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ABSTRACT:Etoposide, an important anticancer agent, undergoes glucuronidation both in vitro and in vivo. In this study, three isomeric glucuronides of etoposide, including one phenolic (EPG) and two alcoholic glucuronides (EAG1 and EAG2), were biosynthesized in vitro with human liver microsomes (HLMs), and identified by liquid chromatography-electrospray ionization-mass spectrometry and confirmed by ␤-glucuronidase cleavage. In vitro UDP-glucuronosyltransferase (

ABSTRACT:7-Ethyl-10-hydroxy-camptothecin (SN-38), the active metabolite of the anti-cancer agent irinotecan, contains a lactone ring that equilibrates with a carboxylate form. Since SN-38 lactone is the active and toxic form, it is prudent to examine whether the more soluble carboxylate is a surrogate for SN-38 lactone conjugation. Therefore, relative rates of glucuronidation and isoform specificity of SN-38 lactone and carboxylate were characterized. The stability of SN-38 lactone and carboxylate in incubation mixtures of microsomes and UDP-glucuronosyltransferase (UGT) isoforms was used to determine optimal incubation times. Microsomal incubations were conducted using rat and human intestinal and hepatic microsomes and human and rat recombinant UGT1A isoforms. Where estimates of lactone and carboxylate glucuronidation rates could not be established due to short incubation times and detection limits, kinetic modeling was used to recover these rate constants. The stability experiments revealed that the lactone was stabilized by rat microsomes, however, the opposite was observed in human microsomes and recombinant isoforms. For all tissues and most UGT isoforms examined, the lactone consistently had catalytic rates up to 6-fold greater than the carboxylate. The rank order of glucuronidation for both SN-38 lactone and carboxylate was 1A7 > 1A1 > 1A9 > 1A8 and 1A7 > 1A8 > 1A1 for human and rat isoforms, respectively. This study provides further support that SN-38 lactone and carboxylate may be considered pharmacokinetically distinct agents. The in vivo impact of this conjugation difference is unknown, since variations in protein binding and transport proteins may affect intracellular concentrations of the lactone or carboxylate.

Purpose-The effects of omeprazole on indinavir when administered alone or in combination with ritonavir were evaluated.Methods-Fourteen men and women age 18-55 years not infected with human immunodeficiency virus who met study qualifications were randomized to receive placebo, 20 mg of omeprazole, or 40 mg of omeprazole daily. After seven days, the single-dose pharmacokinetic profile of an 800-mg dose of indinavir alone or in combination with 200 mg of ritonavir was evaluated. Study participants received each of four study regimens in one of four randomly assigned orders. Blood samples were collected, and plasma indinavir and ritonavir concentrations were analyzed using high-performance liquid chromatography. Conclusion-The AUC of indinavir was substantially decreased in healthy volunteers who received omeprazole 20 or 40 mg daily for seven days before the administration of a single 800-mg dose of indinavir. Concomitant administration of ritonavir 200 mg with indinavir in participants receiving omeprazole led to a significant increase in the AUC of indinavir. Results-The Index termsAntiretroviral agents; Blood levels; Dosage; Drug interactions; Gastrointestinal drugs; HIV infections; Indinavir; Omeprazole; Pharmacokinetics; Ritonavir Protease inhibitors (PIs) exhibit a high degree of pharmacokinetic variability in patients infected with the human immunodeficiency virus (HIV). 1,2 Large interindividual differences in drug absorption and elimination in HIV-infected patients have been primarily attributed to constitutive or altered drug metabolizing enzymes, P-glycoprotein transporter activities, and poor drug solubility. 3 With the PIs indinavir and atazanavir, changes in gastric pH can alter drug absorption. 4,5 Specifically, when these PIs are administered with medications that Address correspondence to Dr. Kashuba at the School of Pharmacy, CB7360, University of North Carolina, Chapel Hill, NC 27599-7360 (akashuba@unc.edu increase gastric pH, such as histamine (H 2 )-receptor antagonists and proton-pump inhibitors (PPIs), bioavailability can decrease by up to 76%. 4,6 A survey of 200 HIV-infected patients was performed to assess their use of drugs that affect gastric acidity. 7 Fifty-six percent of HIV-infected patients who had recently begun highly active antiretroviral therapy (HAART) had taken nonprescription acid-reducing agents, and 39% had used both nonprescription and prescription products for acid reduction. Forty-six percent of patients on a PI-containing regimen had used PPIs or H 2 -receptor antagonists once they started HAART, and 35% had used them within the previous 12 months. This widespread use of acid-reducing agents among HIV-infected patients has implications for drug interactions with antiretrovirals and other medications that require an acidic environment for adequate dissolution and absorption.Ritonavir is a cytochrome P-450 (CYP) isoenzyme 3A and P-glycoprotein inhibitor which, when used in low doses, can increase the exposure of concomitantly administered PIs. The concomitant ad...

