Genotoxicity test battery - An assessment of its utility in early drug development

Formal requirements for genotoxicity testing of drug candidates to support clinical entry have been in place since the issue of initial regulatory guidance over 25 years ago and subsequent update a decade ago. An evaluation of such testing, supporting first clinical entry of 108 small molecule drug candidates over the last decade, showed that the most common approach (75 % of tested compounds) was for a Good Laboratory Practice test battery in the form of 2 in vitro (a bacterial reverse mutation and a mammalian cell) assays and one in vivo assay. The majority of other tested compounds involved in vitro testing only in bacterial reverse mutation and mammalian cell assays. Testing using a bacterial reverse mutation assay and an in vivo assessment of genotoxicity with 2 different tissues was limited to 2 occasions. For in vitro mammalian cell testing, the chromosome aberration test was most commonly used (70 % occasions), followed by a micronucleus test (16 % occasions) or a mouse lymphoma assay (14 % occasions). For in vivo evaluation, the most common test was a rodent bone marrow micronucleus test (87 % occasions). A positive in vitro mammalian cell assay result was seen on 13 % occasions but was not confirmed with further in vivo testing and the drug candidates were taken into the clinic. In conclusion, the present evaluation showed that the current test battery paradigm for genotoxicity testing has an integral part in supporting clinical entry to confirm candidate drugs taken into the clinic are unlikely to have genotoxic activity.

Genotoxicity studies with an ethanolic extract of Kalanchoe pinnata leaves

Kalanchoe pinnata is a medicinal plant, used mainly in African, Brazilian, and Indian traditional medicine for the treatment of several human disorders. Whole leaf extracts, crude juice of the leaves, and aqueous and organic extracts of the leaves are used. Over the last decade, ethanolic extracts have become the most popular form of Kalanchoe medicinal preparation. In this study, an ethanolic extract of this plant leaf was tested in a battery of standard regulatory genetic toxicology tests. This extract did not induce reverse mutations in the Salmonella/microsome assay but induces a weak genotoxic response in the mouse lymphoma assay and the in vivo micronucleus assay in mice. Our results indicate that this material may cause DNA damage, and its use should be restricted.

Detection of Loss of Heterozygosity in Tk-Deficient Mutants from L5178Y Tk+/--3.7.2C Mouse Lymphoma Cells

The mouse lymphoma assay (MLA), a forward mutation assay using the Tk^+/--3.7.2C clone of the L5178Y mouse lymphoma cell line and the Thymidine kinase (Tk) gene, has been widely used as an in vitro genetic toxicity assay for more than four decades. The MLA can evaluate the ability of mutagens to induce a wide range of genetic events including both gene mutations and chromosomal mutations and has been recommended as one component of several genotoxicity test batteries. Tk-deficient mutants often exhibit chromosomal abnormalities involving the distal end of chromosome 11 where the Tk gene is located, in mice, and the type of chromosome alteration can be analyzed using a loss of heterozygosity (LOH) approach. LOH has been considered an important event in human tumorigenesis and can result from any of the following several mechanisms: large deletions, mitotic recombination, and chromosome loss. In this chapter, the authors describe the procedures for the detection of LOH in the Tk mutants from the MLA, and apply LOH analysis for understanding the types of genetic damage that is induced by individual chemicals.

In vitro mutagenicity of gas-vapour phase extracts from flavoured and unflavoured heated tobacco products

The in vitro mutagenic and genotoxic potential of Heated Tobacco Products (HTPs) has already been studied with the particulate phase and reported previously. This study has been designed to complement the in vitro assessment of the HTP and to determine whether the inclusion of potential flavourings would alter the in vitro response by testing the other phase of the aerosol, the gas-vapour phase (GVP). Both flavoured and unflavoured Neostik GVP samples did not show any sign of mutagenic activity in the Ames test but induced a mutagenic response in the mouse lymphoma assay (MLA), however, these responses were significantly less than those of the reference cigarette, 3R4F. The results demonstrated that GVP emissions of this HTP did not induce either new qualitative or quantitative mutagenic hazards compared to 3R4F, as assessed by the Ames test (no new responsive strains) and MLA (a lower mutagenic response), respectively. A statistical comparative analysis of the responses showed that the addition of flavourings that may thermally decompose under the conditions of use did not add to the in vitro baseline responses of the unflavoured Neostik.

