UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No.     )

Filed by the Registrant  [X]

☒                             Filed by a Party other than the Registrant  [ ]☐

Check the appropriate box:

[ ]    Preliminary Proxy Statement

[ ]    Confidential, for Use of the Commission Only (as permitted by Rule14a-6(e)(2))

[X]  Definitive Proxy Statement

[ ]    Definitive Additional Materials

[ ]    Soliciting Material Under Rule14a-12

Spero Therapeutics, Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

675 Massachusetts Avenue, 14^th Floor

Cambridge, Massachusetts 02139

April 27, 201826, 2019

To Our Stockholders:

You are cordially invited to attend the 20182019 annual meeting of stockholders of Spero Therapeutics, Inc. to be held at 9:00 a.m. ETEastern Time on Tuesday,Wednesday, June 5, 20182019 at the offices of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. at One Financial Center, Boston, MA 02111.

Details regarding the meeting, the business to be conducted at the meeting, and information about Spero Therapeutics, Inc. that you should consider when you vote your shares are described in this proxy statement.

At the annual meeting,two people will be elected to our board of directors. In addition, we will ask stockholderstostockholders to ratify the selection of PricewaterhouseCoopers LLP as our independent registered public accounting firm for our fiscal year ending December 31, 2018.2019. The board of directors recommends the approval of each of the proposals. Such other business will be transacted as may properly come before the annual meeting.

We hope you will be able to attend the annual meeting. Whether you plan to attend the annual meeting or not, it is important that you cast your vote either in person or by proxy. You may vote over the Internet as well as by telephone or by mail. When you have finished reading the proxy statement, you are urged to vote in accordance with the instructions set forth in this proxy statement. We encourage you to vote by proxy so that your shares will be represented and voted at the meeting, whether or not you can attend.

Thank you for your continued support of Spero Therapeutics, Inc. We look forward to seeing you at the annual meeting.

Sincerely,

Ankit Mahadevia, M.D.

President and Chief Executive Officer

SPERO THERAPEUTICS, INC.

675 Massachusetts Avenue, 14^th Floor

Cambridge, Massachusetts 02139

April 27, 201826, 2019

NOTICE OF 20182019 ANNUAL MEETING OF STOCKHOLDERS

TIME: 9:00 a.m. Eastern Time

DATE: Tuesday,Wednesday, June 5, 20182019

PLACE: Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., One Financial Center, Boston, MA 02111

PURPOSES:

WHO MAY VOTE:

You may vote if you were the record owner of Spero Therapeutics, Inc. common stock at the close of business on April 16, 2018.8, 2019.

If you are a stockholder of record, you may vote in one of the following ways:

If your shares are held in “street name,” that is, held for your account by a broker or other nominee, you will receive instructions from the holder of record that you must follow for your shares to be voted.

A list of stockholders of record will be available at the annual meeting at Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., One Financial Center, Boston, MA 02111, and during the 10 days prior to the annual meeting at our principal executive offices located at 675 Massachusetts Avenue, 14^th Floor, Cambridge, Massachusetts 02139.

All stockholders are cordially invited to attend the annual meeting.Whether you plan to attend the annual meeting or not, we urge you to vote by following the instructions in the Notice of Internet Availability of Proxy Materials that you previously received and submit your proxy by the Internet, telephone or mail in order to ensure the presence of a quorum.You may change or revoke your proxy at any time before it is voted at the meeting.

BY ORDER OF THE BOARD OF DIRECTORS

Joel Sendek

Stephen Garbacz

Chief Financial Officer and Secretary

TABLE OF CONTENTS

i

SPERO THERAPEUTICS, INC.

675 Massachusetts Avenue, 14^th Floor

Cambridge, Massachusetts 02139

PROXY STATEMENT FOR SPERO THERAPEUTICS, INC.

20182019 ANNUAL MEETING OF STOCKHOLDERS TO BE HELD ON JUNE 5, 20182019

This proxy statement, along with the accompanying Notice of 20182019 Annual Meeting of Stockholders, contains information about the 20182019 annual meeting of stockholders of Spero Therapeutics, Inc. (“Annual Meeting”), including any adjournments or postponements of the annual meeting.Annual Meeting. We are holding the annual meetingAnnual Meeting at 9:00 a.m. ETEastern Time on Tuesday,Wednesday, June 5, 20182019 at the offices of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. at One Financial Center, Boston, MA 02111.

In this proxy statement, we refer to Spero Therapeutics, Inc. as “Spero,” “the Company,” “we” and “us.”

This proxy statement relates to the solicitation of proxies by our board of directors (“Board of Directors”) for use at the annual meeting.Annual Meeting.

On or about May 2, 2018,3, 2019, we intend to begin sending this proxy statement, the attached Notice of Annual Meeting of Stockholders and the enclosed proxy card to all stockholders entitled to vote at the annual meeting.Annual Meeting.

Although not part of this proxy statement, we are also sending, along with this proxy statement, our Annual Report onForm10-K, which includes our financial statements for the fiscal year ended December 31, 2017.2018.

IMPORTANT NOTICE REGARDING THE AVAILABILITY OF PROXY MATERIALS FOR THE

SHAREHOLDER MEETING TO BE HELD ON JUNE 5, 20182019

This proxy statement and our Annual Report onForm10-K are available for viewing, printing and downloading at www.investorvote.com/sprowww.edocumentview.com/SPRO. To view these materials, please have your12-digit control number(s) available that appears on your proxy card. On this website, you can also elect to receive future distributions of our proxy statements and annual reports to stockholders by electronic delivery.

Additionally, you can find a copy of our Annual Report onForm10-K, which includes our financial statements, for the fiscal year ended December 31, 20172018 on the website of the Securities and Exchange Commission, or the SEC, atwww.sec.gov, or in the “SEC Filings” section of the “Investors & Media” section of our website at www.sperotherapeutics.com. You may also obtain a printed copy of our Annual Report onForm10-K, including our financial statements, free of charge, from us by sending a written request to: Spero Therapeutics, Inc., Attn: Investor Relations, 675 Massachusetts Avenue, 14^th Floor, Cambridge, Massachusetts 02139. Exhibits will be provided upon written request and payment of an appropriate processing fee.

IMPORTANT INFORMATION ABOUT THE ANNUAL MEETING AND VOTING

Why is the Company Soliciting My Proxy?

The boardBoard of directorsDirectors of Spero Therapeutics, Inc. is soliciting your proxy to vote at the 2018 annual meeting2019 Annual Meeting of stockholders to be held at the offices of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. at One Financial Center, Boston, MA 02111 on Tuesday,Wednesday, June 5, 20182019 at 9:00 a.m. ET,Eastern Time, and any adjournments of the meeting, which we refer to as the annual meeting. The proxy statement along with the accompanying Notice of Annual Meeting of Stockholders summarizes the purposes of the meeting and the information you need to know to vote at the Annual Meeting.

