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An explanation of a few general principles allows improved understanding of anticancer chemotherapy:

• In a tumour, only the cells which have the capacity to indefinitely reproduce themselves (stem cells) are dangerous. These are the cells that we intend to kill. The other cells, those which multiply themselves but cannot reproduce more than a few generations and those which are very well differentiated and cannot divide, will eventually die naturally without chemotherapy.

Theoretical composition of a tumour: from the top: stem cells which renew themselves indefinitely, then multiplying cells, then differentiating cells which result in cell death.

• We do not possess genuinely specific drugs for cancer cells: each drug that we use is more or less toxic for normal cells.
  
• Most of the drugs (at least in classical chemotherapy) will kill the cancer cells when they are dividing by altering the delicate mechanisms of cell division. Slowly developing tumours are therefore less concerned. Conversely, highly proliferating healthy tissue (like blood cell lineages, mucosa cells, skin) are particularly concerned and should regenerate in order to maintain the patient alive. All our drugs have a more or less marked haematological toxicity and most often other form(s) of chronic toxicity.

In relation to the cell proliferation capacity, the number of dead cells (on the vertical axis) is more important for proliferating tumours (in red). However most drugs only act during a period of the cell cycle. Thus, a cell cycle dependant antimitotic drug will produce a plateau after the initial decrease in the number of the cells. (on the vertical axis: number of cells, on the horizontal axis: time after chemotherapy treatment).