Identification by proton NMR of N-(hydroxymethyl)-N-methylformamide as the major urinary metabolite of N,N-dimethylformamide in mice

Effects of ketoconazole on the biodistribution and metabolism of [11C]loperamide and [11C]N-desmethyl-loperamide in wild-type and P-gp knockout mice

INTRODUCTION

[(11)C]Loperamide and [(11)C]N-desmethyl-loperamide ([(11)C]dLop) have been proposed as radiotracers for imaging brain P-glycoprotein (P-gp) function. A major route of [(11)C]loperamide metabolism is N-demethylation to [(11)C]dLop. We aimed to test whether inhibition of CYP3A4 with ketoconazole might reduce the metabolism of [(11)C]loperamide and [(11)C]dLop in mice, and thereby improve the quality of these radiotracers.

METHODS

Studies were performed in wild-type and P-gp knockout (mdr-1a/b -/-) mice. During each of seven study sessions, one pair of mice, comprising one wild-type and one knockout mouse, was pretreated with ketoconazole (50 mg/kg, ip), while another such pair was left untreated. Mice were sacrificed at 30 min after injection of [(11)C]loperamide or [(11)C]dLop. Whole brain and plasma samples were measured for radioactivity and analyzed with radio-high-performance liquid chromatography.

RESULTS

Ketoconazole increased the plasma concentrations of [(11)C]loperamide and its main radiometabolite, [(11)C]dLop, by about twofold in both wild-type and knockout mice, whereas the most polar radiometabolite was decreased threefold. Furthermore, ketoconazole increased the brain concentrations of [(11)C]loperamide and the radiometabolite [(11)C]dLop by about twofold in knockout mice, and decreased the brain concentrations of the major and most polar radiometabolite in wild-type and knockout mice by 82% and 49%, respectively. In contrast, ketoconazole had no effect on plasma and brain distribution of administered [(11)C]dLop and its radiometabolites in either wild-type or knockout mice, except to increase the low plasma [(11)C]dLop concentration. The least polar radiometabolite of [(11)C]dLop was identified with LC-MS(n) as the N-hydroxymethyl analog of [(11)C]dLop and this also behaved as a P-gp substrate.

CONCLUSION

In this study, ketoconazole (50 mg/kg, ip) proved partially effective for inhibiting the N-demethylation of [(11)C]loperamide in mouse in vivo but had relatively smaller or no effect on [(11)C]dLop.

N-Methylcarbamoyl-lysine adduct in globin: A new metabolic product and potential biomarker of N,N-dimethylformamide in humans

Metabolism of the solvents N,N-dimethylformamide (DMF) and N-methylformamide (MF) results in the formation of N-methylcarbamoyl adducts at the N-terminal valine and lysine in blood protein globin, of which the lysine adduct has so far only been reported in rats given high doses of both solvents [Mráz, J., Simek, P., Chvalová, D., Nohová, H., Smigolová, P., 2004. Studies on the methyl isocyanate adducts in globin. Chem. Biol. Interact. 148, 1-10]. Here we examined whether the lysine adduct is produced, and accessible to analysis, in humans occupationally or experimentally exposed to DMF. Globin from exposed subjects (n=35) and unexposed controls (n=5) was analyzed by two methods. Edman degradation was used as a sensitive reference method to measure the valine adduct by converting it to 3-methyl-5-isopropylhydantoin (MVH). The MVH levels in globin of the exposed subjects were in the range of 1-441 nmol/g, in controls <1 nmol/g. The principal method of globin analysis consisted of enzymatic hydrolysis with pronase to release free N(epsilon)-(N-methylcarbamoyl)lysine (MLU) and N-methylcarbamoylvaline (MVU), which were determined by HPLC/MS/MS, with no clean-up or preconcentration steps needed. For MLU, the parent and product ions were m/z 204-->173, and the limit of detection was approximately 5 nmol/g globin. MLU was found in most globins from the exposed subjects but not in the controls. A close correlation between the MLU and MVH levels (nmol/g) was observed: MLU=7+0.48 MVH (R(2)=0.84, n=32). In conclusion, MLU can be easily measured in globin of workers exposed to DMF. The findings also indicate a long-term persistence of MLU in the human body, and consequently, its potential as a biomarker of chronic exposure to DMF.