We have made available to you on the Internet or have sent you this proxy statement, the Notice of Annual Meeting of Stockholders, the proxy card and a copy of our Annual Report onForm10-K for the fiscal year ended December 31, 2017because2018 because you owned shares of Spero Therapeutics, Inc. common stock on the record date. The Company intends to commence distribution of the proxy materials to stockholders on or about May 2, 2018.3, 2019.

Who Can Vote?

Only stockholders who owned our common stock at the close of business on April 16, 20188, 2019 are entitled to vote at the annual meeting.Annual Meeting. On this record date, there were 14,369,18217,422,878 shares of our common stock outstanding and entitled to vote. Our common stock is our only class of voting stock.

You do not need to attend the annual meetingAnnual Meeting to vote your shares. Shares represented by valid proxies, received in time for the annual meetingAnnual Meeting and not revoked prior to the annual meeting,Annual Meeting, will be voted at the annual meeting.Annual Meeting. For instructions on how to change or revoke your proxy, see “May I Change or Revoke My Proxy?” below.

How Many Votes Do I Have?

Each share of ourcommonour common stock that you own entitles you to one vote.

How Do I Vote?

Whether you plan to attend the annual meetingAnnual Meeting or not, we urge you to vote by proxy. All shares represented by valid proxies that we receive through this solicitation, and that are not revoked, will be voted in accordance with your instructions on the proxy card or as instructed via Internet or telephone. You may specify whether your shares should be voted for or withheld for each nominee for director, and whether your shares should be voted for, against or abstain with respect to the ratification of the appointment of PricewaterhouseCoopers LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2018.2019. If you properly submit a proxy without giving specific voting instructions, your shares will be voted in accordance with the Board’sBoard of Directors’ recommendations as noted below. Voting by proxy will not affect your right to attend the annual meeting.Annual Meeting. If your shares are registered directly in your name through our stock transfer agent, Computershare Trust Company, N.A., or you have stock certificates registered in your name, you may vote:

Telephone and Internet voting facilities for stockholders of record will be available 24 hours a day and will close at 11:59 p.m. Eastern Time on June 4, 2018.2019.

If your shares are held in “street name” (held in the name of a bank, broker or other holder of record), you will receive instructions from the holder of record. You must follow the instructions of the holder of record in order for your shares to be voted. Telephone and Internet voting also will be offered to stockholders owning shares through certain banks and brokers. If your shares are not registered in your own name and you plan to vote your shares in person at the annual meeting,Annual Meeting, you should contact your broker or agent to obtain a legal proxy or broker’s proxy card and bring it to the annual meetingAnnual Meeting in order to vote.

How Does the Board of Directors Recommend That I Vote on the Proposals?

The boardBoard of directorsDirectors recommends that you vote as follows:

If any other matter is presented at the annual meeting,Annual Meeting, your proxy provides that your shares will be voted by the proxy holder listed in the proxy in accordance with his best judgment. At the time this proxy statement was first made available, we knew of no matters that needed to be acted on at the annual meeting,Annual Meeting, other than those discussed in this proxy statement.

May I Change or Revoke My Proxy?

If you give us your proxy, you may change or revoke it at any time before the annual meeting.Annual Meeting. You may change or revoke your proxy in any one of the following ways:

Your most current vote, whether by telephone, Internet or proxy card is the one that will be counted.

What if I Receive More Than One Notice or Proxy Card?

You may receive more than one Notice or proxy card if you hold shares of our common stock in more than one account, which may be in registered form or held in street name. Please vote in the manner described above under “How Do I Vote?” for each account to ensure that all of your shares are voted.

Will My Shares be Voted if I Do Not Vote?

If your shares are registered in your name or if you have stock certificates, they will not be counted if you do not vote as described above under “How Do I Vote?” If your shares are held in street name and you do not provide voting instructions to the bank, broker or other nominee that holds your shares as described above, the bank, broker or other nominee that holds your shares has the authority to vote your unvoted shares only on certain of the proposals set forth in this proxy statement without receiving instructions from you. Therefore, we encourage you to provide voting instructions to your bank, broker or other nominee. This ensures your shares will be voted at the annual meetingAnnual Meeting and in the manner you desire. A “brokernon-vote” will occur if your broker cannot vote your shares on a particular matter because it has not received instructions from you and does not have discretionary voting authority on that matter or because your broker chooses not to vote on a matter for which it does have discretionary voting authority.

What Vote is Required to Approve Each Proposal and How are Votes Counted?

Is Voting Confidential?

We will keep all the proxies, ballots and voting tabulations private. We only let our Inspectors of Election, Computershare Trust Company, N.A.,examine these documents. Management will not know how you voted on a specific proposal unless it is necessary to meet legal requirements. We will, however, forward to management any written comments you make on the proxy card or otherwise provide.

Where Can I Find the Voting Results of the Annual Meeting?

The preliminary voting results will be announced at the annual meeting,Annual Meeting, and we will publish preliminary, or final results if available, in a Current Report onForm8-K within four business days of the annual

meeting.Annual Meeting. If final results are unavailable at the time we file theForm8-K, then we will file an amended report onForm8-K to disclose the final voting results within four business days after the final voting results are known.

What Are the Costs of Soliciting these Proxies?

We will pay all of the costs of soliciting these proxies. Our directors and employees may solicit proxies in person or by telephone, fax or email. We will pay these employees and directors no additional compensation for these services. We will ask banks, brokers and other institutions, nominees and fiduciaries to forward these proxy materials to their principals and to obtain authority to execute proxies. We will then reimburse them for their expenses.

What Constitutes a Quorum for the Annual Meeting?

The presence, in person or by proxy, of the holders of a majority of the voting power of all outstanding shares of our capital stock entitled to vote at the annual meetingAnnual Meeting is necessary to constitute a quorum at the annual meeting.Annual Meeting. Votes of stockholders of record who are present at the annual meetingAnnual Meeting in person or by proxy, abstentions, and brokernon-votes are counted for purposes of determining whether a quorum exists.

Attending the Annual Meeting

The annual meetingAnnual Meeting will be held at 9:00 a.m. Eastern Time on Tuesday,Wednesday, June 5, 20182019 at the offices of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. at One Financial Center, Boston, MA 02111. When you arrive at One Financial Center, signs will direct you to the appropriate meeting rooms. You need not attend the annual meetingAnnual Meeting in order to vote.

Householding of Annual Disclosure Documents

The SEC has adopted rules that permit companies and intermediaries such as brokers to satisfy delivery requirements for proxy statements and annual reports with respect to two or more stockholders sharing the same address by delivering a single proxy statement or annual report, as applicable, addressed to those stockholders. This process, which is commonly referred to as “householding,” potentially provides extra convenience for stockholders and cost savings for companies.

