Issues in the Design and Interpretation of Chronic Toxicity and Carcinogenicity Studies in Rodents: Approaches to Dose Selection

A new approach methodology using kinetically-derived maximum dose levels in risk assessment - A case study with afidopyropen

We present a case study for afidopyropen (AF; insecticide) to characterize chronic dietary human health risk using a Risk 21-based approach. Our objective is to use a well-tested pesticidal active ingredient (AF) to show how a new approach methodology (NAM), using the kinetically-derived maximum dose (KMD) and with far less animal testing, can reliably identify a health-protective point of departure (PoD) for chronic dietary human health risk assessments (HHRA). Chronic dietary HHRA involves evaluation of both hazard and exposure information to characterize risk. Although both are important, emphasis has been placed on a checklist of required toxicological studies for hazard characterization, with human exposure information only considered after evaluation of hazard data. Most required studies are not used to define the human endpoint for HHRA. The information presented demonstrates a NAM that uses the KMD determined by saturation of a metabolic pathway, which can be used as an alternative POD. In these cases, the full toxicological database may not need to be generated. Demonstration that the compound is not genotoxic and that the KMD is protective of adverse effects in 90-day oral rat and reproductive/developmental studies is sufficient to support the use of the KMD as an alternative POD.

Hazards of current concentration-setting practices in environmental toxicology studies

The setting of concentrations for testing substances in ecotoxicological studies is often based on fractions of the concentrations that cause 50% mortality (LC50 or LD50) rather than environmentally relevant levels. This practice can result in exposures to animals at test concentrations that are magnitudes of order greater than those experienced in the environment. Often, such unrealistically high concentrations may cause non-specific biochemical or morphologic changes that primarily reflect the near-lethal health condition of the animal subjects, as opposed to effects characteristic of the particular test compound. Meanwhile, it is recognized that for many chemicals, the toxicologic mode of action (MOA) responsible for lethality may differ entirely from the MOAs that cause various sublethal effects. One argument for employing excessively high exposure concentrations in sublethal studies is to ensure the generation of positive toxicological effects, which can then be used to establish safety thresholds; however, it is possible that the pressure to produce exposure-related effects may also contribute to false positive outcomes. The purpose of this paper is to explore issues involving some current usages of acute LC50 data in ecotoxicology testing, and to propose an alternative strategy for performing this type of research moving forward. Toward those ends, a brief literature survey was conducted to gain an appreciation of methods that are currently being used to set test concentrations for sublethal definitive studies.

10% Body weight (gain) change as criterion for the maximum tolerated dose: A critical analysis

The concept of the Maximum Tolerated Dose (MTD) was introduced in the seventies for carcinogenicity testing and was defined as the highest dose inducing clear toxicity, but not mortality by causes other than cancer. As estimation of the MTD in a carcinogenicity study, the highest dose that causes a 10% decrease in body weight compared to control animals over the course of a 90-day study, was formulated as a suitable criterion. This criterion was not seen as indicator of excessive toxicity but as a means to avoid false negative outcomes in a carcinogenicity study, as tumor formation may be reduced when body weight is significantly decreased. The body weight-based MTD criterion, however, turned up in carcinogenicity test guidelines and guidance (e.g., from OECD) as the highest dose that causes a 10% decrease in body weight gain relative to controls. Moreover, the 10% decrease in body weight gain criterion for MTD also ended up in test guidelines and guidances for toxicity endpoints other than carcinogenicity, so outside the context it was intended for. A 10% decrease in body weight gain relative to controls is however not a biologically relevant effect as it corresponds to less than 3% body weight reduction relative to controls in a 90-day study, which is within the normal variation in body weight. It therefore should certainly not be considered as a condition of excessive toxicity. Using the 10% lower weight gain criterion and incorrectly associating it with excessive toxicity has major implications for top dose selection in regulatory safety studies, resulting in tests performed at doses too low to elicit toxicity. This negatively impacts the reliability of studies and their regulatory usability; moreover, it results in a waste of experimental animals, which is ethically highly undesirable. Hence, our plea is to remove this MTD criterion for top dose selection in test guidelines and guidances for toxicity endpoints other than carcinogenicity and to reinstall the original 10% decrease in body weight criterion in test guidelines and guidances for carcinogenicity.

