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Exhibit 10.24    
    

 
 

OPTIMER PHARMACEUTICALS, INC.
  EMPLOYEE STOCK PURCHASE PLAN    
    

        1.    Purpose.    The purpose of the Plan is to provide employees of the Company and its Designated Subsidiaries with
an opportunity to purchase Common Stock of the Company through accumulated payroll deductions. It is the intention of the Company to have the Plan qualify as an "Employee Stock Purchase Plan" under
Section 423 of the Code. The provisions of the Plan, accordingly, shall be construed so as to extend and limit participation in a uniform and nondiscriminatory basis consistent with the
requirements of Section 423. 

        2.    Definitions.    

        (a)   "Administrator" shall mean the Board or any Committee designated by the Board to administer the plan pursuant to
Section 14. 

        (b)   "Board" shall mean the Board of Directors of the Company. 

        (c)   "Change in Control" means the occurrence of any of the following events: 

          (i)  Any
"person" (as such term is used in Sections 13(d) and 14(d) of the Exchange Act) becomes the "beneficial owner" (as defined in
Rule 13d-3 of the Exchange Act), directly or indirectly, of securities of the Company representing fifty percent (50%) or more of the total voting power represented by the Company's
then outstanding voting securities; or 

         (ii)  The
consummation of the sale or disposition by the Company of all or substantially all of the Company's assets; or 

        (iii)  A
change in the composition of the Board occurring within a two-year period, as a result of which fewer than a majority of the directors are Incumbent
Directors. "Incumbent Directors" means directors who either (A) are Directors as of the effective date of the Plan, or (B) are elected, or nominated for election, to the Board with the
affirmative votes of at least a majority of the Directors at the time of such election or nomination (but will not include an individual whose election or nomination is in connection with an
actual or threatened proxy contest relating to the election of directors to the Company); or 

        (iv)  The
consummation of a merger or consolidation of the Company with any other corporation, other than a merger or consolidation which would result in the voting
securities of the Company outstanding immediately prior thereto continuing to represent (either by remaining outstanding or by being converted into voting securities of the surviving entity or its
parent) at least fifty percent (50%) of the total voting power represented by the voting securities of the Company or such surviving entity or its parent outstanding immediately after such merger or
consolidation. 

        (d)   "Code" shall mean the Internal Revenue Code of 1986, as amended. 

        (e)   "Committee" means a committee of the Board appointed in accordance with Section 14 hereof. 

        (f)    "Common Stock" shall mean the common stock of the Company. 

        (g)   "Company" shall mean Optimer Pharmaceuticals, Inc., a Delaware corporation. 

        (h)   "Compensation" shall mean all base straight time gross earnings, commissions, overtime and shift premium, but exclusive
of payments for incentive compensation, bonuses and other compensation. 

        (i)    "Designated Subsidiary" shall mean any Subsidiary selected by the Administrator as eligible to participate in
the Plan. 

 

        (j)    "Director" shall mean a member of the Board. 

        (k)   "Eligible Employee" shall mean any individual who is a common law employee of the Company or any Designated Subsidiary
and whose customary employment with the Company or Designated Subsidiary is at least twenty (20) hours per week and more than five (5) months in any calendar year. For purposes of the
Plan, the employment relationship shall be treated as continuing intact while the individual is on sick leave or other leave of absence approved by the Company. Where the period of leave exceeds
90 days and the individual's right to reemployment is not guaranteed either by statute or by contract, the employment relationship shall be deemed to have terminated on the 91st day of
such leave. 

        (l)    "Exchange Act" shall mean the Securities Exchange Act of 1934, as amended. 

        (m)  "Exercise Date" shall mean the first Trading Day on or after May 15 and November 15 of each year. The first
Exercise Date under the Plan shall be November 15, 2007. 

        (n)   "Fair Market Value" shall mean, as of any date and unless the Administrator determines otherwise, the value of Common
Stock determined as follows: 

          (i)  If
the Common Stock is listed on any established stock exchange or a national market system, including without limitation the Nasdaq Global Market, its Fair Market
Value shall be the closing sales price for such stock (or the closing bid, if no sales were reported) as quoted on such exchange or system on the date of determination, as reported in  The Wall Street
Journal or such other source as the Board deems reliable; 

         (ii)  If
the Common Stock is regularly quoted by a recognized securities dealer but selling prices are not reported, its Fair Market Value shall be the mean of the closing
bid and asked prices for the Common Stock on the date of determination, as reported in The Wall Street Journal or such other source as the Board
deems reliable; 

        (iii)  In
the absence of an established market for the Common Stock, the Fair Market Value thereof shall be determined in good faith by the Board; or 

        (iv)  For
purposes of the Offering Date of the first Offering Period under the Plan, the Fair Market Value shall be the initial price to the public as set forth in the final
prospectus included within the registration statement on Form S-1 filed with the Securities and Exchange Commission for the initial public offering of the Company's Common Stock
(the "Registration Statement"). 

        (o)   "Fiscal Year" means the fiscal year of the Company. 

        (p)   "Offering Date" shall mean the first Trading Day of each Offering Period. 

        (q)   "Offering Periods" shall mean the periods of approximately six (6) months during which an option granted pursuant
to the Plan may be exercised, commencing on the first Trading Day on or after May 15 and November 15 of each year and terminating on the first Trading Day on or after the subsequent
Offering Period commencement date approximately six months later; provided, however, that the first Offering Period under the Plan shall commence with the first Trading Day on or after the date on
which the Securities and Exchange Commission declares the Company's registration statement on Form S-1 effective and end on the first Trading Day on or after November 15,
2007 and the second Offering Period under the Plan shall commence with the first Trading Day on or after November 16, 2007. The duration and timing of Offering Periods may be changed pursuant
to Section 4 of this Plan. 

        (r)   "Plan" shall mean this Optimer Pharmaceuticals, Inc. Employee Stock Purchase Plan. 

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        (s)   "Purchase Price" shall mean an amount equal to eighty-five percent (85%) of the Fair Market Value of a share
of Common Stock on the Offering Date or on the Exercise Date, whichever is lower; provided however, that the Purchase Price may be determined for subsequent Offering Periods by the Administrator
subject to compliance with Section 423 of the Code (or any successor rule or provision or any other applicable law, regulation or stock exchange rule) or pursuant to Section 20. 

        (t)    "Subsidiary" shall mean a "subsidiary corporation," whether now or hereafter existing, as defined in
Section 424(f) of the Code. 

        (u)   "Trading Day" shall mean a day on which the national stock exchange upon which the Company Common Stock is listed is open
for trading. 

        3.    Eligibility.    

        (a)   First Offering Period.    Any individual who is an Eligible Employee immediately prior to the first Offering
Period shall be automatically enrolled in the first Offering Period. 

        (b)   Subsequent Offering Periods.    Any Eligible Employee on a given Offering Date shall be eligible to participate
in the Plan. 

        (c)   Limitations.    Any provisions of the Plan to the contrary notwithstanding, no Eligible Employee shall be
granted an option under the Plan (i) to the extent that, immediately after the grant, such Eligible Employee (or any other person whose stock would be attributed to such Eligible
Employee pursuant to Section 424(d) of the Code) would own capital stock of the Company and/or hold outstanding options to purchase such stock possessing five percent (5%) or more of the total
combined voting power or value of all classes of the capital stock of the Company or of any Subsidiary, or (ii) to the extent that his or her rights to purchase stock under all employee stock
purchase plans of the Company and its subsidiaries accrues at a rate which exceeds Twenty-Five Thousand Dollars ($25,000) worth of stock (determined at the fair market value of the shares
at the time such option is granted) for each calendar year in which such option is outstanding at any time. 

        4.    Offering Periods.    The Plan shall be implemented by consecutive Offering Periods with a new Offering Period
commencing on the first Trading Day on or after May 15 and November 15 each year, or on such other date as the Board shall determine; provided, however, that the first Offering Period
under the Plan shall commence with the first Trading Day on or after the date upon which the Company's Registration Statement is declared effective by the Securities and Exchange Commission and end on
the first Trading Day on or after November 15, 2007. The Board shall have the power to change the duration of Offering Periods (including the commencement dates thereof) with respect to future
offerings without stockholder approval if such change is announced prior to the scheduled beginning of the first Offering Period to be affected thereafter. 

        5.    Participation.    

        (a)   First Offering Period.    An Eligible Employee shall be entitled to participate in the first Offering Period
only if such individual submits a subscription agreement authorizing payroll deductions in a form determined by the Administrator (which may be similar to the form attached hereto as Exhibit A)
to the Company's designated plan administrator (i) no earlier than the effective date of the Form S-8 registration statement with respect to the issuance of Common
Stock under this Plan and (ii) no later than twenty (20) business days following the effective date of such S-8 registration statement (the "Enrollment
Window"). An Eligible Employee's failure to submit the subscription agreement during the Enrollment Window shall result in the automatic termination of such individual's participation in the Offering
Period. 

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        (b)   Subsequent Offering Periods.    An Eligible Employee may become a participant in the Plan by completing a
subscription agreement in a form determined by the Administrator (which may be similar to the form attached hereto as Exhibit A) and filing it with the Company's designated Plan
administrator prior to the applicable Offering Date. 

        6.    Payroll Deductions.    

        (a)   At
the time a participant files his or her subscription agreement, he or she shall elect to have payroll deductions made on each pay day during the Offering Period in an
amount not exceeding 10% of the Compensation which he or she receives on each pay day during the Offering Period; provided, however, that should a pay day occur on an Exercise Date, a participant
shall have the payroll deductions made on such day applied to his or her account under the new Offering Period. A participant's subscription agreement shall remain in effect for successive Offering
Periods unless terminated as provided in Section 10 hereof. 

        (b)   Payroll
deductions for a participant shall commence on the first pay day following the Offering Date and shall end on the last pay day in the Offering Period to which
such authorization is applicable, unless sooner terminated by the participant as provided in Section 10 hereof; provided, however, that for the first Offering Period, payroll deductions shall
commence on the first pay day on or following the end of the Enrollment Window. 

        (c)   All
payroll deductions made for a participant shall be credited to his or her account under the Plan and shall be withheld in whole percentages only. A participant may
not make any additional payments into such account. 

