Registration Dossier

Administrative data

Endpoint:
basic toxicokinetics
Type of information:
other: expert statement
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: No study covering all the relevant information is available, hence, an extensive assessment of the toxicokinetic behaviour of the substance was performed, taking into account the chemical structure, the available physico-chemical and toxicological data.

Data source

Reference
Reference Type:
other: Expert statement
Title:
Unnamed
Year:
2017
Report Date:
2017

Materials and methods

Objective of study:
absorption
distribution
excretion
metabolism
toxicokinetics
Test guideline
Qualifier:
no guideline required
Principles of method if other than guideline:
The toxicokinetic profile of the substance was predicted using the physical chemical properties of the substance, the data obtained from acute and repeated-dose toxicity studies, and information gained from genotoxicity assays. In addition, published information on the main hydrolysis products was also used in the assessment.
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent

Administration / exposure

Route of administration:
other: all relevant routes of administration are discussed in the expert statement

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:
The toxicokinetic profile of the substance was predicted using the physical chemical properties of the substance, the data obtained from acute and repeated-dose toxicity studies, and information gained from genotoxicity assays. In addition, published information on the main hydrolysis products was also used in the assessment.
Physico-chemical properties
The substance is a mono constituent substance with a molecular weight of 414.5. The substance is a dry powder and is poorly water soluble; the water solubility was determined to be 0.048 mg/L at 22°C. The octanol/water partition coefficient is 3.37 at 22 °C, although the solubility in fat is quite low at 4.49 x 10-3 g/100g fat at 37°C. The vapour pressure is extremely low at 1.23 x 10-6 Pa @ 25°C. The surface tension of the substance determined to be 72.4 mN/m at 24.5°C and the substance is considered to be not surface active. Particle size data acquired for the test material are as follows:
- Proportion of the test material having an inhalable particle size less than 100 µm: 13.2%
- Proportion of the test material having a thoracic particle size less than ~10 µm: 9.72%
- Proportion of the test material having a respirable particle size less than ~4 µm: 1.40%
Absorption – Oral Route
The physical chemical properties described above indicate that the substance has a molecular size that is within the range that may be expected to be absorbed within the mammalian gastrointestinal tract, should that material be ingested (<500). However, with a very low water solubility and low lipophilicity the substance may be expected not to easily cross gastrointestinal epithelial barriers. Being non-surface active it will not participate in micellar transport into the hepatic portal system.
An acute oral gavage toxicity study identified no evidence of systemic toxicity at 5000 mg/kg bw. A 90-day repeat dose dietary toxicology study using the oral route gave a NOAEL of 406.5 mg/kg bw/day. However, at the maximum dose level (1261.3 to 1479.2 mg/kg bw/day for males and females respectively) the only observed effect was a minor reduction in food consumption and bodyweight gain. The possibility that this was caused by an effect on the palatability of the diet cannot be excluded. The absence of adverse findings following oral dosing may be due to limited gastrointestinal absorption of the test material after dosing, and/or a very low index of inherent toxicity for this substance, and/or its hydrolysis products and metabolites. The absence of systemic effects was taken to indicate the likely absence of, or very low, potential for absorption of the substance or its metabolites.
As the substance may hydrolyse in acid conditions (the hydrolysis half-life is 1.78 hours at pH 4 and 50°C), some of the swallowed material, and any orally administered material, will be hydrolysed in the conditions prevailing in the stomach (pH2). In the repeat dose study, the amount of substance that was likely to have been hydrolysed is difficult to estimate because the transit time through the rat stomach is about 2 hours and the temperature is 37°C, which is lower than that used to determine the hydrolysis rate, however, perhaps 50% is a reasonable estimate. The hydrolysis products are sorbitol and 3,4-dimethylbenzaldehyde. Published data indicate that sorbitol is poorly absorbed from the gastro-intestinal tract following oral administration. In humans, labelled sorbitol is absorbed much more slowly than glucose. At least 75% of the dose is metabolized to carbon dioxide and less than 3% appears in the urine. In rats, intraperitoneal injection of labelled sorbitol resulted in 57.4% of the radioactivity being excreted as carbon dioxide, with some radioactive material appearing in liver glycogen and fatty acids. Sorbitol is oxidized to glucose, either directly or via fructose, and hence taken into intermediary metabolism. For the metabolism of 3,4-dimethylbenzaldehyde and 4-methylbenzaldehyde (p-tolualdehyde), it is known that benzaldehyde is oxidized to benzoic acid and that benzoic acid is excreted quantitatively and very rapidly in urine as the glycine conjugate (hippuric acid). The rate limiting step is probably the oxidation of the aldehyde to the acid. In humans, over 97% of the excreted radioactivity was recovered as 14C-hippuric acid in urine within 4 hours of oral administration of a 70 mg dose of 14C-benzoic acid, and over 99% of the administered radioactivity was recovered in urine within 24 hours. Tolualdehyde (methylbenzaldehyde) is metabolized similarly to benzaldehyde. The products are toluic acid and toluylglycine. In mechanistic terms the compounds are oxidized to the relevant aromatic acid and then conjugated, principally with the amino acid, glycine. This mechanism is likely also to apply to 3,4-dimethylbenzaldehyde.
Absorption – Dermal Route
Regarding the dermal absorption of the substance, its rate of uptake into the stratum corneum and its rate of transfer between the stratum corneum and the epidermis are likely to be very slow considering both the average MW and the log Pow. Moreover, it is assumed that the dermal uptake of the substance is also limited because of its low water and fat solubility. These assumptions were supported by the absence of observed systemic effects following dermal application of the substance in the acute dermal toxicity study at 2000 mg/kg bw. The substance is not classified as a skin irritant and in a skin irritation study only grade 1 erythema was observed in 2 animals at the 1 hour observation only, which suggests that dermal absorption is likely to be trivial.
Absorption – Inhalation Route
The proportion of substance having an inhalable particle size less than 100µm is 13.2%; the proportion having a thoracic particle size of less than 10.2µm is 9.72%; and the proportion having a respirable particle size of less than 5.4µm is just 1.40%. Consequently, whilst the potential for inhalation toxicity was not studied directly in a toxicology study, the physical nature of the substance (particle size distribution), and its’ very low vapour pressure, indicate a very low propensity to enter atmospheric air in a respirable form. Thus, respiratory absorption under normal use, and based on the life-cycle information of this substance, is expected to be inconsequential. Any inhaled material will behave as insoluble particulates; the vast majority of deposited material will be cleared from the lung through the mucociliary clearance system and be expectorated or swallowed.
Details on distribution in tissues:
Systemic distribution of substance can be predicted from the physical chemical properties of this substance. The high log Pow and poor water solubility suggests that this substance, upon systemic absorption, may be transported through the circulatory system in association with a carrier molecule such as a lipoprotein or another macromolecule. The slightly lipophilic character of the substance, with moderate Log Pow, low water solubility and MW of 414.5 suggests that a major proportion of the substance will not readily traverse cellular barriers or distribute into fatty tissues. There is no evidence of systemic toxicity and/or histopathological changes or increasing severity of clinical observations from repeated dose studies, nor of cumulative toxicity, as would be manifested by an accumulation of the substance or its’ metabolites in body tissues. As described above, hydrolysis of the substance may occur in the stomach but the hydrolysis products, if absorbed will be excreted rapidly.
Details on excretion:
After oral or inhalation exposure, the majority of the substance is expected to pass through the intestinal tract and be excreted unchanged in the faeces. However, a proportion may hydrolyse in the stomach and there may be some absorption of the hydrolysis products. The structural characteristics of the hydrolysis products indicate that they will undergo phase I and phase II metabolic transformation. The resulting metabolic by-products will undergo routine and rapid renal and/or biliary excretion.

