Biological effects of acetamide, formamide, and their monomethyl and dimethyl derivatives

The industrial use of certain acetamides and formamides (particularly DMAC and DMF) for their solvent properties has resulted in rather extensive examination of their biological properties. Both DMAC and DMF are rapidly absorbed through biological membranes and are metabolized by demethylation first to monomethyl derivatives and then to the parent acetamide or formamide. Relatively high single doses to various species following oral, dermal, i.p., i.v., or inhalation exposures generally are required to produce mortality. The liver is the primary target following acute high level exposure, but massive doses can also produce damage to other organs and tissues. Repeated sublethal treatment by various routes also shows the liver to be the target organ with the degree of damage being proportional to the amount absorbed. With MMF, the potential usefulness as a cancer chemotherapeutic agent needs to be measured against the hepatotoxic effects produced in man. Acetamides and formamides are generally inactive in mutagenicity tests. Mammalian test systems do not appear to be genetically sensitive and DMF has been recommended for use as the vehicle in microbial assays designed to test for genetic activity of hard-to-dissolve chemicals. Embryotoxicity can be demonstrated at high doses; doses which generally show toxicity to the maternal animals. Structural abnormalities in sensitive species such as the rabbit are produced following exposure at near-lethal levels. The spectrum of abnormalities seen is broad and fails to show any time or site specificity in terms of developing organs/organ systems. Inhalation exposures to DMAC and DMF at levels producing some maternal toxicity in rats have produced no teratogenic response and only slight evidence of embryotoxicity. Long-term feeding of relatively high levels of acetamide produces liver cancer in rats. DMAC and DMF appear to be noncarcinogenic. The environmental toxicity of these chemicals is low. Liver damage can be produced by overexposure to these chemicals in man. Airborne concentrations need to be controlled and care should be taken to avoid excessive liquid contact as the chemicals are absorbed through the skin. A relationship exists between the amount of DMAC or DMF absorbed and the amount of MMAC or MMF excreted in the urine so that biomonitoring of the urinary metabolites can indicate situations in which total exposures, both dermal and inhalation, are excessive. An interaction between DMF and ethanol occurs such that signs, including severe facial flushing, appear when DMF-exposed individuals consume alcoholic beverages.

Chemistry, biological activity, and uses of formamidine pesticides

The formamidines, a relatively new group of acaricide-insecticides, are novel both in their range of biological activities and in their mode of action, which is presently unknown. This paper is a review of the historical development, properties, structures, uses, and chemistry of this group of pesticides, with particular emphasis on chlordimeform (Galecron or Fundal), N'-4-chloro-o-tolyl-N,N-dimethylformamidine, and amitraz, 1,3=di-(2,4-dimethylphenylimino)-2-methyl-2-azapropane. Their biological activity and uses are defined by their toxicity to spider mites, ticks, and certain insects, and they are particularly effective against juvenile and resistant forms of these organisms. A significant, but poorly understood feature of their field effectiveness is their breadth of toxic action which includes direct lethality, excitant-repellant behavioral effects, and chemosterilization. They are generally of low hazard for nontarget species with the significant exception of predaceous mites. Several aspects of the chemistry of these compounds are considered, including structure--activity relations, synthetic pathways, isomerism and configuration, and their chemical and environmental stability. A significant feature of the metabolism and toxicity of these agents is the possible activation of chlordimeform by N-demethylation in vivo. Strong evidence for this has been presented with the cattle tick, but recent results discussed here suggest that in other species, i.e., mice, German cockroaches or black cutworm eggs, N-demethylation is neither a strong activation nor a detoxication reaction.