        (d)   A
participant may discontinue his or her participation in the Plan as provided in Section 10 hereof, or may increase or decrease the rate of his or her
payroll deductions during the Offering Period by completing or filing with the Company a new subscription agreement authorizing a change in payroll
deduction rate. The Administrator may, in its discretion, limit the nature and/or number of participation rate changes during any Offering Period. The change in rate shall be effective with the first
full payroll period following five (5) business days after the Company's receipt of the new subscription agreement unless the Company elects to process a given change in participation
more quickly. 

        (e)   Notwithstanding
the foregoing, to the extent necessary to comply with Section 423(b)(8) of the Code and Section 3(c) hereof, a participant's payroll
deductions may be decreased to zero percent (0%) at any time during an Offering Period. Payroll deductions shall recommence at the rate provided in such participant's subscription agreement at the
beginning of the first Offering Period which is scheduled to end in the following calendar year, unless terminated by the participant as provided in Section 10 hereof. 

        (f)    At
the time the option is exercised, in whole or in part, or at the time some or all of the Company's Common Stock issued under the Plan is disposed of, the participant
must make adequate provision for the Company's or its Subsidiary's federal, state, or any other tax liability payable to any authority, national insurance, social security or other tax withholding
obligations, if any, which arise upon the exercise of the option or the disposition of the Common Stock. At any time, the Company or its Subsidiary may, but shall not be obligated to, withhold from
the participant's compensation the amount necessary for the Company or its Subsidiary to meet applicable withholding obligations, including any withholding required to make available to the Company or
its Subsidiary any tax deductions or benefits attributable to sale or early disposition of Common Stock by the Eligible Employee. 

        7.    Grant of Option.    On the Offering Date of each Offering Period, each Eligible Employee participating in such
Offering Period shall be granted an option to purchase on each Exercise Date during such Offering Period (at the applicable Purchase Price) up to a number of shares of the 

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Company's
Common Stock determined by dividing such Eligible Employee's payroll deductions accumulated prior to such Exercise Date by the applicable Purchase Price; provided that in no event shall an
Eligible Employee be permitted to purchase during each Offering Period more than 1,000 shares of the Company's Common Stock (subject to any adjustment pursuant to Section 19), and provided
further that such purchase shall be subject to the limitations set forth in Sections 3(c) and 13 hereof. The Eligible Employee may accept the grant of such option by turning in a
completed Subscription Agreement (attached hereto as Exhibit A) to the Company on or prior to an Offering Date, or with respect to the
first Offering Period, prior to the last day of the Enrollment Window. The Administrator may, for future Offering Periods, increase or decrease, in its absolute discretion, the maximum number of
shares of the Company's Common Stock an Eligible Employee may purchase during each Offering Period. Exercise of the option shall occur as provided in Section 8 hereof, unless the participant
has withdrawn pursuant to Section 10 hereof. The option shall expire on the last day of the Offering Period. 

        8.    Exercise of Option.    

        (a)   Unless
a participant withdraws from the Plan as provided in Section 10 hereof, his or her option for the purchase of shares shall be exercised automatically on
the Exercise Date, and the maximum number of full shares subject to option shall be purchased for such participant at the applicable Purchase Price with the accumulated payroll deductions in his or
her account. No fractional shares shall be purchased; any payroll deductions accumulated in a participant's account which are not sufficient to purchase a full share shall be retained in the
participant's account for the subsequent Offering Period, subject to earlier withdrawal by the participant as provided in Section 10 hereof. Any other funds left over in a participant's account
after the Exercise Date shall be returned to the participant. During a participant's lifetime, a participant's option to purchase shares hereunder is exercisable only by him or her. 

        (b)   If
the Administrator determines that, on a given Exercise Date, the number of shares with respect to which options are to be exercised may exceed (i) the number
of shares of Common Stock that were available for sale under the Plan on the Offering Date of the applicable Offering Period, or (ii) the number of shares available for sale under the Plan on
such Exercise Date, the Administrator may in its sole discretion provide that the Company shall make a pro rata allocation of the shares of Common Stock available for purchase on such Exercise
Date in as uniform a manner as shall be practicable and as it shall determine in its sole discretion to be equitable among all participants exercising options to purchase Common Stock on such Exercise
Date. The Company may make a pro rata allocation of the shares available on the Offering Date of any applicable Offering Period pursuant to the preceding sentence, notwithstanding any
authorization of additional shares for issuance under the Plan by the Company's stockholders subsequent to such Offering Date. 

        9.    Delivery.    As soon as reasonably practicable after each Exercise Date on which a purchase of shares occurs,
the Company shall arrange the delivery to each participant the shares purchased upon exercise of his or her option in a form determined by the Administrator. 

        10.    Withdrawal.    

        (a)   A
participant may withdraw all but not less than all the payroll deductions credited to his or her account and not yet used to exercise his or her option under the Plan
at any time by giving written notice to the Company in the form determined by the Administrator (which may be similar to the form attached as  Exhibit B to this Plan). All of the participant's
payroll deductions credited to his or her account shall be paid to such participant
promptly after receipt of notice of withdrawal and such participant's option for the Offering Period shall be automatically terminated, and no further payroll deductions for the purchase of shares
shall be made for such Offering Period. If a participant withdraws from an Offering Period, payroll deductions shall not resume at 

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the
beginning of the succeeding Offering Period unless the participant delivers to the Company a new subscription agreement. 

        (b)   A
participant's withdrawal from an Offering Period shall not have any effect upon his or her eligibility to participate in any similar plan which may hereafter be
adopted by the Company or in succeeding Offering Periods which commence after the termination of the Offering Period from which the participant withdraws. 

        11.    Termination of Employment.    Upon a participant's ceasing to be an Eligible Employee, for any reason, he or
she shall be deemed to have elected to withdraw from the Plan and the payroll deductions credited to such participant's account during the Offering Period but not yet used to purchase shares of Common
Stock under the Plan shall be returned to such participant or, in the case of his or her death, to the person or persons entitled thereto under Section 15, and such participant's option shall
be automatically terminated. 

        12.    Interest.    No interest shall accrue on the payroll deductions of a participant in the Plan. 

        13.    Stock.    

        (a)   Subject
to adjustment upon changes in capitalization of the Company as provided in Section 19 hereof, the maximum number of shares of the Company's Common Stock
which shall be made available for sale under the Plan shall be 200,000 shares (which has been determined after adjustment to reflect the stock split which will be completed by the Company prior
to the initial registration of the Company's Common Stock under Section 12 of the Exchange Act) plus an annual increase to be added on the first day of each Company Fiscal Year beginning with
the 2008 Fiscal Year, equal to the lesser of (i) 300,000 shares of Common Stock (which has been determined after adjustment to reflect the stock split which will be completed by the Company
prior to the initial registration of the Company's Common Stock under Section 12 of the Exchange Act), (ii) three percent (3%) of the outstanding shares of Common Stock on the last day
of the immediately preceding Fiscal Year or (iii) an amount determined by the Board. 

        (b)   Until
the shares are issued (as evidenced by the appropriate entry on the books of the Company or of a duly authorized transfer agent of the Company), a
participant shall only have the rights of an unsecured creditor with respect to such shares, and no right to vote or receive dividends or any other rights as a stockholder shall exist with respect to
such shares. 

        (c)   Shares
to be delivered to a participant under the Plan shall be registered in the name of the participant or in the name of the participant and his or her spouse. 

        14.    Administration.    The Plan will be administered by an Administrator which shall be either (A) the Board
or (B) a Committee appointed by the Board, which committee will be constituted to satisfy all applicable law. The Administrator shall administer the Plan and shall have full and exclusive
discretionary authority to construe, interpret and apply the terms of the Plan, to determine eligibility and to adjudicate all disputed claims filed under the Plan. Every finding, decision and
determination made by the Administrator shall, to the full extent permitted by law, be final and binding upon all parties. Notwithstanding any provision to the contrary in this Plan, the Administrator
may adopt rules or procedures relating to the operation and administration of the Plan to accommodate the specific requirements of local laws and procedures for jurisdictions outside of the
United States. Without limiting the generality of the foregoing, the Administrator is specifically authorized to adopt rules and procedures regarding eligibility to participate, the definition
of Compensation, handling of payroll deductions, making of contributions to the Plan (including, without limitation, in forms other than payroll deductions), establishment of bank or trust accounts to
hold payroll deductions, payment of interest, conversion of local currency, obligations to pay payroll tax, determination of beneficiary designation requirements, withholding procedures and handling
of stock certificates which vary with local requirements. 

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        15.    Designation of Beneficiary.    

        (a)   A
participant may file a designation of a beneficiary who is to receive any shares and cash, if any, from the participant's account under the Plan in the event of
such participant's death subsequent to an Exercise Date on which the option is exercised but prior to delivery to such participant of such shares and cash. In addition, a participant may file a
designation of a beneficiary who is to receive any cash from the participant's account under the Plan in the event of such participant's death prior to exercise of the option. If a participant is
married and the designated beneficiary is not the spouse, spousal consent shall be required for such designation to be effective. 

        (b)   Such
designation of beneficiary may be changed by the participant at any time by notice in a form determined by the Administrator. In the event of the death of a
participant and in the absence of a beneficiary validly designated under the Plan who is living at the time of such participant's death, the Company shall deliver such shares and/or cash to the
executor or administrator of the estate of the participant, or if no such executor or administrator has been appointed (to the knowledge of the Company), the Company, in its discretion, may
deliver such shares and/or cash to the spouse or to any one or more dependents or relatives of the participant, or if no spouse, dependent or relative is known to the Company, then to such other
person as the Company may designate. 

        (c)   All
beneficiary designations shall be in such form and manner as the Administrator may designate from time to time. 

        16.    Transferability.    Neither payroll deductions credited to a participant's account nor any rights with regard
to the exercise of an option or to receive shares under the Plan may be assigned, transferred, pledged or otherwise disposed of in any way (other than by will, the laws of descent and distribution or
as provided in Section 15 hereof) by the participant. Any such attempt at assignment, transfer, pledge or other disposition shall be without effect, except that the Company may treat such act
as an election to withdraw funds from an Offering Period in accordance with Section 10 hereof. 