Spero has not instituted householding for stockholders of record. However, certain brokerage firms may have instituted householding for beneficial owners of shares of Spero’s common stock held through brokerage firms. If your household has multiple accounts holding shares of Spero’s common stock, you may have already received householding notification from your broker. Please contact your broker directly if you have any questions or require additional copies of this joint proxy statement/prospectus. The broker will arrange for delivery of a separate copy of this joint proxy statement/prospectus promptly upon your request. Spero stockholders may decide at any time to revoke a decision to household, and thereby receive multiple copies.

Electronic Delivery of Company Stockholder Communications

Most stockholders can elect to view or receive copies of future proxy materials over the Internet instead of receiving paper copies in the mail.

You can choose this option and save the Company the cost of producing and mailing these documents by going towww.computershare.com orwww.investorvote.com/SPROand logging into the Investor Center to follow the instructions provided.

SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

The following table sets forth certain information with respect to the beneficial ownership of our common stock as of March 1, 2018April 15, 2019 for (a) the executive officers named in the Summary Compensation Table on page 18 of this proxy statement, (b) each of our directors and director nominees, (c) all of our current directors and executive officers as a group and (d) each stockholder known by us to own beneficially more than 5% of our common stock. Beneficial ownership is determined in accordance with the rules of the SEC and includes voting or investment power with respect to the securities. We deem shares of common stock that may be acquired by an individual or group within 60 days of March 1, 2018April 15, 2019 pursuant to the exercise of options to be outstanding for the purpose of computing the percentage ownership of such individual or group, but not outstanding for the purpose of computing the percentage ownership of any other person shown in the table. Except as indicated in footnotes to this table, we believe that the stockholders named in this table have sole voting and investment power with respect to all shares of common stock shown to be beneficially owned by them based on information provided to us by these stockholders. Percentage of ownership is based on 14,369,18217,455,749 shares of common stock outstanding on March 1, 2018.April 15, 2019.

MANAGEMENT AND CORPORATE GOVERNANCE MATTERS

The Board of Directors

Our Bylawsamended and restatedBy-Laws provide that our business is to be managed by or under the direction of our boardBoard of directors.Directors. Our boardBoard of directorsDirectors is divided into three classes for purposes of election. One class is elected at each annual meeting of stockholders to serve for a three-year term. Our boardBoard of directorsDirectors currently consists of seven members, classified into three classes as follows:

On March 30, 2018,29, 2019, our boardBoard of directorsDirectors accepted the recommendation of the Nominating and Corporate Governance Committee and voted to nominate Casper BreumPatrick Vink, M.D. and David SouthwellFrank E. Thomas for election at the annual meetingAnnual Meeting for a term of three years to serve until the 20212022 annual meeting of stockholders, and until their respective successors have been elected and qualified or subject to their earlier death, resignation or removal.

Set forth below are the names of the persons nominated as directors and directors whose terms do not expire this year, their ages, their offices in the Company, if any, their principal occupations or employment for at least the past five years, the length of their tenure as directors and the names of other public companies in which such persons hold or have held directorships during the past five years. Additionally, information about the specific experience, qualifications, attributes or skills that led to our boardBoard of directors’Directors’ conclusion at the time of filing of this proxy statement that each person listed below should serve as a director is set forth below:

Our boardBoard of directorsDirectors has reviewed the materiality of any relationship that each of our directors has with Spero Therapeutics, Inc., either directly or indirectly. Based upon this review, our Board of Directors has determined that the following members of the Board of Directors are “independent directors” as defined by The Nasdaq Stock Market: Casper Breum, Milind Deshpande, Ph.D., Jean-François Formela, M.D., David Southwell,John C. Pottage, Jr., M.D., Cynthia Smith, Frank E. Thomas, and Patrick Vink, M.D.

Casper Breumhas served on our board of directors since June 2015. Mr. Breum is a Senior Partner at Lundbeckfond Ventures, where he has been since 2009. Previously, Mr. Breum was Chief Executive Officer of Ilochip A/S, a venture-backed company focused on the development of diagnostic biochips. Prior to Ilochip, Mr. Breum held different positions at H. Lundbeck A/S, most recently in corporate business development and strategy. While working at H. Lundbeck A/S, he was a member of the board of directors of Lundbeckfond and Lundbeckfond Invest A/S, the main shareholder of H. Lundbeck A/S andALK-Abello A/S, as an employee representative. Mr. Breum is currently a member of the board of directors of Atox Bio Inc., Aura Biosciences,

Vice President of Chemistry, was named Head of Drug Discovery in April 2002, Senior Vice President of Drug Discovery in December 2002, Senior Vice President and Chief Scientific Officer in December 2004 and Executive Vice President of Research and Chief Scientific Officer in June 2007. He was promoted to President of Research and Development in October 2010. In May 2013, Dr. Deshpande was appointed President and Chief Executive Officer of Achillion and joined its board of directors on which he continues to serve.served until May 2018. Prior to joining Achillion, Dr. Deshpande was Associate Director of Lead Discovery and Early Discovery Chemistry at the Pharmaceutical Research Institute at Bristol-Myers Squibb Co. from 1991 to 2001, where he managed the identification of new clinical candidates to treat infectious and neurological diseases. From 1988 to 1991, he held a faculty position at Boston University Medical School. Dr. Deshpande received his Ph.D. in Organic Chemistry from Ohio University, following his undergraduate education in India. We believe that Dr. Deshpande is qualified to serve on our boardBoard of directorsDirectors due to his extensive experience in the life sciences industry.

Jean-François Formela, M.D. has served on our boardBoard of directorsDirectors since March 2013. Dr. Formela is currently a partner at Atlas Venture and focuses on new advances in biology and drug discovery technologies as well as novel therapeutics. Dr. Formela joined Atlas Venture in 1993 to build its U.S. life sciences franchise. Prior to joining Atlas, he worked at Schering-Plough Corporation, where he directed U.S. Phase 4 clinical trials in all therapeutic areas. Before that, he was responsible for the marketing of Intron A, Schering-Plough’s alpha-interferon. Dr. Formela began his career as a medical doctor and practiced emergency medicine at Necker University Hospital in Paris. Dr. Formela serves as chair of the board of directors of IFM Therapeutics, and serves on the boards of directors of Intellia Therapeutics,F-Star Biotechnology Ltd., KynKDAC, Korro, KYN Therapeutics, Inc., Korro, Triplet, and Translate Bio, which heco-founded. Dr. Formela received his M.D. from Paris University School of Medicine and his M.B.A. from Columbia University. We believe Dr. Formela’s experience in the life sciences industry, as well as his practice of medicine, provides him with the qualifications and skills to serve as a director of our company.Company.

