General Information

Fusilev is a folate analog injection. Fusilev protects against the toxic effects of methotrexate, which is a type of chemotherapy that prevents folates from being metabolized to the reduced active form , therefore leading to cell death. Specifically, methotrexate is a folic acid antagonist which binds and inhibits one enzyme involved in an early step of the activation of folic acid to folinic acid. The administration of Fusilev, therefore, bypasses the metabolic block effected by methotrexate.

Fusilev is specifically indicated after high-dose methotrexate therapy in osteosarcoma and to diminish the toxicity and counteract the effects of impaired methotrexate elimination and of inadvertent overdosage of folic acid antagonists.

Fusilev is supplied as a powder for reconsitution into a solution for intravenous administration. The recommended dosages are based on a methotrexate dose of 12 grams/m2 administered by intravenous infusion over 4 hours. Fusilev rescue at a dose of 7.5 mg (approximately 5 mg/m2) every 6 hours for 10 doses starts 24 hours after the beginning of the methotrexate infusion. Fusilev administration, hydration, and urinary alkalinization (pH of 7.0 or greater) should be continued until the methotrexate level is below 5 x 10-8 M (0.05 micromolar).

Clinical Results

FDA Approval

The safety and efficacy of Fusilev rescue following high-dose methotrexate were evaluated in 16 patients age 6-21 who received 58 courses of therapy for osteogenic sarcoma. High-dose methotrexate was one component of several different combination chemotherapy regimens evaluated across several trials. Methotrexate 12 g/m2 IV over 4 hours was administered to 13 patients, who received Fusilev 7.5 mg every 6 hours for 60 hours or longer beginning 24 hours after completion of methotrexate. Three patients received methotrexate 12.5 g/m2 IV over 6 hours, followed by Fusilev 7.5 mg every 3 hours for 18 doses beginning 12 hours after completion of methotrexate. The mean number of Fusilev doses per course was 18.2 and the mean total dose per course was 350 mg. The efficacy of Fusilev rescue following high-dose methotrexate was based on the adverse reaction profile.

Mechanism of Action

Fusilev is a folate analog injection containing the active isomer of leucovorin (also called folinic acid), which is made up of a mixture of two isomers, one active, and one inactive. Folates are a group of vitamins that allow cells to reproduce. Specifically, they fuel the synthesis of purinic and pyrimidinic bases, the building blocks of DNA. Cells need to duplicate their own DNA before they can divide. Folate deficiency hinders DNA synthesis and cell division, most notably affecting rapidly proliferating tissues, such as normal bone marrow and cancer tissue. Fusilev was designed to protect against the toxic effects of methotrexate, a type of chemotherapy that prevents folates from being metabolized to the reduced active form, therefore leading to cell death.