Binding Of Carbamazepine Research Articles

Molecular modelling of the interactions of carbamazepine and a nicotinic receptor involved in the autosomal dominant nocturnal frontal lobe epilepsy.

1. The normal and a mutant (S248F) human neuronal alpha4beta2 nicotinic receptors, and their interaction with the channel blocker carbamazepine (CBZ) have been modelled. The mutant, responsible for the autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), has an enhanced sensitivity to and a slower recovery from desensitization, a lower conductance, short open times, reduced calcium permeability, and is 3 fold more sensitive to CBZ, a drug used in the treatment of partial epilepsies. 2. Mutant channel properties are explained by the physicochemical properties of the two Phe248 side chains, including size and cation-pi interaction, and their dynamic behaviour. A defective mechanism of dehydration might be responsible for the reduced calcium influx. 3. Phe248 residues are the main component of CBZ binding sites in the mutant, while this is not true for Ser248 in the normal receptor. 4. A higher number of blocking binding sites and a predicted higher affinity found for CBZ in the mutant account for its differential sensitivity to CBZ. 5. Aromatic-aromatic interactions between CBZ and the two Phe248 account for the difference in affinity, which is at least 12 times higher for the mutant, depending on the method used for calculating K(i). 6. Normal vs mutant differences in K(i), enhanced by the higher number of blocking binding sites in the mutant, seem excessive compared to the differential sensitivities to CBZ experimentally found. The negative cooperativity suggested by a predicted overlapping of blocking and non-blocking binding sites gives an explanation, as overlapping is higher in the mutant. 7. For both types of receptors we found that the carbamyl group of the best blocking conformers of CBZ forms hydrogen bonds with serine residues, which may explain the fundamental role of that moiety for this molecule to act as antiepileptic drug.

Pharmacokinetic interactions between carbamazepine and the traditional Chinese medicine Paeoniae Radix.

The present study was conducted to evaluate the effects of Paeoniae Radix (PR), one of the most famous tonic traditional Chinese medicines, on the pharmacokinetics of carbamazepine (CBZ) in rats and to determine the possible interactions between PR and CBZ. The significant decrease in Tmax indicated that simultaneous oral administration of PR contributed to more rapid absorption of CBZ. It is suggested that the faster absorption of CBZ might lead to the rapid onset of its clinical effect. There were no significant differences in maximum concentration (Cmax), area under the plasma concentration-time curve (AUC), half-life (t1/2), mean residence time (MRT), clearance/bioavailability (CL/F), and apparent volume of distribution/bioavailability (Vd/F) of CBZ between the two groups, showing that PR did not significantly affect the absorption extent, distribution, metabolism, and elimination of CBZ. A significant decrease in protein binding rate was found when CBZ was coadministered with PR. Further studies are in progress to clarify the clinical significance and the mechanism underlying the effects of PR on the protein binding of CBZ observed in the present study.

Studies on interactions between traditional herbal and Western medicines. IV: lack of pharmacokinetic interactions between Saiko-ka-ryukotsu-borei-to and carbamazepine in rats.

The possibility of pharmacokinetic interactions between Saiko-ka-ryukotsu-borei-to extract powder (TJ-12), a widely used traditional Chinese herbal (Kampo) medicine, and carbamazepine (CBZ), an important anti-epileptic drug, was examined in rats. There were no significant differences in the serum protein binding of CBZ and carbamazepine- 10,11-epoxide (CBZ-E), its active metabolite, at two concentrations (1 and 10 Bg/ml) between twogroups pretreated orally with the vehicle andTJ-12 suspension (1 g/kg/d, p.o.) for 1 week. One-week repeated pretreatment with TJ- 12 (1 g/kg/d) did not influence liver weight, contents of cytochromes P450 and b5 in hepatic microsomes or the formation rate of CBZ-E from CBZ by its microsomes, while pretreatment with phenobarbital (80 mg/kg/d, i.p.) significantly increased these parameters. Neither a single nor 1-week repeated oral pretreatment with TJ-12 (1 g/kg/d) affected the plasma concentration-time profile and any pharmacokinetic parameter of CBZ or CBZ-E after oral administration of CBZ (50 mg/kg). These results indicated that oral co-administration of TJ-12 with CBZ has no effect ofthe pharmacokinetics of CBZ or CBZ-E in rats. Concomitant treatment with TJ- 12 and CBZ appears to be pharmacokinetically safe in humans.

