Ketamine, a dissociative anesthetic commonly used in pediatric medicine, may induce neurotoxic effects when administered during early neonatal life. Although ketamine-induced neuronal apoptosis has been previously documented, less is known about glial cell responses or temporal evolution of injury. This study investigated the neurotoxic effects of ketamine (20 mg/kg, 5 intraperitoneal injections administered 90 minutes apart) in postnatal day 7 Sprague-Dawley rats of both sexes, compared with age-matched controls. Brains were collected 2 and 16 hours after the last injection and evaluated by histology, immunohistochemistry, and immunofluorescence using markers of apoptosis, DNA damage, and cell populations (Cleaved Caspase-3, γH2AX, NeuN, Iba-1, GFAP, and Olig2). Quantification of apoptotic and immunostained cells in the cortex, hippocampus, and corpus callosum was performed using standardized digital image analysis. Ketamine induced a significant increase in apoptosis and DNA damage at 2 hours, concomitant with an elevation in Olig2-positive cells, suggesting a potential compensatory response. Neuronal, astrocytic, and microglial cells were unaffected. Intrinsic apoptosis primarily involved neurons and oligodendrocytes, with microglia and astrocytes engaged in phagocytosis of apoptotic cells. At 16 hours, a return to baseline levels was observed. These findings offer a temporal and cellular characterization of ketamine-induced neurotoxicity in the developing rat brain.

In April 2025, the US Food and Drug Administration (FDA) introduced a bold roadmap to phase out animal testing in drug development by promoting the integration of New Approach Methodologies (NAMs), beginning with monoclonal antibodies (mAbs). Similar roadmaps have also been created by European health authorities and chemical safety agencies around the globe. This opinion paper from the European Society of Toxicologic Pathology and US Society of Toxicologic Pathology complex in vitro model and pathology working groups provides a balanced and expert perspective on the promise and limitations of these initiatives. NAMs, including in vitro, in silico, and refined in vivo models, offer exciting opportunities to improve human relevance, efficiency, and ethical standards in nonclinical safety assessment. While these approaches are already making meaningful contributions, current scientific and technical limitations prevent them from fully replacing animal studies in all contexts. At this time, NAMs should be viewed as powerful complementary tools whose appropriate integration depends on the context of use, validation, and continued collaboration across disciplines. Pathologists and investigative molecular pathology leaders are uniquely positioned to support this transition by bridging traditional methods with emerging technologies to enhance the safety and effectiveness of future therapeutics.

Intraocular inflammation (IOI) following intravitreal (IVT) administration of large molecules (LM) is commonly seen in preclinical species and often relates to the presence of systemic anti-drug antibodies (ADA) as a reaction to the application of human proteins into a non-human species. This retrospective study aimed to characterize histopathological findings of IOI after IVT administration of LMs to non-human primates (NHP) and minipigs. Three independent safety studies were analyzed using immunohistochemical and special stains for inflammatory cell markers (CD68, Iba1, CD163, CD3, CD20, CD138), glial cell markers (glial fibrillary acidic protein [GFAP], vimentin, glutamine synthetase [GS]), and extracellular matrix (ECM) (fibronectin, collagen I, trichrome staining). Mononuclear cell infiltrates, membrane formation, and retinal degeneration were assessed. Infiltrates in various ocular compartments consisted mainly of monocytes/macrophages, T cells, and plasma cells, indicating an ongoing adaptive immune response. Membranes were primarily composed of glial cells (GFAP, vimentin positive) with a lack of substantial ECM deposition. Retinal degeneration correlated with increased GFAP/vimentin and decreased GS staining in the retina. Overall, these results correlated with the presence, location, and severity of the histopathologic diagnoses and were consistent between the two species. This study provides a detailed characterization of IOI consistent with an adaptive immune response following IVT administration of LM, enhancing our understanding of the ongoing immunological processes.

Drug-induced taste dysfunctions are an underrecognized yet clinically significant class of adverse effects that can substantially impair patients' quality of life and compromise adherence to therapeutic regimens. Despite the potential for many pharmacologic agents to alter taste perception, these effects are frequently overlooked in both clinical practice and drug development. Gustatory disorders are rarely assessed in toxicology studies, and current knowledge regarding the pathophysiology of dysgeusia, the relevant organ systems involved, and nonclinical evaluation methods remains limited. This review details the anatomy of the gustatory system including taste buds and papillae, the functional characteristics of taste cells, the molecular mechanisms of taste transduction, and the neural components (peripheral and central). We emphasize the multimodal nature of flavor perception encompassing gustatory, olfactory and somatosensory systems, including the role of saliva in modulating flavor. Furthermore, we propose a comprehensive framework for nonclinical histopathological assessment of the key organ systems involved in flavor perception, with relevant case examples. By advancing awareness and providing optimized approaches for gustatory system evaluation in nonclinical toxicology studies, we aim to enable earlier detection and mechanistic investigation of taste-related adverse effects and improve safety profiling during drug development.

