CONTINGENT VALUE RIGHTS AGREEMENT

Exhibit 2.3

CONTINGENT VALUE RIGHTS AGREEMENT

THIS CONTINGENT VALUE RIGHTS AGREEMENT, dated as of [            ], 2014 (this “Agreement”), is entered into by and between Daiichi Sankyo Company, Limited, a Japanese corporation with its principal office at 3-5-1, Nihonbashi-honcho, Chuo-ku, Tokyo, Japan (“Parent”), and [                    ], as Rights Agent (together with Parent, the “Parties”).

RECITALS

WHEREAS, Parent, Charge Acquisition Corp., a Delaware corporation (“Sub”), and Ambit Biosciences Corporation, a Delaware corporation (the “Company”), have entered into an Agreement and Plan of Merger dated as of September 28, 2014 (as it may be amended or supplemented from time to time pursuant to the terms thereof, the “Merger Agreement”), pursuant to which Sub (a) has made a tender offer (the “Offer”) to acquire all of the outstanding shares of common stock, par value $0.001 per share, of the Company (“ Company Common Stock”) and (b) following acceptance of the shares of Company Common Stock pursuant to the Offer, will merge with and into the Company, with the Company surviving the Merger as a subsidiary of Parent;

WHEREAS, pursuant to the Merger Agreement, in each of the Offer and the Merger, Parent has agreed to provide to Company’s stockholders the right to receive contingent cash payments as hereinafter described; and

WHEREAS, pursuant to this Agreement, the maximum potential amount payable per CVR is $4.50.

NOW, THEREFORE, in consideration of the foregoing and the consummation of the transactions referred to above, Parent and Rights Agent agree, for the equal and proportionate benefit of all Holders (as hereinafter defined), as follows:

1. DEFINITIONS; CERTAIN RULES OF CONSTRUCTION

1.1. Definitions. Capitalized terms used but not otherwise defined herein will have the meanings ascribed to them in the Merger Agreement. As used in this Agreement, the following terms will have the following meanings:

“Acting Holders” means, at the time of determination, Holders of at least thirty-five percent (35%) of the outstanding CVRs.

“Affiliate” shall mean, with respect to any Person, any other Person that directly or indirectly controls, is controlled by, or is under common control with such Person; “control” means the ownership, directly or indirectly, of more than fifty percent (50%) of the voting securities entitled to vote for the directors (or similar officials) of a Person or the possession, by contract or otherwise, of the authority to direct the management and policies of a Person.

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“AML” means acute myeloid leukemia.

“Assignee” has the meaning set forth in Section 6.3.

“Board of Directors” means the board of directors of Parent.

“Board Resolution” means a copy of a resolution certified by the secretary or an assistant secretary of Parent to have been duly adopted by the Board of Directors and to be in full force and effect on the date of such certification, and delivered to the Rights Agent.

“Business Day” means any day other than a Saturday, Sunday or a day on which banking institutions in New York, New York or Tokyo, Japan are authorized or obligated by law or executive order to remain closed.

“Compound” shall mean the compound known as quizartinib or AC220 or any salt, ester, regioisomer, stereoisomer, solvate, hydrate, polymorph, co crystal, metabolite or pro-drug of such compound.

“CVRs” means the rights of Holders to receive contingent cash payments pursuant to the Merger Agreement and this Agreement.

“CVR Register” has the meaning set forth in Section 2.3(b).

“CVR Shortfall” has the meaning set forth in Section 4.5.

“Diligent Efforts” means, with respect to any Compound or Product, the level of effort, expertise and resources consistent with those efforts, expertise and resources that Parent and its Affiliates would apply, in the exercise of reasonable and good faith business judgment, to developing, seeking Marketing Authorization and making the first sale in the United States of America of a pharmaceutical product that is of similar market potential at a similar stage in its development or product life, taking into account issues of patent and regulatory exclusivity, product profile, including efficacy, safety, tolerability and convenience, the competitiveness of alternate products in the marketplace or under late-stage development, the launch or sales of a generic product containing the same or equivalent (by FDA standards) active pharmaceutical ingredient(s) as contained in such product sold by Parent or its Affiliate, the regulatory environment and the profitability of the product (including pricing and reimbursement status but excluding the payments under this Agreement) and other relevant considerations, including technical, commercial, legal, scientific and/or medical factors.

“DTC” means The Depository Trust Company or any successor thereto.

“FDA” means the United States Food and Drug Administration or any successor thereto.

“First Commercial Sale” means the first sale of a Product by or on behalf of Parent, its Affiliate or its Sublicensee (including without limitation its co-promotion partner or distributor)

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for use or consumption of such Product for any indication in the United States of America following the receipt of Marketing Authorization of such Product. Sale of a Product by Parent to an Affiliate of Parent or a Sublicensee of Parent shall not constitute a First Commercial Sale unless such Affiliate or such Sublicensee is the end user of the Product, but the subsequent sale of the Product by such Affiliate or such Sublicensee shall constitute a First Commercial Sale. In no event shall any sales for pre-marketing, testing or sampling purposes be deemed a First Commercial Sale.

