National Toxicology Program

Human Data:
No epidemiological studies or case reports investigating the association of exposure to methylamine and cancer risk in humans were identified in the available literature. However, the formation of the carcinogen N-nitrosodimethylamine (NDMA) when large amounts of nitrate were added to human gastric fluid which contained methylamine was reported by Zeisel et al. (1988). The ACGIH (1993) summarized available human study information as follows. Transient eye, nose, and throat irritation was produced by brief exposure at 20 to 100 ppm methylamine. No evidence of irritation was produced from exposure at less than 10 ppm. In an unpublished report, allergic or chemical bronchitis was reported in a worker exposed to methylamine at concentrations ranging from 2 to 60 ppm; and some irritation was noted at about 25 ppm. It is unclear from the report what the actual exposure concentrations were. No accounts of long-term effects, systemic reactions, and skin sensitization have been reported in the literature. Although there is limited human exposure data, it appears that there is evidence of irritation at 25 ppm and no or minimal irritation at 10 ppm.

Animal Data:

Acute

Methylamine has not been evaluated for skin absorption potential; however, it has been shown to be irritating to the skin of guinea pigs and the eyes of rabbits, and may be irritating to the gastrointestinal tract of guinea pigs following oral administration (Goffman & McGuire, 1980; ACGIH, 1993).

Reported acute toxicity values are presented in Table 7..

Recent studies on the comparative pulmonary toxicity of methyl isocyanate (MIC) and its hydrolytic derivatives in Wistar rats found that single exposure by both the inhalation (19mmol/l of methylamine vapors for 30 minutes) and subcutaneous (sc) routes (5.75 mmol/kg) caused interstitial pneumonitis at the acute (24 hours), subacute (4 weeks) and chronic (10 weeks) phases progressing to fibrosis, suggesting involvement in the subsequent inflammatory response and contribution to the long-term pulmonary damage of MIC (Jeevaratnam & Sriramachari, 1994; Sriramachari & Jeevaratnam, 1994).

Subchronic

Groups of 10 male rats were exposed by nose-only inhalation (6 hours/day, 5 days/week) for 2 weeks to 75, 250, or 750 ppm of methylamine (99.9% pure). Rats were sacrificed immediately following exposure or following a 14-day recovery period. Exposure to 75 ppm produced mild irritation to the nasal turbinate. Exposure to 250 ppm produced mild, irreversible focal erosion and/or ulceration of the respiratory mucosa of the nasal turbinates. Exposure to 750 ppm produced toxic effects including mortality, severe body weight loss, clinical pathologic changes suggestive of liver damage, nasal degenerative changes, and hematopoietic changes; not all effects were reversible during the recovery period (Kinney et al., 1990).

Chronic/Carcinogenicity

No 2-year carcinogenicity studies of methylamine in animals were identified in the available literature. However, the in vivo conversion of amines to nitrosamines has been reported in the literature. Nitrosamines are known animal carcinogens, and there is evidence to suggest that nitrosamines have carcinogenic potential for humans. The extent of this conversion and relevance to human cancer have not yet been determined (ACGIH, 1993).

Several chemical reaction studies and animal studies have reported that methylamine reacts to form carcinogens and precursor chemicals, including NDMA, N-nitrosomethyl-methoxymethylamine (NMMA), methylurea, and azoxymethane. These studies are summarized below.

• Obiedzinski and coworkers (1980) found that methylamine can react with acidic nitrite under a variety of conditions to form N-nitrosodialkylamines. They determined that NDMA was formed when methylamine was reacted with acidic nitrite. At pH 5, there was a 3-fold increase in the yield of NDMA when the reaction was carried out in the presence of formaldehyde and a 4-fold increase in the presence of thiocyanate. At pH 2, the yield of NDMA in the uncatalyzed reaction was about half that at pH 5, and there was no catalysis by the thiocyanate ion but the yield of NDMA was increased more than 4-fold by formaldehyde. NMMA, a moderately potent lung carcinogen in Sprague-Dawley rats, was also formed in the presence of formaldehyde. They noted that the probable reaction pathways for these transformations involve intermediates identical to those postulated to occur during the metabolic activation of dialkylnitrosamines to carcinogens. They suggested that if nitrosation of a primary amine were to occur at or near a site of biological action, subsequent interactions with biological macromolecules could be indistinguishable from those of the metabolites of the corresponding N-nitrosodialkylamine. They also noted that, although this condition of proximity would not often be met, it is conceivable that chronically high levels of nitrite and amine in, for example, the stomach, might contribute to the metaplasia that precedes tumors in some populations at high risk of gastric cancer. The authors further suggested that, if human exposure to N-nitroso compounds is partly due to in vivo nitrosation of amines, then it may be important to consider the contribution of primary amines when assessing the significance of these reactions.
  

