Automated high-pressure liquid chromatographic assay for antiepileptic drugs and their major metabolites by direct injection of serum samples

We present a fully automated high-pressure liquid chromatographic (HPLC) assay for simultaneous quantitation of five antiepileptic drugs [ethosuximide (ES), primidone, phenobarbital, phenytoin, and carbamazepine] and their major metabolites [phenylethylmalondiamide (PEMA), carbamazepine-10,11-dihydro-10,11-diol (CBZD), and carbamazepine-10,11-epoxide (CBZE)] in serum samples without sample pretreatment. Two other metabolites, p-hydroxyphenobarbital (HPB) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH), can be separated as well. Interference with serum constituents prevents a quantitative determination. Serum samples are injected onto a pre-column, the biological matrix is eluted with water, and the compounds of interest are subsequently separated on an analytical column. The anticonvulsants are eluted at 60 degrees C with a mobile phase containing acetonitrile:water (21:80 by vol) at a flow rate of 1.0-2.0 ml/min (flow-step gradient). The eluted compounds are monitored at 200 nm. Each analysis requires about 30 min. Analytical recoveries varied from 90 to 98%. Within-day coefficients of variation (CVs) ranged from 0.9 to 4.9%, between-day CVs from 2.8 to 7.6%. The lower limit of detection for all drugs is 0.2 microgram/ml serum, with a sample size of 100 microliters. At therapeutic concentrations we achieved baseline separation for all eight compounds, and there was no interference from other drugs.     

Measurement of carbamazepine and its main biotransformation products in plasma by HPLC.

We present a method that permits the simultaneous analysis of carbamazepine (CBZ) and its major biotransformation products, carbamazepine-10,11-epoxide (CBZ-E) and carbamazepine-10,11-dihydroxide (CBZ-diOH), in plasma samples. The method consists of plasma extraction in alkaline medium with NaCl added using chloroform-ethyl acetate (1:1, v/v) and later purification with n-hexane. The samples were submitted to reversed-phase chromatography (RP-18) using acetonitrile-water (3:7, v/v) as the mobile phase and detection at 220 nm. Recoveries of 62.0, 99.9, and 105.4% were obtained for CBZ-diOH, CBZ-E, and CBZ, respectively, with sensitivities of 0.32 micrograms/mL for CBZ-E and CBZ-dOH and of 0.64 micrograms/mL for CBZ. The method proved to be specific, thus permitting measurements in situations of drug combinations.     

High-speed simultaneous determination of nine antiepileptic drugs using liquid chromatography-mass spectrometry

Therapeutic drug monitoring of antiepileptic drugs (AEDs) is important and widely practiced. However, simultaneous AED assays usually concentrate only on old or new AEDs. A new simultaneous assay was developed to monitor both older and newer AEDs in the same sample. This assay measures zonisamide (ZNS), lamotrigine (LTG), topiramate (TPM), phenobarbital (PB), phenytoin (PHT), carbamazepine (CBZ), carbamazepine-10,11-diol (CBZ-Diol), 10-hydroxycarbamazepine (MHD), and carbamazepine-10,11-epoxide (CBZ-E). Sample pretreatment consisted of a single solid-phase extraction (SPE) for all AEDs in a 100-microL plasma sample. HPLC separation was achieved on a Shimdazu Shimpack XR-ODS (4.6 id x 50 mm, 2.2-mum) column with a gradient mobile phase of acetate buffer, methanol, acetonitrile, and tetrahydrofuran. Four internal standards were used. Detection was achieved by atmospheric pressure chemical ionization mass spectrometry (APCI-MS) in selected-ion monitoring (SIM) mode with constant polarity switching. High recovery (88%-96%) was obtained for all compounds by SPE. Linearity was observed throughout an 80-fold concentration range, with correlation coefficient (r) values higher than 0.99 for all AEDs. For the standards, the accuracy ranged from 89.3% to 111.8%. The within-run coefficient of variation (CVw) value was < or =9.7%, the between-run coefficient of variation (CVb) was < or =16.2%, and the total variability (CVt) was < or =16.8%. For the quality controls (QCs), accuracy ranged from 89.3% to 108.8%, CVw was < or =9.6%, CVb was < or =14.1%, and CVt was < or =15.1%. The correlation r values for comparison of this assay with existing validated assays in our laboratory (GC-MS or LC-MS) were 0.95, 0.91, 0.87, 0.95, and 0.95 for PHT, LTG, CBZ, CBZ-E, and CBZ-Diol, respectively.     

