Filgrastim
**Filgrastim** is a recombinant human granulocyte colony-stimulating factor (G-CSF) utilized in veterinary medicine to stimulate the bone marrow to produce neutrophils. Key clinical points: * **Primary Indication:** Used for severe, life-threatening neutropenia (e.g., secondary to parvovirus, chemotherapy-induced myelosuppression, or feline leukemia virus-associated cyclic neutropenia) when intrinsic response is inadequate. * **Immunogenicity Risk:** Because filgrastim is derived from human DNA, it is not species-specific but **can be highly immunogenic** in dogs and cats. Patients may develop neutralizing antibodies against both the exogenous drug and their own endogenous G-CSF, leading to profound, prolonged, and potentially fatal neutropenia. * **Clinical Strategy:** To minimize antibody formation, it is typically administered as a short **"pulse" therapy** (3 to 5 days) only during the critical nadir of neutropenia. * **Cost:** Treatment is highly expensive, which often limits its use in routine veterinary practice.
Mechanism: Filgrastim acts as a hematopoietic growth factor. * It binds specifically to **G-CSF receptors** located on the surface of myeloid progenitor cells in the bone marrow. * Receptor activation triggers intracellular signaling pathways (such as the **JAK/STAT pathway**) โ stimulates the proliferation, differentiation, and activation of progenitor cells exclusively within the **neutrophil-granulocyte lineage**. * **Clinical Pearl:** In addition to increasing the absolute number of circulating mature neutrophils, filgrastim also enhances the functional capacity of these cells, including their phagocytic and bactericidal activities, thereby improving the patient's ability to fight off bacterial infections during periods of severe immunosuppression.
Dosing by species
- Adjunctive therapy of neutropenia (secondary to drug induced aplastic pancytopenia) ยท 5 micrograms/kg SC daily ยท SC ยท daily ยท Pulse therapy of no more than 5 days ยท To avoid autoantibody formation, pulse therapy is recommended.
- Neutropenia ยท 1-5 micrograms/kg SC daily ยท SC ยท daily ยท Pulse therapy of no more than 5 days ยท To avoid autoantibody formation, pulse therapy is recommended.
- Neutropenia ยท 100,000-500,000 IU (1-5 micrograms)/kg ยท SC ยท q24h ยท 3-5 days ยท Neutrophil counts typically rise within 24 hours.
- Neutropenia secondary to drug toxicity, infectious diseases, FeLV associated cyclic neutropenia or idiopathic causes ยท 5 micrograms/kg SC twice daily ยท SC ยท twice daily ยท Pulse therapy of no more than 5 days ยท Cost and/or development of antibodies usually limit usefulness to a few weeks, but often it is effective for acute or lifethreatening neutropenia.
- Neutropenia ยท 1-5 micrograms/kg SC daily ยท SC ยท daily ยท Pulse therapy of no more than 5 days ยท To avoid autoantibody formation, pulse therapy is recommended.
- Adjunctive therapy of neutropenia ยท 5 micrograms/kg SC daily until neutrophil count exceeds 3,000/mcl for 2 days ยท SC ยท daily ยท Until neutrophil count exceeds 3,000/mcl for 2 days (ideally โค 5 days) ยท To avoid autoantibody formation, pulse therapy is recommended.
- Neutropenia ยท 100,000-500,000 IU (1-5 micrograms)/kg ยท SC ยท q24h ยท 3-5 days ยท Neutrophil counts typically rise within 24 hours.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Hypersensitivity to filgrastim
- Patients that have previously developed antibodies to filgrastim with resultant neutropenia
Adverse effects
- Severe neutropenia (due to autoantibody formation against G-CSF)
- Myelofibrosis (theoretical/reported concern)
- Medullary histiocytosis (theoretical/reported concern)
- Injection site irritation
- Bone pain
- Splenomegaly
- Hypotension
Drug interactions
- Antineoplastics ยท Avoid using filgrastim within 24 hours of administering antineoplastic agents that target rapidly proliferating cells, as it may exacerbate toxicity to the newly stimulated myeloid cells.
- Corticosteroids ยท May potentiate the release of neutrophils during G-CSF therapy. ยท moderate
- Lithium ยท May potentiate the release of neutrophils during G-CSF therapy. ยท moderate
- Cytotoxic Chemotherapy ยท Concurrent administration may increase the incidence of adverse effects. The stimulatory effects of G-CSF on haemopoietic precursors render them more susceptible to the effects of chemotherapy. ยท major
Monitoring
- CBC with platelets, routinely (to monitor neutrophil response and detect potential antibody-induced neutropenia)
Overdose
Limited information is available regarding acute toxicity. Because of the high expense of the drug and its apparent limited acute toxic potential, clinically significant overdoses are considered unlikely. If an overdose occurs, monitor CBC closely and provide supportive care.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.