Studies on the metabolism of tolmetin to the chemically reactive acyl-coenzyme A thioester intermediate in rats.

Carboxylic acids may be metabolized to acyl glucuronides and acyl-coenzyme A thioesters (acyl-CoAs), which are reactive metabolites capable of reacting with proteins in vivo. In this study, the metabolic activation of tolmetin (Tol) to reactive metabolites and the subsequent formation of Tol-protein adducts in the liver were studied in rats. Two hours after dose administration (100 mg/kg i.p.), tolmetin acyl-CoA (Tol-CoA) was identified by liquid chromatography-tandem mass spectrometry in liver homogenates. Similarly, the acyl-CoA-dependent metabolites tolmetin-taurine conjugate (Tol-Tau) and tolmetin-acyl carnitine ester (Tol-Car) were identified in rat livers. In a rat bile study (100 mg/kg i.p.), the S-acyl glutathione thioester conjugate was identified, providing further evidence of the formation of reactive metabolites such as Tol-CoA or Tol-acyl glucuronide (Tol-O-G), capable of acylating nucleophilic functional groups. Three rats were treated with clofibric acid (150 mg/kg/day i.p. for 7 days) before dose administration of Tol. This resulted in an increase in covalent binding to liver proteins from 0.9 nmol/g liver in control rats to 4.2 nmol/g liver in clofibric acid-treated rats. Similarly, levels of Tol-CoA increased from 0.6 nmol/g to 4.4 nmol/g liver after pretreatment with clofibric acid, whereas the formation of Tol-O-G and Tol-Tau was unaffected by clofibric acid treatment. However, Tol-Car levels increased from 0.08 to 0.64 nmol/g after clofibric acid treatment. Collectively, these results confirm that Tol-CoA is formed in vivo in the rat and that this metabolite can have important consequences in terms of covalent binding to liver proteins.