Significance of Leydig Cell Neoplasia in Rats Fed Lactitol or Lactose

Carcinogenicity of Monochloro-1,2-Propanediol (α-Chlorohydrin, 3-MCPD)

3-Monochloro-1,2-propanediol (3-MCPD) is a by-product found in trace amounts, generally less than 1 mg/kg (<1 ppm), in hydrolyzed vegetable protein produced through acid hydrolysis. In a chronic study with F344 rats, high doses of 3-MCPD produced benign renal tumors in both sexes and Leydig-cell and mammary tumors in males. 3-MCPD is genotoxic in vitro, but there is no evidence of genotoxicity in vivo. There is some question about the mechanism responsible for the carcinogenicity of 3-MCPD in certain species. Here we present a critical review of the toxicological, metabolic, and mechanistic data on 3-MCPD. On the basis of this review, the tumors reported in F344 rats are concluded to have developed as a result of nongenotoxic mechanisms and are considered not to be relevant to humans exposed to trace amounts of 3-MCPD. This conclusion was based on the lack of carcinogenicity of 3-MCPD in mice or Sprague-Dawley rats; the benign nature of the tumors involved; the dependence of the Leydig-cell and mammary tumors on species-and strain-dependent mechanisms involving chronic changes in hormone balance; the association of the renal tumors with chronic nephropathy and nephrotoxicity; and differences between bacterial and mammalian systems in the metabolism of 3-MCPD that likely account for its genotoxic activity in certain in vitro test systems. At trace levels in foods, 3 MCPD is considered not to pose a carcinogenic risk to humans.

Successful integration of nonclinical and clinical findings in interpreting the clinical relevance of rodent neoplasia with a new chemical entity

Canagliflozin, a sodium glucose co-transporter 2 (SGLT2) inhibitor, has been developed for the treatment of adults with type 2 diabetes mellitus (T2DM). During the phase 3 program, treatment-related pheochromocytomas, renal tubular tumors, and testicular Leydig cell tumors were reported in the 2-year rat toxicology study. Treatment-related tumors were not seen in the 2-year mouse study. A cross-functional, mechanism-based approach was undertaken to determine whether the mechanisms responsible for tumorigenesis in the rat were of relevance to humans. Based on findings from nonclinical and clinical studies, the treatment-related tumors observed in rats were not deemed to be of clinical relevance. Here, we describe the scientific and regulatory journey from learning of the 2-year rat study findings to the approval of canagliflozin for the treatment of T2DM.

Carcinogenicity in rats of the SGLT2 inhibitor canagliflozin

The carcinogenicity potential of canagliflozin, an inhibitor of SGLT2, was evaluated in a 2-year rat study (10, 30, and 100 mg/kg). Rats showed an increase in pheochromocytomas, renal tubular tumors, and testicular Leydig cell tumors. Systemic exposure multiples at the highest dose relative to the maximum clinical dose were 12- to 21-fold. Pheochromocytomas and renal tubular tumors were noted in both sexes at 100 mg/kg. Leydig cell tumors were observed in males in all dose groups and were associated with increased luteinizing hormone levels. Hyperplasia was increased in the adrenal medulla at 100 mg/kg, but only a limited increase in simple tubular hyperplasia was observed in the kidney of males at 100 mg/kg. Hyperostosis occurred and was accompanied by substantial effects on calcium metabolism, including increased urinary calcium excretion and decreased levels of calcium regulating hormones (1,25-dihydroxyvitamin D and parathyroid hormone). A separate study with radiolabeled calcium confirmed that increased urinary calcium excretion was mediated via increased calcium absorption from the gastrointestinal tract. It was hypothesized that, at high doses, canagliflozin might have inhibited glucose absorption in the intestine via SGLT1 inhibition that resulted in glucose malabsorption, which increased calcium absorption by stimulating colonic glucose fermentation and reducing intestinal pH. Pheochromocytomas and adrenal medullary hyperplasia were attributed to altered calcium homeostasis, which have a known relationship in the rat. In conclusion, Leydig cell tumors were associated with increased luteinizing hormone levels and pheochromocytomas were most likely related to glucose malabsorption and altered calcium homeostasis. Renal tubular tumors may also have been linked to glucose malabsorption.