Total body burden arising from a week's repeated dermal exposure to N,N-dimethylformamide

BACKGROUND

Hazardous chemicals and their metabolites may accumulate in the body following repeated airborne exposures and skin contact.

AIMS

To estimate the contribution of skin absorption to total body burden of N,N-dimethylformamide (DMF) across a working week in two groups with similar levels of respiratory exposure but dissimilar skin contact.

METHODS

Twenty five workers in a synthetic leather (SL) factory, 20 in a copper laminate circuit board (CLCB) factory, and 20 age and sex matched non-DMF exposed subjects, were recruited. Environmental monitoring of DMF exposure via respiratory and dermal routes, as well as biological monitoring of pre-shift urinary N-methylformamide (U-NMF), were performed for five consecutive working days.

RESULTS

Environmental and biological monitoring showed no detectable exposure in controls. The average airborne DMF concentration (geometric mean (GM) 3.98 ppm, geometric standard deviation (GSD) 1.91 ppm), was insignificantly lower for SL workers than for CLCB workers (GM 4.49, GSD 1.84 ppm). Dermal DMF exposure and U-NMF values, however, were significantly higher for SL workers. A significant pattern of linear accumulation was found across a five day work cycle for SL workers but not for CLCB workers.

CONCLUSIONS

Dermal exposure to DMF over five consecutive days of occupational exposure can result in the accumulation of a significant DMF body burden. The long term exposure response under both repeated and intermittent conditions of substantial skin exposure is worthy of note.

The effects of simultaneous exposure to methyl ethyl ketone and toluene on urinary biomarkers of occupational N,N-dimethylformamide exposure

General regulations and risk assessment regarding toxicants are single-compound oriented even though humans are exposed to multi-chemicals in the general environment. This study investigated the effects of different levels of N,N-dimethylformamide (DMF) and co-exposure levels of methyl ethyl ketone (MEK) and toluene (TOL) on two biomarkers of DMF exposure: non-metabolized urinary (U-)DMF and the DMF metabolite urinary N-methylformamide (NMF). Thirty-five workers were selected from a two-stage field investigation strategy and were classified into four groups based on DMF exposure and co-exposure levels. Breathing-zone air concentrations of DMF, MEK, and TOL as well as dermal DMF exposure were determined. Post-shift U-DMF and U-NMF levels were determined for each individual. U-DMF concentrations were significantly higher in high-DMF groups than in low-DMF groups, but U-NMF concentrations were significantly (P<0.05) lower in the high-DMF-high-co-exposure group than in the high-DMF-low-co-exposure group; there were no significant differences between two low-DMF groups. The ratio of U-NMF to U-DMF showed the biotransformation from DMF to NMF was significantly suppressed at high co-exposure (P<0.001) for high-DMF exposure groups, possibly because of competitive inhibition of CYP2E1, the responsible enzyme involved. Due to the ubiquity of MEK/TOL in DMF-exposed occupational settings, the biological exposure index for occupational DMF exposure should be re-evaluated at high co-exposure levels.

Urinary biomarkers of occupational N,N-dimethylformamide (DMF) exposure attributed to the dermal exposure

OBJECTIVE

To evaluate whether the dermal exposure to N,N-dimethylformamide (DMF) exerts significant effects and to determine the unit increment of dermal exposure on the total body burden of two biomarkers in urine: metabolism-required N-methylformamide (U-NMF) and non-metabolized DMF (U-DMF) in actual occupational environments.

METHODS

Exposure via respiratory and dermal routes was assessed on an individual basis for 75 workers from four DMF-related factories directly exposed to DMF. Respiratory exposure was determined by breathing-zone sampling for a full-work shift, and dermal exposure was assessed on the palms and forearms of both hands by an adhesive tape-patch method. U-NMF and U-DMF collected immediately postshift were measured.

