Irreversible binding of zomepirac to plasma protein in vitro and in vivo.

Abstract

Zomepirac is a nonsteroidal anti-inflammatory drug recently withdrawn from use because of an unexplained high incidence of immunological reactions. It is metabolized in humans to a reactive, unstable acyl glucuronide which accumulates in plasma. Because of the similarity of zomepirac glucuronide to bilirubin glucuronide in structure and stability and the documented irreversible binding of bilirubin to albumin through its acyl glucuronide, we studied the reaction of zomepirac acyl glucuronide with albumin in vitro from pH 5 to 9 and in vivo in six healthy human volunteers who had received a single 100-mg oral dose of zomepirac. Irreversible binding of zomepirac to protein was determined by exhaustive washing of protein, followed by hydrolysis of bound zomepirac-protein adduct with base, extraction of the liberated drug, and chromatographic measurement. Irreversible binding was observed both in vitro and in vivo. The extent of binding in vitro was time-and pH-dependent. In vitro drug binding was also observed for the isomers of zomepirac glucuronide which were formed by intramolecular acyl migration. Irreversible binding in vivo correlated with overall exposure to zomepirac glucuronide when exposure was expressed as the area under the plasma concentration vs. time curve. When probenecid (500 mg, twice daily), which decreases the plasma clearance of zomepirac glucuronide, was administered concurrently with zomepirac, irreversible binding of zomepirac was increased. The nature of the zomepirac protein binding is probably covalent. Formation of irreversibly protein-bound zomepirac occurs via the acyl glucuronide as previously shown for bilirubin glucuronide, and the reaction may be general for other drugs that are metabolized to acyl glucuronides.

Predictability of Idiosyncratic Drug Toxicity Risk for Carboxylic Acid-Containing Drugs Based on the Chemical Stability of Acyl Glucuronide

“…The reactivity of the isomers has also been reported (Smith et al, 1986;Dickinson and King, 1991), and the half-life in KPB is considered to reflect the formation of the isomers more than that in HSA solution or human fresh plasma. On the other hand, the half-life in HSA solution or human fresh plasma is considered to overestimate the reactivity of AGs because the half-life includes the degradation by hydrolysis.…”

Development of an in Vitro Screening Model for the Biosynthesis of Acyl Glucuronide Metabolites and the Assessment of Their Reactivity toward Human Serum Albumin

“…The ranking according to the amount of drug irreversibly bound observed for the eight compounds studied was consistent with the literature data. Indeed, tolmetin, zomepirac, and diclofenac (Smith et al, 1986;Hyneck et al, 1988a;Munafo et al, 1989) are considered as reactive products, whereas ibuprofen, ketoprofen, and furosemide (Dubois et al, 1993;Presle et al, 1996;Mizuma et al, 1999) are mentioned as safer products.…”

“…These drugs have been extensively studied and represent a large scale of reactive products (Smith et al, 1986;Hyneck et al, 1988a;Castillo and Smith, 1991;Volland et al, 1991;Hayball et al, 1992;Dubois et al, 1993Dubois et al, , 1994Smith and Liu, 1993;Ebner et al, 1999;Mizuma et al, 1999). Tolmetin and zomepirac were withdrawn from the market because of hypersensitivity reactions; ibuprofen is considered to be the safest nonsteroidal anti-inflammatory drug, and furosemide shows very low levels of covalent binding.…”

Studies on the Metabolism of Tolmetin to the Chemically Reactive Acyl-Coenzyme A Thioester Intermediate in Rats

“…An acyl glucuronide is one example of a reactive carboxylic acid metabolite (Smith et al, 1986), but recent studies on acyl-CoAs and acyl-glutathione conjugates have shown that thioester metabolites react more rapidly with thiols (glutathione/N-acetylcysteine) than the corresponding ester glucuronides Li et al, 2002;Olsen et al, 2002;Grillo et al, 2003).…”

“…The protein pellets were washed seven times with 6 ml of methanol/ether (3:1, v/v) following the procedure reported previously (Smith et al, 1986). Subsequently, the pellets were washed as described by Bailey and Dickinson (1996) with slight modifications.…”