Use of Isotopes and LC-MS-ESI-TOF for Mechanistic Studies of Tienilic Acid Metabolic Activation

Tienilic Acid (TA) is a uricosuric drug marketed in 1978 and which caused a number of rare immunoallergic hepatitis. It was withdrawn in US in 1980, in France in 1992. Early batches of tienilic acid also contained 0.1–0.5% tienilic acid isomer. Tienilic acid isomer (TAI), has been shown to be metabolised by Cytochrome P450 into a reactive thiophene 1-oxide which either binds to proteins, or can be trapped by sulfur nucleophiles (Valadon et al. 1996). Tienilic acid is metabolized by human cytochrome P450 2C9 into 5-hydroxytienilic acid (a major metabolite representing 70% of the dose excreted in human urine) but it also forms (a) reactive metabolite(s) which binds covalently to CYP 2C9 and it is a mechanism based inhibitor of CYP 2C9. Adding glutathione to incubations decreases the covalent binding, but only to 1 mol/mol P450. However the reactive metabolite of tienilic acid is still unknown. Recently Koenigs et al. have shown using ESI-LC-MS that CYP 2C9 binds ≈ 2 mol of TA in absence of GSH and only one in presence of 3 mM GSH (Koenigs et al. 1999).