UDS induction by an array of standard carcinogens in human and rodent hepatocytes: effect of cryopreservation

Investigation of in vivo unscheduled DNA synthesis in rabbit corneas following instillation of genotoxic agents

PURPOSE

An unscheduled DNA synthesis (UDS) test is used for in vitro or in vivo genotoxicity evaluation. The UDS test with hepatocytes is well established; however, drug exposure levels at the application site for topically administered drugs (e.g. ophthalmic drugs) often exceed the exposure levels for systemic administration. To establish in vivo genotoxicity on the ocular surface, we performed the UDS test using rabbit corneas from eyes subjected to instillation of genotoxic agents.

MATERIALS AND METHODS

Five genotoxic agents - 1,1'-dimethyl-4,4'-bipyridinium dichloride (paraquat); acridine orange; ethidium bromide; acrylamide; and 4-nitroquinoline 1-oxide (4-NQO) - were instilled once onto both eyes of male Japanese white rabbits. Physiological saline or a general vehicle for ophthalmic solution were instilled as the negative controls. Dimethyl sulfoxide was instilled as the vehicle control. Isolated corneas were incubated with tritium-labelled thymidine and the number of sparsely labelled cells (SLCs, cells undergoing UDS) was counted by autoradiography.

RESULTS

Statistically significant increases in the mean appearance rates of SLCs in the corneal epithelium were noted in paraquat-, acridine orange-, ethidium bromide-, and 4-NQO-treated eyes compared with those of the controls. These increases generally appeared in a dose-dependent manner. Acrylamide did not induce an increase in the mean appearance rates of SLCs, presumably because it caused the generation of fewer metabolites in the cornea.

CONCLUSIONS

UDS tests revealed DNA damage in the cornea epitheliums treated with well-known genotoxic agents. These results suggest that the UDS test is one of the useful tools for the assessment of in vivo genotoxicity on the ocular surface in the development of ophthalmic drugs.

Structure-dependent genotoxic potencies of selected pyrrolizidine alkaloids in metabolically competent HepG2 cells

1,2-unsaturated pyrrolizidine alkaloids (PAs) are natural plant constituents comprising more than 600 different structures. A major source of human exposure is thought to be cross-contamination of food, feed and phytomedicines with PA plants. In humans, laboratory and farm animals, certain PAs exert pronounced liver toxicity and can induce malignant liver tumors in rodents. Here, we investigated the cytotoxicity and genotoxicity of eleven PAs belonging to different structural classes. Although all PAs were negative in the fluctuation Ames test in Salmonella, they were cytotoxic and induced micronuclei in human HepG2 hepatoblastoma cells over-expressing human cytochrome P450 3A4. Lasiocarpine and cyclic diesters except monocrotaline were the most potent congeners both in cytotoxicity and micronucleus assays with concentrations below 3 μM inducing a doubling in micronuclei counts. Other open di-esters and all monoesters exhibited weaker or much weaker geno- and cytotoxicity. The findings were in agreement with recently suggested interim Relative Potency (iREP) factors with the exceptions of europine and monocrotaline. A more detailed micronuclei analysis at low concentrations of lasiocarpine, retrorsine or senecionine indicated that pronounced hypolinearity of the concentration–response curves was evident for retrorsine and senecionine but not for lasiocarpine. Our findings show that the genotoxic and cytotoxic potencies of PAs in a human hepatic cell line vary in a structure-dependent manner. Both the low potency of monoesters and the shape of prototype concentration–response relationships warrant a substance- and structure-specific approach in the risk assessment of PAs.

Investigation of comet assays under conditions mimicking ocular instillation administration in a three-dimensional reconstructed human corneal epithelial model

Purpose: A comet assay is one of the genotoxicity methods for evaluating the potential of chemicals to induce DNA strand breaks. To investigate the usefulness of comet assays for evaluating the genotoxic potential of ophthalmic solutions, a three-dimensional (3D) reconstructed human corneal epithelial model (3D corneal model) was exposed to conditions mimicking topical ocular instillation administration. Methods: The 3D corneal model was exposed to acridine orange, ethidium bromide, hydrogen peroxide, 1,1'-dimethyl-4,4'-bipyridinium dichloride (paraquat), 4-nitroquinoline 1-oxide (4-NQO), acrylamide and methyl methanesulfonate (MMS). To mimic the ocular surface condition to which ophthalmic solutions are administered, the exposure time was set to 1 minute. Likewise, human corneal epithelial (HCE-T) cells, as monolayer cultured cells, were exposed to the same chemicals, for comparison. Results: In the 3D corneal model, the amount of DNA fragments was statistically significantly increased in cells treated with each of the test chemicals except acrylamide. In HCE-T cells, the amount of DNA fragments was statistically significantly increased in acridine orange-, ethidium bromide-, hydrogen peroxide-, 4-NQO- and MMS-treated cells but not in paraquat- or acrylamide-treated cells. In the 3D corneal model, the lowest concentrations at which we observed DNA damage were about 100 times higher than the concentrations in HCE-T cells. Since the 3D corneal model is morphologically similar to human corneal tissue, form a multilayer and having tight junctions, it may be that the test chemicals only permeated about 1% into the 3D corneal model. Conclusion: These results suggest that the comet assay using 3D cell culture models may reflect in vivo conditions better than do monolayer cultured cells, and that the comet assay may be useful for the evaluation of genotoxic potential of topical ophthalmic solution.

