Differences in the biokinetics of inhaled nano- versus micrometer-sized particles

Abstract

Researchers need to study the biokinetics of inhaled biopersistent nano- and micrometer-sized particles (NPs and μPs) to assess their toxicity and to develop an understanding of their potential risks. When particles are inhaled, they do not necessarily remain at their sites of deposition in the respiratory tract. Instead they can undergo numerous transport processes within the various tissues of the lungs, including clearance from the lungs. In this context, we would like to understand how the biokinetic studies performed in animals can be extrapolated to humans. Interestingly, the particle retention is much shorter in rodent lungs and declines much faster than it does in human, simian, and canine lungs. The predominant long-term clearance pathway for both NPs and μPs in humans and other animal species is macrophage-mediated particle transport from the peripheral lungs toward ciliated airways and the larynx. However, the transport rate is 10 times higher in rodents than in other species. In addition to particle clearance out of the lung, we also observe particle redistribution from the epithelium toward and within the interstitium and lymph nodes of the lung and particle translocation to blood circulation leading to subsequent accumulation in secondary organs. While μPs have limited access to interstitial spaces in the rodent lungs, NPs rapidly relocate in the epithelium and the underlying interstitium. By contrast, indirect evidence shows that both NPs and μPs are relocated into the epithelium and interstitial spaces of the human, simian, and canine lungs. Only NPs translocate into the circulatory system and subsequently accumulate in the secondary organs and tissues of the body. Translocated NP fractions are rather low, but they depend strongly on the physicochemical properties of the NP and their surface properties. Growing evidence indicates that the binding and conjugation of proteins to NPs play an essential role in translocation across cellular membranes and organ barriers. In summary, particle biokinetics result from a multitude of highly dynamic processes, which depend not only on physicochemical properties of the particles but also on a multitude of cellular and molecular responses and interactions. Given the rather small accumulation in secondary organs after acute inhalation exposures, it appears likely that adverse effects caused by NPs accumulated in secondary organs may only occur after chronic exposure over extended time periods. Therefore adverse health effects in secondary organs such as the cardiovascular system that are associated with chronic exposure of ambient urban air pollution are less likely to result from particle translocation. Instead, chronic particle inhalation could trigger or modulate the autonomous nervous system or the release of soluble mediators into circulation leading to adverse health effects.

Figure 1

Concept of quantitative particle biokinetics assessment. Particles are administered at time t=0. From this time point on, the entire urinary and fecal excretions are collected separately. Animals are euthanized at times t1, t2, t3, etc., and organs and tissues of interest and the entire remaining carcass are sampled (100% balanced sampling). Special sample preparation is required before chemical analysis. When particles are radio-labeled, total organ samples will be analyzed directly using gamma spectrometry without any further preparation.

Figure 2

Alveolar-macrophage (AM) associated fractions of instilled μP (0.5, 3 and 10 μm polystyrene particles) and inhaled NP (radio-labeled 20 and 80nm iridium NP) found in BAL of rats 24 hours after administration^,. The low fractions of the iridium NP are similar to those of 20nm gold NP, titanium dioxide NP and elemental carbon NP (data not shown).

Figure 3

Fractions of μP (either inhaled 3.5 μm ⁸⁵Sr-labeled polystyrene particles (μPSL) or intratracheally instilled fluorescent 2 μm μPSL versus inhaled NP (radio-labeled 20nm iridium NP) found in broncho-alveolar lavage fluids of rats at various time points from day 3 through six months after administration^,,,,. All fractions are relative to the actual lung burden.

Figure 4

Pathways of relocation of inhaled NP after deposition on the alveolar epithelium of the rodent lungs: rapid transport into the epithelium and interstitial spaces for long-term retention; upon clearance NP can re-entrain back onto the epithelium for macrophage-mediated transport towards ciliated airways and the larynx.

Figure 5

Upper panel shows the increasing particle fraction in the canine lung interstitium during a six-month retention period after a short-term inhalation of 1μm fluorescent polystyrene particles (μPSL). The line corresponds to the particle relocation kinetics with a half-life of 150 d into pulmonary tissues as previously determined in biokinetics studies^-. Lower panels show morphologic locations of μPSL in lung tissues during the six-month period (μPSL, false color).

Figure 5

Upper panel shows the increasing particle fraction in the canine lung interstitium during a six-month retention period after a short-term inhalation of 1μm fluorescent polystyrene particles (μPSL). The line corresponds to the particle relocation kinetics with a half-life of 150 d into pulmonary tissues as previously determined in biokinetics studies^-. Lower panels show morphologic locations of μPSL in lung tissues during the six-month period (μPSL, false color).

Figure 6

Graphical scheme of long-term relocation and retention of NP versus μP showing the differences between rodent lungs versus canine and simian lungs. We hypothesize that the human pattern may be quite similar to that of the large animal species.

Figure 7

Twenty-four-hour translocated fractions of alveolar NP deposit towards blood and subsequent organs and tissues; three different materials (iridium, elemental carbon and titanium dioxide) were inhaled as freshly generated 20nm NP aerosols for 1-2 hours by healthy adult rats. The iridium NP translocation is significantly higher than those of elemental carbon and titanium dioxide NP. Iridium:^,; elemental carbon:; titanium dioxide: Kreyling & Semmler-Behnke, personal communication.