Folinic acid is an adjuvant used in cancer chemotherapy involving the drug methotrexate. It is also used in synergistic combination with the chemotherapy agent 5-fluorouracil. Levofolinic acid and its salts are the enantiopure drugs (in this case, the levo form), and are the only molecules that are biologically active. Folinic acid is a 5-formyl derivative of tetrahydrofolic acid. It is readily converted to other reduced folic acid derivatives, and, thus, has vitamin activity that is equivalent to that of folic 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 Folinic acid rescue therapy. Folinic acid is administered at the appropriate time following methotrexate as part of a total chemotherapeutic plan, where it may "rescue" bone marrow and gastrointestinal mucosa cells from methotrexate.
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