        17.    Use of Funds.    All payroll deductions received or held by the Company under the Plan may be used by the
Company for any corporate purpose, and the Company shall not be obligated to segregate such payroll deductions. Until shares are issued, participants shall only have the rights of an unsecured
creditor. 

        18.    Reports.    Individual accounts shall be maintained for each participant in the Plan. Statements of account
shall be given to participating Eligible Employees at least annually, which statements shall set forth the amounts of payroll deductions, the Purchase Price, the number of shares purchased and the
remaining cash balance, if any. 

        19.    Adjustments Upon Changes in Capitalization, Dissolution, Liquidation, Merger or Change in Control.    

        (a)   Changes in Capitalization.    Subject to any required action by the stockholders of the Company, the maximum
number of shares of the Company's Common Stock which shall be made available for sale under the Plan, the maximum number of shares each participant may purchase each Offering Period (pursuant to
Section 7), as well as the price per share and the number of shares of Common Stock covered by each option under the Plan which has not yet been exercised shall be proportionately adjusted for
any increase or decrease in the number of issued shares of Common Stock resulting from a stock split, reverse stock split, stock dividend, combination or reclassification of the Common Stock, or any
other change in the number of shares of Common Stock effected without receipt of consideration by the Company; provided, however, that conversion of any convertible securities of the Company shall not
be deemed to have been "effected without receipt of consideration." Such adjustment shall be made by the Administrator, 

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whose
determination in that respect shall be final, binding and conclusive. Except as expressly provided herein, no issuance by the Company of shares of stock of any class, or securities convertible
into shares of stock of any class, shall affect, and no adjustment by reason thereof shall be made with respect to, the number or price of shares of Common Stock subject to an option. 

        (b)   Dissolution or Liquidation.    In the event of the proposed dissolution or liquidation of the Company, the
Offering Period then in progress shall be shortened by setting a new Exercise Date (the "New Exercise Date"), and shall terminate immediately prior to the consummation of such proposed
dissolution or liquidation, unless provided otherwise by the Administrator. The New Exercise
Date shall be before the date of the Company's proposed dissolution or liquidation. The Administrator shall notify each participant in writing, at least ten (10) business days prior to the
New Exercise Date, that the Exercise Date for the participant's option has been changed to the New Exercise Date and that the participant's option shall be exercised automatically on the
New Exercise Date, unless prior to such date the participant has withdrawn from the Offering Period as provided in Section 10 hereof. 

        (c)   Merger or Change in Control.    In the event of a merger or Change in Control, each outstanding option
shall be assumed or an equivalent option substituted by the successor corporation or a Parent or Subsidiary of the successor corporation. In the event that the successor corporation refuses to assume
or substitute for the option, the Offering Period then in progress shall be shortened by setting a New Exercise Date and shall end on the New Exercise Date. The New Exercise Date
shall be before the date of the Company's proposed merger or Change in Control. The Administrator shall notify each participant in writing, at least ten (10) business days prior to the
New Exercise Date, that the Exercise Date for the participant's option has been changed to the New Exercise Date and that the participant's option shall be exercised automatically on the
New Exercise Date, unless prior to such date the participant has withdrawn from the Offering Period as provided in Section 10 hereof. 

        20.    Amendment or Termination.    

        (a)   The
Administrator may at any time and for any reason terminate or amend the Plan. Except as provided in Section 19 and this Section 20 hereof, no
amendment may make any change in any option theretofore granted which adversely affects the rights of any participant unless their consent is obtained. To the extent necessary to comply with
Section 423 of the Code (or any successor rule or provision or any other applicable law, regulation or stock exchange rule), the Company shall obtain stockholder approval of any
amendment in such a manner and to such a degree as required. 

        (b)   Without
stockholder approval and without regard to whether any participant rights may be considered to have been "adversely affected," the Administrator shall be
entitled to change the Offering Periods, limit the frequency and/or number of changes in the amount withheld during an Offering Period, establish the exchange ratio applicable to amounts withheld in a
currency other than U.S. dollars, permit payroll withholding in excess of the amount designated by a participant in order to adjust for delays or mistakes in the Company's processing of
properly completed withholding elections, establish reasonable waiting and adjustment periods and/or accounting and crediting procedures to ensure that amounts applied toward the purchase of Common
Stock for each participant properly correspond with amounts withheld from the participant's Compensation, and establish such other limitations or procedures as the Administrator determines in its sole
discretion advisable which are consistent with the Plan. 

        (c)   Without
regard to whether any participant's rights may be considered to have been "adversely affected", in the event the Administrator determines that the ongoing
operation of the Plan may result in unfavorable financial accounting consequences, the Board may, in its discretion 

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and,
to the extent necessary or desirable, modify or amend the Plan to reduce or eliminate such accounting consequence including: 

          (i)  increasing
the Purchase Price for any Offering Period including an Offering Period underway at the time of the change in Purchase Price; 

         (ii)  shortening
any Offering Period so that Offering Period ends on a new Exercise Date, including an Offering Period underway at the time of the Board action; and 

        (iii)  reducing
the number of shares that may be purchased upon exercise of outstanding options. 

        Such
modifications or amendments shall not require stockholder approval or the consent of any Plan participants. 

        21.    Notices.    All notices or other communications by a participant to the Company under or in connection with the
Plan shall be deemed to have been duly given when received in the form and manner specified by the Company at the location, or by the person, designated by the Company for the receipt thereof. 

        22.    Conditions Upon Issuance of Shares.    Shares shall not be issued with respect to an option unless the exercise
of such option and the issuance and delivery of such shares pursuant thereto shall comply with all applicable provisions of law, domestic or foreign, including, without limitation, the Securities Act
of 1933, as amended, the Exchange Act, the rules and regulations promulgated thereunder, and the requirements of any stock exchange upon which the shares may then be listed, and shall be further
subject to the approval of counsel for the Company with respect to such compliance. 

        As
a condition to the exercise of an option, the Company may require the person exercising such option to represent and warrant at the time of any such exercise that the shares are being
purchased only for investment and without any present intention to sell or distribute such shares if, in the opinion of counsel for the Company, such a representation is required by any of the
aforementioned applicable provisions of law. 

        23.    Term of Plan.    The Plan shall become effective upon the earlier to occur of its adoption by the Board of
Directors or its approval by the stockholders of the Company. It shall continue in effect until the later of (i) the date it is terminated under Section 20 hereof or (ii) the date
which is ten (10) years after the date such Plan is approved by the Board. 

        24.    Stockholder Approval.    The Plan will be subject to approval by the stockholders of the Company within twelve
(12) months after the date the Plan is adopted by the Board. Such stockholder approval will be obtained in the manner and to the degree required under Applicable Laws. 

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EXHIBIT A
  
    OPTIMER PHARMACEUTICALS, INC.
  EMPLOYEE STOCK PURCHASE PLAN
  SUBSCRIPTION AGREEMENT    
    

	                        	Original Application	 	Offering Date:	                                        
        
	
                         	

Change in Payroll Deduction Rate	
 	

 	

 
	
                         	

Change of Beneficiary(ies)	
 	

 	

 

	1.
	                                    hereby
elects to participate in the Optimer Pharmaceuticals, Inc. Employee Stock Purchase Plan (the "Employee Stock Purchase Plan") and subscribes to
purchase shares of Optimer Pharmaceuticals, Inc.'s (the "Company") Common Stock in accordance with this Subscription Agreement and the Employee Stock Purchase Plan.

	2.
	I
hereby authorize payroll deductions from each paycheck in the amount of                        % of my Compensation on each pay
day (from 0 to 10%) during the Offering Period in accordance
with the Employee Stock Purchase Plan. (Please note that no fractional percentages are permitted.)

	3.
	I
understand that said payroll deductions shall be accumulated for the purchase of shares of Common Stock at the applicable Purchase Price determined in accordance with the Employee
Stock Purchase Plan. I understand that if I do not withdraw from an Offering Period, any accumulated payroll deductions will be used to automatically exercise my option and purchase Common Stock under
the Employee Stock Purchase Plan.

	4.
	I
have received a copy of the complete Employee Stock Purchase Plan and its accompanying prospectus. I understand that my participation in the Employee Stock Purchase Plan is in all
respects subject to the terms of the Plan.

	5.
	Shares
purchased for me under the Employee Stock Purchase Plan should be issued in the name(s) of (Eligible Employee or Eligible Employee and Spouse only).

	6.
	I
understand that if I dispose of any shares received by me pursuant to the Employee Stock Purchase Plan within 2 years after the Offering Date (the first day of the
Offering Period during which I purchased such shares) or one year after the Exercise Date, I will be treated for federal income tax purposes as having received ordinary income at the time of such
disposition in an amount equal to the excess of the fair market value of the shares at the time such shares were purchased by me over the price which I paid for the shares. I
hereby agree to notify the Company in writing within 30 days after the date of any disposition of my shares and I will make adequate provision for Federal, state or other tax withholding
obligations, if any, which arise upon the disposition of the Common Stock. The Company may, but will not be obligated to, withhold from my compensation the amount necessary to
meet any applicable withholding obligation including any withholding necessary to make available to the Company any tax deductions or benefits attributable to sale or early disposition of Common Stock
by me. If I dispose of such shares at any time after the expiration of the 2-year and 1-year holding periods, I understand that I will be treated for federal income tax
purposes as having received income only at the time of such disposition, and that such income will be taxed as ordinary income only to the extent of an amount equal to the lesser of (1) the
excess of the fair market value of the shares at the time of such disposition over the purchase price which I paid for the shares, or (2) 15% of the fair market value of the shares on the first
day of the Offering Period. The remainder of the gain, if any, recognized on such disposition will be taxed as capital gain.

	7.
	I
hereby agree to be bound by the terms of the Employee Stock Purchase Plan. The effectiveness of this Subscription Agreement is dependent upon my eligibility to participate in the
Employee Stock Purchase Plan. 