Studies on interactions between traditional herbal and Western medicines. I. Effects of Sho-seiryu-to on the pharmacokinetics of carbamazepine in rats.

The effects of oral co- and pre-administration of Sho-seiryu-to extract powder (TJ-19, 1 g/kg), a widely used Kampo (traditional Chinese herbal) medicine, on the pharmacokinetics of an anti-epileptic drug, carbamazepine (CBZ), and its active metabolite (carbamazepine-10,11-epoxide, CBZ-E) after oral administration of CBZ (50 mg/kg) were examined in male rats. The simultaneous administration of TJ-19 significantly lengthened the time to reach the peak plasma concentration (Tmax), but did not influence the peak plasma concentration, area under the plasma concentration-time curve or terminal elimination half-life (t1/2). Each parameter for CBZ or CBZ-E with a single pretreatment with TJ-19 was not significantly different from that with the vehicle. Tmax and the elimination rate constant for CBZ were significantly increased by 1-week repeated pretreatment with TJ-19, by 83% (p<0.01) and 88% (p<0.001), respectively. t1/2 and the mean residence time from zero to infinity (MRT0-infinity) in the TJ-19 pretreatment group were significantly shortened, by 52 and 34% (p<0.005), respectively. No significant difference in the bound fraction of each drug at two concentrations (1 and 10 microg/ml) was observed between the control and TJ-19 pretreatment groups. These results indicate that simultaneous oral administration of TJ-19 delays the oral absorption of CBZ, while 1-week repeated pretreatment with TJ-19 accelerates the metabolism of CBZ in rats, without affecting the protein binding of CBZ.

Microsomal metabolism of carbamazepine and oxcarbazepine in liver and placenta.

Metabolism of both carbamazepine (CBZ) and oxcarbazepine (OCBZ) were catalyzed by human liver microsomes and microsomes from livers of CBZ-induced or non-induced C57BL/6 mice. Human placental microsomes metabolized only OCBZ. Mouse liver microsomes metabolized CBZ to carbamazepine-10,11-epoxide (CBZ-E), 10-hydroxy-10,11-dihydro-carbamazepine (10-OH-CBZ), 3hydroxy-carbamazepine (3-OH-CBZ), 10,11-trans-dihydroxy-10,11-dihydro-carbamazepine (10,11-D) and to an unidentified metabolite. CBZ-pretreatment of mice increased both ethoxyresorufin O-deethylase activity in the liver and the amount of CBZ-E in microsomal incubations regardless of the age of mice. Human liver microsomes catalyzed the formation of CBZ to 9-hydroxymethyl-10-carbamoyl acridan (9-AC) in addition to CBZ-E, 3-OH-CBZ and 10-OH-CBZ. OCBZ was metabolized to its active metabolite in all incubations. An unknown metabolite was also present in some of the incubations. Human liver microsomes catalyzed only minute covalent binding of CBZ and OCBZ to DNA. Binding of OCBZ was, however, one order of magnitude greater than binding of CBZ. Human placental microsomes from the mothers on CBZ therapy did not catalyze CBZ metabolism. The same microsomes catalyzed OCBZ metabolism to 10-OH-CBZ and to an unknown metabolite. These results indicate autoinduction in CBZ metabolism in mouse liver. Due to the higher binding of OCBZ than CBZ to DNA in vitro, further studies on the potential mutagenicity of OCBZ may be warranted.