• Lin & Chang (1983b) reported that reaction of nitrite in acidic medium with aqueous extracts of squid, which contains high levels of methylamine and dimethylamine, yielded appreciable amounts of NDMA. They commented that endogenous production of N-nitroso compounds by dietary amines and nitrite in the gastrointestinal tract is a likely factor in the etiology of stomach cancer and other gastrointestinal tumors.
  

• Kodama & Saito (1980) reported that methylurea, a precursor of the carcinogen methylnitrosourea, was formed by incubating methylamine and carbamyl phosphate in neutral buffer. They noted that the presence of methylamine and carbamyl phosphate in preserved, fermented foods provided a suitable condition for the formation of methylurea.
  

• Fiala (1980) reported that simple oxidation of methylamine in aqueous solution or in methanol leads to the formation of significant amounts of azoxymethane, a strong carcinogen in rodents. However, the reaction conditions (0^oC with perbenzoic acid or a monopersulfate) did not appear relevant to the usual physiological conditions.
  

Short-Term Tests: Methylamine was not mutagenic with or without metabolic activation (S9) in the Salmonella preincubation assay when tested at doses up to 10,000mg/plate in strains TA98, TA100, TA1535, and TA1537 (Mortelmans et al., 1986); when tested in the dose range 0.037­29.44 mg/plate (corrected dose for 0.08-64 mg/plate of methylamine hydrochloride) in strains TA98, TA100, and TA104; or when tested in strains TA97a or TA102 (Meshram et al., 1992).

Methylamine (dose range 0.25, 0.5, 1.0 M) in combination with nitrite (0.25 or 0.5 M) was mutagenic in Escherichia coli Sd-4 (Hussain & Ehrenberg, 1974). Methylamine and 2­aminoethanol also enhanced the mutagenic effect of ethyl nitrite in E. coli. The authors noted that the synergistic action of primary amines could be interpreted as a mutagenic action of monoalkylnitrosamines which are rapidly converted to the corresponding highly reactive diazonium ions (Ehrenberg et al., 1980).

Methylamine induced mutagenic responses at the tk locus in the mouse lymphoma cell forward mutation assay in the absence of S9 at concentrations in the range of 200-300 nl/ml (3-4 mM) and became lethal at approximately 400 nl/ml (5mM) (Caspary & Myhr, 1986; Shelby et al., 1987). A rat inhalation dominant lethal test found that methylamine was mutagenic at 10 µg/m³ (NLM, 1995).

Huber & Lutz (1984a,b) showed in vitro and in vivo methylation of DNA, indicative of DNA damage, from the reaction of methylamine and nitrite. Increased amounts of 7-methylguanine were detected when DNA from calf thymus was incubated with 1.2 mM methylamine (as the hydrochloride) and up to 66.0 mM sodium nitrite (78.8, 78.8, 50.4 mM) and in the stomach and small intestine of male Sprague-Dawley rats gavaged with methylamine (30µmol/kg bw as the hydrochloride) and sodium nitrite (700µmol/kg bw). Methylation of DNA in vivo was at least 330 times lower than after an in vitro incubation of DNA with the reactants.

Tsimis & Yarosh (1990) demonstrated the induction of the adaptive response to DNA alkylation, in which DNA repair genes are coordinately induced to express enzymes which reduce the toxic and mutagenic effects of DNA damage, in E. coli MV1601 cells treated with methylamine and nitrite. The adaptive response was induced in proportion to the concentration of methylamine up to a peak at 40 mM. No induction was observed either with nitrite and no methylamine, or with methylamine and no nitrite. Inhibition of bacterial nitrosation provided additional evidence that the induction of the adaptive response was due to nitrosation of methylamine. The authors suggested that the adaptive response evolved as a defense against environmental mutagens produced by bacteria themselves.

