Nervous System Sampling for General Toxicity and Neurotoxicity Studies in the Laboratory Minipig With Emphasis on the Göttingen Minipig

The Role of Neuropathology Evaluation in the Nonclinical Assessment of Seizure Liability

Test article (TA)-induced seizures represent a major safety concern in drug development. Seizures (altered brain wave [electrophysiological] patterns) present clinically as abnormal consciousness with or without tonic/clonic convulsions (where "tonic" = stiffening and "clonic" = involuntary rhythmical movements). Neuropathological findings following seizures may be detected using many methods. Neuro-imaging may show a structural abnormality underlying seizures, such as focal cortical dysplasia or hippocampal sclerosis in patients with chronic epilepsy. Neural cell type-specific biomarkers in blood or cerebrospinal fluid may highlight neuronal damage and/or glial reactions but are not specific indicators of seizures while serum electrolyte and glucose imbalances may induce seizures. Gross observations and brain weights generally are unaffected by TAs with seizurogenic potential, but microscopic evaluation may reveal seizure-related neuron death in some brain regions (especially neocortex, hippocampus, and/or cerebellum). Current globally accepted best practices for neural sampling in nonclinical general toxicity studies provide a suitable screen for brain regions that are known sites of electrical disruption and/or display seizure-induced neural damage. Conventional nonclinical studies can afford an indication that a TA has a potential seizure liability (via in-life signs and/or microscopic evidence of neuron necrosis), but confirmation requires measuring brain electrical (electroencephalographic) activity in a nonclinical study.

Toxicologic Pathology Forum: Apoptosis/Single Cell Necrosis as a Possible Procedural Effect in Primate Brain Following Ice-Cold Saline Perfusion

Nonclinical studies of test articles (TAs) in nonhuman primates are often designed to assess both biodistribution and toxicity. For this purpose, studies commonly use intravenous perfusion of ice-cold (2°C-8°C) saline to facilitate measurements of TA-associated nucleic acids and proteins, after which tissues undergo later fixation by immersion for histological processing and microscopic evaluation. Intriguingly, minimal apoptosis/single cell necrosis (A/SCN) of randomly distributed neural cells is evident in the cerebral cortex and less often the hippocampus in animals from all groups, including vehicle-treated controls. Affected cells exhibit end-stage features such as cytoplasmic hypereosinophilia, nuclear condensation or fragmentation, and shape distortions, so their lineage(s) generally cannot be defined; classical apoptotic bodies are exceedingly rare. In addition, A/SCN is not accompanied by glial reactions, leukocyte infiltration/inflammation, or other parenchymal changes. The severity is minimal in controls but may be slightly exacerbated (to mild) by TA that accumulate in neural cells. One plausible hypothesis explaining this A/SCN exacerbation is that cold shock (perhaps complicated by concurrent tissue acidity and hypoxia) drives still-viable but TA-stressed cells to launch a self-directed death program. Taken together, these observations indicate that A/SCN in brain processed by cold saline perfusion with delayed immersion fixation represents a procedural artifact and not a TA-related lesion.

Genomic evidence for the suitability of Göttingen Minipigs with a rare seizure phenotype as a model for human epilepsy

Epilepsy is a complex genetic disorder that affects about 2% of the global population. Although the frequency and severity of epileptic seizures can be reduced by a range of pharmacological interventions, there are no disease-modifying treatments for epilepsy. The development of new and more effective drugs is hindered by a lack of suitable animal models. Available rodent models may not recapitulate all key aspects of the disease. Spontaneous epileptic convulsions were observed in few Göttingen Minipigs (GMPs), which may provide a valuable alternative animal model for the characterisation of epilepsy-type diseases and for testing new treatments. We have characterised affected GMPs at the genome level and have taken advantage of primary fibroblast cultures to validate the functional impact of fixed genetic variants on the transcriptome level. We found numerous genes connected to calcium metabolism that have not been associated with epilepsy before, such as ADORA2B, CAMK1D, ITPKB, MCOLN2, MYLK, NFATC3, PDGFD, and PHKB. Our results have identified two transcription factor genes, EGR3 and HOXB6, as potential key regulators of CACNA1H, which was previously linked to epilepsy-type disorders in humans. Our findings provide the first set of conclusive results to support the use of affected subsets of GMPs as an alternative and more reliable model system to study human epilepsy. Further neurological and pharmacological validation of the suitability of GMPs as an epilepsy model is therefore warranted.

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).

Scientific and Regulatory Policy Committee Points to Consider: Sampling, Processing, Evaluation, Interpretation, and Reporting of Test Article-Related Ganglion Pathology for Nonclinical Toxicity Studies

Certain biopharmaceutical products consistently affect dorsal root ganglia, trigeminal ganglia, and/or autonomic ganglia. Product classes targeting ganglia include antineoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, and anti-nerve growth factor agents. This article outlines "points to consider" for sample collection, processing, evaluation, interpretation, and reporting of ganglion findings; these points are consistent with published best practices for peripheral nervous system evaluation in nonclinical toxicity studies. Ganglion findings often occur as a combination of neuronal injury (e.g., degeneration, necrosis, and/or loss) and/or glial effects (e.g., increased satellite glial cell cellularity) with leukocyte accumulation (e.g., mononuclear cell infiltration or inflammation). Nerve fiber degeneration and/or glial reactions may be seen in nerves, dorsal spinal nerve roots, spinal cord, and occasionally brainstem. Interpretation of test article (TA)-associated effects may be confounded by incidental background changes or experimental procedure-related changes and limited historical control data. Reports should describe findings at these sites, any TA relationship, and the criteria used for assigning severity grades. Contextualizing adversity of ganglia findings can require a weight-of-evidence approach because morphologic changes of variable severity occur in ganglia but often are not accompanied by observable overt in-life functional alterations detectable by conventional behavioral and neurological testing techniques.