Simultaneous determination of carbamazepine and carbamazepine 10,11-epoxide by using microcolumn HPLC: study of pharmacokinetics of carbamazepine in a volunteer.

A highly-sensitive microcolumn HPLC method for the simultaneous determination of carbamazepine and carbamazepine-10,11-epoxide in human serum and saliva is described. The method was successfully employed for the study of pharmacokinetics of carbamazepine in humans. After oral administration of 100 and 200 mg of carbamazepine to a volunteer, multiple peaks were observed on the kinetic curves. They were symbathic in the serum and saliva. This indicated the presence of multiple peaks which characterize both free and protein-bound fractions of the drug. The existence of multiple peaks on the kinetic curves implies that the kinetic of carbamazepine cannot be described with the one-compartment linear model. Nevertheless, each peak was treated within the range of a one-compartment linear model of absorption and the results obtained were compared with published data. For the elucidation of the nature of multiple peaks the graphical differentiation of ascending and descending branches of all peaks were carried out. On this basis of the dependence of the absorption and elimination rates on time was constrained. The analysis of experimental data resulted in the following conclusions: (a) the presence of multiple peaks on the kinetic curves is induced by the interrupted character of carbamazepine absorption that is caused by the very poor solubility of carbamazepine; (b) the elimination of the drug from blood serum occurs in two phases. Binding of carbamazepine with tissues takes place in the first phase, and biotransformation and excretion occurs in the second phase. It is possible that the presence of multiple peaks on the kinetic curves is partially caused also by redistribution of the drug from the comparatively easily accessible to the less accessible tissues. This requires further investigation.

Simultaneous Determination of Carbamazepine and Carbamazepine 10,11-Epoxide by Using Microcolumn HPLC: Study of Pharmacokinetics of Carbamazepine in a Volunteer

A highly-sensitive microcolumn HPLC method for the simultaneous determination of carbamazepine and carbamazepine-10,11-epoxide in human serum and saliva is described. The method was successfully employed for the study of pharmacokinetics of carbamazepine in humans. After oral administration of 100 and 200 mg of carbamazepine to a volunteer, multiple peaks were observed on the kinetic curves. They were symbathic in the serum and saliva. This indicated the presence of multiple peaks which characterize both free and protein-bound fractions of the drug. The existence of multiple peaks on the kinetic curves implies that the kinetic of carbamazepine cannot be described with the one-compartment linear model. Nevertheless, each peak was treated within the range of a one-compartment linear model of absorption and the results obtained were compared with published data. For the elucidation of the nature of multiple peaks the graphical differentiation of ascending and descending branches of all peaks were carried out. On this basis the dependence of the absorption and elimination rates on time was constrained. The analysis of experimental data resulted in the following conclusions: (a) the presence of multiple peaks on the kinetic curves is induced by the interrupted character of carbamazepine absorption that is caused by the very poor solubility of carbamazepine; (b) the elimination of the drug from blood serum occurs in two phases. Binding of carbamazepine with tissues takes place in the first phase, and biotransformation and excretion occurs in the second phase. It is possible that the presence of multiple peaks on the kinetic curves is partially caused also by redistribution of the drug from the comparatively easily accessible to the less accessible tissues. This requires further investigation. © 1997 by John Wiley & Sons, Ltd.

Lack of influence of histopathological changes on carbamazepine and carbamazepine-10,11-epoxide concentrations in the brain cortex of epileptic patients

