Anti-Cancer Drugs Used in Chemotherapy

'Cancer' is a diverse range of conditions characterised by new, unregulated growth of cells and spread of these abnormal cells to distant sites in the body. Chemotherapy is an attempt to treat cancers of various types by using chemical agents, namely specific medications and drugs.

The purpose of most chemotherapy agents is to interfere with DNA replication or division, preventing the cancerous cells from multiplying and spreading. These drugs target cells which are actively undergoing division. Many malignant cancers undergo division at a high rate, but so do many normal cell types such as intestinal cells, hair follicles, skin and bone marrow. These normal cells will also be affected by chemotherapy, producing most of the debilitating side effects of chemotherapy. For this reason chemotherapy is usually administered as large doses with some time between them, to allow normal cells in the body to recover.

Chemotherapy drugs are usually administered in a combination with other chemotherapy drugs that have different mechanisms of action to reduce the likelihood of resistance developing in the cancer. The main types of chemotherapy drugs are: alkylating agents, antimetabolites, cytotoxic antibiotics, plant derivatives and miscellaneous agents such as hormones.

Alkylating Agents: This class of drugs form strong bonds within the strands of DNA, damaging it so that the cell cannot replicate and dies. This class of drugs includes Cyclophosphamide, Chlorambucil, Cisplatin and mustard gas. It is not only cancerous cells which are affected, but normal cells too. This causes side effects which vary from drug to drug, but can include immunosuppression, nephrotoxicity and haemorrhagic cystitis.

Antimetabolites: These drugs take the place of normal molecules in biological reactions, stopping the reaction from proceeding and often damaging enzymes in the process. In chemotherapy, these antimetabolites typically replace molecules used to construct DNA. These molecules affect all cells, but because the cancerous cells are multiplying at a faster rate they are more affected by these drugs. Toxicity can still occur as immunosuppression, pancreatitis or hepatotoxicity.

Cytotoxic Antibiotics: These are a selection of antibiotics which are rarely used to treat infections because they are toxic to DNA. This makes them more useful for treating cancers.

Plant Derivatives: The Vicna alkaloids are isolated from periwinkle-myrtle plants, and act by binding to tubulin molecules and preventing cells from dividing. This only effects cells which are dividing, so normal cells (most of which have a low rate of division) are largely unaffected.

Hormones: Hormones can be used to treat some cancers, such as those of the reproductive tract, but this is variable because each cancer is slightly different and may or may not be responsive.