Mitigation of Total Body Irradiation-Induced Mortality and Hematopoietic Injury of Mice by a Thrombopoietin Mimetic (JNJ-26366821)

The threat of a nuclear attack has increased in recent years highlighting the benefit of developing additional therapies for the treatment of victims suffering from Acute Radiation Syndrome (ARS). In this work, we evaluated the impact of a PEGylated thrombopoietin mimetic peptide, JNJ-26366821, on the mortality and hematopoietic effects associated with ARS in mice exposed to lethal doses of total body irradiation (TBI). JNJ-26366821 was efficacious as a mitigator of mortality and thrombocytopenia associated with ARS in both CD2F1 and C57BL/6 mice exposed to TBI from a cobalt-60 gamma-ray source. Single administration of doses ranging from 0.3 to 1 mg/kg, given 4, 8, 12 or 24 hours post-TBI (LD70 dose) increased survival by 30 – 90% as compared to saline control treatment. At the conclusion of the 30-day study, significant increases in bone marrow colony forming units and megakaryocytes were observed in animals administered JNJ-26366821 compared to those administered saline. In addition, enhanced recovery of FLT3-L levels was observed in JNJ-26366821-treated animals. Probit analysis of survival in the JNJ-26366821- and saline-treated cohorts revealed a dose reduction factor of 1.113 and significant increases in survival for up to 6 months following irradiation. These results support the potential use of JNJ-26366821 as a medical countermeasure for treatment of acute TBI exposure in case of a radiological/nuclear event when administered from 4 to 24 hours post-TBI.

Eltrombopag for patients with moderate aplastic anemia or uni-lineage cytopenias

There is no standard or widely effective treatment of patients with moderate aplastic anemia (MAA) or hypo-productive uni-lineage cytopenias (UC). Eltrombopag (EPAG), a small molecule thrombopoietin mimetic, has previously been shown to result in durable multi-lineage hematologic responses with low toxicity in patients with refractory severe aplastic anemia (SAA). Its safety and efficacy in MAA are unknown. This prospective phase 2 study enrolled previously untreated and treated MAA and UC patients with clinically relevant cytopenias. EPAG was administered at doses escalating from 50 to 300 mg/d. Hematologic responses were assessed at 16 to 20 weeks. Responding patients were continued on EPAG until reaching defined robust or stable blood counts. EPAG was reinstituted for relapse. Thirty-four patients were enrolled between 2012 and 2017, including 31 with MAA and 3 with UC. Seventeen patients responded in at least 1 eligible lineage by the primary end point. A striking improvement in anemia was observed in a patient with Diamond-Blackfan anemia. EPAG was well tolerated, and it was discontinued for robust or stable blood counts in 12 of 17 patients after a median of 8 months. A majority required re-initiation of EPAG for declining counts, and all regained response. Two of 34 patients developed non–chromosome 7 bone marrow cytogenetic abnormalities while taking EPAG, without dysplasia or increased blasts. Somatic mutation allele frequencies in cancer genes did not increase overall on EPAG. EPAG is a well-tolerated oral treatment of cytopenias in patients with MAA/UC. This trial was registered at www.clinicaltrials.gov as #NCT01328587.

Romiplostim in patients undergoing hematopoietic stem cell transplantation: results of a phase 1/2 multicenter trial

Persisting severe thrombocytopenia more than 6 weeks after allogeneic stem cell transplantation is a common problem associated with adverse prognosis. Peffault de Latour et al report the results of the first trial of the thrombopoietin mimetic, romiplostim, in this setting, demonstrating recovery of platelet counts in 18 of 24 patients.

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

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.