Amended and Restated Promissory Note, dated February 17, 2024, by and

THIS NOTE HAS NOT BEEN REGISTERED UNDER THE UNITED STATES SECURITIES ACT OF 1933, AS AMENDED (THE “SECURITIES ACT”), OR ANY STATE SECURITIES LAWS AND MAY NOT BE OFFERED, SOLD, PLEDGED, ASSIGNED, OR OTHERWISE TRANSFERRED UNLESS (1) A REGISTRATION STATEMENT WITH RESPECT HERETO IS EFFECTIVE UNDER THE SECURITIES ACT AND ANY APPLICABLE STATE SECURITIES LAWS OR (2) THE COMPANY RECEIVES AN OPINION OF COUNSEL TO THE HOLDER OF THIS NOTE, WHICH COUNSEL AND OPINION ARE REASONABLY SATISFACTORY TO THE COMPANY, THAT THIS NOTE MAY BE OFFERED, SOLD, PLEDGED, ASSIGNED, OR OTHERWISE TRANSFERRED IN THE MANNER CONTEMPLATED WITHOUT AN EFFECTIVE REGISTRATION STATEMENT UNDER THE SECURITIES ACT OR APPLICABLE STATE SECURITIES LAWS.

WHEREAS, the parties hereto entered into that certain Promissory Note, dated as of January 19, 2023 (the “Original Note”), pursuant to which Summit Therapeutics Inc., a Delaware corporation (“Borrower”), promised to pay to Robert Duggan or his successors or assigns (“Lender”), $100,000,000 and any interest, fees, charges and late fees on the Maturity Date (as defined therein), subject to, and in accordance with, the terms set forth therein;

WHEREAS, this Amended and Restated Promissory Note (hereinafter, this “Note”) amends and restates (as well as supersedes and replaces) the Original Note. This Note represents a renewal of, and is issued in substitution and exchange for, and not in satisfaction, repayment or as a novation of, the indebtedness outstanding under the Original Note (the “Existing Debt”). Nothing herein contained shall be construed to deem such Existing Debt paid. Payment in full of, and the satisfaction of all obligations under, this Note shall also be deemed to be payment in full and satisfaction of the Existing Debt. Should there be any conflict between any of the terms of the Original Note and the terms of this Note, the terms of this Note shall control; and

WHEREAS, following the execution of this Note, the Original Note shall be marked “cancelled” on its face and replaced in its entirety by this Note.

NOW, THEREFORE, the Original Note is amended and restated by this Note as follows:

FOR VALUE RECEIVED, Borrower, promises to pay to Lender, $100,000,000 (the “Principal Amount”) and any interest, fees, charges and late fees on April 1, 2025 (the “Maturity Date”), subject to, and in accordance with, the terms set forth herein, and to pay interest on the Outstanding Balance (as defined below) at the rate equal to the Interest Rate (as defined below) per annum (subject to Section 2.2) from the Effective Date until the Outstanding Balance is paid in full. This Note is issued and made effective as of February 17, 2024 (the “Effective Date”). This Note is issued pursuant to, and subject to the terms of, that certain Note Purchase Agreement, dated as of December 6, 2022 (as the same may be amended, restated, amended and restated, supplemented, modified or replaced from time to time, the

“Purchase Agreement”; capitalized terms used, but not otherwise defined, herein shall have the meanings ascribed to such terms in the Purchase Agreement), by and between Borrower, Lender and the other lenders party thereto from time to time. All interest calculations hereunder shall be computed on the basis of a 365-day year, and shall be payable in accordance with the terms of this Note.

