THE TERMS AND CONDITIONS OF THE RIGHTS OFFERING ARE SET FORTH IN THE COMPANY'S PROSPECTUS SUPPLEMENT DATED APRIL 21, 2021 (THE "PROSPECTUS SUPPLEMENT") AND ARE INCORPORATED HEREIN BY REFERENCE. COPIES OF THE PROSPECTUS A

THE TERMS AND CONDITIONS OF THE RIGHTS

OFFERING ARE SET FORTH IN THE COMPANY'S PROSPECTUS SUPPLEMENT DATED APRIL 21, 2021 (THE "PROSPECTUS SUPPLEMENT")

AND ARE INCORPORATED HEREIN BY REFERENCE. COPIES OF THE PROSPECTUS AND PROSPECTUS SUPPLEMENT ARE AVAILABLE UPON REQUEST FROM BROADRIDGE

CORPORATE ISSUER SOLUTIONS, INC., THE SUBSCRIPTION AGENT, BY CALLING (855) 793-5068.

FORM OF INSTRUCTIONS AS TO USE OF

SUMMIT THERAPEUTICS INC. NON-TRANSFERABLE SUBSCRIPTION RIGHTS CERTIFICATES

PLEASE CONSULT THE SUBSCRIPTION AND INFORMATION

YOUR BANK OR BROKER FOR ANY QUESTIONS

The following instructions

relate to a rights offering by Summit Therapeutics Inc., a Delaware corporation ("we," "us,"

"our," or the "Company"), to the stockholders (the "holder" or "you")

of its Common Stock, par value $0.01 per share (the "Common Stock"), as described in the Company's prospectus

dated October 15, 2020 (as supplemented by the Prospectus Supplement, the "Prospectus"). Holders of our Common

Stock as of 5:00 p.m., Eastern Time, on April 9, 2021 (the "Record Date") are receiving, at no charge, non-transferable

subscription rights (each, a "Subscription Right") to purchase up to an aggregate of $75,000,000 of shares of

Common Stock at an initial price per share equal to the lesser of (i) $5.24 per share (the "Initial Price")

and (ii) the volume weighted-average price of the Common Stock for the ten (10) consecutive trading days through and including

the Expiration Date (as defined below) (the "Alternate Price"). Each stockholder will receive one Subscription

Right for each share of Common Stock owned on the Record Date and each Subscription Right will entitle its holder to purchase 0.172598

shares of Common Stock at the Initial Price (the "Basic Subscription Right"). To the extent the Alternate Price

is lower than the Initial Price, we will issue additional shares in the rights offering to you.

Rights will be evidenced by non-transferable subscription rights certificates (the "Non-Transferable Subscription Rights

Certificate"). The number of Subscription Rights to which you are entitled is printed on the face of your Non-Transferable

Subscription Rights Certificate.

Over-Subscription Right

If a holder purchases

all of the shares of Common Stock available to it pursuant to its Basic Subscription Right, it may also exercise an over-subscription

right (the "Over-Subscription Right") to purchase a portion of any shares of Common Stock that are not purchased

by other stockholders through the exercise of their Basic Subscription Rights (the "Unsubscribed Shares"),

subject to the availability and pro rata allocation of the Unsubscribed Shares among all persons exercising their Over-Subscription

exercise your Over-Subscription Right, you should indicate the number of additional shares that you would like to purchase in

the space provided on your Non-Transferable Subscription Rights Certificate, as well as the number of shares that you beneficially

own without giving effect to any shares to be purchased in this rights offering. When you send in your Non-Transferable Subscription

Rights Certificate, you must also send the full purchase price in cash, as provided herein, for the number of additional shares

that you have requested to purchase, at the Initial Price (in addition to the payment in cash, as provided herein, due for shares

purchased through your Basic Subscription Right).

shares remaining after the exercise of all Basic Subscription Rights is not sufficient to satisfy all requests for shares pursuant

to Over-Subscription Rights, you will be allocated additional shares (subject to elimination of fractional shares) in the proportion

which the number of shares you purchased through the Basic Subscription Right bears to the total number of shares that all oversubscribing

stockholders purchased through the Basic Subscription Right. Broadridge Corporate Issuer Solutions, Inc. (the "Subscription

Agent") will return any excess payments in the form in which made.

As soon as practicable

after the Expiration Date, the Subscription Agent will determine the number of shares of Common Stock that you may purchase pursuant

to the Over-Subscription Right. We will issue to you the shares in book-entry, or uncertificated, form as soon as practicable after

the Expiration Date and after all allocations and adjustments have been effected. If you request and pay for more shares than are

allocated to you, we will refund the overpayment in the form in which made. In connection with the exercise of the Over-Subscription

Right, banks, brokers and other nominee holders of Subscription Rights who act on behalf of beneficial owners will be required

to certify to us and to the Subscription Agent as to the aggregate number of Subscription Rights exercised, and the number of shares

of Common Stock requested through the Over-Subscription Right, by each beneficial owner on whose behalf the nominee holder is acting.

fractional shares, but rather will round down the aggregate number of shares you are entitled to receive to the nearest whole number,

with the total exercise price being adjusted accordingly. If, on the Expiration Date, the Alternate Price is lower than the Initial

Price, any excess subscription amounts paid by a subscriber (the "Excess Subscription Amount") will be put towards

the purchase of additional shares in the rights offering (either towards your Basic Subscription Right, if available, or towards

the Over-Subscription Right if you have already exercised your Basic Subscription Right in full). Otherwise, any excess subscription

payments received by the Subscription Agent will be returned, without interest, as soon as practicable.

