Glutathione and N-acetylcysteine conjugates of 2-chloroethyl isocyanate. Identification as metabolites of N,N'-bis(2-chloroethyl)-N-nitrosourea in the rat and inhibitory properties toward glutathione reductase in vitro

Direct Detection of Glutathione Biosynthesis, Conjugation, Depletion and Recovery in Intact Hepatoma Cells

Nuclear magnetic resonance (NMR) spectroscopy was used to monitor glutathione metabolism in alginate-encapsulated JM-1 hepatoma cells perfused with growth media containing [3,3′-13C2]-cystine. After 20 h of perfusion with labeled medium, the 13C NMR spectrum is dominated by the signal from the 13C-labeled glutathione. Once 13C-labeled, the high intensity of the glutathione resonance allows the acquisition of subsequent spectra in 1.2 min intervals. At this temporal resolution, the detailed kinetics of glutathione metabolism can be monitored as the thiol alkylating agent monobromobimane (mBBr) is added to the perfusate. The addition of a bolus dose of mBBr results in rapid diminution of the resonance for 13C-labeled glutathione due to a loss of this metabolite through alkylation by mBBr. As the glutathione resonance decreases, a new resonance due to the production of intracellular glutathione-bimane conjugate is detectable. After clearance of the mBBr dose from the cells, intracellular glutathione repletion is then observed by a restoration of the 13C-glutathione signal along with wash-out of the conjugate. These data demonstrate that standard NMR techniques can directly monitor intracellular processes such as glutathione depletion with a time resolution of approximately < 2 min.

pH-dependent general base catalyzed activation rather than isocyanate liberation may explain the superior anticancer efficacy of laromustine compared to related 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine prodrugs

Laromustine (also known as cloretazine, onrigin, VNP40101M, 101M) is a prodrug of 90CE, a short-lived chloroethylating agent with anticancer activity. The short half-life of 90CE necessitates the use of latentiated prodrug forms for in vivo treatments. Alkylaminocarbonyl-based prodrugs such as laromustine exhibit significantly superior in vivo activity in several murine tumor models compared to analogs utilizing acyl, and alkoxycarbonyl latentiating groups. The alkylaminocarbonyl prodrugs possess two exclusive characteristics: (i) They are primarily unmasked by spontaneous base catalyzed elimination; and (ii) they liberate a reactive carbamoylating species. Previous speculations as to the therapeutic superiority of laromustine have focused upon the inhibition of enzymes by carbamoylation. We have investigated the therapeutic interactions of analogs with segregated chloroethylating and carbamoylating activities (singly and in combination) in the in vivo murine L1210 leukemia model. The combined treatment with chloroethylating and carbamoylating prodrugs failed to result in any synergism and produced a reduction in the therapeutic efficacy compared to the chloroethylating prodrug alone. Evidence supporting an alternative explanation for the superior tumor selectivity of laromustine is presented that is centered upon the high pH sensitivity of its base catalyzed activation, and the more alkaline intracellular pH values commonly found within tumor cells.

Identification of novel reaction products of methylene-bis-phenylisocyanate ("MDI") with oxidized glutathione in aqueous solution and also during incubation of MDI with a murine hepatic S9 fraction

Methylene diphenyl diisocyanate (MDI) is an important industrial chemical and asthmagenic respiratory sensitizer, however its metabolism remains unclear. In this study we used LC-MS and LC-MS/MS to identify novel reaction products of MDI with oxidized glutathione (GSSG), including an 837m/z [M+H](+) ion corresponding to GSSG bound (via one of its N-termini) to partially hydrolyzed MDI, and an 863m/z [M+H](+) ion corresponding to GSSG cross-linked by MDI (via its two γ-glutamate N-termini) [corrected]. Further studies with heavy isotope labeled and native reduced glutathione (GSH) identified an [M+H](+) ion corresponding to previously described mono(GSH)-MDI, and evidence for "oligomeric" GSH-MDI conjugates. This study also investigated transformational changes in MDI after incubation with an S9 fraction prepared from murine liver. LC-MS analyses of the S9 reaction products revealed the formation of [M+H](+) ions with m/z's and retention times identical to the newly described GSSG-MDI (837 and 863) conjugates and the previously described mono(GSH)-MDI conjugate. Together the data identify novel biological transformations of MDI, which could have implications for exposure-related health effects, and may help target future in vivo studies of metabolism.

In vitro cleavage of diisocyanate-glutathione conjugates by human gamma-glutamyl transpeptidase-1

Isocyanates differ from many other xenobiotics in their ability to form S-linked conjugates with glutathione (GSH) through direct nucleophilic addition reactions (e.g. without enzymatic "preactivation" and/or transferase activity), potentially predisposing them to metabolism via the mercapturic acid pathway. In vivo, mono-isocyanates are metabolized via the mercapturic acid pathway and excreted as N-acetylated cysteine conjugates, however, the metabolism of di-isocyanates remains unclear. We assessed the ability of purified human gamma-glutamyl transpeptidase-1 (GGT-1), a primary enzyme of the mercapturic acid pathway, to cleave S-linked GSH conjugates of 4,4'-methylene diphenyl diisocyanate (MDI) and 1,6-hexamethylene diisocyanate (HDI), two widely used industrial chemicals. A combination of liquid chromatography (LC), tandem mass spectrometry (MS/MS) and hydrogen-deuterium exchange studies confirmed GGT-1 mediated formation of the 607.2 and 525.2 m/z (M + H)(+) ions corresponding to bis(cys-gly)-MDI and bis(cys-gly)-HDI, respectively, the cleavage products expected from the corresponding bis(GSH)-diisocyanate conjugates. Additional intermediate metabolites and mono(cys-gly)-conjugates with partially hydrolyzed diisocyanate were also observed. Consistent with GGT enzyme kinetics, metabolism proceeded more rapidly under conditions that favored transpeptidation versus hydrolytic mechanisms of cleavage. Together the data demonstrate the capacity of human GGT-1 to cleave GSH conjugates of both aromatic and aliphatic diisocyanates, suggesting a potential role in their metabolism.

