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Antineoplastic (antitumor) chemotherapy is often used to treat dogs and cats, and it is routinely used for selected tumors in horses. Antineoplastic drugs can be grouped into general categories, based on their biochemical method of action: alkylating agents, antimetabolites, mitotic inhibitors, antineoplastic antibiotics, hormonal agents, and other miscellaneous drugs (see Drugs and Vaccines: Drugs Used to Treat Cancers and Tumors ).
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| Drugs Used to Treat Cancers and Tumors |
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Drug
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Mechanism of Action
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Main Tumor Types Treated
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Alkylating agents
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Cyclophosphamide
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Metabolized by the liver to an active form that affects duplication of DNA in actively dividing cells
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Lymphoma, sarcomas, mammary adenocarcinoma, lymphocytic leukemia
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Melphalan
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Affects duplication of DNA in actively dividing cells
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Multiple myeloma
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Chlorambucil
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Affects duplication of DNA in actively dividing cells; slowest-acting alkylating agent
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Chronic lymphocytic leukemia, lymphoma
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Carmustine
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Affects duplication of DNA and RNA in actively dividing cells; not cross resistant with other alkylating agents
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Central nervous system cancers (astrocytomas and gliomas), gastrointestinal carcinomas, multiple myeloma
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Streptozocin*
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Inhibits duplication of DNA; high affinity for pancreatic beta cells
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Temporary remission of hypoglycemia resulting from functional pancreatic islet cell tumor
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Dacarbazine
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Metabolized by the liver to an active form that affects duplication of DNA and RNA in actively dividing cells
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Lymphoma (for use in protocols after relapse)
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Busulfan
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Affects duplication of DNA in actively dividing cells
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Chronic myelogenous leukemia, polycythemia vera
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Antimetabolites
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Methotrexate
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Inhibits metabolism of actively dividing cells
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Lymphoma, Sertoli cell tumor, osteosarcoma, metastatic transmissible venereal tumor
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5-Fluorouracil†
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Interferes with duplication of DNA and may be incorporated into RNA to cause toxic effects
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Gastrointestinal, lung, liver, and mammary carcinomas (by injection); skin carcinomas (topical)
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Cytarabine
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Interferes with duplication of DNA
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Lymphoma (including central nervous system), leukemias
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Dactinomycin (Actinomycin D)
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Interferes with duplication of DNA; inhibits enzymes that affect DNA and RNA duplication; generates free radicals that break up DNA damage and cell membranes
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Choriocarcinoma, testicular carcinoma, rhabdomyosarcoma, lymphoma
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Antibiotic Antineoplastics
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Doxorubicin
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Interferes with duplication of DNA; inhibits enzymes that affect DNA and RNA duplication; generates free radicals that break up DNA and damage cell membranes
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Lymphoma, acute lymphocytic and granulocytic leukemia, sarcomas (osteosarcoma, hemangiosarcoma, rhabdomyosarcoma) and carcinomas (mammary, ovarian, small cell lung, thyroid, testicular, prostatic, transitional cell, squamous cell of the head and neck, cervical), plasma cell myeloma, hepatoma, neuroblastoma
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Mitoxantrone
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Degrades and breaks up DNA
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Lymphoma, carcinomas (squamous cell, transitional cell, breast, thyroid, kidney), fibrosarcoma, hemangiopericytoma
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Bleomycin
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Generates oxygen radicals that break up DNA
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Carcinomas (testicular, squamous cell of head and neck, cervical, penile) lymphoma, seminoma, malignant teratoma
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Mitotic Inhibitors
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Vinblastine
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Disrupts duplication of cells
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Lymphoma and leukemias, mastocytoma
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Vincristine
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Disrupts duplication of cells
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Transmissible venereal cell tumors, lymphoma and leukemias, central nervous system tumors, mast cell tumors, mammary adenocarcinoma, soft-tissue sarcomas, immune-mediated thrombocytopenia
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Miscellaneous
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Cisplatin‡
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Disrupts duplication of DNA
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Osteosarcoma, carcinomas (transitional cell, testicular, squamous cell of head and neck, ovarian, cervical, bladder, and lung), mesothelioma
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l-Asparaginase
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Inhibits protein synthesis in tumor cells by binding the amino acid asparagine
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Acute lymphocytic and lymphoblastic leukemia and lymphoma
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Mitotane (o,p´DDD)
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Destroys outer layers of the adrenal gland
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Pituitary hyperadrenocorticism, relieves signs of adrenal cortical tumors
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Hydroxyurea
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Inhibits duplication of DNA by destroying an enzyme that is part of the process
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Polycythemia vera, mastocytoma, granulocytic and basophilic leukemia, thrombocythemia
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Etoposide
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Breaks down DNA
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Carcinomas (testicular, small cell lung)
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Selegiline
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Increases levels of the neurotransmitter dopamine in the pituitary gland
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Pituitary hyperadrenocorticism, relieves signs caused by pituitary tumor
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Hormones
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Prednisolone
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Inhibits duplication of lymphocytes
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Lymphoma, mast cell tumors, palliative treatment of brain tumors
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Tamoxifen
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Antiestrogenic; blocks the effects of estrogen on target tissues
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Estrogen-receptor-positive breast carcinomas
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Flutamide
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Antiandrogenic; competes with testosterone for binding to androgen receptors
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Testosterone-receptor-positive prostatic tumors; surgical castration preferred
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Leuprolide
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Mimics a brain hormone, which leads to reduced pituitary hormones and eventually to decreased concentration of testosterone (males) and estrogen (females)
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Testosterone-receptor positive prostatic carcinomas or perianal tumors; surgical castration preferred
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*Toxicities are so severe that use is very limited.
†Use on the skin in cats has resulted in fatal brain and liver toxicity.
‡May cause severe, potentially fatal buildup of fluid in the lungs in cats.
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Chemotherapy drugs are usually given in various combinations of dosages and timing, which are referred to as regimens or protocols. This offers many advantages, because when drugs with different targets or mechanisms of actions are combined, the chances for success are greater.
Chemotherapy may be used in addition to surgery and radiation. Sometimes chemotherapy is started before surgery in an attempt to decrease tumor size or stage of malignancy, thus improving the chances of successful surgery.
The decision to use chemotherapy depends on the type of tumor to be treated, the stage of malignancy, the condition of the animal, and financial constraints. Responses to cancer chemotherapy can range from a decrease in day-to-day signs but no overall increased survival time to a complete remission. Some tumors respond better to chemotherapy than others. For example, canine lymphoma is usually very responsive to chemotherapy. You should discuss the options and the risks and benefits of each with your veterinarian if you are considering chemotherapy to treat cancer in your pet.
Last full review/revision July 2011 by Philip T. Reeves, BVSc, PhD, FACVSc; Jörg M. Steiner, DrMedVet, PhD, DACVIM, DECVIM-CA; Dawn Merton Boothe, DVM, PhD, DACVIM, DACVCP; Maya M. Scott, BS, DVM; Ian Tizard, BVMS, PhD, DACVM; Jozef Vercruysse, DVM, DEVPC
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