Ankit Mahadevia, M.D. has served as our Chief Executive Officer and President since March 2015 and has been a member of our boardBoard of directorsDirectors since September 2013. Heco-founded the Company in 2013. Prior to joining us, Dr. Mahadevia was formerly a Venture Partner in the life sciences group at Atlas Venture, located in Cambridge, Massachusetts, whereMassachusetts. In that capacity he supported the formation of eight companies focused on novel drug discovery platforms and therapeutic products, including SynlogicNimbus Therapeutics, Arteaus Therapeutics (acquired by Lilly), and Translate Bio.Bio (Nasdaq: TBIO). He has led three of these companies as acting or full-time CEO.CEO, including Synlogic (Nasdaq: SYBX). Prior to joining Atlas Venture in 2008, Dr. Mahadevia worked on product and business development with the founding team at Arcion Therapeutics, Inc. He has also held positions in business development both at Genentech, Inc. and at Vanda Pharmaceuticals Inc. Previously, he worked in the health care groups of McKinsey & Company and Monitor Group. Dr. Mahadevia began his career in health care policy, with roles in the U.S. Senate Health, Education, Labor, and Pensions committees, the U.S. Government Accountability Office and the Mexican Institute of Social Security. He has spoken widely on entrepreneurship, including at Harvard University, Columbia University, Northwestern University, and the Berkeley Forum. Dr. Mahadevia has also been active in the policy of life science innovation, including service on the Advisory Council at the NIH National Center for Advancing Translational Sciences. Dr. Mahadevia holds an M.D. from the Johns Hopkins School of Medicine, an M.B.A. from the Wharton School at the University of Pennsylvania and a B.A. in Economics and Biology from Northwestern University. We believe that Dr. Mahadevia is qualified to serve on our boardBoard of directorsDirectors due to his experience serving as our Chief Executive Officer and President and his extensive experience in the life sciences industry.

David SouthwellJohn C. Pottage, Jr., M.D. has served on our Board of Directors since September 2018. Dr. Pottage has served as Senior Vice President and Chief ExecutiveScientific and Medical Officer of ViiV Healthcare since November 2009. From September 2008 to November 2009, Dr. Pottage served as Senior Vice President, Head of Infectious Disease Medicine Development Center and, from June 2007 to September 2008, as the Vice President, Global Clinical Development of Antivirals, at GlaxoSmithKline. Prior to joining GlaxoSmithKline, Dr. Pottage served as Chief Medical Officer and Senior Vice President of Drug Development of Achillion Pharmaceuticals from May 2002 to May 2007. From July 1998 to May 2002, Dr. Pottage served as Medical Director of Vertex Pharmaceuticals. We believe that Dr. Pottage’s extensive industry and executive experience, his broad

experience within the biopharmaceutical sector and his knowledge of the life sciences industry qualifies him to serve on our Board of Directors.

Cynthia Smith has served on our Board of Directors since March 2019. Ms. Smith was Chief Commercial Officer of ZS Pharma, from June 2013 to December 2016. ZS Pharma became a subsidiary of AstraZeneca after its acquisition in December 2015. Prior to joining ZS Pharma, Ms. Smith was Vice President, Market Access & Commercial Development at Affymax, Inc., a biotechnology company focused on the development and commercialization of novel renal therapies, including a new anemia drug for chronic kidney disease patients. Ms. Smith was employed at Affymax from October 2008 to March 2013. Prior to Affymax, Ms. Smith was Executive Director of Healthcare System and Medicare Strategy at Merck. During her tenure at Merck from June 2000 to October 2008, she also held various leadership positions in corporate strategy, public policy, and external affairs, including global crisis management for the Vioxx recall. Before joining the pharmaceutical industry, she served in the White House Office of Management and Budget (OMB) in the Clinton Administration. Ms. Smith earned an MBA from the Wharton School of the University of Pennsylvania, an MS in public policy from the Eagleton Institute of Politics at Rutgers University, and a BA from the University of North Carolina at Chapel Hill. Ms. Smith also serves on the boards of directors of Dicerna Pharmaceuticals, Akebia Therapeutics, and Nivalis Therapeutics. We believe that Ms. Smith’s extensive management experience in the healthcare industry and her experience as a member of the board of directors of Inotek Pharmaceuticals from July 2014 until its merger with Rocket Pharmaceuticals in January 2018. From March 2010 to October 2012, Mr. Southwell servedother publicly traded biotechnology companies, as Executive Vice President, Chief Financial Officer of

Sandoz; Vice-President International Business for Biogen, Inc.; and Head of Worldwide Marketing, Cardiovascular and Thrombosis for Sanofi-Synthélabo SA. Dr. Vink served as a member of the Executive Committee of the European Federation of Pharmaceutical Industries and Associations (EFPIA) between 2013 and 2015. Dr. Vink graduated as a medical doctor from the University of Leiden, Netherlands in 1988 and obtained his M.B.A. in 1992 from the University of Rochester. Dr. Vink serves on the boards of directors of Acacia Pharma Ltd, Arch Biopartners, Concordia International Corp., NMD Pharma, Piqur AG, and Santhera Pharmaceuticals Ag., and Arch Biopartners and is Chairman of the board of directors of Targovax Oy. We believe that Dr. Vink is qualified to serve on our boardBoard of directorsDirectors because of his extensive operational business experience, significant knowledge of the activities of our company, and diverse background serving on the board of directors of various public and private life science companies.

SEC and the corporate governance rules of The Nasdaq Stock Market, Pearl Meyer provided the Compensation Committee with a letter addressing each of the six independence factors. Their responses affirm the independence of Pearl Meyer and the partners, consultants, and employees who service the Compensation Committee on executive compensation matters and governance issues.

A copy of the Compensation Committee’s written charter is publicly available on our website at www.sperotherapeutics.com.

Nominating and Corporate Governance Committee.Our Nominating and Corporate Governance Committee (“Nominating Committee”) did not meetmet twice during fiscal 20172018 and has three members, Milind Deshpande, Ph.D. (Chairman), Jean-François Formela, M.D., and Frank E. Thomas. Our boardBoard of directorsDirectors has determined that

The recommendation must also be accompanied by the following information concerning the proposed nominee:

objective oversight of management’s performance and enhances the effectiveness of our boardBoard of directorsDirectors as a whole. As such, Dr. Mahadevia serves as our Chief Executive Officer while Dr. Deshpande serves as the chairman of our boardBoard of directorsDirectors but is not an officer.

Our boardBoard of directorsDirectors oversees the management of risks inherent in the operation of our business and the implementation of our business strategies. Our boardBoard of directorsDirectors performs this oversight role by using several different levels of review. In connection with its reviews of the operations and corporate functions of our company,Company, our boardBoard of directorsDirectors addresses the primary risks associated with those operations and corporate functions. In addition, our boardBoard of directorsDirectors reviews the risks associated with the Company’s business strategies periodically throughout the year as part of its consideration of undertaking any such business strategies.

