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                                                                   EXHIBIT 10.21

      EXECUTIVE EMPLOYMENT AGREEMENT, dated as of April 27, 2005 (this
      "Agreement"), by and between ImaRx Therapeutics, Inc., a Delaware (the
      "Company") and Greg Cobb (the "Executive").

      The Company and the Executive desire to set forth in this Agreement the
terms and conditions upon which the Executive will be employed by the Company as
the Chief Financial Officer of the Company. In consideration of the foregoing
recital and the mutual promises, representations, warranties, and covenants
hereinafter set forth, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereby
agree as follows:

      1.    CERTAIN DEFINITIONS.

            For the purposes of this Agreement, the following terms shall have
the following meanings:

            "$" means dollars in the lawful currency of the United States of
America.

            "Base Salary" shall have the meaning assigned to such term in
Section 4.1.

            "Board" means the Board of Directors of the Company.

            "Cause" means Executive shall have (i) been indicted for a felony;
(ii) committed an act of willful misconduct or negligence resulting in a
material loss to the Company; (iii) materially violated any material written
Company policy or rules of the Company, unless cured by Executive within 30 days
following written notice thereof to Executive, (iv) refused to follow the
reasonable written directions given by the President and CEO or his designee or
materially breached any covenant or obligation under this Agreement or other
agreement with the Company, unless cured by Executive within 30 days following
written notice thereof to Executive, or (v) the death or Disability of
Executive.

            "Change of Control" means any consolidation or merger of the Company
with or into any other Company or other entity or person, or any other corporate
reorganization, in which the stockholders of the Company immediately prior to
such consolidation, merger or reorganization, own less than fifty percent (50%)
of the Company's voting power immediately after such consolidation, merger or
reorganization, or any transaction or series of related transactions to which
the Company is a party resulting in the sale of substantially all of the
Company's assets or a transfer of the Company's voting power in excess of fifty
percent (50%), excluding any consolidation or merger effected exclusively to
change the domicile of the Company.

            "Common Stock" means the common stock, par value $0.001 per share,
of the Company.

            "Disability" means Executive's adjudication as mentally incompetent,
or the occurrence of a mental or physical disability preventing Executive from
performing his duties for

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one hundred and eighty (180) or more days within any period of three hundred and
sixty five (365) days or any period of ninety (90) consecutive days. Any
question as to the existence of Disability as to which Executive and the Company
cannot agree shall be determined in writing by a qualified independent physician
selected by the Company. The determination of Disability made in writing by such
physician to Executive and the Company shall be final and conclusive for all
purposes.

            "Effective Date" means the first date written above.

            "Good Reason" shall mean (i) a material reduction in Executive's
title, status, authority, or responsibility at the Company; provided however,
that naming Evan C. Unger, M.D. as Chairman of the Board following the hiring of
a new Chief Executive Officer shall not be considered a reduction in Executive's
title, status, or authority or Good Reason under this Agreement or (ii) any
breach by the Company of any material term of this Agreement (including, without
limitation, failure to make any payment or to grant any stock options required
by the terms of this Agreement), which breach is not cured within thirty (30)
days of the Company's receipt from Executive of written notice of such breach.

            "Person" means an individual, partnership, corporation, limited
liability company, limited liability partnership, trust, joint venture, joint
stock company or unincorporated organization.

      1.    "Term" means this Agreement and Executive's employment hereunder
shall commence on the Effective Date, and shall continue until this Agreement
and Executive's employment hereunder is terminated as provided in Section 5.

            "Termination Date" means the first date on which the Executive is no
longer employed by the Company for any reason.

            "Transferee" means any person or entity to whom or which shares of
Executive Stock have been sold, transferred or assigned, by agreement or
operation of law.

      2.    TERM.

            The Company shall employ Executive for the Term or until the
Termination Date, this being an "at-will" employment agreement.

      3.    POSITION AND DUTIES.

            3.1. General. Executive shall serve as the Chief Financial Officer
and shall be subject to the supervision of, and shall report to, the President
and CEO. Executive shall have such duties and authority as are customarily
exercised by a Chief Financial Officer of a business corporation of similar size
and in the same industry as the Company and as may be determined from time to
time by the President and CEO.

            3.2. Location. Executive shall perform duties in accordance with
Section 3.1 throughout Term from the Company offices located in Tucson, Arizona.

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      4.    COMPENSATION.

            4.1. Base Salary. Commencing on the Effective Date, the Company
shall pay to Executive a base salary at an annual rate of $150,000 (as may be
adjusted from time to time, the "Base Salary"), payable in accordance with the
payroll practices of the Company in effect from time to time. Base Salary
payments shall be subject to withholding and other applicable taxes.

