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Developmental toxicity / teratogenicity

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Administrative data

Endpoint:
developmental toxicity
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
weight of evidence
Justification for type of information:
Hypothesis: Fetal effects observed with 4,4’-MDI and pMDI are consistent and secondary to maternal toxicity as a consequence of respiratory irritation. Respiratory irritation, in turn, is consistent with the hypothesized MoA and direct electrophilic reactions of bioaccessible NCO groups.

Justification: All substances of the MDI category share this common hypothesized MoA and since 4,4’-MDI and pMDI have the highest bioaccessible NCO content, they are considered the worst-case substance and can be used as the source for read-across to all substances of the MDI category with high confidence.

Data source

Referenceopen allclose all

Reference Type:
publication
Title:
Prenatal toxicity if inhaled polymeric methylenediphenyl diisocyanate (MDI) aerosols in pregnant Wistar rats.
Author:
Gamer A, Hellwig J, Doe J and Tyl R
Year:
2000
Bibliographic source:
Toxicological Sciences, 54, 431-440.
Reference Type:
study report
Title:
Unnamed
Year:
1994
Report date:
1994

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

Test animals

Species:
rat

Results and discussion

Results: maternal animals

Effect levels (maternal animals)

Key result
Dose descriptor:
NOAEC
Effect level:
4 mg/m³ air (nominal)
Basis for effect level:
body weight and weight gain
clinical signs
mortality

Results (fetuses)

Effect levels (fetuses)

Key result
Dose descriptor:
NOAEC
Effect level:
4 mg/m³ air
Basis for effect level:
fetal/pup body weight changes
other: skeletal variations and retardations associated with reduced growth

Fetal abnormalities

Key result
Abnormalities:
no effects observed

Overall developmental toxicity

Key result
Developmental effects observed:
yes
Lowest effective dose / conc.:
12 mg/m³ air
Treatment related:
not specified
Relation to maternal toxicity:
developmental effects as a secondary non-specific consequence of maternal toxicity effects

