Determination of oxcarbazepine and its related substances using UHPLC method with UV detection

A simple and rapid ultra high performance liquid chromatographic (UHPLC) method for the separation and determination of oxcarbazepine and its related substances in tablets, was developed. Chromatographic separation of oxcarbazepine from its related substances (degradation and by-products) was achieved on a reversed phase C18 column (Pinnacle DB C18, RESTEK, 100 x 2.1 mm, 1.9 μm particle size) using gradient program with water (as mobile phase A) and acetonitrile (as mobile phase B), at a flow rate 0.5 mL min and UV detection at 254 nm. The column temperature was 30°C.The method has good selectivity towards oxcarbazepine and its related substances. The accuracy of the method for oxcarbazepine assay, expressed as mean recovery was 101.8 %, and of the method for quantification of degradation products was 95.9 % – 103.3 %. Limit of quantification for oxcarbazepine and its degradation products ranged from 0.60 μg mL to 1.30 μg mL. The linearity range for oxcarbazepine assay was from 2.4 to 3.60 mg mL (R=0.999). Mean value for oxcarbazepine assay from the tablets was 299.92 mg (label claim 300 mg). The method can be used for routine analysis and the quality control of oxcarbazepine drug substance and its formulated products.


Introduction
10,11-dihydro-10-oxo-5H-dibenz[b,f]azepine-5-carboxamide (Oxcarbazepine) is an anticonvulsant and mood stabilizing drug used in the treatment of epilepsy and reduction in anxiety symptoms accompanying the epilepsy seizures.Oxcarbazepine is structurally a derivative of carbamazepine which is also an effective anticonvulsant [1][2][3].Oxcarbazepine is not official in any Pharmacopoeia.Considering the biological significance of oxcarbazepine, several quantitative analytical procedures have been reported for its determination in various matrices [4].In pharmaceutical formulations such methods include spectroscopic [5][6][7][8][9], HPTLC (High Performance Thin Layer Chromatography) [10,11], HPLC (High Performance Liquid Chromatography) [12,13], GC (Gas Chromatography) [14], and voltammetry [15].The process synthesis of oxcarbazepine involves many steps so that many by-products could be carried over to the final product.Also the storage condition and temperature variations may lead to the formation of degradation products in formulations.The fast and reliable method is required for analysis of related substances in oxcarbazepine formulations.The focus of the present study was to develop and accurate, selective and reproducible method for the determination of oxcarbazepine in the presence of low levels of its degradation and byproducts.Examined by products were: CGP 52118, CGP 83596, CGP 16554 and CGP 84426 and degradation products were: CGP26202, GP 47705, CGP18671, G 32883, GP477 3 and GP 51045.
The developed method was UHPLC with gradient elution.The mobile phase was composed of water and acetonitrile without addition of any additives.

Materials and reagents
All reagents were of analytical reagent grade.Gradient grade acetonitrile was obtained from Merck AG, Darmstadt, Germany.Deionized water (EasyPure) was used throughout the study.Diluent was mixture of acetonitrile and water (80:20 V/V).Standard substances of oxcarbazepine and its related substances as well as Trileptal® film tablets, 300 mg label claim were supplied by Novartis Pharma Stein AG, Switzerland.

Apparatus
Experiments were done on the Nexera X2 UPLC modular system composed LC-30 AD pump, an SPD-M 20A diode array detector an SIL-30AC auto-injector, a DGU 20 A5 degasser and CBM 20A system controller (all from Shimadzu, Kyoto, Japan).A reversed phase C 18 (Pinnacle DB C18, 100 x 2.1 mm, 1.9 µm particle size, RESTEK) column was used for separation.Maximum applied pressure was 1000 bar.Chromatographic data were analysed with Shimadzu Lab Solutions software.

Chromatographic conditions
The mobile phase was water (mobile phase A) and acetonitrile (mobile phase B).Before delivering into the system it was filtered through 0.45 µm PTFE filter and degassed using vacuum.The analysis was carried out under gradient conditions using a flow rate of 0.5 mL min -1 at 30⁰C temperature.Gradient conditions are given in Table I.Chromatograms were recorded at 254 nm for both oxcarbazepine and related substances.

Analytical procedure
For testing suitability of chromatographic system test solution (System Suitability Test -SST) consisting of oxcarbazepine and 10 related substances (Fig. 1) was prepared.Concentration of each compound in this solution was approximately 3 µg mL - 1 which corresponds to 0.1 % of working concentration for assay.SST solution was used to determine resolution factor.
For assay of oxcarbazepine in Trileptal® film tablets standard solution was prepared by weighing and dissolving standard substance oxcarbazepine to yield concentration approximately 3 mg mL -1 .
For quantification of related substances mix solution consisting of oxcarbazepine and its degradation products was prepared.Concentration of oxcarbazepine, GP 47703, GP 47705 and GP 51045 were approximately 3 µg mL -1 which corresponds to 0.1 % of working concentration for assay.Concentration of CGP 26202 and CGP 18671 were approximately 6 µg mL -1 which corresponds to 0.2 % of working concentration for assay.Concentration of G 32883 was approximately 9 µg mL -1 which corresponds to 0.3 % of working concentration for assay.
In order to simulate situation in real sample and to prove selectivity of UHPLC method solution of a synthetic mixture of oxcarbazepine and its 10 related substances was made in which oxcarbazepine concentration was 3 mg mL -1 and all related substances were at the level 0.1 % (approximately 3 µg mL -1 ).

Sample preparation
20 tablets were accurately weighed and finally powdered.An accurately weighed portion of the powder equivalent to 300 mg of oxcarbazepine was transferred to a 100 mL volumetric flask.A 70 mL of diluent was added and sonicated for 15 min., than thoroughly shaken for 15 minutes and brought to volume with diluent and filtered.

