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.

Comparative hepatotoxicity and metabolism of N-methylformamide in rats and mice

N-methylformamide (NMF) produced dose-dependent zone 3 haemorrhagic necrosis in mice; the threshold dose was 100-200 mg/kg. In rats a dose of 1000 mg/kg caused hepatic damage in some animals and slight elevations of plasma transaminases. A species difference in susceptibility to NMF-induced hepatotoxicity is clearly indicated. NMF depleted liver non-protein sulphydryl (NPSH) in a dose-dependent manner in mice, but not in rats. Depletion of liver glutathione by buthionine sulphoximine or diethylmaleate potentiated the hepatotoxicity of NMF in mice. [14C]-methyl NMF was metabolised by mice and rats and a number of urinary metabolites including an N-acetylcysteine conjugate, methylamine and N-hydroxymethylformamide were detected. There were no qualitative differences in the metabolites between rats and mice but mice metabolised NMF much faster and more extensively than rats.

Metabolism of N-methylformamide in mice: primary kinetic deuterium isotope effect and identification of S-(N-methylcarbamoyl)glutathione as a metabolite

S-(N-Methylcarbamoyl)glutathione has been identified by cesium ion liquid secondary ion mass spectrometry as a biliary metabolite in mice of the experimental antitumor agent and hepatotoxin N-methylformamide. Metabolism of N-methylformamide to urinary methylamine, urinary N-acetyl-S-(N-methylcarbamoyl)-cysteine and biliary S-(N-methylcarbamoyl)glutathione was found to be subject to large intermolecular primary kinetic isotope effects when hydrogen was replaced by deuterium in the formyl group (kH/kD = 5.5 +/- 0.2, 4.5 +/- 1.0 and 7 +/- 2, respectively), as shown by mass spectrometry of derivatives of these metabolites. These values indicate the existence of a common metabolic precursor for each of these metabolites. In particular, methylamine is shown not to arise from simple enzymatic hydrolysis of N-methylformamide but is associated with an oxidative process. Therefore, it is highly likely that N-methylformamide is oxidized and conjugated to form S-(N-methylcarbamoyl)glutathione which is metabolized further to N-acetyl-S-(N-methylcarbamoyl) cysteine. Either of these thiocarbamates could be hydrolyzed to give the parent thiol and the observed metabolic end products, methylamine and carbon dioxide. The presence of deuterium in the formyl moiety of N-methylformamide reduced markedly the hepatotoxicity of the compound, as shown by measurements of the activities of appropriate hepatic enzymes in plasma.

Hepatotoxicity of N-methylformamide in mice--II. Covalent binding of metabolites of [14C]-labelled N-methylformamide to hepatic proteins

Incubation of the hepatotoxin N-methylformamide (NMF) labelled either in the methyl group (OHCNH14CH3) or the formyl group (OH14CNHCH3) with mouse hepatic microsomes in the presence of NADPH, but not in its absence, led to covalent binding of metabolites to microsomal proteins. When [14C]NMF was injected into BALB/c mice radioactivity was found to be associated with liver and, to a much lesser extent, with kidney proteins. Association of radioactivity derived from OHCNH14CH3 with hepatic proteins was higher in BALB/c mice than in CBA/CA mice and in these it was higher than in BDF1 mice. Association of label derived from either isotopomer was significantly reduced but not abolished by pretreatment of mice with cycloheximide suggesting both covalent binding and metabolic incorporation of NMF metabolites. Depletion of hepatic glutathione by pretreatment of mice with buthionine sulfoximine or diethyl maleate prior to administration of OH14CNHCH3 enhanced the association of label with hepatic proteins measured 1 hr after drug injection. Covalent binding of [14C]NMF to hepatic microsomes in vitro was abolished in the presence of glutathione. It is argued that the generation of the toxic lesion and the association of NMF metabolites with hepatic proteins may be causally related even though certain mechanistic and enzymatic details of this link remain obscure.

