The Toxicologist - Society of Toxicology

847 ROBUST CHARACTERIZATION OF NANOMATERIALS
IS NECESSARY BEFORE TOXICITY
STUDIES/ASSESSMENT CAN BE UNDERTAKEN.
D. B. Warheit. DuPont Haskell Global Centers, Newark, DE.
Numerous scientific organizations have strongly recommended that investigators
should adequately characterize physicochemical properties of the Nanoparticletypes
that are being evaluated for hazard testing. This is because, in the absence of
a rigorous characterization of the nanoparticle-types being studied, the published
results will have limited value, significance or biological relevance. This is due to 1)
the uncertainty of the specific substance identity of the test substance; and 2) the
inability of others to conduct comparable or confirmatory studies with that particular
nanoparticulate-type. Thus, robust characterization of nanoscale particle-types
is a necessary prerequisite prior to implementing hazard assessments. However, too
often this requirement results in confusion associated with a “laundry list” of numerous
“required”, physicochemical characteristics which may not have adequate
prioritization. In an effort to obviate this relative state of confusion and to provide
some form of pragmatism to this process, it is proposed that, at a minimum, toxicologist
should characterize the following (prioritized) physicochemical properties
prior to conducting hazard studies with nanoparticle-types: Particle size and size
distribution (wet state) and surface area (dry state) in the relevant media being utilized
– with strong consideration of the route of anticipated exposure; Crystal structure;
Aggregation status in the relevant media; Composition/surface coatings;
Surface reactivity; Method of nanomaterial synthesis and/or preparation including
post-synthetic modifications; and Purity of samples. This represents a minimum,
standardized assessment of physiochemical properties that should be analyzed and
documented prior to the development of toxicity testing with nanoparticles. More
importantly, this will permit analyses and comparisons of studies with similar or
identical particulate-types and contribute to the coherence of the nanoparticle hazard
database.
848 MINIMUM MATERIAL CHARACTERIZATIONS FOR
NANOTOXICOLOGY STUDIES: A NECESSITY OR A
NUISANCE?
N. J. Walker. National Toxicology Program, NIEHS, Research Triangle Park, NC.
Nanoscale materials (nanomaterials, nanoparticles), are a broadly defined set of
substances that have at least one critical dimension less than 100 nanometers and
possess unique optical, magnetic, or electrical properties. The unique and diverse
physicochemical properties of nanoscale materials suggest that toxicological properties
may differ from materials of similar composition but different size. Ongoing research
is showing that decreasing particle size below barrier cutoffs for portals of
entry can lead to new unintended routes of exposures. In addition, once internalized,
surface-based interactions can have profound impacts on the kinetics and biodistribution
of materials of similar size and shape leading to large differences in target
organ dosimetry. Finally, for some nanomaterials, “dose” for certain responses
can scale with a size-dependent property such as surface area, thus assessments of
relative risk based on mass-based dose may lead to erroneous assertions of relative
risks. Because of the need to understand how the physical and chemical aspects of a
given nanomaterial impact these issues, its has been suggested that a specified minimum
set of nanomaterial characteristics should be required for all papers that are
published on nanomaterials. However what that minimum set of characterizations
should be, in what context they should be applied and whether they should be a applied
or are appropriate for a specific nanomaterial are still questions under debate.
849 THE SELECTION AND CHARACTERIZATION OF
NANOMATERIALS FOR DEVELOPING
TOXICOLOGICAL QSARS.
S. Oldenburg 1 , A. Neigh 1 , O. Nguyen 1 and S. Ali 2 . 1 nanoComposix, San Diego,
CA and 2 NCTR/FDA, Jefferson, AR.
The number and variety of nanomaterials that pose an exposure risk is rapidly increasing.
Since it is not possible to perform in-depth toxicity studies on each of
these emerging nanomaterials, it is important to identify the physical and chemical
properties of nanoparticulates that are linked to increased toxicity in biological systems.
A silver nanoparticle panel has been developed that contains over 80 different
nanoparticle variants with precisely controlled size, shape, charge and surface. The
particles are unagglomerated, concentrated and purified from residual reactants. By
performing experiments where only a single nanoparticle characteristic is altered,
toxicological quantitative structure activity relationships (QSARs) can be determined
in the model biological system. Initial studies with this panel have demon-
182 SOT 2011 ANNUAL MEETING
strated that both the EC50 minimum inhibition concentration for E. coli as well as
cytotoxicity in RAW 264.7 cells correlates strongly with surface area across
nanoparticle sizes that range from 20 to 100 nm. However, there is only a small
subset of material systems where the nanoparticle characteristics can be so precisely
controlled. For the other material systems, the nanoparticle properties are dictated
by manufacturing methods. Strategies for generating nanoparticles sets made from
less controllable material systems that are still useful for linking toxicity to physical
and chemical properties will be presented.
