UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

(Rule 13d-101)

INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT

TO § 240.13d-1(a) AND AMENDMENTS THERETO FILED PURSUANT TO

§ 240.13d-2(a)

(Amendment No. 2)¹

Summit Therapeutics PLC

(Name of Issuer)

American Depositary Shares and Ordinary Shares, par value £0.01 per share

(Title of Class of Securities)

86627R102

(CUSIP Number)

ADAM W. FINERMAN, ESQ.

OLSHAN FROME WOLOSKY LLP

1325 Avenue of the Americas

New York, New York 10019

(212) 451-2300

(Name, Address and Telephone Number of Person

Authorized to Receive Notices and Communications)

December 24, 2019

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box ¨.

Note:  Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits.  See § 240.13d-7 for other parties to whom copies are to be sent.

1              The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

* The 269,368,810 Ordinary Shares beneficially owned consist of 15,657,641 American Depositary Shares, which may be exchanged for Ordinary Shares at a 1:5 ratio, 166,157,050 Ordinary Shares and includes the exercise of warrants to buy 24,923,555 Ordinary Shares, which are first exercisable on June 24, 2020.

The following constitutes the Schedule 13D/A filed by the undersigned (as amended hereby, the “Schedule 13D”).

Item 3 is hereby amended to add the following:

The cost of the Subscription Shares (as defined below) is approximately $47,371,000. Such shares were acquired with personal funds.

Item 4 is hereby amended to add the following:

On December 6, 2019, the Issuer entered into a securities purchase agreement (the “Securities Purchase Agreement”) with the Reporting Person, pursuant to which the Reporting Person agreed to subscribe for an aggregate of 166,157,050 ordinary shares (the “Subscription Shares”) of the Issuer, par value £0.01 per share (the “Ordinary Shares”) and warrants to purchase an aggregate of 24,923,555 Ordinary Shares at an exercise price of £0.243 per Ordinary Share (the “Subscription Warrant”) in a private placement in the United States for an aggregate purchase price of $47.371 million (collectively, the “Subscription”), or an approximate per share price of £0.221 (which is based on an exchange rate of $1.29 to £1.00) for a Subscription Share plus a Subscription Warrant.

The Subscription closed on December 24, 2019, upon the satisfaction of all closing conditions.

The Reporting Person beneficially owns 15,657,641 American Depositary Shares (the “ADS Shares”), representing 78,288,205 of the Ordinary Shares, and 191,080,605 Ordinary Shares, or an aggregate of approximately 74.66% of the outstanding Ordinary Shares, including 24,923,555 Ordinary Shares issuable upon exercise of the Subscription Warrant.

Following the closing, the Reporting Person entered into agreements to sell an aggregate of 6,664,374 ADS Shares to certain purchasers pursuant to two Stock Purchase Agreements, each dated December 24, 2019, each by and among Robert Duggan and the parties listed on each Exhibit A thereto (each an “SPA”). Subject to the satisfaction of certain customary closing conditions, the SPAs are expected to close on or about January 10, 2020.

The foregoing description does not purport to be complete and is qualified in its entirety by reference to the full text of the Securities Purchase Agreement, the Warrant Instrument and each SPA, included as Exhibits 99.1, 99.2, 99.3 and 99.4 respectively and incorporated herein by reference.

Item 5 is hereby amended and restated as follows:

Percentage: Approximately 74.66%

Item 6 is hereby amended to add the following:

Following the closing, the Reporting Person entered into agreements to sell an aggregate of 6,664,374 ADS Shares to certain purchasers pursuant to the SPAs. Subject to the satisfaction of certain customary closing conditions, the SPAs are expected to close on or about January 10, 2020.

The foregoing description of the SPAs does not purport to be complete and is qualified in its entirety by reference to the full text of each SPA, attached hereto as Exhibits 99.3 and 99.4 and incorporated by reference herein.

* Filed herewith.

SIGNATURES

After reasonable inquiry and to the best of his knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

Dated: December 27, 2019

SCHEDULE A

Transactions in the Shares During the Past Sixty Days

ROBERT W. DUGGAN

* Represents Ordinary Shares issuable upon exercise of the Subscription Warrant. The Subscription Warrant is exercisable from June 24, 2020 to December 24, 2029 at a price of £0.243 per Ordinary Share.

