Pharmacokinetics of spiramycin after intravenous, intramuscular and subcutaneous administration in lactating cows.

Spiramycin is a macrolide antibiotic that is active against most of the microorganisms isolated from the milk of mastitic cows. This work investigated the disposition of spiramycin in plasma and milk after intravenous, intramuscular and subcutaneous administration. Twelve healthy cows were given a single injection of spiramycin at a dose of 30,000 IU/kg by each route. Plasma and milk were collected post injection. Spiramycin concentration in the plasma was determined by a high performance liquid chromatography method, and in the milk by a microbiological method. The mean residence time after intravenous administration was significantly longer (P less than 0.01) in the milk (20.7 +/- 2.7 h) than in plasma (4.0 +/- 1.6 h). An average milk-to-plasma ratio of 36.5 +/- 15 was calculated from the area concentration-time curves. Several pharmacokinetic parameters were examined to determine the bioequivalence of the two extravascular routes. The dose fraction adsorbed after intramuscular or subcutaneous administration was almost 100% and was bioequivalent for the extravascular routes, but the rates of absorption, the maximal concentrations and the time to obtain them differed significantly between the two routes. Spiramycin quantities excreted in milk did not differ between the two extravascular routes but the latter were not bioequivalent for maximal concentration in the milk. However, the two routes were bio-equivalent for the duration of time the milk concentration exceeded the minimal inhibitory concentration (MIC) of various pathogens causing infections in the mammary gland.(ABSTRACT TRUNCATED AT 250 WORDS)