How, indeed, do the medicines we take find their way to their intended targets? As they course through our body, the medicines encounter and probably affect other cells. If their effect on normal cells is deleterious, then we have untoward "side effects." The ideal medication would be one that has an effect only on the target cells and no other.
There are medications that are very selective. For example, penicillin disrupts cell-wall formation in Gram-positive bacteria and since our cells do not have cell walls, our cells are spared the action of penicillin (although some of us develop an allergic reaction to it). But penicillin kills all Gram-positive bacteria – even the "good" ones. (Yes! Not all bacteria are bad. In fact, we harbor good bacteria in our gut, for example.)
So if we could identify a distinguishing feature in a cell that is diseased, we could focus on that feature and target that cell and other cells that have it. Some such "guided missiles" are currently being developed against cancer.
Cancer arises when a cell somehow escapes normal restraints – restraints that will cause the cell to die when it is time for it to die and keep it in the tissue or organ where it belongs. Instead, a cancerous cell proliferates (it keeps multiplying) uncontrollably and spreads to other parts of the body. Obviously, a cure for cancer would be one that kills such errant cells, or at least prevents their proliferation and spread. There are various procedures that are currently being used to accomplish that. One is by physical removal of the cancer cells, e.g. by surgery. Another is by blasting cancer cells with radiation. Yet another is by poisoning cancer cells with toxic drugs. A new technique that is being developed would deprive the cancer cells of nourishment so that they cannot grow. While these procedures have increased survival rates significantly, there are drawbacks. Surgery works very well when the cancerous tissue is localized and accessible, but not when the cancer has spread to too many parts of the body. Radiation treatment has to be very focused, otherwise too many of the normal cells in the vicinity will also be killed.
Similarly, treatment with anti-cancer drugs usually also kills any cell that shares the targeted characteristic (e.g. fast growth) of cancer cells. (The latter is the reason cancer patients undergoing chemotherapy usually lose their hair. A more serious side effect of chemotherapy is the concomitant killing of fast-growing cells in the bone marrow – cells that are essential components of our immune system – so that the patients become susceptible to infection.) In order to minimize collateral damage and other side effects, "molecular guided missiles" are being developed that target the cancer cells while sparing all others. This strategy works when a difference exists between cancer cells and normal cells. There are a number of known cases where that is true.
Chronic myeloid leukemia (CML) is a cancer of white blood cells. In most cases of CML, a chromosomal damage results in the production of an unusual molecule – an enzyme called bcr-abl – and it is this enzyme that causes the uncontrolled proliferation of the cells. A chemical compound has been designed that inserts itself, in a precise and tight way, into a groove in the enzyme and specifically inhibits the enzyme. The compound, marketed as Gleevec (Glivec in Europe), has been found to be effective in treating a very large number of CML patients.
Other molecules have been found that distinguish cancer cells from normal cells. These molecules, usually referred to as "tumor-associated antigens" or simply as "tumor markers," may be present only on the surface of cancer cells. Or more commonly, they are present in significantly larger amounts on cancer cells compared to normal cells – in the words of biologists, the molecules are "over-expressed" in cancer cells. Those surface tumor markers could be targeted by antibodies.
Antibodies are molecules of the immune system that bind tightly and specifically to their target molecules (antigens). The binding of antibodies to cancer cells can cause the lysis of those cells. Another way of killing cancer cells using a specific antibody is to attach a radionuclide to the antibody. The antibody would seek out the cancer cells and these would then be destroyed by radiation. Yet another way is to attach a highly toxic drug to the antibody and the internalization of the drug would then kill the targeted cancer cell. Alternatively, the antibody could be attached to a synthetic vesicle – a liposome, for example – that is filled with a toxic drug, or a combination of toxic drugs for greater effect.