Each of our board committees also oversees the management of the Company’s risk that falls within the committee’s areas of responsibility. In performing this function, each committee has full access to management,

Joel Sendek has served as our Chief Financial Officer since May 2017. Mr. Sendek has more than 25nearly 30 years of experience in the life sciences sector, including 18 years as a senior sell-side research analyst covering biotechnology. Prior to joining us, Mr. Sendek was the Chief Financial Officer at Forward Pharma A/S since August 2014. As an analyst, he served as a Managing Director at Stifel Financial Corp. from January 2012 to July 2014, where he led the firm’s healthcare equity research group, and previously he was a Managing Director at Lazard Ltd. since January 2000, where he established the firm’s healthcare equity research effort. Prior to his career in equity research, Mr. Sendek worked as Senior Director of Corporate Development at Progenics Pharmaceuticals, Inc. and as an investment banking analyst at Goldman, Sachs & Co. Mr. Sendek holds a B.A. in Biochemistry from Rice University.

Cristina Larkin was promoted tohas served as our Chief Operating Officer as ofsince September 2017 and had previously served as our Chief Commercial Officer since March 2016. Ms. Larkin has over 24 years of experience developing strategic

EXECUTIVE OFFICER AND DIRECTOR COMPENSATION

Summary Compensation Table

The following table shows the total compensation paid or accrued during the last two fiscal years ended December 31, 20172018 and 20162017 to our President and Chief Executive Officer and our two next most highly compensated executive officers who earned more than $100,000 during the fiscal year ended December 31, 20172018 and were serving as executive officers as of such date.

medical benefits with another employer. Further, the new agreement provides that upon termination by us without Cause or by Dr. Mahadevia for Good Reason within 90 days prior to or one year following the earlier to occur of a Change of Control (as defined below) or the execution of a definitive agreement the consummation of

David Melnick, M.D.

On December 13, 2017, we entered into an agreement with Dr. Melnick with respect to his employment as our Chief Medical Officer commencing on January 4, 2018. The terms of Mr. Melnick’s offer letter provided for an annual base salary of $380,000 prorated for fiscal year 2018, and eligibility for an annual incentive bonus, with a target bonus opportunity of 35% of his then-current base salary. In addition, Dr. Melnick was granted an option to purchase 135,000 shares of common stock and received asign-on bonus of $10,000. In December 2018, Dr. Melnick’s base salary was increased, effective February 1, 2019, to $390,000 with a target bonus opportunity of 40% of base salary.

The agreement also provides for the following severance payments upon termination by us without Cause or by Dr. Melnick for Good Reason: (i) payment of his then-current base salary for a period of nine months following termination; (ii) apro-rated target bonus for the period during which Dr. Melnick was employed in the year of termination; and (iii) continued coverage under our group health insurance plan until the earlier of 12 months from termination or the date Dr. Melnick becomes eligible for medical benefits with another employer. Further, the agreement provides that upon termination by us without Cause or by Dr. Melnick for Good Reason within 90 days prior to the earlier to occur of a Change of Control or the execution of a definitive agreement the consummation of which would result in a Change of Control or one year following a Change of Control (a “Change of Control Termination”), Dr. Melnick will be entitled to receive: (i) a lump sum payment equal to 12 months of his then-current base salary plus apro-rated target bonus for the period during which Dr. Melnick was employed in the year of termination; (ii) acceleration of (A) all unvested equity awards as of the date of termination if Dr. Melnick’s employment commenced at least 24 months prior to a Change of Control (B) 50% of all unvested equity awards as of the date of termination if Melnick’s employment commenced fewer than 24 months but at least 12 months prior to a Change of Control and (C) 25% of all unvested equity awards as of the date of termination if Dr. Melnick’s employment commenced fewer than 12 months prior to a Change of Control; and (iii) continued coverage under our group health insurance plan until the earlier of 12 months from termination or the date Dr. Melnick becomes eligible for medical benefits with another employer. Payment in each case is subject to Dr. Melnick’s execution of a release satisfactory to us following such termination. In addition, if Dr. Melnick’s employment terminates as a result of disability or death, he shall be entitled to receive apro-rated target bonus for the period during which Dr. Melnick was employed in the year of termination.

Under each of the employment agreements, Cause means (i) the executive’s conviction of (A) a felony or (B) any misdemeanor involving moral turpitude, deceit, dishonesty or fraud; (ii) the executive’s willful failure or refusal to comply with lawful directions of our boardBoard of directors,Directors, with respect to Dr. Mahadevia, or of our Chief Executive Officer, with respect to Mr. SendekDr. Melnick and Ms. Larkin, which failure or refusal continues for more than thirty days after written notice is given to the executive by our boardBoard of directors,Directors, with respect to Dr. Mahadevia, or by our Chief Executive Officer, with respect to Mr. SendekDr. Melnick and Ms. Larkin, which notice sets forth in reasonable detail the nature of such failure or refusal; (iii) willful and material breach by the executive of a written company policy applicable to the executive or the executive’s covenants and/or obligations under his or her employment agreement or the material breach of the executive’s proprietary information and inventions assignment agreement; and/or (iv) material misconduct by the executive that seriously discredits or damages us or any of our affiliates.

Under each of the employment agreements, Good Reason means (i) relocation of the executive’s principal business location to a location more than thirty (30) miles from the executive’s then-current business location; (ii) a material diminution in the executive’s duties, authority or responsibilities; (iii) a material reduction in the executive’s base salary; (iv) willful and material breach by us of our covenants and/or obligations under the executive’s employment agreement; or (v) within one year following a Change of Control, the executive is not an executive of the parent company, provided that the executive’s roles responsibilities and scope of authority within the subsidiary is not comparable to the executive’s roles, responsibilities and scope of authority with us prior to the Change of Control.

Under each of the employment agreements, Change of Control means (i) any person (as such term is used in Sections 13(d) and 14(d) of the Exchange Act) becomes the beneficial owner, directly or indirectly, of securities of the Company representing fifty percent (50%) or more of the total voting power represented by the Company’s then outstanding voting securities (excluding for this purpose any such voting securities held by the Company, or any affiliate, parent or subsidiary of the Company, or by any employee benefit plan of the Company) pursuant to a transaction or a series of related transactions; (ii) a merger or consolidation of the Company other than a merger or consolidation which would result in the voting securities of the Company outstanding immediately prior thereto continuing to represent (either by remaining outstanding or by being converted into voting securities of the surviving entity or the parent of such corporation) at least fifty percent (50%) of the total voting power represented by the voting securities of the Company or such surviving entity or parent of such corporation, as the case may be, outstanding immediately after such merger or consolidation; (iii) our stockholders approve an agreement for the sale or disposition by the Company of all or substantially all of our assets; or (iv) a change in the composition of our boardBoard of directors,Directors, as a result of which fewer than a majority of the directors are incumbent directors

150,000 performance-based options and restricted stock units (“RSUs”) to our senior executives. These options and RSUs (the “Performance Awards”) are subject to performance-based vesting eligibility and a subsequent partial time-based vesting schedule. Specifically, the Performance Awards are eligible for vesting based on the achievement of performance criteria, each representing a 25% vesting opportunity if achieved within a specified time during the performance period (the “Performance Period”), and relating to (i) the release of SPR994top-line data; (ii) FDA acceptance of a SPR994 New Drug Application;(iii) non-dilutive financing; and (iv) equity financing. Following the Performance Period, Performance Awards determined to be eligible for vesting as a result of achievement of the performance criteria will vest as follows: (a) 50% of the eligible award will vest immediately, and (b) the remaining eligible award will vest (i) in the case of options, in equal monthly installments ending two years after the Performance Period expiration, and (ii) in the case of RSUs, on such two year anniversary. The Performance Awards will be subject to provisions of the executives’ employment agreements regarding acceleration of vesting in the event of certain termination events following a change in control only to the extent previously determined to be eligible for vesting as a result of achievement of the performance criteria. We believe the achievement of the performance criteria will require significant execution and effort by the executives with no assurance of achievement guaranteed. Awards for which the performance criteria has not been achieved as specified during the Performance Period will lapse.

