Free Fractions Of Carbamazepine Research Articles

Carbamazepine clearance and seizure stability during pregnancy

The aims of this study were to characterize the alterations in total and free carbamazepine (CBZ) and in total and free carbamazepine-epoxide (CBZ-EPO) clearances during pregnancy, to calculate the change in free fractions of CBZ and CBZ-EPO during pregnancy, and to determine whether seizure worsening is associated with a low ratio to nonpregnant baseline concentration of total or free CBZ or CBZ-EPO. Women on CBZ were enrolled before conception or during pregnancy in this prospective, observational study. Concomitant medications and seizure frequency were recorded. Serum total and free CBZ and CBZ-EPO were collected at each visit. Changes in the clearance of all four compounds and free fractions of CBZ and CBZ-EPO were compared with nonpregnant baseline. During pregnancy, the ratios to baseline concentrations of total and free CBZ and CBZ-EPO were compared for months with and without increased seizure frequency. Total and free CBZ and CBZ-EPO clearances were calculated in 15 pregnancies in 12 women. Clearances did not change for any of these compounds during pregnancy. The free fraction of CBZ increased from 0.23 at baseline to a maximum of 0.32 in the third trimester (p=0.008). In the six women on CBZ monotherapy with adequate seizure diaries and blood sampling, seizure worsening did not correspond to a ratio to baseline concentration of less than 0.65 for total or free CBZ or CBZ-EPO. In conclusion, total and free CBZ and CBZ-EPO clearances did not change substantially during pregnancy, and seizure frequency worsening was not associated with decreased concentrations of total or free CBZ; therefore, therapeutic drug monitoring may not be necessary for all women on CBZ during pregnancy. Further studies with larger sample sizes are needed before definitive recommendations can be made. Carbamazepine monotherapy may be a relatively safe and cost effective treatment option for women with focal epilepsy syndromes during pregnancy.

Cloud-Point Extraction for the Determination of the Free Fraction of Antiepileptic Drugs in Blood Plasma and Saliva

A method for the determination of the free fraction of antiepileptic drugs in plasma and saliva was developed. The separation of free and protein-bound fractions was obtained by cloud-point extraction under optimum conditions of pH, surfactant type, and concentration. It is shown that the free fraction of carbamazepine and of phenobarbital in plasma coincides with the drug concentration in saliva. The dependence of the free fraction in plasma on the administered dose was studied. The influence of the simultaneous administration of the two drugs on their free concentrations was revealed: In the presence of carbamazepine the free fraction of phenobarbital is decreased markedly whereas phenobarbital does not influence the behavior of carbamazepine.

Evaluation of carbamazepine and carbamazepine-epoxide protein binding in patients undergoing epilepsy surgery.

Carbamazepine (CBZ) serum concentrations increase after epilepsy surgery. A possible mechanism may be acute changes in protein binding, specifically those involving the acute phase reactant alpha(1)-acid glycoprotein (AAG). We prospectively evaluated 16 adults (11 receiving CBZ) with epilepsy (mean age 30 +/- 9.9 years, 8 women and 8 men) undergoing temporal lobe resections and characterized AAG, albumin, CBZ, and CBZ-epoxide (CBZ-E) free fractions over time. AAG, ALB, CBZ, and CBZ-E free fractions were determined before surgery (baseline) and on postoperative days 1-5, 14, and 30. AAG was measured with a radial immunodiffusion assay method, CBZ and CBZ-E serum concentrations were determined by high-performance liquid chromatography (HPLC). Free fractions of CBZ and CBZ-E were calculated as the ratio of unbound (determined after ultracentrifugation) to total serum drug concentrations. Statistical analysis included analysis of variance (ANOVA) and Student's t test for paired data when appropriate, with significance assigned at p<0.05. All data are mean +/- SD. AAG concentrations increased significantly from baseline 61.9 +/- 21.3 mg/dl), peaking at postoperative day 3 (116.8 +/- 20.6 mg/gl) and decreasing to baseline levels between days 14 and 30. CBZ serum concentrations were significantly increased in the immediate postoperative period (day 3), but albumin concentrations and CBZ and CBZ-E free fractions did not differ significantly between baseline and the postoperative time points. Temporal lobe resection results in an acute phase reaction which is manifested in part by significant changes in AAG. Although CBZ and CBZ-E total serum concentrations increased significantly in the immediate postoperative period, epilepsy surgery did not appear to result in significant overall changes in drug binding to plasma proteins.

