A new, specific and sensitive reversed-phase high performance liquid chromatographic method was developed for the simultaneous determination of two diuretic drugs; metolazone (MET) and spironolactone (SPL). Good chromatographic separation was achieved within 5.0 min on 150 mm × 4.6 mm i.d., 5 μm particle size Spherisorb-ODS 2 C18 column. A mobile phase containing a mixture of methanol and 0.02 M phosphate buffer (70 : 30) v/v at pH 3.0 was used. The analysis was performed at a flow rate of 1 mL min−1 with UV detection at 235 nm. Xipamide (XPM) was used as an internal standard (IS). The proposed method was rectilinear over the ranges of 0.05–1.0 μg mL−1 and 0.5–10.0 μg mL−1 with limits of detection (LOD) of 0.009, 0.04 ng mL−1 and limits of quantification (LOQ) of 0.03, 0.11 μg mL−1 for MET and SPL, respectively. The suggested method was successfully applied for the simultaneous analysis of the studied drugs in their laboratory prepared mixtures, single tablets and co-formulated tablets. The method was further extended to the determination of both drugs in spiked human urine. The mean percentage recoveries of MET and SPL in spiked human urine were 99.33 ± 2.37 and 99.72 ± 3.27, respectively. The proposed method was also applied for the determination of the studied drugs in the presence of some co-administered or co-formulated drugs without any interference. Statistical evaluation and comparison of the data obtained by the proposed and comparison methods revealed no significant difference between the two methods regarding accuracy and precision.
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