Evaluation of developmental toxicity of Methyl Chloride (Chloromethane) in rats, mice, and rabbits.

Methyl Chloride (MeCl; Chloromethane) is a high production volume chemical (>1000 t/a) and is used as an industrial solvent. Based on cardiac lesions reported in developmental toxicity studies in mice, but not in rats, manufacturers decided to classify MeCl as a developmental toxicant, cat. 2. Recently, the European Chemical Agency required a developmental toxicity study in a non-rodent species. No developmental toxicity was observed in rabbits in the recently completed, GLP, OECD 414 guideline study. In view of the absence of cardiac effects in rats and rabbits, the purpose of this review is to consider whether the cardiac effects reported in mice should be considered real effects and, if so, their potential for relevance to humans. This paper provides substantive new evidence with data from a third species and shows that an evaluation of the integrated scientific evidence indicates the reported developmental cardiac effects in mice, if not an artifact, are unlikely to be relevant to humans. As such the classification of MeCl for developmental toxicity was reconsidered.

Adjustment factors for toluene, styrene and methyl chloride by population modeling of toxicokinetic variability.

The availability of experimental data suitable as a basis to quantify human variability in response to chemical exposure has increased in recent years. It has enabled scientifically based, data driven adjustment factors (AF) to be deployed in the risk assessment process. As part of this development, we derive AF for human toxicokinetic variability (HK) for three lipophilic organic solvents; toluene, styrene and methyl chloride using physiologically based pharmacokinetic (PBPK) models in a population framework. The Monte Carlo simulations cover the influence of age and gender on toxicokinetic variability in the general population, as well as workplace ventilation rates and fluctuations in exposure level and workload in adult male and female workers. The derived AFHK are below 2.2 (95th percentile) for all subpopulations, exposure scenarios and chemicals, except for markers of acute effects in workers, where the factors are up to 5.0.

Physical and physicochemical factors effecting transport of chlorohydrocarbon gases from lung alveolar air to blood as measured by the causation of narcosis.

This systematic investigation examines gas transport in the lung for two sets of chlorohydrocarbons (CHCs): the chloromethanes (C1) and chloroethanes (C2). The C1 series includes chloromethane, methylene chloride, chloroform, and carbon tetrachloride, and the C2 series includes chloroethane, 1,2-dichloroethane, 1, 1, 2-trichloroethane, and 1, 1, 2, 2-tetrachloroethane. Most CHC gases cause narcosis. The comprehensive narcosis work of Lehmann and colleagues on CHCs was used as a basis for the narcosis endpoint in the present examination. The sites for narcosis are located in the brain (midline cortex and posterior parietal area), the spine, and at many peripheral nerve sites. Central nervous system (CNS) exposure executes a multisite, neural transmission set of inhibitions that promotes rapid loss of consciousness, sensory feeling, and current and stored memory while providing temporary amnesia. Absorption into the system requires dissolution into many lipid membranes and binding to lipoproteins. Lipophilicity is a CHC property shared with many anesthetics according to the Meyer-Overton Rule. Many structurally different lipid chemicals produce the narcosis response when the lipid concentration exceeds -67 mM. This suggests narcotic or anesthetic dissolution into CNS membranes until the lipid organization is disrupted or perturbed. This perturbation includes loading of Na(+)- and K(+)-channel transmembrane lipoprotein complexes and disrupting their respective channel functional organizations. The channel functions become attenuated or abrogated until the CHC exposure ceases and CHC loading reverses. This investigation demonstrates how the CHC physical and chemical properties influence the absorption of these CHCs via the lung and the alveolar system on route to the blood. Narcosis in test animals was used here as an objective biological endpoint to study the effects of the physical factors Bp, Vp, Kd (oil: gas) partition, Henry's constant (HK), and water solubility (S%) on gas transport. Narcosis is immediate after gas exposure and requires no chemical activation only absorption into the blood and circulation to CNS narcotic sites. The three physical factors Bp, K(d) (oil: air), and S% vary directly with unitary narcosis (UN) whereas Vp and HK vary inversely with UN in linear log-log relationships for the C2 series but not for the C1 series. Physicochemical properties of C1 series gases indicate why they depart from what is usually assumed to be an Ideal Gas. An essential discriminating process in the distal lung is the limiting alveolar film layer (AFL) and the membrane layer of the alveolar acini. The AFL step influences gas uptake by physically limiting the absorption process. Interaction with and dissolution into aqueous solvent of the AFL is required for transport and narcotic activity. Narcotics or anesthetics must engage the aqueous AFL with sufficient strength to allow transport and absorption for downstream CNS binding. CHCs that do not engage well with the AFL are not narcotic. Lipophilicity and amphipathicity are also essential solvency properties driving narcotics' transport through the alveolar layer, delivery to the blood fats and lipoproteins, and into critical CNS lipids, lipoproteins, and receptor sites that actuate narcosis. AFL disruption is thought to be strongly related to a number of serious pulmonary diseases such acute respiratory distress syndrome, infant respiratory distress syndrome, emphysema, chronic obstructive pulmonary disease, asthma, chronic bronchitis, pneumonia, pulmonary infections, and idiopathic pulmonary fibrosis. The physical factors (Bp, Vp, Kd [oil: gas] partition, Henry's constant, and water solubility [S%]) combine to affect a specific transport through the AFL if lung C > C(0) (threshold concentration for narcosis). The degree of blood CHC absorption depends on dose, lipophilicity, and lung residence time. AFL passage can be manipulated by physical factors of increased pressure (kPa) or increased gas exposure (moles). Molecular lipophilicity facilitates narcosis but lipophilicity alone does not explain narcosis. Vapor pressure is also required for narcosis. Narcotic activity apparently requires stereospecific processing in the AFL and/or down-stream inhibition at stereospecific lipoproteins at CNS inhibitory sites. It is proposed that CHCs likely cannot proceed through the AFL without perturbation or disruption of the integrity of the AFL at the alveoli. CHC physicochemical properties are not expected to allow their transport through the AFL as physiological CO(2) and O(2) naturally do in respiration. This work considers CHC inspiration and systemic absorption into the blood with special emphasis on the CHC potential perturbation effects on the lipid, protein liquid layer supra to the alveolar membrane (AFL). A heuristic gas transport model for the CHCs is presented as guidance for this examination. The gas transport model can be used to study absorption for other gas delivery endpoints of environmental concern such as carcinogens.

