Extended Dosing With Eltrombopag in Refractory Severe Aplastic Anemia

Severe aplastic anemia (SAA) is a life-threatening blood disease that can be successfully treated with immunosuppressive drug regimens or allogeneic stem cell transplantation. However, 20-40% of patients are ineligible for transplant due to lack of an appropriate donor, age, or comorbidities. Immunosuppression can be more broadly utilized, but about 1/3 of patients do not respond to a single course of horse ATG and cyclosporine and have persistent severe cytopenias. Among patients who do respond to immunosuppression, responses may be partial, with persistent thrombocytopenia, neutropenia, and/or anemia. About 30% of responding patients either relapse or are dependent on continued cyclosporine administration. Patients with refractory severe cytopenias are at risk of dying from infection or bleeding, and they require regular platelet and/or red blood cell transfusions, which are expensive and inconvenient, Patients with refractory SAA are also at risk for progression to other hematologic disorders, including myelodysplasia and leukemia.

Thrombopoietin (TPO) was first identified as the principal protein regulating platelet production, and it stimulates the proliferation of megakaryocytes and release of platelets. TPO was later shown to stimulate proliferation of more primitive bone marrow stem and progenitor cells in vitro and in animal models, suggesting it could have an impact of production of red and white blood cells as well as platelets.

The 2nd generation oral small molecule TPO-agonist eltrombopag (Promacta ) has been shown to increase platelets in healthy subjects and in thrombocytopenic patients with chronic immune thrombocytopenic purpura (ITP) and hepatitis C virus (HCV)-infection. Eltrombopag has been well-tolerated in clinical trials, and unlike recombinant TPO, it does not induce autoantibodies. Eltrombopag received FDA accelerated approval on November 20, 2008 for the treatment of thrombocytopenia in patients with chronic immune (idiopathic) thrombocytopenic purpura who have had an insufficient response to corticosteroids, immunoglobulins, or splenectomy. In November 2012, FDA approval was received for hepatitis C associated thrombocytopenia.

We conducted a pilot dose finding study in patients with severe aplastic anemia who had refractory thrombocytopenia following standard immunosuppressive therapy. Patients began at a dose of 50 mg/day and escalated every two weeks to a maximum dose of 150 mg/day. We reported that 11 of 25 patients (44%) achieved hematological response in at least one lineage following 12 weeks of dose-escalating eltrombopag therapy, with minimal toxicity. Responding patients as assessed at 12 weeks were invited to continue on drug in an extension phase. With a median follow-up of 27 months on drug, 7 eventually became tri-lineage responders. Nine became transfusion-independent for platelets (median increase in platelet count 34,000/micro l), six had improved hemoglobin levels (median increase of 3.8g/dL), including three previously dependent on red cell transfusions achieving transfusion-independence, and eight exhibiting increased neutrophil counts (median increase 590 cells/mico L). Serial bone marrow biopsies demonstrated normalization of tri-lineage hematopoiesis in responders, without increased fibrosis.

In the previous study, response assessment occurred at 12 weeks, and patients not fulfilling response criteria at that time had the drug discontinued. Several patients began to have detectable changes in transfusion requirements or blood counts by 12 weeks, but did not fulfill response criteria by that time point and therefore had to discontinue eltrombopag. Other patients who barely met response criteria at 12 weeks showed very marked further improvements in blood counts in all lineages during the extension phase, in some cases not reaching maximal responses until one year after initiating eltrombopag. We hypothesize that a larger fraction of patients may respond if eltrombopag is continued for longer than 12 weeks.