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Description of key information

N, N-dimethylbutylamine (DMBA) is a tertiary aliphatic amine that is readily absorbed via the oral and inhalation route. Dermal absorption is significantly lower. Local skin effects may occur even at low dose levels, and these appear to reduce absorption rates at higher dose levels (>200 mg/kg bw, > 50 mg/L). Absorbed material will be present in the protonated form at physiological pH values.

Metabolic pathways are expected to include (i) formation and urinary excretion of the tertiary amine N-oxide, (ii) dealkylation to dimethylamine and butanal (iii) butanal is further oxidised to butyric acid which is then utilised, (iv) rapid urinary excretion of dimethylamine, either without (95%) or with (5%) further oxidative metabolism to monomethylamine and CO2. Virtually no radioactivity (approx. 1% of administered dose) was retained in the carcass 3 days after dosing rats and mice with dimethylamine, i.e. bioaccumulation is unlikely to occur.

The overall toxicological profile of DMBA is governed by the corrosive properties. As to systemic toxicity, data on dimethylamine and butanal are suitable for filling data gaps by cross reading. Since butanol and butanal are both rapidly oxidised to butyric acid, data on butanol, butyl esters, and butyric acid may also be taken into consideration.

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential
Absorption rate - oral (%):
100
Absorption rate - dermal (%):
10
Absorption rate - inhalation (%):
100

Additional information

The toxicokinetic behaviour of primary, secondary and tertiary aliphatic amines is quite complex. It is therefore outlined below in general and also with reference to N, N dimethylbutylamine (DMBA) because this is believed to be essential for the understanding of the toxicity profile and the rationale underlying Read Across.

1.) Absorption:

Primary, secondary and tertiary amines are well absorbed from the gut and the respiratory tract. Significant dermal absorption is considered for the short chain primary and secondary amines (ligands up to C6), whereas dermal absorption is considered to be much lower for higher molecular weight amines, tertiary amines, and their protonated forms (OECD 2012; OECD 2011; Bingham 2000; Greim 1998; references in section 7.4.1). Free amines are strong bases that are corrosive to the skin what may also affect dermal absorption. In case of low exposures to dilute solutions, aerosols and vapours the base capacity might not be sufficient to overwhelm the skin’s natural acidic barrier, and only few molecules would remain as the uncharged free base that can cross the biological membranes. Once internalised, these molecules would also be protonated because the physiological pH of 7.0 – 7.5 is far below the pKa of any of the amines (N,N-dimethylbutylamine: pKa = 10.2). The charged molecule could then be distributed, metabolised, and could exert systemic effects.  

Exposure to large quantities would result in local corrosion and tissue damage, either superficial (skin, oesophagus, stomach, gut) or in deeper sections (cell walls, lymph and blood vessels, nerves, organ linings, connecting tissue etc.).

The observation that the acute dermal toxicity of N,N-dimethylbutylamine in rats is low (LD50>2000 mg/kg bw; Safepharm (Hewitt&Collier 1985; section 7.2.3) compared to the oral toxicity (LD50= 184 mg/kg bw; Hoechst (Markert&Weigand) 1985; section 7.2.1), and the lack of signs of systemic toxicity following the dermal exposure, suggests that the dermal absorption of N,N-dimethylbutylamine does occur but is of minor importance.  

Based on the above, the oral and inhalation absorption rates are estimated as follows. Absorption is expected to be less at higher exposure levels which are acutely toxic via the oral and inhalation route, but not on the dermal route. The estimates are made in accordance with the ECHA Guidance document R.7C, Figure R.7.12 -5, and refined by expert judgement on the basis of the available relevant data on N,N-dimethylbutylamine.  

Estimated absorption rates:  

oral: 100%; at doses up to 200 mg/kg bw (LD50, rat: 184 mg/kg bw)  

inhalation: 100%; at concentrations up to 50 mg/L (approx. 12 ppm) (50 mg/L = 0.25*LC100)  

dermal: 10%; at doses up to 2000 mg/kg bw (no mortality in-vivo at 2000 mg/kg bw)  

2.) Distribution:

N,N-dimethylbutylamine is a strong base (pKa 10.2) and, therefore, is expected to be present almost exclusively in the protonated form at physiological pH values as described above. Both the free amine and the protonated form are water soluble and, therefore, will distribute in the aqueous compartment of the body after absorption from the point of contact where the caustic properties will lead to local corrosion.

3.) Metabolism

Saturated linear and branched tertiary aliphatic amines may undergo a variety of metabolic reactions in the mammalian organism, catalysed by a variety of enzymes. The most important reaction schemes are outlined below.  

3.1) Oxygenases  

1.    Cytochrome P450 monoxygenases are located in the endoplasmatic reticulum of all cells, especially high contents are present in liver, kidneys and lungs. These are enzymes with a very broad substrate spectrum with a preference for lipophilic substances, including aliphatic amines. The affinity is in the order tertiary>secondary > primary amines, and long chain > short chain amines.    

Oxygenation may occur either at the carbon oft the ligand (C-oxygenation), leading to dimethyl amine and butanal in the case of N,N-dimethylbutylamine according to the general equation

 

R, R’, R’’-N + O2+ NADPH2    ----->        R, R’-N-H + R’’-CHO + H20 + NADP+  

N-oxygenation will result in the formation of stable N-oxide which may be excreted via urine:

R, R’, R’’-N   -------> R, R’, R’’-N+-O-  

In addition, cytochromes P450 may also catalyse chain oxidations, i.e. hydroxylation of the butyl side chain may also occur.  

