Particle Toxicology - Chapter 22 (end) pdf

Mechanistic studies; or put simply, “How do harmful particles cause harm?” Studies aimed at refining the dose metric Knowledge is incomplete and we are not fully aware of the nature of th

22 The Toxicology of Inhaled

Particles: Summing Up an

Emerging Conceptual

Framework

Ken Donaldson

MRC/University of Edinburgh Centre for Inflammation Research,

University of Edinburgh

Lang Tran

Institute of Occupational Medicine

Paul J A Borm

Centre of Expertise in Life Sciences, Hogeschool Zuyd

CONTENTS

22.1 Overview 413

22.2 Defining the Particle Toxicology Endeavor 414

22.3 Classical Toxicology 415

22.4 Exposure 415

22.4.1 Exposure at Portal of Entry 415

22.4.2 Toxicokinetics and Translocation from the Portal of Entry 415

22.4.3 Brain 416

22.4.4 Blood 416

22.5 Dose and the Concept of Biologically Effective Dose (BED) 416

22.5.1 A Generalized Paradigm of Particle Toxicity Based on BED 418

22.5.2 Oxidative Stress as an Early Biological Effect Marker of BED for Different Pathogenic Particles 418

22.6 Response 420

22.6.1 The Occupational Setting 420

22.6.2 The Environmental Setting 420

22.7 Conclusions 421

References 421

22.1 OVERVIEW

The chapters in this book set out the state-of-the-science for particle toxicology as it pertains in the early twenty-first century It points out the maturity of this area of applied science, and toxicology

is, above all, an applied science Toxicology’s primary aim is to provide hazard data for risk

413

assessment towards safe ways of working and living with the chemicals we encounter on a daily basis, which all have inherent toxicity for biological systems In particle toxicology, we seek to provide the data that will allow us to manage the risk associated with living in atmospheres that are often complex mixtures of particles of varying toxicity However, mechanistic particle toxicology crosses over into mainstream molecular medicine and can provide important clues to the basis of other types of disease Our studies of the cardiovascular system, oxidative stress and molecular signaling in relation to particles offer an understanding that is generally applicable This is eminently clear from the pages of this book where high quality and innovative research approaches are demonstrated in addressing the issues relating to particle effects This chapter aims to draw together the various threads in the fabric of particle toxicology and present a new unifying concept, albeit simplified and incomplete, for this discipline

It is clear that the rise in nanotechnological applications and products has and will have a huge impact on particle toxicology Nanoparticle research has become the key area for study by particle toxicologists It represents a considerable challenge with new portals of entry, such as the skin and the gut and new target organs such as the blood and brain Additionally, pharmacological uses of nanoparticles have caused a realignment and whole new areas of research are opening up that build

on the kind of expertise owned by particle toxicologists

22.2 DEFINING THE PARTICLE TOXICOLOGY ENDEAVOR

Particle toxicologists study particles in two main ways:

1 Studies aimed at refining the dose-metric; or put simply, “What is it about particles that makes them harmful or not?”

2 Mechanistic studies; or put simply, “How do harmful particles cause harm?”

Studies aimed at refining the dose metric Knowledge is incomplete and we are not fully aware

of the nature of the harmful dose (the quantity of the particle’s physicochemistry that drives adverse effects; see below) for many particle types If we fully understood what made any particle type harmful, we could focus in on that parameter for measurement and so improve risk management Historically, there have been frequent mismatches between the current dose-metrics and our existing knowledge of the toxicology of some types of particles For example, asbestos and other fibers are measured as mass of all airborne fibers visible by light microscopy longer than

5 mm (with diameter !3 mm and aspect ratio O3:1) However, toxicological research has shown that fibers that are both biopersistent and longer than about 20 mm are the pathogenic ones (Donaldson 2004) Despite this, the “old” fiber standard remains, and it takes no account of the issues of length or solubility In the case of PM10, the mass of particles around 10 mm are measured but much of this mass is harmless, e.g salt However, smaller particles (PM2.5) or transition metals (Donaldson et al 2004a) or oxidant generation (Schaumann et al 2004) might be better predictors for health effects