RESULTS

The average concentrations of airborne DMF, DMF on hands and on forearms, U-NMF, and U-DMF (GM) were 1.51 ppm, 0.04 microg/cm(2), 0.03 microg/cm(2), 0.47 mg/l, and 0.38 mg/l, respectively. In multiple linear regression tests, only airborne DMF and DMF on hands remained significantly (P<0.001) associated with U-NMF and U-DMF. Based on model estimates, the unit increment of hands' exposure (microg/cm(2)) could contribute to 0.53 and 0.46 mg/l of the increment of U-NMF and U-DMF, respectively, given a daily occupational airborne exposure to DMF at about 1.5 ppm.

CONCLUSIONS

Dermal exposure provides a substantial contribution to the total body burden of DMF. A control remedy such as the enforcement of wearing impermeable gloves by workers occupationally exposed to DMF should be implemented with the highest priority.

The effects of co-exposure to methyl ethyl ketone on the biological monitoring of occupational exposure to N,N-dimethylformamide

OBJECTIVES

(1) To assess whether urinary N,N-dimethylformamide (U-DMF) is suitable as a biomarker when co-exposure to methyl ethyl ketone (MEK) exists, and to evaluate whether it is suitable as an exposure biomarker of DMF. (2) To examine whether the co-exposure to MEK affects the characteristics of U-NMF and U-DMF. (3) To investigate if the difference in creatinine-adjusted and non-adjusted measurements of urinary biomarkers of DMF exposure is substantial.

METHODS

Personal exposure monitoring of N,N-dimethylformamide (DMF) and MEK on 11 synthetic-leather workers was performed for 5 consecutive days. Daily post-shift urine for each individual was collected and was analyzed for urinary N-methylformamide (U-NMF) and U-DMF levels on both non-adjusted and creatinine-adjusted bases.

RESULTS

Both U-NMF and U-DMF showed significant associations with airborne DMF. Positive and significant associations between U-NMF and U-DMF on either a non-adjusted basis or a creatinine-adjusted basis were found. Satisfactory linear associations ( P<0.01) between all kinds of urinary biomarkers and DMF exposure were found. The co-exposure to MEK exerted more effect on the relationship of airborne DMF to U-DMF than to U-NMF.

CONCLUSIONS

U-DMF is detectable when occupational DMF exposure is near or below the occupational exposure limit of 10 ppm. In view of the performance of sensitivity, specificity, and positive predictive value, U-NMF, in general, is superior to U-DMF. However, on a par with other findings in this and previous studies, U-DMF might be considered as a complimentary biomarker of exposure to DMF in addition to U-NMF. No distinction between creatinine-adjustment or non-adjustment for urine specimens was found in the biological monitoring of DMF exposure. Further exploration of the influence of co-exposure to MEK at higher exposure is warranted.

Measurement of urinary N-acetyl-S-(N-methylcarbamoyl)cysteine by high-performance liquid chromatography with direct ultraviolet detection

A new high-performance liquid chromatographic (HPLC) method is described for the determination of urinary N-acetyl-S-(N-methylcarbamoyl)cysteine (AMCC), the final product of the conjugation reaction between a metabolic intermediate of N,N-dimethylformamide (DMF) and glutathione. Urine samples were purified by C(18) solid-phase extraction and then directly analysed by HPLC with an Aminex Ion Exclusion HPX-87H column maintained at 25 degrees C and a UV detector set at 196 nm. Under isocratic conditions (2.4 mM sulphuric acid, flow-rate=0.6 ml/min) AMCC eluted at 20.2 min. The reproducibility (C.V.%) was 1.3-2.7% (intra- and inter-assay, N = 5); the accuracy was 98.0+/-1.7% at 10 mg/l and 101.9+/-1.5% at 800 mg/l (mean+/-SD, N = 3). AMCC was measured in urine from 22 exposed subjects. A strong correlation was found between AMCC and environmental DMF [AMCC (mg/g creatinine)=3.40xDMF (mg/m(3)) + 3.07; r=0.95], while in the urine of 20 unexposed subjects the concentration of AMCC was constantly below the detection limit of the method (0.9 mg/l in urine). The method described appears to be useful for the biological monitoring of DMF exposure.

