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.

Cytotoxicity of a series of mono- and di-substituted thiourea in freshly isolated rat hepatocytes: a preliminary structure-toxicity relationship study

The cytotoxicity of a series of 12 mono- and 4 di-substituted thiourea containing compounds in freshly isolated rat hepatocytes was investigated. It was found that thiourea toxicity, as evidenced by an increase in LDH-leakage from the cells, was accompanied by a depletion of intracellular glutathione (GSH). No increase in lipid peroxidation was observed with any of the thiourea. Burimamide and thioperamide, thiourea-containing histamine receptor ligands, were also found to deplete intracellular GSH. A clear structure-toxicity relationship was uncovered among a homologous series of N-phenylalkylthiourea. N-benzylthiourea (BTU) and N-phenylethylthiourea (PETU) were found to be non-toxic at a concentration of 1 mM, while N-phenylpropylthiourea (PPTU) and N-phenylbutylthiourea (PBTU) were found to cause significant LDH-leakage from the cells, accompanied by a depletion of intracellular GSH. This structure-toxicity relationship was further investigated using hepatocytes of differentially induced rats, however, no significantly different results were obtained when using hepatocytes of rats induced with phenobarbital (PB) or beta-naphthoflavone (BNF). Oxidation of the thiourea moiety is thought to be the first step in the bioactivation of thiourea containing compounds. The oxidation of thiocholine sulfenic acids, produced by FMO-mediated oxidation of the thiourea moiety, was used to determine whether the compounds examined are substrates for the FMO enzymes in rat liver. No clear relationship was found between cytotoxicity of the mono-substituted thiourea and lipophilicity of the N-substituent, nor with the FMO-mediated oxidation of the thionosulfur atom of the mono-substituted thiourea. It is concluded from this study, that thiourea toxicity in rat hepatocytes is structure-dependent and manifests itself as LDH-leakage and as a depletion of intracellular non-protein sulfhydryls, notably GSH, most likely followed by alkylation of vital macromolecular structures.

Urinary metabolite profile of phenyl and o-cresyl glycidyl ether in rats: identification of a novel pathway leading to N-acetylserine O-conjugates

The urinary excretion of metabolites of phenyl glycidyl ether (PGE) and o-cresyl glycidyl ether (o-CGE) was investigated in rats. Urine was collected, in fractions, from rats intraperitoneally administered PGE or o-CGE in doses ranging from 0.033 to 1.0 mmol/kg. The metabolites were extracted from acidified urine with ethyl acetate or diethyl ether, and their identity was elucidated by GC/MS analysis. The epoxide of PGE can be inactivated by glutathione (GSH) conjugation or epoxide hydrolysis. After further metabolism, these routes lead to the urinary excretion of phenyl glycidyl ether mercapturic acid (PGEMA) and 3-(phenyloxy)lactic acid (POLA). The excretion of PGEMA and POLA was described before and is confirmed in this study. Additionally, a new metabolite was identified as N-acetyl-O-phenylserine (NAPS), which is proposed to be formed from POLA by subsequent oxidation, transamination, and N-acetylation. For PGEMA a linear dose-excretion relationship was found (r2 = 0.988), and the percentage of the dose excreted declined from 27% to 10% with increasing PGE dose. For NAPS also a linear dose-excretion relationship was found (r2 = 0.985), and NAPS accounted for 27% of the PGE dose. The excretion of PGEMA and NAPS was rather fast: 93% and 75%, respectively, of the respective total cumulative amounts excreted was already collected within 6 h after administration. The urinary metabolite profile of o-CGE was not investigated in rats before. Three urinary metabolites of o-CGE were identified, namely, 3-(o-cresyloxy)lactic acid (COLA), o-cresyl glycidyl ether mercapturic acid (o-CGEMA), and N-acetyl-O-(o-cresyl)serine (NACS), showing that the metabolite profiles of PGE and o-CGE are comparable. Up to a o-CGE dose of 0.333 mmol/kg, the excretion of o-CGEMA was linear (r2 = 0.997), while above this dose the excretion did not increase anymore. The percentage of the o-CGE dose excreted as o-CGEMA declined from 31% to 11% with increasing dose. Again 93% of the total cumulative amount of o-CGEMA excreted was collected within 6 h after administration of o-CGE. Analytical methods were developed for the quantitative determination of mercapturic acid metabolites of PGE and o-CGE. These methods were sufficiently sensitive for their determination in urine of rats administered PGE or o-CGE in the dose range applied. It is anticipated that the analytical methods developed are also sufficiently sensitive to investigate excretion of the mercapturic acid metabolites in humans occupationally exposed to low air concentrations (<6 mg/m3 of air, 8h-TWA) of PGE or o-CGE.

