We examined the morphological effects of letrozole on placental development in pregnant rats. Letrozole was orally administered at a repeat dose to pregnant rats at 0 mg/kg (control group) and 0.04 mg/kg (letrozole group) from gestation day (GD) 6 to GD 20. In the letrozole group, fetal mortality and placental weight increased from GD 15 onwards and GD 13 onwards, respectively. Fetal weights increased on GDs 15 and 17 but decreased on GD 21. Histopathologically, letrozole treatment induced multiple cysts lined with undifferentiated syncytiotrophoblasts in the trophoblastic septa on GD 13. These cysts then develop into dilated maternal sinusoids with congestive hyperemia, resulting in an enlarged placenta. In the metrial gland, there was a dilated lumen of the spiral artery and interstitial edema throughout the experimental period, resulting in thickened metrial gland. These changes are considered to be due to maternal blood circulation stagnation in the metrial gland, which is associated with dilated maternal sinusoids in the labyrinth zone. Thus, although letrozole induces an enlarged placenta due to congestive hyperemia of the labyrinth zone and transient increases in fetal weight, these placentas are thought to decline in function as the pregnancy progresses, leading to intrauterine growth restriction at the end of pregnancy.

We have previously reported on thymomas in Wistar Hannover rats with medullary differentiation and revealed that two different cytokeratin (CK) immunohistochemical types of thymic epithelia (TE), CK18 and CK14, lead to the formation of cortical-medullary structures. In aged F344 rats, epithelial-type thymoma rarely occurs, and thymic epithelial hyperplasia is common. However, CK expression in these F344 rat lesions is unknown. We investigated three hyperplasia and four thymomas in F344 for histopathological features and CK18 and CK14 expression. Hyperplasia was characterized by an increase in tubular structures in the medulla. Thymomas were nodular in shape, with tubular structures similar to those observed in hyperplasia, along with irregular structures such as cord, papillary, and spindloid. Immunohistochemical analysis revealed that the tubular structures consisted of two layers: inner cuboidal-to-columnar TE and outer round-to-oval TE, positive for CK18 and CK14, respectively. The two-layer pattern was maintained to some extent in the irregular structures.

Tuberculosis (TB) is a major health threat for humans and for non-human primates used for toxicology or research purposes. Emerging mycobacterial species represent a major challenge for diagnosis and surveillance programs. Here, we report a natural outbreak of in imported cynomolgus macaques () that occurred at AnaPath Research S.A.U. (APR). The macaques underwent repeated negative intradermal tuberculin tests (IDT) before importation and at the European quarantine station. Exhaustive TB screening was started at APR after confirmation of one positive case at another facility. The animal in question belonged to the same colony received at APR. Diagnostic approaches included clinical examination, PCR, culture, spoligotyping, IDT testing, interferon-γ release assay (IGRA), and thoracoabdominal ultrasound (US). Three regulatory toxicity studies and stock animals were affected. The macaques lacked clinical signs, except for one showing a fistulizing nodule in the right inguinal area, which tested positive for the Mycobacterium tuberculosis complex by PCR. All animals were necropsied and 10 macaques (n=114) showed gross and histologic findings compatible with TB confirmed by PCR and culture. was identified as the etiological agent by Direct Variable Repeat spacer oligonucleotide typing (DVR spoligotyping). The infection was traced to Asia via the SB1622 spoligotype involved, confirming that the animals were infected prior to their import into Europe. Tuberculin skin test (TST), IGRA, and US were only sensitive in detecting advanced cases of infection. One staff member showed a positive TST reaction, which was handled in accordance with the Spanish government's health regulations. All the sanitary measures implemented were effective in eradicating the disease.

In a past study, we proposed a modified Comparative Thyroid Assay (CTA) with additional examinations of brain thyroid hormone (TH) concentrations and brain histopathology but with smaller group sizes. The results showed that the modified CTA in Sprague Dawley rats detected 10 ppm 6-propylthiouracil (6-PTU)-induced significant suppressions of serum/brain TH concentrations in offspring. To confirm the reliability of qualitative brain histopathology and identify the optimal testing time for heterotopia (a cluster of ectopic neurons) in the modified CTA, brain histopathology together with serum/brain TH concentrations were assessed in GD20 fetuses and PND2, 4, 21, and 28 pups using a similar study protocol but with a smaller number of animals (N=3-6/group/time). Significant hypothyroidism was observed and brain histopathology revealed cerebral heterotopia formation in PND21 and PND28 pups, with likely precursor findings in PND2 and PND4 pups but not in GD20 fetuses. This study confirmed that the optimal testing time for cerebral heterotopia in rat CTA was PND21 and thereafter. These findings suggest that cerebral heterotopia assessment at appropriate times may be a useful alternative to the original CTA design.

