Folinic acid (5-formyltetrahydrofolate ([R, S]5-CHOFH4)) is a naturally occurring, reduced form, of folic acid commonly known in clinical practice as leucovorin. Often, clinicians use the terms folic acid and folinic acid interchangeably, but they are, in fact, not the same. Folic acid is a synthetic, oxidized, and water-soluble form of folate (vitamin B9) that is used therapeutically and does not exist in nature, whereas folinic acid exists naturally and is biologically active.[1] Both compounds are classified under the general family of “folates” and are dietary supplements to vitamin B9 found in foods such as leafy green vegetables.[1]

Folate supplementation indications include the prevention of several diseases such as ulcerative colitis, neural tube defects, and cognitive dysfunction in the elderly.[1] Folinic acid represents over 90% of functional folate derivatives in plasma, and as such therapeutic uses of folinic acid have become important in modern clinical practice, especially because humans cannot synthesize folates de novo.[1] Clinicians can consider folinic acid supplementation as superior to folic acid supplementation because folinic acid can reach higher concentrations in plasma, and it can function in the face of defective folate metabolism.[1]

There are several FDA, and non-FDA approved indications of folinic acid. It is most famously known for its use as an antidote for the toxic effects of methotrexate and for its use in conjunction with 5-fluorouracil (5-FU), both of which are FDA approved indications.[2]

Increased levels of folinic acid potentiate the cytotoxic effects of 5-FU in the cell.[3] This finding has revolutionized the treatment of several cancers, most notably, colorectal cancer,[4] which represents the second leading cause of death secondary to malignancies.[1] The combination of folinic acid with 5-FU is an FDA approved indication for the palliative treatment of colorectal cancer. Specifically, patients with advanced, nonresectable, adenocarcinoma of the colon, receiving treatment with folinic acid and 5-FU, display more prolonged progression-free survival and response rates.[3] High dose folinic acid/5-FU regimens can be used as adjuvant therapy for resectable colorectal cancer as well. Folinic acid aids in producing a significant increase in disease-free survival compared to patients that receive treatment with surgical resection alone.[5]

As a derivative of folic acid, folinic acid is useful as an antidote to folic acid antagonists (i.e., methotrexate, pyrimethamine).[6] Frequently referred to as “leucovorin rescue,” folinic acid is used to counteract the toxic effects of high dose methotrexate therapy.[6] Methotrexate has a variety of indications in the treatment of various malignancies and immunologic disorders. Thus, all such indications requiring methotrexate, especially in high dosages, benefit from folinic acid as rescue therapy.[6] As an antidote, folinic acid functions to limit myelosuppression, gastrointestinal toxicity, nephrotoxicity and neurotoxicity that can result secondary to high dosages of methotrexate and other folic acid antagonists.[6] Studies show that the administration of folinic acid after weekly doses of methotrexate reduces the incidence of transaminitis (hepatic damage), gastrointestinal complications, and stomatitis (oral ulcers).[7] The inclusion of folinic acid with methotrexate therapy is so essential that chemotherapy protocols using methotrexate also include detailed recommendations in regards to folinic acid rescue administration.[6] For diminishing toxicity and counteracting the effects of high dose methotrexate, folinic acid is FDA approved in both adults and children.

The intravenous formulation of folinic acid (leucovorin calcium), is also FDA-approved for use in adults and children (recommendation IIa, strength of evidence category B) for the treatment of megaloblastic anemia in patients who have normal vitamin B12 levels and in whom oral therapy is not possible.[8]

The list of non-FDA approved indications of folinic acid are far more extensive and include the following:  

• Similar to its use in colorectal cancer, folinic acid has also been shown to potentiate the effects of 5-FU in the treatment of breast carcinoma. This regimen is currently non-FDA approved, partly because there is insufficient evidence that folinic acid can increase the therapeutic index of 5-FU in breast cancer and because many such clinical trials are presently ongoing.[8]

• Regimens including folinic acid and 5-FU also have non-FDA approved indications for the treatment unresectable/advanced gallbladder and biliary tree carcinoma,[9] gastric cancer,[10] squamous cell carcinoma of the head and neck,[11] and resectable pancreatic cancer.[12]

• Combination chemotherapy regimens that include folinic acid have been effective in various non-Hodgkin lymphomas. Methotrexate and rescue folinic acid, in conjunction with other chemotherapy agents (i.e., doxorubicin, cyclophosphamide) have shown a complete response rate of 84% in the treatment of diffuse large cell lymphoma.[13]

• For patients who require prophylactic therapy for toxoplasmosis and who are unable to tolerate sulfamethoxazole/trimethoprim, folinic acid is the recommendation in combination with clindamycin and pyrimethamine as an alternative. Folinic acid, in conjunction with pyrimethamine and sulfadoxine, is another option.[14]

• For patients that require prophylactic treatment for Pneumocystis jirovecii pneumonia and who are unable to tolerate sulfamethoxazole/trimethoprim, an alternative treatment can be used that includes folinic acid in combination with pyrimethamine and dapsone; among other combinations.[15]

• Serum plasma levels of homocysteine and folate derivatives have an inverse relationship.[1] In particular, the methionine cycle, which uses homocysteine as a substrate, is highly sensitive to folate deficiency.[1] Thus, when cells have functional depletion of folinic acid, plasma homocysteine levels are markedly increased.[1] In patients with hyperhomocysteinemia, folinic acid has been found to reduce the plasma level of homocysteine, particularly in patients on hemodialysis.[16] This finding suggests that folinic acid intake may reduce the risk of cardiovascular disease. Furthermore, folinic acid levels can be an indirect indicator of homocysteine levels.[1]

• There is evidence that patients who have repeated exposures to nitrous oxide, or who require treatment with long term nitrous oxide may benefit from folinic acid prophylaxis to prevent bone marrow suppression.[17]

