How Do Targeted Therapies Work?

Targeted cancer therapies interfere with cancer cell growth and division in several ways and at various points during the development, growth, and spread of cancer. By blocking cellular communication that tells cancer to grow and divide uncontrollably, targeted cancer therapies can help to stop the growth and division of cancer cells.

Categories of Targeted Therapies:

The two main categories of targeted therapy are the following:

• Small molecules: In pharmacology, a small molecule is a small organic compound that is biologically active and not a protein. Most drugs are small molecules.
• Monoclonal antibodies (mAb): These are proteins that bind specific targets called antigens. Monoclonal antibodies are engineered and grown in laboratories to bind only to cancer cells-specific antigens and induce an immunological response against the target cancer cell. When used as medications, the generic name ends with "mab" (example Bevacizumab).

Monoclonal antibodies are identical to each other because they are produced by one type of immune cell that are all clones of a single parent cell. Given (almost) any substance, it is possible to create monoclonal antibodies that specifically bind to that substance. This has become an important tool in medicine.

Once monoclonal antibodies for a given substance have been produced, they can be used to detect the presence and quantity of this substance, such as in a:

1. Western blot test (used to detect a protein on a membrane of a cell or within the cell itself).
2. Immunofluorescence test (used to detect a substance inside a cell).

Different Classes of Targeted Therapies

Within the two categories of small molecules and monoclonal antibodies are different classes of targeted therapies. The names of the major classes of targeted therapies typically include the word "anti-" or "inhibitor", together with the name of the target itself, which means that the drug used blocks that particular target.

It is important to realize that a single drug can have several names, including a generic name and a brand name. This can be confusing, because the generic and brand names are used interchangeably in the literature and by the media. We will list the brand name first often followed by the generic name.

Tyrosine Kinase Receptor Inhibitors

A tyrosine kinase receptor is a molecular structure or site on the surface of a cell that binds with substances such as ligands. Hormones, antigens, drugs, growth factors and neurotransmitters are examples of ligands. Ligands function as cell signaling molecules by binding to receptors on the surface of the cell. Receptors and ligands are necessary for the cells to function and interact with it's environment.

When the receptor binds with one of these ligands, a chemical reaction occurs which, in turn, triggers a series of reactions inside the cell. These reactions ultimately influence cell division, survival, growth, maturation, death, and migration.

These pathways are critical for normal cell activity and are very well regulated by the body. In tumor cells, regulation necessary for normal activity has been altered, the pathways become abnormally triggered which then allows tumor cells to survive, thrive, and spread all over the body. The goal of blocking the receptor is to prevent the cascade of reactions and, thus, prevent tumor survival.

	Gleevec (Small Molecule), generic name - Imatinib mesylate Gleevec, for example, is tailored to halt the action of an abnormal, cancercausing protein in chronic myeloid leukemia cells.
Image and explanation courtesy of Ohio State

(1) This protein is called Bcr-Abl.

(2) It first binds with a molecule of ATP, which transfers the energy to the cancer protein.

(3) The activated Bcr-Abl then binds with a signaling protein, GRB-2, and transfers its energy to that molecule.

(4) Gleevec is designed to bind with Bcr-Abl, which keeps out the ATP and inhibits the activation of GRB-2.

Gleevec was the first member of a new class of agents that act by inhibiting particular tyrosine kinase enzymes, instead of non-specifically inhibiting rapidly dividing cells. Gleevec received FDA approval in May 2001. That same month, it made the cover of TIME magazine as the "magic bullet" that could cure cancer.

EGFR (HER1/neu) inhibitors

The epidermal growth factor receptor EGFR (ErbB-1, HER1 in humans) is the cell-surface receptor for members of the epidermal growth factor family (EGF-family) of extracellular protein ligands.

Mutations that lead to EGFR overexpression (known as upregulation) or overactivity have been associated with a number of cancers, including lung, colon, anal and glioblastoma multiforme. Mutations, amplifications or misregulations of EGFR or family members are implicated in about 30% of all epithelial cancers.

The identification of EGFR as an oncogene has led to the development of anticancer therapeutics directed against EGFR, including gefitinib and erlotinib for lung cancer, and cetuximab for colon cancer.

• Iressa (Small Molecule) - generic name Gefitinib
• Tarceva (Small Molecule) - generic name Erlotinib
• Erbitux (monoclonal antibody) - generic name Cetuximab