Severe aplastic anemia (SAA) is a life-threatening bone marrow failure disorder characterized by pancytopenia and a hypocellular bone marrow. Allogeneic bone marrow transplantation offers the opportunity for cure in younger patients, but most are not suitable candidates for transplantation due to advanced age or lack of a histocompatible donor. Comparable long-term survival in SAA is attainable with immunosuppressive treatment with horse anti-thymocyte globulin (h-ATG) and cyclosporine (CsA). However, of those patients treated with h-ATG/CsA, one quarter to one third will not respond, and 30-40% of responders relapse. The majority of the hematologic responses observed following initial h-ATG/CsA are partial, with only a few patients achieving normal blood counts. Furthermore, analysis of our own extensive clinical data suggests that poor blood count responses to a single course of ATG (non-robust responders), even when transfusion-independence is achieved, predicts a worse prognosis than when robust hematologic improvement is achieved (protocol 90-H-0146). The explanation for partial recovery and relapse are not fully understood, but incomplete elimination of auto-reactive T cells and insufficient stem cell reserve are both possible. Furthermore, 10-15% of SAA patients treated with standard immunosuppression will develop an abnormal karyotype in follow-up, with monosomy 7 being most common, which portends progression to myelodysplasia and leukemia. In contrast, malignant clonal evolution is rare in complete responders to immunosuppression. Although horse ATG/CsA represented a major advance in the treatment of SAA, refractoriness, incomplete responses, relapse, and clonal evolution limit the success of this modality. Thus, newer regimens are needed to address these limitations, and provide a better alternative to stem cell transplantation.

One approach to augment the quality of hematologic responses is to improve underlying stem cell function. Previous attempts to improve responses in SAA with hematopoietic cytokines including erythropoietin, G-CSF, and stem cell factor, have failed. Thrombopoietin (TPO) is the principal endogenous regulator of platelet production. In addition, TPO also has stimulatory effects on more primitive multilineage progenitors and stem cells in vitro and in animal models. Eltrombopag (Promacta ), an oral 2nd generation small molecule TPO-agonist, is currently approved for treatment of chronic immune thrombocytopenic purpura (ITP), chronic hepatitis C-associated thrombocytopenia, and severe aplastic anemia who have had an insufficient response to immunosuppressive therapy. Eltrombopag increases platelets in healthy subjects and in thrombocytopenic patients with chronic ITP and hepatitis C virus (HCV) infection. Our Branch recently completed a pilot study of eltrombopag in refractory SAA. We saw encouraging clinical results in a cohort of patients who have failed on average two prior immunosuppressive regimens (Olnes et al. ASH Annual Meeting Abstracts, San Diego, CA, 2011, oral presentation and N Engl J Med 2012;367:11-9.1). Of the twenty-five SAA patients treated with eltrombopag by mouth for three months, eleven (44%) patients met protocol criteria of clinically meaningful hematologic responses, without significant toxicity. Nine patients demonstrated an improvement in thrombocytopenia (\>20k/ L increase or transfusion independence), hemoglobin improved in two patients (\>1.5g/dL or achieved transfusion independence, and four patients had a significant response in their neutrophil count. When responders continued the drug beyond three months, the hematologic response to eltrombopag increased; a trilineage response was observed in four patients, and a bilineage response occurred in another four, with median follow-up of 13 months. These results suggest that stem cell depletion, a major component of the pathophysiology of SAA, might be directly addressed by eltrombopag administration. The aim of the current study would be to improve the hematologic response rate and its quality, as well as prevent late complications such as relapse and clonal progression, by addition of eltrombopag to standard immunosuppressive therapy.

This trial will evaluate the safety and efficacy of combining eltrombopag with standard hATG/CSA as first line therapy in patients with SAA. The primary endpoint will be the rate of complete hematologic response at six months. Secondary endpoints are relapse, robust hematologic blood count recovery at 3, 6, and 12 months, survival, clonal evolution to myelodysplasia and leukemia, marrow stem cell content and hematological response of relapse patients that re-start treatment.