Exhibit 99.3

STOCK PURCHASE AGREEMENT

This Stock Purchase (this “Agreement”) is dated as of December 24, 2019, and entered into between Robert Duggan, an individual (the “Seller”) and each of the parties listed on Exhibit A hereto (each such party, a “Buyer”, and each of the Buyers and the Seller, a “Party” to this Agreement, and collectively, the “Parties”) for the purchase of certain equity interests of Summit Therapeutics plc, a public limited company incorporated in England and Wales with the Registrar of Companies of England and Wales (the “Company”).

As of the date hereof and pursuant to the closing of that certain Securities Purchase Agreement (“SPA”) by and between the Company and the Seller, the Seller beneficially owns 166,157,050 ordinary shares, par value £0.01 per share of the Company (the “Ordinary Shares”) and warrants to purchase an aggregate of 24,923,555 Ordinary Shares (the “Warrants”). In addition, the Seller owns 15,657,641 American Depository Shares (the “ADS Shares”), which may be exchanged for Ordinary Shares at a 1:5 ratio.

The Parties hereby agree as follows:

Section 1.01        Sale of Shares. On the terms and subject to the conditions set forth in this Agreement, the Seller agrees to sell, assign, transfer and deliver to each Buyer at the Closing, and each Buyer agrees to purchase and redeem from the Seller at the Closing, ADS Shares held by the Seller in the amount set forth opposite each Buyer’s name in Exhibit A (the “Shares”).

Section 1.02        Purchase Price. Each Buyer will purchase their respective Shares for an aggregate purchase price as set forth opposite each Buyer’s name on Exhibit A (each such aggregate price, the “Purchase Price”).

Section 1.03        Closing. Subject to the satisfaction or waiver of the conditions to closing of this Agreement, the closing of the transactions contemplated hereby (the “Closing”) will take place on January 10, 2020, or at such other time as may be agreed in writing by the Seller and each respective Buyer with respect to such Buyer (the “Closing Date”) upon satisfactory delivery of the Shares to each Buyer and payment to the Seller of the Purchase Price by wire transfer of immediately available funds to the account(s) designated via email by the Seller. Delivery of the Shares shall be made by electronic delivery to each Buyer of the Shares held in street name and purchased from the Seller pursuant hereto through the Depository Trust Company to an account maintained by the Company’s transfer agent for the benefit of the Company and, with respect to Shares held of record, by delivery of a certificate or certificates representing such Shares duly endorsed for transfer or accompanied by appropriate stock powers duly executed in blank and other appropriate instructions to the Company’s transfer agent. In either case, the Parties mutually agree to execute and deliver any other documents as are reasonably required by the Company’s transfer agent to consummate the Closing.

Section 1.04        Conditions to Closing.

Section 1.05        Representations and Warranties of the Seller. The Seller hereby makes the following representations and warranties to each Buyer, each of which is true and correct as of the date hereof and as of the Closing Date and each of which will survive the Closing and the transactions contemplated hereby:

Section 1.06        Representations and Warranties of each Buyer. Each Buyer hereby makes the following representations and warranties to the Seller with respect to itself (and not on behalf of any other Buyer), each of which is true and correct as of the date hereof and as of the Closing Date and each of which will survive the Closing and the transactions contemplated hereby:

Section 1.07        Substitution. At the Closing, the Seller may, in his sole discretion, substitute (which shall be at a ratio of five (5) Ordinary Shares for each ADS Share, with such number of Purchased Warrants (as defined in the SPA) as included with each Ordinary Share pursuant to the SPA) any or all of the Shares to be sold hereunder with an equivalent proportion of Purchased Securities (as defined in the SPA) for the same Purchase Price. In the event of a substitution by Seller, (i) the Seller’s representations and warranties in Section 1.05 applicable to the Shares shall apply to any and all Purchased Securities substituted therefor, (ii) Seller shall notify Buyer prior to the Closing of the Purchased Securities being substituted and (iii) all references to “Shares” herein shall be deemed to include any substituted Purchased Securities, mutatis mutandis.

Section 2.01        Notices. All notices, requests and other communications to any Party hereunder will be in writing (including prepaid overnight courier) and will be given to such Party at its address or email address set forth on Exhibit A (or, in the case of Seller, on the signature page attached hereto), or at such other address or email address as such Party may hereafter specify in writing. Each such notice, request or other communication will be effective (a) if given by email, when transmitted to the email address specified in this Section and confirmation of receipt is received by the sender, (b) one business day after deposit with a reputable overnight courier properly addressed and with all charges prepaid or (c) when received, if by any other means. The Parties will promptly notify each other in the manner provided in this Section of any change in their respective addresses. A notice of change of address will not be deemed to have been given until received by the addressee.

