in epileptogenic brain tissue. Increased expression of drug efflux transporters such as P-glycoprotein has been found in brain tissue surgically resected from patients with medically intractable epilepsy, but it is not known whether this leads to decreased extracellular (interstitial) AED concentrations in affected brain regions. This prompted us to measure concentrations of AEDs in the extracellular space of human neocortical tissue by using intraoperative microdialysis (IOMD) in those parts of the brain that had to be removed for therapeutic reasons. For comparison, AED levels were determined in brain tissue, subarachnoid CSF, and serum. METHODS: Concentrations of carbamazepine (CBZ), 10-hydroxy-carbazepine (10-OH-CZ, metabolite of oxcarbazepine), lamotrigine (LTG), levetiracetam (LEV), topiramate, or phenytoin were determined by using one to four catheters during IOMD in the medial temporal gyrus. Furthermore, to calculate the individual recovery of every catheter, an in vitro microdialysis was performed with ultrafiltrate of serum concurrently obtained from the respective patient. In addition, AED levels were determined in the resected brain tissue, CSF, and serum of the same patients. Altogether 22 pharmacoresistant epilepsy patients (nine male, 13 female patients; age 15-54 years) with complex partial seizures or secondarily generalized seizures were involved. In a first series, IOMD samples 40 min after beginning of the microdialysis (flow rate, 1 microl/min), and in a second series, continuous measurements 25, 30, 35, and 40 min from the beginning were evaluated (flow rate, 2 microl/min). With in vitro recovery data of the individual catheters, the concentration in the extracellular space (ECS) was estimated. RESULTS: AED concentrations in the ECS of the cortex measured by catheters located at a distance of 0.6 cm differed markedly in some patients, whereas concentrations in the ultrafiltrate of the serum of the respective patients measured with the same catheters varied only slightly. Furthermore, ECS concentrations related to the ultrafiltrate of serum showed considerable interindividual variations. The high intra- and interindividual variation of ECS concentrations is demonstrated by the low correlation between concentrations in ECS and the ultrafiltrate of serum (CBZ, r= 0.41; 10-OH-CZ, r= 0.42; LTG, r= 0.27) in contrast to the high correlation between brain tissue concentration and the ultrafiltrate of serum (CBZ, r= 0.97; 10-OH-CZ, r= 0.88; LTG, r= 0.98) in the same group of patients. When comparing AED concentrations in the ECS with those in the CSF, ECS concentrations were significantly lower for CBZ, 10-OH-CZ, LTG, and LEV. CONCLUSIONS: The data demonstrate that AED concentrations show a considerable intraindividual and interindividual variation in the ECS of cortical regions. Furthermore, the ECS concentration of several AEDs is significantly lower than their CSF concentration in patients with intractable epilepsy. However, in the absence of data from nonepileptic tissues, it is not possible to judge whether the present findings relate to overexpression of multidrug transporters in the brain. Instead, the present study illustrates the methodologic difficulties involved in performing IOMD studies in patients and may thus be helpful for future approaches aimed at elucidating the role of multidrug transporters in epilepsy.

In vitro responsiveness of human-drug-resistant tissue to antiepileptic drugs: insights into the mechanisms of pharmacoresistance.

Pharmacoresistance in epileptic patients may be ascribed to at least two, not mutually exclusive, mechanisms: a pharmacokinetic mechanism and a decreased sensitivity or availability of targets to antiepileptic drugs (AEDs; i.e., carbamazepine and phenytoin (CBZ, PHT)). Brain:plasma drug concentration ratios were determined intraoperatively during lobectomies performed to alleviate drug-resistant seizures. The brain:plasma ratio of CBZ was 1.48 when therapeutic serum levels (15-34 microM) were achieved. When concentrations of CBZ found in multiple-drug-resistant brain were directly applied to human cortical slices from drug-resistant patients made hyperexcitable and hypersynchronous by Mg(2+)-free media, bursting frequency was not significantly affected and overall excitability was reduced by 40%. Similar results were obtained for PHT. At higher AED concentrations (60-200 microM), a dose-dependent decrease of bursting frequency and amplitude was observed. Slices from drug-resistant epileptic patients made hypersynchronous/hyperexcitable by elevated potassium or inhibition of GABA-A receptors behaved similarly. Of note is the response of slices from human multiple-drug-resistant brain, which was greater than in rodent cortex from naive animals. Taken together, our results support the hypothesis that multiple drug resistance to AEDs involves cerebrovascular changes that impede the achievement of appropriate drug levels in the central nervous system.

The pharmacokinetic inter-relationship of tiagabine in blood, cerebrospinal fluid and brain extracellular fluid (frontal cortex and hippocampus).

