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.

Antisense Oligonucleotide-Related Macrovesicular Vacuolation of Hippocampal Neurons in Nonhuman Primates.

2'-methoxyethyl (MOE) antisense oligonucleotides (ASOs) tested in multidose intrathecal nonhuman primate (NHP) toxicity studies have consistently revealed the presence of single large vacuoles in pyramidal neurons of the hippocampus in the absence of any cellular response. Termed "macrovesicular," these vacuoles were characterized by immunohistochemistry and transmission electron microscopy which showed that these vacuoles are dilated lysosomes in neurons containing accumulated ASO. Additionally, two NHP studies were conducted to investigate the role of tissue fixation on their histogenesis. In Fixation Study 1, 6 doses of 5 mg 2'-MOE ASO with a full phosphorothioate backbone were administered by lumbar puncture over 5 weeks; in Fixation Study 2, 5 doses of 35 mg 2'-MOE ASO with a mixed phosphorothioate/phosphodiester backbone were administered over 12 weeks. At necropsy in each study, brain slices were either immersion fixed in neutral buffered 10% formalin or Carnoy's fixative; frozen at -80 °C; or perfusion fixed with modified Karnovsky's fixative. Fixed samples were processed to paraffin, sectioned, and stained with hematoxylin and eosin (H&E) and compared with H&E cryosections prepared from the frozen tissue of the same brain. The presence of vacuoles in fixed brain tissue but never in fresh frozen tissue showed that they arose during postmortem tissue fixation, and as such represent a processing artifact that is not relevant to human safety assessment of intrathecally administered 2'-MOE ASOs.

Opinion on Performing Pathology Peer-Review During the Global Pandemic: Challenges and Opportunities.

The coronavirus disease 2019 pandemic has affected business on numerous fronts in unprecedented and abrupt ways. From site closures and local "stay-at-home orders" to travel advisories and restrictions, the day-to-day practice of toxicologic pathology has been impacted dramatically and rapidly. A critical function of Toxicologic Pathologists is performing pathology peer review for nonclinical studies. Traditionally, corroborating the findings of histological assessment could be achieved through shipment of histopathological slides to the peer review pathologist, or by the peer review pathologist traveling to the location of the slides (eg, the test facility). Since early 2020, many pathologists have been unable to perform the latter due to local, regional, national, test facility, company, and/or personal restrictions. The disruption for some has been minimal, while others are working from home for the first time. We recommend that contingency plans for all peer review procedures and personnel should be in-place to accommodate sudden and unexpected workflow transitions. Now, more than ever, approaching peer reviews with enhanced adaptability will help ensure success.

Use of Severity Grades to Characterize Histopathologic Changes.

The severity grade is an important component of a histopathologic diagnosis in a nonclinical toxicity study that helps distinguish treatment-related effects from background findings and aids in determining adverse dose levels during hazard characterization. Severity grades should be assigned based only on the extent (i.e., amount and complexity) of the morphologic change in the examined tissue section(s) and be clearly defined in the pathology report for critical lesions impacting study interpretation. However, the level of detail provided and criteria by which severity grades are assigned can vary, which can lead to inappropriate comparisons and confusion when evaluating pathology results. To help address this issue, a Working Group of the Society of Toxicologic Pathology's Scientific and Regulatory Policy Committee was formed to provide a "points to consider" article on the assignment and application of pathology severity grades. Overall, the Working Group supports greater transparency and consistency in the reporting of grading scales and provides recommendations to improve selection of diagnoses requiring more detailed severity criteria. This information should enhance the overall understanding by toxicologic pathologists, toxicologists, and regulatory reviewers of pathology findings and thereby improve effective communication in regulatory submissions.

A Diagnostic Approach for Rodent Progressive Cardiomyopathy and Like Lesions in Toxicology Studies up to 28 Days in the Sprague Dawley Rat (Part 1 of 2).

Spontaneous rodent progressive cardiomyopathy (PCM) in the Sprague Dawley rat may confound identification and/or interpretation of potential test article (TA)-related cardiotoxicity. Pathologists apply diagnostic term(s) and thresholds for diagnosing and assigning severity grades for PCM and/or PCM-like (PCM/like) lesions consistently within a study, which is necessary to identify and interpret TA-related findings. Due to differences in training and/or experiences, diagnostic terms and thresholds may vary between pathologists. Harmonized terminology and thresholds across studies will generate better historical control data, will likely enhance interpretation of study data, and may further enhance our understanding of the spontaneous change. An assessment of the diagnostic approaches of a group of 37 pathologists identified an approach that is relatively easily applied; and if adopted, it could enhance diagnostic consistency across studies. This approach uses the single "slash" term "necrosis/inflammatory cell infiltrate (NICI)" as the diagnosis for the spectrum of lesions seen in younger rats, uses no threshold for diagnosis (e.g., diagnose all lesions clearly identifiable as PCM/like), and uses aggregate lesion size of approximately ≥45% of the field of view (FOV) using a 10×/22 eyepiece and the 40× objective or approximately ≥100% of the FOV using the 60× objective as the criterion separating minimal from mild severities.

A Diagnostic Approach for Rodent Progressive Cardiomyopathy and Like Lesions in Toxicology Studies up to 28 Days in the Sprague Dawley Rat (Part 2 of 2).

To test the diagnostic approach described in part 1 of this article, 2 exercises were completed by pathologists from multiple companies/agencies. Pathologist's examination of whole slide image (WSI) heart sections from rats using personal diagnostic approaches (exercise #1) corroborated conclusions from study #1. Using the diagnostic approach described in part 1, these pathologists examined the same WSI heart sections (exercise #2) to determine whether that approach increased consistency of diagnosis of rodent progressive cardiomyopathy (PCM) lesions. In exercise #2, there was improved consistency of categorization of small borderline morphologies and mild lesions, but a decrement in consistency of categorizing minimal lesions. Exercises 1 and 2 suggest the described diagnostic approach is representative of that in use by the majority of toxicologic pathologists across companies/agencies and that application by all may improve diagnostic consistency of PCM/like lesions. Additionally, a criterion of approximately 5% heart section involvement is suggested for separating mild from moderate or greater severity. While evidence is not absolute, until further investigation shows otherwise, microscopic changes resembling PCM, but located in the epicardial and subepicardial region of the right ventricle, may be considered as part of the spectrum of PCM.

Scientific and Regulatory Policy Committee Review: Review of the Organisation for Economic Co-operation and Development (OECD) Guidance on the GLP Requirements for Peer Review of Histopathology.

In 2014, the Organisation for Economic Co-operation and Development (OECD) issued guidance no. 16, Guidance on the GLP Requirements for Peer Review of Histopathology. The stated purpose of the guidance document is "to provide guidance to pathologists, test facility management, study directors and quality assurance personnel on how the peer review of histopathology should be planned, managed, documented, and reported in order to meet Good Laboratory Practice (GLP) expectations and requirements." On behalf of and in collaboration with the global societies of toxicologic pathology, the Society of Toxicologic Pathology initiated a review of OECD guidance no. 16. The objectives of this review are to provide a unified interpretation of the guidance, to recommend compliant processes for organizations to implement, and to avoid inconsistent process adaptations across the industry. This review of the guidance document is the product of a global collaboration with other societies of toxicologic pathology and provides a section-by-section international consensus view and interpretation of the OECD guidance on peer review.