Poorly soluble particulates: Searching for a unifying denominator of nanoparticles and fine particles for DNEL estimation
Introduction
Results from numerous short-term inhalation/aspiration/instillation studies with various types of carbon nanotubes (CNT) have been published (for reviews see Donaldson et al., 2006, Madl and Pinkerton, 2009, Maynard, 2007, Oberdörster et al., 2005, Oberdörster et al., 2007). The degree and kind of aggregation of CNT structures is determined by the rigidity and pliancy of nanotubes and whether their diameters are thin enough to allow their buckling and self-aggregation into low-density, particle-like, intertwined, and often coiled assemblages (Pauluhn, 2009a). Given the differences in the physical shape of agglomerate structures, a categorization into rigid and flexible CNT appears to be among the most straightforward discriminative variable. In addition, the type of assemblage structure and whether it is stabilized by mere agglomeration or some kind of inter-tubular aggregation (physical entanglement) needs to be appreciated. Hence, depending on these characteristics, agglomerate structures of nanotubes may differ appreciably from thin-walled to thick-walled, rigid MWCNT. These properties may be decisive for hazard assessment as the critical toxic principle may either emerge from the individual tube structure (e.g., fiber) or the collective behavior of inhalable assemblages of nanotubes.
Unlike conventional poorly soluble crystalline particle structures, MWCNT are present as submicronsized agglomerated arrangements of closely packed CNT which increase the void-space volume creating a novel type of composite low-density PM-structures. Consequently, when phagocytized by alveolar macrophages, much less particle mass is needed to exceed the volumetric overload limit for the diminution of macrophage-mediated clearance (Morrow, 1988, Morrow, 1992, Morrow, 1994). Based on Morrow's hypothesis of the volumetric overload of alveolar macrophages, the particle displacement volume rather than surface area appears to be the most critical metric for these types of materials. Hence, agglomerated nanoparticles present in submicronsized form may cause a volumetric overload of alveolar macrophages at lower exposure doses as compared to their micron-sized crystalline counterparts. Yet, no single particle characteristic as a hallmark indicator directing fate and pulmonary toxicity has been identified (Madl and Pinkerton, 2009). However, emerging views suggest that the assemblage displacement volume of MWCNT, which is critical to the impairment of alveolar macrophage-mediated clearance and elicitation of pulmonary inflammation, may dictate the fate and pulmonary response to this type of structures. Considerable efforts have been expended in measuring and modeling pulmonary deposition of inhaled particles in rodents and other species, and several comprehensive reviews have been published (Miller, 2000, Brown et al., 2005, Oberdörster et al., 1992, Oberörster, 2002). This retrospective comparison focuses on a PM-volume-based metric as the most apt denominator to compare PMs of different size, effective density, and structure. The focus of previous approaches was limited to submicronsized, high-density particles (Pauluhn, 2009b) whereas this analysis utilized additional data from recent repeated exposure inhalation studies on rats with nanostructured, low-density materials.
In the context of pulmonary toxicity of poorly soluble particles surface area is often considered to be the leading metric. However, it is hard to believe that the gauge commonly used to determine surface area (N2) (Klobes et al., 2006) is at any rate reflective of the competitive adsorption of the numerous peptides and proteins present in the lining fluids of the lung. In other words, the biologically effective surface areas are dependent on the gauge size of the most avidly binding endogenous polypeptide, including its competitive displacement. Likewise, especially for particle structures in the submicron to nanometer range, the aggregation properties of assemblages of PM may be changed by preparation and collection. For such complex and irregular shape three-dimensional aggregated objects it becomes increasingly difficult to assign one single physical qualifier for an unequivocal characterization. The phenomena occurring during the contact between nanoparticles and cellular media or biological fluids (dispersion, agglomeration/aggregation, protein adsorption) in relation to the surface properties of the nanoparticles considered are discussed elsewhere in detail (Fubini et al., 2010). From that perspective, the three-dimensional characteristics ‘volume’ appears to be a better qualifier than PM number or surface area. In this context and following the hypothesis of AM overload, the most directly accessible and mechanism-based variable is the displacement volume of PM within the available pool of phagocyte or AM (volume of distribution, Vd). Therefore, the following risk analysis is solely focused on this variable. Accordingly, surface area has not been considered in this paper as a previous analysis with higher-density particles of different surface areas has demonstrated that the PM mass concentrations and pulmonary inflammation correlated better than surface area concentrations (Pauluhn, 2009b).
The objective of this paper is to analyze whether the somewhat unique pulmonary inflammatory potency of MWCNT assemblages share some unifying characteristics with their higher-density granular biopersistent counterparts when using a volume metric. Based on this rationale, the composite volume of aggregates appears to be the most critical variable of dose. Hence, it is timely to analyze as to which extent current testing paradigms need to be modified to the advancement in toxicological knowledge to appropriately identify and rank the hazards of PMs. Computational toxicology (modeling) has been utilized to better design and predict the outcome of repeated rat inhalation studies. This latter aspect serves the additional objective to improve the design and dose-rationalization of inhalation bioassays across different laboratories. Especially for lung toxicity, the Guidance given in the new REACH regulation (ECHA, 2008a, ECHA, 2008b, ECHA, 2008c, ECHA, 2008d) is lacking prescriptive instructions to arrive at scientifically reasonable assessment factors and NELs (no-effect levels) for the derivation of Occupational Exposure Levels (OELs) for these types of substances. This paper attempts to rationalize a mode-of-action-based, scientific approach.
Section snippets
Study design and experimental variables
It is important to keep in mind that the model system or testing regimen themselves may influence measured responses irrespective of the particulate material investigated. At present, most of the studies are concerned with pulmonary pathology patterns following single to short-term high-dose bolus dosing. These types of studies have ample room for experimental artifacts which include localized pathology due to particle clumping and irregular particle distribution as a result of dosing of poorly
Pulmonary biokinetics, volume of distribution, and particle clearance
The retention t1/2 of nanosized (uTiO2, AlOOH, MWCNT) and submicronsized (pTiO2, CB, Fe3O4) particles from 4 and 13 weeks inhalation studies (see Table 1) on the cumulative PM-volume concentrations are compared in Fig. 11. This illustration delineates a clear association of an increase in the cumulative volume concentrations and the increased retention t1/2. Thus, using the volume metric, all data converge into one single relationship yielding a threshold estimate of t1/2 = 64 days (Fig. 11)
Conclusions
In regard to particle-related pulmonary toxicity diverse views have been articulated over the past decades on the most critical physical/physicochemical property determining the biopersistence and ensuing cumulative dose of deposited nano- and/or micrometer-sized particles in the lung (Maynard, 2007). This also includes the lead mechanism to initiate and sustain pulmonary inflammation and possible chronic sequelae thereof under conditions of various degrees of lung overload obtained in
Conflict of interest
There are none.
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