“FLT3” means FMS-like tyrosine kinase 3.

“Front Line Milestone” means the First Commercial Sale in the United States of America following Marketing Authorization of a Product for remission induction in patients with AML, either alone or in combination with any other therapy, subject to

Section 2.4(b). For the avoidance of doubt, a Front Line Milestone can only be achieved if the Product’s Marketing Authorization does not require that a patient have received at least one (1) prior systemic therapy for AML.

“Governmental Entity” means any foreign or domestic arbitrator, court, nation, government, any state or other political subdivision thereof and an entity exercising executive, legislative, judicial regulatory or administrative functions of, or pertaining to, government.

“Holder” means a Person in whose name a CVR is registered in the CVR Register at the applicable time.

“Independent Accountant” means an independent certified public accounting firm of nationally recognized standing designated either (i) jointly by the Acting Holders and Parent, or (ii) if such parties fail to make a designation, jointly by an independent public accounting firm selected by Parent and an independent public accounting firm selected by the Acting Holders.

“Marketing Authorization” shall mean any approval required from the FDA to market and sell a product in the United States of America following submission of a new drug application for the product to the FDA.

“Milestone” shall mean the Front Line Milestone or the Second Line Milestone, and when plural it shall mean both the Front Line Milestone and the Second Line Milestone.

“Milestone Notice” has the meaning set forth in Section 2.4.

“Officer’s Certificate” means a certificate signed by the chief executive officer, president, chief financial officer, any vice president, the controller, the treasurer or the secretary, in each case of Parent, in his or her capacity as such an officer, and delivered to the Rights Agent.

“Party” means the Parent, Sub, Company or Rights Agent.

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“Payment Amount” means $2.25 per CVR upon achievement of the Front Line Milestone and $2.25 per CVR upon achievement of the Second Line Milestone; provided that the aggregate Payment Amount will not exceed $4.50 per CVR.

“Permitted Transfer” means: a transfer of CVRs (a) upon death of a Holder by will or intestacy; (b) pursuant to a court order; (c) by operation of law (including by consolidation or merger) or without consideration in connection with the dissolution, liquidation or termination of any corporation, limited liability company, partnership or other entity; (d) in the case of CVRs held in book-entry or other similar nominee form, from a nominee to a beneficial owner and, if applicable, through an intermediary, to the extent allowable by DTC; or (e) pursuant to Section 2.6.

“Person” shall mean any natural person, corporation, limited liability company, trust, unincorporated association, partnership, joint venture or other entity.

“Phase III Trial” shall mean the Phase III clinical trial of a Product for the treatment of relapsed or refractory FLT3-positive AML that is ongoing as of the date of this Agreement (referred to as the QUANTUM-R trial).

“Product” means any pharmaceutical preparation for human use containing or comprising the Compound, whether as the sole active pharmaceutical ingredient or in combination with one (1) or more other active pharmaceutical ingredients, in any dosage form or formulation, presentation and line extension and in any mode of administration.

“Rights Agent” means the Rights Agent named in the first paragraph of this Agreement, until a successor Rights Agent will have become such pursuant to the applicable provisions of this Agreement, and thereafter “Rights Agent” will mean such successor Rights Agent.

“Second Line Milestone” means the First Commercial Sale in the United States of America following Marketing Authorization of a Product for the treatment of relapsed or refractory FLT3-positive AML in patients who have received at least one (1) prior systemic therapy for AML, subject to Section 2.4(b).

“Sublicensee” shall mean an authorized or permitted licensee or sublicensee of rights to any Product.

1.2. Rules of Construction. Except as otherwise explicitly specified to the contrary, (a) references to a Section means a Section of this Agreement unless another agreement is specified, (b) the word “including” (in its various forms) means “including without limitation,” (c) references to a particular statute or regulation include all rules and regulations thereunder and any successor statute, rules or regulation, in each case as amended or otherwise modified from time to time, (d) words in the singular or plural form include the plural and singular form, respectively, (e) references to a particular Person include such Person’s successors and assigns to the extent not prohibited by this Agreement and (f) all references to dollars or “$” refer to United States dollars.

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2. CONTINGENT VALUE RIGHTS

2.1. CVRs. The CVRs represent the rights of Holders to receive contingent cash payments pursuant to this Agreement. The initial Holders will be determined pursuant to the terms of the Merger Agreement.

2.2. Nontransferable. The CVRs may not be sold, assigned, transferred, pledged, encumbered or in any other manner transferred or disposed of, in whole or in part, other than through a Permitted Transfer.

2.3. No Certificate; Registration; Registration of Transfer; Change of Address.

(a) The CVRs will not be evidenced by a certificate or other instrument.

