Hypomethylating Agents: Azacitidine and Decitabine

How They Work

At the low doses used clinically, these drugs act mainly through hypomethylation rather than direct cytotoxicity. Incorporated into DNA in place of cytidine, they form a covalent, irreversible bond with DNMT1 as it attempts to copy the methylation pattern to the new strand.

DNMT1 is then degraded, so each subsequent cell division dilutes methylation further. Silenced genes, including some tumour suppressors and genes that make leukaemic cells more visible to the immune system, can be re-expressed.

Where They Are Used

Azacitidine and decitabine are standard for higher-risk myelodysplastic syndromes and for chronic myelomonocytic leukaemia. In acute myeloid leukaemia, azacitidine combined with the BCL2 inhibitor venetoclax is now widely used for patients not suitable for intensive chemotherapy.

An oral combination of decitabine with cedazuridine, which blocks its breakdown in the gut, allows outpatient dosing.

What to Expect

Responses are typically slow, often taking four to six cycles, and treatment is continued as long as it is working. Early worsening of blood counts is common and does not necessarily indicate failure.

These agents modify disease course and can improve survival in the studied populations but are not curative on their own, and TP53-mutant disease tends to respond less durably.

Practical Use and Limitations

Azacitidine is given by subcutaneous injection or infusion for seven days of a 28-day cycle; decitabine uses a five-day schedule, and the oral decitabine-cedazuridine combination reproduces the exposure without an intravenous line. The main toxicity is myelosuppression, with injection-site reactions, nausea and fatigue also common.

Two limits shape expectations. Benefit takes months to appear, so treatment is not stopped early for a slow response, and it is continued while it works because stopping usually leads to relapse. TP53-mutant myeloid disease responds but seldom durably, and adding venetoclax is now the more active option in acute myeloid leukaemia.

Key Takeaways

• ·Azacitidine and decitabine trap and deplete DNMT1, causing progressive loss of DNA methylation.
• ·They are standard for higher-risk myelodysplastic syndromes and, with venetoclax, for AML.
• ·Responses develop over several months and are generally not curative alone.