Competitive binding of sulfamethazine and its N-acetylated metabolite

The protein binding of sulfamethazine (SMZ) was studied in rabbit serum. Serum solutions comprising various concentrations (approximately 0.01-3 mM) of SMZ and its major metabolite, N4-acetylsulfamethazine (AcSMZ), were prepared. These 'control' samples were also mixed via a Latin square design to study the interactive binding of SMZ and AcSMZ to the proteins. Equilibrium dialysis was conducted for 8 h. Post-dialysis measurement of SMZ/AcSMZ radioactivity in the buffer and serum chambers provided the free and bound fractions based upon equilibrium total serum concentrations. There was no significant change of protein concentration before and after dialysis. The 'control' data revealed concentration-dependent binding for both drugs while the interaction study clearly indicated the presence of competitive binding. Scatchard plots suggested the presence of more than one binding site. As a result, three different competitive binding models were examined via computer analysis of the observations. The most appropriate binding model was identified to consist of specific binding (protein concentration, Pt; dissociation constant, Kd) and nonspecific binding (Nsp). The mean (SD) 'control' parameter estimates for PtSMZ, PtAcSMZ, KdSMZ, KdAcSMZ, NspSMZ, NspAcSMZ were: 0.562(0.041), 0.605(0.024), 0.078(0.006), 0.031(0.002) [mM], 0.205(0.054), and 0.229(0.043), respectively. In the interaction study these values were: 0.599(0.022), 0.479(0.019), 0.091(0.004), 0.023(0.001), 0.228(0.020), and 0.434(0.061), respectively. The findings indicate that both drugs bind to albumin but the affinity of AcSMZ is greater than SMZ. Theoretically, the metabolite can therefore alter the in vivo SMZ binding thereby in turn causing apparent nonlinear acetylation based upon the pharmacokinetics of total SMZ concentrations.