Simultaneous quantitation of etoricoxib, salicylic acid, valdecoxib, ketoprofen, nimesulide and celecoxib in plasma by high-performance liquid chromatography with UV detection

A specific, accurate, precise and reproducible high performance liquid chromatography (HPLC) method was developed and validated for the simultaneous quantitation of etoricoxib, salicylic acid, valdecoxib, ketoprofen, nimesulide and celecoxib in human plasma. The method employed a simple liquid_liquid extraction of etoricoxib, salicylic acid, valdecoxib, ketoprofen, nimesulide and celecoxib and internal standard (IS, DRF‐4367) from human plasma (500 µL) into acetonitirile. The organic layer was separated and evaporated under a gentle stream of nitrogen at 40°C. The residue was reconstituted in the mobile phase and injected onto a Kromasil KR 100‐5C18 column (4.6 × 250 mm, 5 µm). The chromatographic separation was achieved by gradient elution consisting of 0.05 M formic acid (pH 3)‒acetonitrile‒methanol‒water at a flow rate of 1.0 mL/min. The eluate was monitored using an ultraviolet (UV) detector set at 235 nm. The ratio of peak area of each analyte to IS was used for quantification of plasma samples. Nominal retention times of etoricoxib, salicylic acid, valdecoxib, ketoprofen, nimesulide, IS and celecoxib were 15.63, 17.20, 21.66, 24.95, 26.27, 30.24 and 32.22 min, respectively. The standard curve for etoricoxib, salicylic acid, valdecoxib, ketoprofen and celecoxib was linear (r2 > 0.999) in the concentration range 0.1‒50 µg/mL and for nimesulide (r2 > 0.999) in the concentration range 0.5‒50 µg/mL. Absolute recovery was >83% from human plasma for all the analytes and IS. The lower limit of quantification (LLOQ) of nimesulide was 0.5 µg/mL and for etoricoxib, salicylic acid, valdecoxib, ketoprofen and celecoxib the LLOQ was 0.1 µg/mL. The inter‐ and intra‐day precisions in the measurement of QC samples, 0.1, 0.3, 15.0 and 40.0 µg/mL (for all analytes except nimesulide), were in the range 2.29‒9.37% relative standard deviation (RSD) and 0.69‒10.28% RSD, respectively. For nimesulide the inter‐ and intra‐day precisions in the measurement of quality control (QC) samples, 0.5, 1.5, 15.0 and 40.0 µg/mL, were in the range 3.21‒7.37% RSD and 0.97‒7.06% RSD, respectively. Accuracy in the measurement of QC samples for all analytes was in the range 91.03‒106.38% of the nominal values. All analytes including IS were stable in the battery of stability studies, viz. bench top, autosampler and freeze_thaw cycles. Stability of all analytes was established for 21 days at −20°C. The application of the assay in an oral pharmacokinetic study in rats co‐administered with cele