Potential Payments upon Termination orChange-In-Control

The employment agreements provide for the following severance payments upon termination by us without Cause or by the employee for Good Reason: (i) payment of the employee’s then-current base salary for a period of nine months following termination (12 months in the case of the Chief Executive Officer); (ii) apro-rated target bonus for the period during which the employee was employed in the year of termination; and (iii) continued coverage under our group health insurance plan until the earlier of 12 months from termination or the date the employee becomes eligible for medical benefits with another employer.

Further, the agreements provide that upon termination by us without Cause or by the employee for Good Reason within 90 days prior to or one year following the earlier to occur of a Change of Control (as defined below)in the executive’s employment agreements) or the execution of a definitive agreement the consummation of which would result in a Change of Control, the employee will be entitled to receive: (i) a lump sum payment equal to 12 months of the employee’s then-current base salary; (ii)salary plus apro-rated target bonus for the period during which the employee was employed in the year of termination; (iii)(ii) acceleration of all unvested equity awards as of the date of termination;termination in accordance with the terms of the executive’s employment agreement, as described above under “Narrative Disclosure to Summary Compensation Table;” and (iv)(iii) continued coverage under our group health insurance plan until the earlier of 12 months from termination or the date the employee becomes eligible for medical benefits with another employer. Payment in each case is subject to the employee’s execution of a release satisfactory to us following such termination. In addition, if the employee’s employment terminates as a result of disability or death, he or she shall be entitled to receive apro-rated target bonus for the period during which the employee was employed in the year of termination.

Under each of the employment agreements, Cause means (i) the executive’s conviction of (A) a felony or (B) any misdemeanor involving moral turpitude, deceit, dishonesty or fraud; (ii) the executive’s willful failure or refusal to comply with lawful directions of our board of directors, with respect to Dr. Mahadevia, of our Chief Executive Officer, with respect to Mr. Sendek and Ms. Larkin, which failure or refusal continues for more than thirty days after written notice is given to the executive by our board of directors, with respect to Dr. Mahadevia, or by our Chief Executive Officer, with respect to Mr. Sendek and Ms. Larkin, which notice sets forth in reasonable detail the nature of such failure or refusal; (iii) willful and material breach by the executive of a written company policy applicable to the executive or the executive’s covenants and/or obligations under his or her employment agreement or the material breach of the executive’s proprietary information and inventions assignment agreement; and/or (iv) material misconduct by the executive that seriously discredits or damages us or any of our affiliates.

Under each of the employment agreements, Good Reason means (i) relocation of the executive’s principal business location to a location more than thirty (30) miles from the executive’s then-current business location; (ii) a material diminution in the executive’s duties, authority or responsibilities; (iii) a material reduction in the executive’s base salary; (iv) willful and material breach by us of our covenants and/or obligations under the executive’s employment agreement; or (v) following a Change of Control, the executive is not an executive of the parent company, provided that the executive’s roles responsibilities and scope of

authority within the subsidiary is not comparable to the executive’s roles, responsibilities and scope of authority with us prior to the Change of Control.

Under each of the employment agreements, Change of Control means (i) any person (as such term is used in Sections 13(d) and 14(d) of the Exchange Act) becomes the beneficial owner, directly or indirectly, of securities of the Company representing fifty percent (50%) or more of the total voting power represented by the Company’s then outstanding voting securities (excluding for this purpose any such voting securities held by the Company, or any affiliate, parent or subsidiary of the Company, or by any employee benefit plan of the Company) pursuant to a transaction or a series of related transactions; (ii) a merger or consolidation of the Company other than a merger or consolidation which would result in the voting securities of the Company outstanding immediately prior thereto continuing to represent (either by remaining outstanding or by being converted into voting securities of the surviving entity or the parent of such corporation) at least fifty percent (50%) of the total voting power represented by the voting securities of the Company or such surviving entity or parent of such corporation, as the case may be, outstanding immediately after such merger or consolidation; (iii) our stockholders approve an agreement for the sale or disposition by the Company of all or substantially all of our assets; or (iv) a change in the composition of our board of directors, as a result of which fewer than a majority of the directors are incumbent directors.

Director Compensation

The following table shows the total compensation paid or accrued during the fiscal year ended December 31, 20172018 to each of our current and formernon-employee directors. Cynthia Smith joined our Board of Directors in March 2019. Directors who are employed by us are not compensated for their service on our boardBoard of directors.Directors.

(4) On July 6, 2017, Mr. Vink was granted options under the 2017 Plan to purchase 21,236 shares of our common stock at an exercise price of $5.90 per share, which vest over four years with 25% of the options vesting on the first anniversary of the grant date, with an additional 1/36^th of the remaining shares vesting monthly thereafter until the option is fully vested. As part of the Company’s 2017 Reorganization, Mr. Vink was also granted options to replace his previously awarded incentive units in Spero Therapeutics, LLC, which represented 9,278 shares of underlying common stock. The replacement options vest in accordance with the vesting terms of Mr. Vink’s previously held incentive units: 25% of the underlying shares were deemed vested on August 24, 2016, the first anniversary of the vesting commencement date, with an additional 1/36^th of the remaining shares vesting monthly thereafter until the option is fully vested.

(5) 25% of the options will vest on July 17,

(7)

As of December 31, 2018, and an additional 1/36th of the remaining shares vest monthly until the option is fully vested. (6) As of December 31, 2017, the aggregate number of options held by each of ournon-employee directors was as follows (representing both exercisable and unexercisable option awards, none of which have been exercised):

Non-Employee Director Compensation Policy

Under ourNon-Employee Director Compensation Policy, eachnon-employee director is eligible to receive compensation for his or her service consisting of annual cash retainers and equity awards. Ournon-employee directors receive the following annual retainers for their service:

Equity awards fornon-employee directors consist of (i) an initial equity award consisting of anon-qualified stock option to purchase 12,146 shares of our common stock upon first appointment to our boardBoard of directorsDirectors and vesting in equal monthly installments until the third anniversary of the grant date, subject to thenon-employee director’s continued service, on our board of directors, and (ii) annual equity awards commencing in 2018 consisting of anon-qualified stock option to purchase 6,073 shares of our common stock vesting on the first anniversary of the grant date, subject to thenon-employee director’s continued service on our board of directors.service.

