Decitabine (DAC) is successfully used for treatment of patients (pts) with myelodysplastic syndromes and AML. Following cellular uptake, DAC is thought to be activated to DAC-TP and incorporated into DNA. The DAC-TP/DNA complex binds and inactivates DNA methyltransferases (DNMTs), thereby leading to hypomethylation and re-expression of epigenetically silenced tumor suppressor genes and ultimately anti-leukemia activity. However, direct evidence of in vivo DAC-TP occurrence in DAC-treated pts has been difficult to demonstrate due to a lack of suitable validated analytical methodology. Thus, we developed and validated a sensitive and specific LC-MS/MS method for quantification of DAC-TP. The assay exhibited excellent accuracy and precision. The accuracy values were 83.7–109.4%, as determined by calculating the percentage of measured DAC-TP relative to the respective nominal concentrations (50, 500 and 5,000 nM) of the quality control samples. The within-day coefficients of variation (CVs) were 19.9 % (n=6) at 50 nM and 4.7–7.0 % between 500–5,000 nM; the between-day CVs (n=3) were 15.2 % at 50 nM and 7.5–10.2 % between 500–5,000 nM.
Following DAC treatment, we detected DAC-TP in parental and DAC-resistant MV4–11, and in THP-1 and FDC-P1/Kitmut cells (in vitro); and in bone marrow (BM) and spleen of normal and FDC-P1/Kitmut-driven AML mice (in vivo). DAC-TP reached peak levels (0.8, 1.4 and 0.5 pmol/106 cells) in 1–4 hours and declined to 20 % of its peak concentration after 24 hours incubation with 2.5 μM DAC in MV4–11, THP-1 and FDC-P1/Kitmut cells, respectively. Inhibition of hENT1 that mediates DAC transport into the cells and dCK that phosphorylates DAC into DAC-TP by NBTI and 2-thio-2′-deoxycytidine, respectively, significantly inhibited DAC-TP accumulation in AML cells. DAC-TP decay was instead blocked by tetrahydrouridine (THU)-induced inhibition of CDA, the catabolizing enzyme for cytidine and deoxycytidine and analogs. Consistent with these results, low dCK and hENTs but not CDA expression were detected in DAC-resistant MV4–11 cells, which showed 60 % decrease in DAC-TP levels as compared to their parental counterparts. DAC/DAC-TP-mediated downregulation of DNMT proteins (preferentially DNMT1 and DNMT3a) was also demonstrated in the AML cells even at DAC-TP concentrations as low as 0.1–1.3 pmol/106 cells in vitro after 4 hours DAC incubation. In the in vivo experiments, DAC-TP levels in leukemic mice were comparable to that in normal C57BL/6 mice, 0.3 pmol/106 cells in BM and 199.2 pmol/g tissue in spleen at 4-hours and 0.2 pmol/106 cells in BM and 165.3 pmol/g tissue in spleen at 24-hours following an i.v. bolus of 6.5 mg/kg DAC. In BM of leukemic mice, not only DNMT1 and DNMT3a but also DNMT3b protein expression reduced 80 % (DNMT3a) or diminished (DNMT1 and DNMT3b).
The clinical applicability of this method was proven by measuring DAC-TP level in BM and blood mononuclear cells (PBMC) from AML pts treated with a 10-day regimen of DAC given 20 mg/m2/day i.v. over 1 hour. In BM samples, the mean DAC-TP levels were 0.8 ± 0.6 (Day 1) and 0.9 ± 0.5 pmol/106 cells (Day∼5) in complete responsive (CR) pts (n=4); and 0.4 ± 0.3 (Day 1) and 0.12 ± 0.02 pmol/106 cells (Day∼5) in non-responsive (NR) pts (n=3). In PBMC samples, the mean DAC-TP levels were 0.5 ± 0.2 (Day 1) and 1.2 ± 0.4 pmol/106 cells (Day∼5) in CR pts (n=3); and 0.02 ± 0.02 (Day 1) and 0.21 ± 0.04 pmol/106 cells (Day∼5) in NR pts (n=3). These data suggested that higher levels are seemingly associated with clinical response, but a larger number of pts need to be tested. In conclusion, monitoring the intracellular concentration of DAC-TP is feasible, and DAC-TP levels correlate with DNMT downregulation and may serve as a novel pharmacological endpoint for designing more effective DAC-based regimens.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.