Carbamazepine and its major metabolites in plasma: a summary of eight years of therapeutic drug monitoring

For a period of 8 years, carbamazepine (CBZ) and its two major metabolites carbamazepine-10,11-epoxide (CBZE) and 10,11-dihydroxy-carbamazepine (CBZH) were monitored in plasma of black and white patients visiting outpatient epilepsy clinics. Mean CBZ, CBZE, and CBZH levels found in the black group (n = 451) were found to be significantly lower (p less than 0.0005) than in white patients (n = 2,225). The ratio of CBZE to CBZ levels was found to be significantly lower (p less than 0.0005) for blacks, with CBZ levels within the therapeutic range (5-10 micrograms/ml); the ratio of CBZH to CBZ levels was higher (p less than 0.0005), while there was no significant difference in the ratio of CBZH to CBZE between the two groups. This may indicate a difference in the monooxygenase enzyme activities, whereas the epoxide hydrase activity is probably the same for both groups. A poor linear correlation exists between CBZH and CBZ levels in both groups (for blacks, r = 0.614, n = 451; for whites, r = 0.338, n = 2,225); the correlation is better between CBZE and CBZ (for blacks, r = 0.745; for whites, r = 0.553) and good between CBZH and CBZE (for blacks, r = 0.752; for whites, r = 0.710). Abnormal ratios of CBZH to CBZE were found to be useful indicators of noncompliance in difficult cases presenting with therapeutic levels of CBZ.     

Effects of carbamazepine on hepatic glutathione level in rats and determination of carbamazepine and its epoxide metabolite in plasma by HPLC

We investigated whether carbamazepine, which is known to be metabolized to an electrophilic epoxide derivative in the body, causes any decrease, analogous to the action of epoxides, of hepatic glutathione (GSH) level in rats. Carbamazepine was administered to rats and liver GSH levels were determined spectrophotometrically. Neither a single low nor repeated low doses (30 mg/kg) of carbamazepine (CBZ) produced a statistically significant difference in GSH levels relative to controls. A single high dose of CBZ (100 mg/kg) produced a large and significant decrease relative to control (GSH level 3.82 +/- 0.64 vs 6.54 +/- 0.45 mumol GSH/g liver). CBZ and its metabolite carbamazepine-10,11-epoxide were determined in plasma by HPLC after the high dose of carbamazepine administration. The concentrations of carbamazepine and carbamazepine-10,11-epoxide were 18.9 +/- 2.9 micrograms/ml and 10.7 +/- 2.8 micrograms/ml, respectively.     

Dual effects of carbamazepine-phenytoin interaction

By intrapatient comparison at constant phenytoin (PHT) dose, the effect of increased carbamazepine (CBZ) dose was studied in 32 epileptic outpatients treated with a combination of PHT and CBZ. The mean PHT plasma concentration, as well as the concentration/dose ratio for PHT, became significantly higher secondary to increased doses of CBZ (14.1 +/- 3.5 vs. 19.3 +/- 3.6 micrograms/ml and 2.8 +/- 1.0 vs. 3.9 +/- 1.4 micrograms/ml plasma per milligram/kilogram daily dose, respectively; p less than 0.001). Concomitantly, in spite of CBZ dose higher by 17.6%, the CBZ concentration increased by only 6.4%, and the CBZ concentration/dose ratio actually decreased by 10%. In contrast, by intrapatient comparison at constant CBZ dose, the effect of reduced PHT dose on CBZ was studied in 22 patients. The mean CBZ plasma concentration as well as the concentration/dose ratio for CBZ appeared significantly higher, with a concomitant reduction of PHT (6.7 +/- 1.6 vs. 8.6 +/- 1.6 micrograms/ml and 0.37 +/- 0.1 vs. 0.49 +/- 0.2 micrograms/ml plasma per milligram/kilogram daily dose, respectively; p less than 0.001). This simultaneous dual effect--inhibition of PHT metabolism by CBZ and induction of CBZ metabolism by PHT--can result in PHT intoxication along with a fall in CBZ plasma concentration to a subtherapeutic range. This effect may be avoided or reduced if the PHT concentration is adjusted to approximately 13 micrograms/ml before CBZ is added or increased.     