Mouse lymphoma assay

Conduct of the mouse lymphoma assay (MLA) is underpinned by Organisation for Economic Co-operation and Development (OECD) Test Guideline 490 and International Conference on Harmonisation S2(R1) guidance and is a recognised in vitro genotoxicity test battery assay. It has been used on a limited number of occasions for the assessment of some tobacco and nicotine products, such as e-cigarettes and tobacco heating products (THP). The aim of this study was to assess the suitability of the MLA for genotoxicity testing with a variety of tobacco and nicotine products. Total particulate matter (TPM) from a 3R4F cigarette was compared against a commercial electronic cigarette liquid (e-liquid), electronic cigarette (e-cigarette) aerosol matter captured from the same e-liquid, and TPM from a commercial THP. Treatment conditions included 3 h exposures with and without metabolic activation and a longer 24 h exposure without metabolic activation (-S9) at concentrations up to 500 μg/mL. Under all treatment conditions, 3R4F produced a clear positive response with regard to induction of mutation. In contrast, no marked induction of mutation was observed for the e-liquid, e-cigarette aerosol or THP. Additionally, data are presented as a function of nicotine equivalents for comparisons between these different tobacco products and test matrices.

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.

Benchmark Dose Modeling of In Vitro Genotoxicity Data: a Reanalysis

The methods of applied genetic toxicology are changing from qualitative hazard identification to quantitative risk assessment. Recently, quantitative analysis with point of departure (PoD) metrics and benchmark dose (BMD) modeling have been applied to in vitro genotoxicity data. Two software packages are commonly used for BMD analysis. In previous studies, we performed quantitative dose-response analysis by using the PROAST software to quantitatively evaluate the mutagenicity of four piperidine nitroxides with various substituent groups on the 4-position of the piperidine ring and six cigarette whole smoke solutions (WSSs) prepared by bubbling machine-generated whole smoke. In the present study, we reanalyzed the obtained genotoxicity data by using the EPA's BMD software (BMDS) to evaluate the inter-platform quantitative agreement of the estimates of genotoxic potency. We calculated the BMDs for 10%, 50%, and 100% (i.e., a two-fold increase), and 200% increases over the concurrent vehicle controls to achieve better discrimination of the dose-responses, along with their BMDLs (the lower 95% confidence interval of the BMD) and BMDUs (the upper 95% confidence interval of the BMD). The BMD values and rankings estimated in this study by using the EPA's BMDS were reasonably similar to those calculated in our previous studies by using PROAST. These results indicated that both software packages were suitable for dose-response analysis using the mouse lymphoma assay and that the BMD modeling results from these software packages produced comparable rank orders of the mutagenic potency.

Mutagenic and genotoxic potential of pure Cylindrospermopsin by a battery of in vitro tests

Cylindrospermopsin (CYN) is a cyanobacterial toxin with an increasing world-wide occurrence. The main route of human exposure is through the ingestion of contaminated food and water. The European Food Safety Authority has identified the need to further characterize the toxicological profile of cyanotoxins and in this regard the genotoxicity is a key toxicological effect. The data available in the scientific literature show contradictory results. Thus, the aim of this study was to investigate the mutagenic and genotoxic effects of pure CYN using a battery of different in vitro assays including: the bacterial reverse-mutation assay in Salmonella typhimurium (Ames test) (0-10 μg/mL), the mammalian cell micronucleus (MN) test (0-1.35 μg/mL and 0-2 μg/mL in absence or presence of S9 fraction, respectively) and the mouse lymphoma thymidine-kinase assay (MLA)(0-0.675 μg/mL) on L5178YTk ± cells, and the standard and enzyme-modified comet assays (0-2.5 μg/mL) on Caco-2 cells. Positive results were obtained only when the metabolic fraction S9 was employed in the MN test, suggesting pro-genotoxic properties of CYN. Also, DNA damage was not mediated by oxidative stress as CYN did not induced changes in the modified comet assay. These data could contribute to a better risk assessment of this cyanotoxin.

