In vivo determinations of carbamazepine and carbamazepine-10, 11-epoxide binding parameters to serum proteins in monotherapy patients

Pharmacokinetics and Pharmacogenetics of Carbamazepine in Children

Although carbamazepine is one of the oldest anticonvulsant drugs, it is still heavily utilized for treatment of epilepsy in children. The aim of this article was to review the current knowledge about pharmacokinetics and pharmacogenetics of carbamazepine in children. The literature for this review was systematically searched for in the MEDLINE and SCINDEKS databases. Oral bioavailability of carbamazepine in children is about 75-85%, and it is approximately 75-85% bound to plasma proteins. Apparent volume of distribution is 1.2-1.9 l/kg and total clearance between 0.05 and 0.1 l/h/kg. Pharmacokinetics of carbamazepine in children is age and body weight dependent and highly variable due to influence of dosing regimen and co-medication. The current evidence on the importance of pharmacogenetics for carbamazepine efficacy and safety in children supports the association of PXR*1B, HNF4a rs2071197, CYP1A2*1F, ABCC2 1249G>A, and PRRT2 c.649dupC with either pharmacokinetics or pharmacodynamics of carbamazepine. The importance of human leukocyte antigen (HLA) typing for prediction of adverse drug reactions to carbamazepine in children is also confirmed. Both genetic and environmental factors are responsible for shaping pharmacokinetics and pharmacodynamics of carbamazepine in children. To ensure safe and effective use of carbamazepine in this population, physicians should adjust dosing regimen according to existing pattern of genetic and environmental influences.

Effects of poloxamer 407-induced hyperlipidemia on the pharmacokinetics of carbamazepine and its 10,11-epoxide metabolite in rats: Impact of decreased expression of both CYP3A1/2 and microsomal epoxide hydrolase

The pharmacokinetics of carbamazepine (CBZ) and its active 10,11-epoxide metabolite (CBZ-E) were evaluated after intravenous and oral administration of 5 mg/kg CBZ to rats with hyperlipidemia induced by poloxamer 407 (HL rats) and controls. The total area under the plasma concentration-time curve (AUC) of CBZ in HL rats after intravenous administration was significantly greater than that in controls due to their slower non-renal clearance (CL(NR)). This was due to slower hepatic CL(int) for metabolism of CBZ to CBZ-E in HL rats via CYP3A1/2. This result was consistent with a previous study indicating reduced hepatic CYP3A1/2 expression in HL rats. Interestingly, the AUC of CBZ-E was also increased in HL rats, while AUC(CBZ-E)/AUC(CBZ) ratios remained unchanged. These results suggested that further metabolism of CBZ-E to the inactive metabolite trans-10,11-dihydoxyl-10,11-dihydro-CBZ (CBZ-D) via microsomal epoxide hydrolase (mEH) was also slowed in HL rats. The significantly reduced hepatic mRNA level and expression of mEH protein in HL rats compared to controls confirmed the above hypothesis. Similar pharmacokinetic changes were observed in HL rats after oral administration of CBZ. These findings have potential therapeutic implications assuming that the HL rat model qualitatively reflects similar changes in patients with hyperlipidemia. Caution is required regarding pharmacotherapy in the hyperlipidemic state in cases where drugs that are metabolized principally by CYP3A1/2 or mEH and have a narrow therapeutic range are in use.

Human alpha-1-glycoprotein and its interactions with drugs

For about half a century, the binding of drugs to plasma albumin, the "silent receptor," has been recognized as one of the major determinants of drug action, distribution, and disposition. In the last decade, the binding of drugs, especially but not exclusively basic entities, to another plasma protein, alpha 1-acid glycoprotein (AAG), has increasingly become important in this regard. The present review points out that hundreds of drugs with diverse structures bind to this glycoprotein. Although plasma concentration of AAG is much lower than that of albumin, AAG can become the major drug binding macromolecule in plasma with significant clinical implications. Also, briefly reviewed are the physiological, pathological, and genetic factors that influence binding, the role of AAG in drug-drug interactions, especially the displacement of drugs and endogenous substances from AAG binding sites, and pharmacokinetic and clinical consequences of such interactions. It can be predicted that in the future, rapid automatic methods to measure binding to albumin and/or AAG will routinely be used in drug development and in clinical practice to predict and/or guide therapy.

