The biology of thrombopoietin and thrombopoietin receptor agonists

Thrombopoietin (TPO) is the major physiological regulator of platelet production. TPO binds the TPO receptor, activates JAK and STAT pathways, thus stimulating megakaryocyte growth and platelet production. There is no “sensor” of the platelet count; rather TPO is produced in the liver at a constant rate and cleared by TPO receptors on platelets. TPO levels are inversely proportional to the rate of platelet production. Early recombinant TPO molecules were potent stimulators of platelet production and increased platelets in patients with immune thrombocytopenia, chemotherapy-induced thrombocytopenia, myelodysplastic syndromes and platelet apheresis donors. Neutralizing antibodies formed against one recombinant protein and ended their development. A second generation of TPO receptor agonists, romiplostim and eltrombopag, has been developed. Romiplostim is an IgG heavy chain into which four TPO agonist peptides have been inserted. Eltrombopag is an oral small molecule. These activate the TPO receptor by different mechanisms to increase megakaryocyte growth and platelet production. After administration of either to healthy volunteers, there is a delay of 5 days before the platelet count rises and subsequently reaches a peak after 12–14 days. Both have been highly effective in treating ITP and hepatitis C thrombocytopenia. Studies in a wide variety of other thrombocytopenic conditions are underway.     

Thrombopoietin receptor agonists for preparing adult patients with immune thrombocytopenia to splenectomy: results of a retrospective,observational GIMEMA study

In patients with immune thrombocytopenia (ITP) refractory to corticosteroids and intravenous immunoglobulins (IVIG), splenectomy may result at higher risk of peri‐operative complications and, for this reason, potentially contraindicated. The thrombopoietin receptor agonists (TPO‐RAs) romiplostim and eltrombopag have shown high therapeutic activity in primary ITP, but data of efficacy and safety regarding their use in preparation for splenectomy are missing. Thirty‐one adult patients, median age 50 years, with corticosteroids and/or IVIG refractory persistent and chronic ITP who were treated with TPO‐RAs (romiplostim= 24; eltrombopag= 7) with the aim to increase platelet count and allow a safer execution of splenectomy were retrospectively evaluated. Twenty‐four patients (77%) responded to the use of TPO‐RAs with a median platelet count that increased from 11 × 10⁹/L before starting TPO‐RAs to 114 × 10⁹/L pre‐splenectomy, but a concomitant treatment with corticosteroids and/or IVIG was required in 19 patients. Twenty‐nine patients underwent splenectomy while two patients who responded to TPO‐RAs subsequently refused surgery. Post‐splenectomy complications were characterized by two Grade 3 thrombotic events (1 portal vein thrombosis in the patient with previous history of HCV hepatitis and 1 pulmonary embolism), with a platelet count at the time of thrombosis of 260 and 167 × 10⁹/L, respectively and one Grade 3 infectious event. TPO‐RAs may represent a therapeutic option to improve platelet count and reduce the risk of peri‐operative complications in ITP candidates to splenectomy. An increased risk of post‐splenectomy thromboembolic events cannot be ruled out and thromboprophylaxis with low‐molecular weight heparin is generally recommended. Am. J. Hematol. 91:E293–E295, 2016. © 2016 Wiley Periodicals, Inc.     

Use of eltrombopag after romiplostim in primary immune thrombocytopenia

The thrombopoietin receptor agonists (THPO‐RAs), romiplostim and eltrombopag, are effective and safe in immune thrombocytopenia (ITP). However, the value of their sequential use when no response is achieved or when adverse events occur with one THPO‐RA has not been clearly established. Here we retrospectively evaluated 51 primary ITP adult patients treated with romiplostim followed by eltrombopag. The median age of our cohort was 49 (range, 18–83) years. There were 32 women and 19 men. The median duration of romiplostim use before switching to eltrombopag was 12 (interquartile range 5–21) months. The reasons for switching were: lack of efficacy (n = 25), patient preference (n = 16), platelet‐count fluctuation (n = 6) and side‐effects (n = 4). The response rate to eltrombopag was 80% (41/51), including 67% (n = 35) complete responses. After a median follow‐up of 14 months, 31 patients maintained their response. Efficacy was maintained after switching in all patients in the patient preference, platelet‐count fluctuation and side‐effect groups. 33% of patients experienced one or more adverse events during treatment with eltrombopag. We consider the use of eltrombopag after romiplostim for treating ITP to be effective and safe. Response to eltrombopag was related to the cause of romiplostim discontinuation.     

