Organ burden of inhaled nanoceria in a 2-year low-dose exposure study: dump or depot?

Nanoceria: an innovative strategy for cancer treatment

Nanoceria or cerium oxide nanoparticles characterised by the co-existing of Ce³⁺ and Ce⁴⁺ that allows self-regenerative, redox-responsive dual-catalytic activities, have attracted interest as an innovative approach to treating cancer. Depending on surface characteristics and immediate environment, nanoceria exerts either anti- or pro-oxidative effects which regulate reactive oxygen species (ROS) levels in biological systems. Nanoceria mimics ROS-related enzymes that protect normal cells at physiological pH from oxidative stress and induce ROS production in the slightly acidic tumour microenvironment to trigger cancer cell death. Nanoceria as nanozymes also generates molecular oxygen that relieves tumour hypoxia, leading to tumour cell sensitisation to improve therapeutic outcomes of photodynamic (PDT), photothermal (PTT) and radiation (RT), targeted and chemotherapies. Nanoceria has been engineered as a nanocarrier to improve drug delivery or in combination with other drugs to produce synergistic anti-cancer effects. Despite reported preclinical successes, there are still knowledge gaps arising from the inadequate number of studies reporting findings based on physiologically relevant disease models that accurately represent the complexities of cancer. This review discusses the dual-catalytic activities of nanoceria responding to pH and oxygen tension gradient in tumour microenvironment, highlights the recent nanoceria-based platforms reported to be feasible direct and indirect anti-cancer agents with protective effects on healthy tissues, and finally addresses the challenges in clinical translation of nanoceria based therapeutics.

Comparison of biodistribution of cerium oxide nanoparticles after repeated oral administration by gavage or snack in Sprague Dawley rats

The rate of translocation of ingested nanoparticles (NPs) and how the uptake is affected by a food matrix are key aspects of health risk assessment. In this study, female Sprague Dawley rats (N = 4/group) received 0, 1.4, or 13 mg of cerium oxide (CeO₂ NM-212) NPs/rat/day by gavage or in a chocolate spread snack 5 days/week for 1 or 2 weeks followed by 2 weeks of recovery. A dose and time-dependent uptake in the liver and spleen of 0.1-0.3 and 0.004-0.005 parts per million (ng/mg) of the total administered dose was found, respectively. There was no statistically significant difference in cerium concentration in the liver or spleen after gavage compared to snack dosing. Microscopy revealed indications of necrotic changes in the liver and decreased cellularity in white pulp in the spleen. The snack provided precise administration and a more human-relevant exposure of NPs and could improve animal welfare as alternative to gavage.

Nanosafety: An Evolving Concept to Bring the Safest Possible Nanomaterials to Society and Environment

The use of nanomaterials has been increasing in recent times, and they are widely used in industries such as cosmetics, drugs, food, water treatment, and agriculture. The rapid development of new nanomaterials demands a set of approaches to evaluate the potential toxicity and risks related to them. In this regard, nanosafety has been using and adapting already existing methods (toxicological approach), but the unique characteristics of nanomaterials demand new approaches (nanotoxicology) to fully understand the potential toxicity, immunotoxicity, and (epi)genotoxicity. In addition, new technologies, such as organs-on-chips and sophisticated sensors, are under development and/or adaptation. All the information generated is used to develop new in silico approaches trying to predict the potential effects of newly developed materials. The overall evaluation of nanomaterials from their production to their final disposal chain is completed using the life cycle assessment (LCA), which is becoming an important element of nanosafety considering sustainability and environmental impact. In this review, we give an overview of all these elements of nanosafety.

