How do different cancer fighting drugs kill cancer cells by interrupting the cell cycle?

How does chemotherapy work?
Normal cells in the body are constantly dividing to produce new cells in order to replace cells which are damaged or worn out. This process of normal cell division is very precisely controlled so that the number of new cells produced exactly matches the number of cells that need to be replaced.

A cancer develops when a cell, or group of cells, escape from the normal control process and begin to divide and multiply in a random fashion which leads to an excessive number of cells being produced. Chemotherapy simply means treatment with drugs and can be used to describe the use of drugs in any illness. In cancer treatment 'chemotherapy' is shorthand for 'cytotoxic chemotherapy', the 'cytotoxics' being the main group of drugs used against cancer.

The word cytotoxic means 'cell poison' and this actually describes how the drugs work. Cytotoxic drugs interfere with the process of cell reproduction so that when cancer cells come to reproduce (by dividing into two) they are unable to do so and die off.

At the present time there are about a hundred different cytotoxic drugs approved for cancer treatment. All of these act in one way or another to interfere with the process of cell division.

Unfortunately none of these drugs can tell the difference between normal cells and cancer cells. So they will all affect the reproduction of normal cells as well as cancer cells. This is why cytotoxic chemotherapy often causes unpleasant side-effects.

Normal cells will usually recover from the effects of a dose of cytotoxic chemotherapy far more quickly than cancer cells. This is why chemotherapy is often given a number of short courses, with several weeks in between each course to allow for normal cells to recover. In this way treatment can be given which destroys cancer cells but causes no lasting injury to normal cells.

Different cancers respond differently to cytotoxic chemotherapy. Some are very sensitive and can be cured with drug treatment. Others often respond to chemotherapy but are not sensitive enough to achieve cure. In these situations chemotherapy is given with the hope of prolonging good quality life. Others, despite all the different drugs available, remain almost completely resistant and unaffected by cytotoxic treatment.

The great majority of drug treatment for cancer involves the use of cytotoxic drugs but there are a few types of cancer where treatment with hormones may also be helpful. This is considered in a separate question on hormone therapy for cancer.