Nα-acetyl-DL-tryptophan

Endpoint summary

Description of key information

There are no experimental studies available in which the toxicokinetic behavior of N-acetyl-DL-tryptophan (CAS 87-32-1) has been assessed.

In accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017), assessment of the toxicokinetic behavior of the substance is conducted to the extent that can be derived from the relevant available information. This comprises a qualitative assessment of the available substance specific data on physico-chemical and toxicological properties according to the relevant Guidance (ECHA, 2017).

Oral absorption is likely for N-acetyl-DL-tryptophan (CAS 87-32-1), whereas dermal and inhalation absorption potential is assumed to be low. N-acetyl-DL-tryptophan (CAS 87-32-1) will be distributed in the body and excretion is expected via urine.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Basic toxicokinetics: N-acetyl-DL-tryptophan

There are no studies available in which the toxicokinetic behaviour of N-acetyl-DL-tryptophan (CAS 87-32-1) has been investigated.

Therefore, in accordance with Annex VIII, Column 1, Section 8.8.1, of Regulation (EC) No 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017), assessment of the toxicokinetic behaviour of N-acetyl-DL-tryptophan (CAS 87-32-1) is conducted to the extent that can be derived from the relevant available information. This comprises a qualitative assessment of the available substance specific data on physico-chemical and toxicological properties according to the Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017).

N-acetyl-DL-tryptophan (CAS 87-32-1) is a solid at 20 °C with a molecular weight of 246.27 g/mol and a water solubility of 0.88 g/L. The determined log Pow value is -1.7 (at pH 7 and 23 °C) and the vapour pressure is determined to be <4.6E-08 Pa at 20 °C. The substance is a racemic mixture of the stereoisomers N-acetyl-D-tryptophan and N-acetyl-L-tryptophan.

 

Absorption

Absorption is a function of the potential for a substance to diffuse across biological membranes. The most useful parameters providing information on this potential are the molecular weight, the octanol/water partition coefficient (log Pow) value and the water solubility. The log Pow value provides information on the relative solubility of the substance in water and lipids (ECHA, 2017).

 

Oral

In general, molecular weights below 500 and log Pow values between -1 and 4 are favourable for absorption via the gastrointestinal (GI) tract, provided that the substance is sufficiently water soluble (> 1 mg/L). Furthermore, moderate log P values (between -1 and 4) are favourable for absorption by passive diffusion (ECHA, 2017).

The moderate water solubility (0.88 g/L) and the low log Pow value of -1.7 of N-acetyl-DL-tryptophan (CAS 87-32-1) indicate that absorption is likely to occur by dissolving into the GI fluids and the subsequent absorption by passive diffusion. The molecular weight of 246.27 g/mol also favours the absorption of N-acetyl-DL-tryptophan (CAS 87-32-1) via the GI tract.

The available data on teratological studies following the oral administration with the source substance N-acetyl-L-tryptophan revealed no signs of systemic toxicity and no mortality, resulting in a LD50 value of 2500 mg/kg bw (Kadota et al., 1980). The lack of systemic toxicity of the structurally related analogue substance indicates that the target substance N-acetyl-DL-tryptophan (CAS 87-32-1) will have a low toxic potential. 

In conclusion, the oral absorption potential of the target substance is expected to be high and is expected to be the same for both the L-and the D-enantiomer.

 

Dermal

There are no data available on dermal absorption or on acute dermal toxicity of N-acetyl-DL-tryptophan (CAS 87-32-1). On the basis of the following considerations, the dermal absorption of the substance is considered to be low.

To partition from the stratum corneum into the epidermis, a substance must be sufficiently soluble in water. With a water solubility of 0.88 g/L, dermal uptake of the target substance is likely. In addition, the low molecular weight of 246.27 g/mol is favourable for dermal uptake. However, log Pow values <-1 suggest that a substance is not likely to be sufficiently lipophilic to cross the stratum corneum. In addition, as the test substance is a solid, reduced dermal absorption has to be considered as dry particulates first have to dissolve into the surface moisture of the skin before uptake via the skin is possible (ECHA, 2017).

The dermal permeability coefficient (Kp) can be calculated from log Pow and molecular weight (MW) applying the following equation described in US EPA (2014):

log(Kp) = -2.80 + 0.66 log Pow – 0.0056 MW

The Kp was calculated for N-acetyl-DL-tryptophan (CAS 87-32-1) (please refer to Table 1). A dermal flux rate of 0.00327 mg/cm2 per h was calculated indicating only very low dermal absorption potential for N-acetyl-DL-tryptophan (CAS 87-32-1) (please refer to Table 1, Dermwin v2.02, EpiSuite 4.1).

