Relationship between Lipophilicity and Binding Affinity with Human Serum Albumin for Penicillin and Cephem Antibiotics

Relationship between structure and binding affinity to human serum albumin (HSA) has been studied for penicillin and cephem antibiotics. For penicillin ana logs, a good correlation between the apparent affinity constants, Kapp, for HSA binding and the partition coefficient, Papp, determined in isobutyl alcohol-pH 7.4 phosphate buffer system was observed, indicating that the hydrophobic interaction of 6-substituent of penicillins with amino acid of HSA would play an important role for the binding. However, no correlation between the Kapp and Papp values was observed for cephem antibiotics. Mutual competitive displacement effects were demonstrated for the primary binding sites of cephalothin, cefazolin, cefotetan and cefatrizine, suggesting the presence of a common binding region in HSA among these cephem antibiotics examined. Significant differences were observed for the Kapp value among cephems having the same 3-substitute of N-methylthiotetrazole in the molecule, i.e., cefpiramide, cefotetan, cefoperazone, cefamandole, cefmenoxime, cefmetazole and cefbupera-zone, suggesting that 7-substitute of cephem would play an important role for the binding with HSA.Moreover, comparing the binding affinity and the structure of 3-substitute for cephems, all of the analogs having a heterocycle bind strongly with HSA in spite of their low lipophilicity. These observations suggest that an interaction between heterocycle at the position 3 and HSA would contribute to an additional binding force for the binding of cephem antibiotics to HSA.