Structural studies on bioactive compounds. 4. A structure-antitumor activity study on analogues of N-methylformamide

A series of derivatives of N-methylformamide (NMF), an experimental antitumor agent, has been prepared, having the general formula R3C(X)NR1R2 where R1 = H, CH3, CD3, CH2CF3, CH2CH2Cl, cyclopropyl, C2H5, CH2OH, CH2OR, CH2N(CH3)2; R2 = H, CH3; R3 = H, CF3, CCl3, CH3, Ph, NHCH3, N(CH3)2; and X = O, S, NH. A further short series of "push-pull" olefins of the general formula R1R2C = CHNR3R4 has been synthesized where R1 = H, CH3 and R2 = H, NO2, CN, CHO, CH3 and R3 = H and R4 = H, CH3, morpholino. These compounds have been tested for activity against the M5076 ovarian sarcoma and the TLX5 lymphoma in mice. NMF was by far the most potent agent of both series with activity against both tumors. Some other compounds showed weak activity, but there is a rigorous structural requirement for activity and most analogues were inactive. Certain members of the series exist as equilibrium mixtures of rotamers about the amide or pro-amide bonds as shown by NMR.

The effects of diethyldithiocarbamate on the hepatotoxic action and antitumor activity of N-methylformamide in mice

The oral administration of diethyldithiocarbamate (DTC) prevented hepatic necrosis induced by N-methylformamide (NMF) in ddY-strain mice, in more susceptible BALB/c mice and in diethylmaleate-treated mice in which NMP-hepatotoxicity was potentiated, as evidenced by suppression of increases of plasma glutamic pyruvic transaminase activity and liver calcium content or by histological observations. Early depletion of liver glutathione following NMF administration was also prevented by DTC. DTC markedly delayed the in vivo metabolism of NMF as indicated by a prolonged retention of plasma and liver NMF levels and an enhancement of urinary excretion of NMF. These observations support a bioactivation mechanism for NMF hepatotoxicity, and the hepatoprotective action of DTC may be due to an inhibition of the metabolic activation of NMF. Hepatotoxic manifestations after repeated administration of NMF also tended to be ameliorated by simultaneous treatment with DTC. Cotreatment with DTC, however, decreased the antitumor activity of NMF against Ehrlich ascites tumors, and Sarcoma 180. This also implies the involvement of a bioactivation mechanism in the antitumor action of NMF, but further studies are necessary to confirm this point. The possible therapeutic value of DTC as a hepatoprotector may be diminished by the suppression of the antitumor activity of NMF.

The fate of N-methylformamide in mice. Routes of elimination and characterization of metabolites

The fate of N-methylformamide has been investigated in male CBA/CA mice following the administration of this compound labeled with 14C either in the methyl or in the formyl group. The major route of elimination was found to be via the kidneys although a substantial quantity (39% of the dose) was eliminated via the lungs as CO2 in the case of [14C]formyl-labeled N-methylformamide. In addition to the unchanged compound three metabolites were found in the urine by TLC autoradiography. One of these metabolites was identified as methylamine after conversion to its 2,4-dinitrophenyl derivative. The derivative was isolated and shown to be N-methyl-2,4-dinitroaniline by mass spectrometry. Further evidence that methylamine was a metabolite of N-methylformamide was provided by ion pair HPLC analysis of urine from mice dosed with [14C]methyl-labeled N-methylformamide. The second metabolite was tentatively identified as N-hydroxymethylformamide which was present in the urine of mice dosed with either [14C]methyl- or [14C]formyl-labeled N-methylformamide. Formate was not a urinary metabolite of N-methylformamide. The identity of the third urinary metabolite remains unknown.

Permeability of the squid giant axon to organic cations and small nonelectrolytes

The permeability of the Na channel of squid giant axon to organic cations and small nonelectrolytes was studied. The compounds tested were guanidinium, formamidinium, and 14C-labeled urea, formamide, thiourea, and acetone. Permeability was calculated from measurements of reversal potential and influx on internally perfused, voltage clamped squid axons. The project had two objectives: (1) to determine whether different methods of measuring the permeability of organic cations yield similar values and (2) to see whether neutral analogs of the organic cations can permeate the Na channel. Our results show that the permeability ratio of sodium to a test ion depends upon the ionic composition of the solution used. This finding is consistent with the view put forward previously that the Na channel can contain more than one ion at a time. In addition, we found that the uncharged analogs of permeant cations are not measurably permeant through the Na channel, but instead probably pass through the lipid bilayer.

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.