Post-mortem concentrations of carbamazepine (CBZ) and its anticonvulsive metabolite carbamazepine-10,11-epoxide (CE) were determined in different lesions of the cerebral cortex and in the serum (total and free) from 13 epileptic patients. Twenty cortical specimens were obtained from the superior frontal gyrus, the temporopolar region and the neocerebellum. The cortical samples showed various pathological changes characterized by augmented glial cells, fibre gliosis or ulegyria as well as abundant corpora amylacea or encephalitic signs of viral type besides neuronal depletion. The CBZ and CE concentrations in the 20 cortical lesions were not significantly decreased when compared to the control specimens of 32 epileptic patients without essential histopathological alterations of the specified cortical areas ( p < 0.05). A comparable result had been found in our former study on phenytoin (PHT) and phenobarbital (PB). Six patients with cortical lesions of the present series had already been included in this PHT/PB study. Five of these patients revealed unchanged CBZ and CE as well as PHT and PB concentrations. Only in one neocerebellar specimen the CE concentration was just above the upper 95% confidence limit of the control group. But, most probably this finding has no further relevance. The results greatly favour the nonspecific binding of CBZ and CE to cerebral tissue constituents.

Carbamazepine metabolism to a reactive intermediate by the myeloperoxidase system of activated neutrophils

Carbamazepine is an anticonvulsant which is associated with a significant incidence of hypersensitivity reactions including agranulocytosis. We have postulated that many drug hypersensitivity reactions, especially agranulocytosis and lupus, are due to reactive metabolites generated by the myeloperoxidase (MPO) (EC 1.11.1.7) system of neutrophils and monocytes. This led to a study of the metabolism and covalent binding of carbamazepine with MPO/H 2O 2/Cl − and neutrophils. Metabolism and covalent binding were observed in both systems and the same pathway appeared to be involved; however, the metabolism observed with the MPO system was approximately 500-fold greater than that observed with neutrophils. The metabolites identified were an intermediate aldehyde, 9-acridine carboxaldehyde, acridine, acridone, choloroacridone, and dichloroacridone. We postulate that the first intermediate in the metabolism of carbamazepine is a carbonium ion formed by reaction of hypochlorous acid (HOCl) with the 10,11 double bond. Although we have no direct proof for the proposed carbonium ion, it provides the most likely mechanism for the observed ring contraction. Iminostilbene, a known metabolite of carbamazepine, was also metabolized by a similar pathway leading to ring contraction; however, the rate was much faster and the first step may involve N-chlorination and a nitrenium ion intermediate. These data confirm that carbamazepine is metabolized to reactive intermediates by activated leukocytes. Such metabolites could be responsible for some of the adverse reactions associated with carbamazepine, especially reactions such as agranulocytosis and lupus which involve leukocytes.

Carbamazepine Metabolism in Humans

Free and total carbamazepine (CBZ) and carbamazepine-epoxide (CBZ-EP) plasma levels were obtained on 113 patients with epilepsy (18-61 years old) controlled on either monotherapy or coadministration with either phenobarbital (PB), phenytoin (PHT), valproic acid (VPA), or all three. A subset of patients were administered tetradeuterium labeled CBZ to evaluate the effects of autoinduction and coadministration of VPA on the kinetics of CBZ and its metabolite CBZ-EP. Polytherapy had variable effect on free and total CBZ plasma levels compared to monotherapy. Coadministered PHT (co-PHT), or all three anticonvulsants together (PHT, PB, and VPA: co-AEDs) decreased free and total CBZ plasma levels. No change was noted for coadministered VPA (co-VPA). Compared to monotherapy the free and total CBZ-EP levels increased with co-VPA, less with coadministered PB (co-PB), and no change with co-PHT or co-AEDs. Protein binding of CBZ and CBZ-EP was not affected by any antiepileptic drugs studied. The free and total CBZ-EP/CBZ ratio was tripled with co-VPA or co-AED's, and doubled with co-PHT or co-PB. Isotope labeling did not demonstrate any differences in half-life (t1/2), plasma clearance (Cl), or volume of distribution (Vd). Compared to naive controls, monotherapy and co-VPA decreased CBZ t1/2 by 50%, and more than doubled the CBZ Cl without a significant change in the Vd. Autoinduction is one explanation for these changes with chronic CBZ therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic interaction between imipramine and carbamazepine in vivo and in vitro in rats.

