Form of Class B Preferred Share Purchase Warrants

In the event of a change, subdivision, redivision, reclassification, reduction, combination or consolidation thereof or any other adjustment under Section 6 or Section 10, or successive changes, subdivisions, redivisions, reclassifications, reductions, combinations, consolidations or other adjustments, then subject to the adjustments, if any, having been made in accordance with the provisions of this Warrant Certificate, “Shares” shall thereafter mean the shares, other securities or other property resulting from such change, subdivision, redivision, reclassification, reduction, combination, consolidation or other adjustment.

At any time, or from time to time, on or prior to the Expiry Time (the “Exercise Period”), the Holder may exercise all or any number of whole Warrants represented hereby, upon delivering to the Corporation at its principal office above (or such other address as may be notified in writing by the Corporation) a duly completed and executed exercise notice in the form attached hereto (the “Exercise Notice”) evidencing the election (which on delivery to the Corporation shall be irrevocable) of the Holder to exercise the number of Warrants set forth in the Exercise Notice (which shall not be greater than the number of Warrants represented by this Warrant Certificate, as adjusted from time to time in accordance with this Warrant Certificate) and a certified cheque or bank draft payable to the Corporation for the aggregate Exercise Price of all Warrants being exercised. If the Holder is not exercising all Warrants represented by this Warrant Certificate, the Holder shall be entitled to receive, without charge, a certificate representing the number of Warrants which is the difference between the number of Warrants represented by the then original Warrant Certificate and the number of Warrants being so exercised (the “Replacement Certificate”). For greater clarity, the Replacement Certificate shall have the same Expiry Time and Exercise Price as this original Warrant Certificate.

The Holder shall be deemed to have become the holder of record of Shares on the date (the “Exercise Date”) on which the Corporation has received both a duly completed Exercise Notice and payment in full in respect of the Shares (including all applicable taxes) being subscribed for in connection with such Exercise Notice; provided, however, that if such date is not a business day in the City of Toronto, Ontario (a “Business Day”), then the Shares shall be deemed to have been issued and the Holder shall be deemed to have become the holder of record of the Shares on the next following Business Day. Notwithstanding the foregoing, as a condition to receiving any Shares upon the exercise of the Warrants and if not already a party thereto, the Holder shall become a party to, and be bound by, the Shareholders Agreements by signing an adoption agreement to each of the Shareholders Agreements. The Corporation shall promptly issue and deliver (or cause to be delivered) to the Holder, by registered mail to his, her or its address specified in such Exercise Notice, one or more certificates for the appropriate number of issued and outstanding Shares.

Notwithstanding Sections 2 and 3, in lieu of exercising the Warrants pursuant to Section 2, the Holder may elect to receive, without payment by the Holder of any additional consideration, Shares equal to the value of this Warrant Certificate (or the portion thereof being cancelled) by surrender of this Warrant Certificate at the principal office of the Corporation above (or such other address in Canada as may be notified in writing by the Corporation), together with notice of such election, in which event the Corporation shall issue to the Holder hereof a number of Shares computed using the following formula:

The Corporation covenants and agrees that, until the Expiry Time, while any of the Warrants represented by this Warrant Certificate shall be outstanding, it shall reserve and there shall remain unissued out of its authorized capital a sufficient number of Shares to satisfy the right of purchase herein provided, as such right of purchase may be adjusted pursuant to Sections 6 and 7. All Shares which shall be issued upon the exercise of the right to purchase herein provided for, upon payment therefor of the amount at which such Shares may at the time be purchased pursuant to the provisions hereof, shall be issued as fully paid and non-assessable shares and the holders thereof shall not be liable to the Corporation or its creditors in respect thereof.

