Tumours are produced by mutations of normal cells which make them grow out of control. Broadly, chemotheraputic drugs work by selectively targeting fast-dividing cells. Being damaging to cells makes them cytotoxic. There are a variety of different drugs employed in chemotherapy. Some drugs cause cells to commit apoptosis (effectively cell suicide), but generally they all block some essential feature of the dividing cells. Unfortunately scientists have been unable to locate specific features of cancerous cells which would make them uniquely targetable, so other fast dividing cells such those responsible for hair? growth are also affected. However by chance some drugs seem to affect cells from different tissues more intensely than others, and this has been used to the advantage of some patients.
Because the drugs target fast dividing cells it is the tumours with high growth fractions which are more susceptible. This is the case for tumours resulting from leukemia, lymphoma? and [Hodgkins disease]?. The reason for this is that all the drugs can only affect cancerous cells which are at a certain stage in the cell cycle (see mitosis), this is why only a fraction of the cells are killed in each treatment. Chemotheraputic drugs affect younger tumours more effectively because as tumours get older their mass-doubling times increase - meaning their growth fraction has decreased and they are less susceptible. Some of the cells of solid tumours are not dividing at all, so the drugs would not touch these.
Larger tumours may also not be able to be cured by chemotherapy because the drugs would be unable to actually penetrate the the growths to kill cells inside.
Adjuvant chemotherapy is intended to kill any cancerous cells which have spread to other parts of the body. This is often effective as the newly growing tumours are fast-dividing, and therefore very susceptible.
The majority of chemotheraputic drugs can be divided in to: alkylating agents, anti-metabolites, plant alkaloids, and antitumour agents. As all of the drugs affects DNA synthesis or function in some way, they are categorized by their effect on the cell cycle.
Alkylating agents
[Alkylating agent]?s are so named because of their ability to add alkyl groups to many electronegative? groups under conditions present in cells. They stop tumour growth by cross-linking guanine bases in DNA double-helix strands - directly attacking DNA. This makes the strands unable to uncoil and separate. As this is necessary in DNA replication, the cells can no longer divide.
Anti-metabolites
These substances masquerade as purine or pyridimine? - the building blocks of DNA. They prevent these substances becoming incorporated in to DNA during the "S" phase (of mitosis), stopping normal development and reproduction.
Plant (vinca) alkaloids
These alkaloids are derived from plants and block cell division by preventing microtubules being synthesized. These are vital for cell division and without them it can not occur.
Antitumour antibiotics
There are many differing antitumour antibiotics, but generally they prevent cell division by two ways: (1) binding to DNA making it unable to separate (2) inhibiting ribonucleic acid (RNA), preventing enzyme synthesis.
Steroid hormones
These act only on certain hormone-dependant cancers, although their specific mechanism is still unclear.
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