Immune thrombocytopenia: antiplatelet autoantibodies inhibit proplatelet formation by megakaryocytes and impair platelet production in vitro

Primary immune thrombocytopenia is an autoimmune disease mediated by antiplatelet autoantibodies that cause platelet destruction and suppression of platelet production. In vitro effects of autoantibodies on megakaryocyte production and maturation have been reported recently. However, the impact of these autoantibodies on crucial megakaryocyte functions, proplatelet formation and subsequent platelet release, has not been evaluated. We examined the effects of serum and IgG from 19 patients with immune thrombocytopenia using day 8 or 9 megakaryocytes (66.3 ± 10.6% CD41⁺), derived from cord blood hematopoietic stem cells (CD34⁺). The number of proplatelet-bearing megakaryocytes, the number of platelets released in the culture, total megakaryocyte numbers, ploidy pattern and caspase activation were measured at various times after treatment. After 5 days of treatment the number of proplatelet-bearing megakaryocytes was significantly decreased by 13 immune thrombocytopenia autoantibodies relative to the control group (P<0.0001) and this decrease was accompanied by a corresponding reduction of platelet release. Other features, including total megakaryocyte numbers, maturation and apoptosis, were not affected by immune thrombocytopenia antibodies. Treating the megakaryocytes with the thrombopoietin receptor agonists romiplostim and eltrombopag reversed the effect of the autoantibodies on megakaryocytes by restoring their capacity to form proplatelets. We conclude that antiplatelet antibodies in immune thrombocytopenia inhibit proplatelet formation by megakaryocytes and hence the ability of the megakaryocytes to release platelets. Treatment with either romiplostim or eltrombopag regenerates proplatelet formation from the megakaryocytes.     

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.     

Alternate use of thrombopoietin receptor agonists in adult primary immune thrombocytopenia patients: A retrospective collaborative survey from Italian hematology centers

Sequential use of the TPO‐RAs romiplostim and eltrombopag in ITP patients failing either agent was retrospectively evaluated to assess efficacy and impact of clinical characteristics on outcome. Patients were grouped into 5 categories: efficacy issues: 1^st TPO‐RA failure; loss of response; non‐efficacy issues: platelet fluctuations; patient's preference; adverse event development. Either one TPO‐RA sequence was analyzed at 3 month and at last follow‐up. 106/546 patients on TPO‐RA underwent switch and 65% achieved, regained or maintained a short‐ term response independent of switch sequence, gender or age; lower response rates were associated with lines of previous therapy; disease duration lowers probability to respond. Clinically, patients switched for efficacy issue did not differ from those switched for non‐efficacy issues. Response was achieved/regained in 57.8% of patients switched for efficacy issues, the lowest response rates were observed in non‐responders to 1^st TPO‐RA; 80% of patients switched for non‐efficacy issues maintained a response. Platelet fluctuation resolved in 44.4%. Of the 49 patients evaluable for long‐term outcome, 27 were in response on therapy; 16 discontinued the TPO‐RA for reasons other than efficacy, while only 6 were non responders. We confirm the efficacy of TPO‐RA switch; once achieved, response to the 2^nd TPO‐RA seems durable.     

Treatment efficacy for adult persistent immune thrombocytopenia: a systematic review and network meta-analysis

Persistent immune thrombocytopenia (ITP) patients require second-line treatments, for which information on clinical outcomes are lacking. A systematic review and network meta-analysis (NMA) were conducted. Only randomised controlled trials (RCT) of second-line drugs in adult persistent ITP patients with platelet response, platelet count, any bleeding or serious adverse events (SAE) outcome were eligible. Twelve RCTs (n = 1313) were included in NMA. For platelet response outcome, eltrombopag and romiplostin were the best relative to placebo; the former had a non-significant advantage [risk ratio (RR) = 1·10 (95% confidence interval: 0·46, 2·67)]. Both treatments were superior to rituximab and recombinant human thrombopoietin (rhTPO)+rituximab, with corresponding RRs of 4·56 (1·89, 10·96) and 4·18 (1·21, 14·49) for eltrombopag; 4·13 (1·56, 10·94) and 3·79 (1·02, 14·09) for romiplostim. For platelet count, romiplostim ranked highest, followed by eltrombopag, rhTPO+rituximab, and rituximab. For bleeding, rituximab had lowest risk, followed by eltrombopag and romiplostim. For SAEs, rhTPO+rituximab had highest risk, followed by rituximab, eltrombopag and romiplostim. From clustered ranking, romiplostim had the best balance between short-term efficacy and SAEs, followed by eltrombopag. In conclusion, romiplostim and eltrombopag may yield high efficacy and safety. Rituximab may not be beneficial due to lower efficacy and higher complications compared with the thrombopoietin receptor agonists. RCTs with long-term clinical outcomes are required.     

