Conventional chemotherapy (cytotoxic drugs)

Cytotoxic drugs interfere with different steps a cell must complete to copy its DNA and divide. Alkylating agents — and the platinum compounds that act in a similar way — chemically damage the DNA itself. Antimetabolites mimic the natural building blocks a cell needs, so DNA copying stalls. Plant alkaloids, including vinca alkaloids and taxanes, block the apparatus a cell uses to split into two, while topoisomerase inhibitors disable enzymes needed to uncoil DNA strands. Antitumour antibiotics such as doxorubicin bind to DNA and prevent it from being read correctly. A cell damaged in any of these ways usually dies or permanently stops dividing. Because the drugs cannot tell a dividing cancer cell from a dividing healthy one, tissues that renew constantly — bone marrow, the gut lining, hair follicles — are affected as well, which explains the typical side effects. Chemotherapy is therefore given in cycles: a period of treatment is followed by a rest period that gives healthy cells time to recover before the next dose.