2-{3-[(4-amino-2-methylpyrimidin-5-yl)methyl]-4-methyl-1,3-thiazol-3-ium-5-yl}ethyl hydrogen phosphate

Endpoint summary

Description of key information

3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) is expected to be absorbed after oral administration, distributed to organs and tissues and extensively metabolized.

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Additional information

Basic toxicokinetics: 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium

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 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) 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 Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017).

There are no studies available in which the toxicokinetic behaviour of 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) has been investigated. There are many reviews and publications assessing the toxicokinetic behavior of thiamine (vitamin B1) and its derivatives and salts (like thiamine hydrochloride and thiamine mononitrate). A selection of these publications were used as supporting information to consider the absorption, distribution, metabolism and excretion of 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium.

Table 1: Chemical names and abbreviations of thiamine derivatives

CAS No./ EC No.	Chemical name	Abbreviation
10023-48-0/600-039-9	3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium	TMP
67-03-8/ 200-641-8	Thiamine hydrochloride	-
136-09-4/ 205-230-7	2-[3-[(4-amino-2-methylpyrimidin-5-yl)methyl]-4-methyl-1,3-thiazoniol-5-yl]ethyl dihydrogen diphosphate	TPP
532-43-4/ 208-537-4	Thiamine mononitrate	-

 

3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) is a solid at 20 °C with a molecular weight of 344.33 g/mol, a water solubility of 286 ± 36 g/L and a vapour pressure of 0.0011 Pa at 20 °C. The log Pow was calculated to be 0.39 by QSAR analysis (VEGA 1.2.3, MLogP v.1.0.0).

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, 2017b).

Oral

Generally the smaller the molecule the more easily it may be taken up. Molecular weights below 500 are favourable for absorption. In addition, water-soluble substances will readily dissolve into the gastrointestinal fluids. Absorption of very hydrophilic substances by passive diffusion may be limited by the rate at which the substance partitions out of the gastrointestinal fluid (ECHA, 2017b).

The high water solubility, the low octanol/water partition coefficient and the low molecular weights of the target and source substances 1, 2 and 3 indicate that absorption potential by the oral route is very likely. For source substance 4 the oral absorption potential can be assumed, since cleavage of the parent substance is predicted after oral absorption and the resulting metabolites might readily be absorbed.

There is acute oral toxicity data available with the source substance thiamine hydrochloride (CAS no. 67-03-8), showing a LD50 value of 3710 mg/kg bw in rats (Sprince et al., 1974). The clinical signs included marked tremors followed by a characteristic jumping behaviour. Death occurred within 15 – 45 min after intubation from respiratory failure, indicating that absorption has occurred.

Gregory (1997) reported that the thiamine in most foods tested is highly available for absorption and utilization in humans and that the chemical form of thiamine, such as the hydrochloride or the mononitrate salt, has little effect on its bioavailability. Thiamine hydrochloride (source 3) and thiamine mononitrate (source 4) are nutritionally equivalent to thiamine for use in supplements and food fortification and therefore considered to be absorbed from the gastrointestinal tract at a similar rate. However, the non-phosphorylated form (thiamine) is the one that is absorbed from the gastrointestinal tract. Rindi (1996) reported that thiamine is found in the intestinal lumen in the free form as alcohol, as its phosphoesters (including the target substance) are completely hydrolyzed by different intestinal phosphatases, and described that single oral doses of thiamine above 2.5 – 5 mg were largely unabsorbed and that intestinal uptake in vivo followed saturation kinetics. Marcus and Coulston (1996) reported that absorption of the usual dietary amounts of thiamine from the gastrointestinal tract occurs by Na+-dependent active transport and is limited to a maximal daily amount of 8 – 15 mg. In vitro investigations in human tissues showed that thiamine is absorbed mainly by an active, carrier-mediated system at concentrations < 1 µmol/L and that passive diffusion is the main absorption mechanism at higher concentrations. In general, the absorption takes place primarily in the jejunum (Rindi, 1996). Gregory (1997) reported that the intestinal absorption of thiamine by a saturable transport mechanism at low (micromolar) concentrations appears to be facilitated by metabolic trapping through intracellular phosphorylation.

The reviews/opinions prepared by the SCF (2001) and FDA (1978), and the references therein, confirm that ingested thiamine is well absorbed and, that this involves two mechanisms: the first is an active rate-limited jejunal uptake mechanism. When the active transport is saturated, at an intestinal concentration greater than 3 μmol/L, there is passive uptake. Above an oral intake of 5 mg thiamine absorption rapidly declines. Absorption occurred rapid as shown by a rise in the thiamine level of blood within 6 min after oral administration of 15 mg to healthy human subjects (FDA review, 1978 and references therein).

Overall, taken into account the available data, for the target and the source substances 1, 2, 3 and 4 high oral absorption potential is assumed.

