Decitabine

Myelodysplastic syndromes (MDS) are a group of hematopoietic neoplasms that manifest in peripheral cytopenias and may eventually progress to secondary acute myeloid leukemia (sAML). Included in the over 45 genes commonly mutated in MDS patients are those involved in DNA methylation and histone modification, and it is well-established that alteration of the epigenetic landscape is a feature of myeloid leukemias.

Decitabine is considered a prodrug, as it requires transport into cells and subsequent phosphorylation by distinct kinases to generate the active molecule 5-aza-2'-deoxycytidine-triphosphate, which is incorporated by DNA polymerase during DNA replication. Once incorporated into DNA, decitabine is recognized as a substrate by DNA methyltransferase enzymes (DNMTs), specifically DNMT1, but due to the presence of an N5 rather than C5 atom, traps the DNMT through the irreversible formation of a covalent bond. At low concentrations, this mode of action depletes DNMTs and results in global DNA hypomethylation while at high concentrations, it additionally results in double-strand breaks and cell death.

The general hypothesis regarding decitabine's therapeutic efficacy is that the global hypomethylation it induces results in the expression of previously silent tumour suppressor genes. However, there are other putative mechanisms also related to this change in DNA methylation, including indirect alteration of transcription through effects on transcription factors, indirectly altering histone modifications and chromatin structure, and activating pathways involved in DNA damage response. The overall effect of decitabine is a decrease in neoplastic cell proliferation and an increase in the expression of tumour suppressor genes.