Inactivation of rat liver cytochrome P450 (P450) by N,N-dimethylformamide and N,N-dimethylacetamide

N,N-dimethylformamide (DMF), an organic solvent widely used in industry, is bioactivated by cytochrome P450 (P450) to reactive metabolites which are believed to be responsible for the hepatotoxicity observed in animals and humans. A decrease of the activating enzyme has been reported in rats treated with DMF, although the specific P450 isoform(s) involved and the nature of the reactive species responsible for this and the other toxic effects are still being investigated. In the present work, the effect of DMF and of the structurally related N,N-dimethylacetamide (DMAc) on the activating enzyme and the nature of the reactive species involved in the mechanism of P450 inactivation by the two chemicals were investigated in vitro. Incubation of liver microsomes from pyridine-induced rats with either substrate resulted in a dose-dependent (0-20 mM) loss of P450 (up to 28 and 24% for DMF and DMAc, respectively), microsomal haem (up to 24 and 20% for DMF and DMAc, respectively), but not protoporphyrin IX content. Moreover, bubbling of CO through the incubation mixture gave almost complete protection against substrate-dependent P450 inactivation, and the spin trapping agent N-tert-butyl-alpha-phenylnitrone, but neither glutathione nor vitamin C, provided a significant protection against DMF- or DMAc-dependent haem loss. Finally, electron spin resonance analysis of microsomal incubations in presence of DMF or DMAc showed spectral evidence for a carbon centered radical intermediate. The results indicate, overall, that both compounds are metabolized in vitro by P450, probably CYP2E1, to free radical metabolites which attack the haem prosthetic group, leading to suicidal enzyme inactivation.

Simultaneous determination of two human urinary metabolites of N,N-dimethylformamide using gas chromatography-thermionic sensitive detection with mass spectrometric confirmation

Two human urinary metabolites of the industrial solvent N,N-dimethylformamide (DMF), N-hydroxymethyl-N-methylformamide (HMMF) and N-acetyl-S-(N-methylcarbamoyl)cysteine (AMCC), were assayed using a new analytical method (gas chromatography and thermionic sensitive detection). Clean-up of urine samples includes a liquid-liquid extraction step followed by a solid-phase extraction step to separate HMMF and AMCC from other urine components. During clean-up, AMCC is converted into ethyl-N-methylcarbamate (EMC), and during gas chromatography, HMMF is degraded in the injector to N-methylformamide (NMF). All the validation data necessary for a quantitative procedure are given. The method was applied to urine samples from workers exposed to DMF and from the general population. The results were confirmed by mass spectrometric determination. For this purpose a further liquid-liquid extraction step was introduced in the clean-up procedure. Background levels of AMCC in the general population were identified.

Biological monitoring of workers exposed to N-N-dimethylformamide. II. Dimethylformamide and its metabolites in urine of exposed workers

N,N-Dimethylformamide (DMF) exposure was monitored in a synthetic leather factory; at the same time, urinary dimethylformamide and its metabolites were measured in urine samples collected before and at the end of workshifts. The study was run during two different periods. During the first phase ten workers were observed for 3 days (Monday, Tuesday and Wednesday) in the same week. In the second phase 16 workers were involved in the study on a Friday and on the following Monday. Urinary DMF, as well as hydroxymethyl-N-methylformamide and hydroxymethylformamide [measured as N-methylformamide (NMF) and formamide, respectively], were measured as a "physiological" product in subjects not exposed to dimethylformamide. Environmental exposure to DMF ranged between 10 and 25 mg/m3. The unmodified solvent found in urine collected at the end of the exposure was significantly related to the environmental concentrations of DMF; its urinary concentrations were found to range between 0.1 and 1 mg/l. Higher concentrations of NMF (mean 23.3 mg/l) and formamide (24.7 mg/l) were measured in urine samples collected at the end of workshifts. The same concentrations were related to individual exposures to DMF. N-Acetyl-S-(N-methylcarbamoyl)cysteine in the urine of workers exposed to DMF showed a mean concentration of 40.4 mg/l on Friday (before and after the workshift) and a mean concentration of 10.3 mg/l on Monday. Its slow kinetic profile favours its body accumulation during the working week.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological monitoring of workers exposed to N,N-dimethylformamide by determination of the urinary metabolites, N-methylformamide and N-acetyl-S-(N-methylcarbamoyl) cysteine

