Analysis of loxoprofen in tablets, patches, and equine urine as tertbutyldimethylsilyl derivative by gas chromatography-mass spectrometry

Loxoprofen is a non-steroidal anti-inflammatorydrug of the 2-arylpropionic acid type, which has used totreat musculoskeletal disorders in the horse racing industry. However, it has also used illicitly to mask clinical signs ofinflammation and pain in racehorses. Thus, its accurateanalysis has become an important issue in horse dopinglaboratories. In this study, an analytical method of loxoprofenwas developed as tert-butyldimethylsilyl (TBDMS)derivative by gas chromatography-mass spectrometry(GC–MS). Characteristic fragment ions of [M-15], [M-57],and [M-139] permitted the accurate and selective detectionof loxoprofen. Under optimal conditions, this methodshowed good linearity (r C 0.999) in the range of10–500 ng/mL, repeatability (% relative standard deviation= 5.6–8.5), and accuracy (% relative error =- 0.3–0.9) with a detection limit of 1.0 ng. When appliedto the analysis of loxoprofen in tablet and patch products,loxoprofen was positively identified as TBDMS derivativeby GC–MS. The present method provided rapid andaccurate determination of loxoprofen in patch and tabletproducts. Levels of loxoprofen were highest in equine urineat 0.5 and 1 h after oral administration with single dose(3 mg/kg) to three horses, and then rapidly reduced tobelow the lower limit of quantification at 24 h. Therefore,the present method will be useful for the pharmacokineticstudy and doping tests for loxoprofen and other similaracidic drugs in horses.