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

Page 7 of 8 – SEC Filing

Exhibit A

AGREEMENT

The undersigned agree that this Schedule 13D/A,
dated September 20, 2017, 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: September 20, 2017

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

Follow Summit Therapeutics Inc. (NASDAQ:SMMT)