(b) The Rights Agent will keep a register (the “CVR Register”) for the purpose of registering CVRs and transfers of CVRs as herein provided. The CVR Register will initially show one position for [                    ] representing all the shares of Company Common Stock held by DTC on behalf of the street holders of the shares of Company Common Stock tendered by such holders in the Offer or held by such holders as of immediately prior to the Effective Time. The Rights Agent will have no responsibility whatsoever directly to the street name holders with respect to transfers of CVRs. With respect to any payments to be made under Section 2.4 below, the Rights Agent will accomplish the payment to any former street name holders of shares of Company Common Stock by sending one lump payment to DTC. The Rights Agent will have no responsibilities whatsoever with regard to the distribution of payments by DTC to such street name holders.

(c) Subject to the restrictions on transferability set forth in Section 2.2, every request made to transfer a CVR must be in writing and accompanied by a written instrument of transfer in form reasonably satisfactory to the Rights Agent pursuant to its guidelines, duly executed by the Holder thereof, the Holder’s attorney duly authorized in writing, the Holder’s personal representative or the Holder’s survivor, and setting forth in reasonable detail the circumstances relating to the transfer. Upon receipt of such written notice, the Rights Agent will, subject to its reasonable determination that the transfer instrument is in proper form and the transfer otherwise complies with the other terms and conditions of this Agreement (including the provisions of Section 2.2), register the transfer of the CVRs in the CVR Register. Parent and Rights Agent may require payment of a sum sufficient to cover any stamp or other tax or governmental charge that is imposed in connection with any such registration of transfer. The Rights Agent shall have no duty or obligation to take any action under any section of this Agreement that requires the payment by a Holder of a CVR of applicable taxes or charges unless and until the Rights Agent is satisfied that all such taxes or charges have been paid. All duly transferred CVRs registered in

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the CVR Register will be the valid obligations of Parent and will entitle the transferee to the same benefits and rights under this Agreement as those held immediately prior to the transfer by the transferor. No transfer of a CVR will be valid until registered in the CVR Register.

(d) A Holder may make a written request to the Rights Agent to change such Holder’s address of record in the CVR Register. The written request must be duly executed by the Holder. Upon receipt of such written notice, the Rights Agent will, subject to its reasonable determination that the transfer instrument is in proper form, promptly record the change of address in the CVR Register.

2.4. Payment Procedures.

(a) Upon achievement of a Milestone, Parent will deliver to the Rights Agent a notice (the “Milestone Notice”) indicating the achievement of such Milestone along with any letter of instruction required by the Rights Agent.

(b) If the Front Line Milestone is achieved before the achievement of the Second Line Milestone, then the Second Line Milestone shall be deemed to be achieved either (i) upon receipt of first Marketing Authorization of a Product for the treatment of relapsed or refractory FLT3-positive AML in patients who have received at least one prior systemic therapy for AML if such Marketing Authorization has not occurred prior to achievement of the Front Line Milestone or (ii) concurrently with achievement of the Front Line Milestone if receipt of first Marketing Authorization of a Product for the treatment of relapsed or refractory FLT3-positive AML in patients who have received at least one prior systemic therapy for AML has occurred prior to achievement of the Front Line Milestone. If the Second Line Milestone is achieved before the achievement of the Front Line Milestone, then the Front Line Milestone shall be deemed to be achieved either (i) upon receipt of first Marketing Authorization of a Product for remission induction in patients with AML, either alone or in combination with any other therapy, if such Marketing Authorization has not occurred prior to achievement of the Second Line Milestone or (ii) concurrently with achievement of the Second Line Milestone if receipt of first Marketing Authorization of a Product for remission induction in patients with AML, either alone or in combination with any other therapy, has occurred prior to achievement of the Second Line Milestone.

(c) The Rights Agent will promptly, and in any event within ten (10) Business Days of receipt of a Milestone Notice as well as any letter of instruction required by the Rights Agent, send each Holder at its registered address a copy of such Milestone Notice. If a Payment Amount is payable to the Holders, then at the time the Rights Agent sends a copy of the Milestone Notice to the Holders, the Rights Agent will also pay the Payment Amount to each of the Holders (the amount to which each Holder is entitled to receive will be the Payment Amount multiplied by the number of CVRs held by such Holder as reflected on the CVR Register) by check mailed to the address of each Holder as reflected in the CVR Register as of the close of business on the date of the Milestone Notice.

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(d) The Payment Amounts shall be payable only once following the initial achievement of each Milestone (i.e. the Front Line Milestone or Second Line Milestone) and no amounts shall be due or payable for subsequent or repeated achievement of the same Milestone.

