Scientific and Regulatory Policy Committee Points to Consider: Review of the United States Food and Drug Administration (FDA) Guidance on Pathology Peer Review in Nonclinical Toxicology Studies.

In December 2021, the United States Food and Drug Administration (FDA) issued the final guidance for industry titled Pathology Peer Review in Nonclinical Toxicology Studies: Questions and Answers. The stated purpose of the FDA guidance is to provide information to sponsors, applicants, and nonclinical laboratory personnel regarding the management and conduct of histopathology peer review as part of nonclinical toxicology studies conducted in compliance with good laboratory practice (GLP) regulations. On behalf of and in collaboration with global societies of toxicologic pathology and the Society of Quality Assurance, the Scientific and Regulatory Policy Committee (SRPC) of the Society of Toxicologic Pathology (STP) initiated a review of this FDA guidance. The STP has previously published multiple papers related to the scientific conduct of a pathology peer review of nonclinical toxicology studies and appropriate documentation practices. The objectives of this review are to provide an in-depth analysis and summary interpretation of the FDA recommendations and share considerations for the conduct of pathology peer review in nonclinical toxicology studies that claim compliance to GLP regulations. In general, this working group is in agreement with the recommendations from the FDA guidance that has added clear expectations for pathology peer review preparation, conduct, and documentation.

Historical Control Data of Spontaneous Pathological Findings in C57BL/6J Mice Used in 18-Month Dietary Carcinogenicity Assays.

A retrospective analysis in C57BL6/J mice used in dietary carcinogenicity studies was performed to determine the survival rate, causes of death and incidences of spontaneous non-tumoral and tumoral findings. Data were collected from 1600 mice from control dose groups of sixteen 18-month carcinogenicity assays performed between 2003 and 2021 at the same test facility with similar environmental conditions and experimental procedures. The survival rate was high in both sexes (81%-85%) and the causes of humane euthanasia or death were mainly non-tumoral (chronic ulcerative dermatitis, atrial thrombosis). Benign tumors were more frequent than malignant tumors and females were more affected than males. Pituitary gland adenoma in females, lymphoma, bronchioloalveolar adenoma, and harderian gland adenoma in both sexes were the most common tumors. Systemic amyloidosis, the most frequent non-tumoral lesion, was observed variably across studies without sex predilection. The analysis by cohort (3 time periods of 6 years) showed a tendency toward higher incidences of lymphoma and pituitary gland adenoma and lower incidences of amyloidosis over time. The results presented here provide for the first time a robust set of control historical data in untreated C57BL/6J mice kept for 18 months contributing to build in depth knowledge of this animal model.

Toxicologic Pathology Forum: Opinion on Not Euthanizing Control Animals in the Recovery Phase of Non-Rodent Toxicology Studies.

Nonclinical toxicology studies that are required to support human clinical trials of new drug candidates are generally conducted in a rodent and a non-rodent species. These studies typically contain a vehicle control group and low, intermediate, and high dose test article groups. In addition, a dosing-free recovery phase is sometimes included to determine reversibility of potential toxicities observed during the dosing phase and may include additional animals in the vehicle control and one or more dose groups. Typically, reversibility is determined by comparing the test article-related changes in the dosing phase animals to concurrent recovery phase animals at the same dose level. Therefore, for interpretation of reversibility, it is not always essential to euthanize the recovery vehicle control animals. In the absence of recovery vehicle control tissues, the pathologist's experience, historical control database, digital or glass slide repositories, or literature can be used to interpret the findings in the context of background pathology of the species/strain/age. Therefore, in most studies, the default approach could be not to euthanize recovery vehicle control animals. This article provides opinions on scenarios that may or may not necessitate euthanasia of recovery phase vehicle control animals in nonclinical toxicology studies involving dogs and nonhuman primates.

Artificial Intelligence in Toxicological Pathology: Quantitative Evaluation of Compound-Induced Follicular Cell Hypertrophy in Rat Thyroid Gland Using Deep Learning Models.