Biological monitoring of workers exposed to N,N-dimethylformamide (DMF) was carried out by determination of the urinary metabolites, N-methylformamide (MF, mainly from N-hydroxymethylformamide) and N-acetyl-S-(N-methylcarbamoyl)cysteine (AMCC), which were derived from two different routes of metabolism of the solvent. The urinary levels of MF increased rapidly at the start of the work shift, and decreased almost to zero within 24 h after the beginning of the last exposure. The highest level was found between the end of the afternoon shift and bedtime. AMCC levels remained constant over the consecutive work days and increased after the cessation of exposure, with the peak concentration being observed at 16-40 h after the cessation of exposure. AMCC levels at the beginning of the next morning shift were closely correlated with personal exposure levels of DMF in air, although the correlation of MF and DMF in air was highest in the urine at the end of the shift. Hence urinary AMCC represents an index of the average exposure during several preceding work days and may indicate the internal dose. By contrast, MF represents an index of daily exposure.

Metabolism and hepatotoxicity of N,N-dimethylformamide, N-hydroxymethyl-N-methylformamide, and N-methylformamide in the rat

The metabolism and hepatotoxicity of N,N-dimethylformamide (DMF) and two of its metabolites, N-hydroxymethyl-N-methylformamide (HMMF) and N-methylformamide (NMF) were evaluated over a 4-day period in rats. DMF toxicity was dose dependent and delayed toxicity after the administration of a high DMF dose (13.7 mmol/kg) in comparison to a lower dose (4.1 mmol/kg) was observed. Treatment of rats with 13.7 mmol/kg DMF, HMMF, or NMF showed i) that DMF is more toxic than HMMF or NMF, and ii) that hepatotoxicity occurs later for DMF than for HMMF or NMF. Analysis of serum and urine samples demonstrated that DMF is first metabolized to HMMF, which is then partially converted to NMF. After HMMF administration, NMF was found both in serum and in urine. The time course of DMF and HMMF toxicity in relation to NMF formation fitted the hypothesis that the hepatotoxicity of DMF and HMMF is mediated via NMF. The degree of hepatotoxicity after HMMF and NMF treatment is similar. However, the degree of DMF hepatotoxicity is much higher than in the case of NMF or HMMF. The role of NMF as an obligatory intermediate in DMF and HMMF hepatotoxicity is discussed.

The microsomal dealkylation of N,N-dialkylbenzamides

The in vitro metabolism of N,N-dialkylamides by phenobarbital-induced rat liver microsomes yields an N-alkylamide and the corresponding aldehyde. Although, N-hydroxymethyl-N-alkylamide intermediates can be detected from N-methyl-N-alkylamides, no N-hydroxyalkyl-N-alkylamide intermediates are detected from the N,N-dialkylamide substrates. Vmax values were independent of amide structure, whereas Vmax/Km values were dependent on the lipophilicity of the N,N-dialkylbenazamide studied. These results suggest that diffusion of substrate into the membrane-bound enzyme active site limits the rate of microsomal oxidation of the amides. Metabolism of N-alkyl-N-methylamides reveals identical values of Vmax for demethylation and dealkylation. Values of Vmax/Km for demethylation depend upon the lipophilicity of the N-alkyl group, whereas Vmax/Km values for dealkylation appear to be dependent upon the steric bulk of the alkyl group, particularly around the alpha-carbon. Moreover, Vmax/Km values for demethylation are larger than for dealkylation, implying the reactions are under kinetic control. Comparison of the kinetic data with theoretical AM1 semi-empirical molecular orbital calculations suggests a mechanism involving formation of a carbon-centred radical. Use of an N-cyclopropylmethylbenzamide substrate to trap such a radical failed, presumably because oxygen rebound is faster than radical rearrangement. An N-cyclopropylamide substrate did not undergo metabolism of the cyclopropyl ring, consistent with carbon-centred radical, but not nitrogen radical cation, formation.