B-cell lymphoma is generally observed in the spleen, mesenteric lymph nodes, and Peyer's patches in aged mice and rarely appears in other organs. Herein, we report a case of spontaneous B-cell lymphoma originating from the cranial mediastinal lymph node in a male 75-week-old C57BL/6J mouse. Macroscopically, a white mass was found at the base of the heart with no connection to the thymus. Microscopic examination revealed a solid proliferation of tumor cells with large nuclei at the center of the mass. Some macrophages, normal-sized lymphocytes, and lymphatic sinuses were found in both central and peripheral areas. Immunohistochemical analysis showed positive staining for cluster of differentiation 19, paired box protein 5, immunoglobulin M, and Ki-67 but not for cytokeratin AE1/AE3. These findings were not completely consistent with the established mouse lymphoma classification, leading to a diagnosis of B-cell lymphoma originating from the cranial mediastinal lymph node. This case report is the first to document a B-cell lymphoma in the cranial mediastinal lymph nodes in an aged C57BL/6J mouse.

Linalool oxide is frequently used as a flavoring agent, however, data on its toxicity is limited. In this study, we performed a 13-week subchronic toxicity study of linalool oxide (furanoid) in male and female Crl:CD(SD) rats. Doses of 0, 80, 250, and 800 mg/kg body weight (bw) per day were orally administered by gavage, using corn oil as the vehicle. Abnormal gait in both sexes and decreased locomotor activity in males were observed in the 800 mg/kg group. Reduced body weight gain was noted in both sexes at 800 mg/kg and at 250 mg/kg in males. In the 800 mg/kg group, serum biochemistry showed increased γ-glutamyl transpeptidase and decreased glucose in both sexes, increased total protein in males, and increased total cholesterol and phospholipids in females, suggesting that linalool oxide may have adverse effects on the liver. Increased relative and/or absolute liver weights, centrilobular hepatocellular hypertrophy in both sexes, and periportal microvesicular fatty changes in females were observed in the 800 mg/kg group. Increased relative liver weights and decreased serum glucose levels were observed in the 250 mg/kg male and female groups, respectively. Increased serum magnesium levels and relative kidney weights were observed in both sexes in the 800 mg/kg group, suggesting possible adverse effects of linalool oxide. Although histopathology showed accumulation of hyaline droplets in the male kidneys, immunohistochemistry revealed α-globulin nephropathy, which was not considered toxicologically significant. These results indicate that the no-observed-adverse-effect level of linalool oxide was 80 mg/kg bw/day for both sexes.

Gene therapy (GT) products created using adeno-associated virus (AAV) vectors tend to exhibit toxicity via immune reactions, but other mechanisms of toxicity remain incompletely understood. We examined the cardiotoxicity of an overexpressed transgenic protein. Male C57BL/6J mice were treated with a single intravenous dose of product X, an AAV-based GT product, at 2.6 × 10 vg/kg. Necropsies were performed at 24 h, 7 days, and 14 days after dosing. Pathological examination and gene expression analysis were performed on the heart. Histopathologically, hypertrophy and vacuolar degeneration of cardiomyocytes and fibrosis were observed 14 days after dosing. Immunohistochemistry for endoplasmic reticulum (ER) stress-related proteins revealed increased positive reactions for glucose-regulated protein 78 and C/EBPR homologous protein in cardiomyocytes 7 days after dosing, without histopathological abnormalities. Fourteen days after dosing, some cardiomyocytes showed positivity for PKR-like endoplasmic reticulum kinase and activating transcription factor 4 expression. Ultrastructurally, increases in the ER and cytosol were observed in cardiomyocytes 7 days after dosing, along with an increase in the number of Golgi apparatus compartments 14 days after dosing. The tissue concentration of the transgene product protein increased 7 days after dosing. Gene expression analysis showed upregulation of ER stress-related genes 7 days after dosing, suggesting activation of the PKR-like ER kinase pathway of the unfolded protein reaction (UPR). Thus, the cardiotoxicity induced by product X was considered to involve cell damage caused by the overexpression of the product protein accompanied by UPR. Marked UPR activation may also cause toxicity of AAV-based GT products.