Safety and efficacy of thrombopoietin‐receptor agonists in myelodysplastic syndromes: a systematic review and meta‐analysis of randomized controlled trials

Thrombocytopenia is common (40–65%) and potentially serious in myelodysplastic syndromes (MDS). A systematic review was conducted to determine the safety and efficacy of adding a thrombopoietin‐receptor (THPO‐R) agonist to standard MDS treatment. MEDLINE, EMBASE and CENTRAL databases were searched. We included randomized controlled trials comparing a THPO‐R agonist to placebo. A meta‐analysis of the effects was performed. Endpoints included bleeding and platelet transfusion rates, risk of progression to acute myeloid leukaemia (AML) and mortality. Three hundred and eighty four patients from five trials were included, four using romiplostim and one using eltrombopag. Overall, the relative risk (RR) of bleeding with romiplostim versus placebo was 0·84 [95% confidence interval (CI): 0·57–1·24]. However, compared to placebo, romiplostim significantly decreased the exposure‐adjusted bleeding rate (RR 0·92; 95% CI: 0·86–0·99), as well as the exposure‐adjusted platelet transfusion rate (RR 0·69; 95% CI: 0·53–0·88). The RR of AML progression with romiplostim was 1·36 (95% CI: 0·54–3·40), however the outcome data were judged as higher risk of bias. Romiplostim is promising in its ability to decrease patient‐important outcomes: bleeding and platelet transfusion need. Although the risk of AML progression was not increased, due to unclear risk of bias in the data, this safety concern is difficult to assess. Therefore, romiplostim cannot yet be routinely recommended. Early eltrombopag data is promising.     

What is the potential for thrombopoietic agents in acute leukemia?

In the 16 years since thrombopoietin was identified and cloned, much has been learned about its biochemistry, how it is regulated, and its involvement in a wide range of functions in a variety of cell lineages. The first generation of recombinant human thrombopoietins, rHuTPO and pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF), were shown to increase platelet counts in patients with immune thrombocytopenia, in platelet apheresis donors, and in patients receiving nonmyeloablative chemotherapy. Their effects in patients with acute myeloid leukemia (AML) showed no benefit at a wide range of doses and schedules. The two second-generation TPO mimetics approved by the US Food and Drug Administration (FDA) for the treatment of ITP, romiplostim and eltrombopag, are now being studied in a number of thrombocytopenic disorders including those due to chemotherapy and hepatitis C. Since romiplostim is comparable to the first-generation recombinant thrombopoietins, it may not be beneficial in AML treatment; however, given its novel mechanism of action, eltrombopag may be a TPO potentiator and if given at the proper time during chemotherapy, may enable AML patients to recover platelet counts sooner.     

Have splenectomy rate and main outcomes of ITP changed after the introduction of new treatments? A monocentric study in the outpatient setting during 35 years

In the last years, rituximab (RTX) and agonists of the thrombopoietin receptor (TPO‐R) eltrombopag and romiplostim have provided new treatment options in persistent and chronic immune thrombocytopenia (ITP). Here, we analyzed the changes in therapeutic choices over time and their impact on clinical outcomes in a cohort of 557 ITP outpatients followed at the “L. and A. Seràgnoli” Institute of Hematology, Bologna, Italy, from 1980 to 2015. Overall 397 patients (71%) required front‐line corticosteroids, mainly prednisone. Over the decades, splenectomy was delayed from second to third‐line, but was steadily used in around 15–25% of patients refractory or relapsing after first‐line treatment. Consensually, RTX and TPO‐R agonists emerged as second and third‐line therapy of choice, respectively. Splenectomy was associated with the best response rates and the lower incidences of relapse, while the relapse rate after RTX was comparable to that observed with corticosteroids and other immunosuppressive agents. The introduction of TPO‐R agonists gave an alternative to the administration of immunosuppressive drugs and probably contributed to moderate the incidence of infectious complications that remained stable over the decades, despite an increasing use of RTX from the 2000s onwards. Overall responses were similar over time, with over 97% achieving a response in all time‐periods. However, the cumulative risk of bleeding significantly decreased [14.3% (1980–89) vs. 7% (1990–99) vs. 5.6% (2000–09) vs. 0.2% (2010–15)] (P < 0.001), mainly thanks to the optimization of front‐line corticosteroids therapy and to the wider availability of second and third‐line therapies. Am. J. Hematol. 91:E267–E272, 2016. © 2016 Wiley Periodicals, Inc.     

