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Carcinogenicity

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Description of key information

The pulmonary effects described in rats after chronic inhalation exposure to either polymeric or monomeric MDI include interstitial fibrosis, hyperplasia and bronchiolo-alveolar adenomas, the latter occurring at low incidence in the highest exposure groups.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Endpoint conclusion
Endpoint conclusion:
no study available

Carcinogenicity: via inhalation route

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEC
1 mg/m³
Study duration:
chronic
Species:
rat
System:
respiratory system: lower respiratory tract
Organ:
lungs

Carcinogenicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

The test substance is covered by the category approach of methylenediphenyl diisocyanates (MDI). Hence, data of the category substances can be used to cover this endpoint. The read-across category justification document is attached in IUCLID section 13. It is important to note that the MDI category approach for read-across of environmental and human hazards between the MDI substances belonging to the MDI category is work in progress under REACH. Therefore the document should be considered a draft.

Two carcinogenicity studies are available, which showed a very low incidence of tumour formation. In both studies rats were exposed via inhalation. The summaries of these studies are presented below. The studies support the results observed from other studies that long-term local toxicity from MDI aerosols lead to hyperplasia and adenoma formation. The concentrations tested in these studies are only possible to form in the laboratory due to the physical-chemical properties of MDI. Therefore the exposure conditions are not representative for human exposure. This is supported by epidemiological studies that show no increase in cancer in MDI exposed worker population. This conclusion is supported by numerous organisations and regulatory groups including WHO, EU RAC, IARC, NTP, and US OSHA that do not classify MDI as a human carcinogen.

 

In a combined chronic toxicity and carcinogenicity study, rats were exposed for 6 hours/day, 5 days/week for 2 years to polymeric MDI aerosol concentrations of 0, 0.2, 1.0 or 6.0 mg/m3 (analytical conc.: 0, 0.19, 0.98, 6.03 mg/m3) (Reuzel et al.,1990; 1994). This GLP reliability 2 key study was conducted according to OECD Guideline 453 (Combined Chronic Toxicity / Carcinogenicity Studies). Histopathology of the organs/tissues investigated showed that exposure to 6.0 mg/m3 was related to the occurrence of pulmonary tumours in males (6 adenomas and 1 adenocarcinoma) and females (2 adenomas). Therefore, polymeric MDI was carcinogenic in rats after long-term inhalation to aerosol concentrations of 6.0 mg/m3. It was also concluded that exposure to polymeric MDI at concentrations not leading to recurrent lung tissue damage will not produce pulmonary tumours.

 

The chronic toxicity and the carcinogenicity of 4,4’-MDI were investigated by Hoymann et al. (1995) in a long-term inhalation study over a maximum of 24 months including satellite groups with 3, 12, and 20 months exposure. Female Wistar rats in groups of 80 animals, were exposed for 18 hours/day, 5 days/week to 0, 0.23, 0.70, and 2.05 mg/m3 MDI in aerosol form. A dose-dependent impairment of the lung function in the sense of an obstructive-restrictive malfunction with diffusion disorder, increased lung weights, an inflammatory reaction with increased appearance of lymphocytes (but not of granulocytes) in the lung in the high dose group as a sign of specific stimulation of the immune system by MDI, a moderately retarded lung clearance in the high dose group as well as dose-dependent interstitial and peribronchiolar fibrosis, alveolar bronchiolisations and a proliferation of the alveolar epithelium as well as a bronchiolo-alveolar adenoma (at 2.05 mg/m3) were reported. There was no MDI-related increase in the organ-specific tumour rate. The number of tumour-bearing rats was identical, and the total number of tumours did not significantly differ between the control and the high dose group.

It has to be noted that there was an untypically high mortality due to tumours of the pituitary gland, in all groups (including control), which limits the evaluation of this study. With this study it cannot be clarified whether prolonged exposure would have resulted in tumour formation. Another factor is the exposure duration of 17 hours/day. This long daily exposure is unusual and prevents normal homeostatic healing processes after contact with irritating substances. As the concentrations used are locally damaging (local toxicity was shown in all dose groups), this again limits the study for relevance to worker risk assessment.

 

The two carcinogenic studies (Reuzel et al. 1990 and Hoymann et al. 1995) have been compared by Feron et al. (2001), with the aim of providing a definitive overview of the chronic pulmonary toxicity/carcinogenicity of MDI. In this study, the test materials and study designs were compared, and an in-depth review of observed histopathological lesions was provided (with many lung slides re-examined). The extensive comparative analysis of both carcinogenicity studies showed that qualitatively, the results were comparable.   