Section 2.02        Expenses. Each Party shall bear its own expenses incurred in connection with the preparation of this Agreement.

Section 2.03        Specific Performance. Each of the Parties acknowledges and agrees that irreparable injury to the other Parties would occur if any of the provisions of this Agreement were not performed in accordance with their specific terms or were otherwise breached and that such injury would not be adequately compensable by the remedies available at law (including the payment of money damages). It is accordingly agreed that each individual Party herein will each be entitled to specific enforcement of, and injunctive relief to prevent any violation of, the terms hereof, and the other Parties will not take action, directly or indirectly, in opposition to the moving Party seeking such relief on the grounds that any other remedy or relief is available at law or in equity. This Section 2.04 is not the exclusive remedy for any violation of this Agreement.

Section 2.04        Further Assurances. Each Party will execute and deliver such additional documents, instruments, conveyances and assurances and take such further actions as may be reasonably required to carry out the provisions of this Agreement and give effect to the transactions contemplated by this Agreement.

Section 2.05        Assignment. No Party will assign this Agreement or any rights, interests or obligations hereunder, or delegate performance of any of its obligations hereunder, without the prior written consent of the other Parties.

Section 2.06        Entire Agreement. This Agreement constitutes the entire agreement and understanding of the Parties in respect of the subject matter contained herein. This Agreement supersedes all prior agreements and understandings between the Parties with respect to the subject matter hereof.

Section 2.07        Severability. If any term, provision, covenant or restriction of this Agreement is held by a court of competent jurisdiction to be invalid, void or unenforceable, the remainder of the terms, provisions, covenants and restrictions of this Agreement will remain in full force and effect and will not be affected, impaired or invalidated. In addition, the Parties agree to use commercially reasonable efforts to agree upon and substitute a valid and enforceable term, provision, covenant or restriction for any of such that is held invalid, void or enforceable by a court of competent jurisdiction.

Section 2.08        Waiver, Amendment, etc. This Agreement may not be amended or supplemented, and no waivers of or consents to or departures from the provisions hereof will be effective, unless set forth in a writing, and delivered to, the other applicable Parties. No failure or delay of any Party in exercising any power or right under this Agreement will operate as a waiver thereof, nor will any single or partial exercise of any right or power, or any abandonment or discontinuance of steps to enforce such right or power, preclude any other or further exercise thereof or the exercise of any other right or power.

Section 2.09        Binding Agreement; No Third Party Beneficiaries. This Agreement will be binding upon and inure to the benefit of the Parties and their successors and permitted assigns. Nothing expressed or implied herein is intended or will be construed to confer upon or to give to any third party any rights or remedies by virtue hereof.

Section 2.10        Governing Law; Exclusive Jurisdiction. This Agreement is governed by and is to be construed and enforced in accordance with the laws of the State of New York without reference to the conflict of laws principles thereof. Any legal action or proceeding with respect to this Agreement and the rights and obligations arising hereunder, or for recognition and enforcement of any judgment in respect of this Agreement and the rights and obligations arising hereunder brought by a Party or its successors or assigns, will be brought and determined exclusively in the state and federal courts located in New York. Each of the Parties hereby irrevocably submits with regard to any such action or proceeding for itself and in respect of its property, generally and unconditionally, to the personal jurisdiction of the aforesaid courts and agrees that he or it will not bring any action relating to this Agreement in any court other than the aforesaid courts. Each of the Parties hereby irrevocably waives, and agrees not to assert in any action or proceeding with respect to this Agreement, (a) any claim that he or it is not personally subject to the jurisdiction of the above-named courts for any reason, (b) any claim that he or it or his or its property is exempt or immune from jurisdiction of any such court or from any legal process commenced in such courts (whether through service of notice, attachment prior to judgment, attachment in aid of execution of judgment, execution of judgment or otherwise) and (c) to the fullest extent permitted by applicable legal requirements, any claim that (1) the suit, action or proceeding in such court is brought in an inconvenient forum, (2) the venue of such suit, action or proceeding is improper or (3) this Agreement, or the subject matter hereof, may not be enforced in or by such courts.

Section 2.11        Counterparts. This Agreement may be executed in one or more counterparts each of which when so executed and delivered will be deemed an original but all of which will constitute one and the same Agreement (including by means of electronic delivery).

[Signature Pages Follow]

The Seller and each party listed on Exhibit A hereto have caused this Stock Purchase to be duly executed as of the date first above written.

