Elucidation of Cefmetazole Binding to Human Serum Albumin In Vitro and Prediction of Unbound Cefmetazole Concentrations in Clinical Patients.

Cefmetazole (CMZ), a cephamycin, has a protein-binding rate of approximately 85%. The unbound concentration is microbiologically active and may vary greatly depending on variations in human serum albumin (HSA) concentration. However, the binding parameters and sites of HSA have not been clarified, and the predicted unbound concentrations using these parameters have not been evaluated. This study aimed to clarify these binding characteristics and evaluate unbound CMZ concentrations predicted using the calculated binding parameters. Total and unbound CMZ concentrations in the HSA solutions were measured at several HSA concentrations (10 g/L, 25 g/L, and 40 g/L) and total CMZ concentrations (10, 60, and 150 mg/L). A Scatchard plot was constructed. To identify specific binding sites, competitive binding assays were performed using the site markers warfarin, dansylsarcosine, bicalutamide, and bilirubin. Unbound CMZ concentrations were predicted using binding parameters from total CMZ and albumin concentrations in 37 patients. The association constant (K) for CMZ to HSA was determined to be 6.59 × 103 M-1, with number of binding sites (n) of 1.08. The binding rate decreased as the CMZ concentration increased or the HSA concentration decreased. Competitive binding assays revealed that CMZ binds to Subdomain IB on HSA. A strong correlation (r=0.907) was observed between predicted and measured unbound CMZ concentrations. This study successfully defined the binding parameters and specific binding sites of CMZ on HSA, and unbound CMZ concentrations were accurately predicted from the total CMZ concentrations in clinical patients using these binding parameters.