Are lifespan rodent carcinogenicity studies defensible for pharmaceutical agents?

The failure of rodent carcinogenesis as a model for Man

Recent advances in our understanding of the process of carcinogenesis in Man have required revision of our thinking about the classical initiation/promotion sequence; understanding must now encompass the roles of both genetic and epigenetic change, realisation of the importance of the variable genetic backgrounds of the tumour bearers in any group and an understanding of the importance of random genetic events over time. The behavior of tumours, once established, is more complex than has been thought. Current views of the processes involved are not modelled in toxicity testing programmes.

Reassessing the two-year rodent carcinogenicity bioassay: a review of the applicability to human risk and current perspectives

The 2-year rodent carcinogenicity test has been the regulatory standard for the prediction of human outcomes for exposure to industrial and agro-chemicals, food additives, pharmaceuticals and environmental pollutants for over 50 years. The extensive experience and data accumulated over that time has spurred a vigorous debate and assessment, particularly over the last 10 years, of the usefulness of this test in terms of cost and time for the information obtained. With renewed interest in the United States and globally, plus new regulations in the European Union, to reduce, refine and replace sentinel animals, this review offers the recommendation that reliance on information obtained from detailed shorter-term, 6 months rodent studies, combined with genotoxicity and chemical mode of action can realize effective prediction of human carcinogenicity instead of the classical two year rodent bioassay. The aim of carcinogenicity studies should not be on the length of time, and by obligation, number of animals expended but on the combined systemic pathophysiologic influence of a suspected chemical in determining disease. This perspective is in coordination with progressive regulatory standards and goals globally to utilize effectively resources of animal usage, time and cost for the goal of human disease predictability.

Evaluation of carcinogenicity studies of medicinal products for human use authorised via the European centralised procedure (1995-2009)

Carcinogenicity data of medicinal products for human use that have been authorised via the European centralised procedure (CP) between 1995 and 2009 were evaluated. Carcinogenicity data, either from long-term rodent carcinogenicity studies, transgenic mouse studies or repeat-dose toxicity studies were available for 144 active substances contained in 159 medicinal products. Out of these compounds, 94 (65%) were positive in at least one long-term carcinogenicity study or in repeat-dose toxicity studies. Fifty compounds (35%) showed no evidence of a carcinogenic potential. Out of the 94 compounds with positive findings in either carcinogenicity or repeat-dose toxicity studies, 33 were positive in both mice and rats, 40 were positive in rats only, and 21 were positive exclusively in mice. Long-term carcinogenicity studies in two rodent species were available for 116 compounds. Data from one long-term carcinogenicity study in rats and a transgenic mouse model were available for eight compounds. For 13 compounds, carcinogenicity data were generated in only one rodent species. One compound was exclusively tested in a transgenic mouse model. Six compounds were tumourigenic in repeat-dose toxicity studies in rats. The majority of tumour findings observed in rodent carcinogenicity studies were considered not to be relevant for humans, either due to a rodent-specific mechanism of carcinogenicity, a high safety margin between exposures at the NOAEL (No Observed Adverse Effect Level) in rodents and recommended therapeutic doses in humans, or based on historical control data, a small effect size and lack of dose-response relationship and tumours typically observed in rodent strains used, or were considered not to be relevant for humans based on literature and clinical data or likely differences in metabolism/local concentrations between rodents and humans. Due to the high number of rodent tumour findings with unlikely relevance for humans, the value of the currently used testing strategy for carcinogenicity appears questionable. A revision of the carcinogenicity testing paradigm is warranted.

Cytotoxicity and genotoxicity reporter systems based on the use of mammalian cells

With the dramatic increase in the number of new agents arising from the chemical, pharmaceutical, and agricultural industries, there is an urgent need to develop assays for rapid evaluation of potential risks to man and environment. The panel of conventional tests used for cytotoxicity and genotoxicity and the strategies to progress from small scale assays to high content screening in toxicology are discussed. The properties of components necessary as sensors and reporters for new reporter assays, and the application of genetic strategies to design assays are reviewed. The concept of cellular reporters is based on the use of promoters of chemical stress-regulated genes ligated to a suitable luminescent or fluorescent reporter gene. Current reporter assays designed from constructs transferred into suitable cell lines are presented.

