Leucovorin

Mechanism of Action

Methotrexate is a folate antagonist that inhibits the dihydrofolate reductase enzyme in cells. Leucovorin is a folate derivative that does not require reduction by dihydrofolate reductase for use in various one-carbon transfer reactions in the body, such as DNA synthesis.[7] By replenishing reduced folate intermediates, leucovorin restores purine and thymidylate biosynthesis in rapidly dividing normal tissues, including bone marrow progenitors and gastrointestinal epithelium, which are the tissues most vulnerable to methotrexate toxicity.

In contrast, leucovorin can enhance both the therapeutic effects and toxicities of fluoropyrimidines, such as 5-FU, by increasing drug binding to thymidylate synthase. As 5,10-methylenetetrahydrofolate, leucovorin stabilizes the covalent ternary complex formed between fluorodeoxyuridine monophosphate (FdUMP) and thymidylate synthase, thereby prolonging enzyme inhibition and enhancing 5-FU cytotoxicity. This mechanism provides the pharmacologic basis for the inclusion of leucovorin in FOLFOX, FOLFIRI, and FOLFIRINOX. Clinical data indicate that leucovorin pretreatment followed by 5-FU bolus administration optimizes intracellular folate availability and improves ternary complex formation compared with other sequencing approaches.

The onset of action is less than 5 minutes after intravenous (IV) administration, 10 to 20 minutes after intramuscular (IM) administration, and 20 to 30 minutes after oral administration. Leucovorin is rapidly absorbed after oral administration; however, absorption becomes saturated in the gut at doses above 25 mg. Leucovorin is cleared renally, primarily as 10-formyl tetrahydrofolate and 5,10-methenyl tetrahydrofolate.

Levoleucovorin is the pharmacologically active L-enantiomer of racemic leucovorin. Because only the levo isomer participates in folate metabolism, levoleucovorin is administered at half the milligram equivalent of racemic leucovorin (eg, levoleucovorin 50 mg = leucovorin 100 mg). Levoleucovorin may be substituted for leucovorin at half the dose during shortages or per institutional formulary. Otherwise, the indications, monitoring parameters, and adverse effects are similar.

Administration

Leucovorin may be administered orally, intramuscularly, or intravenously. IV administration is preferred in patients with vomiting or malabsorption.

In the setting of methotrexate toxicity, IV leucovorin administration is preferred and should be initiated immediately. Methotrexate undergoes progressive polyglutamation; the longer it remains in the cell, the more polyglutamated methotrexate accumulates in the nucleus, rendering it less susceptible to reversal with leucovorin.[8] Tissue toxicity may become irreversible if leucovorin therapy is delayed beyond 40 hours. Monitoring methotrexate levels is recommended to determine the appropriate dosage and duration of leucovorin rescue therapy. Dosing may require adjustment in the presence of renal insufficiency, dehydration, or third spacing. Protocol-based methotrexate rescue guided by serial methotrexate levels may reduce toxicity compared with a fixed-dose approach.[9][10]

In cases of severe or delayed methotrexate elimination, glucarpidase (carboxypeptidase G2, Voraxaze) should be considered. FDA-approved in 2012, glucarpidase cleaves methotrexate into inactive metabolites, reducing plasma concentrations by more than 98% within minutes. Glucarpidase does not eliminate intracellular methotrexate or methotrexate within the central nervous system. Leucovorin should be continued after glucarpidase but must not be administered within 2 hours before or after the dose, as leucovorin is also a glucarpidase substrate.[11][12]

When used as part of chemotherapeutic regimens, leucovorin is not administered along with methotrexate. It is typically administered 24 hours after a course of methotrexate. Methotrexate may also be administered intrathecally, such as in patients with leptomeningeal carcinomatosis. Leucovorin rescue must not be administered intrathecally because administration by this route may be fatal.[13] Elimination of methotrexate from the cerebrospinal fluid occurs by slow systemic diffusion and can be countered by IV methotrexate. Monitoring methotrexate levels is recommended.

Leucovorin enhances the binding of 5-FU to thymidylate synthase, and when both are administered together, a lower dosage of 5-FU is required. Leucovorin administration usually occurs midway through or after the fluorouracil administration.

Adverse Effects

Allergic reactions, including anaphylactoid reactions and urticaria, have been reported in patients receiving both oral and parenteral leucovorin.[14][15]

Leucovorin enhances the effects of 5-FU and may cause toxicity. High doses of folic acid may counteract the pharmacologic effects of antiepileptic drugs such as primidone, phenobarbital, and phenytoin. In addition, concomitant administration of leucovorin during treatment of Pneumocystis jirovecii infection with trimethoprim-sulfamethoxazole (TMP-SMX) has been associated with higher rates of treatment failure and morbidity.

