Current knowledge regarding lung overload
The concept of `lung overload` was first introduced by Morrow in 1988 who defined it as an impairment of alveolar macrophage mediated lung clearance following exposure to high concentrations of low soluble particles of low inherent toxicity (PSP), thereby triggering accumulation of particles in the deep lung, persistent pulmonary inflammation, epithelial cell proliferation as well as non-neoplastic and in the case of rats also to neoplastic lung lesions. In the context of the rat lung response to PSPs a more refined definition had been formulated following an ILSI expert workshop (ILSI 2000) as follows: "For chronic inhalation of PSPs, particle overload is a consequence of exposure that results in a retained lung burden of particles that is greater than the steady-state burden predicted from the deposition rates and clearance kinetics of particles inhaled during exposure.
The present comprehensive review on the available scientific evidence on `lung overload` generally does support the conclusions drawn by the ILSI expert group but allow also additional insights into the mechanistic understanding and obvious species-specific differences important for risk characterisation exercises. In this respect the available data clearly demonstrate that the rat represents a particularly sensitive model with regard to pulmonary non-neoplastic effects and, moreover, a unique model with regard to lung neoplastic responses under conditions of particle overload. In fact, lung tumours following chronic exposure to PSP have been reported exclusively in rats, but not in mice, hamster, non-human primates or humans.
Epidemiological studies to date have not found comparable `lung overload` conditions in workers exposed chronically to PSPs, not even in former coal miners having experienced worst-case exposure conditions. Furthermore, well-conducted epidemiological studies thus far have not been able to detect an association between occupational exposure to PSPs and an increased risk for cancer.
The synopsis of current experimental data allow us to conclude that `lung overload` contributes to a cascade of species independent pathogenic events leading to non-neoplastic lung responses in all animal species investigated so far but to a specific neoplastic pathogenesis only in rats. Although finally resulting in different adverse outcomes, the induction of pulmonary inflammation, generation of reactive oxygen and nitrogen species and increased cell proliferation seems to be common and recurring effects described for all animal species during lung overload conditions. As such they can be considered as `intermediate steps` in the sense of an adverse outcome pathway (AOP).
Lung tumours have to be regarded the final phenotypic `adverse outcome` only in rats whereas in other species non-neoplastic lesions seem to be the respective `adverse outcome`. Key events for this divergence in the largely common mechanistic sequence of the AOP may be seen in a species specific biological diversity of detoxification systems, e.g. anti-oxidant defences reducing `oxidative stress` as well as differences of particle translocation processes. In fact, it could be demonstrated that the rat has greater propensity for generating a pro-inflammatory response, whereas mice and hamsters show an increased anti-inflammatory response which may not only contribute to the higher inflammatory reactions seen in rats, but do also indicate less potential for scavenging secondary mutagenic effects in rats.
The state of knowledge on particle deposition in the respiratory tract of the experimental animals (rats) and humans is sufficiently well developed to be used in dosimetry models for extrapolation of animal data on humans. The regional deposition fraction is one of the key parameters for the calculation of the retained dose from exposure concentrations and the determination of HECs. AS well as the MPPD model providing average values for the deposition fraction, there is experimental evidence (supported by CFD calculations) of significant heterogeneity in lung deposition on the surface within each compartment. This is particularly the case for the trachea-bronchial and extrathoracic region but less of an issue in the alveolar region. Translocation seems also to be species dependent. Whereas in rats most of the particulate matter is located in the lumen of the alveolar ducts and alveoli, the larger part of particles in humans is translocated into the interstitium.
Beside physical translocation of deposited particles, chemical dissolution is one of the basic clearance principles of the respiratory tract. Clearance of biosoluble particles by dissolution can occur in all three major regions of the respiratory tract, i.e. the nasopharyngeal, tracheobronchial and/or alveolar regions. In contrast, physical translocation of inhaled particles of low (bio)solubility is different in these regions and depends on particle characteristics such as size and shape. As the most prevailing clearance mechanism for solid particles in the alveolar region is mediated by alveolar macrophages, (bio)solubility of particles is considered a major determinant in the establishment of `particle lung overload’ conditions.
The Task Force also concluded that there is no clear evidence showing that particles below 100 nm exhibit any kind of step-change in their hazard status or for the onset of any novel nano-specific hazard. A higher biological activity of smaller particles is not necessarily to be expected and notwithstanding their smaller size, nanoparticles are no more hazardous than conventional particles. Normal toxicological principles can therefore be applied equally, and conventional particle toxicology data are useful and relevant to the determination of nanoparticle hazard and risk evaluation.
It is now scientifically accepted that all kind of PSPs (micro-sized and nano-sized) are eliciting comparable localised pulmonary toxicity via processes that causes oxidative stress and are pro-inflammatory in nature, and which are initiating an acute, neutrophil driven inflammatory response. In this respect, there is more and more evidence that all particles of low solubility and no/low inherent toxicity, independent of the particle size (i.e. whether in the nano or microsize) are following the same mode of action and basic mechanism. Oxidative stress is evidentially implicated in this process and experimental evidence suggests a relationship between the induction of inflammation and the severity of the oxidative insult. It should be noted that the mechanisms leading to an oxidative and inflammatory pulmonary status is clearly threshold related as exemplified in a number of studies.
As discussed within this Report, there is substantial evidence that poorly soluble particles of low toxicity, whether nanosized or microsized, exert toxicological relevant adverse effects - even the tumourigenic action of high exposure concentrations of PSPs, such as TiO2 in rats - via a threshold mode of action. Hence, the derivation of DNELs based on NOAELs/NOAECs for poorly soluble (nano)particles of low toxicity is toxicologically justified.
The mechanisms of toxicity of inhaled PSPs are well understood, allowing the establishment of safe exposure levels and identification and use of adequate risk management measures. This knowledge already results in practical application for the setting of occupational exposure limits and, under the EU REACH Regulation, the setting of derived no effect levels (DNELs). DNELs based on NOAELs/NOAECs as derived in animal inhalation studies and adjusted for human equivalent concentrations by appropriate dosimetry modelling, is recommended. Due to the higher sensitivity of the rat compared to humans with regard to lung overload driven effects and based on comparable biokinetics, an overall assessment factor of 1 for intra- as well as interspecies differences is considered suitable and sufficient. While within the scope of this work it was not possible to scientifically justify a default assessment factor for exposure duration extrapolation, the Task Force recommends further investigations on the need of such AF and reconsideration as new knowledge becomes available.
Since the ILSI 2000 report, there has been a vast amount on in vivo and in vitro work on poorly soluble particles of low toxicity but there have been no compelling studies or a weight of evidence that would allow the Task Force to conclude that the rat lung overload findings is a reliable predictive model, in particular for neoplasia, with regard to hazard or risk assessment for humans who are exposed to poorly soluble particles of low toxicity.