Metabolism: Zeisel and coworkers (1983) reported that humans and rats excrete methylamine in their urine after eating choline or lecithin, compounds found in many common foodstuffs. They found that almost 1 mmol per day of methylamines was excreted in the urine of humans who consumed a normal diet, almost 2 mmol/day after consumption of 27 mmol of choline chloride, and 0.8 mmol/day after ingestion of lecithin. Rats excreted 0.015 to 0.018 mmol/day of methylamine after consuming a choline-free diet. Excretion of methylamine was similar after administration of 2 mmol/kg b.w. of choline chloride or lecithin. They noted that these methylamines could be substrates for the formation of carcinogenic nitrosamines.

In a later study, Zeisel and coworkers (1988) showed that biological fluids from fasting humans and experimental animals contained methylamine. In humans that had fasted overnight, the concentration of methylamine in gastric fluid (3.7 nmol/ml) was similar to that in saliva (5.0 nmol/ml) and blood (3.8 nmol/ml), but was lower than that in urine (156.4 nmol/ml). The concentration of methylamine in the gastric fluid of dogs (11.8 nmol/ml), ferrets (17.4 nmol/ml), or rats (23.1 nmol/ml) was considerably higher, possibly reflecting differences in metabolism or in the amine content of the diet. When large amounts of nitrite were added to the human gastric fluid, NDMA was formed.

The toxicokinetics of methylamine has been studied in the rat. Streeter and coworkers (1990) observed biphasic first-order elimination following a single intravenous (iv) bolus dose of 18.9 µmol/kg [¹⁴C]- methylamine with a terminal half-life of 19.1 minutes. The apparent steady state volume of distribution, systemic blood clearance, and renal blood clearance were 1.21 liter/kg, 53.4 ml/min/kg, and 5.72 ml/min/kg, respectively. The amount of unchanged methylamine excreted in the urine within 24 hours was 10%. Urinary excretion of total radioactivity was 12.3%, in good agreement with a value of 12% reported for intraperitoneal (ip) doses of 7.5 µmol/kg in the rat by Krishna & Casida (1966). In another study in rats, Schwartz (1966) found 24% of the unchanged compound in the urine following an ip dose of 400µmol/kg. Streeter et al. (1990) commented that this was probably the result of saturation of the metabolic capacity of the animal leading to more methylamine being eliminated unchanged in the urine. Streeter and coworkers (1990) also administered a single intragastric (ig) dose of 81.9 µmol/kg [¹⁴C]-methylamine to male rats and found that 69% of the dose reached the systemic circulation unchanged out of a total of 93% absorbed from the gut.

Reported metabolites of methylamine include monomethylurea (Dar & Bowman, 1985) and formaldehyde and formate as metabolic intermediates in the conversion of methylamine to carbon dioxide (Keefer et al., 1987). Carbon dioxide has been reported to account for approximately 50% of the elimination of methylamine in rats administered the compound ip (Krishna & Casida, 1966; Dar et al., 1985). Streeter and coworkers (1990) noted that a similar extensive conversion to this metabolite would be expected following an iv dose. Methylamine can be metabolized in the rat to dimethylamine to a small extent (Asatoor & Simenhoff, 1965).

Studies by Krishna & Casida (1966) and Schwartz (1966) determined that negligible amounts of unchanged methylamine were excreted in the expired air following ip doses of 7.5 or 400 µmol/kg to male rats.

Krishna & Casida (1966) did not observe accumulation of radioactivity in the fat of rats at 48 hours after dosing with [¹⁴C]-methylamine. Steeter and coworkers (1990) commented that high lipophilicity with resulting accumulation in the fat is not likely to occur since methylamine would be expected to be ionized at physiological pH values.