Teratogenic assessment of four solvents using the Frog Embryo Teratogenesis Assay--Xenopus (FETAX)

The Frog Embryo Teratogenesis Assay--Xenopus (FETAX) was used to assess the teratogenic potential of four solvents. Embryos of the South African clawed frog, Xenopus laevis, were exposed for 96 h to ethanol, dimethyl sulfoxide (DMSO), formamide or glycerol formal. Exposure groups were maintained using a static renewal system in which the exposure media were changed at 24-h intervals. Survival was monitored at 24-h intervals. Length, as an indicator of growth effects, and developmental malformations were determined at the end of the assay (96 h). Using this information, the 96-h LC50, the 96-h EC50 (Malformation), and the no observable effect levels (NOELs) for mortality, malformation and length were determined for each solvent. The teratogenic index [TI = 96-h LC50/96-h EC50 (Malformation)] also was calculated for each of the solvents. DMSO appeared to be the least toxic or teratogenic solvent examined, with a pooled LC50 of 1.92%, a pooled EC50 (Malformation) of 1.57% and TI values of 1.20 and 1.24 in replicate trials. Formamide appeared to be the most toxic solvent, with a pooled LC50 of 1.04%. Data trends suggested that ethanol was the most teratogenic solvent tested, with a pooled EC50 (Malformation) of 1.04% and TI values of 1.42 and 1.50. The results obtained in the present work for ethanol and DMSO were compared to previously published FETAX results for these two solvents. The present results are in close agreement with these results from other laboratories, thus providing further evidence supporting the interlaboratory reproducibility of FETAX results.

Studies on N4-(2-deoxy-D-pentofuranosyl)-4,6-diamino-5-formamidopyrimidine (Fapy.dA) and N6-(2-deoxy-D-pentofuranosyl)-6-diamino-5-formamido-4-hydroxypyrimidine (Fapy.dG)

Exposure of DNA to oxidative stress produces a variety of DNA lesions including the formamidopyrimidines, which are derived from the purines. These lesions may play important roles in carcinogenesis. We achieved the first chemical syntheses of a monomeric form of Fapy-dA (1) and oligonucleotides containing this lesion or Fapy-dG at a defined site. Monomeric Fapy-dA readily epimerized at 25 degrees C in phosphate buffer (pH 7.5). The beta-anomer was favored by a ratio of 1.33:1.0, and equilibration was achieved in less than 7 h. Deglycosylation of Fapy-dA in the monomer follows first-order kinetics from 37 to 90 degrees C. The rate constants for deglycosylation of Fapy-dA in the monomeric and oligonucleotide substrates were measured at a common temperature (55 degrees C) and found to be the same within experimental error (t(1/2) = 20.5 h). Implementation of the activation parameters measured for the deglycosylation of 1 indicates that the half-life for deglycosylation of Fapy-dA at 37 degrees C is approximately 103 h. Analysis of the rate constant for deglycosylation of Fapy-dG in an oligonucleotide, revealed that this lesion is approximately 25 times more resistant to hydrolysis than Fapy-dA at 55 degrees C. These results indicate that Fapy-dA and Fapy-dG will be sufficiently long-lived in DNA so as to warrant investigation of their genotoxicity, and both anomers will be present during this time.

Chemistry and toxicology of quinoxaline, organotin, organofluorine, and formamidine acaricides

Quinoxaline, organotin, organofluorine, and formamidine compounds are among the newer pesticide chemicals used for acarine control. Included in these four classes are some of the most selective synthetic organic toxicants currently in the acaricide/insecticide arsenal. Oxythioquinox, Plictran (tricyclohexylhydroxytin), Nissol [2-fluoro-N-methyl-N-(1-naphthyl)acetamide], and chlordimeform are examples of quinoxaline, organotin, organofluorine, and formamidine acaricides, respectively. The chemistry and toxicology of these and related compounds are discussed.

Biological monitoring of workers exposed to N,N-dimethylformamide in the synthetic fibre industry

OBJECTIVES

Monitoring of workplace air and biological monitoring of 23 workers exposed to N,N-dimethylformamide (DMF) in the polyacrylic fibre industry was carried out on 4 consecutive days. The main focus of the investigation was to study the relationship between external and internal exposure, the suitability of the metabolites of DMF for biological monitoring and their toxicokinetic behaviour in humans.