Deuterium NMR spectroscopy of biofluids for the identification of drug metabolites: application to N,N-dimethylformamide

The metabolism of N,N-dimethylformamide in the Sprague-Dawley rat has been reinvestigated using NMR spectroscopy of urine. In particular, through the use of N,N-dimethylformamide-d7 (DMF-d7) and 2H-NMR spectroscopy, the principal metabolites of this compound have been confirmed in a direct manner. The use of inverse-detected two-dimensional 2H-13C correlation with 13C decoupling aided metabolite identification through the provision of 13C chemical shifts.

Simultaneous determination of N,N-dimethylformamide, N-monomethylformamide and N-hydroxymethyl-N-methylformamide in rat plasma by capillary gas chromatography

A capillary gas chromatographic method with nitrogen-phosphorus detection for the simultaneous quantitative determination of N,N-dimethylformamide, and its two metabolites, N-monomethylformamide and N-hydroxymethyl-N-methylformamide, in rat plasma has been developed. The method involves a single extraction step with ethyl acetate-acetone (4:1, v/v). The extract is injected into a fused-silica capillary column coated with Carbowax 20M. A temperature gradient (65-110 degrees C) is applied, and the three products can be separated within 10 min. The quantitation limits, using 25 microliters of rat plasma, for N,N-dimethylformamide, N-monomethylformamide and N-hydroxymethyl-N-methylformamide are 0.4, 0.4 and 2 micrograms/ml, respectively. This method is suitable for toxicokinetic studies in rats.

Dimethylformamide pharmacokinetics following inhalation exposure in monkeys

Male and female cynomolgus monkeys received whole-body inhalation exposures to dimethylformamide (DMF) at concentrations of 30, 100, and 500 ppm for 6 hours a day, 5 days a week over a 13-week period. Serial blood samples were drawn at the conclusion of the first day of exposure and following 15, 29, 57, and 85 days of testing. Area under the plasma concentration curve (AUC) values were determined for DMF and "NMF" [N-methylformamide (NMF) plus N-(hydroxymethyl)-N-methylformamide (DMF-OH)]. Urine samples were also collected and assayed for DMF, NMF and DMF-OH. The systemic exposure to DMF increased disproportionately as the airborne DMF concentrations increased. DMF AUC values increased 19- to 37-fold in male and 35- to 54-fold in female monkeys as the inhalation concentrations increased 5-fold (100 to 500 ppm). These data are consistent with saturation of DMF metabolism as inhaled DMF concentrations increased from 100 to 500 ppm. AUC values, peak plasma concentrations, and plasma half-lives were essentially unaltered over the duration of the study within each exposure concentration tested. Estimated plasma half-lives ranged from 1 to 2 hours and 4 to 15 hours for DMF and "NMF" respectively. DMF was rapidly converted to "NMF" following 30 ppm exposures, with "NMF" plasma concentrations higher than DMF plasma concentrations at the 0.5 hour time-point. In plasma samples simultaneously assayed for DMF-OH and NMF, the concentration of DMF-OH exceeded, was equal to, or was less than NMF concentrations depending upon the plasma sample. DMF-OH was always the main urinary metabolite (56 to 95 percent) regardless of exposure level or time on study.

Dimethylformamide pharmacokinetics following inhalation exposures to rats and mice

Whole-body inhalation exposures to N,N-dimethylformamide (DMF) were conducted with rats and mice. The exposure concentrations were 10, 250, and 500 ppm DMF. The exposure routines consisted of single 1-, 3-, or 6-hour exposures and ten 6-hour exposures (ten exposure days in 2 weeks). Area under the plasma concentration curve (AUC) values were determined following exposure for DMF and "N-methylformamide" ["NMF" represented N-methylformamide plus N-(hydroxymethyl)-N-methylformamide (DMF-OH)]. The DMF AUC values increased 8- and 29-fold for rats and mice, respectively, following single six-hour exposures to 250 and 500 ppm DMF. These data are indicative of saturation of DMF metabolism. Peak "NMF" plasma concentrations for rats and mice, following single 6-hour exposures, did not increase as DMF exposure concentrations increased from 250 to 500 ppm. In addition, the "NMF" plasma levels in rats following a single 6-hour 500 ppm DMF exposure did not decay by 24 hours post exposure. These "NMF" plasma data also indicate saturation of DMF metabolism. Multiple exposures to 500 ppm DMF resulted in a 3- and 4-fold reduction in DMF AUC values for rats and mice, respectively, compared to AUC values following a single six-hour 500 ppm DMF exposure. This indicates enhanced metabolism of DMF resulting from multiple 500 ppm DMF exposures and together with saturation of DMF metabolism suggest using exposure levels below 500 ppm in a chronic bioassay. Selected plasma samples were simultaneously assayed for NMF and DMF-OH. The "NMF" values consisted of between 30 to 60 percent DMF-OH depending upon the exposure group (conversely NMF represented 30 to 60 percent of the "NMF" levels). Urinary analysis of all samples revealed DMF-OH represented over 90 percent of the summed DMF, DMF-OH and NMF quantities.