Animal carcinogenicity studies: 3. Alternatives to the bioassay

Conventional animal carcinogenicity tests take around three years to design, conduct and interpret. Consequently, only a tiny fraction of the thousands of industrial chemicals currently in use have been tested for carcinogenicity. Despite the costs of hundreds of millions of dollars and millions of skilled personnel hours, as well as millions of animal lives, several investigations have revealed that animal carcinogenicity data lack human specificity (i.e. the ability to identify human non-carcinogens), which severely limits the human predictivity of the bioassay. This is due to the scientific inadequacies of many carcinogenicity bioassays, and numerous serious biological obstacles, which render profoundly difficult any attempts to accurately extrapolate animal data in order to predict carcinogenic hazards to humans. Proposed modifications to the conventional bioassays have included the elimination of mice as a second species, and the use of genetically-altered or neonatal mice, decreased study durations, initiation-promotion models, the greater incorporation of toxicokinetic and toxicodynamic assessments, structure-activity relationship (computerised) systems, in vitro assays, cDNA microarrays for detecting changes in gene expression, limited human clinical trials, and epidemiological research. The potential advantages of non-animal assays when compared to bioassays include the superior human specificity of the results, substantially reduced time-frames, and greatly reduced demands on financial, personnel and animal resources. Inexplicably, however, the regulatory agencies have been frustratingly slow to adopt alternative protocols. In order to decrease the enormous cost of cancer to society, a substantial redirection of resources away from excessively slow and resource-intensive rodent bioassays, into the further development and implementation of non-animal assays, is both strongly justified and urgently required.

Animal carcinogenicity studies: 1. Poor human predictivity

The regulation of human exposure to potentially carcinogenic chemicals constitutes society's most important use of animal carcinogenicity data. Environmental contaminants of greatest concern within the USA are listed in the Environmental Protection Agency's (EPA's) Integrated Risk Information System (IRIS) chemicals database. However, of the 160 IRIS chemicals lacking even limited human exposure data but possessing animal data that had received a human carcinogenicity assessment by 1 January 2004, we found that in most cases (58.1%; 93/160), the EPA considered animal carcinogenicity data inadequate to support a classification of probable human carcinogen or non-carcinogen. For the 128 chemicals with human or animal data also assessed by the World Health Organisation's International Agency for Research on Cancer (IARC), human carcinogenicity classifications were compatible with EPA classifications only for those 17 having at least limited human data (p = 0.5896). For those 111 primarily reliant on animal data, the EPA was much more likely than the IARC to assign carcinogenicity classifications indicative of greater human risk (p < 0.0001). The IARC is a leading international authority on carcinogenicity assessments, and its significantly different human carcinogenicity classifications of identical chemicals indicate that: 1) in the absence of significant human data, the EPA is over-reliant on animal carcinogenicity data; 2) as a result, the EPA tends to over-predict carcinogenic risk; and 3) the true predictivity for human carcinogenicity of animal data is even poorer than is indicated by EPA figures alone. The EPA policy of erroneously assuming that tumours in animals are indicative of human carcinogenicity is implicated as a primary cause of these errors.

Unexpected tumour findings in lifetime rodent bioassay studies--what to do?

Currently, the majority of substances tested in lifetime bioassays in rodents are not mutagenic and, therefore, at the most weakly carcinogenic, generally by epigenetic mechanisms. It thus appears obvious that only marginal increases of tumour incidences can be expected in lifetime bioassays and that, therefore, every aspect of a potential carcinogenic effect must be thoroughly evaluated. This paper describes a series of key factors, which should be looked at in order to exclude that the lifetime bioassay in question is flawed for design, technical or qualification reasons. It also provides some hints whether there is indeed a real effect and not just a variation of the spontaneous tumour incidences. Tumour findings must be seen in the context of the animal model, the pharmcokinetics and pharmcodynamics of the test substance, as well as any other observation in the present or other studies with the test substance, including non-tumour findings and--in particular--potential precursor lesions and effects on feed intake and survival. The possibility that the observed carcinogenic effects may be species-specific and not relevant for man is discussed. It is also important to check what findings are reported with similar substances or substances with the same pharmacological effect. Data from additional investigations on material of the same study and/or mechanistic studies are often needed to support the final risk assessment.

