In vitro irreversible binding of ketoprofen glucuronide to plasma proteins

Many aryl alkanoic acids are cleared as ester glucuronide excreted in urine. While conjugation with glucuronic acid is generally considered as a detoxication process, this conjugate has been shown over the past decade to be a potentially reactive metabolite, undergoing hydrolysis, intramolecular rearrangement, and irreversible binding to proteins. This study describes the in vitro degradation of biosynthetic ketoprofen glucuronide after incubation with human plasma, human serum albumin solutions at various concentrations (290 and 580 microM), and in protein-free buffer, in physiological conditions (pH = 7.4, 37 degrees C). The protein concentrations chosen correspond to that found in synovial fluid and plasma, respectively. Albumin catalyzed the hydrolysis of the glucuronide, but the extent of the reaction was not dependent on the protein concentration. The irreversible binding of ketoprofen was investigated in identical conditions. Maximal ketoprofen-adduct concentrations were achieved after 3 and 10 hr incubation, and were 6.65, 3.2, and 2.6% of initial ketoprofen in plasma and albumin solutions at 580 and 290 microM, respectively. The difference in binding between plasma and albumin (580 microM) could not be totally attributed to the other major plasma proteins, because no irreversible binding was detected with fibrinogen and gamma globulins, and only 0.14% of ketoprofen was bound to alpha and beta globulins after 3 hr incubation. The covalent interaction with albumin was proportional to conjugate concentration over the range studied (from 5 to 30 micrograms/ml or 11.62 to 69.72 microM).