|Chemotherapy for Cats and Dogs with Cancer
|What is chemotherapy?
Chemotherapy refers to drugs that kill cancer cells. These drugs are usually given the pet intravenously (into the vein) but
some may be available as a pill or some other formulation. Some chemotherapy drugs must be administered over a long
period of time so a pet may spend several hours in the hospital to complete the treatment. The goal of chemotherapy is to
control the cancer in your pet and to prolong survival while maintaining a good quality of life for the pet. In some cases,
chemotherapy may result in a cure, but in most cases, it 'buys' time. It is critical that pet owners understand the difference
between remission (resolution) and cure. For example, dogs diagnosed with lymphoma experience a rapid remission after
initiating chemotherapy. Many pet owners might consider this initial response as cure, however, the effect is not long term
and many dogs will inevitably experience recurring cancer.
When is chemotherapy used?
Chemotherapy is typically used for the following circumstances in cats and dogs:
1. For pets with a tumor known to respond to chemotherapy (e.g. canine lymphoma)
2. As complementary therapy to eradicate micrometastasis (any remaining cancer cells left behind after surgery)
3. As complementary therapy to prevent the recurrence of local tumor after its incomplete surgical removal.
4. To ease the clinical signs associated with tumor that couldn't be removed surgically (e.g. pressure or pain) or has
spread to other organs
5. To sensitize tissues to radiation therapy (referred to as radiosensitizing chemotherapy)
6. After surgery if the localized tumor has a high likelihood of metastasizing to other organs (e.g. spleen tumors to
minimize the risk of metastasis)
How does chemotherapy work?
Chemotherapeutic drugs act by several different mechanisms depending on the specific class of drugs but the ultimate
goal of these drugs is to kill the cancer cell or prevent it from further multiplying. For example, alkylating agents (e.g.
cyclophosphamide) interfere with the cell's DNA; antimetabolite drugs (e.g. methotrexate) interfere with the production of
nucleic acids (RNA and DNA); anthracyclines (e.g. doxorubicin) interfere with enzymes important for DNA replication; or
mitotic inhibitors are drugs that interfere with the mechanics of cell division. These are just representative examples but
many other (sub)classes of chemotherapy drugs exist.
Unfortunately, the drugs also target normal cells which can lead to side effects in the treated patient. The goal of veterinary
oncologists is to use as much drug as possible to increase the likelihood of killing cancer cells but without causing
substantial harm to the pet along the process.
Chemotherapy efficacy and resistance in cats and dogs with cancer
There are many factors that will affect how effectively chemotherapy will work for your pet and resistance remains as one of
the biggest challenges in controlling cancer. Some tumors will contain cells that will be resistant to the treatment from the
beginning or will become resistant after being exposed to the treatment. Some tumor cells will begin to produce molecules
that will inactivate the chemotherapeutic drugs, or prevent the drugs from gaining access to the cells, or prevent the cells
from dying, all of which will render the therapy ineffective.
Pet owners should familiarize themselves with the commonly used terms relating to chemotherapy treatment in order to
gain better understanding of the procedures performed on their pets.
Maintenance therapy - Continuation of a less intensive chemotherapy treatment for the purpose to maintain a remission
(resolution) and to prevent relapse (recurrence).
Complete remission (CR) - Complete resolution of measurable tumor after chemotherapy based on physical examination,
monitoring or imaging.
Partial remission (PR) - Reduction in the measurable tumor after chemotherapy without development of new tumors.
Stable disease (SD) - Stable disease shows neither worsening nor improvement.
Progressive disease (PR) - Progressive disease shows signs of tumor growth and/or development of new tumors.
Median duration of response/median survival - The point at which half the pets have relapsed or died and half are still in
remission and alive.
Adjuvant chemotherapy - Administration of chemotherapy after surgery or radiation therapy in cases with high risk of
recurrence of metastases.
Primary (neoadjuvant) chemotherapy - The initial treatment (typically for localized cancer) by chemotherapy, followed by
surgery or irradiation.
Maximal tolerated dose (MTD) - The maximum recommended dose of a chemotherapeutic agent based on its toxicity.
It is generally believed that treatment with one drug is not curative and the combination of several drugs has a better
chance of improving survival for pets with cancer. By the time the tumor is detected, it will contain many different types of
cancer cells, each with a differential response to a particular drug. By combining several drugs, we are increasing the
chances of killing all the different types of cells present in the tumor. Combination therapies are designed so that maximum
efficacy can be achieved with acceptable toxic side effects.
Toxicity of chemotherapeutic drugs
Most chemotherapeutic drugs are well tolerated by most pets. Less than 5% of pets will have to be hospitalized and less
than 1% of pets will die as a direct result of drug-induced toxicity. There are, however, commonly experienced side effects
within one week of beginning therapy such as nausea, vomiting, diarrhea and/or lack of appetite. This is because the
normal cells in the gastrointestinal tract are rapidly dividing and thus become an unwanted target for chemotherapy drugs.
The symptoms are generally mild and use of anti-diarrhea or antiemetics usually offset some of these side effects.
Another commonly experienced side effect is myelosuppression, a term referring to the killing of blood stem cells in the
bone marrow. When the blood cell count becomes very low, chemotherapy is usually delayed until the count is restored to
acceptable levels, since low white blood cell count can increase the pet's susceptibility to infection. When the pet
experiences severe side effects, the dose of chemotherapy is typically lowered but at the same time, lowering of the dose
also lowers the efficacy of the drugs in killing tumor cells.
Some chemotherapy (e.g. vincristine or doxorubicin) drugs can irritate the skin if they leak outside the vein during its
administration, causing severe inflammation, ulceration and swelling. To minimize this potential side effect, chemotherapy
should be administered by experienced technicians and veterinarians.
Some dogs with rapidly growing hair coats (e.g. poodles, terriers, Old English sheepdogs) lose hair during chemotherapy
and cats may lose their guard hairs and whiskers while undergoing chronic chemotherapy. The hair will usually regrow
after discontinuation of chemotherapy, although it may differ in color and texture.
The following table summarizes commonly used classes of chemotherapeutic drugs and their associated toxicities.
Source: Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
Additional online resources:
|PET CANCER CENTER
Comprehensive guide to cancer diagnosis and treatment in cats and dogs