alkylating agent

An alkylating agent is a chemotherapy drug that chemically glues parts of the DNA together. Picture trying to copy a book, but someone has welded random pages shut: you can no longer turn them, so the copying stalls. By attaching small carbon-containing (alkyl) groups to DNA, these drugs jam the molecule so the cell cannot replicate it.

Chemically, alkylating agents are reactive molecules that bond to DNA bases, especially the nitrogen at position 7 of guanine. Many can react twice, creating cross-links that tie the two DNA strands together or link bases within one strand. A cell facing this damage either repairs it, halts at a checkpoint, or triggers apoptosis (programmed cell death). Because they act on existing DNA, they can hit cells in any phase, making them largely cell-cycle nonspecific.

This same DNA-damaging power has a dark side: alkylating agents are themselves mutagenic and can cause secondary cancers years later, particularly leukaemias. They also suppress bone marrow and may impair fertility. Examples include cyclophosphamide, cisplatin (a platinum compound that acts in a closely related way), and the historic nitrogen mustards.