Prediction of human CNS pharmacokinetics using a physiologically-based pharmacokinetic modeling approach.

Knowledge of drug concentration-time profiles at the central nervous system (CNS) target-site is critically important for rational development of CNS targeted drugs. Our aim was to translate a recently published comprehensive CNS physiologically-based pharmacokinetic (PBPK) model from rat to human, and to predict drug concentration-time profiles in multiple CNS compartments on available human data of four drugs (acetaminophen, oxycodone, morphine and phenytoin). Values of the system-specific parameters in the rat CNS PBPK model were replaced by corresponding human values. The contribution of active transporters for the four selected drugs was scaled based on differences in expression of the pertinent transporters in both species. Model predictions were evaluated with available pharmacokinetic (PK) data in human brain extracellular fluid and/or cerebrospinal fluid, obtained under physiologically healthy CNS conditions (acetaminophen, oxycodone, and morphine) and under pathophysiological CNS conditions where CNS physiology could be affected (acetaminophen, morphine and phenytoin). The human CNS PBPK model could successfully predict their concentration-time profiles in multiple human CNS compartments in physiological CNS conditions within a 1.6-fold error. Furthermore, the model allowed investigation of the potential underlying mechanisms that can explain differences in CNS PK associated with pathophysiological changes. This analysis supports the relevance of the developed model to allow more effective selection of CNS drug candidates since it enables the prediction of CNS target-site concentrations in humans, which are essential for drug development and patient treatment.

Development of a permeability-limited model of the human brain and cerebrospinal fluid (CSF) to integrate known physiological and biological knowledge: Estimating time varying CSF drug concentrations and their variability using in vitro data.

A 4-compartment permeability-limited brain (4Brain) model consisting of brain blood, brain mass, cranial and spinal cerebrospinal fluid (CSF) compartments has been developed and incorporated into a whole body physiologically-based pharmacokinetic (PBPK) model within the Simcyp Simulator. The model assumptions, structure, governing equations and system parameters are described. The model in particular considers the anatomy and physiology of the brain and CSF, including CSF secretion, circulation and absorption, as well as the function of various efflux and uptake transporters existing on the blood-brain barrier (BBB) and blood-CSF barrier (BCSFB), together with the known parameter variability. The model performance was verified using in vitro data and clinical observations for paracetamol and phenytoin. The simulated paracetamol spinal CSF concentration is comparable with clinical lumbar CSF data for both intravenous and oral doses. Phenytoin CSF concentration-time profiles in epileptic patients were simulated after accounting for disease-induced over-expression of efflux transporters within the BBB. Various 'what-if' scenarios, involving variation of specific drug and system parameters of the model, demonstrated that the 4Brain model is able to simulate the possible impact of transporter-mediated drug-drug interactions, the lumbar puncture process and the age-dependent change in the CSF turnover rate on the local PK within the brain.

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.

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.

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.

Use of antiepileptic drugs in nontraumatic neurosurgical procedures. Is there any best route and time of administration?

We assessed in 15 consecutive patients the best route and time of administration for phenytoin (PHT) prophylaxis in neurosurgical procedures. We also correlated PHT levels in serum and cerebrospinal fluid after oral and parenteral loading doses. The mean PHT level was 13.9 micrograms/ml in serum and 2.03 micrograms/ml in cerebrospinal fluid (CSF), with a significant correlation between levels in both compartments (r = 0.73, p < 0.01). Mean PHT levels among the different groups were not statistically significant. We conclude that therapeutic levels of PHT in CSF can be achieved independently of the route of administration, as long as accepted loading doses are used.

Antiepileptic drug pharmacokinetics and neuropharmacokinetics in individual rats by repetitive withdrawal of blood and cerebrospinal fluid: phenytoin.

