Hypomethylation Induction and Molecular Response after Decitabine Therapy in Chronic Myelomonocytic Leukemia (CMML).

Decitabine has shown therapeutic activity in patients with MDS and CMML. The mechanisms of response to therapy remain incompletely understood. In particular, the relative contribution of this drug’s ability to induce hypomethylation and cytotoxicity remains unclear. To address this issue, we studied the dynamics of neoplastic cell clearance during decitabine treatment determined by quantitative monitoring of the mutant allele using pyrosequencing. DNA extracted from peripheral blood mononuclear cells from consented patients with CMML in a decitabine phase II study were first screened for JAK2 and NPM1 mutations as previously reported. We identified three patients with mutations (two with JAK2 mutation, one with NPM1 mutation) and samples at multiple points during therapy were available. All three carried normal karyotype. LINE repetitive element methylation and several other gene specific methylations were also assessed. In the three patients, LINE methylation decreased after each cycle of therapy, and recovered to near baseline after the drug was stopped (e.g. during the first cycle, average relative hypomethylation from baseline was 13.9% at day 12 and 6.5% at day 28). At the same time, the proportion of circulating neoplastic cells decreased slowly after the first cycle (decrease by 19.3% at day 12 and 13.5% at day 28). A substantial decrease in mutant allele percentage was observed after cycles 2, 3, and 2 in patients 1, 2, and 3, respectively. Clinical complete responses were achieved along with molecular responses at cycles 5, 4 and 2, respectively. Patients 1 and 2 showed complete disappearance of detectable neoplastic clones, and had sustained remissions (duration 1.5 and 2.5 years). In patient 3, the proportion of neoplastic cells was lower than baseline but still detectable at clinical remission, and the remission only lasted 8 months. We conclude that neoplastic cell clearance after decitabine therapy in CMML is observed after several courses of therapy, and is initially seen concurrently with hypomethylation. While LINE methylation returns to its steady state values after completion of decitabine infusion, the tumor elimination process slowly continues. Our data suggest a non-cytotoxic mechanism of action for the drug, whereby the biology of the neoplastic clone is altered by hypomethylation, leading to delayed clearances of unknown mechanism. Possibilities include an immune response and effects on the neoplastic (or normal) stem cells.

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