The effects of leucovorin during pregnancy have not been adequately studied. Under the former FDA pregnancy risk classification system, leucovorin was designated as pregnancy category C. It is not known whether leucovorin is excreted into human milk; therefore, caution is warranted when administering leucovorin to breastfeeding patients.

Contraindications

If leucovorin is used alone to treat megaloblastic anemia, clinicians must first exclude vitamin B12 deficiency. Treatment of patients with vitamin B12 deficiency with leucovorin may reverse the megaloblastic anemia; however, neurologic manifestations of vitamin B12 deficiency may persist or worsen.

Monitoring

In the setting of inadvertent methotrexate overdose, leucovorin should be initiated as soon as possible. A dose of 10 mg/m² should be administered every 6 hours until the serum methotrexate concentration decreases below 10⁻⁸ M on the assay.

Serum methotrexate levels should be monitored every 24 hours until levels fall below the target threshold. Renal function should also be monitored with daily serum creatinine measurements.

If the serum creatinine concentration increases by more than 50% from the previous day's baseline, if the serum methotrexate concentration exceeds 5 × 10⁻⁶ M at 24 hours after admission, or if the serum methotrexate concentration exceeds 9 × 10⁻⁷ M at 48 hours, the leucovorin dose should be increased to 100 mg/m² administered intravenously until the serum methotrexate concentration falls below 10⁻⁸ M. Adequate hydration is essential. Sodium bicarbonate infusion should be initiated and titrated to maintain a urine pH greater than 7.0. If standard leucovorin rescue proves insufficient or serum creatinine continues to rise, consultation regarding glucarpidase eligibility is recommended.

For patients receiving leucovorin rescue after high-dose methotrexate therapy, daily monitoring of methotrexate and serum creatinine levels is required. Hydration and urinary alkalinization are essential. Leucovorin must be continued until the methotrexate level falls below 0.05 µM. If delayed excretion is suspected or creatinine levels rise, the leucovorin dose should be increased.

Patients receiving leucovorin in combination with 5-FU should undergo serial complete blood counts with differential. Complete blood counts with differential counts should be obtained after the first 2 cycles of chemotherapy and repeated weekly thereafter. Counts should be repeated after courses until the lowest anticipated white blood cell (WBC) count is reached. Liver function tests and serum electrolyte levels should be monitored before each treatment cycle.

In the presence of severe stomatitis, severe gastrointestinal toxicity, a WBC nadir of 1000 to 1900/mm³, or a platelet nadir of 25,000 to 75,000/mm³, the dose should be reduced by 20%. In the presence of severe gastrointestinal toxicity, a WBC count below 1000/mm³, or a platelet count below 25,000/mm³, the dose should be reduced by 30%. If none occurs during a course, the doses of leucovorin and fluorouracil may be increased by 10%.

When used to treat folate deficiency, leucovorin may be administered intramuscularly at doses of up to 1 mg daily. Monitoring serum leucovorin levels is not required. Reticulocyte counts assess treatment response. Serial hemoglobin, indirect bilirubin, lactate dehydrogenase, serum iron, and potassium levels may also be considered.

Toxicity

Hypersensitivity reactions to leucovorin, levoleucovorin, or other folate derivatives have been reported but are uncommon. As noted previously, intrathecal administration of leucovorin may be fatal.

When leucovorin is administered in combination with fluorouracil, the following adverse effects may occur:

• Dermatologic: alopecia, dermatitis
• Gastrointestinal: stomatitis, nausea, diarrhea, vomiting, anorexia
• Central nervous system: fatigue, malaise
• Hematologic: myelosuppression
• Constitutional: fatigue, malaise

Enhancing Healthcare Team Outcomes

Effective leucovorin management, whether for chemotherapy support or methotrexate toxicity, requires early recognition, precise dosing, and close interprofessional monitoring. The prescribing clinician should involve a clinical pharmacist early to verify dosing, review potential drug interactions (particularly with antiepileptic drugs, TMP-SMX, and 5-FU), and assess eligibility for glucarpidase in cases of impaired methotrexate clearance. Pharmacists monitoring daily methotrexate concentrations are well positioned to identify when leucovorin dose-escalation thresholds have been exceeded.

Nursing staff are responsible for timely drug administration, verification of the appropriate route (IV or oral only; never intrathecal), and monitoring for hypersensitivity reactions. Any change in patient tolerance or unexpected clinical deterioration should prompt direct communication with the pharmacy and the physician team.

In outpatient oncology settings, infusion nurses managing fluorouracil-based regimens, such as FOLFOX, FOLFIRI, or FOLFIRINOX, should systematically document hematologic nadirs, stomatitis severity, and gastrointestinal toxicity across treatment cycles. These coordinated interprofessional efforts help optimize therapeutic outcomes while minimizing adverse effects associated with leucovorin therapy.