Interactions of Phenobarbital and Phenytoin with Carbamazepine and Its Metabolites' Concentrations, Concentration Ratios, and Level/Dose Ratios in Epileptic Children

The effects of phenytoin (PHT) or phenobarbital (PB) comedication on the concentrations, concentration ratios, and level/dose ratios of carbamazepine (CBZ) and its metabolites were investigated. The hetero-induction effects of CBZ metabolism by PHT or PB were clearly demonstrated. Serum CBZ level/dose ratios in patients with CBZ polytherapy were decreased while CBZ-10,11-epoxide (CBZ-E) and trans-10,11-dihydroxy-10,11-dihydro-CBZ (CBZ-H) concentrations were increased as compared with those of patients receiving CBZ alone. The concentration ratios of CBZ-H/CBZ and CBZ-E/CBZ were also greater in patients receiving CBZ+PHT or CBZ+PB than in patients receiving CBZ alone. In addition, positive correlations between serum PHT concentration and CBZ-H/CBZ or CBZ-E/CBZ concentration ratios were observed. There were no significant differences in CBZ-H/CBZ-E concentration ratios, the free fractions of CBZ and its metabolites, and CBZ-E or CBZ-H level/dose ratios among the three groups of patients. Because this approach investigates the in vivo relation between the substrates and products of the enzymes involved in CBZ biotransformation, more detailed information about the drug interactions was obtained. The results suggest that the PHT has a potent induction effect on CBZ epoxidase, whereas PB is a moderate inducer.

Influence of additional therapy with zonisamide (Excegran) on protein binding and metabolism of carbamazepine.

The influence of comedication with zonisamide (ZNS) on protein binding and carbamazepine (CBZ) metabolism was studied in 16 pediatric epileptic patients. Correlations were evaluated between ZNS daily dose and individual change of level per dose ratio (L/D ratio), free fraction, and carbamazepine-10,11-epoxide/CBZ level ratio (CBZE/CBZ). Statistical significant negative correlation was observed between L/D ratio and ZNS daily dose. Despite alteration in the L/D ratio, the free fraction of CBZ was unaltered in combination with ZNS. To assess the effect of addition of ZNS on CBZ metabolism, the CBZE/CBZ level ratio was compared; this ratio showed a tendency to increase with increase in ZNS dose. Consequently, ZNS is considered to have a significant effect on CBZ metabolism but not on protein binding.

Improved therapeutic monitoring of drug interactions in epileptic children using carbamazepine polytherapy.

Drug interactions in epileptic children with carbamazepine (CBZ) polytherapy were investigated by analysis of total and free CBZ and its metabolites simultaneously. Heteroinduction effects of CBZ metabolism by other antiepileptic drugs (AEDs), including phenytoin (PHT), phenobarbital (PB), or primidone (PRM), were clearly demonstrated. Serum CBZ level/dose ratios in patients taking CBZ plus other AEDs were decreased while CBZ-10,11-epoxide (CBZ-E) and trans-10,11-dihydroxy-10,11-dihydro-CBZ (CBZ-H) concentrations were significantly increased compared to patients with CBZ alone. Concentration ratios of CBZ-H/CBZ and CBZ-E/CBZ were also significantly higher in patients taking CBZ plus other AEDs. Interactions between CBZ and valproic acid (VPA) involved both protein binding displacement and metabolic inhibition. Patients taking CBZ plus VPA showed significantly increased free fractions of CBZ and CBZ-E and substantially increased serum CBZ-E concentrations and CBZ-E level/dose ratios, while CBZ-H/CBZ-E concentration ratios were decreased compared with patients on CBZ alone. Since this approach investigates the in vivo relationship between substrates and products of the enzymes involved in CBZ biotransformation (the ratios between CBZ and its metabolites), detailed information about the activities of the enzymes may be obtained. This approach appears to be a practical way to improve the monitoring of CBZ metabolism influenced by various physiological or pathological conditions and achieve a better understanding of the drug interactions under different drug regimens (coadministered inhibitor or inducer). This principle may also be adopted for other drugs with similar metabolic characteristics.

Determination of Total and Free Carbamazepine and the Principal Metabolites in Serum by High-Performance Liquid Chromatography with Photodiode-Array Detection