An unsupervised, single-class anomaly detection approach based on a vision transformer architecture was developed to aid histopathology evaluation of rat liver from nonclinical toxicology studies. The approach was designed to detect any histopathology finding without requiring specific histopathology examples for training. The model was developed by training a vision transformer with approximately 135,000 whole slide images (WSIs) and then extracting high-dimensional feature representations from 750 unremarkable vehicle control WSIs, creating a rat liver histomorphology reference dataset representing typical biological variability. Each test WSI was compared against the reference dataset at the tile level, and the resulting anomaly detections were aggregated to derive an overall image-level anomaly score. Anomaly scores were higher in affected groups treated with established toxicants than in control groups, and based on comparing anomaly scores at the individual animal level with pathologist diagnoses, model sensitivity was 95.9% and specificity was 70.8%. Utility as a pathologist aiding tool was further studied by comparing tile-level anomaly annotations with diagnosis. For one treated group, histomorphology feature extraction was used to explore discordance between anomaly detection and diagnosis. Specificity was impacted by pre-analytical variables as well as detection of anomalies not considered noteworthy by pathologists.

Mononuclear cell infiltrates (MCIs) and inflammation in the central nervous system (CNS) can be challenging in nonclinical studies, particularly in nonrodent species. A retrospective study was performed to determine the incidences of spontaneous MCI and inflammation in the brain and spinal cord of rodent and nonrodent species considering age (rodents), administration routes and geographical origin (primates). Historical control data (HCD) were collected from rats, mice, beagle dogs, Göttingen minipigs, cynomolgus macaques, and New Zealand White (NZW) rabbits. Perivascular MCI in the brain (choroid plexus and meninges most frequently affected) had the highest incidence in cynomolgus macaques (8.25% males/7.38% females), followed by Göttingen minipigs (3.23% males/3.27% females), beagle dogs (3.18% males/3.14% females). CD-1 mice, Sprague-Dawley rats, Han Wistar rats and NZW rabbits were the species with the least commonly identified perivascular MCI in the brain. Brain inflammation was a rare event. Perivascular MCI in the spinal cord (meninges most frequently affected) had the highest incidence in cynomolgus macaques, Göttingen minipigs, and beagle dogs. This HCD review will help in the interpretation of inflammatory CNS findings in toxicological safety studies and should allow discrimination of spontaneous and induced lesions in nonclinical studies.

Environmental toxicants are increasingly implicated in male reproductive disorders, highlighting the need for physiologically relevant in vitro testicular models for toxicological testing. We previously developed a testis organoid system that recapitulates key architectural and endocrine features of native tissue; however, its suitability for toxicological applications requires validation. In this study, cadmium, a well-characterized environmental heavy metal and testicular toxicant, was used as a positive control to evaluate model responsiveness. Testis organoids were exposed to increasing concentrations of cadmium (0.01, 1, 10, and 100 μM) to assess dose-dependent toxicological effects. Cadmium bioaccumulation within organoids was quantified, and multiple endpoints were evaluated, including testis cell assembly, cord formation, apoptosis/necrosis, germ cell number, DNA damage, cellular proliferation, and endocrine function. Cadmium accumulated in a concentration-dependent manner, with greater accumulation at 10 and 100 μM. At these concentrations, cadmium exposure reduced cord formation, germ cell numbers, and cellular proliferation, and decreased testosterone and estradiol production, while increasing apoptosis/necrosis and DNA damage. Disruption of testis cell assembly was observed only at 100 μM. Collectively, these findings demonstrate dose-dependent toxicological responses and support the use of this testis organoid system as a relevant in vitro model for reproductive toxicology and environmental toxicant evaluation.

