THIS ANNOUNCEMENT IS NOT FOR RELEASE, PUBLICATION OR DISTRIBUTION, DIRECTLY OR INDIRECTLY, IN OR INTO THE UNITED STATES, AUSTRALIA, CANADA, JAPAN, THE REPUBLIC OF SOUTH AFRICA OR ANY JURISDICTION WHERE TO DO SO MIGHT CON

THIS ANNOUNCEMENT IS NOT FOR RELEASE, PUBLICATION OR DISTRIBUTION, DIRECTLY OR INDIRECTLY, IN OR INTO THE UNITED STATES,

AUSTRALIA, CANADA, JAPAN, THE REPUBLIC OF SOUTH AFRICA OR ANY JURISDICTION WHERE TO DO SO MIGHT CONSTITUTE A VIOLATION OF LOCAL SECURITIES LAWS OR REGULATIONS. THIS ANNOUNCEMENT, WHICH SHOULD BE READ IN ITS ENTIRETY INCLUDING THE APPENDICES, IS FOR

INFORMATION PURPOSES ONLY AND DOES NOT ITSELF CONSTITUTE AN OFFER FOR SALE OR SUBSCRIPTION OF ANY SECURITIES IN THE COMPANY.

Summit Therapeutics plc

( Summit or the Company )

Summit Therapeutics Announces a Proposed Placing To Raise Up To 15.0 Million

Oxford, UK, 27 March 2018 Summit Therapeutics plc (NASDAQ: SMMT, AIM: SUMM) announces a proposed placing of up to 8,333,333

new Ordinary Shares of 1 penny each in the Company (the Placing Shares ) to raise gross proceeds of up to 15.0 million, before expenses, for the Company (the Placing ). It is intended that the price at which the

Placing Shares are to be issued ( Placing Price ) will be 180 pence per Placing Share. The Placing Shares will be issued on a non-pre-emptive basis, within the

Company s existing share allotment authorities, to existing and new investors.

Glyn Edwards, Chief Executive

Officer of Summit, commented: Summit recently announced positive interim 24-week data from our PhaseOut DMD clinical trial which showed that ezutromid significantly reduced muscle damage and

inflammation in patients with DMD. We look forward to the full 48-week trial data in the third quarter of 2018 which could further support the disease modifying potential of ezutromid as a treatment for all

The funds from this placing will provide us clinical and regulatory flexibility as we seek to rapidly advance

ezutromid and bring this potentially transformational therapy to patients as quickly as possible.

Market abuse regulation

Market Soundings, as defined in the Market Abuse Regulation ( MAR ), were taken in respect of the proposed Placing with the result that certain

persons became aware of inside information, as permitted by MAR. That inside information in relation to the Placing is set out in this Announcement and has been disclosed as soon as possible in accordance with paragraph 7 of article 17 of MAR.

Therefore, those persons that received inside information in a Market Sounding are no longer in possession of inside information relating to the Company and its securities.

Regulation S of the US Securities Act

does not constitute an offer to sell or the solicitation of an offer to buy the securities, nor shall there be any sale of the securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to the registration or

qualification under the securities laws of such jurisdiction. The securities were offered and are being sold outside the United States in offshore transactions pursuant to Regulation S of the US Securities Act of 1933. The securities have not been

registered under the US Securities Act of 1933. The securities may not be offered or sold in the United States of America absent registration or an exemption from registration requirements under the US Securities Act of 1933.

The important notice and the Appendices below form part of this Announcement. Further information about the Placing is set out in Appendix I. The Terms and

Conditions of the Placing are included in Appendix II, and capitalised terms not otherwise defined in the text of this Announcement are defined in Appendix III.

Neither the contents of the Company s website nor the contents of any website accessible from hyperlinks on the Company s website (or any other

website) is incorporated into, or forms part of, this Announcement.

This Announcement does not constitute, or form part of, a prospectus relating to the

Company, nor does it constitute or contain any invitation or offer to any person, or any public offer, to subscribe for, purchase or otherwise acquire any shares in the Company or advise persons to do so in any jurisdiction, nor shall it, or any

part of it form the basis of or be relied on in connection with any contract or as an inducement to enter into any contract or commitment with the Company.

The content of this Announcement has not been approved by an authorised person within the meaning of the Financial Services and Markets Act 2000

( FSMA ). This Announcement has been issued by and is the sole responsibility of the Company. The information in this Announcement is subject to change.

The Placing Shares are being offered and are being sold outside the United States in offshore transactions pursuant to Regulation S of the US Securities Act

of 1933. This Announcement does not constitute an offer to sell, or a solicitation of offers to purchase or subscribe for, securities in the United States. The securities referred to herein have not been and will not be registered under the U.S.

