Bacterial mutagenicity screening in the pharmaceutical industry.

Genetic toxicity testing is used as an early surrogate for carcinogenicity testing. Genetic toxicity testing is also required by regulatory agencies to be conducted prior to initiation of first in human clinical trials and subsequent marketing for most small molecule pharmaceutical compounds. To reduce the chances of advancing mutagenic pharmaceutical candidates through the drug discovery and development processes, companies have focused on developing testing strategies to maximize hazard identification while minimizing resource expenditure due to late stage attrition. With a large number of testing options, consensus has not been reached on the best mutagenicity platform to use or on the best time to use a specific test to aid in the selection of drug candidates for development. Most companies use a process in which compounds are initially screened for mutagenicity early in drug development using tests that require only a few milligrams of compound and then follow those studies up with a more robust mutagenicity test prior to selecting a compound for full development. This review summarizes the current applications of bacterial mutagenicity assays utilized by pharmaceutical companies in early and late discovery programs. The initial impetus for this review was derived from a workshop on bacterial mutagenicity screening in the pharmaceutical industry presented at the 40th Annual Environmental Mutagen Society Meeting held in St. Louis, MO in October, 2009. However, included in this review are succinct summaries of use and interpretation of genetic toxicity assays, several mutagenicity assays that were not presented at the meeting, and updates to testing strategies resulting in current state-of the art description of best practices. In addition, here we discuss the advantages and liabilities of many broadly used mutagenicity screening platforms and strategies used by pharmaceutical companies. The sensitivity and specificity of these early mutagenicity screening assays using proprietary compounds and their concordance (predictivity) with the regulatory bacterial mutation test are discussed.

Predictive models for carcinogenicity and mutagenicity: frameworks, state-of-the-art, and perspectives.

Mutagenicity and carcinogenicity are endpoints of major environmental and regulatory concern. These endpoints are also important targets for development of alternative methods for screening and prediction due to the large number of chemicals of potential concern and the tremendous cost (in time, money, animals) of rodent carcinogenicity bioassays. Both mutagenicity and carcinogenicity involve complex, cellular processes that are only partially understood. Advances in technologies and generation of new data will permit a much deeper understanding. In silico methods for predicting mutagenicity and rodent carcinogenicity based on chemical structural features, along with current mutagenicity and carcinogenicity data sets, have performed well for local prediction (i.e., within specific chemical classes), but are less successful for global prediction (i.e., for a broad range of chemicals). The predictivity of in silico methods can be improved by improving the quality of the data base and endpoints used for modelling. In particular, in vitro assays for clastogenicity need to be improved to reduce false positives (relative to rodent carcinogenicity) and to detect compounds that do not interact directly with DNA or have epigenetic activities. New assays emerging to complement or replace some of the standard assays include Vitotox, GreenScreenGC, and RadarScreen. The needs of industry and regulators to assess thousands of compounds necessitate the development of high-throughput assays combined with innovative data-mining and in silico methods. Various initiatives in this regard have begun, including CAESAR, OSIRIS, CHEMOMENTUM, CHEMPREDICT, OpenTox, EPAA, and ToxCast. In silico methods can be used for priority setting, mechanistic studies, and to estimate potency. Ultimately, such efforts should lead to improvements in application of in silico methods for predicting carcinogenicity to assist industry and regulators and to enhance protection of public health.

An assessment of the carcinogenic potential of ezetimibe using nonclinical data in a weight-of-evidence approach.

Ezetimibe blocks intestinal absorption of sterols via interaction with the Neimann-Pick C1-Like 1 (NPC1L1) transporter and is approved for use in the treatment of primary hyperlipidemia (heterozygous familial and non-familial), homozygous familial hypercholesterolemia, and homozygous sitosterolemia. A recently completed randomized clinical trial [simvastatin and ezetimibe in aortic stenosis (SEAS)] testing the effectiveness of Vytorin (a combination of simvastatin and ezetimibe) in patients with aortic stenosis reported an unexpected safety finding: an increase in overall cancer incidence and cancer-associated mortality (all types) in the treated groups relative to the placebo control. A subsequent meta-analysis utilizing a much larger database from two ongoing clinical trials indicated that the observed findings in the SEAS trial were likely due to chance and not a true drug-induced effect. Nonetheless, it has been suggested by various commentators on the SEAS trial that ezetimibe may be carcinogenic. The extensive nonclinical database for ezetimibe was used to test the hypothesis that ezetimibe may be a direct or indirect carcinogen. Using two different in silico approaches, ezetimibe showed no structural alerts for genetic toxicity or carcinogenicity. Ezetimibe was not genotoxic in two reverse mutation assays, one in vitro clastogenicity assay, and two mouse micronucleus assays. No evidence of proliferative lesions was observed in three species in studies of 1-12 months in duration. Ezetimibe was not carcinogenic in standard 2-year bioassays in mice and rats. Additionally, in these 2-year bioassays, no drug-related non-neoplastic lesions were noted. The absence of drug-induced non-neoplastic or proliferative lesions in these studies indicates that ezetimibe treatment was not associated with findings characteristic of carcinogens (i.e., DNA reactivity or cell proliferation) Administration of pharmacologic doses of ezetimibe to mice did not alter hepatic expression patterns of genes associated with apoptosis, cell proliferation, or epithelial-mesenchymal transition. No evidence of drug-induced tumors was observed in mice in which the molecular target of ezetimibe (NPC 1L1) was knocked out over the life span of the animal. In conclusion, the nonclinical data do not support the proposed hypothesis based on the single observation from the SEAS trial and, rather, support the conclusion that ezetimibe does not represent a carcinogenic hazard to humans using this drug in a therapeutic setting.