Folates and folic acid are not biologically active and must be converted into tetrahydrofolate by way of dihydrofolate reductase. Folinic acid does not require dihydrofolate reductase for conversion into tetrahydrofolate.[1] Tetrahydrofolate and its derivatives then go on to participate in thymidylate and purine synthesis as they are essential in carrying out one-carbon transfer reactions in vivo.[18] These reactions are especially important in the generation of nucleic acids, the regulation of gene expression, and the overall stability of the genome.[1][18] The metabolism of serine, methionine, histidine, and glycine also depends on such substrates and reactions. Thus, folinic acid ultimately plays a crucial role in normal metabolism and gene regulation.[1] Furthermore, folinic acid is essential for the synthesis of methionine from homocysteine, as this is a methylation reaction.[1]

The two main roles of folinic acid in pharmacology is to counteract the effects of folic acid antagonists and to enhance the effects of fluoropyrimidines.[19] The former function becomes possible because folinic acid can enter cells through the reduced folate carrier and subsequently be converted to tetrahydrofolate despite the presence of methotrexate, thereby “rescuing” these cells from methotrexate toxicity.[6]

Most folic acid antagonists share a similar mechanism of action that includes the inhibition of dihydrofolate reductase: the enzyme responsible for generating the functional tetrahydrofolate molecule. As mentioned earlier, folinic acid does not require dihydrofolate reductase to convert into its functional derivatives. In this setting, folinic acid functions as an antidote by rescuing these cells from the chemotherapeutic toxicities of folate antagonists such as methotrexate.[18]

While counteracting the side effects of methotrexate, folinic acid functions to enhance the effects of 5-FU. In the cell, 5-FU converts to fluoro-deoxy uridylic acid, a molecule that functions to inhibit thymidylate synthase. Thymidylate synthase is an enzyme that is important in DNA repair and replication. The functional derivative of folinic acid, 5,10 methylenetetrahydrofolate stabilizes the bound fluoro-deoxy uridylic acid to thymidylate synthase. This interaction yields a ternary complex known as the thymidylate synthase 5-fluorodeoxyuridine monophosphate-methylenetetrahydrofolate complex, which ultimately functions to inhibit thymidylate synthase. Increased cellular amounts of functional folinic acid derivatives lead to increased stability of the aforementioned inhibitory complex, which leads to a depletion of thymidylate synthesis and disrupts DNA synthesis and repair.[19][2]

Due to the wide range of folinic acid indications and administration guidelines, specific protocols are available, which clinicians should follow. In general, folinic acid is compounded with calcium in the form of leucovorin calcium (especially for FDA approved indications), and can be administered intramuscularly, intravenously, or orally.  The timing, dosage, and route of folinic acid administration depend on the desired outcome for the desired indication.[19]

Adult:

• As an antidote for the toxic effects of methotrexate, folinic acid should not be administered concurrently. Most commonly, folinic acid is initiated 24 hours after starting methotrexate therapy. Furthermore, the toxic effects of methotrexate may not be reversible without initiation of folinic acid within approximately 40 hours of methotrexate. As rescue therapy, folinic acid is generally administered orally, intramuscularly or as an IV bolus.[6]

• For combination therapy with 5-FU, clinicians should follow specific administration schedules as individual protocols vary. In general, when used for combination therapy, folinic acid is administered as IV bolus or short IV infusions (minutes-hours).[2]

Pediatric:

• Folinic acid usually gets compounded with calcium, and as such, it should not be administered intravenously at rates over 160mg/min. Folinic acid is also not to be administered intrathecally.[20] As with adult dosing recommendations, individual protocols require close adherence, as guidelines differ depending on the desired outcome and particular indication. However, there are similar generalities in both patient populations. For both methotrexate toxicity and combination therapy with 5-FU, the before mentioned adult administration recommendations remain the same for the pediatric patient population.[2]

Because folinic acid administration is usually in combination with chemotherapy agents, it is challenging to discern causative relationships between folinic acid and adverse effects. The most commonly reported adverse reactions to folinic acid include GI upset (nausea, vomiting diarrhea), leukopenia, alopecia, and stomatitis. Other adverse outcomes include erythema, pruritus, skin rash, and urticaria.[21]

Hypocalcemia can result in patients treated with folinic acid. Studies suggest that there is evidence of decreased vitamin D levels in such patients, which may contribute to hypocalcemia. Thus, there is a recommendation for calcium monitoring and appropriate supplementation as needed.[22]

Gastrointestinal complications are found to be more frequent and severe, with the addition of folinic acid to 5-FU therapy, particularly in the treatment of colorectal cancer. Patients treated with folinic acid in conjunction with 5-FU were found to have a higher incidence of stomatitis and diarrhea than patients treated with 5-FU alone.  Patients who experience gastrointestinal side effects should not receive further combination treatment until GI toxicity resolves, regardless of severity. It merits noting that patients with GI toxicity, in particular, diarrhea, have the capability of deteriorating rapidly. These patients should be monitored closely, especially in the elderly or debilitated.[23] Elderly patients have the potential to develop severe enterocolitis, diarrhea, and dehydration that may result in death.[24]

Although rare, anaphylactic reactions are possible following the administration of folinic acid.[25]

As mentioned earlier, patients who develop GI toxicity with combination 5-FU and folinic acid therapy should no longer receive such treatment until symptoms have resolved.[23]

Intrathecal administration and hypersensitivity to folinic acid or its components are contraindications to its administration.[20]

Caution is necessary for patients receiving folinic acid therapy for the treatment of megaloblastic anemia, particularly in the presence of concurrent vitamin B12 deficiency or pernicious anemia. In such cases, neurologic manifestations may progress and get masked by hematologic remission.[8]