How do chemotherapy drugs work?
The object of all chemotherapy drugs is to kill the cancerous cells and not harm the adjacent healthy cells. To that end, scientists tried to identify characteristics that are unique to cancer cells and are not found on normal tissue. A distinct cancer trait could serve as a potential target for a chemotherapy drugs and thereby fulfill this goal. One feature that is truly unique for most cancer cells is that they grow at a rate faster than normal cells. Therefore targeting some aspect of the cell growth cycle seems reasonable. Fast growing cells would be affected the most and slow growing cells would be least disturbed. In fact, that is the basis for many chemotherapeutics. This seems obvious when considering the side effect profiles of most chemotherapy drugs. Hair follicles, skin, and the cells that line the gastrointestinal tract are some of the fastest growing cells in the human body, and therefore are most sensitive to the effects of chemotherapy. It is for this reason that patients may experience hair loss, diarrhea, and rashes.
The human body processes and excretes all drugs through either the liver or the kidneys. Therefore, when a patient has kidney or liver damage, giving chemotherapy becomes a balancing act. Administering the recommended amount of drug may prove to be too toxic in a patient unable to metabolize and excrete it. The pharmacokinetics (a branch of pharmacology concerned with the rate at which drugs are absorbed, distributed, metabolized and eliminated by the body) for cancer patients are very complex and chemotherapy pharmacology is a subspecialty on its own. Unfortunately, kidney and liver damage often result due to cancer invasion, limiting the patient's chemotherapy options.
Pharmacokinetics is further complicated in the cancer patient, as they are often taking multiple medications, some of which have overlapping metabolic pathways and side effect profiles. An example of this difficult situation is in the brain cancer patient. Because brain tumors often present as seizures, many of these patients take anti-seizure medications. Anti-seizure medications are metabolized by the liver and affect the metabolism of many chemotherapy drugs. Dose adjustments are an absolute necessity to avoid toxicities or sub-therapeutic dosing.
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