Effects of chronic dietary nitrate supplementation on longevity, vascular function and cancer incidence in rats

RATIONALE

Dietary nitrate and nitrite have a notoriously bad reputation because of their proposed association with disease, in particular cancer. However, more recent lines of research have challenged this dogma suggesting that intake of these anions also possess beneficial effects after in vivo conversion to the vital signaling molecule nitric oxide. Such effects include improvement in cardiovascular, renal and metabolic function, which is partly mediated via reduction of oxidative stress. A recent study even indicates that low dose of dietary nitrite extends life span in fruit flies.

METHODS

In this study, 200 middle-aged Wistar rats of both sexes were supplemented with nitrate or placebo in the drinking water throughout their remaining life and we studied longevity, biochemical markers of disease, vascular reactivity along with careful determination of the cause of death.

RESULTS

Dietary nitrate did not affect life span or the age-dependent changes in markers of oxidative stress, kidney and liver function, or lipid profile. Ex vivo examination of vascular function, however, showed improvements in endothelial function in rats treated with nitrate. Neoplasms were not more common in the nitrate group.

CONCLUSION

We conclude that chronic treatment with dietary nitrate does not affect life span in rats nor does it increase the incidence of cancer. In contrast, vascular function was improved by nitrate, possibly suggesting an increase in health span.

Opportunities and challenges related to saturation of toxicokinetic processes: Implications for risk assessment

Top dose selection for repeated dose animal studies has generally focused on identification of apical endpoints, use of the limit dose, or determination of a maximum tolerated dose (MTD). The intent is to optimize the ability of toxicity tests performed in a small number of animals to detect effects for hazard identification. An alternative approach, the kinetically derived maximum dose (KMD), has been proposed as a mechanism to integrate toxicokinetic (TK) data into the dose selection process. The approach refers to the dose above which the systemic exposures depart from being proportional to external doses. This non-linear external-internal dose relationship arises from saturation or limitation of TK process(es), such as absorption or metabolism. The importance of TK information is widely acknowledged when assessing human health risks arising from exposures to environmental chemicals, as TK determines the amount of chemical at potential sites of toxicological responses. However, there have been differing opinions and interpretations within the scientific and regulatory communities related to the validity and application of the KMD concept. A multi-stakeholder working group, led by the Health and Environmental Sciences Institute (HESI), was formed to provide an opportunity for impacted stakeholders to address commonly raised scientific and technical issues related to this topic and, more specifically, a weight of evidence approach is recommended to inform design and dose selection for repeated dose animal studies. Commonly raised challenges related to the use of TK data for dose selection are discussed, recommendations are provided, and illustrative case examples are provided to address these challenges or refute misconceptions.

Afidopyropen: Challenges and impact of a toxicokinetic study designed to identify a point of inflection from dose-proportionality

Afidopyropen is an insecticide that acts as a transient receptor potential vanilloid subtype (TRPV) channel modulator in chordotonal organs of target insects and has been assessed for a wide range of toxicity endpoints including chronic toxicity and carcinogenicity in rats and mice. The current study evaluates the toxicokinetic properties of afidopyropen and its plasma metabolites in rats at dose levels where the pharmacokinetics (PK) are linear and nonlinear in an attempt to identify a point of inflection. Based on the results of this study and depending on the analysis method used, the kinetically derived maximum dose (KMD) is estimated to be between 2.5 and 12.5 mg/kg bw/d with linearity observed at doses below 2.5 mg/kg bw/d. A defined point of inflection could not be determined. These data demonstrate that consideration of PK is critical for improving the dose-selection in toxicity studies as well as to enhance human relevance of the interpretation of animal toxicity studies. The study also demonstrates the technical difficulty in obtaining a defined point of inflection from in vivo PK data.

A Proposal for New Strategies in Dose Selection for 26-Week Tg.Rash2 Carcinogenicity Studies

Our experience indicates that extrapolation of doses from the maximum tolerated doses (MTD) derived from 4-week dose range finding (DRF) studies conducted in CByB6F1 may overpredict tolerability and undermine utility of the high-dose groups in 26-week carcinogenicity studies conducted in Tg.rasH2. In the 26-week carcinogenicity studies conducted in Tg.rasH2 mice, we analyzed the initial body weights, food consumption (FC), terminal body weights, body weight gain (BWG), mortality, and tumor incidence for vehicle and test article-treated dose groups for 26 studies conducted from 2014 to 2018. Although not statistically significant compared to the control dose group, the % BWG decreased in male mice of mid- and high-dose groups by >10%, whereas in females there were no differences. The mortality increased in a statistically significant manner for medium and high doses of males. In female mice, the mortality increased in the high-dose group but not in a statistically significant manner. When the cause of death (COD) was analyzed in all dose groups of both sexes, the COD due to tumors was highest in the control groups, whereas it was lowest in high-dose groups of both sexes. At the same time, the COD due to undetermined causes, which is possible indication of test article-induced toxicity, was highest in high-dose groups of both sexes. These findings together indicate that MTD derived from earlier DRF studies was exceeded when applied to 26-week carcinogenicity studies and did not serve any purpose in the outcome of these studies.

