Most anticancer drugs belong to one of three general types: cytotoxic, targeted, or immunotherapy. Cytotoxic drugs kill all types of cells since the drugs cannot distinguish cancer cells from healthy cells. This group includes most traditional chemotherapy drugs. Targeted agents, a newer type, interact with and disrupt the activity of specific molecules that promote the growth of cancer cells. Immunotherapy drugs stimulate the body’s immune system to fight cancer.
For patients, the main difference in the types of drugs is their side effects. Because cytotoxic drugs affect all the cells in the body, these drugs may have severe side effects. Not all cytotoxic drugs cause the same side effects, but hair loss, nausea, and fatigue are common side effects of some of these drugs. Some cytotoxic drugs can cause nerve damage. Others cause difficulty with concentration and memory. In contrast, targeted drugs affect mostly cancer cells and tend to have less severe side effects, which may include fever, diarrhea, problems with blood clotting and wound healing, skin problems, high blood pressure, or liver problems. And some immunotherapies may cause flu-like symptoms, rashes, or a drop in blood pressure.
Cytotoxic drugs kill cancer cells or damage them so that they cannot replicate (divide into two copies). Cytotoxic agents are categorized according to their function, structure, and relationship with other therapies. It is important to note that some drugs can be classified in more than one of the groups below.
- Alkylating agents damage cells’ DNA so that the cells cannot replicate.
- Antimetabolites substitute for the building blocks of RNA or DNA and interfere with RNA or DNA replication.
- Antitumor antibiotics attack the DNA of cancer cells so that the cells cannot continue to grow and replicate.
- Topoisomerase inhibitors disrupt the activities of enzymes (a type of protein) that aid in the separation of DNA strands during DNA replication.
- Mitotic inhibitors stop mitosis, a process where a cell splits into two daughter cells.
Targeted therapies, also known as molecularly targeted agents or precision medicines, are designed to block the actions of a specific target—usually a protein or gene that is directly or indirectly involved in the growth and progression of cancer. There are two main forms of targeted therapies, small-molecule drugs and monoclonal antibodies, and each form targets cancer cells in a different way.
Small-molecule drugs have the ability to enter cells and interact with target proteins within the cells. Many small-molecule drugs have generic names that end in –ib.
Monoclonal antibody drugs function like natural antibodies, which are proteins that bind to target proteins on the surfaces of bacteria, viruses, or cancer cells so that the body’s immune system can locate and destroy the invading cells. Monoclonal antibody drugs have generic names that end in –mab.
Targeted drugs can also be sorted into different types according to their function. As cancer research progresses, the number of types increases. A few common types of targeted drugs are listed below.
- Signal transduction inhibitors block enzymes that signal cancer cells to replicate.
- Apoptosis inducers cause controlled cell death in cancer cells.
- Angiogenesis inhibitors block the formation of new blood vessels to tumors.
- Hormone therapies stop the body from making hormones that certain types of cancer need to survive. Although these drugs work against a particular hormone, most do not target a specific molecule the way small-molecule drugs and monoclonal antibodies do.
- Antibody-drug conjugates are drugs in which a molecule of a cytotoxic drug or radioactive agent is linked to a monoclonal antibody that targets cancer cells.
Immunotherapies help the body’s natural immune system fight cancer. Some targeted drugs, such as monoclonal antibodies, are also considered immunotherapy drugs because they stimulate an immune response against a specific protein. Other immunotherapies stimulate a general response by the immune system. Two common types of these nonspecific immunotherapies are listed below.
- Interferons boost the ability of the body’s immune cells to fight cancer or infections.
- Interleukins help immune cells grow and replicate more quickly.
Cytotoxic, targeted, and immunotherapy drugs can be used with each other and other forms of cancer treatment, such as surgery and radiation therapy. Since cancer cells can develop a resistance to cytotoxic or targeted therapies, such combinations of treatments are often recommended.
For more information, ask your physician, visit www.mdanderson.org, or call askMDAnderson at 877-632-6789.
OncoLog, September 2015, Volume 60, Issue 9