chemotherapy resistance

Chemotherapy resistance is a tumour's ability to survive a drug that once worked. At first the medicine shrinks the cancer, but over time it stops responding, like weeds that come back after spraying because the survivors were the ones the weedkiller could not touch. The tumour is not consciously adapting; rather, the few cells that happen to resist are the ones that keep growing.

Cancers evade drugs through many tricks. They can pump the drug back out using efflux transporters such as P-glycoprotein, switch on enzymes that detoxify the drug, repair the damage faster, mutate the drug's target so it no longer binds, or block the apoptosis machinery so damaged cells refuse to die. Resistance can be present from the start (intrinsic) or develop under the selective pressure of treatment (acquired), and it may apply to one drug or to many at once (multidrug resistance).

Understanding resistance shapes how cancer is treated. Combining drugs with different mechanisms makes it less likely that any single cell resists all of them at once, and switching to a next-generation agent can overcome a specific resistance mutation. Even so, resistance remains a central reason that initially effective cancer therapy eventually fails.