Our Board of Directors (the “Board”) is soliciting your proxy to vote at the 20222024 Annual Meeting to be held via live webcast, on June 8, 2022,20, 2024, at 8:30 a.m., Eastern Time, and any adjournments or postponements of the meeting, which we refer to as the 2022 Annual Meeting.meeting. This proxy statement,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 20222024 Annual Meeting.

You received the Notice because you owned shares of our common stock on April 26, 2022.25, 2024 (the “Record Date”).

As permitted by the rules of the U.S. Securities and Exchange Commission or the SEC,(the “SEC”) we may furnish our proxy materials to our stockholders by providing access to such documents on the Internet, rather than mailing printed copies of these materials to each stockholder. Stockholders will not receive printed copies of the proxy materials unless they request them. We believe that this process will expedite stockholders’ receipt of proxy materials, lower the costs of the 20222024 Annual Meeting and help to conserve natural resources. If you received the Notice by mail or electronically, you will not receive a printed or email copy of the proxy materials, unless you request one by following the instructions included in the Notice. Instead, the Notice instructs you as toprovides instructions on how you may access and review all of the proxy materials and submit your proxy onvia the Internet. If you requested a paper copy of the proxy materials, you may authorize the voting of your shares by following the instructions on the proxy card, in addition to the other methods of voting described in this Proxy Statement.

The 2022To attend the virtual 2024 Annual Meeting, visit www.virtualshareholdermeeting.com/SLS2024 on June 20, 2024 and follow the instructions included on your proxy card, your Notice, or voting instruction card. The meeting will be held on Wednesday, June 8, 2022,begin promptly at 8:30 a.m., Eastern Time via live webcast. You will be able to attend and participate in the 2022 Annual Meeting online via live webcast and vote your shares electronically by visiting: www.meetnow.global/MQAFWZ9 on the meeting date and time described in this Proxy Statement. There is no physical location for the 2022 Annual Meeting. You are entitled to participate in the 2022 Annual Meeting only if you were a stockholder of the Company as of the close of business on the Record Date or if you hold a valid proxy for the 2022 Annual Meeting.

Whether you plan to attend the 20222024 Annual 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 the Internet or telephone. You may specify (i) whether your shares should be voted FOR or WITHHELD for each of our nominees for Class III director in Proposal 1, and (ii) whether your shares should be voted for, against or abstain with respect to Proposals 2 and 3. If you properly submit a proxy without giving specific voting instructions, your shares will be voted in accordance with our Board of Directors’ recommendations as noted below. Voting by proxy will not affect your right to attend the 20222024 Annual Meeting. You

If, on the Record Date, your shares were registered directly in your name with our transfer agent, Computershare, then you are a stockholder of record. If, on the Record Date, your shares were not held in your name, but rather in an account at a brokerage firm, bank dealer or other similar organization, then you are the beneficial owner of shares held in “street name.”

If you are a beneficial owner of shares held in “street name” and you do not provide instructions on how to vote your shares, your broker, bank, or other nominee may still be able to vote your shares in its discretion. A broker, bank, or other nominee may generally vote in their discretion on routine matters. In this regard, Proposals 1 and 3 are considered to be “non-routine”,“non-routine,” meaning that if your broker, bank, or other nominee does not receive instructions from you on how to vote your shares on such non-routine matter, the brokerit will not have the

The following table sets forth certain information with respect to the beneficial ownership of our common stock as of April 13, 202215, 2024 for (i) the named executive officers named in the Summary Compensation Table on page 1822 of this proxy statement,Proxy Statement, (ii) each of our directors and director nominees, (iii) all of our current directors and named executive officers as a group and (iv) 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 April 13, 202215, 2024 pursuant to the exercise of options or warrants or the vesting of restricted stock units to be outstanding for the purpose of computing the percentage ownership of such individual or group, but those shares are not deemed to be 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 20,535,62956,267,670 shares of common stock outstanding on April 13, 2022.15, 2024.

As required under Nasdaq listing standards, a majority of the members of a listed company’s board of directors must qualify as “independent,” as affirmatively determined by the board of directors of such listed company. Our Board consults with our internal and outside counsel to ensure that its determinations are consistent with relevant securities and other laws and regulations regarding the definition of “independent,” including those set forth in pertinent listing standards of Nasdaq, as in effect from time to time.

