Energy metabolism modulation by biguanides in comparison with rotenone in rat liver and heart.

The biguanide metformin, a widely used antidiabetic drug, has received great interest in oncology research in recent years after an epidemiological study showed a link between metformin treatment and a reduced cancer risk in diabetic patients. Since mitochondrial metabolism has become a target for possible cancer therapeutic approaches, especially for tumors relying on oxidative metabolism, mitochondrial complex I inhibition is under discussion to be responsible for the anti-cancer effect of metformin. Rotenone, a well-known strong mitochondrial complex I inhibitor, yet associated with toxic effects, has also shown anti-cancer activity. Thus, we compared metformin and phenformin, another biguanide previously on the market as antidiabetic, with rotenone, to elucidate potential mechanisms rendering biguanides apparently less toxic than rotenone. Therefore, we conducted in vivo rat studies with metformin and phenformin, based on an experimental design previously described for mechanistic investigations of the effects of rotenone, including blood and tissue analysis, histopathology and gene expression profiling. These investigations show that the mechanistic profile of phenformin appears similar to that of rotenone, yet at a quantitatively reduced level, whereas metformin displays only transient similarities after one day of treatment. A potential reason may be that metformin, but not rotenone or phenformin, self-limits its entry into mitochondria due to its molecular properties. Thus, our detailed molecular characterization of these compounds suggests that inhibition of mitochondrial functions can serve as target for an anti-cancer mode of action, but should be self-limited or balanced to some extent to avoid exhaustion of all energy stores.

Mechanistic Investigations of the Mitochondrial Complex I Inhibitor Rotenone in the Context of Pharmacological and Safety Evaluation.

Inhibitors of the mitochondrial respiratory chain complex I are suggested to exert anti-tumor activity on those tumors relying on oxidative metabolism and are therefore of interest to oncology research. Nevertheless, the safety profile of these inhibitors should be thoroughly assessed. Rotenone, a proven complex I inhibitor, has shown anti-carcinogenic activity in several studies. In this context rotenone was used in this study as a tool compound with the aim to identify suitable biomarker candidates and provide enhanced mechanistic insights into the molecular and cellular effects of complex I inhibitors. Rats were treated with 400 ppm rotenone daily for 1, 3 or 14 consecutive days followed by necropsy. Classical clinical endpoints, including hematology, clinical chemistry and histopathology with supporting investigations (FACS-analysis, enzymatic activity assays) were examined as well as gene expression analysis. Through these investigations, we identified liver, bone marrow and bone as target organs amongst approx. 40 organs evaluated at least histopathologically. Our results suggest blood analysis, bone marrow parameters, assessment of lactate in serum and glycogen in liver, and especially gene expression analysis in liver as useful parameters for an experimental model to help to characterize the profile of complex I inhibitors with respect to a tolerable risk-benefit balance.

ESTP comments on the draft updated OECD test guideline 407.

The revision of the OECD TG 407 test guideline (repeated dose 28-day oral toxicity study in rodents) focuses on endpoints to detect endocrine activities of chemicals. The new endpoints are likely to influence other previously established core endpoints of this study type. An expert group of pathologists and toxicologists within the European Society of Toxicologic Pathologists (ESTP) has contributed to the scientific discussion of the draft guideline. The advantages and disadvantages of methodical changes as necropsy of all females in dioestrus, blood collection for clinical chemistry and haematology at the same cycle stage, weighing of the thyroid gland and separate weighing of ventral and dorsolateral lobes of the prostate are considered. Possible alternatives are pointed out covering scientific as well as practical aspects.

Prevalidation of a rat liver foci bioassay (RLFB) based on results from 1600 rats: a study report.

A rat liver foci bioassay (RLFB) based on an initiation-promotion protocol employing preneoplastic foci of altered hepatocytes (FAH) as an endpoint, was prevalidated in 5 different laboratories. FAH were identified by immunohistochemical demonstration of glutathione-S-transferase (placental form, GSTP) and by staining with hematoxilin/eosin (H&E), and their area fraction was quantified morphometrically. The four model hepatocarcinogens N-nitrosomorpholine, 2-acetylaminofluoren, phenobarbital, and clofibrate were selected according to characteristic differences in their presumed mode of action, and tested in a total of 1,600 male and female rats at 2 different dose levels. The chemicals were found to differ characteristically in their potency and dose-response relationship to induce FAH when given alone or when administered following initiation with diethylnitrosamine. The interlaboratory variation was small for results obtained with the GSTP-stain and somewhat larger with respect to H&E. The assessment of the carcinogenic potential of the four chemicals by the different laboratories was in the same range and the nature of their dose-response relationships did not differ essentially between laboratories. Our results suggest that this RLFB is a sensitive bioassay, providing potentially valuable information for risk assessment including the classification of carcinogenic chemicals according to their mode of action.