LIGAND-EXCHANGE CHROMATOGRAPHY FOR THE CHIRAL SEPARATIONS OF OFLOXACIN ENANTIOMERS

Acta Poloniae Pharmaceutica ñ Drug Research, Vol. 74 No. 6 pp. 1659ñ1665, 2017 ISSN 0001-6837 Polish Pharmaceutical Society LIGAND-EXCHANGE CHROMATOGRAPHY FOR THE CHIRAL SEPARATIONS OF OFLOXACIN ENANTIOMERS ALEKSANDRA CHMIELEWSKA and TOMASZ B CZEK* Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Medical University of GdaÒsk, Hallera 107, 80-416 GdaÒsk, Poland Abstract: The method presented here is a chiral ligand-exchange chromatography (CLEC) with UV detection method for the determination (R)- and (S)-enantiomers of ofloxacin in drug substance and pharmaceutical formulations (tablets) using an elaborated chiral procedure. Spectrophotometric detector wavelength was set at 293 nm. The separation was achieved by using a 150 4.6 mm, 5 µm Phenomenex Luna C-18 column with the mobile phase consisting of methanol-cmpa solution (20 : 80, v/v). The solution of CMPA (chiral mobile phase additives) consisted of CuSO 4 (0.4 mm) mixed with L-phenylalanine (0.6 mm) in acetate buffer (ph 5.23; 20 mm). The flow rate of the mobile phase was set at 0.7 ml/min, and temperature of the analysis equalled 25 O C. Under optimal conditions, baseline separation of two enantiomers was obtained with an enantioseparation factor (α) of 1.35 and resolution (R s ) of 4.87. The calibration curves were linear for both (R)- and (S)-enantiomers of ofloxacin in the concentration range of 0.25-50 µg/ml. The average regression coefficients (R 2 ) equalled 0.9974 and 0.9975 for (R)- and (S)-ofloxacin, respectively. The method provided a high sensitivity and good precision (RSD < 10%). Keywords: enantioseparation, ofloxacin enantiomers, chiral ligand-exchange chromatography, Cu(II)-Lphenylalanine complex, C-18 column Ofloxacin, (±)-9-fluoro-2,3-dihydro-3-methyl- 10-(4-methyl-1-piperazinyl)-7-oxo-7H-pyrido[1,2, 3-de]-1,4-benzoxazine-6-carboxylic acid, is a totally synthetic fluoroquinolone with gyrase-inhibiting action in bacteria. Pharmaceutical research has shown that the antibacterial activity of the S-(-)- isomer is 8ñ128 times higher than that of the R-(+)- isomer (1, 2). Currently, the drug is either marketed as a racemic mixture, i.e., consisting of equal amounts of (R)- and (S)-enantiomers, or only the S- isomer. Therefore, studies on the separation of racemic ofloxacin and its application in pharmaceutical formulations are important. Chiral ligand-exchange chromatography (CLEC) involves the use of a chiral ligand and a suitable transition metal ion in the mobile phase. The optically active counterion produces a diastereoisomer complex during the passage of the racemate through the column. It can then be easily separated on a conventional reversed-phase bonded column (3). CLEC can be applied to compounds possessing electron-donating heteroatoms or π electron donating double bonds as is the case in fluoroquinolone compounds (4). Generally, copper cations are preferred, owing to the rapid formation and good stability of their diastereoisomeric complex. Enantioselective ligand exchange chromatography is a liquid chromatography technique that has provided complete and reliable separation of stereoisomers of the most important classes of natural and synthetic compounds, such as amino acids, hydroxy acids, amino alcohols, and some others (5). HPLC methods based on chiral mobile phase additives (CMPA) are not only efficient in separating racemic analytes but also relatively inexpensive and feasible, in which the chiral ligand-exchange chromatography method plays an important role in the separation of enantiomeric mixtures (6). Few reports on the