Mechanistic studies These studies aim to understand the cellular and molecular basis of the toxic effects of particles and the sum of their interactions with biological systems Such studies contribute to risk assessment by providing a more complete framework for our understanding of how particles behave in the body, the effects they have on cells and how being in the body changes them In combination with toxicokinetics, which describe how fast and to which extent particles get distributed to different organs or tissues, such studies allow the entire “life history” of particles in the body to be traced These mechanistic studies offer the possibility of therapeutic intervention in the process of disease An example is the current exploration of soluble TNF-receptors in the treatment of IPF, as a result of initial work on TNF-a in fibrotic disorders caused by quartz-containing dust (Piguet et al 1990)

22.3 CLASSICAL TOXICOLOGY

The classical toxicology paradigm of exposure–dose–response can be used for particles as for any other toxin Exposure, dose and response together with ADME/toxicokinetics allow us to describe the detailed history of a toxin in the body Complete toxicokinetics analysis is not available for any pathogenic particle Most pathogenic particles have not been considered to be metabolized and excreted in any conventional way However, nanoparticles, because of their smallness, may undergo such changes (Oberdo¨rster et al 2005b) The fuller our understanding of these processes, the better we will be able to interpret toxicology studies and understand the nature of the toxic effect

of any particle Of exposure, dose and response, we are especially focused on “dose” as a key to understanding molecular and cellular toxicity as well as contributing to understanding the best metric For nanoparticles, the dose-metric is not yet elucidated and is likely to vary with different particle types since they can be composed of a range of materials and can be different sizes and shapes The concept of a “biologically effective dose” (BED) is an important one for particles since all particle exposures are mixed We can hypothesize that there is one or more actual component of this total dose that actually drives the adverse effect, and this is the BED

22.4 EXPOSURE

In the past, particle toxicology was concerned almost exclusively the lungs With the advent of nanoparticle toxicology there has been a sea change in how inhalation particle toxicology is viewed (Donaldson et al 2004b) Following inhalation of nanoparticles, the blood and brain are seen as secondary targets for particle effects (Oberdo¨rster et al 2005a, 2005b)

The aerodynamic diameter is the key particle parameter that predicts whether any particle gains access to the lungs and it also determines the site of particle deposition in the lungs (Gehr et al 2000) Aerodynamic diameter is important for deposition of bigger particles with impaction and sedimentation being the main processes, whilst these become less important as size diminishes and diffusion comes to dominate the deposition process For fibers, interception is an important depo-sition process whereby the extremities of the fiber make contact with airspace walls while the center

of gravity of the fiber is following the airstream at a bifurcations (Morgan and Seaton 1995) Deposition of compact particles occurs as particles fall out in accord with their weight, i.e., sedimentation They also deposit by impaction as particles fail to negotiate bifurcations and collide with the bronchial wall; deposition at bifurcations is increased also by the normal turbulence that results from the disruption to the even flow of air at these points Finally, the smallest particles reach the distal lungs to the point where the net flow of air is zero, where they move by molecular (Brownian) motion and they deposit efficiently by diffusion For these reasons, deposition in the lung is highly focal as a result of the dose being applied to certain anatomical areas/hot-spots, such

as the bifurcations of airways and the centriacinar region At these hot-spots, deposition can be 100-fold higher than in adjacent areas (Balashazy et al 2003)

Up until recently, the translocation of particles from their site of entry to other target organs was not considered a major issue However, because of data showing nanoparticle translocation from the lungs (Hoet et al 2004; Nemmar et al 2004), there is increasing concern in this regard To date, animal studies show some translocation of radioactive nanoparticles from the lungs to the blood following instillation exposure (Nemmar et al 2001) and to the brain following inhalation exposure (Oberdo¨rster et al 2004) (Figure 22.1) There is no evidence for this type of translocation following inhalation of any nanoparticle type in man at the time of writing A flow diagram of the hypothetical

fate and effects of nanoparticles is shown inFigure 22.2, based on limited animal studies with a few selected nanoparticle types

