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Acceleron Provides Updated Results from Phase 2 Studies of Luspatercept in Beta-Thalassemia at the 22nd Congress of the European Hematology Association

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Acceleron Pharma Inc. (NASDAQ:XLRN), a clinical stage biopharmaceutical
company focused on the discovery, development, and commercialization of
innovative therapeutics to treat serious and rare diseases, today
announced preliminary results from the ongoing Phase 2 study of
luspatercept in patients with beta-thalassemia during an oral
presentation at the 22nd Congress of the European Hematology
Association (EHA) in Madrid, Spain. Luspatercept is being developed to
treat a range of hematologic diseases including beta-thalassemia,
myelodysplastic syndromes (MDS), and myelofibrosis as part of a global
collaboration between Acceleron and Celgene.

"With beta-thalassemia patients now remaining on study for over two
years, we continue to be highly encouraged by luspatercept's long-term
efficacy results and safety profile," said Habib Dable, President and
Chief Executive Officer of Acceleron. "Combined with the rapid
completion of enrollment in the BELIEVE Phase 3 trial, the program's
momentum continues to build alongside our enthusiasm to potentially
transform the treatment of beta-thalassemia patients globally."

Phase 2 Results

A total of 32 transfusion-dependent beta-thalassemia patients have been
treated with therapeutic dose levels of luspatercept in the ongoing
study.

  • 69% (22 of 32) achieved a reduction in red blood cell (RBC)
    transfusion burden of at least 33% in any 12-week treatment interval
    as compared to baseline.

A 12-week fixed interval analysis was conducted to review RBC
transfusion reduction during weeks 13 to 24 and weeks 37 to 48 compared
to the baseline 12-week period pre-treatment in order to evaluate
durability of response. The ongoing BELIEVE Phase 3 trial will use this
12-week fixed interval analysis for evaluating the proportion of
patients achieving at least a 33% reduction in RBC transfusion burden.

  • 50% (12 of 24 patients with 6-20 units RBC / 24 weeks estimated
    pre-treatment) achieved a reduction in RBC transfusion burden of at
    least 33% in the fixed 12-week interval from weeks 13 to 24 as
    compared to baseline.
  • 46% (11 of 24 patients with 6-20 units RBC / 24 weeks estimated
    pre-treatment) achieved a reduction in RBC transfusion burden of at
    least 33% in the fixed 12-week interval from weeks 37 to 48 as
    compared to baseline.

A total of 31 non-transfusion-dependent beta-thalassemia patients have
been treated with therapeutic dose levels of luspatercept in the ongoing
study.

  • 71% (22 of 31) achieved a clinically meaningful increase in hemoglobin
    of at least 1.0 g/dL compared to baseline (mean increase over 12
    weeks).

There are patients who remain on luspatercept with clinically meaningful
increases in hemoglobin and reductions in RBC transfusion burden for up
to 24 months.

Phase 2 Safety Summary

A total of 64 beta-thalassemia patients have been treated with
luspatercept in the ongoing Phase 2 studies (all dose levels).

  • The majority of adverse events (AEs) were Grade 1 or 2. The most
    common related AEs (occurring in ≥ 10% of patients) were bone pain,
    headache, myalgia, arthralgia, musculoskeletal pain, asthenia,
    injection site pain, and back pain.
  • Grade 3 AEs probably related to study drug were bone pain (n=3),
    asthenia (n=2) and headache (n=1).
  • There were no serious AEs related to study drug.

"Beta-thalassemia remains an area of critical medical need for many
patients around the world," said Michael Pehl, President,
Hematology/Oncology for Celgene. "These longer-term results continue to
illustrate the potential for luspatercept to affect transfusion
dependence and hemoglobin levels, making a meaningful impact for
patients with this serious blood disease."

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

The BELIEVE trial, a global Phase 3 study of luspatercept in
transfusion-dependent beta-thalassemia patients, is fully enrolled and
top-line results are expected in mid-2018.