LA-ICP-MS and Immunohistochemical Staining with Lanthanide-Labeled Antibodies to Study the Uptake of CeO2 Nanoparticles by Macrophages in Tissue Sections

Due to the increasing use and production of CeO₂ nanoparticles (NPs), the likelihood of exposure especially via the air rapidly grows. However, the uptake of CeO₂ NPs via the lung and the resulting distribution into various cell types of remote organs are not well understood because classical analytical methods provide limited spatial information. In this study, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was combined with immunohistochemical (IHC) staining with lanthanide-labeled antibodies to investigate the distribution of intratracheally instilled CeO₂ NPs from the rat lung to lymph nodes, spleen, and liver after 3 h, 3 days, and 21 days. We selected regions of interest after fast imaging using LA-ICP-MS in low-resolution mode and conducted high-resolution LA-ICP-MS in combination with IHC for cellular localization. The lanthanide labeling, which was largely congruent with conventional fluorescent labeling, allowed us to calculate the association rates of Ce to specific cell types. Major portions of Ce were found to be associated with phagocytic cells in the lung, lymph nodes, spleen, and liver. In the lung, almost 94% of the Ce was co-localized with CD68-positive alveolar macrophages after 21 days. Ce was also detected in the lymph nodes outside macrophages 3 h post instillation but shifted to macrophage-associated locations. In the liver, Ce accumulations associated with Kupffer cells (CD163-positive) were found. Ce-containing populations of metallophilic and marginal zone macrophages (both CD169-positive) as well as red pulp macrophages (CD68-positive) were identified as major targets in the spleen. Overall, high-resolution LA-ICP-MS analysis in combination with IHC staining with lanthanide-labeled antibodies is a suitable tool to quantify and localize Ce associated with specific cell types and to estimate their particle burden under in vivo conditions.

The Road to Achieving the European Commission's Chemicals Strategy for Nanomaterial Sustainability-A PATROLS Perspective on New Approach Methodologies

The European Green Deal outlines ambitions to build a more sustainable, climate neutral, and circular economy by 2050. To achieve this, the European Commission has published the Chemicals Strategy for Sustainability: Towards a Toxic-Free Environment, which provides targets for innovation to better protect human and environmental health, including challenges posed by hazardous chemicals and animal testing. The European project PATROLS (Physiologically Anchored Tools for Realistic nanOmateriaL hazard aSsessment) has addressed multiple aspects of the Chemicals Strategy for Sustainability by establishing a battery of new approach methodologies, including physiologically anchored human and environmental hazard assessment tools to evaluate the safety of engineered nanomaterials. PATROLS has delivered and improved innovative tools to support regulatory decision-making processes. These tools also support the need for reducing regulated vertebrate animal testing; when used at an early stage of the innovation pipeline, the PATROLS tools facilitate the safe and sustainable development of new nano-enabled products before they reach the market.

Determination of metallic nanoparticles in biological samples by single particle ICP-MS: a systematic review from sample collection to analysis

A systematic review of the use of single particle ICP-MS to analyse engineered nanomaterials (ENMs) in biological samples (plants, animals, body fluids) has highlighted that efforts have focused on a select few types of ENMs (e.g., Ag and TiO₂) and there is a lack of information for some important tissues (e.g., reproductive organs, skin and fatty endocrine organs). The importance of sample storage is often overlooked but plays a critical role. Careful consideration of the ENM and matrix composition is required to select an appropriate protocol to liberate ENMs from a tissue whilst not promoting the transformation of them, or genesis of new particulates. A 'one size fits all' protocol, applicable to all possible types of ENM and biological matrices, does not seem practical. However, alkaline-based extractions would appear to show greater promise for wide applicability to animal tissues, although enzymatic approaches have a role, especially for plant tissues. There is a lack of consistency in metrics reported and how they are determined (e.g. size limit of detection, and proportions of recovery), making comparison between some studies more difficult. In order to establish standardised protocols for regulatory use, effort is needed to: develop certified reference materials, achieve international agree on nomenclature and the use of control samples, and to create a decision tree to help select the best sample preparation for the type of tissue matrix.

An Integrated Approach to Testing and Assessment to Support Grouping and Read-Across of Nanomaterials After Inhalation Exposure

Introduction: Here, we describe the generation of hypotheses for grouping nanoforms (NFs) after inhalation exposure and the tailored Integrated Approaches to Testing and Assessment (IATA) with which each specific hypothesis can be tested. This is part of a state-of-the-art framework to support the hypothesis-driven grouping and read-across of NFs, as developed by the EU-funded Horizon 2020 project GRACIOUS.