Carmustine enhances the anticancer activity of selenite in androgen-independent prostate cancer cells

Apoptosis is one of the major mechanisms targeted in the development of therapies against various cancers, including prostate cancer. Resistance to chemotherapy poses a significant problem for the effective treatment of androgen-independent (hormone-refractory) prostate cancer. Although high concentrations of sodium selenite exert strong anticarcinogenic effects in several cell culture systems and animal models, the therapeutic potential of selenite in patients with advanced or metastatic prostate cancer is extremely limited by the genotoxicity of high-dose selenite. We examined the ability of nontoxic concentrations of selenite to promote apoptosis and inhibit proliferation in carmustine-sensitized androgen-independent human prostate cancer cells. Androgen-dependent LNCaP cells exhibited a significant decrease in cell viability when exposed to nontoxic concentrations of selenite, whereas androgen-independent PC-3 and DU145 cells showed a significant decrease in cell viability only at higher concentrations. Treatment of PC-3 cells with a combination of nontoxic selenite and carmustine resulted in greater increases in cytotoxicity, reactive oxygen species generation, growth inhibition, apoptosis, and DNA double-strand breaks, with concomitant decreases in DNA synthesis, glutathione, glutathione reductase, and antiapoptotic proteins. Combination treatment with carmustine and selenite triggered caspase-dependent apoptosis in PC-3 cells, which was not apparent when these cells were treated with selenite or carmustine alone. Genotoxicity in normal prostate epithelial cells was completely absent in the combination treatment of carmustine and selenite. In addition, carmustine decreased the induction of DNA double strand breaks by high-dose selenite in normal prostate epithelial cells. This is the first study to demonstrate that a nontoxic dose of selenite, in combination with carmustine, significantly induces apoptosis and growth inhibition in androgen-independent prostate cancer cells without causing undesirable genotoxicity in normal prostate epithelial cells, suggesting that this combination therapy may be a promising therapeutic approach in the treatment of patients with metastatic hormone-refractory prostate cancer.

Toluene diisocyanate reactivity with glutathione across a vapor/liquid interface and subsequent transcarbamoylation of human albumin

Glutathione has previously been identified as a reaction target for toluene diisocyanate (TDI) in vitro and in vivo, and has been suggested to contribute to toxic and allergic reactions to exposure. In this study, the reactivity of reduced glutathione (GSH) with TDI in vitro was further investigated using a mixed phase (vapor/liquid) exposure system to model the in vivo biophysics of exposure in the lower respiratory tract. HPLC/MS/MS was used to characterize the observed reaction products. Under the conditions tested, the major reaction products between TDI vapor and GSH were S-linked bis(GSH)-TDI and to a lesser extent mono(GSH)-TDI conjugates (with one N═C═O hydrolyzed). The vapor-phase-generated GSH-TDI conjugates were capable of transcarbamoylating human albumin in a pH-dependent manner, resulting in changes in the self-protein's conformation/charge, on the basis of electrophoretic mobility under native conditions. Specific sites of human albumin-TDI conjugation, mediated by GSH-TDI, were identified (Lys(73), Lys(159), Lys(190), Lys(199), Lys(212), Lys(351), Lys(136/137), Lys(413/414), and Lys(524/525)) along with overlap with those susceptible to direct conjugation by TDI. Together, the data extend the proof-of-principle for GSH to act as a "shuttle" for a reactive form of TDI, which could contribute to clinical responses to exposure.

5-oxo-ETE is a major oxidative stress-induced arachidonate metabolite in B lymphocytes

B lymphocytes convert arachidonic acid (AA) to the 5-lipoxygenase products leukotriene B4 (LTB4) and 5-hydroxy-6,8,11,14-eicosatetraenoic acid (5-HETE) when subjected to oxidative stress. 5-HETE has little biological activity, but can be oxidized by a selective dehydrogenase in some cells to 5-oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE), a potent eosinophil chemoattractant. We found that CESS cells, a B lymphocyte cell line, convert AA to 5-oxo-ETE and this is selectively stimulated by oxidative stress. In the presence of H2O2, 5-oxo-ETE is a major AA metabolite in these cells (5-oxo-ETE≈5-HETE>LTB4). The cyclooxygenase product 12-hydroxy-5,8,10-heptadecatrienoic acid is also formed, but is not affected by H2O2. Diamide had effects similar to those of H2O2 and both substances had similar effects on human tonsillar B cells. H2O2 also stimulated 5-oxo-ETE formation from its direct precursor 5-HETE in tonsillar B and CESS cells, and this was inhibited by the glutathione reductase inhibitor carmustine. H2O2 concomitantly induced rapid increases in GSSG and NADP+ and reductions in GSH and NADPH. We conclude that oxidative stress stimulates 5-oxo-ETE synthesis in B lymphocytes by two mechanisms: activation of 5-lipoxygenase and increased oxidation of 5-HETE by NADP+-dependent 5-hydroxyeicosanoid dehydrogenase. B lymphocyte-derived 5-oxo-ETE could contribute to eosinophilic inflammation in asthma and other allergic diseases.

BCNU-sequestration by metallothioneins may contribute to resistance in a medulloblastoma cell line

PURPOSE

Resistance of neoplastic cells to the alkylating drug BCNU [1,3-bis(2-chloroethyl)-1-nitrosourea] has been correlated with expression of O (6)-methylguanine-DNA methyltransferase, which repairs the O (6)-chloroethylguanine produced by the drug. Other possible mechanisms of resistance include raised levels of glutathione or increased repair of the DNA interstrand cross-links formed by BCNU. Transcriptional profiling revealed the upregulation of several metallothionein (MT) genes in a BCNU-resistant medulloblastoma cell line [D341 MED (OBR)] relative to its parental line. Previous studies have shown that MTs, through their reactive thiol groups can quench nitrogen mustard-derived alkylating drugs. In this report, we evaluate whether MTs can also quench BCNU.

