CHEMOTHERAPY IN PETS
Chemotherapy by definition includes any anticancer agent used to manage cancer. Chemotherapy may be in the form of pills given by mouth or liquid given in the vein, under the skin or sometimes directly into the mass. The goal of using chemotherapy is to control cancer and prolong survival of the patient while maintaining a good to excellent quality of life. In general, chemotherapy works by killing rapidly dividing cells. A fundamental problem is that the cancer cells are not the only cells in the body undergoing rapid division, therefore, normal cells may be affected as well. The bone marrow and the lining of the gastrointestinal tract are the most common sites in the body that may be adversely affected by chemotherapy drugs causing unwanted side effects.

In veterinary medicine, chemotherapy is used at dosages and in combinations which have been shown to have very minimal side effects. We use these drugs with a goal that the patient will not experience even 1 bad day. If significant problems do occur, the protocol will be changed so that the patient can maintain a good quality of life. Rarely, an animal will not tolerate chemotherapy well, in which case treatment can always be discontinued. The most common adverse effects of chemotherapy include the following:

Bone marrow toxicity
White blood cells and platelets are produced in the bone marrow at a rapid rate with a relatively short life in the bloodstream, and therefore, are the cell lines which can be affected by chemotherapy. A low white blood cell count, which usually is most severe at 5-7 days post treatment, can cause an animal to be more susceptible to infection by bacteria that live in the gut or elsewhere. In the worst case, these bacteria can enter the bloodstream and cause sepsis (systemic infection) which leads to lethargy, decreased appetite, fever, and depression. This severe side effect is very uncommon and is usually treatable with supportive care, including intravenous fluids and antibiotics. The effect of chemotherapy is usually short, and therefore, the bone marrow will regenerate the needed cells within a few days to a week. We check the white blood cell and platelet counts of patients before each treatment to make sure they are safe to continue.

Gastrointestinal toxicity
The lining of the stomach and intestines consists of epithelial cells which have a very high turnover rate. Killing these cells can cause signs of nausea such as decreased appetite, lethargy, and vomiting. Effects on the large intestines may cause diarrhea. These signs can usually be managed with gastroprotective agents such as Pepcid AC, antivomiting medication, and antidiarrheal medications. If not, the treatment protocol will be adjusted.

Hair loss
This is very uncommon in dogs as opposed to people. Dogs with coarse fast growing hair coats, such as poodles and schnauzers tend to be more susceptible. Tactile hairs or whiskers may be affected, especially in cats. Also, if the hair has been shaved, it may be slower to grow back while a patient is receiving chemotherapy.

Local tissue reaction
Some chemotherapy drugs can be irritating to surrounding tissues if they are accidentally administered outside of the vein. Vincristine reactions tend to be fairly mild, including redness and irritation, which often causes the animal to lick the area excessively. The animal's licking can exacerbate the problem, leading to a major wound. If even a small amount of doxorubicin (or Adriamycin) leaks out of the vein, severe sloughing of the skin can occur. Extreme care is used in the administration of these drugs in order to prevent this rare complication. If a tissue slough does occur, the best treatment is strict management to keep the animal from licking the area, and time.

Acute tumor lysis syndrome
In animals with a high tumor cell burden which declines rapidly following administration of chemotherapy, severe systemic reactions may occur. Clinical signs of this would include vomiting, depression, and diarrhea, and usually start shortly after administration of treatment. This reaction is very rare and can be severe or fatal. Treatment includes aggressive intensive care.

Other Side Effects
Some drugs have specific potential effects which will be discussed individually. For instance, doxorubicin (or Adriamycin) can affect the heart muscle function after cumulative doses, and therefore, the heart function is monitored during treatment using periodic echocardiograms (ultrasound of the heart). Also, cyclophosphamide (or Cytoxan) can rarely cause sterile hemorrhagic cystitis, which is inflammation of the bladder leading to blood in the urine and increased frequency of urination. L-Asparaginase (Elspar) can cause a hypersensitivity reaction after administration, which can in the extreme cases be severe. These animals are pretreated with antihistamines to help prevent reactions and monitored after treatment for 30 minutes in the hospital. Other effects which can rarely occur include liver toxicity, pancreatitis, neurotoxicity, and skin reactions.

If you have any questions or concerns regarding this information, or if you have questions about side effects your pet may be experiencing, please call.

Elizabeth A. Kergosien, DVM
Diplomate ACVIM (Oncology)

Jennifer West
Oncology Technical Assistant

Phone (985) 626-4862 - Voicemail box 156