ACE-536 Increases Hemoglobin and Decreases Transfusion Burden and Serum Ferritin in Adults with Beta-Thalassemia: Preliminary Results from a Phase 2 Study

Introduction. ACE-536, a recombinant fusion protein containing modified activin receptor type IIB and IgG Fc, is being developed for the treatment of anemias due to ineffective erythropoiesis (IE), such as beta-thalassemia (β-thal). In β-thal, IE is due to intracellular α-globin aggregates causing premature death of late-stage erythroid precursors. Patients with β-thal often have elevated levels of EPO and are unresponsive to erythropoiesis-stimulating agents (ESAs). ACE-536 binds to ligands in the TGF-β superfamily, such as GDF11, and promotes late-stage erythroid differentiation via a mechanism distinct from ESAs. In a healthy volunteer study, ACE-536 was well-tolerated and increased hemoglobin (Hb) levels (Attie K et al., Am J Hematol 2014). Murine ACE-536 (RAP-536) increased Hb levels and decreased RBC inclusion bodies and hemolysis in a mouse model of β-thal (Suragani R et al., Blood 2014).

Aims. This is an ongoing, phase 2, multicenter, open-label, dose-finding study to evaluate the effects of ACE-536 in adults with transfusion-dependent (TD, defined as ≥4 units RBCs/8 weeks prior to baseline) or non-transfusion dependent (NTD, defined as <4 units RBCs/8 weeks prior to baseline) β-thalassemia. Study outcomes include erythroid response (either Hb increase in NTD patients or reduced RBC transfusion burden in TD patients), safety, tolerability, PK, and pharmacodynamic biomarkers.

Methods. Inclusion criteria included age ≥ 18 yr and anemia, defined as either being TD patient or having baseline Hb <10.0 g/dL in NTD patient. Transfusion burden in TD patients for any 12 weeks on treatment was compared with the 12 weeks prior to treatment. ACE-536 was administered by subcutaneous injection once every 3 weeks for up to 5 doses with a 2-month follow-up. Study design includes sequential cohorts (n=6 patients/cohort) at dose levels of 0.2, 0.4, 0.6, 0.8, 1.0, 1.25 and 1.5 mg/kg. An expansion cohort (n=30) is planned, and all patients completing this study may be eligible to enroll in a 12-month extension study.

Results. Preliminary data were available for the 30 patients (23 NTD/7 TD) enrolled in the first 5 cohorts as of 18 Jul 2014. Median age was 34.5 yr (range: 20-57 yr), 16 (53%) were male and 83% had prior splenectomy. Mean (SD) baseline Hb for the NTD patients was 8.3 (0.9) g/dL.

Efficacy data for the NTD patients in the 0.6 to 1.0 mg/kg cohorts demonstrated a maximum increase from baseline in Hb ≥ 1.5 g/dL in 7 of 11 (64%) patients. Efficacy data for the 7 TD patients treated in the 0.6 to 1.0 mg/kg cohorts demonstrated a >50% reduction in transfusion burden for all 7 patients. This was accompanied by a maximum decrease from baseline in serum ferritin levels ranging from 12-60%. Reductions in liver iron concentration by MRI were observed in both NTD and TD patients. Two patients with long-standing leg ulcers at baseline (one NTD, one TD) healed within approximately 3 months of ACE-536 treatment.

ACE-536 was generally well tolerated. No related serious adverse events have been reported to date. The most frequent related adverse events included bone pain, headache and myalgia. No notable changes in platelets or WBC were observed.

Conclusions. Based on preliminary data, ACE-536 administered SC every 3 weeks for up to 5 doses increased Hb levels in NTD patients and decreased transfusion requirement and serum ferritin levels in TD patients with β-thalassemia via a novel mechanism of action, and demonstrated a favorable safety profile. These data strongly support further evaluation of ACE-536 in patients with β-thalassemia.