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Leucovorin is a folate analog and methylating agent used to reduce the toxicity of methotrexate and enhance the efficacy of 5-fluorouracil in cancer treatment. It provides a source of reduced folate to bypass metabolic blocks.

Dosage for Leucovorin varies significantly depending on the reason for use and the specific chemotherapy protocol being followed.

Methotrexate Rescue (Post-High Dose Therapy)

The standard protocol usually begins 24 hours after the start of the methotrexate infusion. A common starting dose is 15 mg (approximately 10 mg/m²) every 6 hours for 10 doses. This is continued until the methotrexate blood level falls below a specific threshold (typically 0.05 micromolar). If the patient shows signs of delayed methotrexate elimination (e.g., rising creatinine levels), the dose of Leucovorin may be increased to 100 mg/m² every 3 hours intravenously until the levels are safe.

Advanced Colorectal Cancer (with 5-FU)

There are two primary regimens used in the United States:

1. 1The Mayo Clinic Regimen: Leucovorin 20 mg/m² followed by 5-FU 425 mg/m² intravenously for 5 consecutive days, repeated every 4 to 5 weeks.
2. 2The Roswell Park Regimen: Leucovorin 500 mg/m² as a 2-hour infusion, with 5-FU administered midway through the infusion, repeated once a week for 6 weeks.

Megaloblastic Anemia

For folate deficiency, the typical dose is up to 1 mg per day via intramuscular or intravenous injection. There is no evidence that doses higher than 1 mg/day are more effective.

Leucovorin is frequently used in pediatric oncology, particularly for the treatment of osteosarcoma (bone cancer) as part of high-dose methotrexate protocols. The dosing is generally calculated based on the child's Body Surface Area (BSA) in square meters (m²).

• Rescue Dosing: Similar to adults, 10-15 mg/m² every 6 hours is common.
• Adjustments: Pediatric patients require intensive monitoring of renal function, as children can experience rapid changes in methotrexate clearance.

Renal Impairment

Leucovorin itself is not nephrotoxic (toxic to the kidneys). However, because it is used to rescue patients from methotrexate (which is nephrotoxic), dosage adjustments are critical. If a patient's creatinine clearance drops or their serum creatinine rises by 50% or more, the Leucovorin dose must be increased significantly (often to 100-150 mg every 3 hours) to prevent catastrophic methotrexate toxicity.

Hepatic Impairment

Specific dose adjustments for Leucovorin in patients with liver disease are not well-defined in the manufacturer's labeling. However, since the conversion to the active metabolite occurs partly in the liver, patients with severe hepatic dysfunction should be monitored closely for efficacy.

Elderly Patients

Clinical studies have shown that elderly patients are at a higher risk for severe gastrointestinal toxicity when Leucovorin is combined with 5-fluorouracil. Healthcare providers often start at the lower end of the dosing range and monitor for severe diarrhea and dehydration.

• Oral: Tablets can be taken with or without food. If the dose is higher than 25 mg, your doctor will likely switch you to an injectable form due to the absorption limits of the gut.
• Injection: This must be administered by a healthcare professional in a clinical setting. It should not be administered intrathecally (into the spinal canal), as this can be fatal.
• Storage: Tablets should be stored at room temperature (20°C to 25°C / 68°F to 77°F) in a dry place. Injectable vials should be protected from light.

In the context of chemotherapy rescue, a missed dose is a medical emergency. Leucovorin must be taken exactly on schedule to prevent the methotrexate from killing healthy cells. If you miss a dose at home, call your oncologist or go to the emergency room immediately. For non-cancer uses, take the missed dose as soon as you remember, unless it is almost time for the next dose.

Leucovorin is generally considered to have low toxicity. However, excessive amounts can neutralize the chemotherapeutic effect of methotrexate entirely, potentially allowing the cancer to grow. In combination with 5-FU, an overdose of Leucovorin can lead to severe, life-threatening diarrhea and white blood cell suppression. If an overdose is suspected, contact a Poison Control Center or seek emergency care.

> Important: Follow your healthcare provider's dosing instructions. Do not adjust your dose without medical guidance. Timing is critical for the success and safety of this medication.

When Leucovorin is used alone at standard doses, side effects are remarkably rare because it is a vitamin derivative. However, when used in combination with chemotherapy, it significantly increases the toxicity of the other drugs.