Interim relative potency factors for the toxicological risk assessment of pyrrolizidine alkaloids in food and herbal medicines

Pyrrolizidine alkaloids (PAs) are among the most potent natural toxins occurring in a broad spectrum of plant species from various families. Recently, findings of considerable contamination of teas/herbal infusions prepared from non-PA plants have been reported. These are obviously due to cross-contamination with minor amounts of PA plants and can affect both food and herbal medicines. Another source of human exposure is honey collected from PA plants. These findings illustrate the requirement for a comprehensive risk assessment of PAs, hampered by the enormous number of different PA congeners occurring in nature. Up to now, risk assessment is based on the carcinogenicity of certain PAs after chronic application to rats using the sum of detected PAs as dose metric. Because of the well-documented large structure-dependent differences between sub-groups of PA congeners with respect to their genotoxicity and (cyto)toxicity, however, this procedure is inadequate. Here we provide an overview of recent attempts to assess the risk of PA exposure and the available literature on the toxic effects and potencies of different congeners. Based on these considerations, we have derived interim Relative Potency (REP) factors for a number of abundant PAs suggesting a factor of 1.0 for cyclic di-esters and open-chain di-esters with 7S configuration, of 0.3 for mono-esters with 7S configuration, of 0.1 for open-chain di-esters with 7R configuration and of 0.01 for mono-esters with 7R configuration. For N-oxides we suggest to apply the REP factor of the corresponding PA. We are confident that the use of these values can provide a more scientific basis for PA risk assessment until a more detailed experimental analysis of the potencies of all relevant congeners can be carried out.

Development of Safe and Effective Botanical Dietary Supplements

Regulated differently than drugs or foods, the market for botanical dietary supplements continues to grow worldwide. The recently implemented U.S. FDA regulation that all botanical dietary supplements must be produced using good manufacturing practice is an important step toward enhancing the safety of these products, but additional safeguards could be implemented, and unlike drugs, there are currently no efficacy requirements. To ensure a safe and effective product, botanical dietary supplements should be developed in a manner analogous to pharmaceuticals that involves identification of mechanisms of action and active constituents, chemical standardization based on the active compounds, biological standardization based on pharmacological activity, preclinical evaluation of toxicity and potential for drug-botanical interactions, metabolism of active compounds, and finally, clinical studies of safety and efficacy. Completing these steps will enable the translation of botanicals from the field to safe human use as dietary supplements.

Current industrial practices in assessing CYP450 enzyme induction: preclinical and clinical

Induction of drug metabolizing enzymes, such as the cytochromes P450 (CYP) is known to cause drug-drug interactions due to increased elimination of co-administered drugs. This increased elimination may lead to significant reduction or complete loss of efficacy of the co-administered drug. Due to the significance of such drug interactions, many pharmaceutical companies employ screening and characterization models which predict CYP enzyme induction to avoid or attenuate the potential for drug interactions with new drug candidates. The most common mechanism of CYP induction is transcriptional gene activation. Activation is mediated by nuclear receptors, such as AhR, CAR, and PXR that function as transcription factors. Early high throughput screening models utilize these nuclear hormone receptors in ligand binding or cell-based transactivation/reporter assays. In addition, immortalized hepatocyte cell lines can be used to assess enzyme induction of specific drug metabolizing enzymes. Cultured primary human hepatocytes, the best established in vitro model for predicting enzyme induction and most accepted by regulatory agencies, is the predominant assay used to evaluate induction of a wide variety of drug metabolizing enzymes. These in vitro models are able to appropriately predict enzyme induction in patients when compared to clinical drug-drug interactions. Finally, transgenic animal models and the cynomolgus monkey have also been shown to recapitulate human enzyme induction and may be appropriate in vivo animal models for predicting human drug interactions.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

The Liverbeads as a tool for the comet assay

Liverbeads, cryopreserved hepatocytes entrapped within an alginate matrix, were examined for their relevance in the comet assay. It was estimated by their capacity to activate the indirectly acting mutagens, cyclophosphamide (CP), benzo[a]pyrene (BP), dimethylbenzanthracene (DMBA) and 2-acetylaminofluorene (2-AAF), into DNA reactive metabolites. The comet assay performed in alkaline condition is a sensitive method for detecting strand breaks at the level of individual cells and allows use of quiescent cells. Experimental conditions as treatment time, cell density, beads dissociation and viability were investigated. Significant statistical positive results assessed by the tail extent moment (TEM) were observed with both human and rat Liverbeads after 12h duration incubation compared to metabolic non-competent cells, HeLa S3. Due to the maintenance of specific functions assessed by the observed capacity to metabolize xenobiotics, Liverbeads represent a suitable tool system, easy to handle, for the detection of promutagens using the comet assay.