Limited studies indicate that nanoparticles can gain access to the brain via the nose and the nerves that run from the olfactory epithelium into the olfactory lobes of the brain (Oberdo¨rster et al 2004) Nothing is known of the dosimetry in relation to exposure, nor whether this is generic property of nanoparticles However, given the ubiquitousness of combustion-derived nanoparticles in our environment, if this is a general property of NP, then it is likely that we all have a burden of nanoparticles in our brain Indirect evidence that this is indeed true and that there might be adverse effects on the brain comes from studies in Mexico City describing unusual brain pathology in the young (Calderon-Garciduenas et al 2004) It is not known if nanoparticles can generally cross the blood/brain barrier, but medical nanoparticles have been designed to efficiently translocate

to the brain from the blood (Kreuter et al 2002)

Limited data indicates that nanoparticles can pass from the lungs to the blood (Kreyling et al 2002a; Kreyling et al 2002b; Nemmar et al 2004) In the blood, particles can have a range of effects on blood components and associated cells, such as endothelial cells, monocytes and platelets (Hoet, Nemmar, and Nemery 2004) Exposure to combustion-derived nanoparticles and PM have shown effects at the level of the endothelium to impair vasomotion in a human model (Mills et al 2005), which is known to be a risk factor for myocardial infarction Concomitant pro-thrombotic effects that would favor thrombus propagation in the event of atherothrombosis were also reported (Mills et al 2005) Few studies have reported the effect

of engineered nanoparticles on the cardiovascular system (Radomski et al 2005), but several studies have identified that exposure to PM or CAPS enhances severity of atherogenesis in Watanabe rabbits and ApoE mice (Suwa et al 2002; Chen and Nadziejko 2005; Sun et al 2005) Since particle based systems are being explored for molecular imaging in atherosclerotic disease, more research in this area is needed

22.5 DOSE AND THE CONCEPT OF BIOLOGICALLY EFFECTIVE DOSE (BED)

The internal dose is the quantity of a toxin that gains access to the body For inhalation exposure, because of particle clearance, of course, the fraction of the deposited dose that remains in a long-lived compartment like the interstitium or in long-lived macrophage accumulations can take

Brain Blood Nanoparticles Respiratory tract

Degeneration Atherothrombosis Inflammation, etc.

FIGURE 22.1 Outline of the potential toxicokinetic pathways for inhaled nanoparticles to translocate and have effects in the body following inhalation exposure

part in toxic reactions, and it is less than the “total dose” that is breathed in and that deposits This is then acted on by the milieu in the interstitium or in cells to cause dissolution of non-biopersistent particles or components This may release harmful soluble components as well as harmless soluble components, which are cleared or metabolized The BED is a useful concept, being that fraction of the total dose that is sufficiently biopersistent and also sufficiently active to cause an adverse effect such as oxidative stress, adduct formation, etc This can be understood in that all realistic particle exposures are particles that are multi-component and poly-dispersed and within this total dose, we can identify sub-fractions that are likely to be more harmful or effective than others For example, PM10 is measured by the mass metric, yet a variable and often substantial quantity of the mass of PM10 is sea salt, which is likely to be completely invisible to the lung following exposure at ambient levels, i.e., a harmless dose Conversely, the transition metals can be seen to be driving the oxidative stress and inflammatory effects of PM in human (Schaumann et al 2004) and animal models (Jimenez et al 2000; Campen et al 2001; Campen et al 2002; Molinelli et al 2002), yet this component would contribute very little to mass Thus the transition metals can be seen as the BED, and their ability to cause oxidative stress is an early biological effect Other examples of BED are the amount of “free” or “clean” quartz surfaces that are available to interact with cells that drive quartz’s inflammatory effects (Donaldson and Borm 1998) and the proportion of biopersistent, long (O20 mm) fibers, which drive the pathogenicity of a fiber sample (Donaldson and Tran 2004c) The concept of the BED