 

Table 1: Dermal absorption value for N-acetyl-DL-tryptophan (CAS 87-32-1) (calculated with Dermwin v 2.02, Epiweb 4.1)

Component	Structural formula	Flux (mg/cm2/h)
N-acetyl-DL-tryptophan	C13 H14 N2 O3	0.00327

 

Skin irritation effects of the substance may cause an increase in the uptake of the test substance through the damaged skin. No increased uptake of the test substance is expected, as the irritation studies with N-acetyl-DL-tryptophan (CAS 87-32-1) showed no irritating effects (references 7.3.1-1 and 7.3.1-2).

Overall, taking into account the physico-chemical properties of N-acetyl-DL-tryptophan (CAS 87-32-1) and the QSAR calculations, the dermal absorption potential of the substance is anticipated to be low and is expected to be the same for both the L-and the D-enantiomer.

 

Inhalation

In humans, particles with aerodynamic diameters below 100 μm have the potential to be inhaled. Particles with aerodynamic diameters below 50μm may reach the thoracic region and those below 15 μm the alveolar region of the respiratory tract. Moderate log P values (between -1 and 4) are favourable for absorption directly across the respiratory tract epithelium by passive diffusion (ECHA, 2017).

The test substance is a solid with a very low determined vapour pressure of <4.6E-08 Pa thus being of low volatility and inhalation as a vapour is limited (ECHA, 2017). Particles with aerodynamic diameters below 50 μm may reach the thoracic region and those below 15 μm the alveolar region of the respiratory tract. The granulometric data shows, D10: 14.1 µm, meaning that a very low fraction of the particles are likely to reach the alveoli of the lungs. The D50: 94.0 µm and the D90: 292.7 µm, indicating that a fraction of the particles may be inhaled and are likely to be deposited before reaching the alveolar region. Particles deposited in the nasopharyngeal/thoracic region will mainly be cleared from the airways by the mucocilliary mechanism and swallowed or coughed out. The low fraction of particles that reach the alveoli of the lung can easily be metabolized to the essential, endogenous amino acid tryptophan by esterases (Neuhäuser-Berthold et al., 1988), and is expected to be systemically available after resorption within the respiratory tract. Considering that the available experimental data do not indicate systemic toxicity at doses of 500, 600 and 900 mg/kg bw/day in pregnant rabbits, rats and mice after intraperitoneal injection, systemic available N-acetyl-L tryptophan is not considered to exhibit harmful properties following inhalation.

Overall, taking into account the physico-chemical properties of N-acetyl-DL-tryptophan (CAS 87-32-1) and available toxicological data on the source substance, the inhalation potential of N-acetyl-DL-tryptophan is considered to be low and is expected to be the same for both the L-and the D- enantiomer.

 

Distribution and accumulation

Distribution of a compound within the body depends on the rates of the absorption and the physico-chemical properties of the substance; especially the molecular weight, the lipophilic character and the water solubility. In general, the smaller the molecule, the wider is the distribution. Small water-soluble molecules and ions will diffuse through aqueous channels and pores. The rate at which very hydrophilic molecules diffuse across membranes could limit their distribution (ECHA, 2017).

N-acetyl-DL-tryptophan (CAS 87-32-1) has a low molecular weight and is of moderate water solubility. Based on the physico-chemical properties and the absorption potential, distribution within the body can be considered as very likely. After absorption, N-acetyl-DL-tryptophan (CAS 87-32-1) will enter the blood circulating system through which it will be distributed within the body. Substances with moderate water solubility and low log Pow like N-acetyl-DL-tryptophan (CAS 87-32-1) are unlikely to accumulate in adipose tissue.

 

