Product Name: Calcium Folinate
Molecular Formula: C20H21CaN7O7
Molecular Weight: 511.5
CAS Registry Number: 1492-18-8
EINECS:  216-082-8
Melting point: 245°C

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Folinic acid (INN) or leucovorin (USAN), generally administered as calcium or sodium folinate(or leucovorin calcium/sodium), is an adjuvant used in cancer chemotherapy involving the drug methotrexate. It is also used in synergistic combination with the chemotherapy agent 5-fluorouracil.
 
Folinic acid was first discovered in 1948 as citrovorum factor and occasionally is still called by that name. Folinic acid should be distinguished from folic acid (vitamin B9). However, folinic acid is a vitamer for folic acid, and has the full vitamin activity of this vitamin. It is on the World Health Organization's List of Essential Medicines, a list of the most important medication needed in a basic health system.

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. No apparent effect is seen on pre-existing methotrexate-induced nephrotoxicity. Folinic acid can be taken as a pill (orally) or injected into a vein (intravenously) or muscle (intramuscularly). 
While not specifically an antidote for methotrexate, folinic acid may also be useful in the treatment of acute methotrexate overdose. Different dosing protocols are used, but folinic acid should be redosed until the methotrexate level is less than 5 x 10−8 M.
 
Additionally, folinic acid is sometimes used to reduce the side effects of methotrexate in rheumatoid arthritispatients. This includes reductions in nausea, abdominal pain, abnormal liver blood tests, and mouth sores. 
 
Folinic acid is also used in combination with the chemotherapy agent 5-fluorouracil in treating colon cancer. In this case, folinic acid is not used for "rescue" purposes; rather, it enhances the effect of 5-fluorouracil by inhibiting thymidylate synthase.
 
Folinic acid is also sometimes used to prevent toxic effects of high doses of antimicrobial dihydrofolate reductase inhibitors such as trimethoprim and pyrimethamine. It may be prescribed in the treatment oftoxoplasmosis retinitis, in combination with the folic acid antagonists pyrimethamine and sulfadiazine.
Folinic acid has dextro- and levorotary isomers, only the latter one being pharmacologically useful. As such, levoleucovorin was approved by the FDA in 2008.