Microdialysis in the study of drug transporters in the CNS.

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

Kinetic model for gentamicin dosing with the use of individual patient parameters.

Multiple-infusion dosing regimens for gentamicin were established for 84 patients with the use of individually calculated values of elimination kinetic parameters. Serum level-time data obtained after a single infusion were used to determine the patient's gentamicin half-life (t 1/2) and distribution volume. Patients with serum creatinine (Cr) less than 1.2 mg per 100 ml had t 1/2 (mean, 2.25 hr) and total body clearances (mean, 0.082 L/hr/kg) significantly different from those with Cr greater than or equal to 1.2 mg/100 ml (means, 5.3 and 0.039, respectively). Distribution volumes were not significantly different (means, 0.22 and 0.21 L/kg, respectively). Calculations of dosing intervals and infusion rates, based on each patient's kinetic parameters and desired steady-state peaks and nadirs, assumed a one-compartment model with first-order elimination and 1-hr constant-rate input at fixed intervals. Follow-up steady-state peak and nadir levels were measured in 63 of the regimens. Differences between predicted and measured peak levels averaged --0.05 mug/ml with 60% of the measured values falling within 1 mug/ml of that predicted. Predicted-measured nadir differences averaged --0.62 mug/ml (significantly different from zero) indicating slight bias in the model. Fifty-six percent of these nadirs were within 1 mug/ml of that predicted.

Altered phenytoin clearance with febrile illness.

A prospective study was performed of antiepileptic drug levels in 14 boys resident in a pediatric chronic care facility. Blood samples and 24-hour urine collections were obtained monthly. During febrile illness (temperature greater than 101 degrees F for more than 24 hours), six additional blood samples and two urine collections were obtained for each child. During 8 of 10 febrile illnesses, phenytoin (PHT) decreased more than 40% from pre-illness baseline. Mean PHT level before illness was 16.7 (+/- 4.5 micrograms/ml) and during illness, 8.2 (+/- 3.6 micrograms/ml), significantly lower (p less than 0.001). Neither PHT binding nor absorption was altered by illness, so the most probable cause of the drop in PHT levels was induction of the hepatic oxidative enzyme system.

Pharmacokinetics of valproic acid in children: I. Multiple antiepileptic drug therapy.

We studied the pharmacokinetics of valproic acid (VPA) in 37 children who were taking other antiepileptic drugs. Thirteen children were studied both after initial and while on maintenance valproic acid therapy. Significant differences occurred between initial and maintenance therapy in the mean apparent volume of distribution and in apparent VPA clearance, whereas VPA half-life remained relatively constant. Analysis of data in the 13 children studied on two occasions demonstrated wide intrapatient variability of VPA pharmacokinetics. Children taking VPA together with other antiepileptic medications generally require higher doses of VPA given more frequently.

Severe methanol poisoning. Application of a pharmacokinetic model for ethanol therapy and hemodialysis.

Two patients with extremely high blood methanol concentrations (260 and 282 mg/dl) were successfully treated using pharmacokinetic dosing of ethanol, hemodialysis and supportive measures. Both patients recovered completely without residual ophthalmologic deficits. Early hemodialysis and inhibition of methanol metabolism with effective ethanol concentrations were attributed to the patients' full recovery. Methanol elimination was enhanced by hemodialysis as evidenced by a decrease in half-life from eight to two and a half hours. Methanol dialysance was 98 ml/min. A dosage regimen for ethanol was devised, utilizing dose-dependent pharmacokinetic parameters and the ethanol dialysance (100 to 120 ml/min) from these two patients. An ethanol loading dose of 0.6 g/kg should be administered to an adult with an acute methanol ingestion. This dose will produce a blood ethanol concentration of approximately 100 mg/dl which can be maintained by an ethanol infusion of 66 mg/kg/hour for nondrinkers to 154 mg/kg/hour for chronic ethanol drinkers. Hemodialysis should be initiated if the blood methanol concentration is greater than 50 mg/dl. If hemodialysis is initiated, the ethanol infusion should be increased by 7.2 g/hour.