Comprehensive analysis of chronic rodent inhalation toxicity studies for methyl acrylate with attention to test conditions exceeding a maximum tolerated concentration

MA is a chemical intermediate, manufactured and processed within closed systems. While so far available subacute to chronic inhalation toxicity studies performed in compliance with OECD TG principles gave no indication of any carcinogenic potential for MA, a recent 2-year inhalation study with F344/DuCrlCrlj rats published in 2017 by the JBRC showed a statistically significant increase of squamous cell carcinoma in the nasal cavity of male rats at the highest tested concentration of 160 ppm. However, the results of the different studies in total indicate that this high concentration exceeded the MTC. As MA has a low potential for genotoxic and mutagenic activity, the increased tumour incidence can be attributed to a non-genotoxic mechanism, namely to a strong inflammatory response observed in this study. Together with mechanistic and epidemiologic data for other compounds related to nasal carcinogenesis via this mode of action, it can be concluded that the relevance of this increased tumour incidence in male rats for humans is questionable. Also, a long-term exposure to higher concentrations of MA is highly unlikely to be reached in the environment or at workplaces. Therefore, a risk for humans including cancer hazard is considered implausible.

The limitations of using the NTP chronic bioassay on vanadium pentoxide in risk assessments

Regulatory interest in assessing the health effects of vanadium compounds is hindered by the limited chronic toxicity data available. The National Toxicology Program (NTP) conducted a robust chronic inhalation bioassay of crystalline vanadium pentoxide (V₂O₅), but this study has noteworthy limitations. Multiple dose range-finding studies were conducted at two separate laboratories that showed cross-laboratory differences in lung pathology (inflammation) in both species and likely complicated dose-selection. In mice, the only tissue pathology (inflammation and tumors) was at the site of entry, the respiratory system. Although significantly different from control, because lung tumor incidences were at a maximal level across all concentrations tested, the ability to extrapolate risks to the public is problematic. In rats, lung inflammation and vanadium lung burdens were comparable to those of mice, but lung tumorigenicity was not substantiated, further raising questions about appropriate species extrapolation. Open questions also exist regarding test material chemical characterization, as the laboratory relied on vanadium measurement in test chambers as a surrogate for V₂O₅. In sum, the NTP V₂O₅ study does not provide an appropriate dataset for purposes of classification and risk assessment. Additional repeat exposure studies of vanadium compounds are needed and recommendations for future studies are provided.

Cell proliferation analysis is a reliable predictor of lack of carcinogenicity: Case study using the pyrethroid imiprothrin on lung tumorigenesis in mice

In the mouse carcinogenicity study, an apparent increase in lung adenocarcinoma was observed in male mice at 7000 ppm. Based on the overall evaluation of toxicology, oncology, pathology and statistics, we concluded that the apparent increase in lung tumors is not relevant for evaluation of carcinogenicity of imiprothrin (Regul Toxicol Pharmacol, 105, 1-14, 2019). To investigate whether imiprothrin has any mitogenic effect on mouse Club cells, the present study examined its effects on replicative DNA synthesis of Club cells and lung histopathology in male mice treated with imiprothrin for 7 days at 3500 and 7000 ppm in the diet. Isoniazid, a known mouse lung mitogen and tumor inducer, was also examined at 1000 ppm in the diet as a positive control of Club cell mitogenesis and morphological changes. Neither imiprothrin nor isoniazid caused any necrotic changes in lung by light or electron microscopy. There were no increases observed in the bromodeoxyuridine (BrdU) labeling index in the imiprothrin groups, while there was a statistically significant increase in the BrdU labeling index in the isoniazid group. These findings demonstrate that imiprothrin does not induce mouse Club cell proliferation or morphologic changes, supporting our previous conclusion described above. Thus, imiprothrin should not be classified as a carcinogen. Furthermore, this study indicates that short-term studies focusing on cell proliferation can be reliable for predicting a lack of carcinogenic potential of test chemicals.