(e) Parent shall be entitled to deduct or withhold, or cause the Rights Agent to deduct or withhold, from any Payment Amount otherwise payable pursuant to this Agreement such amounts as may be required to be deducted or withheld therefrom under the Code, the Treasury Regulations thereunder, or any other applicable Tax Law, as may be determined by Parent or the Rights Agent. Prior to making any such Tax withholdings or causing any such Tax withholdings to be made with respect to any Holder, Parent shall instruct the Rights Agent to solicit from such Holder an IRS Form W-9 or other applicable Tax form within a reasonable amount of time in order to provide the opportunity for the Holder to provide any necessary Tax forms (including an IRS Form W-9 or an applicable IRS Form W-8) in order to avoid or reduce such withholding amounts. To the extent such amounts are so deducted or withheld, such amounts shall be treated for all purposes under this Agreement as having been paid to the person to whom such amounts would otherwise have been paid, and prior to the 15^th day of February in the year following any payment of such taxes by Parent or the Rights Agent, Parent shall deliver (or shall cause the Rights Agent to deliver) to the person to whom such amounts would otherwise have been paid the original Form 1099 or other reasonably acceptable evidence of such withholding.

(f) Any portion of any Payment Amount that remains undistributed to the Holders six (6) months after the date of the applicable Milestone Notice will be delivered by the Rights Agent to Parent, upon demand, and any Holder will thereafter look only to Parent for payment of such Payment Amount, without interest, but such Holder will have no greater rights against Parent than those accorded to general unsecured creditors of Parent under applicable law.

(g) Neither Parent nor the Rights Agent will be liable to any person in respect of any Payment Amount delivered to a public official pursuant to any applicable abandoned property, escheat or similar law. If, despite Parent’s and/or the Rights Agent’s commercially reasonable efforts to deliver a Payment Amount to the applicable Holder, such Payment Amount has not been paid immediately prior to the date on which such Payment Amount would otherwise escheat to or become the property of any Governmental Entity, any such Payment Amount will, to the extent permitted by applicable law, become the property of Parent, free and clear of all claims or interest of any person previously entitled thereto. In addition to and not in limitation of any other indemnity obligation herein, Parent agrees to indemnify and hold harmless Rights Agent with respect to any liability, penalty, cost or expense Rights Agent may incur or be subject to in connection with transferring such property to Parent.

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2.5. No Voting, Dividends or Interest; No Equity or Ownership Interest in Parent.

(a) The CVRs will not have any voting or dividend rights, and interest will not accrue on any amounts payable on the CVRs to any Holder.

(b) The CVRs will not represent any equity or ownership interest in Parent or in any constituent company to the Merger.

2.6. Ability to Abandon CVR. A Holder may at any time, at such Holder’s option, abandon all of such Holder’s remaining rights in a CVR by transferring such CVR to Parent without consideration therefor. Nothing in this Agreement is intended to prohibit Parent from offering to acquire CVRs for consideration in its sole discretion.

3. THE RIGHTS AGENT

3.1. Certain Duties and Responsibilities. The Rights Agent will not have any liability for any actions taken or not taken in connection with this Agreement, except to the extent of its willful misconduct, bad faith or gross negligence.

3.2. Certain Rights of Rights Agent. The Rights Agent undertakes to perform such duties and only such duties as are specifically set forth in this Agreement, and no implied covenants or obligations will be read into this Agreement against the Rights Agent. In addition:

(a) the Rights Agent may rely and will be protected and held harmless by Parent in acting or refraining from acting upon any resolution, certificate, statement, instrument, opinion, report, notice, request, direction, consent, order or other paper or document believed by it to be genuine and to have been signed or presented by the proper party or parties;

(b) whenever the Rights Agent will deem it desirable that a matter be proved or established prior to taking, suffering or omitting any action hereunder, the Rights Agent may rely upon an Officer’s Certificate, which certificate shall be full authorization and protection to the Rights Agent, and the Rights Agent shall, in the absence of bad faith, gross negligence or willful misconduct on its part, incur no liability and be held harmless by Parent for or in respect of any action taken, suffered or omitted to be taken by it under the provisions of this Agreement in reliance upon such certificate;

(c) the Rights Agent may engage and consult with counsel of its selection and the written advice of such counsel or any opinion of counsel will be full and complete authorization and protection and shall be held harmless by Parent in respect of any action taken, suffered or omitted by it hereunder in good faith and in reliance thereon;

(d) the permissive rights of the Rights Agent to do things enumerated in this Agreement will not be construed as a duty;

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(e) the Rights Agent will not be required to give any note or surety in respect of the execution of such powers or otherwise in respect of the premises;

(f) the Rights Agent shall not be liable for or by reason of, and shall be held harmless by Parent with respect to any of the statements of fact or recitals contained in this Agreement or be required to verify the same, but all such statements and recitals are and shall be deemed to have been made by the Parent only;

(g) the Rights Agent will have no liability and shall be held harmless by Parent in respect of the validity of this Agreement or the execution and delivery hereof (except the due execution and delivery hereof by the Rights Agent and the enforceability of this Agreement against the Rights Agent assuming the due execution and delivery hereof by Parent); nor shall it be responsible for any breach by the Parent of any covenant or condition contained in this Agreement;

(h) Parent agrees to indemnify Rights Agent for, and hold Rights Agent harmless against, any loss, liability, claim, demands, suits or expense arising out of or in connection with Rights Agent’s duties under this Agreement, including the costs and expenses of defending Rights Agent against any claims, charges, demands, suits or loss, unless such loss has been determined by a court of competent jurisdiction to be a result of Rights Agent’s gross negligence, bad faith or willful or intentional misconduct;

