Genotoxicity evaluation of titanium dioxide nanoparticles using the Ames test and Comet assay

Potential Artifacts and Control Experiments in Toxicity Tests of Nanoplastic and Microplastic Particles

To fully understand the potential ecological and human health risks from nanoplastics and microplastics (NMPs) in the environment, it is critical to make accurate measurements. Similar to past research on the toxicology of engineered nanomaterials, a broad range of measurement artifacts and biases are possible when testing their potential toxicity. For example, antimicrobials and surfactants may be present in commercially available NMP dispersions, and these compounds may account for toxicity observed instead of being caused by exposure to the NMP particles. Therefore, control measurements are needed to assess potential artifacts, and revisions to the protocol may be needed to eliminate or reduce the artifacts. In this paper, we comprehensively review and suggest a next generation of control experiments to identify measurement artifacts and biases that can occur while performing NMP toxicity experiments. This review covers the broad range of potential NMP toxicological experiments, such as in vitro studies with a single cell type or complex 3-D tissue constructs, in vivo mammalian studies, and ecotoxicity experiments testing pelagic, sediment, and soil organisms. Incorporation of these control experiments can reduce the likelihood of false positive and false negative results and more accurately elucidate the potential ecological and human health risks of NMPs.

Effects of Particle Size and Surface Charge on Mutagenicity and Chicken Embryonic Toxicity of New Silver Nanoclusters

Though there are many toxicological studies on metal nanoparticles (NPs), it remains difficult to explain discrepancies observed between studies, largely due to the lack of positive controls and disconnection between physicochemical properties of nanomaterials with their toxicities at feasible exposures in a specified test system. In this study, we investigated effects of particle size and surface charge on in vitro mutagenic response and in vivo embryonic toxicity for newly synthesized silver nanoclusters (AgNCs) at human or environmental relevant exposure and compared the new findings with one of the most common nanoscale particles, titanium dioxide NPs (TiO₂ NPs as a positive control). We hypothesized that the interaction of the test system and physicochemical properties of nanomaterials are critical in determining their toxicities at concentrations relevant with human or environmental exposures. We assessed the mutagenicity of the AgNCs (around 2 nm) and two sizes of TiO₂ NPs (i.e., small: 5-15 nm, big: 30-50 nm) using a Salmonella reverse mutation assay (Ames test). The smallest size of AgNCs showed the highest mutagenic activity with the Salmonella strain TA100 in the absence and presence of the S9 mixture, because the AgNCs maintained the nano-size scale in the Ames test, compared with two other NPs. For TiO₂ NPs, the size effect was interfered by the agglomeration of TiO₂ NPs in media and the generation of oxidative stress from the NPs. The embryonic toxicity and the liver oxidative stress were evaluated using a chicken embryo model at three doses (0.03, 0.33, and 3.3 μg/g egg), with adverse effects on chicken embryonic development in both sizes of TiO₂ NPs. The non-monotonic response was determined for developmental toxicity for the tested NPs. Our data on AgNCs was different from previous findings on AgNPs. The chicken embryo results showed some size dependency of nanomaterials, but they were more well correlated with lipid peroxidation (malondialdehyde) in chicken fetal livers. A different level of agglomeration of TiO₂ NPs and AgNCs was observed in the assay media of Ames and chicken embryo tests. These results suggest that the test nanotoxicities are greatly impacted by the experimental conditions and the nanoparticle's size and surface charge.

Studies on safety and efficacy of particles containing a mixture of hydroxyapatite–argentum–titanium oxide (HAT) and sheets coated with HAT particles to be used in masks to improve nasal allergy: II. Cellular, in vivo, and clinical studies

Purpose

We report the manufacture of particles containing a mixture of hydroxyapatite–argentum–titanium oxide (HAT), followed by attachment to nonwoven polyester fabrics to produce HAT-coated sheets (HATS) for use in masks. The purpose of the present study was to perform cellular, in vivo, and clinical studies to further examine the safety of HATS for use in masks to improve nasal allergy.

Methods

Reverse mutation tests for HAT were performed using five bacterial strains. A cellular toxicity test was performed using a Chinese hamster cell line incubated with the HATS extracts. Skin reactions after intradermal administration were examined in rabbits. Skin sensitization tests in guinea pigs were performed using the HATS extracts. HAT was administered to the nasal cavity and conjunctival sac of the rabbits. An oral administration study was performed in rats. Finally, a human skin patch test was performed using the HATS.

Results

Reverse mutation tests showed negative results. The cellular toxicity test showed that the HATS extract had moderate cytotoxicity. The intradermal skin reaction and skin sensitization tests were all negative. The administration of HAT to the nasal cavity and intraocular administration showed negative results. No toxicity was observed after oral administration of HAT powder up to a dose of 2000 mg/kg. Finally, the skin patch test result was negative.

Conclusion

Although HAT showed moderate cytotoxicity, in vivo results indicated that HAT is safe because it does not come in direct contact with cells in normal usage, and HATS is safe when used in masks.

Genotoxicity of aluminium oxide, iron oxide, and copper nanoparticles in mouse bone marrow cells

The aim of this study was to evaluate the genotoxic effects of Al₂O₃, Fe₂O₃, and Cu nanoparticles with chromosomal aberration (CA), micronucleus (MN), and comet assays on the bone marrow of male BALB/c mice. Three doses of Al₂O₃, Fe₂O₃ (75, 150, and 300 mg/kg), or Cu (5, 10, and 15 mg/kg) nanoparticles were administered to mice through intraperitoneal injection once a day for 14 days and compared with negative control (distilled water) and positive control (mitomycin C and methyl methanesulphonate). Al₂O₃ and Fe₂O₃ did not show genotoxic effects, but Cu nanoparticles induced significant (P<0.05) genotoxicity at the highest concentration compared to negative control. Our findings add to the health risk information of Al₂O₃, Fe₂O₃, and Cu nanoparticles regarding human exposure (occupational and/or through consumer products or medical treatment), and may provide regulatory reference for safe use of these nanoparticles. However, before they can be used safely and released into the environment further chronic in vivo studies are essential.