enantiomeric separation of ofloxacin by the CLEC method have been published (4, 6-11). The efficiency of ligand exchange chromatography is affected by several factors, such as ligand type and concentration, concentration of organic modifier, and temperature. Also, ionic liquids have been used as additives of the mobile phase during the separation of the enantiomers of ofloxacin in the CLEC method. In the result, a reduced tailing and an enhanced resolution were obtained (8). Their use for the * Corresponding author: tbaczek@gumed.edu.pl; phone: +48 58 349 16 33; fax: +48 58 349 16 35 1659

1660 ALEKSANDRA CHMIELEWSKA and TOMASZ B CZEK improvement of ligand-exchange efficiency in chromatography separation remains a rarity. The previous reports concern the determination of the ofloxacin enantiomers by a CLEC in the plasma and urine (12, 13). Furthermore, this method has been used to determine the stereoselectivity of glucuronidation metabolism of ofloxacin optical isomers in rat liver microsomes (14). The latest report refers to the simultaneous enantioseparation of ofloxacin and its related substances using the CLEC method (15). Considering the successful application of ligand-exchange chromatography and the excellent enantioseparation capability of HPLC, the application of the chiral ligand-exchange chromatography method is worthy of attention for further analytic merits. The aim of this study was to develop and validate a simple and rapid HPLC assay for measuring ofloxacin enantiomers in pharmaceutical formulation by using a low concentration of chiral mobile phase additives on a conventional C-18 column. Stereospecificity was achieved in the ligand exchange mode by incorporating chiral reagents directly into the mobile phase. A relatively lower concentration of L-phenylalanine was used as a ligand agent and Cu 2+ as a ligand ion. The solution of A B Figure 1. Representative chromatograms of chiral separation of ofloxacin enantiomers standard solution containing 100 µg/ml of levofloxacin (S(-)-ofloxacin) standard solution containing 100 µg/ml of racemic ofloxacin

Ligand-exchange chromatography for the chiral separations of... 1661 A B Figure 2. Representative chromatograms of pharmaceutical dosage forms an assay sample solution of Tarivid tablets containing 200 mg of (RS)-ofloxacin an assay sample solution of Oflodinex tablets containing 200 mg of (RS)-ofloxacin chiral mobile phase additive (CMPA) consisted of those mentioned ligands in acetate buffer. The effects of different separation conditions, especially such as the ph of the mobile phase and the type of column on enantioseparation, were investigated. The method was validated in compliance with International Conference on Harmonization Guidelines (16). The proposed method was successfully applied to investigate the stereoselectivity of ofloxacin enantiomers in tablet dosage forms. MATERIALS AND METHODS Chemicals and reagents L-phenylalanine, levofloxacin, racemic ofloxacin and cupper (II) sulfate pentahydrate were obtained from Sigma-Aldrich (Saint Louis, MO, USA). Methanol LiChrosolv Æ was provided by Merck (Darmstadt, Germany). Sodium acetate and acetic acid were procured from POCH (Gliwice, Poland). LC water from Milliporeís Milli-Q System was used throughout the study. Instrumentation The chromatographic system consisted with the following components which all were obtained from Knauer (Berlin, Germany). Namely, solvent pump (Mini-Star K-500), column thermostat jet stream 2 plus with injection valve (20 µl loop) (D- 14163), a variable wavelength UV detector (K- 2500) and a computer system for data acquisition (Eurochrom 2000).

