The Making of a Monoclonal Antibody
In cancer treatment today, three letters have infiltrated the oncology dialect: "mab," as in bevacizumab (Avastin), cetuximab (Erbitux), or trastuzumab (Herceptin). This suffix is reserved for monoclonal antibodies - agents that have become a growing component of the oncologist's arsenal.
Currently, eight monoclonal antibodies, or MAbs, are approved by the FDA to treat cancer. And many more are being tested in phase II and III clinical trials for a variety of tumor types.
To understand how these drugs work, one must take a careful look at the human immune system. The immune system comprises a diverse collection of specialized cell types that circulate through the bloodstream and lymphatic system, and monitors the body for bacterial, viral, fungal, and parasitic invaders. The "innate" immune response provides the first line of defense, and includes cells such as granulocytes, macrophages, and dendritic cells, which are always ready to recognize and attack foreign particles in the body. For persistent or recurrent infections, the sophisticated "adaptive" immune response comes into play.
Dendritic cells (DCs) form the bridge between the innate immune response and the antibodies of the adaptive immune response. When DCs digest a pathogen, they save and display foreign proteins found on the surface of the pathogen - known as antigens - for the lymphocytes of the adaptive immune system to recognize. When a B lymphocyte (also called a B cell) recognizes an antigen displayed by a DC as foreign, it changes into a plasma cell that secretes antibodies - small proteins that recognize and bind to the antigen, either interfering with the invader's ability to attack the cells of the body or marking them for destruction by other immune cells.
Cancer cells exhibit many of the behaviors of foreign pathogens, such as invading and damaging healthy tissue. However, explains Dr. Robert Kreitman of NCI's CCR, "In general, the immune system recognizes cancer cells as self and does not attack them as foreign antigens." This is because they carry many of the same surface proteins as normal cells, which mark them as part of the body. The development of MAbs circumvented this obstacle.
To create an MAb, researchers first identify a protein that is overexpressed on the surface of cancer cells and that the cells depend on to survive or grow. Trastuzumab, for example, is used to treat breast cancers that overexpress a protein called HER2, and rituximab (Rituxan) is used to treat blood cancers that overexpress the protein CD20. The more important the protein is to the cancer cell, the more effective the MAb will be as a therapy.
"Successful targeting of the HER2 protein has been the reason why Herceptin has been so effective," explains Dr. Charles Geyer, the National Surgical Adjuvant Breast and Bowel Project's director of Medical Affairs. "HER2 gene amplification and the resultant abnormal protein production clearly play a very important, central role in the aggressive malignant phenotype of HER2-positive breast cancers, and Herceptin appears to shut down the activity of the HER2 protein."
Once a target protein is isolated, it is injected into mice. Unlike the human immune system, the mouse immune system recognizes the protein as foreign and produces antibodies to fight it. These mouse antibodies are then isolated, but cannot be used directly as a therapy because they would be recognized as foreign and attacked as invaders if injected into the human body. So they must first go through a process called humanization.
In humanization, the variable domain of the mouse antibody - the very small portion that recognizes the human protein - is removed and grafted onto a human antibody. When introduced back into the human body, the MAb recognizes and binds to the target protein, disrupting a vital signaling pathway in the cancer cell.
Some MAbs are further engineered to carry a toxin or radionuclide, to increase the damage done after the MAb binds to a cancer cell, such as 131 I tositumomab (Bexxar), which is used to treat B-cell lymphoma. Because the MAbs bind predominantly to cancer cells, very few normal cells are damaged either by the antibodies or by their toxic conjugates.
Another class of therapeutic drugs called small - molecule inhibitors also target proteins on the surface of cancer cells. But MAbs are distinct from these agents because they also may play a role in activating an immune response called antibody-dependent cellular cytotoxicity. Researchers are now trying to better understand how MAbs interact with the rest of the immune system and how they can best be used with other treatments to maximize their effectiveness as an anticancer therapy.
"I think that discovering better combinations with other agents and the timing of how to give them to get the best response will be the future," says Dr. Kreitman. "In the 1950s and 1960s, we had chemotherapy drugs, but we didn't know how to combine them, so diseases like Hodgkin's disease were not curable. But as people learned how to combine drugs and do it safely… diseases started becoming curable. I think the same will happen with monoclonal antibodies and other agents."
By Sharon Reynolds