Eltrombopag inhibition of acute myeloid leukemia cell survival does not depend on c-Mpl expression

Xanthotoxin, a novel inducer of platelet formation, promotes thrombocytopoiesis via IL-1R1 and MEK/ERK signaling

BACKGROUND

Thrombocytopenia is a common hematological disease caused by many factors. It usually complicates critical diseases and increases morbidity and mortality. The treatment of thrombocytopenia remains a great challenge in clinical practice, however, its treatment options are limited. In this study, the active monomer xanthotoxin (XAT) was screened out to explore its medicinal value and provide novel therapeutic strategies for the clinical treatment of thrombocytopenia.

METHODS

The effects of XAT on megakaryocyte differentiation and maturation were detected by flow cytometry, Giemsa and phalloidin staining. RNA-seq identified differentially expressed genes and enriched pathways. The signaling pathway and transcription factors were verified through WB and immunofluorescence staining. Tg (cd41: eGFP) transgenic zebrafish and mice with thrombocytopenia were used to evaluate the biological activity of XAT on platelet formation and the related hematopoietic organ index in vivo.

RESULTS

XAT promoted the differentiation and maturation of Meg-01 cells in vitro. Meanwhile, XAT could stimulate platelet formation in transgenic zebrafish and recover platelet production and function in irradiation-induced thrombocytopenia mice. Further RNA-seq prediction and WB verification revealed that XAT activates the IL-1R1 target and MEK/ERK signaling pathway, and upregulates the expression of transcription factors related to the hematopoietic lineage to promote megakaryocyte differentiation and platelet formation.

CONCLUSION

XAT accelerates megakaryocyte differentiation and maturation to promote platelet production and recovery through triggering IL-1R1 and activating the MEK/ERK signaling pathway, providing a new pharmacotherapy strategy for thrombocytopenia.

Clinical spectrum, outcome and management of immune thrombocytopenia associated with myelodysplastic syndromes and chronic myelomonocytic leukemia

Myelodysplastic syndromes (MDS) and chronic myelomonocytic leukemia (CMML) are associated with systemic inflammatory or autoimmune diseases in 10-20 % of cases. Among them, immune thrombocytopenia (ITP) has been reported but large studies assessing this association are missing. Whether such patients have a particular phenotype and require particular management is unclear. This study analyzes the clinical spectrum, outcome and therapeutic management of patients with ITP associated with MDS or CMML, in comparison (i) to patients with primary ITP without MDS/CMML and (ii) to patients with MDS/CMML without ITP. Forty-one MDS/CMML-associated ITP patients were included, with chronic ITP in 26 (63%) patients, low-risk myelodysplasia in 30 (73%) patients and CMML in 24 (59%) patients. An associated autoimmune disease was noted in 10 (24%) patients. In comparison to primary ITP patients, MDS/CMML-associated ITP patients had a higher occurrence of severe bleeding despite similar platelet counts at diagnosis. First-line treatment consisted of glucocorticoids (98%) and intravenous immunoglobulin (IVIg) (56%). Response achievement with IVIg was more frequent in primary ITP than in MDS/CMML-associated ITP patients. Response rates to second-line therapies were not statistically different between primary ITP and MDS/CMMLassociated ITP patients. Ten percent (n=4) of patients with MDS/CMML-associated ITP had multirefractory ITP versus none in primary ITP controls. After a median follow-up of 60 months, there was no difference in overall survival between MDS/CMML-associated ITP and primary ITP patients. Leukemia-free-survival was significantly better in MDS/CMMLassociated ITP patients than in MDS/CMML without ITP MDS/CMML-associated ITP have a particular outcome with more severe bleeding and multirefractory profile than primary ITP, similar response profile to primary ITP therapy except for IVIg, and less progression toward acute myeloid leukemia than MDS/CMML without ITP.

Mechanisms and therapeutic prospects of thrombopoietin receptor agonists

The second-generation thrombopoietin (TPO) receptor agonists eltrombopag and romiplostim are potent activators of megakaryopoiesis and represent a growing treatment option for patients with thrombocytopenic hematological disorders. Both TPO receptor agonists have been approved worldwide for the treatment of children and adults with chronic immune thrombocytopenia. In the EU and USA, eltrombopag is approved for the treatment of patients with severe aplastic anemia who have had an insufficient response to immunosuppressive therapy and in the USA for the first-line treatment of severe aplastic anemia in combination with immunosuppressive therapy. Eltrombopag has also shown efficacy in several other disease settings, for example, chemotherapy-induced thrombocytopenia, selected inherited thrombocytopenias, and myelodysplastic syndromes. While both TPO receptor agonists stimulate TPO receptor signaling and enhance megakaryopoiesis, their vastly different biochemical structures bestow upon them markedly different molecular and functional properties. Here, we review and discuss results from preclinical and clinical studies on the functional and molecular mechanisms of action of this new class of drug.

