Chemotherapy Types

Cancer chemotherapy agents are classified according to their chemical nature and function. Some of the well known classes of cancer chemotherapy agents include alkylating agents, plant alkaloids, antimetabolites, anthracyclines, topoisomerase inhibitors and corticosteroids.

Normal cell cycle

All cells form through the division of their parent cells, a procedure called mitosis. The cell cycle can be divided into five phases:

Gap 0 phase - This is the resting phase, when the cell is not dividing. The cell only comes out of this phase when it receives the signal to divide.

Gap 1 phase - The cell enlarges and makes proteins and mRNA to ensure it is ready for the steps that lead to mitosis.

S or synthesis phase - During this phase, DNA replication produces two new copies of DNA that can be passed onto the daughter cells produced by mitosis.

Gap 2 phase - This phase is the period between the synthesis stage and the mitosis stage during which the cell continues to grow. A checkpoint control mechanism ensures that the cell is ready to progress to mitosis.

M phase or mitosis - The cell no longer grows and begins to divide into two daughter cells. Another checkpoint mechanism ensures the cell is ready to completely divide.

Chemotherapy agents act on different phases of the cell cycle. Alkylating agents, for example, inhibit the cell cycle at the mitosis stage to stop the production of new cells. Some of the chemotherapy agents and their functions include:

Alkylating agents

These agents damage DNA and prevent mitosis. They are used to treat leukemia, lymphomas, multiple myeloma, sarcoma and lung, breast, and ovarian cancer. Examples of drugs in this class include:

• Nitrogen mustards such as chlorambucil, cyclophosphamide, ifosfamide, and melphalan
• Alkylsulfonates such as busulfan
• Nitrosoureas such as streptozotocin, carmustine, and lomustine
• Triazines such as dacarbazine
• Ethylenimines such as thiotepa and altretamine
• Platinum drugs such as cisplatin, carboplatin and oxalaplatin

Antimetabolites

These agents interrupt the S phase and substitute normal DNA and RNA with other amino acids, thereby interfering with cell replication and proliferation. Antimetabolites are used to treat leukemia and cancers of the breast and ovary.

Examples of drugs in this class include 5-fluorouracil (5-FU), 6-mercaptopurine (6-MP), cytarabine, capecitabine, fludarabine, gemcitabine, methotrexate, pemetrexed, pentostatin and thioguanine.

Anthracyclines

Anthracyclines are anti-tumor antibiotics that inhibit the enzymes that bring about DNA replication. Examples of drugs in this class include doxorubicin, daunorubicin, idarubicin and epirubicin.

Topoisomerase inhibitors

These agents inhibit the enzyme topoisomerase which would usually help untangle DNA strands so they can be replicated. They are used to treat leukemia and lung, ovarian and gut cancer. Examples include topotecan, irinotecan, etoposide and teniposide.

Plant alkaloids

Also called mitotic inhibitors, these drugs interrupt the M phase of the cell cycle and inhibit mitosis. They are used to treat breast and lung cancers and myeloma, lymphoma, and leukemia. Examples of drugs in this class include taxanes such as paclitaxel and docetaxel, vinca alkaloids such as vinblastine, vincristine and vinorelbine.

Corticosteroids

This drug class includes naturally occurring hormones such as the steroid hormones as well as artificially synthesized analogues of these hormones. They are used to treat lymphoma, leukemia and multiple myeloma and examples include prednisone, methylprednisolone and dexamethasone.