Identification of S-(n-butylcarbamoyl)glutathione, a reactive carbamoylating metabolite of tolbutamide in the rat, and evaluation of its inhibitory effects on glutathione reductase in vitro

The inactivation of human CYP2E1 by phenethyl isothiocyanate, a naturally occurring chemopreventive agent, and its oxidative bioactivation

Phenethylisothiocyanate (PEITC), a naturally occurring isothiocyanate and potent cancer chemopreventive agent, works by multiple mechanisms, including the inhibition of cytochrome P450 (P450) enzymes, such as CYP2E1, that are involved in the bioactivation of carcinogens. PEITC has been reported to be a mechanism-based inactivator of some P450s. We describe here the possible mechanism for the inactivation of human CYP2E1 by PEITC, as well as the putative intermediate that might be involved in the bioactivation of PEITC. PEITC inactivated recombinant CYP2E1 with a partition ratio of 12, and the inactivation was not inhibited in the presence of glutathione (GSH) and not fully recovered by dialysis. The inactivation of CYP2E1 by PEITC is due to both heme destruction and protein modification, with the latter being the major pathway for inactivation. GSH-adducts of phenethyl isocyanate (PIC) and phenethylamine were detected during the metabolism by CYP2E1, indicating formation of PIC as a reactive intermediate following P450-catalyzed desulfurization of PEITC. Surprisingly, PIC bound covalently to CYP2E1 to form protein adducts but did not inactivate the enzyme. Liquid chromatography mass spectroscopy analysis of the inactivated CYP2E1 apo-protein suggests that a reactive sulfur atom generated during desulfurization of PEITC is involved in the inactivation of CYP2E1. Our data suggest that the metabolism of PEITC by CYP2E1 that results in the inactivation of CYP2E1 may occur by a mechanism similar to that observed with other sulfur-containing compounds, such as parathion. Digestion of the inactivated enzyme and analysis by SEQUEST showed that Cys 268 may be the residue modified by PIC.

Central role of mitochondria in drug-induced liver injury

A frequent mechanism for drug-induced liver injury (DILI) is the formation of reactive metabolites that trigger hepatitis through direct toxicity or immune reactions. Both events cause mitochondrial membrane disruption. Genetic or acquired factors predispose to metabolite-mediated hepatitis by increasing the formation of the reactive metabolite, decreasing its detoxification, or by the presence of critical human leukocyte antigen molecule(s). In other instances, the parent drug itself triggers mitochondrial membrane disruption or inhibits mitochondrial function through different mechanisms. Drugs can sequester coenzyme A or can inhibit mitochondrial β-oxidation enzymes, the transfer of electrons along the respiratory chain, or adenosine triphosphate (ATP) synthase. Drugs can also destroy mitochondrial DNA, inhibit its replication, decrease mitochondrial transcripts, or hamper mitochondrial protein synthesis. Quite often, a single drug has many different effects on mitochondrial function. A severe impairment of oxidative phosphorylation decreases hepatic ATP, leading to cell dysfunction or necrosis; it can also secondarily inhibit ß-oxidation, thus causing steatosis, and can also inhibit pyruvate catabolism, leading to lactic acidosis. A severe impairment of β-oxidation can cause a fatty liver; further, decreased gluconeogenesis and increased utilization of glucose to compensate for the inability to oxidize fatty acids, together with the mitochondrial toxicity of accumulated free fatty acids and lipid peroxidation products, may impair energy production, possibly leading to coma and death. Susceptibility to parent drug-mediated mitochondrial dysfunction can be increased by factors impairing the removal of the toxic parent compound or by the presence of other medical condition(s) impairing mitochondrial function. New drug molecules should be screened for possible mitochondrial effects.