Semicarbazide-sensitive amine oxidase (SSAO) in homogenates of rat aorta, porcine aorta, human umbilical artery, and rat white and brown adipose tissue showed deaminating activity towards methylamine. Formaldehyde was the metabolic product of methylamine deamination by SSAO from rat and porcine aorta (Precious et al., 1988; Boor et al., 1992; Conforti et al., 1993). Measurement of urinary levels in rats, before and after treating them with drugs capable of inhibiting either SSAO or mitochondrial monoamine oxidase (MAO) activities, indicated that MAO is not involved in methylamine degradation. These results were consistent with the possibility that SSAO, or related enzymes, may be involved in endogenous methylamine turnover (Lyles & McDougall, 1989). Streeter and coworkers (1990) commented that if SSAO is able to metabolize methylamine in vivo in the rat, not all of the dose would reach the venous sampling site following iv administration, with a consequent overestimation of the apparent volume of distribution at steady state. They also noted that an overestimation of the apparent volume of distribution at steady state might occur if an uptake process were occurring in the liver or other organs of the rat because the concentration at the sampling site would not be representative of that within the tissues. Solheim & Seglen (1983) found that isolated hepatocytes can accumulate methylamine intracellularly to concentrations in excess of those in the extracellular medium.

Methylamine is also a substrate for the related soluble enzyme, human plasma oxidase (McEwen, 1965).

Methylamine is formed in rats from the metabolism of endogenous compounds such as epinephrine (Schayer et al., 1952), sarcosine, glycine, and creatine (Davis & deRopp, 1961). It is also a metabolite of a large number of xenobiotics such as nicotine (McKennis et al., 1962), carbaryl in rats (Krishna & Casida, 1966), N-methylformamide in rats and mice (Threadgill et al., 1987; Tulip & Timbrell, 1988), dazomet in rats and mice (Lam et al., 1993), metham in mice (Lam et al., 1993), methylhydrazine in rats (Schwartz, 1966), azoxymethane in rats (Fiala et al., 1978), and NDMA in rats (Heath & Dutton, 1958; Burak et al., 1991).

Other Biological Effects:

Reproductive Effects/Teratology

In reproduction studies in which 6 female Wistar rats were orally administered 5 mg/kg bw methylamine daily and mated to untreated males, Sarkar and Sastry (1990) found no effect on the estrous cycle, reproductive indices of fertility, gestation, live birth, lactation, the average weight of pups at birth, and weaning. However, the average litter size of the treated group decreased significantly (P < 0.05) from the control group. The investigators noted that this effect may be due to either resorption of the fetus or some other reason. In an earlier study in rats, Miller (1971) found that a single intracardial injection of methylamine hydrochloride (dose not stated) on day 13 of gestation did not result in any gross malformations.

The reproductive toxicity of methylamine has also been studied in mice. Methylamine did not exert any maternal or fetal toxicity when injected ip (3 mmol/kg) during midgestation (day 8) to pregnant Swiss mice or when injected ip at levels up to 5 mmol/kg (as the hydrochloride salt) from days 1 to 17 of gestation to groups of pregnant CD-1 mice (6-8 animals per group) (Varma et al., 1990; Guest & Varma, 1991). When cultured for 48 hours with 8-day-old mouse embryo cells, however, methylamine (0.75, 1.0, 2.0 mM) caused dose-dependent decreases in size, DNA, RNA, and protein content as well as embryo survival, suggesting teratogenic potential. The authors speculated that methylamine may act as an endogenous teratogen under certain conditions (Guest & Varma, 1991).

Structure/Activity Relationships: Four structurally related chemicals were selected for evaluation of relative biological effects. A summary of information found in the available literature is presented in Table 8 followed by a more detailed discussion. No information on carcinogenicity or mutagenicity for the structurally related compound n-propylamine [107-10-8] was found. Information on carcinogenicity was identified for only one of the compounds. Dimethylamine was nontumorigenic in rats by the oral route and negative results were also seen in rats and mice following inhalation assays. Mutagenicity data were available on three of the structurally related compounds. Test results were negative for two of these compounds, trimethylamine and ethylamine. Although most indicators of genotoxicity were negative for dimethylamine, three studies reported some activity. It was weakly mutagenic in S. typhimurium strain TA1530, positive in Saccharomyces cerevisiae strain D7, and marginally active in Chinese hamster ovary (CHO) cells for chromosomal aberrations (CA) and sister chromatid exchange (SCE). The formation of carcinogenic nitrosamines from the interaction of nitrite and dimethylamine or trimethylamine has been reported. In addition, pyrolysates of dimethylamine and trimethylamine have exhibited mutagenicity in S. typhimurium strains TA98 and TA100.