 

The pathogenesis as proposed by Reuzel et al. (1990), for the polymeric MDI-associated lung effects including tumours is as follows: The initial event is cytotoxicity to cells, especially Type I pneumocytes, lining the centriacinar region of the lung following contact with polymeric MDI aerosol. Alveolar macrophages phagocytise the foreign material. However, focal denudation of the epithelium leaves basement membranes exposed and/or damages the interstitium. Repair of both interstitium (by fibroblasts) and the epithelium ensue. Phagocytosis of test material by activated macrophages may lead to elaboration of growth factors for both fibroblasts and epithelial cells, primarily Type II pneumocytes. Fibroblast and epithelial proliferation occur. Hyperplasia of Type II pneumocytes progresses, perhaps mediated by factors from alveolar macrophages. This process continues until a small number of lung adenomas occur in the areas of most marked Type II pneumocyte hyperplasia and macrophage accumulation. The progression to adenoma and/or adenocarcinoma may involve expression of an increased background mutation rate secondary to the prolonged Type II cell proliferation.

 

The EU Risk Assessment (EC 2005) noted that in the 2-year animal inhalation study by Reuzel et al., (1990, 1994) there was no adverse effect found on the distribution and incidence of tumours apart from tumours in the lungs. According to the authors, and compatible with the hypothesis of Pauluhn et al. (1999), Pauluhn (2000) and Kilgour et al. (2002), the pulmonary tumours developed secondarily to the local toxicity by polymeric MDI aerosol. Hyperplasia of Type II alveolar cells is a common non-specific reaction to many forms of toxic lung injury. It is commonly accepted (though not proven for the lung) that such processes can produce tumours through non-genotoxic (epigenetic) mechanisms.

 

Furthermore, the experimental evidence does not support a genotoxic mode of action. It has been proposed that tumours may arise from formation of the mutagenic diamine in either the experimental exposure chambers or metabolically from MDI. However, analytical methods have shown the former demonstrably not to occur (Sepai et al 1995, Gledhill 2005). A guideline ADME study using radiolabel MDI could not detect MDA as a metabolite in blood, bile or urine (Gledhill 2005). Others have reported no MDA in urine or blood of workers (Sennbro 2003). The proposal that MDI is a metabolite of MDA arises from the presence of acetylated MDA as a metabolite. However, recent understanding of the interaction of MDI with glutathione and proteins, in vivo, indicate that such acetylated metabolites can be formed from the thiocarbamates without MDA as an intermediate metabolite (sections 7.1, 7.9.3).Further, experimental results of reliable guidelines studies from an in vivo micronucleus assay (systemic genotoxicity) and an in vivo inhalation comet assay (portal of entry genotoxicity) were negative (Pauluhn and Gallapudi, 2001; Randazzo, 2017). Thus the weight of scientific evidence supports the conclusion that MDI (or its metabolites or degradation products) is not mutagenic or genotoxic both systemically and at the portal of entry.

 

The mode of action for carcinogenicity of MDI reviewed in detail by Greim (2008), in the report from the MAK Collection for Occupational Health and Safety. Greim ( 2008) concluded that the NO(A)EC and LO(A)EC from the short-term and chronic inhalation studies are remarkably similar without evidence of cumulative dose effects. Greim (2008) considered that based on all available data, a mechanism related to the local lung-irritating effect of MDI to be relevant. The initial event stems from interaction of MDI or pMDI with the surfactant systems of the lungs. Here MDI apparently reacts with nucleophilic scavenger molecules (i.e. glutathione). Depletion of glutathione levels may allow reaction with surfactant, resulting in surfactant destabilisation. Complexes consisting of reacted MDI and precipitated surfactant are phagocytosed by alveolar macrophages or eliminated from the deposition site via mucociliary clearance (Pauluhn 2000, 2002). The initial event occurs along with alveolar protein exudation, where increased concentrations of overall protein serve as a measuring parameter for acute alveolar irritating effects (Kilgour et al. 2002; Pauluhn 2000; Pauluhn et al. 1999). This means that the available findings support the hypothesis that pulmonary tumours are caused by chronic regenerative cell proliferation and not by genotoxic initiation and promotion. It should also be pointed out that the acute irritating effects and the associated chronic endpoints correspond, i.e. there are no indications of cumulative or overadditive effects. Alternatively, hypertrophy or hyperplasia of the type-II pneumocytes may also occur due to the reactions of MDI with the surfactant and/or with glutathione or due to cytotoxicity of type-I pneumocytes. This aetiopathology would not contradict the above hypothesis. The increased regenerative proliferation of type-II cells, regardless of whether this is the result of irritating damage to type-I pneumocytes or a chronically increased surfactant synthesis capacity (compensation for the fraction of surfactant removed by MDI), is therefore considered to be the cause of the preneoplastic changes in rats, which is a known chronic reaction of rat lung to irritating substances. The documentation of the MAK values summarises thus: “On the basis of all available data, a mechanism via the local irritating effects of MDI in the lungs is considered relevant (AGS 2000; EU 2005; Gledhill et al. 2005; Kilgour et al. 2002; Pauluhn et al. 1999 a; Reuzel et al. 1994 b).