is shown in Figure 22.2

Deposited particles Cleared by

mucociliary clearance

Particles interstitialised/

sequestered in long-lived macrophages

Biopersistent particles

Ineffective dose Biological

effective dose Adverse effect Dimension factor

Soluble toxins

Non-biopersistent particles

Metabolism

to urine/bile*

*may be

relevant for

nanoparticles

FIGURE 22.2 The relationship between deposited particles, biopersistence and the BED

22.5.1 A GENERALIZEDPARADIGM OFPARTICLETOXICITYBASED ONBED

These factors can be assembled into a small number of sources of BED that allow us to present a generalized paradigm for the total BED in the lungs for any specific particle type The paradigm may

be similar or different for pathogenicity of particles in other sites, such as blood or brain For insoluble particles, it is only the surface layer that makes contact with the biological system and

so can be considered to mediate the harm For this reason, the surface area has come to the fore as a dose-metric that effectively describes the potential toxicity of particles, rather than total mass or particle number The particle’s surface is likely to vary in intrinsic reactivity/toxicity, depending on the material and (nanoscale) the pattern of which the particle is made Therefore, intrinsic reactivity

is a multiplier of surface area to obtain the total reactive surface produced by any mass of particles In addition, we know from the asbestos and SVF experience that, for fibers, length can be important and

so we can add “shape” as a factor In addition, studies of PM10, welding fume, etc tell us that many particles are complex and contain soluble components that can have considerable toxic potential Additionally, the length of time that the particle is likely to persist and not be cleared determines the length of time that the BED is applied to the system; thus a biopersistence factor is required Taking these factors together, we have a paradigm that could predict the toxicity of a particle in the lungs The three main attributes for the biologically effective dose of a particle to a cell are:

1 Surface attributeZsurface area!specific surface reactivity (i.e., reactivity per unit SA)!surface availability

2 Dimension attributeZlengthCdiameter (mainly length if greater than a critical length)

3 Composition attributeZVolume!specific volumetric reactivity (i.e., the toxic material per unit volume)!availability (Zrelease rate i.e., amount per unit time)

For acute effects the BED is related to the Potency, which can be best described as the sum of (1)C(2)C(3) For chronic effects the biopersistence plays a dominant role and the BED is best described by the product of biopersistence and potency:

Biologically effective dose Z Bio-persistence!Potency This equation reflects the issue of translocation but deals specifically with the toxic outcome of the interaction between a particle and a cell We do not think we are yet in a position where we can even approach the development of a paradigm for translocation that is based on structure Table 22.2shows attributes contributing to Bio for several particle types:

PATHOGENICPARTICLES

Inspection ofTable 22.1raises questions as to how so many different chemical and physical entities (length, soluble toxins of different types, different surfaces) could all constitute some common form

of “harmful dose” to the machinery of the cell However, this may be understood in the findings that the ability to deliver oxidative stress is a common property of harmful particles This oxidative stress can emanate from the particle itself and it can also be a consequence of the cellular and/or the inflammatory response induced by the particle (Donaldson et al 2003; Knaapen et al 2004) There is a highly plausible link between oxidative stress and inflammation (Barrett et al 1999; Tao et al 2003; Donaldson et al 2005b), with many oxidative stress-responsive pathways signaling for pro-inflammatory gene transcription (Piette et al 1997; Rahman and MacNee 2000) There is also a clear link between oxidative stress and adducts such as 8-hydroxy-deoxyguanosine, the hydroxyl radical-induced adduct of guanine, which is involved in carcinogenesis (Lloyd et al 1998; Tsurudome et al 1999; Maeng et al 2003) Many pathogenic particle types have been

shown to activate NF-kB (Schins and Donaldson 2000) and cause inflammation (Donaldson and Tran 2002) The role of the surface is emphasized by studies showing that the ability of quartz to deliver oxidative stress is dramatically lowered by surface coating (Knaapen et al 2002) Combustion-derived NP have their effects by oxidative stress and inflammation (Donaldson

et al 2005a), and several engineered NP types have been described as having oxidative stressing