METHODS

To demonstrate the binding of BCNU to MT, we used an assay that measured the release of the MT-bound divalent cations (Zn(2+), Cd(2+)) upon their displacement by the drug. We also measured the decomposition rates of BCNU at those reaction conditions.

RESULTS

The rate of release of the cations was higher in pH 7.4 than at pH 7.0, which is likely a result of more rapid decomposition of BCNU (thus faster release of MT-binding intermediate) at pH 7.4 than at pH 7.0.

CONCLUSION

We demonstrate that resistance to BCNU may be a result of elevated levels of MTs which act by sequestering the drug's decomposition product(s).

Transient cerebral hypoperfusion enhances intraarterial carmustine deposition into brain tissue

We hypothesized that bolus injections of lipid soluble chemotherapeutic drugs during transient cerebral hypoperfusion could significantly boost regional drug delivery. In the first two groups of New Zealand White rabbits we measured brain tissue carmustine concentrations after intravenous infusion, intraarterial infusion with normal perfusion, and after intraarterial injections during transient cerebral hypoperfusion. In the third group of animals we assessed the safety of the technique by assessing electroencephalographic changes for 6 h after flow arrest carmustine administration and subsequent histological examination. The brain tissue carmustine concentrations were fivefold to sevenfold higher when the drug was injected during cerebral hypoperfusion compared to a conventional intracarotid infusion (68.4 +/- 24.5 vs. 14.2 +/- 8.3 microg/g, n = 5 each, respectively, P < 0.0001). The brain tissue carmustine concentrations (y) were a linear function of the bolus dose (x) injected during cerebral hypoperfusion, y = 10.4 x x - 21 (R = 0.84, P < 0.001). Stable EEGs were recorded several hours after flow arrest carmustine exposure and histological examinations did not reveal any gross evidence of cerebral injury. Transient cerebral hypoperfusion during intraarterial bolus injection of carmustine significantly increases drug delivery. Clinical techniques that decrease CBF, such as, transient arterial occlusion by balloon tipped catheters, hyperventilation, hypothermia, induced hypotension, or transient circulatory arrest, could enhance intraarterial drug delivery to the brain. We believe that the mechanisms for improved drug delivery is the decrease in drug dilution by reduced or absent blood flow, decreased protein binding and a longer time for high concentrations of free drugs to transit through the blood brain barrier.

Acute depletion of reduced glutathione causes extensive carbonylation of rat brain proteins

This study was aimed at establishing whether oxidative stress induced by acute depletion of brain glutathione (GSH) is sufficient to generate protein carbonyls (PCOs). To this end, rat brain slices were incubated separately with the GSH depletors 1,3-bis[2-chloroethyl]-1-nitrosourea (BCNU) and diethyl maleate (DEM), and protein carbonylation was assessed on Western blots after derivatization with dinitrophenyl hydrazine. Incubation with 1 mM BCNU or 10 mM DEM for 2 hr decreased GSH levels by > 70%. Under these conditions the carbonylation of several proteins (40-120 kDa) increased by 2-3 fold. Isolation of carbonylated proteins showed that augmented PCOs represents a rise in the amount of oxidized protein. The iron chelator deferoxamine, the superoxide scavenger rutin and the H2O2 quencher dimethylthiourea all prevented DEM-induced protein carbonylation and lipid peroxidation (TBARS), indicating that the underlying mechanism involves the iron-catalyzed generation of hydroxyl radicals from H(2)O(2) (Fenton reaction). Inhibition of catalase activity with sodium azide and aminotriazole, and glutathione peroxidase activity with mercaptosuccinic acid did not increase PCOs or TBARS, suggesting that increased production of reactive oxygen species (ROS) rather than compromised cellular antioxidant defenses is the cause for the accumulation of H2O2 after GSH depletion. PCO formation was not affected by the xanthine oxidase inhibitor oxypurinol but it was reduced by SKF-525A and carbonyl cyanide 3-chlorophenylhydrazone, indicating that the microsomal monooxygenase system and the mitochondrial electron transport system are the major sources of ROS. Consistent with these findings, subcellular fractionation studies showed that mitochondria and synaptosomes are the major PCO-containing organelles. These results were also supported by the anatomic distribution of PCOs in brain. Our observations may be important in the context of multiple sclerosis where decreased GSH, mitochondrial dysfunction, excessive production of ROS, and increased protein carbonylation have all been reported.

Differential inhibition of cellular glutathione reductase activity by isocyanates generated from the antitumor prodrugs Cloretazine™ and BCNU

The antitumor, DNA-alkylating agent 1,3-bis[2-chloroethyl]-2-nitrosourea (BCNU; Carmustine), which generates 2-chloroethyl isocyanate upon decomposition in situ, inhibits cellular glutathione reductase (GR; EC 1.8.1.7) activity by up to 90% at pharmacological doses. GR is susceptible to attack from exogenous electrophiles, particularly carbamoylation from alkyl isocyanates, rendering the enzyme unable to catalyze the reduction of oxidized glutathione. Evidence implicates inhibition of GR as a cause of the pulmonary toxicity often seen in high-dose BCNU-treated animals and human cancer patients. Herein we demonstrate that the prodrug Cloretazine (1,2-bis[methylsulfonyl]-1-[2-chloroethyl]-2-[(methylamino)carbonyl]hydrazine; VNP40101M), which yields methyl isocyanate and chloroethylating species upon activation, did not produce similar inhibition of cellular GR activity, despite BCNU and Cloretazine being equally potent inhibitors of purified human GR (IC(50) values of 55.5 microM and 54.6 microM, respectively). Human erythrocytes, following exposure to 50 microM BCNU for 1h at 37 degrees C, had an 84% decrease in GR activity, whereas 50 microM Cloretazine caused less than 1% inhibition under the same conditions. Similar results were found using L1210 murine leukemia cells. The disparity between these compounds remained when cells were lysed prior to drug exposure and were partially recapitulated using purified enzyme when 1mM reduced glutathione was included during the drug exposure. The superior antineoplastic potential of Cloretazine compared to BCNU in animal models could be attributed in part to the contribution of the methyl isocyanate, which is synergistic with the co-generated cytotoxic alkylating species, while at the same time unable to significantly inhibit cellular GR.