METHODS

Air samples were collected using personal air samplers. The limit of detection (LOD) for DMF using an analytical method recommended by the Deutsche Forschungsgemeinschaft (DFG) was 0.1 ppm. The urinary metabolites, N-hydroxymethyl-N-methylformamide (HMMF), N-methylformamide (NMF), and N-acetyl-S-(N-methylcarbamoyl)-cysteine (AMCC), were determined in one analytical run by gas chromatography with thermionic sensitive detection (GC/TSD). The total sum of HMMF and NMF was determined in the form of NMF. The LOD was 1.0 mg/l for NMF and 0.5 mg/l for AMCC.

RESULTS AND CONCLUSIONS

The external exposure to DMF vapour varied greatly depending on the workplace (median 1.74 ppm, range < 0.1-159.77 ppm). Urinary NMF concentrations were highest in post-shift samples. They also covered a wide range (< 1.0-108.7 mg/l). This variation was probably the result of different concentrations of DMF in the air at different workplaces, dermal absorption and differences in the protective measures implemented by each individual (gloves, gas masks etc.). The urinary NMF concentrations had decreased almost to zero by the beginning of the next shift. The median half-time for NMF was determined to be 5.1 h. The concentrations of AMCC in urine were determined to be in the range from < 0.5 to 204.9 mg/l. Unlike the concentrations of NMF, the AMCC concentrations did not decrease during the intervals between the shifts. For the exposure situation investigated in our study, a steady state was found between the external exposure to DMF and the levels of AMCC excreted in urine about 2 days after the beginning of exposure. AMCC is therefore excreted more slowly than NMF. The half-time for AMCC is more than 16 h. Linear regression analysis for external exposure and urinary excretion of metabolites was carried out for a sub-group of 12 workers. External exposure to 10 ppm DMF in air (the current German MAK value) corresponds to an average NMF concentration of about 27.9 mg/l in post-shift urine from the same day and an average AMCC concentration of 69.2 mg/l in pre-shift urine from the following day. NMF in urine samples therefore represents an index of daily exposure to DMF, while AMCC represents an index of the average exposure over the preceding working days. AMCC is considered to be better suited for biomonitoring purposes because (1) it has a longer half-time than NMF and (2) its formation in humans is more closely related to DMF toxicity.

In vivo metabolism of dimethylformamide and relationship to toxicity in the male rat

After in vivo administration of dimethylformamide (DMF) to male rats, about 50% of the dose is excreted in urine as N-hydroxymethyl-N-methylformamide (DMF-OH) and about 4% as N-methylformamide (NMF). NMF is not a product of DMF-OH biotransformation but is directly formed from DMF. Comparison of the acute toxicity of DMF, DMF-OH and NMF shows that NMF is more toxic than DMF-OH, which is itself more toxic than DMF. This study explains the different toxicity profile of DMF and NMF which until recently was believed to represent the main metabolite of DMF.

Possible role of thioformamide as a proximate toxicant in the nephrotoxicity of thiabendazole and related thiazoles in glutathione-depleted mice: structure-toxicity and metabolic studies

In mice depleted of GSH by treatment with buthionine sulfoximine (BSO), thiabendazole (TBZ) causes renal injury characterized by an increase in serum urea nitrogen (SUN) concentration and by tubular necrosis. Previous studies have shown that TBZ requires metabolic activation before it produces nephrotoxicity and that the structure contributing to the toxicity of TBZ is the thiazole moiety of the molecule. TBZ and its thiazole analogues were examined for the ability to increase SUN concentration and serum alanine aminotransferase activity in GSH-depleted mice. Unsubstituted thiazole and thiazoles with 4- and/or 5-, and no 2-, substituents caused marked increases in SUN concentration, suggesting nephrotoxicity. Furthermore, the nephrotoxic potency of these thiazoles decreased with the increasing number and bulk of the 4- and/or 5-substituents. On the other hand, the target organ (the kidney or liver) and the toxic potency of 4-methylthiazoles were markedly altered with the type of substituents at the 2-position. These observations and the known toxicity of thiono-sulfur compounds led us to the hypothesis that the nephrotoxic thiazoles, which lack 2-substituents, would undergo microsomal epoxidation of the C-4,5 double bond and, after being hydrolyzed, the resulting epoxide would then be decomposed to form thioformamide, a possibly toxic metabolite. Evidence for this hypothesis was provided by the results that thioformamide and tert-butylglyoxal as the accompanying fragment were identified as urinary metabolites in mice dosed with 4-tert-butylthiazole and that thioformamide caused a marked increase in SUN concentration when administered to mice in combination with BSO.