1662 ALEKSANDRA CHMIELEWSKA and TOMASZ B CZEK Preparation of stock and standard solutions Stock solutions of levofloxacin and racemic ofloxacin (1 mg/ml) were prepared in methanol. These solutions were stored in the dark under refrigeration at 4 O C. The stability of the standard solutions was checked periodically by injecting a solution of the analyte. A series of standard working solutions with the concentration in the range of 0.5ñ100 µg/ml for racemic ofloxacin were obtained by a further dilution of the stock solution with mixture of methanol and water at a proportion of 20 : 80 (v/v). Working standard solution of levofloxacin was prepared in the same manner. Sample preparation Ten ofloxacin tablets (label claim: 200 mg of racemic ofloxacin/tablet) were accurately weighed, their mean weight were determined, and they were then finely powdered. An amount equivalent to 20 mg was transferred into a 20 ml volumetric flask, added 10 ml of methanol, sonicated for 15 min, diluted to 20 ml with methanol and a 5 ml of the sample taken from this solution was centrifuged at 9000 rpm for 15 min. Then a 3-mL aliquot from supernatant was decanted to another test tube and was centrifuged again at 12 000 rpm for 5 min. A 50 µl aliquot was transferred into the Eppendorf tube Table 1. Parameters of the system suitability (a) and validation of the analytical method. Parameter (S)-ofloxacin (R)-ofloxacin Selectivity factor (α) 1.35 Retention factor (k) 11.29 15.27 Resolution (R s ) 4.87 No. of theoretical plates (N) 5461 5214 Tailing factor (T) 1.002 1.002 Linearity range, µg/ml 0.25 ñ 50 0.25 ñ 50 LOD, µg/ml (b) 0.1 0.1 LOQ, µg/ml (c) 0.25 0.25 Slope (±SD) 2.9316 ( ± 0.056) 2.9306 ( ± 0.056) Intercept (±SD) 0.0532 ( ± 1.278) 0.0536 ( ± 1.288) Correlation coefficient, R (R 2 ) 0.9990 (0.9975) 0.9990 (0.9974) Standard error of the regression (s y/x ) 2.906 2.928 Number of data points (n) 9 9 (a) Required limits α > 1; N > 2000; R s > 2; T < 1.5; (b) LOD- limit of detection; (c) LOQ- limit of quantitation Table 2. Summary of (S)-ofloxacin and (R)-ofloxacin calibration standards (n = 6). Nominal Mean concentration ± SD Precision, as RSD Accuracy, as recovery concentration [µg/ml] [%] [%] [µg/ml] ( * ) (S)-ofloxacin (R)-ofloxacin (S)-ofloxacin (R)-ofloxacin (S)-ofloxacin (R)-ofloxacin 0.25 0.253 ± 0.024 0.252 ± 0.024 9.63 9.68 101.40 100.95 0.5 0.509 ± 0.043 0.505 ± 0.043 8.48 8.56 101.87 100.97 1.25 1.198 ± 0.088 1.208 ± 0.089 7.31 7.33 95.87 96.62 2.5 2.389 ± 0.153 2.395 ± 0.155 6.43 6.46 95.55 95.81 5.0 4.893 ± 0.276 4.896 ± 0.277 5.64 5.66 97.85 97.93 12.5 11.783 ± 0.528 11.799 ± 0.529 4.48 4.48 94.26 94.40 25.0 26.209 ± 0.950 26.192 ± 0.959 3.63 3.66 104.83 104.77 37.5 39.109 ± 0.864 39.148 ± 0.869 2.21 2.22 104.29 104.40 50.0 48.483 ± 0.723 48.468 ± 0.720 1.49 1.49 96.97 96.94 ( * ) the concentration of each ofloxacin enantiomers in the series of standard working solutions for racemic ofloxacin in the range 0.5-100 µg/ml.