1.1 Payment of Interest. Subject to Section 2.1, from and including the date hereof to, but excluding, the Maturity Date, interest on this Note shall accrue daily and compound quarterly in arrears, commencing with the Effective Date, on the Outstanding Balance of this Note outstanding at a rate of interest per annum (the “Interest Rate”) equal to the greater of (x) 12% and (y) the prime rate announced to be then in effect as published as the average rate in the Wall Street Journal (Northeast Edition) (the “Prime Rate”) plus 350 basis points, to be determined on the first day of each calendar month for the succeeding calendar month. Default Interest shall be paid as provided for in the Purchase Agreement. All accrued and unpaid interest from and including January 1, 2024 to, but excluding, the Effective Date shall be payable on the date that is five Business Days following the Effective Date. All accrued and unpaid interest from and including the date hereof to, but excluding, the Maturity Date shall be payable on the Maturity Date, or if such day is not a Business Day, on the immediately succeeding Business Day. Interest shall be calculated on the basis of a 365-day year and actual days elapsed.

1.2 Payment of Principal and Interest. On the Maturity Date, the Outstanding Balance, including all principal under this Note, together with all accrued and unpaid interest thereon and all other sums evidenced by this Note, shall be due and payable in cash. Upon payment in full of the Outstanding Balance of this Note and all accrued and unpaid interest thereon, this Note will be automatically cancelled, whether or not this Note has been surrendered.

As used herein, “Outstanding Balance” means, as of any date of determination, the outstanding principal amount of this Note, as reduced or increased, as the case may be, pursuant to the terms hereof for payment, offset or otherwise, plus accrued, but unpaid, interest, collection and enforcements costs (including attorneys’ fees) incurred by Lender, transfer, stamp, issuance and similar taxes and any other fees or charges incurred under this Note.

2.1 Defaults. As used herein, the term “Event of Default” has the meaning given such term in the Purchase Agreement.

2.2 Remedies. At any time and from time to time after Lender becomes aware of the occurrence of any Event of Default that is continuing, Lender may exercise all of its rights and remedies provided for in the Purchase Agreement or otherwise available under applicable law or equity, including, subject to the Purchase Agreement, the right to accelerate this Note by written notice to Borrower, with the Outstanding Balance becoming immediately due and payable in cash. Notwithstanding the foregoing, upon the occurrence of any Event of Default described in clause (b), (c), (d), (e) or (f) of Section 9.1 of the Purchase Agreement, the Outstanding Balance, as of the date of acceleration, shall become immediately and automatically due and payable in cash, without any written notice required by Lender or any other person. Upon the occurrence of an Event of Default, interest shall accrue on the Outstanding Balance at an interest rate equal to the Default Interest (as defined in the Purchase Agreement) rate. In connection with acceleration described herein, Lender need not provide, and Borrower hereby waives, any presentment,

demand, protest or other notice of any kind, and Lender may immediately and without expiration of any grace period enforce any and all of its rights and remedies hereunder and all other remedies available to it under applicable law. Such acceleration may be rescinded and annulled by Lender in accordance with the Purchase Agreement at any time prior to payment hereunder and Lender shall have all rights as a holder of the Note until such time, if any, as Lender receives full payment pursuant to this Section 2.2. No such rescission or annulment shall affect any subsequent Event of Default or impair any right consequent thereon. Nothing herein shall limit Lender’s right to pursue in one or more arbitrations any other remedies available to it at law or in equity including, without limitation, a decree of specific performance and/or injunctive relief. In the event the Lender accelerates, and receives any payment (including any payment in full) of the Outstanding Balance and other monetary obligations of the Borrower under the Transaction Documents, the Borrower shall not be entitled to any refund of the Prepaid Interest.

4. Waiver. No waiver of any provision of this Note shall be effective unless it is in the form of a writing signed by the party granting the waiver. No waiver of any provision or consent to any prohibited action shall constitute a waiver of any other provision or consent to any other prohibited action, whether or not similar. No waiver or consent shall constitute a continuing waiver or consent or commit a party to provide a waiver or consent in the future except to the extent specifically set forth in writing.

5. Borrower Prepayment. Borrower shall have the right, exercisable at any time following its completion of a capital raise, in its sole and absolute discretion without penalty or fee, to prepay an amount of this Note equal to the lesser of (i) the net proceeds of the capital raise or (ii) the full amount outstanding of this Note (such amount, the “Prepayment Amount”), in either case by providing written notice to Lender at least two Business Days in advance of such prepayment. On the prepayment date Borrower shall pay the Prepayment Amount in cash to Lender.