Many antibody-based therapeutics are in the process of development and some have already been approved for human use. One is Rituxan, a chimeric (part murine/part human) antibody that is directed against CD20, a molecule that is over-expressed on the surface of B lymphocytes (a white blood cell) in patients with non-Hodgkin’s lymphoma. Other therapeutic antibodies that have been approved for the treatment of non-Hodgkin’s lymphoma are Zevalin and Bexxar, both radiolabeled murine antibodies (Zevalin with Yttrium-90, Bexxar with Iodine-131) and both also directed against CD20. Another therapeutic antibody is Herceptin, a "humanized" murine antibody that is directed against HER-2 (or erb B2), a molecule that is over-expressed on the surface of breast cancer cells in 35-40 percent of breast cancer patients. (Antibodies are more easily obtained from nonhuman sources, usually the mouse, and have to be made to look like human molecules so as not to be rejected by the patient’s immune system. To find out how this is done, read next week’s Star Science article on antibody humanization.)
Two antibodies have recently been approved for the treatment of colorectal cancer: Erbitux, a chimeric murine/human antibody directed against epidermal growth factor receptor (EGFR), and Avastin, a humanized murine antibody directed against vascular endothelial growth factor (VEGF). EGFR is important in transmitting signals from the outside to the inside of the cancer cell, instructing it to grow and multiply, while VEGF stimulates the growth of new blood vessels needed to support the fast-growing cancer cells. And so, Erbitux prevents the growth of the tumor and Avastin inhibits the growth of new blood vessels to the growing tumor, thereby "starving" the tumor. Other therapeutic antibodies in the pipeline are directed against other tumor markers.
An effective strategy of antibody-based therapy makes use of an antibody directly linked to a drug or linked to a drug delivery system that contains drugs. Myelotarg is an antibody conjugate used as a last resort in the treatment of Acute Myeloid Leukemia (AML). AML is the most serious of the leukemias and has the poorest prognosis. Myelotarg consists of a humanized antibody directed against the AML marker CD33 and that is linked to a highly toxic drug known as calicheamicin. For the treatment of breast cancer, researchers are developing an anti-HER2 antibody linked to a liposome that contains the anticancer drug doxorubicin.
A tumor marker that is a very promising target is TAG 72 (Tumor-Associated Glycoprotein No. 72), discovered by the group of Jeffrey Schlom at the National Cancer Institute of the US National Institutes of Health (NIH). TAG 72 is a "pancarcinomic" antigen, meaning it is present in many different cancer types and in a very high percentage of the cancer cells, but not in normal tissues (except in the endometrium and transitional mucosa of the gastrointestinal tract). A number of murine antibodies against TAG 72 have been generated; the one that is the most studied is "CC49." Various forms of CC49 have been constructed to improve its effectiveness. For example, I designed a ‘humanized’ version of CC49. Further, the effective affinity of CC49 for TAG 72 has been dramatically increased by the construction of a tetravalent version of the molecule (done by Dr. Ameurfina D. Santos of NIMBB, UP Diliman, while on a fellowship in my laboratory at the US NIH). (Antibody Engineering is the subject of another forthcoming article in Star Science.) Furthermore, improved versions of humanized CC49 have been constructed (by Dr. Noreen R. Gonzales-McCurdy, formerly of the Ateneo de Manila University and also a former fellow in my laboratory).
There is currently a group of scientists in the Philippines called AMOR that is exploring the use of CC49 against breast cancer. The AMOR program was conceptualized and organized in 1998 when I first met Dr. Gisela P. Concepcion, who works on anticancer marine natural products at the UP Marine Science Institute. (She coined the term AMOR which stands for "Antibody and Molecular Oncology Researchers.") I had proposed that we combine antitumor antibodies with anticancer marine natural products to act like molecular guided missiles. She invited key researchers in UP Diliman and other universities to work with her and form the AMOR team. In 2000, the Department of Science and Technology approved major funding for the six-year AMOR program. AMOR is now in its fifth year. AMOR is exploring the use of CC49 by itself to target and kill breast cancer cells. AMOR is also using CC49, combined with cytotoxic Philippine marine and plant natural products, to target TAG72 in breast cancer cells and kill those cells. (Await still another Star Science article on anticancer marine natural products.) To date, Dr. Concepcion appears to show some encouraging results in the testing of AMOR prototype products in animals. The hope is that one day, AMOR will produce an effective targeted therapy using local natural products that can be used in combination with surgery for the benefit of Filipino women afflicted with breast cancer.