(i) Rights Agent shall not be liable for consequential damages under any provision of this Agreement or for any consequential damages arising out of any act or failure to act hereunder in the absence of bad faith, gross negligence or willful misconduct on its part;

(j) Parent agrees (i) to pay the fees and expenses of the Rights Agent in connection with this Agreement as agreed upon in writing by the Rights Agent and Parent on or prior to the date hereof, and (ii) to reimburse the Rights Agent for all taxes and governmental charges, reasonable expenses and other charges of any kind and nature incurred by the Rights Agent in the execution of this Agreement (other than taxes imposed on or measured by the Rights Agent’s net income and franchise or similar taxes imposed on it). The Rights Agent will also be entitled to reimbursement from Parent for all reasonable and necessary out-of-pocket expenses paid or incurred by it in connection with the administration by the Rights Agent of its duties hereunder; and

(k) No provision of this Agreement shall require the Rights Agent to expend or risk its own funds or otherwise incur any financial liability in the performance of any of its duties hereunder or in the exercise of its rights if there shall be reasonable grounds for believing that repayment of such funds or adequate indemnification against such risk or liability is not reasonably assured to it.

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3.3. Resignation and Removal; Appointment of Successor.

(a) The Rights Agent may resign at any time by giving written notice thereof to Parent specifying a date when such resignation will take effect, which notice will be sent at least sixty (60) days prior to the date so specified but in no event will such resignation become effective until a successor Rights Agent has been appointed. Parent has the right to remove Rights Agent at any time by notice specifying a date when such removal will take effect but no such removal will become effective until a successor Rights Agent has been appointed. Such notice of removal will be given by Parent to Rights Agent, which notice will be sent at least 60 days prior to the date so specified.

(b) If the Rights Agent provides notice of its intent to resign, is removed or becomes incapable of acting, Parent, by a Board Resolution, will as soon as is reasonably possible appoint a qualified successor Rights Agent who, unless otherwise consented to in writing by the Holders of not less than a majority of the outstanding CVRs, shall be a stock transfer agent of national reputation or the corporate trust department of a commercial bank. The successor Rights Agent so appointed will, forthwith upon its acceptance of such appointment in accordance with Section 3.4, become the successor Rights Agent.

(c) Parent will give notice of each resignation and each removal of a Rights Agent and each appointment of a successor Rights Agent by mailing written notice of such event by first-class mail to the Holders as their names and addresses appear in the CVR Register. Each notice will include the name and address of the successor Rights Agent. If Parent fails to send such notice within ten days after acceptance of appointment by a successor Rights Agent, the successor Rights Agent will cause the notice to be mailed at the expense of Parent.

3.4. Acceptance of Appointment by Successor. Every successor Rights Agent appointed hereunder will execute, acknowledge and deliver to Parent and to the retiring Rights Agent an instrument accepting such appointment and a counterpart of this Agreement, and thereupon such successor Rights Agent, without any further act, deed or conveyance, will become vested with all the rights, powers, trusts and duties of the retiring Rights Agent. On request of Parent or the successor Rights Agent, the retiring Rights Agent will execute and deliver an instrument transferring to the successor Rights Agent all the rights, powers and trusts of the retiring Rights Agent.

4. COVENANTS

4.1. List of Holders. Parent will furnish or cause to be furnished to the Rights Agent in such form as Parent receives from the Company’s transfer agent (or other agent performing similar services for the Company), the names and addresses of the Holders within ten (10) Business Days of the Effective Time.

4.2. Payment of Payment Amount. If a Milestone has been achieved in accordance with this Agreement, Parent will, promptly following the delivery of the Milestone Notice for such Milestone, deposit with the Rights Agent, for payment to the Holders in accordance with

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Section 2.4, the aggregate amount necessary to pay the applicable Payment Amount to all Holders. For the avoidance of doubt, the Payment Amount shall only be paid, if at all, for the specific Milestone(s) achieved (or deemed to be achieved pursuant to Section 2.4(b)): the Front Line Milestone, the Second Line Milestone or both.

4.3. Diligent Efforts. Parent shall, and shall cause its Affiliates and any Sublicensees, to, use Diligent Efforts to achieve the Milestones. Without limiting the foregoing, Parent shall, and shall cause its Affiliates and any Sublicensees, as applicable, to apply Diligent Efforts to conduct the Phase III Trial.

4.4 Books and Records. Parent shall, and shall cause its Affiliates to, keep true, complete and accurate records in sufficient detail to enable the Holders and their consultants or professional advisors to confirm the applicable Payment Amount payable hereunder in accordance with the terms specified in this Agreement.