The pharmacokinetic consequences of the combination of carbamazepine with imipramine in male Wistar rats have been investigated. It was found that a 2-week treatment with the combination resulted in the increase of the concentrations of the parent compounds and a simultaneous decrease in their metabolites in blood plasma i.e. carbamazepine inhibited imipramine demethylation in the side chain while imipramine inhibited carbamazepine 10,11-epoxidation. The velocity of imipramine 2-hydroxylation and 10,11-epoxy-carbamazepine hydration did not seem to be changed by the combination. On the basis of studies in vitro it is concluded that the observed metabolic interaction between carbamazepine and imipramine is due to the competition of the drugs for the active centre of cytochrome P 450 and to a certain qualitative alteration of the enzyme by imipramine as can be deducted from the decrease of carbamazepine binding to the cytochrome.

Effects of josamycin on carbamazepine kinetics.

The steady state pharmacokinetics of oral carbamazepine in epileptic patients (n = 8) was compared before and after one week of treatment with josamycin (2 g/day). There was a small but statistically significant decrease in oral clearance of total (17%) and unbound (21.5%) drug. In spite of an unchanged AUC of 10,11-epoxide carbamazepine the ratio of metabolite to parent drug AUC was significantly decreased (20.2%). The plasma protein binding of carbamazepine and its 10,11-epoxide metabolite did not vary. The results demonstrate impairment by josamycin of the apparent clearance of carbamazepine. Care should be taken in patient receiving both carbamazepine and josamycin.

A study on carbamazepine levels, including estimation of 10-11 epoxy-carbamazepine and levels in free plasma and saliva.

Blood and saliva were collected from 48 children, aged three to 17 years, before a morning dose of carbamazepine and then at three, six and eight hours afterward. There was no significant difference in the ratio of 10-11 epoxy-carbamazepine to total carbamazepine for children on carbamazepine alone (0.18) and those on an additional anticonvulsant (0.19). 11 children were studied on their first day of therapy. Though four of them achieved levels of total carbamazepine which would have given a readily measurable 10-11 epoxide on the above ratio, 10 of the 11 consistently had levels less than 4 mumol/l. This may be of help in detecting non-compliance. For 16 children free carbamazepine also was estimated and there was an over-all correlation coefficient of 0.85 between saliva and free-drug levels. The proportion of free to total carbamazepine varied between 19 and 54 per cent. There was a significantly higher proportion of free drug at the pre-dose sampling time than at other times. Protein binding of carbamazepine therefore is very variable and saliva estimations can contribute usefully to the monitoring of drug levels.

Total and free serum concentrations of carbamazepine and carbamazepine-10,11-epoxide in children with epilepsy.

Simultaneous steady-state serum total and free (non-protein-bound) concentrations of carbamazepine and carbamazepine-10,11-epoxide were measured in 68 patients under the age of 21 years with epilepsy (44 males, 24 females; mean age, 11.8 +/- 4.5 years). Thirty patients were maintained on monotherapy with carbamazepine. Mean serum total carbamazepine and carbamazepine-10,11-epoxide concentrations obtained were 7.0 +/- 2.4 mg/L (29.5 +/- 10.0 mumol/L) and 1.5 +/- 0.6 mg/L (5.9 +/- 2.6 mumol/L), respectively. Mean serum-free carbamazepine and carbamazepine-10,11-epoxide concentrations obtained were 1.3 +/- 0.5 mg/L (5.7 +/- 2.1 mumol/L) and 0.5 +/- 0.3 mg/L (2.2 +/- 1.1 mumol/L), respectively. Binding of carbamazepine and carbamazepine-10,11-epoxide was 81% +/- 3% and 62% +/- 10%, respectively. There was no significant difference in binding between male and female patients or those maintained on monotherapy and polytherapy. Age correlated significantly with carbamazepine binding but not with carbamazepine-10,11-epoxide binding. Free concentrations of carbamazepine and carbamazepine-10,11-epoxide correlated significantly with total carbamazepine and total carbamazepine-10,11-epoxide concentrations, respectively, indicating that the binding capacities of both carbamazepine and carbamazepine-10,11-epoxide are constant at serum total carbamazepine concentrations within the quoted therapeutic range.