Drug kinetics in burn patients.

Thermal trauma reportedly induces a spectrum of intricate physiological alterations that often involve cardiovascular, hepatic, renal, and dermatological functions. Individual responses to varying degrees of burn and secondary complications may produce unpredictable changes in the pharmacokinetics of drugs. A number of reports describing the disposition of aminoglycoside antibiotics in burn patients have demonstrated clinially significant alterations in the pharmacokinetics of this class of drugs. Several authors have found significantly reduced serum half-lives in burn patients as compared with average values in non-burned subjects. Increased aminoglycoside clearances in some burn patients have been attributed to increased glomerular filtration secondary to elevated cardiac output, prostaglandin formation, and glucagon secretion: others attribute this to enhanced elimination of drug across damaged skin tissue. Monitoring of serum aminoglycoside concentrations is recommended because of the low therapeutic index of these agents and the variable course of renal function following burn injury. Topical application of mafenide acetate, providone iodine, and gentamicin to burn wounds has resulted in varying amounts of systemic absorption. Various systemic toxicities have been attributed to these topical therapies, especially in patients with compromised renal function. The extent and area of the burn, degree of hydration, and the amount of drug applied are factors influencing transcutaneous absorption. Sulphonamide derivatives are excreted in the urine subsequent to the application of silver sulphadiazine cream, but silver ions appear to be localised on surface tissue and are thus unavailable to the subeschar space. Additional studies are needed to characterise the pharmacokinetics of medications commonly administered to burn patients. Unfortunately, the marked variability and fluctuations in pathophysiological status following burn trauma often confound the interpretation of such kinetic investigations.

The necessity of increased doses of amikacin in burn patients.

Extended periods (6 to 9 hours) of subtherapeutic serum amikacin levels were observed in five burn patients receiving the recommended intravenous dose of 7.5 mg/kg every 12 hours. Kinetic studies revealed an unusually short half-life, especially in younger patients. This more rapid elimination necessitated a shorter dosing interval (every 6 hours), resulting in an increased daily dose (30 mg/kg/day). In this study of 10 patients with normal renal function, an intravenous dosage regimen of 7.5 mg/kg every 6 hours resulted in therapeutic peak concentrations and shortened intervals of subtherapeutic serum concentrations. A transient episode of tinnitus without cochlear damage occurred in one patient after the first dose. Neither ototoxicity nor nephrotoxicity occurred in any of the patients. Because of interpatient variability in amikacin elimination, frequent monitoring of serum levels with necessary dosage changes is imperative to provide optimal serum concentrations. However, it must be emphasized that these increased dosage regimens of amikacin are not suggested for routine use without previously measuring serum levels.

Phenytoin metabolism in pregnancy.

Approximately 45% of patients with epilepsy experience an increase in seizure frequency during pregnancy. Despite the availability of effective anticonvulsant medications, there has been no decrease in reported seizure frequency accompanying pregnancy in the past 35 years. The authors prospectively studied concentrations of phenytoin (DPH) and its metabolites in 5 patients throughout pregnancy and during the postpartum period. The concentrations of phenytoin, as well as both unconjugated and conjugated forms of its principal metabolite, 5-(4-hydroxyphenyl)-5-phenylhydantoin (4-OH-DPH), were measured in plasma and urine by a precise high-pressure liquid chromatography method. Four patients experienced a decrease in plasma DPH during gestation, 3 of whom developed seizures at a time of low plasma DPH concentration. Within the therapeutic range, small increments in dosage resulted in large increments in plasma DPH levels, and in the postpartum period high levels of DPH were encountered if the dosage was not reduced. As pregnancy progressed, there was a decrease in the percentage of the daily dosage of DPH excreted as 4-OH-DPH. Based on these observations, the authors conclude that 1) pregnancy is associated with altered DPH absorption or metabolism or both, and 2) periodic measurement of plasma DPH concentration is valuable when managing a pregnant epileptic patient.

