(R)-(-)-butane-1,3-diol

Genetic toxicity: in vitro

Administrative data

Endpoint:

in vitro cytogenicity / micronucleus study

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

an in vitro cytogenicity study in mammalian cells or in vitro micronucleus study does not need to be conducted because adequate data from an in vivo cytogenicity test are available

Cross-reference

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

in vivo mammalian somatic cell study: cytogenicity / bone marrow chromosome aberration

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
Data for butane-1,3-diol (CAS No. 107-88-0) was used to address the toxicological data requirements for (R)-(-)-butane-1,3-diol (CAS No. 6290-03-5) in an analogue read-across approach. The basis for this read-across approach is the extreme structural similarity of the source and target substances, in that the source substance is a racemic mixture of a pair of enantiomers, whereas the target substance is solely the R-enantiomer of that source pair. Two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds (ECHA, 2008). Passive absorption of a substance into a test species and distribution through its tissues are governed by the physical-chemical properties of the substance, particularly its molecular size, log P, and water solubility (ECHA, 2014), and are therefore expected to be exactly the same for both enantiomers. The R-enantiomer half of the source substance and all of the target substance have been shown to metabolise in a mammalian system to a physiological ketone body, whereas the S-enantiomer of that ketone body derived from the other half of the source substance has been shown to metabolise into a compound that is not naturally present, but which can still be utilized by a less direct pathway (Desrochers et al., 1992). On the premise that a less direct metabolic pathway must be more energy-expensive, and therefore may be more likely to perturb the system and potentially produce an adverse effect, toxicity data on the source substance will represent a very similar or slightly worse case than, and provide a sound basis for a slightly conservative assessment of, the toxicity of the target substance.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Target Chemical: (R)-(-)-butane-1,3-diol (228-532-0; 6290-03-5)
Source Chemical: butane-1,3-diol (203-529-7; 107-88-0)
For further details refer to attached Justification For Read-Across Of Toxicity Data

The target substance is known to be of high purity (≥99 % w/w), so the low levels of impurities it could contain are not expected to substantially affect its physical-chemical properties. The purities of the samples of source material that were tested are not specifically known, but it is assumed that they would not have been sufficiently impure as to substantially affect the study results. On this basis, the applicability of the data on the source substance to the target substance is not expected to be compromised by the presence of impurities in either substance.

3. ANALOGUE APPROACH JUSTIFICATION
The basis for this read-across approach is the extreme structural similarity of the source and target substances. Specifically, the source substance is a racemic mixture of a pair of enantiomers, whereas the target substance is solely the R-enantiomer of that source pair. The source substance is therefore nominally comprised 50% of the target substance itself (the R-enantiomer), and 50% of its mirror image (the S-enantiomer), which differs from the target substance only in the chirality of one carbon atom. The selection of this source substance is justified on the basis that there is no other source substance that could possess a greater degree of structural similarity to the target substance.

Enantiomers are two stereoisomers that are related to each other by a reflection: they are mirror images of each other. Every stereocentre in one has the opposite configuration in the other. Two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds (ECHA, 2008). Passive absorption of a substance into a test species and distribution through its tissues are governed by the physical-chemical properties of the substance, particularly its molecular size, log P, and water solubility (ECHA, 2014), and are therefore expected to be exactly the same for both enantiomers.

In a mammalian system, both enantiomers have been shown to be taken up by the liver and converted to their respective 3-hydroxybutyrate (beta-hydroxybutyrate; BHB) at identical rates. The target substance and one half of the source substance are converted into R-BHB, and the other half of the source substance is converted into S-BHB. R-BHB is a physiological ketone body, whereas S-BHB is not naturally present, but can still be utilized by a less direct pathway (Desrochers et al., 1992). On the premise that a less direct metabolic pathway is more energy-expensive, and may therefore be more likely to perturb the system and potentially produce an adverse effect, toxicity data on the source substance will represent a very similar or slightly worse case than, and provide a sound basis for a slightly conservative assessment of, the toxicity of the target substance.

4. CONCLUSION
Values generated on the source substance will represent a very similar or slightly worse case than the target substance

REFERENCES
Desrochers S, David F, Garneau M, Jetté M, Brunengraber H (1992). Metabolism of R- and S-1,3-butanediol in perfused livers from meal-fed and starved rats. Biochem J 285:647-653.

ECHA (2008). Guidance on information requirements and chemical safety assessment. Chapter R.6: QSARs and grouping of chemicals. May 2008. Available at: https://echa.europa.eu/documents/10162/13632/information_requirements_r6_en.pdf

ECHA (2014). Guidance on information requirements and chemical safety assessment. Chapter R.7c: Endpoint specific guidance. Volume 2.0, November 2014. Available at: https://echa.europa.eu/documents/10162/13632/information_requirements_r7c_en.pdf/e2e23a98-adb2-4573-b450-cc0dfa7988e5

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

read-across: supporting information

Principles of method if other than guideline:

genotoxicity test in vivo after subchronic oral exposure over 3 generations

Type of assay:

other: chromosome aberration assay

Specific details on test material used for the study:

(R)-(-)-Butane-1,3-diol value is read-across from supporting (R/S)-butane-1,3-diol (203-529-7; 107-88-0) data.

Key result

Sex:

male/female

Genotoxicity:

negative

Toxicity:

no effects

Remarks:

only slight depression of body weight gain

Vehicle controls validity:

not applicable

Negative controls validity:

valid

Positive controls validity:

not applicable

Remarks on result:

other:

Remarks:

Values generated on the source substance will represent a very similar or slightly worse case than the target substance

Additional information on results:

Dietary concentrations of 5, 10 and 24% (R/S)-1,3-butanediol correspond with body doses of 2000, 4000 and 9600 mg/kg bw for males and 2500, 5000 and 12000 mg/kg bw for females (based on a daily food consumption of 40 and 50 g/kg bw for males and females, respectively, according to the Guidance on Information Requirements R.8).

The number of abnormal cells was not increased with respect to the normal range of aberrant cells in untreated F1A, F2A and F3A animals. No specific abnormalities were observed in the treated animals and no dose-related effects were noted. Values generated on the source substance will represent a very similar or slightly worse case than the target substance.

Conclusions:

The test substance (R/S)-1,3-butanediol did not induce chromosomal aberrations after subchronic oral exposure of rats over 3 generations with dietary concentrations of up to 24%. Values generated on the source substance will represent a very similar or slightly worse case than the target substance.

Executive summary:

Rats were fed butane-1,3-diol in concentrations up to 24% of the diet and paired to produce F1A, F2A and F3A litters. Analysis of the femur bone marrow of at least two animals per sex and dose of these litters revealed no increase in chromosomal aberrations.

Values generated on the source substance will represent a vey similar or slightly worse case than the target substance. Therefore, it is predicted that consumption of the target substance (R)-1,3 -butanediol would not result in an increase in chromosomal aberrations.

This study was well performed with doses high enough to cause a reduced body weight gain. Despite some conceptional deficiencies (no positive controls, low numbers of cells per dose group examined) as well as incomplete data reporting (e.g. with respect to substance purity, time point of examination, statistical analysis of the results) this study is judged to be reliable and sensitive, due to the repeated application of high doses over long time periods and several generations.