Pharmacokinetics and tissue residues of spiramycin in cattle after intramuscular administration of multiple doses

Pharmacokinetic variables of spiramycin and its distribution in muscle, liver, kidney, and injection sites were studied in 18 mixed-sex 1-year-old calves to assess drug withdrawal time after 2 IM administrations of 100,000 IU of spiramycin/kg of body weight at 48-hour intervals. Presence of a compound, other than spiramycin I (ie, neospiramycin), was observed in tissues used for withdrawal time determination. High concentrations observed at the injection sites decreased slowly to maximal residue limit with half-life of 109.5 hours for neospiramycin and 77.5 hours for spiramycin. At 14 days, neospiramycin concentrations were higher in kidney than in liver and half-life was different between these 2 tissues. Two methods of withdrawal time determination were used and the part of the samples without residue detected, in the calculation, was discussed. Withdrawal time of 35 days can be proposed on the basis of average daily intake determined for spiramycin, with concentration at injection sites representing 10% of the whole muscle concentration.