Effects of rifampicin on plasma pharmacokinetics of tulathromycin in goats.

We investigated the pharmacokinetic profile of co-administration of tulathromycin with rifampicin. Healthy male goats were allocated to three groups (n = 8) as Group A (single dose 2.5 mg/kg tulathromycin s.c.), B (10 mg kg-1  day-1 rifampicin p.o. daily for 7 days and single dose 2.5 mg/kg tulathromycin s.c. on 8th day), and C (10 mg kg-1  day-1 rifampicin p.o. daily for 21 days and single dose 2.5 mg/kg tulathromycin s.c. on 8th day). Blood samples were collected from jugular veins. Plasma samples were analyzed for tulathromycin by liquid chromatography-mass spectrometry (LC-MS/MS). Peak plasma concentration (Cmax ) values of tulathromycin were 1,390 ± 173, 958 ± 106, and 807 ± 116 ng/ml in groups A, B, and C, respectively. Cmax value of group A was greater than other groups (p < .05). Mean residence time based on time zero to last sample time (MRTlast ) values were 52 ± 1, 56 ± 4 and 66 ± 4 hr in A, B, and C groups, respectively whereas mean residence time based on time zero extrapolated to infinity (MRTINF_obs ) values were 69 ± 4, 85 ± 5, and 86 ± 4 hr, respectively. MRTlast and MRTINF_obs values were greater in B and C groups than group A (p < .05). These findings suggest that rifampicin administration may change several pharmacokinetic parameters of tulathromycin in goats.