Use of toxicokinetic data for afidopyropen to determine the dose levels in developmental toxicity studies

Afidopyropen is an insecticide that acts as a TRPV channel modulator in chordotonal organs of target insects and has been assessed for a wide range of toxicity endpoints including developmental toxicity in rats and rabbits. The GLP developmental toxicity study in rabbits did not produce evidence of maternal or fetal toxicity at the highest dose tested (32 mg/kg/day) but pharmacokinetics (PK) in pregnant rabbits in this study exhibited onset of PK nonlinearity from 5 mg/kg/day on, as measured by plasma C_max and AUC. The NOAEL (32 mg/kg/day) is 9000X higher than maximum expected human dietary exposures to afidopyropen; the dose range where nonlinear PK were observed (5-15 mg/kg/day) is 1400-4200X higher. As nonlinearity occurred between 5 and 15 mg/kg/day, 32 mg/kg/day is concluded to be a sufficiently high dose (kinetically derived maximum dose) for a prenatal developmental toxicity study. As recognized by regulatory dose-selection guidance, onset of saturated PK is evidence of excessive biological stress to test animals rendering any effects at such doses of questionable relevance for human risk assessment. These data demonstrate that consideration of PK is critical for improving the dose-selection in developmental toxicity studies to enhance human relevance of animal toxicity studies.

Critical evaluation of 2-ethylhexyl acrylate dermal carcinogenicity studies using contemporary criteria

Skin tumors have been observed in C3H/HeJ mice following treatment with high and strongly irritating concentrations of 2-ethylhexyl acrylate (2-EHA). Dermal carcinogenicity studies performed with 2-EHA are reviewed, contrasting the results in two mouse strains (C3H/HeJ and NMRI) under different dosing regimens. Application of contemporary evaluation criteria to the existing dermal carcinogenicity dataset demonstrates that 2-EHA induces skin tumors only at concentrations exceeding an maximum tolerated dose (MTD) and in the immune-dysregulated C3H/HeJ mouse model. Overall, the available chronic toxicity and genotoxicity data on 2-EHA support a non-genotoxic chemical irritant mechanism, whereby chronic irritation leads to inflammation, tissue injury, and wound repair, the latter of which is disrupted in C3H/HeJ mice and leads to tumor formation. Tumor response information in excess of an MTD should not be considered in a human hazard or risk assessment paradigm. For the purposes of an appropriate hazard assessment, 2-EHA did not cause or initiate dermal carcinogenesis in an immune competent (NMRI) mouse model, and, even in the immune compromised C3H/HeJ model, did not induce skin tumors at doses which did not exceed the MTD.

Establishing maximum tolerated doses for a 2-year combined chronic/carcinogenicity rat study based on toxicokinetic and toxicity gender differences

For agrochemicals tested in a carcinogenicity rodent study, it is often not possible to use the same high dose to achieve maximum tolerated dose (MTDs) without overdosing or insufficiently challenging one gender if significant gender differences are known. Toxicokinetic (TK) data for pesticide FR from a 28-day rat study showed that males required a 3-fold higher external dose compared to females to produce similar internal exposure levels of the parent compound. In the 90-day study, 8%/17% (M/F) decrease in bodyweight gain (BWG) and 15%/15% (M/F) increase in relative liver weights were observed in the 6000 ppm males and 2000 ppm females, respectively. Based on the above TK and toxicity data, different high dose levels were selected for females (1600 ppm) and males (4800 ppm) for a 2-year combined chronic/carcinogenicity study in rats. In the 2-year study, 14%, 13%, 13% and 21% reduction in BWG of males and 10%, 12%, 19% and 20% reduction in BWG of females were observed at weeks 13, 26, 52 and 104, respectively in the highest dose tested. Similar reductions in bodyweight gain in males and females at the different high dose levels clearly demonstrated that appropriate MTDs were reached. Therefore, it is scientifically sound and practical to use TK and toxicity data to use different high dose levels to achieve MTDs for a pesticide with large gender differences.

