Cytarabine
Cytarabine (often referred to as **Ara-C**) is a synthetic pyrimidine nucleoside antimetabolite primarily used in veterinary medicine for the treatment of lymphoreticular neoplasms, myeloproliferative diseases (leukemias), and CNS lymphoma in dogs and cats. > **Clinical Pearl:** Beyond traditional oncology, cytarabine is widely utilized in veterinary neurology as a cornerstone immunosuppressive therapy for **Meningoencephalitis of Unknown Etiology (MUE)** and Granulomatous Meningoencephalomyelitis (GME) in dogs. Its ability to cross the blood-brain barrier, especially when administered as a continuous rate infusion (CRI) or via high-dose subcutaneous protocols, makes it highly effective for CNS inflammatory diseases.
Mechanism: Cytarabine is a cell-cycle phase-specific antimetabolite that acts principally during the **S-phase** (active DNA synthesis). * **Intracellular Conversion:** Cytarabine enters the cell and is phosphorylated into its active form, **cytarabine triphosphate**. * **Enzyme Inhibition:** This active metabolite competes with deoxycytidine triphosphate → incorporates into DNA → potently inhibits **DNA polymerase**. * **Result:** Inhibition of DNA synthesis and repair, leading to cell death in rapidly dividing cells. Under certain conditions, it may also block cells from transitioning from the G1 phase to the S phase.
Dosing by species
- Susceptible neoplastic diseases · 100 mg/m2 · IV/SC · continuous infusion over 2-3 days or divided and given IV or SC for 2-4 days · 2-4 days · Dosed in mg/m2 (NOT mg/kg). Cats are generally dosed similarly to dogs.
- Neoplastic diseases · 200-300 mg/m2 (usually 1-1.5 grams per horse per dose) · SC/IM/IV · every 1-2 weeks · Consultation with a veterinary oncologist is encouraged before use.
- Susceptible neoplastic diseases · 100 mg/m2 · IV/SC · continuous infusion over 2-3 days or divided and given IV or SC for 2-4 days · 2-4 days · Dosed in mg/m2 (NOT mg/kg). Use only as a general guide.
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 cytarabine
- Pregnancy (potentially teratogenic and embryotoxic)
Adverse effects
- Myelosuppression (leukopenia, anemia, thrombocytopenia)
- Anorexia
- Nausea
- Vomiting
- Diarrhea
- Conjunctivitis
- Oral ulceration
- Neurotoxicity
- Hepatotoxicity
- Fever
- Anaphylaxis (rare)
Drug interactions
- Digoxin · Cytarabine may decrease the amount of digoxin (tablets only) absorbed after oral dosing due to intestinal mucosa alterations; effect may persist for several days after cytarabine discontinuation.
- Flucytosine (5-FC) · Cytarabine may antagonize the anti-infective activity of flucytosine; monitor for decreased efficacy.
- Gentamicin · Cytarabine may antagonize the anti-infective activity of gentamicin; monitor for decreased efficacy.
Monitoring
- Efficacy (tumor response or resolution of neurologic signs)
- Hemograms (mandatory; monitor for myelosuppression, nadir typically at 5-7 days)
- Liver function tests (periodic)
- Kidney function tests (periodic)
Overdose
Efficacy and toxicity are dependent on both the dose and the rate of administration. In dogs, the IV LD50 is 384 mg/kg when given over 12 hours, but drops to 48 mg/kg when infused IV over 120 hours. In the event of an inadvertent overdose, aggressive supportive therapy should be instituted.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.