DRUG FORMULATIONS AND CLINICAL METHODS Reversed-Phase Ultra-Performance Liquid Chromatographic Method Development and Validation for Determination of Impurities Related to Torsemide Tablets HITESH B. PATEL Torrent Research Centre, Bhat-382 428, Gujarat, India and Saurashtra University, Department of Chemistry, Rajkot-360 005, Gujarat, India ARIVOZHI MOHAN Shasun Pharmaceuticals, PIMS Rd, Pondicherry-605 014, India HITENDRA S. JOSHI Saurashtra University, Department of Chemistry, Rajkot-360 005, Gujarat, India A simple RP-ultra-performance LC method was developed and validated for determination of impurities related to torsemide tablets. The rapid method provided adequate separation of all known related impurities and degradation products. Separation was achieved on a Zorbax SB-C18 column (50 ´ 4.6 mm id, 1.8 mm particle size) with binary gradient elution, and detection was performed at 288 nm. The drug product was subjected to oxidative, hydrolytic, photolytic, and thermal stress conditions to prove the specificity of the proposed method. The linearity and recovery were investigated for known impurities in the range of 0.025 to 1.0%, with respect to the drug concentration in the prepared sample. The linearity of the calibration curve for each of the impurities and torsemide was found to be very good (r 2 > 0.999). Relative response factors for each of the known impurities were established by the slope ratio method from the linearity study. T orsemide, N-[[(1-methylethyl)amino]carbonyl]-4- [(3-methylphenyl)amino]-3- pyridinesulfonamide, 1- isopropyl-3-[(4-m-toludino-3-pyridyl)sulfonyl]urea, is a loop diuretic used for edema associated with heart failure, including pulmonary edema, and with renal and hepatic disorders. It is also used in the treatment of hypertension, either alone or with other antihypertensives (1–3). Torsemide drug substance is official in the U.S. Pharmacopeia (USP) as well as the British Pharmacopoeia, but its tablet formulation is not official in any pharmacopeia (4, 5). An assay method has been described for determination of torsemide in a tablet formulation by HPLC with an amperometric detector (6). A spectrophotometric method for determination of torsemide in bulk drug and formulation using Folin-Ciocalteu color-forming reagent is reported (7). Some methods are also available for determination of torsemide in urine and biological fluids (8–13), but no method is available for determination of related impurities in torsemide tablets. This paper describes a method for determination of related impurities in torsemide tablets with established relative response factor (RRF) and relative retention time (RRT) values for known impurities. Moreover, an analytical column with sub-2 mm particle size technology was explored for better chromatography because, according to the van Deemter equation, as the particle size decreases to less than 2 mm there is a significant gain in efficiency (N), which is inversely proportional to particle size (dp). So, by keeping the L/dp ratio constant or almost similar, the chromatographer can change from a 150 mm column length (L) with a 5 to 7 mm particle size to a 50 mm column length with a 1.7 to 1.8 mm particle size to achieve a shorter run time with similar resolution (14–18). An ultra-performance liquid chromatography (UPLC) system was used with this column. Experimental Standards and Samples (a) Torsemide, USP RS.—99.6% (U.S. Pharmacopeial Convention, Rockville, MD). (b) Torsemide working standard.—99.5% (TRC, Bhat, India). (c) Torsemide-related compound A, USP RS (Impurity A).—[4-[(3-methylphenyl)amino]-3-pyridinesul- fonamide], 100.0% (U.S. Pharmacopeial Convention). (d) Torsemide-related compound B, USP RS (Impurity B).—[N-[(n-butylamino)-carbonyl]-4-[(3-methyl phenyl) amino]-3-pyridinesulfonamide], 100.0% (U.S. Pharmacopeial Convention). (e) Torsemide-related compound C, USP RS (Impurity C) .—[N-[(ethylamino)carbonyl]-4-[(3-methylphenyl) PATEL ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 94, NO. 1, 2011 143 Received May 12, 2009. Accepted by SW February 21, 2010. Corresponding author’s e-mail: chemhbp@yahoo.com Downloaded from https://academic.oup.com/jaoac/article/94/1/143/5655388 by guest on 30 June 2021