Safety Assessment of Food and Feed from GM Crops in Europe: Evaluating EFSA's Alternative Framework for the Rat 90-day Feeding Study

Regulatory-compliant rodent subchronic feeding studies are compulsory regardless of a hypothesis to test, according to recent EU legislation for the safety assessment of whole food/feed produced from genetically modified (GM) crops containing a single genetic transformation event (European Union Commission Implementing Regulation No. 503/2013). The Implementing Regulation refers to guidelines set forth by the European Food Safety Authority (EFSA) for the design, conduct, and analysis of rodent subchronic feeding studies. The set of EFSA recommendations was rigorously applied to a 90-day feeding study in Sprague-Dawley rats. After study completion, the appropriateness and applicability of these recommendations were assessed using a battery of statistical analysis approaches including both retrospective and prospective statistical power analyses as well as variance-covariance decomposition. In the interest of animal welfare considerations, alternative experimental designs were investigated and evaluated in the context of informing the health risk assessment of food/feed from GM crops.

Origin of the TTC values for compounds that are genotoxic and/or carcinogenic and an approach for their re-evaluation

The threshold of toxicological concern (TTC) approach is a resource-effective de minimis method for the safety assessment of chemicals, based on distributional analysis of the results of a large number of toxicological studies. It is being increasingly used to screen and prioritize substances with low exposure for which there is little or no toxicological information. The first step in the approach is the identification of substances that may be DNA-reactive mutagens, to which the lowest TTC value is applied. This TTC value was based on the analysis of the cancer potency database and involved a number of assumptions that no longer reflect the state-of-the-science and some of which were not as transparent as they could have been. Hence, review and updating of the database is proposed, using inclusion and exclusion criteria reflecting current knowledge. A strategy for the selection of appropriate substances for TTC determination, based on consideration of weight of evidence for genotoxicity and carcinogenicity is outlined. Identification of substances that are carcinogenic by a DNA-reactive mutagenic mode of action and those that clearly act by a non-genotoxic mode of action will enable the protectiveness to be determined of both the TTC for DNA-reactive mutagenicity and that applied by default to substances that may be carcinogenic but are unlikely to be DNA-reactive mutagens (i.e. for Cramer class I-III compounds). Critical to the application of the TTC approach to substances that are likely to be DNA-reactive mutagens is the reliability of the software tools used to identify such compounds. Current methods for this task are reviewed and recommendations made for their application.

Regulatory Forum Opinion Piece*

High doses in Tg.rasH2 carcinogenicity studies are usually set at the maximum tolerated dose (MTD), although this dose selection strategy has not been critically evaluated. We analyzed the body weight gains (BWGs), mortality, and tumor response in control and treated groups of 29 Tg.rasH2 studies conducted at BioReliance. Based on our analysis, it is evident that the MTD was exceeded at the high and/or mid-doses in several studies. The incidence of tumors in high doses was lower when compared to the low and mid-doses of both sexes. Thus, we recommend that the high dose in male mice should not exceed one-half of the estimated MTD (EMTD), as it is currently chosen, and the next dose should be one-fourth of the EMTD. Because females were less sensitive to decrements in BWG, the high dose in female mice should not exceed two-third of EMTD and the next dose group should be one-third of EMTD. If needed, a third dose group should be set at one-eighth EMTD in males and one-sixth EMTD in females. In addition, for compounds that do not show toxicity in the range finding studies, a limit dose should be applied for the 26-week carcinogenicity studies.

Scientific evaluation of the acute toxicity and 13-week subchronic toxicity of Rheum emodi rhizome extracts in Sprague Dawley rats

Rheum emodi has been used as an edible and medicinal plant in Tibet and Kashmir for a long period of time, while safety evaluation of this plant has not yet been investigated. In this study, acute and subchronic oral toxicity studies of aqueous extract of R. emodi (AERE) rhizome were conducted in SD rats. Animals were treated with a single dose of 1000, 2000, 4000 or 10,000 mg/kg of AERE in the acute toxicity. In subchronic oral toxicity, animals were randomly divided into four groups (10 rats/sex/group) and received doses of 0, 1000, 2000, and 4000 mg/kg/d of AERE for 90 days. Daily clinical observations, weekly measurement of body weight and food consumption were conducted. Blood and urine were collected on days 91 to measure changes. At necropsy, selected organs were weighed and recorded, and histological examination was performed. During the subchronic oral toxicity study, no mortality, obvious treatment-related clinical signs and urinalysis parameters were observed. Differences in weight gain, food consumption, hematology, biochemistry, relative organ weight and histopathology examinations between the treated group and the control group were not considered treatment-related. Our results indicated that the no-observed-adverse-effect level (NOAEL) for AERE was 4000 mg/kg/d in both genders.