A randomized trial of amifostine and carmustine-containing chemotherapy to assess lung-protective effects

We conducted a randomized, double blind, placebo-controlled multi-institutional trial to assess the ability of amifostine to protect patients against acute lung injury associated with cyclophosphamide/cisplatin/carmustine (BCNU) (STAMP I), a BCNU-containing high dose chemotherapy regimen used with hematopoietic cell transplantation. Amifostine was administered in a dose of 740 mg/m(2) for 2 doses preceding administration of BCNU, the presumed pulmonary-toxic component of the regimen. The trial was stopped after 79 patients were randomized and a planned interim analysis demonstrated that it was unlikely that pulmonary cytoprotection would be detected with further accrual. We conclude that amifostine, used in the dose and schedule we tested, does not reduce the incidence of acute lung injury produced by STAMP I. Further, we suggest that amifostine use with BCNU in other contexts and with clinically achievable doses is unlikely to protect the lung from BCNU-associated acute injury.

Peroxide toxicity in conditioned lens epithelial cells – evaluation of multi-defense systems

Immortal murine lens epithelial cells which were conditioned to survive peroxide stress were found to have a remarkable increase in catalase activity as well as lesser changes in a number of other antioxidative defense systems [Invest. Ophthalmol. Vis. Sci. 43 (2002) 3251]. Furthermore, the gene expression of hundreds of other genes was altered. In order to determine the relative importance of catalase, other enzyme systems which maintain the reducing environment of the cell and the involvement of Fenton chemistry, an analysis of the effect of inhibiting catalase, disruption of the cells' reducing environment by inhibition of GSSG reductase (GR) and chelation of metal ion was investigated. It was found that inhibition of catalase caused peroxide resistant cells to die within 48-72 hr when exposed to normally tolerated concentrations of peroxide. If 1,10-phenanthroline (OP), an effective metal ion chelator was present, the cells were not affected by catalase inhibition and survived peroxide stress. Peroxide vulnerable unconditioned control cells were similarly protected by the chelator. The results demonstrate that H2O2 itself has minimal toxicity and that it is the products resulting from interaction with metal ion that produces lethal toxicity. In stark contrast, however, metal chelation did not protect the cells when GR was inhibited by BCNU. Examination of non-protein thiol (NP-SH), which is primarily GSH, indicated that rapid and extensive oxidation occurred almost immediately after exposure to peroxide under all conditions. However, NP-SH returns to the normal range in the conditioned cells even though later cell death is observed in some cases, suggesting fatal damage during the period when the cell is exposed to an oxidizing environment. Examination of DNA damage by alkaline elution indicated that H2O2 caused little observed strand breakage in peroxide resistant cells even if catalase is inhibited, suggesting that such cells have developed other systems to protect DNA and that H2O2 induced death is probably not related to DNA single strand breaks. In contrast, unconditioned cells (C cells) show extensive H2O2 induced DNA damage which is prevented by OP. Thus, depending on the conditions, DNA damage may contribute to cell death. The overall results indicate that the conditioned cell lines are not simply dependent on catalase activity but have developed a complex defense which includes GSH dependent systems and possibly more effective regulation of metal ion concentrations to resist oxidative stress.

Biomedical and biological mass spectrometry

This review focuses on biological and biomedical mass spectrometry, and covers a selection of publications in this area included in the MEDLINE database for the period 1987-2001. Over the last 15 years, biological and biomedical mass spectrometry has progressed out of all recognition. The development of soft ionization methods, such as electrospray ionization and matrix-assisted laser desorption ionization, has mainly contributed to the remarkable progress, because they can easily produce gas-phase ions of large, polar, and thermally labile biomolecules, such as proteins, peptides, nucleic acids and others. The innovations of ionization methods have led to remarkable progress in mass spectrometric technology and in biochemistry, biotechnology and molecular biology research. In addition, mass spectrometry is one of the powerful and effective technologies for drug discovery and development. It is applicable to studies on structural determination, drug metabolism, including pharmacokinetics and toxicokinetics, and de novo drug discovery by applying post-genomic approarches. In the present review, the innovative soft ionization methods are first discussed along with their features. Also, the characteristics of the mass spectrometers which are active in the biological and biomedical research fields are also described. In addition, examples of the applications of biological and biomedical mass spectrometry are provided.

Synthesis, characterization, and solvolysis of mono- and bis-S-(glutathionyl) adducts of methylene-bis-(phenylisocyanate) (MDI)

Bifunctional isocyanates are highly reactive compounds that undergo nucleophilic attack by a variety of functional groups available in the biological system. While the etiology of the respiratory disease caused by diisocyanates is not fully understood, a great deal of research has been performed to elucidate the chemical mechanisms involved in the direct and indirect effects of these compounds. Since adducts of isocyanates are found not only to proteins along the entire respiratory tree but also to proteins in the circulatory system, it is likely that a transport mechanism for the isocyanate from the respiratory to the circulatory system exists. The initial reaction of isocyanates with cellular thiols to form thiocarbamates, which are known to release the isocyanate under physiological conditions, is believed to provide a possible carrier mechanism for the isocyanate functional group. Previous work with aliphatic mono-isocyanates and the aromatic diisocyanate toluene diisocyanate has demonstrated the feasibility of this mechanism. Adding to this database, the products of the reaction of the highly water-insoluble, low vapor pressure, methylene-bis-(phenylisocyanate) (MDI) with glutathione were synthesized, and their chemical stability under various pH and buffer conditions was tested. Novel synthetic routes were developed for both the mono- and bis-S-(glutathionyl) adducts with MDI that yielded each compound in analytically pure form. Both compounds were found to be unstable under mild basic conditions (phosphate-buffered saline, pH 7.4, and NaHCO(3), pH 8.2), however to a different degree. Furthermore, a significant influence of the pH value (the rate of degradation increases with pH) and the concentration of free glutathione (increasing thiol stabilizes the adduct) on the stability was observed, indicating a base-catalyzed mechanism of the degradation/formation of the thiocarbamate bond. Unlike the monoadduct, which forms almost exclusively the polyurea upon degradation, a variety of products were formed upon degradation of the bis adduct. Though the disappearance of the bis adduct was complete as measured by HPLC, (1)H NMR spectra showed the existence of residual thiocarbamate bonds in the final mixture. In both cases, no evidence of the free methylene-bis-phenylamine (MDA) could be detected under the applicable conditions.

