Development of adverse outcome pathways relevant for the identification of substances having endocrine disruption properties Uterine adenocarcinoma as adverse outcome

Development of adverse outcome pathways (AOPs) for uterine adenocarcinoma can provide a practical tool to implement the EFSA-ECHA Guidance (2018) for the identification of endocrine disruptors in the context of Regulations (EU) No 528/2012 and (EC) No 1107/2009. AOPs can give indications about the strength of the relationship between an adverse outcome (intended as a human health outcome) and chemicals (pesticides but not only) affecting the pathways. In this scientific opinion, the PPR Panel explored the development of AOPs for uterine adenocarcinoma. An evidence-based approach methodology was applied, and literature reviews were produced using a structured framework assuring transparency, objectivity, and comprehensiveness. Several AOPs were developed; these converged to a common critical node, that is increased estradiol availability in the uterus followed by estrogen receptor activation in the endometrium; therefore, a putative AOP network was considered. An uncertainty analysis and a probabilistic quantification of the weight of evidence have been carried out via expert knowledge elicitation for each set of MIEs/KEs/KERs included in individual AOPs. The collected data on the AOP network were evaluated qualitatively, whereas a quantitative uncertainty analysis for weight of the AOP network certainty has not been performed. Recommendations are provided, including exploring further the uncertainties identified in the AOPs and putative AOP network; further methodological developments for quantifying the certainty of the KERs and of the overall AOPs and AOP network; and investigating of NAMs applications in the context of some of the MIEs/KEs currently part of the putative AOP network developed.

Research-Relevant Conditions and Pathology of Laboratory Mice, Rats, Gerbils, Guinea Pigs, Hamsters, Naked Mole Rats, and Rabbits

Animals are valuable resources in biomedical research in investigations of biological processes, disease pathogenesis, therapeutic interventions, safety, toxicity, and carcinogenicity. Interpretation of data from animals requires knowledge not only of the processes or diseases (pathophysiology) under study but also recognition of spontaneous conditions and background lesions (pathology) that can influence or confound the study results. Species, strain/stock, sex, age, anatomy, physiology, spontaneous diseases (noninfectious and infectious), and neoplasia impact experimental results and interpretation as well as animal welfare. This review and the references selected aim to provide a pathology resource for researchers, pathologists, and veterinary personnel who strive to achieve research rigor and validity and must understand the spectrum of "normal" and expected conditions to accurately identify research-relevant experimental phenotypes as well as unusual illness, pathology, or other conditions that can compromise studies involving laboratory mice, rats, gerbils, guinea pigs, hamsters, naked mole rats, and rabbits.

GH3 and RC-4BC cell lines are not suitable as in vitro models to study prolactin modulation and AHR responsiveness in rat pituitary

Some environmental contaminants and pharmaceuticals increase the incidence of uterine tumors in toxicological studies with rats. These tumors can result from a hormonal imbalance due to rat-specific disrupted pituitary prolactin regulation, and are therefore of questionable relevance for humans. In this study we compared in vitro prolactin regulation in rat primary pituitary cells to that in pituitary cell lines, GH3 and RC-4BC. Moreover, we assessed the potential effects of aryl hydrocarbon receptor (AHR) activation on prolactin regulation by using two different AHR agonists, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and DELAQ, the N-deethylated minor metabolite of the pharmaceutical laquinimod. In rat primary pituitary cells, known prolactin stimulant thyrotropin-releasing hormone (TRH) marginally increased prolactin secretion (1.2-fold) and gene expression (1.3-fold). In contrast, synthetic dopamine receptor agonist quinpirole, a known inhibitor of prolactin release, significantly inhibited prolactin secretion (2.6-fold) and gene expression (3.6-fold). In GH3 cells, TRH strongly increased prolactin secretion (6.8-fold) and gene expression (30.8-fold), whereas quinpirole did not affect prolactin secretion nor gene expression. In RC-4BC cells, both TRH and quinpirole did not modulate prolactin secretion nor gene expression. Prolactin secretion and gene expression did not significantly change upon exposure to TCDD or DELAQ. However, DELAQ, but not TCDD, attenuated quinpirole-inhibited prolactin gene expression by 51% in primary pituitary cells. This study shows that pituitary prolactin regulation in rat primary pituitary cells in vitro is distinctly different from rat pituitary cell lines GH3 and RC-4BC. Therefore, effects on pituitary prolactin regulation in vitro should best be performed using rat primary pituitary cells. Additionally, AHR ligands may interact with rat pituitary prolactin regulation, but this appears to depend on the ligand and constitutive prolactin secretion. However, interpretation of the in vitro results with respect to occurrence of uterine tumors in rats should take the complex regulation of prolactin release in the pituitary into account as well as the in vivo hypothalamus-pituitary-gonadal (HPG) axis and its feedback loops.

