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Toxicological information

Acute Toxicity: dermal

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Administrative data

Endpoint:
acute toxicity: dermal
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
weight of evidence
Justification for type of information:
Hypothesis: In the case of dermal exposure, before the reactive NCO groups present on MDI substances have opportunity to be absorbed to any significant extent through the stratum corneum they react with proteins and moisture at the skin surface leading to the formation of an insoluble polymerized mass thereby limiting dermal absorption and systemic availability. Modified MDI substances, having a higher molecular weight than mMDI isomers and due to their higher molecular volume, increased octanol-water partition coefficient and decreased water solubility will in any event not be able to penetrate the stratum corneum.

Justification: Considering that there is only data available for two substances of the MDI category this is insufficient to serve as the sole basis for concluding consistency of effects within the data matrix. However, since the formation of inert polyureas is dependent on the presence of reactive NCO groups and all substances acting via this common mechanism contain high levels of such groups, this justifies the claim of consistency within the matrix with high confidence.

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
1964

Materials and methods

Test material

Constituent 1
Reference substance name:
4,4'-Methylenediphenyl diisocyanate, oligomeric reaction products with butane-1,3-diol, 2,4'-diisocyanatodiphenylmethane, [(methylethylene)bis(oxy)]dipropanol and propane-1,2-diol
EC Number:
500-312-1
EC Name:
4,4'-Methylenediphenyl diisocyanate, oligomeric reaction products with butane-1,3-diol, 2,4'-diisocyanatodiphenylmethane, [(methylethylene)bis(oxy)]dipropanol and propane-1,2-diol
Cas Number:
123714-19-2
Test material form:
liquid: viscous
Details on test material:
Batch # 290832

Results and discussion

Effect levels
Key result
Sex:
not specified
Dose descriptor:
LD50
Effect level:
> 9 400 mg/kg bw
Remarks on result:
other: Weight of evidence, Based on the available study data all members of the MDI category are regarded as relatively non-toxic for the endpoint acute dermal toxicity and

Applicant's summary and conclusion

Interpretation of results:
GHS criteria not met
Conclusions:
No acute dermal data exist for the target substance 44MDI/1,3-BD/TPG/PG. Accordingly, this endpoint is satisfied by a weight of evidence and read-across from valid acute oral toxicity studies on category substances . The study used (Wazeter et al. 1964) ) for read-across did not find lethality up to the maximum dose tested, and the LD50 was greater than 9,400 mg/kg bw (Wazeter et al., 1964a). A lack of acute dermal toxicity is confirmed by the available acute dermal toxicity studies of the MDI category (see table attached background material). Based on the available study data all members of the MDI category are regarded as relatively non-toxic for the endpoint acute dermal toxicity and are not classified according EU GHS 1272/2008 CLP.
Executive summary:

The available acute dermal toxicity studies indicate that all substances of the MDI category have low acute dermal toxicity. The study describing the acute dermal toxicity of pMDI in rabbits did not find lethality up to the maximum dose tested, and the LD50 was greater than 9,400 mg/kg bw (Wazeter et al., 1964a). Other less reliable studies on pMDI or 4,4’-MDI are consistent with this.  Observed differences in LD50 values between pMDI and 4,4'-MDI/TPG are not considered to be significant or represent a trend since they are significantly higher than the limit for classification and are indicative of a lack of systemic exposure. The available data for the substances of the MDI category is consistent with the hypothesis that NCO groups present on MDI substances react with proteins and moisture at the skin surface leading to the formation of an insoluble polymerized mass resulting in limited dermal absorption and systemic availability. The available data and hypothesis is supported by the key dermal absorption study that shows that MDI substances have very low systemic bioavailability (<1 %) (Leibold et al., 1999). By comparison, modified MDI substances, with molecular weight greater than mMDI, will


demonstrate even further reduced dermal absorption based upon physico-chemical properties (i.e. increased octanol-water partition coefficient, decreased water solubility, and increased molecular weight), and this has been confirmed with GastroPlus™ modeling (Bartels, 2021). Although a reliable, acute dermal in vivo toxicity data is only available on two category substances, it is considered sufficient for assessment of this endpoint for the category.


All substances of the MDI category share similar chemical features namely that they a) all contain a significant amount of mMDI, and b) contain at least two NCO functional groups per molecule which is bound to an aromatic ring and this ring is connected to a second aromatic ring by a methylene group. It is the NCO value (driven by the bioaccessible groups on monomeric MDI and low molecular weight constituents (e.g. three-ring oligomer) which is responsible for chemical and physiological reactivity and subsequent toxicological profile. As reactive NCO groups are a common feature of all substances of the MDI category, it is predicted that these have a similar reactivity profile and a read across within the category is warranted (detailed information on the Mode of Action is available in Category Justification Document).