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.

Methyltertiary-Butyl Ether: Studies for Potential Human Health Hazards

When methyl tertiary-butyl ether (MTBE) in gasoline was first introduced to reduce vehicle exhaust emissions and comply with the Clean Air Act, in the United States, a pattern of complaints emerged characterised by seven "key symptoms." Later, carefully controlled volunteer studies did not confirm the existence of the specific key symptoms, although one study of self-reported sensitive (SRS) people did suggest that a threshold at about 11-15% MTBE in gasoline may exist for SRSs in total symptom scores. Neurobehavioral and psychophysiological studies on volunteers, including SRSs, found no adverse responses associated with MTBE at likely exposure levels. MTBE is well and rapidly absorbed following oral and inhalation exposures. Cmax values for MTBE are achieved almost immediately after oral dosing and within 2 h of continuous inhalation. It is rapidly eliminated, either by exhalation as unchanged MTBE or by urinary excretion of its less volatile metabolites. Metabolism is more rapid humans than in rats, for both MTBE and tert-butyl alcohol (TBA), its more persistent primary metabolite. The other primary metabolite, formaldehyde, is detoxified at a rate very much greater than its formation from MTBE. MTBE has no specific effects on reproduction or development, or on genetic material. Neurological effects were observed only at very high concentrations. In carcinogenicity studies of MTBE, TBA, and methanol (included as an endogenous precursor of formaldehyde, without the presence of TBA), some increases in tumor incidence have been observed, but consistency of outcome was lacking and even some degree of replication was observed in only three cases, none of which had human relevance: alpha(2u)-globulin nephropathy-related renal tubule cell adenoma in male rats; Leydig-cell adenoma in male rats, but not in mice, which provide the better model of the human disease; and B-cell-derived lymphoma/leukemia of doubtful pathogenesis that arose mainly in lungs of orally dosed female rats. In addition, hepatocellular adenomas were significantly higher in female CD-1 mice and thyroid follicular-cell adenomas were increased in female B6C3F1 mice treated with TBA, but these results lack any independent confirmation, which would have been possible from a number of other studies.

PPARα Agonist-Induced Rodent Tumors: Modes of Action and Human Relevance

Widely varied chemicals--including certain herbicides, plasticizers, drugs, and natural products--induce peroxisome proliferation in rodent liver and other tissues. This phenomenon is characterized by increases in the volume density and fatty acid oxidation of these organelles, which contain hydrogen peroxide and fatty acid oxidation systems important in lipid metabolism. Research showing that some peroxisome proliferating chemicals are nongenotoxic animal carcinogens stimulated interest in developing mode of action (MOA) information to understand and explain the human relevance of animal tumors associated with these chemicals. Studies have demonstrated that a nuclear hormone receptor implicated in energy homeostasis, designated peroxisome proliferator-activated receptor alpha (PPARalpha), is an obligatory factor in peroxisome proliferation in rodent hepatocytes. This report provides an in-depth analysis of the state of the science on several topics critical to evaluating the relationship between the MOA for PPARalpha agonists and the human relevance of related animal tumors. Topics include a review of existing tumor bioassay data, data from animal and human sources relating to the MOA for PPARalpha agonists in several different tissues, and case studies on the potential human relevance of the animal MOA data. The summary of existing bioassay data discloses substantial species differences in response to peroxisome proliferators in vivo, with rodents more responsive than primates. Among the rat and mouse strains tested, both males and females develop tumors in response to exposure to a wide range of chemicals including DEHP and other phthalates, chlorinated paraffins, chlorinated solvents such as trichloroethylene and perchloroethylene, and certain pesticides and hypolipidemic pharmaceuticals. MOA data from three different rodent tissues--rat and mouse liver, rat pancreas, and rat testis--lead to several different postulated MOAs, some beginning with PPARalpha activation as a causal first step. For example, studies in rodent liver identified seven "key events," including three "causal events"--activation of PPARalpha, perturbation of cell proliferation and apoptosis, and selective clonal expansion--and a series of associative events involving peroxisome proliferation, hepatocyte oxidative stress, and Kupffer-cell-mediated events. Similar in-depth analysis for rat Leydig-cell tumors (LCTs) posits one MOA that begins with PPARalpha activation in the liver, but two possible pathways, one secondary to liver induction and the other direct inhibition of testicular testosterone biosynthesis. For this tumor, both proposed pathways involve changes in the metabolism and quantity of related hormones and hormone precursors. Key events in the postulated MOA for the third tumor type, pancreatic acinar-cell tumors (PACTs) in rats, also begin with PPARalpha activation in the liver, followed by changes in bile synthesis and composition. Using the new human relevance framework (HRF) (see companion article), case studies involving PPARalpha-related tumors in each of these three tissues produced a range of outcomes, depending partly on the quality and quantity of MOA data available from laboratory animals and related information from human data sources.