Exhibit A

Exhibit 99.4

STOCK PURCHASE AGREEMENT

This Stock Purchase (this “Agreement”) is dated as of December 24, 2019, and entered into between Robert Duggan, an individual (the “Seller”) and each of the parties listed on Exhibit A hereto (each such party, a “Buyer”, and each of the Buyers and the Seller, a “Party” to this Agreement, and collectively, the “Parties”) for the purchase of certain equity interests of Summit Therapeutics plc, a public limited company incorporated in England and Wales with the Registrar of Companies of England and Wales (the “Company”).

As of the date hereof and pursuant to the closing of that certain Securities Purchase Agreement (“SPA”) by and between the Company and the Seller, the Seller beneficially owns 166,157,050 ordinary shares, par value £0.01 per share of the Company (the “Ordinary Shares”) and warrants to purchase an aggregate of 24,923,555 Ordinary Shares (the “Warrants”). In addition, the Seller owns 15,657,641 American Depository Shares (the “ADS Shares”), which may be exchanged for Ordinary Shares at a 1:5 ratio.

The Parties hereby agree as follows:

Section 1.01        Sale of Shares. On the terms and subject to the conditions set forth in this Agreement, the Seller agrees to sell, assign, transfer and deliver to each Buyer at the Closing, and each Buyer agrees to purchase and redeem from the Seller at the Closing, ADS Shares held by the Seller in the amount set forth opposite each Buyer’s name in Exhibit A (the “Shares”).

Section 1.02        Purchase Price. Each Buyer will purchase their respective Shares for an aggregate purchase price as set forth opposite each Buyer’s name on Exhibit A (each such aggregate price, the “Purchase Price”).

Section 1.03        Closing. Subject to the satisfaction or waiver of the conditions to closing of this Agreement, the closing of the transactions contemplated hereby (the “Closing”) will take place on January 10, 2020, or at such other time as may be agreed in writing by the Seller and each respective Buyer with respect to such Buyer (the “Closing Date”) upon satisfactory delivery of the Shares to each Buyer and payment to the Seller of the Purchase Price by wire transfer of immediately available funds to the account(s) designated via email by the Seller. Delivery of the Shares shall be made by electronic delivery to each Buyer of the Shares held in street name and purchased from the Seller pursuant hereto through the Depository Trust Company to an account maintained by the Company’s transfer agent for the benefit of the Company and, with respect to Shares held of record, by delivery of a certificate or certificates representing such Shares duly endorsed for transfer or accompanied by appropriate stock powers duly executed in blank and other appropriate instructions to the Company’s transfer agent. In either case, the Parties mutually agree to execute and deliver any other documents as are reasonably required by the Company’s transfer agent to consummate the Closing.

Section 1.04        Conditions to Closing.

Section 1.05        Representations and Warranties of the Seller. The Seller hereby makes the following representations and warranties to each Buyer, each of which is true and correct as of the date hereof and as of the Closing Date and each of which will survive the Closing and the transactions contemplated hereby:

Section 1.06        Representations and Warranties of each Buyer. Each Buyer hereby makes the following representations and warranties to the Seller with respect to itself (and not on behalf of any other Buyer), each of which is true and correct as of the date hereof and as of the Closing Date and each of which will survive the Closing and the transactions contemplated hereby:

Section 1.07        Substitution. At the Closing, the Seller may, in his sole discretion, substitute (which shall be at a ratio of five (5) Ordinary Shares for each ADS Share, with such number of Purchased Warrants (as defined in the SPA) as included with each Ordinary Share pursuant to the SPA) any or all of the Shares to be sold hereunder with an equivalent proportion of Purchased Securities (as defined in the SPA) for the same Purchase Price. In the event of a substitution by Seller, (i) the Seller’s representations and warranties in Section 1.05 applicable to the Shares shall apply to any and all Purchased Securities substituted therefor, (ii) Seller shall notify Buyer prior to the Closing of the Purchased Securities being substituted and (iii) all references to “Shares” herein shall be deemed to include any substituted Purchased Securities, mutatis mutandis.

Section 2.01        Notices. All notices, requests and other communications to any Party hereunder will be in writing (including prepaid overnight courier) and will be given to such Party at its address or email address set forth on Exhibit A (or, in the case of Seller, on the signature page attached hereto), or at such other address or email address as such Party may hereafter specify in writing. Each such notice, request or other communication will be effective (a) if given by email, when transmitted to the email address specified in this Section and confirmation of receipt is received by the sender, (b) one business day after deposit with a reputable overnight courier properly addressed and with all charges prepaid or (c) when received, if by any other means. The Parties will promptly notify each other in the manner provided in this Section of any change in their respective addresses. A notice of change of address will not be deemed to have been given until received by the addressee.

