Binding of antibiotics to bovine and ovine serum

The degree of binding of 37 antibiotics to bovine and ovine serum, after treatment at therapeutic doses, was determined by equilibrium dialysis and ultrafiltration methods. In general, binding values obtained by the two methods were comparable. The extent of binding varied from 0% for cephaloridine and kanamycin to >95% for novobiocin and fusidic acid. Of the 37 drugs studied, one-fourth were less than 25% bound, one-fourth were more than 75% bound, and the percentage binding of about half of the antibiotics ranged from 25 to 75%. Animal to animal variations in the extent of binding of a particular antibiotic were very small. The capacity of bovine or ovine serum to bind antibiotics was, with a few exceptions, similar to the reported capacity of human serum. At drug concentration ranges usually achieved during therapy, variations in drug levels in serum did not influence the degree of binding except with cephalexin, lincomycin, clindamycin, and chloramphenicol. With these antibiotics, the extent of binding increased two- to sevenfold with the decrease in drug concentration in serum.