Azacitidine with or without eltrombopag for first-line treatment of intermediate- or high-risk MDS with thrombocytopenia

Azacitidine treatment of myelodysplastic syndromes (MDSs) generally exacerbates thrombocytopenia during the first treatment cycles. A Study of Eltrombopag in Myelodysplastic Syndromes Receiving Azacitidine (SUPPORT), a phase 3, randomized, double-blind, placebo-controlled study, investigated the platelet supportive effects of eltrombopag given concomitantly with azacitidine. International Prognostic Scoring System intermediate-1, intermediate-2, or high-risk MDS patients with baseline platelets <75 × 10⁹/L were randomized 1:1 to eltrombopag (start, 200 mg/d [East Asians, 100 mg/d], maximum, 300 mg/d [East Asians, 150 mg/d]) or placebo, plus azacitidine (75 mg/m² subcutaneously once daily for 7 days every 28 days). The primary end point was the proportion of patients platelet transfusion-free during cycles 1 through 4 of azacitidine therapy. Based on planned interim analyses, an independent data monitoring committee recommended stopping the study prematurely because efficacy outcomes crossed the predefined futility threshold and for safety reasons. At termination, 28/179 (16%) eltrombopag and 55/177 (31%) placebo patients met the primary end point. Overall response (International Working Group criteria; complete, marrow, or partial response) occurred in 20% and 35% of eltrombopag and placebo patients, respectively, by investigator assessment. There was no difference in hematologic improvement in any cell lineage between the 2 arms. There was no improvement in overall or progression-free survival. Adverse events with ≥10% occurrence in the eltrombopag vs placebo arm were febrile neutropenia and diarrhea. Compared with azacitidine alone, eltrombopag plus azacitidine worsened platelet recovery, with lower response rates and a trend toward increased progression to acute myeloid leukemia. This trial was registered at www.clinicaltrials.gov as #NCT02158936.

c-MPL provides tumor-targeted T-cell receptor-transgenic T cells with costimulation and cytokine signals

Adoptively transferred T-cell receptor (TCR)-engineered T cells depend on host-derived costimulation and cytokine signals for their full and sustained activation. However, in patients with cancer, both signals are frequently impaired. Hence, we developed a novel strategy that combines both essential signals in 1 transgene by expressing the nonlymphoid hematopoietic growth factor receptor c-MPL (myeloproliferative leukemia), the receptor for thrombopoietin (TPO), in T cells. c-MPL signaling activates pathways shared with conventional costimulatory and cytokine receptor signaling. Thus, we hypothesized that host-derived TPO, present in the tumor microenvironment, or pharmacological c-MPL agonists approved by the US Food and Drug Administration could deliver both signals to c-MPL-engineered TCR-transgenic T cells. We found that c-MPL⁺ polyclonal T cells expand and proliferate in response to TPO, and persist longer after adoptive transfer in immunodeficient human TPO-transgenic mice. In TCR-transgenic T cells, c-MPL activation enhances antitumor function, T-cell expansion, and cytokine production and preserves a central memory phenotype. c-MPL signaling also enables sequential tumor cell killing, enhances the formation of effective immune synapses, and improves antileukemic activity in vivo in a leukemia xenograft model. We identify the type 1 interferon pathway as a molecular mechanism by which c-MPL mediates immune stimulation in T cells. In conclusion, we present a novel immunotherapeutic strategy using c-MPL-enhanced transgenic T cells responding to either endogenously produced TPO (a microenvironment factor in hematologic malignancies) or c-MPL-targeted pharmacological agents.

From chronic immune thrombocytopenia to severe aplastic anemia: recent insights into the evolution of eltrombopag

