The modified electron spin resonance (ESR)-based photosafety test (ESR-PT) is a non-animal prediction test using ESR spectroscopy that is applicable to hydrophobic and colored chemicals and the results show high concordance with existing photosafety reference information. The modified ESR-PT is based on the detection of singlet oxygen and free radical photoproducts generated from chemicals in the presence of 4-hydroxy-2,2,6,6-tetramethyl-piperidine (4-hydroxy-TEMP). We obtained false-negative results caused by signal increment of the control solution, which is used as the denominator of the classifier in the modified ESR-PT, when we used a different type of lighting unit (type I) to the previously used type of lighting unit (type II). Spectral measurement of the irradiated light from the light sources revealed that the type I lighting unit emitted stronger UV-C light than the type II lighting unit. Consequently, as UV-C absorption of 4-hydroxy-TEMP (λ_max<210 nm) was confirmed, we repeated the modified ESR-PT using a type I lighting unit equipped with a UV-C cut filter, which led to an apparent decrease of the signal increment in the control solution, and all the false-negative-judged chemicals correctly tested positive but a false positive result was also noted. Therefore, installation of a UV-C cut filter in the lighting unit in modified ESR-PT appears to be a reliable solution for avoiding UV-C light-mediated false-negative results. However, it may also be necessary to reconsider the classifier used in ESR-PT to avoid obtaining false-positive results when using a UV-C cut filter.

The applicability of the in vitro to in vivo extrapolation (IVIVE) approach for the quantitative risk assessment of chemicals was evaluated using the results of mouse uterotrophic bioassays conducted by the Japanese Ministry of Health, Labour and Welfare. Five chemicals were selected. In the uterotrophic bioassay, three chemicals exhibited positive estrogenic activity, whereas two exhibited negative activity. These chemicals were active in 16 in vitro assays that investigated the key events from estrogen receptor (ER) binding (initiating event) to ER-induced proliferation, enabling us to derive IVIVE conversion factors using a physiologically based kinetic model. The oral equivalent doses (OEDs) that were extrapolated from the activity concentrations at the half-maximal response (AC₅₀) and the cutoff point (ACC) were compared with those of other key events to determine the critical key event that plays the most important role in the occurrence of uterotrophic responses. For the three chemicals that exhibited positive estrogenic activity, the OEDs from the in vitro AC₅₀ values for the determined critical events were within a factor of 2 of the lowest observed effect levels in the uterotrophic bioassay. In addition, the OEDs from the ACC values for the critical key events of the two chemicals that exhibited negative activity were higher than the highest dose tested in the bioassay. Based on these findings, the IVIVE approach was largely valid. However, the critical key events that significantly affect in vivo responses need to be appropriately determined to apply the IVIVE approach for the quantitative risk assessment of chemicals.

To promote the 3Rs in toxicological assessment, the recommendation for blood microsampling in toxicokinetic evaluation is noted in the ICH Harmonized Guideline S3A Q&As. However, there are only a few articles reporting the practical application of microsampling in the toxicological assessment with toxic drugs. In this study, we investigate the effect of microsampling on toxicological assessment in rats treated with tacrine, which is known to have toxic effects that induce an increase in neutrophils and behavioral abnormalities. Thirty female Sprague-Dawley rats were divided into microsampling (MS) and non-microsampling (non-MS) groups, and orally administered tacrine once daily at dose levels of 0 (vehicle only), 3 and 10 mg/kg bw for 28 days (each group: n=5). In the MS group, blood samples (50 μL/time point) were collected at 6 time points on day 1 and 7 time points on day 28 to 29. All the animals underwent necropsy on day 29. By comparing the results of toxicological and toxicokinetic analysis between the MS and non-MS groups, we validated effects of microsampling for toxicological assessment. Although increase in neutrophils and repeated stereotypic behaviors were observed as toxic effects in the rats administered tacrine, we could not find any difference between the MS and non-MS groups, and also found that microsampling did not affect any other data from toxicological and toxicokinetic analysis. In conclusion, blood microsampling appeared to be a feasible technique for the toxicity study of tacrine and was considered to be applicable in the toxicity study of even drugs with toxic effects on hematological parameters, such as an increase in neutrophils.

Toxicity enhancement mediated by plasma protein binding during intoxication remains poorly understood. It is known that in mice, brain penetration of amoxapine (AMX) increases nonlinearly with increasing doses; therefore, this study investigated its potential to enhance toxicity via plasma protein binding. AMX was added to mouse or human plasma and adjusted to therapeutic, toxic, and lethal concentrations. The plasma protein-binding ratio and free AMX concentration were measured using ultrafiltration and equilibrium dialysis. Furthermore, the binding ratios of varying concentrations of chlorpromazine, which is often co-administered with AMX in cases of overdose, was analyzed in the presence of therapeutic AMX concentrations. The binding ratio of AMX exceeded 90%, thereby demonstrating a high binding rate; however, this ratio was lower in human plasma than in mice. A nonlinear increase in free AMX concentration was observed in mouse plasma, particularly at high concentrations. In contrast, free AMX concentration showed a linear increase in human plasma. Neither the therapeutic nor toxic concentration of chlorpromazine produced any visible effects on the plasma protein binding ratio or the free AMX concentration. These results suggest that protein binding of AMX is more readily saturated in mouse plasma than in human plasma. In addition, chlorpromazine inhibits AMX binding to α₁-acid glycoprotein; however, AMX may alternatively binds to albumin, which results in no apparent change in the total binding ratio. Further insights into the toxicokinetic interactions mediated by plasma protein binding are also needed for various toxic substances other than AMX.