Comprehensive quantitative and qualitative liquid chromatography-radioisotope-mass spectrometry analysis for safety testing of tolbutamide metabolites without standard samples

Liquid chromatography-radioisotope-mass spectrometry (LC-RI-MS) analysis was used to determine the structures of 12 (four previously unknown) (14) C-tolbutamide (TB) metabolites in rat biological samples (plasma, urine, bile, feces, and microsomes). The four novel metabolites are ω-carboxy TB, hydroxyl TB (HTB)-O-glucuronide, TB-ortho or meta-glutathion, and tolylsulphoaminocarbo-glutathion. In rat plasma, after oral administration of (14) C-TB at therapeutic dose (1 mg/kg) and microdose (1.67 µg/kg), the total RI and six metabolites [HTB, carboxy TB (CTB), M1: desbutyl TB, M2: ω-hydroxyl TB, M3: α-hydroxyl TB, and M4: ω-1-hydroxyl TB] were quantified by LC-RI-MS. The plasma concentrations were calculated using their response factors (MS-RI intensity ratio) without standard samples, and the area under the curve (AUC) of plasma concentration per time for evaluation of Safety Testing of Drug Metabolites (MIST) was calculated using the ratio of TB metabolites AUC/total RI AUC. The ratios were as follows: TB 94.5% and HTB 2.5% for the microdose (1.67 µg/kg) and TB 95.6%, HTB 0.96%, CTB 0.065%, M1 0.62%, M2 0.0035%, M3 0.077%, and M4 0.015% for the therapeutic dose (1 mg/kg). These values were less than 10% of the MIST criteria.

Characterization of a novel dithiocarbamate glutathione reductase inhibitor and its use as a tool to modulate intracellular glutathione

Thiol redox state (TRS) is an important parameter to reflect intracellular oxidative stress and is associated with various normal and abnormal biochemical processes. Agents that can be used to increase intracellular TRS will be valuable tools in TRS-related research. Glutathione reductase (GR) is a critical enzyme in the homeostasis of TRS. The enzyme catalyzes the reduction of GSSG to GSH to maintain a high GSH:GSSG ratio. Inhibition of the enzyme can be used to increase TRS. Despite the reports of various GR inhibitors, N,N-bis(2-chloroethyl)-N-nitrosourea, an anticancer drug with IC(50) = 647 microm against yeast GR, remains the most commonly used GR inhibitor in the literature. However, the toxicity caused by nonspecific interactions, as well as inhibition of DNA synthesis, complicates the use of N,N-bis(2-chloroethyl)-N-nitrosourea as a GR inhibitor. We report 2-acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylthiocarbonylamino)phenylthiocarbamoylsulfanyl]propionic acid (2-AAPA) as a novel irreversible GR inhibitor. 2-AAPA was prepared by one-step synthesis from commercially available reagents. The K(i) and k(inact) of 2-AAPA against yeast GR were determined to be 56 microm and 0.1 min(-1), respectively. At the concentration that produced >80% yeast GR inhibition, 2-AAPA showed no inhibition against glutamylcysteine synthetase, glutathione synthetase, catalase, and superoxide dismutase, but minimal inhibition against glutathione S-transferase and glutathione peroxidase. In CV-1 cells, 2-AAPA (0.1 mm) produced 97% GR inhibition, 25% GSH reduction, and a 5-fold increase in GSSG in 20 min. The compound can be a useful tool in TRS-related research.

2-Acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino)- phenylcarbamoylsulfanyl]propionic acid and its derivatives as a novel class of glutathione reductase inhibitors

Glutathione reductase (GR) catalyzes the reduction of oxidized glutathione to reduced glutathione. The enzyme is an attractive target for the development of antimalarial agents, agents to decrease malarial drug resistance and anticancer agents. In addition, inhibition of the enzyme has been employed as a tool in research for various purposes. In this paper, we present a rational design of 2-acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino)phenylcarbamoylsulfanyl]propionic acid and its derivatives as irreversible GR inhibitors. The K(i) and k(inact) values of 2-acetylamino-3-[4-(2-acetylamino-2-carboxyethylsulfanylcarbonylamino)phenylcarbamoylsulfanyl]propionic acid, the most potent derivative of the series, are 88 muM and 0.1 min(-1), respectively. Although the K(i) value of the inhibitor is in the micromolar range, it is more potent than N,N-bis(2-chloroethyl)-N-nitrosourea, which is currently the most commonly employed irreversible GR inhibitor with a reported IC(50) value of 646 microM. Additional attractive features of the inhibitor include its ready availability through a one-step synthesis and good solubility in both organic and aqueous solutions.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Glutathione and mercapturic acid conjugates of sulofenur and their activity against a human colon cancer cell line