Method development
During the method development top priority was given to complete separation of oxcarbazepine from its related substances.We required resolution ≥ 1.4 between each pair of peaks.Also, repeatability of peak area should give relative standard deviation ≤ 3 % for each peak in solution for quantification of degradation products (n = 6) and ≤ 1 % for oxcarbazepine in solution for assay.
Molecular structures of oxcarbazepine and its related substances studied in this work are presented in Figure 1.All these materials were subjected to separation by reversed phase UHPLC.We found that separation and resolution were pH independent, so the pH of the mobile phase was not adjusted.Acetonitrile was used as solvent modifier to improve separation.Since we could not met the required quality chromatographic separation using isocratic elution we optimized the gradient elution with water as phase A and acetonitrile as phase B. Several gradient conditions were tried, applying One Factor at Time Approach, in which gradient slope was varied, bearing in mind that total time of analysis should be less than 15 min.The temperature of column was varied between 25⁰C and 40⁰C with 30⁰C giving the best resolution.The flow rate was subjected to optimization with the aim to shorten the total time of analysis without adversely affecting the resolution.Column characteristics and high applied pressure allowed good flow rate at 0.5 ml/min.At this flow rate the retention of oxcarbazepine and related substances is satisfactory so that subsequent analysis was made with flow rate of the mobile phase 0.5 mL min -1 .Injected volume of the analyte was varied from 0.1 µL to 1 µL and obtained peak areas and relative standard deviations were satisfactory with 1 µL of injected volume.A typical chromatogram of a SST solution is given in Figure 2.
The peaks were identified according to their retention times by comparison with co-injected individual standards.Reproducible peak shapes were obtained under the optimum conditions.Therefore the C 18 column choice was good.The retention time (t R ), the relative retention time (RRT), relative response factors and resolutions were determined and recorded in Table II.Standard mixture containing known amounts of degradation products were analysed by UHPLC under optimal conditions.The accuracy of the method was checked for 5 different concentration levels spinning the range 20 % -120 % from specification limits.Defined quantity of each of the degradation product was added to the placebo and chromatographed.Mean recoveries are given in the Table III.

Table III
Accuracy of the method expressed as mean Recovery

Accuracy for oxcarbazepine assay
The accuracy of the method for quantification of drug substance was determined by applying the method to placebo to which known amount of oxcarbazepine corresponding to 80 %, 100 % and 120 % of label claim had been added.At each level five determinations were performed.Mean Recoveries at each level are 102.0%, 101.5 % and 102.0 %, respectively and 101.8 % mean value.

Selectivity
To demonstrate the selectivity of the method the placebo was prepared and spiked with oxcarbazepine and each of the related substances and it was found that the excipients don't interfere either with oxcarbazepine or any of related substances.This indicated that method is selective for separation and determination of oxcarbazepine and its degradation products in film tablets (Figure 3).

Linearity
Calibration graphs (concentration vs. peak area) were constructed at 5 concentration levels for oxcarbazepine and its degradation products.At each concentration level 3 determinations were made.Table IV

Method application
The validated UHPLC method was applied for determination of oxcarbazepine and its degradation products in Trileptal® film tablets, 300 mg (Novartis, Pharmastein AG, Stein, Swiss).Specification limits for assay of oxcarbazepine were 285.0 -315.0 mg/tablet.Shelf-life specification included determination of CGP 26202 (≤ 0.2 %), CGP 18671 (≤ 0.2 %), G 32883 (≤ 0.5 %), other individual (≤ 0.1 %), sum of other degradation products (≤ 0.5 %) and total of degradation products (≤ 1.0 %).Samples are prepared in triplicate and analysed.Typical chromatogram is presented in Figure 4. Mean value, expressed as the percentage of the label claim, was 99.76 % indicating that the amount of oxcarbazepine in the tablets met the requirements (95 % -105 % of the label claim).Amount of degradation produced in sample solution could be calculated either using peak areas of corresponding peaks in quantification solution or using RRT and RRF values given in Table II.In analysed sample of Trileptal® film tablets amount of all related substances were bellow specification limits.

Conclusion
The described gradient reversed phase UHPLC method for the determination of oxcarbazepine and its degradation products in dosage form has been evaluated for accuracy, precision, selectivity, linearity and LOQ.The developed method has an advantage over existing literature methods with respect to the total time of analysis and consumption of organic solvents.It is suitable not only for separation and determination of degradation products but also for monitoring of synthesis by-products and it has applicability in routine analysis of oxcarbazepine.

Figure 2 .
Figure 2. Chromatogram of SST solultion under gradient conditions with water as mobile phase A and acetonitrile as mobile phase B at 30 0 C and UV detection at 254 nm Slika 2. Hromatogram rastvora za proveru pogodnosti sistema u uslovima gradijentnog eluiranja sa dodom kao mobilnom fazom A i acetonitrilom kao mobilnom fazom B na 30 0 C i detekcijom na 254 nm

Table II
Chromatographic parameters of oxcarbazepine and its related substances Tabela II Hromatografski parametri okskarbamazepina i njegovih srodnih supstanci *DP -Degradation product, BP -By-product, DS -Drug substance 2.

2 Accuracy for degradation products
. gives concentration range, linearity equation, correlation coefficient (R 2 ), precision expressed as RSD (Relative Standard Deviation) and LOQ for all compounds.Table IV Linear regression data for determination of oxcarbazepine and its related substances Tabela IV Podaci linearne regresije za metodu za određivanje sadržaja okskarbamazepina i