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.

Studies on the metabolism of troglitazone to reactive intermediates in vitro and in vivo. Evidence for novel biotransformation pathways involving quinone methide formation and thiazolidinedione ring scission

Therapy with the oral antidiabetic agent troglitazone (Rezulin) has been associated with cases of severe hepatotoxicity and drug-induced liver failure, which led to the recent withdrawal of the product from the U.S. market. While the mechanism of this toxicity remains unknown, it is possible that chemically reactive metabolites of the drug play a causative role. In an effort to address this possibility, this study was undertaken to determine whether troglitazone undergoes metabolism in human liver microsomal preparations to electrophilic intermediates. Following incubation of troglitazone with human liver microsomes and with cDNA-expressed cytochrome P450 isoforms in the presence of glutathione (GSH), a total of five GSH conjugates (M1-M5) were detected and identified tentatively by LC-MS/MS analysis. In two cases (M1 and M5), the structures of the adducts were confirmed by NMR spectroscopy and/or by comparison with an authentic standard prepared by synthesis. The formation of GSH conjugates M1-M5 revealed the operation of two distinct metabolic activation pathways for troglitazone, one of which involves oxidation of the substituted chromane ring system to a reactive o-quinone methide derivative, while the second involves a novel oxidative cleavage of the thiazolidinedione (TZD) ring, potentially generating highly electrophilic alpha-ketoisocyanate and sulfenic acid intermediates. When troglitazone was administered orally to a rat, samples of bile were found to contain GSH conjugates which reflected the operation of these same metabolic pathways in vivo. The finding that metabolism of the TZD ring of troglitazone was catalyzed selectively by P450 3A enzymes is significant in light of the recent report that troglitazone is an inducer of this isoform in human hepatocytes. The implications of these results are discussed in the context of the potential for troglitazone to covalently modify hepatic proteins and to cause oxidative stress through redox cycling processes, either of which may play a role in drug-induced liver injury.

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

Tolbutamide (TOLB), a widely used hypoglycemic agent in the therapy of non-insulin-dependent diabetes mellitus, has been reported to be teratogenic and/or embryotoxic in several animal species and humans. It has been proposed that the teratogenic effects of TOLB are linked to drug-mediated depletion of glutathione (GSH) through inhibition of the enzyme glutathione reductase (GR), although the mechanism by which this inhibition occurs remains unknown. In the study presented here, rats were injected with TOLB (200 mg/kg ip), and bile was collected for analysis by liquid chromatography/tandem mass spectrometry (LC/MS/MS). This led to the identification of S-(n-butylcarbamoyl)glutathione (SBuG), a reactive GSH conjugate derived from n-butyl isocyanate, as a minor metabolite of TOLB in bile. Upon incubation of SBuG (0.25-1.0 mM) with GR from either yeast or bovine intestinal mucosa in the presence of NADPH (0.20 mM), enzyme activity was lost in a time- and concentration-dependent manner. No inhibition was observed when NADPH was omitted from incubations, or when the natural substrate for the enzyme, glutathione disulfide (GSSG, 0.05 mM), was added. TOLB itself did not inhibit GR over the concentration range of 0.8-2.0 mM. It is concluded that metabolic activation of TOLB in vivo leads to the generation of reactive intermediates (n-butyl isocyanate and SBuG) which carbamoylate and thereby inhibit GR. At critical periods of organogenesis, the resulting perturbation of GSH homeostasis in exposed tissues may play a key role in the teratogenic and/or embryotoxic effects of TOLB.

Toxicity of fotemustine in rat hepatocytes and mechanism-based protection against it

Fotemustine is a relatively novel DNA-alkylating 2-chloroethyl-substituted N-nitrosourea (CENU) drug, clinically used for the treatment of disseminated malignant melanoma in different visceral and non-visceral tissues. Thrombocytopenia has been observed in patients treated with fotemustine and liver and renal toxicities as well. In this study, firstly the metabolism of fotemustine was investigated in vitro and secondly the undesired cytotoxicity of fotemustine as well as different ways of protection against it. In rat hepatocytes, chosen as a model system, fotemustine was shown to cause lactate dehydrogenase (LDH) leakage, glutathione (GSH) depletion, GSSG-formation and lipid peroxidation (LPO). A reactive metabolite, DEP-isocyanate, is most likely responsible for these undesired cytotoxic effects. Based on the observed cytotoxicity mechanisms, chemoprotection with several sulfhydryl-containing nucleophiles and antioxidants was investigated. The sulfhydryl nucleophiles; GSH, N-acetyl-L-cysteine (NAC) and glutathione isopropylester (GSH-IP) protected almost completely against fotemustine-induced LDH-leakage and LPO. NAC and GSH protected partly against fotemustine-induced GSH-depletion. The antioxidant, vitamin E protected completely against fotemustine-induced LPO, but only partly against fotemustine-induced LDH-leakage and not against GSH-depletion. Ebselen, a peroxidase-mimetic organoselenium compound, did not show protective effects against the cytotoxicity of fotemustine, possibly because GSH is required for the bioactivation of ebselen. It is concluded that co-administration of sulfhydryl nucleophiles, in particular NAC and GSH-IP, possibly in combination with antioxidants, such as vitamin E, are effective against the toxicity of fotemustine in vitro. It might, therefore, be worthwhile to investigate the cytoprotective potency of these agents against undesired toxicities of fotemustine in vivo as well.