Digital pathology has recently been more broadly deployed, fueling artificial intelligence (AI) application development and more systematic use of image analysis. Here, two different AI models were developed to evaluate follicular cell hypertrophy in hematoxylin and eosin-stained whole-slide-images of rat thyroid gland, using commercial AI-based-software. In the first, mean cytoplasmic area measuring approach (MCA approach), mean cytoplasmic area was calculated via several sequential deep learning (DL)-based algorithms including segmentation in microanatomical structures (separation of colloid and stroma from thyroid follicular epithelium), nuclear detection, and area measurements. With our additional second, hypertrophy area fraction predicting approach (HAF approach), we present for the first time DL-based direct detection of the histopathological change follicular cell hypertrophy in the thyroid gland with similar results. For multiple studies, increased output parameters (mean cytoplasmic area and hypertrophic area fraction) were shown in groups given different hypertrophy-inducing reference compounds in comparison to control groups. Quantitative results correlated with the gold standard of board-certified veterinary pathologists' diagnoses and gradings as well as thyroid hormone dependent gene expressions. Accuracy and repeatability of diagnoses and grading by pathologists are expected to be improved by additional evaluation of mean cytoplasmic area or direct detection of hypertrophy, combined with standard histopathological observations.

Opinion on Maintaining In-House GLP Status for Toxicologic Pathology in Pharmaceutical and (Agro)Chemical Development.

Many pharmaceutical companies have recently elected to stop maintaining good laboratory practices (GLP) status of their R&D sites. Similar discussions have also been engaged in the (agro)chemical industry. This opinion paper examines the pros and cons of maintaining facility GLP status for the purposes of performing the pathology interpretation or peer reviews of GLP studies internally. The toxicologic pathologist provides gross and histomorphologic evaluation and interpretation of nonclinical exploratory and regulatory studies during drug and (agro)chemical development. This assessment significantly contributes to human risk assessment by characterizing the toxicological profile and discussing the human relevance of the findings. The toxicologic pathologist is a key contributor to compound development decisions (advancement or termination) and in the development of de-risking strategies for backup compounds, thus playing a critical role in helping to reduce the late attrition of drugs and chemicals. Maintaining GLP compliance is often perceived as a costly and cumbersome process; a common and short-term strategy to reduce the costs is to outsource regulatory toxicity studies. However, there are significant advantages in maintaining the GLP status for toxicologic pathology activities in-house including the sustainable retention of internal pathology expertise that has maintained the necessary training needed to manage GLP studies. [Box: see text].

Artificial Intelligence in Toxicologic Pathology: Quantitative Evaluation of Compound-Induced Hepatocellular Hypertrophy in Rats.

Digital pathology evolved rapidly, enabling more systematic usage of image analysis and development of artificial intelligence (AI) applications. Here, combined AI models were developed to evaluate hepatocellular hypertrophy in rat liver, using commercial AI-based software on hematoxylin and eosin-stained whole slide images. In a first approach, deep learning-based identification of critical tissue zones (centrilobular, midzonal, and periportal) enabled evaluation of region-specific cell size. Mean cytoplasmic area of hepatocytes was calculated via several sequential algorithms including segmentation in microanatomical structures (separation of sinusoids and vessels from hepatocytes), nuclear detection, and area measurements. An increase in mean cytoplasmic area could be shown in groups given phenobarbital, known to induce hepatocellular hypertrophy when compared to control groups, in multiple studies. Quantitative results correlated with the gold standard: observation and grading performed by board-certified veterinary pathologists, liver weights, and gene expression. Furthermore, as a second approach, we introduce for the first time deep learning-based direct detection of hepatocellular hypertrophy with similar results. Cell hypertrophy is challenging to pick up, particularly in milder cases. Additional evaluation of mean cytoplasmic area or direct detection of hypertrophy, combined with histopathological observations and liver weights, is expected to increase accuracy and repeatability of diagnoses and grading by pathologists.

Towards a mechanism-based approach for the prediction of nongenotoxic carcinogenic potential of agrochemicals.