 

An overview on the interrelation of acute and chronic mode of action and respective key events is published by Pauluhn (2011). In this report, a computational approach was used to interrelate data from (sub)acute and chronic studies. The no-observed-adverse effect levels (NOAELs) in rats from two published whole-body exposure chronic inhalation bioassays was compared with the lung irritation-based point of departures (PODs) from acute and subacute nose-only inhalation studies. Acute irritation was related to elevated concentrations of protein in bronchoalveolar lavage fluid (short-term studies), whilst the chronic events were characterized by histopathology. In the chronic bioassay the exposure duration was either 6 or 18h/day while in all other studies a 6h/day regimens were applied. The computational analysis supports the conclusion that the C×T(day) relative to the acute pulmonary irritation threshold is more decisive for the chronic outcome than the concentration per se or the time-adjusted cumulative dose. For MDI aerosols, the acute threshold C×T(day) was remarkably close to the NOAELs of the chronic inhalation studies, independent on their differing exposure mode and regimens. This evidence is supportive of a simple, direct MoA at the site of initial deposition of aerosol (Pauluhn, 2011).

 

Finally, it needs to be appreciated that the test material in the rodent bioassays was delivered as respirable aerosols. Such atmospheres are technically very difficult to achieve and maintain, and in practice are not formed outside of the laboratory. For instance, in inhalation toxicity tests, the test substance often needs to be heated to allow for nebulization so that aerosols are formed. As a general rule particles are described as inhalable or respirable depending on size. Inhalable particles can enter the nose but because of the relatively large size deposit entirely in this region. Finer, respirable particles, (10 µm MMAD or less), can penetrate to lower regions of the respiratory tract and deposit in terminal alveolar regions. As respirable particles are deposited throughout the lung, but the tumours were only seen in the bronchiole-alveolar region, the regional deposition of respirable particles in the lower regions of the lung is a key event in tumour development. For MDI to be present in the workplace atmosphere in respirable form for prolonged durations seems inconceivable. It is also to be noted that epidemiological data do not demonstrate any carcinogenic risk for workers in the MDI using (polyurethane) industry (section 7.10.2). Evaporation of MDI can occur, but only to a very low extent because of its very low vapour pressure of <0.000002 kPa at room temperature. If the workplace atmosphere would be completely saturated with the test substance, it would contain around 0.1-0.2 mg/m3 MDI, which is close to the 8h-TWA occupational exposure limit of 0.05 mg/m3.

 

Based on the available information it can be concluded that:

- Polymeric MDI was carcinogenic in rats after long-term inhalation to aerosol concentrations of 6.0 mg/m3.

- Exposure to polymeric MDI at concentrations not leading to recurrent lung tissue damage will not produce pulmonary tumours.

- Experimental evidence does not support that the tumour formation has a genotoxic mode of action.

- The increased regenerative proliferation of type-II cells is considered to be the cause of the preneoplastic changes in rats, which is a known chronic reaction of rat lung to irritating substances.

- The testatmospheres used in the rodent bioassays are technically very difficult to achieve and maintain, and in practice are not formed outside of the laboratory. For MDI to be present in the workplace atmosphere in respirable form for prolonged durations seems inconceivable.

Justification for classification or non-classification

Classified as Carcinogen Cat 3 by the DSD incl 30 and 31 ATP. R40 Limited evidence of a carcinogenic effect.

By CLP as Carc Cat 2 (H351): Suspected of causing cancer by inhalation.