Genetic variation in metabolic genes, occupational solvent exposure, and risk of non-hodgkin lymphoma.

Using 1996-2000 data among Connecticut women, the authors evaluated whether genetic variation in 4 metabolic genes modifies organic solvent associations with non-Hodgkin lymphoma and 5 major histologic subtypes. P(interaction) values were determined from cross-product terms between dichotomous (ever/never) solvent variables and genotypes at examined loci in unconditional logistic regression models. The false discovery rate method was used to account for multiple comparisons. Overall associations between the chlorinated solvents dichloromethane (odds ratio (OR) = 1.69, 95% confidence interval (CI): 1.06, 2.69), carbon tetrachloride (OR = 2.33, 95% CI: 1.23, 4.40), and methyl chloride (OR = 1.44, 95% CI: 0.94, 2.20) and total non-Hodgkin lymphoma were increased among women TT for rs2070673 in the cytochrome P4502E1 gene, CYP2E1 (dichloromethane: OR = 4.42, 95% CI: 2.03, 9.62; P(interaction) < 0.01; carbon tetrachloride: OR = 5.08, 95% CI: 1.82, 14.15; P(interaction) = 0.04; and methyl chloride: OR = 2.37, 95% CI: 1.24, 4.51; P(interaction) = 0.03). In contrast, no effects of these solvents were observed among TA/AA women. Similar patterns were observed for diffuse large B-cell lymphoma and follicular lymphoma, as well as marginal zone lymphoma for dichloromethane. The weak, nonsignificant overall association between benzene and diffuse large B-cell lymphoma (OR = 1.29, 95% CI: 0.84, 1.98) was increased among women AA for rs2234922 in the microsomal epoxide hydrolase gene, EPHX1 (OR = 1.77, 95% CI: 1.06, 2.97; P(interaction) = 0.06). In contrast, no effect was observed among AG/GG women. Additional studies with larger sample size are needed to replicate these findings.

An improved system for exposure of cultured mammalian cells to gaseous compounds in the chromosomal aberration assay.

A gas exposure system using rotating vessels was improved for exposure of cultured mammalian cells to gaseous compounds in the chromosomal aberration assay. This system was composed of 12 square culture vessels, a device for preparation of air containing test gas, and positive and negative control gases at target concentrations and for supplying these gases to the culture vessels, and a roller apparatus in an incubator. Chinese hamster lung cells (CHL/IU) were grown on one side of the inner surface of the square culture vessel in the MEM medium. Immediately prior to exposure, the medium was changed to the modified MEM. Air in the culture vessel was replaced with air containing test gas, positive or negative control gas. Then, the culture vessels were rotated at 1.0 rpm. The monolayered culture cells were exposed to test gas during about 3/4 rotation at upper positions and alternatively immersed into the culture medium during about 1/4 rotation at lower positions. This system allowed the chromosomal aberration assay simultaneously at least at three different concentrations of a test gas together with positive and negative control gases with and without metabolic activations, and duplicate culture at each exposure concentration. Seven gaseous compounds, 1,3-butadiene, chlorodifluoromethane, ethyl chloride, methyl bromide, methyl chloride, propyne, and vinyl chloride, none of which has been tested to date, were tested on CHL/IU for the chromosomal aberration assay using this gas exposure system. All the compounds except chlorodifluoromethane showed positive responses of the structural chromosomal aberrations, whereas polyploidy was not induced by any of these gases. This improved gas exposure system proved to be useful for detecting chromosomal aberrations of gaseous compounds.