Dermal

To partition from the stratum corneum into the epidermis, a substance must be sufficiently soluble in water. Due to the high water solubility and low molecular weight, dermal uptake potential is favourable. However, for substances with log Pow values <0, poor lipophilicity will limit penetration into the stratum corneum and thereby dermal absorption. Log Pow values between 1 and 4 favour dermal absorption (values between 2 and 3 are optimal) particularly if water solubility is high. 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, 2017b). The physico-chemical properties of the target and the source substances 2 and 4 indicate low dermal absorption potential, whereas for source substance 1 and 3 a moderate dermal absorption potential can be assumed.

Skin irritation may lead to an increased potential for dermal absorption through the damaged skin. If the substance has been identified as a skin sensitizer then, provided the challenge application was to intact skin, some uptake must have occurred although it may only have been a small fraction of the applied dose (ECHA, 2017b). The available data with the target substance did not show a potential for skin irritating properties in vitro, but revealed a skin sensitisation potential, indicating that some absorption might be possible.

The dermal permeability coefficient (Kp) can be calculated from log Pow and molecular weight (MW) applying the following equation described in US EPA (2012):

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

The Kp was calculated for 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CASno.10023-48-0) (please refer to Table 2). A dermal flux rate of 0.00168 mg/cm2 per h was calculated indicating only very low absorption potential for 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CASno.10023-48-0) (please refer Table 2, Dermwin v2.02, Epiweb 4.1).

Table 2: Dermal absorption values for the components of 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) (calculated with Dermwin v 2.02, Epiweb 4.1)

Component	Structural formula	Flux (mg/cm^2/h)
3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium	C12 H19 N4 O4 P1 S1	0.00168

 

Skin irritation may lead to an increased potential for dermal absorption through the damaged skin. The test substance did not show a potential for skin irritating properties in vitro. Therefore, the available data are not indicative for local effects after dermal exposure.

Overall, taking into account the physico-chemical properties and available toxicological data of 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0), the dermal absorption potential of the substance is predicted to be low.

Inhalation

In humans, particles with aerodynamic diameters below 100μm have the potential to be inhaled. Particles with aerodynamic diameters below 50μm may reach the thoracic region and those below 15μm the alveolar region of the respiratory tract (ECHA, 2017).

Inhalation of 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) is considered as negligible as the inhalation of the test substance is unlikely with regard to the particle size (median particle size L50: 1000 µm). Thus, the contained particles are far above the inhalable size. Moreover, the test substance has a low determined vapour pressure of 0.0011 Pa thus being of low volatility (ECHA, 2017). Therefore, under normal use and handling conditions, inhalation exposure and thus availability for respiratory absorption of the substance in the form of vapours, gases, or mists is not significant.

Based on the physical state and the physico-chemical properties of 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0), absorption via the lung is considered to be negligible.

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).

3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) has a low molecular weight and high water solubility. Based on the physico-chemical properties and the high absorption potential, distribution within the body can be considered as very likely. After oral absorption, 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) will enter the blood circulating system through which it will be distributed within the body.

In healthy volunteers thiamine plasma levels rose only marginally and it was actively excreted in the urine for up to six hours following an oral test dose of 10 mg. Thiamine is phosphorylated when it crosses the intestinal epithelium, but enters the blood principally as free thiamine and diffuses down a concentration gradient in the liver, heart, kidneys and brain. In the blood thiamine is distributed between the plasma (10%) and cells (90%) and is poorly stored (SCF opinion, 2001 and references therein). The times to reach steady state after administration of 500 mg thiamine i.m. once a day for 11 days and of 250 mg thiamine orally twice daily for 11 days, respectively, were similar and the mean elimination half-life was 1.8 days (Royer-Morrot, 1992). In adult humans the thiamine content is estimated to be approximately 30 mg and follows a rank order liver > hear > kidney > skeletal muscle > brain > small intestine. When thiamine intake exceeds the minimum requirement, which is approximately 1 mg per day, the tissue stores are first saturated before the excess appears quantitatively in the urine (Marcus and Coulston, 1996).

The systemic toxicity findings observed in the acute oral toxicity study with the source substance thiamine hydrochloride (CAS 67-03-8) and the available toxicokinetic data likewise indicate the target substance will be distributed extensively to organs and tissues and storage in the tissues cannot be excluded.