Thrombopoietin and platelet aggregation in patients with stable coronary artery disease

Thrombopoietin (TPO) may facilitate platelet activation and aggregation. However, data on the impact of TPO on platelet aggregation in patients with stable coronary artery disease (CAD) are scarce. We aimed to investigate associations between TPO and platelet aggregation and activation in patients with stable coronary artery disease (CAD). We studied 900 stable CAD patients. Serum TPO was assessed by ELISA. Platelet aggregation was evaluated using the Multiplate Analyzer (agonists: arachidonic acid [AA] and collagen) and the VerifyNow Aspirin Assay. Platelet activation was evaluated by soluble (s)P-selectin. Cyclooxygenase-1 inhibition was evaluated by serum thromboxane B₂ (TXB₂). We found that TPO correlated weakly with platelet aggregation evaluated by Multiplate using AA (r = ?0.09, p = 0.01) and collagen as agonists (r = ?0.03, p = 0.43) and by VerifyNow (r = 0.07, p = 0.03). We found no correlation between TPO and sP-selectin (r = ?0.01, p = 0.70). Independent predictors of AA-induced platelet aggregation by Multiplate included high levels of sP-selectin and serum TXB₂, high platelet count, increasing age and body mass index, female sex, and active smoking. Independent predictors of TPO included low AA-induced platelet aggregation by Multiplate, high levels of hs-CRP, active smoking, and high platelet aggregation evaluated by VerifyNow. In conclusion, TPO levels did not correlate with platelet activation and only weak associations were found between TPO and platelet aggregation, suggesting that TPO did not substantially facilitate platelet aggregation in stable CAD patients.     

GPIbα-selective activation of platelets induces platelet signaling events comparable to GPVI activation events

Platelet glycoprotein (GP)Ib-IX-V, which binds von Willebrand factor (VWF), and GPVI, which binds collagen, form an adhesion-signaling complex on platelets and mediate platelet adhesion in flowing blood. Platelet activation following engagement of GPIb-IX-V/GPVI by VWF/collagen is critical for initiation and development of a protective thrombus across a site of damaged or exposed endothelium. We examined platelet aggregation and signaling following selective engagement of platelet GPIbα (the major ligand-binding subunit of GPIb-IX-V) by a multivalent surface-expressed GPIbα-binding VWF-A1 domain on COS-7 cells. COS-7 cells expressing the VWF-A1 domain containing an R543W mutation (a gain-of-function mutation found in Type 2B von Willebrand's Disease) were used as a selective agonist for GPIb-IX-V. When incubated in a cell-to-platelet ratio of up to 1 : 1200, VWF-A1/R543W cells caused rapid, spontaneous aggregation of washed platelets that was GPIbα- and α_IIbβ₃-dependent (blocked by inhibitory anti-VWF-A1, anti-GPIbα and anti-α_IIbβ₃ antibodies). Platelet aggregation was also sensitive to inhibitors of Src, phosphoinositide 3-kinase (PI3-kinase) or Syk, confirming a role for these proteins in GPIbα-mediated signal transduction. Platelet tyrosine phosphorylation patterns and specific tyrosine phosphorylation of Syk after GPIbα engagement by VWF-A1/R543W was comparable to that induced by engagement of GPVI by collagen or collagen-related peptide (CRP). These data indicate signaling events triggered by specific ligation of GPIbα can lead to robust platelet activation and help define GPIb-IX-V as both an adhesion and signaling receptor on platelets.     

Cyproheptadine‐induced myeloma cell apoptosis is associated with inhibition of the PI3K/AKT signaling

Recent studies revealed that the anti‐allergic cyproheptadine displays anti‐blood cancer activity. However, its mechanism is still elusive. In this study, cyproheptadine was found to decrease the expression of anti‐apoptotic proteins, including Bcl‐2, Mcl‐1, and XIAP. More importantly, cyproheptadine‐induced apoptosis was accompanied by suppressing AKT activation in myeloma cells. In the subsequent study, cyproheptadine was found to inhibit insulin‐like growth factor 1‐triggered AKT activation in a time‐ and concentration‐dependent manner. Specifically, cyproheptadine blocked AKT translocation from nuclei for phosphorylation. This inhibition led to suppressed activation of p70S6K and 4EBP1, two key downstream signaling proteins in the PI3K/AKT pathway. However, cyproheptadine did not display inhibition on activation of IGF‐1R or STAT3, possible upstream signals of AKT activation. These results further demonstrated that cyproheptadine suppresses the PI3K/AKT signaling pathway, which is probably critical for cyproheptadine‐induced MM cell apoptosis.     