	8.
	In
the event of my death, I hereby designate the following as my beneficiary(ies) to receive all payments and shares due me under the Employee Stock Purchase Plan: 

	
 	

NAME: (Please print)	

 
	 	 	
 (First)
                                (Middle)
                                (Last)

	
 	

 Relationship	
 	

	
 	

 Percentage of Benefit	
 	

 (Address)

	
 	

NAME: (Please print)	

 
	 	 	
 (First)
                                (Middle)
                                (Last)

	
 	

 Relationship	
 	

	
 	

 Percentage of Benefit	
 	

 (Address)

	
 	

Employee's Social Security Number:	

 
	 	 	

	
 	

Employee's Address:	

 
	
 	

 	

	
 	

 	

	
 	

 	

I UNDERSTAND THAT THIS SUBSCRIPTION AGREEMENT SHALL REMAIN IN EFFECT THROUGHOUT SUCCESSIVE OFFERING PERIODS UNLESS TERMINATED BY ME. 

	
 Dated:	
 	

 	
 	

 
	 	 	
	 	
 Signature of Employee
	
 	
 	

 	
 	

 Spouse's Signature (If beneficiary other than spouse)

 
 

EXHIBIT B
  
    OPTIMER PHARMACEUTICALS, INC.
  EMPLOYEE STOCK PURCHASE PLAN
  NOTICE OF WITHDRAWAL    
    

        The undersigned participant in the Offering Period of the Optimer Pharmaceuticals, Inc. Employee Stock Purchase Plan that began
on                                    ,
                        (the "Offering Date") hereby notifies the Company that he or she hereby withdraws from the Offering
Period. He or she hereby directs the Company to pay to the undersigned as
promptly as practicable all the payroll deductions credited to his or her account with respect to such Offering Period. The undersigned understands and agrees that his or her option for such Offering
Period will be automatically terminated. The undersigned understands further that no further payroll deductions will be made for the purchase of shares in the current Offering Period and the
undersigned shall be eligible to participate in succeeding Offering Periods only by delivering to the Company a new Subscription Agreement. 

	
 	
 	

Name and Address of Participant:
	
 	
 	

	 	 	

	 	 	

	 	 	Signature:
	
 	
 	

	 	 	Date:
	
 	
 	

QuickLinks

Exhibit 10.24

OPTIMER PHARMACEUTICALS, INC. EMPLOYEE STOCK PURCHASE PLAN

EXHIBIT A OPTIMER PHARMACEUTICALS, INC. EMPLOYEE STOCK PURCHASE PLAN SUBSCRIPTION AGREEMENT

EXHIBIT B OPTIMER PHARMACEUTICALS, INC. EMPLOYEE STOCK PURCHASE PLAN NOTICE OF WITHDRAWALQuickLinks
 -- Click here to rapidly navigate through this document

 
 

Exhibit 10.25    
    

EXECUTION COPY 

 
 

PROSPECTIVE BUY-BACK AGREEMENT    
    

        This Prospective Buy-Back Agreement (this "Agreement") is hereby entered into as of January 19, 2007 (the "Effective Date") by and between
Optimer Pharmaceuticals, Inc. ("Optimer"), a Delaware corporation with offices located at 10110 Sorrento Valley Rd., Suite C, San Diego, CA 92121 and Par Pharmaceutical, Inc.
("Par"), a Delaware corporation with offices located at 300 Tice Boulevard, Woodcliff Lake, NJ 07677. 

        WHEREAS, the Parties entered into that certain Collaboration Agreement dated April 29, 2005 (the "Collaboration Agreement") under
which the Parties agreed to collaborate regarding, and granted to each other certain rights including with respect to, the development, promotion, distribution and sale of certain products under
development by Optimer. 

        WHEREAS, the Parties now wish to enter into an agreement granting to each other certain rights relating to the Collaboration Agreement and
the subject matter thereof, including providing for the termination of the Collaboration Agreement under certain circumstances, as described in further detail below. 

        NOW, THEREFORE, for good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the Parties hereby
agree as follows: 

        1.    Definitions    

        1.1   "Acquisition Cost" means, as applicable, direct cost of finished, labeled and packaged Product including actual
out-of-pocket expenses paid to non-Affiliate manufacturers and suppliers and amounts paid by Optimer for supply of Product, API or other supplies or materials used
in manufacture of the Product, including, specifically, the write-off of unusable materials, start up costs and unsold inventory and batches, but excluding general corporate and other
overhead. 

        1.2   "Affiliate(s)" means any Person (defined below) that, directly or indirectly, controls, is controlled by or under common
control with a Party. For purposes of this definition of Affiliate, the term "control" (including with correlative meaning, the terms "controlling", "controlled by", and "under common control with")
as used with respect to any Person, shall mean (i) ownership directly or indirectly of fifty percent (50%) or more of the voting stock of the controlled entity (or, for nonstock organizations,
the right to receive over fifty percent (50%) of the profits of the controlled entity by contract or otherwise), or (ii) possession, directly or indirectly, of the power to direct or cause the
direction of the management and policies of such Person, whether through ownership of voting securities, by contract, or otherwise. 

        1.3   "Applicable Laws" means all Applicable Laws, rules, and regulations that apply to the manufacture, development,
marketing, sale or commercialization of Product or the performance of either Party's obligations under this Agreement including laws and regulations governing the import, export, development,
marketing, distribution and sale of Product, to the extent applicable and relevant, and including all current Good Manufacturing Practices or current Good Clinical Practices or similar standards or
guidelines promulgated or adopted by the United States Food and Drug Administration where applicable, as well as U.S. export control laws and the U.S. Foreign Corrupt Practices Act. 

        1.4   "Confidential Information" means, with respect to a Party, all information of any kind whatsoever (including without
limitation, data, compilations, formulae, models, patent disclosures, procedures, processes, projections, protocols, results of experimentation and testing, specifications, strategies, techniques and
all non-public intellectual property rights), and all tangible and intangible embodiments thereof of any kind whatsoever (including without limitation, apparatus, compositions, documents,
drawings, machinery, patent applications, records and reports), that is 

 

disclosed
by such Party (the "Disclosing Party") to the other Party (the "Receiving Party") and is marked, identified or otherwise communicated to be confidential at the time of disclosure to the
Receiving Party. Notwithstanding the foregoing, Confidential Information shall not include information that can be established by competent proof (a) to have been publicly known prior to
disclosure of such information to the Receiving Party, (b) to have become publicly known, without fault on the part of the Receiving Party, (c) to have been received by the Receiving
Party free of an obligation of confidentiality at any time from a source, other than the Disclosing Party, rightfully having possession of and the right to disclose such information free of an
obligation of confidentiality, (d) to have been independently developed either by personnel of the Receiving Party prior to receipt from the Disclosing Party or by personnel of the Receiving
Party who had no access to and made no use of the Confidential Information provided by the Disclosing Party, or (e) to have been otherwise known by the Receiving Party prior to disclosure of
such information by the Disclosing Party. 

        1.5   "Commercial Launch" means the first commercial sale of the Product by Optimer or its Affiliate or Licensee to a Third
Party, on a country by country basis. 

        1.6   "Equity Financing" means an initial public offering of Optimer's common stock. 

        1.7   "Fully-Burdened Costs" shall mean the fully allocated cost for Par to provide the Product to Optimer, calculated using a
reasonable method to allocate direct and indirect costs associated with the manufacture, packaging, storage, releasing, handling, shipping, quality assurance and control testing and all costs related
to maintaining FDA approval to manufacture the Product, including, but not limited to, labor costs, out-of-pocket expenses for materials (including API, excipient raw material
ingredients and packaging components), services and external contractors, and all indirect costs including attributable overheads. 

        1.8   "GAAP" means generally accepted accounting principles in the United States, consistently applied. 

        1.9   "Licensee" means a Third Party to whom Optimer has granted a license or right to make, sell, have sold, use, import or
distribute the Product. 

        1.10 "Net Sales" means the dollar amount determined by deducting from the gross invoiced price billed for Product to Third
Parties in arm's length transactions, the following amounts: (i) all applicable sales credits accrued in accordance with GAAP, (ii) payments or rebates actually incurred with respect to
Products pursuant to federal, state and local government assistance programs, whether in existence now or enacted at any time hereafter, (iii) costs for transit insurance, freight, handling or
other transportation as billed to customers (to the extent included in the amount invoiced for Products), (iv) sales, use or excise taxes and (v) any actually allowed
write-offs or credits for inventory or batches unsold by purchasers of the Product. Sales credits accrued in accordance with accounting principles generally accepted in the United States
may include credits or discounts related
to the following: (i) customer returns, returned goods allowances, rejected goods and damaged goods not covered by insurance, (ii) cash or terms discounts, (iii) customer rebate
programs, (iv) chargebacks and administration fees or similar credits or payments granted to customers pursuant to contract or other purchases, (v) sales promotions, trade show discounts
and stocking allowances, (vi) price adjustments, including those on customer inventories following price changes and (vii) product recalls. Net Sales shall also include the fair market
value of all non-cash consideration received by Optimer or its Affiliates for sale of Product, including payment in kind, exchange or another form. 

        1.11 "Net Revenues" received by Optimer means all consideration received by Optimer and its Affiliates in connection with an
applicable license, transfer, covenant, assignment, sale or any grant related to the Product in the ROW (including without limitation, up-front payments, license fees, milestone payments
and running royalties), but excluding: (i) amounts received as payment 

2

 

specifically
for performance of research and development activities, to the extent such payments are reasonably related to the costs of such activities; (ii) any amounts received as payment for
equity and (iii) amounts received as payment for Product or API transferred to a Licensee, to the extent such amounts do not exceed the Acquisition Costs therefor. Net Revenues shall also
include the fair market value of all non-cash consideration received by Optimer or its Affiliates, including payment in kind, exchange or another form. In the event that the Parties do
not, after good faith discussions, agree upon the fair market value of Net Revenues received as non-cash consideration by Optimer or its Affiliates for purposes of determining the amounts
payable to Par under Section 2.3(c) of this Agreement, the Parties shall select one neutral expert having expertise in determining the fair market value of such non-cash
consideration to determine its fair market value, with each Party's agreement on the identity of the expert not to be unreasonably withheld and both Parties sharing equal in the costs of for such
expert, and the decision of such expert regarding the fair market value of such non-cash consideration shall be final and binding on the Parties for purposes of determining the amounts
payable to Par under Section 2.3(c) of this Agreement. For avoidance of doubt, amounts received from a Licensee for Product or API in excess of the applicable Acquisition Costs shall be
included within Net Revenues. Notwithstanding any of the foregoing, in no event shall Net Revenues be deemed to include amounts received by Optimer in consideration for a sale of all or substantially
all of the business or assets of Optimer to which this Agreement pertains (whether by merger, sale of stock, sale of assets or otherwise). For Net Revenues that are received by Optimer and its
Affiliates that are not readily allocable to the Territory or the ROW, such Net Revenue shall be allocated as sixty percent (60%) as Net Revenues for the Territory and forty percent (40%) as Net
Revenues for the ROW. 