Epigenetic dysregulation of key developmental genes in radiation-induced rat mammary carcinomas

With the increase in the number of long-term cancer survivors worldwide, there is a growing concern about the risk of secondary cancers induced by radiotherapy. Epigenetic modifications of genes associated with carcinogenesis are attractive targets for the prevention of cancer owing to their reversible nature. To identify genes with possible changes in functionally relevant DNA methylation patterns in mammary carcinomas induced by radiation exposure, we performed microarray-based global DNA methylation and expression profiling in γ-ray-induced rat mammary carcinomas and normal mammary glands. The gene expression profiling identified dysregulation of developmentally related genes, including the downstream targets of polycomb repressive complex 2 (PRC2) and overexpression of enhancer of zeste homolog 2, a component of PRC2, in the carcinomas. By integrating expression and DNA methylation profiles, we identified ten hypermethylated and three hypomethylated genes that possibly act as tumor-suppressor genes and oncogenes dysregulated by aberrant DNA methylation; half of these genes encode developmental transcription factors. Bisulfite sequencing and quantitative PCR confirmed the dysregulation of the polycomb-regulated developmentally related transcription-factor genes Dmrt2, Hoxa7, Foxb1, Sox17, Lhx8, Gata3 and Runx1. Silencing of Hoxa7 was further verified by immunohistochemistry. These results suggest that, in radiation-induced mammary gland carcinomas, PRC2-mediated aberrant DNA methylation leads to dysregulation of developmentally related transcription-factor genes. Our findings provide clues to molecular mechanisms linking epigenetic regulation and radiation-induced breast carcinogenesis and underscore the potential of such epigenetic mechanisms as targets for cancer prevention.

Historical control background incidence of spontaneous pituitary gland lesions of Han-Wistar and Sprague-Dawley rats and CD-1 mice used in 104-week carcinogenicity studies

The aim of this study was to determine the range and incidences of spontaneous microscopic lesions of the pituitary gland in control Han-Wistar and Sprague-Dawley rats and CD-1 mice from 104-week carcinogenicity studies carried out between 1998 and 2010 at Charles River Edinburgh. In both strains of rats and in CD-1 mice, non-proliferative lesions of the pituitary gland were generally uncommon, excluding cysts/pseudocysts (6.42% in Han-Wistar rats, 5.85% in Sprague-Dawley rats, and 2.08% in CD-1 mice). Primary proliferative lesions were most frequently found in the pars distalis of the pituitary gland. Adenomas and carcinomas of the pars distalis were more common in Sprague-Dawley rats (49.33% and 2.85%, respectively) than in Han-Wistar rats (27.29% and 0.21%, respectively), and adenomas in both strains of rats and CD-1 mice exhibited a marked sex predisposition, with females more commonly affected.

Analysis of genes involved in the PI3K/Akt pathway in radiation- and MNU-induced rat mammary carcinomas

The PI3K/AKT pathway is one of the most important signaling networks in human breast cancer, and since it was potentially implicated in our preliminary investigations of radiation-induced rat mammary carcinomas, our aim here was to verify its role. We included mammary carcinomas induced by the chemical carcinogen 1-methyl-1-nitrosourea to determine whether any changes were radiation-specific. Most carcinomas from both groups showed activation of the PI3K/AKT pathway, but phosphorylation of AKT1 was often heterogeneous and only present in a minority of carcinoma cells. The negative pathway regulator Inpp4b was significantly downregulated in both groups, compared with in normal mammary tissue, and radiation-induced carcinomas also showed a significant decrease in Pten expression, while the chemically induced carcinomas showed a decrease in Pik3r1 and Pdk1. Significant upregulation of the positive regulators Erbb2 and Pik3ca was observed only in chemically induced carcinomas. However, no genes showed clear correlations with AKT phosphorylation levels, except in individual carcinomas. Only rare carcinomas showed mutations in PI3K/AKT pathway genes, yet these carcinomas did not exhibit stronger AKT phosphorylation. Thus, while AKT phosphorylation is a common feature of rat mammary carcinomas induced by radiation or a canonical chemical carcinogen, the mutation of key genes in the pathways or permanent changes to gene expression of particular signaling proteins do not explain the pathway activation in the advanced cancers. Although AKT signaling likely facilitates cancer development and growth in rat mammary carcinomas, it is unlikely that permanent disruption of the PI3K/AKT pathway genes is a major causal event in radiation carcinogenesis.