Ligand-exchange chromatography for the chiral separations of... 1663 Table 3. Analysis of two ofloxacin commercial pharmaceutical preparations Pharmaceutical Label claim Recovery ± SD [%] y preparation x (mg per tablet) (S)-ofloxacin (R)-ofloxacin Oflodinex tablets a 200 98.93 ± 0.50 98.77 ± 0.53 Tarivid tablets b 200 99.16 ± 0.54 99.13 ± 0.55 x Marketed by: a Polpharma S.A.; b Hoechst Marion Roussel; y Average of three determinations and diluted to 1 ml with the mobile phase. 20 µl of the prepared sample was injected into chromatographic system. The amounts of S(-) and R(+)- ofloxacin in dosage forms were individually calculated using the related linear regression equations. Chromatographic conditions Chromatographic separation was carried out using a Phenomenex Luna C-18 column (5 µm particle size, 150 4.6 mm id.) and Nucleosil 100 C-18 (5 µm particle size, 250 4.0 mm id.) with a mobile phase consisting of methanol-cmpa solution (20 : 80, v/v). The solution of CMPA consisted of CuSO 4 (0.4 mm) mixed with L-phenylalanine (0.6 mm) in acetate buffer (20 mm; ph range of 4.8-5.57). The flow-rate of mobile phase was 0.7 ml/min. Spectrophotometric detector wavelength was set at 293 nm. Chromatograms were performed at 25 O C. In this study, chromatographic retention factors, k were determined taking NaNO 3 peak as a marker of a column dead volume. ph value of acetate buffer was measured before the preparation of the mobile phases. The mobile phases consisting methanol-cmpa solution were vacuum-filtered through 0.45 µm membrane filter and degassed in the ultrasonic bath before use. It is worth mentioning that a new column needs to be rinsed with a large volume of mobile phase (approximately 1 L) before a stable baseline can be reached. However, before running the analysis with the eluent on a modified composition, the analytical column was always cleaned with a water-methanol (30 : 70, v/v) solution and then conditioned by flowing the selected mobile phase through the column for about 12 h with a flow rate of 0.7 ml/min. RESULTS AND DISCUSSION Optimization of chiral separation conditions In this study the chiral separation of ofloxacin enantiomers on the conventional reversed-phase bonded column is based on the stereospecificity of the (R)- or (S)-ofloxacinñCu(II)ñphenylalanine complex. As a result, two kinds of ternary complexes were obtained with different configurations. An adequate concentration of Cu(II) and L-phenylalanine is required in the mobile phase to maintain equilibrium of the two components on the separation column. It was determined that a concentration of around 0.4 mm Cu(II) and 0.6 mm L-phenylalanine allowed for a stable chromatographic baseline and at the same time provided sufficient Cu(II) and L-phenylalanine to satisfy the needs of the column (4). In the process of optimizing the conditions for the separation of ofloxacin enantiomers into account were taken the effect of ph of the mobile phase and type of conventional C18 column. Also, for a balance between good and rapid chromatographic separation of the ofloxacin enantiomers, 20% methanol was selected as the organic modifier (8). In the previous studies, the mobile phase was prepared by dissolving amino acid and copper salt in distilled water and the solution was adjusted to the required ph with phosphoric acid, acetic acid, trifluoroacetic acid (4, 8, 10) or hydrochloric acid (10, 15) and sodium hydroxide (15). Then, the desired percentage of organic modifier was added. In these works, the ph dependence of enantioseparation of ofloxacin was investigated in a ph range of 3.0-5.4 (4, 8, 10). The ph of the CMPA solution value as measured by our team, containing only CuSO 4 (0.4 mm) mixed with L-phenylalanine (0.6 mm) in water was 4.1. Thus in this case, it was impossible to use one of said acids (4, 8, 10) as an adjuster to the higher value ph of CMPA solution. In comparison to the previous study, the novelty of the procedure is associated with the application of acetate buffer as component of the mobile phase and more precisely as a component of CMPA solution. In the study, chromatographic separation was carried out using conventional C-18 column, i.e., Phenomenex Luna and Nucleosil 100 (for details, see the Experimental section), with a mobile phase consisting of methanol-cmpa solution (20 : 80, v/v). The solution of CMPA (chiral mobile phase additive) consisted of CuSO 4 (0.4 mm) mixed with L- phenylalanine (0.6 mm) in acetate buffer (20 mm; ph range of 4.8ñ5.57). The flow rate of the mobile phase was set at 0.7 ml/min, and temperature of the

