Distribution and blood-to-milk transfer of labeled antibiotics.

RADIOACTIVITY DISTRIBUTION WAS DETERMINED IN SERUM AND MILK OF LACTATING EWES AFTER PARENTERAL ADMINISTRATION OF FIVE LABELED ANTIBIOTICS: (14)C-benzylpenicillin G, (3)H-dihydrostreptomycin, (3)H-tetracycline, (14)C-chloramphenicol, and (14)C-spiramycin. Antibiotic levels were measured simultaneously by microbiological assay. Radiochemical and microbiological assay procedures presented similar kinetic patterns for uptake in serum and penetration into milk, except for tetracycline. Small reductions in milk pH markedly increased the excretion of spiramycin and slightly influenced the milk passage of penicillin, dihydrostreptomycin, and tetracycline but did not alter the transfer of chloramphenicol into milk. Thus, it appears that the five antibiotics penetrate milk in accordance with the nonionic passive diffusion principle, and that good agreement is achieved between the calculated and observed milk/serum ultrafiltrate concentration ratios obtained during equilibrium.