Effects of the Janus Kinase Inhibitor, Tofacitinib, on Testicular Leydig Cell Hyperplasia and Adenoma in Rats, and on Prolactin Signaling in Cultured Primary Rat Leydig Cells

Tofacitinib is an oral Janus kinase (JAK) inhibitor for the treatment of rheumatoid arthritis. Tofacitinib preferentially inhibits receptor signaling through JAK3 and JAK1, relative to JAK2. In the 2-year rat carcinogenicity study, there were tofacitinib, dose-related increases in the incidences of testicular Leydig cell hyperplasia and benign adenomas in male rats, and decreased incidences of mammary tumors and duct dilatation/galactocele in female rats. Such findings in rats are typical of agents, such as dopamine agonists, which decrease prolactin (PRL) activity. Since prolactin signals through the JAK2 pathway, we hypothesized that these findings were off-target effects due to inhibition of PRL signaling via JAK2. The studies reported here were designed to investigate the interruption of PRL signaling pathways in Leydig cells. In isolated primary rat Leydig cells, PRL increased phosphorylated Signal Transducer and Activator of Transcription-5 protein, and mRNA levels for luteinizing hormone receptor. Tofacitinib, at concentrations observed in the rat carcinogenicity study, dose-dependently inhibited these effects. These observations illustrate a novel mechanism, the inhibition of prolactin signaling by which modulation of JAK activity can modulate PRL signaling pathways to induce Leydig cell tumors in rats. Since human Leydig cells lack this PRL dependence for normal function, these rodent tumors do not indicate a health risk to human patients.

Results from oral gavage carcinogenicity studies of ruxolitinib in Tg.rasH2 mice and Sprague-Dawley (Crl:CD) rats

Ruxolitinib is a selective and potent inhibitor of Janus kinase (JAK) 1 and JAK2. It is approved for the treatment of patients with intermediate or high-risk myelofibrosis, or those with polycythemia vera who have had an inadequate response to or are intolerant of hydroxyurea. To investigate its carcinogenic potential, ruxolitinib was administered by oral gavage once daily to Tg.rasH2 mice for 6 months at doses of 15, 45 or 125 mg/kg/day, and to Sprague-Dawley (Crl:CD) rats for 2 years at 10, 20 or 60 mg/kg/day. Ruxolitinib had no effect on survival, and did not increase the incidence of any neoplastic findings in either species. Exposure (AUC) was similar to or exceeded that associated with therapeutic use. Lymphoid depletion and a decrease in extramedullary hematopoiesis in the spleen occurred in rats, which were attributed to the pharmacologic activity of ruxolitinib. In Tg.rasH2 mice, increased inflammation in the nasal cavity was observed. Dose-dependent decreases in a number of spontaneous neoplastic/preneoplastic lesions were observed in rats, including mammary tumors in females, adrenal pheochromocytomas in males, hepatocellular adenomas/carcinomas in males, and hepatic basophilic (males and females) and eosinophilic (males) foci. Peribiliary fibrosis was also decreased. Clear cell foci in the liver were increased in females. Based on the results of these studies, ruxolitinib is not considered to be carcinogenic.

A high dose mode of action for tetrabromobisphenol A-induced uterine adenocarcinomas in Wistar Han rats: A critical evaluation of key events in an adverse outcome pathway framework