4.5 Audits.

(a) Upon the written request of the Acting Holders provided to Parent not less than forty-five (45) days in advance (such request not be made more than once in any twelve (12) month period), Parent shall permit, and shall cause its Affiliates to permit, the Independent Accountant to have access during normal business hours to such of the records of Parent, the Company or such other Affiliates or Sublicensees of Parent as may be reasonably necessary to determine whether a Milestone has been achieved. Parent shall, and shall cause to its Affiliates to, furnish to the Independent Accountant such access, work papers and other documents and information reasonably necessary for the Independent Accountant’s evaluation of the occurrence of a Milestone; provided that Parent may, and may cause its Affiliates to, redact documents and information not relevant for such evaluation pursuant to this Section 4.5. as the Independent Accountant may request and as are available to Parent. The Independent Accountant shall disclose to Parent and the Acting Holders any matters directly related to its findings to the extent necessary to verify the achievement of the applicable Milestone(s).

(b) If the Independent Accountant concludes that a Payment Amount that was properly due was not paid to the Holders, Parent shall pay to the Rights Agent (for further distribution to the Holders) or to each Holder the applicable Payment Amount, plus interest on such Payment Amount at the “prime rate” as published in the Wall Street Journal or similar reputable data source from time to time calculated from when the Payment Amount should have been paid (if Parent had given notice of achievement of the applicable Milestone(s) following its actual achievement, as determined by the Independent Accountant, at the time required pursuant to the terms of this Agreement), as applicable, to the date of actual payment (such amount including interest being the “CVR Shortfall”). The CVR Shortfall shall be paid within ten (10) days of the date the Independent Accountant delivers to Parent and the Acting Holders the Independent Accountant’s written report. The decision of the Independent Accountant shall be

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final, conclusive and binding on Parent and the Holders, shall be non-appealable and shall not be subject to further review. The fees charged by the Independent Accountant shall be paid by the Acting Holders; provided, however, that if the Independent Accountant concludes that a Payment Amount that was properly due was not paid to the Holders, the fees charged by such Independent Accountant shall be paid by Parent.

(c) Each person seeking to receive information from Parent in connection with a review pursuant to this Section 4.5 shall enter into, and shall cause its accounting firm to enter into, a reasonable and mutually satisfactory confidentiality agreement with Parent or any controlled Affiliate obligating such party to retain all such information disclosed to such party in confidence pursuant to such confidentiality agreement.

(d) Parent shall not, and shall cause its Affiliates not to, enter into any license or distribution agreement with any third party (other than Parent or its Affiliates) with respect to a Product unless such agreement contains provisions that would allow any Independent Accountant appointed pursuant to this Section 4.5 such access to the records of the other party to such license or distribution agreement as may be reasonably necessary to perform its duties pursuant to this Section 4.5; provided that (i) Parent and its Affiliates shall not be required to amend any of its existing licenses and (ii) Parent and its Affiliates may redact documents and information not relevant for such evaluation pursuant to this Section 4.5. The parties hereto agree that, if Parent or its Affiliates have exercised audit rights under any license or distribution agreement prior to the Acting Holders’ request for an audit under this Section 4.5 and under such license or distribution agreement Parent and its Affiliates cannot request another audit, the results of Parent’s prior audit of such licensee or distributor shall be used for purposes of the audit requested by the Acting Holders under this Section 4.5 and that Parent shall not have any further obligation to provide access to an Independent Accountant with respect to such licensee until such time as Parent may again exercise its rights of audit under the license agreement with such licensee.

5. AMENDMENTS

5.1. Amendments without Consent of Holders.

(a) Without the consent of any Holders or the Rights Agent, Parent, when authorized by a Board Resolution, at any time and from time to time, may enter into one or more amendments hereto, to evidence the succession of another Person to Parent and the assumption by any such successor of the covenants of Parent herein as provided in Section 6.3.

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(b) Without the consent of any Holders, Parent, when authorized by a Board Resolution, and the Rights Agent, in the Rights Agent’s sole and absolute discretion, at any time and from time to time, may enter into one or more amendments hereto, for any of the following purposes:

(i) to evidence the succession of another Person as a successor Rights Agent and the assumption by any such successor of the covenants and obligations of the Rights Agent herein;

(ii) to add to the covenants of Parent such further covenants, restrictions, conditions or provisions as Parent and the Rights Agent will consider to be for the protection of the Holders; provided that, in each case, such provisions do not adversely affect the interests of the Holders;

(iii) to cure any ambiguity, to correct or supplement any provision herein that may be defective or inconsistent with any other provision herein, or to make any other provisions with respect to matters or questions arising under this Agreement; provided that, in each case, such provisions do not adversely affect the interests of the Holders;

(iv) as may be necessary or appropriate to ensure that the CVRs are not subject to registration under the Securities Act or the Exchange Act;

(v) to reduce the number of CVRs, in the event any Holder agrees to renounce such Holder’s rights under this Agreement in accordance with Section 6.4 or to transfer such CVRs to Parent pursuant to Section 2.6; or

(vi) any other amendments hereto for the purpose of adding, eliminating or changing any provisions of this Agreement, unless such addition, elimination or change is adverse to the interests of the Holders.