850 DOSIMETRIC CONSIDERATIONS FOR EXPLORING THE
CYTOTOXICITY OF SILICA NANOPARTICLES IN VITRO.
D. F. Lison 1 , V. Rabolli 1 and L. Thomassen 2 . 1 Louvain Centre for Toxicology and
Applied Pharmacology, Brussels, Belgium and 2 Centrum voor Oppervlaktechemie en
Katalyse, Katholieke Universiteit Leuven, Leuven, Belgium.
There is currently a strong pressure to develop reliable methods allowing to assess
the health hazards of nanomaterials. In vitro tests are essential tools for this purpose.
However, the use of cell culture models poses some specific challenges when
applied to nanomaterials. To address some of these issues, we used a set of amorphous
silica nanoparticles (NP, 2-335 nm) as a model and examined (1) how to express
the dose of NP in cytotoxicity assays, (2) the influence of the physicochemical
characteristics of NP on cytotoxicity and (3) the role of NP aggregation in the cytotoxic
response. Several cell lines and erythrocytes were used in these studies.
We found that, despite their limited diffusion and/or gravitational sedimentation
rate in culture medium, all suspended silica NP reach the cells at the bottom of the
culture well and contribute to the cytotoxic response. Using the nominal dose to design
and/or report the results of in vitro studies with NP is, therefore, appropriate.
The response to these silica NP is governed by different physicochemical parameters
which vary with cell type : in J774 cells, the cytotoxic activity increases with external
surface area and decreases with micropore volume; in EAHY926 and 3T3
cells, the cytotoxic activity of the SNP increases with surface roughness and small
diameter, and in erythrocytes, the lytic activity increases with the diameter of the
NP. It is, therefore, possible to predict the in vitro cytotoxic potential of silica NP
with good accuracy on the basis of their physicochemical characteristics. These determinants
are, however, complex and vary with cell type.
Finally, we generated stable aggregates of these silica NP with increasing size but
no change in specific surface area. We found in J774 and 3T3 cells that the degree
of aggregation did not significantly modify the cytotoxic response, which is consistent
with the major role of surface area in governing the cytotoxic response to insoluble
NP.
851 ENABLING PREDICTION AND EXTRAPOLATION: A
NEW PARADIGM FOR NANOMATERIAL DOSIMETRY
IN VITRO.
J. G. Teeguarden 1 , P. M. Hinderliter 2 , G. Orr 3 , K. R. Minard 2 , B. D. Thrall 2
and J. G. Pounds 2 . 1 Biological Monitoring and Modeling, Pacific Northwest
National Laboratory, Richland, WA, 2 Biological Sciences, Pacific Northwest National
Laboratory, Richland, WA and 3 Environmental and Molecular Sciences Laboratory,
Pacific Northwest National Laboratory, Richland, WA.
Current practice in in vitro nanomaterial toxicity studies is to ignore the issue of
cellar dose in favor of measures of exposure (media concentration). While appealing
for their simplicity and ease of calculation, use of exposure metrics such as mass
media concentration (μg/ml) introduce significant errors the hazard assessment
process and obstruct extrapolation of in vitro findings equivalent in vivo exposure
levels for risk assessment purposes. Like particles and chemicals in vivo, nanoparticles
and their larger relatives have unique kinetic processes in vitro. Particles diffuse
and sediment at rates that depend on their size, density and agglomeration state, as
well as the properties of the media. Thus, the dose-rate for all particles are not
equal, but can vary significantly, in direct contrast to the assumption of dose-equivalency
implicit in the use of mass based media concentrations as metrics of exposure
for dose-response assessment. Accounting for particle dependent dosimetry either
by computational kinetics or direct measurement would overcome this major barrier
to effective application of in vitro studies for nanomaterials. Most importantly,
replacing exposure measures with measures of target cell dose would bring the field
in line with current practice for chemicals and particles and enable extrapolation of
in vitro findings to in vivo exposures. This talk will present a new, flexible paradigm
for nanomaterial dosimetry in vitro which is consistent with current practice for
chemical risk assessment, reflects particokinetics in vitro, and can be used to extrapolate
accurately between in vitro systems, across particle types and from in vitro
to in vivo.