You are not required

to exercise any or all of your Subscription Rights. However, if you do not exercise your Subscription Rights and the rights offering

is completed, the number of shares of our Common Stock you own will not change but your percentage ownership of our total outstanding

voting stock will decrease because shares will be purchased by other stockholders in the rights offering. Your percentage ownership

of our voting stock may also decrease if you do not exercise your Subscription Right in full. Please see the discussion of risk

factors related to the rights offering, including dilution, under the heading "Risk Factors-Risks Related to The Rights

Offering," in the Prospectus Supplement.

RIGHTS WILL EXPIRE AND WILL HAVE NO VALUE AT 5:00 P.M., EASTERN TIME, ON MAY 10, 2021, SUBJECT TO EXTENSION OR EARLIER TERMINATION

(THE "EXPIRATION DATE"). YOUR NON-TRANSFERABLE SUBSCRIPTION RIGHTS CERTIFICATE AND SUBSCRIPTION PAYMENT FOR EACH RIGHT

THAT IS EXERCISED PURSUANT TO THE SUBSCRIPTION RIGHT MUST BE RECEIVED BY THE SUBSCRIPTION AGENT ON OR BEFORE THE EXPIRATION DATE.

ONCE YOU HAVE EXERCISED YOUR SUBSCRIPTION RIGHT, SUCH EXERCISE MAY NOT BE REVOKED, EVEN IF YOU LATER LEARN INFORMATION THAT YOU

CONSIDER TO BE UNFAVORABLE TO THE EXERCISE OF YOUR SUBSCRIPTION RIGHTS. SUBSCRIPTION RIGHTS NOT EXERCISED PRIOR TO THE EXPIRATION

DATE OF THE RIGHTS OFFERING WILL EXPIRE WITHOUT VALUE.

If you do not exercise

your Subscription Rights prior to that time, your Subscription Rights will expire and will no longer be exercisable. We will not

be required to issue shares of our Common Stock to you if the Subscription Agent receives your Non-Transferable Subscription Rights

Certificate(s) or your subscription payment after the Expiration Date, regardless of when the Non-Transferable Subscription Rights

Certificate(s) and subscription payment were sent. If you send your Non-Transferable Subscription Rights Certificate(s) and Initial

Price payment by mail, we recommend that you send them by registered mail, properly insured, with return receipt requested, and

that a sufficient number of days be allowed to ensure delivery to the Subscription Agent and clearance of payment before the expiration

of the subscription period. See "The Rights Offering-Expiration of this rights offering and Extensions, Amendments

and Termination" in the Prospectus Supplement.

upon exercise of the Subscription Rights will be delivered as soon as practicable after the Expiration Date, and after all pro

rata allocations and adjustments have been completed.

questions concerning the rights offering, please contact the Subscription Agent, Broadridge Corporate Issuer Solutions, Inc., at

the following address and number:

Broadridge Corporate Issuer Solutions,

Attn: BCIS Re-Organization Dept.

Brentwood, NY 11717-0718

(855) 793-5068 (toll free)

Subscription Rights, please: (1) complete Section 1 on your Non-Transferable Subscription Rights Certificate, attached to these

instructions; (2) sign Section 1 of your Non-Transferable Subscription Rights Certificate; and (3) mail the properly completed

and executed Non-Transferable Subscription Rights Certificate evidencing such Basic Subscription Rights and, if applicable, Over-Subscription

Rights subscribed, together with payment in full of the Initial Price for each share of Common Stock subscribed for pursuant to

the Basic Subscription Right and, if applicable, Over-Subscription Rights, to the Subscription Agent, on or prior to the Expiration

the Common Stock to be issued pursuant to the Subscription Rights are to be issued in a name other than that of the registered

holder, or sent to an address other than that shown on the front of the Non-Transferable Subscription Rights Certificate, please

complete Section 2 of the Non-Transferable Subscription Rights Certificate and obtain a signature guarantee as described below

prior to mailing the Non-Transferable Subscription Rights Certificate to the Subscription Agent, prior to the Expiration Date.

Payment of the Initial Price will be held in a segregated account to be maintained by the Subscription Agent.

Registered Holder. Your signature on the Non-Transferable Subscription Rights Certificate must correspond with the name of

the registered holder exactly as it appears on the face of the Non-Transferable Subscription Rights Certificate without any alteration

or change whatsoever. Persons who sign the Non-Transferable Subscription Rights Certificate in a representative or other fiduciary

capacity must indicate their capacity when signing and, unless waived by the Subscription Agent in its sole and absolute discretion,