(c) Promptly after the execution by Parent and the Rights Agent of any amendment pursuant to the provisions of this Section 5.1, Parent will mail (or cause the Rights Agent to mail) a notice thereof by first class mail to the Holders at their addresses as they appear on the CVR Register, setting forth such amendment.

5.2. Amendments with Consent of Holders.

(a) Subject to Section 5.1 (which amendments pursuant to Section 5.1 may be made without the consent of the Holders), with the consent of the Holders of not less than a majority of the outstanding CVRs, whether evidenced in writing or taken at a meeting of the Holders, Parent, when authorized by a Board Resolution, and the Rights Agent may enter into one or more amendments hereto for the purpose of adding, eliminating or changing any provisions of this Agreement, even if such addition, elimination or change is materially adverse to the interest of the Holders; provided, however, that no such amendment shall, without the consent of the Holders of sixty six (66%) percent of the outstanding CVRs:

(i) modify in a manner adverse to the Holders (A) any provision contained herein with respect to the termination of this Agreement or the CVRs, (B) the time for, and amount of, any payment to be made to the Holders pursuant to this Agreement, or (C) the definition of Milestone, Front Line Milestone or Second Line Milestone;

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(ii) reduce the number of CVRs (except as provided in Section 5.1(b)(v)), or

(iii) modify any provisions of this Section 5.2, except to increase the percentage of Holders from whom consent is required or to provide that certain provisions of this Agreement cannot be modified or waived without the consent of the Holder of each outstanding CVR affected thereby.

(b) Promptly after the execution by Parent and the Rights Agent of any amendment pursuant to the provisions of this Section 5.2, Parent will mail (or cause the Rights Agent to mail) a notice thereof by first class mail to the Holders at their addresses as they appear on the CVR Register, setting forth such amendment.

5.3. Execution of Amendments. In executing any amendment permitted by this Section 5, the Rights Agent will be entitled to receive, and will be fully protected in relying upon, an opinion of counsel selected by Parent stating that the execution of such amendment is authorized or permitted by this Agreement. The Rights Agent may, but is not obligated to, enter into any such amendment that affects the Rights Agent’s own rights, privileges, covenants or duties under this Agreement or otherwise.

5.4. Effect of Amendments. Upon the execution of any amendment under this Section 5, this Agreement will be modified in accordance therewith, such amendment will form a part of this Agreement for all purposes and every Holder will be bound thereby.

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6. OTHER PROVISIONS OF GENERAL APPLICATION

6.1. Notices to Rights Agent and Parent. Any notice or other communication required or permitted hereunder shall be in writing and shall be deemed given when delivered in person, by overnight courier, by facsimile transmission (with receipt confirmed by telephone or by automatic transmission report) or by electronic mail, or two (2) business days after being sent by registered or certified mail (postage prepaid, return receipt requested), as follows:

If to the Rights Agent, to it at:

With a copy to:

If to Parent, to it at:

Yuichi Kubo

Vice President, Business Development & Licensing

Daiichi Sankyo Company, Limited

Daiichi Sankyo Building A

3-5-1, Nihonbashi-honcho, Chuo-ku, Tokyo 103-8426, Japan

Email: ***@***

Yoshihiro Aoyagi

General Counsel

Vice President, Legal Affairs

Daiichi Sankyo Company, Limited

Daiichi Sankyo Building A

3-5-1, Nihonbashi-honcho, Chuo-ku, Tokyo 103-8426, Japan

Email: ***@***

with a copy to (which shall not constitute notice):

Simpson Thacher & Bartlett LLP

425 Lexington Avenue

New York, NY 10017

Attention: Gary I. Horowitz and Patrick J. Naughton

Email: ***@***; ***@***

The Rights Agent or Parent may specify a different address or facsimile number by giving notice in accordance with this Section 6.1.

6.2. Notice to Holders. Where this Agreement provides for notice to Holders, such notice will be sufficiently given (unless otherwise herein expressly provided) if in writing and mailed, first-class postage prepaid, to each Holder affected by such event, at the Holder’s address as it appears in the CVR Register, not later than the latest date, and not earlier than the earliest date, if any, prescribed for the giving of such notice. In any case where notice to Holders is given by mail, neither the failure to mail such notice, nor any defect in any notice so mailed, to any particular Holder will affect the sufficiency of such notice with respect to other Holders.