Pharmaceutical excipient development: the need for preclinical guidance

Pharmaceutical excipients have a vital role in drug formulations, a role that has tended to be neglected as evidenced by the lack of mechanisms to assess excipient safety outside a new drug application process. Currently, it is assumed that an excipient is "approved" when the new drug formulation, of which it is a constituent, receives regulatory acceptance. Existing regulations and guidelines indicate that new (novel) excipients should be treated as new chemical entities with full toxicological evaluation. No guidance is available for potentially useful materials (essentially new excipients) available from other industries, e.g., food additives or for established excipients with a new application, e.g., dose route change. However, despite this situation, drug companies are actively evaluating new materials or applying new uses to established excipients. Recently developed excipients (e.g., materials giving "sugar-free" status to medical preparations, the cyclodextrins, and the hydrofluoroalkane inhalation propellants) and excipients undergoing development (e.g., chitosan, various enteric coating substances, liposomes, polymers derived from glycolic and lactic acids, and vaccine adjuvants) are all discussed. In light of many other areas of drug development having recently benefited from new or updated regulatory guidance, specific guidance to assist companies in the development of their excipients is urgently needed. Also, an excipient testing strategy would be an excellent topic for inclusion for International Conference on Harmonisation (ICH) consideration. Such guidance/discussion would complement the current advances in pharmacopoeial standardization of excipient quality. As a consequence, it may be possible to have excipients reviewed by a committee of an international pharmacopoeia with the safety data assessed by elected experts and published.

Rodent Leydig cell tumorigenesis: a review of the physiology, pathology, mechanisms, and relevance to humans

Leydig cells (LCs) are the cells of the testis that have as their primary function the production of testosterone. LCs are a common target of compounds tested in rodent carcinogenicity bioassays. The number of reviews on Leydig cell tumors (LCTs) has increased in recent years because of its common occurrence in rodent bioassays and the importance in assessing the relevance of this tumor type to humans. To date, there have been no comprehensive reviews to identify all the compounds that have been shown to induce LCTs in rodents or has any review systematically evaluated the epidemiology data to determine whether humans were at increased risk for developing LCTs from exposure to these agents. This review attempts to fill these deficiencies in the literature by comparing the cytology and ontogeny of the LC, as well as the endocrine and paracrine regulation of both normal and tumorigenic LCs. In addition, the pathology of LCTs in rodents and humans is compared, compounds that induce LC hyperplasia or tumors are enumerated, and the human relevance of chemical-induced LCTs is discussed. There are plausible mechanisms for the chemical induction of LCTs, as typified by agonists of estrogen, gonadotropin releasing hormone (GnRH), and dopamine receptors, androgen receptor antagonists, and inhibitors of 5alpha-reductase, testosterone biosynthesis, and aromatase. Most of these ultimately involve elevation in serum luteinizing hormone (LH) and/or LC responsiveness to LH as proximate mediators. It is expected that further work will uncover additional mechanisms by which LCTs may arise, especially the role of growth factors in modulating LC tumorigenesis. Regarding human relevance, the pathways for regulation of the hypothalamo-pituitary-testis (HPT) axis of rats and humans are similar, such that compounds that either decrease testosterone or estradiol levels or their recognition will increase LH levels. Hence, compounds that induce LCTs in rats by disruption of the HPT axis pose a risk to human health, except for possibly two classes of compounds (GnRH and dopamine agonists). Because GnRH and prolactin receptors are either not expressed or are expressed at very low levels in the testes in humans, the induction of LCTs in rats by GnRH and dopamine agonists would appear not to be relevant to humans; however, the potential relevance to humans of the remaining five pathways of LCT induction cannot be ruled out. Therefore, the central issue becomes what is the relative sensitivity between rat and human LCs in their response to increased LH levels; specifically, is the proliferative stimulus initiated by increased levels of LH attenuated, similar, or enhanced in human vs. rat LCs? There are several lines of evidence that suggest that human LCs are quantitatively less sensitive than rats in their proliferative response to LH, and hence in their sensitivity to chemically induced LCTs. This evidence includes the following: (1) the human incidence of LCTs is much lower than in rodents even when corrected for detection bias; (2) several comparative differences exist between rat and human LCs that may contribute, at least in part, to the greater susceptibility of the rat to both spontaneous and xenobiotic-induced LCTs; (3) endocrine disease states in man (such as androgen-insensitivity syndrome and familial male precocious puberty) underscore the marked comparative differences that exist between rats and man in the responsiveness of their LC's to proliferative stimuli; and (4) several human epidemiology studies are available on a number of compounds that induce LCTs in rats (1,3-butadiene, cadmium, ethanol, lactose, lead, nicotine) that demonstrate no association between human exposure to these compounds and induction of LC hyperplasia or adenomas. (ABSTRACT TRUNCATED)