We report a histiocytic sarcoma originating from the epididymis observed in a 110-week-old male CD-1 mouse in a carcinogenicity study. At necropsy, no lesions were observed in the epididymis. Histologically, a neoplastic lesion was observed in the cauda of the epididymis that was well demarcated from the surrounding tissues. The lesion mainly consisted of spindle-shaped tumor cells with oval to elongated nuclei and abundant eosinophilic or foamy cytoplasm. The tumor cells were arranged in a fascicular pattern, interlacing bundles, or a whorl pattern. The nuclei showed mild atypia with irregular shapes and varied sizes, whereas few mitotic figures and no typical multinucleated cells were observed. The epididymal ducts remained within the neoplastic lesion, and the tumor cells invaded between the epithelium and the smooth muscle layer of the epididymal duct. Immunohistochemically, the tumor cells were positive for vimentin and macrophage markers (Iba1, CD204, F4/80, and Mac-2) but negative for cytokeratin and other mesenchymal cell (α-smooth muscle actin, desmin, CD31, and platelet-derived growth factor receptor-β), neural cell (S-100 and nestin), or Leydig cell markers (calretinin). Proliferating cell nuclear antigen-positive tumor cells were sporadically observed in the lesion. Based on these results, the tumor was diagnosed as a histiocytic sarcoma originating from the epididymis. This report provides additional histopathological evidence of spontaneous histiocytic sarcomas originating from the epididymis of aged mice.

The historical control database of a multinational laboratory services provider was queried for all histopathologic findings in New Zealand White rabbits which were used as control animals during a ten-year period (2011-2020). The query included all evaluated tissues, with or without microscopic findings, in studies conducted for safety testing for regulatory approval by the U.S. Food and Drug Agency (FDA) or the U.S. Environmental Protection Agency. A second query included studies conducted in the United Kingdom for control rabbits used in studies compliant with the Healthcare Products Regulatory Agency (MHRA) and/or the European Medicines Agency (EMA), which provide regulatory oversight in the United Kingdom and European Union, respectively. Infiltrates of inflammatory (mixed or mononuclear) cells were commonly noted in various organs including heart, digestive tract, muscle, thyroid, kidney, urinary bladder, eyelid, ocular structures, harderian gland, lacrimal gland, and lung. Mineralization was noted in aorta, kidney, urinary bladder, and ovary. Also noted were degeneration/necrosis in the myocardium, and intramuscular injection sites of the skin, degeneration/regeneration of muscle and diaphragm, ectopic tissue in the pancreas and thyroid, basophilic foci in salivary gland, increased/decreased vacuolation in adrenal gland, increased/decreased lymphocytic cellularity of lymph nodes, intrasinusoidal erythrocytes in lymph nodes, thymic atrophy, increased adipocytes in bone marrow, inflammatory cell foci in the liver and gall bladder, lacrimal gland atrophy, renal tubule basophilia, degeneration/regeneration, and dilatation; oviduct cyst; in the testis, degeneration/atrophy, cellular debris, dilatation, decreased sperm and segmental hypoplasia of seminiferous tubules; and squamous metaplasia of the testis and seminal vesicle.

Tissue cross-reactivity (TCR) studies for the development of therapeutic antibodies are conducted to estimate any possible binding sites within the human body that can be affected by the antibody when assessing safety in humans. Any possible binding sites include specific binding sites of the antibody to its target antigen and nonspecific or off-target binding sites. In TCR studies the therapeutic antibodies and immunohistochemistry (IHC) of frozen tissues must be applied in assays. However, there are technical issues with applying a therapeutic antibody or test article to IHC, such as human-on-human staining, difficulty in applying the test article to IHC, and retention of the target antigen in frozen sections. In the current review, we introduce three case studies in which these technical issues were addressed, and propose a practical scheme for points to consider when conducting a TCR study. Information on the target antigen distribution obtained through robust assays and case-by-case strategies were found to be useful for understanding and assessing the relevance of toxic effects between animals and humans. Thus, we anticipate that by considering the points discussed in the current review and combining the data with information on the biological features of the target antigens and therapeutic antibodies, it will be possible to predict safety risks in humans with higher accuracy.