TBBPA is a non-genotoxic flame retardant used to improve fire safety in a wide variety of consumer products. Estimated human exposures to TBBPA are very low (<0.000084 mg/kg-day), relative to the doses (500 and 1000 mg/kg-day of TBBPA) administered in a recent bioassay that resulted in uterine tumors in Wistar Han rats following chronic exposure. As part of an effort to characterize the relevance of the uterine tumors to humans, data and biological knowledge relevant to the progression of events associated with TBBPA-induced uterine tumors in female rats were organized in an adverse outcome pathway (AOP) framework. Based on a review of possible MOAs for chemically induced uterine tumors and available TBBPA data sets, a plausible molecular initiating event (MIE) was the ability of TBBPA to bind to and inhibit estrogen sulfotransferases, the enzymes responsible for sulfation of estradiol. Subsequent key events in the AOP, including increased bioavailability of unconjugated estrogens in uterine tissue, would occur as a result of decreased sulfation, leading to a disruption in estrogen homeostasis, increased expression of estrogen responsive genes, cell proliferation, and hyperplasia. Available data support subsequent key events, including generation of reactive quinones from the metabolism of estrogens, followed by DNA damage that could contribute to the development of uterine tumors. Uncertainties associated with human relevance are highlighted by potential strain/species sensitivities to development of uterine tumors, as well as the characterization of a dose-dependent MIE. For the latter, it was determined that the TBBPA metabolic profile is altered at high doses (such as those used in the cancer bioassay), and thus an MIE that is only operative under repeated high dose, administration. The MIE and subsequent key events for the development of TBBPA-induced uterine tumors are not feasible in humans given differences in the kinetic and dynamic factors associated with high dose exposures in rats relative to human exposure levels to TBBPA.

F344/NTac Rats Chronically Exposed to Bromodichloroacetic Acid Develop Mammary Adenocarcinomas With Mixed Luminal/Basal Phenotype and Tgfβ Dysregulation

Breast cancer is the most common cancer and the second-leading cause of cancer mortality in women in the United States. A recent 2-year National Toxicology Program carcinogenicity study showed an increased incidence of proliferative mammary lesions (hyperplasia, fibroadenoma, adenocarcinoma) in F344/NTac rats exposed to bromodichloroacetic acid (BDCA), a disinfection by-product in finished drinking water with widespread human exposure. We hypothesized that the increase in mammary tumors observed in BDCA-exposed F344/NTac rats may be due to underlying molecular changes relevant for human breast cancer. The objective of the study was to compare (1) gene and protein expression and (2) mutation spectra of relevant human breast cancer genes between normal untreated mammary gland and mammary tumors from control and BDCA-exposed animals to identify molecular changes relevant for human cancer. Histologically, adenocarcinomas from control and BDCA-exposed animals were morphologically very similar, were estrogen/progesterone receptor positive, and displayed a mixed luminal/basal phenotype. Gene expression analysis showed a positive trend in the number of genes associated with human breast cancer, with proportionally more genes represented in the BDCA-treated tumor group. Additionally, a 5-gene signature representing possible Tgfβ pathway activation in BDCA-treated adenocarcinomas was observed, suggesting that this pathway may be involved in the increased incidence of mammary tumors in BDCA-exposed animals.

Reproductive senescence, fertility and reproductive tumour profile in ageing female Han Wistar rats

A study using vehicle administration in 104 female rats investigated reproductive aging in Han Wistar rats as a useful tool to interprete carcinogenicity studies where hormonal patterns are perturbated. From 16 weeks of age oestrous cycles were monitored every 6 weeks to investigate reproductive ageing. A subset of 20 females was used to assess fertility at 21 months of age. The animals were necropsied after 106-107 weeks on study and female reproductive organs, mammary glands and pituitary glands were examined for hyperplasias and/or tumours. The majority of rats had regular oestrous cycles up to 6 months of age. After this age, there was a rapid decline in the number of rats with regular oestrous cycles and an increase in irregular cycles and cycles in persistent di-oestrus with an occasional pro-oestrus. By the end of the study, the majority of animals were acyclic and the few remaining cyclic animals had irregular cycles. In the fertility assessment, 19/20 animals mated but only four animals became pregnant. These pregnant animals had normal numbers of corpora lutea of pregnancy but had high pre-implantation losses and could not sustain a viable pregnancy. 65 animals (62.5%) showed adenomas and/or pituitary hyperplasia in the pituitary gland at necropsy. The pituitary tumours were likely to be prolactin secreting that give rise to pseudopregnancy and mammary tumours, demonstrated by the fact that 43/65 (66%) of the affected animals had histopathological signs of these conditions. Multiple corpora lutea were found in 61% of all animals at time of termination. Only one uterine tumour was seen in this study probably due to lack of persistent oestrus seen in these animals.

