Transplacental transfer of spiramycin was investigated in a rhesus monkey model to study whether the antibiotic reaches therapeutic levels in the fetus. Spiramycin concentrations were measured by bioassay and high-performance liquid chromatography. Pharmacokinetic parameters were determined for bioactive spiramycin as measured by the bioassay. Pharmacokinetic pilot studies showed that spiramycin distribution follows a two-compartment model in rhesus monkeys. Following a single intravenous dose of 50 or 250 mg, dose-dependent kinetics were observed. At a dose of 50 mg, 10% of the dose was excreted unchanged in the urine. At the higher dose of 250 mg, an oliguric effect was observed. Spiramycin concentrations in fetal serum were measured over time while the maternal concentration was maintained at a constant level. During a 5-h experiment, a maximum fetal-maternal serum ratio of 0.27 was found. In three fetuses, concentrations in serum and tissue were measured following intravenous administration of 50 mg of spiramycin twice daily to the mother for at least 7 days. The fetal-maternal serum ratios were found to be 0.4 to 0.58 after intravenous administration of the final dose of 50 mg to the mother. It appeared that spiramycin accumulated in the soft tissues, especially in the liver and spleen, of both the mother and the fetus. The concentration in placental tissue appeared to be 10 to 20 times that of the concentration in fetal serum. The concentration of spiramycin in amniotic fluid was about five times higher than the concentration in fetal serum. Another important observation was that absolutely no spiramycin was found in the brain
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