In vivo and in vitro oxidative biotransformation of dimethylformamide in rat

In rats and in humans, dimethylformamide (DMF) is mainly metabolized into N-hydroxymethyl-N-methylformamide (DMF-OH). The in vitro oxidation of DMF by rat liver microsomes is decreased in the presence of catalase and superoxide dismutase. The radical scavengers, dimethylsulfoxide (DMSO), tertiary butyl alcohol (t-butanol), aminopyrine, hydroquinone and trichloroacetonitrile reduce the oxidation of DMF to DMF-OH in vitro and in vivo. Conversely, DMF inhibits the demethylation of DMSO, t-butanol and aminopyrine. The addition of iron-EDTA to the incubation system induces the production of N-methylformamide (NMF) from DMF. These results support the hypothesis that the metabolic pathway leading from DMF to DMF-OH and NMF involves hydroxyl radicals. Superoxide radical and hydrogen peroxide take part in the metabolic process. DMF is preferentially metabolized into DMF-OH. NMF appears mainly when the production of hydroxyl radicals is stimulated, the methyl group being recovered as formic acid.

Formamide as a substrate of xanthine oxidase

Formamide is a substrate of xanthine oxidase. At pH 8.2 and 1.14 mM-O2, Vmax.(app.) is 3.1 s-1 and Km (app.) is 0.7 M. Mo(V) e.p.r. signals obtained by treating the enzyme with formamide were studied, and these provide new information about the ligation of molybdenum in the enzyme and about the enzymic mechanism. The substrate is the first compound that is not a nitrogen-containing heterocycle to give a Very Rapid signal. This supports the hypothesis that the Very Rapid signal, though it is not detectable with all substrates, represents an essential intermediate in turnover. Formamide also gives the Inhibited signal and is the first non-aldehyde substrate to do so. The Rapid type 1 signal obtained in the presence of formamide was examined in H2O enriched with 2H or with 17O. The single oxygen atom detectable in the signal is shown to be strongly and anisotropically coupled. This indicates that this atom remains as an oxo ligand of molybdenum in this signal-giving species. Other structural features of this species are discussed.

Tissue analysis of N-methylformamide: organ distribution

This report describes a gas chromatographic procedure, utilizing a packed column and flame ionization detector, suitable for the quantitative measurement of N-methylformamide (N-MF) in tissue samples. N-MF is a polar solvent that induces the maturation of cancer cells in vitro and, in vivo, exhibits antitumor activity with human tumors xenografted in nude (athymic) mice. Therapeutic monitoring is essential as toxicology studies have shown this compound to be hepatotoxic. N-MF is currently undergoing phase 1 clinical trials as an anticancer drug. This method of tissue analysis was developed to aid in the understanding of N-MF disposition and distribution in a murine model. The data thus generated may help predict the clinical behavior of this drug.

Phase I trial of N-methylformamide

N-methylformamide was administered iv and later orally to 19 patients in a phase I study, with a starting dose of 300 mg/m2/day X 5. The cycles were planned to be repeated every 2 weeks, and doses were escalated in four steps to 1200 mg/m2/day X 5. The principal toxic effect of the drug was nausea and vomiting, but this was not severe enough to interfere with oral medication. Parallel bioavailability studies confirmed excellent absorption of the drug. Biochemical disturbances included reversible elevation in transaminases not related to dose, peripheral neuropathy in one patient, and drug interaction with alcohol in another. The dose-limiting toxic effect was reversible hyperbilirubinemia. One patient became jaundiced. Among 15 evaluable patients, there were two partial responses (prostate and cervix carcinoma) and two minimal responses (ovarian carcinoma and hypernephroma). The recommended dose for the phase II study is 800 mg/m2/day X 5 orally, repeated every 2 or 3 weeks.