The temporal pharmacokinetic (blood) and neuropharmacokinetic (cerebrospinal fluid, CSF) interrelationship of phenytoin was studied after acute and during chronic (up to 5 days) intraperitoneal administration of phenytoin (30, 50 or 100 mg/kg) using a new freely behaving rat model. After administration, phenytoin rapidly appeared in both serum (Tmax mean range 0.15-0.38 h) and CSF (Tmax mean range 0.9-1.4 h), suggesting ready penetration of the blood-brain barrier. However, transport across the blood-brain barrier may be rate limiting since whilst phenytoin concentrations rose dose dependently in serum, CSF concentrations did not. Further, the divergence between the blood and CSF compartments increased with chronic dosing. Cmax, AUC and t1/2 values for serum increased non-linearly, suggestive of accumulation kinetics. Based on these data, high initial phenytoin blood concentrations are essential if phenytoin entry into the brain is to be facilitated, and this may be important in studies of phenytoin in animal models of status epilepticus.

Drogas anticonvulsivantes en suero y líquido cefalorraquideo de pacientes epilépticos: cuantificación por cromatografía líquida / Anticonvulsant drugs in serum and crebrospinal fluid of epileptical patients: liquid chromatography measurement

Se desarrolló un procedimiento de cromatografía líquida de alta presión para cuantificar simultáneamente varias drogas anticonvulsivantes en el suero y líquido cefalorraquídeo de 20 pacientes. Los coeficientes de correlación para las concentraciones en ambos líquidos decarbamazepina, fenobarbital y fenitoína fueron r=0,8588 (p<0,01), r=0,9721 (p<0,01) y r=0,9289 (p<0,01), respectivamente. Las concentraciones de cada droga en líquido cefalorraquídeo representaron porcentajes de la concentración en suero, comparables a los referidos en la literatura. Las concentraciones séricas de carbamazepina en los pacientes sin barbitúricos asociados fueron mayores que las de aquellos bajo politerapia con barbitúricos, independientemente de la dosis. La fenitoína y las concentraciones de fenobarbital en suero y líquido cefalorraquídeo. Estos resultados aqpoyan el uso de la monoterapia para tratar las epilepsias

Phenytoin prevents epinephrine-induced arrhythmias through central nervous system in halothane-anesthetized dogs.

The authors investigated the effect of phenytoin through the central nervous system on epinephrine-induced arrhythmias in halothane-anesthetized dogs. The arrhythmogenic dose (AD) of epinephrine during halothane anesthesia was determined in the presence of phenytoin (1 mg/kg), vehicle, and saline, which were administered directly into the cisterna magna. Phenytoin increased the AD of epinephrine as compared with vehicle or saline. The cerebrospinal and plasma concentration of phenytoin during the arrhythmias were 23.6 and less than 0.5 micrograms/ml, respectively. There was no significant difference in AD between the vehicle and saline groups. The same dose of phenytoin (1 mg/kg) administered intravenously did not affect the AD of epinephrine, and the plasma concentration of phenytoin during the arrhythmias was 1.2 micrograms/ml. These findings suggested that phenytoin exerts a protective effect against halothane-epinephrine arrhythmias through a central mechanism and that the central nervous system may be involved, at least in part, in the myocardial sensitization by halothane.

Phenytoin disposition in the pregnant rat. Dose-dependent studies and salicylate effects.

Studies were conducted in 19-day gestation Sprague-Dawley rats to investigate the dose dependency and effects of salicylate coadministration on phenytoin disposition during pregnancy. After iv loading doses of both drugs, concurrent iv infusions of 75.6, 151.2, or 302.4 micrograms/min/kg of 14C-phenytoin and 65, 130, or 195 micrograms/min/kg of salicylate were administered for 180 min. Maternal plasma, fetal plasma, and whole fetus samples were obtained during the infusions, and maternal tissue (heart, skeletal muscle, fat, liver, and brain) and cerebrospinal fluid specimens were collected at the termination of drug administration. All samples were assayed for phenytoin and p-hydroxydiphenylhydantoin by liquid scintillation counting following separation by TLC. Systemic and intrinsic phenytoin clearance, which averaged 12 and 41.5 ml/min/kg, respectively, for the three phenytoin infusions, were both dose independent, and were unaltered by the three salicylate treatments. Similarly, the maternal tissue-to-plasma concentration ratios for phenytoin and p-hydroxydiphenylhydantoin were dose and/or concentration independent following the three phenytoin infusions, and were also not affected by salicylate coadministration. Additionally, the fetal distribution ratios for whole fetus-to-maternal plasma and whole fetus-to-fetal plasma were also invariant for the three phenytoin infusions and salicylate treatments. The results showed that the maternal and fetal disposition of phenytoin was dose and concentration independent and unaltered by salicylate coadministration with the dosages studied.

