Hypomethylation Therapy of Decitabine in Patients with Myelodysplastic Syndromes (MDS) Induces Apoptosis and Reduces Proliferation.

Background: MDS is characterized by cytopenias believed to be the direct result of increased apoptosis of the hematopoietic cells in the bone marrow despite an increase in proliferation. Aberrant methylation resulting in leukemogenesis is frequent in MDS and is a potential target for pharmacologic therapy. Decitabine (Dacogen™; DAC), which has been shown to be effective in treating patients with MDS (Saba et al, Kantarjian et al), indirectly depletes methylcytosine, resulting in hypomethylation of target genes. However, it is not known whether this correction of methylation leads to reducing the apoptosis, allowing normal cells to grow, or by increasing the apoptosis killing off the tumor cells.

Methods: We studied apoptosis and proliferation in patients with MDS treated on a randomized protocol to receive either DAC or best supportive care (SC). Apoptosis as measured by annexin V and mitochondrial potential was studied in various subpopulations of cells using multiparameter flow cytometry. Proliferation was also measured in a similar fashion using BrdU incorporation. Bone marrow (BM) and peripheral blood (PB) samples were collected from patients at baseline and at various times on therapy.

Results: At baseline, the DAC group showed no significant difference in apoptosis, proliferation or percent of CD34+ cells from the SC group in BM (n = 39 and 33 patients, respectively) or PB (n = 51 and 50, respectively). After three months on treatment a significant increase in apoptosis (annexin V) in CD34+ cells was noted in the DAC arm but not in the SC arm (Wilcoxon test, P=0.01). Similar results were obtained when disturbance in mitochondrial potential was measured in the blast population (P=0.02). Interestingly, a similar increase in apoptosis was observed in CD8+ cells in the DAC arm (P=0.02). The increase in apoptosis was augmented with additional courses of DAC therapy. A greater reduction in proliferation (BrdU incorporation) in the CD34+ cells in the decitabine arm compared to SC (P=0.01) was also observed. Those DAC-treated patients who had a higher proliferation rate (BrdU incorporation) at diagnosis were more likely to achieve a CR than those with low level of proliferation (P=0.03).

Conclusions: Decitabine therapy in patients with MDS leads to hypomethylation, but the net effects include high levels of apoptosis and the death of the neoplastic cells and a complimentary reduction of proliferation of the leukemic cells.