DHODH inhibition synergizes with DNA demethylating agents in the treatment of myelodysplastic syndromes

Dihydroorotate dehydrogenase (DHODH) catalyzes a rate-limiting step in de novo pyrimidine nucleotide synthesis, the conversion of DHO to orotate. DHODH inhibition has been reported recently as a new approach for treating acute myeloid leukemia (AML) by inducing differentiation. PTC299 represents a novel potent DHODH inhibitor and recently clinical development of PTC299 as a potential treatment option for acute leukemia was initiated. Here, we explore the efficacy of PTC299 for myelodysplastic syndrome (MDS). PTC299 efficiently inhibited the proliferation of MDS cell lines and the inhibitory effect was reversed by an excess of exogenous uridine, which bypasses the requirement for de novo pyrimidine synthesis. In contrast to AML cells, PTC299 was inefficient in inducing differentiation of MDS cells. However, PTC299 synergized with decitabine in inhibiting the growth and inducing apoptosis of MDS cells. This synergistic effect was also exhibited with primary MDS samples. PTC299 prolonged the survival of MDS mice in xenograft models using MDS cell lines, as a single agent, and was more potent in combination with the hypomethylating agent decitabine. Mechanistically, treatment with PTC299 induced an intra-S-phase arrest followed by entry into apoptotic cell death. Of interest, PTC299 enhanced incorporation of decitabine , an analog of cytidine, into DNA during S phase by inhibiting pyrimidine production, thereby enhancing the cytotoxic effect of decitabine. Gene set enrichment analysis of RNA-seq data revealed marked downregulation of MYC target gene sets with PTC299 exposure. Our results indicate that the DHODH inhibitor PTC299 suppresses the growth of MDS cells and acts in a synergistic manner with decitabine. This combination therapy could be a new therapeutic option for the treatment of MDS.