8-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C.  20549

___________________________

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest eventreported):  January 8, 2018

___________________________

ACCELERON PHARMA INC.

(Exact name of Registrant as specified in its charter)

Delaware (State or other jurisdiction of incorporation)	001-36065 (Commission File Number)	27-0072226 (I.R.S. Employer Identification Number)
128 Sidney Street Cambridge, MA  (Address of principal executive offices)		02139 (Zip Code)

Registrant’s telephone number, including area code:  (617) 649-9200

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

o Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth companyo

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.o

Item 7.01    Regulation FD Disclosure.

On January 8, 2018, Acceleron Pharma Inc. (the "Company") uploaded a slide presentation to its website to be presented at the JP Morgan Healthcare Conference. A copy of this slide presentation is furnished herewith as Exhibit 99.1 to this Current Report on Form 8-K.

The furnishing of the attached slide presentation is not an admission as to the materiality of any information therein. The information contained in the slides is summary information that is intended to be considered in the context of more complete information included in the Company's filings with the Securities and Exchange Commission and other public announcements that the Company has made and may make from time to time by press release or otherwise. The Company undertakes no duty or obligation to update or revise the information contained in this report, although it may do so from time to time as its management believes is appropriate.

The information contained in Item 7.01 of this Current Report on Form 8-K and Exhibit 99.1 attached hereto is being furnished and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) or for any other purpose, and shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, regardless of any general incorporation language in any such filing, except as expressly set forth by specific reference in such a filing.

Item 8.01    Other Events.

On January 8, 2018, the Company issued a press release titled "Acceleron Announces Preliminary Results from Part 1 of the ACE-083 Phase 2 Trial in Patients with Facioscapulohumeral Dystrophy." A copy of the press release is attached as Exhibit 99.2 to this Current Report on Form 8-K and is incorporated herein by reference.

Item 9.01    Financial Statements and Exhibits.

(d)    Exhibits.

Exhibit Number	Description of Exhibit
99.1	Slide presentation of Acceleron Pharma Inc. dated January 2018
99.2	Press release of Acceleron Pharma Inc. dated January 8, 2018

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

	ACCELERON PHARMA INC.
	By:	/s/ John D. Quisel, J.D., Ph.D.
		John D. Quisel, J.D., Ph.D.
		Senior Vice President and General Counsel
Date:  January 8, 2018

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EXHIBIT INDEX

Exhibit Number	Description of Exhibit
99.1	Slide presentation of Acceleron Pharma Inc. dated January 2018
99.2	Press release of Acceleron Pharma Inc. dated January 8, 2018

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EX-99.1

JP Morgan Healthcare Conference January 2018 Exhibit 99.1

Acceleron Forward-Looking Statements 2 THIS PRESENTATION 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 PRESENTATION 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.

3 Our mission is to transform the lives of patients with serious and rare diseases

MUSCLE PULMONARY Harnessing the Power of TGF-Beta Biology 4 TGF-Beta HEMATOLOGY Strategic Focus in Three Therapeutic Areas Growth Repair Differentiation C E L L U L A R :

High Unmet Medical Need Across Therapeutic Area Focus HEMATOLOGY NEUROMUSCULAR PULMONARY MDS / Thalassemia / MF FSHD / CMT PAH  No approved anemia therapies  RBC transfusions + off-label meds  Focal muscle disease  Physical therapy and bracing  Multiple drugs approved, yet still only 5-7 year survival 5

Our Vision 6 Leader in neuromuscular and pulmonary Complement in-house research portfolio success with BD efforts to drive therapeutic area leadership Drive life-cycle management Potential multi-billion $ asset Leader in chronic anemia HEMATOLOGY Develop first-in/best-in class assets NEUROMUSCULAR PULMONARY

Building Therapeutic Area Leadership Phase 1 Myelodysplastic Syndromes (MDS) Beta-Thalassemia FSH Dystrophy Neuromuscular Disease Preclinical Phase 2 Phase 3 Product Hematology Luspatercept Sotatercept ACE-2494 Neuromuscular ACE-083 MEDALIST Phase 3 COMMANDS Phase 3 (Planned) BELIEVE Phase 3 Phase 2 Base/Extension Myelofibrosis (MF) Phase 2 BEYOND Phase 2 7 Phase 2 CMT Disease Phase 1 Phase 2 MDS/Beta-Thalassemia Sotatercept Phase 2 Study (Planned) Pulmonary Arterial Hypertension (PAH) Pulmonary ACE-1334 Pulmonary Disease Preclinical Phase 2 (Planned)