The microsomal demethylation of N,N-dimethylbenzamides. Substituent and kinetic deuterium isotope effects

The metabolism of N,N-dimethylbenzamides by phenobarbital-induced rat liver microsomes results in the formation of N-methylbenzamides and formaldehyde. The reaction proceeds via the formation of an intermediate N-hydroxymethyl-N-methylbenzamide, which, for the microsomal oxidation of N,N-dimethylbenzamide, was isolated and characterized. Confirmation of the N-hydroxymethyl-N-methylbenzamide was obtained by its independent synthesis from N-methylbenzamide and formaldehyde. The intermolecular kinetic deuterium isotope effects for the reaction are 0.9 (+/- 0.1) for Vmax and 1.4 (+/- 0.1) for Vmax/Km. The intramolecular kinetic deuterium isotope effect, determined from the relative amounts of N-methylbenzamide and N-trideuteriomethylbenzamide formed in the microsomal demethylation of N-trideuteriomethyl-N-methylbenzamide, is 6.0 +/- 0.3. There is no correlation of Vmax or Vmax/Km with the substituent in the aromatic ring, nor with the calculated ionization potentials of the benzamides. The results are interpreted in terms of a mechanism in which the benzamide undergoes direct hydrogen atom abstraction to form a carbon centred radical. This carbon centred radical subsequently forms an N-hydroxymethyl-N-methylbenzamide that decomposes to formaldehyde and an N-methylbenzamide. Semi-empirical AM1 self consistent field molecular orbital calculations identify that loss of a hydrogen atom from the E-methyl group is thermodynamically more favourable than from the Z-methyl group by ca. 5 kJ/mol.

Absorption, metabolism and elimination of N,N-dimethylformamide in humans

Excretion of N,N-dimethylformamide (DMF) and DMF metabolites N-hydroxymethyl-N-methylformamide ("MF"), N-hydroxymethyl-formamide ("F") and N-acetyl-S-(N-methylcarbamoyl)cysteine (AMCC) has been monitored in the urine of volunteers during and after their 8-h exposure to DMF vapour at a concentration of 10, 30 and 60 mg.m-3. The pulmonary ventilation in these experiments was typically about 10 l.min-1 and the retention in the respiratory tract was 90%. After exposure to 30mg DMF.m-3, the yield of compound determined in the urine represented 0.3% (DMF), 22.3% ("MF"), 13.2% ("F") and 13.4% (AMCC) of the dose absorbed via the respiratory tract. The excretion curves of the particular compounds attained their maximum 6-8h (DMF), 6-8h ("MF"), 8-14h ("F") and 24-34h (AMCC) after the start of the exposure. The half-times of excretion were approximately 2, 4, 7 and 23 h respectively. In contrast to slow elimination of AMCC after exposure to DMF, AMCC was eliminated rapidly after AMCC intake. This discrepancy could be explained by rate-limiting reversible protein binding of a reactive metabolic intermediate of DMF, possibly methylisocyanate.