Duplicate testes lined in series were observed in the right scrotum of a 6-week-old Sprague-Dawley rat in a single-dose toxicity study. Of the two right testicles, one was spherical and less than half the size of a normal testis. The other was oval-shaped, slightly smaller than a normal testis, and possessed clear, tortuous blood vessels similar to those of a normal testis. Each right testis was grossly separated but faced the intertesticular adipose tissue and was sparsely joined by thin cord-like structures. Only one epididymis covered or encompassed the two right testes. The caput epididymis was attached to the smaller spherical testis, whereas the cauda epididymis was attached to the oval testis. Histopathological examination revealed that the smaller spherical testis on the right side and the testis on the left side were normal. The oval-shaped testis on the right exhibited markedly dilated degenerative seminiferous tubules with one to two layers of Sertoli or germ cells, and almost no spermatogenesis was observed. Multinucleated germ cells were observed in the lumen of the degenerated seminiferous tubules. The right epididymis was morphologically normal and contained few sperm in the epididymal duct of the tail. The cord-like structures between duplicate testes comprised fibrous and adipose tissues. Single efferent ductules, ectopic cartilage, and skeletal muscle tissues were buried in the adipose tissue. To our knowledge, this is the first report of spontaneous polyorchidism in a rodent.

In neurosurgical interventions, effective closure of the dura mater is essential to prevent cerebrospinal fluid leakage and minimize post-operative complications. Biodegradable synthetic materials have the potential to be used as dura mater grafts owing to their regenerative properties and low immunogenicity. This study evaluated the safety of ArtiFascia, a synthetic dura mater graft composed of poly(l-lactic-co-caprolactone acid) and poly(d-lactic-co-caprolactone acid), in a rabbit durotomy model. Previously, ArtiFascia demonstrated positive local tolerance and biodegradability in a 12-month preclinical trial. Here, specialized stains were used to evaluate potential brain damage associated with ArtiFascia use. Histochemical and immunohistochemical assessments included Luxol Fast Blue, cresyl Violet, Masson's Trichrome, neuronal nuclei,, Glial Fibrillary Acidic Protein, and ionized calcium-binding adaptor molecule 1 stains. The stained slides were graded based on the brain-specific reactions. The results showed no damage to the underlying brain tissue for either the ArtiFascia or control implants. Neither inflammation nor neuronal loss was evident, corroborating the safety of the ArtiFascia. This approach, combined with previous histopathological analyses, strengthens the safety profile of ArtiFascia and sets a benchmark for biodegradable material assessment in dura graft applications. This study aligns with the Food and Drug Administration guidelines and offers a comprehensive evaluation of the potential neural tissue effects of synthetic dura mater grafts.

Renal tubular epithelial cell (TEC) injury is the most common cause of drug-induced kidney injury (DIKI). Although TEC regeneration facilitates renal function and structural recovery following DIKI, maladaptive repair of TECs leads to irreversible fibrosis, resulting in chronic kidney disease (CKD). CD44 is specifically expressed in TECs during maladaptive repair in several types of rat CKD models. In this study, we investigated CD44 expression and its role in renal fibrogenesis in a cyclosporine (CyA) rat model of CKD. Seven-week-old male Sprague-Dawley rats fed a low-salt diet were subcutaneously administered CyA (0, 15, or 30 mg/kg) for 28 days. CD44 was expressed in atrophic, dilated, and hypertrophic TECs in the fibrotic lesions of the CyA groups. These TECs were collected by laser microdissection and evaluated by microarray analysis. Gene ontology analysis suggested that these TECs have a mesenchymal phenotype, and pathway analysis identified CD44 as an upstream regulator of fibrosis-related genes, including fibronectin 1 (). Immunohistochemistry revealed that epithelial and mesenchymal markers of TECs of fibrotic lesions were downregulated and upregulated, respectively, and that these TECs were surrounded by a thickened basement membrane. hybridization revealed an increase in mRNA in the cytoplasm of TECs of fibrotic lesions, whereas fibronectin protein was localized in the stroma surrounding these tubules. Enzyme-linked immunosorbent assay revealed increased serum CD44 levels in CyA-treated rats. Collectively, these findings suggest that CD44 contributes to renal fibrosis by inducing fibronectin secretion in TECs exhibiting partial epithelial-mesenchymal transition and highlight the potential of CD44 as a biomarker of renal fibrosis.

