OR21, a New Oral DNA Methyltransferase Inhibitor for Treatment of MDS and AML

Myelodysplastic syndromes (MDS) is a clonal disorder caused by mutation in hematopoietic stem cells (HSCs), including several somatic mutations and aberrant DNA hypermethylation, which can induce silence of tumor-suppressor genes and lead to the pathogenesis and/or progression of MDS. Thus, reducing methylation with the aim of genes reestablishing the expression of silenced tumor-suppressor genes represents one promising approach for the treatment of MDS. Azacytidine (AZA) and/or decitabine (DAC), two DNA methyltransferase (DNMT) inhibitors currently available for the treatment of MDS, have dramatically improved the prognosis of MDS. However, these DNMT inhibitors must be administered intravenously or subcutaneously, necessitating that patients must visit a hospital every day for treatment. Thus, oral DNMT inhibitors are desired. OR21 is a novel and potentially oral-absorbable DAC prodrug with an oral absorbability of +2.14 (Log P value).

In this study, we investigated the efficacy of OR21 in MDS (MDS-L) and AML cell lines (HL60, THP1 and KG1a). OR21 inhibited cell growth and induced cell apoptosis in a dose dependent manner in MDS and AML cell lines. Its effects were similar to those of DAC, but not to those of AZA. Western blotting and pyrosequencing revealed that OR21 and DAC were more effective in reducing the DNMT protein level and LINE-1 methylation levels, respectively (Control, 88.9%; OR21, 74.2%; DAC 71.2%; AZA, 88.8%) in MDS-L (100 nM, 96 hr) than AZA. Next, we investigated changes in gene expression after DAC treatment (100 nM, 96 hr) in the MDS-L cell line using RNA microarray analysis. Out of a total of approximately 20000 sequenced genes, 188 genes were up-regulated and 238 genes were down-regulated. DAC dramatically changed gene expression in the MDS-L cell line via hypomethylation. Because MDS is also associated with impairment of cell differentiation in hematopoietic progenitor cells, developing agents that reestablish. Cell differentiation is also promising approach for the treatment of MDS. We assessed the effects of OR21 on the expression of the surface differentiation marker CD11b and stem cell marker HLA-DR in MDS-L cells. CD11b expression was upregulated while HLA-DR expression was downregulated, indicating that OR21 can induce cell differentiation. CEBPE, a key late differentiation driver, is silencing via hypermethylation in MDS. CEBPE mRNA level was up-regulated after OR21 treatment (100 nM, 96hr).

Finally, we used a mouse xenograft model to evaluate the anti-tumor effect of OR21 in vivo. BALB/c Rag-2/JAK3 double-deficient (BRJ) mice were injected intravenously via their tail veins with 5 × 10⁶ HL60 cells. OR21 (2.7 mg/kg), DAC (1.0 mg/kg), or vehicle (1% DMSO) was administered by intraperitoneal injection twice weekly. The AUCs of OR21 and DAC were equal. We evaluated in vivo model using intraperitoneal injection because oral intake of capsule coated OR 21 successfully absorbed in cynomolgus monkey model. OR21 significantly prolonged survival (median survival 49 days and 44 days, P=0.005), whereas DAC did not prolong survival (median 46.5 days and 44 days, P=0.164) in the xenograft mouse model. OR21 significantly lowered LINE-1 methylation levels in mouse bone marrow cells (Vehicle, 83.7%; OR21, 62.8%; P<0.0001). Furthermore, OR21-treated mice tended to show less in anemia than DAC-treated mice (Hb: Vehicle, 17.5 g/dL; OR21, 17.1 g/dL; DAC, 15.8 g/dL). OR21 has a stronger anti-tumor effect with lower toxicity than DAC. Taken together, these results suggest OR21 could be used as an alternative drug to decitabine for the treatment of MDS.