Myelodysplastic syndromes (MDS) are a group of heterogeneous malignant clonal hematopoietic stem cell disorders characterized by abnormal differentiation and maturation of myeloid cells (morphologically pathologic hematopoiesis), ineffective hematopoiesis and refractory hematocrit, hematopoietic failure and high risk of transformation to acute myeloid leukemia (AML). For MDS, only allogeneic hematopoietic stem cell transplantation (allo-HSCT) may cure it, but patients are mostly elderly and it is not widely available due to age, severe comorbidities and donor reasons. Prior to hypomethylating agents (HMAs) and lenalidomide, there were no specific drugs to change the face of MDS as a refractory disease. HMAs currently include decitabine (DAC) and azacitidine, which are 40-60% effective in the treatment of MDS and may alter the natural course of MDS. the optimal dosing regimen for HMAs is still inconclusive and is currently focused on better understanding the mode of action of HMAs, improving efficacy, reducing adverse effects, finding synergistic agents, and predicting efficacy indicators. (A) Mode of action of HMAs Initially, it was found that the oncogene promoters in MDS and AML were silenced due to hypermethylation, and HMAs may act in the following ways: 1 Demethylation of the promoter CpG island region to “wake up” the oncogene. 2 Involvement in DNA damage repair pathways, autophagy and developmental abnormalities in cell differentiation. 3 Induces anti-tumor immunity, immunomodulation, and promotes tumor-associated antigen expression and presentation. 4 Cytotoxic effects. The mechanism of drug resistance of HMAs is related to the abnormal cellular metabolic pathway of HMAs and retarded cellular response. (ii) DAC dose optimization For completed clinical trials and related studies, the following aspects were found to be of concern with respect to decitabine. First, whether decitabine is clinically effective or not is not necessarily related to the methylation status of the genome before and after dosing. Second, there was no linear relationship between clinical efficacy and dose, i.e., it was not better at higher doses. DAC was administered to 48 patients for 5 to 10 days at 20 mg/m2/day and was found to be more effective at lower doses. Subsequent clinical trials validated the efficacy of 15 mg/m2 three times a day for a total of 135 mg/m2 in patients with MDS. The DAC dose was then further optimized and 20 mg/m2/day for 5 days at a total of 100 mg/m2 was found to be more effective with fewer adverse effects, achieving a 15% CR rate and 43% (CR + PR + HI) overall response rate, and demonstrating the ability of DAC to delay the conversion of MDS to AML. Given the refractory hematocrit and transfusion dependence that are the most important problems in the lower risk group (lower risker group) – low risk for IPSS, intermediate risk-1 group – a lower dose and milder regimen was designed for the lower risk group – 20 mg/m2/day for 3 days by subcutaneous injection or intravenous infusion. Very high transfusion/platelet detachment rates were achieved, with 67% or 59% of patients achieving red cell or/and platelet detachment, respectively, and a survival benefit was seen, with approximately 70% of patients surviving longer than 500 days. Myelosuppression improved as DAC dosage was reduced, with drug-related neutropenia of 28% and 36%, anemia of 23% and 18%, and thrombocytopenia of 16% and 32% in the two groups, respectively. We explored a “small 3-day regimen” of 20 mg/m2/day IV for 3 days in 25 patients with transfusion-dependent low-risk (IPSS low-risk or intermediate-risk-1) MDS, with 3 (12%) achieving complete remission, 4 (16%) coming off component blood transfusion, 8 (32%) achieving hematologic improvement, and 2 (8%) achieving hematologic improvement. The overall response rate was 68% (17/25). Of the 11 patients with viable cytogenetic evaluation, one achieved partial cytogenetic remission (PRc). The incidence of grade IV hematologic toxicity was 48% (12/25), the incidence of grade III-IV infection was 20% (5/25), and there was no grade III-IV bleeding, grade III-IV nausea or vomiting, or grade III-IV hepatic impairment. The Karnofsky activity status score (KPS) score was 47 ± 16 before treatment and increased to 66 ± 22 after treatment (P = 0.001). More patients had an improved prognosis after treatment, with a significantly higher proportion of patients with a prognostic score based on the World Health Organization (WHO) classification (WPSS) ≤1 or MD Anderson Cancer Center, USA (MDACC) prognostic score ≤7 (44% versus 16%, P = 0.031; 64% versus 8%, P = 0.022). The median follow-up time was 467 d (14 to 881 d), with 2 deaths during the follow-up period, at days 14 and 156 after reduced-dose decitabine treatment. the expected survival rates were 100% and 95.2% at day 100 and 100% and 90.5% at day 600 after treatment in the low- and intermediate-risk-1 IPSS groups, respectively. Reduced doses of decitabine improved transfusion dependence, low incidence of severe hematologic toxicity and early mortality in low-risk MDS patients, improved prognosis, and possibly prolonged survival. Third, more detailed studies have found that lower concentrations of DAC, show its specific effects. DAC at 1.0 μM, like Ara-C, was inhibiting leukemic cells – RUNX1-ETOCD34 cells, KASUMI-1 cells, and normal CD34 cells – but at 0.5 μM it selectively inhibited abnormal clones, while having no effect on normal CD34 cells. The in vitro colony formation assay also showed that DAC at this concentration promoted proliferation of normal CD34 cells rather than inhibiting it. Of course, in the