Common Chemotherapy Drugs
Central Toronto Veterinary Referral Clinic
Kevin Finora DVM, Diplomate ACVIM (Oncology and Small Animal Internal Medicine)
Common Chemotherapy Drugs
I often receive questions from both concerned clients and veterinarians about the drugs which are being used in a patient’s treatment protocol. Veterinary oncologists use many different anti-neoplastic agents. These agents include chemotherapy drugs, growth inhibitors and immunotherapy. Currently, the most common medical approaches to the treatment of cancer in domestic species are with chemotherapy. While there are many different drugs available, certain drugs, with a wide range of anti-neoplastic activity, are used most commonly. Understanding the mechanisms of action of the drugs, their common side effects, when side effects can be expected and when the CBC nadir (low point) will occur is invaluable in helping a client understand if the changes they see in their pet are associated with the chemotherapy treatments.
As a general rule, GI side effects develop more quickly than bone marrow side effects. Gastrointestinal cells turn over more quickly than WBCs and their precursors, accounting for the time difference. GI effects usually include one or all of vomiting, diarrhea or loss of appetite. These signs typically develop on the 3rd, 4th or 5th day following treatment and, if seen, last for 12 to 24 hours. Most GI side effects can be managed by diet modification and the use of medications such as metronidazole, metoclopramide or maropitant citrate. Approximately 30% of dogs will have GI effects and 10% of cats will be affected. CBC side effects typically include leukopenia, comprised of neutropenia. If the neutrophil count drops significantly below 1.0 then there is a risk of pyrexia, lethargy, malaise, or even sepsis.
Over 70% of animals receiving chemotherapy treatments will not experience any side effects throughout their entire treatment protocol. Following is a summary of the most common chemotherapy drugs used in cancer treatment.
Carboplatin: Most common side effects are GI related and are expected 3 to 5 days after treatment. This medication can rarely be nephrotoxic. The CBC nadir in dogs is at 10 days and potentially again at 21 days. The feline nadir is between 14 and 21 days.
Cyclophosphamide: Most common side effects are GI related and are expected to occur 3 to 5 days after treatment. In dogs this medication can cause sterile hemorrhagic cystitis (in 9% of cases) in the first 24 to 48 hours following treatment. The CBC nadir is expected at 7 days.
Doxorubicin: Most common side effects are GI related and are seen 3 to 5 days following treatment. In dogs this drug has a total lifetime cumulative cardiotoxicity. In cats this drug has the potential to be nephrotoxic. The CBC nadir is expected at 7 days.
L-asparaginase: The most common side effect is an allergic reaction to the drug’s E. coli protein base. Allergic reaction is very uncommon. Allergic reactions typically are noted within 20 minutes of treatment. This medication, when used alone, does not significantly impact the CBC.
Lomustine: The most common side effects are GI related and may develop 3 to 7 days following treatment. The CBC nadir is at 10 days. Elevations in liver values can be noted over time in about 86% of cases. Progressive thrombocytopenia can develop over a period of months, though this change is rarely noted.
Vincristine: The most common side effects are GI related and are seen 3 to 5 days following treatment. The CBC nadir is expected as 7 days.
If I am treating a patient of yours with a drug not mentioned here and you feel you may be seeing a chemotherapy side effect please contact the CTVRC.
Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.
Posted by: Michael Goldstein, DVM, Diplomate ACVIM
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