This study aimed to establish an exposure method that can induce homogeneous lesions with minimal inter-individual variability. The distribution of lesions induced by bleomycin (BLM) administration was also analyzed. C57BL mice were intrabronchially administered 20 µL of BLM (3 mg/mL) using a bronchoscope in the left or right bronchus. The mice were sacrificed 14 days after administration, and their lungs were evaluated histopathologically. BLM-induced inflammatory lesions were widely observed in the lungs. In the left bronchus-treated group, lesions were uniformly observed throughout the lobe, and no individual differences were noted. Meanwhile, in the right bronchus-treated group, individual differences in the distribution of the pulmonary lesions were observed. The distribution of lesions differed among the four lobes of the right lung owing to their anatomical features. Administration into the left bronchus is recommended for highly homogeneous lung exposure and for establishing models that contribute to highly accurate toxicity and efficacy evaluations.

The United States Senate passed the "FDA Modernization Act 2.0." on September 29, 2022. Although the effectiveness of this Bill, which aims to eliminate the mandatory use of laboratory animals in new drug development, is limited, it represents a significant trend that will change the shape of drug applications in the United States and other countries. However, pharmaceutical companies have not taken major steps towards the complete elimination of animal testing from the standpoint of product safety, where they prioritize patient safety. Nonetheless, society is becoming increasingly opposed to animal testing, and efforts will be made to use fewer animals and conduct fewer animal tests as a natural and reasonable response. These changes eventually alter the shape of new drug applications. Based on the assumption that fewer animal tests will be conducted or fewer animals will be used in testing, this study explored bioinformatics and new technologies as alternatives to compensate for reduced information and provide a picture of how future new drug applications may look. The authors also discuss the directions that pharmaceutical companies and nonclinical contract research organizations should adopt to promote the replacement, reduction, and refinement of animals used in research, teaching, testing, and exhibitions.

In drug development, assessment of non-clinical peripheral neurotoxicity is important to ensure human safety. Clarifying the pathological features and mechanisms of toxicity enables the management of safety risks in humans by estimating the degree of risk and proposing monitoring strategies. Published guidelines for peripheral neurotoxicity assessment do not provide detailed information on which endpoints should be monitored preferentially and how the results should be integrated and discussed. To identify an optimal assessment method for the characterization of peripheral neurotoxicity, we conducted pathological, biochemical (biomaterials contributing to mechanistic considerations and biomarkers), and behavioral evaluations of isoniazid-treated rats. We found a discrepancy between the days on which marked pathological changes were noted and those on which biochemical and behavioral changes were noted, suggesting the importance of combining these evaluations. Although pathological evaluation is essential for pathological characterization, the results of biochemical and behavioral assessments at the same time points as the pathological evaluation are also important for discussion. In this study, since the measurement of serum neurofilament light chain could detect changes earlier than pathological examination, it could be useful as a biomarker for peripheral neurotoxicity. Moreover, examination of semi-thin specimens and choline acetyltransferase immunostaining were useful for characterizing morphological neurotoxicity, and image analysis of semi-thin specimens enabled us to objectively show the pathological features.

The pathogenesis of nasal cavity tumors induced in rodents has been critically reviewed. Chemical substances that induce nasal cavity tumors in rats, mice, and hamsters were searched in the National Toxicology Program (NTP), International Agency for Research on Cancer (IARC), and Japan Bioassay Research Center (JBRC) databases, in addition to PubMed. Detailed data such as animal species, administration routes, and histopathological types were extracted for induced nasal cavity tumors. Data on non-neoplastic lesions were also extracted. The relationship between the tumor type and non-neoplastic lesions at equivalent sites was analyzed to evaluate tumor pathogenesis. Genotoxicity data were also analyzed. Squamous cell carcinoma was the most frequent lesion, regardless of the dosing route, and its precursor lesions were squamous metaplasia and/or respiratory epithelial hyperplasia, similar to squamous cell papilloma. The precursor lesions of adenocarcinoma, the second most frequent tumor type, were mainly olfactory epithelial hyperplasia, whereas those of adenoma were respiratory epithelial lesions. These pathways were consistent among species. Our results suggest that the responsible lesions may be commonly linked with chemically-induced cytotoxicity in each tumor type, irrespective of genotoxicity, and that the pathways may largely overlap between genotoxic and non-genotoxic carcinogens. These findings may support the documentation of adverse outcome pathways (AOPs), such as cytotoxicity, leading to nasal cavity tumors and the integrated approaches to testing and assessment (IATA) for non-genotoxic carcinogens.

