Azacitidine (5-azacytidine) is a chemical analogue of the cytosine nucleoside used in DNA and RNA. Azacitidine is thought to induce antineoplastic activity via two mechanisms; inhibition of DNA methyltransferase at low doses, causing hypomethylation of DNA, and direct cytotoxicity in abnormal hematopoietic cells in the bone marrow through its incorporation into DNA and RNA at high doses, resulting in cell death. As azacitidine is a ribonucleoside, it incoporates into RNA to a larger extent than into DNA. The incorporation into RNA leads to the dissembly of polyribosomes, defective methylation and acceptor function of transfer RNA, and inhibition of the production of protein. Its incorporation into DNA leads to a covalent binding with DNA methyltransferases, which prevents DNA synthesis and subsequent cytotoxicity.
Azacitidine is believed to exert its antineoplastic effects by causing hypomethylation of DNA and direct cytotoxicity on abnormal hematopoietic cells in the bone marrow. The concentration of azacitidine required for maximum inhibition of DNA methylation in vitro does not cause major suppression of DNA synthesis. Hypomethylation may restore normal function to genes that are critical for differentiation and proliferation. The cytotoxic effects of azacitidine cause the death of rapidly dividing cells, including cancer cells that are no longer responsive to normal growth control mechanisms. Non-proliferating cells are relatively insensitive to azacitidine. Upon uptake into cells, azacitidine is phosphorylated to 5-azacytidine monophosphate by uridine-cytidine kinase, then to diphosphate by pyrimidine monophosphate kinases and triphosphate by diphosphate kinases. 5-Azacitidine triphosphate is incorporated into RNA, leading to the disruption of nuclear and cytoplasmic RNA metabolism and inhibition of protein synthesis. 5-Azacytidine diphosphate is reduced to 5-aza-deoxycytidine diphosphate by ribonucleotide reductase. The resultant metabolite is phosphorylated to 5-azadeoxycitidine triphosphate by nucleoside diphosphate kinases. 5-azadeoxycitidine triphosphate is then incoporated into DNA, leading to inhibition of DNA synthesis. Azacitidine is most toxic during the S-phase of the cell cycle.
An in vitro study of azacitidine incubation in human liver fractions indicated that azacitidine may be metabolized by the liver. The potential of azacitidine to inhibit cytochrome P450 (CYP) enzymes is not known.
One case of overdose with azacitidine was reported during clinical trials. A patient experienced diarrhea, nausea, and vomiting after receiving a single IV dose of approximately 290 mg/m2, almost 4 times the recommended starting dose.