Developmental milestones in neonatal and juvenile C57Bl/6 mouse - Indications for the design of juvenile toxicity studies

From guideline to practice: three years of ICH S11 insights and recommendations

Juvenile Animal Studies (JAS) may be warranted to ensure the safe clinical use of medicines for children. The ICH S11 guideline was developed to guide the need for and design of JAS, and proposes a weight-of-evidence (WoE) approach. We evaluated how the introduction of the guideline shaped the non-clinical strategy for paediatric medicines in the European Union. Our review included Paediatric Investigation Plans (PIPs) for 127 products approved between 2020 and 2023, along with the associated regulatory assessment and final non-clinical plans. Although in 12 of selected PIPs a JAS was already ongoing or completed at the time of submission, in all other cases (115/127), the PIP was submitted before the initiation of JAS. In 75% (86/115) of these procedures the discussions based on the ICH S11 WoE approach led to an agreement on the proposed non-clinical strategy. In approximately a quarter of PIPs, there was disagreement on the outcome of the WoE analysis leading to the addition (3%), modification (10%), or removal (11%) of JAS. Our review indicates that the implementation of ICH S11 facilitates science-driven discussions about the necessity and design of JAS within the broader non-clinical strategy. A thorough consideration of developmental aspects of the product's pharmacological target, the clinical relevance of notable toxicity findings, and the clinical context of the medicine's use fosters effective dialogue and improves regulatory alignment. The WoE approach in ICH S11 ensures that relevant safety information is generated to support paediatric drug development while balancing the principles of non-clinical replacement, reduction and refinement (the 3Rs).

Weight of Evidence: Is an Animal Study Warranted? Assessments for Carcinogenicity, Drug Abuse Liability, and Pediatric Safety

Nonclinical safety studies are typically conducted to establish a toxicity profile of a new pharmaceutical in clinical development. Such a profile may encompass multiple differing types of animal studies, or not! Some types of animal studies may not be warranted for a specific program or may only require a limited evaluation if scientifically justified. The goal of this course was to provide a practical perspective on regulatory writing of a dossier(s) using the weight of evidence (WOE) approach for carcinogenicity, drug abuse liability and pediatric safety assessments. These assessments are typically done after some clinical data are available and are highly bespoke to the pharmaceutical being developed. This manuscript will discuss key data elements to consider and strategy options with some case studies and examples. Additionally, US FDA experience with dossier(s) including WOE arguments is discussed.

Neonatal Brains Exhibit Higher Neural Reparative Activities than Adult Brains in a Mouse Model of Ischemic Stroke

The neonatal brain is substantially more resistant to various forms of injury than the mature brain. For instance, the prognosis following ischemic stroke is generally poor in the elderly but favorable in neonates. Identifying the cellular and molecular mechanisms underlying reparative activities in the neonatal brain after ischemic injury may provide feasible targets for therapeutic interventions in adults. To this end, we compared the reparative activities in postnatal day 13 and adult (8-12-week-old) mouse brain following middle cerebral artery occlusion. Immunohistochemistry revealed considerably greater generation of ischemia-induced neural stem/progenitor cells (iNSPCs) expressing nestin or Sox2 in ischemic areas of the neonatal brain. The iNSPCs isolated from the neonatal brain also demonstrated greater proliferative activity than those isolated from adult mice. In addition, genes associated with neuronal differentiation were enriched in iNSPCs isolated from the neonatal brain according to microarray and gene ontogeny analyses. Immunohistochemistry further revealed considerably greater production of newborn doublecortin+ neurons at the sites of ischemic injury in the neonatal brain compared to the adult brain. These findings suggest that greater iNSPC generation and neurogenic differentiation capacities contribute to the superior regeneration of the neonatal brain following ischemia. Together, our findings may help identify therapeutic targets for enhancing the reparative potential of the adult brain following stroke.