Predictive modes of action of pesticides in uterine adenocarcinoma development in rats

Endometrial adenocarcinoma in the uterine corpus is a malignant cancer that occurs in menopausal women and aged rodents. Because of the similarities in pathogenesis and morphology of endometrial adenocarcinoma in rodents and humans, prediction of the modes of action (MOA) in uterine carcinogenesis is important for extrapolation of rodent data to humans. Three MOAs have been accepted as major pathways for uterine carcinogenesis in rodents: 1) estrogenic activity, 2) increased serum 17beta-estradiiol (E2) to progesterone (P4) ratio and 3) modulation of estrogen metabolism to produce 4-hydroxyestradiol via P450 induction. Inhibition of estrogen excretion and increased aromatase in situ in the tumor are also a potential pathway. Here, chemicals showing uterine carcinogenicity were chosen from approximately 300 pesticides evaluated in Japan within the past decade, and their mechanisms were predicted using parameters from mechanistic and toxicity studies. Seven pesticides increased uterine tumor formation in rats, and the pathways of 4 pesticides could be predicted based on various mechanistic studies. The MOAs of cyenopyrafen and benthiavalicarb-isopropyl were predicted to be modulation of estrogen metabolism, while those of pyriminobac-methyl and spirodiclofen were predicted to be increased E2 to P4 ratio. The driven pathways of metazosulfuron and isopyrazam could not be predicted using several mechanistic studies. No mechanistic studies have been reported for sedaxane, which has a chemical structure and toxicological profile similar to isopyrazam. Our results indicated that appropriate mechanistic studies are useful for mechanism prediction in risk assessment. From this analysis, a flowchart showing a decision tree for predictive MOAs in uterine carcinogenesis was proposed.

Hormone Receptor Expression in Spontaneous Uterine Adenocarcinoma in Fischer 344 Rats

Most uterine cancers, the most common gynecological malignancies in women in developed countries, are hormone-dependent endometrial adenocarcinomas (EACs) that express estrogen and progesterone receptors. Although rat strains exist with a high spontaneous incidence of EAC, the Fischer 344 (F344) strain, previously one of the most commonly used strains in carcinogenicity testing, is not a high-incidence strain. To better understand the biology of this neoplasm, we assessed estrogen receptor α (ER), progesterone receptor (PR), and Ki-67 expression using immunohistochemistry in spontaneous EAC in 18 F344 rats used as control animals in 2-year National Toxicology Program bioassays. Of the 18 tumors, 9 were well-differentiated and 9 were poorly differentiated. Most tumors, 7/18, were ER+PR+, as observed in women. Of the remainder, 6/18 were ER+PR-, 2/18 were ER-PR+, and 3/18 were ER-PR-. Well-differentiated tumors were ER+ (8/9) more often than poorly differentiated tumors (5/9). The percentage of ER+ tumors (72%) in rats was similar to that seen in women, but rats less frequently had PR+ (50%) tumors than women. The heterogeneous estrogen and progesterone receptor immunophenotypes observed in F344 rats in this study highlight the importance of evaluating hormone receptor expression in animal models used for chemical evaluations.

Two-year carcinogenicity study of acrylamide in Wistar Han rats with in utero exposure

Acrylamide is an important chemical with widespread industrial and other uses in addition to generalized population exposure from certain cooked foods. Previous rat studies to assess the carcinogenic potential of acrylamide have been carried out exclusively in the Fischer 344 rat with identification of a number of tumors amongst which mesotheliomas of the tunica vaginalis is an important tumor endpoint in the classification of acrylamide as a 'probably human carcinogen. In a rat carcinogenicity study to determine the human relevance of mesotheliomas Wistar Han rats were exposed to 0, 0.5, 1.5, or 3.0mg acrylamide/kg body weight/day in drinking water starting at gestation day 6. At the end of two years, mammary gland fibroadenomas in females and thyroid follicular cell tumors in both sexes were the only tumors increased in acrylamide treated rats. These tumor endpoints have rat-specific modes of action suggesting less likelihood of human cancer risk than previously estimated. This study demonstrates that tunica vaginalis mesotheliomas are strain specific and not likely of genotoxic origin.

A peripherally restricted P2Y12 receptor antagonist altered rat tumor incidences with no human relevance: Mode of action consistent with dopamine agonism

Background

Ticagrelor is an orally available, direct acting and reversible P2Y₁₂ receptor antagonist approved for treatment of acute coronary syndrome. The objectives of these studies were to (1) evaluate the Ticagrelor 2-year rat carcinogenicity bioassay data; (2) investigate potential mode of action (MOA) and (3) interpret human relevance.

Methods

The following studies were done (1) rat two-year carcinogenicity study in male and female rats, (2) in vitro and in vivo genotoxicity assays, (3) quantitative whole body autoradiography (QWBA; male and female rats), (4) in vitro pharmacological profiling for more than 300 assays, and (5) in vivo ovariectomized rat assay.