Age-related alteration of carbamazepine-serum protein binding in man

To determine whether biological maturation influences the kinetics of carbamazepine-serum protein binding, the carbamazepine free fraction (%) was investigated in the serum of 66 patients, ranging from 4 to 83 years, with epilepsy or trigeminal neuralgia, treated with carbamazepine alone or carbamazepine in combination with phenytoin, phenobarbital, and/or valproic acid, over a relatively long period. Biochemical parameters such as levels of albumin and non-glycated albumin showed a significant relationship with carbamazepine free fraction (r = -0.521, P < 0.001 for albumin; r = -0.700, P < 0.001 for non-glycated albumin). Non-glycated albumin was more strongly correlated with carbamazepine free fraction. The biochemical parameters showed a significant relationship with age (r =-0.243, P < 0.1 for albumin; r =0.666, P < 0.001 for glycated albumin; r = -0.459, P < 0.001 for non-glycated albumin; r = 0.640, P < 0.001 for carbamazepine free fraction). Glycated albumin (%), non-glycated albumin and carbamazepine free fraction (%) were strongly correlated with age, whereas albumin showed only a weak correlation with age. To evaluate the effects of ageing on carbamazepine-serum protein binding, the patients were divided into three groups according to age: children, 4-15 years; adults, 16-64 years; elderly, 65-83 years. Albumin and non-glycated albumin were much lower, and glycated albumin (%) and carbamazepine free fraction (%) much higher in the elderly group than in the other two groups. The results of this study showed that the major ligand of carbamazepine in the serum was non-glycated albumin, which decreased with age. These observations suggested that in elderly patients, the elevation of free carbamazepine concentrations in the serum caused by reduced non-glycated albumin levels, induces increases in the sensitivity of the pharmacological effects of carbamazepine and the risk of drug interactions.

Effect of glycosylation on carbamazepine-serum protein binding in humans

Effect of glycosylation on carbamazepine-serum protein binding was investigated in vitro using the serum from 24 diabetics and 10 healthy subjects, and in vivo using the serum from 49 patients receiving carbamazepine. In both binding studies, nonglycosylated albumin levels were strongly correlated with the carbamazepine free fraction (%). To evaluate the effect of glycosylation in vivo, the patients were divided into two groups according to glycosylated albumin levels (%): a healthy group (10-15) and a high group (15 and over). The high group had decreased nonglycosylated albumin levels and an increased carbamazepine free fraction. Our results suggest that one should not use total concentrations for the monitoring of serum carbamazepine concentrations, but free concentrations, especially in poorly controlled diabetics.

Comparison of equations for predicting bound serum concentrations of carbamazepine and carbamazepine-10, 11-epoxide after polytherapy in patients with epilepsy

In a previous study, an equation with in vivo population binding parameters of carbamazepine and carbamazepine-10, 11-epoxide (CBZ-E) to serum proteins for the relation between unbound and bound serum concentrations was defined. A review by Pynnönen indicates that the average bound/unbound plasma fraction ratio is 3.0 for carbamazepine and 1.0 for CBZ-E. In this study, the ability of equations with in vivo population binding parameters of the previous study (method 1) or with the average bound/unbound plasma fraction ratio of 3.0 of Pynnönen (method 2) to predict the bound serum carbamazepine concentration was retrospectively evaluated using 85 serum samples from 46 patients with epilepsy taking carbamazepine polytherapy. In 21 serum samples from 16 patients, the ability of these equations to predict bound serum CBZ-E concentration was also determined with in vivo population binding parameters from the previous study (method A) or with the average bound/unbound plasma fraction ratio of 1.0 of Pynnönen (method B). Mean prediction error, mean absolute prediction error (MAE), and root mean squared error (RMSE) were calculated for each method, and these values served as a measure of prediction bias and precision. Method 1 showed a bias to overpredict bound serum carbamazepine. The MAE and RMSE were significantly smaller with method 2 (MAE = 2.4 mumol/L; RMSE = 3.2 mumol/L) than with method 1 (MAE = 4.1 mumol/L; RMSE = 4.8 mumol/L). Method 2 was superior to method 1 in terms of accuracy and precision. For bound CBZ-E prediction, method B had a bias to underprediction. The MAE and RMSE were smaller with method A (MAE = 0.581 mumol/L; RMSE = 0.796 mumol/L) than with method B (MAE = 0.724 mumol/L; RMSE = 0.905 mumol/L). Method A was superior to method B in terms of accuracy and precision.