1664 ALEKSANDRA CHMIELEWSKA and TOMASZ B CZEK analysis equalled 25 O C. The monitoring wavelength was 293 nm and peak areas were used. The highest resolutions on the tested columns were obtained with an acetate buffer at ph 5.23. However, in the case of a column Nucleosil 100 resolution does not exceed the value of 1, and was R s = 0.727. The peaks of ofloxacin enantiomers were fuzzy, and the total analysis time exceeded 2 hours. Reduction of the methanol amount to 14% in the mobile phase, admittedly increased value of the resolution (R s = 1.57), but the retention times of the two enantiomers were significantly lengthened. Therefore, due to the retention times and selectivity, the Phenomenex Luna C-18 column was chosen for further studies. A chromatogram obtained at the optimized conditions for S(ñ)- and R(+)-ofloxacin enantiomers displayed symmetrical peaks, and the substances were separated from the solvent front in the elution order of S(-)- ofloxacin and R(+)-ofloxacin as shown in Figure 1. Under the optimal conditions, baseline separation of two enantiomers was obtained with a enantioseparation factor (α) of 1.35 and resolution (R s ) of 4.87. The system suitability parameters are given in Table 1. Validation of the method The aim of the method validation was to confirm that the present method was suitable for its intended purpose as described in ICH guidelines (16). The described method has been extensively validated in terms of specifity, linearity, accuracy, precision, limits of detection (LOD) and quantification (LOQ) and system suitability. A linear response was obtained for both R(+)- and S(-)-ofloxacin. The complete response function parameters can be found in Table 1. The absorption maximum of ofloxacin was determined at 293 nm. For quantitative determination of (R)- and (S)-ofloxacin, the linear calibration curves were obtained in the range of 0.25 to 50 µg/ml. Calibration curves were constructed using the areas of the chromatographic peak measured at nine increasing concentrations for S(-)- and R(+)-ofloxacin. Coefficients of variation for the lowest concentration of the linear range for both (S) ñ and (R) ñ ofloxacin, do not exceed 10% (amounted to 9,63% and 9,68%) (see Table 2). The sensitivity of the method was determined with respect to LOD and LOQ. In the present study, the LOD and LOQ were calculated for the calibration graphs of ofloxacin enantiomers as three and ten times of the baseline noise level for LOD and LOQ, respectively. The LOD and LOQ values of the developed method are presented in Table 1. After validation, the developed method have been applied to pharmaceutical dosage forms containing ofloxacin. The system suitability tests are an integral part of a liquid chromatographic method, and they were used to verify that the proposed method was able to produce good resolution between the peaks of interest with high reproducibility (17). The system suitability was determined by making six replicate injections from freshly prepared standard solutions. The results of the system suitability test were shown in Table 1. According to the results presented, the proposed method fulfils these requirements within the accepted limits. Application of the validated method to pharmaceutical products On the basis of above results, the proposed method was applied to the determination of S(-)-and R(+)-ofloxacin in tablet dosage forms. The representative chromatograms obtained from the analysis of ofloxacin enantiomers in tablet samples are shown in Figure 2. The mean percentage recoveries obtained after three repeated experiments were found between 98.77 and 99.16 (see Table 3), indicating that the results are accurate and precise and there is no interference from the common excipients used in the pharmaceutical dosage forms. CONCLUSION High accuracy and reproducibility were confirmed in the study and the suitability of the method for testing of pharmaceutical formulations (tablets from two producers) of ofloxacin was discussed. The proposed methodology can be useful for the quality evaluation of ofloxacin enantiomers. The novelty of the procedure is associated with the application of acetate buffer as component of the mobile phase for the separation of ofloxacin enantiomers significantly increasing α and R s values. This method is suitable for the routine analysis of both ofloxacin (S)- and (R)- enantiomers in bulk of drug substance and pharmaceutical formulations. Acknowledgment This project was supported by the Ministry of Science and Higher Education of the Republic of Poland, from the quality ñ promoting subsidy, under the Leading National Research Centre (KNOW) programme for the years 2012-2017. Conflict of interest The authors have declared no conflict of interest.

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