Metabolite characterisation studies

Details on metabolites:
Like most xenobiotics, the substance may be expected to undergo phase I oxidation/reduction, esterase-catalyzed hydrolysis and subsequent Phase II conjugation. Acute and repeated-dose toxicity testing provided no evidence that the substance was metabolized into toxic metabolites. Data from bacterial mutagenicity, mammalian cell mutagenicity, and a mammalian cell chromosomal aberration, in which the substance was subjected to rat hepatic microsomal enzyme systems, did not show any evidence of genotoxic activity from the substance or its metabolites. Furthermore, the in vitro toxicity of the substance was unchanged by the presence of the microsomal enzyme system.

Applicant's summary and conclusion

Conclusions:
The substance has very low solubility in both water and in fat and consequently absorption via the oral, dermal or inhalation is likely to be very low. As the substance can hydrolyse in acid conditions, some of the swallowed material will be hydrolysed in the conditions prevailing in the stomach (pH 2). It is anticipated that the hydrolysis products will either be taken into intermediary metabolism or excreted very rapidly in urine. Furthermore, the hydrolysis products (sorbitol and 3,4-dimethylbenzaldehyde) are considered to have very low inherent toxicity and will be excreted rapidly.
Executive summary:

Millad 3988 has very low solubility in both water and in fat. It is unlikely that the substance itself is absorbed at greater than trivial amounts. As the substance may hydrolyse in acid conditions, some of the substance ingested via the oral route will be hydrolysed in the conditions prevailing in the stomach (pH 2). It is anticipated that the hydrolysis products, which have low inherent toxicity, will either be taken into intermediary metabolism or excreted very rapidly in the urine.