Hematology: Luspatercept

Chronic Anemia Due to Rare Blood Disorders BETA-THALASSEMIA Genetic hemoglobin mutation MYELOFIBROSIS Fibrotic bone marrow disease LOWER-RISK MDS Bone marrow failure disorder CHRONIC ANEMIA 9

High Need for RBC Transfusion Reduction/Elimination 10 Reduces quality of life Negatively impacts progression and survival Inconvenient – 3 to 5 hours every 2 to 3 weeks Causes high spikes and low troughs in Hb levels Human derived product Potential iron overload

Increase and maintain ↑ hemoglobin levels Reduce or eliminate RBC transfusion burden LUSPATERCEPT TREATMENT GOALS: 11

Non-EPO Driven/Ineffective Erythropoiesis Chronic Anemias 12 EPO: erythropoietin Erythropoiesis-stimulating agent Erythroid maturation agent (EMA) Luspatercept-responsive 1 2 3 4 5 6 7 EPO-dependent ESA Luspatercept Hemoglobin (Hb) production

Multiple Phase 3 and Phase 2 Trials Ongoing and Planned 13 Lower-risk MDS PHASE 2 Beta-thalassemia Myelofibrosis First-line / ESA Naive RS+ ESA Refractory / Ineligible Non-Transfusion-Dependent Transfusion-Dependent Monotherapy / Combination PHASE 2

MDS Trials Supported by Luspatercept Phase 2 Results 14 Lower-risk MDS First-line / ESA Naïve RS+ ESA Refractory / Ineligible 55%1 39%2 RBC Transfusion Independence 1. ASH 2017: ESA naïve LR MDS patients, RBC-TI: 17/31 or 55% 2. ASH 2015: RS+ LR MDS patients, RBC-TI: 12/31 or 39% RBC Transfusion Independence Phase 2 Results

Beta-Thalassemia Trials Supported by Luspatercept Phase 2 Results 15 Beta-thalassemia Non-Transfusion-Dependent Transfusion-Dependent 52%1 41%2 Increase in Mean Hb ≥ 1.5 g/dL 1. EHA 2017: NTD patients, ≥ 1.5 g/dL, 16/31 or 52%, over a 12-week period on treatment versus baseline 2. EHA 2017: TD patients, 12/29 or 41%, in weeks 13-24 fixed interval compared to 12 weeks pre-treatment RBC Transfusion Reduction ≥ 33% PHASE 2 Phase 2 Results

MF Trial Supported by Sotatercept Phase 2 MD Anderson IST Results 16 Myelofibrosis Monotherapy 39%1 30%2 Anemia Response 1. ASH 2017: sotatercept, 7/18 or 39% of evaluable patients 2. ASH 2017: sotatercept, 3/10 or 30% of evaluable patients Combination Therapy Anemia Response Phase 2 Results

>40K PATIENTS MDS (RS+) ESA Experienced/Ineligible NTD Beta-Thalassemia >20K PATIENTS Myelofibrosis >15K PATIENTS >20K PATIENTS TD Beta-Thalassemia MDS First-line >20K PATIENTS ADDRESSABLE US/EU PATIENT POPULATION POTENTIAL NEW INDICATION >115K Building a Potential Multi-Billion Dollar Anemia Brand 17

Long-Term Durability of Response (Ongoing Phase 2 Trials) 18 MDS MF 3 YEARS 2 YEARS 1 YEAR Median1 19 Months Median2 14.2 Months Median3 -thal 1. ASH 2017: Mutational Profile and Analysis of Lower-Risk Myelodysplastic Syndromes (MDS) Patients Treated with Luspatercept: Phase 2 PACE-MDS Study 2. EHA 2017: Luspatercept Increases Hemoglobin and Decreases Transfusion Burden in Adults With Beta-Thalassemia 3. ASH 2017: Sotatercept (ACE-011) Alone and with Ruxolitinib in Patients with MPN-associated Myelofibrosis (MF) and Anemia, 7/18 monotherapy

Collaborating with the Leader in Hematology 19 PROGRAM COSTS 100% Celgene funded WORLDWIDE ROYALTY Low- to mid-20% MILESTONES $185M outstanding CO-PROMOTE North America Celebrating 10 years of partnership

