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Potent muscle relaxant (thiocolchicoside, TCC) and nonsteroidal anti-inflammatory drug (etoricoxib, ETXB) fixed-dose combination is formulated at relatively high 1:15 and 1:7.5 ratios for TCC and ETXB, respectively. Since the minor component (TCC) has lower absorptivity, assay of TCC/ETXB tablets presents an analytical challenge. The current study presents two novel methods: first is a micellar electrokinetic capillary chromatography (MEKC). Background electrolyte is borate buffer (40 mM, pH 9.2) containing 30 mM sodium dodecyl sulfate and methanol (ratio 80:20%, v/v), measured at 210 nm. Second is a direct double A max spectrophotometric method; minor component, TCC, is measured directly at its distant λ max (373 nm), at zero absorption of ETXB. Then, a ten-fold dilution step is carried out to eliminate TCC spectral interference and ETXB is determined at its λ max (282 nm). Both drugs’ concentrations disclose obedient linearities at 2–100 μg·mL−1 in MEKC, versus 3–25 and 40–350 μg·mL−1 for TCC and ETXB, respectively, in spectrophotometry. All ICH validation elements have been fulfilled for the developed methods. MEKC and spectrophotometric assays achieve accuracy, precision, selectivity, and robustness to be recommended for industrial quality control routine analysis of TCC/ETXB pills formulated at cited ratios or even higher.

Objective and Significance: Methocarbamol (MET) and aspirin (ASP) are widely used as a muscle relaxant combination. The USP reports guaifenesin (GUA) and salicylic acid (SAL) as related substances and hydrolytic products of MET and ASP, respectively. This work aimed at developing and validating a simple and sensitive RP-HPLC method for the determination of both drugs as well as their related substances (at their pharmacopeial limits) in their bulk powders, laboratory prepared mixtures, and MET-ASP combined tablets. Methods and Results: Chromatographic separation was achieved in less than 9 min with the required resolution, peak symmetry, and accuracy on C18 column using isocratic elution system of diluted acetic acid (pH 3.2): acetonitrile at the ratio of 79: 21, v/v, at a flow rate of 1 mL/min. Detection was achieved with photodiode array at 233 nm for MET, GUA, and SAL and at 273 nm for ASP. The developed method has been validated as per ICH guidelines and the calibration plots were linear over the concentration ranges of 2-150, 0.4-30, 25-450, and 0.2-27 μg/mL for MET, GUA, ASP, and SAL, respectively. Conclusion: The optimized method proved to be specific, robust and precise for the quality control of the studied drugs in pharmaceutical preparations to ascertain that their related substances are not exceeding the permitted pharmacopeial limits. [ABSTRACT FROM AUTHOR]