Information from the Internet: attitudes of Australian oncology patients.

BACKGROUND: Patients require accurate information about their illness to make informed decisions. Many sources of information exist, although reliability is variable. Our objective was to investigate information seeking behaviour and attitudes toward health-related information from the Internet in a sample of Australian oncology patients. METHOD: During their outpatient attendance, 109 patients completed a self-administered paper-pen format questionnaire. They were required to have a recent cancer diagnosis (<6 months ago) adequate English and no cognitive impairment. RESULTS: Seventy-four per cent of questionnaires were returned. The majority of patients (78%) wanted as much information about their cancer diagnosis as possible and 90% reported receiving adequate information from their treating team. Despite this, more than half actively searched for additional information, with 77% using the Internet. Patients were trusting of information obtained from the Internet. More than half of information searchers discussed information obtained in their search with a health professional. The majority of patients did not believe that information searching adversely affected the doctor-patient relationship. CONCLUSION: Information searching is common in ambulatory Australian oncology patients, with the Internet being a frequently used resource. To ensure patients find reliable and relevant information and to minimize the risk of harm, health professionals involved in treating oncology patients should provide guidance in finding information sources and assistance in interpreting the information obtained.

The influence of N-dialkyl and other cationic substituents on DNA intercalation and genotoxicity.

DNA intercalation by small chemical molecules can result in frameshift mutagenesis and chromosomal breakage. With evidence mounting that broadly diverse structures are capable of intercalating between DNA base pairs, it becomes important to better define those structural features that enhance intercalation strength and those that confer genotoxicity particularly among those intercalators that do not have the classical planar tricyclic fused ring structure. A chemical substituent that is present on many pharmaceutical and other biologically active molecules is the N-dialkyl group. In the present study, we investigate if and how the presence of an aromatic N-dialkyl or other cationic group affects the genotoxicity and DNA intercalation ability of 26 selected acridines, phenothiazines, benzophenones, triphenylethylenes and other classes of molecules. The data were obtained from the literature, from experiments using a cell-based DNA intercalation assay, and from modeling studies using a three-dimensional computational DNA docking program. It is demonstrated that cationic substitution can enhance both genotoxicity and electrostatic interactions within a chemical/DNA intercalation complex.

New concepts in breast cancer therapy.

Breast cancer is a common disease in the community. Its incidence is increasing but its mortality is falling. New therapeutic approaches based on a biological understanding of the disease are being investigated to improve the outcome. These include the areas of chemoprevention and therapy in the adjuvant and advanced settings, and some derive from a better understanding of the biology of breast cancer. New prognostic indicators are being studied to enable adjuvant chemotherapy to be directed towards those most likely to benefit. Trastuzumab was the first of the new targeted therapies to be introduced into routine practice. Several other agents targeting a variety of receptors and cell pathways, such as the epidermal growth factor receptor, the vascular epithelial growth factors and various intracellular tyrosine kinase mediated pathways, are under investigation. Increasing attention is being placed on evidence-based supportive care, both for control of symptoms such as hot flushes and for psychosocial problems such as anxiety and depression.

Navajo neurohepatopathy: a mitochondrial DNA depletion syndrome?

Navajo neurohepatopathy (NNH) is an autosomal recessive disease of full-blooded Navajo children living in the Navajo Reservation of southwestern United States. Clinical features of NNH include peripheral and central nervous system involvement, acral mutilation, corneal scarring or ulceration, liver failure, and metabolic and immunologic derangement. The cause of NNH is unknown, but the clinical features of NNH are similar to those of patients with mitochondrial DNA (mtDNA) depletion. Therefore, we studied mtDNA concentration in the liver from 2 patients with NNH. Using histochemical, biochemical, and molecular techniques, we found evidence of mtDNA depletion, and we propose that the primary defect in NNH is in the nuclear regulation of mtDNA copy number.