Litchi chinensis inspired nanoformulations: a synergy guided approach for unraveling promising cytotoxic attributes of metal and nonmetal conjugates

In present study, diverse Litchi chinensis-mediated nanostructures in combination with 5-fluorouracil drug were fabricated viz. Au, Se, Ag, Ag-Se, Ag-Au, 5-FU Ag-Se and 5-FU Ag-Au with subsequent characterization and scrutinization of their anticarcinogenic capabilities. UV-Visible spectroscopic analysis confirmed the state transition for each precursor salt. XRD and transmission electron microscopy analysis revealed spherical/quasispherical nanostructures with monoclinic crystalline organization ranged between 18 nm and 38 nm. FTIR analysis revealed fabricated nanoparticles to be capped with various phytoconstituents. DLS and Zeta potential analysis of unloaded and drug-loaded bielemental nanoparticles (BNPs) showed comparatively large hydrodynamic particle size distribution and sufficient stability of nanoparticles. BNPs showed promising lethality concentrations for brine shrimp (LC50 < 2 μg/ml) and antitumor (LC50 < 10 μg/ml) assessments. These findings were in positive correlation with the antioxidant inhibitory concentrations IC50 (74.2-180.1 μg/ml) of the tested entities. Ag-Se and Ag-Au were loaded with 5-FU (loading efficiency of 47% ± 1.14 and 25% ± 0.32, respectively) in light of their promising cytotoxic actions. All nanostructures showed profound hemocompatibility with maximum hemolytic activity as low as 2.4%. Highly significant difference (P < 0.01) was observed in antineoplastic potentials of unloaded and 5-FU loaded BNPs against HepG2 and HT144, with most substantial IC50 for 5-FU Ag-Au (8.95 ± 2.86 μg/ml). 5-FU Ag-Au was identified as a significant inducer of DNA fragmentation with maximum relative tail moment (HepG2: 3.45 ± 0.21) among all treatments.

U.S. Federal Agency interests and key considerations for new approach methodologies for nanomaterials

Engineered nanomaterials (ENMs) come in a wide array of shapes, sizes, surface coatings, and compositions, and often possess novel or enhanced properties compared to larger sized particles of the same elemental composition. To ensure the safe commercialization of products containing ENMs, it is important to thoroughly understand their potential risks. Given that ENMs can be created in an almost infinite number of variations, it is not feasible to conduct in vivo testing on each type of ENM. Instead, new approach methodologies (NAMs) such as in vitro or in chemico test methods may be needed, given their capacity for higher throughput testing, lower cost, and ability to provide information on toxicological mechanisms. However, the different behaviors of ENMs compared to dissolved chemicals may challenge safety testing of ENMs using NAMs. In this study, member agencies within the Interagency Coordinating Committee on the Validation of Alternative Methods were queried about what types of ENMs are of agency interest and whether there is agency-specific guidance for ENM toxicity testing. To support the ability of NAMs to provide robust results in ENM testing, two key issues in the usage of NAMs, namely dosimetry and interference/bias controls, are thoroughly discussed.

Genotoxicity Assessment of Metal-Based Nanocomposites Applied in Drug Delivery

Nanocomposites as drug delivery systems (e.g., metal nanoparticles) are being exploited for several applications in the biomedical field, from therapeutics to diagnostics. Green nanocomposites stand for nanoparticles of biocompatible, biodegradable and non-toxic profiles. When using metal nanoparticles for drug delivery, the question of how hazardous these "virus-sized particles" can be is posed, due to their nanometer size range with enhanced reactivity compared to their respective bulk counterparts. These structures exhibit a high risk of being internalized by cells and interacting with the genetic material, with the possibility of inducing DNA damage. The Comet Assay, or Single-Cell Gel Electrophoresis (SCGE), stands out for its capacity to detect DNA strand breaks in eukaryotic cells. It has huge potential in the genotoxicity assessment of nanoparticles and respective cells' interactions. In this review, the Comet assay is described, discussing several examples of its application in the genotoxicity evaluation of nanoparticles commonly administered in a set of routes (oral, skin, inhaled, ocular and parenteral administration). In the nanoparticles boom era, where guidelines for their evaluation are still very limited, it is urgent to ensure their safety, alongside their quality and efficacy. Comet assay or SCGE can be considered an essential tool and a reliable source to achieve a better nanotoxicology assessment of metal nanoparticles used in drug delivery.

An integrated approach for assessing the in vitro and in vivo redox-related effects of nanomaterials

Over the last few decades, nanotechnology has risen to the forefront of both the research and industrial interest, resulting in the manufacture and utilization of various nanomaterials, as well as in their integration into a wide range of fields. However, the consequent elevated exposure to such materials raises serious concerns regarding their effects on human health and safety. Existing scientific data indicate that the induction of oxidative stress, through the excessive generation of Reactive Oxygen Species (ROS), might be the principal mechanism of exerting their toxicity. Meanwhile, a number of nanomaterials exhibit antioxidant properties, either intrinsic or resulting from their functionalization with conventional antioxidants. Considering that their redox properties are implicated in the manifestation of their biological effects, we propose an integrated approach for the assessment of the redox-related activities of nanomaterials at three biological levels (in vitro-cell free systems, cell cultures, in vivo). Towards this direction, a battery of translational biomarkers is recommended, and a series of reliable protocols are presented in detail. The aim of the present approach is to acquire a better understanding with respect to the biological actions of nanomaterials in the interrelated fields of Redox Biology and Toxicology.

Genotoxicity Evaluation of Titanium Dioxide Nanoparticles In Vitro: a Systematic Review of the Literature and Meta-analysis

With the wide use of titanium dioxide nanoparticles (TiO₂-NPs), the genotoxicity of TiO₂-NPs, which is a factor for safety assessment, has attracted people's attention. However, their genotoxic effects in vitro remain controversial due to inconsistent reports. Therefore, a systematic review was conducted followed by a meta-analysis to reveal whether TiO₂-NPs cause genotoxicity in vitro. A total of 59 studies were identified in this review through exhaustive database retrieval and exclusion. Meta-analysis results were presented based on different evaluation methods. The results showed that TiO₂-NP exposure considerably increased the percentage of DNA in tail and olive tail moment in comet assay. Gene mutation assay revealed that TiO₂-NPs could also induce gene mutation. However, TiO₂-NP exposure had no effect on micronucleus (MN) formation in the MN assay. Subgroup analysis showed that normal cells were more vulnerable to toxicity induced by TiO₂-NPs. Moreover, mixed form and small particles of TiO₂-NPs increased the percentage of DNA in tail. In addition, short-term exposure could detect more DNA damage. The size, coating, duration, and concentration of TiO₂-NPs influenced MN formation. This study presented that TiO₂-NP exposure could cause genotoxicity in vitro. The physicochemical properties of TiO₂-NPs and experimental protocols influence the genotoxic effects in vitro. Comet and gene mutation assays may be more sensitive to the detection of TiO₂-NP genotoxic effects.