Glutathione transferase T1 phenotype affects the toxicokinetics of inhaled methyl chloride in human volunteers.

The aim of the present study was to investigate how the genetic polymorphism in glutathione transferase T1 (GSTT1) affects the metabolism and disposition of methyl chloride in humans in vivo. The 24 volunteers (13 males and 11 females) who participated in the study were recruited from a group of 208 individuals previously phenotyped for GSTT1 by measuring the glutathione transferase activity with methyl chloride in lysed erythrocytes ex vivo. Eight individuals with high (+/+), eight with medium (+/0) and eight with no (0/0) GSTT1 activity were exposed to methyl chloride gas (10 p.p.m.) in an exposure chamber for 2 h. Uptake and disposition was studied by measuring the concentration of methyl chloride in inhaled air, exhaled air and blood. A two-compartment model with two elimination pathways corresponding to exhalation and metabolism was fitted to experimental data. The average net respiratory uptake of methyl chloride was 243, 158, and 44 micromol in individuals with high, intermediate and no GSTT1 activity, respectively. Metabolic clearance was high (4.6 l/min) in the +/+ group, intermediate (2.4 l/min) in the +/0 group, and close to zero in 0/0 individuals, while the exhalation clearance was similar in the three groups. No exposure related increase in urinary S-methyl cysteine was detected. However, gender and the GSTTl phenotype seemed to affect the background levels. In conclusion, GSTT1 appears to be the sole determinant of methyl chloride metabolism in humans. Thus, individuals with nonfunctional GSTT1 entirely lack the capacity to metabolize methyl chloride.

Inhibitory action of chloramine on formate-metabolizing system. Studies suggested by an unusual case record.

We previously reported on a patient exposed simultaneously to methyl chloride and chloramine gas who developed metabolic acidosis and permanent blindness [M. Minami et al., Hum Exp Toxicol 11: 27-34, 1992]. The case report suggested the possibility of potentiation of methyl chloride toxicity by chloramine. The potentiating mechanism was investigated by exposing mice to methyl chloride followed by ammonia chloramine, and then the level of formate in urine samples was measured with an enzyme coupling method to detect disturbance of formate metabolism. Mice dosed with 0.05 mL 1.0 mM chloramine after methyl chloride exposure excreted a significantly larger amount of urinary formate than mice treated with only methyl chloride. There was no difference in urinary formate levels between mice treated with only 0.05 mL 1.0 mM chloramine and those given only the vehicle (0.1 M phosphate buffer pH 6.0) for chloramine. The underlying biochemical mechanism of deterioration of formate metabolism was found to be the inhibition of the enzyme, N10-formyl tetrahydrofolate (N10-f-THF) dehydrogenase by 0.56-3.35 microM chloramine in the in vitro experiment using the purified enzyme. Positive control mice, given orally 0.1 mL 10% methanol in 0.1 M phosphate buffer (pH 6.0) excreted the same amount of urinary formate as those receiving 0.05 mL 1.0 mM chloramine after methanol administration. This was ascribed to the inhibitory effect of chloramine on formaldehyde dehydrogenase and depletion of substrate for further metabolism. The inhibition of the enzyme by chloramine (2.7-100.8 microM) was confirmed by in vitro experiments, using the purified enzyme, formaldehyde dehydrogenase.

Biochemical effects of methyl chloride in relation to its tumorigenicity.