Sodium valproate and valpromide: differential interactions with carbamazepine in epileptic patients.

To evaluate the comparative effects of valproic acid (VPA) and valpromide (VPM) on plasma levels and protein binding of carbamazepine (CBZ) and CBZ-10,11-epoxide (CBZ-E), 12 adult epileptic patients stabilized on CBZ monotherapy were divided into two groups. One group (n = 6) received sodium valproate (1,100 mg/day) for 2 weeks, while the other group (n = 6) was given, for the same period, a dosage of VPM (1,200 mg/day) expected to produce VPA levels equivalent to those achieved with valproate. Plasma CBZ levels were not affected by either treatment. In the valproate-treated group, plasma CBZ-E levels increased by 101% (range, 29-238%) within 1 week of combined therapy (p less than 0.02) and returned to baseline values after VPA treatment was stopped. In the VPM-treated patients, the elevation of plasma CBZ-E levels was much greater. In this group, plasma CBZ-E increased by 330% (range, 110-864%), and this was associated in two patients with the appearance of adverse effects which subsided after reducing the VPM dosage. The plasma protein binding of CBZ and CBZ-E was not affected significantly by VPM or valproate therapy. Plasma VPA levels were similar in the two groups. It is concluded that VPM is not simply a prodrug of VPA. Although both VPA and VPM increase CBZ-E levels--probably by inhibiting the enzyme epoxide hydrolase--the interaction caused by VPM is of much greater magnitude and potential clinical significance.

Clinical pharmacokinetics of carbamazepine and its epoxy and hydroxy metabolites in humans after an overdose.

In five cases of carbamazepine (CBZ) intoxication, the time curves of the plasma concentration and of the renal excretion rate of carbamazepine and its metabolites carbamazepine 10,11-epoxide (CBZ-epoxide) and trans-10,11-dihydro-10,11-dihydroxy-CBZ (CBZ-diol) were measured. Pseudo-steady-state or plateau-like plasma concentration-time curves were observed when doses of 12 or 18 g of CBZ were ingested. Hemoperfusion lowers the plasma concentration of CBZ and its metabolites by affecting the half-life. The effect of hemoperfusion is reduced by the continuous absorption from the gastrointestinal tract and redistribution from the tissues. The renal clearances of CBZ and of CBZ-epoxide are low (1 and 8 ml/min, respectively); both are flow dependent. The renal clearance of CBZ-diol is approximately 160-350 ml/min and is independent of the urine flow. Although urine stimulation increases the renal clearance of CBZ by 100%, the overall amount excreted increases only 1-2% of the dose. Protein binding of CBZ is approximately 80%, of CBZ-epoxide 50%, and of CBZ-diol 70%.

Altered serum protein binding of carbamazepine in disease states associated with an increased? 1-acid glycoprotein concentration

The protein binding of carbamazepine (CBZ) in vitro was assessed in sera from 47 patients with various diseases known to alter alpha 1-acid glycoprotein (AAG) concentration and from 20 drug-free normal control subjects. In the patient group, AAG and albumin (HSA) concentrations ranged from 6 to 74 mumol/l and from 377 to 652 mumol/l, respectively; in the controls, protein concentrations were less variable, ranging from 11 to 26 mumol/l for AAG and from 623 to 754 mumol/l for HSA. In both the patient and the combined patient and control groups, free CBZ fractions were inversely correlated with the serum AAG concentration (r = -0.62). No significant relationship could be found between the free CBZ fraction and the serum HSA concentration. The free CBZ fraction was moderately but significantly decreased in patients with AAG levels above 26 mumol/l (the highest value found in controls) as compared either to patients with a normal AAG concentration or to control subjects (19 +/- 5% vs 23 +/- 4% and 23 +/- 2%), despite the finding of a higher HSA concentration in the control group. The data confirm AAG as an important determinant of interindividual variability in serum CBZ binding.