The Exercise Price (and the number of Shares purchasable upon exercise in the case of Sections 6(a), 6(b) and 6(c)) shall be subject to adjustment from time to time in the events and in the manner provided as follows:

Share Reorganization. If at any time on or after the date hereof and at or prior to the Expiry Time (the “Adjustment Period”) the Corporation shall:

subdivide, redivide or change its outstanding Shares into a greater number of Shares, or

consolidate, reduce or combine its outstanding Shares into a lesser number of Shares,

Special Distribution. If and whenever during the Adjustment Period the Corporation shall issue or distribute to all or to substantially all the holders of the Shares:

securities of the Corporation including shares, rights, options or warrants to acquire shares of any class or securities exchangeable for or convertible into or exchangeable into any such shares or cash, or property or assets and including evidences of its indebtedness, or

any property or other assets,

Capital Reorganization. If and whenever during the Adjustment Period there shall be a reclassification of Shares at any time outstanding or a change of the Shares into other shares or into other securities (other than a Share Reorganization), or a consolidation, amalgamation, arrangement or merger of the Corporation with or into any other corporation or other entity (other than a consolidation, amalgamation, arrangement or merger which does not result in any reclassification of the outstanding Shares or a change of the Shares into other securities), or a transfer of the undertaking or assets of the Corporation as an entirety or substantially as an entirety to another corporation or other entity (any of such events being herein called a “Capital Reorganization”), the Holder, where he has not exercised the right of subscription and purchase under this Warrant Certificate prior to the effective date or record date, as the case may be, of such Capital Reorganization, shall be entitled to receive, and shall accept upon the exercise of such right for the same aggregate consideration, in lieu of the number of Shares to which such Holder was theretofore entitled upon such exercise, the aggregate number of shares, other securities or other property which such Holder would have been entitled to receive as a result of such Capital Reorganization if, on the effective

If and whenever at any time after the date hereof and prior to the Expiry Time, the Corporation takes any action affecting its Shares to which the foregoing provisions of this Section 6, in the opinion of the board of directors of the Corporation, acting reasonably and in good faith, are not strictly applicable, or if strictly applicable would not fairly adjust the rights of the Holder against dilution in accordance with the intent and purposes thereof, or would otherwise materially affect the rights of the Holder hereunder, then the Corporation shall, subject to any requisite regulatory approval, execute and deliver to the Holder an amendment hereto providing for an adjustment in the application of such provisions so as to adjust such rights as aforesaid in such a manner as the board of directors of the Corporation may determine to be equitable in the circumstances, acting reasonably and in good faith.

The following rules and procedures shall be applicable to the adjustments made pursuant to Section 6:

The adjustments provided for in Section 6 are cumulative, and shall, in the case of adjustments to the Exercise Price be computed to the nearest one-tenth of one cent and shall be made successively whenever an event referred to therein shall occur, subject to the following paragraphs of this Section 7.

No adjustment in the Exercise Price shall be required unless such adjustment would result in a change of at least 2.5% in the prevailing Exercise Price and no adjustment shall be made in the number of Shares purchasable upon exercise of this Warrant unless it would result in a change of at least one one-hundredth of a Share; provided, however, that any adjustments which, except for the provisions of this Section 7(b) would otherwise have been required to be made, shall be carried forward and taken into account in any subsequent adjustment.

No adjustment in the Exercise Price or in the number of Shares purchasable upon exercise of Warrants shall be made in respect of any event described in Section 6, other than the events referred to in Sections 6(a)(i) and (ii), if the Holder is entitled to participate in such event on the same terms, mutatis mutandis, as if it had exercised its Warrants prior to or on the effective date or record date of such event. For greater certainty the right to participate in a financing through the exercise of a pre-emptive right or otherwise, shall not be considered the right to participate in such event as outlined in the preceding sentence.

For greater certainty, no adjustment in the Exercise Price shall be made pursuant to Section 6 in respect of the issue from time to time:

of Shares purchasable on exercise of the Warrants represented by this Warrant Certificate;

of dividends paid in the ordinary course of Shares to holders of Shares who exercise an option or election to receive substantially equivalent dividends in Shares in lieu of receiving a cash dividend pursuant to a dividend reinvestment plan or similar plan adopted by the Corporation in accordance with applicable securities laws; or

of Shares pursuant to any option, share option plan, share purchase plan or other benefit plan in force at the date hereof for directors, officers, employees, advisers or consultants of the Corporation, as such option or plan is amended or superseded from time to time in accordance with applicable securities laws, and such other benefit plans as may be adopted by the Corporation in accordance with applicable securities laws;

If the Corporation shall set a record date to determine the holders of the Shares for the purpose of entitling them to receive any dividend or distribution or any subscription or purchase rights and shall, thereafter and before the distribution to such shareholders of any such dividend, distribution or subscription or purchase rights, legally abandon its plan to pay or deliver such dividend, distribution or subscription or purchase rights, then no adjustment in the Exercise Price or the number of Shares purchasable upon exercise of any Warrant shall be required by reason of the setting of such record date.