A precise and accurate high-performance liquid chromatography (HPLC) method has been established for the simultaneous analysis of carbamazepine (CBZ), carbamazepine-10,11-epoxide (CBZ-E), and trans-10,11-dihydroxy-10,11-dihydro-CBZ (CBZ-H) in serum samples and their ultrafiltrates. CBZ and its metabolites are eluted in a 3-microM ODS-Hypersil column (250 x 2 mm) at a column temperature of 40 degrees C. The mobile phase is a mixture containing potassium phosphate buffer-acetonitrile-methanol (110:50:30, vol/vol/vol) at a flow rate of 0.2 ml/min. Signals are monitored by a photodiodearray detector with a main sample wavelength of 215 nm and a bandwidth of 10 nm. Coefficients of variation (CVs) for within- and between-day are within 5%, with the recovery rates ranging from 98.16 to 104.64%. This method has the necessary sensitivity and linearity for routine therapeutic monitoring of both total and free CBZ and its principal metabolites. Total serum concentrations of CBZ, CBZ-E, and CBZ-H obtained from 55 epileptic children were 12.58 +/- 4.42, 2.45 +/- 1.22, and 5.83 +/- 3.17 (mean +/- SD, micrograms/ml), respectively. Levels of free CBZ, CBZ-E, and CBZ-H were 2.59 +/- 0.93, 1.05 +/- 0.57 and 3.73 +/- 1.87, respectively. Free fractions of CBZ, CBZ-E, and CBZ-H were 20.98 +/- 4.34, 42.63 +/- 8.21, and 65.41 +/- 7.80%, respectively. CBZ-H and CBZ-E had larger CVs than did CBZ (54.34 and 49.75 vs 35.15%, respectively, for total levels, and 50.31 and 54.46 vs 36.22%, respectively, for free levels), as well as higher free fractions. Determination of both total and free concentrations and free fractions of CBZ and its metabolites, as well as their ratios, should provide additional needed information for therapeutic drug monitoring of CBZ.

ANALYSIS OF THE FACTORS INFLUENCING ANTI-EPILEPTIC DRUG CONCENTRATIONS-CARBAMAZEPINE

The factors that influence carbamazepine (CBZ) serum concentrations and level:dose ratios were evaluated retrospectively on 83 consecutive routine CBZ determinations from chronically treated epileptic patients. The total level:dose ratio was lower when the drug was given in combination with phenytoin or with phenytoin plus other anti-epileptic drugs (AED) than when CBZ was given alone. The free level:dose ratio was also decreased during concomitant treatment with phenytoin. In spite of alterations to both level:dose ratios, the free fraction of CBZ was unaltered when in combination with other AEDs. There were statistically significant correlations between the CBZ dose (mg/kg/day) and the serum levels:total (r = 0.372, P less than 0.001) and free (r = 0.335, P less than 0.005). However, the wide scatter in the CBZ serum levels at each given dose were such that the relationships had no predictive values. Stepwise multivariate regression analysis (MVR) was also performed. A comparison of the normalized regression coefficients indicated that the CBZ dose, PHT dose, the time elapsed from the previous dose, the albumin concentration and the triglyceride levels were important factors which influence the CBZ total serum level. Similar analysis for the CBZ free serum level, gave a multiple correlation coefficient of 0.838, which indicates that 70.3% of the variance was explained by nine independent variables. The major factors that significantly affected the free serum level of CBZ were CBZ dose, PHT dose, the time elapsed from the previous dose and the triglyceride and albumin levels. Our observations indicate that it may be possible to predict reliably the CBZ serum concentrations for individual patients before institution of CBZ therapy.

Variations in protein binding of drugs in plasma and serum.

The free fractions of five drugs were determined in plasma treated with EDTA and in serum. The free fractions of phenytoin, sodium valproate, and phenobarbital in serum were significantly higher than in plasma (P less than 0.05). In contrast, the free fractions of carbamazepine and theophylline were significantly lower in serum than in plasma. Although we observed minor differences in the protein patterns obtained with two-dimensional electrophoresis, these did not have an important influence on protein binding. No significant differences were observed between plasma and serum in the physiological data, except for pH. Binding of these drugs by protein was shown to be pH-dependent. We conclude that serum may be better than plasma for determination of the free fraction, because the free fraction in plasma is affected by anticoagulants such as EDTA salts and heparin.

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.

Salivary concentrations and plasma protein binding of carbamazepine and carbamazepine 10,11-epoxide in epileptic patients.

1. The relationships between saliva, free and total plasma concentrations of carbamazepine (CBZ) and carbamazepine 10,11-epoxide (CBZ-EP) were studied in 24 chronically medicated epileptic patients. Four patients were taking CBZ alone, while 20 were taking one or more additional anticonvulsant drugs. 2. The free fraction of CBZ in plasma ranged from 0.19 to 0.33 (mean 0.24) while the saliva:plasma (S:P) concentration ratios ranged from 0.20 to 0.35 (mean 0.27). The free fraction of CBZ-EP in plasma ranged from 0.16 to 0.50 (mean 0.32), while the S:P ratios ranged from 0.14 to 0.70 (mean, 0.43). The plasma protein binding and S:P ratios of these compounds appeared to be independent of age, sampling time and concurrently administered anticonvulsant drugs. 3. Significant linear relationships between saliva and total plasma concentrations and between saliva and free plasma concentrations were observed for both compounds (P less than 0.001). However, salivary concentrations of CBZ and CBZ-EP were significantly more reliable as predictors of their respective free plasma concentrations than of their respective total plasma concentrations (P less than 0.01). 4. It is concluded that measurement of CBZ and CBZ-EP in the saliva of chronically medicated epileptic patients provides a more reliable estimate of the pharmacodynamically active, free concentrations of these compounds in plasma.