Acceleron Announces Third Quarter 2021 REBLOZYL® Net Sales

Acceleron Pharma Inc. (Nasdaq:XLRN), a biopharmaceutical company dedicated to the discovery, development, and commercialization of TGF-beta superfamily therapeutics to treat serious and rare diseases, today announced net sales of REBLOZYL^® (luspatercept-aamt) as reported by its global collaborator, Bristol Myers Squibb, were approximately $160 million for the third quarter ended September 30, 2021. Net sales of REBLOZYL for the third quarter include approximately $20 million to $25 million of sales from an inventory build due to the transition to wholesaler distribution and approximately $13.5 million of net sales outside of the United States.

Acceleron expects to report royalty revenue of approximately $32.0 million from net sales of REBLOZYL in the third quarter ended September 30, 2021. This compares with approximately $25.6 million in royalty revenue from approximately $128 million of net sales of REBLOZYL for the second quarter ended June 30, 2021.

The preliminary unaudited revenue estimate for the quarter ended September 30, 2021 included in this release is the responsibility of management and is subject to the completion of the Company's customary quarter-end financial closing procedures, including management's review and finalization, as well as review procedures by the Company's independent registered public accounting firm, which have not yet been completed. During the course of the Company's review process, items may be identified that would require it to make adjustments, which could result in material changes to the Company's preliminary unaudited estimated financial results. Consequently, this revenue estimate should not be viewed as a substitute for the Company's earnings release and Quarterly Report on Form 10-Q.

About Acceleron

Acceleron is a biopharmaceutical company dedicated to the discovery, development, and commercialization of therapeutics to treat serious and rare diseases. Acceleron's leadership in the understanding of TGF-beta superfamily biology and protein engineering generates innovative compounds that engage the body's ability to regulate cellular growth and repair.

Acceleron focuses its research, development, and commercialization efforts in pulmonary and hematologic diseases. In pulmonary, Acceleron is developing sotatercept for the treatment of pulmonary hypertension (PH). Following positive PULSAR Phase 2 results, Acceleron is executing on its Phase 3 development plan to support its long-term vision of establishing sotatercept as a backbone therapy for patients with pulmonary arterial hypertension (PAH) at all stages of the disease. Acceleron is also expanding the development of sotatercept into Group 2 PH, with the CADENCE Phase 2 trial expected to initiate this year. Acceleron has expanded its rare pulmonary disease pipeline and is investigating the potential of ACE-1334 in a Phase 1b/Phase 2 trial in systemic sclerosis-associated interstitial lung disease (SSc-ILD).

In hematology, REBLOZYL® (luspatercept-aamt) is the first and only erythroid maturation agent approved in the United States, Europe, and Canada for the treatment of anemia in certain blood disorders. REBLOZYL is part of a global collaboration partnership with Bristol Myers Squibb. The Companies co-promote REBLOZYL in the United States and are also developing luspatercept for the treatment of anemia in patient populations of myelodysplastic syndromes, beta-thalassemia, and myelofibrosis.

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 the Company's financial results. 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 other clinical trials, that regulatory approval of the Company's compounds in one indication or country may not be predictive of approval in another indication or country, that the development of the Company's compounds may take longer and/or cost more than planned or accelerate faster than currently expected, that the Company or its collaboration partner, Bristol Myers Squibb ("BMS"), may be unable to successfully complete the clinical development of the Company's compounds, that the Company or BMS may be delayed in initiating, enrolling or completing any clinical trials, and that the Company's compounds may 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.