As a condition precedent to the taking of any action which would require any adjustment in any of the subscription rights pursuant to this Warrant Certificate, including the Exercise Price and the number or class of shares or other securities which are to be received upon the exercise thereof, the Corporation shall take any corporate action which may be necessary in order that the Corporation have unissued and reserved in its authorized capital and may validly and legally issue as fully paid and non-assessable all the shares or other securities which the holder of such Warrant Certificate is entitled to receive on the full exercise thereof in accordance with the provisions hereof.

For the purposes of this Warrant Certificate, “Current Fair Market Value” of a Share is (i) in the case of a Liquidation Event or a Share Sale, the price payable per Share in connection with such Liquidation Event or Share Sale; or (ii) otherwise, the price per Share determined by the board of directors of the Corporation in good faith.

In the absence of a resolution of the board of directors of the Corporation fixing a record date for a Special Distribution, the Corporation shall be deemed to have fixed as the record date therefor the date on which such distribution is effected.

Any question that at any time or from time to time arises with respect to the amount of any adjustment to the Exercise Price or other adjustments pursuant to Section 6 shall be conclusively determined by a firm of independent chartered accountants (who may be the Corporation’s auditors) and shall be binding upon the Corporation and the Holder. In the event that any such determination is made, the Corporation shall notify the Holder in the manner contemplated in Section 21 describing such determination.

On the happening of each and every such event set out in Section 6, the applicable provisions of this Warrant Certificate, including the Exercise Price, shall, ipso facto, be deemed to be amended accordingly and the Corporation shall take all necessary action so as to comply with such provisions as so amended.

In any case in which Section 6 shall require that an adjustment shall be effective immediately after a record date for an event referred to herein, the Corporation may defer, until the occurrence of such an event:

issuing to the holder of any Warrant exercised after such record date and before the occurrence of such event, the additional Shares issuable upon such exercise by reason of the adjustment required by such event, and

delivering to such holder any distributions declared with respect to such additional Shares after such record date and before such event;

If the Holder becomes a Defaulting Purchaser pursuant to the terms of the Subscription Agreement at any time following an Exercise Date, (i) such Holder’s Shares, including any shares in the capital of the Corporation issued upon the conversion or exchange of such Shares (the “Default Shares”) shall automatically convert into Non-Voting Common Shares in the capital of the Corporation (the “Non-Voting Common Shares”), with no further act or formality on the part of the Defaulting Purchaser on a 10-to-1 basis, rounded down to the nearest whole Non-Voting Common Share (as adjusted for any dividends, reclassification, exchange, substitution, or other event that results in a change of the number and/or class of the applicable Default Shares); and (ii) such Defaulting Purchaser shall lose its rights associated with the ownership of any Shares as set forth in the Shareholders Agreements and the Share Exchange and Voting Agreement (as defined in the Subscription Agreement).

At least 10 Business Days prior to: (a) a Liquidation Event; (b) a Share Sale; or (c) the effective date or record date, as the case may be, of any event which requires or might require adjustment in any of the subscription rights pursuant to this Warrant Certificate, including the Exercise Price and the number of Shares which are purchasable upon the exercise thereof, the Corporation shall notify the Holder of the particulars of such event and, if determinable, the required adjustment and the computation of such adjustment. In case any adjustment for which such notice has been given is not then determinable, the Corporation shall promptly after such adjustment is determinable notify the Holder of the adjustment and the computation of such adjustment.

The Corporation shall maintain at its principal office: (a) a register of holders in which shall be entered the names and addresses of the holders of the Warrants and of the number of Warrants held by them; and (b) a register of transfers in which shall be entered the date and other particulars of each transfer of Warrants. Such registers shall be open at all reasonable times for

The Corporation shall not be required to issue fractional Shares in satisfaction of its obligations hereunder. If any fractional interest in a Share would, except for the provisions of this Section 13, be deliverable upon the exercise of a Warrant, the Corporation shall in lieu of delivering the fractional Shares therefor satisfy the right to receive such fractional interest by payment to the holder of such Warrant of an amount in cash equal (computed in the case of a fraction of a cent to the next lower cent) to the value of the right to acquire such fractional interest on the basis of the Current Fair Market Value at the Exercise Date.

