Evaluation of rifamycin SV and rifampin kinetics in lactating ewes.

Serum and milk concentrations of rifamycin SV and rifampin were determined in lactating ewes after a single intravenous injection, and pharmacokinetic parameters were evaluated by the two-compartment open-system model. Rifampin was distributed throughout a greater volume than rifamycin SV and was eliminated more slowly from the body. The concentrations of the two drugs, both lipophilic weak acids, in milk after intravenous or intramuscular injection were lower than in serum, but rifampin was detected in milk sooner and for longer periods than rifamycin SV. Under constant serum drug concentrations, the observed milk/serum ultrafiltrate concentrations ratios (0.19 to 0.29 for rifamycin SV, and 0.90 to 1.28 for rifampin) were close to the calculated ratios derived from the pH-pK passive diffusion concept.