Supplier-origin gut microbiomes affect host body weight and select autism-related behaviors

Autism spectrum disorders (ASD) are complex human neurodiversities increasing in prevalence within the human population. In search of therapeutics to improve quality-of-life for ASD patients, the gut microbiome (GM) has become a promising target as a growing body of work supports roles for the complex community of microorganisms in influencing host behavior via the gut-brain-axis. However, whether naturally-occurring microbial diversity within the host GM affects these behaviors is often overlooked. Here, we applied a model of population-level differences in the GM to a classic ASD model - the BTBR T+ Itpr3tf/J mouse - to assess how complex GMs affect host behavior. Leveraging the naturally occurring differences between supplier-origin GMs, our data demonstrate that differing, complex GMs selectively effect host ASD-related behavior - especially neonatal ultrasonic communication - and reveal a male-specific effect on behavior not typically observed in this strain. We then identified that the body weight of BTBR mice is influenced by the postnatal GM which was potentially mediated by microbiome-dependent effects on energy harvest in the gut. These data provide insight into how variability within the GM affects host behavior and growth, thereby emphasizing the need to incorporate microbial diversity within the host GM as an experimental factor in biomedical research.

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and clear dead cells generated during development or disease. The phagocytic process shapes the microglia phenotype, which affects the local environment. A unique population of microglia resides in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence the neurogenic niche is not well understood. Here, we demonstrate that phagocytosis contributes to a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering a neuroinflammatory microglia phenotype that resembles dysfunctional microglia in neurodegeneration and aging and that reduces neural precursor proliferation via elevated interleukin-1β signaling; interleukin-1 receptor inhibition rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to the maintenance of a pro-neurogenic phenotype in the developing V-SVZ.

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and are responsible for clearing dead cells generated during development or disease. The phagocytic process shapes the phenotype of the microglia, which affects the local environment. A unique population of microglia reside in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence this neurogenic niche is not well-understood. Here, we demonstrate that phagocytosis creates a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering the development of a neuroinflammatory phenotype, reminiscent of neurodegenerative and-age-associated microglia, that reduces neural precursor proliferation via elevated interleukin (IL)-1β signaling; inhibition of IL-1 receptor rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to a phenotype that promotes neurogenesis in the developing V-SVZ.

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.

Mouse models of sarcopenia: classification and evaluation

Sarcopenia is a progressive and widespread skeletal muscle disease that is related to an increased possibility of adverse consequences such as falls, fractures, physical disabilities and death, and its risk increases with age. With the deepening of the understanding of sarcopenia, the disease has become a major clinical disease of the elderly and a key challenge of healthy ageing. However, the exact molecular mechanism of this disease is still unclear, and the selection of treatment strategies and the evaluation of its effect are not the same. Most importantly, the early symptoms of this disease are not obvious and are easy to ignore. In addition, the clinical manifestations of each patient are not exactly the same, which makes it difficult to effectively study the progression of sarcopenia. Therefore, it is necessary to develop and use animal models to understand the pathophysiology of sarcopenia and develop therapeutic strategies. This paper reviews the mouse models that can be used in the study of sarcopenia, including ageing models, genetically engineered models, hindlimb suspension models, chemical induction models, denervation models, and immobilization models; analyses their advantages and disadvantages and application scope; and finally summarizes the evaluation of sarcopenia in mouse models.

Scientific and Regulatory Policy Committee Best Practices: Documentation of Sexual Maturity by Microscopic Evaluation in Nonclinical Safety Studies

The sexual maturity status of animals in nonclinical safety studies can have a significant impact on the microscopic assessment of the reproductive system, the interpretation of potential test article-related findings, and ultimately the assessment of potential risk to humans. However, the assessment and documentation of sexual maturity for animals in nonclinical safety studies is not conducted in a consistent manner across the pharmaceutical and chemical industries. The Scientific and Regulatory Policy Committee of the Society of Toxicologic Pathology convened an international working group of pathologists and nonclinical safety scientists with expertise in the reproductive system, pathology nomenclature, and Standard for Exchange of Nonclinical Data requirements. This article describes the best practices for documentation of the light microscopic assessment of sexual maturity in males and females for both rodent and nonrodent nonclinical safety studies. In addition, a review of the microscopic features of the immature, peripubertal, and mature male and female reproductive system and general considerations for study types and reporting are provided to aid the study pathologist tasked with documentation of sexual maturity.