        1.12 "Non-Qualified Equity Financing" means an Equity Financing which is not a Qualified Equity Financing. 

        1.13 "Party" shall mean individually Optimer or Par, as may be applicable; and "Parties" shall refer to Par and Optimer
collectively. 

        1.14 "Person" means an individual, corporation, partnership, limited liability company, firm, association, joint venture,
estate, trust, governmental or administrative body or agency, or any other entity. 

        1.15 "OPT-80 Data" shall mean: (a) research data, laboratory notebooks, data with respect to the current
dosage form of the Product, analytical chemistry data, stability data, polymorphism data, pharmacology data, chemistry, manufacturing and control (CMC) data, pre-clinical or
non-clinical data, clinical data, safety data; regulatory filings and supporting documentation and data (including information in any drug master files); marketing materials, brochures,
pamphlets, analyst data and market research audits, data and studies; know-how, and any other similar data or documentation; in each foregoing case to the extent (i) that such are
owned, possessed and transferable by Par as of the Effective Date and (ii) that Par has no contractual or legal obligation restricting their transfer or disclosure. 

        1.16 "Phase III Study" means a pivotal human clinical study in any country the results of which could be used to establish
safety and efficacy of a Product as a basis for a regulatory approval application submitted to the FDA and as defined in 21 C.F.R. 312.21(c). 

        1.17 "Product" means the product designated as OPT-80 or PAR-101, or any product containing the same
active moiety, which are the subject of the Collaboration Agreement. 

        1.18 "Qualified Equity Financing" means an initial public offering of Optimer's capital stock, in which the offering price
per share of Optimer capital stock multiplied by the number of shares of stock purchased by the underwriter in connection with the offering equals at least $50,000,000 in exchange for capital stock. 

3

 

        1.19 "ROW" means worldwide, excluding the Territory. 

        1.20 "Successful Completion" means, with respect to any Phase III Study, the completion of such Phase III Study in which any
primary endpoint contained in the study protocol is met; provided, however, that with respect to (i) the North American double-blind, randomized, parallel group Phase 2b/3 clinical study in
CDAD patients being conducted as of the Effective Date which Optimer intends to transition to a Phase 3 study in the first quarter of 2007 and (ii) the pivotal Phase 3 study of the same design
that Optimer intends to conduct in the first quarter of 2007, "Successful Completion" shall mean completion of the clinical studies described in both (i) and (ii) and the meeting of any
primary endpoint contained in each study protocol for each such study. 

        1.21 "Termination Exercise Period" means the period commencing on the Effective Date and ending on April 30, 2007, or
such later date as the Parties may agree upon, each at their own discretion. 

        1.22 "Territory" means Canada, Israel and the United States, and their respective territories and possessions and the
Commonwealth of Puerto Rico. 

        1.23 "Third Party" means any person or entity other than Optimer or Par or any of their Affiliates. 

        2.    Automatic Termination of the Collaboration Agreement.    The Parties hereby agree that,
in the event that Optimer undergoes a Qualified Equity Financing at any time during the Termination Exercise Period, the following provisions of this Article 2 below shall apply and Optimer
shall promptly notify Par that the Qualified Equity Financing has occurred: 

        2.1   Optimer
shall pay to Par a termination fee of Twenty Million Dollars ($20,000,000) (the "Collaboration Agreement Termination Fee") within five (5) Business Days
of its receipt of Twenty Million Dollars ($20,000,000) from such Qualified Equity Financing but in no circumstance later then forty-five (45) days after a Qualified Equity
Financing. 

        2.2   The
Collaboration Agreement shall terminate immediately upon Optimer's payment of the Collaboration Agreement Termination Fee to Par (the "Termination"). Notwithstanding
the terms of Section 13.5 of the Collaboration Agreement, all articles and sections of the Collaboration Agreement and all rights, licenses and obligations of each Party thereunder (including,
without limitation, the rights, licenses and obligations of each Party under Articles 2-6 of the Collaboration Agreement) shall immediately terminate, provided that the rights and
obligations of each Party under Article 9 with respect to the Confidential Information of the other Party disclosed under the Collaboration Agreement shall survive, provided further that
(i) the last sentence of Section 9.1(i) of the Collaboration Agreement shall not apply and Optimer shall have the right to use the Confidential Information of Par solely as
reasonably necessary or useful in the development, manufacture, marketing or commercialization of the Product, and to disclose the same, subject to reasonable obligations of confidentiality, to its
Affiliates and licensees for such use, and (ii) Optimer shall not be obligated under Section 9.4 of the Collaboration Agreement to return or destroy the Confidential Information of Par.
Notwithstanding the foregoing, information related to Par's general business, scientific and clinical operations, protocols, management or financing shall remain the Confidential Information of Par
and Optimer may not be disclosed to a Third Party or in any way used by Optimer. 

        2.3   In
addition to payment to Par of the Collaboration Agreement Termination Fee, Optimer shall make the payments set forth in this Section 2.3 below: 

        (a)   Optimer
shall pay to Par five percent (5%) of all Net Sales of Product sold by Optimer, its Affiliates or Licensees, or any assignee,
successor-in-interest or other Third Party that may otherwise be licensed, transferred, assigned, sold or granted any right to the Product, in the Territory on a
country-by-country basis made during the period commencing on the 

4

 

applicable
Commercial Launch until the seventh (7th) anniversary thereof. Such payments shall be made within forty-five (45) days after the end of the applicable
calendar quarter in which sales of Product have been made and each such payment shall include a report detailing the records used in the calculation of such payment. For clarity, no payments shall be
due by Optimer to Par on the Net Sales of Product in the Territory in a particular country after the seventh (7th) anniversary of the applicable Commercial Launch. 

        (b)   Optimer
shall pay to Par one and one-half percent (1.5%) of the Net Sales of Product sold by Optimer or its Affiliates in the ROW made during the period
commencing on the applicable Commercial Launch until the seventh (7th) anniversary thereof. Such payments shall be made within forty-five (45) days after the end of
the applicable calendar quarter in which sales of Product have been made and each such payment shall include a report detailing the records used in the calculation of such payment. For clarity, no
payments shall be due by Optimer to Par on the Net Sales of Product in the ROW in a particular country after the seventh (7th) anniversary of the applicable Commercial Launch. 

        (c)   In
the event Optimer licenses, transfers, covenants, assigns, sells or grants any right to sell, have sold, market or distribute the Product in the ROW to a Third Party,
Optimer shall pay to Par six and one-quarter percent (6.25%) of all Net Revenues related thereto for the period until the seventh (7th) anniversary of the first Commercial Launch. Such
payments shall be made within forty-five (45) days after the end of the applicable calendar quarter and each such payment shall include a report detailing the records used in the
calculation of such payment. For clarity, no payments shall be due by Optimer to Par on the Net Revenues received by Optimer and its Affiliates from a Licensee after the seventh (7th) anniversary of
Commercial Launch by such Licensee. 

        (d)   Optimer
shall pay to Par a one-time milestone payment of Five Million Dollars ($5,000,000) within thirty (30) days after the earlier to occur of the
following events: (i) the Successful Completion by Optimer or its Affiliate of a Phase III Study, (ii) the grant by Optimer to a Third Party of the right to make, develop, market,
distribute, sell or have sold the Product, or (iii) the submission to the United States Food and Drug Administration (or any successor agency) of a new drug application for Product. 

        2.4   Records and Audits.    Par shall have the right, at its own expense, for any period during which a Product for
which a royalty would be payable to Par under Section 2.3(a) or 2.3(b) above is sold hereunder or Net Revenues of which a share would be payable to Par under Section 2.3(c) are received
in connection with or under any license or right granted, transferred or assigned by Optimer to a Third Party and for one (1) year thereafter, to have an independent certified public
accountant, reasonably acceptable to the party being audited, audit the relevant financial books and records of account of Optimer and its Affiliates, Licensees and any Third Party licensed by Optimer
to sell, have sold, market or distribute the Product, but not more frequently than once in each calendar year, during normal business hours, upon reasonable demand, to determine or verify the
financial information relevant to this Agreement including, for example and as applicable, relevant costs of API, royalty amounts due and payable, Net Sales, Net Revenues and appropriate manufacturing
cost of Product, and the like. If errors in Par's favor are discovered as a result of such audit, Optimer shall pay the deficiency with interest immediately. If errors of more than ten percent (10%)
in Par's favor are discovered as a result of such audit, Optimer shall reimburse Par for the expense of such audit. As a condition to such examination, the independent public accountant selected shall
execute a written agreement, reasonably satisfactory in form and substance to the party being audited, to maintain in confidence all information obtained during the course of any such examination
except for disclosure as necessary for the above purpose and all reasonable documents will be delivered to the auditor under these confidential terms. Additionally no auditor may be employed on a
contingency basis. Notwithstanding anything herein to the 

5

 

contrary,
in the event that either Party wishes to conduct an audit of a Licensee or a Third Party that is auditable by Par hereunder, such Party shall notify the other Party and the Parties shall use
commercially reasonable efforts to agree upon an independent third party auditor to conduct such audit for the benefit of both Optimer and Par in order to reduce the likelihood of multiple audits of
the business of such Licensee or Third Party by both Optimer and Par. Optimer hereby covenants that it will not enter into any agreement, contract or arrangement with any such potential Licensee or
Third Party, unless such agreement names Par as a third party beneficiary and under such agreement such party grants Par the right to inspect its records and accounting books in the manner set forth
in this Section and assumes the payment obligation to Par under Section 2.3, as applicable. The foregoing covenant shall not apply (i) to Contractors as set forth in Section 2.5
below, and (ii) on a country-by-country basis, after the seventh (7th) year following a Commercial Launch in such country. The foregoing assumption by a
Licensee or Third Party to be licensed, transferred, assigned, sold or granted any right to the Product shall not in any way relieve Optimer of its obligation to Par under Section 2.3, and
Optimer shall remain responsible to Par for all royalties under Section 2.3 regardless of whom may make the applicable Product sale. For clarity, Optimer and any Licensee or Third Party which
assumes an obligation under Section 2.3 shall be jointly and severally responsible for such payments and Par may not collect any royalty payments in excess of those due Par under
Section 2.3. In the event Optimer enters into an agreement with a Licensee or licenses, transfers, assigns or grants an interest in the Product to a Third Party during the applicable time
periods specified in Section 2.3 above for which payments would be due to Par based on the activities of such Licensee or Third Party, it shall promptly inform Par and provide Par with a copy
of all provisions of such agreement related to payment obligations to Par under Section 2.3 and the obligations as set forth under this Section. The foregoing sentence shall not apply, on a
country-by-country basis, after the seventh (7th) year following a Commercial Launch in such country. 