Chemical substances are subjected to assessment of genotoxic and carcinogenic effects before being marketed to protect man and the environment from health risks. For agrochemicals, the long-term rodent carcinogenicity study is currently required from a regulatory perspective. Although it is the current mainstay for the detection of nongenotoxic carcinogens, carcinogenicity studies are shown to have prominent weaknesses and are subject to ethical and scientific debate. A transition toward a mechanism-based weight-of-evidence approach is considered a requirement to enhance the prediction of carcinogenic potential for environmental (agro)chemicals. The resulting approach should make optimal use of innovative (computational) tools and be less animal demanding. To identify the various mode of actions (MOAs) underlying the nongenotoxic carcinogenic potential of agrochemicals, we conducted an extensive analysis of 411 unique agrochemicals that have been evaluated for carcinogenicity by the United States Environmental Protection Agency (US EPA) and the European Chemicals Agency (ECHA). About one-third of these substances could be categorized as nongenotoxic carcinogens with an average of approximately two tumor types per substance, observed in a variety of organs. For two-third of the tumor cases, an underlying MOA (network) could be identified. This analysis demonstrates that a limited set of MOA (networks) is underlying nongenotoxic carcinogenicity of agrochemicals, illustrating that the transition toward a MOA-driven approach appears manageable. Ultimately the approach should cover relevant MOAs and its associated key events; this will also facilitate the evaluation of the human relevance. This manuscript describes the results of the analysis while identifying knowledge gaps and necessities to achieve a mechanism-based weight-of-evidence approach.

A comprehensive view on mechanistic approaches for cancer risk assessment of non-genotoxic agrochemicals.

Currently the only methods for non-genotoxic carcinogenic hazard assessment accepted by most regulatory authorities are lifetime carcinogenicity studies. However, these involve the use of large numbers of animals and the relevance of their predictive power and results has been scientifically challenged. With increased availability of innovative test methods and enhanced understanding of carcinogenic processes, it is believed that tumour formation can now be better predicted using mechanistic information. A workshop organised by the European Partnership on Alternative Approaches to Animal Testing brought together experts to discuss an alternative, mechanism-based approach for cancer risk assessment of agrochemicals. Data from a toolbox of test methods for detecting modes of action (MOAs) underlying non-genotoxic carcinogenicity are combined with information from subchronic toxicity studies in a weight-of-evidence approach to identify carcinogenic potential of a test substance. The workshop included interactive sessions to discuss the approach using case studies. These showed that fine-tuning is needed, to build confidence in the proposed approach, to ensure scientific correctness, and to address different regulatory needs. This novel approach was considered realistic, and its regulatory acceptance and implementation can be facilitated in the coming years through continued dialogue between all stakeholders and building confidence in alternative approaches.

Nonproliferative and Proliferative Lesions of the Rat and Mouse Endocrine System.

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) Project (www.toxpath.org/inhand.asp) is a joint initiative among the Societies of Toxicological Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying microscopic lesions observed in the endocrine organs (pituitary gland, pineal gland, thyroid gland, parathyroid glands, adrenal glands and pancreatic islets) of laboratory rats and mice, with color photomicrographs illustrating examples of the lesions. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous and aging lesions as well as lesions induced by exposure to test materials. A widely accepted and utilized international harmonization of nomenclature for endocrine lesions in laboratory animals will decrease confusion among regulatory and scientific research organizations in different countries and provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists.

Regulatory Forum opinion piece: image analysis-based cell proliferation studies using electronic images: the CEPA industry working group's proposal.

Electronic images of histopathological changes are commonly and increasingly used in toxicologic pathology for morphological evaluation, illustration, peer review, or reporting. Toxicity studies in which cell proliferation is an end point are also pivotal in determining the carcinogenic potential of new molecules. In this article, we describe the approach of the European Cell Proliferation and Apoptosis working group (CEPA) for performing cell proliferation studies and morphometry using electronic images. The Society of Toxicologic Pathology (STP) has published a position statement on handling of pathology image data in compliance with 21 Code of Federal Regulations (CFR) Parts 58 and 11. CEPA supports the STP position and shares the issues involved in the use of electronic images in pathology. However, considering the experience and current know-how of members, particularly in conducting cell proliferation studies, CEPA would like to recommend in this article that electronic images acquired using state-of-the-art slide imaging techniques, including whole slide scanning, need not be considered as raw data, and therefore are not subject to 21 CFR Parts 58 and 11 regulations for archiving. In this article, we detail the reasons why we come to this proposal and we describe the measures that are taken to ensure Good Laboratory Practice-compliant execution of cell proliferation studies that include acquisition and validation of imaging and image analysis systems, development and validation of methods for their intended use, formulation, and use of standard operating procedures.

Comparison of early morphological and molecular changes induced by 17-alpha-methyltestosterone and estradiol benzoate in the rat ovary.

