The goal of cancer chemotherapy is to kill tumor cells in a cancer patient without causing any harm to the patient as a side effect of the treatment. The majority of conventional chemotherapies work by poisoning cancer cells. While these drugs are effective against cancer, they do have toxic side effects on non-cancer cells. As cancer scientists learn more about the inner workings of cancer cells, more specific, less toxic therapies will be developed and become available.
Drugs That Block DNA Replication
A number of cancer drugs kill cancer cells by interfering with the ability of cancer cells to reproduce DNA, an essential step of cancer cell growth. Such drugs include daunorubicin, doxorubicin--known commercially as Adriamycin--and actinomycin-D. These drugs often have toxic side effects as they prevent DNA replication in all cell types, not just cancer cells. The strategy of these drugs is to poison and kill fast-growing cancer cells while only having a minor toxicity effect on the more slowly dividing, normal cells in the rest of the patient. A specific class of cancer drugs known as topoisomerase inhibitors also prevents cancer DNA replication. Topoisomerase inhibitors block the ability of DNA to unwind itself during the DNA replication process. Topoisomerase inhibitor drugs include irinotecan (CPT-11), etoposide (VP-16) and mitoxantrone. These drugs are also toxic to non-cancerous cells and also increase a patient's risk of developing secondary cancers, such as leukemia. Many of these chemotherapies are described in depth by Tirgan Oncology Associates, a non-profit resource dedicated to cancer chemotherapy education.
Drugs That Damage DNA
In addition to preventing DNA synthesis and replication, other chemotherapy drugs simply cause irreversible damage to the DNA of cancer cells to block tumor cell growth. Unfortunately, like DNA synthesis inhibitors, these drugs are not very specific and can damage the DNA of normal cells, leading to bone marrow loss, decreased white blood cells and leukemia with long-term use. Despite their toxicities, DNA damaging agents can be highly effective against many cancer types, including lymphoma, breast cancer, lung cancer and ovarian cancers. The American Cancer Society lists a number of these cancer drugs, generally known as alkylating agents. These drugs include nitrogen mustards, such as mechlorethamine, chlorambucil, and cyclophosphamide; nitrosoureas, such as streptozocin, carmustine (BCNU) and lomustine; and the ethylamines thiotepa and altretamine.
Immunotherapies
Immunotherapeutics are more specific than conventional chemotherapeutics that attack DNA. Immunotherapies kill cancer through two general methods. The ideal active immunotherapy would be a cancer vaccine that would stimulate the body's immune system to attack and kill unwanted cancer cells. Such vaccines are in development and undergoing clinic trials. More often, however, immunotherapeutics consist of antibodies that are injected into cancer patients to specifically target and block essential protein components that cancer cells need to grow and proliferate. The majority of these drugs are monoclonal antibodies produced in a laboratory, denoted with the suffix "--mab". Such drugs include rituximab, alemtuzumab and bevacizumab.
Targeted Chemotherapies
Cancer researchers are uncovering a number of mechanisms specific to individual cancer types that may eventually lead to the development of drugs that target and kill only certain cancer types. If a cancer is known to have a single unique cancer gene, for instance, it may be possible to develop a drug specific for that gene to kill the cancer. This exciting area of cancer drug development is based upon the work of Dr. Brian Druker at the Knight Cancer Institute at Oregon Health & Science University. Druker and his colleagues discovered that the drug Gleevec specifically blocks an essential cancer gene in leukemias and other cancers. This research has essentially provided a cure for a specific type of cancer known as chronic myeloid leukemia.
