Chemotherapy involves the use of drugs to destroy cancer cells. Although an ideal drug would destroy cancer cells without harming normal cells, most drugs are not that selective. Instead, drugs are designed to inflict greater damage on cancer cells than on normal cells, typically by using drugs that affect a cell's ability to grow. Uncontrolled and rapid growth is characteristic of cancer cells. However, because normal cells also need to grow, and some grow quite rapidly (such as those in the bone marrow and those lining the mouth and intestine), all chemotherapy drugs affect normal cells and cause side effects.
Although a single chemotherapy drug may be effective against some types of cancer, often doctors give several chemotherapy drugs at the same time (combination chemotherapy—see Combination Cancer Therapy).
One newer approach to limiting side effects and increasing effectiveness uses a variety of "molecularly targeted" drugs. These drugs kill cancer cells by attacking specific pathways and processes vital to the cancer cells’ survival and growth. For example, cancer cells need blood vessels to provide nutrients and oxygen. Some drugs can block blood vessel formation to cancer cells or the master signaling pathways that control cell growth. Imatinib, the first such drug, is highly effective in chronic myelocytic leukemia and certain cancers of the digestive tract. Erlotinib and gefitinib target receptors located on the surface of cells in non–small cell lung cancer. Molecularly targeted drugs have proven useful in treating many other cancers, including breast and kidney cancers.
Not all cancers respond to chemotherapy. The type of cancer determines which drugs are used, in what combination, and at what dose. Chemotherapy may be used as the sole treatment or combined with radiation therapy, surgery, or both.