Applicant's summary and conclusion

Conclusions:
No developmental toxicity test data is available for the target substance and therefore this endpoint is satisfied by read-across to reliable developmental toxicity studies on 4,4’-MDI and pMDI as described in the category justification document attached to Chapter 13. Briefly, the target substance is part of a category based on the hypothesized MoA that predicts local effects in the lungs and no significant systemic exposure to unreacted NCO since it reacts with biological nucleophiles before being absorbed as GHS or protein adducts. With no systemic exposure to unreacted isocyanate or toxic metabolite, no direct effects on developmental toxicity are predicted. This is consistent with the observed lack of systemic toxicity in combined chronic toxicity studies on 4,4’-MDI and pMDI as well as the proposed mechanism for MDI absorption toxicokinetics. Developmental effects noted in experimental testing are confined to fetal growth and secondary to maternal toxicity related to the respiratory irritation.
Studies from two reliable developmental toxicity studies are used as Weight of Evidence source studies to satisfy this endpoint via read-across. In both studies female rats were exposed by the inhalation route to aerosols of 4,4,’-MDI (Buschmann, et al., 1996) or polymeric MDI (Gamer et al., 1994). In the source study with pMDI, exposure at the highest dose (12 mg/m3) resulted in clear signs of maternal toxicity via mortality, damage to the respiratory tract, reduced body weight development and reduced mean gravid uterus weights. At this concentration clear signs of developmental (embryo-/foeto-) toxicity in the form of reduced placental and foetal body weights and an increased occurrence of foetal skeletal (and overall) variations and retardation were recorded; however, no substance-induced teratogenic effects were observed up to and including the highest concentration (12 mg/m³). The fetal effects observed are consistent and secondary to maternal toxicity as a consequence of respiratory irritation. Respiratory irritation, in turn, is consistent with the hypothesized MoA and direct electrophilic reactions of bioaccessible NCO groups. Systemic exposure and toxicity was not observed and therefore direct developmental toxicity is not relevant. This points to the conclusion that toxic effects are determined by respiratory irritation and inflammation.
In support of this conclusion, a study by Buschmann et al. (1996) performed according to OECD 414 using 4,4’-MDI at concentrations of 1, 3, and 9 mg/m3 was assessed.. The lung weights in the high-dose group were significantly increased compared to the sham-treated control animals. Treatment did not influence any other maternal and/or fetal parameters investigated (maternal weight gain, number of corpora lutea, implantation sites, pre- and post-implantation loss, fetal and placental weights, gross and visceral anomalies, degree of ossification), although a slight but significant increase in litters with fetuses displaying asymmetric sternebra(e) (within the limits of biological variability) was observed after treatment with the highest dose of 9 mg/m3. Conservatively, a no effect level of 3 mg/m3 for developmental and maternal toxicity was determined.
This study is considered appropriate as part of a read-across weight of evidence in that both the source and target substance (and MDI category as a whole) share similar chemical features namely that they a) 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 (e.g. point of contact irritation). 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). Accordingly, the category substances that are elicit the highest irritation potential (i.e. 4,4’-MDI and pMDI) would also be the substances most likely to result in secondary developmental effects.
With respect to potential toxic effects not linked to the hypothesized MoA, it is recognized that the oligomeric portion (e.g. non-monomeric) of the target substance constituents, does not contribute to the toxicity.
As described above in previous endpoints and in the Category Justification Document, the non-monomeric MDI constituents lowers the solubility and reduces its availability to react with biological molecules. In vitro reactivity experiment by Zhang et al. (2021) demonstrated that the mMDI constituents in modified MDI substances followed a similar pattern of glutathione adduct formation seen previously (Wisnewski et al., 2019a). However, when pMDI was incubated with GSH, adducts with the higher molecular weight constituents (>3 ring oligomer) were not detected in subsequent GC/MS. Additional solubility experiments by Zhang et al. (2021) confirmed that these constituents are not soluble and thus unable to react. In vivo inhalation experiments by Pauluhn (2002b) with pMDI demonstrates that inhalation exposure with pMDI (50 % of mMDI, 34 % of three-ring MDI, ratio about 3/2) in rats subsequent adducts detected (measured as two- or three-core MDA after hydrolysis) were present in a ratio of approximately 10/1. In other words, the presence of three-core adducts was seven times lower than expected from the composition of the test substance. Thus, even for the most soluble constituent next to mMDI of the MDI category substances, the 3-ring oligomer showed a significant reduction in bioaccessibility. Further increasing the molecular weight (either by condensation, or glycol adduct formation) even further reduces this potential bioaccessibility of the molecule. Taken together, the increasing octanol-water partition coefficients of the non-monomeric MDI constituents quickly attenuates the availability of these constituents to react with nucleophiles and elicit inflammation and irritation. Therefore, these constituents do not need to be considered for the prediction of systemic exposure and toxicity and predictions for all endpoints can be based on bioaccessible NCO as described in the hypothesis
An additional OECD 414 Developmental Toxicity study is proposed for the MDI category boundary substance that is considered the least bioaccessible (4,4,'-MDI/DPG/HMWP) to confirm this hypothesis and complete the weight of evidence. Supporting evidence will be provided by OECD 422 studies on 9 MDI category substances representing each subgroup and key structural/linkage type. These bridging studies will confirm the proposed MoA and that additional structural features to contribute additional toxic potential.
Executive summary:

The principle route of exposure for human concern is via inhalation.  This is supported by the toxicokinetic profile of exposure and indicates oral exposure cannot be extrapolated for the hazard or risk assessment of inhaled aerosols.  Accordingly, inhalation toxicity data on developmental toxicity is the most relevant as the source read-across for the MDI substance category evaluation.


The available reliable study data for the developmental toxicity endpoint are consistent with the hypothesized MoA and based on the high reactivity of the NCO group (Gamer et al., 1994 and Buschmann et al., 1996). The hypothesized MoA predicts local effects in the lungs and no significant systemic exposure to unreacted NCO since it reacts with biological nucleophiles before being absorbed as GHS/protein adducts. Minor fetal effects related to growth were considered secondary to the maternal irritation and toxicity.  No direct toxicity was noted in the developmental toxicity study which is consistent with the observed lack of systemic toxicity in combined chronic toxicity and carcinogenicity studies on 4,4’-MDI and pMDI as well as the proposed mechanism for MDI absorption toxicokinetics. Accordingly, the target substance like the rest of the MDI substance category is not classified as a developmental toxicant by EU GHS 1272/2008 CLP.