Reductive and oxidative metabolism of nitrofurantoin in rat liver

Microsomal oxidative stress induced by NADPH is inhibited by nitrofurantoin redox biotranformation

Nitrofurantoin is used in the antibacterial therapy of the urinary tract. This therapy is associated with various adverse effects whose mechanisms remain unclear. Diverse studies show that the nitro reductive metabolism of nitrofurantoin leads to ROS generation. This reaction can be catalyzed by several reductases, including the cytochrome P450 (CYP450) reductase. Oxidative stress arising from this nitro reductive metabolism has been proposed as the mechanism underlying the adverse effects associated with nitrofurantoin. There is, however, an apparent paradox between these findings and the ability of nitrofurantoin to inhibit lipid peroxidation provoked by NADPH in rat liver microsomes. This work was aimed to show the potential contribution of different enzymatic systems to the metabolism of this drug in rat liver microsomes. Our results show that microsomal lipid peroxidation promoted by NADPH is inhibited by nitrofurantoin in a concentration-dependent manner. This suggests that the consumption of NADPH in microsomes can be competitively promoted by lipid peroxidation and nitrofurantoin metabolism. The incubation of microsomes with NADPH and nitrofurantoin generated 1-aminohidantoin. In addition, the biotransformation of a classical substrate of CYP450 oxidative system was competitively inhibited by nitrofurantoin. These results suggest that nitrofurantoin is metabolized through CYP450 system. Data are discussed in terms of the in vitro redox metabolism of nitrofurantoin.

Effect of pregnancy on nitrofurantoin disposition in mice

We investigated the effect of pregnancy on nitrofurantoin (NFT) disposition in wild-type and Bcrp1(-/-) mice. Pregnant and non-pregnant mice were administered NFT intravenously (5 mg/kg) or orally (10 mg/kg). Blood samples were collected at various times (5-60 min) after drug administration, plasma NFT concentrations determined by HPLC/UV, and pharmacokinetic parameters estimated. Dose-normalized area under the plasma concentration-time curve (AUC), terminal plasma half-life (T(1/2)), total plasma clearance (CL), and steady-state volume of distribution (V(ss)) of intravenous NFT in wild-type or Bcrp1(-/-) mice were not altered by pregnancy. After oral administration, pregnancy did not affect dose-normalized AUC of NFT in wild-type mice; however, dose-normalized AUC in Bcrp1(-/-) mice was decreased by approximately 70% by pregnancy. In conclusion, since Bcrp1 plays a minor role in the systemic clearance of NFT in female mice, pregnancy did not affect disposition of intravenous NFT despite the fact that Bcrp1 expression in the liver and kidney of mice is significantly induced by pregnancy. On the other hand, pregnancy may affect expression and activity of certain intestinal efflux transporters and/or metabolic enzymes in Bcrp1(-/-) mice, resulting in a drastic decrease in the systemic exposure of oral NFT in pregnant Bcrp1(-/-) mice.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Nitrofurazone disposition by perfused rat liver. Effect of dose size and glutathione depletion

The disposition of nitrofurazone was studied in the isolated perfused rat liver using a recirculating system. The drug was administered as a bolus in two different doses (3.5 and 14 mg: initial concentrations 0.35 and 1.4 mM respectively), and its disappearance was monitored by analyzing perfusate samples at various times. Biliary excretion and bile flow were also measured. In all experiments perfusate disappearance was monoexponential, and no significant difference was found between the two doses (T 1/2: 5.34 +/- 2.03 and 6.19 +/- 1.47 min for 14 and 3.5 mg respectively). Bile flow increased more than 2-fold 5-10 min after administration of the drug and subsequently returned to control levels. The increase in bile flow was dose-related and paralleled the excretion of the parent drug in the bile; however, of the total dose administered, only 0.27 +/- 0.04% was excreted unchanged in bile, thus ruling out an osmotic choleresis due to the parent drug. Since nitrofurazone may be excreted in part as a glutathione conjugate, this or other metabolites could have caused an osmotic choleresis. This hypothesis was tested by administering diethylmaleate which causes glutathione depletion. Although the initial bile flow in treated livers was not different from untreated livers, bile flow did not increase after administration of nitrofurazone. In addition, the perfusate half-life of nitrofurazone was increased (18.18 +/- 1.30 min, P less than 0.005). These results suggest that nitrofurazone is cleared rapidly by the liver and that glutathione plays an important role in its disposition.

Reductive metabolism and activation of benznidazole

Benznidazole (Bz) (N-benzyl-2-nitro-1-imidazole-acetamide) is a drug used against Chagas' disease. Rat liver microsomal and cytosolic fractions, but not mitochondria, exhibited Bz nitroreductase activity under anaerobic conditions in the presence of NADPH. Microsomal nitroreductase activity was enhanced by FAD and was inhibited totally by oxygen and partially by carbon monoxide. Liver cystosol fraction was able to reduce Bz nitrogroups in the presence of either N-methylnicotinamide or hypoxanthine as substrates. These enzyme activities were inhibited by menadione or allopurinol respectively. Under every experimental condition leading to enzymatic reduction of Bz nitrogroups and its inhibition or enhancement, reactive metabolites that bind covalently to proteins were also produced. This covalent binding was effectively prevented by reduced glutathione. Results suggest the participation of cytochrome P-450 and cytochrome c reductase in liver microsomal processes and of xanthine oxidase and aldehyde oxidase in liver cytosolic processes of Bz nitroreduction and activation to reactive metabolites that bind covalently to proteins. Possible pharmacological and toxicological implications of the described observations were discussed.

Reduction of nitroheterocyclic compounds by mammalian tissues in vivo

To determine whether nitro group reduction occurs in mammalian tissues, metronidazole (0.021, 0.064 and 10 mg/kg), misonidazole (0.015 mg/kg) and nitrofurazone (0.13 mg/kg), respectively, were administered to germfree rats. A reduced metabolite [1-(2-aminoimidazol-1-yl)-3-methoxypropan-2-ol] and two of its hydrolysis products, urea and (2-hydroxy-3-methoxypropyl)-guanidine, were found in the urines of germfree rats that received misonidazole. When nitrofurazone was administered, a reduced metabolite, 4-cyano-2-oxobutyraldehyde semicarbazone, was detected in the urines. However, acetamide and N-(2-hydroxyethyl)oxamic acid, fragmentation products from the reduction of metronidazole, were not found in significant concentrations in the urine when germfree rats received metronidazole. Apparently metronidazole is reduced so much more slowly than misonidazole and nitrofurazone in the tissues of germfree rats that its reductive metabolites are not detectable. This observation may be explained by the one-electron reduction potentials (E1 7) of these drugs, that of metronidazole (E1 7 = -486 mV) being lower than those of either misonidazole (E1 7 = -389 mV) or nitrofurazone (E1 7 = -257 mV). Under these circumstances, metronidazole reduction is not detected, either because its radical anion forms more slowly than that of the other nitroheterocyclic compounds or because its radical anion interacts more rapidly with oxygen to restore the parent compound.