Occupational liver injury due to N,N-dimethylformamide in the synthetics industry

N,N-dimethylformamide (DMF) is a solvent used extensively in the chemical industry. The main toxic effect reported after exposure to DMF is hepatotoxicity. We encountered four patients with liver injury due to DMF exposure; the severity of the liver injury was related to the exposure levels. After removal of exposure, all patients recovered without specific treatment. A careful evaluation of occupational history is necessary when liver dysfunction develops in industrial workers.

Hepatotoxicity and P-4502E1-dependent metabolic oxidation of N,N-dimethylformamide in rats and mice

A comparative biochemical and histological study on the hepatotoxicity of a single dose of N,N-dimethylformamide (DMF) and N-methylformamide (NMF) in control and acetone-treated SD male rats and CD-1 male mice was performed. In control and acetone-pretreated rats, neither DMF nor NMF caused hepatic damage or elevation of plasma transaminases. In contrast, in acetonized but not in control mice, DMF administration yielded some evidence of liver necrosis and elevation of ALAT (alanine-amino transferase) activity. After a DMF dose of 1000 mg/kg, ALAT activity was found 1215 +/- 832 mU/ml and 47 +/- 18 mU/ml in acetonized and control mice, respectively. NMF treatment was hepatotoxic in control mice and lethal in acetonized mice. In control mice, an NMF dose of 600 mg/kg increased ALAT activity from a basal value of 35 +/- 5 to 2210 +/- 1898 mU/ml. When the oxidative metabolism of DMF was investigated, microsomes from both rats and mice preinduced by acetone increased the demethylation rate of DMF 7 to 10-fold compared to that (about 0.25 nmol/min per mg protein) of the corresponding control microsomes. The enzymatic affinities for DMF oxidation, however, were different: in mice the Km (0.05 mM) was one order of magnitude lower than that (0.56 mM) found in rats. The experiments performed with purified rat and mouse P-450 2E1 in a reconstituted system confirmed that the P-450 2E1 isoforms are very active catalysts towards DMF oxidation (the turnover was about 10 nmol/min per nmol P-450 for both enzymes) but with a strikingly different affinity. Whereas the Km for mouse P-450 2E1 was 0.08 +/- 0.03 mM, that for rat P-450 2E1 was 1.1 +/- 0.2 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Copper(II) complexes with 2-pyridineformamide-derived thiosemicarbazones: spectral studies and toxicity against Artemia salina

The copper(II) complexes [Cu(H2Am4DH)Cl2] (1), [Cu(H2Am4Me)Cl2] (2), [Cu(H2Am4Et)Cl2] (3) and [Cu(2Am4Ph)Cl] (4) with 2-pyridineformamide thiosemicarbazone (H2Am4DH) and its N(4)-methyl (H2Am4Me), N(4)-ethyl (H2Am4Et) and N(4)-phenyl (H2Am4Ph) derivatives were studied by means of infrared and EPR spectral techniques. The crystal structure of 4 was determined. The studied compounds proved to be toxic to Artemia salina, suggesting that they could present cytotoxic activity against solid tumors. Among the free thiosemicarbazones H2Am4Ph presented higher toxicity than all other compounds, which showed comparable effects. In the case of complexes 2 and 3 toxicity is probably attributable to the complex as an entity or to a synergistic effect involving the thiosemicarbazone and copper. H2Am4Ph and complexes 2 and 3 revealed to be the most promising compounds as potential antineoplasic agents.

