Prodrug
A drug given in an inactive form that the body converts into the active medicine. It can make a drug easier to take, longer-lasting, or active only where it is needed.
Capecitabine is a tablet that liver and tumour enzymes convert to 5-fluorouracil, replacing an infusion; irinotecan is converted to the far more potent SN-38, and cyclophosphamide is activated by liver enzymes. The idea is extended in cancer therapy to drugs activated selectively in tumours, for instance masked antibodies whose binding site is uncovered by tumour enzymes, and ADCs, whose payload is inert until the linker is cut inside the target cell. Genetic differences in the activating and clearing enzymes (DPYD for fluoropyrimidines, UGT1A1 for irinotecan) explain why some patients suffer severe toxicity at standard doses and are now tested for in advance.