Consistent with these considerations, after review of all relevant identified transactions or relationships between each director, or any of his or her family members, and us, our senior management and our independent auditors, our Board has affirmatively determined that the following directors are independent directors within the meaning of the applicable Nasdaq listing standards: Jane Wasman, Robert L. Van Nostrand, each Class I directors, David A. Scheinberg, M.D., Ph.D., a and Katherine Bach Kalin, each Class II director,directors, and John Varian, a Class III director. In making this determination, our Board found that none of these directors had a material or other disqualifying relationship with the Company. Dr. Stergiou, who is a Class III director, is an employee and is therefore not “independent.”

Our Board has an independent Chair, Jane Wasman, who has served as Chair since 2017, and has authority, among other things, to preside over Board meetings and stockholder meetings, and shall have such powers and duties as may from time to time be assigned by the Board. Accordingly, the Chair has substantial ability to shape the work of the Board.

In March 2021, our Board of Directors adopted a clawback policy. The clawback policy applies to all incentive-based compensation granted after the policy’s adoption, including cash and equity incentive awards. The policy provides that if both (1) an accounting restatement is required due to our material noncompliance with any financial reporting requirement under the U.S. federal securities laws and (2) our Board of Directors (or a committee thereof), in its sole discretion, determines that an act or omission of a current or former executive officer contributed to the circumstances requiring the restatement and that such act or omission involved fraud or intentional misconduct then we will use reasonable efforts to recover from such person up to 100% of any incentive-based compensation awarded during the three-year period preceding the date on which we are required to prepare such accounting restatement. In light of the adoption by the SEC in October 2022 of new Rule 10D-1 under the Exchange Act implementing Section 954 of the Dodd-Frank Wall Street Reform and Consumer Protection Act of 2010, we amended and restated our clawback policy, effective September 12, 2023, in accordance with updated Nasdaq listing standards.

Our Board reviews the Nasdaq listing standards’ definition of “independence” for Audit Committee members on an annual basis and has determined that all members of our Audit Committee are independent (as currently defined in Rule 5605(c)(2)(A)(i) and (ii) of the Nasdaq listing standards and under Rule 10A-3 under the Exchange Act).

Our Board has also determined that Messrs. Varian and Van Nostrand each qualify as an “audit committee financial expert,” as defined in applicable SEC rules. Our Board made a qualitative assessment of the level of knowledge and experience of both Mr. Varian and Mr. Van Nostrand based on a number of factors, including the formal education and experience of each of Messrs. Van Nostrand and Varian as former chief financial officers for public reporting companies, and the status of Messrs. Van Nostrand and Varian as former Certified Public Accountants.

Our Nominating and Corporate Governance Committee believes that it is in the best position to identify, review, evaluate and select qualified candidates for Board membership, based on thesethe comprehensive criteria for Board membership approved by the CompensationBoard. Generally, our Nominating and Corporate Governance Committee will consider candidates recommended from several sources, such as other directors or officers, stockholders, third party search firms or other appropriate sources. Once identified, the following companiesNominating and Corporate Governance Committee will evaluate a candidate’s qualifications. The Nominating and Corporate Governance Committee believes that candidates for our 2022 peer group: Actinium Pharmaceuticals, Inc., Aileron Therapeutics, Cardiff Oncology, Checkmate Pharmaceuticals, Cidara Therapeutics, Inc., ContraFect Corporation, Cyclacel Pharmaceuticals, Inc., Entasis Therapeutics, GlycoMimetics, Inc., Infinity Pharmaceuticals, Jounce Therapeutics, Leap Therapeutics, MEI Pharma, Oncternal Therapeutics, Inc., PDS Biotechnology, Selecta Biosciences Inc., Sierra Oncology, Tonix Pharmaceuticals, Tyme Technologies Inc.,director should have certain qualifications, including the ability to read and Verastem, Inc.

In the case of incumbent directors whose terms of office are set to expire, our Nominating and Corporate Governance Committee reviews these directors’ overall service to the Company during their terms, including base salarythe number of meetings attended, level of participation, quality of performance and target bonus payments.any other relationships and transactions that might impair the directors’ independence. The Committee also takes into account the results of the Board’s self-evaluation, conducted annually on a group and individual basis.

