An LC–MS/MS method for the quantification of diclofenac sodium in dairy cow plasma and its application in pharmacokinetics studies

: An LC-MS/MS method with internal standard tolfenamic acid for determining diclofenac sodium (DCF) in dairy cow plasma was developed and validated. Samples were processed with protein precipitation by cold formic acid-acetonitrile. Determination of DCF was performed using LC-ESI+ -MS/MS with the matrix-matched calibration curve. The results showed that the method was sensitive (LOD 2 ng mL-1 , LOQ 5 ng mL-1 ), accurate (97.60 ± 5.64%), precise (<10%) and linear in the range of 5-10,000 ng mL-1 . A single intravenous (i.v.) or intramuscular (i.m.) administration of 5% diclofenac sodium injection at a dose of 2.2 mg kg-1 was performed in six healthy dairy cows according to a two-period crossover design. The main pharmacokinetic (PK) parameters after a single i.v. administration were as follows: t1/2β , 4.52 ± 1.71 h; AUC, 77.79 ± 16.76 h μg mL-1 ; mean residence time, 5.16 ± 1.11 h. The main PK parameters after a single i.m. administration were as follows: Tmax , 2.38 ± 1.19 h; Cmax , 7.46 ± 1.85 μg mL-1 ; t1/2β , 9.46 ± 2.86 h; AUC 67.57 ± 13.07 h μg mL-1 . The absolute bioavailability was 87.37 ± 5.96%. The results showed that the diclofenac sodium injection had PK characteristics of rapid absorption and slow elimination, and high peak concentration and bioavailability in dairy cows, and that the recommended clinical dosage of diclofenac sodium injection is 2.2 mg kg-1 .