In vitro genotoxicity testing of (1-chloroethenyl)oxirane, a metabolite of beta-chloroprene.

(1-Chloroethenyl)oxirane (CEO) is a metabolite of beta-chloroprene (2-chloro-1,3-butadiene, CD). The purpose of this study was to evaluate the in vitro mutagenic and clastogenic (chromosome breaking) potential of CEO. For comparative purposes, the study also included an evaluation of the racemic compounds, 3,4-epoxy-1-butene (EB) and 1,2:3,4-diepoxybutane (DEB). Mutagenicity was evaluated in a bacterial reverse mutation test (Ames), using the pre-incubation method in the presence and absence of an exogenous metabolism system (Aroclor)-induced rat liver S9). Four Salmonella typhimurium tester strains, TA97a, TA98, TA100 and TA1535 were used. The exposure concentrations in the sealed incubation vials ranged from 0 to 69 mM for CEO, 0 to 102 mM for EB, and 0 to 83 mM for DEB. All three compounds showed signs of toxicity, with DEB being substantially more toxic than either CEO or EB. Mutagenic activity was observed with all three chemicals in primarily the base pair substitution strains (S. typhimurium TA100 and TA1535), but some activity was also seen in the frameshift elimination strains (S. typhimurium TA97a and TA98). The observed mutagenic responses after exposure with CEO or EB were greater than the observed response for DEB, most likely because of the higher toxicity of DEB. Generally, the mutagenic responses were unchanged in the frameshift strains and base pair substitution strains in the presence of S9 metabolism. In vitro clastogenicity was evaluated using the cytochalasin-B blocked micronucleus test in cultured Chinese hamster V79 cells. The test was conducted without S9 metabolism because of the absence of substantial changes in the Ames test. Exposure concentrations ranged from 0 to 0.943 mM for CEO, 0 to 3.0 mM for EB, and 0 to 0.035 mM for DEB, with the upper exposure concentrations dictated by cytotoxicity. Cytotoxicity, measured as a reduction in the proportion of binucleated cells and altered cell morphology, was observed for CEO at concentrations > or =0.175 mM. Exposure to EB led to a reduced proportion of binucleated cells at concentrations > or =2.0 mM, and cell death was observed after DEB exposure at concentrations > or =0.025 mM. No clastogenicity was observed in the V79 cells when tested up to cytotoxic concentrations of CEO, whereas an elevated frequency of micronuclei was observed after exposure to either EB (> or =1.0 mM) or DEB (> or =0.0125 mM). These results suggest that CEO-induced mutagenicity, but not clastogenicity, may contribute to the observed beta-chloroprene-induced carcinogenicity in the rodent bioassay studies.

A review of the genotoxicity of marketed pharmaceuticals.

Information in the 1999 Physician's Desk Reference as well as from the peer-reviewed published literature was used to evaluate the genotoxicity of marketed pharmaceuticals. This survey is a compendium of genotoxicity information and a means to gain perspective on the inherent genotoxicity of structurally diverse pharmaceuticals. Data from 467 marketed drugs were collected. Excluded from analysis were anti-cancer drugs and nucleosides, which are expected to be genotoxic, steroids, biologicals and peptide-based drugs. Of the 467 drugs, 115 had no published gene-tox data. This group was comprised largely of acutely administered drugs such as antibiotics, antifungals, antihistamines decongestants and anesthetics. The remaining 352 had at least one standard gene-tox assay result. Of these, 101 compounds (28.7%) had at least one positive assay result in the pre-ICH/OECD standard four-test battery (bacterial mutagenesis, in vitro cytogenetics, mouse lymphoma assay (MLA), in vivo cytogenetics). Per assay type, the percentage of positive compounds was: bacterial mutagenesis test, 27/323 (8.3%); in vitro cytogenetics 55/222 (24.8%); MLA 24/96 (25%); in vivo cytogenetics 29/252 (11.5%). Of the supplemental genetic toxicology test findings reported, the sister chromatid exchange (SCE) assay had the largest percentage of positives 17/39 (43.5%) and mammalian mutagenesis assays (excluding MLA) had the lowest percentage of positives 2/91 (2.2%). The predictive value of genetic toxicology findings for 2-year bioassay outcomes is difficult to assess since carcinogenicity can occur via non-genotoxic mechanisms. Nevertheless, the following survey findings were made: 201 drugs had both gene-tox data and rodent carcinogenicity data. Of these, 124 were negative and 77 were equivocal or positive for carcinogenicity in at least 1 gender/1 species. Of the 124 non-carcinogens, 100 had no positive gene-tox findings. Of the remaining 24, 19 were positive in in vitro cytogenetics assays. Among the 77 compounds that exhibited equivocal or positive effects in carcinogenesis studies, 26 were positive in gene-tox assays and 51 were negative. Of the 51 negatives, 47 had multiple negative gene-tox assay results suggesting that these are probably non-genotoxic carcinogens. Statistical analyses suggested that no combination of gene-tox assays provided a higher predictivity of rodent carcinogenesis than the bacterial mutagenicity test itself.