Neuromuscular Disease

ACE-083 Local Agent Building a Neuromuscular Franchise with ACE-083 and ACE-2494 21 ACE-2494 Systemic Agent

 Administered into target muscle for concentrated effect ACE-083: A Locally-Acting Myostatin+ Therapy to Target Focal Muscle Disease 22 FSHD MYOPATHY  20,000 US patients CMT NEUROPATHY  > 100,000 US patients

Preliminary Results for Phase 2 FSHD Study - Part 1, Cohorts 1 and 2 Pooled 23 N=11 N=12 % Ch an ge in T o tal M u sc le V o lu m e Tibialis Anterior Biceps Brachii 12.6% 13.2% 0 5 10 15 20 A b sol u te Ch an ge in F at F rac tio n Tibialis Anterior Biceps Brachii -5.3% -0.6% -10 -5 0 5 N=11 N=12 Safety Summary No serious adverse events, a majority of adverse events were injection-site related and grades 1-2 One patient with related grade 3 non-serious adverse event of “lower leg intramuscular swelling” MRI 3 weeks after last dose

Pulmonary Disease

Building a Pulmonary Platform Starting with PAH 25

Sotatercept Profile and Preclinical Results in PAH SOTATERCEPT IS A TGF-BETA LIGAND TRAP  Blocks activin restoring the BMP signaling pathway  Phase 2 ready with ~400 patients of clinical experience  Subcutaneous injection every three weeks  Outperforms standard of care in preclinical animal models PAH Artery Normal Artery Test Compound 72.5% 67.4% 29.3%** Pulmonary Arterioles (10-50 µm, N=100) Classification (%) % of Completely Muscularized Vessels Vehicle Sildenafil Sotatercept1 26

Where Could Sotatercept Fit in Treatment Paradigm? 27 Endothelin Receptor Antagonists (ERAs) PDE-5 Inhibitors Prostacyclin Standard of Care Options MOA: Vasodilation + + + Goal: Significantly Extend Survival and Quality of Life with Disease Modifying Combination sGC Stimulators + SOTATERCEPT MOA: Vascular Remodeling/ Disease Modifying Sequential or Immediate Combination

Upcoming Corporate Priorities HEMATOLOGY  Luspatercept – MEDALIST and BELIEVE Phase 3 trial top-line results expected in mid-2018 – Initiate the COMMANDS Phase 3 trial in 1H 2018 NEUROMUSCULAR  ACE-083 – FSHD and CMT Part 1 of Phase 2 trial preliminary results from all dose-escalation cohorts in 2H 2018 – Initiate FSHD Part 2 of Phase 2 trial Q2 2018 – Initiate CMT Part 2 of Phase 2 trial YE 2018  ACE-2494 – Phase 1 healthy volunteer trial results in 1H 2019 PULMONARY  Sotatercept – Initiate Phase 2 trial in PAH in 1H 2018 28

Our Commitment to Late-Stage Milestones 29

EX-99.2

Exhibit 99.2

Acceleron Announces Preliminary Results from Part 1 of the ACE-083 Phase 2 Trial

in Patients with Facioscapulohumeral Dystrophy

- Mean total muscle volume increases of over 12% in the tibialis anterior and biceps brachii muscle cohorts -

- Company plans to initiate Part 2 of the FSHD Phase 2 trial in Q2 2018 -

Cambridge, Mass. – January 8, 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 for the first two cohorts in Part 1 of the Phase 2 clinical trial with ACE-083 in patients with facioscapulohumeral dystrophy (FSHD), a rare genetic muscle disorder that results in progressive focal muscle loss and weakness. The Company plans to initiate Part 2 of the ACE-083 FSHD Phase 2 trial during the second quarter of 2018.

“Preliminary results of ACE-083 in FSHD patients demonstrated positive safety and tolerability along with unprecedented mean increases in total muscle volume of over 12% in the two distinct muscles evaluated,” said Matthew Sherman, M.D., Chief Medical Officer of Acceleron. “These data support our decision to advance to Part 2 of the Phase 2 trial, which we expect to initiate in the second quarter of this year. We look forward to fully exploring functional outcomes in the larger, placebo-controlled Part 2 of the trial.”

