Exhibit 10.3

FORM OF LOCK-UP AGREEMENT

March 8, 2018

Sellas Life Sciences Group, Inc.:

Ladies and Gentlemen:

Defined terms not otherwise defined in this letter agreement (the “Letter Agreement”) shall have the meanings set forth in the Purchase Agreement. Pursuant to Section 2.2(a)(v) of the Purchase Agreement and in satisfaction of a condition of the Company’s obligations under the Purchase Agreement, the undersigned irrevocably agrees with the Company that, from the date hereof until the earlier of (i) the six month anniversary of the First Closing and (ii) the initial closing date of a Qualified Offering (such period, the “Restriction Period”), the undersigned will not offer, sell, contract to sell, hypothecate, pledge or otherwise dispose of (or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition due to cash settlement or otherwise) by the undersigned or any Affiliate of the undersigned or any person in privity with the undersigned or any Affiliate of the undersigned), directly or indirectly, including the filing (or participation in the filing) of a registration statement with the Commission in respect of, or establish or increase a put equivalent position or liquidate or decrease a call equivalent position within the meaning of Section 16 of the Exchange Act with respect to, any shares of Common Stock or Common Stock Equivalents beneficially owned, held or hereafter acquired by the undersigned (the “Securities”), other than: (A) exercise of stock options or warrants to purchase shares of Common Stock or the vesting of stock awards of Common Stock and any related transfer of shares of Common Stock to the Company in connection therewith (x) deemed to occur upon the “cashless” or “net” exercise of such options or warrants or (y) for the purpose of paying the exercise price of such options or warrants or for paying taxes due as a result of the exercise of such options or warrants, the vesting of such options, warrants or stock awards, or as a result of the vesting of such shares of Common Stock, it being understood that all shares of Common Stock received upon such exercise, vesting or transfer will remain subject to the restrictions of this Agreement during the Restriction Period; (B) transfers to the spouse, domestic partner, parent, child or grandchild of the undersigned (each, an “Immediate Family Member”) or to a trust formed for the direct or indirect benefit of the undersigned or an Immediate Family Member, in each case, for estate planning purposes; (C) transfers by will, other testamentary document or intestate succession to the legal representative, heir, beneficiary, trustee or Immediate Family Member of the undersigned; (D) as a bona fide gift or gifts; (E) as a distribution to partners, stockholders, members of or owners of similar equity interests in the undersigned, (F) to a corporation, partnership, limited liability company, investment fund or other entity that controls or is controlled by, or is under common control with, the undersigned, or is wholly owned by the undersigned, or, in the case of an investment fund,

that is managed by, or is under common management with, the undersigned (including, for the avoidance of doubt, a fund managed by the same manager or managing member or general partner or management company or by an entity controlling, controlled by, or under common control with such manager or managing member or general partner or management company as the undersigned or who shares a common investment advisor with the undersigned); (G) the establishment of a trading plan pursuant to Rule 10b5-1 under the Securities Exchange Act of 1934, as amended, for the transfer of shares of Common Stock or securities convertible into or exchangeable for Common Stock, provided that such plan does not provide for the transfer of shares of Common Stock during the Restriction Period and no filing or other public announcement shall be made during the Restriction Period; and (H) pursuant to a bona fide third party tender offer, merger, consolidation or other similar transaction made to all holders of Securities involving a Change of Control (as defined below) of the Company; provided, however, that in the event that the tender offer, merger, consolidation or other such transaction is not completed, the Securities owned by the undersigned shall remain subject to the restrictions contained in this Letter Agreement (“Change of Control” shall mean the transfer (whether by tender offer, merger, consolidation or other similar transaction), in one transaction or a series of related transactions, to a person or group of affiliated persons, of the Company’s voting securities if, after such transfer, such person or group of affiliated persons would hold more than 50% of the outstanding voting securities of the Company (or the surviving entity)); provided that, in the case of any transfer or distribution pursuant to clauses (B)—(F), it shall be a condition precedent to any such transfer or distribution that (1) the transferee or recipient agrees to be bound in writing by the same restrictions set forth herein for the duration of the Restriction Period, and (2) any such transfer or distribution shall not involve a disposition for value.

Beneficial ownership shall be calculated in accordance with Section 13(d) of the Exchange Act. In order to enforce this covenant, the Company shall impose irrevocable stop-transfer instructions preventing the Transfer Agent from effecting any actions in violation of this Letter Agreement.

The undersigned acknowledges that the execution, delivery and performance of this Letter Agreement is a material inducement to each Purchaser to complete the transactions contemplated by the Purchase Agreement and that each Purchaser (which shall be a third party beneficiary of this Letter Agreement) and the Company shall be entitled to specific performance of the undersigned’s obligations hereunder. The undersigned hereby represents that the undersigned has the power and authority to execute, deliver and perform this Letter Agreement, that the undersigned has received adequate consideration therefor and that the undersigned will indirectly benefit from the closing of the transactions contemplated by the Purchase Agreement.

This Letter Agreement may not be amended or otherwise modified in any respect without the written consent of the Company and the undersigned. This Letter Agreement shall be construed and enforced in accordance with the laws of the State of New York without regard to the principles of conflict of laws. The undersigned hereby irrevocably submits to the exclusive jurisdiction of the United States District Court sitting in the Southern District of New York and the courts of the State of New York located in Manhattan, for the purposes of any suit, action or proceeding arising out of or relating to this Letter Agreement, and hereby waives, and agrees not to assert in any such suit, action or proceeding, any claim that (i) it is not personally subject to the jurisdiction of such court, (ii) the suit, action or proceeding is brought in an inconvenient

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forum, or (iii) the venue of the suit, action or proceeding is improper. The undersigned hereby irrevocably waives personal service of process and consents to process being served in any such suit, action or proceeding by receiving a copy thereof sent to the Company at the address in effect for notices to it under the Purchase Agreement and agrees that such service shall constitute good and sufficient service of process and notice thereof. The undersigned hereby waives any right to a trial by jury. Nothing contained herein shall be deemed to limit in any way any right to serve process in any manner permitted by law. The undersigned agrees and understands that this Letter Agreement does not intend to create any relationship between the undersigned and each Purchaser and that each Purchaser is not entitled to cast any votes on the matters herein contemplated and that no issuance or sale of the Securities is created or intended by virtue of this Letter Agreement.

This Letter Agreement shall be binding on successors and assigns of the undersigned with respect to the Securities and any such successor or assign shall enter into a similar agreement for the benefit of the Purchasers.

This Letter Agreement shall automatically terminate, and the undersigned shall be released from its obligations hereunder upon the termination of the Purchase Agreement prior to payment for and delivery of the securities of the Company be sold thereunder.

*** SIGNATURE PAGE FOLLOWS***

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This Letter Agreement may be executed in two or more counterparts, all of which when taken together may be considered one and the same agreement.

By signing below, the Company agrees to enforce the restrictions on transfer set forth in this Letter Agreement.

SELLAS LIFE SCIENCES GROUP, INC.

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