6. Payment of Collection Costs. If this Note is placed in the hands of an attorney for collection or enforcement prior to commencing arbitration or legal proceedings, or is collected or enforced through any arbitration or legal proceeding, or Lender otherwise takes action to collect amounts due under this Note or to enforce the provisions of this Note, then Borrower shall pay the costs incurred by Lender for such collection, enforcement or action including, without limitation, attorneys’ fees and disbursements.

7. Opinion of Counsel. In the event that an opinion of counsel is needed for any matter related to this Note, Lender has the right to have any such opinion provided by its counsel; provided that such opinion shall be reasonably acceptable to Borrower.

8. Governing Law; Venue. This Note shall be construed and enforced in accordance with, and all questions concerning the construction, validity, interpretation and performance of this Note shall be governed by, the internal laws of the State of New York, without giving effect to any choice of law or conflict of law provision or rule (whether of the State of New York or any other jurisdiction) that would cause the application of the laws of any jurisdiction other than the State of New York. The provisions set forth in the Purchase Agreement to determine the proper venue for any disputes are incorporated herein by this reference.

9. Cancellation. After repayment of the entire Outstanding Balance, this Note shall be deemed paid in full, shall automatically be deemed canceled and shall not be reissued.

10. Amendments. The prior written consent of both parties hereto shall be required for any change or amendment to this Note.

11. Assignments. Borrower may not assign this Note without the prior written consent of Lender. This Note may be offered, sold, assigned or transferred by Lender without the consent of Borrower.

12. Time is of the Essence. Time is expressly made of the essence with respect to each and every provision of this Note and the documents and instruments entered into in connection herewith.

13. Notices. Whenever notice is required to be given under this Note, unless otherwise provided herein, such notice shall be given in accordance with the subsection of the Purchase Agreement titled “Notices.”

14. Liquidated Damages. Lender and Borrower agree that in the event Borrower fails to comply with any of the terms or provisions of this Note, Lender’s damages would be uncertain and difficult (if not impossible) to accurately estimate because of the parties’ inability to predict future interest rates, future share prices, future trading volumes and other relevant factors. Accordingly, Lender and Borrower agree that any fees, balance adjustments, Default Interest or other charges assessed under this Note are not penalties but instead are intended by the parties to be, and shall be deemed, liquidated damages (under Lender’s and Borrower’s expectations that any such liquidated damages will, if allowed under applicable law, tack back to the Effective Date for purposes of determining the holding period under Rule 144).

15. Waiver of Jury Trial. EACH OF LENDER AND BORROWER IRREVOCABLY WAIVES ANY AND ALL RIGHTS SUCH PARTY MAY HAVE TO DEMAND THAT ANY ACTION, PROCEEDING OR COUNTERCLAIM ARISING OUT OF OR IN ANY WAY RELATED TO THIS NOTE OR THE RELATIONSHIPS OF THE PARTIES HERETO BE TRIED BY JURY. THIS WAIVER EXTENDS TO ANY AND ALL RIGHTS TO DEMAND A TRIAL BY JURY ARISING UNDER COMMON LAW OR ANY APPLICABLE STATUTE, LAW, RULE OR REGULATION. FURTHER, EACH PARTY HERETO ACKNOWLEDGES THAT SUCH PARTY IS KNOWINGLY AND VOLUNTARILY WAIVING SUCH PARTY’S RIGHT TO DEMAND TRIAL BY JURY.

16. Voluntary Agreement. Borrower has carefully read this Note and has asked any questions needed for Borrower to understand the terms, consequences and binding effect of this Note and fully understand them. Borrower has had the opportunity to seek the advice of an attorney of Borrower’s choosing, or has waived the right to do so, and is executing this Note voluntarily and without any duress or undue influence by Lender or anyone else.

17. Severability. If any part of this Note is construed to be in violation of any law, such part shall be modified to achieve the objective of Borrower and Lender to the fullest extent permitted by law and the balance of this Note shall remain in full force and effect.