Application of genetically altered models as replacement for the lifetime mouse bioassay in pharmaceutical development

The international pharmaceutical regulatory academic and industrial toxicology communities are collaborating to improve the efficiency and effectiveness of cancer hazard identification based on dramatic improvements in our understanding of the cancer process. Guidelines emanating from the International Conference on Harmonization provide for use of in vivo alternatives. Standard practices utilizing lifetime rat and mouse studies are recognized as seriously flawed with over 80% false positive rates. Furthermore, tobacco, the most important human carcinogen commercialized by industry, is negative in these traditional lifetime studies. The lifetime mouse bioassay is generally recognized in pharmaceutical development as not adding value in safety assessment. An international consortium under the aegis of ILSI has recently completed an evaluation of alternative mouse cancer models. Transgenic models are less expensive, use fewer animals and take less time than traditional lifetime bioassays. These alternative models have now been sufficiently evaluated to be considered useful in the safety assessment plan for pharmaceuticals in development. Specifically for example, the rasH2 appears useful in detecting nongenotoxic as well as genotoxic rodent tumorigens with improved concordance with human response. The p53+/- heterozygous mouse apparently identifies hormonal carcinogenic mechanisms, immunosuppressive carcinogens, and genotoxic carcinogens. The TG:AC predicts for rodent tumorigens applied topically. Recent experiences at FDA, CPMP, and MHW indicate that with good planning and agency interactions, regulatory acceptability can be anticipated.

Melatonin increases both life span and tumor incidence in female CBA mice

From the age of 6 months until their natural deaths, female CBA mice were given melatonin with their drinking water (20 mg/l) for 5 consecutive days every month. Intact mice served as controls. The results of this study show that the consumption of melatonin did not significantly influence food consumption, but it did increase the body weight of older mice; it did not influence physical strength or the presence of fatigue; it decreased locomotor activity and body temperature; it inhibited free radical processes in serum, brain, and liver; it slowed down the age-related switching-off of estrous function; and it increased life span. However, we also found that treatment with the used dose of melatonin increased spontaneous tumor incidence in mice. For this reason, we concluded that it would be premature to recommend melatonin as a geroprotector for long-term use.

Carcinogenesis bioassays: study duration and biological relevance

Criticisms of the scientific value of rodent carcinogenicity bioassays have focused on the arguments that the studies are too long and that most organ-specific carcinogenic effects observed in experimental animals have little or no relevance to humans. For example, Davies et al. (Davies, T.S., Lynch, B.S., Monro, A.M., Munro, I.C., Nestmann, E.R., 2000. Rodent carcinogenicity tests need be no longer than 18 months: an analysis based on 210 chemicals in the IARC Monographs. Food and Chemical Toxicology 38, 219-235) concluded that the duration of rodent bioassays should be no more than 18 months, based on their analysis of 210 International Agency for Research on Cancer (IARC) rodent carcinogens in which they report that most chemicals showed "tumorigenic effects" at or before 12 months. However, many of these "tumorigenic effects" reflect the occurrence of a single neoplasm, with most tumors occurring much later in the study. Reliance on a single tumor at an early time point as providing definitive evidence of rodent carcinogenicity is a dangerous practice that could produce both false positive and false negative outcomes. An extensive evaluation of the NTP database reveals that many rodent carcinogens produce later-appearing tumors that would not be detected as statistically significant in a 12-18 month study. Such a shortened duration study would be roughly equivalent to evaluating human cancer in subjects 30-50 years of age, which would result in markedly reduced study sensitivity. In fact, many investigators recommend extending the duration of rodent studies to 30 months or to a true lifetime to increase study sensitivity. We also do not agree with the second conclusion of Davies et al. (2000) that the mode of action of rodent carcinogenesis is sufficiently well understood to justify discounting the majority of organ-specific carcinogenic effects found in these studies. The consequences of performing rodent carcinogenicity studies with inadequate sensitivity, and then discounting most of the carcinogenic effects that are observed will be that potential human carcinogens will not be detected, thus forcing near total reliance on human studies for this purpose. This is not prudent public health policy.

