By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
Brand Name: Oncovin, Vincasar
Available as injectable
Because they are transported everywhere that the bloodstream can take them, cancer chemotherapy drugs have the ability to reach tumors that are inaccessible to the surgeon and undetectable to the diagnostician. In order to spare the normal cells of the body, anti-cancer drugs must affect activities that involve cancer cells but spare normal cells. Since cancer cells divide rapidly and uncontrollably while normal cells divide more slowly and in response to controlled cellular messages, most anti-cancer drugs target cell division.As a cell divides, microscopic protein rods called microtubules pull the two cell halves apart. These important structural proteins must be functional or cell division cannot occur. Vincristine is a member of the vinca alkaloid class of chemotherapy drugs. It is extracted from the Vinca rosea plant and is a microtubule poison.
How This Medication Is Used
Vincristine is chiefly used as one drug in multi-drug combination protocols against lymphoid and round cell tumors. Lymphoma, mast cell tumors, and transmissible venereal tumors often include vincristine in their protocols.
Separate from its chemotherapy use, vincristine has the unexplained ability to cause a sudden release of platelets (blood clotting cells) from the bone marrow. For this reason, vincristine is commonly included in treatment of immune-mediated thrombocytopenia. There has been some controversy about whether these platelets are as functional as normal ones but current published protocols for this condition suggest vincristine should be used.
Vincristine is typically used as a weekly injection.
Vincristine must absolutely be given intravenously. Vincristine is highly irritating to the soft tissues and if it is not given intravenously, where the bloodstream rapidly carries it away and dilutes it within the body’s blood volume, it will cause what is called a tissue slough. This means the soft tissues will die and fall away, leaving a large sore. Unlike the tissue sloughs of doxorubicin, a vincristine slough will eventually heal but it will require bandaging and be a source of discomfort.
Vincristine has some potential for neurologic side effects, especially in cats. A neurologically based constipation may result, particularly in cats. Difficulty coordinating the feet may be seen. Humans report paresthesias, which include odd sensations such as occur when your foot is “asleep.”
Vincristine use will increase blood uric acid levels, rarely a concern in animals, but could be a problem in dogs with a history of uric acid bladder stones.
Interactions With Other Drugs
The neurologic toxicities noted above are more likely to occur when vincristine is used in combination with L-Asparaginase, not an uncommon combination in the treatment of lymphoma.
Concerns And Cautions
Vincristine is not able to pass through the blood/brain barrier and thus cannot treat tumors in the nervous system.
Vincristine is removed from the body by the liver. If the pet has poor liver function, a dose adjustment may be needed or a different drug selected. A guideline that has been published is to cut the vincristine dose by 50 percent in patients with a total bilirubin level of higher than 2 mg/dl.
Vincristine should not be used in pregnancy and may cause sterility in males.
Vincristine is also a drug whose metabolism depends on the P-glycoprotein. Collie-type breeds tend to have a mutation of the P-glycoprotein gene that could make them sensitive to neurologic side effects of this drug. (The P-glycoprotein normally provides a block to keep drugs out of the central nervous system; if the glycoprotein is non-functional, drugs that normally do not access the nervous system can then do so, causing toxicity.) There is now a test for P-glycoprotein mutation so that sensitive dogs can be identified. This is a DNA test using an oral swab. Test kits can be ordered directly from the Washington State University Veterinary School.
Border collies appear to have an increased risk for vincristine-related bone marrow suppression that is separate from the P-glycoprotein mutation described above.
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