In silico approaches in carcinogenicity hazard assessment: case study of pregabalin, a nongenotoxic mouse carcinogen

In silico toxicology protocols are meant to support computationally-based assessments using principles that ensure that results can be generated, recorded, communicated, archived, and then evaluated in a uniform, consistent, and reproducible manner. We investigated the availability of in silico models to predict the carcinogenic potential of pregabalin using the ten key characteristics of carcinogens as a framework for organizing mechanistic studies. Pregabalin is a single-species carcinogen producing only one type of tumor, hemangiosarcomas in mice via a nongenotoxic mechanism. The overall goal of this exercise is to test the ability of in silico models to predict nongenotoxic carcinogenicity with pregabalin as a case study. The established mode of action (MOA) of pregabalin is triggered by tissue hypoxia, leading to oxidative stress (KC5), chronic inflammation (KC6), and increased cell proliferation (KC10) of endothelial cells. Of these KCs, in silico models are available only for selected endpoints in KC5, limiting the usefulness of computational tools in prediction of pregabalin carcinogenicity. KC1 (electrophilicity), KC2 (genotoxicity), and KC8 (receptor-mediated effects), for which predictive in silico models exist, do not play a role in this mode of action. Confidence in the overall assessments is considered to be medium to high for KCs 1, 2, 5, 6, 7 (immune system effects), 8, and 10 (cell proliferation), largely due to the high-quality experimental data. In order to move away from dependence on animal data, development of reliable in silico models for prediction of oxidative stress, chronic inflammation, immunosuppression, and cell proliferation will be critical for the ability to predict nongenotoxic compound carcinogenicity.

Survey of tumorigenic sensitivity in 6-month rasH2-Tg mice studies compared with 2-year rodent assays

The pharmacokinetic endpoint of a 25-fold increase in human exposure is one of the specified criteria for high-dose selection for 2-year carcinogenicity studies in rodents according to ICH S1C(R2). However, this criterion is not universally accepted for 6-month carcinogenicity tests in rasH2-Tg mice. To evaluate an appropriate multiple for rasH2-Tg mice, we evaluated data for 53 compounds across five categories of rasH2-Tg mouse-positive [(1) genotoxic and (2) non-genotoxic] carcinogens and rasH2-Tg mouse-negative [(3) non-genotoxic carcinogens with clear or uncertain human relevance; (4) non-genotoxic rodent-specific carcinogens; and (5) non-carcinogens], and surveyed their tumorigenic activities and high doses in rasH2-Tg mice and 2-year rodent models. Our survey indicated that area under the curve (AUC) margins (AMs) or body surface area-adjusted dose ratios (DRs) of tumorigenesis in rasH2-Tg mice to the maximum recommended human dose (MRHD) were 0.05- to 5.2-fold in 6 category (1) compounds with small differences between models and 0.2- to 47-fold in 7 category (2) including three 2-year rat study-negative compounds. Among all 53 compounds, including 40 compounds of the rasH2-Tg mouse-negative category (3), (4), and (5), no histopathologic risk factors for rodent neoplasia were induced only at doses above 50-fold AM or DR in rasH2-Tg mice except for two compounds, which induced hyperplasia and had no relationship with the tumors observed in the rasH2-Tg mouse or 2-year rodent studies. From the results of these surveys, we confirmed that exceeding a high dose level of 50-fold AM in rasH2-Tg mouse carcinogenicity studies does not appear to be of value.

Expression of Hematopoietic Stem and Endothelial Cell Markers in Canine Hemangiosarcoma

Several chemicals and pharmaceuticals increase the incidence of hemangiosarcomas (HSAs) in mice, but the relevance to humans is uncertain. Recently, canine HSAs were identified as a powerful tool for investigating the pathogenesis of human HSAs. To characterize the cellular phenotype of canine HSAs, we evaluated immunoreactivity and/or messenger RNA (mRNA) expression of markers for hematopoietic stem cells (HSCs), endothelial cells (ECs), a tumor suppressor protein, and a myeloid marker in canine HSAs. Neoplastic canine cells expressed EC markers and a myeloid marker, but expressed HSC markers less consistently. The canine tumor expression results were then compared to previously published immunoreactivity results for these markers in human and mouse HSAs. There are 2 noteworthy differences across species: (1) most human HSAs had HSC marker expression, indicating that they were comprised of tumor cells that were less differentiated than those in canine and mouse tumors; and (2) human and canine HSAs expressed a late-stage EC maturation marker, whereas mouse HSAs were negative, suggesting that human and canine tumors may retain greater differentiation potential than mouse tumors. These results indicate that HSA development is variable across species and that caution is necessary when discussing translation of carcinogenic risk from animal models to humans.

Fenretinide treatment accelerates atherosclerosis development in apoE-deficient mice in spite of beneficial metabolic effects

BACKGROUND AND PURPOSE

Fenretinide, a synthetic retinoid derivative first investigated for cancer prevention and treatment, has been shown to ameliorate glucose tolerance, improve plasma lipid profile and reduce body fat mass. These effects, together with its ability to inhibit ceramide synthesis, suggest that fenretinide may have an anti-atherosclerotic action.

EXPERIMENTAL APPROACH

To this aim, nine-week-old apoE-knockout (EKO) female mice were fed for twelve weeks a Western diet, without (control) or with (0.1% w/w) fenretinide. As a reference, wild-type (WT) mice were treated similarly. Growth and metabolic parameters were monitored throughout the study. Atherosclerosis development was evaluated in the aorta and at the aortic sinus. Blood and lymphoid organs were further characterized with thorough cytological/histological and immunocytofluorimetric analyses.

KEY RESULTS

Fenretinide treatment significantly lowered body weight, glucose levels and plasma levels of total cholesterol, triglycerides, and phospholipids. In the liver, fenretinide remarkably reduced hepatic glycogenosis and steatosis driven by the Western diet. Treated spleens were abnormally enlarged, with severe follicular atrophy and massive extramedullary haematopoiesis. Severe renal hemosiderin deposition was observed in treated EKO mice. Treatment resulted in a threefold increase of total leukocytes (WT and EKO) and raised the activated/resting monocyte ratio in EKO mice. Finally, atherosclerosis development was markedly increased at the aortic arch, thoracic and abdominal aorta of fenretinide-treated mice.

CONCLUSIONS AND IMPLICATIONS

We provide the first evidence that, despite beneficial metabolic effects, fenretinide treatment may enhance the development of atherosclerosis.