Only a few years ago, it was generally accepted by pharmaceutical scientists that phase II metabolites of drugs, such as acyl glucuronide conjugates, are rapidly excreted following their formation in the body and that these metabolites are not active or reactive. It has recently become recognized that acyl glucuronides of carboxylic acids are unstable and reactive metabolites. At physiologic pH they can undergo isomerization, facile hydrolysis, reversible and irreversible binding to proteins. The general aims of this grant proposal are to determine the scope and biological significance of the rearrangement and protein binding reactions of acyl glucuronides, and to establish whether irreversible binding of these metabolites to macromolecules is responsible for some of the major toxicological side-effect of NSAIDs. We hypothesize that four determinants, related to disposition of the drug acyl glucuronide, may be predictive of the potential for immunotoxicity. These are: the ability of the acyl glucuronide to lead to irreversible binding with proteins (in vitro); the plasma-concentration-time profile of the acyl glucuronide; and the stability of the in vivo adduct. We will study acyl glucuronides of several commonly used acidic drugs (tolmetin, naproxen, carprofen, ibuprofen and fenoprofen), as well as drugs of known serious toxicity to humans (zomepirac and benoxaprofen), some experimental (flunoxaprofen and oxaprozin), and endogenous (bilirubin) compounds.
Specific aims of the proposal are: 1). elucidate the interrelationship between irreversible protein binding and acyl glucuronide reactivity. Here the acyl glucuronides of a group of five primary compounds (tolmetin, zomepirac, benoxaprofen, flunoxaprofen and carprofen) will be isolated, and their acyl migration, reversible and irreversible binding to human serum albumin, will be determined. For chiral compounds, diastereomeric acyl glucuronides of R and S enantiomers will be investigated individually and compared; 2). investigate the relationship for the group of primary compounds between the reversible binding of acyl glucuronides to plasma protein and their irreversible binding; 3). establish the chemical structures of the irreversibly bound drug/protein complexes, determine the mechanism by which they are formed, pinpoint the site of binding and the role of isomeric glucuronides, and determine whether a single mechanism prevails for most acyl glucuronide; 4). measure the stability and disposition of selected albumin adducts in vivo in animals; 5). undertake dosing of approved drugs in healthy volunteers and patients; 6). determine whether acyl glucuronide adducts with proteins are effective immunogens and whether there is immunological cross reactivity between adducts from different glucuronides; and 7). investigate the relationship between irreversible binding in vivo and immunological toxicity.
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