Safety assessment of titanium dioxide (E171) as a food additive

The present opinion deals with an updated safety assessment of the food additive titanium dioxide (E 171) based on new relevant scientific evidence considered by the Panel to be reliable, including data obtained with TiO2 nanoparticles (NPs) and data from an extended one-generation reproductive toxicity (EOGRT) study. Less than 50% of constituent particles by number in E 171 have a minimum external dimension < 100 nm. In addition, the Panel noted that constituent particles < 30 nm amounted to less than 1% of particles by number. The Panel therefore considered that studies with TiO2 NPs < 30 nm were of limited relevance to the safety assessment of E 171. The Panel concluded that although gastrointestinal absorption of TiO2 particles is low, they may accumulate in the body. Studies on general and organ toxicity did not indicate adverse effects with either E 171 up to a dose of 1,000 mg/kg body weight (bw) per day or with TiO2 NPs (> 30 nm) up to the highest dose tested of 100 mg/kg bw per day. No effects on reproductive and developmental toxicity were observed up to a dose of 1,000 mg E 171/kg bw per day, the highest dose tested in the EOGRT study. However, observations of potential immunotoxicity and inflammation with E 171 and potential neurotoxicity with TiO2 NPs, together with the potential induction of aberrant crypt foci with E 171, may indicate adverse effects. With respect to genotoxicity, the Panel concluded that TiO2 particles have the potential to induce DNA strand breaks and chromosomal damage, but not gene mutations. No clear correlation was observed between the physico-chemical properties of TiO2 particles and the outcome of either in vitro or in vivo genotoxicity assays. A concern for genotoxicity of TiO2 particles that may be present in E 171 could therefore not be ruled out. Several modes of action for the genotoxicity may operate in parallel and the relative contributions of different molecular mechanisms elicited by TiO2 particles are not known. There was uncertainty as to whether a threshold mode of action could be assumed. In addition, a cut-off value for TiO2 particle size with respect to genotoxicity could not be identified. No appropriately designed study was available to investigate the potential carcinogenic effects of TiO2 NPs. Based on all the evidence available, a concern for genotoxicity could not be ruled out, and given the many uncertainties, the Panel concluded that E 171 can no longer be considered as safe when used as a food additive.

Synthesis, characterization, biological activities and molecular docking of Epilobium parviflorum aqueous extract loaded chitosan nanoparticles

Epilobium is a medicinal plant; its extracts are widely used traditional medicine due to their broad range of pharmacological and therapeutic properties. Its most prominent feature is its therapeutic effects on prostatic diseases. The aim of this study is preparation of controlled release system of Epilobium parviflorum, and determination of its potential of anticancer applications. For this purpose, Epilobium parviflorum extract (EPE) loaded chitosan nanoparticles were prepared with ionic gelation method to increase the bioavailability of the extract. The nanoparticles were investigated in terms of size, zeta potential, polydispersity index, encapsulation efficiency, loading capacity and release profile. Besides, scanning electron microscopy (SEM) was used to observe the morphology of the nanoparticles. Moreover, Ames/Salmonella test was used to determine the mutagenicity of EPE, and it was shown that it had no mutagenic effect. It was found that EPE loaded chitosan nanoparticles were with 64.47 nm in average size, 0.168 PdI and 15.2 mV zeta potential. Encapsulation efficiency and loading capacity were found as 92.46% and 8%, respectively. Finally, DNA binding assay and in silico molecular docking studies were performed between EPE and DNA in order to contribute to design of plant based controlled release system for use in cancer therapy.

Toxicologic Evaluation for Amorphous Silica Nanoparticles: Genotoxic and Non-Genotoxic Tumor-Promoting Potential

Amorphous silica nanoparticles (SiO₂NPs) have been widely used in medicine including targeted drug/DNA delivery, cancer therapy, and enzyme immobilization. Nevertheless, SiO₂NPs should be used with caution due to safety concerns associated with unique physical and chemical characteristics. The objective of this study was to determine the effects of SiO₂NPs on genotoxic and non-genotoxic mechanisms associated with abnormal gap junctional intercellular communication (GJIC) in multistage carcinogenesis. The SiO₂NPs exhibited negative responses in standard genotoxicity tests including the Ames test, chromosome aberration assay, and micronucleus assay. In contrast, the SiO₂NPs significantly induced DNA breakage in comet assay. Meanwhile, SiO₂NPs inhibited GJIC based on the results of scrape/loading dye transfer assay for the identification of non-genotoxic tumor-promoting potential. The reduction in expression and plasma membrane localization of Cx43 was detected following SiO₂NP treatment. Particularly, SiO₂NP treatment increased Cx43 phosphorylation state, which was significantly attenuated by inhibitors of extracellular signal-regulated kinases 1/2 (ERK1/2) and threonine and tyrosine kinase (MEK), but not by protein kinase C (PKC) inhibitor. Taken together, in addition to a significant increase in DNA breakage, SiO₂NP treatment resulted in GJIC dysregulation involved in Cx43 phosphorylation through the activation of mitogen-activated protein kinase (MAPK) signaling. Overall findings of the genotoxic and non-genotoxic carcinogenic potential of SiO₂NPs provide useful toxicological information for clinical application at an appropriate dose.

Role of MicroRNAs in regulation of DNA damage in monocytes exposed to polystyrene and TiO2 nanoparticles

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Time and dose dependent DNA damage profile was established to determine the genotoxicity of PSNPs and nano-TiO₂-ARS.

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Expression of DNA damage repairing genes was elevated post nano-TiO₂-ARS, but not post PSNPs exposure.

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miRNA expression screening with different potential activators revealed miR-155-5p as best candidate.

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Transfection of miR-155-5p mimic influenced expression of genes invovled in DNA damage post expousre to TiO₂ and PSNPs.

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miR-155-5p showed the highest potential to be used as biomarker for PSNPs and nano-TiO₂-ARS induced adverse effects.

