ABIC LEUCOVORIN INJECTION

Mechanism of action

Leucovorin acts as a biochemical modulator that provides one-carbon units essential for purine synthesis and the conversion of deoxyuridylate to thymidylate. It bypasses the DHFR inhibition caused by methotrexate by supplying a pre-reduced form of folate, allowing for continued DNA synthesis and repair in healthy cells. In the context of colorectal cancer treatment, leucovorin enhances the activity of fluorouracil by stabilizing the binding of fluorodeoxyuridylic acid (FdUMP) to thymidylate synthase, thus forming a ternary complex that inhibits DNA synthesis more effectively. Additionally, in cases of Cerebral Folate Deficiency, it normalizes levels of 5-MTHF in the central nervous system.

Pharmacodynamics

Leucovorin serves as a salvage agent that restores reduced folate levels necessary for DNA and RNA synthesis, helping to prevent bone marrow suppression and gastrointestinal mucosal damage induced by methotrexate. It can cross the blood-brain barrier, affecting rapidly dividing cell populations. The drug has a wide therapeutic index; however, its oral absorption can become saturable at doses above 25 mg, necessitating intravenous administration for higher doses. When used in combination with 5-fluorouracil, leucovorin can significantly increase toxicity, leading to potential complications such as diarrhea and dehydration, especially in vulnerable populations.

Pharmacokinetics

Leucovorin is rapidly absorbed and has a relatively short duration of action. It distributes well throughout the body and can penetrate the blood-brain barrier. The pharmacokinetics may vary based on the route of administration, with intravenous administration preferred for higher dosing due

Adverse effects

• Diarrhea
• Nausea
• Vomiting
• Abdominal pain
• Rash
• Fever
• Anemia
• Leukopenia

Interactions

• Methotrexate
• 5-Fluorouracil
• Anticonvulsants
• Antibiotics

Precautions

• Monitor blood counts during therapy.
• Use with caution in patients with kidney impairment.
• Caution in patients with a history of hypersensitivity to folic acid or its derivatives.

Pregnancy

Leucovorin is classified as Category A in pregnancy, indicating it is generally considered safe for use.

Breast-feeding

Leucovorin is excreted in breast milk; caution should be exercised when administered to breastfeeding women.

Storage

Store in a cool, dry place, away from light. Do not freeze.

Formulations

• Oral tablets
• Injection solution

Mechanism of action

Leucovorin is a racemic mixture of diastereoisomers, with the L-isomer (levoleucovorin) being the biologically active moiety. It functions at the molecular level by providing a source of "one-carbon" moieties required for the synthesis of purines and the conversion of deoxyuridylate to thymidylate. Methotrexate acts as a competitive inhibitor of the enzyme DHFR, which normally reduces folic acid to tetrahydrofolate (THF). By providing a pre-reduced form of folate, leucovorin bypasses the DHFR block, allowing for continued DNA synthesis and cellular repair in normal host cells. It also competes with methotrexate for the same active transport processes into the cell, further reducing the drug's toxic impact. In the treatment of colorectal cancer, leucovorin acts as a biochemical sensitizer. 5-FU is metabolized into fluorodeoxyuridylic acid (FdUMP), which binds to the enzyme thymidylate synthase. Leucovorin is converted into 5,10-methylenetetrahydrofolate, which binds to and stabilizes the FdUMP-thymidylate synthase complex. This "ternary complex" locks the enzyme in an inactive state, leading to a more profound inhibition of DNA synthesis and enhanced tumor cell death. In Cerebral Folate Deficiency (CFD), leucovorin serves as a direct substrate to normalize levels of 5-MTHF in the central nervous system, particularly when folate transport across the blood-brain barrier is impaired by autoantibodies or genetic defects. Leucovorin is a derivative of tetrahydrofolic acid, the reduced form of folic acid, which is involved as a cofactor for 1-carbon transfer reactions in the biosynthesis of purines and pyrimidines of nucleic acids. Impairment of thymidylate synthesis in patients with folic acid deficiency is thought to account for the defective DNA synthesis that leads to megaloblast formation and megaloblastic and macrocytic anemias. Because of its ready conversion to other tetrahydrofolic acid derivatives, leucovorin is a potent antidote for both the hematopoietic and reticuloendothelial toxic effects of folic acid antagonists (eg, methotrexate, pyrimethamine, trimethoprim). It is postulated that in some cancers leucovorin enters and rescues normal cells from the toxic effects of folic acid antagonists, in preference to tumor cells, because of a difference in membrane transport mechanisms; this principle is the basis of high dose methotrexate therapy with leucovorin rescue.

Pharmacodynamics

Leucovorin acts as a biochemical modulator and salvage agent that bypasses metabolic blocks in the folate cycle. Its primary action is to restore the pool of reduced folates necessary for DNA and RNA synthesis, thereby preventing or reversing bone marrow suppression and gastrointestinal mucosa inflammation caused by methotrexate. Systemically, it affects rapidly dividing cell populations and can readily cross the blood-brain barrier to concentrate in the cerebrospinal fluid. The drug's duration of action is relatively short. Leucovorin has a wide therapeutic index, though its oral absorption is saturable at doses exceeding 25 mg, necessitating intravenous administration for higher dose requirements. When used with 5-fluorouracil, it significantly enhances the toxicity of the chemotherapy, which may lead to severe diarrhea and dehydration, particularly in elderly or debilitated patients.