Protein binding of oxazepam and its glucuronide conjugates to human albumin

The binding of oxazepam and its glucuronide conjugates to human serum albumin (HSA), as well as the binding interactions of the drug and its metabolites, were examined by equilibrium dialysis and kinetic probe studies. Oxazepam and its S(+) glucuronide are bound to the HSA molecule with affinity constants of 3.5 X 10(5) M-1 and 5.5 X 10(4) M-1, respectively, which were independent of protein concentration over a range of 0.1 to 5.0 g/dl. The R(-) glucuronide bound weakly to albumin, with the binding parameter, N X K, increasing at lower albumin concentrations. Pre-acetylation of fatty acid free-HSA resulted in decreased binding of all three compounds, probably by altering the conformation of the binding sites. Kinetic probe studies with p-nitrophenyl acetate indicate that oxazepam and its S(+) glucuronide shared a common binding site on HSA, but that the R(-) glucuronide bound at another site. Oxazepam binding was unaffected by the presence of its glucuronide conjugates but was inhibited by fatty acids. The percentage of oxazepam bound to plasma proteins in patients with renal impairment (94%) was lower than in normal volunteers (97%). This lower binding can neither be attributed to lower albumin concentrations because of the large binding capacity of the protein and linearity of N X K nor to displacement by elevated concentrations of glucuronide conjugates, but it may be ascribed partly to increased plasma fatty acids.