ABSTRACT: In a subacute study, male Wistar rats were treated daily by gavage with 800 mg/kg metformin for 1, 3, or 14 consecutive days, followed by necropsy 24h after the last application. The biguanide phenformin was used as antidiabetic drug due to its antihyperglycaemic effect, but was withdrawn from the market in the 1970s for safety reasons. In recent years, biguanides received great interest in oncology research after an epidemiological study showed a link between the treatment with metformin, another biguanide with a better safety profile, 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. The known strong complex I inhibitor Rotenone has also shown anti-cancer activity, however is associated with toxic effects. Therefore, we compared metformin and phenformin, with rotenone, to elucidate potential mechanisms rendering biguanides apparently less toxic than rotenone. In this context, various blood and tissue parameters as well as histopathology were measured and/or evaluated. Moreover, gene expression profiling was conducted in liver and heart due to the high metabolic activity and high energy demand. All investigations were based on an experimental design previously described for mechanistic investigations of the effects of rotenone. Our examinations regarding gene expression showed that 1630 transcripts were deregulated by rotenone, metformin and/or phenformin in liver, whereas 777 transcripts were deregulated in heart, indicating that the heart is less affected by these compounds. Overall, the mechanistic profile of phenformin appears to be similar to that of rotenone, yet at a quantitatively reduced level, whereas metformin displayed only transient similarities after one day of treatment. These differences are likely due to differential molecular properties of these compounds, especially concerning their effects on mitochondria: Metformin, in contrast to rotenone, requires a certain mitochondrial potential to allow accumulation in this organelle, thereby self-limiting its entry and thus ability to inhibit mitochondrial function, whereas rotenone and to some extent also phenformin can enter mitochondria freely. Thus, our more detailed molecular characterization of these compounds suggests that inhibition of mitochondrial functions can serve as target for an anti-cancer mode of action, yet should be self-limited or balanced to some extent to avoid exhaustion of all energy stores.
