An Open-Label, Phase 2, Dose-Finding Study of Sotatercept (ACE-011) in Patients with Low or Intermediate-1 (Int-1)-Risk Myelodysplastic Syndromes (MDS) or Non-Proliferative Chronic Myelomonocytic Leukemia (CMML) and Anemia Requiring Transfusion

Introduction: Anemia, a hallmark of MDS, is challenging to treat, particularly after failure of erythropoiesis-stimulating agents (ESAs). Sotatercept (ACE-011) is a novel and first-in-class activin type IIA receptor fusion protein that acts on late-stage erythropoiesis to increase mature erythrocyte release into the circulation (Carrancio et al. Br J Haematol 2014;165:870-82). Treatment with sotatercept stimulated erythropoiesis and significantly increased hemoglobin (Hb) levels in healthy volunteers (Sherman et al. J Clin Pharmacol 2013;53:1121-30), supporting its clinical development for the treatment of anemia in patients (pts) with lower-risk MDS.

Methods: The primary objective of this phase 2, open-label, dose-finding study is to determine a safe, tolerable, and effective dose of sotatercept resulting in erythroid hematological improvement (HI-E; modified IWG 2006 criteria) in pts with anemia and IPSS-defined Low or Int-1-risk MDS or non-proliferative CMML (white blood cells < 13,000/µL). Secondary objectives include rate of RBC-transfusion independence (RBC-TI) ≥ 8 weeks. Eligible pts had anemia (≥ 2 RBC units transfusion requirement in the 12 weeks prior to enrollment for Hb ≤ 9.0 g/dL) with no response, loss of response, or low chance of response to ESAs (serum erythropoietin [EPO] > 500 mIU/mL). Pts received subcutaneous sotatercept at dose levels of 0.1, 0.3, 0.5, or 1.0 mg/kg once every 3 weeks. ClinicalTrials.gov identifier: NCT01736683.

Results: As of May 22, 2014, a total of 54 MDS pts were enrolled: 7, 6, 21, and 20 in the sotatercept 0.1, 0.3, 0.5, and 1.0 mg/kg dose groups, respectively. Median age was 71 years (range 56–86) and median time from diagnosis was 4 years (range 0–31); most pts were male (70%). Pts received a median of 6 RBC units (range 0–18) in the 8 weeks prior to treatment start. Forty-five pts (83%) received ≥ 4 RBC units in the 8 weeks prior to treatment start (high transfusion burden; HTB), and 9 pts (17%) received < 4 units in the 8 weeks prior to treatment start (low transfusion burden; LTB). Nineteen pts (35%) had IPSS Low and 34 pts (63%) had IPSS Int-1-risk MDS; IPSS risk data were missing for 1 pt. Fifty-one pts (94%) had prior treatment with ESAs, 30 (56%) with hypomethylating agents, 26 (48%) with lenalidomide, and 26 (48%) with other MDS treatments; 15 pts (28%) had serum EPO > 500 mIU/mL.

Of the 53 pts evaluable for efficacy, HI-E was observed in 21 pts (40%) overall: 0, 4 (67%), 8 (40%), and 9 pts (45%) in the sotatercept 0.1, 0.3, 0.5, and 1.0 mg/kg dose groups, respectively. Nineteen of 44 HTB pts responded with a ≥ 4 RBC units/8 weeks transfusion burden reduction; duration of transfusion response appeared to be dose-dependent. Five HTB pts achieved RBC-TI ≥ 8 weeks, with RBC-TI duration ranging from 59–345+ days.

Eight of 9 LTB pts showed Hb increases, not influenced by transfusion, ranging from 1.3–3.8 g/dL. Of these, 2 pts had a Hb increase ≥ 1.5 g/dL sustained for ≥ 8 weeks. Pts with Hb > 11.0 g/dL were subject to dose delay per protocol, which may have impacted Hb increase sustainability. RBC-TI ≥ 8 weeks was achieved in 6 LTB pts. Increases in platelet and neutrophil levels were seen in pts with baseline thrombocytopenia and pts with baseline neutropenia, respectively.

Sotatercept was generally well tolerated. Twenty pts (37%) reported ≥ 1 suspected treatment-related adverse event (AE); fatigue (11%), headache (9.3%), decreased appetite (7.4%), and nausea (7.4%) were the most common.

Of 35 pts (65%) who discontinued treatment, 28 discontinued due to lack of therapeutic effect and 4 due to AEs. Of those AEs leading to discontinuation, 3 were suspected to be treatment-related: 1 pt with grade 2 hemolytic anemia, 1 pt with grade 3 hypertension, and 1 pt with grade 2 muscular weakness in the sotatercept 0.3, 0.5, and 1.0 mg/kg dose groups, respectively. Other reasons for discontinuation were withdrawal of consent (n = 2; 4%) and pt decision (n = 1; 2%).

Conclusions: Sotatercept was well tolerated in lower-risk MDS pts at the dose levels tested, with promising evidence of clinical activity in this largely HTB cohort of ESA-refractory, anemic, lower-risk MDS pts. Further exploration of higher sotatercept dose levels and longer-term treatment is planned.

PF and AFL contributed equally to this abstract as senior co-authors.