The biochemical effects of methyl chloride were investigated in tissues of F-344 rats and B6C3F1 mice (both sexes). Activities of GST were 2-3 times higher in livers of male B6C3F1 mice, compared with those of female mice, and with rats of both sexes. In kidneys GST activities of (male) mice were about 7 times lower than those found in livers. The activity of FDH was higher in livers of mice (both sexes) than in those of rats. No obvious sex difference was found in livers of rats and mice with respect to FDH. In kidneys, however, (minor) differences in FDH activities occurred between male and female B6C3F1 mice (4.7 vs. 3.1 nmol/min per mg). Sex differences of FDH activity in kidneys were not observed in F-344 rats. The microsomal transformation (by cytochrome P-450) of methyl chloride and S-methyl-L-cysteine to formaldehyde in tissues of B6C3F1 mice occurred preferentially in the liver. More formaldehyde was produced in liver microsomes of male, compared to those of female mice. Kidney microsomes metabolized methyl chloride to formaldehyde much less than liver microsomes. After a single exposure of mice of both sexes to 1000 ppm methyl chloride no elevation in formaldehyde concentrations was observed in livers and kidneys ex vivo. The determination of DNA lesions, using the alkaline elution technique, revealed no DNA-protein crosslinks in kidneys of male B6C3F1 mice after exposure to methyl chloride (1000 ppm, 6 h day-1, 4 days) and gave only minor evidence of single-strand breaks. Lipid peroxidation (production of TBA reactive material), induced by single exposure to methyl chloride (1000 ppm, 6 h), was very pronounced in livers of male and female mice. Smaller increases in peroxidation were observed in the kidneys of exposed mice. The theory that renal tumors observed in male mice after chronic exposure of the test animals to high (1000 ppm) concentrations of methyl chloride, are evoked by intermediates and in situ produced formaldehyde is proven unlikely by our results.

An ultrastructural study of lesions induced in the cerebellum of mice by inhalation exposure to methyl chloride.

Female C57BL/6 mice were exposed for 6 h/day, 5 day/wk for 2 weeks, to 1,500 ppm methyl chloride. Focal and diffuse malacia, involving the cerebellar inner granular layer was found while renal lesions were minimal or absent. The cerebellar lesions were most frequently found in the ventral paraflocculus, and less often in other regions of the cerebellum. The earliest ultrastructural changes were seen in the nuclei of scattered cerebellar granule cells, with progression from slight confluence of heterochromatin, to complete nuclear condensation or karyorrhexis. More severely affected areas exhibited severe watery swelling and disruption of granule cell perikarya with less severe changes in other cell types. Blood vessels appeared normal, even in areas of severe malacia. It was concluded that the lesions in the mouse cerebellum closely resemble methyl chloride induced brain lesions previously described in guinea pigs, and that these lesions are not secondary to the renal toxicity of methyl chloride.

Role of testicular versus epididymal toxicity in the induction of cytotoxic damage in Fischer-344 rat sperm by methyl chloride.

Exposure of male Fischer-344 (F-344) rats to methyl chloride (MeCl) results in testicular and epididymal toxicity and the induction of both pre- and postimplantation embryonic loss; the preimplantation loss is caused by cytotoxic damage to sperm that leads to failure of fertilization (Toxicol Appl Pharmacol 1986; 86:124-130). The present study examined whether the cytotoxicity of MeCl to sperm is due to the testicular or epididymal toxicity of MeCl. Groups of 18 males were exposed to 3000 ppm MeCl 6 h/day for 5 days, with and without concurrent treatment with the anti-inflammatory agent 3-amino-1-[m-(trifluoromethyl)phenyl]-2-pyrazoline (BW755C; 10 mg/kg, i.p. 1 h pre- and postexposure); BW755C was used to inhibit the epididymal toxicity of MeCl. Control groups were untreated or injected as described above with BW755C. Six males from each group were killed weekly for 3 weeks. Toxic effects of MeCl on the testis were demonstrated by decreased relative organ weight (week 3), testicular histopathology (weeks 1-3) and decreased daily sperm production (weeks 1-3); these effects were not prevented by BW755C. In both the MeCl and the MeCl + BW755C treatment groups, tubules devoid of sperm were observed in regions 4 and 5 of the epididymis at week 2, and in regions 6A and 6B at week 3. Sperm were present in the vas deferens of both groups at week 3 in decreased numbers and had decreased motility and more frequent morphologic abnormalities compared to untreated controls. In conjunction with known epididymal transit times for F-344 rat sperm, these data indicate that the induction of preimplantation loss by MeCl at weeks 2 and 3 postexposure is likely to result from cytotoxic effects on sperm located in the testes at the time of exposure.

Assessment of the genotoxic effects of methyl chloride in human lymphoblasts.

The activity of methyl chloride was measured in 4 genotoxicity assays. In an established human lymphoblast line, a 3-h treatment with 0-5% methyl chloride resulted in a dose-related increase in mutant fraction at the thymidine kinase locus and induction of sister-chromatid exchange. No increase in DNA damage, as measured by alkaline elution, was detected in the lymphoblasts at concentrations of methyl chloride shown to be mutagenic. Also, a concentration-related increase in 8-azaguanine-resistant fraction in Salmonella typhimurium was observed following a 3-h treatment with atmospheres containing 0-20% methyl chloride. Thus, methyl chloride is a weak, direct-acting mutagen for bacteria and human cells in culture.

