Table 2. Molecular mechanisms and targets for metformin and methotrexate.

Whereas for most of the drugs commonly used “ primary targets” (e.g., drug receptors) responsible for therapeutics can be identified,62 no such primary target is identified for metformin. It is therefore doubtful if drug agencies would accept metformin today, as a defined target is often conditional for approval.

Possible primary targets for metformin are the transporters. It may compete with the in-and efflux or excretion of endogenous substrates or nutrients and other drugs. If this results in so-called “off-target” effects, or is of no consequence or even contributes to therapeutic efficacy is yet not known.

Established “secondary” targets of metformin, important for therapeutics, are mitochondrial complex I (inhibition), also believed to contribute to lactic acidosis after toxic doses/plasma concentrations and AMPK (activation).

Accepted “primary targets” of methotrexate and its polyglutamates are tetrahydrofolate-dependent enzymes in nucleotide biosynthesis. A secondary target may be AMPK (activation) via increase of AICAR.