Carbamazepine-phenytoin interaction: elevation of plasma phenytoin concentrations due to carbamazepine comedication

By intrapatient comparison at constant phenytoin (PHT) dose, the effect of carbamazepine (CBZ) comedication on PHT was studied in a group of 24 epileptic outpatients. In half of the patients with steady-state PHT plasma concentration, a significant increase of this concentration was noted after CBZ was added to their regimen. Twenty percent showed clinical manifestations of acute drug toxicity initially thought to be CBZ related. The mean PHT plasma concentration for the 12 patients (22.7 +/- 5.64 micrograms/ml) as well as concentration/dose ratio for PHT (4.61 +/- 1.65 micrograms/ml plasma per mg/kg/day dose) was significantly higher (p less than 0.001) with concomitant administration of CBZ than when PHT was given alone: PHT concentration, 12.54 +/- 3.93 micrograms/ml and PHT concentration/dose ratio, 2.52 +/- 0.78 micrograms/ml plasma per mg/kg/day dose. Those patients with higher PHT plasma concentrations seem to be at higher risk of PHT toxicity due to CBZ comedication.     

Simultaneous determination of phenytoin,carbamazepine, and 10,11-carbamazepine epoxide in human plasma by high-performance liquid chromatography with ultraviolet detection

The Bioanalytical Chemistry Department at the Madison facility of Covance Laboratories, has developed and validated a simple and sensitive method for the simultaneous determination of phenytoin (PHT), carbamazepine (CBZ) and 10,11-carbamazepine epoxide (CBZ-E) in human plasma by high-performance liquid chromatography with 10,11 dihydrocarbamazepine as the internal standard. Acetonitrile was added to plasma samples containing PHT, CBZ and CBZ-E to precipitate the plasma proteins. After centrifugation, the acetonitrile supernatant was transferred to a clean tube and evaporated under N₂. The dried sample extract was reconstituted in 0.4 ml of mobile phase and injected for analysis by high-performance liquid chromatography. Separation was achieved on a Spherisorb ODS2 analytical column with a mobile phase of 18:18:70 acetonitrile:methanol:potassium phosphate buffer. Detection was at 210 nm using an ultraviolet detector. The mean retention times of CBZ-E, PHT and CBZ were 5.8, 9.9 and 11.8 min, respectively. Peak height ratios were fit to a least squares linear regression algorithm with a 1/(concentration)² weighting. The method produces acceptable linearity, precision and accuracy to a minimum concentration of 0.050 μg ml⁻¹ in human plasma. It is also simple and convenient, with no observable matrix interferences.     

Possible relationships between plasma carbamazepine-10,11-epoxide levels and antimanic efficacy and side effects in patients with schizoaffective disorde

We examined the relationships of plasma levels of carbamazepine (CBZ) and its two major metabolites, carbamazepine-10,11-epoxide (CBZ-E) and carbamazepine-10,11-diol (CBZ-D), with antimanic efficacy and side effects in patients with schizoaffective disorder. Positive relationships were found among plasma concentrations of CBZ-E and the degree of clinical improvement and side effects, whereas neither plasma CBZ nor CBZ-D levels were correlated with the degree of clinical improvement or side effects. Copyright 2000 John Wiley & Sons, Ltd.     

Simultaneous determination of zonisamide, carbamazepine and carbamazepine-10,11-epoxide in infant serum by high-performance liquid chromatography

This study developed a simple method for the simultaneous determination of zonisamide (ZNS), carbamazepine (CBZ) and its active metabolite, carbamazepine-10,11-epoxide (CBZE) in infant serum using reversed-phase high-performance liquid chromatograph (HPLC). The method involves a single-step protein precipitation procedure that uses no solid-phase or liquid-liquid extraction. The HPLC separation was carried out on a Cadenza CD-C18 column (3 microm, 4.6 mm x 150 mm) with potassium phosphate buffer (pH 4.6; 25 mM)-methanol-acetonitrile (65:20:15 (v/v/v)) as a mobile phase at a 1.0 ml/min flow rate: ZNS was detectable using a UV detector at 235 nm, and both CBZ and CBZE were at 215 nm. The quantification limits were established in accordance with each therapeutic range at 2.5 microg/ml for ZNS, 0.5 microg/ml for CBZ, and 0.25 microg/ml for CBZE. The respective coefficients of variation were 1.3-6.0% and 2.2-7.7% for the intra- and inter-assay.     