6.3. Parent Successors and Assigns. Parent may assign, in its sole discretion and without the consent of any other party, any or all of its rights, interests and obligations hereunder to one or more direct or indirect wholly-owned subsidiaries of Parent or to any purchaser or licensee of substantial rights to the Product (each, an “Assignee”) provided that the Assignee agrees to assume and be bound by all of the terms of this Agreement. Any such Assignee may thereafter assign, in its sole discretion and without the consent of any other party, any or all of its rights, interests and obligations hereunder to one or more additional Assignees who agree to assume and be bound by all of the terms of this Agreement; provided, however, that in

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connection with any assignment to an Assignee, Parent (or the other assignor) shall agree to remain liable for the performance by each Assignee (and such other assignor, if applicable) of all obligations of Parent and Sub hereunder, with such Assignee substituted for Parent and Sub under this Agreement. This Agreement will be binding upon, inure to the benefit of and be enforceable by Parent’s successors and each Assignee, and this Agreement shall not restrict Parent’s, any Assignee’s or any of their respective successor’s ability to merge or consolidate. Each of Parent’s successors and assigns shall expressly assume by an instrument supplemental hereto, executed and delivered to the Rights Agent, the due and punctual payment of the CVRs and the due and punctual performance and observance of all of the covenants and obligations of this Agreement to be performed or observed by Parent.

6.4. Benefits of Agreement. Nothing in this Agreement, express or implied, will give to any Person (other than the Rights Agent, Parent, Parent’s successors and assignees, and the Holders) any benefit or any legal or equitable right, remedy or claim under this Agreement or under any covenant or provision herein contained, all such covenants and provisions being for the sole benefit of the, Rights Agent, Parent, Parent’s successors and assignees, and the Holders. The rights of Holders are limited to those expressly provided in this Agreement and the Merger Agreement. Notwithstanding anything to the contrary contained herein, any Holder may agree to renounce, in whole or in part, such Holder’s rights under this Agreement by written notice to the Rights Agent and Parent, which notice, if given, shall be irrevocable.

6.5. Severability. If any provision of this Agreement is held invalid or unenforceable by any court of competent jurisdiction, the other provisions of this Agreement shall remain in full force and effect. Any provision of this Agreement held invalid or unenforceable only in part or degree shall remain in full force and effect to the extent not held invalid or unenforceable. The parties further agree to replace such invalid or unenforceable provision of this Agreement with a valid and enforceable provision that will achieve, to the extent possible, the economic, business and other purposes of such invalid or unenforceable provision.

6.6. Counterparts and Signature. This Agreement may be executed in two or more counterparts (including by facsimile or by an electronic scan delivered by electronic mail), each of which shall be deemed an original but all of which together shall be considered one and the same agreement and shall become effective when counterparts have been signed by each of the parties hereto and delivered to the other party, it being understood that the parties need not sign the same counterpart.

6.7. Termination. This Agreement will be terminated and of no force or effect, the parties hereto will have no liability hereunder (other than with respect to monies due and owing by Parent to Rights Agent), and no payments will be required to be made, upon the earlier of (a) following the mailing by the Rights Agent to the address of each Holder as reflected in the CVR Register the full amount of all potential Payment Amounts required to be paid under the terms of this Agreement and (b) ten years from the date hereof.

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6.8. Entire Agreement. This Agreement contains the entire understanding of the parties hereto and thereto with reference to the transactions and matters contemplated hereby and thereby and supersedes all prior agreements, written or oral, among the parties with respect hereto and thereto. If and to the extent that any provision of this Agreement is inconsistent or conflicts with the Merger Agreement, this Agreement will govern and be controlling.

6.9. Applicable Legal Requirements; Jurisdiction.

(a) This Agreement shall be governed by, and construed in accordance with, the laws of the State of Delaware, regardless of the laws that might otherwise govern under applicable principles of conflicts of laws thereof. Subject to Section 6.9(b), in any action or proceeding arising out of or relating to this Agreement, its negotiation or any of the transactions contemplated by this Agreement: (i) each of the Parties irrevocably and unconditionally consents and submits to the exclusive jurisdiction and venue of the Chancery Court of the State of Delaware and any state appellate court therefrom or, if such court lacks subject matter jurisdiction, the United States District Court sitting in New Castle County in the State of Delaware, (it being agreed that the consents to jurisdiction and venue set forth in this Section 6.9(a) shall not constitute general consents to service of process in the State of Delaware and shall have no effect for any purpose except as provided in this paragraph and shall not be deemed to confer rights on any Person other than the Parties hereto); and (ii) each of the Parties irrevocably consents to service of process by first class certified mail, return receipt requested, postage prepaid, to the address at which such Party is to receive notice in accordance with Section 6.1. The Parties hereto agree that a final judgment in any such action or proceeding shall be conclusive and may be enforced in other jurisdictions by suit on the judgment or in any other manner provided by applicable Legal Requirements; provided, however, that nothing in the foregoing shall restrict any Party’s rights to seek any post-judgment relief regarding, or any appeal from, such final trial court judgment.

(b) EACH OF THE PARTIES HERETO IRREVOCABLY WAIVES ANY AND ALL RIGHT TO TRIAL BY JURY IN ANY LEGAL PROCEEDING BETWEEN THE PARTIES HERETO ARISING OUT OF OR RELATING TO THIS AGREEMENT OR THE TRANSACTIONS CONTEMPLATED HEREBY.

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IN WITNESS WHEREOF, each of the parties has caused this Agreement to be executed on its behalf by its duly authorized officers as of the day and year first above written.

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