Repeated exposure to 17-α-methyltestosterone (17MT) and estradiol benzoate (EB) for 28 or 90 days in rats induce similar ovarian atrophy. The objective of the present work was to identify and compare the early effects induced by 17MT and EB on the ovary using molecular and histopathological tools. Female rats were evaluated after 1, 3 or 7 days following an oral exposure by gavage at a daily dose of 600 mg/kg/day for 17MT and 5 mg/kg/day for EB. All animals were found to be acyclic after 3 or 7 days of treatment with 17MT and EB. Histopathological changes were present in the ovary, uterus, vagina and mammary gland after both treatments. Ovarian atrophy known as the long term effect of 17MT and EB was not yet detected after 7 days of treatment. But non regressive corpora lutea and cystic follicles were identically observed in the ovary of 17MT and EB treated females. Both compounds induced a decrease of LH transcripts together with an increase of plasma progesterone and prolactin levels. Differences in the profile of regulation of the aromatase were noted after 1 and 3 days of treatment in 17MT treated animals (upregulated) when compared to EB treated animals (downregulated). In summary, we have shown that despite the different nature of hormonal activity, EB and 17MT induce very early endocrine perturbation which presents several similarities. Our work indicated that the detection of early key hormonal markers in short term studies can help to predict the adverse long term effects on target tissues.

Historical control data of neoplastic lesions in the Wistar Hannover Rat among eight 2-year carcinogenicity studies.

Incidences of neoplastic lesions were evaluated in untreated Hannover Wistar Rats RjHan: WI (470 males and 470 females) used as control animals in eight carcinogenicity studies. All these studies were performed in a similar environment either for the in vivo and the postmortem evaluation. The major neoplastic lesions were found in the endocrine, integumentary and reproductive systems. Pituitary adenoma was the most frequent neoplasm and occurred in 33.9% of the males and 54.6% of the female rats. The other most frequent tumors in males were thyroid C-cell adenoma (8.6%), pancreatic islet cell adenoma (8.1%), subcutaneous fibrosarcoma (6.6%), subcutaneous fibroma (4.7%), benign pheochromocytoma (3.4%), and cutaneous keratoacanthoma (3.4%). In females, the other highest incidences were mammary fibroadenoma (29%), uterine endometrial stromal polyp (18.1%), mammary adenocarcinoma (14.2%), mammary fibroadenoma with atypia (13.7%), thyroid C-cell adenoma (7.5%), benign thymoma (3.7%), and subcutaneous fibrosarcoma (3.6%). All these data were compared to previously published historical control data. This retrospective analysis was undergone in order to illustrate the result of a stable organization which guarantees a robust historical data base for neoplastic and non neoplastic findings.

Potential new targets involved in 1,3-dinitrobenzene induced testicular toxicity.

1,3-Dinitrobenzene (DNB) causes testicular injury, particularly to Sertoli cells, and induces apoptosis in the surrounding germinal cells in rodents; however, the mechanisms causing this toxicity are poorly understood. Our studies, using standard and molecular tools, were conducted to better understand the pathogenesis of the testicular effects. Four daily oral doses of 0.1-8mg/kg/day caused marked testicular lesions in rats from 4mg/kg/day. Global transcriptomics revealed cell cycle and cell death as the major biological processes affected with the expression of genes associated with cell cycle progression ("mitotic roles of polo-like kinase") being particularly altered. In a single dose time course study (4mg/kg), no adverse changes were recorded; however, in contrast to the data from the multiple dose study, plasma testosterone and testicular steroidogenesis-related gene expression were affected. These steroid hormone effects were confirmed in vitro using the H295R steroidogenesis assay. With this global approach we show that DNB not only induces apoptosis and interferes with cell cycle in the testes but that DNB can also modulate steroid hormone biosynthesis, suggesting an interference with the endocrine system. However, the contribution of the endocrine changes to the severe testicular lesions is presently unknown and requires further investigation.

A molecular and phenotypic integrative approach to identify a no-effect dose level for antiandrogen-induced testicular toxicity.