Acute phenytoin and primidone intoxication: a pharmacokinetic analysis.

Serial plasma levels of phenytoin, primidone, and phenobarbital were determined in a patient following massive overdose of phenytoin and primidone. The patient's neurologic status improved slowly over a period of 10 days and correlated best with the rise and fall of phenytoin plasma concentrations. The pharmacokinetics of all three agents were characterized by nonlinear regression analysis of their respective plasma concentration-time profiles during the elimination phase, followed by analog computer simulations of their entire plasma concentration-time profile closely resembled the observed values. Average values of Km and Vmax obtained from patients undergoing chronic therapy were used in the simulation and appear to adequately describe phenytoin elimination in this overdose situation. The elimination half-lives of primidone and phenobarbital of 6.2 and 83.5 hours, respectively, were within the "normal range" for patients on chronic therapy. Two distinct absorption phases for primidone and three for phenytoin were noted. The marked decrease in the estimated absorption rate constant between phases 1 and 2 for each drug may have been due to slow dissolution of a large congealed mass of phenytoin and primidone in the gut. The analysis of serial plasma samples following a massive overdose is recommended to provide a reliable data base for therapeutic decisions.

Modeling the route of administration-based enhancement in the brain delivery of EAB 515, studied by microdialysis.

EAB 515 (S-alpha-amino-5-phosphonomethyl[1,1'biphenyl]-3-propanoic acid) is an extremely hydrophilic N-methyl-D-aspartate antagonist. It shows marked CNS activity, in that it is a potent neuroprotector in models of cerebral ischemia, and also demonstrates social and non-social behavioral alteration following systemic administration in animals. Because of its high degree of ionization at physiologic pH, one would not expect appreciable brain uptake of EAB 515 across tight junctions of the blood-brain barrier. This is in contrast to its pharmacologic effect as well as brain/plasma ratios measured during systemic administration in rats. These observations lead us to investigate other transport pathways that might account for its brain uptake. Such mechanistic information is imperative in rational drug delivery and drug design strategies. Upon intracerebroventricular administration, the observed steady-state cortical extracellular fluid concentrations of EAB 515 were over 100-fold higher than those observed following intravenous administration, when normalized for the dosing rate. This increased distribution into the brain, based upon the route of administration, suggests the transport of drug directly between the cerebrospinal fluid and the brain extracellular space. The parameters of the model that adequately describes the data obtained from the two routes of administration in individual animals were estimated. The clinical significance of these results is in the use of intracerebroventricular administration for enhanced brain delivery of hydrophilic drugs that poorly cross the blood-brain barrier.

Theophylline absorption and disposition in rabbits: oral, intravenous, and concentration-dependent kinetic studies.

Theophylline pharmacokinetics following oral and intravenous administration were studied, and the absolute bioavailability of five commercially available products was determined using the rabbit as an in vivo model. Concentration-dependent clearance studies were performed by multiple constant-rate infusion and multiple bolus dose administration of aminophylline. Theophylline pharmacokinetics following the oral administration of these products obeyed the one-compartment open model adequately. However, the data obtained following rapid intravenous aminophylline administration in the rabbit fit either the one-compartment model (half-life = 2.8 hr and the volume of distribution = 4.4 hr and Vd(beta) = 0.708 liter/kg). There were no significant product-to-product differences in the time to peak (tmax), the rate constant of absorption (ka), or the percent of dose absorbed at 1 hr (F1); however, differences in the absolute bioavailability (F), dose-normalized peak serum concentration (Cmax(n)), and percent of dose absorbed at 6 hr (F6) were significant. There was no evidence of concentration-dependent clearance for theophylline in the rabbit in the serum concentration range studied, but the results of the multiple constant-rate infusion study suggest that total clearance decreases at higher serum theophylline concentrations.

Absorption of cyclosporine from rabbit small intestine in situ.