Occupational dimethylformamide exposure. 2. Monomethylformamide excretion in urine after occupational dimethylformamide exposure

The relationship between the 8-h time-weighted average (TWA) intensity of exposure to N,N-dimethylformamide (DMF) vapor (with little possibility of skin contact with liquid DMF) and the subsequent excretion of N-monomethylformamide (MMF) precursor in shift-end urine samples was examined in 116 workers exposed to DMF and 92 workers exposed to DMF in combination with toluene. Urinary MMF level was examined also in 42 non-exposed subjects. The TWA vapor concentration in breathing zone air of each worker was successfully measured by means of a recently developed diffusive sampler in which water was used as an absorbent. The examination of gas chromatographic (GC) conditions for MMF determination showed that the formation of MMF was not saturated when the injection port temperature was set at 200 degrees C, reached a plateau at 250 degrees C, and showed no additional increase at 300 degrees C. There was a linear relationship between DMF in air and MMF in urine with a regression equation of y = 1.65 x + 1.69 (r = 0.723, P less than 0.01), where y is MMF (unit; mg/l, uncorrected for urine density) in urine and x is DMF (ppm) in air, when only those exposed to DMF were selected, and the injection port temperature was set at 250 degrees C. From this equation, it was possible to estimate that about 10% of the DMF absorbed will be excreted into urine as the MMF precursor. The slope of the regression line was significantly smaller among those exposed to DMF and toluene in combination as compared with those with DMF exposure only.

NMR studies of drug metabolism and disposition

The ways in which NMR is being used in vivo to study drug metabolism and disposition are reviewed. We also assess the role of this technique as a non-invasive method for monitoring the fate of drugs in human and animals, and for providing information about pharmacology and toxicity.

The application of nuclear magnetic resonance spectroscopy to drug metabolism studies

1. The applications of n.m.r. spectroscopy studies to drug metabolism, both in vitro (biofluids, isolated cells, excised tissue samples, isolated organs) and in vivo (animals, humans) are reviewed. 2. N.m.r. is a relatively insensitive technique, but it has the great advantage of being non-invasive and non-destructive, i.e. it allows a direct study of intact biological samples. 3. The majority of studies examined deal with 19F-n.m.r. spectroscopy mainly because of the favourable n.m.r. characteristics of this nucleus, and the low level of endogenous fluorine which gives no detectable 19F signal. However, the potential utility of 1H-31P- and 13C-n.m.r. is also emphasized.

Differences between rodents and humans in the metabolic toxification of N,N-dimethylformamide

The widely used industrial solvent N,N-dimethylformamide (DMF) causes liver damage in occupationally exposed persons and is suspected of involvement in the generation of certain occupational malignancies. Here the extent of the biotransformation of DMF to three urinary metabolites has been compared in humans and rodents. The metabolites, which were quantified by gas chromatography (GC) are N-(hydroxymethyl)-N-methylformamide (HMMF), which yielded N-methylformamide on GC analysis, a species which decomposed to formamide on GC analysis, and N-acetyl-S-(N-methylcarbamoyl) cysteine (AMCC), measured after derivatization with ethanol to give ethyl N-methylcarbamate. Ten volunteers who absorbed between 28 and 60 mumol/kg DMF during an 8-hr exposure to DMF in the air at 60 mg/m3 excreted in the urine within 72 hr between 16.1 and 48.7% of the dose as HMMF, between 8.3 and 23.9% as formamide, and between 9.7 and 22.8% as AMCC. AMCC, together with HMMF, was also detected in the urine of workers after occupational exposure to DMF. The portion of the dose (0.1, 0.7, or 7.0 mmol/kg given ip) which was metabolized in mice, rats, or hamsters to HMMF varied between 8.4 and 47.3% of the dose; between 7.9 and 37.5% were excreted as formamide and only between 1.1 and 5.2%, as AMCC. The results suggest that there is a quantitative difference between the metabolic pathway of DMF to AMCC in humans and rodents. It is argued that the hepatotoxic potential of DMF may be linked to the extent of its metabolic conversion to AMCC.

Dimethylformamide (DMF) hepatotoxicity

Scattered case reports of accidental exposure and a few epidemiological studies have indicated that the liver is the main target organ following acute and chronic exposure to dimethylformamide (DMF). This has been confirmed in several animal species. In humans, ethanol intolerance is one of the earliest manifestations of (excessive) exposure to DMF, followed at higher exposure levels by various complaints (nausea, vomiting, abdominal pain) and the release of liver cytolytic enzymes in the plasma. The metabolic pathway of DMF has been recently clarified, but the primary cellular lesion responsible for its hepatotoxicity is still unknown.