Biotransformation of the naturally occurring isothiocyanate sulforaphane in the rat: identification of phase I metabolites and glutathione conjugates

Sulforaphane (SFN) is a naturally occurring isothiocyanate present in cruciferous vegetables, such as broccoli, that has been identified as a potent inducer of glutathione S-transferase activities in laboratory animals. The present studies were carried out to elucidate the metabolic fate of SFN in the rat. Particular emphasis was placed on glutathione (GSH)-dependent pathways because conjugation with GSH is a major route by which many isothiocyanates are eliminated in mammals. Male Sprague-Dawley rats were administered a single dose of SFN (50 mg kg-1 ip), and bile and urine were collected over ascorbic acid. Analysis of biological fluids was carried out by ionspray LC-MS/MS using the neutral loss (129 Da) and precursor ion (m/z 164) scan modes to detect GSH and N-acetylcysteine (NAC) conjugates, respectively. In bile, five thiol conjugates (designated M1-M5) were detected. Metabolites M2 and M4 were identified as the GSH conjugates of SFN and erucin (ERN, the sulfide analog of SFN), respectively, by comparing their LC-MS/MS properties with those of standards obtained by synthesis. M1 was characterized as the GSH conjugate of a desaturated metabolite of SFN (tentatively assigned the structure of delta 1-SFN), suggesting that the parent compound also undergoes oxidative metabolism. Metabolites M3 and M5 were identified as the NAC conjugates of SFN and ERN, respectively, and together with the NAC conjugate of delta 1-SFN, these species also were detected in urine. Quantitative determination of the former two mercapturates in urine indicated that approximately 60% and approximately 12% of a single dose of SFN is eliminated in 24 h as the NAC conjugates of SFN and ERN, respectively. The corresponding figures in rats dosed with ERN were approximately 67% and approximately 29%. When the GSH conjugate of SFN was incubated with phosphate buffer (pH 7.4, 37 degrees C), < 1% of the conjugate dissociated to liberate free SFN. On the other hand, the conjugate underwent a facile thiol exchange reaction (> 70% conversion) when incubated in the presence of excess cysteine, thereby acting as an effective carbamoylating agent. It is concluded that SFN undergoes metabolism by S-oxide reduction and dehydrogenation and that GSH conjugation is the major pathway by which the parent compound and its phase I metabolites are eliminated in the rat.

Inhibition of carbamyl phosphate synthetase-I and glutamine synthetase by hepatotoxic doses of acetaminophen in mice

The primary mechanisms proposed for acetaminophen-induced hepatic necrosis should deplete protein thiols, either by covalent binding and thioether formation or by oxidative reactions such as S-thiolations. However, in previous studies we did not detect significant losses of protein thiol contents in response to administration of hepatotoxic doses of acetaminophen in vivo. In the present study we employed derivatization with the thiol-specific agent monobromobimane and separation of proteins by SDS-PAGE to investigate the possible loss of specific protein thiols during the course of acetaminophen-induced hepatic necrosis. Fasted adult male mice were given acetaminophen, and protein thiol status was examined subsequently in subcellular fractions isolated by differential centrifugation. No decreases in protein thiol contents were indicated, with the exception of a marked decrease in the fluorescent intensity, but not of protein content, as indicated by staining with Coomassie blue, of a single band of approximately 130 kDa in the mitochondrial fractions of acetaminophen-treated mice. This protein was identified by isolation and N-terminal sequence analysis as carbamyl phosphate synthetase-I (CPS-I) (EC 6.3.4.16). Hepatic CPS-I activities were decreased in mice given hepatotoxic doses of acetaminophen. In addition, hepatic glutamine synthetase activities were lower, and plasma ammonia levels were elevated in mice given hepatotoxic doses of acetaminophen. The observed hyperammonemia may contribute to the adverse effects of toxic doses of acetaminophen, and elucidation of the specific mechanisms responsible for the hyperammonemia may prove to be useful clinically. However, the preferential depletion of protein thiol content of a mitochondrial protein by chemically reactive metabolites generated in the endoplasmic reticulum presents a challenging and potentially informative mechanistic question.

Formation, solvolysis, and transcarbamoylation reactions of bis(S-glutathionyl) adducts of 2,4- and 2,6-diisocyanatotoluene

During our ongoing studies of the reactions of toluene diisocyanate (2,4- and 2,6-diisocyanatotoluene, TDI) in vivo, it became apparent that reactive form(s) of these diisocyanates reach(es) the circulatory system after passage through the respiratory system. Based on recent work by others regarding the transcarbamoylation reactions of monoisocyanates, we hypothesized that the reactive form could be masked as an S-thiocarbamoylglutathione adduct of one or more of the isocyanato moieties. In this study, the glutathione adducts of 2,4- and 2,6-diisocyanatotoluene were synthesized under physiological conditions. Bis adducts were the major products when near-equimolar amounts of glutathione and the individual diisocyanato compounds were mixed at physiological pH, and were formed in high yield. Little to no mono adducts formed under these reaction conditions. The masses of the bis adducts were confirmed by electrospray mass spectrometry (MS), and 1H NMR analysis strongly suggested that the thiol of the cysteine residue of glutathione was the nucleophile in each case. The rates of solvolysis of the two bis adducts in aqueous buffer under conditions of physiological temperature and pH were determined, and electrospray MS analysis showed that the corresponding mono(glutathionyl)-TDIs were formed in these reactions. Incubation in vitro of each of the bis(glutathionyl)-TDI adducts with a 12 amino acid peptide (Thr-Cys-Val-Glu-Trp-Leu-Arg-Arg-Tyr-Leu-Lys-Asn) at pH 7.5 resulted in transfer of one mono(glutathionyl)-toluylisocyanato moiety to the peptide as detected by HPLC and on-line electrospray MS analyses. In both the solvolysis and transfer experiments, the 2,4-TDI-derived bis(glutathionyl) adduct reacted most quickly, while both the bis(glutathionyl)-2,6-TDI adduct and its transfer product with the peptide were more stable than their 2,4-TDI-derived counterparts. The results indicate high stoichiometry in formation and ready transfer to nucleophilic sites of protein, and suggest that the isocyanato moiety of both 2,4- and 2,6-TDI may be regenerated in vivo from their bis(glutathionyl) adducts. As a consequence, the thiol status of particular tissues may be a contributing factor to individual TDI toxicity susceptibility, and a mechanism by which toxicity at sites distant to the initial point of contact may be proposed.