Short- and intermediate-term carcinogenicity testing--a review. Part 1: the prototypes mouse skin tumour assay and rat liver focus assay

Carcinogenicity testing is by far the most expensive and time-consuming study type of toxicology. For many years, the lifetime exposure with the maximum tolerated dose in two rodent species has been the gold standard of carcinogenicity testing of pharmaceuticals. Major change was introduced by the Fourth International Conference on Harmonization in July 1997; a chronic rodent bioassay in one species and a short-term carcinogenicity assay are regarded as sufficient for registration. Such requirements provide the opportunity to redirect the vast resources previously spent on the lifetime study in the second species. Numerous experimental protocols for short- and intermediate-term carcinogenicity testing in many target tissues have been available for years. The first part of this review describes the basic principles of short- and intermediate-term carcinogenicity testing using the examples of the widely used mouse skin tumour assay and the rat liver foci assay. In the context of these experimental models, the discrimination and quantification of initiating and promoting activity and the use of preneoplastic lesions as endpoints in carcinogenicity testing are described. The review includes the limitations of the models with regard to the extrapolation from effects observed in animal experiments to a potential exposure of humans.

A toxicological review of lactose to support clinical administration by inhalation

Although lactose is widely used in the pharmaceutical industry as an excipient in preparations given by several routes of administration, including by inhalation, there is no comprehensive review of its toxicological properties. This document seeks to review the available oral preclinical and clinical safety data from the literature, together with that generated by Fisons in animals using the inhalation route. In animal toxicity studies, lactose has been administered primarily by the inhalation and dietary routes to the rat, dog and/or primate. Adverse findings, such as abdominal distension and diarrhoea, have been demonstrated in rodent feeding studies. However, these changes are considered to be due to non-specific effects associated with high dietary doses of lactose, with a subsequent production of a dietary imbalance which results in physiological disturbances and an overload in the metabolic processes particularly involving calcium. These changes at high dietary intakes of lactose are considered to be of little relevance for man under the normal conditions of use of the material as an excipient in pharmaceutical formulations. No adverse local effects to the lung have been demonstrated in the animal studies using the inhalation route. Although the inhalation dose of lactose in the animal studies, of which most is subsequently swallowed, is markedly higher than the clinical dose, it is considerably less than consumed in animal studies using the dietary route. Consequently, it is not surprising that lactose is well tolerated by the inhalation route. In a small number of susceptible humans, intolerance to lactose is generally observed with oral intake of lactose, usually as a constituent of milk and is associated with lactase deficiency. Swallowed lactose at the levels present in inhaled preparations is unlikely to present any significant problems in patients with lactase deficiency. In conclusion, lactose is well recognized as a safe pharmaceutical excipient for use in oral or inhalation formulations and is not likely to constitute any significant toxicological hazard to man.

