Neuropathology Evaluation in Juvenile Toxicity Studies in Rodents: Comparison of Developmental Neurotoxicity Studies for Chemicals With Juvenile Animal Studies for Pediatric Pharmaceuticals

Behavioral and transcriptional effects of repeated electroconvulsive seizures in the neonatal MK-801-treated rat model of schizophrenia

RATIONALE

Electroconvulsive therapy (ECT) is an effective treatment modality for schizophrenia. However, its antipsychotic-like mechanism remains unclear.

OBJECTIVES

To gain insight into the antipsychotic-like actions of ECT, this study investigated how repeated treatments of electroconvulsive seizure (ECS), an animal model for ECT, affect the behavioral and transcriptomic profile of a neurodevelopmental animal model of schizophrenia.

METHODS

Two injections of MK-801 or saline were administered to rats on postnatal day 7 (PN7), and either repeated ECS treatments (E10X) or sham shock was conducted daily from PN50 to PN59. Ultimately, the rats were divided into vehicle/sham (V/S), MK-801/sham (M/S), vehicle/ECS (V/E), and MK-801/ECS (M/E) groups. On PN59, prepulse inhibition and locomotor activity were tested. Prefrontal cortex transcriptomes were analyzed with mRNA sequencing and network and pathway analyses, and quantitative real-time polymerase chain reaction (qPCR) analyses were subsequently conducted.

RESULTS

Prepulse inhibition deficit was induced by MK-801 and normalized by E10X. In M/S vs. M/E model, Egr1, Mmp9, and S100a6 were identified as center genes, and interleukin-17 (IL-17), nuclear factor kappa B (NF-κB), and tumor necrosis factor (TNF) signaling pathways were identified as the three most relevant pathways. In the V/E vs. V/S model, mitophagy, NF-κB, and receptor for advanced glycation end products (RAGE) pathways were identified. qPCR analyses demonstrated that Igfbp6, Btf3, Cox6a2, and H2az1 were downregulated in M/S and upregulated in M/E.

CONCLUSIONS

E10X reverses the behavioral changes induced by MK-801 and produces transcriptional changes in inflammatory, insulin, and mitophagy pathways, which provide mechanistic insight into the antipsychotic-like mechanism of ECT.

Toxicologic Neuropathology of Novel Biotherapeutics

Biotherapeutic modalities such as cell therapies, gene therapies, nucleic acids, and proteins are increasingly investigated as disease-modifying treatments for severe and life-threatening neurodegenerative disorders. Such diverse bio-derived test articles are fraught with unique and often unpredictable biological consequences, while guidance regarding nonclinical experimental design, neuropathology evaluation, and interpretation is often limited. This paper summarizes key messages offered during a half-day continuing education course on toxicologic neuropathology of neuro-targeted biotherapeutics. Topics included fundamental neurobiology concepts, pharmacology, frequent toxicological findings, and their interpretation including adversity decisions. Covered biotherapeutic classes included cell therapies, gene editing and gene therapy vectors, nucleic acids, and proteins. If agents are administered directly into the central nervous system, initial screening using hematoxylin and eosin (H&E)-stained sections of currently recommended neural organs (brain [7 levels], spinal cord [3 levels], and sciatic nerve) may need to expand to include other components (e.g., more brain levels, ganglia, and/or additional nerves) and/or special neurohistological procedures to characterize possible neural effects (e.g., cell type-specific markers for reactive glial cells). Scientists who evaluate the safety of novel biologics will find this paper to be a practical reference for preclinical safety testing and risk assessment.

Toxicologic Pathology Forum Opinion: Rational Approaches to Expanded Neurohistopathology Evaluation for Nonclinical General Toxicity Studies and Juvenile Animal Studies

Existing nervous system sampling and processing "best practices" for nonclinical general toxicity studies (GTS) were designed to assess test articles with unknown, no known, or well-known neurotoxic potential. Similar practices are applicable to juvenile animal studies (JAS). In GTS and JAS, the recommended baseline sampling for all species includes brain (7 sections), spinal cord (cervical and lumbar divisions [cross and longitudinal sections for each]), and 1 nerve (sciatic or tibial [cross and longitudinal sections]) in hematoxylin and eosin-stained sections. Extra sampling and processing (ie, an "expanded neurohistopathology evaluation" [ENHP]) are used for agents with anticipated neuroactivity (toxic ± therapeutic) of incompletely characterized location and degree. Expanded sampling incorporates additional brain (usually 8-15 sections total), spinal cord (thoracic ± sacral divisions), ganglia (somatic ± autonomic, often 2-8 total), and/or nerves (2-6 total) depending on the species and study objectives. Expanded processing typically adds special neurohistological procedures (usually 1-4 for selected samples) to characterize glial reactions, myelin integrity, and/or neuroaxonal damage. In my view, GTS and JAS designs should sample neural tissues at necropsy as if ENHP will be needed eventually, and when warranted ENHP may incorporate expanded sampling and/or expanded processing depending on the study objective(s).

Considerations for and against dosing rodent pups before 7 days of age in juvenile toxicology studies

This review focuses on preweaning ontogenic and developmental processes that can influence the selection of the appropriate age at which to start dosing rodent pups in juvenile animal studies (JAS). The ICH S11 guideline on 'Nonclinical Safety Testing in Support of Development of Paediatric Medicines' highlights the need to adapt the age from which animals are dosed according to the stage of development in the target organs/tissues of concern in the youngest pediatric patients. Rodents (rat or mouse) are the most common species for JAS. Despite previous practices, based on comparative ontogeny, it is rarely necessary to dose rodents younger than one week of age since postnatal day (PND)7 is appropriate to address concern for the vast majority of organs. In exceptional cases, earlier dosing (e.g., PND4) can be appropriate to address specific concern in preterm neonates and when a tissue of concern has a particularly early developmental trajectory in the rodent compared to humans. The comparative development of the CNS is particularly complex. While exposure of rodents from PND10 covers most CNS development stages relevant to human neonates, a later dosing start (yet, not later than PND14) can sometimes be appropriate to reflect specific aspects (e.g., transformation of GABAergic transmission). An extended study design including subsets of several ages can be helpful to address multiple concerns within a preweaning JAS. Such design can allow for individual assessment of each concern, whilst minimizing (potentially irrelevant) signals from tissues exposed at a developmental stage that do not match the human situation.

Regulatory Perspectives on Juvenile Animal Toxicologic Pathology

The Society of Toxicologic Pathology's Annual Virtual Symposium (2021) included a session on "Regulatory Perspectives on Juvenile Animal Toxicologic Pathology." The following narrative summarizes the key concepts from the four talks included in this symposium session chaired by Drs Deepa Rao and Alan Hoberman. These encompass an overview of various global regulations impacting the conduct of juvenile animal studies in pharmaceutical drug development and chemical toxicity assessments in a talk by Dr Alan Hoberman. Given the numerous regulatory guidances and legal statutes that have covered the conduct of juvenile animal studies and the recent harmonization of these guidances for pharmaceuticals, Dr Paul Brown provided an update on the harmonization of these guidances for pharmaceuticals, in the recently finalized version of the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use S11 guidance document, "Nonclinical Safety Testing in Support of Development of Pediatric Medicines." The first two talks on regulations were followed by two talks focused on an evaluation of the postnatal development of two major organ systems relevant in juvenile animals. Dr Aurore Varela covered study design and endpoints impacting the skeletal system (bone), while Dr Brad Bolon presented a talk on the study design and conduct of neuropathology evaluations for the developing nervous system.