Summit Therapeutics plc ( Summit , the Company or the Group ) SUMMIT THERAPEUTICS REPORTS FINANCIAL RESULTS FOR THE THIRD QUARTER AND NINE MONTHS ENDED 31 OCTOBER 2017 AND OPERATIONAL PROGRESS Oxford, UK, 6 December 2017

Summit Therapeutics plc

Company or the Group )

SUMMIT THERAPEUTICS REPORTS FINANCIAL RESULTS FOR THE THIRD QUARTER AND NINE MONTHS ENDED

31 OCTOBER 2017 AND OPERATIONAL PROGRESS

Oxford, UK, 6 December 2017 Summit Therapeutics plc (AIM: SUMM,

NASDAQ: SMMT), the drug discovery and development company advancing therapies for Duchenne muscular dystrophy ( DMD ) and C. difficile infection ( CDI ), today reports its financial results for the third quarter and nine

months ended 31 October 2017, and reports on operational progress.

Glyn Edwards, Chief Executive Officer of Summit, commented: Next

year holds the potential to be transformative for our utrophin modulation programme. We remain on track to report during the first quarter of 2018 the 24-week data from PhaseOut DMD, our ongoing Phase 2

clinical trial evaluating our lead utrophin modulator ezutromid. Ezutromid is a potentially disease-modifying treatment for all patients with DMD and we look forward to reporting these initial data from this proof of concept trial.

In this period there has also been continued momentum in the development of our precision CDI antibiotic, ridinilazole. Ridinilazole achieved another

positive Phase 2 clinical trial result, further highlighting its potential to both treat the infection and preserve the microbiome to reduce the risk of recurrent disease. The award of a contract worth up to $62 million from BARDA during the

period will support in part the Phase 3 clinical and regulatory development of ridinilazole was a major achievement, one which we believe provides important validation of its potential. We are looking forward to initiating the Phase 3 clinical

programme for ridinilazole in the first half of 2018 as we seek to bring this urgently needed treatment to patients.

The ongoing support of

our shareholders is allowing us to continue to advance these two therapies that have the potential to enhance the quality of life of patients and families living with the burden of DMD and CDI.

Utrophin Modulation Programme: A Universal Treatment for DMD

CDI Programme: Ridinilazole, a Precision Investigational Antibiotic

Operational Highlights

Financial Highlights

This announcement contains inside information for the purposes of Article 7 of EU Regulation 596/2014 (MAR).

About Summit Therapeutics

Summit is a biopharmaceutical

company focused on the discovery, development and commercialisation of novel medicines for indications for which there are no existing or only inadequate therapies. Summit is conducting clinical programmes focused on the genetic disease Duchenne

muscular dystrophy and the infectious disease C. difficile infection. Further information is available at www.summitplc.com and Summit can be followed on Twitter (@summitplc).

For more information, please contact:

Forward Looking Statements

Any statements in this press release about our future expectations, plans and prospects, including statements about the development and potential

commercialisation of our product candidates, the therapeutic potential of our product candidates, the timing of initiation, completion and availability of data from clinical trials, the potential benefits and future operation of the collaboration

with Sarepta including any potential future payments thereunder, the potential benefits and future operation of the BARDA contract including any potential future payments thereunder, any other potential third-party collaborations and expectations

regarding the sufficiency of our cash balance to fund operating expenses and capital expenditures, and other statements containing the words anticipate, believe, continue, could, estimate,

expect, intend, may, plan, potential, predict, project, should, target, would, and similar expressions, constitute

forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995. Actual results may differ materially from those indicated by such forward-looking statements as a result of various important factors, including:

the uncertainties inherent in the initiation of future clinical trials, availability and timing of data from ongoing and future clinical trials and the results of such trials, whether preliminary results from a clinical trial will be predictive of

the final results of that trial or whether results of early clinical trials will be indicative of the results of later clinical trials, the ability of BARDA to terminate its contract with us for convenience at any time, expectations for regulatory

approvals, availability of funding sufficient for our foreseeable and unforeseeable operating expenses and capital expenditure requirements and other factors discussed in the Risk Factors section of filings that we make with the

Securities and Exchange Commission, including our Annual Report on Form 20-F for the fiscal year ended 31 January 2017. In addition, any forward-looking statements included in this press release represent

our views only as of the date of this release and should not be relied upon as representing our views as of any subsequent date. We specifically disclaim any obligation to update any forward-looking statements included in this press release.

Revenue was 1.7 million for the three

months ended 31 October 2017 compared to 0.6 million for the three months ended 31 October 2016. Revenue was 22.4 million for the nine months ended 31 October 2017 compared to 0.6 million for the

nine months ended 31 October 2016. These increases were principally due to income received pursuant to Summit s exclusive licence and collaboration agreement with Sarepta Therapeutics, Inc. ( Sarepta ). During the three months

ended 31 October 2017, 1.7 million relating to the upfront payment of 32.8 million ($40.0 million) made by Sarepta in October 2016 was recognised. To date, an aggregate of 7.5 million of the upfront payment

has been recognised while the remaining 25.3 million is classified as deferred revenue and will continue to be recognised as revenue over the development period. Revenue during the nine months ended 31 October 2017 reflects the

receipt of a development milestone of 17.2 million ($22.0 million) paid by Sarepta which was recognised in full.