Rodent carcinogenicity tests need be no longer than 18 months: an analysis based on 210 chemicals in the IARC monographs

The IARC Monographs (Vols 1-70) were studied to determine the time of onset of treatment-related tumorigenicity in long-term rodent studies for chemicals classified by IARC as having sufficient evidence of carcinogenicity in animals. The analysis excluded studies on metals and their salts, studies on particulates, studies by parenteral routes of administration that resulted in tumours only at the site of exposure, and studies that did not approximate to the current standard long-term rodent carcinogenicity bioassay, for instance transplacental or multigeneration studies, initiator-promoter studies, lung tumour assays in Strain A mice and studies in newborn animals. Data from a total of 210 chemicals revealed that, overall, evidence of treatment-related tumorigenicity was first apparent within 12 months for 66% of the chemicals and for only 7% were studies of longer than 18 months necessary. All IARC Group 1 chemicals were detected in animals within 18 months, and most within 12 months. Most of the tumour types that required more than 18 months for detection were of dubious relevance to human risk assessment. Termination of rodent carcinogenicity studies at 18 months or earlier would greatly reduce the complications that arise in interpreting the findings in aged animals which often have defective hepatic or renal function and would also markedly reduce the time required for histopathological examination of dozens of tissues taken from the approximately 500 animals routinely employed in these studies.

Oxymetholone: II. Evaluation in the Tg-AC transgenic mouse model for detection of carcinogens

Several rodent models are under examination as possible alternatives to the classical 2-yr carcinogenicity bioassay. The Tg.AC transgenic mouse has been proposed as a shorter term model offering the possibility of detecting nongenotoxic and genotoxic carcinogenic agents. Retrospective studies of chemicals with established carcinogenic potential have revealed a close correlation between classical bioassay results and the production of skin tumors in the Tg.AC mouse model. Oxymetholone is a synthetic testosterone derivative that is a suspected carcinogen but has shown no evidence of genotoxic activity in a comprehensive battery of genetic toxicity assays. It currently is being tested by the National Toxicology Program (NTP) in a 2-yr rat carcinogenicity bioassay. Because of its nongenotoxicity and the ongoing chronic bioassay, oxymetholone was considered an ideal candidate for a prospective evaluation of the predictive validity of the Tg.AC dermal carcinogenicity model. Consequently, a 6-mo dermal study with oxymetholone in the Tg.AC mouse model was initiated and completed prior to disclosure of the NTP rat bioassay results. In this study, male and female hemizygous Tg.AC mice, 7-8 wk old, were housed individually in suspended plastic cages. An area of dorsal skin was shaved to accommodate dermal applications of 200-microl doses of vehicle control (acetone), drug (1.2, 6.0, or 12 mg oxymetholone in dimethylsulfoxide:acetone, 20:80), or positive control (1.25 microg 12-o-tetradecanoyl-phorbol-13-acetate [TPA]) solutions. Mice received oxymetholone or acetone daily or TPA twice weekly for 20 wk followed by a 6-wk recovery period. The acetone control groups exhibited low spontaneous incidences of papillomas, whereas dermal application of oxymetholone produced dose-related increases in the numbers of papilloma-bearing mice and the numbers of papillomas per animal. Females showed a somewhat greater response to the androgen than did the males. TPA caused an unequivocal increase in papillomas, with males exhibiting a greater response than females. The results of this study indicate that this nongerotoxic androgenic compound possesses proliferative properties. The results predict that chronic systemic administration of oxymetholone will most likely be associated with increased incidences of neoplasms.