Effect of four-day treatment with carbamazepine at different dose levels on microsomal enzyme induction, drug metabolism and drug toxicity

The effect of intraperitoneal injections of 0, 30, 60 and 100 mg/kg of carbamazepine (CBZ), twice a day for 4 days, was studied in 4 groups of 6 male Sprague-Dawley rats per group to evaluate its hepatic enzymatic induction, toxicity and metabolism. Rats were sacrificed on the fifth day and the urines of the last 24 hours were collected. While the activities of hepatic microsomal aminopyrine N-demethylase and epoxide hydratase tended to increase with the dose of CBZ, the cytochrome P-450 content and the activity of aniline hydroxylase however reached a maximum at 60 mg/kg. The percentage of the administered daily dose of CBZ excreted as unchanged CBZ in the urine increased considerably with the dose, while that of metabolites such as carbamazepine-10,11-epoxide (CBZ-E), trans-10,11-dihydrodihydroxycarbamazepine (TDC), and thioethers (T) did not markedly change. These data not only corroborate a maximum in enzyme induction but also suggest a saturation of the induced hepatic enzymes. Urinary T and TDC, representing more than 50% and less than 10%, respectively of the total amount recovered, tend to demonstrate that the glutathione conjugation with the intermediates of CBZ leading to the formation of higher mercapturates could be more important than the epoxide-diol pathway for the metabolism of CBZ under conditions of repeated dosing.     

Simplified high performance liquid chromatographic method for the determination of clonazepam and other benzodiazepines in serum

A convenient and sensitive high performance liquid chromatographic method was developed for determination of clonazepam in serum using a C-18 reverse-phase column, and mobile phase consisting of a 50:35:15 mixture by volume of pH 6.0 phosphate buffer:methanol:acetonitrile. Quantitation was performed at 313 nm with flunitrazepam as the internal standard. Using 1 ml of serum for extraction, the assay is linear for clonazepam concentrations between 10 and 250 ng/ml. The relative recovery averaged 100.3%, and the coefficient of variation for between-day and within-day assays was less than 7%. A simple modification permits analysis of 200 microliter of serum, with little loss of precision and with a detection limit of 20 ng/ml. Only three drugs tested (nitrazepam, methaqualone, and norchlordiazepoxide) interfered with the assay, and none are likely to be used therapeutically with clonazepam. Importantly, carbamazepine and carbamazepine-10,11-epoxide do not interfere under the conditions of the assay. The method is equally suitable for the determination of nitrazepam. By adjusting the mobile phase so that volume ratios of phosphate buffer, methanol, and acetonitrile are 45:35:20, and using nitrazepam as the internal standard, seven other benzodiazepines (demoxepam, oxazepam, chlordiazepoxide, norchlordiazepoxide, temazepam, diazepam, and nordiazepam) can be resolved at 254 nm.     

Serum concentrations of carbamazepine and its epoxide and diol metabolites in epileptic patients: the influence of dose and comedication

The influence of carbamazepine (CBZ) dose, CBZ preparation used, comedication (phenobarbital, phenytoin, primidone, valproate), and factors such as age, weight, and sex on the concentration of CBZ and its metabolites carbamazepine-10,11-epoxide (CBZ-epoxide) and 10,11-dihydro-10,11-dihydroxy-carbamazepine (CBZ-diol) in serum was investigated. A non-linear regression analysis using the data of 609 patients shows that other anti-epileptic drugs can influence the metabolism of CBZ in various ways. The mean serum concentration of CBZ is lower when the drug is given in combination with phenytoin (59.4%), primidone (58.2%), phenobarbital (65.7%), and valproate (83.0%) than when CBZ is given alone (100%), whereas the mean concentration of CBZ-epoxide is increased by valproate (144.8%), by primidone (118.5%), and by a combination of the latter (167.4%). The CBZ-diol concentrations are also increased during concomitant treatment with the other antiepileptic drugs. Our results indicate a nonlinear relationship between the CBZ dose and the CBZ concentration, but a linear relationship between the CBZ dose and the CBZ-diol concentration.     