The release of nanoparticles into the environment can interfere with the health of the exposed organisms. MicroRNAs have been suggested as potential toxicology biomarkers. The expression of potential zebrafish nano-toxicity biomarker miRNAs in our previous study was validated in THP-1 human monocytic cell line after exposure to polystyrene (PSNPs) and ARS labeled Titanium dioxide nanoparticles (nano-TiO₂-ARS). miRNAs expression post exposure to PLGA nanoparticles and E. coli BioParticles was used to exclude potential activation and engagement of miRNAs through phagocytosis or pro-inflammatory specific responses. miR-155-5p showed the highest potential to be used as biomarker for PSNPs and nano-TiO₂-ARS induced toxicity. To determine effects of PSNPs and nano-TiO₂-ARS on genotoxicity, time and dose dependent DNA damage profile was established. Severe DNA damage was triggered by both nanoparticles, and expression of DNA damage repairing genes was elevated post nano-TiO₂-ARS, but not post PSNPs exposure, questioning the utility of the comet assay as universal assessment tool for genotoxicity induced by nanoparticles in general. Transfection of miR-155-5p mimic influenced the expression of miR-155-5p related, DNA damage responsible genes post both nano-TiO₂-ARS and PSNPs exposure. Transfection results suggest significant involvement of miR-155-5p in gene repair mechanisms triggered by adverse effects of PSNPs and nano-TiO₂-ARS on monocytes.

Cytotoxicity and genotoxicity of cadmium oxide nanoparticles evaluated using in vitro assays

Cadmium oxide nanoparticles (CdO NPs) are among some of the most studied and industrially used metal oxide NPs. They have been widely used for industrial application, such as paint pigments and electronic devices, and medical therapeutics. With increasing use of CdO NPs and concerns for their potential adverse effects on the environment and public health, evaluation of the cytotoxicity and genotoxicity of CdO NPs becomes very important. To date, there is a limited understanding of the potential hazard brought by CdO NPs and a lack of information and research, particularly on the genotoxicity assessment of these NPs. In this study, 10 nm CdO core-PEG stabilized NPs were synthesized, characterized and used for evaluation of CdO NPs' cytotoxicity and genotoxicity. Release of cadmium ions (Cd⁺²) from the CdO NPs in cell culture medium, cellular uptake of the NPs, and the endotoxin content of the particles were measured prior to the toxicity assays. Cytotoxicity was evaluated using the MTS assay, ATP content detection assay, and LDH assay. Genotoxicity was assessed using the Ames test, Comet assay, micronucleus assay, and mouse lymphoma assay. The cytotoxicity of cadmium chloride (CdCl₂) was also evaluated along with that of the CdO NPs. The results showed that endotoxin levels within the CdO NPs were below the limit of detection. CdO NPs induced concentration-dependent cytotoxicity in TK6 and HepG2 cells with the MTS, ATP and LDH assays. Although the genotoxicity of CdO NPs was negative in the Ames test, positive results were obtained with the micronucleus, Comet, and mouse lymphoma assays. The negative response of CdO NPs with the Ames test may be the result of unsuitability of the assay for measuring NPs, while the positive responses from other genotoxicity assays suggest that CdO NPs can induce chromosomal damage, single or double strand breaks in DNA, and mutations. The toxicity of the CdO NPs results from the NPs themselves and not from the released Cd⁺², because the ions released from the NPs were minimal. These results demonstrate that CdO NPs are cytotoxic and genotoxic and provide new insights into risk assessment of CdO NPs for human exposure and environmental protection.

Combined 2D-QSAR, Principal Component Analysis and Sensitivity Analysis Studies on Fluoroquinolones' Genotoxicity

In this paper, two-dimensional quantitative structure-activity relationship (2D-QSAR) and principal component analysis (PCA) methods were employed to screen the main parameters affecting the genotoxicity of fluoroquinolones (FQs), and the rules affecting the genetic toxicity of FQs were investigated by combining 2D-QSAR and PCA with the sensitivity analysis method. First, four types of parameters were calculated, namely, the geometric parameters (7), electronic parameters (5), physical and chemical parameters (8), and spectral parameters (7), but the physical and chemical parameters heat of formation (HF) and critical volume (CV) were excluded after the establishment of the 2D-QSAR model. Then, after PCA, it was found that the first principal component represented the main driving factors affecting the molecular genetic toxicity of FQs. In addition, after comprehensive analysis of the factor loading of the first, second, and third principal components, seven parameters affecting the genotoxicity of the FQs were screened out, namely, total energy (TE), critical temperature (CT), and molecular weight (Mol Wt) (increased with increasing genotoxicity of the FQs) and steric parameter (MR), quadrupole moment QXX (QXX), quadrupole moment QYY (QYY), and boiling point (BP) (decreased with increasing genotoxicity of the FQs); the above key parameters were also verified by sensitivity analysis. The obtained rules could be used to determine the substitution sites and the substitution groups associated with higher genotoxicity in the process of FQ modification, and these rules agreed well with the hologram quantitative structure-activity relationship (HQSAR) model. Finally, it was also found through SPSS analysis that the parameters screened in this paper were significantly correlated with FQ derivatives' genetic toxicity.

Genotoxicity Assessment of Nanomaterials: Recommendations on Best Practices, Assays, and Methods

Nanomaterials (NMs) present unique challenges in safety evaluation. An international working group, the Genetic Toxicology Technical Committee of the International Life Sciences Institute's Health and Environmental Sciences Institute, has addressed issues related to the genotoxicity assessment of NMs. A critical review of published data has been followed by recommendations on methods alterations and best practices for the standard genotoxicity assays: bacterial reverse mutation (Ames); in vitro mammalian assays for mutations, chromosomal aberrations, micronucleus induction, or DNA strand breaks (comet); and in vivo assays for genetic damage (micronucleus, comet and transgenic mutation assays). The analysis found a great diversity of tests and systems used for in vitro assays; many did not meet criteria for a valid test, and/or did not use validated cells and methods in the Organization for Economic Co-operation and Development Test Guidelines, and so these results could not be interpreted. In vivo assays were less common but better performed. It was not possible to develop conclusions on test system agreement, NM activity, or mechanism of action. However, the limited responses observed for most NMs were consistent with indirect genotoxic effects, rather than direct interaction of NMs with DNA. We propose a revised genotoxicity test battery for NMs that includes in vitro mammalian cell mutagenicity and clastogenicity assessments; in vivo assessments would be added only if warranted by information on specific organ exposure or sequestration of NMs. The bacterial assays are generally uninformative for NMs due to limited particle uptake and possible lack of mechanistic relevance, and are thus omitted in our recommended test battery for NM assessment. Recommendations include NM characterization in the test medium, verification of uptake into target cells, and limited assay-specific methods alterations to avoid interference with uptake or endpoint analysis. These recommendations are summarized in a Roadmap guideline for testing.