Part 1 of the Phase 2 trial is an open-label, dose-escalation study of ACE-083 designed to evaluate safety as well as changes in total muscle volume in up to 36 patients with FSHD. Preliminary results include data from 23 patients evaluable for magnetic resonance imaging (MRI) among two different cohorts (11 patients with tibialis anterior weakness and 12 patients with biceps brachii weakness). Each patient received ACE-083 (150 mg or 200 mg) as a unilateral intramuscular injection once every three weeks for 12 weeks. Total muscle volume changes were measured by MRI relative to baseline at 3 weeks after the last injection of ACE-083. Based on overlap in dosing on a milligram per gram muscle analysis, dose cohorts were pooled for the analyses of each muscle.

Tibialis Anterior Part 1 Cohorts (150 mg and 200 mg pooled) Preliminary Results (n=11):

The tibialis anterior (TA), which is located in the lower leg, is the main muscle responsible for ankle dorsiflexion, or the ability to lift the front of the foot when taking a step. Over 70% of FSHD patients experience tibialis anterior weakness over the course of their disease, which can lead to general decreased mobility and an increased frequency of falling.

• The TA cohorts generated a mean total muscle volume increase of 12.6%.

• The TA cohorts produced a mean decrease or improvement in muscle fat fraction of 5.3%

Biceps Brachii Part 1 Cohorts (150 mg and 200 mg pooled) Preliminary Results (n=12):

The biceps brachii (BB), which is located in the upper arm, is a major muscle responsible for elbow flexion, or the ability to lift the lower arm. A majority of FSHD patients experience biceps brachii weakness early in their disease, which leads to the inability to lift objects or perform other important daily activities without assistance.

• The BB cohorts generated a mean total muscle volume increase of 13.2%.

• The BB cohorts produced a mean decrease or improvement in muscle fat fraction of 0.6%.

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In the BB cohorts, the majority of patients had less intramuscular fat at baseline relative to the patients in the TA cohorts. Patients with higher fat fraction in the BB cohorts at baseline demonstrated larger decreases in fat fraction with treatment.

“I am encouraged by the preliminary safety and tolerability results of ACE-083 in Part 1 of the trial. There are currently no FDA approved therapies for FSHD, therefore limiting our treatment of patients to supportive options such as physical therapy and bracing,"said Dr. Jeffrey Statland, M.D., Associate Professor of Neurology at the University of Kansas Medical Center and the ACE-083 FSHD Phase 2 trial principal investigator. “ACE-083 is demonstrating encouraging activity in its ability to increase muscle volume, and I look forward to its advancement in Part 2 of the trial.”

Strength and Function Tests:

Strength and function tests are being explored in Part 1 to assist with the design of the randomized, double-blind, placebo-controlled Part 2 of the study. Given the lack of placebo control and small sample size of patients in Part 1, no conclusions can be made on the strength and function assessments at this time. Effects of ACE-083 on strength and function versus a placebo-control will be evaluated in Part 2 of the study.

Part 1 Preliminary Safety Results (n=25):

There were no serious adverse events reported. The most common adverse events were injection site related and grades 1-2. One patient experienced a related grade 3 non-serious adverse event of lower leg intramuscular swelling. This patient fully recovered and was discontinued from the study.

Acceleron plans to present an in-depth review of Part 1 data at a medical conference in 2018.

For additional information on this clinical trial, please visit clinicaltrials.gov, identifier NCT02927080.

About ACE-083

ACE-083 is a 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 diseases such as facioscapulohumeral dystrophy (FSHD) and Charcot-Marie-Tooth (CMT) disease, in which improved muscle strength in target muscles may provide a clinical benefit and enhance quality of life.

About Facioscapulohumeral Dystrophy (FSHD)

FSHD is a rare genetic muscle disorder affecting approximately 20,000 people in the United States for which there are currently no approved treatments. The primary clinical presentation of FSHD is debilitating skeletal muscle weakness and loss. The symptoms of FSHD develop in a descending pattern, beginning with the face and upper body and progressing to the lower body in a "muscle by muscle" fashion. The disease is typically diagnosed by a characteristic pattern of muscle weakness and other clinical symptoms, as well as through genetic testing.

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.

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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

Cautionary Note Regarding 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:

For Acceleron:

Investors:

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

Media:

BMC Communications

Brad Miles, (646) 513-3125

Source: Acceleron Pharma

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