European Society of Toxicologic Pathology (Pathology 2.0 Molecular Pathology Special Interest Group): Review of In Situ Hybridization Techniques for Drug Research and Development

In situ hybridization (ISH) is used for the localization of specific nucleic acid sequences in cells or tissues by complementary binding of a nucleotide probe to a specific target nucleic acid sequence. In the last years, the specificity and sensitivity of ISH assays were improved by innovative techniques like synthetic nucleic acids and tandem oligonucleotide probes combined with signal amplification methods like branched DNA, hybridization chain reaction and tyramide signal amplification. These improvements increased the application spectrum for ISH on formalin-fixed paraffin-embedded tissues. ISH is a powerful tool to investigate DNA, mRNA transcripts, regulatory noncoding RNA, and therapeutic oligonucleotides. ISH can be used to obtain spatial information of a cell type, subcellular localization, or expression levels of targets. Since immunohistochemistry and ISH share similar workflows, their combination can address simultaneous transcriptomics and proteomics questions. The goal of this review paper is to revisit the current state of the scientific approaches in ISH and its application in drug research and development.

Toxicologic Pathology Forum Opinion: Interpretation of Gliosis in the Brain and Spinal Cord Observed During Nonclinical Safety Studies

Gliosis, defined as a nonneoplastic reaction (hypertrophy and/or proliferation) of astrocytes and/or microglial cells, is a frequent finding in the central nervous system (CNS [brain and/or spinal cord]) in nonclinical safety studies. Gliosis in rodents and nonrodents occurs at low incidence as a spontaneous finding and is induced by various test articles (e.g., biomolecules, cell and gene therapies, small molecules) delivered centrally (i.e., by injection or infusion into cerebrospinal fluid or neural tissue) or systemically. Several CNS gliosis patterns occur in nonclinical species. First, gliosis may accompany degeneration and/or necrosis of cells (mainly neurons) or neural parenchyma (neuron processes and myelin). Second, gliosis often follows inflammation (i.e., leukocyte accumulation causing parenchymal damage) or neoplasm formation. Third, gliosis may appear as variably sized, randomly scattered foci of reactive glial cells in the absence of visible parenchymal damage or inflammation. In interpreting test article-related CNS gliosis, adversity is indicated by parenchymal injury (e.g., degeneration, necrosis, or inflammation) and not the mere existence of a glial reaction. In the absence of clear structural damage to the parenchyma, gliosis as a standalone CNS finding should be interpreted as a nonadverse reaction to regional alterations in microenvironmental conditions rather than as evidence of a glial reaction associated with neurotoxicity.

A Technical Guide to Sampling the Beagle Dog Nervous System for General Toxicity and Neurotoxicity Studies

Beagle dogs are a key nonrodent species in nonclinical safety evaluation of new biomedical products. The Society of Toxicologic Pathology (STP) has published "best practices" recommendations for nervous system sampling in nonrodents during general toxicity studies (Toxicol Pathol 41[7]: 1028-1048, 2013), but their adaptation to the Beagle dog has not been defined specifically. Here we provide 2 trimming schemes suitable for evaluating the unique neuroanatomic features of the dog brain in nonclinical toxicity studies. The first scheme is intended for general toxicity studies (Tier 1) to screen test articles with unknown or no anticipated neurotoxic potential; this plan using at least 7 coronal hemisections matches the STP "best practices" recommendations. The second trimming scheme for neurotoxicity studies (Tier 2) uses up to 14 coronal levels to investigate test articles where the brain is a suspected or known target organ. Collection of spinal cord, ganglia (somatic and autonomic), and nerves for dogs during nonclinical studies should follow published STP "best practices" recommendations for sampling the central (Toxicol Pathol 41[7]: 1028-1048, 2013) and peripheral (Toxicol Pathol 46[4]: 372-402, 2018) nervous systems. This technical guide also demonstrates the locations and approaches to collecting uncommonly sampled peripheral nervous system sites.

High Prevalence of Recombinant Porcine Endogenous Retroviruses (PERV-A/Cs) in Minipigs: A Review on Origin and Presence

Minipigs play an important role in biomedical research and they have also been used as donor animals for preclinical xenotransplantations. Since zoonotic microorganisms including viruses can be transmitted when pig cells, tissues or organs are transplanted, virus safety is an important feature in xenotransplantation. Whereas most porcine viruses can be eliminated from pig herds by different strategies, this is not possible for porcine endogenous retroviruses (PERVs). PERVs are integrated in the genome of pigs and some of them release infectious particles able to infect human cells. Whereas PERV-A and PERV-B are present in all pigs and can infect cells from humans and other species, PERV-C is present in most, but not all pigs and infects only pig cells. Recombinant viruses between PERV-A and PERV-C have been found in some pigs; these recombinants infect human cells and are characterized by high replication rates. PERV-A/C recombinants have been found mainly in minipigs of different origin. The possible reasons of this high prevalence of PERV-A/C in minipigs, including inbreeding and higher numbers and expression of replication-competent PERV-C in these animals, are discussed in this review. Based on these data, it is highly recommended to use only pig donors in clinical xenotransplantation that are negative for PERV-C.