Other Operating Income

Other operating income was 1.6 million for the three and nine months ended 31 October 2017, compared to nil for the three

months ended 31 October 2016 and 0.1 million for the nine months ended 31 October 2016. These increases resulted from the recognition of 0.7 million pursuant to Summit s funding contract with the Biomedical

Advanced Research and Development Authority ( BARDA ) that was awarded to the Group in September 2017 and 0.9 million resulting from the derecognition of a part of Summit s financial liabilities on funding arrangements,

which is further discussed in Note 6 Financial liabilities on funding arrangements.

Other operating income recognised in comparative

periods related to the Innovate UK funding agreement, from which the Company withdrew in May 2016 to take advantage of more tax efficient opportunities related to research and development expenditure, and the Wellcome Trust Translational Award

and Development Expenses

Research and development expenses increased by 3.4 million to 7.4 million for the three months ended

31 October 2017 from 4.0 million for the three months ended 31 October 2016. Research and development expenses increased by 4.9 million to 19.1 million for the nine months ended 31 October 2017 from

14.2 million for the nine months ended 31 October 2016. These increases reflected the greater investment in both the DMD and CDI clinical programmes, as well as an increase in research and development related staffing costs.

General and Administration Expenses

administration expenses increased by 0.1 million to 2.0 million for the three months ended 31 October 2017 from 1.9 million for the three months ended 31 October 2016. General and administration expenses

increased by 1.6 million to 6.9 million for the nine months ended 31 October 2017 from 5.3 million for the nine months ended 31 October 2016. These increases were primarily due to a net negative movement

in exchange rate variances and increased staff-related costs, offset by a decrease in legal and professional fees.

Finance income was 3.1 million for the three and nine months ended 31 October 2017 and related to the derecognition of a part of Summit s

financial liabilities on funding arrangements, specifically the re-measurements and discounts associated with the liabilities since initial recognition, which is further discussed in Note 6

Financial liabilities on funding arrangements. Finance income recognised in comparative periods relates to interest received.

Finance costs relate to the subsequent re-measurement and unwinding of the discounts associated with the

financial liability recognised in respect of charitable funding arrangements. Finance costs remained

consistent at 0.2 million for the three

months ended 31 October 2017 and for the three months ended 31 October 2016. Finance costs remained consistent at 0.7 million for the nine months ended 31 October 2017 and 0.6 million for the nine months ended

credit increased by 0.6 million to 1.5 million for the three months ended 31 October 2017 from 0.9 million for the three months ended 31 October 2016. The income tax credit increased by

1.0 million to 4.0 million for the nine months ended 31 October 2017 from 3.0 million for the nine months ended 31 October 2016. These increases were the result of higher research and development

loss for the three months ended 31 October 2017 was 1.8 million with a basic loss per share of 3 pence compared to a total comprehensive loss of 4.6 million for the three months ended 31 October 2016 and a basic

loss per share of 8 pence. Total comprehensive income for the nine months ended 31 October 2017 was 4.4 million with a basic earnings per share of 7 pence compared to a total comprehensive loss of 16.4 million for the

nine months ended 31 October 2016 and a basic loss per share of 27 pence.

Operating Activities

For the nine months ended

31 October 2017, net cash used in operating activities was 8.9 million. This compares to net cash generated from operating activities of 17.9 million for the nine months ended 31 October 2016. This net negative

movement of 26.8 million was primarily driven by the receipt of the 32.8 million ($40.0 million) upfront payment from Sarepta during the nine months ended 31 October 2016, offset by the receipt of a

17.2 million ($22.0 million) development milestone payment from Sarepta during the nine months ended 31 October 2017; this resulted in a net reduction in cash received from Sarepta during the nine months ended 31 October 2017

of 15.6 million, compared to during the nine months ended 31 October 2016. In addition, an increase of 6.5 million in research and development and general and administration expenses, and a 3.0 million

reduction in research and development tax credits received (due to timing) during the nine months ended 31 October 2017 as compared to the nine months ended 31 October 2016, contributed to the net movement.

Investing Activities

Net cash used in investing

activities for the nine months ended 31 October 2017 and the nine months ended 31 October 2016 includes the net amount of bank interest received on cash deposits less amounts paid to acquire property, plant and equipment. Amounts paid to

acquire property, plant and equipment during the nine months ended 31 October 2017 of 0.4 million related primarily to the Company s relocation of its UK offices, for which the Company signed a

ten-year lease in February 2017.

Financing Activities

Net cash generated from financing activities for the nine months ended 31 October 2017 includes 13.5 million of proceeds, net of transaction

costs, received following the Company s underwritten public equity offering in September 2017 and 0.4 million received following the exercise of warrants and share options. For the nine months ended 31 October 2016, the Company

received net proceeds of 0.4 million following the exercise of warrants and share options.

As at 31 October 2017, cash and cash equivalents were 31.8 million compared to 28.1 million as at 31 January 2017.

Glyn Edwards Erik Ostrowski

Chief Executive Officer Chief Financial Officer

FINANCIAL STATEMENTS

CONSOLIDATED STATEMENT OF

COMPREHENSIVE INCOME (unaudited)

For the three months ended 31 October 2017

CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME

For the nine months ended 31 October 2017

CONSOLIDATED STATEMENT OF FINANCIAL POSITION