The EHA beta-thalassemia presentation is available under the Science
page of the Company's website at www.acceleronpharma.com/.

About the Phase 2 Study

Data from two open-label Phase 2 studies were presented at the
conference: the base study in which patients received treatment with
luspatercept for three months and the ongoing long-term safety extension
study in which patients may receive treatment with luspatercept for up
to an additional five years. In both the three-month base study and the
long-term extension study, red blood cell (RBC) transfusion-dependent
patients (≥ 4 units RBC / 8 weeks) and non-transfusion-dependent
patients (< 4 units RBC / 8 weeks) were enrolled and treated with
open-label luspatercept, dosed subcutaneously once every three weeks.

The primary outcome measure of the three-month base study was the
proportion of patients who have an erythroid response, defined as 1) a
hemoglobin increase of ≥ 1.5 g/dL from baseline for ≥ 14 days (in the
absence of RBC transfusions) in non-transfusion dependent patients, or
2) ≥ 20% reduction in RBC transfusion burden compared to pretreatment in
transfusion-dependent patients. The primary outcome for the long-term
extension study is to evaluate the long-term safety and tolerability of
luspatercept.

About Luspatercept

Luspatercept is a modified activin receptor type IIB fusion protein that
acts as a ligand trap for members in the transforming growth factor-beta
superfamily involved in the late stages of erythropoiesis (red blood
cell production). Luspatercept regulates late-stage erythrocyte (red
blood cell) precursor cell differentiation and maturation. This
mechanism of action is distinct from that of erythropoiesis stimulating
agents (ESAs), which stimulate the proliferation of early-stage
erythrocyte precursor cells. Acceleron and Celgene are jointly
developing luspatercept as part of a global collaboration. Phase 3
clinical trials are underway to evaluate the safety and efficacy of
luspatercept in patients with myelodysplastic syndromes (the "MEDALIST"
study) and in patients with beta-thalassemia (the "BELIEVE" study). 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 innovative therapeutics
to treat serious and rare diseases. Its pioneering research platform
leverages the powerful biology behind the body's ability to rebuild and
repair its own cells and tissues. This approach to drug discovery has
generated four therapeutic candidates that are currently in clinical
trials. The Company's lead therapeutic candidate, luspatercept, is being
evaluated in Phase 3 studies for the treatment of the hematologic
diseases myelodysplastic syndromes (MDS) and beta-thalassemia under a
global partnership with Celgene. Acceleron is also advancing its ACE-083
clinical program in the field of neuromuscular disease, and has a
comprehensive preclinical research effort targeting fibrotic and other
serious diseases.

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

Cautionary Note on Forward-Looking Statements

This press release contains forward-looking statements about Acceleron's
strategy, future plans and prospects, including statements regarding the
development of luspatercept, the timeline for clinical development and
regulatory approval of Acceleron's compounds, the expected timing for
the reporting of data from ongoing trials, and the structure of
Acceleron's planned or pending clinical trials. The words "anticipate,"
"appear," "believe," "continue," "could," "estimate," "expect,"
"forecast," "goal," "intend," "may," "plan," "potential," "predict,"
"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.

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 preclinical testing of Acceleron's
compounds and 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 Acceleron expects it to be, that Acceleron or its
collaboration partner, Celgene, will be unable to successfully complete
the clinical development of Acceleron's compounds, that the development
of Acceleron's compounds will take longer or cost more than planned,
that Acceleron or Celgene may be delayed in initiating or completing any
clinical trials, and that Acceleron'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 Acceleron's Annual Report on Form 10-K which
was filed with the Securities and Exchange Commission (SEC) on March 1,
2017, and other filings that Acceleron has made and may make with the
SEC in the future. The forward-looking statements contained in this
press release reflect Acceleron's current views with respect to future
events, and Acceleron does not undertake and specifically disclaims any
obligation to update any forward-looking statements.

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