proliferation and differentiation of megakaryocytes, DAC also showed its unique effect at lower concentrations, inducing downward differentiation and maturation of megakaryocyte lineages to form polyploid platelet-producing megakaryocytes, which produce more platelets. This effect is also achieved by demethylation. Clinical results also show that platelet response is a good prognostic factor for efficacy and survival. Saunthararajah et al. further explored and found that DAC at lower doses – as low as 5 mg/m2/d – could achieve targeted inhibition of DNMT1 with minimal clinical cytotoxicity with epigenetic modifications. Clinical to basic research has shown that lower doses of DAC have unique pharmacological effects and low cytotoxicity. This is a clear advantage for MDS, especially for patients in the lower risk group – those with more manifestations and characteristics of bone marrow hematopoietic failure. Given that small doses (3-day regimen) still have a 48% grade IV hematologic toxicity, the author’s Hematopoietic Failure Clinic is exploring ultra-low dose DAC for MDS, 5-7 mg/m2/d for 6 doses, with an eye toward relieving transfusion dependence in MDS patients. Twenty-three cases have been completed, including 1 RA, 3 RAS, 1 RN, 9 RCMD, 2 RAEB1, 4 RAEB2, and 3 MDS/MPN, using 1-6 courses of DAC with a median of 3 courses. Results were effective in 8 of 14 cases (57%) in the lower risk group for IPSS, 3 of 6 cases (50%) in the higher risk group for IPSS, and all 3 cases of MDS/MPN, while 7 cases (30%) had grade IV hematologic toxicity. Preliminary results show that this regimen does have lower hematologic toxicity when good efficacy is achieved, reducing the cost and risk and lowering the threshold for the use of DAC in MDS. (iii) DAC to overcome drug resistance Clinical results have shown that DAC will work well in MDS with chromosome 7, 5 and complex chromosomal abnormalities, but in fact these are karyotypes with poor prognosis, especially complex chromosomal alterations, mostly combined with P53 mutations or deletions, which generate drug-resistant cells. DAC can target the removal of DNA methyltransferase-1 activity without inducing P53 phosphorylation and early apoptotic molecule expression, but rather induces late expression of key differentiation factors CCAAT enhancer binding protein and p27/cyclin dependent kinase inhibitor 1B (CDKN1B), allowing differentiation and apoptosis of p53 and p16/CDKN2A deficient resistant AML cells. DAC can induce apoptosis via the P73 pathway, a P53 alternative pathway. While most chemotherapeutic drugs remove tumor cells via the P53 pathway, this role of DAC has shown that it can replace chemotherapeutic drugs to overcome refractory/relapsed AML to the extent that it becomes an important drug for bridging and maintenance therapy before and after transplantation. A German multicenter comparison of 231 cases with chromosome 7 monosomy (-7, 7q-) MDS/AML treatment outcomes included the use of supportive therapy (49%), low-dose chemotherapy (4%), high-dose chemotherapy (8%), demethylating agents (HMA, 54%), transplantation (20%), and others (14%). Survival was better in transplants than non-transplants, 924 days vs 361 days, p<0.01; however, in patients in the non-transplanted IPSS high-risk group or IPSS-R very high-risk group, survival was significantly longer in the HMAs-treated group compared to the non-HMAs-treated group: 444 days vs 201 days or 444 days vs 203 days. It indicates that DAC can indeed overcome the refractory relapse status. Regarding cytogenetic analysis also confirmed that DAC can overcome the prognostic impact of poor karyotype. (iv) Immunomodulatory effects of DAC In the lower risk groups of MDS - low risk and intermediate risk-1 groups of IPSS - immune hyperactivity plays an important role in apoptosis, ineffective hematopoiesis and refractory hematocrit links, but given that MDS is a clonal neoplastic disease, immunosuppressive therapy is fraught with controversy. Can DAC inhibit malignant clones of MDS while simultaneously acting as an immunomodulatory agent to improve hematopoiesis in MDS from such a pathway? Both in vitro and in vivo assays have demonstrated that DAC induces CD4+CD25-T cells to express FOXP3, producing Treg capable of acting as immunosuppressive via perforin 1 by direct cell contact, and induces homozygous reactive T cells to become Treg by demethylating genes downstream of FOXP3. similarly, for naive T cells, DAC can interact with transforming growth factor, interleukin-2 and TCR stimulators synergistically to convert them into alfa/beta Tregs (iTregs). In γδ T cells are not only involved in adaptive immunity, but also have an important role in natural immunity. In vitro assays showed that DAC, in combination with transforming growth factor and interleukin-15, induced γδ T cells to express FOXP3 and promoted the expression of the negative costimulatory molecules ICOS and TGF-b1, IL-10, and inhibited the proliferation of anti-CD3/anti-CD28-stimulated peripheral blood individual nucleated cells. Basic immunological studies have revealed that sustained and stable production of Foxp3+ Treg by the thymus depends on the demethylation of the Treg-specific demethylated region (TSDR) on the DNA. Summary DAC is an effective drug for many MDS that are not suitable for allo-HSCT, delaying conversion to white, improving transfusion dependence, improving quality of life and prolonging survival. The dose and regimen of DAC for MDS are being optimized, and a new chapter of DAC for MDS will be opened for different patient populations and different therapeutic purposes, using different mechanisms of the drug.