Exhibit 10.20.3

ADDENDUM TO ASSET PURCHASE AGREEMENT

This Addendum to Asset Purchase Agreement (“Addendum”) is entered into as of April 12, 2016 by and between Novartis Pharma AG, a Swiss company (“Novartis”) and Mereo BioPharma 1 Limited, a private limited company incorporated in England and Wales, and a wholly owned subsidiary of Mereo BioPharma Group Limited, a company incorporated in England and Wales (collectively ‘‘Mereo”). Hereinafter “Parties” shall mean Novartis and Mereo Biopharma 1, and “Party” shall mean either Novartis or Mereo Biopharma l, as the context requires.

WHEREAS the Parties entered into an asset purchase agreement on July 28, 2015 entitled “BCT Asset Purchase Agreement” (hereinafter “the APA”);

WHEREAS subsequent to the execution of the APA the Parties have jointly filed patent applications at the UK Patent Office identified as PCT/GB2016/050635 and PCT/GB2016/050636 (hereinafter “the Patent Applications”).

WHEREAS Novartis wishes to assign and Mereo Biopharma 1 wishes to receive all rights in the jointly conceived inventive concepts.

NOW, THEREFORE, in consideration of the ongoing premises and the mutual covenants contained herein, Mereo Biopharma 1 and Novartis hereby agree as follows:

1. The Parties acknowledge and agree that the Patent Applications shall be considered Purchased IP as defined in the APA and as such all of the terms and conditions of the APA shall govern the rights and obligations of the Parties as they relate to the Patent Applications.

2. Novartis acknowledges that it shall have no further rights to the Patent Applications unless specifically enumerated in the APA.

3. This Agreement shall be governed by the laws of the State of New York, without regard to the conflict of law principles thereof.

SIGNATURE PAGE TO FOLLOW

SIGNATURE PAGE

/s/ Denise Scots-Knight

Mereo BioPharma I Limited

Print Name: DENISE SCOTS KNIGHT

CEO AND DIRECTOR

/s/ Charles Sermon

Mereo BioPharma I Limited

Print Name: C. SERMON

                    DIRECTOR

/s/ Paul Fehlner

Novartis Pharma AG

Print Name: Paul Fehlner

                     Head Intellectual Property Pharma

                     Novartis Pharma AG

/s/ Matt Owens

Novartis Pharma AG

Print Name: Matt Owens

                    Global Head Legal-

                    Strategic Partnership & Digital Medicine

2