Enantiomeric composition tests of ketoprofen in equine plasma and urine as diastereomeric (S)-(-)-1-phenylethylamides by achiral GC-MS.

The enantiomeric composition of ketoprofen in equine plasma and urine after administration of a commercial racemic ketoprofen product were determined as diastereomeric (S)-(-)-1-phenylethylamidation by achiral gas chromatography–mass spectrometry in selected ion monitoring mode. This method showed linearity (r ≥ 0.9986) over the range tested (5.0–5,000 ng), with acceptable precision (% RSD ≤ 9.5) and accuracy (% RE = −3.7–7.3). The ratio of (S)-ketoprofen in plasma after 6.0 h and in urine after 71.0 h increased progressively to final values of 67.3 ± 0.1 and 91.9 ± 2.2%, respectively, attributable to the inversion of (R)-ketoprofen to (S)-ketoprofen.