            4.2. Annual Bonus. With respect to each full fiscal year until the
Termination Date, Executive shall be eligible to receive an annual bonus award
of up to 50% of Base Salary, as determined by annual pre-determined milestones
which shall be mutually agreeable to Executive and the Board in its sole
discretion.

            4.3. Option Grant. The Executive shall be granted an option pursuant
to an option agreement, in substantially the form attached hereto as Exhibit B
(the "Stock Option Agreement") to purchase 150,000 shares of Company Common
Stock. The Executive Option may be exercised, from time to time, with respect to
all or any part of the then vested shares in accordance with the following
vesting schedule: 15,000 shares shall vest immediately; 33,750 shares shall vest
over four years on each anniversary date following the Effective Date; provided,
however, all shares subject to the Executive Option shall immediately vest upon
the consummation of a Change of Control..

            4.4. Employee Benefits. Until the Termination Date, or such later
time as provided in Section 5, Executive shall be entitled to receive health and
disability insurance and the opportunity to participate in any 401(k) plan to
the same extent as such benefits are in effect from time to time for other
executive officers of the Company ("Benefits").

            4.5. Moving Expenses Allowance. Executive shall be entitled to
reimbursement by the Company to relocate from San Ramon, CA to Tucson, AZ.
Reimbursement for all actual and reasonable moving expenses up to $35,000 is to
be paid after the move is completed. To be eligible for this moving expenses
allowance you much complete your move by the end of August 2005. Up until the
earlier of your move or August 31, 2005, the Company will reimburse to you,
subject to presentment of appropriate supporting documentation, for up to $3,000
per month in reasonable apartment, auto rental and travel expenses incurred by
you prior to relocating to Tucson. If you decide to leave Company's employment
within 18 months after your move, you must repay these moving expenses, within
30 days after you leave the Company, prorated by the number of full months
remaining in this 18-month period. (For example, if you leave Company after 6
months, you would be obligated to repay 12/18 of the moving expenses.) In such
an event, Company may offset these moving expenses against amounts the Company
owes you at the time of your departure.

            4.6. Expense Reimbursement and Housing Allowance. Executive shall be
entitled to reimbursement by the Company for all reasonable travel, lodging, and
other expenses actually incurred by Executive in connection with the performance
of his duties, against receipts or other appropriate written evidence of such
expenditures as required by the appropriate United States Internal Revenue
Service regulations and, for any expenses in excess of $1,000

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individually or $10,000 in the aggregate, subject to the Chief Executive
Officer's prior approval.

      5.    TERMINATION.

            5.1. Disability. At the election of the Company, Executive's
employment and this Agreement shall terminate upon Executive's becoming totally
or permanently disabled. For purposes of this Agreement, the term "totally or
permanently disabled" or "total or permanent disability" means any physical or
mental condition that prevents Executive from performing Executive's employment
duties and responsibilities under this Agreement for a period of more than 90
consecutive days or for more than 180 days during any twelve (12) month period.
A reasonable determination by the Company of the existence of a disability shall
be conclusive for all purposes hereunder. In making such determination of
disability, the Company may utilize such advice and consultation as it deems
appropriate, but there is no requirement of procedure or formality associated
with the making of a determination of disability.

            5.2. Death of or Resignation by Executive. Executive's employment
and this Agreement shall terminate immediately upon the death of or resignation
by Executive.

            5.3. Termination for Cause or without Good Reason. If (a) the
Company terminates Executive's employment at any time for Cause or (b) Executive
terminates his employment at any time other than for Good Reason, Executive
shall be entitled to receive a pro rata portion of his Base Salary through the
Termination Date. Other than the payments described in this subsection,
Executive shall be entitled to no other payments from the Company.

            5.4. Termination Without Cause. The Company may terminate
Executive's employment and this Agreement at any time without cause or reason
upon written notice to Executive. Termination "without cause" shall mean
termination by the Company of employment on any basis other than termination of
Executive's employment hereunder pursuant to Section 5.3.

            5.5. No Additional Effect on Executive Option. The right of
Executive to retain the vested portion of his rights under the Executive Option
shall be governed by the Stock Option Agreement entered into by and between the
Company and Executive.

      6.    PROPRIETARY RIGHTS AGREEMENT AND ARBITRATION AGREEMENT. The parties
acknowledge and incorporate that certain Invention and Confidential Information
Agreement ("Invention and Confidential Information Agreement") and Arbitration
Agreement ("Arbitration Agreement") executed by Executive, copies of which are
attached hereto as Exhibit A.

      7.    MISCELLANEOUS.

            7.1. Survival. The representations, warranties, covenants and
agreements made herein shall survive the execution of this Agreement.

            7.2. Assignment. This Agreement and the rights and obligations
hereunder shall be assignable only with the prior written consent of each of the
parties; provided, however,

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that the Company may assign its rights under this Agreement to the surviving
entity pursuant to a sale of substantially all of the Company's assets, by
merger or consolidation or by a sale of all of the Company's capital stock
(provided that the surviving entity shall remain liable as to all obligations of
the Company under this Agreement).