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.

Selection of appropriate tumour data sets for Benchmark Dose Modelling (BMD) and derivation of a Margin of Exposure (MoE) for substances that are genotoxic and carcinogenic: considerations of biological relevance of tumour type, data quality and uncertainty assessment

This article addresses a number of concepts related to the selection and modelling of carcinogenicity data for the calculation of a Margin of Exposure. It follows up on the recommendations put forward by the International Life Sciences Institute - European branch in 2010 on the application of the Margin of Exposure (MoE) approach to substances in food that are genotoxic and carcinogenic. The aims are to provide practical guidance on the relevance of animal tumour data for human carcinogenic hazard assessment, appropriate selection of tumour data for Benchmark Dose Modelling, and approaches for dealing with the uncertainty associated with the selection of data for modelling and, consequently, the derived Point of Departure (PoD) used to calculate the MoE. Although the concepts outlined in this article are interrelated, the background expertise needed to address each topic varies. For instance, the expertise needed to make a judgement on biological relevance of a specific tumour type is clearly different to that needed to determine the statistical uncertainty around the data used for modelling a benchmark dose. As such, each topic is dealt with separately to allow those with specialised knowledge to target key areas of guidance and provide a more in-depth discussion on each subject for those new to the concept of the Margin of Exposure approach.

Interpreting Stress Responses during Routine Toxicity Studies

Stress often occurs during toxicity studies. The perception of sensory stimuli as stressful primarily results in catecholamine release and activation of the hypothalamic–pituitary–adrenal (HPA) axis to increase serum glucocorticoid concentrations. Downstream effects of these neuroendocrine signals may include decreased total body weights or body weight gain; food consumption and activity; altered organ weights (e.g., thymus, spleen, adrenal); lymphocyte depletion in thymus and spleen; altered circulating leukocyte counts (e.g., increased neutrophils with decreased lymphocytes and eosinophils); and altered reproductive functions. Typically, only some of these findings occur in a given study. Stress responses should be interpreted as secondary (indirect) rather than primary (direct) test article–related findings. Determining whether effects are the result of stress requires a weight-of-evidence approach. The evaluation and interpretation of routinely collected data (standard in-life, clinical pathology, and anatomic pathology endpoints) are appropriate and generally sufficient to assess whether or not changes are secondary to stress. The impact of possible stress-induced effects on data interpretation can partially be mitigated by toxicity study designs that use appropriate control groups (e.g., cohorts treated with vehicle and subjected to the same procedures as those dosed with test article), housing that minimizes isolation and offers environmental enrichment, and experimental procedures that minimize stress and sampling and analytical bias.

This article is a comprehensive overview of the biological aspects of the stress response, beginning with a Summary (Section 1) and an Introduction (Section 2) that describes the historical and conventional methods used to characterize acute and chronic stress responses. These sections are followed by reviews of the primary systems and parameters that regulate and/or are influenced by stress, with an emphasis on parameters evaluated in toxicity studies: In-life Procedures (Section 3), Nervous System (Section 4), Endocrine System (Section 5), Reproductive System (Section 6), Clinical Pathology (Section 7), and Immune System (Section 8). The paper concludes (Section 9) with a brief discussion on Minimizing Stress-Related Effects (9.1.), and a final section explaining why Parameters routinely measured are appropriate for assessing the role of stress in toxicology studies (9.2.).

Rodent models for the preclinical evaluation of drugs suitable for pharmacological intervention in aging

INTRODUCTION

There is a growing scientific and public interest in the development of new antiaging drugs for the purposes of extending mean and/or maximum life span, maintaining normal physiological function, and alleviating the onset and severity of age-associated diseases. This review looks at the current screening approaches used to evaluate the efficacy of such compounds, with a particular focus on those that extend life span.

AREAS COVERED

This article reviews the current preclinical approaches for assessing longevity therapy including the assessment of antiaging drugs (aging reversal) and geroprotectors (drugs that prevent premature aging and/or slowdown or postpone aging). This article also discusses the methods and the importance in evaluating the anticarcinogenic potential and safety of antitumor drugs.