Brain interstitial fluid and intracellular distribution of phenytoin.

After intravenous (i.v.) administration (10 mg/kg), the biodisposition of phenytoin (PHT) in serum (total and free concentration), cerebrospinal fluid (CSF), brain, and the interstitial fluid (IF) of the normal brain were determined in dogs. A sufficient volume of IF was obtained through a multiperforated polypropylene ball implanted into the left parietotemporal region for 4-5 weeks. PHT brain distribution coefficient values ranged between 1.9 and 3.75, while the ratios of IF to free serum PHT concentrations ranged between 0.19 and 1.04; thus, our data indicate that most of the free unbound PHT which enters the brain parenchyma accumulates in the cellular compartment. Furthermore, at 60 and 90 min the peak CSF and IF concentrations are delayed; thus, for PHT, an apparent diffusion front from the CSF into the extracellular space of the brain seems to occur.

Effect of hypercapnia and/or hypoxemia and metabolic acidosis on kinetics and concentrations of phenytoin in the cerebrospinal fluid of conscious rabbits.

The present study was designed to determine the effect of changes in gases and pH in the blood on kinetics and passage to the cerebrospinal fluid (CSF) of phenytoin (DPH). Five groups of 6 rabbits were used, a control [with a mean partial pressure (Pa) of oxygen of 84 +/- 2 (SEM) mmHg, partial pressure of carbon dioxide (PaCO2) of 23 +/- 1 mmHg and pH = 7.512 +/- 0.018], a second group with hypercapnia (PaCO2 = 65 +/- 3 mmHg, pH = 7.244 +/- 0.008), a third group with hypoxemia (PaO2 = 48 +/- 2 mmHg), a fourth group with hypercapnia combined with hypoxemia (PaCO2 = 72 +/- 3 mmHg, PaO2 = 51 +/- 1 mmHg and pH = 7.252 +/- 0.008) and a fifth group with metabolic acidosis (pH = 7.232 +/- 0.011). All animals were conscious during the experiments following the administration of 10 mg/kg (i.v.) of phenytoin, hypoxemia decreased the clearance of phenytoin from 4.20 +/- 0.55 to 2.65 +/- 0.44 ml/min per kg (P less than 0.05) and consequently the area under the plasma concentration/time curve (AUC) for phenytoin increased (2575 +/- 319 to 4316 +/- 740 micrograms min/ml; P less than 0.05). Metabolic acidosis increased the volume of distribution of phenytoin from 780 +/- 70 to 1103 +/- 65 ml/kg (P less than 0.01). The protein binding of phenytoin was not affected by any of the experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein binding and CSF penetration of phenytoin following acute oral dosing in man.

Prophylactic phenytoin (DPH) has been evaluated in 20 patients undergoing diagnostic myelography. DPH (0.75 g) was ingested at 20.00 h the night before and 0.5 g at 08.00 h on the morning of the procedure. Total DPH concentrations at myelography (mean +/- s.d.: 12.7 +/- 4.3 mg l-1; range 6.3-21.5 mg l-1) correlated with CSF values (1.3 +/- 0.46 mg l-1; range 0.7-2.2 mg l-1; r = 0.83, P less than 0.001). DPH protein binding at that time varied two-fold (9.2-18.5%) and free drug levels (1.7 +/- 0.6 mg l-1) correlated with CSF (r = 0.83, P less than 0.001) and total (r = 0.89, P less than 0.001) plasma DPH concentrations. There were significant negative correlations between patient weight (n = 17) and total (r = 0.57, P less than 0.05) and CSF (r = -0.55, P less than 0.05) DPH concentrations at myelography. Total plasma DPH levels 8 h (14.5 +/- 3.9 mg l-1; range 7.3-20.6 mg l-1) and 24 h (12.3 +/- 3.8 mg l-1; range 5.0-19.8 mg l-1) after myelography were largely within the 'therapeutic range' of 10-20 mg l-1 for the drug. No patient suffered a seizure although, in two, spike discharges were seen on a post-myelography electroencephalogram. A simple regime involving two doses of DPH would provide acceptable plasma CSF concentrations as a basis for controlled studies in seizure prophylaxis following neuroradiological investigations involving intrathecal contrast.