The formation and metabolism of N-hydroxymethyl compounds--I. The oxidative N-demethylation of N-dimethyl derivatives of arylamines, aryltriazenes, arylformamidines and arylureas including the herbicide monuron

The metabolism of the N-methyl moieties of aryldimethylamines and N-methyl compounds of the general formula Aryl-X-N(Me)2, where X is either -N=N-(3-aryl-1, 1-dimethyltriazenes). -NHCO- (N'-aryl-N,N-dimethylureas) or -N=CH- (N'-aryl-N,N-dimethylformamidines) was studied using mouse liver microsomes. Products of microsomal metabolism were reincubated with mouse liver homogenate devoid of microsomes and assayed colourimetrically for formaldehyde. This allows metabolically generated formaldehyde to be distinguished from formaldehyde precursors. Whereas the N-methyl moieties of the aryldimethyltriazenes, formamidines and amines were metabolised to formaldehyde, the aryldimethylureas formed stable formaldehyde precursors upon metabolism. The products of metabolism of one such aryldimethylurea, the herbicide monuron (N'-(4-chlorophenyl)-N, N-dimethylurea) were investigated using a high pressure liquid chromatographic method. Two metabolites were found on incubation of monuron with microsomes, one of which was identified as the N-desmethyl compound by mass spectrometry. The other product showed chromatographic properties similar to 4-chlorophenylurea but resembled the monomethylaryl urea on mass spectral analysis. It is concluded that this metabolite is likely to be N'-(4-chlorophenyl)-N-hydroxymethyl-N-methylurea. A urinary product of conjugative metabolism obtained after the administration of monuron to mice also gave the mass spectrum of the monomethyl compound after deconjugation which suggests that a conjugated N-hydroxymethyl compound may have been formed in vivo.

Studies of the pharmacology of N-methylformamide in mice

When 400 mg/kg of 14C-methyl-labeled N-methylformamide (NMF) was injected ip into mice, the curve for plasma concentration of radioactivity versus time was superimposable on the curve obtained by measuring unmetabolized NMF with gas-liquid chromatography during the first 24 hrs. Radioactivity in plasma was measurable for 8 days after NMF administration, but NMF was not measurable by gas chromatography beyond 24 hrs after administration. Radioactivity was eliminated from the plasma after 60 hrs, with an apparent half-life of 71.1 hrs. Of the radioactivity injected with NMF, 73.6% was recovered in the urine in 24 hrs; 26.4% of this was unchanged NMF. Three percent of the administered radioactivity was exhaled as 14CO2 in 7 hrs at a constant rate of 0.007% per min. One urinary metablite was a stable precursor of formaldehyde, which decomposed to formaldehyde only after alkaline hydrolysis and may well be N-(hydroxymethyl)-formamide. The areas under the plasma concentration versus time curve were estimated after ip, iv, and oral administration of NMF. The bioavailability of NMF was 1.01 after oral administration and 1.10 after ip administration.

N-methylformamide: antitumour activity and metabolism in mice

The antitumour activities of N-methylformamide, N-ethylformamide and formamide against a number of murine tumours in vivo (Sarcoma 180, M5076 ovarian sarcoma and TLX5 lymphoma) have been estimated. In all cases N-methyl-formamide had significant activity, formamide had marginal or no activity and N-ethylformamide had no significant activity. N-methylformamide and N-ethylformamide were equitoxic to the TLX5 lymphoma in vitro. Formamide was found as a metabolite in the plasma and urine of animals given N-methylformamide and N-ethylformamide, but excretion profiles do not support the hypothesis that formamide is an active antitumour species formed from N-alkylformamides. No appreciable metabolism of N-methylformamide occurred under a variety of conditions with liver preparations in vitro. N-methylformamide, but not N-ethylformamide or formamide, reduced liver soluble non-protein thiols by 59.8% 1 h after administration of an effective antitumour dose.

Jack bean urease (EC 3.5.1.5). V. On the mechanism of action of urease on urea, formamide, acetamide, N-methylurea, and related compounds

Acetamide and N-methylurea have been shown for the first time to be substrates for jack bean urease. In the enzymatic hydrolysis of urea, formamide, acetamide, and N-methylurea at pH 7.0 and 38 degrees C, kcat has the values 5870, 85, 0.55, and 0.075 s-1, respectively. The urease-catalyzed hydrolysis of all these substrates involves the active-site nickel ion(s). Enzymatic hydrolysis of the following compounds could not be detected: phenyl formate, p-nitroformanilide, trifluoroacetamide, p-nitrophenyl carbamate, thiourea, and O-methylisouronium ion. In the enzymatic hydrolysis of urea, the pH dependence of kcat between pH 3.4 and 7.8 indicates that at least two prototropic forms are active. Enzymatic hydrolysis of urea in the presence of methanol gave no detectable methyl carbamate. A mechanism of action for urease is proposed which involves initially an O-bonded complex between urea and an active-site Ni2+ ion and subsequently an O-bonded carbamato-enzyme intermediate.

