The use of cytotoxic (anticancer) drugs in isolation or in combination is an important,
if relatively recent, addition to the armoury of treatments for neoplasia in cats
and dogs. However these are potentially harmful agents and are best administered
by experienced oncologists.
As with radiotherapy the clinical skill necessary for the successful use of these
drugs is to deliver a therapeutic dose to the site of the cancer, and at the same
time minimise side-effects due to normal tissues being affected by these non-selective
drugs - which usually affect the growth or cell division phase of tissues. The most
common complication is myelosuppression leading to leucopenia, increased risk of
infection and sepsis.
Toxicity is the main limiting factor in the application of chemotherapy and for this
reason combination therapy employing lower doses of drugs with different modes of
action is usually preferred. Even with this approach multiple drug resistance can
occur with some neoplasms.
In all cases the general physical condition of the animal must be good and pre-treatihent
screening for renal and hepatic impairment is essential. Providing there is no evidence
of major organ disease the absolute age of an animal is less important, except in
as much as it might limit the period of lifespan that can be expected following chemotherapy.
Usually one should aim to provide an animal with a normal quality of life for a period
of at least 6 months to 1 year to justify the use of this form of treatment.
White blood cell counts should be performed every 2-6 weeks during treatment with
cytotoxic drugs as myelosuppression is their main side effect. If the white cell
count falls below 3 x 109/litre the drug dose should be halved. If
it falls below 2 x 109/litre the drug should be withheld.
Some of the drugs commonly used for chemotherapy in small animal veterinary medicine
are listed below in more detail.
Other cytotoxic drugs
Success of chemotherapy