In vitro evaluation of the genotoxicity of poly(anhydride) nanoparticles designed for oral drug delivery

In the last years, the development of nanomaterials has significantly increased due to the immense variety of potential applications in technological sectors, such as medicine, pharmacy and food safety. Focusing on the nanodevices for oral drug delivery, poly(anhydride) nanoparticles have received extensive attention due to their unique properties, such as their capability to develop intense adhesive interactions within the gut mucosa, their modifiable surface and their biodegradable and easy-to-produce profile. However, current knowledge of the possible adverse health effects as well as, toxicological information, is still exceedingly limited. Thus, we investigated the capacity of two poly(anhydride) nanoparticles, Gantrez^® AN 119-NP (GN-NP) and Gantrez^® AN 119 covered with mannosamine (GN-MA-NP), and their main bulk material (Gantrez^® AN 119-Polymer), to induce DNA damage and thymidine kinase (TK^+/-) mutations in L5178Y TK^+/- mouse lymphoma cells after 24h of exposure. The results showed that GN-NP, GN-MA-NP and their polymer did not induce DNA strand breaks or oxidative damage at concentrations ranging from 7.4 to 600μg/mL. Besides, the mutagenic potential of these nanoparticles and their polymer revealed no significant or biologically relevant gene mutation induction at concentrations up to 600μg/mL under our experimental settings. Considering the non-genotoxic effects of GN-NP and GN-MA-NP, as well as their exceptional properties, these nanoparticles are promising nanocarriers for oral medical administrations.

Genotoxic effects of synthetic amorphous silica nanoparticles in the mouse lymphoma assay

Synthetic amorphous silica nanoparticles (SAS NPs) have been used in various industries, such as plastics, glass, paints, electronics, synthetic rubber, in pharmaceutical drug tablets, and a as food additive in many processed foods. There are few studies in the literature on NPs using gene mutation approaches in mammalian cells, which represents an important gap for genotoxic risk estimations. To fill this gap, the mouse lymphoma L5178Y/Tk^+/− assay (MLA) was used to evaluate the mutagenic effect for five different concentrations (from 0.01 to 150 μg/mL) of two different sizes of SAS NPs (7.172 and 7.652 nm) and a fine collodial form of silicon dioxide (SiO₂). This assay detects a broad spectrum of mutational events, from point mutations to chromosome alterations. The results obtained indicate that the two selected SAS NPs are mutagenic in the MLA assay, showing a concentration-dependent effect. The relative mutagenic potencies according to the induced mutant frequency (IMF) are as follows: SAS NPs (7.172 nm) (IMF = 705.5 × 10⁻⁶), SAS NPs (7.652 nm) (IMF = 575.5 × 10⁻⁶), and SiO₂ (IMF = 57.5 × 10⁻⁶). These in vitro results, obtained from mouse lymphoma cells, support the genotoxic potential of NPs as well as focus the discussion of the benefits/risks associated with their use in different areas.

In vitro genotoxicity testing of carvacrol and thymol using the micronucleus and mouse lymphoma assays

Currently, antimicrobial additives derived from essential oils (Eos) extracted from plants or spices, such as Origanum vulgare, are used in food packaging. Thymol and carvacrol, the major EO compounds of O. vulgare, have demonstrated their potential use as active additives. These new applications use high concentrations, thereby increasing the concern regarding their toxicological profile and especially their genotoxic risk. The aim of this work was to investigate the potential in vitro genotoxicity of thymol (0-250 μM) and carvacrol (0-2500 μM) at equivalent doses to those used in food packaging. The micronucleus (MN) test and the mouse lymphoma (MLA) assay on L5178Y/Tk(±) mouse lymphoma cells were used. The negative results for thymol with the MN with and without the S9 fraction and also with the MLA assay reinforce the view that this compound is not genotoxic in mammalian cells. However, carvacrol presented slight genotoxic effects, but only in the MN test at the highest concentration assayed (700 μM) and in the absence of metabolic activation. The lack of genotoxic response in the MLA assay after 4 and 24h of exposure indicates a low genotoxic potential for carvacrol. Alternatively, the general negative findings observed in both assays suggest that the MN results of carvacrol are marginal data without biological relevance. These results can be useful to identify the appropriate concentrations of these substances to be used as additives in food packaging.