Summit Therapeutics plc ( Summit or the Company ) SUMMIT THERAPEUTICS REPORTS FINANCIAL RESULTS FOR THE SECOND QUARTER ENDED 31 JULY 2016 AND OPERATIONAL PROGRESS Conference call to be held 1:00pm BST / 8:00am EDT Oxford

Summit Therapeutics plc

( Summit or the Company )

THERAPEUTICS REPORTS FINANCIAL RESULTS FOR THE

SECOND QUARTER ENDED 31 JULY 2016 AND OPERATIONAL PROGRESS

Oxford, UK, 8 September 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 second quarter and half year ended 31 July 2016.

Mr Glyn Edwards, Chief Executive Officer of Summit

commented: During the first half of the year, our DMD programme has continued to gain momentum. We initiated our Phase 2 proof of concept clinical trial, PhaseOut DMD, of ezutromid and are actively enrolling patients in the UK.

This trial represents the first long-term clinical trial of a utrophin modulator, a potential disease-modifying therapy applicable to all patients with DMD. We look forward to receiving data from the first group of 24-week biopsies in Q2/Q3 2017.

We also announced results from a Phase 1 trial in DMD patients that showed a new formulation of ezutromid achieved over a six-fold increase

in maximum plasma levels but with a reduced oral dose compared to the current formulation. With these results, we plan to incorporate the new formulation, subject to regulatory approval, into PhaseOut DMD alongside the current formulation. Including

both formulations allows us to further explore the potential therapeutic effect of ezutromid with an expanded range of blood concentrations.

In our CDI programme, additional results from our Phase 2 CoDIFy clinical trial support ridinilazole as a highly selective, microbiome-sparing

antibiotic that has great promise in both the treatment of the initial infection and in reducing CDI recurrences, the key issue in the treatment of CDI. We continue to evaluate our options for advancing ridinilazole into Phase 3 clinical

Ezutromid Highlights

Ridinilazole Highlights

Financial Highlights

Conference Call and Webcast Information

a conference call and webcast to review the financial results for the second quarter ended 31 July 2016 today at 1:00pm BST / 8:00am EDT. To participate in the conference call, please dial +44(0)20 3427 1911 (UK and international participants)

or +1 646 254 3367 (US local number) and use the conference confirmation code 9792163. Investors may also access a live audio webcast of the call via the investors section of the Company s website www.summitplc.com. A replay of the

webcast will be available shortly after the presentation finishes.

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 clinical development and commercialisation

of our product candidates, the timing of clinical results, 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 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, 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 United States and Europe.

Summit s CDI therapy 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 by the US Food and Drug Administration

Duchenne Muscular Dystrophy: Utrophin Modulation Programme

Ezutromid: Phase 2 Proof of Concept Trial

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

concept for ezutromid and utrophin modulation through measurements of muscle fat infiltration, as well as through measurements of utrophin protein and muscle fibre regeneration in muscle biopsies. A primary endpoint of the trial is the change from

baseline in magnetic resonance imaging parameters related to fat infiltration and inflammation of the leg muscles. Functional endpoints, including the six-minute walk test and North Star Ambulatory Assessment, and patient reported outcomes, are also

Enrolment and dosing of patients into PhaseOut DMD at sites in the United Kingdom is now underway, with patients expected to be enrolled

into sites in the United States shortly. The Company anticipates reporting data periodically during this trial with 24-week muscle biopsy data from the first group of patients expected to be reported in Q2/Q3 2017.

Ezutromid: Phase 1 New Formulation Trial

Summit announced in August 2016 the top-line data from a Phase 1 clinical trial testing a new formulation of ezutromid, referred to as F6, in patients with

DMD. 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 represented a greater than six-fold increase in maximum plasma levels but with a reduced oral dose compared to the current clinical

formulation of ezutromid referred to as F3. In an earlier Phase 1 trial of the F3 formulation in patients who followed the same modified diet, an average maximum plasma concentration of 63ng/mL (2,500mg, twice a day) on the final day of dosing (day

Subject to regulatory approval, Summit now plans to incorporate the F6 formulation into the ongoing PhaseOut DMD proof of concept trial.

Summit anticipates testing up to ten of the 40 patients expected to be enrolled on F6 to compare the safety and efficacy of both formulations of ezutromid. This will help to determine which formulation to use in future clinical trials. These two

formulations of ezutromid are expected to modulate utrophin and with a wider range in blood plasma exposure, are expected to allow Summit to further explore the potential therapeutic effect of ezutromid in patients with DMD.