TABLE 22.1

Table Shows the BED for a Number of Particle Types That Are Well Studied and the Mismatch with Their Exposure Metric

Quartz Area of reactive (unblocked or

unpassivated) surface

Respirable mass Asbestos Biopersistent fibers longer than w20 mm Fibers longer then 5 mm, O3 mm diameter and

aspect ratioO3

Welding fume (NP) Soluble transition metals Respirable mass

Diesel soot (NP) Organics/metals/surfaces Contained in PM 10

Carbon black (NP) Surface area Nuisance dust standard of respirable mass

TABLE 22.2

Relative Importance of Properties Contributing to the BED of Different Particle Types

Surface Attribute DimensionAttribute Composition Attribute Particle type Surface Area ReactivitySurface Length a Soluble Toxins b Biopersistence c

a Longer than 20 mm.

b E.g., metals, organics.

c More plusses equals more SA, reactivity, soluble toxins or biopersistence.

TABLE 22.3

Oxidative Stress Mechanism for Different Particle Types

Source of oxidative stress Exemplar Particle Mechanism of GenerationOxidative Stress Reference

surfaces (Vallyathan et al 1994) Soluble metals ROFA, welding fume Fenton chemistry (Shi 2003 3196/id)

Organics DEP, PM 10 Redox cycling of quinones etc (Squadrito et al 2001)

effects (Hussain et al 2005; Manna et al 2005; Sayes et al 2005) Attributes contributing to oxidative stressing potential of different particle types is shown inTable 22.3

22.6 RESPONSE

Particle-related lung diseases of various types can be seen in both occupational and environmental settings, and both the pattern and intensity of the exposure can differ quite distinctly between these two (Table 22.4)

Traditional particle-associated lung diseases are those seen in occupational settings and the clas-sical particles are quartz, asbestos, coalmine dust, etc High airborne mass exposures, characteristic

of historic workplaces, leads to the responses of pneumoconiosis and COPD (Table 22.4) In addition, there are cancers and asthma arising in workplaces due to particle exposures The worker population can generally be seen as a healthy, predominantly male population that can in general tolerate such exposures well, at least at the commencement of their exposure, because of the

“healthy worker” status The “healthy” status within the workforce is conserved by the simple process that those who are adversely affected by exposure to the dusty atmosphere leave to take up alternative employment leaving only a healthy workforce

There is a well-documented link between exposure to environmental particles (PM10) and morta-lity/morbidity in airways and cardiovascular disease and cancer (Pope and Dockery 1999; Pope

et al 2003) These low mass exposures commonly affect susceptible populations of patients with existing lung disease (asthma and COPD) or cardiovascular disease to produce quite a different exposure pattern and response (Table 22.4) Aged populations and those with airways disease and cardiovascular disease have pre-existing oxidative stress as part of the inflammatory components of their conditions, and this could be a factor in making them susceptible to particle effects driven by oxidative stress

Common features of the two paradigms are encapsulated inFigure 22.3, where the common roles of oxidative stress and inflammation are emphasized With NP, translocation and effects distal

to the site of deposition come into play and render the whole equation more complex However, the

TABLE 22.4

Characteristics of Inhalation Exposure to Particles

Typical Particle Types Exposed Population Exposure Typical Responses Occupational Silica, asbestos, welding fume,

manufactured nanoparticles, organic particles (grain, cotton)

Predominantly healthy males !65 years old High Pneumoconiosis,COPD, cancer,

asthma Environmental PM 10 containing

combustion-derived nanoparticles

Everyone including susceptible and O65 years old, ill populations with pre-existing inflammation and oxidative stress

