Small molecule compounds are medicines that interfere with the molecular process of cancer cells that can cause them to die or stop their growth. Many of these medicines can be taken by mouth.

One example of a small molecule compound is imatinib. It's used to treat a rare stomach cancer called gastrointestinal stromal tumor (GIST) and certain types of leukemia.

Chronic myelogenous leukemia (CML) is an unusual type of cancer in that only one molecular defect is needed to turn a normal cell into a cancerous one. The problem arises when two genes fuse together and make an abnormal protein. This protein sends a signal to the cell that tells it to grow in an uncontrolled manner. Imatinib controls the growth of CML tumors by preventing the abnormal protein from signaling the cancer cells to grow.

Imatinib also works against other tumors that have defects in proteins similar to the one involved in CML.

Many other small molecularly targeted therapies are being created. Erlotinib was approved by the FDA to treat metastatic non-small cell lung cancer and pancreatic cancer in certain cases or tumor types. Erlotinib works by blocking a protein called the epidermal growth factor receptor (EGFR) from signaling the cell to grow. For this reason, it is classified as an EGFR inhibitor. EGFR is made in large amounts by many tumors, such as those found in lung, breast, head and neck, pancreas, and colon cancer. But tumors that have certain mutations in EGFR are treated best with erlotinib. In advanced stage lung cancer, for example, EGFR mutated lung cancer is better treated with erlotinib than with standard (nontargeted) chemotherapy. Whether a lung cancer has this EGFR mutation can be learned by a special molecular test done on the tumor (biopsy). Because other molecular defects cause these cancers, more than just one medicine will likely be needed to control or destroy these tumors.

The second category of targeted therapies is monoclonal antibodies. Antibodies are normal parts of the immune system that help rid the body of foreign invaders or infectious germs such as bacteria. Antibodies recognize abnormal surface patterns or antigens on the invader.

Antibodies trigger the body's immune response to an invader. They are programmed to remember previous invaders so that they can quickly destroy them if they attack the body again.

Monoclonal antibodies are made in a lab. They work in a similar way to the body's natural antibodies. They find and bind to antigens found on cancer cells and get rid of them. Monoclonal antibodies can be used alone to stimulate an immune response. Or they can be used to deliver medicines, toxins, or radioactive material directly to a tumor.

Here are a few monoclonal antibody therapies approved by the FDA:

• Bevacizumab has been approved the first treatment for metastatic colorectal cancer, meaning the cancer has spread. It also used to treat other tumors, including certain lung, brain and kidney cancers. It is the first medicine to be approved that works by targeting angiogenesis,. This is the formation of new blood vessels to the tumor.  

• Cetuximab has been approved for some cases of head and neck cancers, and colorectal cancers. It is thought to work by targeting the epidermal growth factor receptor (EGFR) on the surface of cancer cells.

• Trastuzumab is used to treat breast cancer. This medicine targets a protein on breast cancer cells called the human epidermal growth factor receptor-2 (HER-2). Trastuzumab only works against breast tumors that make too much HER-2 protein. It has also been approved to treat some gastric and gastroesophageal junction tumors.

• Rituximab is used to treat B-cell non-Hodgkin lymphomas that carry a protein called CD20.

• Ibritumomab tiuxetan binds to the same CD20 target that Rituximab does, so it's used to treat the same types of cancer. But ibritumomab carries an additional punch because its monoclonal antibody is tied to a radioactive compound called yttrium-90, which can kill cancer cells.  By delivering this damaging compound directly to the tumor, ibritumomab allows larger and more deadly doses of the radioactive medicine to reach the tumor. But it keeps any damage to healthy cells to a minimum.

• Ofatumumab is used to treat chronic lymphocytic leukemia. It is directed against the CD20 cell surface antigen.

• Ipilimumab is used to treat advanced melanoma. It is directed against a T-lymphocyte-associated antigen-4 (CTLA-4), located on the surface of activated T cells.