Impact of metal oxide nanoparticles on in vitro DNA amplification

Polymerase chain reaction (PCR) is used as an in vitro model system of DNA replication to assess the genotoxicity of nanoparticles (NPs). Prior results showed that several types of NPs inhibited PCR efficiency and increased amplicon error frequency. In this study, we examined the effects of various metal oxide NPs on inhibiting PCR, using high- vs. low-fidelity DNA polymerases; we also examined NP-induced DNA mutation bias at the single nucleotide level. The effects of seven major types of metal oxide NPs (Fe₂O₃, ZnO, CeO₂, Fe₃O₄, Al₂O₃, CuO, and TiO₂) on PCR replication via a low-fidelity DNA polymerase (Ex Taq) and a high-fidelity DNA polymerase (Phusion) were tested. The successfully amplified PCR products were subsequently sequenced using high-throughput amplicon sequencing. Using consistent proportions of NPs and DNA, we found that the effects of NPs on PCR yield differed depending on the DNA polymerase. Specifically, the efficiency of the high-fidelity DNA polymerase (Phusion) was significantly inhibited by NPs during PCR; such inhibition was not evident in reactions with Ex Taq. Amplicon sequencing showed that the overall error rate of NP-amended PCR was not significantly different from that of PCR without NPs (p > 0.05), and NPs did not introduce single nucleotide polymorphisms during PCR. Thus, overall, NPs inhibited PCR amplification in a DNA polymerase-specific manner, but mutations were not introduced in the process.

Genotoxicity evaluation of titanium dioxide nanoparticles using the mouse lymphoma assay and the Ames test

Titanium dioxide nanoparticles (TiO₂-NPs) are widely used in the cosmetics, health, and food industries, but their safety and genotoxicity remain a matter of debate. We investigated whether TiO₂-NPs could induce gene mutations in mouse lymphoma L5178Y cells and Salmonella typhimurium strains TA97a, TA98, TA100, TA102, and TA1535. Following preliminary tests, 2 mg/mL for the mouse lymphoma gene mutation assay and 1.25 mg/plate for the in vitro bacterial reverse mutation assay (Ames test) were selected as the highest concentrations. Exposure to TiO2-NPs for 4 or 24 h with or without S9 metabolic activation did not increase mutation frequency for any of the concentrations tested in L5178Y cells. In the Ames test, TiO₂-NPs did not induce reverse mutation in the bacterial strains. No positive mutagenic responses were observed in either test system, and therefore we cannot classify TiO₂-NPs as mutagenic; further testing will be required to determine conclusively whether TiO₂-NPs are genotoxic.

From the Cover: An Investigation of the Genotoxicity and Interference of Gold Nanoparticles in Commonly Used In Vitro Mutagenicity and Genotoxicity Assays

The suitability of 4 in vitro assays, commonly used for mutagenicity and genotoxicity assessment, was investigated in relation to treatment with 14 nm citrate-stabilized gold nanoparticles (AuNPs). Specifically, the Ames test was conducted without metabolic activation, where no mutagenic effects were observed. High resolution transmission electron microscopy and Cytoviva dark-field image analysis showed that AuNPs did not enter the bacterial cells, thus confirming the unreliability of the Ames test for nanoparticle mutagenicity studies. In addition, the Chinese hamster ovary (CHO) cell line was used for Comet, Chromosome aberration and Micronucleus assays. CHO cells were treated with AuNPs for 20 h at 37 °C. Cytotoxicity was not detected by cell impedance studies even though AuNP uptake was confirmed using Cytoviva image analysis. The DNA damage was statistically significant in treated cells when assessed by the Comet assay. However, minimal and nonstatistically significant chromosomal DNA damage was observed using the chromosome aberration and micronucleus assays. In this study, we showed that false positive results obtained with Comet assay may have been due to the possibility of direct contact between the residual, intracellular AuNPs and DNA during the assay procedure. Therefore, the chromosome aberration and micronucleus assays are better suited to assess the genotoxic effects of nanoparticles due to low probability of such direct contact occurring. Genotoxic effect of 14 and 20 nm citrate-stabilized, as well as, 14 nm PCOOH AuNPs were also investigated using chromosome aberration and micronucleus assays. Based on our acceptance criteria for a positive genotoxic response, none of the AuNPs were found to be genotoxic in either of these assays.

Effects on human bronchial epithelial cells following low-dose chronic exposure to nanomaterials: A 6-month transformation study

The most plausible exposure route to manufactured nanomaterials (MNM) remains pulmonary inhalation. Yet, few studies have attempted to assess carcinogenic properties in vitro following long-term exposure of human pulmonary cells to low and occupationally relevant doses. The most advanced in vitro tests for carcinogenicity, the cell transformation assay (CTA), rely mostly on rodent cells and short-term exposure. We hypothesized that long-term exposure of human bronchial epithelial cells with a normal phenotype could be a valuable assay for testing carcinogenicity of nanomaterials. Therefore, this study (performed within the framework of the FP7-NANoREG project) assessed carcinogenic potential of chronic exposure (up to 6months) to low doses of multi-walled carbon nanotubes (MWCNT, NM-400 and NM-401) and TiO₂ materials (NM62002 and KC7000). In order to harmonize and standardize the experiments, standard operating protocols of MNM dispersion (NANOGENOTOX) were used by three different NANoREG project partners. All nanomaterials showed low cytotoxicity in short-term tests for the tested doses (0.96 and 1.92μg/cm²). During long-term exposure, however, NM-401 clearly affected cell proliferation. In contrast, no cell transformation was observed for NM-401 by any of the partners. NM-400 and NM62002 formed some colonies after 3months. We conclude that agglomerated NM-401 in low doses affect cell proliferation but do not cause cell transformation in the CTA assay used.

In vitro uptake and toxicity studies of metal nanoparticles and metal oxide nanoparticles in human HT29 cells

In this paper, we investigated the toxicological behavior of metal nanoparticles (gold, silver) and metal oxide nanoparticles (copper oxide, zinc oxide, titanium dioxide) in vitro in human colorectal adenocarcinoma cells (HT29). We analyzed the cellular uptake by ICP-MS and TEM, the influence on cell viability by MTT assay and trypan blue exclusion test, their effect on DNA damage and/or generation of oxidized bases by alkaline comet assay, and their potential to induce apoptosis by flow cytometry after 24-h nanoparticle treatment with concentrations between 2 and 10 µg/ml. We determined the amount of metal taken up by a single HT29 cell, ranging from 0.02 pg/cell up to 1.39 pg/cell. Cell viability assays showed a significantly decrease for metal oxide nanoparticles using trypan blue exclusion test and for all nanoparticles, except titanium dioxide, using MTT assay. Genotoxic effects after nanoparticle treatment were not observed for the tested concentrations. Apoptosis induction was significantly increased for silver nanoparticles (tested for two sizes) as well as for titanium dioxide and zinc oxide nanoparticles. Our results indicate potential health risks of oral NP uptake by food ingredients or food contamination, making further mechanistic investigations on cellular uptake and toxicity necessary.

