Page 1300 - Small Animal Internal Medicine, 6th Edition

1272 PART XII Oncology

As a general rule, chemotherapy is considered to be pal-
TABLE 76.2 liative in pets with cancer. Although the cure rate of some
VetBooks.ir Conversion of Body Weight to Body Surface Area human cancers treated with chemotherapy is high (e.g.,
>75% in high-grade lymphomas and pediatric acute lym-
in Cats
BODY WEIGHT BODY WEIGHT BODY SURFACE phoid leukemias), the cost and toxicity associated with the
high doses of chemotherapy is unacceptable in pets. For
(lb) (kg) AREA (m ) example, a dose of cyclophosphamide for a dog is rarely
2
2
higher than 300 mg/m , whereas in humans doses of 2 to 3 g/
5 2.3 0.165 m are occasionally used. Similarly, we have used doses of
2
6 2.8 0.187 300 to 600 mg/m of cytarabine once every 1 to 2 weeks in
2
7 3.2 0.207 dogs, whereas in humans, some protocols call for 3 g/m
2
8 3.6 0.222 q12h for 6 or 7 days. Finally, conventional dosages of doxo-
2
9 4.1 0.244 rubicin in dogs are 30 mg/m q2 or 3 weeks, whereas some
2
10 4.6 0.261 human protocols use 75 mg/m q3 weeks.
Chemotherapy should not be used as a substitute for
11 5.1 0.278 surgery or radiotherapy nor should it be used in animals
12 5.5 0.294 with severe underlying multiple-organ dysfunction (or it
13 6.0 0.311 should be used cautiously, with a dose modification) because
14 6.4 0.326 this increases the risk of systemic toxicity.
15 6.9 0.342
16 7.4 0.356 MECHANISM OF ACTION OF
17 7.8 0.371 ANTICANCER DRUGS
18 8.2 0.385
19 8.7 0.399 The effects of anticancer drugs on a neoplastic cell popula-
20 9.2 0.413 tion follow first-order kinetic principles (i.e., the number of
cells killed by a drug or drug combination is directly propor-
tional to the dose used). These drugs kill a constant propor-
tion of cells rather than a constant number of cells. Therefore
INDICATIONS AND the efficacy of a drug or drug combination depends on the
CONTRAINDICATIONS OF number of cells in a given tumor (e.g., a drug combination
CHEMOTHERAPY that kills 99% of the cells in a tumor containing 100 million
9
[10 ] cells leaves 1 million [10 ] viable cells).
6
Chemotherapy is primarily indicated for animals with sys- As discussed in the following paragraphs, different types
temic (e.g., lymphoma, leukemias) or metastatic neoplasms, of anticancer drugs kill tumor cells by different mechanisms.
although it can also be used for the management of nonre- Drugs that kill only dividing tumor cells (i.e., that do not kill
sectable, chemoresponsive neoplasms that have historically cells in the G 0 phase) by acting on several phases of the cycle
proved refractory to radiotherapy (primary chemotherapy). are termed cell cycle phase-nonspecific drugs. Alkylating
It can also be used as an adjuvant treatment after partial agents belong to this group. Drugs that selectively kill tumor
surgical debulking of a neoplasm (e.g., partial excision of an cells during a given phase of the cell cycle are termed cell
undifferentiated sarcoma) and is indicated for the control of cycle phase-specific drugs. Most antimetabolites and plant
micrometastatic disease after the surgical excision of some alkaloids are phase-specific drugs. Finally, drugs that kill
primary neoplasms (e.g., carboplatin or doxorubicin therapy neoplastic cells regardless of their cycle status (i.e., they kill
after limb amputation in dogs with osteosarcoma; VAC or both dividing and resting cells) are termed cell cycle-
doxorubicin after splenectomy for dogs with hemangiosar- nonspecific drugs. These latter drugs are extremely myelosup-
coma). Chemotherapy can also be administered intracavi- pressive (e.g., nitrosoureas).
tarily in dogs and cats with malignant effusions or neoplastic
involvement of the cavity/area in question (e.g., intrapleu-
rally administered cisplatin or 5-fluorouracil in dogs with TYPES OF ANTICANCER DRUGS
pleural carcinomatosis). Finally, neoadjuvant or primary
chemotherapy is the approach used in animals with bulky Anticancer drugs are commonly classified into six categories
tumors not amenable to surgical excision or radiotherapy. (Box 76.1). Most of these drugs are currently available as
After the drugs cause the tumor to shrink, the tumor can be generic products at a reasonable cost.
surgically excised; chemotherapy is then continued to elimi- Alkylating agents cross-link DNA, thus preventing its
nate any residual neoplastic cells (e.g., VAC chemotherapy duplication. Because they mimic the effects of radiotherapy,
for dogs with subcutaneous hemangiosarcomas, multiple they are also referred to as radiomimetics. These drugs are
chemotherapy protocols for dogs with mast cell tumors). active during several phases of the cell cycle (i.e., they are