Biotransformation of allyl chloride in the rat. Influence of inducers on the urinary metabolic profile

Allyl chloride (AC) is used as intermediate in the synthesis of epichlorohydrin (ECH). We investigated the biotransformation of AC in rats to select potential urinary biomarkers of exposure. For this purpose, we developed analytical methods to measure different selected urinary metabolites of AC. The earlier described urinary metabolites of AC [allyl mercapturic acid (ALMA) and 3-hydroxypropyl mercapturic acid (HPMA)], as well as two urinary metabolites of ECH [alpha-chlorohydrin (alpha-CH) and 3-chloro-2-hydroxypropyl mercapturic acid (CHPMA)], were determined in this study. After intraperitoneal administration of AC, in doses ranging from 66 to 590 mumol/kg, control rats excreted 30 +/- 6.5% of the AC dose as ALMA. HPMA was a minor urinary metabolite of AC (< 3% of the AC dose), and, for this metabolite, no clear dose-excretion relationship was found. Two other minor urinary metabolites were also found as well, namely CHPMA and alpha-CH, suggesting the formation of ECH. CHPMA excretion was linear from 66 to 330 mumol/kg AC and amounted to 0.21 +/- 0.08% of the AC dose. alpha-CH excretion was linear in the dose range used and was excreted for 0.13 +/- 0.02% of the AC dose. In addition, we investigated the influence of three different enzyme inducers on the urinary metabolite profile of AC, namely pyrazole, beta-naphthoflavone, and phenobarbital. Pyrazole only increased the urinary excretion of alpha-CH. beta-Naphthoflavone induction only enhanced the ALMA excretion significantly. Phenobarbital inducted both the excretion of CHPMA and alpha-CH. From these studies, we conclude that urinary excretion of ALMA, CHPMA, and alpha-CH can be used as biomarkers in humans potentially exposed to AC. However, ALMA seems to be the more appropriate biomarker, because enzyme induction may play a confounding role if CHPMA or alpha-CH is used.

Molecular mechanisms of toxic effects of fotemustine in rat hepatocytes and subcellular rat liver fractions

Fotemustine is a clinically used DNA-alkylating 2-chloro-ethyl-substituted N-nitrosourea, which sometimes shows signs of haematotoxicity and reversible liver and renal toxicity as toxic side-effects. Mechanistic data on these side-effects are scarce and incomplete. In this study, firstly the cytotoxicity of fotemustine in freshly isolated rat hepatocytes was investigated and secondly the metabolism of fotemustine and possible mechanisms involved in the observed cytotoxicity. Fotemustine caused concentration- and time-dependent cytotoxic effects in rat hepatocytes. Extensive GSH-depletion and formation of GSSG were first observed, followed by lipid peroxidation and finally by cell death measured as LDH-leakage. 2-Chloroethyl analogues of fotemustine, which in contrast to fotemustine have no carbamoylating potency, were not toxic to rat hepatocytes. The data suggest that the cytotoxicity of fotemustine is resulting from its reactive decomposition product, DEP-isocyanate. GSH-conjugation of DEP-isocyanate was shown to protect against the cytotoxicity of fotemustine, however, only temporary and not completely. Synthetical DEP-SG, the GSH-conjugate of DEP-isocyanate, was also toxic to rat hepatocytes, albeit to a significantly lesser extent than fotemustine. In rat liver microsomes, no fotemustine-induced LPO was observed, suggesting that reactive decomposition products of fotemustine do not directly cause peroxidation of cellular membranes. Fotemustine did not affect the antioxidant enzymes superoxide dismutase, catalase, GSH-peroxidase, GSSG-reductase and GSH S-transferases. Thus, direct effects on these antioxidant enzymes are not likely to explain the cytotoxic effects of fotemustine in hepatocytes. In conclusion, it is proposed that the cytotoxicity of fotemustine in rat hepatocytes is caused by rapid and extensive depletion of GSH by DEP-isocyanate, a reactive decomposition product of fotemustine, consequently hampering the endogenous protection against its own toxicity. Knowledge of molecular mechanisms of the cytotoxicity of fotemustine may contribute to a more rational design of selective protection against toxic side-effects which occur upon therapy of patients with fotemustine.

