The use of cytotoxic agents in the treatment of immune-mediated diseases of dogs and cats.

Cytotoxic drugs are often combined with glucocorticoids in the therapy of immune-mediated diseases. Cyclophosphamide, azathioprine, chlorambucil, and methotrexate are cytotoxic drugs used most commonly for the purpose of immunosuppression. Cyclophosphamide and chlorambucil are alkylating agents, which cause cross-linking of DNA resulting in altered protein production, decreased cell division, and cell death. Azathioprine and methotrexate are anti-metabolites, which cause inhibition of DNA synthesis and a cascade of other effects. These drugs, in general, are more effective if administered during the initial phases of disease when immunocompetent lymphocytes are in a phase of rapid proliferation. This is often impractical in the clinical situation and may explain therapeutic failures. This article focuses on the use of these drugs in the treatment of immune-mediated diseases in dogs and cats and covers mechanisms of action, dosages, and side effects of individual cytotoxic agents.