        2.5   Optimer
agrees not to grant a right or license to make, develop, market, distribute, sell or have sold the Product anywhere in the Territory to a Third Party, or assign
or sell all or some of it rights to the Product during the period from the Termination until the first anniversary thereof without the prior written consent of Par. Such consent may be withheld by Par
in its discretion during the six month period immediately following the Termination and such consent shall not be unreasonably withheld or delayed during the remainder of the one-year
period following the Termination. Notwithstanding the above, Optimer shall be free to hire contractors (including but not limited to contract manufacturers, development and/or formulation services
providers, pre-clinical and clinical service providers and clinical trial sites) to perform services for Optimer, and to grant all necessary rights and licenses to such contractors, in
each case in the normal course of development and commercialization of the Product ("Contractors") so long as Optimer is not receiving any financial consideration or other payments from such third
party contractors in exchange for rights associated with the Product. 

        2.6   Transition Assistance.    Par agrees to use commercially reasonable efforts to cooperate with Optimer and/or
its designee(s) to facilitate a smooth, orderly and prompt transition of the development of the Product to Optimer during the period commencing on the date of the Termination and continuing until
April 30, 2007 or until such earlier date that may be requested by Optimer in writing (the "Transition Period"). Without limiting the foregoing, Par agrees to provide the following transition
assistance to Optimer during the Transition Period: 

        (a)   Upon
the request of Optimer, Par agrees to provide to Optimer a summary of all development (including formulation, manufacturing, pre-clinical and clinical
development work) work being conducted by Par, its Affiliates and/or Third Parties contractors with respect to the Product, including a list of all Third Party contractors conducting such work. 

6

 

        (b)   Upon
the request of Optimer, Par agrees to continue Product development activities, as previously conducted by Par under the Collaboration Agreement, as requested during
the Transition Period or until assumed by Optimer, whichever is earlier, and Par shall be responsible for all of its out-of-pocket costs incurred during the Transition Period
for such activities. 

        (c)   With
respect to any ongoing clinical trials for the Product being conducted by or on behalf of Par or its Affiliates, Par agrees to (i) use commercially
reasonable efforts to promptly transition to Optimer or its designee some or all of such clinical trials and the activities related to or supporting such trials or (ii) use commercially
reasonable efforts to continue to conduct such clinical trials during the Transition Period, in each case as requested by Optimer. To the extent that the foregoing involves the assignment or transfer
of services of, or agreements with, any Third Party, Optimer shall receive assignment or transfer of such services or agreements in its sole discretion. The foregoing notwithstanding, Optimer agrees
that, notwithstanding that certain letter to Advanced Biologics, LLC dated March 22, 2006, it will be responsible for and will assume all costs and payments Par may be responsible for under
such letter
after the Termination or related to services that will be provided by Advanced Biologics, LLC in connection with the Product after the Termination under that certain agreement between Advanced
Biologics, LLC and Optimer related to the Product dated November 16, 2005 (as amended) (the "AB Agreement"), and (i) will send a written letter to Advanced Biologics, LLC (with a copy
going to Par) stating the forgoing, and (ii) indemnify and hold Par harmless with respect to any liability or obligation (including payments) to Advanced Biologics, LLC in connection with, or
arising out of, the AB Agreement arising after the Termination for work performed after the Termination. In the event Optimer elects in writing to receive assignment or transfer of certain services or
agreements, Optimer shall reasonably cooperate with Par to execute and all applicable documentation to effect such transfer or assignment to Optimer, and Optimer shall assume sole and complete
responsibility and liability for such Third Party arrangements. At the request of Optimer, Par will continue to manufacture Product to support the clinical trials for the Product with such Product to
be manufactured, stored, handled, packaged and shipped in accordance with Applicable Laws and the specifications and standards established by the Parties under the Collaboration Agreement, and to
provide quality control and quality assurance support for such Product as requested by Optimer. Optimer agrees to pay Par for such Product at a price equal to Par's Fully-Burdened Costs plus twenty
percent (20%) for Product requested by Optimer under this Section 2.6(c). Par agrees, without any explicit or implied warranty, that all such Product: (i) has been manufactured and
tested in full compliance with Applicable Laws, (ii) fully meets the release specifications established in the certificate of analysis for such Product and (iii) has and will be handled,
stored, packaged and shipped by Par and/or its designee in accordance with Applicable Laws and established procedures. In the event Product does not meet the above criteria, Optimer may, as its sole
remedy, reject such Product received from Par under this Section 2.6(c), by providing written notice to Par of such rejection within thirty (30) days of Optimer's receipt of such
Product. Par's obligation under this Section 2.6(c) shall extend beyond the Transition Period and shall end on December 31, 2007 or until such earlier date that may be requested by
Optimer (or if Optimer fails to request Product for any sixty (60) day period). 

        (d)   Par
agrees to use commercially reasonable efforts to assign to Optimer, and Optimer agrees to assume sole and complete responsibility and liability for all obligations
and liabilities under, that certain Supply Agreement for tiacumicin active pharmaceutical ingredient ("API") between Par and Biocon Limited dated July 15, 2005 (the "Biocon Agreement"). Par
represents and warrants that it has the necessary rights and authority to make such assignment and to Par's knowledge (i) the Biocon Agreement is in full force and effect and (ii) Par is
not 

7

 

in
material breach of any of its material obligations under the Biocon Agreement and all payments for purchase orders of API and all payments under Section 5.2 of the Biocon Agreement have or
will be paid by Par in accordance with the terms of the Biocon Agreement. 

        (e)   Optimer
agrees to purchase all of Par's on-hand inventory of Product at the time of Termination, at a price equal to Par's Fully-Burdened Costs for such
Product, within thirty (30) days of the Termination but Optimer will not be obligated to purchase Product in excess of Two Million Dollars ($2,000,000) in the aggregate. In addition, in the
event Optimer needs any raw material or goods related to the Product (including for example, comparative drugs), other than Product or API ("Materials"), Optimer shall purchase such Materials from
Par's on-hand inventory of Materials, before purchasing any such Materials from a Third Party, at a price equal to the lower of Par's out-of-pocket costs for such
Materials or the price quoted in a bona
fide offer by a bona fide Third Party supplier for such Materials. Par agrees, without any explicit or implied warranty, that such Product and Materials provided by Par to Optimer hereunder:
(i) have been manufactured and tested in full compliance with Applicable Laws, (ii) fully meets the release specifications established in the certificate of analysis for such Product or
Materials, as applicable, and (iii) has and will be handled, stored, packaged and shipped by Par and/or its designee in accordance with Applicable Laws and established procedures. In the event
Product or Materials do not meet the above criteria, Optimer may, as its sole remedy, reject such Product or Materials received from Par under this Section 2.6(e), by providing written notice
to Par of such rejection within thirty (30) days of Optimer's receipt of such Product or Materials. 

        (f)    Optimer
agrees to purchase three (3) approximately thirty-five kilogram (35 kg) batches of API (approximately one hundred and five kilograms (105 kg)
of API in total) from Par at a price equal to Eighteen Thousand Dollars ($18,000) per kilogram within thirty (30) days of the Termination. Par agrees, without any explicit or implied warranty,
that all such API: (i) has been manufactured and tested in full compliance with Applicable Laws, (ii) fully meets the release specifications established in the certificate of analysis
for such API, and (iii) has and will be handled, stored, packaged and shipped by Par and/or its designee in accordance with Applicable Laws and established procedures. In the event API does not
meet the above criteria, Optimer may, as its sole remedy, reject such API received from Par under this Section 2.6(f), by providing written notice to Par of such rejection within thirty
(30) days of Optimer's receipt of such API. All executed batch records and control records for API will be made available to Optimer upon request. Par agrees to store and manage all such API
until such time as Optimer identifies a designee for Par to ship such API to. 

