TET2 Inhibitory Effects of Eltrombopag Contribute Its Hematopoietic Activity

Severe aplastic anemia (sAA) is bone marrow (BM) failure syndrome characterized by immune mediated BM hypoplasia and pancytopenia usually treated with immunosuppressive therapy (IST) or allogeneic BM transplant. Despite the successes of IST in treating acquired, idiopathic sAA, most of the available treatment options only convert severe into non-severe forms of disease. Recently, eltrombopag (Epag), a small molecule thrombopoietin receptor (TPOR) agonist has been introduced into the routine treatment of de-novo and refractory AA with robust clinical response. Epag treatment showed remarkable tri-lineage responses in AA, suggesting an effect on hematopoietic stem and progenitor cells (HSPCs) amplification. The use of hematopoietic growth factors in AA that increases HSPCs has been associated with justified fears of facilitating AA evolution to MDS. Similarly, Epag has parallel effects on the amplification of HSPCs and there has been recent signals that long term treatment of Epag may increase the MDS risk. Using an in-house developed highly stable cell free high throughput screening of bioactive small molecule chemical library, we identified Epag as one of the most potent inhibitor of TET2. TET2 is the most abundant DNA dioxygenase in HSPCs. The loss-of-function mutations of TET2 frequently occurs in myeloid neoplasia. Here using in vitro and in vivo model system we report that effect of Epag, in part, is mediated by its ability to inhibit TET-dioxygenase activity.

We demonstrate that Epag directly binds to TET2 in the presence of Fe²⁺ and inhibits its dioxygenase function in cell free system. In cell culture model we observed that nearly 40% of Epag partitioned into to nucleus suggesting its potential TET2 inhibitory role in cells. To test if TPOR signaling activation by Epag has any impact on TET activity we engineered murine cell lines BaF3 and 32D by introducing human TPOR. Irrespective of the status of TPOR-JAK-STAT pathway activation, as reflected in STAT5 phosphorylation, Epag demonstrated a robust TET-inhibition activity. However, in similar experiments peptide agonist of TPOR or another small molecule TPOR agonist Avotrombopag did not have any TET inhibitory effects. Consistent with these observations, Epag but not TPO or Avotrombopag inhibited TET activity in murine BM. Thus, Epag mediated TET2 inhibition in these cells are specific and independent of TPOR signaling. Consistent with previous reports, we observed that Epag treatment can significantly expand murine HSPCs even in the absence of TPOR signaling activation. The hematopoietic activity of Epag in murine system is dependent on its ability to inhibit TET2 as confirmed by the lack of Epag effects on Tet2^-/- HSPCs.

We further confirmed that TET-inhibition by Epag is central for its ability to expand HSPCs in murine model under in vivo settings using BM transplant experiments. We transplanted a mixture of cells derived from Tet2^+/+ Pep Boy (CD45.1, 95%) and Tet2^-/- C57BL/6 (CD45.2, 5%) into lethally irradiated recipient PepBoy mice. Epag treatment of mice reconstituted with the mixture of BM cells increases the fraction of Tet2^+/+ BM cells only. Interestingly, Epag mimic loss of Tet2 in vivo as observed in the expansion of myeloid compartments of Tet2^+/+ fractions reflected in CD11b^-CD11c⁺, CD11b⁺CD11c^-, CD11b⁺CD11c^-Ly6C⁺Ly6G^- (monocytes) and CD11b⁺CD11c^-Ly6C^lowLy6G⁺ (neutrophils). We did not observe any significant change in B220⁺, CD4⁺ or CD8⁺ lymphoid populations. On the contrary, we observed a growth restrictive effect of Epag treatment on Tet2^-/- myeloid cells. Consistent with our hypothesis that Epag inhibits TET-dioxygenase activity, we observed hypermethylation in the mononuclear cells derived AA patients (n= 16) after Epag treatment. This was further confirmed in ex vivo Epag treatment of BM cells derived from healthy donor. Epag treatment significant increase clonogenic expansion of normal BM cells but not TET2 mutant myeloid cells.

In summary, here we demonstrate that Epag is a potent inhibitor of TET-dioxygenase and its biological consequences are independent of TPOR-JAK-STAT activation. Epag treatment mimics TET2 loss of function in AA cells, however it restricts clonal expansion of TET2 mutant myeloid cells.