Catalytic inhibition of DNA topoisomerase IIalpha by sodium azide.

It has been demonstrated previously that sodium azide reduces the clastogenicity of several DNA topoisomerase II (topo II) poisons in cultured mammalian cells. These studies suggested that azide may be a catalytic topo II inhibitor. Azide interferes with mitochondrial production of ATP and is also known to inhibit cellular ATPases. Since topo II requires ATP for catalytic activity (enzyme turnover), it seemed likely that interference with ATP levels or ATP catabolism was the underlying mechanism of topo II inactivation; however, this has not been examined in living cells under conditions where the endogenous topo II is active on genomic DNA. The present studies were carried out to verify that azide inhibits endogenous topo II in cells. We show that azide blocks both decatenation and relaxation activity of purified topo II in a concentration dependent manner and reduces topoII/DNA covalent complex formation in cells. From these studies, it is concluded that sodium azide catalytically inactivates topo II via an ATP-sensitive process.

Comparison of the results of a modified miniscreen and the standard bacterial reverse mutation assays.

The bacterial reverse mutation assay (Ames test) provides a rapid assessment of the mutagenic potential of chemicals. The assay is widely used in the pharmaceutical industry for early assessment during candidate compound selection and for regulatory drug submissions. Early in development, many candidate compounds are available in only very small quantities. The use of the standard plate incorporation bacterial reverse mutation assay for screening, using only a single petri plate per concentration, requires the use of approximately 140 mg of test compound to test up to a stock concentration of 100 mg/ml (5000 microg/plate) in five strains of bacteria. A modification of the existing Ames Miniscreen assay has been developed using six-well cell-culture dishes that requires only 21 mg of compound to test a stock concentration of up to 100 mg/ml (2000 microg/well) in three strains of bacteria. The standard plate incorporation assay and the modified Miniscreen assays conducted on proprietary compounds without and with metabolic activation have yielded a high degree of concordance in findings.

The bleomycin amplification assay in V79 cells predicts frameshift mutagenicity of intercalative agents.

We have recently reported on the use of a cell-based bleomycin amplification assay for the detection of DNA intercalating agents. In order to further validate this assay, two series of proprietary compounds were evaluated for frameshift mutagenesis in the Ames bacterial reversion system and for bleomycin amplification in the Chinese hamster V79 micronucleus system. It is shown that 10 of 11 frameshift-positive compounds were bleomycin amplifiers. These studies indicate that positive frameshift mutagenicity findings are consistent with expectations from the results of the bleomycin amplification assay, providing additional validation of the amplification assay for the detection of DNA intercalating agents. The studies also demonstrate that intercalation is necessary but not sufficient for frameshift mutagenesis since bleomycin amplifiers lacking frameshift mutagenic activity were also identified.

Further evidence that the radioprotective aminothiol, WR-1065, catalytically inactivates mammalian topoisomerase II.

It has recently been proposed that the thiol form of the cytoprotective drug amifostine that is designated WR-1065 [2-((aminopropyl)amino)ethanethiol] exerts its cytoprotective effects in part via a catalytic inhibition of DNA topoisomerase II (topo II)alpha. This in turn leads to the subsequent accumulation of cells in G2 phase and a prolongation of the cell cycle. We have used a Chinese hamster V79 cell-based micronucleus assay to further evaluate this hypothesis. It is demonstrated that WR-1065 strongly inhibits the clastogenesis of the topo II poisons etoposide and clinafloxacin at clinically attained exposure levels while having no effect on clastogenesis induced by topo II-noninteractive chemicals. These findings are consistent with the hypothesis that WR-1065 is a catalytic inhibitor of topo II in mammalian cells. These studies also suggest that WR-1065 might be expected to reduce the toxicity and clastogenicity in clinical applications of etoposide or quinolone antibiotics in dose-limiting normal tissues.

Considerations on photochemical genotoxicity: report of the International Workshop on Genotoxicity Test Procedures Working Group.