EXPERT OPINION

Based on more than 30 years of experience in the field, the authors believe that the standard testing protocols for antiaging drugs should include the simultaneous evaluation of the drug's safety, as well as its antitumor and anticarcinogenic activity potential. The authors also believe that the principles of international programs for the expert critical evaluation of pharmacological interventions should be created to improve the range of antiaging interventions available for human studies.

Recent and remote spatial memory in mice treated with cytosine arabinoside

Clinical studies suggest that chemotherapy is associated with long-term cognitive impairment in some patients. A number of underlying mechanisms have been proposed, however, the etiology of chemotherapy-related cognitive dysfunction remains relatively unknown. As part of a multifaceted approach, animal models of chemotherapy-induced cognitive impairment are being developed. Thus far, the majority of animal studies have utilized a rat model, however, mice may prove particularly beneficial in studying genetic risk factors for developing chemotherapy-induced cognitive impairment. Various chemotherapy agents, including cytosine arabinoside (Ara-C), have been found to impair remote spatial memory in rats in the Morris water maze. The present study evaluated the effects of Ara-C on remote (30 d) spatial memory in mice. In addition, the possibility that time relative to chemotherapy treatment may modulate the effect of chemotherapy on spatial learning and/or recent (1 d) memory was explored. Male C57BL/6J mice received either Ara-C (275 mg/kg i.p. daily for 5 days) or saline. Spatial learning and memory was assessed using the Morris water maze. Half the mice performed a remote (30 d) memory version of the task and the other half performed a recent (1 d) memory version of the task. The experiment was designed such that the probe trial for the recent memory version occurred on the same day relative to chemotherapy treatment as the remote memory version. Despite significant toxic effects as assessed by weight loss, Ara-C treated mice performed as well as control mice during acquisition, recent memory, and remote memory portions of the task. As are some humans, C57BL/6J mice may be resistant to at least some aspects of chemotherapy induced cognitive decline.

Critical Review of Preclinical Approaches to Evaluate the Potential of Immunosuppressive Drugs to Influence Human Neoplasia

Many immunosuppressive drugs are associated with an increased risk of B-cell lymphoma, squamous cell carcinoma, and Kaposi sarcoma. Thirteen immunosuppressive drugs have been tested in 2-year carcinogenicity studies (abatacept; azathioprine; busulfan; cyclophosphamide; cyclosporine; dexamethasone; everolimus; leflunomide; methotrexate; mycophenolate mofetil; prednisone; sirolimus; and tacrolimus) and in additional models including neonatal and genetically modified mice; chemical, viral, ultraviolet, and ionizing radiation co-carcinogenesis, and in models with transplanted tumor cells. The purpose of this review is to outline the mechanisms by which immunosuppressive drugs can influence neoplasia, to summarize the available preclinical data on the 13 drugs, and to critically review the performance of the models. A combination of primary tumor and metastasis assays conducted with transplanted cells may provide the highest value for hazard identification and can be applied on a case-by-case basis. However, for both small molecules and therapeutic proteins, determining the relative risk to patients from preclinical data remains problematic. Classifying immunosuppressive drugs based on their mechanism of action and hazard identification from preclinical studies and a prospective pharmacovigilance program to monitor carcinogenic risk may be a feasible way to manage patient safety during the clinical development program and postmarketing.

Logarithmic dose transformation in epidemiologic dose-response analysis: use with caution

Dose-response models based on the logarithm of dose or exposure are utilized in the analysis of both toxicological and epidemiologic data. While the applications are similar, fundamental differences in the studies conducted by each discipline affects the interpretation of the use of the logarithmic dose transformation. Using numerical illustrations and examples from the literature, this paper explores the implications of using logarithmic dose transformation in epidemiologic studies and provides some caveats for consideration in interpreting epidemiologic studies based upon logarithmic dose transformation.

Evaluation of Possible Carcinogenic Risk to Humans Based on Liver Tumors in Rodent Assays

The two-year rodent bioassay remains the mainstay for carcinogenicity testing, although numerous difficulties have been identified. Fundamentally, a chemical can increase the risk of cancer (1) by damaging DNA directly (DNA reactive) or (2) indirectly by increasing the number of DNA replications (non–DNA reactive). Mechanistic research has identified numerous precursor lesions in the sequence of key events necessary for neoplasia development. Based on these concepts, the author has proposed a short-term (thirteen-week) assay for screening for carcinogenic potential based on a mode of action analysis and on readily available, identifiable preneoplastic changes. A screening assay that detects all potential rodent hepatocarcinogens has been previously identified ( Toxicol Pathol32 [2004], 393–401) including increased liver weight, hepatocellular necrosis, hypertrophy, and cytomegaly. Labeling index for DNA replication might supply additional support. These markers have high sensitivity but low specificity. However, most chemicals can be appropriately classified as to their mode(s) of action for hepatocarcinogenesis with follow-up mechanistic studies, and a rational evaluation of their relevance to humans can be made. A similar process can be envisioned for other tissues for evaluation for carcinogenic potential.