Our Nominating and Corporate Governance Committee believes that it is in the best position to identify, review, evaluate and select qualified candidates for Board membership, based on the comprehensive criteria for Board membership approved by the Board. Generally, our Nominating and Corporate Governance Committee will consider candidates recommended from several sources, such as other directors or officers, stockholders, third party search firms or other appropriate sources. Once identified, the Nominating and Corporate Governance Committee will evaluate a candidate’s qualifications. The Nominating and Corporate Governance Committee believes that candidates for director should have certain qualifications, including the ability to read and understand basic financial statements and the possession of the highest personal integrity and ethics. Candidates for director should also be over 21 years of age. Our Nominating and Corporate Governance Committee also intends to considerconsiders such factors as possessing relevant expertise enabling the candidate to offer advice and guidance to management, having sufficient time to devote to the affairs of the Company, demonstrated excellence in his or her field, having the ability to exercise sound business judgment and having the commitment to rigorously represent the long-term interests of our stockholders. However, our Nominating and Corporate Governance Committee retains the right to modify these qualifications from time to time. Candidates for director nominees are reviewed in the context of the current composition of the Board, the operating requirements of the Company and the long-term interests of stockholders. In conducting this assessment, our Nominating and Corporate Governance Committee typically considers diversity, age, skills and such other factors as it deems appropriate, given our current needs and the needs of our Board, to maintain a balance of knowledge, experience and capability. The Nominatingcapability and Corporate Governance Committee considers issues of diversity among its members in identifying and considering nominees for director, and strivesstriving where appropriate to achieve a diverse balance of backgrounds, perspectives, experience, age, gender, ethnicity and country of citizenship on our Board of Directors and its committees. Our Nominating and Corporate Governance Committee also determines whether the nominee is independent for Nasdaq purposes, which determination is based upon applicable Nasdaq listing standards, applicable SEC rules and regulations and the advice of counsel, if necessary.

In the case of incumbent directors whose terms of office are set to expire, our Nominating and Corporate Governance Committee reviews these directors’ overall service to the Company during their terms, including the number of meetings attended, level of participation, quality of performance and any other relationships and transactions that might impair the directors’ independence. The Committee also takes into account the results of the Board’s self-evaluation, conducted annually on a group and individual basis. In the case of new director candidates, our Nominating and Corporate Governance Committee also determines whether the nominee is independent for Nasdaq purposes, which determination is based upon applicable Nasdaq listing standards, applicable SEC rules and regulations and the advice of counsel, if necessary. The Nominating and Corporate Governance Committee then uses its network of contacts to compile a list of potential candidates, but may also engage, if it deems appropriate, a professional search firm. The Nominating and Corporate Governance Committee conducts any appropriate and necessary inquiries into the backgrounds and qualifications of possible candidates after considering the function and needs of the Board.

Our Nominating and Corporate Governance Committee believes that it is in the best position to identify, review, evaluate and select qualified candidates for Board membership, based on the comprehensive criteria for Board membership approved by the Board. The Nominating and Corporate Governance Committee meets to discuss and consider the candidates’ qualifications and then selects a nominee for recommendation to the Board by majority vote. Although the Nominating and Corporate Governance Committee does not have a formal policy with regard to the consideration of director candidates recommended

The Company may require any proposed nominee to furnish such other information as it may reasonably require to determine the eligibility of such proposed nominee to serve as an independent director of the Company or that could be material to a reasonable stockholder’s understanding of the independence, or lack thereof, of such proposed nominee.

The Board has a Science Committee (formerly known as the Research & Development Committee) that meets from time to timeat least quarterly to review the Company’s clinical programs and is currently comprised of Dr. David A. Scheinberg, as Chair, and Dr. Angelos M. Stergiou and Mr. John Varian, as members. The Science Committee met four times in 2021.2023.

The Board expects that the views of our stockholders pertinent to matters relating to our business will be heard by the Board, its committees, or individual directors, as applicable, and that appropriate responses be provided to stockholders on a timely basis. Stockholders wishing to formally communicate with the