<unk>  is a 5-formyl derivative of tetrahydrofolic acid. It is readily converted to other reduced<unk> ic acid derivatives (e.g., 5,10-methylenetetrahydrofolate, 5-methyltetrahydrofolate), thus has vitamin activity equivalent to that of<unk> ic acid. Since it does not require the action of dihydrofolate reductase for its conversion, its function as a vitamin is unaffected by inhibition of this enzyme by drugs such as methotrexate. This is the classical view of<unk>  rescue therapy. In 1980s, however,<unk>  was found to reactivate the dihydrofolate reductase itself even when methotrexate exists. Although the mechanism is not very clear, the polyglutamylation of methotrexate and dihydrofolate in malignant cells is considered to play an important role in the selective reactivation of dihydrofolate reductase by<unk>  in normal cells.<unk> , therefore, allows for some purine/pyrimidine synthesis to occur in the presence of dihydrofolate reductase inhibition, so some normal DNA replication processes can proceed.<unk>  has dextro- and levorotary isomers. Both levoleucovorin (the levorotary isomer) and racemic<unk>  (a mixture of both isomers) have similar efficacy and tolerability. Levoleucovorin was approved by the FDA in 2008.
Folinic acid
Tienilic acid
Kaurenoic acid
Alpha-Zearalenol
Ioxaglic acid
Beta-Zearalenol
Sakacin
Ellagic acid