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EC number: 219-799-4
CAS number: 2536-05-2
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).
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