RC-0315 is a chemotherapy-exposed mesenchymal stem cell-derived secretome developed as a potential advanced therapy for idiopathic pulmonary fibrosis (IPF). A Good Laboratory Practice (GLP)-compliant repeated-dose toxicity study was conducted to evaluate the safety of RC-0315 following intratracheal (ITR) instillation in immunocompetent ICR and SCID mice. Animals received two cycles of three repeated ITR instillations of RC-0315 performed within a 7-day period and separated by a two-week interval. Mice were assigned to saline, vehicle, low-dose, or high-dose groups and were monitored for 3 days (main phase) or 13 weeks (recovery phase) post last administration. No test article-related mortality or toxic clinical signs were observed in either strain. Clinical pathology parameters, body weight, food consumption, and ophthalmologic findings remained within normal limits, with no dose-dependent alterations. Histopathological examination revealed no RC-0315-related adverse findings in the lungs or other organs, and no local toxicity was evident at the site of administration. These findings support the favorable safety profile of RC-0315 when administered via the clinically intended route.

The impact of non-fasting on clinical pathology parameters in pharmacologic safety assessment has been incompletely characterized. The aim of this study was to characterize changes in hematology, coagulation, and clinical chemistry parameters between healthy fasted and fed rats and macaques. Adult Sprague-Dawley rats (41 males, 40 females) and cynomolgus macaques (10 per sex) were sampled after free access to food and after 8 hours of fasting. Fed rats demonstrated lower reticulocyte and neutrophil counts in males only. Higher alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase activities and higher glucose, triglyceride, cholesterol, and globulin concentrations were observed with lower urea nitrogen, albumin, and phosphorus concentrations in fed compared with fasted rats of both sexes. Fed macaques of both sexes demonstrated higher triglyceride concentrations and lower phosphorus and total bilirubin concentrations. All changes in fed compared with fasted rats and macaques were considered minor. Non-fasting did not impact coagulation parameters in rats and macaques or hematology parameters in macaques. This study supports that non-fasting has a minimal impact on standard clinical pathology parameters in Sprague-Dawley rats and cynomolgus macaques; however, fasting is still recommended when there are anticipated effects on glucose and lipid metabolism or on endocrine systems related to energy metabolism.

Timely tissue fixation is critical to prevent autolysis, which confounds histopathologic assessment. This study systematically mapped autolysis progression in ex vivo rat tissues under room temperature and refrigerated (4°C) storage conditions to establish an evidence-based, tissue-specific timeline for practical guidance. Thirty-six Sprague-Dawley rats were euthanized; major organs were rapidly collected and maintained in sterile saline at either room temperature or 4°C for intervals ranging from 0.5 to 16 hours before fixation in 10% neutral buffered formalin and H&E evaluation. The data provide a clear timeframe of autolytic change in rat tissues. At room temperature, most tissues exhibited minimal autolysis within the first 2 hours, and refrigeration at 4°C extended the time to autolysis. Immunohistochemistry for the endothelial marker CD31 suggested that CD31 antigenicity remained detectable in most tissues maintained at room temperature out to 16 hours with the exception of intestinal tissues demonstrating severe autolysis, indicating that advanced autolysis can compromise immunohistochemical interpretation. In conclusion, this work provides guidance for handling tissues that require temporary preservation in saline. The documented tissue-specific timelines serve as a valuable reference for pathologists.

The second session of the 2nd Joint BSTP/ESTP Toxicologic Pathology Congress, Manchester, UK, September 23-26, 2025, entitled "New Approach Methodologies for Carcinogenicity Evaluation," was dedicated to innovative strategies for assessing carcinogenic risks in various substances, particularly in drug development and agrochemicals. The two-year rodent cancer bioassay in two species (generally rats and mice) is currently the standard method for assessing the carcinogenic potential of agrochemicals for humans. However, this method has some weaknesses and is subject to ethical and scientific debate. Attempts to waive those studies have been proposed, but more relevant methods using less or preferably no animals are still being sought. The session featured five key presentations that explored cutting-edge methodologies aimed at improving the accuracy and reliability of carcinogenicity predictions. Below is a summary of each talk presented.

The International Academy of Toxicologic Pathology (IATP) Satellite Symposium on Pathology Working Groups (PWGs) in Toxicologic Pathology, organized in the United Kingdom, addressed the value and importance of PWGs and expert working groups in assuring the accurate evaluation of chronic/carcinogenicity studies. These specialized panels of expert pathologists play a crucial role in confirming data quality and accurate interpretation in support of human safety and risk decisions of xenobiotics. While not every carcinogenicity bioassay may require a PWG to evaluate the quality and accuracy of study interpretation, it was the consensus of the workshop that it can be a highly valuable addition to traditional peer review. Performing a PWG should be given serious consideration, especially when it is anticipated that the chronic/carcinogenicity study will be used for making significant regulatory or business decisions.