Carbamoylation of glutathione reductase by N,N-bis(2-chloroethyl)-N- nitrosourea associated with inhibition of multidrug resistance protein (MRP) function

Intracellular glutathione (GSH) concentrations have been implicated recently as a regulatory determinant of multidrug resistance protein (MRP)-mediated drug efflux. Inhibition of glutathione reductase (GR) activity of N,N-bis(2-chloroethyl)-N-nitrosourea (BCNU) has been employed extensively to investigate the role of GSH redox cycle in cellular function. The present study examined the effect of BCNU on the MRP-mediated efflux of doxorubicin in the multidrug-resistant human fibrosarcoma cell line HT1080/DR4 overexpressing MRP. No significant difference in GR activity between HT1080 (parental) and multidrug-resistant HT1080/DR4 cells was detected (38.6 +/- 2.2 and 37.8 +/- 5.28 nmol/min/10(6) cells, respectively). Exposure of HT1080 and HT1080/DR4 cells to 100-500 microM BCNU decreased GR activity concentration dependently with subsequent reduction in cellular GSH pools in both cell lines. Inhibition of GSH biosynthesis by D,L-buthionine-(S,R)-sulfoximine (D,L-BSO), a specific inhibitor of gamma-glutamylcysteine synthetase, significantly reduced MRP-mediated drug efflux and potentiated the cytotoxicity of doxorubicin in MRP-expressing HT1080/DR4 cells (dose modifying factor 20.8). While equally effective inhibition of GR activity by BCNU was observed in parental and resistant cells, a significant increase in intracellular retention of doxorubicin was only achieved in MRP-expressing HT1080/DR4 cells. Furthermore, inhibition of MRP function following treatment with BCNU or D,L-BSO was directly related to the degree of GSH depletion in MRP-expressing tumor cells [r = 0.94 (P < 0.001) and 0.99 (P < 0.001), respectively]. Based on northern blot analysis of MRP mRNA levels, exposure of HT1080/DR4 cells to BCNU did not produce down-regulation of MRP gene expression. The results reported herein indicate that derivatives of nitrosourea with carbamoylating properties are potent inhibitors of MRP function. Depletion of intracellular GSH pools by inhibition of the GSH redox cycle or GSH de novo biosynthesis significantly inhibited MRP-mediated doxorubicin transport and restored intracellular drug concentrations in vitro.

In vivo and in vitro reactions of toluene diisocyanate isomers with guinea pig hemoglobin

Guinea pig hemoglobin (Hb) adducts of 2,4- and 2,6-toluene diisocyanate (2,4- and 2,6-TDI) were individually prepared, each at a Hb tetramer to diisocyanate ratio of 1:1, and compared with adducts of Hb from animals exposed to 1 ppm 2,4-TDI vapor. Each Hb sample was subjected to C4 HPLC chain/heme separation with UV detection. Survey of the LC fractions using an antiserum prepared to a heterologous TDI-protein indicated a difference in chain specificity and product types formed in vitro by the two isomers. Ionspray mass spectrometry (MS) revealed the chemistry of the adduction products. Carbamoylated products, formed from adduction by one TDI molecule (with one isocyanato group hydrolyzed to an amino group), were detected by MS with 2,4-TDI on each chain and with 2,6-TDI on the beta chain. Additionally, a quasi-molecular ion of a bis carbamoylation adduct was noted by MS in the in vitro 2,4-TDI-Hb adduct in the form of TDI-cross-linked alpha and beta chains. Ionspray MS analysis of the Hb isolated from guinea pigs exposed in vivo to 2,4-TDI indicated carbamoylation products with both the alpha and beta chains in which one of the two original isocyanato groups had been hydrolyzed to the amine. We also found evidence of an amine-nitroso adduct on the alpha chain in the in vivo sample. These results indicate that at least one of the isocyanato moieties (or a masked derivative) of 2,4-TDI survived passage through the lung, into the serum, and through the erythrocyte membrane to form adducts with Hb that were stable to dialysis, gel filtration, and reversed phase HPLC separation under acidic conditions. The presence of an apparent amine-nitroso adduct indicated in vivo formation of 2,4-diaminotoluene, a recognized animal carcinogen.

