13D Filing: Lansdowne Partners and Summit Therapeutics PLC (ADR) (SMMT)

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Exhibit A

AGREEMENT

The undersigned agree that this Schedule
13D/A, dated January 30, 2018, relating to the Ordinary Shares, par value £0.01 per share and the American Depositary
Shares, each representing five Ordinary Shares, of Summit Therapeutics PLC shall be filed on behalf of the undersigned.

Date: January 30, 2018

Lansdowne Partners (UK) LLP /s/ Hugh Orange
Signature
Hugh Orange
Chief Compliance Officer
Lansdowne Developed Markets Master Fund Limited /s/ Hugh Orange
Signature
Hugh Orange
Chief Compliance Officer of its investment adviser

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