Leydig cell hyperplasia and adenoma formation: mechanisms and relevance to humans

Leydig cell adenomas are observed frequently in studies evaluating the chronic toxicity of chemical agents in laboratory animals. Doubts have been raised about the relevance of such responses for human risk assessment, but the question of relevance has not been evaluated and presented in a comprehensive manner by a broad group of experts. This article reports the consensus conclusions from a workshop on rodent Leydig cell adenomas and human relevance. Five aspects of Leydig cell biology and toxicology were discussed: 1) control of Leydig cell proliferation; 2) mechanisms of toxicant-induced Leydig cell hyperplasia and tumorigenesis; 3) pathology of Leydig cell adenomas; 4) epidemiology of Leydig cell adenomas; and 5) risk assessment for Leydig cell tumorigens. Important research needs also were identified. Uncertainty exists about the true incidence of Leydig cell adenomas in men, although apparent incidence is rare and restricted primarily to white males. Also, surveillance databases for specific therapeutic agents as well as nicotine and lactose that have induced Leydig cell hyperplasia or adenoma in test species have detected no increased incidence in humans. Because uncertainties exist about the true incidence in humans, induction of Leydig cell adenomas in test species may be of concern under some conditions. Occurrence of Leydig cell hyperplasia alone in test species after lifetime exposure to a chemical does not constitute a cause for concern in a risk assessment for carcinogenic potential, but early occurrence may indicate a need for additional testing. Occurrence of Leydig cell adenomas in test species is of potential concern as both a carcinogenic and reproductive effect if the mode of induction and potential exposures cannot be ruled out as relevant for humans. The workgroup focused on seven hormonal modes of induction of which two, GnRH agonism and dopamine agonism, were considered not relevant to humans. Androgen receptor antagonism, 5 alpha-reductase inhibition, testosterone biosynthesis inhibition, aromatase inhibition, and estrogen agonism were considered to be relevant or potentially relevant, but quantitative differences may exist across species, with rodents being more sensitive. A margin of exposure (MOE; the ratio of the lowest exposure associated with toxicity to the human exposure level) approach should be used for compounds causing Leydig cell adenoma by a hormonal mode that is relevant to humans. For agents that are positive for mutagenicity, the decision regarding a MOE or linear extrapolation approach should be made on a case-by-case basis. In the absence of information about mode of induction, it is necessary to utilize default assumptions, including linear behavior below the observable range. All of the evidence should be weighed in the decision-making process.

Safety of propellants

Chlorofluorocarbon (CFC) propellants used in metered dose inhalers (MDI) must be replaced under the terms of the Montreal Protocol. Following a review of the available data, HFA134a (1,1,1,2 tetrafluoroethane) was selected as a potential alternative propellant. However, because this data was insufficient to satisfy the stringent requirements for pharmaceuticals, additional toxicological assessments were performed. These included genotoxicology, animal inhalation studies, reproductive toxicology, local tolerability and safety pharmacology studies. A special grade of HFA134a, containing relatively high concentrations of all likely impurities, was used for all pivotal studies. HFA134a was devoid of genotoxicity and had exceptional low acute toxicity. No toxicity was seen in rats or dogs exposed to concentrations of up to 5% or 12% respectively for up to one year. No fetotoxicity, effects on reproductive performance, peri- or postnatal development was demonstrated. HFA134a was devoid of oncogenic potential in rats and mice. There was no evidence of sensitisation or local irritation to the skin or eyes. All species demonstrated high systemic HFA134a concentrations. An extensive toxicological evaluation, at very high multiples of the likely patient exposure, has indicated that HFA134a is a suitable alternative to CFC propellants for MDIs.

HFA-134a (1,1,1,2-tetrafluoroethane); lack of oncogenicity in rodents after inhalation

1. Groups of 60 male and 60 female B6C3F1 mice or HanIbm Wistar rats were exposed to HFA-134a using snout-only inhalation exposure techniques for periods of one hour daily for at least 104 weeks. HFA-134a was delivered directly from cylinders at vapour concentrations of 2500, 15,000 and 75,000ppm for mice and from metered-dose inhalers at vapour concentrations of 2500, 10,000 and 50,000ppm for rats. 2. Intended dosages were achieved. 3. Evidence of absorption was found at each dose level and was dose related. 4. Neither species suffered treatment related effects on survival, clinical signs, body weights, haematology nor on the type, incidence, site or severity of gross lesions. 5. There was no effect of treatment on the type, incidence, site or severity of neoplasms in mice or rats. 6. There were no non-neoplastic findings related to treatment in mice. 7. HFA-134a was considered not to be oncogenic and to provide a safe alternative to chlorofluorocarbons for use in pharmaceutical metered-dose inhalers.

