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Acceleron Announces Preliminary Results from Part 1 of the ACE-083 Phase 2 Trial in Patients with Charcot-Marie-Tooth Disease at the 2018 Annual Meeting of the Peripheral Nerve Society

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- Mean total muscle volume increases of more than 12% seen in the
tibialis anterior –

- Company plans to initiate Part 2 of the CMT Phase 2 trial in the
third quarter of 2018 –

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 positive preliminary results
from Part 1 of the Phase 2 clinical trial with ACE-083 in patients with
Charcot-Marie-Tooth (CMT) disease at the Peripheral Nerve Society (PNS)
annual meeting held in Baltimore, Maryland. CMT is one of the most
common inherited neurological diseases and is associated with
significant focal muscle weakness. The Company plans to initiate Part 2
of the ACE-083 CMT Phase 2 trial in the third quarter of 2018.

"Preliminary Phase 2 results of ACE-083 in patients with CMT show robust
mean increases in total and contractile muscle volume, reductions in fat
fraction, and an encouraging safety profile," said Robert K Zeldin,
M.D., Chief Medical Officer of Acceleron. "We now look forward to
initiating the randomized, placebo-controlled portion of the Phase 2
trial in which we will evaluate ACE-083's potential to improve function
over a six-month treatment period."

Part 1 of the trial was an open-label, dose-escalation study that
enrolled a total of 18 patients (in three cohorts of six patients each)
with CMT1 or CMTX who received ACE-083 at dose levels of 150 mg, 200 mg,
or 240 mg. ACE-083 was administered by injection into the tibialis
anterior (TA) muscle bilaterally once every three weeks for three months
to evaluate safety and increases in muscle volume. The TA, which is
located in the lower leg, is the primary muscle responsible for ankle
dorsiflexion, or the ability to lift the front of the foot when taking a
step. TA weakness can result in foot drop and increased risk of falls.

Muscle volume was measured by magnetic resonance imaging (MRI) three
weeks after the last injection of ACE-083. Muscle volume and fat
fraction are represented as the changes from baseline averaged for each
side.

  • Mean total muscle volume (TMV) increases ranged from 12.6% to 14.2%.
  • Mean absolute decreases (improvement) in fat fraction ranged from 1.7%
    to 3.5%.

In addition, treatment with ACE-083 resulted in mean increases from
baseline in contractile muscle volume (an MRI-derived calculation to
measure viable, functional muscle volume as part of the TMV), ranging
from 15.8% to 19.6%.

The most common adverse events—injection-site reactions, muscle spasms,
and myalgia—were mild or moderate (grades 1-2).

"There are currently no FDA-approved therapies for patients with CMT
with muscle weakness. The muscle volume and fat fraction changes
demonstrated in Part 1 of the ACE-083 trial are encouraging," said
Florian P. Thomas, M.D., neurologist and principal investigator at
Hackensack Meridian School of Medicine at Seton Hall University. "We're
hopeful that ACE-083 may become an important option for patients."

The 2018 PNS ACE-083 Part 1 oral and poster presentations are available
in the "Science" section on Acceleron's website, www.acceleronpharma.com.

The double-blind, placebo-controlled Part 2 of the CMT Phase 2 trial
will enroll approximately 40 patients who will be randomized (1:1) to
receive either placebo or ACE-083. Patients will be evaluated for
changes in muscle volume, fat fraction, strength, function and safety
over a six-month primary treatment period, followed by a six-month
open-label treatment period. Preliminary results are expected by the end
of 2019.

For additional information on this clinical trial, please visit
clinicaltrials.gov, identifier NCT03124459.

About ACE-083

ACE-083 is a locally-acting therapeutic candidate, based on the
naturally-occurring protein follistatin, which utilizes the Myostatin+
approach to inhibit multiple TGF-beta ligands. It is designed to have a
concentrated effect along targeted muscles to maximize growth and
strength selectively in the muscles into which the drug is administered.
Acceleron is developing ACE-083 for disorders such as CMT disease and
facioscapulohumeral muscular dystrophy (FSHD), in which improved muscle
strength in target muscles may provide a clinical benefit and enhance
quality of life. For more information, please visit www.clinicaltrials.gov.

About Charcot-Marie-Tooth Disease (CMT)

CMT is one of the most common inherited neurologic diseases. It is
estimated to affect more than 125,000 people in the United States. The
primary clinical manifestations of CMT include muscle weakness in the
lower legs and arms. The lower leg muscle weakness can result in foot
drop leading to a high-stepped gait and frequent tripping or falls. The
disease is typically diagnosed by the presence of a characteristic
pattern of muscle weakness, nerve conduction studies, and genetic
testing. There are no FDA-approved drug therapies for CMT.

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 Phase 2 pulmonary program with sotatercept 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 factors, 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 results of any clinical trial may not be predictive of
the results or success of other clinical trials of the same product
candidate, that the development of the Company's compounds will take
longer and/or cost more than planned, that the Company will be unable to
successfully complete the clinical development of the Company's
compounds, that the Company 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.

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