The cytotoxicity of mitomycin C and adriamycin in genetically engineered V79 cell lines and freshly isolated rat hepatocytes

The objective of the present study was to investigate the cytotoxicity of Adriamycin (ADR) and mitomycin C (MMC) in tumor and non-tumor cells with respect to the role of cytochrome P450 (P450). Therefore, genetically engineered V79 Chinese hamster fibroblasts expressing only single enzymes of P450 were used. SD1 and XEM2 cells expressed rat P450IIB1 and P450IA1, respectively, whereas the V79 parental cells contained no detectable P450 levels. The cytotoxicity of ADR and MMC in the V79 cell system was compared with that in freshly isolated hepatocytes from phenobarbital (PB-hepatocytes)- and beta-naphthoflavone (beta NF-hepatocytes)-induced rats. Following 24 h of exposure to ADR equal cytotoxicity was observed in V79, SD1 and XEM2 cells. Addition of metyrapone (MP, an inhibitor of P450IIB1) and alpha-naphthoflavone (alpha NF, an inhibitor of P450IA1) had no effect on the ADR-induced cytotoxicity in SD1 and XEM2 cells, respectively. Likewise, MMC was equitoxic in V79 and SD1 cells. Co-incubation of SD1 cells with MP did not alter MMC-induced cytotoxicity. MMC, however, showed a decreased cytotoxicity in XEM2 cells when compared to the parental V79 cells. Unexpectedly, the cytotoxicity of MMC in XEM2 cells was increased by alpha NF to the same level as observed in the parental V79 cells. In contrast to V79- and V79-derived cells, in freshly isolated hepatocytes from PB or beta NF-induced rats, MMC was cytotoxic (measured as lactate dehydrogenase leakage) within 3 h of incubation. ADR, however, was only cytotoxic to the hepatocytes when intracellular glutathione was first depleted by diethylmaleate. The MMC- and ADR-induced cytotoxicity was found to be more pronounced in PB-hepatocytes than in beta NF-hepatocytes. Contrary to the findings in the V79-derived cells, MP afforded complete protection against both MMC- and ADR-induced cytotoxicity in PB-hepatocytes, whereas alpha NF only partially inhibited the cytotoxicity of MMC in beta NF-hepatocytes. In conclusion, we have demonstrated that PB-inducible P450s play a role in the cytotoxicity of both MMC and ADR in freshly isolated PB-hepatocytes but that P450IIB1 does not in genetically reconstituted SD1 cells. P450IA1, however, decreased the cytotoxicity of MMC in the XEM2 cells. The ADR-induced cytotoxicity, which was observed in XEM2 cells, was not mediated by P450IA1. The present study underscores the complexity in the comparison of ADR- and MMC-induced cytotoxicities in normal and tumor cells.

Cytotoxicity of mitomycin C and adriamycin in freshly isolated rat hepatocytes: the role of cytochrome P450

The role of cytochrome P450 (P450) in the cytotoxicity of mitomycin C (MMC) and Adriamycin (ADR) was investigated in freshly isolated hepatocytes from phenobarbital-induced rats. The loss of cell viability [measured as lactate dehydrogenase (LDH) leakage] upon MMC exposure was accompanied by a rapid and extensive intracellular glutathione (GSH) depletion and followed by minor lipid peroxidation (LPO). Coincubation of the hepatocytes with the P450 inhibitors, metyrapone and SK&F 525-A, strongly protected against MMC-induced LDH leakage, GSH depletion, and LPO. Inasmuch as the depletion of intracellular GSH by MMC, which is considered as a critical event in the development of MMC cytotoxicity, was not accompanied by a stoichiometric oxidation to oxidized GSH (GSSG), the formation of a MMC-GSH conjugate after one-electron reductive bioactivation of MMC by P450 was anticipated. In contrast to MMC, ADR was only cytotoxic to the hepatocytes upon prior depletion of intracellular GSH with diethylmaleate. Addition of metyrapone and SK&F 525-A completely protected the hepatocytes against ADR-induced LDH leakage and LPO. Moreover, the ADR-induced LDH leakage and LPO were strongly inhibited by dimethyl sulfoxide and ethanol, indicating that hydroxyl radicals were involved in the cytotoxicity of ADR. In conclusion, the present investigations indicate that P450 plays a major role in the cytotoxicity of both MMC and ADR in freshly isolated hepatocytes from phenobarbital-induced rats. The present findings lead to a better understanding of the mechanism of the cytotoxic actions of both MMC and ADR.

