Summit Therapeutics plc ( Summit , the Company or the Group ) SUMMIT THERAPEUTICS REPORTS FINANCIAL RESULTS FOR THE THIRD QUARTER ENDED 31 OCTOBER 2016 AND OPERATIONAL PROGRESS Oxford, UK, 15 December 2016 Summit Therape

Summit Therapeutics plc

( Summit , the Company or the Group )

SUMMIT THERAPEUTICS REPORTS FINANCIAL RESULTS FOR THE THIRD QUARTER ENDED 31 OCTOBER 2016 AND OPERATIONAL PROGRESS

Oxford, UK, 15 December 2016 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 ended 31 October 2016.

Mr Glyn Edwards, Chief Executive Officer of Summit commented: During the third quarter, we strengthened our DMD programme by entering into an

exclusive collaboration and license agreement with Sarepta Therapeutics, granting European rights to our utrophin modulator pipeline, including our lead candidate, ezutromid. This collaboration gives us access to Sarepta s DMD development,

regulatory and commercial expertise, while strengthening our financial position with global research and development cost sharing and the potential for future milestones. Ultimately, we believe that combining our strengths through this collaboration

could help to bring utrophin modulators to market, where we have the potential to benefit all patients with this muscle wasting disease.

PhaseOut DMD, our Phase 2 proof of concept trial of ezutromid, is enrolling now in the UK and the US, and we are on track to report data from the

first group of 24-week biopsies in the second or third quarter of 2017. These biopsies have the potential to demonstrate proof of mechanism for ezutromid through a change in the pattern of utrophin expression

from baseline and an association of utrophin with mature muscle fibres a phenomenon that we expect would only occur with drug treatment. This trial is expected to evaluate the F3 and F6 formulations of ezutromid that both have the potential

to modulate utrophin over a wide range of exposures that could help us to maximise safety and efficacy in patients over longer-term dosing.

With our CDI programme, we continue preparations for Phase 3 trials of ridinilazole, a novel antibiotic with potential as a front-line treatment for

patients suffering from this serious bacterial infection. We look forward to a productive 2017 with both programmes.

Utrophin Modulation Programme for DMD

Collaboration and License Agreement with Sarepta Therapeutics Inc. ( Sarepta )

Ezutromid Clinical Development

Financial Highlights

About Summit Therapeutics

Summit is a biopharmaceutical company focused on the discovery, development and commercialization of novel medicines for indications for which there are no

existing or only inadequate therapies. Summit is conducting clinical programs 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 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

Therapeutics Inc., 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, 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 2016. 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.

Summit is seeking to develop a treatment for all patients affected with the fatal disorder DMD using its utrophin modulation technology. Summit is also

advancing the development of a highly selective antibiotic to treat CDI.

Summit s DMD utrophin modulation programme is a treatment approach

independent of the underlying mutations in the dystrophin gene that cause the disease. Therefore, we believe this approach has the potential to benefit the entire patient population. Summit has established a leadership position in the field of

utrophin modulation and is developing a pipeline of orally administered small molecule utrophin modulator product candidates. Summit s most advanced utrophin modulator is ezutromid which is currently being evaluated in a Phase 2 proof of

concept trial. Ezutromid has received orphan drug designation in the US and Europe, and Fast Track and Rare Pediatric Disease designations in the US.

Summit s CDI product candidate is ridinilazole, a novel class antibiotic that has the potential to treat the initial infection and reduce recurrent

disease, the key clinical issue in CDI. Ridinilazole markedly reduced recurrence rates and had a statistically superior rate of sustained clinical response ( SCR ) compared to vancomycin in a Phase 2 proof of concept trial. Summit is

currently evaluating its options for advancing ridinilazole into Phase 3 clinical trials. Ridinilazole has received Qualified Infectious Disease Product, or QIDP, designation and has been granted Fast Track designation in the US.

Duchenne Muscular Dystrophy: Utrophin Modulation Programme

Exclusive Collaboration and License Agreement with Sarepta Therapeutics Inc. ( Sarepta )

In October 2016, Summit announced an exclusive collaboration and license agreement with Sarepta. This granted Sarepta rights to Summit s utrophin

modulator pipeline in the European Union, Switzerland, Norway, Iceland, Turkey and the Commonwealth of Independent States, with an option for Latin American rights. Summit retains commercialization rights in all other countries, including the key

markets of the US and Japan.

Under the terms of the agreement, Summit has received an upfront payment of $40 million, and will be eligible for

future ezutromid-related development, regulatory and sales milestone payments totalling up to $522 million. This includes $42 million in respect of specified development milestones (including a $22 million milestone upon the first

dosing of the last patient in Summit s PhaseOut DMD trial, payable on or after 1 April 2017), $150 million in respect of specified regulatory milestones and a potential $330 million from specified sales milestones. In addition,

Summit is eligible for escalating royalties ranging from a low to high teens percentage of net sales in the licensed territories. Beginning in 2018, Summit and Sarepta will share at a 55%/45% split specified global research and development costs

related to ezutromid and future generation utrophin modulators. If Sarepta elects to exercise its option for Latin American rights, Summit would be entitled to additional fees, milestones and royalties. Summit will also be eligible to receive

development and regulatory milestones related to potential future generation utrophin modulator candidate(s).