A Data-Based Assessment of Alternative Strategies for Identification of Potential Human Cancer Hazards

The two-year cancer bioassay in rodents remains the primary testing strategy for in-life screening of compounds that might pose a potential cancer hazard. Yet experimental evidence shows that cancer is often secondary to a biological precursor effect, the mode of action is sometimes not relevant to humans, and key events leading to cancer in rodents from nongenotoxic agents usually occur well before tumorigenesis and at the same or lower doses than those producing tumors. The International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute (HESI) hypothesized that the signals of importance for human cancer hazard identification can be detected in shorter-term studies. Using the National Toxicology Program (NTP) database, a retrospective analysis was conducted on sixteen chemicals with liver, lung, or kidney tumors in two-year rodent cancer bioassays, and for which short-term data were also available. For nongenotoxic compounds, results showed that cellular changes indicative of a tumorigenic endpoint can be identified for many, but not all, of the chemicals producing tumors in two-year studies after thirteen weeks utilizing conventional endpoints. Additional endpoints are needed to identify some signals not detected with routine evaluation. This effort defined critical questions that should be explored to improve the predictivity of human carcinogenic risk.

Application of toxicokinetics to improve chemical risk assessment: implications for the use of animals

While toxicokinetics has become an integral part of pharmaceutical safety assessment over the last two decades, its use in the chemical industry is relatively new. However, it is recognised as a potentially important tool in human health risk assessment and recent initiatives have advocated greater application of toxicokinetics as part of an improved assessment strategy for crop protection chemicals that could offer greater efficiency, use fewer animals and provide better data for risk assessment purposes. To explore the potential scientific and animal welfare benefits of increased use of toxicokinetic data across the chemical industry, an international workshop was held in 2008. Experts from a wide range of chemical industry sectors, including industrial chemicals, agrochemicals and consumer products, participated in the meeting as well as representatives from relevant regulatory authorities. Pharmaceutical industry experts were also invited, in order to share experiences from the extensive use of toxicokinetics in drug development. Given that increased generation of toxicokinetic data could potentially result in an increased number of animals undergoing testing, technologies and strategies to reduce and refine animal use for this purpose were also considered. This paper outlines and expands upon the key themes that emerged from the workshop.

The safety of PolyGlycopleX (PGX) as shown in a 90-day rodent feeding study

BACKGROUND

This study was designed to evaluate the safety of PolyGlycopleX (PGX), a novel viscous dietary polysaccharide (fiber), when administered to Sprague Dawley(R) rats in the diet for 90 days.

METHODS

Groups of ten male and ten female rats each consumed PGX mixed in the diet at levels of 0, 1.25, 2.5 or 5.0% for 90 days, then evaluated for toxicological effects on parameters that included neuromotor activity, body weight, clinical chemistry, urinalysis, hematology, and histopathology.

RESULTS

Mean body weight, mean feed consumption and food efficiency in the treated groups were generally comparable to controls for both male and female rats. No changes were noted in neuromotor behavior, and histopathological analysis revealed no significant changes between treated and control animals. There were no differences in mean organ weight, organ-to-body weight or organ-to-brain weight values between controls and treated animals. Decreased red blood cell count occurred in the high dose males and increases in aspartate and alanine aminotransferase enzyme levels and triglycerides, while significant decreases in serum sodium, potassium and chloride concentrations were observed in the females fed 5.0% PGX. However, the decreased mineral concentrations may be the result of significantly increased urinary volume in both males and females at the high dose, with a concomitant decrease in urinary specific gravity (males and females) and protein concentration (females). These results were within historical control values, did not correlate with any histopathological changes, and were not considered adverse.

CONCLUSION

The results indicate a no observed adverse effect level (NOAEL) for PGX at 5.0% of the diet, corresponding to an average daily intake of 3219 and 3799 mg/kg bw/day in male and female rats, respectively.