Securities Act of 1933, as amended (the Securities Act ), and may not be offered or sold, directly or indirectly, in or into the United States or to, or for the account or benefit of, a U.S. Person (as defined in Regulation S under

the Securities Act), except pursuant to an applicable exemption from, or in a transaction not subject to, the registration requirements of the Securities Act. No public offering of securities is being made in the United States. Any public offering

of securities to be made in the United States may be made only by means of a prospectus obtained from the issuer that contains detailed information about the issuer and its management and financial statements. This Announcement is not for release,

publication or distribution, directly or indirectly, in or into the United States, Australia, Canada, Japan, the Republic of South Africa or any jurisdiction where to do so might constitute a violation of local securities laws or regulations (a

Prohibited Jurisdiction ). This Announcement and the information contained herein are not for release, publication or distribution, directly or indirectly, to persons in a Prohibited Jurisdiction unless permitted pursuant to an

exemption under the relevant local law or regulation in any such jurisdiction.

This Announcement is directed only at persons whose ordinary activities

involve them in acquiring, holding, managing and disposing of investments (as principal or agent) for the purposes of their business and who have professional experience in matters relating to investments and: (i) if in a member state of the

European Economic Area, are, unless otherwise agreed with the Joint Bookrunners, qualified investors within the meaning of article 2(1)(e) of the Prospectus Directive ( Qualified Investors ); and (ii) if in the United Kingdom,

fall within: (a) article 19(5) (investment professionals) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended (the Order ); (b) article 49(2)(a) to (d) (high net worth companies,

unincorporated associations, etc.) of the Order; or (c) any other person to whom it may lawfully be communicated (all such persons together being referred to as Relevant Persons ). This Announcement must not be acted on or

relied on by persons who are not Relevant Persons. Any investment or investment activity to which this Announcement relates is available only to Relevant Persons and will be engaged in only with Relevant Persons.

Panmure Gordon is authorised and regulated by the Financial Conduct Authority in the United Kingdom. Panmure Gordon is acting solely as joint broker and joint

bookrunner exclusively for the Company and no one else in connection with the contents of this Announcement and will not regard any other person (whether or not a recipient of this Announcement) as its client in relation to the contents of this

Announcement nor will it be responsible to anyone other than the Company for providing the protections afforded to its clients or for providing advice in relation to the contents of this Announcement. Apart from the responsibilities and liabilities,

if any, which may be imposed on Panmure Gordon by FSMA or the regulatory regime established thereunder, Panmure Gordon accepts no responsibility whatsoever, and makes no representation or warranty, express or implied, for the contents of this

Announcement including its accuracy, completeness or verification or for any other statement made or purported to be made by it, or on behalf of it, the Company or any other person, in connection with the Company and the contents of this

Announcement, whether as to the past or the future.

Panmure Gordon accordingly disclaims all and any liability whatsoever, whether arising in tort, contract or otherwise (save as referred to above), which it might otherwise have in respect of the

contents of this Announcement or any such statement.

N+1 Singer is authorised and regulated by the Financial Conduct Authority in the United Kingdom. N+1

Singer is acting solely as joint broker and joint bookrunner exclusively for the Company and no one else in connection with the contents of this Announcement and will not regard any other person (whether or not a recipient of this Announcement) as

its client in relation to the contents of this Announcement nor will it be responsible to anyone other than the Company for providing the protections afforded to its clients or for providing advice in relation to the contents of this Announcement.

Apart from the responsibilities and liabilities, if any, which may be imposed on N+1 Singer by FSMA or the regulatory regime established thereunder, N+1 Singer accepts no responsibility whatsoever, and makes no representation or warranty, express or

implied, for the contents of this Announcement including its accuracy, completeness or verification or for any other statement made or purported to be made by it, or on behalf of it, the Company or any other person, in connection with the Company

and the contents of this Announcement, whether as to the past or the future. N+1 Singer accordingly disclaims all and any liability whatsoever, whether arising in tort, contract or otherwise (save as referred to above), which it might otherwise have

in respect of the contents of this Announcement or any such statement.

Cairn is authorised and regulated by the Financial Conduct Authority in the United

Kingdom. Cairn is acting solely as nominated adviser exclusively for the Company and no one else in connection with the contents of this Announcement and will not regard any other person (whether or not a recipient of this Announcement) as its

client in relation to the contents of this Announcement nor will it be responsible to anyone other than the Company for providing the protections afforded to its clients or for providing advice in relation to the contents of this Announcement. Apart

from the responsibilities and liabilities, if any, which may be imposed on Cairn by FSMA or the regulatory regime established thereunder, Cairn accepts no responsibility whatsoever, and makes no representation or warranty, express or implied, for

the contents of this Announcement including its accuracy, completeness or verification or for any other statement made or purported to be made by it, or on behalf of it, the Company or any other person, in connection with the Company and the

contents of this Announcement, whether as to the past or the future. Cairn accordingly disclaims all and any liability whatsoever, whether arising in tort, contract or otherwise (save as referred to above), which it might otherwise have in respect

of the contents of this Announcement or any such statement.

