The human organism is quite adept at handling potentially harmful xenobiotic substances, such as drugs, environmental pollutants and food contaminants, that have been ingested, inhaled or absorbed either actively or passively. While many organs and tissues contribute to the detoxication process, the liver is the major site of metabolism, where foreign materials are detoxified before normal biochemical processes are disrupted. Metabolism cannot be equated with detoxication in all cases, however. Many xenobiotic substances are metabolically transformed into compounds that are more toxic than the original substance. Since these metabolic processes are prevalent in the liver, this organ is particularly subject to the toxicity of such bioactivated intermediates. The proposed research will investigate novel therapeutic routes to alleviate the hepatotoxicity associated with high doses of the common analgesic agent, acetaminophen. Acetaminophen overdose depletes hepatic glutathione (GSH) and allows damage to normal cellular constituents. Supplying L-cysteine, one of the amino acid precursors of GSH, stimulates GSH biosynthesis and amplifies the existing protective mechanism. New prodrug forms of this amino acid, synthesized with disaccharide components, are proposed that allow selective delivery to the liver by capitalizing on the high level of carbohydrate receptors in this organ. Numerous parameters will be investigated to determine the biochemical basis for the efficacy of the prodrugs. Hepatic localization and the fate of the administered prodrugs will be studied using radio-labeled compounds. Hepatic levels of important thiols will be determined after the administration of the prodrugs. The activities of enzymes involved in GSH synthesis and metabolism may be important and will be investigated, as will the content of cytochrome P-450. All of these parameters will be studied both in the absence and the presence of acetaminophen to assess the prodrugs potential when the toxic insult is present. Protection by the prodrugs against the potent hepatotoxicity of acetaminophen will be determined in vivo. These studies offer the possibility of improving the management of acetaminophen overdose, which is a clinically relevant situation. The proposed research will provide important insight into the precesses of GSH regulation and may provide new agents with therapeutic utility.
