Pharmacokinetics of sulphadoxine and trimethoprim in sows: influence of lactation.

A potentiated sulpha drug was administered intravenously to 12 sows on the 17th day of lactation and to 4 sows in early pregnancy to study the influence of lactation on its disposition kinetics. The dose-rate of sulphadoxine (SDX) used was 12 mg/kg b.w. while that of trimethoprim (TMP) was 2.4 mg/kg b.w. The pharmacokinetic parameters of SDX showed no significant difference between lactating and pregnant sows (Vss, 0.24 +/- 0.04 L/kg; Cls, 0.25 +/- 0.05 ml/min per kg: MRT, 17.08 +/- 4.48 h). SDX did not accumulate in milk, the concentrations in milk being less than the concentrations in serum at the same time. Of the pharmacokinetic parameters for TMP, only the mean residence time was significantly different between the two groups (Vss, 1.60 +/- 0.31 L/kg; Cls, 4.62 +/- 1.07 ml/min per kg: MRTlactating, 5.43 +/- 1.26 h; MRTpregnant, 7.74 +/- 1.72 h). TMP was excreted in milk to a considerable extent, the ratio of its concentration in milk to that in serum at the same time being over 2.2. These two substances show a completely different pharmacokinetic behaviour. Even though TMP is excreted more quickly in lactating sows, adjusting the dose of this potentiated sulpha drug does not seem to be appropriate.