The safety assessment of chemicals for humans relies on identifying no-observed adverse effect levels (NOAELs) in animal toxicity studies using standard methods. With the advent of high information content technologies, especially microarrays, it is pertinent to determine the impact of molecular data on the NOAELs. Consequently, we conducted an integrative study to identify a no-transcriptomic effect dose using microarray analyses coupled with quantitative reverse transcriptase PCR (RT-qPCR) and determined how this correlated with the NOAEL. We assessed the testicular effects of the antiandrogen, flutamide (FM), in a rat 28-day toxicity study using doses of 0.2-30 mg/kg/day. Plasma testosterone levels and testicular histopathology indicated a NOAEL of 1 mg/kg/day. A no-effect dose of 0.2 mg/kg/day was established based on molecular data relevant to the phenotypic changes. We observed differential gene expression starting from 1 mg/kg/day and a deregulation of more than 1500 genes at 30 mg/kg/day. Dose-related changes were identified for the major pathways (e.g., fatty acid metabolism) associated with the testicular lesion (Leydig cell hyperplasia) that were confirmed by RT-qPCR. These data, along with protein accumulation profiles and FM metabolite concentrations in testis, supported the no-effect dose of 0.2 mg/kg/day. Furthermore, the microarray data indicated a dose-dependent change in the fatty acid catabolism pathway, a biological process described for the first time to be affected by FM in testicular tissue. In conclusion, the present data indicate the existence of a transcriptomic threshold, which must be exceeded to progress from a normal state to an adaptative state and subsequently to adverse toxicity.

Standard and molecular NOAELs for rat testicular toxicity induced by flutamide.

An important step in the safety assessment of chemicals for humans is to determine the no observed adverse effect level (NOAEL) in toxicity studies conducted in animal models. With the increasing use of molecular tools in toxicity studies, a question often posed is how a NOAEL derived from molecular data compares to a NOAEL established using standard methods. The objective of the present study was to address this question when considering testicular toxicity. To do this, we assessed the effects of the reference antiandrogen flutamide on rat testes in a standard 28-day toxicity study using doses of 0.04-150 mg/kg/day. At necropsy, blood samples were collected for testosterone measurements. The testes were collected for histopathological assessment as well as for the evaluation of gene expression changes using quantitative PCR analyses. Results showed that increases in plasma testosterone level and Leydig cell hyperplasia were detected from 6 mg/kg/day. An alteration in the level of accumulation of a selection of genes was also detected from 6 mg/kg/day. This was the case for genes functionally associated with the testicular lesion, such as lipid metabolism and cell death/cell proliferation, as well as for genes not functionally associated with the lesion. Contrary to the misgivings, these data show that, using a standard 28-day toxicity study and a well-characterized adverse effect, the NOAEL based on transcript changes is similar to the NOAELs based on testosterone levels and histopathological examination.

A mammary gland adenomyoepithelioma in a C57BL/6 mouse.

Mammary gland adenomyoepitheliomas are benign complex mammary gland tumors composed of neoplastic cells of epithelial and myoepithelial origins, described in many species (humans, dogs, cats, rats) and rarely in mice. We report here an adenomyoepithelioma in a C57BL/6 female mouse. Histologically, tubes and cords formed by neoplastic epithelial cells were separated by bundles of neoplastic myoepithelial cells in a clear and partially mucinous matrix. The tumor displayed characteristics of a benign neoplastic proliferation with a compressive growth pattern, and moderate cellular pleomorphism and mitotic index. At immunohistochemistry, the epithelial cells were strongly cytokeratin positive; the myoepithelial cells were weakly cytokeratin positive and strongly smooth muscle actin positive. This is to our knowledge, the first report of a mammary gland adenomyoepithelioma in a C57BL/6 mouse.

An atypical case of histiocytic sarcoma in a Wistar rat (Rattus norvegicus).

Histiocytic sarcoma is the most frequent hematopoietic tumor in rats. We report here a histiocytic sarcoma infiltrating the liver, the spleen and the pancreas from a Wistar rat. In the liver, the tumor was associated with oval cell and bile duct hyperplasia. The cells looked like neoplastic histocytic cells described in this species but with some particularities (e.g. lack of multinucleated giant cells). At immunohistochemistry, neoplastic cells in the liver were vimentine positive but lysozyme and CD68 negative. In the kidney, lysozyme-positive cytoplasmic droplets were observed. We describe here an atypical case of histiocytic sarcoma in the rat and we compare the nature of these neoplastic cells to other species.