Thrombopoietin (TPO) is the most potent cytokine stimulating thrombopoiesis. Therapy with exogenous TPO is limited by the formation of antibodies cross-reacting with endogenous TPO. Mimetics of TPO are compounds with no antigenic similarity to TPO. Eltrombopag is an orally-active nonpeptide small molecule that binds to the transmembrane portion of the TPO receptor MPL. Initial trials of eltrombopag have centered on immune thrombocytopenia (ITP), which is due to both increased destruction and decreased production of platelets. Eltrombopag at 25-75 mg/day has been shown to be highly effective in raising the platelet count in ITP with suboptimal response to immunosuppression and splenectomy. These successful results led to the exploration of eltrombopag in other thrombocytopenic disorders. In hepatitis C viral infection, eltrombopag raises the platelet count sufficiently enough to allow treatment with ribavirin and pegylated interferon. Because MPL is expressed on hematopoietic cells, eltrombopag use in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) might enhance leukemic proliferation. Clinical trials of eltrombopag in MDS and AML, however, have shown amelioration of thrombocytopenia without promoting disease progression. In severe aplastic anemia (SAA) not responding to immunosuppression with anti-thymocyte globulin (ATG) and cyclosporine, eltrombopag as a single agent at 150-300 mg/day results in an overall response rate of 40-70%. At high doses, adverse effects including pigmentation, gastrointestinal upset and hepatic derangement have become evident. Current studies have examined the first-line use of eltrombopag in combination with ATG in SAA. In a recent study, eltrombopag used at 150 mg/day with horse ATG resulted in an overall response rate of 90% in newly diagnosed SAA patients, with a complete response rate of about 50%. Clonal karyotypic aberrations are, however, found in 10-20% of SAA patients treated with eltrombopag. The safety and efficacy of eltrombopag in SAA require further evaluation, particularly when it is used with less intensive immunosuppression.

Thrombocytopenia in MDS: epidemiology, mechanisms, clinical consequences and novel therapeutic strategies

Thrombocytopenia is commonly seen in myelodysplastic syndrome (MDS) patients, and bleeding complications are a major cause of morbidity and mortality. Thrombocytopenia is an independent factor for decreased survival and has been incorporated in newer prognostic scoring systems. The mechanisms of thrombocytopenia are multifactorial and involve a differentiation block of megakaryocytic progenitor cells, leading to dysplastic, hypolobated and microscopic appearing megakaryocytes or increased apoptosis of megakaryocytes and their precursors. Dysregulated thrombopoietin (TPO) signaling and increased platelet destruction through immune or nonimmune mechanisms are frequently observed in MDS. The clinical management of patients with low platelet counts remains challenging and approved chemotherapeutic agents such as lenalidomide and azacytidine can also lead to a transient worsening of thrombocytopenia. Platelet transfusion is the only supportive treatment option currently available for clinically significant thrombocytopenia. The TPO receptor agonists romiplostim and eltrombopag have shown clinical activity in clinical trials in MDS. In addition to thrombopoietic effects, eltrombopag can inhibit leukemic cell proliferation via TPO receptor-independent effects. Other approaches such as treatment with cytokines, immunomodulating drugs and signal transduction inhibitors have shown limited activity in selected groups of MDS patients. Combination trials of approved agents with TPO agonists are ongoing and hold promise for this important clinical problem.

Eltrombopag modulates reactive oxygen species and decreases acute myeloid leukemia cell survival

Previous studies have demonstrated that the small molecule thrombopoietin (TPO) mimetic, eltrombopag (E), induces apoptosis in acute myeloid leukemia (AML) cells. Here, we sought to define the mechanism of the anti-leukemic effect of eltrombopag. Our studies demonstrate that, at a concentration of 5 μM E in 2% serum, E induces apoptosis in leukemia cells by triggering PARP cleavage and activation of caspase cascades within 2–6 hours. The induction of apoptotic enzymes is critically dependent on drug concentration and the concentration of serum. This effect is not associated with an alteration in mitochondrial potential but is associated with a rapid decrease in a reactive oxygen species (ROS) in particular hydrogen peroxide (H₂O₂). Interestingly, E also decreases mitochondrial maximal and spare respiratory capacities suggesting an induced mitochondrial dysfunction that may not be readily apparent under basal conditions but becomes manifest only under stress. Co-treatment of MOLM14 AML cells with E plus Tempol or H₂O₂ provides a partial rescue of cell toxicity. Ferric ammonioum citrate (FAC) also antagonized the E induced toxicity, by inducing notable increase in ROS level. Overall, we propose that E dramatically decreases ROS levels leading to a disruption of AML intracellular metabolism and rapid cell death.

Eltrombopag in aplastic anemia

The treatment of aplastic anemia is currently with immunosuppressive therapy (IST) with anti-thymocyte globulin (ATG) and cyclosporine, to which two thirds of patients respond. However, a significant proportion of these responders relapse and many have persistent cytopenias. The management of these patients is challenging. Modifications to this standard approach using alternative immunosuppressive agents or adding hematopoietic cytokines such as granulocyte colony-stimulating factor (G-CSF) and erythropoietin (EPO) have not improved outcome. A recent trial has shown that eltrombopag, a thrombopoeitin mimetic, is efficacious in the treatment of patients with severe aplastic anemia (SAA) refractory to IST. There is evidence that this drug works by directly stimulating marrow stem and progenitor cells thereby promoting hematopoietic recovery in patients with bone marrow failure. Several trials are ongoing in our institution using this very promising drug in combination therapy in the upfront treatment of SAA, in IST-refractory SAA and in moderate disease.