Acceleron Announces Preclinical Results in Pulmonary Arterial Hypertension at the American Heart Association 2017 Scientific Sessions

– Preclinical results show potential first-in-class disease-modifying
properties of sotatercept in pulmonary arterial hypertension –

– Company expects to initiate a Phase 2 trial in 1H 2018 –

CAMBRIDGE, Mass.–(BUSINESS WIRE)–$XLRN–Acceleron Pharma Inc. (NASDAQ:XLRN), a leading biopharmaceutical company
in the discovery and development of TGF-beta therapeutics to treat
serious and rare diseases, today announced new preclinical results with
sotatercept in pulmonary arterial hypertension (PAH) during an oral
presentation at the American Heart Association (AHA) 2017 Scientific
Sessions in Anaheim, California.

“We are extremely encouraged by these positive preclinical results,
which support our mechanistic approach for sotatercept and its potential
to be a first-in-class disease-modifying therapy for patients with
pulmonary arterial hypertension. We look forward to initiating a Phase 2
trial with sotatercept in PAH during the first half of 2018,” said Ravi
Kumar, Chief Scientific Officer for Acceleron.

The oral presentation by Paul Yu, M.D., Ph.D., Associate Professor of
Medicine, Brigham and Women’s Hospital, entitled “ACTRIIA-Fc rebalances
BMP and activin/TGFβ signaling to attenuate experimental pulmonary
hypertension,” provided data from three animal models of PAH
demonstrating potent effects of sotatercept on many aspects of vascular
and cardiac disease.

  • In animals treated early in disease progression, sotatercept blocked
    the development of pulmonary vascular remodeling and right heart
    failure, and prevented the increase in pulmonary arterial pressure.
  • For animals treated after established disease, sotatercept achieved a
    marked reduction in intimal-medial thickness of pulmonary arterioles
    relative to untreated animals, decreased vessel muscularization,
    improved pulmonary arterial pressures and decreased indicators of
    right heart failure.

“The sotatercept preclinical results show remarkable biological
activity, particularly on the pulmonary vasculature, across multiple,
well-established animal models of PAH,” said Dr. Yu. “These results are
very exciting, and further establish the importance of signaling by the
TGF-beta superfamily as a target for therapies to treat PAH.”

Sotatercept is an investigational product that is not approved for use
in any country.

The AHA preclinical presentation is available under the Science page of
the Company’s website at www.acceleronpharma.com.

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
superfamily involved in remodeling and regeneration of a variety of
different tissues, including the vasculature and fibrosis.

About Pulmonary Arterial Hypertension

Pulmonary Arterial Hypertension (PAH) is a rare and chronic, rapidly
progressing disorder characterized by the constriction of small
pulmonary arteries and elevated blood pressure in the pulmonary
circulation. PAH results in significant strain on the heart, often
leading to limited physical activity, heart failure, and reduced life
expectancy. The 5-year survival rate for patients with PAH is
approximately 57%. Available therapies generally act by promoting the
dilation of pulmonary vessels without addressing the underlying cause of
the disease. As a result, PAH often progresses rapidly for many patients
despite standard of care treatment. A growing body of research has
implicated imbalances in BMP and TGF-beta signaling as a primary driver
of PAH in familial, idiopathic and acquired forms of the disease.

About Acceleron

Acceleron is a Cambridge-based, clinical-stage biopharmaceutical company
dedicated to the discovery, development, and commercialization of
therapeutics to treat serious and rare diseases. The Company’s
leadership in the understanding of TGF-beta biology and protein
engineering generates innovative compounds that engage the body’s
ability to regulate cellular growth and repair.

Acceleron focuses its research and development efforts in hematologic,
neuromuscular, and pulmonary diseases. In hematology, the Company and
its global collaboration partner, Celgene, are developing luspatercept
for the treatment of chronic anemia in myelodysplastic syndromes,
beta-thalassemia, and myelofibrosis. Acceleron is also advancing its
neuromuscular franchise with two distinct Myostatin+ agents, ACE-083 and
ACE-2494, and a pulmonary program with a Phase 2 trial of sotatercept
planned in pulmonary arterial hypertension.

For more information, please visit www.acceleronpharma.com.
Follow Acceleron on Social Media: @AcceleronPharma and
LinkedIn.

Forward-Looking Statements

This press release contains forward-looking statements about the
Company’s strategy, future plans and prospects, including statements
regarding the development of the Company’s compounds, the timeline for
clinical development and regulatory approval of the Company’s compounds
and the expected timing for reporting of data from ongoing clinical
trials. The words “anticipate,” “believe,” “could,” “estimate,”
“expect,” “goal,” “intend,” “may,” “plan,” “potential,” “project,”
“should,” “target,” “will,” “would,” and similar expressions are
intended to identify forward-looking statements, although not all
forward-looking statements contain these identifying words.

Actual results could differ materially from those included in the
forward-looking statements due to various risks and uncertainties,
including, but not limited to, that preclinical testing of the Company’s
compounds and data from clinical trials may not be predictive of the
results or success of ongoing or later clinical trials, that the
development of the Company’s compounds will take longer and/or cost more
than planned, that the Company or its collaboration partner, Celgene,
will be unable to successfully complete the clinical development of the
Company’s compounds, that the Company or Celgene may be delayed in
initiating, enrolling or completing any clinical trials, and that the
Company’s compounds will not receive regulatory approval or become
commercially successful products. These and other risks and
uncertainties are identified under the heading “Risk Factors” included
in the Company’s most recent Annual Report on Form 10-K, and other
filings that the Company has made and may make with the SEC in the
future.

The forward-looking statements contained in this press release are based
on management’s current views, plans, estimates, assumptions and
projections with respect to future events, and the Company does not
undertake and specifically disclaims any obligation to update any
forward-looking statements.

Contacts

Acceleron Pharma Inc.
Todd James, IRC, 617-649-9393
Vice
President, Investor Relations and Corporate Communications
Or
Candice
Ellis, 617-649-9226
Manager, Investor Relations and Corporate
Communications
Or
Media:
BMC Communications
Brad
Miles, 646-513-3125