An investigation of the mechanism of hepatotoxicity of the antitumour agent N-methylformamide in mice

N-Methylformamide (NMF) has been reported to cause liver damage in animals and man. This hepatotoxicity was characterized in BALB/c mice by the release of liver enzymes into the plasma and by histopathological examination of livers after single and repeated administration of NMF. Whereas plasma levels of sorbitol dehydrogenase were elevated dramatically 24 hr after 400 mg/kg given as a single dose, the glutathione content of the livers was not different from controls even after repeated administration. Liver damage was apparent on gross inspection and was defined as periacinar necrosis on histopathology. A dose of 100 mg/kg did not cause damage even after repeated injections on five consecutive days. The hypothesis that NMF is metabolized to a chemically reactive species was tested. Incubation of mouse hepatocytes with 7 mM NMF for 80 min produced a decrease in intracellular glutathione. Exposure of hepatocytes to NMF for 240 min led to the production of breakdown products of lipid peroxides at levels significantly above controls. However, incubation of microsomes or mitochondria with NMF and NADPH did not lead to raised levels of lipid peroxides. The effects described were specific to NMF as incubation of N,N-dimethylformamide, N-hydroxymethylformamide or formamide with hepatocytes did not result in glutathione depletion or increased lipid peroxidation. NMF undergoes extensive metabolism in vivo and the results indicate that NMF forms a chemically reactive metabolite, even though incubation of the drug with liver fractions or hepatocytes did not lead to metabolites at levels which were analytically identifiable.

Liver and heart toxicity due to 90-day oral exposure of ICR mice to N,N-dimethylformamide

N,N-dimethylformamide (DMF) is a colorless liquid with a faint amine odor, which is widely used in the world. DMF exposure may induce adverse effects on liver, but few studies showed damage to heart after exposure to DMF. In the present study, DMF was administered to ICR mice with the doses of 0.32, 0.63 and 1.26 g/kg of body weight by gavage for 90 days. The increase in the relative liver weight is accompanied with the presence of the centrilobular hepatocellular hypertrophy as well as increased serum levels of aspartate transaminase (AST) and alanine transaminase (ALT). An increase of malondialdehyde (MDA) level was shown in liver homogenate, while superoxide dismutase (SOD) and glutathione (GSH) activities decreased. Heart damage was also shown in mice exposed to DMF for 90 days, although pathological examination showed only slight inflammatory cell infiltration. Increased levels of serum lactate dehydrogenase (LDH), isoenzymes of creatine kinase (CK-MB) and cardiac troponin I (cTnI) were shown. Increased level of MDA was also shown in heart homogenate, in contrast with the decreased activity of SOD. These data suggested that the administration of DMF could induce liver and heart injuries and oxidative stress was involved in the toxic effects.

Formamide and dimethylformamide: reproductive assessment by continuous breeding in mice

Reproductive toxicity in Swiss mice, during chronic exposure to formamide (FORM) or dimethylformamide (DMF), was evaluated using the Reproductive Assessment by Continuous Breeding Protocols. FORM administered in drinking water at 0, 100, 350, and 750 ppm (approximately 20 to 200 mg/kg/d) reduced fertility and litter size in F0 animals without generalized toxicity at 750 ppm FORM. Crossover matings suggested that females were the affected sex. After F1 mating, FORM reduced F2 litter size, increased days to litter, reduced relative ovarian weight, and lengthened estrous cycles at 750 ppm. The No-Observed-Adverse-Effect-Level for generalized toxicity was 750 ppm for the F0 and 350 ppm for the F1 generation. Reproductive performance was normal at 350 ppm for both F0 and F1 mice. Chronic exposure to DMF in drinking water at 0, 1000, 4000, and 7000 ppm (approximately 200 to 1300 mg/kg/d) reduced fertility by the first litter at 4000 ppm, reduced body weight in F0 females at 7000 ppm, and increased liver weights at all doses in both sexes. A crossover mating at 7000 ppm identified F0 females as the affected sex. F1 postnatal survival was reduced at > or =4000 ppm DMF. F1 mating reduced F2 litter size and live pup weight at > or =1000 ppm. At necropsy, body weight of F1 males and females was reduced at > or =4000 ppm. DMF-treated pups (both F1 and F2) and F1 adults had cranial and sternebral skeletal malformations. Only DMF caused overt developmental toxicity. A No-Observed-Adverse-Effect-Level for DMF was not established.

