Acceleron Earns $7 Million Milestone Payment for Celgene's Initiation of Sotatercept Phase 2 Clinical Trial in Patients with End Stage Renal Disease on Hemodialysis

Acceleron Earns $7 Million Milestone Payment for Celgene's Initiation of Sotatercept Phase 2 Clinical Trial in Patients with End Stage Renal Disease on Hemodialysis

December 18, 2013

CAMBRIDGE, Mass.--(BUSINESS WIRE)--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 announced that its collaboration partner, Celgene, has initiated a phase 2 clinical trial of sotatercept in patients with end stage renal disease on hemodialysis. Sotatercept is a novel and first-in-class activin type IIA receptor (ActRIIA) fusion protein that acts during late-stage erythropoiesis (red blood cell production) to increase the production of mature red blood cells into circulation. Acceleron will receive a $7 million milestone payment from Celgene for the achievement of this event and is also eligible to receive future development, regulatory and commercial milestones of up to $360 million for the sotatercept program.

"In previous clinical studies, sotatercept has shown encouraging activity by increasing red blood cells as well as bone mass in patients, " said Matthew Sherman, M.D., Chief Medical Officer at Acceleron. "We believe that the distinct profile of sotatercept could benefit many patients, including those who have end stage renal disease, the most advanced stage of chronic kidney disease. We look forward to the ongoing development of this promising protein therapeutic candidate, which is also being studied in phase 2 trials in several rare hematologic diseases, including beta-thalassemia and myelodysplastic syndromes."

About the Phase 2 Clinical Trial

The phase 2 clinical trial in patients with end stage renal disease (ESRD) who are on hemodialysis is designed as a two-part study to assess the safety and efficacy of sotatercept as a therapy to treat anemia and potentially to control the adverse manifestations of chronic kidney disease mineral and bone disorder (CKD-MBD). Patients in both parts of the study must first be on a stable dose of an erythropoiesis stimulating agent (ESA) to maintain hemoglobin levels and will then be switched to treatment with sotatercept after an ESA treatment free period of approximately five days.

The first part is a dose-escalation study of intravenous (IV) and subcutaneous (SC) routes of administration of sotatercept in approximately 60 patients to evaluate pharmacokinetics, safety and tolerability. Patients in the dose escalation part of the study will be given sotatercept once every fourteen days up to a total of eight doses and followed for approximately four months after their last dose. The first part of the study is designed to inform the dosing regimens to be tested in the second part of the clinical trial. The second part will be a randomized, controlled study of approximately 230 patients to evaluate the efficacy and safety of sotatercept versus an erythropoiesis stimulating agent (ESA). Efficacy measures for part two of the study include the change in mean hemoglobin concentration from baseline and the ability of sotatercept to maintain patients' hemoglobin levels within a target range after switching from an ESA to sotatercept. Measures of biomarkers for bone remodeling and mineral metabolism also will be studied.

For additional information on this clinical trial, please visit www.clinicaltrials.gov, identifier NCT01999582.

About Chronic Kidney Disease

Chronic Kidney Disease (CKD) is a serious condition characterized by the progressive loss of kidney function and is most often caused by diabetes or high blood pressure. CKD is classified in five stages according to the degree of kidney impairment. End Stage Renal Disease (ESRD), also known as kidney failure, is the most advanced stage of CKD and a life-threatening condition in which patients require a either kidney transplant or dialysis for survival. Anemia, low levels of red blood cells, is a common secondary disorder to Chronic Kidney Disease and worsens with progression of the disease. Disturbance in mineral and bone metabolism, known as Chronic Kidney Disease Mineral and Bone Disorder (CKD-MBD), is also a common complication in CKD patients that affects almost all patients who are on dialysis. CKD-MBD can lead to bone abnormalities and/or calcification in soft tissue, especially blood vessels, and is associated with increased morbidity and mortality in CKD patients.

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-) 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 TGF- 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, ACE-536, dalantercept, or ACE-083 and the Company's TGF- 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 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, 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 Registration Statement on Form S-1 which was declared effective by the Securities and Exchange Commission (SEC) on September 18, 2013, 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.