Comparison of ionic selectivity of batrachotoxin-activated channels with different tetrodotoxin dissociation constants

The purpose of these experiments is to test whether the differences between normal and tetrodotoxin-resistant Na+ channels reside in the selectivity filter. To do this, we have compared the selectivity of batrachotoxin-activated channels for alkali cations, organic cations, and nonelectrolytes in two neuroblastoma clonal cell lines: N18, which has normal tetrodotoxin (TTX) sensitivity, and C9, which is relatively TTX-resistant. We have also studied the effect of H+ on Na+ permeability and on the interaction between TTX and its receptor site in both cell lines. There is no qualitative difference between the two cell lines in any of these properties. In both cell lines the batrachotoxin-activated Na+ channels have a selectivity sequence of Tl+ greater than Na+ greater than K+, guanidinium greater than Rb+ greater than Cs+, methylamine. Also, in both cell lines H+ blocks Na+ channels with a pKa of 5.5 and inhibits the action of TTX with the same pKa. These observations indicate that the selectivity filters of the Na+ channels in C9 and N18 do not differ significantly despite the 100-fold difference in TTX-affinity. Our selectivity studies of batrachotoxin-activated Na+ channels for both cell lines suggest that these toxin-activated Na+ channels have a limiting pore size of 3.8 x 6.0 A, as compared to a pore size of 3.0 x 5.0 A for potential-activated Na+ channels.

Oxalate, formate, formamide, and methanol metabolism in Thiobacillus novellus

Thiobacillus novellus was able to grow with oxalate, formate, formamide, and methanol as sole sources of carbon and energy. Extensive growth on methanol required yeast extract or vitamins. Glyoxylate carboligase was detected in extracts of oxalate-grown cells. Ribulose bisphosphate carboxylase was found in extracts of cells grown on formate, formamide, and thiosulfate. These data indicate that oxalate is utilized heterotrophically in the glycerate pathway, and formate and formamide are utilized autotrophically in the ribulose bisphosphate pathway. Nicotinamide adenine dinucleotide-linked formate dehydrogenase was present in extracts of oxalate-, formate-, formamide-, and methanol-grown cells but was absent in thiosulfate- and acetate-grown cells.

Cation permeability ratios of sodium channels in normal and grayanotoxin-treated squid axon membranes

Permeabilities of squid axon membranes to various cations at rest and during activity have been measured by voltage clamp before and during internal perfusion of 4 X 10(-5) M grayanotoxin I. The resting sodium and potassium permeabilities were estimated to be 6.85 X 10(-8) cm/sec and 2.84 X 10(-6) cm/sec, respectively. Grayanotoxin I increased the resting sodium permeability to 7.38 10(-7) cm/sec representing an 11-fold increase. The potassium permeability was increased only by a factor of 1.24. The resting permeability ratios as estimated by the voltage clamp method before application of grayanotoxin I were Na (1): Li (0.83): formamidine (1.34): guanidine (1.49): Cs (0.87): methylguanidine (0.86): methylamine (0.78). Grayanotoxin I did not drastically the resting permeability ratios with a result of Na (1): Li (0.95): formamidine (1.27): guanidine (1.16): Cs (0.47): methylguanidine (0.72): methylamine (0.46). The membrane potential method gave essentially the same resting permability ratios before and during application of grayanotoxin I if corrections were made for permeability to choline as the cation substitute and for changes in potassium permeability caused by test cations. The permeability ration choline/Na was estimated to be 0.72 by the voltage clamp method and 0.65 by the membrane potential method. Grayanotoxin I decreased the ration to 0.43. The permeability ratios during peak transient current were estimated to be Na (1): Li (1.12): formamidine (0.20): guanidine (0.20): Cs (0.085): methylguanidine (0.061): methylamine (0.036). Thus the sodium channels for the peak current are much more selective to cation than the resting sodium channels. It appears that the resting sodium channels in normal and grayanotoixn I-treated axons are operationally different from the sodium channels that undergo a conductance increase upon stimulation.