A novel thrombopoietin mimetic RWJ-800088 increases megakaryopoiesis without causing malignant proliferation in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML).

Abstract

e18527 Background: In patients (pts) with AML and MDS, thrombocytopenia is a source of morbidity/mortality and there is a need for safer treatment options. RWJ-800088 (RWJ) is a novel pegylated thrombopoietin (TPO) mimetic peptide that increases megakaryopoiesis preclinically and in humans. Before using a TPO mimetic agent in AML and MDS pts, it is important to evaluate if the agent has potential stimulatory effects on malignant myeloid cells. To assess whether RWJ is likely to stimulate malignant myeloid cells, we evaluated its effects in vitro on primary human samples from healthy controls and AML/MDS pts. Methods: We studied the effect of various RWJ doses on proliferation of AML/MDS cell lines; clonogenic capacity for leukemic colony forming units (CFU) and megakaryocytic CFU (CFU-Mk) of mononuclear cells (MNC) from controls and several MDS and AML pts; and expression of differentiation markers in MNC cultures. Results: RWJ treatment had no effect on proliferation of THP1, MOLM13, MV411 and MDSL cell lines at doses up to 1000 ng/ml. A slight increase in proliferation of CMK, a megakaryocytic cell line, was seen at doses > 5xEC50 (mean 12%, p = 0.03). Primary clonogenic assays from 18 AML/MDS pts did not show increased growth of leukemic CFU at RWJ concentrations up to 10 ng/ml and RWJ had a nonsignificant inhibitory effect in 4 pts. Notably, RWJ significantly increased normal CFU-Mk formation from MNC in 6 of 8 AML and all MDS pts (mean 2.6-fold, p = 0.02), as well as from healthy CD34+ stem cells and healthy MNC controls (mean 1.7-fold, p = 0.03). FACS analysis of primary cultures treated with RWJ showed no increase in the malignant blast compartment, no change in differentiation markers of granulocyte/monocyte or erythrocyte lineages but a significant increase in the number of CD41a+ megakaryocytic cells (mean 1.8-fold, p < 0.01). RWJ treatment increased phosphorylation of STAT5 but not STAT3, demonstrating on target stimulation of the TPO receptor in hematopoietic cells. Conclusions: Our results show that the novel TPO mimetic RWJ stimulates megakaryopoiesis without causing proliferation of the malignant myeloid clone in MDS and AML.