Impact of anatase titanium dioxide nanoparticles on mutagenic and genotoxic response in Chinese hamster lung fibroblast cells (V-79): The role of cellular uptake

The unique physico-chemical properties of nano crystalline anatase titanium dioxide nanoparticles (TiO₂ NPs) render them with different biological and chemical activities. Hence, it is widely used in industrial and consumer applications. Previous studies have shown the genotoxicity of TiO₂ NPs. However, there is a paucity of data regarding mutagenicity of these NPs. In the present study, the cellular uptake, sub-cellular localization, cytotoxicity and short term DNA interaction of TiO₂ NPs (1-100 μgmL^-1) of diameter ranging from 12 to 25 nm on mammalian lung fibroblast cells (V-79) has been studied. The flow cytometric analysis and electron micrographs of V-79 monolayer showed the internalization of TiO₂ NPs in the cytoplasm with the confirmation of elemental composition through SEM/EDX analysis. TEM analysis also showed TiO₂ NPs induced ultra-structural changes such as swollen mitochondria and nuclear membrane disruption in V-79 cells. TiO₂ NPs generated free radicals, which induced indirect mutagenic and genotoxic responses. Apart from measuring the genotoxicity by Comet assay, the mutagenic potential of TiO₂ NPs in V-79 cells was evaluated by mammalian HGPRT gene forward mutation assay, showing a 2.98- fold increase in 6TG^R HGPRT mutant frequency (*p < 0.05, **p < 0.01, ***p < 0.001) by culture plate method, which is an early indicator of potential carcinogenicity. Hence, TiO₂ NPs should be closely monitored and there should be a judicious use and disposal of NPs.

Titanium dioxide nanoparticles: an in vitro study of DNA binding, chromosome aberration assay, and comet assay

Engineered titanium dioxide nanoparticles (TiO2 NPs) are extensively used in cosmetic, pharmaceutical and other industries globally due to their unique properties, which has raised concern for biosafety. Genotoxicity assessment is an important part of biosafety evaluation; we report in vitro cytogenetic assays for NPs considering their unique physicochemical characteristics to fill the gap of laboratory data regarding biological safety along with mechanistic study for mode of interaction of NP with genetic material. Comet and chromosome aberration assay (CA assay) using short-term human peripheral blood cultures following exposure to TiO2 NPs; along with physicochemical parameters for stability of nano form in cultures; and DNA binding activity were carried out. The dynamic light scattering and zeta potential measurements revealed mono dispersion in media. The fluorescence spectroscopy for binding affinity of TiO2 NPs and human genomic DNA showed binding constant (Kb), 4.158 × 106 M-1 indicating strong binding affinity and negative ΔG0 value suggesting spontaneous DNA binding supporting its genotoxic potential. Following in vitro exposure to TiO2 NPs for 24 h, the cultures were analyzed for comet and CA assays, which showed significant results (p < 0.05) for % DNA intensity in tail, Olive Tail Moment and frequency of Chromosomal aberrations (CA) at 75 and 125 μM but not at 25 μM.

Fine and ultrafine atmospheric particulate matter at a multi-influenced urban site: Physicochemical characterization, mutagenicity and cytotoxicity

Particulate Matter (PM) air pollution is one of the major concerns for environment and health. Understanding the heterogeneity and complexity of fine and ultrafine PM is a fundamental issue notably for the assessment of PM toxicological effects. The aim of this study was to evaluate mutagenicity and cytotoxicity of a multi-influenced urban site PM, with or without the ultrafine fraction. For this purpose, PM_2.5-0.3 (PM with aerodynamic diameter ranging from 0.3 to 2.5 μm) and PM_2.5 were collected in Dunkerque, a French coastal industrial city and were extensively characterized for their physico-chemical properties, including inorganic and organic species. In order to identify the possible sources of atmospheric pollution, specific criteria like Carbon Preference Index (CPI) and PAH characteristic ratios were investigated. Mutagenicity assays using Ames test with TA98, TA102 and YG1041 Salmonella strains with or without S9 activation were performed on native PM sample and PM organic extracts and water-soluble fractions. BEAS-2B cell viability and cell proliferation were evaluated measuring lactate dehydrogenase release and mitochondrial dehydrogenase activity after exposure to PM and their extracts. Several contributing sources were identified in PM: soil resuspension, marine emissions including sea-salt or shipping, road traffic and industrial activities, mainly related to steelmaking or petro-chemistry. Mutagenicity of PM was evidenced, especially for PM_2.5, including ultrafine fraction, in relation to PAHs content and possibly nitro-aromatics compounds. PM induced cytotoxic effects at relatively high doses, while alteration of proliferation with low PM doses could be related to underlying mechanisms such as genotoxicity.

Factors affecting the in vitro micronucleus assay for evaluation of nanomaterials

A number of in vitro methodologies have been used to assess the genotoxicity of different nanomaterials, including titanium dioxide nanoparticles (TiO₂ NPs) and silver nanoparticles (AgNPs). The in vitro micronucleus assay is one of the most commonly used test methods for genotoxicity evaluation of nanomaterials. However, due to the novel features of nanomaterials, such as high adsorption capacity and fluorescence properties, there are unexpected interactions with experimental components and detection systems. In this study, we evaluate the interference by two nanoparticles, AgNPs and TiO₂ NPs, with the in vitro micronucleus assay system and possible confounding factors affecting cytotoxicity and genotoxicity assessment of the nanomaterials including cell lines with different p53 status, nanoparticle coatings and fluorescence, cytochalasin B, fetal bovine serum in cell treatment medium and different measurement methodologies for detecting micronuclei. Our results showed that micronucleus induction by AgNPs was similar when evaluated using flow cytometry or microscope, whereas the induction by TiO₂ NPs was different using the two methods due to TiO₂'s fluorescence interference with the cytometry equipment. Cells with the mutated p53 gene were more sensitive to micronucleus induction by AgNPs than the p53 wild-type cells. The presence of serum during treatment increased the toxicity of AgNPs. The coatings of nanoparticles played an important role in the genotoxicity of AgNPs. These collective data highlight the importance of considering the unique properties of nanoparticles in assessing their genotoxicity using the in vitro micronucleus assay.