        (g)   Subject
to the occurrence of the conditions precedent of the termination of the Collaboration Agreement per the terms of this Agreement, Par: (i) hereby assigns
to Optimer all of its right, title and interest in all OPT-80 Data solely and exclusively related to the Product; and (ii) hereby grants to Optimer a non-exclusive,
fully-paid up, perpetual, royalty-free worldwide license, including the right to grant and authorize sublicenses, to use all OPT-80 Data primarily related to the
Product but not included in part (i) above for any and all uses in connection with the Product. Par further agrees to grant the foregoing assignment and license, subject to the occurrence of
the conditions precedent of the termination of the Collaboration Agreement per the terms of this Agreement. For clarity, the Parties agree that the OPT-80 Data assigned by Par to Optimer
in part (i) above shall include but not be limited to the following OPT-80 Data related to the Product: research data, data with respect to the current dosage form of the Product,
analytical chemistry data, stability data, polymorphism data, pharmacology data, chemistry, manufacturing and control (CMC) data, pre-clinical and non-clinical data, clinical
data, safety data and regulatory filings and supporting documentation and data (including information in any drug master files). At the request of Optimer, Par shall 

8

 

transfer
to Optimer any and all OPT-80 Data. The Parties agree that the OPT-80 Data described in part (i) above shall be the Confidential Information of Optimer and the
OPT-80 Data described in part (ii) above shall be the Confidential Information of Par, and in each case shall be subject to the provisions of Article 9 of the Collaboration
Agreement, provided that Optimer shall have the right to use and disclose such OPT-80 Data in connection with the development and commercialization of the Product. Par shall, at the
request of Optimer, make available such available Par personnel reasonably familiar with the OPT-80 Data to provide reasonable assistance to Optimer at mutually agreeable times to enable
Optimer to utilize the OPT-80 Data during the Transition Period. The Parties agree that Par shall be responsible for its out-of-pocket costs incurred during the
Transition Period in connection with transferring the OPT-80 Data and providing assistance to Optimer with respect thereto. 

        (h)   Notwithstanding
anything to the contrary, except as specifically provided in Section 2.6(c) Par's obligations with respect to the Product, including any clinical
trials, and the OPT-80 Data shall end at the conclusion of the Transition Period, including any obligation to Optimer under this Agreement; provided, however, that the following
obligations of Par shall continue as contemplated in this Agreement: (i) Par's confidentiality obligations under Section 2.2, (ii) Par's obligations to reasonably cooperate in
audits as described in Section 2.4, (iii) the representations made by Par in Section 2.6(d) with respect to the Biocon Agreement and (iv) Par's obligations under Articles
5, 6, 8 and 12 and Section 9.2. 

        3.    Elective Termination of the Collaboration Agreement.    The Parties hereby agree that,
in the event that Optimer undergoes a Non-Qualified Equity Financing, Optimer may terminate the Collaboration Agreement upon written notice and payment of the Collaboration Agreement
Termination Fee to Par at any time during the Termination Exercise Period, and that the provisions of Sections 2.2-2.6 shall apply in the event of such termination. 

        4.    No Shop.    Par agrees that, during the Termination Exercise Period, it shall not enter
into any agreements granting any rights or licenses to Third Parties with respect to the rights licensed by Optimer to Par under the Collaboration Agreement, the Products or the OPT-80
Data. Despite the foregoing, Par has disclosed to Optimer that Par has engaged in prior discussions with certain Third Parties and may continue such discussions concerning potential business
relationships related to the Product including without limitation potential licenses or other grants of rights related to the Product. Par will use commercially reasonable efforts to involve Optimer
in such discussions. 

        5.    Termination of Section 3(d) of the Amended and Restated Investors' Rights Agreement Dated November 30,
2005.    Optimer and Par hereby agree that Section 3(d) of Optimer's Amended and Restated Investors' Rights Agreement, and all rights and obligations of the
Parties therein, are terminated contingent upon and effective as of the closing of the Equity Financing. Such termination shall be binding upon Par and its Affiliates, successors and assigns. 

        6.    Participation in Over-Allotment Exercise, If Any.    Optimer agrees that, in
the event of an Equity Financing during the Termination Exercise Period, Optimer and Par shall negotiate and enter into an Underwriting Agreement (the "Underwriting Agreement") with the underwriters
of such Equity Financing (the "Underwriters"). Optimer, Par and the Underwriters shall include in the Underwriting Agreement provisions (a) requiring the Underwriters to purchase all or a
portion of Par's shares of Optimer capital stock in the event the Underwriters exercise their option to purchase additional shares of Optimer common stock, and (b) providing that in the event
the Underwriters exercise such option, no other shares of Optimer common stock shall be purchased by the Underwriters pursuant to such option until all of Par's shares of Optimer common stock have
purchased by the Underwriters. The Underwriting Agreement shall contain such customary terms and conditions as needed to effectuate the intentions of Par and Optimer under this Article 6,
including without limitation, customary selling stockholder representations and warranties and indemnification provisions, and as may be negotiated and agreed to by the Parties and the Underwriters.
Par acknowledges and agrees that Par shall be a 

9

 

party
to such Underwriting Agreement as a selling stockholder for purposes of selling shares to the Underwriters in connection with the sale of its shares. Optimer hereby covenants not to enter into
any agreement for the sale of Optimer capital stock to the Underwriters during the Termination Exercise Period other than the foregoing described Underwriting Agreement. This Article 6 shall
not be effective, and no rights granted in this Article 6 shall be enforceable, in the event that Optimer consummates an Equity Financing after the Termination Exercise Period. 

        7.    Non-Participation in Over-Allotment Exercise.    In the event
Par does not sell any or all of its shares of Optimer common stock pursuant to Article 6 above, at the request of Par, Optimer shall promptly deliver, subject to compliance with applicable
securities laws and at its own expense, to Optimer's transfer agent and Par, an opinion of its legal counsel authorizing and allowing for the removal of the legend from Par's stock certificates by
Optimer's transfer agent; in addition, upon a written request by Par to file a registration statement on Form S-3 for a public offering of its shares of Optimer common stock
following the Equity Financing, if any, Optimer shall use its reasonable best efforts to (i) to the extent required, obtain a waiver or amendment Optimer's
Amended and Restated Investor Rights Agreement dated November 30, 2005 in order to permit Optimer to grant the registration right contemplated by this Article 7 and (ii) subject
to (1) obtaining a waiver or amendment as set forth in (i) above, if required, and (2) Optimer's qualification and eligibility for Form S-3 registration at the
time of such filing, cause such registration statement to be filed with the U.S. Securities and Exchange Commission. 

        8.    Release and Disclaimer.    Effective upon the consummation of a Qualified Equity
Financing or the termination of the Collaboration Agreement pursuant to Article 3, each Party, on behalf of itself and its Affiliates, hereby forever releases and discharges the other Party and
its Affiliates, and their respective successor, assigns, servants, agents, employees, officers, directors and shareholders (the "Releasees"), of and from any and all claims and demands of every kind
and nature, known and unknown, suspected and unsuspected, anticipated and unanticipated, disclosed and undisclosed and in particular of and from all claims and demands of every kind and nature, known
and unknown, suspected and unsuspected, disclosed and undisclosed, for damages, whether actual or consequential, past, present and future, arising out of or in any way connected with the Collaboration
Agreement or the Product, except with respect to the past, present and future obligations under this Agreement. The foregoing release shall be binding upon each Party and its Affiliates, and their
respective heirs, personal representatives, successors and assigns, and shall inure to the benefit of each Releasee and their respective heirs, personal representatives, successors and assigns. Par
makes no representation or warranty, and expressly disclaims any liability with respect to the Product, the OPT-80 Data, the API, the Materials or any action or non-actions
related to Par's activity, obligations or rights under the Collaboration Agreement, including but not limited to errors or omissions. Par provides the OPT-80 Data, its interests in the
Product, any Product transferred or sold to Optimer, the API and the Materials "as is" and Par disclaims all other warranties, express or implied, with regard to the foregoing, including the warranty
of merchantability and the warranty of fitness for a particular purpose. For avoidance of doubt, Par makes no representation or warranty of any kind whatsoever with respect to any other use Product or
the likelihood that Optimer will be able to develop or commercialize the Product, and disclaims any and all liabilities in connection with the use of the Product the API or the Materials. 

        9.    Indemnification.    

        9.1   Effective
upon the consummation of a Qualified Equity Financing or the termination of the Collaboration Agreement pursuant to Article 3, Optimer shall promptly
reimburse, defend and indemnify Par and its Affiliates, and their respective successors, assigns, servants, agents, employees, officers, directors and shareholders (the "Par Indemnitees"), and hold
each of them harmless from and against, any and all liabilities and obligations, whether accrued or fixed, absolute or contingent, matured or unmatured, or determined or determinable, including those 

10

 

arising
under any laws, action or governmental order, including any judicial decision and those arising under any contract, agreement, arrangement, commitment or undertaking, or otherwise and any and
all damages, fines, penalties, deficiencies, losses and expenses (including interest, court costs, amounts paid in settlement, reasonable fees and expenses of attorneys, accountants and other experts
or other reasonable expenses of litigation or other proceedings or of any claim, default or assessment) (collectively, a "Liability") suffered, incurred, or sustained by any Par Indemnitee or to which
a Par Indemnitee becomes subject, resulting from, arising out of, or relating to: 

        (a)   any
sale or registration of Optimer stock pursuant to which Par is a selling stockholder in the registration statement in an Equity Financing; 

        (b)   the
Product; 

        (c)   any
agreement, including the Biocon Agreement or any agreement or other arrangement contemplated in connection with Section 2.6(c), assigned or otherwise
transferred to Optimer by Par or its Affiliates; 

        (d)   any
misrepresentation or breach of warranty by Optimer made or contained in this Agreement; and 

        (e)   any
failure of Optimer to perform or observe any covenant or agreement to be performed or observed by Optimer pursuant to this Agreement. 

Notwithstanding
the foregoing, in no event shall Optimer be required to reimburse, defend, indemnify or hold harmless any Par Indemnitee from or against any liabilities or obligations resulting from,
arising out of, or relating to (x) the willful misconduct of Par and (y) with respect to 9.1(a) above, any information or materials provided in writing by Par to be incorporated into
such registration statement. 

        9.2   Indemnification Procedure.    In the event that Par intends to claim indemnification under this
Article 9, Par shall promptly notify Optimer in writing of any claim, complaint, suit, proceeding or cause of action
in respect of which Par intends to claim such indemnification (for purposes of this Article 8, each a "Claim"), and Optimer shall have sole control of the defense and/or settlement thereof;
provided that Par shall have the right to participate, at its own expense, with counsel of its own choosing in the defense and/or settlement of such Claim. The indemnification under this
Article 9 shall not apply to amounts paid with respect to settlement of any Claim if such settlement is effected without the consent of Optimer, which consent will not be unreasonably withheld
or delayed. The failure to deliver written notice to Optimer within a reasonable period of time after the commencement of any such claim, suit or proceeding, if prejudicial to its ability to defend
such action, shall relieve Optimer of any liability to Par under this Article 9, but the omission to so deliver written notice to Optimer shall not relieve Optimer of any liability to Par
otherwise than under this Article 9. Without limiting the foregoing, Par shall keep Optimer fully informed of the progress of any Claim for which it intends to claim indemnification under this
Article 9. Par under this Article 9, and its employees, at Optimer's request and expense, shall provide full information and reasonable assistance to Optimer and its legal
representatives with respect to such Claims covered by this indemnification. 