Ezutromid: Phase 2 Proof of Concept Trial

PhaseOut DMD is a Phase 2 clinical trial evaluating ezutromid in patients with DMD, and it aims to establish proof of concept for this utrophin modulator. The 48-week open-label trial is expected to enrol up to 40 boys ranging in age from their fifth to their tenth birthdays.

Enrolment of patients into PhaseOut DMD is ongoing at sites in the UK and US. The Company anticipates reporting

24-week muscle biopsy data from the first group of patients in Q2 or Q3 2017.

DMD is characterised by high levels

of muscle degeneration caused by the absence of functional dystrophin. Muscle fibres consequently enter into a cycle of repair and degeneration that over time leads to fat infiltrating into muscle, loss of ambulation and loss of other functional

abilities. Ezutromid aims to maintain production of utrophin so that it can substitute for the missing dystrophin. This has potential to allow muscle fibres to mature and so reduce the level of muscle degeneration, reduce the rate of fat

infiltration and reduce the rate of decline in functional abilities. PhaseOut DMD is assessing all these factors through various techniques including use of muscle biopsy to evaluate utrophin expression and muscle fibre regeneration and maturity,

magnetic resonance imaging to measure fat infiltration, and various functional tests including the North Star Ambulatory Assessment and the six minute walk test.

Ezutromid: Phase 1 New Formulation Trial

announced in August 2016 results from a Phase 1 clinical trial that showed a new formulation of ezutromid called F6 achieved a substantial increase in plasma exposure in patients compared to the current clinical formulation called F3. The trial

evaluated the pharmacokinetics and safety of three fixed doses (250mg, 500mg and 1,000mg twice a day) of the F6 formulation in patients aged between five and nine years who followed a modified diet. At the highest dose, the five evaluable patients

achieved an average maximum concentration of 390ng/mL on day 7, the final day of dosing. This was a six-fold increase in maximum plasma levels compared to formulation F3 but were achieved with two-fifths of the dose of ezutromid.

Summit plans to test the F6 formulation alongside the F3 formulation in the

ongoing PhaseOut DMD clinical trial. It is anticipated that up to ten of the patients enrolled in the US will be dosed with F6 to evaluate safety and efficacy. The two formulations of ezutromid have the potential to modulate the expression of

utrophin, and the inclusion of F6 will provide a greater understanding of the potential relationship between drug exposure and clinical benefit.

Ezutromid: Commercialisation Strategy

August 2016 its strategy for the future development of ezutromid. The Company plans to conduct a randomised, placebo controlled trial designed with the potential to support accelerated and conditional regulatory approvals in the United States and

Europe respectively. Assuming positive interim data from PhaseOut DMD, it is anticipated that this trial would start in the second half of 2017, with data available for potential regulatory filings in 2019. Summit also expects to conduct a separate

confirmatory clinical trial designed to support full product approvals in major territories.

Development of Biomarkers

As highlighted, a key endpoint in the PhaseOut DMD trial is measurement of utrophin and muscle regeneration biomarkers from muscle biopsies. Summit, in

collaboration with Flagship Biosciences Inc., has been developing an automated, digital analysis tool to precisely measure muscle maturity and integrity and utrophin expression in individual fibres, and data from this research were presented at the

21st Congress of the World Muscle Society held in Granada, Spain, in October 2016. This research represents an important step in helping to further our understanding of the potential benefits of

utrophin modulator therapies such as ezutromid.

Fast Track and Rare Pediatric Disease Designations

In September, ezutromid was granted two separate designations by the US FDA in the treatment of DMD: Fast Track and Rare Pediatric Disease. Fast Track

designation provides the Company with advantages such as opportunities for more frequent interactions with the FDA during all aspects of development, submission of a New Drug Application ( NDA ) on a rolling basis, and eligibility for

accelerated approval and priority review. Rare Pediatric Disease designation could qualify Summit for a Priority Review Voucher upon the approval of ezutromid, which could be used for a subsequent marketing application or sold or transferred an

unlimited number of times (although only used once). The Priority Review Voucher programme was extended on 13 December 2016 through the enactment of the 21st Century Cures Act, under which a

drug designated as a Rare Pediatric Disease can receive a voucher if approved before 1 October 2022.

DMD Community Website

On 12 September 2016, Summit launched www.utrophintrials.com, an online resource on utrophin and Summit s utrophin modulator clinical trials,

in an effort to broaden the Company s relationship with the Duchenne community as it advances ezutromid and other utrophin modulators.

C. difficile Infection Programme

Summit has generated a comprehensive package of data supporting the potential of the novel class antibiotic ridinilazole as a new front-line treatment of

infections caused by the bacteria Clostridium difficile. The Phase 2 proof of concept trial called CoDIFy showed ridinilazole outperformed the antibiotic vancomycin, which is the current standard of care. Ridinilazole demonstrated a large

numerical reduction in rates of recurrent disease compared to vancomycin with this difference driven by ridinilazole outperforming vancomycin in the preservation of the gut microbiome.

Recurrence of CDI, and the failure to subsequently achieve a sustained clinical response after treatment, is a major issue in the management of the disease,

as collateral damage to the gut microbiome by antibiotics such as vancomycin leaves patients vulnerable to disease recurrence.

Phase 3 preparations are

ongoing as the Company continues to explore options to support the future development of ridinilazole with a view to maximising the potential commercial value of this antibiotic. Summit s preference remains finding a third party collaborator