Carcinogenic Effects

Dimethylamine. In a 2-year inhalation study, groups of 95 male and female F344 rats and B6C3F1 mice were exposed to 0, 10, 50, or 175 ppm dimethylamine for 6 hours/day, 5 days/week. Histopathological examinations at 6, 12, 18, and 24 months found no evidence of a carcinogenic response. Concentration-dependent toxicity was characterized by decreased body weight (175 ppm only) and progressive inflammatory, degenerative, and hyperplastic lesions of the nasal passages (Buckley et al., 1985; CIIT, 1990). Dimethylamine was nontumorigenic when 27 noninbred rats (sex not stated) were fed 1.6 g/kg diet for 2.5 years (PHS-149, 1979-1980; ACGIH, 1993).

A secondary or tertiary amine may react with nitrite under acidic conditions to give the carcinogenic nitroso compound (Ohshima & Kawabata, 1978).

Mutagenic Effects

Dimethylamine. At concentrations of 0.05-0.5 M with metabolic activation, dimethylamine was weakly mutagenic in S. typhimurium strain TA1530. It was not mutagenic in TA1530 without activation or in TA1531, TA1532, or TA1964 with or without activation. At 800 mg/kg it was also negative in the host-mediated assay with strains TA1950, TA1951, TA1952, and TA1964 (Green & Savage, 1978). At up to 4500 µg/plate, dimethylamine was negative with and without activation in S. typhimurium TA100, TA1535, TA1537 and TA98 (Zeiger et al., 1987). Dimethylamine was negative in strain Sd-4-73 of   E. coli (dose not stated) (Szybalski, 1958). A dose dependent increase in convertants and revertants was observed in S. cerevisiae strain D7 when dimethylamine was tested at a maximal dose of 4 mM with S9 (Galli et al., 1993). In CHO cells, dimethylamine did not exhibit cytotoxic or mutagenic effects at up to 22 mM even with S9, and marginal effects on SCE and chromosome aberrations were seen in the presence of S9 (Hsie et al., 1987). Dimethylamine (dose not stated) was negative for chromosome aberrations in Chinese hamster lung fibroblasts with or without activation (Ishidate et al., 1981). It was also negative for UDS in rat hepatocytes at 3.3 mM (Martelli et al., 1983).

When dimethylamine hydrochloride was pyrolysed at 300° to 600°C for 3 minutes, the pyrolysates were mutagenic in S. typhimurium strains TA98 and TA100 with activation. The pyrolysates were also slightly mutagenic in TA100 without activation. At doses of 5-20 µmol, the mutagenic activity began to appear from the pyrolysates at 400°C and the pyrolysates at 600°C showed the highest mutagenic activity (Ohe, 1982).

Trimethylamine. No evidence for mutagenic activity of trimethylamine in S. typhimurium strains TA1535, TA1537, TA98, and TA100 was detected at doses up to 1,000 <µg/plate with or without activation (Mortelmans et al., 1986).

When trimethylamine hydrochloride was pyrolysed at 300° to 600°C for 3 minutes, the pyrolysates were mutagenic in S. typhimurium strains TA98 and TA100. The pyrolysates at 600°C showed the highest mutagenic activity (Ohe, 1982).

Ethylamine. Ethylamine was negative when tested for mutagenicity in S. typhimurium strains TA100, TA1535, TA1537, and TA98 at doses up to 10,000 µg/plate. The preincubation assay was performed both with and without activation (Mortelmans et al., 1986). Ethylamine was inactive when administered in a mouse in vivo system to assess testicular DNA synthesis inhibition following intraperitoneal doses of 5, 15, or 50 mg/kg (Seiler, 1981).