A review of drug-induced Leydig cell hyperplasia and neoplasia in the rat and some comparisons with man

This paper describes control of normal Leydig cell function and testosterone production. The macroscopic and histopathological appearances of spontaneous Leydig cell hyperplasias and tumors (LCT) in the rat are reviewed together with their incidence and hormonal status. Drugs which induce LCTs in chronic studies are discussed and include busereline, carbamazepine, cimetidine, finasteride, flutamide, gemfibrozil, histrelin, hydralazine, indomethacin, isradipine, lactitol, leuprolide, metronidazole, mesulergine, nafarelin, norprolac and vidarabine. The known mechanisms of LCT induction in the rat are reviewed together with other possible etiological factors. The incidence, clinical picture and etiological factors of LCTs in man are also surveyed. Hormone production in Leydig cells and LCTs in rats and man are compared. Differences between the two species are considered, particularly with regard to Leydig cell control mechanisms. The paper concludes that drug-induced LCTs in rats are most probably not predictive for man and their occurrence has little relevance in human safety assessment.

Evaluation of the genotoxicity potential and chronic inhalation toxicity of 1,1-dichloro-1-fluoroethane (HCFC-141b)

A battery of in vitro and in vivo tests were conducted on HCFC-141b as a vapour. Bacterial gene mutation assays with Escherichia coli and Salmonella typhimurium were negative in all tester strains. In vitro chromosomal aberration assays were positive on CHO cells but negative on human lymphocytes. Moreover, HCFC-141b was negative in vivo in a mouse micronucleus inhalation assay. On the basis of these data and previously reported genotoxicity testing, HCFC-141b is considered non-genotoxic. Groups of 80 male and 80 female Sprague-Dawley rats were exposed, by inhalation (6 hr/day, 5 days/wk) to vapours of HCFC-141b for 104 wk at target concentrations of 0 (control), 1500, 5000 and 20,000 ppm (increased from 15,000 ppm after 17 wk of exposure). No exposure-related effects of toxicological significance were noted with respect to survival, clinical signs, ophthalmoscopy, haematology, clinical chemistry, urinalysis or organ weight analysis. Reduced food intake and body weight gain were noted in both sexes of the 15,000 ppm group during the first 16 wk; thereafter, body weight gains in all groups were similar although the intergroup differences in body weight remained evident. Reduced food intake persisted in both sexes through wk 52 and in females during the second year of exposure. Treatment-related effects on macroscopic pathology were confined to increased incidences of testicular masses and altered appearance. Microscopic pathology examinations confirmed the testes as the target organ with findings of increased incidences of benign interstitial cell tumours and hyperplasia at 5000 and 20,000 ppm. The no-observable-adverse-effect level (NOAEL) was 1500 ppm. The testicular changes at high exposure levels were considered to be due to a change of the senile hormonal imbalance in geriatric rats and of little significance for the assessment of human health effects.

Interstitial cell adenoma of the rat testis: why is it such a problem in toxicology studies using Sprague-Dawley rats?

Arbitrary criteria can give unreliable diagnoses. Conditions that occupy the same marginal diagnostic space, such as interstitial cell hyperplasia and interstitial cell adenoma, sometimes will be mistaken for one another. When the true prevalence is low, the risk of false positives, as a percentage of all positives, is high. The preceding analysis predicts that, with currently recommended diagnostic criteria, any compound that causes nonneoplastic interstitial cell hyperplasia will be interpreted to cause interstitial cell neoplasia if SD rats are used and only microscopic diagnoses are considered. Conversely, the analysis predicts that no compound that actually causes neoplasia in interstitial cells will be identified as an interstitial cell tumorigen if F-344 rats are used and only microscopic diagnoses are considered. These predictions are consistent with experience.