Assessment of romiplostim immunogenicity in adult patients in clinical trials and in a global postmarketing registry

Antibodies to first-generation recombinant thrombopoietin (TPO) neutralized endogenous TPO and caused thrombocytopenia in some healthy subjects and chemotherapy patients. The second-generation TPO receptor agonist romiplostim, having no sequence homology to TPO, was developed to avoid immunogenicity. This analysis examined development of binding and neutralising antibodies to romiplostim or TPO among adults with immune thrombocytopenia (ITP) in 13 clinical trials and a global postmarketing registry. 60/961 (6·2%) patients from clinical trials developed anti-romiplostim-binding antibodies post-baseline. The first positive binding antibody was detected 14 weeks (median) after starting romiplostim, at median romiplostim dose of 2 µg/kg and median platelet count of 29.5 × 10⁹/l; most subjects had ≥98·5% of platelet assessments showing response. Neutralising antibodies to romiplostim developed in 0·4% of patients, but were unrelated to romiplostim dose and did not affect platelet count. Thirty-three patients (3·4%) developed anti-TPO-binding antibodies; none developed anti-TPO-neutralising antibodies. In the global postmarketing registry, 9/184 (4·9%) patients with spontaneously submitted samples had binding antibodies. One patient with loss of response had anti-romiplostim-neutralising antibodies (negative at follow-up). Collectively, anti-romiplostim-binding antibodies developed infrequently. In the few patients who developed neutralising antibodies to romiplostim, there was no cross-reactivity with TPO and no associated loss of platelet response.     

Are adverse drug reaction patterns different between romiplostim and eltrombopag? 2009–2013 French PharmacoVigilance assessment

BackgroundRomiplostim and eltrombopag, the two marketed thrombopoietin receptor agonists (TPO-RAs), have distinct binding sites and might have distinct pharmacodynamic mechanisms. The aim of this study was to compare their adverse drug reaction (ADR) patterns.MethodsWe selected in the French PharmacoVigilance Database all ADRs associated with TPO-RAs from TPO-RA marketing until the 31st of December 2013. Medical charts were reviewed. We conducted disproportionality analyses comparing romiplostim exposure in the reports of a given ADR pattern (thrombosis, neurological, cutaneous, gastrointestinal or hematological) to romiplostim exposure in all other TPO-RA-related ADR reports. Reporting Odds Ratios (RORs) were adjusted for age and gender. We also compared the number of reports of a given ADR pattern per million daily defined doses (DDDs) dispensed in France during the study period.ResultsWe described 45 reports (53 ADRs) with romiplostim and 26 reports (37 ADRs) with eltrombopag. There were 19 venous thromboses. At least one other risk factor was present in 83.3% of the cases. Ten (55.6%) patients had been splenectomized previously. There were eight arterial thromboses. Another risk factor was noticed in all cases. There was no signal for an excess risk of thrombosis with romiplostim versus eltrombopag (ROR: 1.45, 95% CI [0.48–4.45]). There was a signal for a higher risk of gastrointestinal ADRs with eltrombopag (ROR: 30.28, 95% CI [3.23–383.86]) and of hematological ADRs with romiplostim (ROR: 14.36, 95% CI [1.73–119.08]). Dispensing data-adjusted comparisons led to similar results.ConclusionsThis study suggests different ADR patterns between romiplostim and eltrombopag.     