Except as herein expressly provided, all or any of the rights conferred upon the Holder by the terms hereof may be enforced by the Holder by appropriate legal proceedings.

The Holder may at any time up to and including the Expiry Time upon the surrender hereof to the Corporation at its principal office exchange this Warrant Certificate for one or more Warrant Certificates entitling the Holder to subscribe in the aggregate for the same number of Shares as is expressed in this Warrant Certificate. Any Warrant Certificate tendered for exchange shall be surrendered to the Corporation and cancelled.

If this Warrant Certificate becomes stolen, lost, mutilated or destroyed, the Corporation shall, on such terms as it may in its discretion acting reasonably impose, issue and deliver to the Holder a new Warrant Certificate of like denomination, tenor and date as the Warrant Certificate so stolen, lost, mutilated or destroyed.

This Warrant Certificate, the Warrants represented hereby and the Shares issuable upon exercise of the Warrants or any interest therein or portion thereof shall be transferable by the registered holder hereof subject to any restriction under the articles of the Corporation, the terms and conditions of the Shareholders Agreements (as if the Warrants were fully-exercised) and any applicable law (including applicable hold periods).

The transferee of this Warrant Certificate shall, after the Transfer Form attached hereto or any other form of transfer acceptable to the Corporation, acting reasonably, is duly completed and the Warrant Certificate is lodged with the Corporation and upon compliance with all other conditions in that regard required by this Warrant Certificate or by law, be entitled to have its name entered on the register of holders as the owner of the Warrants represented thereby free from all equities or rights of set-off or counterclaim between the Corporation and the transferor or any previous holder of such Warrant, save in respect of equities of which the Corporation or the transferee is required to take notice by statute or by order of a court of competent jurisdiction. The costs of any such transfer registration shall be borne by the Corporation.

Nothing contained herein shall confer any right upon the Holder or any other person to subscribe for or purchase any shares of the Corporation or any securities of any other corporation at any time subsequent to the Expiry Time. After the Expiry Time this Warrant Certificate and all rights hereunder shall be void and of no effect.

Except as expressly set out herein, the holding of this Warrant Certificate or the Warrants represented hereby shall not constitute a holder hereof a holder of Shares nor entitle it to any right of interest in respect thereof.

Unless herein otherwise expressly provided, any notice to be given hereunder to the Holder shall be deemed to be validly given if such notice is given by personal delivery or registered mail to the attention of the Holder at its registered address recorded in the registers maintained by the Corporation. Any notice so given shall be deemed to be validly given, if delivered personally, on the day of delivery and if sent by post or other means, on the fifth Business Day next following the sending thereof. In determining under any provision hereof the date when notice of any event must be given, the date of giving notice shall be included and the date of the event shall be excluded. All costs in connection with the giving of notices contemplated by this Warrant Certificate shall be borne by the Corporation.

Time is of the essence hereof.

This Warrant Certificate is binding upon the Corporation and its successors.

This Warrant Certificate and the Warrants represented hereby shall be governed by and construed in accordance with laws of the State of New York.

subscribes for                                      Shares (as defined in the Warrant Certificate) (or such number of other securities or property to which such subscription entitles the undersigned in lieu thereof or in addition thereto under the Warrant Certificate) of FUSION PHARMACEUTICALS INC. at the price per share in United States dollars equal to US$1.5154 (or such adjusted price which may be in effect under the provisions of the Warrant Certificate) and <circle one>:

in payment of the exercise price encloses a certified cheque in lawful money of the United States of America payable to the order of FUSION PHARMACEUTICALS INC. or its successor corporation; or

The undersigned wishes to exercise its Warrants pursuant to Section 4 and the number above reflects the net exercise of warrants to purchase                                          Shares at a Fair Market Value of US$                             per Share

delivers herewith the above-mentioned Warrant Certificate entitling the undersigned to subscribe for the above-mentioned number of Shares.

Address(es)

(including Postal Code)

Number(s)
of Shares