Metabolism

In an in vivo study the use of N-acetyl-L-tryptophan as a source of L-tryptophan following intravenous administration was assessed in rats receiving total parenteral nutrition for twelve days (Neuhäuser-Berthold, 1988). The results of this study showed that the utilisation of N-acetyl-L-tryptophan and free L-tryptophan was comparable, as the analysis of the amino acid concentration in the plasma revealed no significant differences between the treatment group administered with L-tryptophan and the treatment group receiving N-acetyl-L-tryptophan. The acetylation had no influence on the metabolic availability, although N-acetyl-L-tryptophan has to be deacetylated as a first step. The target substance is a racemic mixture of D/L-isomers, therefore it is expected that the L-isomer of N-acetyl-tryptophan will have the same metabolic fate as that determined for the source substance. In general, D-isomers of amino acids are not physiologically active in humans and only the L-isomers of amino acids are proteinogenic (Stryer, 2015). The D-isomer of acetyl-tryptophan is expected to be metabolised to the D-isomer of tryptophan, which is further metabolised as a xenobiotic via phase II enzymes and excreted. The principal role of L-tryptophan in the human body is as a constituent of protein synthesis, the second most prevalent metabolic pathway of L-tryptophan is for the synthesis of kynurenine following for the synthesis of serotonin (Richard et al., 2009). For degradation of L-tryptophan several oxygenases are needed. Tryptophan-2,3-dioxygenase cleaves the pyrol ring and kynurenine-2-monooxygenase hydroxylates the remaining benzene ring. After release of alanine, 3-hydroxyanthranilat is cleaved by a further dioxygenase and subsequently degraded into acetyl CoA (Stryer, 2015).

The potential metabolites following enzymatic metabolism were predicted using the QSAR OECD toolbox (v4.0, OECD, 2017). This QSAR tool predicts which metabolites may result from enzymatic activity in the liver and in the skin, and by intestinal bacteria in the gastrointestinal tract. As the SMILES code used to perform the prediction does not distinguish between stereoisomers, the results do not distinguish between the D-isomer and the physiologically active L-isomer. The results will therefore not reflect this known difference between the D- and L-isomer, but gives an indication of which metabolites are most likely to occur. Up to 14 hepatic and 5 dermal metabolites were predicted for the test substance, respectively. Primarily, oxidation of the side chain and cleavage of the pyrol ring following an oxidation of the carbon and nitrogen of the benzene and pyrol ring may occur in the liver, respectively. In the skin cleavage and oxidation of the side chain are the main metabolic pathways. In general, the hydroxyl groups make the substances more water-soluble and susceptible to metabolism by phase II-enzymes. Up to 58 metabolites were predicted to result from all kinds of microbiological metabolism for the test substance. Most of these metabolites were found to be a consequence of the degradation of the molecule by microbial metabolism.

Available genotoxicity data do not show any genotoxic properties of the test substance. An Ames-test, an in vitro chromosomal aberration test and an in vitro mammalian gene mutation assay with N-acetyl-DL-tryptophan (CAS 87-32-1) were consistently negative and therefore no indication of that the test substance may cause genotoxic reactivity is indicated.

 

Excretion

The major routes of excretion for substances from the systemic circulation are the urine and/or the faeces (via bile and directly from the GI mucosa) (ECHA, 2017).

Based on the metabolism described above, N-acetyl-DL-tryptophan (CAS 87-32-1) is expected to be metabolised in the body to a high extent. While N-acetyl-L-tryptophan is likely to be metabolised extensively in the above-mentioned metabolic pathways, N-acetyl-D-tryptophan is predicted to be excreted rapidly. Due to the moderate water solubility and the low molecular weight, the estimated metabolites are probably not excreted to a significant degree via the exhaled air or faeces but excreted via urine.

 

References

Aungst B. and Shen D.D. (1986). Gastrointestinal absorption of toxic agents. In Rozman K.K. and Hanninen O. Gastrointestinal Toxicology. Elsevier, New York, US.

ECHA (2017). Guidance on information requirements and chemical safety assessment, Chapter R.7c: Endpoint specific guidance. Version 3.0, June, 2017.

US EPA. (2012) Estimation Programs Interface Suite™ for Microsoft® Windows, v 4.1.; Prediction of dermal flux rate: DERMWIN v.2.02 (September 2012); United States Environmental Protection Agency, Washington, DC, USA; Prediction performed on 05 May 2017.

OECD, 2017: OECD QSAR Toolbox v4.0, April 2017, Laboratory of Mathematical Chemistry Oasis.Downloaded from https://qsartoolbox.org/ Prediction performed on 31 July 2017.

Neuhäuser-Berthold M., Schneider R., Musil H.E. and Bässler K.H., 1988: Die Verwertung von N-Acetyl-L-Tryptophan bei langfristiger intravenöser Verabreichung in wachsenden Ratten.Infusionstherapie 15:164-168.

Stryer (2015). Stryer Biochemie.Jeremy M. Berg,Lubert Stryer,John L. Tymoczko, Springer Verlag, ISBN: 978-3-8274-2988-9.

Richard D.M., Dawes M.A., Mathias C.W., Acheson A., Hill-Kapturczak N. and Dougherty D.M. (2009). L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications.Int J Tryptophan Res. 2:45-60.