Occurrence of a cytochrome P-450-containing mixed-function oxidase system in the pond snail, Lymnaea stagnalis

1. The occurrence of an as yet unidentified cytochrome P-450 in the microsomal fraction of the digestive gland of the snail, Lymnaea stagnalis was studied. 2. Studies in vivo and in vitro (digestive gland homogenates or the 170,000g fraction) of the cytochrome P-450-mediated metabolism of substrates such as biphenyl, pentoxy- and ethoxy-resorufin and aminopyrine have been made. 3. Cytochrome P-450 concentration in the digestive gland calculated from CO-difference spectra was 0.30 +/- 0.05 nmol/g tissue. This amount was not increased either by phenobarbital or by 3-methylcholanthrene. 4. Aniline binding spectra resembled normal type II spectra, while type I model substrates such as hexobarbital and 2,2'-dichlorobiphenyl showed type II- or reversed type I-like spectra. 5. O-Deethylation of 7-ethoxyresorufin did not occur, but 7-pentoxyresorufin O-depentylation activity (80.4 +/- 28.6 pmol resorufin/g per hour) and aminopyrine N-demethylation activity (375 +/- 96 pmol formaldehyde/g per minute) were demonstrated. 6. 4-Hydroxybiphenyl was the major metabolite of biphenyl, while minor amounts of 2-hydroxybiphenyl were formed (in vivo: 63.7 nmol 4-hydroxybiphenyl and 3.33 nmol 2-hydroxybiphenyl per snail per 24 h, after an oral dose of 778.2 nmol biphenyl; in vitro 118 +/- 21 pmol and 21 +/- 9 nmol, respectively (digestive gland homogenate/mg protein, per hour). 7. The results indicate that the isoenzymes involved in the observed MFO activities resemble isoenzymes P-450b/e.

Reduction by acetylsalicylic acid of paracetamol-induced hepatic glutathione depletion in rats treated with 4,4'-dichlorobiphenyl, phenobarbitone and pregnenolone-16-alpha-carbonitrile

The role of enzyme induction in the reduction by acetylsalicylic acid (ASA) of paracetamol-induced hepatic glutathione (GSH) depletion has been studied in rats. Administration of an overdose of paracetamol to control rats resulted in an appreciable decrease of GSH concentration. Pretreatment with the enzyme inducers phenobarbitone, 3-methylcholanthrene (3-MC), pregnenolone-16-alpha-carbonitrile (PCN) and 4,4'-dichlorobiphenyl (4,4'-DCB) significantly potentiated the paracetamol-induced depletion of GSH. Simultaneous administration of an equimolar dose of ASA resulted in a reduction of the paracetamol-induced depletion of GSH in all instances except for those rats that were not pretreated and those given 3-MC. Benorylate, the ASA ester of paracetamol, depressed rat liver GSH to levels comparable to those produced by the combination of paracetamol and ASA. ASA itself caused only minor changes in liver GSH concentrations. The results demonstrate that ASA causes a diminution of paracetamol-induced GSH depletion in rats with phenobarbitone type of enzyme induction. Inhibition of the formation of the reactive metabolite of paracetamol or reduction of the absorption rate of paracetamol seem to be unlikely as mechanisms underlying the ASA-induced effect. An ASA-mediated effect via changes of the hepatic thiol status is proposed.

Paracetamol, 3-monoalkyl- and 3,5-dialkyl derivatives: comparison of their hepatotoxicity in mice

The effect of 3-monoalkyl and 3,5-dialkyl substitution (R = CH3, C2H5, and i-C3H7) on hepatotoxicity of the analgesic paracetamol was studied in vivo. To that purpose, varying doses of paracetamol and six alkyl-substituted derivatives were orally administered to male DAP mice. Paracetamol caused hepatotoxicity as judged from elevation of plasma transaminase activities and liver histopathology at a dose of 3.95 mmol/kg. All 3-monoalkyl-substituted derivatives of paracetamol caused centrilobular necrosis at oral doses of 4.40, 4.85, and 5.30 mmol/kg of 3-methyl-, 3-ethyl-, and 3-isopropyl derivatives, respectively. Oral dosage of the 3,5-dialkyl-substituted derivatives up to 6.25 mmol/kg did not result in hepatotoxicity. Since 3,5-dialkyl substitution of paracetamol does not reduce the analgesic activity, the observed prevention of paracetamol-induced hepatic necrosis by 3,5-dialkyl substitution may offer perspectives for the design of safer analgesics.

Pharmacokinetics of 2-phenyl-1,3-indandione in the rat after i.v. and oral administration

The pharmacokinetics of the anticoagulant drug, 2-phenyl-1,3-indandione, after i.v and oral administration in the rat might be best described as a non-linear open two-compartment model with elimination fromthe peripheral compartment. The volume of the central compartment comprises the extracellular fluid.