Kinetics study of metaxalone degradation under hydrolytic, oxidative and thermal stress conditions using stability-indicating HPLC method

An isocratic stability indicating RP-HPLC–UV method is presented for the determination of metaxalone (MET) in the presence of its degradation products. The method uses Dr. Maisch C18 column (250 mm×4.6 mm, 5 μm) with mobile phase consisting of acetonitrile–potassium dihydrogen orthophosphate buffer with 4 mL of 0.4% triethyl amine (pH 3.0; 10 mM) (58:42, v/v) at a flow rate of 1.0 mL/min. pH of the buffer was adjusted with o-phosphoric acid. UV detection was performed at 225 nm. The method was validated for specificity, linearity, precision, accuracy, limit of detection, limit of quantification and robustness. The calibration plot was linear over the concentration range of 1–100 μg/mL having a correlation coefficient (r(2)) of 0.999. Limits of detection and quantification were 0.3 and 1 μg/mL, respectively. Intra-day and inter-day precision (% RSD) was 0.65 and 0.79 respectively. The proposed method was used to investigate the degradation kinetics of MET under different stress conditions employed. Degradation of MET followed a pseudo-first-order kinetics, and rate constant (K), time left for 50% potency (t(1/2)), and time left for 90% potency (t(90)) were calculated.