Anticancer drugs in dogs and cats

The effects of anticancer drugs in dogs and cats on a neoplastic cell population follow first-order kinetic principles (i.e., the number of cells killed by a drug or drug combination is directly proportional to one variable-the dose used). These drugs kill a constant proportion of cells, rather than a constant number of cells. Therefore the efficacy of a drug or drug combination depends on the number of cells in a given tumor (e.g., a drug combination that kills 99% of a cell in a tumor containing 100,000,000 cells leaves 1,000,000 viable cells).

Different types of anticancer drugs kill tumor cells by different mechanisms. Drugs that kill only dividing tumor cells (i.e., that do not kill cells in the G0 phase) by acting on several phases of the cycle are termed cell cycle phase-nonspecific drugs. Alkylating agents belong to this group. Drugs that selectively kill tumor cells during a given phase of the cell cycle are termed cell cycle phase-specific drugs.

Most antimetabolites and plant alkaloids are phase-specific drugs. Finally, drugs that kill neoplastic cells regardless of their cycle status (i.e., they kill both dividing and resting cells) are termed cell cycle-nonspecific drugs. These latter drugs are extremely myelosuppressive (e.g., nitrosoureas) and are infrequently used in veterinary medicine.

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