Development of Grouping Hypotheses and IATA: Respirable NFs, depending on their physicochemical properties, may dissolve either in lung lining fluid or in acidic lysosomal fluid after uptake by cells. Alternatively, NFs may also persist in particulate form. Dissolution in the lung is, therefore, a decisive factor for the toxicokinetics of NFs. This has led to the development of four hypotheses, broadly grouping NFs as instantaneous, quickly, gradually, and very slowly dissolving NFs. For instantaneously dissolving NFs, hazard information can be derived by read-across from the ions. For quickly dissolving particles, as accumulation of particles is not expected, ion toxicity will drive the toxic profile. However, the particle aspect influences the location of the ion release. For gradually dissolving and very slowly dissolving NFs, particle-driven toxicity is of concern. These NFs may be grouped by their reactivity and inflammation potency. The hypotheses are substantiated by a tailored IATA, which describes the minimum information and laboratory assessments of NFs under investigation required to justify grouping.

Conclusion: The GRACIOUS hypotheses and tailored IATA for respiratory toxicity of inhaled NFs can be used to support decision making regarding Safe(r)-by-Design product development or adoption of precautionary measures to mitigate potential risks. It can also be used to support read-across of adverse effects such as pulmonary inflammation and subsequent downstream effects such as lung fibrosis and lung tumor formation after long-term exposure.

Creating sets of similar nanoforms with the ECETOC NanoApp: real-life case studies

The ECETOC NanoApp was developed to support industry in the registration of sets of nanoforms, as well as regulators in the evaluation of these registration dossiers. The ECETOC NanoApp uses a systematic approach to create and justify sets of similar nanoforms, following the ECHA guidance in a transparent and evidence-based manner. The rational and decision rules behind the ECETOC NanoApp are described in detail in "Janer, G., R. Landsiedel, and W. Wohlleben. 2021. [Rationale and Decision Rules Behind the ECETOC NanoApp to Support Registration of Sets of Similar Nanoforms within REACH. Nanotoxicology 15 (2): 145-122. https://doi.org/10.1080/17435390.2020.1842933]". The decision criteria apply to human health and environmental hazards and risks. Here, we focus mostly on human health hazards; the decision rules are applied to a series of case studies, each consisting of real nanoforms: two barium sulfate nanoforms, four colloidal silica nanoforms, eight ceria nanoforms, and four copper phthalocyanine nanoforms. For each of them, we show step by step how the ECETOC NanoApp rules are applied. The cases include nanoforms that are justified as members of the same set of similar nanoforms based on sufficient similarity of their intrinsic properties (Tier 1). They also include other nanoforms with a relatively high (but insufficient) similarity of intrinsic properties; their similarity could be justified by functional properties (Tier 2). The case studies also include nanoforms that are concluded not to belong to the same set of similar nanoforms. These outcomes of the NanoApp were overall consistent (sometimes conservative) with available in vivo data. We also noted that datasets for various nanoforms were limited and use of the NanoApp may require the generation of data relevant to the decision criteria.

Combinatory Effects of Cerium Dioxide Nanoparticles and Acetaminophen on the Liver-A Case Study of Low-Dose Interactions in Human HuH-7 Cells

The interactions between pharmaceuticals and nanomaterials and its potentially resulting toxicological effects in living systems are only insufficiently investigated. In this study, two model compounds, acetaminophen, a pharmaceutical, and cerium dioxide, a manufactured nanomaterial, were investigated in combination and individually. Upon inhalation, cerium dioxide nanomaterials were shown to systemically translocate into other organs, such as the liver. Therefore we picked the human liver cell line HuH-7 cells as an in vitro system to investigate liver toxicity. Possible synergistic or antagonistic metabolic changes after co-exposure scenarios were investigated. Toxicological data of the water soluble tetrazolium (WST-1) assay for cell proliferation and genotoxicity assessment using the Comet assay were combined with an untargeted as well as a targeted lipidomics approach. We found an attenuated cytotoxicity and an altered metabolic profile in co-exposure experiments with cerium dioxide, indicating an interaction of both compounds at these endpoints. Single exposure against cerium dioxide showed a genotoxic effect in the Comet assay. Conversely, acetaminophen exhibited no genotoxic effect. Comet assay data do not indicate an enhancement of genotoxicity after co-exposure. The results obtained in this study highlight the advantage of investigating co-exposure scenarios, especially for bioactive substances.

