Sotatercept Data from the Phase 2a Clinical Trial in Hemodialysis Patients Presented at the American Society of Nephrology Kidney Week 2014

Acceleron Pharma Inc. (NASDAQ:XLRN), a clinical stage biopharmaceutical company focused on the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases, today reported that investigators on the sotatercept program presented interim clinical data demonstrating encouraging effects of sotatercept on vascular calcification, bone mineral density, and hemoglobin levels in patients with end-stage renal disease on hemodialysis. The data were presented at the American Society of Nephrology (ASN) Kidney Week 2014 meeting in Philadelphia, PA. Acceleron’s collaboration partner, Celgene, is conducting the phase 2 clinical trial of sotatercept in patients with end-stage renal disease on hemodialysis.

“The sotatercept data showing a reduction in the rate of vascular calcification suggests a novel potential treatment for patients with end-stage renal disease,” said Matthew L. Sherman, M.D., Chief Medical Officer of Acceleron. “Cardiovascular disease is the leading cause of death in these ESRD patients on hemodialysis and we’re encouraged by the potential of sotatercept to help address this unmet need.”

Vascular Calcification:

Many patients on hemodialysis suffer from rapidly increasing vascular calcification which can lead to serious and often fatal cardiovascular disease. Sotatercept slowed progression of vascular calcification in the abdominal aorta.

• Dose dependent change from baseline in total Agatston score over approximately 8 months was 58.4%, 24.9%, 17.3% and 3.4% in the placebo, 0.3, 0.5 and 0.7 mg/kg cohorts, respectively.

Renal Osteodystrophy

Hemodialysis patients also suffer from renal osteodystrophy with aberrant bone metabolism characterized by increases in trabecular and decreases in cortical bone mineral density leading to increased risk of fractures. Sotatercept treatment led to decreases in trabecular and increases in cortical bone mineral density over approximately 8 months.

• Dose dependent decreases in trabecular bone mineral density
  • Changes in the lumbar spine bone mineral density were 12.6%, 8.0%, 0.5% and -2.7% in the placebo, 0.3, 0.5 and 0.7 mg/kg cohorts, respectively.
• Dose dependent increases in cortical bone mineral density
  • Changes in the femoral neck cortical bone mineral density were -0.9%, -1.4%, 1.6% and 3.0% in the placebo, 0.3, 0.5 and 0.7 mg/kg cohorts, respectively.
  • Changes in the total hip cortical bone mineral density were -0.1%, -1.1%, 0.5% and 2.7% in the placebo, 0.3, 0.5 and 0.7 mg/kg cohorts, respectively.

Anemia

Sotatercept also produced dose dependent increases in hemoglobin in these patients on hemodialysis during the first 28-day dose cycle.

• A hemoglobin increase of ≥1.0 g/dL was achieved by 13%, 38%, 43% and 60% of the patients in the placebo, 0.3, 0.5 and 0.7 mg/kg cohorts, respectively.

• Treatment with EPO for hemoglobin levels below 9 g/dL was given to 63%, 25%, 29% and 0% of the patients in the placebo, 0.3, 0.5 and 0.7 mg/kg cohorts, respectively.

Sotatercept was generally well-tolerated and most treatment emergent adverse events were mild or moderate in severity, unrelated to study drug, relatively similar between groups, and generally consistent with subjects’ medical histories. The most common treatment emergent adverse events were fatigue, pain, constipation, nausea, viral infection, hypertension, fall, dizziness and increased blood phosphorus.

The clinical posters containing these and other data are available on Acceleron’s website (www.acceleronpharma.com) in the “Publications” tab.

About Sotatercept

Sotatercept is an activin receptor type IIA fusion protein that acts as a ligand trap for members in the Transforming Growth Factor-Beta (TGF-β) superfamily involved in the late stages of erythropoiesis (red blood cell production). Sotatercept regulates late-stage erythrocyte (red blood cell) precursor cell differentiation and maturation. This mechanism of action is distinct from that of erythropoietin (EPO), which stimulates the proliferation of early-stage erythrocyte precursor cells. Acceleron and Celgene are jointly developing sotatercept as part of a global collaboration. Sotatercept is currently in multiple phase 2 clinical trials. For more information, please visit www.clinicaltrials.gov.

About Acceleron

Acceleron is a clinical stage biopharmaceutical company focused on the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases. The company is a leader in understanding the biology of the Transforming Growth Factor-Beta (TGF-β) protein superfamily, a large and diverse group of molecules that are key regulators in the growth and repair of tissues throughout the human body, and in targeting these pathways to develop important new medicines. Acceleron has built a highly productive R&D platform that has generated innovative clinical and preclinical protein therapeutic candidates with novel mechanisms of action. These protein therapeutic candidates have the potential to significantly improve clinical outcomes for patients with cancer and rare diseases.

For more information, please visit www.acceleronpharma.com.

Cautionary Note on Forward-Looking Statements

This press release includes forward-looking statements about the Company’s strategy, future plans and prospects, including statements regarding the development of the Company’s compounds, including sotatercept, luspatercept, dalantercept, or ACE-083 and the Company’s TGF-β superfamily program generally, the timeline for clinical development and regulatory approval of the Company’s compounds, the expected timing for the reporting of data from ongoing trials, and the structure of the Company’s planned or pending clinical trials. The words “anticipate,” “appear,” “believe,” “estimate,” “expect,” “intend,” “may,” “plan,” “predict,” “project,” “target,” “potential,” “will,” “would,” “could,” “should,” “continue,” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. Each forward-looking statement is subject to risks and uncertainties that could cause actual results to differ materially from those expressed or implied in such statement. Applicable risks and uncertainties include the risks that the Company’s cash position will be insufficient to fund operations into the second half of 2017, that preclinical testing of the Company’s compounds and preliminary data from clinical trials may not be predictive of the results or success of ongoing or later clinical trials, that data may not be available when we expect it to be, that the Company or its collaboration partner, Celgene, will be unable to successfully complete the clinical development of its compounds, that the development of the Company’s compounds will take longer or cost more than planned, that the Company may be delayed in initiating or completing any clinical trials, and that the Company’s compounds will not receive regulatory approval or become commercially successful products. Other risks and uncertainties include those identified under the heading “Risk Factors” included in the Company’s Annual Report on Form 10-K which was filed with the Securities and Exchange Commission (SEC) on March 17, 2014, and other filings that the Company may make with the SEC in the future. The forward-looking statements contained in this press release reflect the Company’s current views with respect to future events, and the Company does not undertake and specifically disclaims any obligation to update any forward-looking statements.