Simultaneous determination of lamotrigine, phenobarbitone, carbamazepine and phenytoin in human serum by high-performance liquid chromatography

A simple reversed-phase high-performance liquid chromatography (HPLC) method was developed for the simultaneous estimation of the antiepileptic drugs (AEDs) lamotrigine (LTG), phenobarbitone (PB), carbamazepine (CBZ) and phenytoin (PHT) in human serum. The procedure involves extraction of the AEDs by mixing 200 microl of serum with 200mul of acetonitrile containing 10 microg/ml of pentobarbitone as internal standard (IS). After centrifugation, 10 microl of the supernatant was injected onto a NOVA PAK C-18 column (250 mm x 4.6mm, 5 microm Hypersil ODS) and eluted with a mobile phase consisting of phosphate buffer (10 mM)-methanol-acetonitrile-acetone in the ratio of 55:22:12:11 (v/v) adjusted to pH 7.0. A UV detector set at 210 nm was employed for detection. The AEDs were well resolved from the human serum constituents and the internal standard. The method can quantify LTG, PB, CBZ, and PHT at concentrations as low as 0.2 microg/ml. The method was quantitatively evaluated in terms of linearity, accuracy, precision, recovery, selectivity, sensitivity, and specificity. The method is simple, convenient, and suitable for the analysis of AEDs from human serum.     

Quantification of carbamazepine, carbamazepine-10,11-epoxide, phenytoin and phenobarbital in plasma samples by stir bar-sorptive extraction and liquid chromatography

A sensitive and reproducible stir bar-sorptive extraction and high-performance liquid chromatography-UV detection (SBSE/HPLC-UV) method for therapeutic drug monitoring of carbamazepine, carbamazepine-10,11-epoxide, phenytoin and phenobarbital in plasma samples is described and compared with a liquid:liquid extraction (LLE/HPLC-UV) method. Important factors in the optimization of SBSE efficiency such as pH, extraction time and desorption conditions (solvents, mode magnetic stir, mode ultrasonic stir, time and number of steps) assured recoveries ranging from 72 to 86%, except for phenytoin (62%). Separation was obtained using a reverse phase C(18) column with UV detection (210nm). The mobile phase consisted of water:acetonitrile (78:22, v/v). The SBSE/HPLC-UV method was linear over a working range of 0.08-40.0mugmL(-1) for carbamazepine, carbamazepine-10,11-epoxide and phenobarbital and 0.125-40.0mugmL(-1) for phenytoin, The intra-assay and inter-assay precision and accuracy were studied at three concentrations (1.0, 4.0 and 20.0mugmL(-1)). The intra-assay coefficients of variation (CVs) for all compounds were less than 8.8% and all inter-CVs were less than 10%. Limits of quantification were 0.08mugmL(-1) for carbamazepine, carbamazepine-10,11-epoxide and phenobarbital and 0.125mugmL(-1) for phenytoin. No interference of the drugs normally associated with antiepileptic drugs was observed. Based on figures of merit results, the SBSE/HPLC-UV proved adequate for antiepileptic drugs analyses from therapeutic levels. This method was successfully applied to the analysis of real samples and was as effective as the LLE/HPLC-UV method.     

高效液相色谱法同时测定人血浆中卡马西平、奥卡西平及其活性代谢产物的浓度和临床应用

目的：建立同时测定人血浆中卡马西平（CBZ）、10,11-环氧卡马西平（CBZE）、奥卡西平（OXC）和单羟基卡马西平（MHD）浓度的高效液相色谱法,并将其应用于临床中卡马西平、奥卡西平及其活性代谢产物血药浓度的测定。方法：以苯巴比妥为内标,血浆经乙醚-二氯甲烷（2∶1）提取。色谱柱为WondaSil C₁₈柱（150 mm×4.6 mm,5 μm）,流动相为甲醇∶水=50∶50,柱温30℃,流速1.0 mL·min^-1,检测波长215 nm,进样量20 μL。结果：卡马西平、10,11－环氧卡马西平、奥卡西平和单羟基卡马西平标准曲线范围分别为0.1~20,0.05~10,0.05~20,0.2~50 mg·L^-1,最低检测限分别为0.1,0.05,0.05,0.2 mg·L^-1,日内、日间精密度均小于10%。结论：该方法灵敏准确,简便快速,适用于卡马西平、奥卡西平及其代谢产物血药浓度检测。     

Charcoal hemoperfusion in the treatment of two cases of acute carbamazepine poisoning