Short- and intermediate-term carcinogenicity testing--a review. Part 2: available experimental models

Numerous experimental protocols for short- and intermediate-term carcinogenicity assays have been available for many years. This paper surveys various of these test systems in rodents, fish species, non-vertebrates and avian embryos in ovo. The mouse skin tumour assay and the rat liver foci assay were used to introduce the basic concepts of short- and intermediate-term carcinogenicity testing in the previous part of the review. The focus of this second part of the review is on rodent assays for carcinogenicity testing in the lung, kidney, urinary bladder, pancreas, stomach, oral cavity, small intestine, colon, and on the possibility to combine several target organs in multi-organ models. The potential use of various fish species, non-vertebrates and hatching eggs for carcinogenicity testing is outlined and the advantages and limitations are discussed. This review also presents the problem of validation of any carcinogenicity test system and proposes a strategy for contemporary safety assessment of chemicals with regard to the detection and evaluation of carcinogenicity.

Veterinary drugs no longer need testing for carcinogenicity in rodent bioassays

The putative carcinogenic risk to humans from ingestion of edible tissues containing traces of nongenotoxic veterinary drugs is so slight that the routine application of rodent cancer bioassays cannot be justified. This argument is based, first, on the pharmacological similarity of veterinary and human drugs: many of the latter that are carcinogenic to rodents have been deemed on mechanistic and/or potency grounds not to pose a cancer risk to humans. Second, the distribution of a veterinary drug through the target animal body before ingestion of a portion of edible tissue by humans days or weeks later means that the human dose from a residue is several orders of magnitude lower than the normal dose of human drugs. The dose of residue is also much lower than the exposure of humans to the most potent carcinogens.

The utility of two rodent species in carcinogenic risk assessment of pharmaceuticals in Europe

For the past 20-30 years, lifespan carcinogenicity studies for pharmaceuticals have been required to be carried out in two rodent species. Due to scientific progress, the necessity/justification of lifespan studies in two species for the assessment of carcinogenic risk of pharmaceuticals is currently under discussion. A study in one species (either rat or mouse) might suffice. To appraise the need for a study in a second species, a database was compiled of all pharmaceuticals tested for carcinogenicity for which a marketing authorization was applied for in Germany and The Netherlands since 1980. The incidence of treatment-related tumor findings was determined in either rat or mouse or in both. Tumor findings occurred for nearly 50% of all compounds, with the rat being more sensitive than the mouse. Specific attention was given to the question whether tumor findings in mice ever caused the regulatory authorities to refuse registration, to restrict the proposed therapeutic indication of a pharmaceutical, or to apply a cautionary label. It was found that no tumor findings in mice alone ever led to such a regulatory action. In addition, whether mouse studies had been important in interpreting the results of rat studies was determined. A negative mouse study (no tumors found) was rarely used to declare the rat findings irrelevant to humans. A mechanistic explanation was used as a much more important argument in the assessment of tumor findings in rats. In case of transspecies findings, the target organs were the usual ones, such as lung and liver, or the tumors occurred as a result of an exaggerated pharmacodynamic action expected from the pharmacology of the compound. The results of the database thus question the need of maintaining the requirement of rodent carcinogenicity studies in two species.

Safety assessment of pharmaceuticals: examples of inadequate assessments and a mechanistic approach to assuring adequate assessment

For a conventional organic new chemical entity (NCE) being developed as a pharmaceutical, standard regulatory safety assessment studies are required. Early in development, an NCE should undergo a safety/benefit analysis to justify further development. This analysis is made easier and more effective when comprehensive nonclinical data are available. One of the most important aspects of nonclinical toxicologic studies is to provide information on absence of potential carcinogenicity in humans. To avoid human exposures to potentially carcinogenic agents, even in early development of an NCE, the Decision Point Approach to carcinogen testing provides a useful guide to acquisition of mechanistically relevant data for risk assessment.