Directors may be reimbursed for travel, food, lodging and other expenses directly related to their service as directors. Directors are also entitled to the protection provided by their indemnification agreements and the indemnification provisions in our amended and restated certificate of incorporation and amended and restatedBy-Laws.

EQUITY COMPENSATION PLANS AND OTHER BENEFIT PLANS

Equity Compensation Plan Information

The following table provides certain aggregate information with respect to all of the Company’s equity compensation plans in effect as of December 31, 2017.2018.

(1) This plan category consists of our

Benefits Programs

Each named executive employee is eligible to participate in our benefits programs, which include health, life, disability and dental insurance and a 401(k) retirement savings plan.

Spero Therapeutics, Inc.’s 2017 Stock Incentive Plan

We adopted the Spero Therapeutics, Inc.’s 2017 Stock Incentive Plan on June 30,28, 2017, as amended on October 18, 2017. The 2017 Plan will expire on June 30, 2027. Under the 2017 Plan, we may grant incentive stock options,non-qualified stock options, restricted and unrestricted stock awards and other stock-based awards. There are 2,696,401Since its adoption, there have been 3,303,7254 shares of our common stock authorized for issuance under the 2017 Plan. Additionally,Plan, which amount includes the numberautomatic increase effective as of January 1, 2019. As of April 15, 2019, a total of 153,059 shares of common stock that may be issuedare available for future grant under the 2017 Plan will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2019 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, by an amount equal to the lowest of:Plan.

Our boardBoard of directorsDirectors is authorized to administer the 2017 Plan. In accordance with the provisions of the 2017 Plan, our boardBoard of directorsDirectors determines the terms of the options and other awards issued pursuant thereto, including the following:

No participant may receive awards for more than 1,000,000 shares of our common stock in any fiscal year.

In addition, our boardBoard of directorsDirectors or any committee to which our boardBoard of directorsDirectors delegates authority may, with the consent of the affected plan participants, amend outstanding awards consistent with the terms of the 2017 Plan.

Upon a merger, consolidation, or sale of all or substantially all of our assets, our boardBoard of directorsDirectors or any committee to which our boardBoard of directorsDirectors delegates authority, or the boardBoard of directorsDirectors of any corporation assuming the our obligations, may, in its sole discretion, take any one or more of the following actions pursuant to the 2017 Plan, as to some or all outstanding awards, to the extent not otherwise agreed under any individual agreement:

In connection with the Company’s 2017 Reorganization, all outstanding incentive units issued under Spero Therapeutics, LLC’s operating agreement were cancelled. Any incentive unit holders who were employees, directors or consultants of the Company at the time of the 2017 Reorganization were issued options under the 2017 Plan with continued vesting on the same schedule and the same terms as such person’s incentive units.

Spero Therapeutics, Inc.’s 2019 Inducement Equity Incentive Plan

On March 11, 2019, the Board of Directors adopted Spero Therapeutics, Inc.’s 2019 Inducement Equity Incentive Plan (the “Inducement Plan”) and reserved 331,500 shares of our common stock to be used exclusively for grants of awards to individuals that were not previously employees or directors of the Company, as an inducement to the individual’s entry into employment with the Company within the meaning of Rule 5635(c)(4) of the Nasdaq Listing Rules. The Inducement Plan was adopted without stockholder approval pursuant to Rule 5635(c)(4). The Inducement Plan provides for the grant of equity-based awards, including options, restricted and unrestricted stock awards, and other stock-based awards, and its terms are substantially similar to the 2017 Plan, but with such other terms and conditions intended to comply with the Nasdaq inducement award exception. As of April 15, 2019, there were no shares outstanding and 331,500 shares available for grant under the Inducement Plan.

Rule10b5-1 Sales Plans

Our directors and executive officers may adopt written plans, known asRule10b5-1 plans, in which they will contract with a broker to buy or sell shares of our common stock on a periodic basis. Under aRule10b5-1 plan, a broker executes trades pursuant to parameters established by the director or officer when entering into the plan, without further direction from the director or officer. The director or officer may amend or terminate the plan in limited circumstances. Our directors and executive officers may also buy or sell additional shares of our common stock outside of a Rule10b5-1 plan when they are not in possession of material, nonpublic information.

REPORT OF AUDIT COMMITTEE

The Audit Committee of the boardBoard of directors,Directors, which consists entirely of directors who meet the independence and experience requirements of The Nasdaq Stock Market has furnished the following report:

The Audit Committee assists the Board of Directors in overseeing and monitoring the integrity of our financial reporting process, compliance with regulatory requirements and the quality of internal and external audit processes. This committee’s role and responsibilities are set forth in our charter adopted by the Board of Directors, which is available on our website at www.sperotherapeutics.com. This committee reviews and reassesses our charter annually and recommends any changes to the Board of Directors for approval. The Audit Committee is responsible for overseeing our overall financial reporting process, and for the appointment, compensation, retention, and oversight of the work of PricewaterhouseCoopers LLP. In fulfilling its responsibilities for the financial statements for fiscal year ended December 31, 2017,2018, the Audit Committee took the following actions:

Based on the Audit Committee’s review of the audited financial statements and discussions with management and PricewaterhouseCoopers LLP, the Audit Committee recommended to the Board of Directors that the audited financial statements be included in our Annual Report onForm10-K for the fiscal year ended December 31, 20172018 for filing with the SEC.

Members of the Spero Therapeutics, Inc.AuditInc. Audit Committee

Frank E. Thomas (Chairman)

David SouthwellJohn C. Pottage, Jr., M.D.

Patrick Vink, M.D.

SECTION 16(a) BENEFICIAL OWNERSHIP REPORTING COMPLIANCE

Section 16(a) of the Securities Exchange Act of 1934, as amended, or the Exchange Act, requires our directors and executive officers, and persons who own more than ten percent of a registered class of our equity securities, to file with the SEC initial reports of ownership and reports of changes in ownership of our common stock and other equity securities. Officers, directors and greater than ten percent stockholders are required by SEC regulations to furnish us with copies of all Section 16(a) forms they file.

To our knowledge, based solely on a review of the copies of such reports furnished to us and written representations regarding the filing of required reports, we believe that all Section 16(a) filing requirements applicable to our directors, executive officers andgreater-than-ten-percent beneficial owners with respect to fiscal year ended December 31, 20172018 were met.

