| Phase II metabolism, including sulfation and glucuronidation, occurs primarily in the liver, and is an important step in drug elimination. Phase II metabolites exhibit increased molecular weight and hydrophilicity, and undergo biliary or hepatic basolateral (and subsequent urinary) excretion. Much progress has been made recently in the mechanistic understanding of hepatic transport systems; however, transport mechanisms responsible for excretion of glucuronide, and especially sulfate conjugates from hepatocytes into bile and sinusoidal blood have not been elucidated completely. The long-term objective of this research was to characterize the transport systems responsible for the hepatic excretion of model sulfate and glucuronide conjugates, and to examine species differences in biliary excretion.; In rats, biliary excretion of acetaminophen sulfate and 4-methylumbelliferyl sulfate was mediated by both Bcrp1 and Mrp2, whereas the excretion of the glucuronide conjugates was mediated only by Mrp2. In contrast, in mice the biliary excretion of the sulfate conjugates was mediated by Bcrp1, whereas the glucuronide conjugates were excreted by Bcrp1, and to a lesser extent by Mrp2. Further species differences in the mechanisms of biliary excretion were evident for harmol sulfate and glucuronide, which were excreted by Bcrp1 in mice, but in rats were excreted by both Mrp2 and a non-Mrp2 and non-GF 120918-sensitive transport system. Thus, Mrp2 plays a more important role in the biliary excretion of these model glucuronide and sulfate conjugates in rats than in mice.; Hepatic basolateral transport of sulfate and glucuronide conjugates was examined using relevant gene knockout mice, which indicated that hepatic basolateral excretion of sulfate conjugates was mediated by multiple transport mechanisms, primarily Mrp3 and Mrp4. In contrast, basolateral excretion of glucuronide metabolites was mediated primarily by Mrp3. This project improved current mechanistic understanding of hepatic excretion that follows the formation of sulfate and glucuronide conjugates in the liver. Results of this dissertation research provide a framework for predicting the hepatobiliary disposition of active phase II conjugates, which may dictate the pharmacodynamics, hepatotoxicity, and/or systemic toxicity, of these metabolites, and will advance significantly the rational selection of pre-clinical species for biliary excretion studies in drug development. |