Life-threatening diarrhea afflicts a considerable percentage of patients treated with irinotecan, an anticancer agent with effects elicited through its active metabolite 7-ethyl-10-hydroxycamptothecin (SN-38). The primary detoxification pathway for SN-38 is glucuronidation. The purpose of this study was to evaluate the role that intestinal UDP-glucuronosyltransferases (UGTs) have from hepatic UGTs in modulating this diarrhea. To investigate this, Gunn rats devoid of UGT1A activity were injected with recombinant adenoviral vectors expressing UGT1A1, 1A6, and 1A7, resulting in reconstituted hepatic UGT expression comparable to a heterozygote. Hepatic microsome studies indicated that 4 to 7 days after adenoviral injection, transfected Gunn rats (j/jAV) had SN-38 glucuronide (SN-38G) formation rates three times higher than control heterozygote rats (jϩAV). The adenovirus did not impart any glucuronidating capacity to the intestine in j/jAV rats, whereas jϩAV rats possessed intestinal UGT function. After the administration of 20 mg/kg/day irinotecan i.p. to j/jAV rats 4 days after adenovirus injection, diarrhea ensued before the fourth irinotecan dose. jϩAV rats were spared the diarrhea, and the toxicity was mild compared with the j/jAV rats, as measured by diarrhea scores, weight loss, and histological assessments of the cecum and colon. The pharmacokinetics of irinotecan, SN-38, and SN-38G indicate that the systemic exposure of SN-38 and SN-38G was higher and lower, respectively, in j/jAV rats. Despite this, the biliary excretion of irinotecan and metabolites was similar. Because intestinal UGTs are the main discriminating factor between j/jAV and jϩAV rats, their presence seems to be critical for the gastrointestinal protection observed in jϩAV rats.The UDP-glucuronosyltransferases (UGTs) are a superfamily of metabolic enzymes that catalyze the transfer of UDP-glucuronic acid to endogenous and xenobiotic substrates. Glucuronidation is a detoxification mechanism from several standpoints. It decreases the apparent volume of distribution, increases the molecular weight, and increases substrate specificity for active transport by imparting a negative charge, which are all processes that facilitate substrate elimination from the body (Guillemette, 2003). Furthermore, in most cases, glucuronidation renders the substrate inactive with respect to its pharmacological or physiological target. A Wistar-derived rat model of UGT1A subfamily deficiency, the Gunn rat, has allowed tremendous insight into the importance of the UGT1A family in the metabolism and toxicity of substrates (Wells et al., 2004). The abolition of UGT1A glucuronidation in these rats stems from a frameshift mutation that yields a truncated, nonfunctional protein unable to bind UDP-glucuronic acid (Iyanagi et al., 1989).Although the primary organ of glucuronidation receiving most attention has been the liver, research on intestinal UGTs has shown their importance. Large differences in protein levels of the UGT1A family are not observed between these ...