Sulofenur is one of the diarylsulfonylureas developed as an anticancer agent. Sulofenur possesses a broad spectrum of activity in several solid tumor models and has undergone extensive clinical trials based on its impressive preclinical activity. However, the clinical response of sulofenur has been disappointing because of the side effect of anemia. Furthermore, the anticancer mechanism of sulofenur and its diarylsulfonylurea analogs still remains unknown. Elucidation of the metabolic fates of sulofenur may help to delineate the mechanism and provide information to guide the structural modification for more potent anticancer agents with less side effects. We have identified a glutathione conjugate and a mercapturic acid conjugate from sulofenur-dosed rats with the aid of liquid chromatography/mass spectrometry. The fraction of the dose of sulofenur as the glutathione conjugate in the dosed-rat bile over 5 h was 0.12 +/- 0.03%, and the mercapturic acid conjugate in urine over 24 h was 1.4 +/- 0.7%. Protein binding of the glutathione conjugate and mercapturic acid conjugate was determined to be 20 +/- 3 and 84 +/- 2%, respectively, as opposed to >99% of sulofenur. The high protein binding of sulofenur requires a higher than in vitro dose, which is believed to cause the side effect of anemia. The significance of this metabolic pathway is that both conjugates were found to be glutathione reductase inhibitors and to possess anticancer activity comparable to sulofenur against human colon adenocarcinoma GC(3)/c1 cells, a sulofenur-sensitive cell line. These conjugates may serve as new leads for the development of novel anticancer agents.

Glutathione-dependent metabolism of the antitumor agent sulofenur. Evidence for the formation of p-chlorophenyl isocyanate as a reactive intermediate

The antitumor agent sulofenur (LY186641), which has shown promising activity against a wide range of cancers, causes hemolytic anemia and methemoglobinemia at dose-limiting toxicities. The antitumor and toxicological mechanism(s) of action of the drug is (are) not well understood, but unlike other antineoplastic agents, sulofenur does not interfere with DNA, RNA, or protein synthesis, or with polynucleotide function. In the present study, we evaluated the hypothesis that sulofenur undergoes bioactivation in vivo to generate p-chlorophenyl isocyanate (CPIC), which could carbamoylate biological macromolecules directly or form a conjugate with glutathione (GSH) which would serve as a latent form of CPIC. The objectives of this study, therefore, were to determine if the GSH and N-acetylcysteine conjugates of CPIC were excreted into bile and urine, respectively, after an i.p. dose of sulofenur to rats. In addition, the chemical stability and thiol exchange properties of these S-linked conjugates were determined. The results of this study indicate that sulofenur does undergo metabolism in vivo to yield the GSH conjugate of CPIC, and that this conjugation reaction is reversible and subject to thiol exchange in buffered aqueous solution (pH 7.4, 37 degrees C). In contrast, sulofenur itself was stable under these same conditions, even in the presence of GSH and glutathione-S-transferase (GST), thus raising the possibility that bioactivation of sulofenur is necessary for liberation of CPIC. These findings suggest that the generation of this isocyanate in vivo and subsequent carbamoylation of biological macromolecules may play a role in the toxicity and/or antitumor activity of sulofenur and related diarylsulfonylureas.

Glutathione conjugation as a bioactivation reaction

In general, glutathione conjugation is regarded as a detoxication reaction. However, depending on the properties of the substrate, bioactivation is also possible. Four types of activation reaction have been recognized: direct-acting compounds, conjugates that are activated through cysteine conjugate beta-lyase, conjugates that are activated through redox cycling and lastly conjugates that release the original reactive parent compound. The glutathione S-transferases have three connections with the formation of biactivated conjugates: they catalyze their formation in a number of cases, they are the earliest available target for covalent binding by these conjugates and lastly, the parent alkylating agents are regularly involved in the induction of the enzymes. Individual susceptibility for each of these agents is determined by individual transferase subunit composition and methods are becoming available to assess this susceptibility.