Common effects when combined with 5-Fluorouracil include:

• Diarrhea: This is often severe and can lead to rapid dehydration. It feels like intense abdominal cramping followed by frequent, watery stools.
• Stomatitis (Mouth Sores): Painful ulcers in the mouth and throat that can make eating or drinking difficult. These usually appear 5-7 days after treatment.
• Nausea and Vomiting: Generally manageable with anti-nausea medications, but can be persistent.
• Leukopenia: A drop in white blood cell counts, which increases the risk of infections. You may feel unusually tired or develop a low-grade fever.

Leucovorin is a potent biological agent. While it is a form of a B-vitamin, it must be handled with the same caution as the chemotherapy drugs it accompanies. It is not a 'supplement' and should never be used without strict oncological supervision. The most critical safety point is that Leucovorin enhances the toxicity of 5-fluorouracil. While it makes the chemotherapy more effective at killing cancer, it also makes it more effective at damaging the lining of the gut and the bone marrow.

No FDA black box warnings for Leucovorin. However, clinical warnings regarding its use with 5-FU and its potential to mask Vitamin B12 deficiency are considered 'Major Precautions' in the clinical literature.

• Allergic Reactions: Though rare, anaphylaxis can occur. Patients with a known allergy to folic acid or folinic acid should not take this medication.

There are few absolute contraindications for Leucovorin, but it should never be used as the sole treatment for Pernicious Anemia or other megaloblastic anemias where Vitamin B12 deficiency is present. The clinical consequence is the progression of neurological disability while the hematologic (blood) markers appear to recover.

Anticonvulsants (Phenytoin, Phenobarbital, Primidone)

Leucovorin can decrease the blood levels of these anti-seizure medications.

• Mechanism: It is believed that folates increase the metabolism of these drugs in the liver.
• Consequence: Increased risk of breakthrough seizures.

1. 1Hypersensitivity: Anyone with a known life-threatening allergic reaction to Leucovorin Calcium or any component of the formulation must not receive the drug. The mechanism is an IgE-mediated immune response that can lead to anaphylaxis.
2. 2Pernicious Anemia / Vitamin B12 Deficiency: As discussed, using Leucovorin for these conditions is absolutely contraindicated because it allows neurological degeneration to continue. The mechanism is that folate can bypass the need for B12 in the DNA synthesis pathway of red blood cells, but it cannot replace B12 in the maintenance of the myelin sheath (the coating of nerves).

1. 1Seizure Disorders

FDA Pregnancy Category C (under the old system). Animal reproduction studies have not been conducted with Leucovorin. It is also not known whether Leucovorin can cause fetal harm when administered to a pregnant woman. However, since Leucovorin is a form of a naturally occurring vitamin (folate), it is generally not thought to be a potent teratogen (substance that causes birth defects). In fact, folates are usually protective. However, because it is almost always used with toxic chemotherapy (methotrexate or 5-FU), the overall treatment regimen is highly dangerous to a fetus. Leucovorin should be used in pregnancy only if clearly needed.

It is not known whether Leucovorin is excreted in human milk. Because many drugs are excreted in milk and because of the potential for serious adverse reactions in nursing infants from the chemotherapy drugs usually given with Leucovorin, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

Leucovorin is widely used in children, especially for bone cancer (osteosarcoma). It is considered safe when used according to weight-based or BSA-based protocols. The main concern in children is the risk of increased seizures, particularly in those with CNS tumors or a history of epilepsy. Growth effects have not been specifically linked to Leucovorin, though the underlying cancer and chemotherapy can affect development.

Leucovorin (folinic acid) is a 5-formyl derivative of tetrahydrofolic acid. Its primary molecular mechanism is to provide a source of reduced folates for purine and pyrimidine synthesis.

1. 1In Methotrexate Rescue: It competes with methotrexate for the same transport carrier into the cell. Once inside, it provides the folate pool necessary for the enzyme thymidylate synthase to continue functioning, even if dihydrofolate reductase is inhibited.
2. 2In 5-FU Potentiation: It is converted to 5,10-methylene tetrahydrofolate, which binds to the complex of 5-FU and thymidylate synthase. This 'ternary complex' is much more stable than the 5-FU/enzyme complex alone, leading to more prolonged inhibition of DNA synthesis in cancer cells.