Size- and coating-dependent cytotoxicity and genotoxicity of silver nanoparticles evaluated using in vitro standard assays

The physicochemical characteristics of silver nanoparticles (AgNPs) may greatly alter their toxicological potential. To explore the effects of size and coating on the cytotoxicity and genotoxicity of AgNPs, six different types of AgNPs, having three different sizes and two different coatings, were investigated using the Ames test, mouse lymphoma assay (MLA) and in vitro micronucleus assay. The genotoxicities of silver acetate and silver nitrate were evaluated to compare the genotoxicity of nanosilver to that of ionic silver. The Ames test produced inconclusive results for all types of the silver materials due to the high toxicity of silver to the test bacteria and the lack of entry of the nanoparticles into the cells. Treatment of L5718Y cells with AgNPs and ionic silver resulted in concentration-dependent cytotoxicity, mutagenicity in the Tk gene and the induction of micronuclei from exposure to nearly every type of the silver materials. Treatment of TK6 cells with these silver materials also resulted in concentration-dependent cytotoxicity and significantly increased micronucleus frequency. With both the MLA and micronucleus assays, the smaller the AgNPs, the greater the cytotoxicity and genotoxicity. The coatings had less effect on the relative genotoxicity of AgNPs than the particle size. Loss of heterozygosity analysis of the induced Tk mutants indicated that the types of mutations induced by AgNPs were different from those of ionic silver. These results suggest that AgNPs induce cytotoxicity and genotoxicity in a size- and coating-dependent manner. Furthermore, while the MLA and in vitro micronucleus assay (in both types of cells) are useful to quantitatively measure the genotoxic potencies of AgNPs, the Ames test cannot.

Applications of the comet assay in particle toxicology: air pollution and engineered nanomaterials exposure

Exposure to ambient air particles is associated with elevated levels of DNA strand breaks (SBs) and endonuclease III, formamidopyrimidine DNA glycosylase (FPG) and oxoguanine DNA glycosylase-sensitive sites in cell cultures, animals and humans. In both animals and cell cultures, increases in SB and in oxidatively damaged DNA are seen after exposure to a range of engineered nanomaterials (ENMs), including carbon black, carbon nanotubes, fullerene C60, ZnO, silver and gold. Exposure to TiO2 has generated mixed data with regard to SB and oxidatively damaged DNA in cell cultures. Nanosilica does not seem to be associated with generation of FPG-sensitive sites in cell cultures, while large differences in SB generation between studies have been noted. Single-dose airway exposure to nanosized carbon black and multi-walled carbon nanotubes in animal models seems to be associated with elevated DNA damage levels in lung tissue in comparison to similar exposure to TiO2 and fullerene C60. Oral exposure has been associated with augmented DNA damage levels in cells of internal organs, although the doses have been typically very high. Intraveneous and intraperitoneal injection of ENMs have shown contradictory results dependent on the type of ENM and dose in each set of experiments. In conclusion, the exposure to both combustion-derived particles and ENMs is associated with increased levels of DNA damage in the comet assay. Particle size, composition and crystal structure of ENM are considered important determinants of toxicity, whereas their combined contributions to genotoxicity in the comet assay are yet to be thoroughly investigated.

Genotoxicity of metal oxide nanomaterials: review of recent data and discussion of possible mechanisms

Nanotechnology has rapidly entered into human society, revolutionized many areas, including technology, medicine and cosmetics. This progress is due to the many valuable and unique properties that nanomaterials possess. In turn, these properties might become an issue of concern when considering potentially uncontrolled release to the environment. The rapid development of new nanomaterials thus raises questions about their impact on the environment and human health. This review focuses on the potential of nanomaterials to cause genotoxicity and summarizes recent genotoxicity studies on metal oxide/silica nanomaterials. Though the number of genotoxicity studies on metal oxide/silica nanomaterials is still limited, this endpoint has recently received more attention for nanomaterials, and the number of related publications has increased. An analysis of these peer reviewed publications over nearly two decades shows that the test most employed to evaluate the genotoxicity of these nanomaterials is the comet assay, followed by micronucleus, Ames and chromosome aberration tests. Based on the data studied, we concluded that in the majority of the publications analysed in this review, the metal oxide (or silica) nanoparticles of the same core chemical composition did not show different genotoxicity study calls (i.e. positive or negative) in the same test, although some results are inconsistent and need to be confirmed by additional experiments. Where the results are conflicting, it may be due to the following reasons: (1) variation in size of the nanoparticles; (2) variations in size distribution; (3) various purities of nanomaterials; (4) variation in surface areas for nanomaterials with the same average size; (5) differences in coatings; (6) differences in crystal structures of the same types of nanomaterials; (7) differences in size of aggregates in solution/media; (8) differences in assays; (9) different concentrations of nanomaterials in assay tests. Indeed, due to the observed inconsistencies in the recent literature and the lack of adherence to appropriate, standardized test methods, reliable genotoxicity assessment of nanomaterials is still challenging.

Genotoxic and cell-transforming effects of titanium dioxide nanoparticles

The in vitro genotoxic and the soft-agar anchorage independent cell transformation ability of titanium dioxide nanoparticles (nano-TiO2) and its microparticulated form has been evaluated in human embryonic kidney (HEK293) and in mouse embryonic fibroblast (NIH/3T3) cells. Nano-TiO2 of two different sizes (21 and 50 nm) were used in this study. The comet assay, with and without the use of FPG enzyme, the micronucleus assay and the soft-agar colony assay were used. For both the comet assay and the frequency of micronuclei a statistically significant induction of DNA damage, was observed at the highest dose tested (1000 µg/mL). No oxidative DNA damage induction was observed when the comet assay was complemented with the use of FPG enzyme. Furthermore, long-term exposure to nano-TiO2 has also proved to induce cell-transformation promoting cell-anchorage independent growth in soft-agar. Results were similar for the two nano-TiO2 sizes. Negative results were obtained when the microparticulated form of TiO2 was tested, indicating the existence of important differences between the microparticulated and nanoparticulated forms. As a conclusion it should be indicated that the observed genotoxic/tranforming effects were only detected at the higher dose tested (1000 µg/mL) what play down the real risk of environmental exposures to this nanomaterial.