Section 2.02        Expenses. Each Party shall bear its own expenses incurred in connection with the preparation of this Agreement.

Section 2.03        Specific Performance. Each of the Parties acknowledges and agrees that irreparable injury to the other Parties would occur if any of the provisions of this Agreement were not performed in accordance with their specific terms or were otherwise breached and that such injury would not be adequately compensable by the remedies available at law (including the payment of money damages). It is accordingly agreed that each individual Party herein will each be entitled to specific enforcement of, and injunctive relief to prevent any violation of, the terms hereof, and the other Parties will not take action, directly or indirectly, in opposition to the moving Party seeking such relief on the grounds that any other remedy or relief is available at law or in equity. This Section 2.04 is not the exclusive remedy for any violation of this Agreement.

Section 2.04        Further Assurances. Each Party will execute and deliver such additional documents, instruments, conveyances and assurances and take such further actions as may be reasonably required to carry out the provisions of this Agreement and give effect to the transactions contemplated by this Agreement.

Section 2.05        Assignment. No Party will assign this Agreement or any rights, interests or obligations hereunder, or delegate performance of any of its obligations hereunder, without the prior written consent of the other Parties.

Section 2.06        Entire Agreement. This Agreement constitutes the entire agreement and understanding of the Parties in respect of the subject matter contained herein. This Agreement supersedes all prior agreements and understandings between the Parties with respect to the subject matter hereof.

Section 2.07        Severability. If any term, provision, covenant or restriction of this Agreement is held by a court of competent jurisdiction to be invalid, void or unenforceable, the remainder of the terms, provisions, covenants and restrictions of this Agreement will remain in full force and effect and will not be affected, impaired or invalidated. In addition, the Parties agree to use commercially reasonable efforts to agree upon and substitute a valid and enforceable term, provision, covenant or restriction for any of such that is held invalid, void or enforceable by a court of competent jurisdiction.

Section 2.08        Waiver, Amendment, etc. This Agreement may not be amended or supplemented, and no waivers of or consents to or departures from the provisions hereof will be effective, unless set forth in a writing, and delivered to, the other applicable Parties. No failure or delay of any Party in exercising any power or right under this Agreement will operate as a waiver thereof, nor will any single or partial exercise of any right or power, or any abandonment or discontinuance of steps to enforce such right or power, preclude any other or further exercise thereof or the exercise of any other right or power.

Section 2.09        Binding Agreement; No Third Party Beneficiaries. This Agreement will be binding upon and inure to the benefit of the Parties and their successors and permitted assigns. Nothing expressed or implied herein is intended or will be construed to confer upon or to give to any third party any rights or remedies by virtue hereof.

Section 2.10        Governing Law; Exclusive Jurisdiction. This Agreement is governed by and is to be construed and enforced in accordance with the laws of the State of New York without reference to the conflict of laws principles thereof. Any legal action or proceeding with respect to this Agreement and the rights and obligations arising hereunder, or for recognition and enforcement of any judgment in respect of this Agreement and the rights and obligations arising hereunder brought by a Party or its successors or assigns, will be brought and determined exclusively in the state and federal courts located in New York. Each of the Parties hereby irrevocably submits with regard to any such action or proceeding for itself and in respect of its property, generally and unconditionally, to the personal jurisdiction of the aforesaid courts and agrees that he or it will not bring any action relating to this Agreement in any court other than the aforesaid courts. Each of the Parties hereby irrevocably waives, and agrees not to assert in any action or proceeding with respect to this Agreement, (a) any claim that he or it is not personally subject to the jurisdiction of the above-named courts for any reason, (b) any claim that he or it or his or its property is exempt or immune from jurisdiction of any such court or from any legal process commenced in such courts (whether through service of notice, attachment prior to judgment, attachment in aid of execution of judgment, execution of judgment or otherwise) and (c) to the fullest extent permitted by applicable legal requirements, any claim that (1) the suit, action or proceeding in such court is brought in an inconvenient forum, (2) the venue of such suit, action or proceeding is improper or (3) this Agreement, or the subject matter hereof, may not be enforced in or by such courts.

Section 2.11        Counterparts. This Agreement may be executed in one or more counterparts each of which when so executed and delivered will be deemed an original but all of which will constitute one and the same Agreement (including by means of electronic delivery).

[Signature Pages Follow]

The Seller and each party listed on Exhibit A hereto have caused this Stock Purchase to be duly executed as of the date first above written.

Exhibit A