CERTAIN RELATIONSHIPS AND RELATED PERSON TRANSACTIONS

The following is a description of transactions since January 1, 2017, to which we have been a party, in which the amount involved exceeds $120,000, and in which any of our directors, executive officers or holders of more than 5% of our capital stock, or an affiliate or immediate family member thereof, had or will have a direct or indirect material interest. We refer to such transactions as “related party transactions” and such persons as “related parties.” With the approval of our boardBoard of directors,Directors, we have engaged in the related party transactions described below. We believe the terms obtained or consideration that we paid or received, as applicable, in connection with the transactions described below were comparable to terms available or the amounts that would be paid or received, as applicable, from unaffiliated third parties.

Equity Financings

Class C Preferred Unit Financing

In March 2017, we issued an aggregate of 29,647,582 Class C preferred units, consisting of (i) 5,321,112 Class C preferred units in exchange for 8,500 bridge units and (ii) 24,326,470 Class C preferred units at a price per unit of $1.7749 for an aggregate purchase price of approximately $43,177,052. The following table sets forth the number of Class C preferred units purchased by our directors, executive officers and 5% stockholders and their affiliates at the time of or as a result of such issuance and the aggregate purchase price paid for such units.

Right of First Refusal andCo-Sale Agreement

We entered into a right of first refusal andco-sale agreement with certain holders of our common stock and preferred stock, including entities with which certain of our directors are affiliated. This agreement provided for rights of first refusal andco-sale relating to the shares of our common stock held by the parties to the agreement. The right of first refusal andco-sale agreement has terminated.

with BVF Inc. (“BVF”), which beneficially owns more than 5% of our outstanding common stock, purchased all 2,220 shares of Series A Preferred stock for a total purchase price of $27,500,000, before deducting underwriting discounts and commissions and expenses related to the offering. On November 15, 2018, we entered into an agreement with such BVF entities to exchange 1,000,000 shares of our common stock held by BVF for 1,000 shares of our Series B Convertible Preferred Stock, each share of which is convertible into 1,000 shares of our common stock, subject to certain ownership restrictions.See footnote 2 to the beneficial ownership table in the section “Security Ownership of Certain Beneficial Owners and Management.”

Indemnification Agreements with Officers and Directors and Directors’ and Officers’ Liability Insurance

We have entered into indemnification agreements with each of our executive officers and directors. The indemnification agreements, our amended and restated certificate of incorporation and our amended and restatedBy-Laws require us to indemnify our directors to the fullest extent not prohibited by Delaware law. Subject to certain limitations, our amended and restated certificate of incorporation also requires us to advance expenses incurred by our directors and officers, subject to limited exceptions. We also maintain a general liability insurance policy which covers certain liabilities of directors and officers of the Company arising out of claims based on acts or omissions in their capacities as directors or officers.

PROPOSAL 1: ELECTION OF DIRECTORS

The boardBoard of directorsDirectors has nominated Casper BreumPatrick Vink, M.D. and David SouthwellFrank E. Thomas forre-election at the annual meeting.Annual Meeting. The boardBoard of directorsDirectors currently consists of seven members, classified into three classes as follows: (i) Casper BreumCynthia Smith and David SouthwellconstituteJohn C. Pottage, Jr., M.D. constitute our Class I directors with a term ending at the upcoming2021 annual meeting; (ii) Patrick Vink, M.D. and Frank E. Thomas constitute our Class II directors with a term ending at the 2019upcoming annual meeting; and (iii) Milind Deshpande, Ph.D., Jean-François Formela, M.D., and Ankit Mahadevia, M.D. constitute our Class III directors with a term ending at the 2020 annual meeting. At each annual meeting of stockholders, directors are elected for a full term of three (3) years to succeed those directors whose terms are expiring.

The boardBoard of directorsDirectors has voted to nominate Casper BreumPatrick Vink, M.D. and David SouthwellFrank E. Thomas for election at the annual meetingAnnual Meeting as Class III directors for a term of three years to serve until the 2021 Annual Meeting2022 annual meeting of Stockholders, and until their respective successors are elected and qualified. The Class III directors (Patrick Vink(Cynthia Smith and Frank Thomas)John C. Pottage, Jr., M.D.) and the Class III directors (Milind Deshpande, Ph.D., Jean-François Formela, M.D., and Ankit Mahadevia) will serve until the Annual Meetingsannual meeting of Stockholders to be held in 20192021 and 2020, respectively, and until their respective successors have been elected and qualified.

Unless authority to vote for any of these nominees is withheld, the shares represented by the enclosed proxy will be votedFOR the election as directors of Casper BreumPatrick Vink, M.D. and David Southwell.Frank E. Thomas. In the event that either nominee becomes unable or unwilling to serve, the shares represented by the enclosed proxy will be voted for the election of such other person as the boardBoard of directorsDirectors may recommend in that nominee’s place. We have no reason to believe that any nominee will be unable or unwilling to serve as a director.

A plurality of the votes castFOR each nominee at the Annual Meeting is required to elect each nominee as a director.

We have no formal policy regarding diversity of our board members, but our Corporate Governance Guidelines provide that the background and qualifications of the members of our boardBoard of directorsDirectors considered as a group should provide a significant breadth of experience, knowledge, and ability to assist our boardBoard of directorsDirectors in fulfilling its responsibilities. Our priority in selection of board members is identification of members who will further the interests of our stockholders through their established records of professional accomplishment, the ability to contribute positively to the collaborative culture among our board members, knowledge of our business, understanding of the competitive landscape in which we operate and adherence to high ethical standards.

THE BOARD OF DIRECTORS RECOMMENDS THE ELECTION OF CASPER BREUMPATRICK VINK, M.D. AND DAVID SOUTHWELLFRANK E. THOMAS AS DIRECTORS, AND PROXIES SOLICITED BY THE BOARD OF DIRECTORS WILL BE VOTED IN FAVOR THEREOF UNLESS A STOCKHOLDER HAS INDICATED OTHERWISE ON THE PROXY.

PROPOSAL 2: INDEPENDENT PUBLIC ACCOUNTANTS

The Audit Committee has appointed PricewaterhouseCoopers LLP, or PwC, as our independent registered public accounting firm, to audit our financial statements for the fiscal year ending December 31, 2018.2019. PricewaterhouseCoopers LLP was our independent registered public accounting firm for the fiscal yearyears ended December 31, 2017.The2017 and 2018. The Board of Directors proposes that the stockholders ratify this appointment. PricewaterhouseCoopers LLP audited our financial statements for the fiscal year ended December 31, 2017. We expect that representatives of PricewaterhouseCoopers LLP will be present at the annual meeting,Annual Meeting, will be able to make a statement if they so desire, and will be available to respond to appropriate questions.

In deciding to appoint PricewaterhouseCoopers LLP, the Audit Committee reviewed auditor independence issues and existing commercial relationships with PricewaterhouseCoopers LLP and concluded that PricewaterhouseCoopers LLP has no commercial relationship with the Company that would impair its independence for the fiscal year ending December 31, 2018.2019.

The following table presents fees for professional audit services and other services rendered by PricewaterhouseCoopers LLP to the Company for the fiscal years ended December 31, 20172018 and December 31, 2016.2017.