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Chemotherapy is a general term for treatments that use chemical agents (drugs) to kill cancer cells. Many different kinds of drugs are used, either alone or in combination, to treat different cancers. The specific drug or combination used is chosen to best combat the type and extent of cancer present.
Chemotherapy drugs are given for several reasons:
- To treat cancers that respond well to chemotherapy
- To decrease the size of tumors for easier and safer removal by surgery
- To enhance the cancer-killing effectiveness of other treatments, such as radiation therapy
- In higher dosages, to overcome the resistance of cancer cells
- To control the cancer and enhance the patient's quality of life
Normally, cells live, grow and die in a predictable way. Cancer occurs when certain cells in the body keep dividing and forming more cells without the ability to stop this process. Chemotherapy protocols involve destroying cancer cells by keeping the cells from further multiplying. Unfortunately, in the process of undergoing chemotherapy protocols, healthy cells can also be affected, especially those that naturally should divide quickly.
Chemotherapy protocols strive to maximize the elimination of cancer cells while minimizing the negative effects that these protocols have on healthy cells. Much progress in developing successful chemotherapy protocols has been made, including the identification of many different types of cancer and the corresponding development of effective chemotherapy protocol solutions. But there is still much work to be done.
The discussion of chemotherapy protocols is incomplete without a thorough understanding of additional topics regarding chemotherapy.
Each chemotherapy drug works in a different way to prevent cells from growing. Often a combination of drugs will be used, with each drug attacking the cancer cells in a different way. This decreases the possibility that cancer cells will survive, become resistant and continue to grow.
Chemotherapy is given in different ways depending on the cancer type and the drugs used.
Methods of giving chemotherapy drugs include:
Intravenously (IV) – injected into a vein
Intrathecally (IT) – injected into the spinal canal during a lumbar puncture
Intramuscular (IM) – injected into a muscle
Intraperitoneal (IP) – injected into the abdominal cavity
Intracavitary (IC) – injected into a body cavity
Subcutaneous (sub.q.) – injected just under the skin
Oral (PO) – as a pill or a liquid to be swallowed
Alkylating agents
Alkylating agents are most active in the resting phase of the cell. These types of drugs are cell-cycle non-specific. There are several types of alkylating agents used in chemotherapy treatments:
Mustard gas derivatives: Mechlorethamine, Cyclophosphamide, Chlorambucil, Melphalan, and Ifosfamide.
Ethylenimines: Thiotepa and Hexamethylmelamine.
Alkylsulfonates: Busulfan.
Hydrazines and Triazines: Altretamine, Procarbazine, Dacarbazine and Temozolomide.
Nitrosureas: Carmustine, Lomustine and Streptozocin. Nitrosureas are unique because, unlike most types of chemo treatments, they can cross the blood-brain barrier. They can be useful in treating brain tumors.
Metal salts: Carboplatin, Cisplatin, and Oxaliplatin.
Plant Alkaloids
Plant alkaloids are chemotherapy treatments derived made from certain types of plants. The vinca alkaloids are made from the periwinkle plant (catharanthus rosea). The taxanes are made from the bark of the Pacific Yew tree (taxus). The vinca alkaloids and taxanes are also known as antimicrotubule agents. The podophyllotoxins are derived from the May apple plant. Camptothecan analogs are derived from the Asian "Happy Tree" (Camptotheca acuminata). Podophyllotoxins and camptothecan analogs are also known as topoisomerase inhibitors, which are used in certain types of chemotherapy. The plant alkaloids are cell-cycle specific. This means they attack the cells during various phases of division.
Vinca alkaloids: Vincristine, Vinblastine and Vinorelbine.
Taxanes: Paclitaxel and Docetaxel.
Podophyllotoxins: Etoposide and Tenisopide.
Camptothecan analogs: Irinotecan and Topotecan.
Antitumor Antibiotics
Antitumor antibiotics are chemo treatments made from natural products produced by species of the soil fungus Streptomyces. These drugs act during multiple phases of the cell cycle and are considered cell-cycle specific. There are several types of antitumor antibiotics:
Anthracyclines: Doxorubicin, Daunorubicin, Epirubicin, Mitoxantrone, and Idarubicin.
Chromomycins: Dactinomycin and Plicamycin.
Miscellaneous: Mitomycin and Bleomycin.
Antimetabolites
Antimetabolites are types of chemotherapy treatments that are very similar to normal substances within the cell. When the cells incorporate these substances into the cellular metabolism, they are unable to divide. Antimetabolites are cell-cycle specific. They attack cells at very specific phases in the cycle. Antimetabolites are classified according to the substances with which they interfere.
Folic acid antagonist: Methotrexate.
Pyrimidine antagonist: 5-Fluorouracil, Foxuridine, Cytarabine, Capecitabine, and Gemcitabine.
Purine antagonist: 6-Mercaptopurine and 6-Thioguanine.
Adenosine deaminase inhibitor: Cladribine, Fludarabine, Nelarabine and Pentostatin.
Topoisomerase inhibitors
Toposiomerase inhibitors are types of chemotherapy drugs that interfere with the action of topoisomerase enzymes (topoisomerase I and II). During the process of chemo treatments, topoisomerase enzymes control the manipulation of the structure of DNA necessary for replication.
Topoisomerase I inhibitors: Ironotecan, topotecan
Topoisomerase II inhibitors: Amsacrine, etoposide, etoposide phosphate, teniposide
Miscellaneous Antineoplastics
Several useful types of chemotherapy drugs are unique:
Ribonucleotide reductase inhibitor: Hydroxyurea.
Adrenocortical steroid inhibitor: Mitotane
Enzymes: Asparaginase and Pegaspargase.
Antimicrotubule agent: Estramustine
Retinoids: Bexarotene, Isotretinoin, Tretinoin (ATRA)
Why are there side effects from chemotherapy?
Chemotherapy drugs target rapidly dividing cells, including normal ones. When normal cells are damaged, it can cause side effects. But normal cells can repair the damage or be replaced by other healthy cells, which is why side effects are usually temporary.
Factors influencing side effects include:
The specific chemotherapy drug
The dose of the drug
The health of the patient
Despite monitoring the effects of chemotherapy very closely, some long-term effects can occur, sometimes years after therapy is completed. Therefore, it is important that every patient be followed throughout his or her life by a physician who is aware of the late effects of treatment.
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