Results

The carcinogenicity study indicated Ticagrelor increased uterine tumor incidence while decreasing mammary and pituitary tumors/hyperplasia incidences in only high dose female rats. However, this altered tumor incidences were not P2Y₁₂ target related since marketed non-reversible P2Y₁₂ receptor antagonists were not associated with alter tumor incidences. MOA studies determined Ticagrelor exposure in the anterior pituitary and Ticagrelor was (1) non-genotoxic, (2) peripherally-restricted, (3) a dopamine transport (DAT) inhibitor with an IC₅₀ lower than systemic free exposure in the rat carcinogenic study and more than a log higher than the free systemic exposure seen in clinical trials and (4) an inhibitor of estradiol-induced prolactin secretion.

Discussion

Similar to Ticagrelor, centrally active dopamine agonists induce the same altered tumor incidence patterns that according to literature do not translate into the clinical setting, with a MOA involving decreased prolactin secretion. The Ticagrelor MOA data and literature suggest that altered dopamine levels in the hypophyseal part of the hypothalamus–hypophyseal axis (by Ticagrelor) will result in similar altered tumor incidences in rat that do not translate into the clinical setting, based on qualitative species differences. In conclusion Ticagrelor increased uterine tumors in the rat carcinogenesis study by a MOA consistent with reduced dopamine inhibition of prolactin, which is not a patient safety risk.

Differential responses to JNJ-37822681, a specific and fast dissociating dopamine D2 receptor antagonist, in cynomolgus monkey and Sprague-Dawley rat general toxicology studies: clinical observations, prolactin levels, mammary histopathology findings and toxicokinetics

JNJ-37822681 is a potent, specific and fast dissociating dopamine D2 receptor antagonist intended for the treatment of schizophrenia. Its nonclinical toxicological profile was investigated in a series of general repeat dose toxicity studies in cynomolgus monkeys and Sprague-Dawley rats. The maximum duration of treatment was 9 and 6 months, respectively. Interspecies differences were noted in the response to JNJ-37822681 in terms of extrapyramidal (EPS)-like clinical signs and prolactin-mediated tissue changes in the mammary gland. Monkeys showed severe EPS-like clinical signs such as abnormal posture, abnormal eye movements and hallucination-like behavior at relatively low exposures compared to those associated with EPS in patients with schizophrenia. The high sensitivity of the monkey to JNJ-37822681-induced EPS-like signs was unexpected based on the fast dissociating properties of the compound. Rats, however, were not prone to EPS. Elevated serum prolactin levels were found in rats and monkeys. While rats showed slight to moderate prolactin-related tissue changes upon histopathological examination in all studies, which among others affected the mammary gland, only minor mammary gland tissue changes were noted in monkeys. Prolactin levels were only slightly increased in patients with schizophrenia receiving relatively high dose levels of JNJ-37822681. The monkey toxicology studies did not provide an exposure-based safety margin, while in rats adverse effects were only noted at exposures considerably higher than those achieved at efficacious plasma concentrations in the clinic. Overall, the available data suggest that the cynomolgus monkey showed better predictivity towards the nature of JNJ-37822681-associated adverse events in humans than the Sprague-Dawley rat.

Scientific and Regulatory Policy Committee (SRPC) Paper

Hormonally mediated effects on the female reproductive system may manifest as pathologic changes of endocrine-responsive organs and altered reproductive function. Identification of these effects requires proper assessment, which may include investigative studies to profile female reproductive hormones. Here, we briefly describe normal hormonal patterns across the estrous or menstrual cycle and provide general guidance on measuring female reproductive hormones and characterizing hormonal disturbances in nonclinical toxicity studies. Although species used in standard toxicity studies share basic features of reproductive endocrinology, there are important species differences that affect both study design and interpretation of results. Diagnosing female reproductive hormone disturbances can be complicated by many factors, including estrous/menstrual cyclicity, diurnal variation, and age- and stress-related factors. Thus, female reproductive hormonal measurements should not generally be included in first-tier toxicity studies of standard design with groups of unsynchronized intact female animals. Rather, appropriately designed and statistically powered investigative studies are recommended in order to properly identify ovarian and/or pituitary hormone changes and bridge these effects to mechanistic evaluations and safety assessments. This article is intended to provide general considerations and approaches for these types of targeted studies.