The absorption of the immunosuppressive drug, cyclosporine, from intestinal segments in the anesthetized rabbit in situ is reported. The experimental technique allows serial blood sampling with simultaneous sampling of the intestinal perfusate. The method of Loo and Riegelman for estimating the systemically absorbed fraction of the drug was used in conjunction with the in situ method. Taking the length of the rabbit small intestine as 300 cm and using the apparent permeability of cyclosporine determined in polyethylene glycol 400 (PEG 400) at a bulk fluid flow rate of 0.27 mL/min, the anatomical reserve length for cyclosporine was calculated to be -302 cm. The studies demonstrate that a significant portion of the loss of cyclosporine from rabbit small intestine cannot be accounted for as systemically absorbed drug. Furthermore, the apparent permeability coefficient of cyclosporine in these studies is so low that the length of small intestine in the rabbit is not sufficient for complete absorption of the drug from the lumen under the conditions used.

Zidovudine transport in the rabbit brain during intravenous and intracerebroventricular infusion.

The distribution of zidovudine (AZT) between plasma, brain extracellular fluid (ECF), and cerebrospinal fluid (CSF) was investigated in a crossover design study (n = 5) in unanesthetized rabbits. Drug was administered by intravenous (iv) and intracerebroventricular (icv) infusions at the same infusion rate (1.5 mg/h.kg). The concentrations of AZT in ECF and CSF were measured by HPLC with microdialysis sampling. Plasma concentrations of AZT were quantitated by HPLC. Following iv infusion, the ECF- and CSF-to-plasma concentration ratios at steady state (SS), were 0.19 +/- 0.05 and 0.29 +/- 0.06, respectively. These values were less than unity, indicating the existence of active transport processes for the transport of AZT from brain to plasma across the blood-brain barrier (BBB) or blood-CSF barrier (BCB). The transport processes were modeled by compartmental model analysis, and the results suggest that the transport efficiency of AZT across the BBB is asymmetric; that is, the efflux clearance was five times greater than the influx clearance. Similarly, the efflux clearance from CSF is three times larger than the influx clearance into CSF. The SS concentrations of AZT in brain ECF in the same animals that received an icv infusion of AZT in the crossover design study were approximately two orders of magnitude greater than those in animals following iv infusion at the same dosing rate. Nevertheless, the SS plasma concentrations of AZT were similar for both routes of administration (1.2 +/- 0.19 and 1.2 +/- 0.13 micrograms/mL for iv and icv routes, respectively), confirming that the brain is not an organ that exhibits first-pass metabolism under the present experimental conditions.

A partial area difference analysis for estimating elimination rate constants and distribution volumes of metabolites.

The true elimination rate constant of a metabolite is calculated using a partial area difference analysis. A linear pharmacokinetic model which describes clearances of parent and metabolite is developed. The model makes no restrictions on the route of administration of the parent but requires time-invariant kinetics for both parent and metabolite. Carbamazepine (CBZ), an antiepileptic, is metabolized to the 10,11-epoxide (CBZE), which is further hydrated to the trans-dihydrodiol (CBZD). Plasma data of the metabolites after a 200-mg single oral dose of Tegretol chewable tablets is used to demonstrate the applicability of the method. Elimination rate constants calculated using partial area analysis are 0.087 +/- 0.015 (h-1) and 0.056 +/- 0.014 (h-1) for CBZE and CBZD, respectively. These correspond to an average half-life of 8.0 h for CBZE and 12.4 h for CBZD, which suggest elimination-rate-limited disappearance of CBZD. The elimination rate constants for CBZE and CBZD calculated using partial areas are comparable to those in the literature determined after oral administration of CBZE. Volumes of distribution of CBZE and CBZD are also estimated to be 0.57 +/- 0.11 L/kg and 0.76 +/- 0.24 L/kg, respectively. The higher values observed with CBZD volumes of distribution are attributed to the higher plasma free fraction of the diol relative to CBZE.

In vitro-in vivo correlation and dissolution studies with oral theophylline dosage forms.

