Zidovudine
Zidovudine (AZT) is a **nucleoside reverse transcriptase inhibitor (NRTI)** used primarily in veterinary medicine as an adjunctive treatment for **feline immunodeficiency virus (FIV)** and **feline leukemia virus (FeLV)**. * **Clinical Utility**: While it can reduce the viral load in infected cats and improve clinical signs (such as stomatitis or neurological manifestations), it is not a cure and may not significantly alter the natural long-term course of the disease. * **Clinical Pearl**: Because retroviruses integrate into the host genome, NRTIs like zidovudine are most effective at preventing the infection of new cells rather than clearing already infected cells.
Mechanism: Zidovudine is a synthetic thymidine analogue. * **Mechanism**: It is converted *in vivo* by cellular kinases into its active metabolite, **zidovudine triphosphate**. * **Pathway**: Zidovudine triphosphate competes with natural thymidine triphosphate for incorporation into viral DNA by **viral RNA-directed DNA polymerase** (→ **reverse transcriptase**). * **Effect**: Because zidovudine lacks a 3'-OH group, its incorporation prevents the formation of phosphodiester linkages, leading to **DNA chain termination** and a virustatic effect against retroviruses. * It also possesses some activity against gram-negative bacteria and can exhibit host cell cytotoxicity (particularly in rapidly dividing cells like bone marrow).
Dosing by species
- Adjunctive therapy of FeLV and FIV · 5-10 mg/kg PO or SC q12h · PO, SC · q12h · Higher dose should be used carefully as side effects can develop. For SC injection, dilute lyophilized product in isotonic sodium chloride to prevent local irritation. For PO, syrup or gelatin capsules can be used.
- Adjunctive therapy of FeLV and FIV · 5-10 mg PO or SC q12h · PO, SC · q12h · Note: Dose is in mg per cat, not mg/kg. Higher dose should be carefully used in FeLV-infected cats because side effects, particularly non-regenerative anemia, can develop.
- FeLV · 5 mg/kg PO or SC q12h · PO, SC · q12h · If giving SC, dilute in sterile normal saline to prevent local irritation. Check CBC weekly the first month.
- FIV encephalopathy · 20 mg/kg PO q12h · PO · q12h
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Patients with a history of life-threatening hypersensitivity reactions to zidovudine
Adverse effects
- Non-regenerative anemia (most common in cats, dose-dependent)
- Reductions in RBCs, PCV, and hemoglobin
- Diarrhea
- Weakness
- Granulocytopenia (reported in humans)
- Gastrointestinal effects
Drug interactions
- Azole Antifungals (e.g., ketoconazole) · May increase zidovudine levels
- Atovaquone · May increase zidovudine levels
- Doxorubicin · May antagonize each other's effects; avoid concurrent use. Also increases risk of hematologic toxicity.
- Interferon alfa · Increased risk for hematologic and hepatotoxicity
- Probenecid · May increase zidovudine levels
- Myelo-/Cytotoxic Drugs (e.g., chloramphenicol, flucytosine, vincristine, vinblastine) · Administered with zidovudine may increase the risk of hematologic toxicity
- Rifampin · May decrease blood levels (AUC) of zidovudine
Monitoring
- CBC (Complete Blood Count): Weekly for the first month of treatment (non-regenerative anemia is common, especially at higher doses). If stable, monthly checks are sufficient.
- Hematocrit: If PCV drops below 20%, treatment should be discontinued (anemia usually resolves within a few days).
- CD4/CD8 ratios (if possible)
- Clinical efficacy and improvement of signs
Overdose
Human adults and children have survived massive oral overdoses (up to 50 grams) without permanent sequelae. * **Clinical Signs**: Vomiting and transient hematologic effects (e.g., bone marrow suppression) are the most consistent adverse effects reported with acute overdoses. * **Management**: Treatment is largely supportive and symptomatic, focusing on gastrointestinal decontamination if caught early, and monitoring hematologic parameters.
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.