Charcoal hemoperfusion is effective in the treatment of acute carbamazepine (CBZ) poisoning, its efficacy depending on the metabolic capacity of the patient involved. This was assessed in two cases of CBZ poisoning in which CBZ and its metabolite carbamazepine-10,11-epoxide (CBZO) were monitored. One patient had not been treated with CBZ or other enzyme-inducing drugs before the overdose ingestion. The CBZO/CBZ plasma concentration ratio of this patient was 0.15 +/- 0.01 (mean +/- s.d.), indicating a normal metabolic capacity. The average clearance values obtained with the Haemocol were 85 ml/min for CBZ and 81 ml/min for CBZO. The other patient had been on long-term treatment with anticonvulsive drugs before. The CBZO/CBZ ratio was 1.58 +/- 0.16, indicating a high metabolic capacity and, consequently, a high intrinsic clearance. The average clearances obtained with the Adsorba 300 C were 129 ml/min for CBZ and 133 ml/min for CBZO. Saturation of a charcoal column can occur during a four hours treatment, in particular if the plasma CBZO concentration is high. CBZ and CBZO were also monitored in erythrocytes. The erythrocyte/plasma concentration ratios of CBZ were 0.90 +/- 0.11 (mean +/- s.d.) and 1.36 +/- 0.10. CBZO was 30-40 per cent more concentrated in erythrocytes than was CBZ. The erythrocyte/plasma concentration ratios of CBZO were 1.36 +/- 0.10 and 1.80 +/- 0.23.     

A new herb-drug interaction of Polygonum cuspidatum, a resveratrol-rich nutraceutical, with carbamazepine in rats

Carbamazepine (CBZ), an antiepileptic with narrow therapeutic window, is a substrate of CYP 3A which metabolizes CBZ to carbamazepine-10,11-epoxide (CBZE), an active metabolite. This study investigated the acute and chronic effects of Polygonum cuspidatum (PC), a resveratrol‐rich nutraceutical, on the pharmacokinetics of CBZ in rats and the underlying mechanisms. Rats were orally administered CBZ (200 mg/kg) alone and coadministered with a single dose and the 7th dose of PC (2 g/kg) in a crossover design. The concentrations of CBZ and CBZE in serum and various tissues were determined by HPLC method. The results showed that PC significantly increased the AUC_0-t of CBZ and CBZE, whereas the formation rate of CBZE was decreased. Tissue analysis showed that the concentrations of CBZ and CBZE in brain, liver and kidney were significantly increased by PC. Cell studies indicated that the efflux function of MRP 2 was inhibited by the serum metabolites of PC. In conclusion, PC markedly increased the systemic exposure and brain concentration of CBZ and CBZE through inhibiting the activities of CYP 3A and MRP 2.     

Factors influencing simultaneous concentrations of total and free carbamazepine and carbamazepine-10, 11-epoxide in serum of children with epilepsy

Various factors that may influence the simultaneous concentration of total and free carbamazepine (CBZ) and carbamazepine-10,11-epoxide (CBZ-E) in serum of 68 children (mean age 11.8 +/- 4.5 years) with epilepsy were assessed. Separation of free and bound drug fractions was achieved by ultrafiltration, and CBZ and CBZ-E concentrations were determined using a sensitive high pressure liquid chromatographic technique. Thirty children were on CBZ monotherapy. Both total CBZ and CBZ-E serum concentrations correlated significantly with their respective free serum concentrations. CBZ was 81 +/- 3% and CBZ-E 63 +/- 9% bound. There was no correlation between the CBZ dose and either CBZ total or free serum concentrations. A statistically significant correlation was, however, observed between CBZ dose and simultaneous CBZ-E total and free concentrations. CBZ total and free concentrations correlated significantly with those of total CBZ-E. A significant negative correlation was observed between age and total (r = -0.49, p less than 0.01) and free (r = -0.43, p less than 0.025) CBZ-E/CBZ ratios. Concomitant drug therapy (phenytoin, phenobarbitone, and sodium valproate) significantly elevated CBZ-E/CBZ ratios.     

Primidone-carbamazepine interaction: clinical consequences

A 15-year old boy, suffering from partial complex seizures, was treated with primidone (PR) and carbamazepine (CBZ). In spite of daily doses in the usual range (PR = 12 mg/kg, CBZ = 30 mg/kg), he was not free from seizures and serum levels of CBZ were remarkably low (4.8 micrograms/ml). A good control of seizures was obtained after gradually stopping treatment with PR. This lead to a substantial increase of CBZ serum levels to a decrease of carbamazepine-10, 11-epoxide levels and a 60% reduction in total CBZ clearance.