Acceleron Pharma Presents Positive Phase 1 Results Demonstrating ACE-011 Increases Markers of Bone Formation

Encouraging safety and biological activity supports advancement into phase 2 studies

Honolulu, HI – September 18, 2007 - Acceleron Pharma, Inc., a biopharmaceutical company developing novel therapeutics that modulate the growth of tissues including bone and muscle, today announced positive results for the phase 1 clinical trial of ACE-011 at The American Society for Bone and Mineral Research (ASBMR) 29th Annual Meeting held in Honolulu, Hawaii, USA, September 16 – 19, 2007. The oral presentation was titled, “A Single Dose of ACE-011 Is Associated with Increases in Bone Formation and Decreases in Bone Resorption Markers in Healthy, Postmenopausal Women,” and was presented this morning at the Hawaii Convention Center.

This presentation highlighted that ACE-011 is a first-in-class novel bone forming agent with biological evidence of both an increase in markers of bone formation and a decrease in markers of bone resorption. It was the first time clinical data for an activin antagonist was presented at a scientific meeting.

Results of the study:

ACE-011 was well-tolerated in this study and demonstrated an encouraging safety profile. There were no serious adverse events reported in this study and the adverse events were generally mild and transient.
The pharmacokinetic (PK) properties showed a linear profile with dose and a mean half-life of approximately 24-32 days. The absorption after subcutaneous (SC) administration was 100%.
A single dose of ACE-011 caused a rapid, sustained dose-dependent increase of up to 35% in serum levels of the marker of bone formation bone-specific alkaline phosphatase with a slight decrease in serum levels of markers of bone resorption. These bone biomarker changes were sustained for at least 29 days at the highest dose levels tested. There was also a dose-dependent decrease of nearly 50% in serum follicle stimulating hormone (FSH) levels consistent with inhibition of activin.

“We are extremely encouraged with the promising safety, efficacy and PK profile demonstrated with ACE-011 in this phase 1 study” said Matthew Sherman, M.D., Chief Medical Officer at Acceleron. “To see statistically significant effects on biomarkers of bone formation following a single dose of ACE-011 is very exciting and we look forward to advancing ACE-011 to phase 2 trials later this year.”

“We look forward to building upon the positive results from the ACE-011 phase 1 study and begin phase 2 studies with ACE-011 later this year. Furthermore, we are planning to start a phase 1 study with ACE-031, our exciting muscle building compound, early in 2008. We’re thrilled with the progress we’ve made with our pipeline,” said John Knopf, Chief Executive Officer at Acceleron.

About the Phase 1 Study

The phase 1 study was a randomized, double-blind, placebo-controlled study in healthy, postmenopausal women. Forty-eight subjects were randomized in cohorts of 6 to receive either a single dose of ACE-011 or placebo (5 active: 1 placebo). Dose levels ranged from 0.01 to 3.0 mg/kg intravenously and 0.03 to 0.1 mg/kg subcutaneously. All subjects were followed for 120 days. Subjects were excluded from study participation if they took medications affecting bone metabolism within 6 months of study entry. Safety evaluations were conducted following each cohort to determine dose escalation. In addition to
pharmacokinetic (PK) analyses, the biologic activity of ACE-011 was assessed by measurement of FSH levels and biochemical markers of bone formation and resorption.

About ACE-011

ACE-011, a protein therapeutic based on the activin receptor type IIA, is a novel bone forming agent that is expected to begin phase 2 clinical trials in Q4 2007. In numerous preclinical models of bone loss, ACE-011 demonstrated beneficial effects on both trabecular and cortical bone. ACE-011 increased bone mineral density, improved bone architecture, increased the mineral apposition and bone formation rates and improved bone mechanical strength. These effects have been demonstrated in therapeutic models of bone loss in which ACE-011 stimulated bone formation – a significant unmet medical need that is underserved by current treatments for bone loss.

About Acceleron Pharma

Acceleron is a privately held biopharmaceutical company committed to discover, develop, manufacture and commercialize novel biotherapeutics that modulate the growth of bone, muscle, fat and the vasculature to treat musculoskeletal, metabolic and cancer-related diseases. Acceleron’s scientific approach takes advantage of its unique insight into the regenerative powers of two protein families: the Growth and Differentiation Factors (GDFs) and Bone Morphogenetic Proteins (BMPs). ACE-011, an anabolic bone agent, is the company’s lead program, and is being developed to reverse bone loss in diseases such as cancer-related bone loss and osteoporosis. In addition, the company is advancing through preclinical development with product candidates that increase muscle mass, control angiogenesis and inhibit fat accumulation. Acceleron utilizes proven biotherapeutic technologies and capitalizes on the company’s internal GMP manufacturing capability to rapidly and efficiently advance its therapeutic programs. The investors in Acceleron are Advanced Technology Ventures, Flagship Ventures, OrbiMed Advisors, Polaris Ventures, Sutter Hill Ventures and Venrock.

Contact: Steven Ertel, Vice President, Corporate Development, Acceleron Pharma, tel: 617-520-1334 and Paul Kidwell (Media) Suda Communications, tel: 617-296-3854.

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