Acceleron

Acceleron and Celgene Initiate Phase 2 Study of ACE-011 to Treat Chemotherapy-Induced Anemia

-- Innovative Approach Through Activin Receptor Type IIA May Lead to New Treatment
Option for Patients with Chemotherapy-Induced Anemia --

CAMBRIDGE, Mass. Acceleron Pharma, Inc., a biopharmaceutical company developing
novel therapeutics that modulate the growth of cells and tissues including red blood cells, bone and muscle,
and Celgene Corporation (NASDAQ: CELG) today announced the initiation of a second Phase 2 clinical
study of ACE-011. This Phase 2 clinical trial is a randomized, double-blind, placebo-controlled study
designed to evaluate the potential of ACE-011 to treat chemotherapy-induced anemia in patients with
metastatic breast cancer. ACE-011 is a novel therapeutic agent that has been shown to increase levels of
red blood cells and hemoglobin and stimulate new bone formation. ACE-011 works through a novel
mechanism by inhibiting certain members of the TGF-beta superfamily while potentially avoiding the
erythropoietin pathway.

“We are pleased to build on the robust hematologic effects observed in earlier studies of ACE-011 by
initiating this Phase 2 clinical trial in breast cancer patients with chemotherapy-induced anemia,” said
Matthew Sherman, M.D., Chief Medical Officer of Acceleron. “This study, along with the nearly completed
Phase 2 clinical trial in patients with multiple myeloma, will provide insight into the therapeutic potential of
ACE-011.”

“Our discovery of a novel class of anemia therapies comes at a critical time for cancer patients. Many of
these patients suffer from an inadequate response to existing therapies or remain untreated due to safety
concerns with erythropoietin-based therapies for chemotherapy-induced anemia,” said John Knopf, Ph.D.,
Chief Executive Officer of Acceleron. “We are excited to be working with a pre-eminent global
biopharmaceutical company like Celgene, which is experienced in maximizing the clinical and commercial
potential of disease-altering compounds, to advance the development of this unique molecule, which could
replenish red blood cells and has been shown to inhibit the growth and metastasis of tumors in nonclinical
animal models of breast cancer.”

About Chemotherapy-Induced Anemia

Anemia is one of the most common and debilitating complications associated with cancer chemotherapy,
which depletes and prevents formation of red blood cells, leading to incapacitating fatigue and weakness.
The vast majority of therapies approved and in development for anemia target the erythropoietin pathway.
However, recent studies have suggested an increased risk of mortality associated with this class of therapy,
arising from exposure to high levels of recombinant erythropoietin and its derivatives, which may stimulate
tumor progression and increase the occurrence of thromboembolic events.

About ACE-011

ACE-011, a soluble form of the activin receptor type IIA (ActRIIA), is a biologic therapeutic that binds to and
prevents signaling of several members of the TGF-beta protein super family, and has been shown to
stimulate red blood cell production, promote bone formation, and inhibit tumor growth and metastasis. ACE-
011 is the first in a novel class of anemia therapies. In Phase 1 clinical studies in healthy volunteers, ACE-
011 had an encouraging safety profile, increased levels of hemoglobin and hematocrit, biomarkers of bone
formation, and bone mineral density. ACE-011 is currently being studied in two Phase 2 clinical trials in
cancer patients and is being jointly developed by Acceleron and Celgene Corporation.