Genotoxicity in primary human peripheral lymphocytes after exposure to lithium titanate nanoparticles in vitro

Lithium titanate (Li2TiO3) nanoparticles (LTT NPs; <100 nm) are widely used in battery technology, porcelain enamels, and ceramic insulating bodies. With the increased applications of LTT NPs, the concerns about their potential human toxicity effects and their environmental impact were also increased. However, toxicity data for LTT NPs relating to human health are very limited. Therefore, the purpose of this study was to evaluate whether LTT NPs are able to induce genetic damage in human peripheral lymphocytes in vitro when taking into consideration that DNA damage plays an important role in carcinogenesis. With this aim, the chromosome aberrations (CA), sister chromatid exchanges (SCE), and micronucleus (MN) assays were used as genotoxicity end points. Human peripheral lymphocytes obtained from five healthy male volunteers were exposed to LTT NPs at final dispersed concentrations ranging from 0 to 1000 μg/mL for 72 h at 37°C. The obtained results indicated that LTT NPs compound did not induce DNA damage in human peripheral lymphocytes as depicted by CA/cell, SCE/cell, and MN/1000 cell values in all concentrations tested. In summary, our results revealed that exposure to LTT NPs is not capable of inducing DNA lesions in human peripheral lymphocytes for the first time.

Genotoxicity assessment of magnetic iron oxide nanoparticles with different particle sizes and surface coatings

Magnetic iron oxide nanoparticles (IONPs) have been widely used for various biomedical applications such as magnetic resonance imaging and drug delivery. However, their potential toxic effects, including genotoxicity, need to be thoroughly understood. In the present study, the genotoxicity of IONPs with different particle sizes (10, 30 nm) and surface coatings (PEG, PEI) were assessed using three standard genotoxicity assays, the Salmonella typhimurium reverse mutation assay (Ames test), the in vitro mammalian chromosome aberration test, and the in vivo micronucleus assay. In the Ames test, SMG-10 (PEG coating, 10 nm) showed a positive mutagenic response in all the five test bacterial strains with and without metabolic activation, whereas SEI-10 (PEI coating, 10 nm) showed no mutagenesis in all tester strains regardless of metabolic activation. SMG-30 (PEG coating, 30 nm) was not mutagenic in the absence of metabolic activation, and became mutagenic in the presence of metabolic activation. In the chromosomal aberration test, no increase in the incidence of chromosomal aberrations was observed for all three IONPs. In the in vivo micronucleus test, there was no evidence of increased micronuclei frequencies for all three IONPs, indicating that they were not clastogenic in vivo. Taken together, our results demonstrated that IONPs with PEG coating exhibited mutagenic activity without chromosomal and clastogenic abnormalities, and smaller IONPs (SMG-10) had stronger mutagenic potential than larger ones (SMG-30); whereas, IONPs with SEI coating (SEI-10) were not genotoxic in all three standard genotoxicity assays. This suggests that the mutagenicity of IONPs depends on their particle size and surface coating.

DNA damage caused by inorganic particulate matter on Raji and HepG2 cell lines exposed to ultraviolet radiation

Epidemiological studies have correlated exposure to ultraviolet-irradiated particulate matter with cardiovascular, respiratory, and lung diseases. This study investigated the DNA damage induced by two major inorganic particulate matter compounds found in diesel exhaust, ammonium nitrate and ammonium sulfate, on Burkitt's lymphoma (Raji) and hepatocellular carcinoma (HepG2) cell lines. We found a dose-dependent positive correlation of accumulated DNA damage at concentrations of ammonium nitrate (25 μg/ml, 50 μg/ml, 100 μg/ml, 200 μg/ml, 400 μg/ml) with ultraviolet exposure (250 J/m(2), 400 J/m(2), 600 J/m(2), 850 J/m(2)), as measured by the comet assay in both cell lines. There was a significant difference between the treated ammonium nitrate samples and negative control samples in Raji and HepG2 cells (p<0.001). Apoptosis was shown in Raji and HepG2 cells when exposed to high concentrations of ammonium nitrate (200 μg/ml and 400 μg/ml) for 1h in samples without ultraviolet exposure, as assessed by the comet assay. However, the level of apoptosis greatly diminished after ultraviolet exposure at these concentrations. Over a 24h period, at intervals of 1, 4, 8, 12, 18, and 24h, we also observed that ammonium nitrate decreased viability in Raji and HepG2 cell lines and inhibited cell growth. Ammonium sulfate-induced DNA damage was minimal in both cell lines, but there remained a significant difference (p<0.05) between the ultraviolet radiation treated and negative control samples. These results indicate that the inorganic particulate compound, ammonium nitrate, induced DNA strand breaks at all concentrations, and indications of apoptosis at high concentrations in Raji and HepG2 cells, with ultraviolet radiation preventing apoptosis at high concentrations. We hypothesize that ultraviolet radiation may inhibit an essential cellular mechanism, possibly involving p53, thereby explaining this phenomenon. Further studies are necessary to characterize the roles of apoptosis inhibition induced by DNA damage caused by inorganic particulate matter.

Genotoxicity of titanium dioxide nanoparticles

Titanium dioxide nanoparticles (TiO₂-NPs, <100 nm) are increasingly being used in pharmaceuticals and cosmetics due to the unique properties derived from their small sizes. However, their large surface-area to mass ratio and high redox potential may negatively impact human health and the environment. TiO₂-NPs can cause inflammation, pulmonary damage, fibrosis, and lung tumors and they are possibly carcinogenic to humans. Because cancer is a disease involving mutation, there are a large number of studies on the genotoxicity of TiO₂-NPs. In this article, we review the results that have been reported in the literature, with a focus on data generated from the standard genotoxicity assays. The data include genotoxicity results from the Ames test, in vitro and in vivo Comet assay, in vitro and in vivo micronucleus assay, sister chromatid exchange assay, mammalian cell hypoxanthine-guanine phosphoribosyl transferase gene assay, the wing somatic mutation and recombination assay, and the mouse phosphatidylinositol glycan, class A gene assay. Inconsistent results have been found in these assays, with both positive and negative responses being reported. The in vitro systems for assessing the genotoxicity of TiO₂-NPs have generated a greater number of positive results than the in vivo systems, and tests for DNA and chromosome damage have produced more positive results than the assays measuring gene mutation. Nearly all tests for measuring the mutagenicity of TiO₂-NPs were negative. The current data indicate that the genotoxicity of TiO₂-NPs is mediated mainly through the generation of oxidative stress in cells.