        10.    Non-Termination of the Collaboration Agreement.    In the event that
Optimer does not undergo a Qualified Equity Financing during the Termination Exercise Period or does not otherwise terminate the Collaboration Agreement under Article 3 above, all of the
Parties' rights and obligations under this Agreement shall terminate and shall not have any effect on the terms and conditions of the Collaboration Agreement, and the Parties' rights and obligations
under the Collaboration Agreement shall continue as set forth therein. 

11

 

        11.    LIMITATION OF LIABILITY.    NOTWITHSTANDING ANYTHING TO THE CONTRARY IN THIS AGREEMENT,
EXCEPT WITH RESPECT TO THIRD PARTY CLAIMS PURSUANT TO THE INDEMNIFICATION OBLIGATIONS SET FORTH IN THIS AGREEMENT, NEITHER PARTY SHALL BE LIABLE TO THE OTHER FOR ANY CONSEQUENTIAL, INCIDENTAL OR
INDIRECT DAMAGES, INCLUDING FOR LOST PROFITS, OR LOSS OF OPPORTUNITY OR USE OF ANY KIND SUFFERED BY THE OTHER PARTY, WHETHER IN CONTRACT, TORT OR OTHERWISE. 

        12.    Miscellaneous.    

12.1    Interpretation.    The article and section headings contained in this Agreement are for convenience of reference only and
shall not affect the meaning or interpretation of this Agreement. As used in this Agreement, any reference to the masculine, feminine or neuter gender shall include all genders, the plural shall
include the singular, and the singular shall include the plural. References herein to a Party or other Person include their respective successors and assigns. The words "include," "includes" and
"including" when used herein shall be deemed to be followed by the phrase "but not limited to." Unless the context otherwise requires, (i) references herein to Articles, or Sections shall be
deemed references to Articles and Sections to this Agreement, and (ii) the words "hereof," "hereby" and "herein" and words of similar meaning when used in this Agreement refer to this Agreement
in its
entirety and not to any particular Article, Section or provision hereof. Except as otherwise expressly provided herein: (a) any reference in this Agreement to any agreement shall mean such
agreement as amended, restated, supplemented or otherwise modified from time to time; (b) any reference in this Agreement to any statute shall include corresponding provisions of any successor
statute and any regulations and rules promulgated pursuant to such statute or such successor statute; (c) all payments required to be made by Optimer hereunder shall be paid in U.S. Dollars by
wire transfer of immediately available funds to the account designated in writing by Par to Optimer from time to time; and (d) all terms of an accounting or financial nature shall be construed
in accordance with GAAP, consistently applied, as in effect from time to time. Neither the captions to Sections or subdivisions thereof shall be deemed to be a part of this Agreement. The provisions
of this Agreement, and the documents and instruments referred to herein, have been prepared, examined, negotiated and revised by each Party hereto and their respective lawyers, and no implication
shall be drawn and no provision shall be construed against any Party hereto by virtue of the purported identity of the drafter of this Agreement, or any portion thereof. 

12.2    Disputes.    In the event of any controversy or claim arising out of, relating to or in connection with any provision of
this Agreement, or the rights or obligations of the Parties hereunder, the Parties shall try in good faith to settle their differences amicably between themselves. If the Parties are unable to settle
a dispute within thirty (30) days the Parties understand and agree that to the extent that Optimer initiates litigation, the venue for such litigation shall be the federal courts in the State
of New Jersey; and to the extent that Par initiates litigation, the venue for such litigation shall be the federal courts in the State of California. 

12.3    Governing Law.    This Agreement shall be governed by and construed in accordance with the substantive law of the State of
Delaware, without regard to the conflicts of law provisions thereof and applicable federal law of the United States. 

12.4    Independent Contractor Status.    It is understood and agreed that the Parties hereto are independent contractors and are
engaged in the operation of their own respective businesses, and neither Party hereto is to be considered the agent of the other Party for any purpose whatsoever, and neither Party shall have any
authority to enter into any contracts or 

12

 

assume
any obligations for the other Party nor make any warranties or representations on behalf of that other Party. 

12.5    Waiver.    The waiver by either Party of a breach of any provisions contained herein may only be pursuant to written
instrument, signed by the waiving Party, and any such waiver shall in no way be construed as a waiver of any succeeding breach of such provision or the general waiver of the provision itself. 

12.6    Assignment.    This Agreement may not be assigned, conveyed or transferred whether by operation of law or otherwise, to any
other party by either Party without the prior written consent of the other; provided, however, that Par may assign this Agreement without such consent to an Affiliate or in connection with a merger
with a Third Party or the sale or transfer or conveyance of all or substantially all of Par's assets or business, provided that written notice of such assignment is given to Optimer and the assignee
consents to, and undertakes full liability under, this Agreement in writing for all the obligations of the assignor hereunder. 

12.7    Entire Agreement.    This Agreement contains the entire agreement of the Parties regarding the subject matter hereof and
supersedes all prior agreements, understandings and negotiations regarding the same, including the Collaboration Agreement and any side letters related thereto. This Agreement may not be changed,
modified, amended or supplemented except by a written instrument signed by both Parties. Furthermore, it is the intention of the Parties that this Agreement is controlling over additional or different
terms of any order, confirmation, invoice or similar document, even if accepted in writing by both Parties, and that waivers and amendments shall be effective only if made by
non-pre-printed agreements clearly understood by both Parties to be an amendment or waiver. 

12.8    Severability.    If any provision of this Agreement shall be held illegal or unenforceable, that provision shall be limited
or eliminated to the minimum extent necessary, and the remaining provisions of this Agreement shall otherwise remain in full force and effect and enforceable. 

12.9    Further Assurances.    Each Party hereto agrees to execute, acknowledge and deliver such further instruments, and to do or
cause to be done such other acts, including without limitation the filing of such assignments, agreements, documents and instruments, as may be necessary or appropriate, in order to carry out the
purposes and intent of this Agreement. 

12.10    Use of Party's Name.    No right, express or implied, is granted by this Agreement to either Party to use the name of the
other or any other trade name or trademark of the other in any manner for the promotion or advertisement of the Product, except as mutually agreed by the Parties. For clarity, it is understood that
nothing herein shall prohibit either Party from using the name of the other Party (i) in certain of such Party's disclosure documents including those provided to investors, potential investors,
advisors, partners, potential acquirers and Affiliates of such Party, or filed or disclosed in order to comply with its obligations under Applicable Law or the listing standards or agreements of any
national or international securities exchange or The NASDAQ Stock Market or other similar laws of a governmental authority, (ii) to respond to an inquiry of a governmental authority, or
(iii) in a judicial, administrative or arbitration proceeding, or from disclosing the fact that it has granted or obtained a license to the intellectual property rights of the other Party so
long as such use of the other's name is limited to statements of fact and is not done in a manner to suggest or imply endorsement by the other Party. 

12.11    Notice and Reports.    All notices, consents or approvals required by this Agreement shall be in writing sent by certified
or registered air mail, postage prepaid or by confirmed facsimile (also to be confirmed by such certified or registered mail) to the Parties at the 

13

 

following
addresses or such other addresses as may be designated in writing by the respective Parties: 

To Optimer:

Optimer
Pharmaceuticals, Inc.

10110 Sorrento Valley Rd, Suite C

San Diego, CA 92121

Attention: Michael Chang 

With
copies to: 

Martin
Waters, Esq.

Wilson Sonsini Goodrich & Rosati

12235 El Camino Real, Suite 200

San Diego, CA 92130

Fax: (858)-350-2399 

To Par:

Par
Pharmaceutical, Inc.

300 Tice Boulevard

Woodcliff Lake, New Jersey 07677

Attention: Tom Haughey, Esq. & Paul Campanelli 

With
copies to: 

Frommer
Lawrence & Haug LLP

745 Fifth Avenue

New York, New York 10151

Attn: Arthur Hoag, Esq.

Fax: (212) 588-0800 

Notices
given to the Parties shall be deemed effective on the date of receipt if sent by mail, and on the date of transmission if sent by confirmed facsimile. It is understood that notice to the
Parties' counsel as set forth above is for courtesy only, and shall not serve as effective notice hereunder independently of notice to the Party. 

        12.12    Both Parties hereby warrant and represent that: (i) they have the power and authority to enter into this
Agreement and perform the responsibilities and obligations herein and the execution and delivery of this Agreement has been duly authorized; (ii) they have the power to carry out their
obligations under this Agreement; and (iii) nothing in this Agreement or in the execution or performance thereof shall constitute a breach, violation or default of any provision contained in
such Party's certificate or articles of incorporation or other organizing instruments nor violate any contract or other commitment made by such Party.    

        12.13    Force Majeure.    A Party shall not be liable for nonperformance or delay in
performance (other than of obligations regarding payment of money or confidentiality or non-performance due to such Party's negligence) caused by any event reasonably beyond the control of
such Party including, but not limited to wars, hostilities, revolutions, riots, civil commotion, national emergency, strikes, lockouts, unavailability of supplies, epidemics, fire, flood, earthquake,
force of nature, explosion, embargo, or any other Act of God, or any law, proclamation, regulation, ordinance, or other act or order of any court, government or governmental agency. 

14

 

EXECUTION COPY 

        IN
WITNESS WHEREOF the Parties hereto have caused this Prospective Buy-Back Agreement to be executed by their duly authorized officers upon the Effective Date. 

	 OPTIMER PHARMACEUTICALS, INC.	 	PAR PHARMACEUTICAL, INC.
	

By:	

	
 	

By:	

	

Name:	

	
 	

Name:	

	

Title:	

	
 	

Title:	

15

QuickLinks

Exhibit 10.25

PROSPECTIVE BUY-BACK AGREEMENT

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