Ezutromid: Commercialisation Strategy

clinical findings from the Phase 1 trial evaluating the F6 formulation of ezutromid, Summit outlined in 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. It is anticipated that this trial would start in the second half of 2017 (assuming positive interim data from PhaseOut DMD),

with data available for potential regulatory filings in 2019. A separate confirmatory clinical trial designed to support full product approvals in major territories is also expected to be conducted.

Summit holds exclusive, worldwide commercialisation rights for ezutromid and its pipeline of utrophin modulators. The Company s existing strategy is to

market ezutromid on its own in the United States and Europe should ezutromid receive marketing approval. The Company may however opportunistically evaluate alternative marketing approaches including potential collaboration, distribution and other

marketing arrangements with third parties.

Utrophin Modulator Pipeline Update

As part of the Company s strategy to maintain its leadership position in the field of utrophin modulation, Summit has a pipeline of second and future

generation utrophin modulators.

The second generation molecules are structurally related to ezutromid, but are designed to have more favourable

pharmaceutical properties to achieve higher plasma levels of drug. The substantial increase in ezutromid plasma levels achieved with the F6 formulation in the recently completed Phase 1 trial fulfilled the key objective for the second generation

utrophin modulator programme. In light of this progress, Summit has put the development of the second generation on hold and will switch focus to its pipeline of future generation utrophin modulators. This research is aiming to build on the promise

of ezutromid to identify new, structurally distinct molecules, including ones that may have possible new utrophin related mechanisms. This research is being conducted as part of the strategic alliance with the University of Oxford.

Summit hosted a Utrophin R&D

Day on 15 June 2016 in New York City. The event was attended by analysts, institutional investors and members of the DMD community and featured presentations from three key opinion leaders in DMD alongside management. A replay of the event is

available on the Company website: www.summitplc.com/investors/presentations.

C. difficile Infection Programme

Phase 2 CoDIFy Clinical Trial: Presentation of Additional Microbiome Data

In April 2016 at ECCMID and June 2016 at ASM Microbe, additional data were reported from CoDIFy, the Phase 2 proof of concept clinical trial of ridinilazole.

CoDIFy was a double-blind, randomised active-control trial evaluating the efficacy of ridinilazole against the current standard of care, the antibiotic vancomycin. The trial enrolled 100 patients with half the patients receiving ten days of dosing

with ridinilazole (200mg, twice a day), and half the patients receiving ten days of dosing with vancomycin (125mg, four times a day). The trial was conducted in the United States and Canada.

In CoDIFy, ridinilazole demonstrated substantial clinical benefit over vancomycin, including a large numerical reduction in rates of recurrent disease.

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.

The additional CoDIFy data from June also showed ridinilazole outperformed vancomycin in the preservation of the gut

microbiome of patients. Stool samples from patients were analysed for five specific bacterial groups associated with a healthy gut microbiome (Bacteriodes, Prevotella, Enterbactericeae, C. coccoides and C. leptum)

and total bacteria present. Treatment with vancomycin resulted in a significant decrease (p<0.001) in four of the five bacterial groups and also resulted in a significant decrease in total bacteria. In contrast, patients treated with ridinilazole

did not experience a decrease in these specific bacterial groups, nor in total bacteria. In summary, these results show ridinilazole may have advantages compared to vancomycin in preserving a healthy gut microbiome during treatment for CDI.

Summit believes these comprehensive data from CoDIFy support the positioning of ridinilazole as a selective antibiotic with potential to eradicate the C.

difficile bacteria while simultaneously preserving the microbiome to protect against disease recurrence.

Separately, an exploratory Phase 2 clinical

trial evaluating ridinilazole against the antibiotic fidaxomicin is ongoing in Europe and the US. The results from this open-label trial are expected to help inform the design of future Phase 3 clinical trials and the commercial positioning of

ridinilazole. Top-line results from this trial are expected in the second half of 2016.

The development of ridinilazole has been financially supported by

Seeding Drug Discovery and Translational Awards from the Wellcome Trust.

The Company continues to explore options to support the future development of

ridinilazole through Phase 3 trials as it seeks to maximise the potential commercial value of this antibiotic. Summit s preference remains finding a third party collaborator although the Company continues to evaluate other potential options