Low Exacerbations of

COPD/asthma, cardiovascular disease, diabetes, cancer

basic principle of particle toxicology will be seen to apply and the responses should be interpretable

in the light of the foregoing discussion

22.7 CONCLUSIONS

The shape of this chapter emerged while we were putting together and reviewing all the chapters

of the book We believe that it represents the first effort to try and develop a single conceptual framework for the adverse effects of pathogenic particles, and it is undoubtedly simplified However, it offers hypotheses for testing and a framework to unify research and thinking in this important and fertile area of applied research As our understanding increases, it will undoubtedly be refined and improved and we look forward to the contributions of our colleagues

in this regard

REFERENCES

Balashazy, I., Hofmann, W., and Heistracher, T., Local particle deposition patterns may play a key role in the development of lung cancer, J Appl Physiol., 94, 1719–1725, 2003

Barrett, E G., Johnston, C., Oberdo¨rster, G., and Finkelstein, J N., Antioxidant treatment attenuates cytokine and chemokine levels in murine macrophages following silica exposure, Toxicol Appl Pharmacol.,

158, 211–220, 1999

Garciduenas, L., Reed, W., Maronpot, R R., Henriquez-Roldan, C., gado-Chavez, R., Calderon-Garciduenas, A., and Dragustinovis, I et al., Brain inflammation and Alzheimer’s-like pathology in individuals exposed to severe air pollution Toxicol Pathol., 32 2004

Campen, M J., Nolan, J P., Schladweiler, M C., Kodavanti, U P., Evansky, P A., Costa, D L., and Watkinson, W P., Cardiovascular and thermoregulatory effects of inhaled PM-associated transi-tion metals: a potential interactransi-tion between nickel and vanadium sulfate, Toxicol Sci., 64, 243–252, 2001

Campen, M J., Nolan, J P., Schladweiler, M C., Kodavanti, U P., Costa, D L., and Watkinson, W P., Cardiac and thermoregulatory effects of instilled particulate matter-associated transition metals in healthy and cardiopulmonary-compromised rats, J Toxicol Environ Health A, 65, 1615–1631, 2002

Particles

Oxidative stress

Oxidative

Cardiovascular disease

Pneumoconiosis

Airways disease

DNA adducts

(8-OH-dG)

FIGURE 22.3 A paradigm describing the events that could occur following the deposition of pathogenic particles

Chen, L C and Nadziejko, C., Effects of subchronic exposures to concentrated ambient particles (CAPs) in mice V CAPs exacerbate aortic plaque development in hyperlipidemic mice, Inhal Toxicol., 17, 217–224, 2005

Donaldson, K and Borm, P J., The quartz hazard: a variable entity 7, Ann Occup Hyg., 42, 287–294, 1998 Donaldson, K and Tran, C L., Inflammation caused by particles and fibers, Inhal Toxicol., 14, 2002 Donaldson, K and Tran, C L., An introduction to the short-term toxicology of respirable industrial fibers, Mutat Res., 553, 5–9, 2004

Donaldson, K., Stone, V., Borm, P J., Jimenez, L A., Gilmour, P S., Schins, R P., and Knaapen, A M et al., Oxidative stress and calcium signaling in the adverse effects of environmental particles (PM(10)), Free Radic Biol Med., 34, 1369–1382, 2003

Donaldson, K., Jimenez, L A., Rahman, I., Faux, S P., MacNee, W., Gilmour, P S., Borm, P J., Schins,

R P F., Shi, T., and Stone, V., Respiratory health effects of ambient air pollution particles: role of reactive species, In Oxygen/Nitrogen Radicals: Lung Injury and Disease, Vallyathan, V., Shi, X., and Castranova, V., eds., Marcel Dekker, New York, 2004a Volk 187 in Lung Biology in Health and Disease, Exec Ed., Lenfant, C

Donaldson, K., Stone, V., Tran, C L., Kreyling, W., and Borm, P J., Nanotoxicology, Occup Environ Med.,

