Mutations of normal cells creates cancerous tumours which can grow out of control. Broadly, chemotheraputic drugs work by selectively targeting these fast-dividing cells. As they cause damage to cells they are termed cytotoxic. Some drugs cause cells to commit apoptosis (effectively cell suicide), but chemotheraputic drugs all block some essential feature of the cell division process which makes cells unable to reproduce.
Unfortunately scientists have been unable to locate specific features of cancerous cells to 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 can be used to the advantage of patients in certain situations.
Because the drugs target fast-dividing cells it is the tumours with high growth fractions which are more susceptible to being killed. Tumours such as those resulting from leukemia, lymphoma and Hodgkins disease are examples of these fast-divinding cells. The explanation is that drugs can only affect cancerous cells when they are at a certain stage in the cell cycle. As only a fraction of the cells are ever at this stage, the faster growing the tumour the more likely a high fraction of its cells are affected.
Chemotheraputic drugs affect younger tumours more effectively because as tumours get older their growth slows down. Some of the cells in solid tumours are not dividing at all, so no chemotheraputic drugs touch these. Drugs can often can not even penetrate the insides to work there.
Types of drugs
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 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.
These substances masquerade as purine or pyrimidine - which become the building blocks of DNA. They prevent these substances becoming incorporated in to DNA during the "S" phase (of the cell cycle), 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.
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.
These act only on certain hormone-dependant cancers, although their specific mechanism is still unclear.
There are a number of strategies in the administration of chemotheraputic drugs used today. Combined Modality Chemotherapy is the use of drugs with other treatments, such as radiation therapy or surgery. Most cancers are now treated in this way. Combination Chemotherapy is a similar practice which involves treating a patient with a number of different drugs simultaneously. The drugs differ in their mechanism and side effects. The biggest advantage is minimising the chances of resistance developing to any one agent. Adjuvant Chemotherapy can be used when there is little evidence of cancer present, but there is risk of recurrence. This can help reduce chances of resistance developing if the tumour does develop.It is also useful in killing 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.
Current chemotheraputic techniques have a range of side effects mainly affecting the fast-dividing cells of the body. These include the mouth, digestive system, skin, hair and bone marrow. The treatment can be exhausting physically for the patient. For some patients almost all the bone marrow stem cells (cells which produce white and red blood cells for the rest of the body) are destroyed. In these cases bone marrow cell transplants are necessary, where cells are removed before the treatment, multiplied and then re-injected afterwards (see stem cells for more on treatment). However some patients still develop diseases such as fungal tuberculosis because of this interference with bone marrow.