            7.3. Benefits of Agreement. This Agreement and all obligations of
the parties shall be binding upon, and inure to the benefit of, their respective
successors and assigns.

            7.4. Severability. In case any provision of the Agreement shall be
invalid, illegal or unenforceable, the validity, legality and enforceability of
the remaining provisions shall not in any way be affected or impaired thereby,
and the court making the determination of invalidity, illegality, or
unenforceability shall have the power to reduce the scope, duration or area of
the term or provision, to delete specific words or phrases, or to replace any
invalid, illegal, or unenforceable term or provision with a term or provision
that is valid, legal and enforceable and that comes closest to expressing the
intention of the invalid or unenforceable term or provision, and this Agreement
shall be enforceable as so modified.

            7.5. Further Assurances. Each party agrees to execute such other
documents, instruments, agreements and consents, and take such other actions as
may be reasonably requested by the other parties hereto to effectuate the
purposes of this Agreement.

            7.6. Amendment and Waiver. This Agreement may be amended, modified
or waived only with prior written consent of each of the parties.

            7.7. Delays or Omissions. It is agreed that no delay or omission to
exercise any right, power or remedy accruing to any party, upon any breach,
default or noncompliance by another party under this Agreement shall impair any
such right, power or remedy, nor shall it be construed to be a waiver of any
such breach, default or noncompliance, or any acquiescence therein, or of or in
any similar breach, default or noncompliance thereafter occurring. All remedies
shall be cumulative and not alternative.

            7.8. Notices. All notices required or permitted hereunder shall be
in writing and shall be deemed effectively given: (a) upon personal delivery to
the party to be notified, (b) when sent by confirmed facsimile if sent during
normal business hours of the recipient, if not, then on the next business day,
(c) five (5) days after having been sent by registered or certified mail, return
receipt requested, postage prepaid, or (d) one (1) day after deposit with a
nationally recognized overnight courier, specifying next day delivery, with
written verification of receipt. Any notice herein required or permitted to be
given shall be given by depositing the same in the United States first class
mail, postage prepaid, or hand delivered or transmitted by facsimile, in any
case with a copy sent by overnight courier service, and addressed to the parties
as follows:

                 If to the Company: ImaRx Therapeutics, Inc.
                                    1635 East 18th Street
                                    Tucson, AZ  85719
                                    Attention: Chief Financial Officer
                                    Facsimile: (520) 791-2437

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                 If to Executive:   Greg Cobb
                                    Bishop Ranch 3
                                    2603 Camino Ramon, Suite 200
                                    San Ramon, CA 94583

or, to such other address or facsimile number as the party to whom notice is to
be given may have furnished to the other parties in writing in accordance
herewith.

            7.9. Titles and Subtitles. The titles of the sections and
subsections of the Agreement are for convenience of reference only and are not
to be considered in construing this Agreement.

            7.10. Counterparts. This Agreement may be executed in any number of
counterparts (facsimile or otherwise), each of which shall be an original, but
all of which together shall constitute one instrument.

            7.11. Pronouns. All pronouns contained herein, and any variations
thereof, shall be deemed to refer to the masculine, feminine or neutral,
singular or plural, as the identity of the parties hereto may require.

            7.12. Governing Law. This Agreement shall be construed in accordance
with, and governed by, the laws of the State of Arizona (without giving effect
to its conflict of laws principles).

            7.13 Prior Agreements. This Agreement, the Stock Option Agreement,
the Proprietary Rights Agreement and the Arbitration Agreement contain the
entire agreement of the parties relating to the subject matter hereof and
supersede all prior agreements and understandings with respect to such subject
matter, and the parties hereto have made no agreements, representations or
warranties relating to the subject matter of this Agreement, the Stock Option
Agreement, the Proprietary Rights Agreement or the Arbitration Agreement that
are not set forth herein or therein.

                            (SIGNATURE PAGE FOLLOWS)

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      IN WITNESS WHEREOF, the parties have executed this Executive Employment
Agreement as of the Effective Date.

                                            THE COMPANY:

                                            IMARX THERAPEUTICS, INC.

                                            By:    /s/ Evan C. Unger
                                                ------------------------------
                                                Name:  Evan C. Unger, M.D.
                                                Title: President and CEO

                                            THE EXECUTIVE:

                                               /S/ GREG COBB
                                            ----------------------------------
                                            GREG COBB

Exhibit A: Attach copy of signed Proprietary Rights Agreement and Arbitration
           Agreement.

Exhibit B: Attach copy of form of Stock Option Agreement.

                  EXECUTIVE EMPLOYMENT AGREEMENT SIGNATURE PAGE