Regulation of the biliary excretion of reduced glutathione (GSH) and glutathione disulfide (GSSG) and responses to selected model toxins were examined in male Sprague-Dawley rats. In control and phenobarbital-pretreated rats in which the intrahepatic concentration of GSH was modulated by the administration of diethyl maleate or acetaminophen, the biliary concentration of GSH was consistently lower than, but directly proportional to, the intrahepatic concentration of GSH. Furthermore, increments in bile flow produced by the infusion of sulfobromophthalein (BSP)-glutathione were associated with proportional increases in the biliary excretion of GSH, suggesting that GSH passes into bile passively along a concentration gradient. In contrast, GSSG appears to be secreted into bile against a steep concentration gradient. An increased hepatic production and biliary excretion of GSSG resulted from the administration of t-butyl hydroperoxide. Measurement of biliary GSSG and BSP during a constant infusion of the GSH adduct of BSP indicated that GSSG shares a common excretory mechanism with GSH adducts. Diquat, nitrofurantoin, and paraquat also markedly stimulated the biliary excretion of GSSG. On a molar basis, these compounds generated much more GSSG than a direct substrate for glutathione peroxidase such as t-butyl hydroperoxide, indicating that the compounds undergo redox-cycling with concomitant production of hydrogen peroxide. Aminopyrine (0.8 mmol/kg) also significantly increased biliary GSSG. This increase, however, was associated with a proportional increase in bile flow and in the biliary excretion of GSH such that the GSSG/GSH ratio in bile did not change. Acetaminophen and chloroform, two compounds generating electrophilic metabolites that deplete intrahepatic GSH, led to a progressive decrease in the biliary excretion of GSH and GSSG. Furosemide and dimethylnitrosamine, the electrophilic metabolites of which do not deplete hepatic GSH, minimally altered biliary GSH and GSSG. Similarly, carbon tetrachloride and iproniazid, which yield organic radical metabolites that can peroxidize membrane lipids, did not increase the biliary excretion of GSSG. This finding indicates that membrane-bound lipid hydroperoxides may not be good substrates for glutathione peroxidases. The measurement of the biliary excretion of GSSG and of the GSSG/GSH ratio in bile is a sensitive index of oxidative stress in vivo and thus complements other in vivo parameters for the study of reactive intermediates of xenobiotics such as the determination of covalent binding, the formation of lipid hydroxy acids, and the depletion of intracellular GSH.

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Biliary excretion of glutathione and glutathione disulfide in the rat. Regulation and response to oxidative stress.

Nitrofurantoin is a commonly used urinary tract antibiotic prescribed to lactating woman. It is actively transported into human and rat milk by an unknown mechanism. Our group has demonstrated an important role of the breast cancer resistance protein (BCRP/ABCG2) in the secretion of xenotoxins into the milk. This ATP-binding cassette drug efflux transporter extrudes xenotoxins from cells in intestine, liver, mammary gland, and other organs, affecting the pharmacological and toxicological behavior of many compounds. We investigated whether Bcrp1 is involved in the pharmacokinetic profile of nitrofurantoin and its active secretion into the milk. Using polarized cell lines, we found that nitrofurantoin is efficiently transported by murine Bcrp1 and human BCRP. After oral administration of 10 mg/kg nitrofurantoin, the area under the plasma concentrationtime curve in Bcrp1 knockout mice was almost 4-fold higher than in wild-type mice (148.8 +/- 30.4 versus 37.5 +/- 6.8 min x microg/ml); and after i.v. administration (5 mg/kg), 2-fold higher (139.2 +/- 22.0 versus 73.9 +/- 9.0 min x microg/ml). Hepatobiliary excretion of nitrofurantoin was almost abolished in Bcrp1 knockout mice (9.6 +/- 3.2 versus 0.2 +/- 0.1% in wild-type and Bcrp1 knockout mice, respectively). The milk-to-plasma ratio of nitrofurantoin was almost 80 times higher in wild-type compared with Bcrp1 knockout lactating female mice (45.7 +/- 16.2 versus 0.6 +/- 0.1). Nitrofurantoin elimination via milk was quantitatively even higher than hepatobiliary elimination. We conclude that Bcrp1 is an important determinant for the bioavailability of nitrofurantoin and the main mechanism involved in its hepatobiliary excretion and secretion into the milk.