61, 727–728, 2004b

Donaldson, K., Tran, L., Jimenez, L., Duffin, R., Newby, D E., Mills, N., MacNee, W., and Stone, V., Combustion-derived nanoparticles: a review of their toxicology following inhalation exposure, Part Fiber Toxicol., 2, 10, 2005a

Donaldson, K., Tran, L., Jimenez, L A., Duffin, R., Newby, D E., Mills, N., MacNee, W., and Stone, V., Combustion-derived nanoparticles: a review of their toxicology following inhalation exposure, Part Fiber Toxicol., 2, 10, 2005b

Gehr, P., Brand, P., and Heyder, J., Particle deposition in the respiratory tract, In Particle Lung Cell Interactions, Gehr, P and Heyder, J., eds., Vol 143, Marcel Dekker, New York, 229–322, 2000 Lung Biology in Health and Disease, Executive Editor, Lenfant, C., Ref Type: Generic

Hoet, P H., Nemmar, A., and Nemery, B., Health impact of nanomaterials? 12, Nat Biotechnol., 22, 19, 2004 Hussain, S M., Hess, K L., Gearhart, J M., Geiss, K T., and Schlager, J J., In vitro toxicity of nanoparticles in BRL 3A rat liver cells 1, Toxicol In Vitro, 19, 975–983, 2005

Jimenez, L A., Thompson, J., Brown, D A., Rahman, I., Antonicelli, F., Duffin, R., Drost, E M., Hay, R T., Donaldson, K., and MacNee, W., Activation of NF-kappaB by PM(10) occurs via an iron-mediated mechanism in the absence of IkappaB degradation, Toxicol Appl Pharmacol., 166, 101–110, 2000 Knaapen, A M., Albrecht, C., Becker, A., Hohr, D., Winzer, A., Haenen, G R., Borm, P J., and Schins, R P., DNA damage in lung epithelial cells isolated from rats exposed to quartz: role of surface reactivity and neutrophilic inflammation, Carcinogenesis, 23, 1111–1120, 2002

Knaapen, A M., Borm, P J., Albrecht, C., and Schins, R P., Inhaled particles and lung cancer Part A: mechanisms, Int J Cancer, 109, 799–809, 2004

Kreuter, J., Shamenkov, D., Petrov, V., Ramge, P., Cychutek, K., Koch-Brandt, C., and Alyautdin, R., Apolipoprotein-mediated transport of nanoparticle-bound drugs across the blood–brain barrier,

J Drug Target., 10, 317–325, 2002

Kreyling, W., Semmler, M., Erbe, F., Mayer, P., Takenaka, S., Oberdo¨rster, G., and Ziesenis, A., Minute translocation of inhaled ultrafine insoluble iridium particles from lung epithelium to extrapulmonary tissues, Ann Occup Hyg., 46 (suppl.1), 223–226, 2002a

Kreyling W G., Semmler M., Erbe F., Mayer P., Takenaka, S., Shculz H., Oberdo¨rster, G., and Ziesenis A., Ultrafine insoluble iridium particles are negligibly translocated from the lung epithelium to extra-pulmonary organs, 2002b

Lloyd, D R., Carmichael, P L., and Phillips, D H., Comparison of the formation of 8-hydroxy-20 -deoxygua-nosine and single- and double-strand breaks in DNA mediated by fenton reactions, Chem Res Toxicol., 11, 420–427, 1998

Lund, L G and Aust, A E., Iron-catalyzed reactions may be responsible for the biochemical and biological effects of asbestos 7, Biofactors, 3, 83–89, 1991

Maeng, S H., Chung, H W., Yu, I J., Kim, H Y., Lim, C H., Kim, K J., Kim, S J., Ootsuyama, Y., and Kasai, H., Changes of 8-OH-dG levels in DNA and its base excision repair activity in rat lungs after inhalation exposure to hexavalent chromium, Mutat Res., 539, 109–116, 2003