PURPOSE: Tiagabine is a unique antiepileptic drug with a novel mechanism of action. Whilst some limited data are available as to the peripheral blood pharmacokinetics of tiagabine, data regarding the kinetics of tiagabine in the central brain compartment are very limited. We therefore sought to investigate serum, cerebrospinal fluid (CSF) and frontal cortex and hippocampal extracellular fluid (ECF) kinetic inter-relationship of tiagabine in a freely moving rat model. METHODS: Adult male rats were implanted with either a jugular vein catheter and a cisterna magna catheter for blood and CSF sampling, respectively, or a blood catheter and a microdialysis probe in the hippocampus and frontal cortex (for ECF sampling). Tiagabine was administered intraperitoneal (i.p.) at 20 or 40 mg/kg and blood, CSF and ECF were collected at timed intervals for the measurement of tiagabine concentrations by high performance liquid chromatography. RESULTS: Tiagabine concentrations in blood and CSF rose linearly and dose-dependently and time to maximum concentration (Tmax) was 15 and 29 min, respectively. Mean CSF/serum tiagabine concentration ratios (range, 0.008-0.01) were much smaller than the mean free/total tiagabine concentration ratios in serum (0.045 +/- 0.003). Entry of tiagabine into brain ECF (frontal cortex and hippocampus) was rapid with Tmax values of 31-46 min. Distribution of tiagabine in brain was not brain region specific with values in the frontal cortex and hippocampus being indistinguishable. Whilst elimination from CSF was comparable to that of serum, half-life (t(1/2)) values in ECF were three times longer. CONCLUSIONS: Tiagabine is associated with linear kinetic characteristics and with rapid brain penetration. However, CSF concentrations are not reflective of free non-protein-bound concentrations in serum. The observation that tiagabine elimination from the brain is threefold slower than that seen in blood, may explain as to the relatively long duration of action of tiagabine.

The plasma pharmacokinetics and cerebral spinal fluid penetration of intravenous topiramate in newborn pigs.

INTRODUCTION: Pre-clinical studies suggest that the anticonvulsant topiramate confers neurologic protection against ischemia. An intravenous formulation of topiramate has been developed for administration during conditions such as hypoxia-ischemia when enteral absorption may be unpredictable. The plasma pharmacokinetics, cerebrospinal fluid (CSF) penetration and pharmacodynamics of intravenous topiramate were studied in an established piglet model of hypoxia-ischemia. METHODS: The plasma pharmacokinetics of topiramate were studied in a group of chronically instrumented conscious piglets (n = 8), and in a group of piglets following an episode of hypoxia-ischemia (n = 8). These groups were divided into equal dose cohorts in which two doses of intravenously administered topiramate, 5 and 40 mg/kg, were studied. The animals' heart rate, arterial pressure and EEG were monitored. Plasma for topiramate concentration was sampled for up to 26 h. A single CSF topiramate concentration was determined 1 h following drug administration. Topiramate was quantified using a specific LC/MS assay. RESULTS: The animals tolerated intravenous topiramate well, with no significant changes in physiologic and neurologic parameters. Plasma topiramate concentrations following an intravenous dose were best described by a bi-exponential equation, with a mean clearance of 39+/-18 ml/h/kg, and a terminal half-life of 14.3 (range 7.5-48.1) h. A dose of 5 mg/kg was sufficient to maintain plasma drug concentrations greater than 10 micro M for 24 h. CSF topiramate concentration at 1 h was 12+/-1 micro M and 109+/-26 micro M at the 5 and 40 mg/kg doses, respectively. CONCLUSION: Topiramate administered intravenously was well tolerated. Slow clearance of the drug allowed for maintenance of potential neuroprotective concentrations following a single dose of drug for 24 h. High drug penetration into the CSF is an ideal pharmacologic characteristic of any potential neuroprotective agent. The pharmacokinetic profile of intravenously administered topiramate, including its penetration into the CSF, appears to achieve this goal.

A comparison of central brain (cerebrospinal and extracellular fluids) and peripheral blood kinetics of phenytoin after intravenous phenytoin and fosphenytoin.

Phenytoin (PHT) is a first-line drug in the treatment of status epilepticus. However, the parenteral PHT formulation is associated with administration difficulties and therefore fosphenytoin (FosPHT), a PHT pro-drug, has been developed. As the peripheral (blood) and central (cerebrospinal fluid [CSF] and brain extracellular fluid [ECF]) kinetic inter-relationship of PHT after i.v. FosPHT administration is unknown we sought to ascertain the relationship and to compare it to that of i.v. PHT. A freely behaving rat model, which allows for the concurrent and temporal sampling of blood (jugular vein), CSF (cisterna magna) and brain ECF (frontal cortex and hippocampus), was used. PHT and FosPHT were administered by i.v. infusion and blood, CSF and microdialysate samples collected at timed intervals up to 6 hours. The pharmacokinetic parameters in plasma of PHT after PHT and FosPHT (30 and 60 mg/kg) administration were indistinguishable. The PHT plasma free fraction (free/total concentration ratio) was 0.25-0.31 and 0.26-0.31 for PHT and FosPHT, respectively. Mean PHT Tmax values for CSF were 9-13 minutes. The equivalent values in the frontal cortex and hippocampal ECF were 29-34 minutes. Cmax values increased dose-dependently and were independent of whether PHT or FosPHT was administered. Furthermore the kinetic profiles of PHT for the frontal cortex and hippocampus were indistinguishable suggesting that PHT distribution in the brain is not brain region specific. Thus, overall, the central and peripheral kinetics of PHT are indistinguishable after PHT and FosPHT.