The breast cancer resistance protein (BCRP/ABCG2) affects pharmacokinetics, hepatobiliary excretion, and milk secretion of the antibiotic nitrofurantoin.

(1988). The metabolic activation of nitroheterocyclic therapeutic agents. Drug Metabolism Reviews: Vol. 19, No. 1, pp. 33-62.

The metabolic activation of nitroheterocyclic therapeutic agents.

Nitrofurans are antimicrobial agents not authorised for use in food-producing animals in the European Union Nitrofurans are rapidly metabolised, occurring in animal tissues as protein-bound metabolites The European Commission requested EFSA to provide a scientific opinion on the risks to human health related to the presence of nitrofurans in food and whether a reference point for action (RPA) of 10 μg/kg for the marker metabolites is adequate to protect public health Data on occurrence of nitrofuran marker metabolites in food were extracted from the national residue monitoring plan results and from the Rapid Alert System for Food and Feed (RASFF) The CONTAM Panel concluded that these data were too limited to carry out a reliable human dietary exposure assessment Instead, human dietary exposure was calculated for a scenario in which a single nitrofuran marker metabolite is present at 10 μg/kg in foods of animal origin, excluding milk and dairy products The mean chronic dietary exposure for this worst-case scenario would range from 33 to 80 and 19 to 43 ng/kg bw per day for toddlers and adults, respectively Nitrofurans and their marker metabolites, generally, are genotoxic and carcinogenic and, also, have non-neoplastic effects in animals Margins of exposure (MOEs) were calculated at 20 × 105 or greater for carcinogenicity and at 25 × 103 or greater for non-neoplastic effects The CONTAM Panel concluded that it is unlikely that exposure to food contaminated with nitrofuran marker metabolites at or below 10 μg/kg is a health concern A scenario in which foods are considered to be contaminated with semicarbazide, from use of carrageenan as a food additive, at 1 μg/kg was used to assess whether it is appropriate to apply the RPA to foods of non-animal origin; MOEs of greater than 104 calculated for non-neoplastic effects do not indicate a health concern

/pdf/scientific-opinion-on-nitrofurans-and-their-metabolites-in-muppj6y6m6.pdf

Scientific Opinion on nitrofurans and their metabolites in food

Breast cancer resistance protein (BCRP/ABCG2) is an ATP-binding cassette efflux transporter, important in drug disposition and in the development of multidrug resistance in cancer. Flavonoids, a large class of natural compounds widely present in the diet and herbal products, have been shown in vitro to be BCRP inhibitors. The flavonoid chrysin is a potent inhibitor of BCRP, inhibiting the efflux of mitoxantrone with an IC(50) of 0.39 microM in BCRP-overexpressing human MCF-7 breast cancer cells. The purpose of this study was to investigate the potential pharmacokinetic interactions between chrysin and nitrofurantoin (a specific BCRP substrate) in rats. In Madin-Darby canine kidney cells expressing human BCRP or murine Bcrp1, the polarized transport of nitrofurantoin was effectively inhibited by chrysin at concentrations of 20 and 100 microM. Compared with the vehicle-treated group, p.o. coadministration of chrysin (200 mg/kg) significantly increased the area under the curve (AUC) and C(max) of nitrofurantoin (10 mg/kg) by 1.76- (p < 0.01) and 1.72-fold (p < 0.05), respectively. When nitrofurantoin (2 mg/kg) was given i.v., administration of chrysin (50 mg/kg i.p.) significantly increased the AUC of nitrofurantoin (123 +/- 34.0 versus 91.5 +/- 18.0 microg/ml x min in controls, p < 0.05). Moreover, the cumulative hepatobiliary excretion of nitrofurantoin (1.5 mg/kg i.v.) was significantly decreased by approximately 75% at the end of 120 min after the coadministration of chrysin (50 mg/kg i.p.). Taken together, these results indicate that the flavonoid chrysin significantly inhibits nitrofurantoin transport mediated by human BCRP and murine Bcrp1. Bcrp1 inhibition by chrysin is likely one potential mechanism for the observed chrysin-nitrofurantoin pharmacokinetic interactions in rats.

https://dmd.aspetjournals.org/content/dmd/early/2006/11/08/dmd.106.011684.full.pdf

M. F. Paul

Papers

Studies on the distribution and excretion of certain nitrofurans.

Inhibition of glutathione reductase by nitrofurantoin.