The dissolution rates of theophylline from six commercially available products (three uncoated and three sustained-release formulations) were determined in distilled water using the USP and rotating-filter dissolution apparatus. The effect of pH on the dissolution of these products was also examined by both methods. In addition, the effect of stirring rate on the dissolution of theophylline from these products was studied using the rotating-filter apparatus. The data obtained under all conditions were reproducible and well-described by a first-order equation. There was no significant difference between the percent of labeled content dissolved in 30 min (D30) and in 60 min (D60) obtained by the USP method and those obtained by the rotating-filter apparatus. The product-to-product variation in D30 and D60 was significant (p less than 0.001) for both the sustained-release and uncoated dosage forms. The pH of the dissolution fluid had a significant effect on the dissolution of theophylline from the products. The data obtained from the dissolution and absolute bioavailability studies in the rabbit were subjected to linear least-squares regression analysis, and good correlations were obtained between the dose-normalized peak serum level, time-to-peak, percent of the dose absorbed at 1 h and at 6 h, or the dose-normalized area under the curve from t = 0 to t = 00 and from t = 0 to t = 6 h and D30, D60, or the rate constant for dissolution. The linear relationship assumed for two of the products was used to predict bioavailability parameters from dissolution variables. The values predicted by this method were not statistically different from the actual values of these parameters.

Effect of probenecid on the renal and nonrenal clearances of zidovudine and its distribution into cerebrospinal fluid in the rabbit.

The effect of probenecid on the renal excretion of zidovudine (3'-azido-3'-deoxythymidine; AZT) and its distribution into CSF was studied in the rabbit. Although probenecid is chemically unrelated to AZT, it has been shown that probenecid inhibits AZT elimination in Acquired Immunodeficiency Syndrome (AIDS) patients. The effect of probenecid on the renal clearance of AZT after an iv bolus dose was studied in crossover experiments in the absence (control) and the presence of a continuous iv infusion of probenecid. Probenecid coadministration increased the AZT plasma AUC by 70%, by proportionally decreasing the total body clearance. The renal clearance decreased by 50%. The effect of probenecid on the renal clearance of AZT at steady state was studied by measuring the renal clearance of AZT at different steady-state plasma probenecid concentrations. The renal clearance of AZT decreased with increasing probenecid concentration, suggesting competitive inhibition of the secretion of AZT in the renal tubule. The relationship between AZT renal clearance and probenecid plasma concentrations, during and after probenecid iv infusion in conscious and in anesthetized uretercannulated rabbits, showed hysteresis, indicating that probenecid plasma concentration is different from the concentration at the site of interaction. This suggests the presence of an effect compartment for the inhibition of AZT renal excretion by probenecid. The effect of probenecid on the CSF distribution of AZT was also studied in the rabbit. Probenecid coadministration caused a sevenfold increase in the AZT AUCCSF in probenecid-treated rabbits when compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Zidovudine transport within the rabbit brain during intracerebroventricular administration and the effect of probenecid.

The elimination of zidovudine (AZT) from cerebrospinal fluid (CSF), its distribution from CSF to brain tissue, and its transport from brain extracellular fluid (ECF) to plasma were studied during intracerebroventricular (i.c.v.) infusion in unanesthetized rabbits. The effect of probenecid (PBD) on these transport processes was also studied. The concentration of AZT in brain ECF was measured by microdialysis with retrodialysis calibration for in vivo recovery. Plasma and CSF were sampled and analyzed for AZT and PBD using HPLC. The elimination of AZT from CSF showed nonlinear characteristics as the i.c.v. infusion rate was increased to 1 mg/h kg. The estimated maximum transport capacity and dissociation constant were 3.5 micrograms/min kg and 127 micrograms/mL, respectively. The total linear elimination clearance from CSF was 0.0073 mL/min kg. The spatial distribution of AZT in brain during i.c.v. infusion was simulated using a mathematical model which describes diffusive solute transport in brain ECF and efflux across the blood-brain barrier. This analysis yielded a brain to plasma efflux rate constant of 0.040/min. This parameter and the elimination clearance from CSF decreased significantly by the end of an 8-hour period during which PBD was infused intravenously at a rate of 15 mg/h kg.