In connection with the Placing, the Bookrunners and any of its respective affiliates, acting

as investors for their own accounts, may subscribe for or purchase ordinary shares in the Company ( Ordinary Shares ) and, in that capacity may retain, purchase, sell, offer to sell or otherwise deal for their own accounts in such

Ordinary Shares and other securities of the Company or related investments in connection with the Placing or otherwise. Accordingly, references to the Ordinary Shares being offered, subscribed, acquired, placed or otherwise dealt in should be read

as including any offer to, or subscription, acquisition, placing or dealing by the Bookrunners and any of its respective affiliates acting as investors for their own accounts. In addition, the Bookrunners or its respective affiliates may enter into

financing arrangements and swaps in connection with which it or its affiliates may from time to time acquire, hold or dispose of Ordinary Shares. The Bookrunners have no intention to disclose the extent of any such investment or transactions

otherwise than in accordance with any legal or regulatory obligations to do so.

BACKGROUND TO THE PLACING

biopharmaceutical company focused on the discovery, development and commercialisation of novel medicines for indications in rare and infectious diseases for which there are no existing or only inadequate therapies. Summit is conducting clinical

programmes focused on the genetic disease Duchenne muscular dystrophy ( DMD ) and the infectious disease C. difficile infection ( CDI ).

DMD is a progressive muscle wasting disease that

affects around 50,000 boys and young men in the developed world. The disease is caused by different genetic faults in the gene that encodes dystrophin, a protein that is essential for the healthy function of all muscles. There is currently no cure

for DMD and life expectancy is into the late twenties. Utrophin protein is structurally and functionally similar to dystrophin. In preclinical studies, the continued expression of utrophin had a meaningful, positive effect on muscle performance.

Summit believes that utrophin modulation has the potential to slow down or even stop the progression of DMD in all patients with the disease, regardless

of their underlying dystrophin gene mutation. Summit also believes that utrophin modulation could potentially be complementary to other therapeutic approaches for DMD.

The Company s lead utrophin modulator, ezutromid, is an orally administered, small molecule. In January and February 2018, Summit announced positive 24-week interim results from its open-label Phase 2 proof of concept trial called PhaseOut DMD.

The key findings from

PhaseOut DMD after 24-weeks of ezutromid treatment were as follows:

Additional findings from the interim data showed that ezutromid has been well tolerated to

date. All patients retained ambulation and achieved plasma levels of ezutromid that are believed to be sufficient to modulate expression of utrophin protein; no exposure response relationship has been observed in the trial to date.

The combination of reduced muscle fibre damage, reduced muscle inflammation and increased levels of utrophin provides the first evidence of ezutromid target

engagement, proof of mechanism and evidence of early impact of ezutromid on downstream muscle health. The top-line data from the full 48-week trial are expected in the

third quarter of 2018.

is a serious healthcare threat in hospitals, long-term care homes and increasingly in the wider community with over one million estimated cases of CDI annually in the United States and Europe. There are an estimated 29,000 deaths annually from CDI

in the United States alone. The United States Centers for Disease Control and Prevention highlighted C. difficile as one of three pathogens that pose an immediate public health threat. The economic impact of CDI is significant with

one study estimating annual acute care costs attributable to CDI of $4.8 billion in the United States.

CDI is a bacterial infection of the colon

that produces toxins that cause inflammation and severe diarrhoea, and in the most serious cases can be fatal. Patients typically develop CDI following the use of broad-spectrum antibiotics that can cause widespread damage to the natural

gastrointestinal (gut) flora and allow overgrowth of C. difficile bacteria. Existing CDI treatments are predominantly broad spectrum antibiotics, which cause further damage to the gut flora and are associated with high rates of

recurrent disease. Recurrent disease is the key clinical issue in CDI, as repeat episodes of the disease are typically more severe and associated with increased mortality rates and healthcare costs.

Ridinilazole is a small molecule precision antibiotic that Summit is developing for the treatment of CDI. In preclinical efficacy studies, ridinilazole

exhibited a targeted spectrum of activity that combined a potent bactericidal effect against all clinical isolates of C. difficile tested, with minimal impact on other bacteria that are typically found in the gut microbiome. In a