Perillyl alcohol (POH) is a natural monoterpene with established antitumor activity. Although its anticancer efficacy is well-documented, its impact on normal blood cells, particularly red blood cells (RBCs), remains underexplored. Given the importance of RBC integrity in maintaining homeostasis, this study aims to investigate the hematological effects of POH, focusing on hemolysis, eryptosis, and systemic blood parameters. Erythrocytes collected from June-August 2023 were treated with POH (0.5–2.5 mM) for 24 hr at 37°C. Hemolysis was assessed using photometric assays. Eryptosis was detected by flow cytometry using annexin V, Fluo4/AM, and H₂DCFDA to quantify phosphatidylserine (PS) translocation, intracellular Ca²⁺, and oxidative stress, respectively. Complete blood count (CBC) parameters were also analyzed. POH induced dose-dependent hemolysis, elevated intracellular Ca²⁺, and significant PS externalization, indicating increased eryptosis. The hemolytic activity of POH was supported by marked increases in LDH, CK, and AST. Notably, polyethylene glycol (PEG) significantly attenuated hemolysis, suggesting a protective effect. In whole blood, POH reduced RBC count, hemoglobin, and hematocrit, while increasing RDW-CV. Reticulocyte profiling showed elevated immature reticulocyte fraction (IRF) and a medium fluorescence ratio (MFR). Moreover, POH induced leukopenia with immature granulocytes and platelet aggregation. POH disrupts RBC integrity, triggering hemolysis and eryptosis independently of oxidative stress. The observed hematological alterations underscore potential systemic toxicity and highlight the need for further preclinical evaluation to guide its therapeutic use in oncology.

Steroidogenesis disruption caused by compounds causes vacuolar lesions in the adrenal cortex. However, research on the reversibility of compound-induced adrenal lesions remains limited. In a 4-week oral repeated-dose toxicity study with a 4-week recovery period in dogs, administration of S-637880—a compound that off-target inhibits steroid hormone synthesis in vitro—resulted in diffuse microvesicular vacuolation, which was considered to reflect excess lipid accumulation in adrenocortical cells. After the 4-week recovery period, these diffuse vacuolar lesions developed into characteristic multifocal macrovesicular vacuolation in the adrenal cortex, making it difficult to evaluate lesion reversibility. Immunohistochemical evaluation suggested that the histopathological changes observed after the recovery period reflected slow degeneration of lipid-laden adrenocortical cells, culminating in cell death accompanied by macrophage activation and aggregation to process apoptotic or necrotic cells and the liberated lipids. These findings are considered to represent a transitional phase in the recovery process. The reversibility of S-637880-induced vacuolar lesions was confirmed in a subsequent 13-week oral repeated-dose toxicity study with a 13-week recovery period. This case study may be helpful in evaluating the reversibility of vacuolar lesions in the adrenal cortex during nonclinical safety assessments and in determining an appropriate recovery periods for assessing adrenal toxicity.

Nucleic acid drugs hold considerable promise; however, their toxicological profiles are often difficult to assess in animal models. Clinical studies have reported adverse effects, including thrombocytopenia, complement activation, hepatotoxicity, and nephrotoxicity. While human cell-based models for hepatotoxicity are advancing, nephrotoxicity assessment remains limited by the scarcity of physiologically relevant kidney cells. In this study, a three-dimensional spheroid model of human primary renal proximal tubule epithelial cells (3D-RPTEC, Nikkiso) was employed to evaluate the nephrotoxicity of nucleic acid drugs. Proteomic profiling revealed enhanced expression of drug transporters and endocytic machinery in 3D-RPTEC compared with two-dimensional cultures. Lipofection enabled efficient intracellular delivery of nucleic acids. Toxicity was assessed using ATP quantification, biomarker analysis (LDH, KIM-1, NGAL), and high-content analysis (HCA). Significant ATP depletion was observed only after prolonged exposure to SPC5001, a nephrotoxic antisense oligonucleotide. In contrast, biomarker expression and HCA facilitated early detection of compound-specific toxicity and implicated endoplasmic reticulum and mitochondrial stress as underlying mechanisms. These findings establish 3D-RPTEC as a sensitive and physiologically relevant platform for predicting the nephrotoxic potential of nucleic acid drugs.