Should medical treatment options be exhausted before splenectomy is performed in adult ITP patients? A debate

Patients with primary immune thrombocytopenia (ITP) may require treatment to reduce the risk of serious bleeding if platelets remain consistently below 30 × 10⁹/L. While approximately 70–80% of patients respond to an initial course of corticosteroids, relapse is common. For steroid-refractory patients, there is a choice between surgical splenectomy and further medical treatments, based on many factors including the patient’s bleeding history, fitness for surgery, comorbidities, tolerance of adverse events, lifestyle and preferences. Treatments that have traditionally been used (corticosteroids, azathioprine, danazol) suppress the immune system, potentially predisposing patients to infection. Recent insights into the underlying pathophysiology of the disease have allowed the development of targeted therapies, including the thrombopoietin (TPO) receptor agonists, which enhance platelet production. Phase III trials have found romiplostim and eltrombopag to be well tolerated and effective in elevating platelet counts and reducing bleeding in both splenectomised and nonsplenectomised patients with chronic ITP. The B-cell targeted monoclonal antibody rituximab has also shown some potential in this setting, although data are currently limited and there are toxicity concerns. The decision whether to proceed to splenectomy or try other medical therapies in corticosteroid-refractory patients remains patient-specific. Splenectomy has its risks (including perioperative and long-term risks), and relapse/nonresponse are relatively common, but it offers the possibility of cure in the majority of patients. However, newer treatments may potentially allow splenectomy to be deferred for prolonged periods, as well as providing alternative treatment options for patients who fail splenectomy.     

Characterization of UBO-QIC as a Gαq inhibitor in platelets

Gα_q plays an important role in platelet activation by agonists such as thrombin, adenosine diphosphate (ADP) and thromboxane. The significance of Gα_q signaling in platelets was established using YM254890, a Gα_q/11-specific inhibitor and Gα_q knockout murine platelets. However, YM-254890 is no longer available for investigators and there is a need to characterize other Gα_q inhibitors. The aim of this study is to characterize the specificity of a compound, {L-threonine,(3R)-N-acetyl-3-hydroxy-L-leucyl-(aR)-a-hydroxybenzenepropanoyl-2,3-idehydro-N-methylalanyl-L-alanyl-N-methyl-L-alanyl-(3R)-3-[[(2S,3R)-3-hydroxy-4-methyl-1-oxo-2-[(1-oxopropyl)amino]pentyl]oxy]-L-leucyl-N,O-dimethyl-,(7→1)-lactone (9CI)} (UBO-QIC), as a Gα_q inhibitor in platelets. Human platelets treated with UBO-QIC showed a concentration-dependent inhibition of platelet aggregation and secretion by protease-activated receptors (PAR) agonists, U46619 and ADP. UBO-QIC also abolished Gα_q pathway signaling events such as calcium mobilization and pleckstrin phosphorylation. UBO-QIC had no nonspecific effects on the Gα_12/13 pathway since platelet shape change was intact in Gα_q knockout murine platelets stimulated with PAR agonists in the presence of the inhibitor. In addition, UBO-QIC-treated platelets did not affect collagen-related peptide-induced platelet activation suggesting that this inhibitor had no non-specific effects on the GPVI pathway. Furthermore, Akt phosphorylation downstream of the Gα_i and Gα_z pathways, and vasodilator-stimulated phosphoprotein phosphorylation downstream of the Gα_s pathway were not inhibited in UBO-QIC-treated platelets. UBO-QIC is a specific inhibitor for Gα_q, which can be a useful tool for investigating Gα_q-coupled receptor signaling pathways in platelets.     

Pathophysiology and management of thrombocytopenia in bone marrow failure: possible clinical applications of TPO receptor agonists in aplastic anemia and myelodysplastic syndromes

Aplastic anemia is a bone marrow failure syndrome that causes pancytopenia and can lead to life-threatening complications. Bone marrow transplantation remains the standard of care for younger patients and those with a good performance status but many patients may not have a suitable donor. Immunosuppressive therapy is able to resolve cytopenias in a majority of patients with aplastic anemia but relapses are not uncommon and some patients remain refractory to this approach. Patients may require frequent blood and platelet transfusion support which is expensive and inconvenient. Life-threatening bleeding complications still occur despite prophylactic platelet transfusion. Thrombopoietin (TPO) mimetics, such as romiplostim and eltrombopag, were developed to treat patients with refractory immune thrombocytopenia but are now being investigated for the treatment of bone marrow failure syndromes. TPO is the main regulator for platelet production and its receptor (c-Mpl) is present on megakaryocytes and hematopoietic stem cells. Trilineage hematopoietic responses were observed in a recent clinical trial using eltrombopag in patients with severe aplastic anemia refractory to immunosuppression suggesting that these agents can provide a new therapeutic option for enhancing blood production. In this review, we discuss these recent results and ongoing investigation of TPO mimetics for aplastic anemia and other bone marrow failure states like myelodysplastic syndromes. Clonal evolution or progression to acute myeloid leukemia remains a concern when using these drugs in bone marrow failure and patients should only be treated in the setting of a clinical trial.