Kinetics of CSF phenytoin in children.

The efficacy of intravenous phenytoin for the treatment of status epilepticus is related to the rapid entry of phenytoin into brain parenchyma. There is no information concerning the correlation between phenytoin serum and CSF concentrations in children, and the application of CSF data to clinical use. We report 7 children (2-11 yrs) who were treated or exposed to phenytoin in doses between 10.5-230 mg/kg. Lumbar puncture was performed 9 times in 6 of the patients. In one patient, an intraventricular catheter permitted successive assessment of CSF phenytoin concentrations. The ratio of CSF/serum phenytoin concentrations was 0.16 +/- 0.08, with gradual increase over the first 8 hours as the serum phenytoin concentration decreased. There was good correlation between therapeutic outcome and CSF phenytoin levels higher than 2 mcg/ml. In one patient the coma state secondary to phenytoin intoxication was associated with high CSF concentration (6 mcg/ml).

Effect of heparin or salicylate infusion on serum protein binding and on concentrations of phenytoin in serum, brain and cerebrospinal fluid of rats.

The purpose of this investigation was to determine if administration of heparin causes displacement of an acidic drug from serum protein binding sites in vivo as has been suggested by several groups of investigators. Rats were injected and infused with phenytoin to produce steady-state serum concentrations of about 25 micrograms/ml. After 2 hr, some of the animals also received injections and infusions of either heparin or salicylic acid. Salicylic acid (about 300 micrograms/ml), a classical inhibitor of phenytoin protein binding, reduced the steady-state serum concentration of total (free plus bound) phenytoin but had no significant effect on the steady-state serum concentration of free phenytoin or on the concentrations of phenytoin in cerebrospinal fluid and brain. The concentrations of phenytoin in cerebrospinal fluid were almost identical to the free phenytoin concentrations in serum. Similar effects were observed with respect to 5-(p-hydroxyphenyl)-5-phenylhydantoin and were found to be due to displacement of this major metabolite of phenytoin from serum protein binding sites by salicylate. Heparin administration caused an apparent increase of the steady-state plasma concentration of free phenytoin when determined as the product of the concentration of total phenytoin and the free fraction measured by in vitro equilibrium dialysis. Since heparin treatment had no significant effect on the total concentrations of phenytoin in plasma, cerebrospinal fluid and brain, it is concluded that the apparent displacement of phenytoin from plasma proteins occurs in vitro, after collection of blood samples from heparinized animals.

Centrally mediated effect of phenytoin on digoxin-induced ventricular arrhythmias.

Studies on the efficacy of phenytoin administered directly into the cerebrospinal fluid in protecting against digoxin-induced arrhythmias were carried out in 20 anesthetized dogs. Phenytoin was administered in an average dose of 10 mg directly into the cisterna magna of 10 dogs. The other control dogs received only the vehicle for phenytoin intrathecally. Both groups of dogs subsequently received a toxic dose of digoxin (0.2 mg/kg) intravenously. The time after intravenous digoxin to the onset of ventricular premature beats, ventricular tachycardia and ventricular fibrillation (VF) was significantly shorter in control animals compared to the phenytoin-treated dogs. In the control group, one dog survived the 3-h observation period without developing ventricular fibrillation, whereas 5 of the 10 phenytoin-treated dogs survived this period without VF (P less than 0.05). Phenytoin had a similar protective effect against digoxin-induced arrhythmias in 10 other dogs that received phenytoin intravenously. In the animals that received phenytoin intrathecally, plasma concentrations of phenytoin were undetectable. Thus, in this experimental model, phenytoin exerts a protective effect against digoxin-induced ventricular arrhythmias which is mediated via the central nervous system.

Diphenylhydantoin and primidone in tears.

Diphenylhydantoin (PHT) and primidone (PRM) were determined by the EMIT technique in the plasma, tears, saliva, and CSF of epileptic patients. Results indicate for PHT that tear values are more strictly correlated than are the saliva values to plasma and CSF concentrations. As for PRM, the data obtained show great interindividual variability of concentration in the different body fluids--in agreement with the wide range of both protein binding and half-life of this drug.