Carbamazepine and carbamazepine-epoxide serum protein binding in newborn infants.

To evaluate the serum protein binding of carbamazepine (CBZ) and its active metabolite carbamazepine-10, 11-epoxide (CBZ-E) in newborns, cord serum was obtained from 20 infants at the time of birth. Each of the samples was spiked to concentrations of CBZ 12 + CBZ-E 3 micrograms/ml, and CBZ 4 + CBZ-E 1 micrograms/ml and then subjected to analysis for total and free drug concentrations. Total drug concentrations were determined by high pressure liquid chromatography (HPLC), and free drug concentrations were determined by ultrafiltration and subsequent HPLC. The mean percentage of CBZ unbound, calculated as (free drug concentration divided by total drug concentration) X 100, was 30.6 and 29.8% at the high and low concentrations, respectively. Results for CBZ-E were 52.0 and 47.5% at the high and low concentrations, respectively. Compared with previously published values for protein binding in older children and adults, neonates appear to bind CBZ to a lesser degree and CBZ-E to the same degree. The lower binding of CBZ suggests that the therapeutic range of total CBZ in neonates may differ from that in older patients; however, this will have to be determined through clinical study. As previously demonstrated in older patients, CBZ and CBZ-E binding was variable. Monitoring of free CBZ serum concentrations in neonates appears advisable because of this variability.

Alpha 1-acid glycoprotein concentration and serum protein binding of carbamazepine and carbamazepine-10,11 epoxide in children with epilepsy.

The relationship between the serum protein binding of carbamazepine (CBZ) and carbamazepine-10,11 epoxide (CBZ-E) and the concentration of alpha 1-acid glycoprotein (AAG) and albumin (HSA) was examined in 39 CBZ-treated epileptic children aged 4 months to 12 years. A significant inverse correlation was found between the free fraction of both compounds and serum AAG, even though changes in AAG concentration explained only part of the variation in binding. No correlation was found between the free fraction of CBZ and CBZ-E and HSA, probably due to the small intersubject variation in HSA concentration. In vitro experiments showed that both CBZ and CBZ-E were bound to HSA and to a lesser extent to AAG. At equivalent HSA concentrations, the binding of CBZ and its metabolite increased proportionately with increasing AAG concentration within the range occurring clinically.

Determinants of carbamazepine and carbamazepine 10,11-epoxide binding to serum protein, albumin and alpha 1-acid glycoprotein.

The binding of carbamazepine and carbamazepine 10,11-epoxide to serum, albumin and alpha 1-acid glycoprotein (AAG) was determined and compared at drug concentrations ranging from 0.5 to 400 mg/l using equilibrium dialysis and liquid chromatography. The total binding of carbamazepine in serum was determined primarily by albumin and to a lesser extent (20-30%) by AAG. Modified Scatchard plots for carbamazepine binding in serum were biphasic, suggesting the presence of two binding sites on serum protein. Association constants characterizing the first (k1 = 2.4 X 10(4) l/mol) and second (k2 = 4.6 X 10(2) l/mol) binding sites agreed with those measured for AAG and albumin respectively. Modified Scatchard plots for carbamazepine 10,11-epoxide binding in serum were linear and serum binding was largely accounted for by binding to albumin. The epoxide metabolite did not bind to AAG. Carbamazepine binding to AAG was drug concentration-dependent over the concentration range considered to be therapeutic, while the percent binding values for carbamazepine and epoxide binding to albumin and serum from a normal individual were constant over this range. Computer simulations showed that physiological extremes in AAG and albumin concentrations can result in a range of carbamazepine unbound fractions of 0.17 to 0.47. These data suggest that normal variations in concentrations of both proteins may be the principal cause of interpatient variability in serum protein binding of carbamazepine.