Pharmacokinetics of propylene glycol in humans during multiple dosing regimens.

The pharmacokinetics of propylene glycol has been examined during multiple oral-dosing regimens. The glycol is rapidly absorbed, with Cpmax observed within 1 h following administration. The terminal elimination half-life is approximately 4 h. After a minimum of 10 half-lives of maintenance dosing on a fixed regimen, the accumulation of propylene glycol differed significantly among individuals because of variability in apparent clearance. The average apparent total body clearance is approximately 0.1 L/kg X h and may be concentration dependent. The apparent volume of distribution is approximately 0.5 L/kg, approximating total body water.

Pharmacokinetic model for subcutaneous absorption of cyclosporine in the rabbit during chronic treatment.

The objective of this study was to examine the concentrations in blood of cyclosporine (CyA) in rabbits during chronic subcutaneous (sc) administration and to propose a model that describes these data. Ten rabbits received sc CyA at 15 mg/kg daily for 1 week, then at 20 mg/kg twice weekly for 3 weeks, and at 15 mg/kg twice weekly for 7 weeks. Concentrations in blood were obtained weekly during the dosing period, and three to five concentrations were obtained over a 5-week period after dosing was terminated. CyA blood concentration-time (concentration in blood versus time) profiles could not be adequately described by absorption from a single dosing compartment. A two-compartment, sc absorption-site model was postulated. Steady-state concentrations in blood from three additional rabbits that had received CyA at 16.8 micrograms/min as a constant-rate intravenous infusion were added to the data set. A nonlinear mixed effects model was used to obtain the following parameter estimates (percent relative standard error): K12 = 0.111 day-1 (10.0), k21 = 0.0109 day-1 (12.6), ka = 0.0807 day-1 (11.8), CL/F = 14.6 L/day/kg (3.8), and Vd/F = 1.52 L/kg (13.4), where k12 and k21 are intercompartmental rate constants between sc compartments, ka is the absorption rate constant into the sampling compartment, CL/F is the apparent blood clearance of CyA from the body (F is bioavailability), and Vd/F is the apparent volume of distribution of CyA. The interindividual variability in CL/F was estimated as 20.5% (41.2), and the residual variability was 25.8% (15.6). The sc administration of CyA appears to provide slow, but very significant, absorption in rabbits.

Simultaneous first- and zero-order absorption of carbamazepine tablets in humans.

The absorption kinetics of carbamazepine (5H-dibenz[b,f]azepine-5-carboxamide) in healthy adult volunteers was investigated following a single 400-mg (2 X 200-mg) oral dose of commercially available conventional tablets (Tegretol). Wagner-Nelson plots of the data from all subjects (n = 10) showed that the fraction remaining to be absorbed declined in a biphasic manner, suggesting a mixed order of absorption. A model assuming the absorption of carbamazepine by simultaneous first-order and zero-order rates was used to describe the overall absorption process. Model parameters (and their mean +/- SD values) were: alpha, the fraction of the dose absorbed at a first-order rate (0.646 +/- 0.070); Ka, the first-order absorption rate constant (0.45 +/- 0.13 h-1); and tdur, the duration of the zero-order absorption component (36.0 +/- 4.4 h). If complete absorption can be assumed, then the corresponding average zero-order rate was 4.0 mg X h-1. The results indicate that 35% of the available dose is absorbed at a zero-order rate. These data suggest a prolonged constant rate of absorption due to continued delivery during its transit in the intestine. In addition, an assessment of the mean absorption time, based on the parameters from the model described above, compared closely (7.95 versus 8.44 h) with the mean absorption time estimated from an analysis of the fraction remaining to be absorbed versus time plot using a noncompartmental approach.