Reversal by L-cysteine of the growth inhibitory and glutathione-depleting effects of N-methylformamide and N,N-dimethylformamide

N-Methylformamide and N,N-dimethylformamide, which can induce differentiation in selected malignant cell lines, are known to increase doubling times, inhibit clonigenicity in agar, and to effect responses against particular human colon carcinomas in vivo. At concentrations which inhibit growth and clonigenicity, N-methylformamide (170 mM) and N,N-dimethylformamide (103 mM) deplete total intracellular glutathione levels of DLD-1 Clone A human colon carcinoma cells in a dose and time dependent manner. In the presence of 0.5 mM 1-cysteine, both the growth and glutathione levels of polar-solvent treated DLD-1 Clone A cells are restored. 1-Cysteine also reverses the inhibition of clonigenicity mediated by NMF. The mechanism of action of N-methylformamide and N,N-dimethylformamide against this cell line, at least in vitro, is therefore related to its effects on cysteine/glutathione metabolism. Furthermore, this evidence suggests that glutathione plays a key role in regulating the growth of these cells.

Formamide sensitivity: a novel conditional phenotype in yeast

Yeast mutants unable to grow in the presence of 3% formamide have been isolated in parallel with mutants sensitive to either 37 degrees or 6% ethanol. The number of formamide-sensitive mutations that affect different genes that can be identified from yeast cells is at least as large as the number of thermosensitive or ethanol-sensitive mutations. These mutations are of two types: those that are sensitive to formamide, temperature and/or ethanol simultaneously; and those that are specific for formamide sensitivity and show no temperature or ethanol sensitivity phenotype. Those genes susceptible to giving rise to formamide-sensitive alleles include the structural gene for DNA ligase, CDC9, and the structural gene for arginine permease, CAN1. The results indicate that formamide sensitivity can be used as a novel conditional phenotype for mutations on both essential and nonessential genes. This work also confirms that ethanol-sensitivity can be used as a conditional phenotype to identify mutations in at least as many genes as those susceptible to temperature or formamide sensitive mutations.

Mitochondrial DNA alterations in blood of the humans exposed to N,N-dimethylformamide

N,N-Dimethylformamide (DMF) has been widely used in industries because of its extensive miscibility with water and solvents. Its health effects include hepatotoxicity and male reproductoxicity, possibly linked with mitochondrial DNA (mtDNA) alterations including mtDNA common deletion (DeltamtDNA(4977)) and mtDNA copy number. The relationship between DMF exposure and mtDNA alterations, however, has not been postulated yet. The purposes of this study were to investigate whether the DMF exposure is associated with DeltamtDNA(4977) and mtDNA copy number and to evaluate the DMF-derived mtDNA alterations are more associated with exposure to the airborne DMF concentrations or to the levels of two urinary DMF biomarkers of N-methylformamide (NMF) and N-acetyl-S-(N-methylcarbamoryl) cysteine(AMCC). Thirteen DMF-exposed workers and 13 age and seniority-matched control workers in a synthetic leather factory were monitored on their airborne DMF, NMF and AMCC in the urine as well as DeltamtDNA(4977) and mtDNA copy number in blood cells. We found that the frequencies of relative DeltamtDNA(4977) in DMF-exposed group were significantly higher than those in the control group. Moreover, elevation in the proportion of DeltamtDNA(4977) of individuals with high urine AMCC (U-AMCC) and airborne DMF levels were significantly higher than those without. We conclude that long-term exposure to DMF is highly associated with the alterations of mtDNA in urine and blood cells. The DeltamtDNA(4977) was more significantly related to repeated exposure to DMF and mtDNA copy number was more closely related to short-term DMF exposure. We also confirmed that U-AMCC is more appropriate to serve as a toxicity biomarker for DMF exposure than U-NMF. Further study with a larger number of subjects is warranted.

