Two-year carcinogenicity studies with the oral direct thrombin inhibitor ximelagatran in the rat and the mouse.

The results of 18 months mouse and 24 months rat carcinogenicity studies with the oral direct thrombin inhibitor ximelagatran are presented. In the mouse, gavage doses of ximelagatran up to 180 µmol/kg per d produced no neoplastic changes in any of the tissues examined. In the rat, gavage doses up to 240 µmol/kg per d produced multiple macroscopically detectable nodules in the pancreas, which are seen to be focal/multifocal acinar cell hyperplasia and focal/multifocal acinar cell adenoma upon histological evaluation. There were no other treatment-related effects on tumor incidence or distribution in the rat. The studies show a clear species difference in pancreatic effects between the rat and the mouse to long-term treatment with ximelagatran.

An analysis of pharmaceutical experience with decades of rat carcinogenicity testing: support for a proposal to modify current regulatory guidelines.

Data collected from 182 marketed and nonmarketed pharmaceuticals demonstrate that there is little value gained in conducting a rat two-year carcinogenicity study for compounds that lack: (1) histopathologic risk factors for rat neoplasia in chronic toxicology studies, (2) evidence of hormonal perturbation, and (3) positive genetic toxicology results. Using a single positive result among these three criteria as a test for outcome in the two-year study, fifty-two of sixty-six rat tumorigens were correctly identified, yielding 79% test sensitivity. When all three criteria were negative, sixty-two of seventy-six pharmaceuticals (82%) were correctly predicted to be rat noncarcinogens. The fourteen rat false negatives had two-year study findings of questionable human relevance. Applying these criteria to eighty-six additional chemicals identified by the International Agency for Research on Cancer as likely human carcinogens and to drugs withdrawn from the market for carcinogenicity concerns confirmed their sensitivity for predicting rat carcinogenicity outcome. These analyses support a proposal to refine regulatory criteria for conducting a two-year rat study to be based on assessment of histopathologic findings from a rat six-month study, evidence of hormonal perturbation, genetic toxicology results, and the findings of a six-month transgenic mouse carcinogenicity study. This proposed decision paradigm has the potential to eliminate over 40% of rat two-year testing on new pharmaceuticals without compromise to patient safety.

Proliferative and molecular effects of the dual PPARalpha/gamma agonist tesaglitazar in rat adipose tissues: relevance for induction of fibrosarcoma.

The dual peroxisome-proliferator-activated receptor (PPAR) α/γ agonist tesaglitazar has been shown to produce fibrosarcomas in rats. Here, the authors studied morphology, proliferation, differentiation, and inflammation markers in adipose tissue from rats exposed to 1, 3, or 10 µmol/kg tesaglitazar for 2 or 12 weeks, including recovery groups (12 weeks treatment followed by 12 weeks recovery), and 3 or 10 µmol/kg tesaglitazar for 24 weeks. Subcutaneous white and brown fat revealed reversible dose-related histopathological alterations and after 12 and 24 weeks developed areas of thickened skin (fatty lumps). There was a dose-dependent increase in proliferation of interstitial cells in white and brown fat as shown by increased mitotic index in all dose groups after 2 weeks. This was limited to the high dose after 12 and 24 weeks in white fat. Gene expression analyses showed that while tesaglitazar induced differentiation of adipose tissue characterized with a switch in cyclin D1 and D3 mRNA by 12 weeks, longer exposure at high doses reversed this differentiation concurrent with a reappearance of early adipocyte and inflammatory markers. These data suggest that sustained increased turnover of mesenchymal cells in adipose tissues, concomitant with onset of inflammation and fibrosis, drives development of fibrosarcomas in rats treated with tesaglitazar.

Phenotypic screening of hepatocyte nuclear factor (HNF) 4-gamma receptor knockout mice.

Using the mouse as a model organism in pharmaceutical research presents unique advantages as its physiology in many ways resembles the human physiology, it also has a relatively short generation time, low breeding and maintenance costs, and is available in a wide variety of inbred strains. The ability to genetically modify mouse embryonic stem cells to generate mouse models that better mimic human disease is another advantage. In the present study, a comprehensive phenotypic screening protocol is applied to elucidate the phenotype of a novel mouse knockout model of hepatocyte nuclear factor (HNF) 4-gamma. HNF4-gamma is expressed in the kidneys, gut, pancreas, and testis. The first level of the screen is aimed at general health, morphologic appearance, normal cage behaviour, and gross neurological functions. The second level of the screen looks at metabolic characteristics and lung function. The third level of the screen investigates behaviour more in-depth and the fourth level consists of a thorough pathological characterisation, blood chemistry, haematology, and bone marrow analysis. When compared with littermate wild-type mice (HNF4-gamma(+/+)), the HNF4-gamma knockout (HNF4-gamma(-/-)) mice had lowered energy expenditure and locomotor activity during night time that resulted in a higher body weight despite having reduced intake of food and water. HNF4-gamma(-/-) mice were less inclined to build nest and were found to spend more time in a passive state during the forced swim test.