DNA Cross-Linking and Alkylation Quiz

DNA Cross-Linking Agents

• Alkylating agents can react with a nucleic acid base on each chain or on the same chain of DNA to form interstrand or intrastrand DNA cross-links.
• DNA alkylation, intrastrand and interstrand DNA cross-linking disrupt the template function of DNA, including replication and transcription, and result in DNA destruction and cell death.
• Potential mechanisms of cell death after DNA-alkylation include disruption of the template function of DNA and activation of apoptosis caused by p53.

Alkylating Agents

• These agents have poor selectivity and toxic side effects, and can also alkylate nucleophilic groups on proteins.
• Alkylating agents have been useful in the treatment of cancer, as tumour cells often divide more rapidly than normal cells and are more drastically affected by disruption of DNA function.
• However, these drugs can be mutagenic and carcinogenic in their own right.

Thiotepa

• Thiotepa is an aziridine-based DNA cross-linker, which is active intact, but is also converted by oxidative desulfuration to an active triethylenephosphoramide metabolite.
• Thiotepa incorporates a less reactive aziridine ring compared with that formed in mechlorethamine, due to the adjacent thiophosphoryl being electron withdrawing and reducing the reactivity of the aziridine ring system.
• Thiotepa is metabolically converted to TEPA by desulfuration in vivo, and forms cross-links with DNA by sequential reactions of thiotepa itself with DNA.

Aromatic (R = aryl) Mustards

• These are much weaker electrophiles, less toxic, and less reactive than other alkylating agents.
• They are most commonly administered orally for the palliative care of patients with leukemia, lymphoma, or multiple myeloma.

Inactivation of Alkylating Agents

• Inactivation of the alkylating agent is possible by reaction with water, which represents the nucleophile that is present in greatest abundance in the body.
• Reaction involves displacement of the leaving group on the electrophile by the nucleophile (water) with the formation of an alcohol.

Organoplatinum Compounds

• Once inside cells, cisplatin undergoes aquation, where the chloro substituents are displaced by neutral water molecules to give reactive, positively charged species trapped in cells.
• These species bind to DNA preferentially at the N-7 of two adjacent guanine units of a same strand, forming intrastrand covalent Pt-DNA cross-links.
• Cross-linking results in altered normal base-pairing, inhibits DNA-transcription and replication, and results in cell apoptosis.

Cyclophosphamide and Ifosfamide

• These agents are nitrogen mustard prodrugs, which are relatively non-toxic and can be taken orally without causing damage to the gut wall.
• However, acrolein, an electrophilic byproduct of cyclophosphamide and ifosfamide activation, has been associated with bladder and kidney toxicity.
• Bladder toxicity can be reduced by co-administrating sulphydryl donors such as mesna, which reacts with and detoxifies acrolein.