The formation and metabolism of N-hydroxymethyl compounds--IV. Cytotoxicity and antitumour activity of N-hydroxymethylformamide, a putative metabolite of N-methylformamide (NSC 3051)

Experiments were conducted to test the hypothesis that N-hydroxymethylformamide (HMF) is the active metabolite of the antitumour agent N-methylformamide (NMF). In an in vitro bioassay against the TLX5 lymphoma HMF was more cytotoxic than NMF; this cytotoxicity was abolished by preincubating the TLX5 cells with semicarbazide, a formaldehyde trapping agent. Similarly, the inhibition of incorporation of radiolabelled thymidine, uridine, formate and leucine into TLX5 cells elicited by HMF was eliminated by preincubation of the cells with semicarbazide. HMF is considerably less toxic to tumour-bearing BDF1 mice than NMF and, unlike NMF, does not reduce hepatic glutathione levels in vivo. HMF has no inhibitory activity against the TLX5 lymphoma or the Sarcoma 180 in mice in vivo and only marginal activity against the M5076 reticulum cell sarcoma; these tumours are highly sensitive to NMF. However, like NMF, HMF inhibits growth of the human mammary tumour MX-1 implanted in the subrenal capsule of mice.

Chemical synthesis and cytotoxic properties of N-alkylcarbamic acid thioesters, metabolites of hepatotoxic formamides

The S-linked cysteine and glutathione conjugates of N-methylformamide and N-ethylformamide, together with a series of methyl ester derivatives thereof, have been synthesized and characterized by 1H NMR, 13C NMR, FAB-MS, and FAB tandem mass spectrometry. In vitro cytotoxicity assays showed that all of the title conjugates were toxic to isolated mouse hepatocytes when incubated at concentrations in excess of 1 mM and that they served as potent growth inhibitors of murine TLX5 lymphoma cells when present at levels of 10-100 microM. Both of these effects were reversed by the addition to incubation media of glutathione (10 mM). The possibility is raised that N-alkylcarbamic acid thioester conjugates, which are formed during the metabolism of N-alkylformamides in mammalian systems, may act as important mediators of the antineoplastic and/or hepatotoxic activity of the parent formamides, possibly through their ability to liberate methyl isocyanate at cell membranes.

Cytotoxicity and metabolism of the hepatotoxin N-methylformamide and related formamides in mouse hepatocytes

Some N-alkylformamides such as N-methylformamide (NMF) possess hepatotoxic properties in vivo. To study the mechanism of this toxicity, suspensions of mouse hepatocytes were tested as an in vitro model system suitable for the study of the relationship between (i) the toxic potential of formamides, (ii) their metabolism to N-alkylcarbamoylating species, and (iii) their ability to deplete hepatic glutathione pools. The effects of NMF were compared with those of its analogs N-ethylformamide (NEF), N,N-dimethylformamide (DMF), formamide (F), N-methylacetamide (NMA), and N-methyldeuteroformamide ([2H]NMF). Only NEF and [2H]NMF share with NMF the ability to cause liver damage in vivo in mice. Hepatocellular toxicity was determined by measuring LDH leakage into the extracellular medium; metabolism to N-alkylcarbamoylating species was measured by GLC after derivatization with propanol to form propyl N-alkylcarbamate; glutathione concentrations were determined spectrophotometrically. Of the formamide analogs studied, only NMF and NEF caused cytotoxicity, being apparently equipotent. NMF, NEF, and [2H]NMF gave rise to the formation of detectable levels of N-alkylcarbamoylating metabolites and depleted glutathione pools. Toxicity, metabolism, and glutathione depletion were dependent on NMF concentration. [2H]NMF was markedly less cytotoxic than NMF, yielding only 35% of the amount of N-methylcarbamoylating metabolite compared to NMF and caused less depletion of glutathione than did NMF. These results parallel closely the in vivo hepatotoxic potential of NMF and its analogs, their metabolism to urinary S-(N-alkylcarbamoyl)mercapturates and their ability to deplete hepatic glutathione in mice. The results provide support for the contention that metabolism is involved with formamide-induced hepatotoxicity and suggest that suspensions of isolated mouse hepatocytes are an appropriate in vitro model for the further study of the mechanism by which formamides cause toxicity.