Biomedical and biochemical applications of liquid chromatography-mass spectrometry

This review centres on the application of various LC-MS and LC-MS-MS techniques to the study and solution of practical problems in biomedical research. For this purpose it covers a selection of publications in this area included in the MEDLINE database for the period 1991-mid-1994. As shown herein, LC-MS is increasingly gaining in importance in the biomedical field, especially after the revolution brought about by the introduction of the new liquid-phase atmospheric pressure ionization (API) techniques, such as electrospray (ES) and ionspray. The information in this database shows that thermospray (TS), which clearly dominated LC-MS coupling in the 1980s, is on a downward trend relative to the more modern API techniques which will certainly dominate this application field in the present decade. Studies on drug metabolism, therapeutic drug monitoring and pharmacology have been traditionally carried out by GC-MS. However, LC-MS has lately been replacing classical GC-MS techniques in many of these applications. For instance, LC-ES-MS has greatly facilitated the application of both qualitative and quantitative LC-MS methods to highly polar molecules. This is possible without the need to resort to elaborate sample handling and derivatization procedures for relatively high-molecular-mass compounds such as drug conjugates, biosynthetic and natural peptides and therapeutic proteins obtained by recombinant DNA technology, all of them formerly totally inaccessible to the standard GC-MS or LC-MS methods. With regard to studies on metabolism and biochemical phenomena of endogenous compounds, LC-ES-MS is also becoming very strong in the analysis of structural biopolymers such as peptides, proteins, glycoproteins and glycolipids, and also lower molecular mass compounds such as fatty acids, vitamins, steroids and nucleic acids. For example, structural verification of post-translational modifications in proteins can be efficiently obtained in the time frame of an LC run and suitable MS methods for the location of glycopeptide-containing fractions in proteolytic digests of glycoproteins have been developed. Interesting examples are also shown of the use of LC-MS in clinical studies and the determination of biological markers of disease.

Determination of xanomeline in human plasma by ion-spray tandem mass spectrometry

Xanomeline is a muscarinic receptor agonist currently in phase II clinical trials for the treatment of Alzheimer's disease. A fast, sensitive and specific assay has been developed to determine xanomeline plasma concentrations using ion-spray tandem mass spectrometry. Xanomeline and a structural analog, LY282122, were extracted from basifed plasma into hexane. The dried hexane extracts were reconstituted and injected onto a 10 x 1 mm C18 reversed-phase column. A mobile phase of 33 mM ammonium acetate and 0.33% acetic acid in 30/70 (v/v) water-acetonitrile was pumped through the column at 50 microliters min-1. The mobile phase eluant was introduced directly into the ion-spray interface. The mass spectrometer was operated in the positive ion mode for specific detection of the product ions of xanomeline and the internal standard. The method has a linear range of 0.075-5.0 ng xanomeline per milliliter of plasma. Sample run times were 2.5 min from one injection to the next.

Effect of carbamate thioester derivatives of methyl- and 2-chloroethyl isocyanate on glutathione levels and glutathione reductase activity in isolated rat hepatocytes

The present study examined the effects of S-(N-methylcarbamoyl)glutathione (SMG), S-(N-methylcarbamoyl)-L-cysteine (L-SMC) and some analogs of these S-linked conjugates of methyl isocyanate (MIC) on the activity of glutathione reductase (GR) in freshly isolated rat hepatocytes and on the levels of reduced and oxidized glutathione (GSH and GSSG) in exposed cells. Both SMG and its monoethyl ester (0.5 mM) were found to inhibit GR weakly, although L-SMC proved to be an effective inhibitor of the enzyme (60 +/- 4% activity remaining after a 4-hr incubation at 0.5 mM). The cysteine adduct (SCC) of 2-chloroethyl isocyanate (CEIC) was a strong inhibitor of GR (27 +/- 1% activity remaining after a 1-hr incubation at 0.1 mM) and was essentially equipotent with the antitumor agent N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU). L-SMC depleted intracellular GSH in a time- and concentration-dependent manner up to 2 hr of incubation, beyond which time GSH levels began to recover. Exposure of cells to the enantiomeric conjugate, D-SMC, led to a similar concentration- and time-dependent inhibition of GR and fall in intracellular GSH, but in this case the depletion of GSH was extensive and was sustained throughout the 5-hr incubation period. Only a small amount (less than 10%) of the GSH that was lost from cells exposed to SMC was recovered in the medium, indicating that SMC did not cause efflux of GSH (most of the free cysteine released during breakdown of SMC was recovered in the medium). Experiments with hepatocytes exposed for 5 hr to SCC (0.1 mM) demonstrated that GSSG levels were elevated by 32 +/- 5% relative to controls. Collectively, these results indicate that carbamate thioester conjugates of MIC and CEIC inhibit GR, probably via release of the free isocyanate at the cell surface, which then penetrates the hepatocyte. The inhibitory effects of the isocyanates on GR, coupled with their propensity to react spontaneously with GSH, combine to deplete significantly intracellular stores of GSH.

Selective and irreversible inhibition of glutathione reductase in vitro by carbamate thioester conjugates of methyl isocyanate

Exposure of yeast glutathione reductase (GR) in vitro to S-(N-methylcarbamoyl)glutathione (SMG) and S-(N-methylcarbamoyl)cysteine (SMC), two carbamoylating metabolites of methylisocyanate (MIC), led to a time-dependent, irreversible loss of enzyme activity (50-90%) over a period of 3 hr. The extent of inhibition was dependent upon the concentration of these carbamate thioester conjugates (0.1 to 1.0 mM) and on the presence of NADPH (100 microM). Omission of NADPH markedly attenuated the inhibitory effects of both SMG and SMC, while oxidized glutathione (GSSG), the natural substrate of the enzyme, protected against the inhibition. Parallel experiments with the antineoplastic drug N,N'-bis-(2-chloroethyl)-N-nitrosourea (BCNU), a carbamoylating agent which is known to inhibit GR selectively, gave results that were similar to those obtained with the above conjugates. When analogs of SMG and SMC labeled with 14C in the carbamoyl group were incubated with GR, radioactivity became bound covalently to the enzyme. These findings, together with the results of kinetic experiments on the release of GSH from SMG and cysteine from SMC, suggested that while both conjugates inhibit GR by carbamoylation of an active-site thiol(s), SMG exhibits a greater affinity for the active site than SMC. In contrast to the studies with GR, SMG and SMC failed to inhibit either glutathione-S-transferase (GST) or glutathione peroxidase (GPO) enzymes in vitro. It is concluded, therefore, that these conjugates most likely inhibit GR by carbamoylating free thiol groups in the active site of this enzyme, which are absent (or inaccessible) at the active-site of GST and GPO.