alkylating agent
An alkylating agent is a cancer drug that chemically welds itself onto DNA. It carries a reactive group that grabs onto the bases of the DNA double helix and forms a permanent covalent bond. Picture jamming a stick into the rungs of a ladder: the cell can no longer separate the two strands to copy them, and replication grinds to a halt.
Many alkylating agents have two reactive arms, so they can bridge between the two DNA strands or between two points on the same strand, creating crosslinks that are especially hard to repair. A cell that meets enough of this damage and cannot fix it usually commits suicide through its programmed cell-death pathway.
Because the damage is to DNA itself and not to a tumor-specific molecule, alkylating agents hit any dividing cell and share the broad toxicities of chemotherapy. They also carry a long-term risk: by mutating DNA they can, years later, cause secondary cancers. Tumors resist them mainly by ramping up their DNA-repair machinery.
Cyclophosphamide and the platinum drug cisplatin both crosslink DNA — cyclophosphamide is even a prodrug, activated in the liver into the reactive species that does the alkylating.
DNA-crosslinking agents; cyclophosphamide needs metabolic activation first.
"Alkylating" means transferring a carbon-containing alkyl group onto another molecule. In these drugs the alkyl group is attached to DNA, which is the source of both their anticancer power and their mutagenic danger.