2. Flavin-dependent Monoogygenase, FMO. The enzyme (several isoenzymes exist per species) is also present in the endoplasmatic reticulum of all cells with high concentrations in liver, kidneys and lungs, i.e. the tissue distribution is very similar to that of cytochrome P450. The affinity to aliphatic amines is in the order tertiary>secondary>primary amines.  

Reactions with tertiary amines give the N-oxide (R, R’, R’’-N  ----> R, R’, R’’-N+-O-),  

and secondary amines give hydroxylamines  (R, R’-N –H ---> R, R’- N-O-H)  

Primary amines are oxidised to the respective aldehyde (R-N –H --->  R-CHO)  

Thus, pathway will also primarily give the DMBA N-oxide.

3.2) Oxidases

Monoamine oxidases (MAO A and B) catalyse the deamination of primary, secondary, and tertiary amines (Beard and Noe 1981) according to the following equation, i.e. one of the aliphatic ligands is removed in the oxidative de-amination reaction, and one molecule of the respective aldehyde and hydrogen peroxide are formed.  

       2 RCH2NR’R’’ + O2+ 2 H2O  -----> 2 RCHO +  2 NHR’R’’ + H2O + H2O2

The wide-spread monoamine oxidases are mainly located in liver, kidney and the intestinal mucosa. In the case of tertiary amines like DMBA the reaction is mainly catalysed by MAO B.  

It should be noted that the longest chain ligand is most likely oxidised and removed from the tertiary amine, i.e. the most likely metabolites are dimethylamine and butanal in the case of DMBA, and the DMBA N-oxide.  

3.3) Further reactions  

The resulting aldehyde (butanal) is then further oxidised chiefly by NADH+-dependent aldehyde dehydrogenases to the respective fatty acids. Generally, these enter directly the ß-oxidation pathway to be degraded stepwise to C2 -bodies and finally to carbon dioxide via the citric-acid cycle. The intermediates may serve as precursors for fatty acid and cholesterol biosynthesis.  

Ammonia released by deamination may be used in the intermediary metabolism, but excess ammonia is excreted as urea into the urine.  

Hydrogen peroxide (H2O2) which also is formed as by-product in the MAO reaction is readily degraded by ubiquitous catalases (e.g. Beard and Noe 1981).  

4.) Excretion

The N-oxide will primarily be excreted via urine, either unchanged or bearing free or conjugated hydroxyl groups in the butyl moiety (Bingham et al. 2000). Dimethylamine is expected to be excreted mainal via urine, based on the observation that 95% dimethylamine was found in the 0-24 h urine in rats and mice whereas the demethylation product, monomethylamine, accounted for only 3-5% of the radioactivity in the 0-24 h urine in both species (Mitchell et al., 1994).  

Butanal, hydrogen peroxide and ammonia are expected to be metabolised in the intermediary metabolism. Excess ammonia is excreted as urea into the urine.    

5.) Justification of cross reading  

In the case of N,N-dimethylbutylamine, the following metabolites are expected:  

The N,N-dimethylbutylamine N-oxide, resulting from the N-oxygenation by cytochromes P450 and by FMO. The N-oxide will primarily be excreted via urine, either unchanged or bearing free or conjugated hydroxyl groups in the butyl moiety.  

Dimethylamine and butanal, resulting from reactions catalysed by cytochrome P450, and monoamine oxidases A and B. Butanal will be oxidised to butyric acid which is further metabolised, but to a certain extent it may also covalently bind to nucleophiles present in proteins and nucleic acids.

It is therefore believed that data on the systemic toxicity of dimethylamine and butanol are suitable to fill data gaps by read across. Butanol and butyl acetate are both suitable because they lack the irritating properties of butyric acid. The ester is rapidly cleaved and liberates butanol. Butanol is rapidly oxidised to butyric acid via butanal.  

6.) References  

Further to the references quoted above and cross-referenced to other sections of this dossier, basic information on the metabolism of aliphatic alcohols, aldehydes, and amines is contained in toxicological textbooks and reviews including those cited below.

Beard R R and Noe JT (1981) Aliphatic and alicyclic amines [in industrial hygiene and toxicology]. Patty's Ind. Hyg. Toxicol. (3rd Revis. Ed.) 2B: 3135-73.

Benedetti, M. S. (2001) Biotransformation of xenobiotics by amine oxidases. Fundamental & Clinical Pharmacology 15: 75-84.

Cavender, FL (2000) Aliphatic and Alicyclic Amines. In:Eula Bingham, Barbara Cohrssen, Charles H. Powell (ed.),Patty's Industrial Hygiene and Toxicology (2000) John Wiley & Sons, Inc.  

Eisenbrand G and Metzler M (2002) Toxikologie. 2nd edition, Wiley-VCH, Weinheim, Germany

Greim H and Demel E (editors; 1996) Toxikologie. VCH Weinheim, Germany

Hayes A W (2001) Principles and Methods of Toxicology. Philadelphia, Taylor & Francis.

Leung H-W and Paustenbach DJ (1990) Organic acids and bases: Review of toxicological studies. Am J Ind Med 18, 717-735

Marquardt H and Schäfer S (editors, 2004) Lehrbuch der Toxikologie. 2nd edition, Wissenschaftliche Verlagsgesellschaft Stuttgart, Germany