Christopher D. King

Genetic predisposition to the metabolism of irinotecan (CPT-11): Role of uridine diphosphate glucuronosyltransferase isoform 1A1 in the glucuronidation of its active metabolite (SN-38) in human liver microsomes

Irinotecan (CPT-11) is a promising antitumor agent, recently approved for use in patients with metastatic colorectal cancer. Its active metabolite, SN-38, is glucuronidated by hepatic uridine diphosphate glucuronosyltransferases (UGTs). The major dose-limiting toxicity of irinotecan therapy is diarrhea, which is believed to be secondary to the biliary excretion of SN-38, the extent of which is determined by SN-38 glucuronidation. The purpose of this study was to identify the specific isoform of UGT involved in SN-38 glucuronidation. In vitro glucuronidation of SN-38 was screened in hepatic microsomes from normal rats (n = 4), normal humans (n = 25), Gunn rats (n = 3), and patients (n = 4) with Crigler-Najjar type I (CN-I) syndrome. A wide intersubject variability in in vitro SN-38 glucuronide formation rates was found in humans. Gunn rats and CN-I patients lacked SN-38 glucuronidating activity, indicating the role of UGT1 isoform in SN-38 glucuronidation. A significant correlation was observed between SN-38 and bilirubin glucuronidation (r = 0.89; P = 0.001), whereas there was a poor relationship between para-nitrophenol and SN-38 glucuronidation (r = 0.08; P = 0.703). Intact SN-38 glucuronidation was observed only in HK293 cells transfected with the UGT1A1 isozyme. These results demonstrate that UGT1A1 is the isoform responsible for SN-38 glucuronidation. These findings indicate a genetic predisposition to the metabolism of irinotecan, suggesting that patients with low UGT1A1 activity, such as those with Gilberts syndrome, may be at an increased risk for irinotecan toxicity.

UDP-GLUCURONOSYLTRANSFERASES IN HUMAN INTESTINAL MUCOSA

While UDP-glucuronosyltransferases (UGTs) are known to be expressed at high levels in human liver, relatively little is known about extrahepatic expression. In the present study, UGT2B family isoforms involved in the glucuronidation of steroid hormones and bile acids have been characterized in microsomes prepared from jejunum, ileum and colon from six human subjects. Glucuronidation of androsterone and testosterone was highly significant and increased from proximal to distal intestine. In contrast, hyodeoxycholic acid was glucuronidated at a low level in jejunum and ileum and activity was barely detectable in colon. No significant glucuronidation of lithocholic acid was found. Small phenols were glucuronidated with much lower activity than found in liver. High levels of UGT protein were detected with polyclonal anti-rat androsterone- and testosterone-UGT antibodies, whereas UGT2B4, a major hepatic hyodeoxycholic acid-specific UGT, was undetectable using a highly specific anti-human UGT2B4 antibody. Screening for RNA expression by RT-PCR confirmed the absence of UGT2B4 and UGT1A6 and showed expression of UGT2B7, a hepatic isoform shown to glucuronidate androsterone, in all intestinal segments. To our knowledge, the presence of functional androsterone and testosterone directed isoforms in human intestine is a novel finding which supports the idea that the intestinal tract functions as a steroid-metabolizing organ and plays a significant role in steroid hormone biotransformation.

Metabolism of Endobiotics and Xenobiotics by UDP-Glucuronosyltransferase

Publisher Summary All organisms are exposed to a number of chemical compounds that are toxic were it not for metabolic mechanisms available to the organism to moderate their effects. Chemical substances may be xenobiotics, such as drugs, or compounds presented to the organism from environmental or dietary sources. The elimination of many such compounds and their detoxification involves different types of metabolic reactions. One of them is conjugation through glucuronic acid catalyzed by UDP-glucuronosyltransferases (UGTs). In addition to xenobiotics; endobiotics, such as steroids or bilirubin, are glucuronidated. Formation of glucuronides from xeno- and endo- biotics generally results in the formation of products that are more hydrophilic and more readily excreted by the kidney or liver. A number of UGTs have been purified to homogeneity from liver of several species, including humans. Recently identified are a number of UGTs that are important for the metabolism of xenobiotics and endobiotics, such as steroids and estrogen catechols. This chapter discusses the metabolism of important amines, opioids, and endobiotics, such as steroids and their catechol derivatives. There are details on glucuronidation of amines, glucuronidation of opioid compounds, and glucuronidation of catechol estrogens by UGTs. Many isoforms of UGT catalyze the glucuronidation of primary and secondary amines. In humans, many important therapeutic agents, such as the antihistamines, antipsychotics, and tricyclic antidepressants, are tertiary amines. Many of these tertiary amines, in humans, are converted to and excreted as quaternary ammonium-linked glucuronides. Stably expressed human UGT1.4 protein has been discovered to catalyze the N-glucuronidation of a variety of tertiary amines. To date, only human UGT1.4 has been identified to catalyze the formation of quaternary ammonium-linked glucuronides from tertiary amines. UGT2B1 and UGT1.1 have been demonstrated to catalyze the glucuronidation of opioid substances. UGT2B1 has high activity toward many opioid substrates, whereas UGT1.1 catalyzes the glucuronidation the oripavine-type opioid compounds.