Unscheduled DNA synthesis in rat tracheal epithelial cells, hepatocytes and spermatocytes following exposure to methyl chloride in vitro and in vivo.

Measurement of DNA repair as unscheduled DNA synthesis (UDS) in vitro following exposure in vivo in multiple tissues from the same treated animal can provide valuable information relating to the tissue- and organ-specificity of chemically induced DNA damage. UDS was evaluated in primary cultures of rat tracheal epithelial cells, hepatocytes and pachytene spermatocytes after exposure in vitro to methyl chloride (MeCl), and after isolation from the same treated animal following inhalation exposure in vivo. Concentrations of 1-10% MeCl in vitro induced UDS in hepatocytes and spermatocytes, but not in tracheal epithelial cells. Inhalation exposure to MeCl in vivo (3000-3500 ppm 6 h/day for 5 successive days) failed to induce DNA repair in any cell type. In vivo exposure to 15 000 ppm MeCl for 3 h also failed to induce UDS in tracheal epithelial cells and spermatocytes, but did cause a marginal increase in UDS in hepatocytes. Thus, MeCl appears to be a weak, direct-acting genotoxicant. While activity could be measured in hepatocytes and spermatocytes directly in vitro, only extremely high concentrations of MeCl elicited a response in the whole animal, and then only in hepatocytes.

Potential halogenated industrial carcinogenic and mutagenic chemicals. II. Halogenated saturated hydrocarbons.

The halogenated saturated hydrocarbons analogously to the previously considered halogenated unsaturated hydrocarbons (Part I) possess considerable utility in a broad spectrum of applications including; solvents, dry-cleaning fluids, refrigerants, fumigants, degreasing agents, propellants and intermediates in the production of other chemicals, textiles and plastics. Methyl chloride, methylene chloride, chloroform, carbon tetrachloride, methyl chloroform, 1,1,2-trichloroethane, hexachloroethane, ethyl chloride and fluorocarbons were reviewed principally in terms of their synthesis (or occurrence), areas of application, stability, distribution, reactivity, levels of exposure, populations at risk, carcinogenicity, mutagenicity and metabolism.

Inhalation carcinogenicity of alpha halo ethers. I. The acute inhalation toxicity of chloromethyl methyl ether and bis(chloromethyl)ether.

A range of acute studies were performed with chloromethyl methyl either (CMME) and bis(chloromethyl)ether (BCME), including 14-day LC50's following single seven-hour inhalation exposures. The LC50's for CMME were 55 ppm for rats and 65 ppm for hamsters. The LC50's for BCME were 7 ppm for both species. All animals showed characteristic changes of acute irritation of the respiratory tract manifested by congestion, edema, and hemorrhage. Severe shortening of life span was seen in 30-day exposures of rats to CMME and in all studies with BCME. Incidences of mucosal changes, including atypia, were generally increased in a dose-related manner in both species. The carcinogenicity of BCME in these range finding experiments was demonstrated by a skin cancer in a rat after three exposures and a nasal tumor in a hamster after one exposure to 1 ppm BCME.

Inhalation carcinogenicity of alpha halo ethers. II. Chronic inhalation studies with chloromethyl methyl ether.

Rats and hamsters were exposed to 1 ppm of chloromethyl methyl ether six hours per day, five days per week, throughout their lifetime. Mortality and weight gain of the exposed animals paralleled that of the control animals. Malignant tumors of the respiratory tract were found in two rats. These were a squamous cell carcinoma of the lung with blood vessel invasion and an esthesloneuroepithelioma originating in the olfactory epithelium and invading the forebrain. One hamster was found to have an adenocarcinoma of the lung and another, a squamous papilloma of the trachea. A single exposed rat had a pituitary tumor of primitive cell type that may well have been coincidental.

Inhalation carcinogenicity of alpha halo ethers. III. Lifetime and limited period inhalation studies with bis(chloromethyl)ether at 0.1 ppm.

Rats and hamsters were exposed to 0.1 ppm bis(chloromethyl)ether (BCME) six hours per day, five days per week throughout their lifetime. Additional groups of rats were given 10, 20, 40, 60, 80, and 100 exposures to 0.1 ppm BCME and then held until death. Forty cancers originating in the respiratory tract were found in the 200 rats involved in these studies. These included 14 cancers of the lung and 26 cancers of the nasal cavity. They occurred in dose-related fashion. A single undifferentiated carcinoma of the lung was seen in a hamster.