Ameloblastoma is a locally aggressive tumor derived from the odontogenic epithelium of the developing tooth germ. It is rarely reported in experimental Sprague-Dawley (SD) rats. In this 90-day percutaneous repeated-dose toxicity study, mandibular nodules were observed from day 56 to 90. Upon necropsy, a well-demarcated nodule, approximately 1.2×1.0×1.0 cm, was found attached to the mandibular bone, alongside the unerupted left incisor. Histopathologically, the epithelial cells formed islands, nests, or anastomosing strands. The epithelial islands were surrounded by a peripheral layer of tall columnar cells with antibasilar nuclei arranged in a palisading pattern. Several mitotic cells were observed. Some epithelial islands displayed cystic degenerative changes and squamous metaplasia. Necrotic tissue with inflammatory cell infiltration was observed at the tumor margins. Immunohistochemically, the neoplastic epithelial islands and mesenchymal components exhibited positivity for pan-cytokeratin and vimentin, respectively. Based on these features, the case was diagnosed as follicular ameloblastoma in an SD rat.

Japan has various death investigation systems; however, external examinations, postmortem computed tomography, macroscopic examinations, and microscopic examinations are performed regardless of the system used. These examinations can reveal morphological abnormalities, whereas the cause of death in cases with non-morphological abnormalities can be detected through additional examinations. Molecular autopsy and postmortem genetic analyses are important additional examinations. They are capable of detecting inherited arrhythmias or inherited metabolic diseases, which are representative non-morphological disorders that cause sudden death, especially in infants and young people. In this review, we introduce molecular autopsy reports from Japan and describe our experience with representative cases. The relationships between drug-related deaths and genetic variants are also reviewed. Based on the presented information, molecular autopsy is expected to be used as routine examinations in death investigations because they can provide information to save new lives.

This study was conducted to visualize the lobular distribution of enhanced mRNA expression levels of heat shock proteins (HSPs) in liver samples from carbon tetra chloride (CCl4)-treated mice using hybridization (ISH). Male BALB/c mice given a single oral administration of CCl4 were euthanized 6 hours or 1 day after the administration (6 h or 1 day). Paraffin-embedded liver samples were obtained, ISH for HSPs was conducted, as well as hematoxylin-eosin staining and immunohistochemistry (IHC). At 6 h, centrilobular hepatocellular vacuolization was observed, and increased signals for , and , which are HSPs, were noted in the centrilobular area using ISH. At 1 day, zonal hepatocellular necrosis was observed in the centrilobular area, but mRNA signal increases for HSPs were no longer observed there. Some discrepancies between ISH and IHC for HSPs were observed, and they might be partly caused by post-transcriptional gene regulation, including the ribosome quality control mechanisms. It is known that CCl4 damages centrilobular hepatocytes through metabolization by cytochrome P450, mainly located in the centrilobular region, and HSPs are induced under cellular stress. Therefore, our ISH results visualized increased mRNA expression levels of HSPs in the centrilobular hepatocytes of mice 6 hours after a single administration of CCl4 as a response to cellular stress, and it disappeared 1 day after the treatment when remarkable necrosis was observed there.

Determining the optimal timing for histopathological examination following exposure to a test article is crucial for assessing neurotoxicity. However, no study has focused on identifying an ideal dataset to define the optimal timing for histopathological examination of central nervous system (CNS) toxicity in monkeys. Therefore, this study aimed to define a predictive endpoint that would guide us in selecting the optimal timing for histopathological examination of CNS toxicity in monkeys. Four cynomolgus monkeys were administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intravenously at a dosage of 0.6 mg/kg twice at 1-week intervals. Necropsies were performed 1 week after the final dose. The Parkinsonian rating (PR) score and temporal changes in neurofilament light chain and glial fibrillary acidic protein concentrations in the cerebrospinal fluid (CSF) and serum were evaluated and compared with the histopathological findings in the brain. The PR score of all animals administered MPTP increased from days 10 to 11, with some degree of individual variability. Microscopically, all animals showed axonal swelling and vacuolation, with or without microgliosis in the nigrostriatal bundle. However, substantial neurodegenerative findings were observed only in animals with high PR scores at necropsy. A slight increase in CSF biomarker levels at necropsy was also observed in animals with high PR scores. However, their correlation with microscopic findings in these animals was unclear. These data suggest that comprehensive clinical observations, such as PR score alone or combined with other CSF biomarkers, could be further evaluated as potential indicators for triggering anatomic CNS evaluations in monkeys following toxic insults.