DRUG FORMULATIONS AND CLINICAL METHODS Selective and Stability-Indicating Methods for the Simultaneous Determination of Mexiletine Hydrochloride and/or Its Related Substance: 2,6-Dimethylphenol TAREK S. BELAL,RIM S. HAGGAG , and RASHA A. SHAALAN University of Alexandria, Faculty of Pharmacy, Pharmaceutical Analytical Chemistry Department, Elmessalah 21521, Alexandria, Egypt Four simple, rapid, sensitive, and selective analytical procedures were developed for determination of mexiletine hydrochloride (MX) and/or its related substance: 2,6-dimethylphenol (DMP). The latter is a synthetic impurity for which a maximum pharmacopeial limit is defined. The first method depends on derivative-ratio spectro- photometry, for which the first-derivative signals of the ratio spectra at 259 nm (Dl = 3 nm) are selected for the determination of MX. The second method is based on the spectrofluorometric measurement of MX in alkaline solution in the presence of 15 mM sodium dodecyl sulfate micellar medium at 292 nm (l Ex 260 nm). The third method is based on liquid chromatographic (LC) separation of MX and DMP on an RP-C8 column with a mobile phase consisting of 50 mM Na 2 HPO 4 –acetonitrile (60 + 40, adjusted to pH 2.4), and quantification of the analytes is achieved with UV detection at 212 nm based on peak area. The fourth method uses the coupling reaction of DMP with 2,6-dibromo- quinone-4-chlorimide (DBQC) in borate buffer to form an intensely colored product that was spectrophotometrically measured using first-derivative amplitudes at 670 nm (Dl = 6 nm) for the determination of DMP. Different variables affecting each method were carefully investigated and optimized. The reliability and analytical performance of the proposed methods, including linearity, range, precision, accuracy, and detection and quantitation limits, were statistically validated. The first 3 methods were successfully applied for the stability-indicating determination of MX in laboratory-prepared mixtures with DMP, as well as for the determination of MX in capsules. Also, the LC and the DBQC spectrophotometric methods permitted the selective determination of DMP in the presence of a large excess of the parent drug at or near the pharmacopeial limit (0.1–1%). M exiletine hydrochloride (MX; Figure 1), 1-(2,6- dimethylphenoxy)-2-propanamine hydrochloride (1), is an orally active class I antiarrhythmic drug used for the prevention and treatment of ventricular arrhythmias, particularly after myocardial infarction (2). MX is an official drug in the U.S. Pharmacopeia (USP; 3), which describes a column high-performance liquid chromatographic (LC) method for its assay as bulk powder as well as in capsules. It is also official in the British Pharmacopoeia (BP; 4), where a potentiometric nonaqueous titration procedure is used for the analysis of the bulk powder and a spectrophotometric method is applied for the quantitation of the capsules and injections. A full analytical bibliography of MX up to 1991 has been published (5) and, in addition, several more recent methods have been published for the determination of the drug in its pharmaceutical preparations. These include fixed pH titration (6), potentiometric titration using an MX selective membrane electrode (7), spectrophotometric and spectrofluorometric measurement of the Hantzsch condensation reaction product (8), various reversed-phase LC (RPLC) methods (9–11), and a normal-phase thin-layer chromatographic (TLC) method with densitometric measurement (12). The determination of the drug and/or its enantiomers and metabolites in biological fluids is based mainly on using separation techniques, including gas chromatography (13–17), high-performance TLC (HPTLC; 18), LC with UV detection (19), or fluorometric detection after derivatization with several fluorogenic reagents (20–23), and capillary electrophoresis (24). Reviewing the literature of MX revealed that only one paper described the LC resolution of MX from its related substance 2,6-dimethylphenol (DMP; Figure 1) using an RP C18 monolithic column (11). In the present work, MX has been determined in the presence of DMP by 3 stability-indicating methods, namely, derivative-ratio spectrophotometry, micelle-enhanced spectrofluorometry in alkaline medium, and RPLC using a C8 column and UV detection at 212 nm for separation and quantitation of both MX and DMP. Since MX is prepared by the condensation of the sodium salt of DMP with chloroacetone with subsequent conversion 720 BELAL ET AL.:JOURNAL OF AOAC INTERNATIONAL VOL. 91, NO. 4, 2008 Received August 26, 2007. Accepted by SW November 30, 2007. Corresponding author’s e-mail: tbelaleg@yahoo.com Downloaded from https://academic.oup.com/jaoac/article-abstract/91/4/720/5656142 by guest on 25 July 2020