The most successful and widely used treatment for cancer is surgery. While physically removing a tumor is optimal, it is often difficult for a surgeon to get all of the tumor cells. (A tumor about 1 centimeter in diameter already has a billion cells!) Tumors are generally embedded in normal tissues. Added to this is the probability that cells of the tumor may have broken off and spread to other organs. This makes it unlikely that localized surgery will be curative. So other approaches are taken to treat or cure cancer, and these generally target the cell cycle (Figure 11.25). The goal is to decrease the rate of cell division and/or increase the rate of apoptosis so that the cancer cell population decreases.

Question

The cell cycle treatments affect the cell cycles of all dividing cells in the body, not just those of the tumor cells. By contrast, targeted drugs affect altered proteins present only in tumor cells. The side effects of the general cell cycle drugs would be on organs and systems that rely on dividing cells. For example, blood cells undergo apoptosis after a period in the bloodstream and must be replaced by dividing cells; if division in these cells is blocked by an anticancer drug, the patient may develop side effects such as poor immunity (too few white blood cells) and anemia (too few red blood cells).

An example of a cancer drug that targets the cell cycle is 5-fluorouracil, which blocks the synthesis of thymine, one of the four bases in DNA. The drug paclitaxel prevents the functioning of microtubules in the mitotic spindle. Both drugs inhibit the cell cycle, and apoptosis causes tumor shrinkage. More dramatic is radiation treatment, in which a beam of high-energy radiation is focused on the tumor. DNA damage is extensive, and the cell cycle checkpoint for DNA repair is overwhelmed. As a result, the cell undergoes apoptosis. A major problem with these treatments is that they affect normal cells as well as the tumor cells. These treatments are toxic to tissues with large populations of normal dividing cells such as those in the intestine, skin, and bone marrow (where blood cells are produced).

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A major effort in cancer research is to find treatments that target only cancer cells. A promising recent example is Herceptin, which targets the HER2 growth factor receptor that occurs at high levels on the surfaces of some breast cancer cells (see Figure 11.24A). Herceptin binds specifically to the HER2 receptor but does not stimulate it. This prevents the natural growth factor from binding and stimulating cell division. Because cell division is blocked and the apoptosis rate remains the same, the tumor shrinks.