Alkylating antineoplastic agent

# Overview
An alkylating antineoplastic agent is an alkylating agent that attaches an alkyl group to DNA.
Since cancer cells generally proliferate unrestrictively more than do healthy cells they are more sensitive to DNA damage, and alkylating agents are used clinically to treat a variety of tumours.

# Agents acting nonspecifically
Some alkylating agents are active under conditions present in cells; and the same mechanism that makes them toxic allows them to be used as anti-cancer drugs. They stop tumour growth by cross-linking guanine nucleobases in DNA double-helix strands - directly attacking DNA. This makes the strands unable to uncoil and separate. As this is necessary in DNA replication, the cells can no longer divide. These drugs act nonspecifically.

# Agents require activation
Some of them require conversion into active substances in vivo (e.g. cyclophosphamide).
Cyclophosphamide is one of the most potent immunosuppressive substances. In small dosages, it is very efficient in the therapy of systemic lupus erythematosus, autoimmune hemolytic anemias, Wegener's granulomatosis and other autoimmune diseases. High dosages cause pancytopenia and hemorrhagic cystitis.

# Dialkylating agents, limpet attachment, and monoalkylating agents
Dialkylating agents can react with two different 7-N-guanine residues and if these are in different strands of DNA the result is cross-linkage of the DNA strands, which prevents uncoiling of the DNA double helix.  If the two guanine residues are in the same strand the result is called limpet attachment of the drug molecule to the DNA.
Monoalkylating agents can react only with one 7-N of guanine.
Limpet attachment and monoalkylation do not prevent the separation of the two DNA strands of the double helix but do prevent vital DNA processing enzymes from accessing the DNA.  The final result is inhibition of cell growth or stimulation of apoptosis, cell suicide.

# Examples
In the Anatomical Therapeutic Chemical Classification System, alkylating agents are classified under L01A.
Alkylating agents activated by cytochrome p-450:
- Alkyl sulfonates
Busulfan (L01AB01 (WHO))
- Busulfan (L01AB01 (WHO))
- Ethyleneimines and methylmelamines
Hexamethylmelamine or altretamine (L01XX03 (WHO))
Thiotepa (L01AC01 (WHO))
- Hexamethylmelamine or altretamine (L01XX03 (WHO))
- Thiotepa (L01AC01 (WHO))
- Nitrogen mustards
Cyclophosphamide (L01AA01 (WHO))
Mechlorethamine or mustine (L01AA05 (WHO))
Uramustine or uracil mustard (no ATC code, Template:PubChem, Template:DrugBank)
Melphalan (L01AA03 (WHO))
Chlorambucil (L01AA02 (WHO))
Ifosfamide (L01AA06 (WHO))
- Cyclophosphamide (L01AA01 (WHO))
- Mechlorethamine or mustine (L01AA05 (WHO))
- Uramustine or uracil mustard (no ATC code, Template:PubChem, Template:DrugBank)
- Melphalan (L01AA03 (WHO))
- Chlorambucil (L01AA02 (WHO))
- Ifosfamide (L01AA06 (WHO))
- Nitrosoureas
Carmustine (L01AD01 (WHO))
Streptozocin (L01AD04 (WHO))
- Carmustine (L01AD01 (WHO))
- Streptozocin (L01AD04 (WHO))
- Triazenes
Dacarbazine (L01AX04 (WHO))
- Dacarbazine (L01AX04 (WHO))
- Imidazotetrazines
Temozolomide (L01AX03 (WHO))
- Temozolomide (L01AX03 (WHO))
Other selected examples: