alkylating agent
Meaning of alkylating agent: The oldest family of chemotherapy drugs, acting by sticking chemical groups onto DNA so it can no longer be read.
Definition of alkylating agent
An alkylating agent is a drug that attaches a small carbon-containing group, an alkyl group, directly onto DNA. The attachment distorts the strand and can lock the two strands together, so the cell can neither read nor copy its instructions. Cells preparing to divide are damaged most, which is why these drugs act on tumors and also on healthy fast-renewing tissue.
How do they damage DNA?
The drug forms a highly reactive fragment that grabs the nearest electron-rich site. In DNA that is usually a particular nitrogen atom on guanine, one of the four bases. Once an alkyl group is attached there, the base pairs incorrectly or fails to pair at all. Agents with two reactive ends can attach to both strands at once and staple them together, a cross-link that stops the strands separating. When the cell next tries to copy its DNA it meets damage it cannot get past, repair systems fail to keep up, and the cell triggers its own death.
What types are there?
| Group | Notable feature |
|---|---|
| Nitrogen mustards | The original family, including cyclophosphamide |
| Nitrosoureas | Fat-soluble enough to reach the brain |
| Alkyl sulfonates | Strong effect on the bone marrow |
| Triazenes | Includes agents used in melanoma and brain tumors |
| Platinum compounds | Cross-link DNA similarly, though listed separately |
What follows from that mechanism?
The drug cannot tell a tumor cell from a healthy one; it simply reaches whatever is dividing. Bone marrow, the lining of the mouth and gut, hair follicles and the tissues producing eggs and sperm are all fast-renewing, and all suffer. Low blood counts, mouth ulcers, nausea and hair loss follow directly. Reduced fertility is common and may be permanent. Because the damage is done to DNA itself, a small long-term risk exists of a second cancer, usually a leukemia appearing years after treatment, and this is weighed when the drugs are used in situations with a good outlook.
Are they used outside cancer?
Yes. Cyclophosphamide at lower doses suppresses the immune system and is used in severe autoimmune disease and in some kidney conditions, where the target is an overactive immune response rather than a tumor. The same class remains part of standard combinations in breast cancer, lymphoma and several other diseases.
Used in a sentence
The protocol combined an alkylating agent with an anthracycline over six cycles.
Good to know
These drugs damage DNA in any dividing cell rather than selecting tumor cells, which explains both their side effects and the small long-term risk of a second cancer.