Role of multidrug transporters in pharmacoresistance to antiepileptic drugs.

Epilepsy, one of the most common neurologic disorders, is a major public health issue. Despite more than 20 approved antiepileptic drugs (AEDs), about 30% of patients are refractory to treatment. An important characteristic of pharmacoresistant epilepsy is that most patients with refractory epilepsy are resistant to several, if not all, AEDs, even though these drugs act by different mechanisms. This argues against epilepsy-induced alterations in specific drug targets as a major cause of pharmacoresistant epilepsy, but rather points to nonspecific and possibly adaptive mechanisms, such as decreased drug uptake into the brain by intrinsic or acquired over-expression of multidrug transporters in the blood-brain barrier (BBB). There is accumulating evidence demonstrating that multidrug transporters such as P-glycoprotein (PGP) and members of the multidrug resistance-associated protein (MRP) family are over-expressed in capillary endothelial cells and astrocytes in epileptogenic brain tissue surgically resected from patients with medically intractable epilepsy. PGP and MRPs in the BBB are thought to act as an active defense mechanism, restricting the penetration of lipophilic substances into the brain. A large variety of compounds, including many lipophilic drugs, are substrates for either PGP or MRPs or both. It is thus not astonishing that several AEDs, which have been made lipophilic to penetrate into the brain, seem to be substrates for multidrug transporters in the BBB. Over-expression of such transporters in epileptogenic tissue is thus likely to reduce the amount of drug that reaches the epileptic neurons, which would be a likely explanation for pharmacoresistance. PGP and MRPs can be blocked by specific inhibitors, which raises the option to use such inhibitors as adjunctive treatment for medically refractory epilepsy. However, although over-expression of multidrug transporters is a novel and reasonable hypothesis to explain multidrug resistance in epilepsy, further studies are needed to establish this concept. Furthermore, there are certainly other mechanisms of pharmacoresistance that need to be identified.

Microdialysis sampling of carbamazepine, phenytoin and phenobarbital in subcutaneous extracellular fluid and subdural cerebrospinal fluid in humans: an in vitro and in vivo study of adsorption to the sampling device.

The purpose of the study was to determine if binding of the drugs to the sampling equipment during microdialysis would influence the results for carbamazepine, phenytoin and phenobarbital. In vitro experiments with microdialysis catheters and separate parts of catheters were performed to estimate the degree of drug binding to the dialysis equipment. A mathematical model to calculate drug binding and recovery is proposed. In vivo protein unbound carbamazepine concentrations in subcutaneous extracellular fluid at different flow rates (6 patients), unbound carbamazepine (1 patient) and unbound phenobarbital (I patient) in subdural cerebrospinal fluid and subcutaneous extracellular fluid were estimated and the in vivo data were compared to the in vitro results and data generated by the mathematical model. Binding to the soft outlet polyurethane tubing was extensive and variable for phenytoin, which precluded in vivo testing, but limited and more predictable for carbamazepine and phenobarbital. None of the three compounds bound to the hard internaltubing. Phenytoin and phenobarbital did not bind to the dialysis membrane, while a small degree of binding may be present for carbamazepine. In vivo estimates of carbamazepine protein unbound subcutaneous extracellular concentrations by microdialysis, adjusted for binding to the plastic tubing, were 81% of protein unbound plasma concentrations. In single case studies, subdural cerebrospinal fluid and subcutaneous extracellular levels of carbamazepine and phenobarbital were similar and when corrected for binding to the plastic tubings they were also close to protein unbound plasma concentrations. Microdialysis can be used for reliable estimations of protein unbound carbamazepine and possibly phenobarbital concentrations when drug binding to the plastic tubing is considered. Reliable estimation of unbound phenytoin is not possible at present.

Plasma concentration of topiramate correlates with cerebrospinal fluid concentration.

The authors examined the ratio between the plasma and the cerebrospinal fluid (CSF) concentration of topiramate in 14 adults with epilepsy. Simultaneous trough samples of venous blood and CSF were collected and analyzed as total and unbound concentrations. Concomitant levels were also analyzed of lamotrigine (n = 5) and the relevant oxcarbazepine metabolite, 10-hydroxycarbazepine (n = 3). There was a close correlation between the plasma and the CSF concentration for both the total and unbound concentration of topiramate. The median CSF/plasma ratio of total topiramate was 0.85. The free topiramate concentration in plasma was not different from the free topiramate concentration in CSF. The CSF/plasma ratios showed little variation and were independent of the plasma level for both the total and the unbound levels. The unbound fraction of topiramate was 84% in plasma and 97% in CSF. The CSF concentrations of lamotrigine and 10-hydroxycarbazepine were 50% and 61% of the plasma concentrations, respectively. For topiramate, there is a close correlation between the plasma concentration and the CSF concentration. There does not seem to be a saturable carrier mechanism restricting topiramate transport across the blood-brain barrier. The concentration of topiramate in CSF is equal to the unbound proportion of topiramate in plasma, implying that the delivery of topiramate to the brain occurs via transfer from the unbound plasma pool. Plasma is thus a relevant matrix for therapeutic drug monitoring of topiramate.