Metabolism

Phosphorylated forms of thiamine include thiamine monophosphate (TMP, CAS 10023-48-0), thiamine pyrophosphate (TPP, CAS 136-09-4) and thiamine triphosphate (TTP) (please refer to Table 1). In blood, thiamine is transported in erythrocytes which contain free thiamine and its phosphorylated forms, and in plasma, which contains only free thiamine and TMP (Rindi, 1996). Thiamine is phosphorylated when it crosses the intestinal epithelium, but enters the blood principally as free thiamine (SCF opinion, 2001, and references therein). In animal tissues the free thiamine and its phosphorylated forms are present in different amounts, TPP being the most abundant (about 80% of total thiamine). Five to ten percent of total thiamine is accounted for by TTP, the remainder is in the form of thiamine and TMP. Thiamine is found in the intestinal lumen in free form, its phosphoesters being completely hydrolyzed by different intestinal phosphatases. In the animal body, the four forms of thiamine are interconvertible by intervention of various enzyme systems (Rindi, 1996). In detail, thiamine pyrophosphokinase phosphorylates thiamine into thiamine pyrophosphate which can be phosphorylated into thiamine triphosphate by thiamine pyrophosphate kinase. However, thiamine pyrophosphate can also be dephosphorylated into thiamine monophosphate (Rindi, 1996). Thiamine pyrophosphate is the physiologically active form of thiamine, and functions in carbohydrate metabolism as a coenzyme in the decarboxylation of α-keto acids such as pyruvate and α-ketoglutarate and in the utilization of pentose in the hexose monophosphate shunt (Marcus and Coulston, 1996).

Ariaey-Nejad and Pearson (1968) proposed a cleavage pathway for the thiamine catabolism in the rat whereby 4-methyl-5-(2-hydroxyethyl) thiazole can be considered as intermediate in the generation of 4-methyl thiazole-5-acetic acid from thiamine. Ariaey-Nejad et al. (1970) investigated the metabolism of 2-¹⁴C-thiazole-labeled thiamine and 2-¹⁴C-pyrimidine-labeled thiamine in four healthy adult men. The administration of pyrimidine-labeled thiamine revealed 10 metabolites of which 4 were major and the use of thiazole-labeled thiamine resulted in 18 metabolites of which 6 were major. One of the major urinary metabolites of thiazole-labeled thiamine was identified as 4-methyl-thiazole-5-acetic acid.

The findings on metabolism indicate, that the target substance 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) is equivalent to the substance required in the body, thiamine monophosphate, and is metabolised via established metabolic pathways.

Excretion

The orally administered 2-¹⁴C-thiazole-labeled thiamine was almost completely absorbed with no measurable amount of ¹⁴CO₂detected in the respiratory air. The half excretion time of the ingested labeled thiamine occurred within 9.5 – 18.5 days (Ariaey-Nejad et al., 1970).

After an oral dose of thiamine, peak excretion occurs in about 2 hours and is nearly complete after 4 hours. The times to reach steady state after administration of 500 mg thiamine i.m. once a day for 11 days and of 250 mg thiamine orally twice daily for 11 days, respectively, were similar and the mean elimination half-life was 1.8 days (Royer-Morrot, 1992). In adults, 1 mg of thiamine per day is completely degraded by the tissues, which is roughly the minimal daily requirement. Little or no thiamine is excreted in the urine, when intake is at a low level (approximately 1 mg thiamine). When intake exceeds the minimum requirement, tissue stores are first saturated. Thereafter, the excess appears quantitatively in the urine as intact thiamine or as pyrimidine, which arises from degradation of the thiamine molecule. As the intake of thiamine is increased further, more of the excess is excreted unchanged (Marcus and Coulston, 1996). Free thiamine, thiamine monophosphate and small amounts of thiamine pyrophosphate are excreted in urine. In addition, different thiamine catabolites, including pyrimidine and thiazole moieties and some 20 – 30 breakdown products, are also found in urine (Rindi, 1996).

Based on the available information, 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-4-methyl-5-[2-(phosphonooxy)ethyl]-thiazolium (CAS 10023-48-0) is shown to be excreted in the urine.

References

Ariaey-Nejad M.R. and Pearson W.N. (1968) 4-Methylthiazole-5-acetic acid--a urinary metabolite of thiamine. J Nutr. 4:445-449.

Ariaey-Nejad M.R.,Balaghi M., Baker E.M.,Sauberlich H.E. (1970) Thiamin metabolism in man.Am J Clin Nutr.6:764-778.

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

Expert Group on Vitamins and Minerals (2002).EVM/00/14.REVISEDAUG2002

FDA (1978). Evaluation of the health aspects of thiamin hydrochloride and thiamin mononitrate as food ingredients. Contract No. FDA 223-75-2004.

Gregory JF (1997). Bioavailability of thiamin. Eur. J. Clin. Nutr.51Suppl 1: S34-7.

Marcus R, Coulston AM (1996). Water-Soluble Vitamins. In: Goodman and Gilman's the pharmacological basis of therapeutics, 9th Edition. Goodman AG, Rall TW, Nies AS, Taylor P (eds) . New York.Pergamon Press.

Rindi G (1996). Thiamin. In “ Present Knowledge in Nutrition”, 7th Edition (Ziegler EE, Filer LJ Eds). ILSI Press, Washington DC, USA

Royer-Morrot MJ, Zhiri A, Paille F, Royer RJ (1992). Plasma thiamine concentrations after intramuscular and oral multiple dosage regimens in healthy men. Eur J Clin Pharmacol 42:219-222.

SCF (2001). Opinion of the Scientific Committee on Food on the Tolerable Upper Intake Level of Vitamin B1. 16 July 2001.