A Complete In Vitro Toxicological Assessment of the Biological Effects of Cerium Oxide Nanoparticles: From Acute Toxicity to Multi-Dose Subchronic Cytotoxicity Study

Engineered nanomaterials (ENMs) are of significant relevance due to their unique properties, which have been exploited for widespread applications. Cerium oxide nanoparticles (CeO₂-NPs) are one of most exploited ENM in the industry due to their excellent catalytic and multi-enzyme mimetic properties. Thus, the toxicological effects of these ENMs should be further studied. In this study, the acute and subchronic toxicity of CeO₂-NPs were assessed. First, an in vitro multi-dose short-term (24 h) toxicological assessment was performed in three different cell lines: A549 and Calu3 were used to represented lung tissue and 3T3 was used as an interstitial tissue model. After that, a sub-chronic toxicity assessment (90 days) of these NPs was carried out on a realistic and well-established reconstituted primary human airway epithelial model (MucilAir™), cultured at the Air–Liquid Interface (ALI), to study the long-term effects of these particles. Results showed minor toxicity of CeO₂-NPs in acute exposures. However, in subchronic exposures, cytotoxic and inflammatory responses were observed in the human airway epithelial model after 60 days of exposure to CeO₂-NPs. These results suggest that acute toxicity approaches may underestimate the toxicological effect of some ENMs, highlighting the need for subchronic toxicological studies in order to accurately assess the toxicity of ENM and their cumulative effects in organisms.

The preparation temperature influences the physicochemical nature and activity of nanoceria

Cerium oxide nanoparticles, so-called nanoceria, are engineered nanomaterials prepared by many methods that result in products with varying physicochemical properties and applications. Those used industrially are often calcined, an example is NM-212. Other nanoceria have beneficial pharmaceutical properties and are often prepared by solvothermal synthesis. Solvothermally synthesized nanoceria dissolve in acidic environments, accelerated by carboxylic acids. NM-212 dissolution has been reported to be minimal. To gain insight into the role of high-temperature exposure on nanoceria dissolution, product susceptibility to carboxylic acid-accelerated dissolution, and its effect on biological and catalytic properties of nanoceria, the dissolution of NM-212, a solvothermally synthesized nanoceria material, and a calcined form of the solvothermally synthesized nanoceria material (ca. 40, 4, and 40 nm diameter, respectively) was investigated. Two dissolution methods were employed. Dissolution of NM-212 and the calcined nanoceria was much slower than that of the non-calcined form. The decreased solubility was attributed to an increased amount of surface Ce4+ species induced by the high temperature. Carboxylic acids doubled the very low dissolution rate of NM-212. Nanoceria dissolution releases Ce3+ ions, which, with phosphate, form insoluble cerium phosphate in vivo. The addition of immobilized phosphates did not accelerate nanoceria dissolution, suggesting that the Ce3+ ion release during nanoceria dissolution was phosphate-independent. Smaller particles resulting from partial nanoceria dissolution led to less cellular protein carbonyl formation, attributed to an increased amount of surface Ce3+ species. Surface reactivity was greater for the solvothermally synthesized nanoceria, which had more Ce3+ species at the surface. The results show that temperature treatment of nanoceria can produce significant differences in solubility and surface cerium valence, which affect the biological and catalytic properties of nanoceria.