Recent toxicological observations have caused concern regarding the need to test, for example, pharmaceuticals and cosmetic products for photochemical genotoxicity. The objective of this report is to give assistance on how to adapt existing test methods to investigate the potential of light-absorbing compounds to induce genotoxic effects on photoactivation. In general, the Organization for Economic Co-Operation & Economic Development (OECD) draft guideline on in vitro phototoxicity testing served as a basis for consideration. Concomitant exposure of the cells to the test compound and solar simulated light was considered appropriate as the initial, basic test condition. Optimization of the exposure scheme, e.g., a change of the irradiation spectrum, might be indicated depending on the initial test results. Selection of test compound concentrations should be based on results obtained with the dark version of the respective test system but might have to be modified if phototoxic effects are observed. Selection of the irradiation dose has to be performed individually for each test system based on dose-effect studies. The irradiation should induce per se a small, reproducible toxic or genotoxic effect. The report includes a specification of necessary controls, discusses factors that might have an impact on the irradiation characteristics, and gives a rationale for the omission of an external metabolic activation system. It also addresses the question that physicochemical and pharmacokinetic properties might trigger the need to test a chemical for photochemical genotoxicity. Relevant experimental observations are presented to back up the recommendations. The working group did not reach a consensus as to whether a single, adequately perfomed in vitro test for clastogenicity would be sufficient to exclude a photogenotoxic liability or whether a test battery including a gene mutation assay would be needed for product safety testing regarding photochemical genotoxicity.

Use of catalytic topoisomerase II inhibitors to probe mechanisms of chemical-induced clastogenicity in Chinese hamster V79 cells.

Determination of the clastogenic potential of new chemical entities, particularly pharmaceuticals, is an important part of the overall safety assessment of such drugs. It is appreciated that clastogenicity can arise from perturbation of many different cellular processes distinct from direct DNA/drug interactions. One such alternative clastogenic process is inhibition of DNA topoisomerase II, during which process the topoisomerase/DNA/drug ternary complex forms stable DNA double-strand breaks (cleavable complex), which become templates for recombinational, mutagenic, and chromosomal fragmentation events. Without extensive experimentation, it is generally not possible to distinguish clastogenicity arising from direct drug/DNA interaction from that arising from inhibition of topoisomerase II. In the present investigation, we demonstrate that specific catalytic inhibitors of DNA topoisomerase II reduce the clastogenicity of topoisomerase poisons but not that arising via non-topoisomerase-dependent mechanisms. In particular, it is shown that catalytic topoisomerase II inhibitors such as chloroquine, sodium azide, and A-74932, as well as certain intercalating agents such as 9-aminoacridine and ethidium bromide, strongly antagonize the formation of micronuclei induced by the DNA gyrase inhibitor clinafloxacin and the antitumor topoisomerase II poison etoposide. These catalytic inhibitors are also shown to antagonize the clastogenicity of experimental compounds and novel pharmaceuticals presumed to be DNA intercalating agents by virtue of their response in a cell-based bleomycin amplification assay. We extend our previous hypothesis, suggesting that the clastogenicity of some nonstructurally alerting drugs may be due to an as yet unappreciated propensity for DNA intercalation. It is further proposed that intercalation-dependent inhibition of DNA topoisomerase II may be responsible for this clastogenicity and that this may be detected in intact mammalian cells with the use of catalytic topoisomerase inhibitors.

Enhancement of bleomycin-induced micronucleus formation in V79 cells as a rapid and sensitive screen for non-covalent DNA-binding compounds.

Non-covalent drug/DNA interactions are difficult to study and because of this, the significance of such interactions from a safety standpoint and their contribution to positive genetic toxicology test findings is poorly understood. It is shown in the present study that such interactions may be detected and quantified in Chinese hamster V79 cells by an adaptation of the bleomycin amplification assay. This assay measures the ability of a test compound to enhance the DNA damaging activity of the antibiotic bleomycin using micronucleus formation as an endpoint. Results are presented examining the bleomycin amplification activity of known intercalating agents, groove-binding agents and other structurally diverse classes of compounds for which intercalative status has not been reported. The assay reveals a strong and predictable SAR for amplification activity based on number and orientation of aromatic rings. Moreover, excellent correlations are observed between DNA binding (viscometric analyses) and DNA amplification in V79 cells for a series of seven experimental compounds. The assay is shown to be useful in understanding the genotoxicity of marketed antihistamines and to help explain genetic toxicology findings observed in a series of novel pharmaceutical entities. It is proposed that assessment of bleomycin amplification activity of novel compounds in early genotoxicity prescreening may provide important information upon which to base synthesis of compounds with minimal or no genotoxic liability.