The Use of Nanomaterial In Vivo Organ Burden Data for In Vitro Dose Setting

Effects of nanomaterials are usually observed at higher concentrations in vitro compared to animal studies. This is pointing to differences between in vivo situations and generally less complex in vitro models. These differences concern toxicodynamics and the internal exposure (at the target cells of the in vitro and in vivo test system). The latter can be minimized by appropriate in vivo to in vitro dose extrapolations (IVIVE). An IVIVE six-step procedure is proposed here: 1) determine in vivo exposure; 2) identify in vivo organ burden at lowest observed adverse effect concentration; 3) extrapolate in vivo organ burden to in vitro effective dose; 4) extrapolate in vitro effective dose to nominal concentration; 5) set dose ranges to establish dose-response relationships; and 6) consider uncertainties and specificities of in vitro test system. Assessing the results of in vitro studies needs careful consideration of discrepancies between in vitro and in vivo models: apart from different endpoints (usually cellular responses in vitro and adverse effects on organs or organisms in vivo), nanomaterials can also have a different potency in relatively simple in vitro models and the more complex corresponding organ in vivo. IVIVE can, nonetheless, reduce the differences in exposures.

Coagulation disorders following an accidental ingestion of cerium dioxide nanoparticles

Clinically, acute cerium dioxide poisoning or damage is very rare. Here, the investigator reported a case about the patient who accidental ingested cerium dioxide nanoparticles during work shift. The patient recovered well after the treatment with removal of cerium and restoring coagulation factor activity. The author also reviewed relative literatures to discuss the potential mechanism of coagulation disorders following cerium dioxide nanoparticles ingestion. This case is the first report in the world about acute oral poisoning due to cerium dioxide nanoparticles, with the exact exposure concentration.

Rationale and decision rules behind the ECETOC NanoApp to support registration of sets of similar nanoforms within REACH

New registration requirements for nanomaterials under REACH consider the possibility to form 'sets of similar nanoforms' for a joined human health and environmental hazard, exposure and risk assessment. We developed a tool to create and justify sets of similar nanoforms and to ensure that each of the nanoforms is sufficiently similar to all other nanoforms. The decision logic is following the ECHA guidance in a transparent and evidence-based manner. For each two nanoforms the properties under consideration are compared and corresponding thresholds for maximal differences are proposed. In tier1, similarity is assessed based on intrinsic properties that mostly correspond to those required for nanoform identification under REACH: composition, impurities/additives, size, crystallinity, shape and surface treatment. Moreover, potential differences in the agglomeration/aggregation state resulting from different production processes are considered. If nanoforms were not sufficiently similar based on tier1 criteria, additional data from functional assays are required in tier2. In rare cases, additional short-term in vivo rodent data could be required in a third tier. Data required by tier 2 are triggered by the intrinsic properties in the first tier that did not match the similarity criteria. Most often this will be data on dissolution and surface reactivity followed by in vitro toxicity, dispersion stability, dustiness. Out of several nanoforms given by the user, the tool concludes which nanoforms could be justified to be in the same set and which nanoforms are outside. It defines the boundaries of sets of similar nanoforms and generates a justification for the REACH registration.

Parametric Optimization of an Air-Liquid Interface System for Flow-Through Inhalation Exposure to Nanoparticles: Assessing Dosimetry and Intracellular Uptake of CeO2 Nanoparticles

Air-liquid interface (ALI) systems have been widely used in recent years to investigate the inhalation toxicity of many gaseous compounds, chemicals, and nanomaterials and represent an emerging and promising in vitro method to supplement in vivo studies. ALI exposure reflects the physiological conditions of the deep lung more closely to subacute in vivo inhalation scenarios compared to submerged exposure. The comparability of the toxicological results obtained from in vivo and in vitro inhalation data is still challenging. The robustness of ALI exposure scenarios is not yet well understood, but critical for the potential standardization of these methods. We report a cause-and-effect (C&E) analysis of a flow through ALI exposure system. The influence of five different instrumental and physiological parameters affecting cell viability and exposure parameters of a human lung cell line in vitro (exposure duration, relative humidity, temperature, CO2 concentration and flow rate) was investigated. After exposing lung epithelia cells to a CeO2 nanoparticle (NP) aerosol, intracellular CeO2 concentrations reached values similar to those found in a recent subacute rat inhalation study in vivo. This is the first study showing that the NP concentration reached in vitro using a flow through ALI system were the same as those in an in vivo study.