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                                                                   Exhibit 10.18

                    AMENDED EXECUTIVE EMPLOYMENT AGREEMENT

      THIS AMENDED EXECUTIVE EMPLOYMENT AGREEMENT (the "Agreement") is made and
entered into as of August 8, 2005, by and between IMARX THERAPEUTICS, INC., a
Delaware corporation (the "Company"), and EVAN C. UNGER ( "Executive") and
replaces the Executive Employment Agreement dated July 12, 2004 (the "Former
Agreement").

      WITNESSETH:

      WHEREAS, the Company desires to retain the services of Executive, and
Executive desires to be employed by the Company, on the terms and conditions of
this Agreement.

      NOW, THEREFORE, in consideration of the premises and the mutual covenants
and agreements set forth herein, the Company and Executive, intending to be
legally bound, hereby agree as follows:

      1. Employment. The Company agrees to employ Executive as President and
Chief Executive Officer of the Company, and Executive accepts such employment
and agrees to perform full-time employment services for the Company, subject to
Section 3.2 and the Third Amended and Restated Certificate of Incorporation and
Bylaws of the Company, as amended from time to time (the "Organizational
Documents"), and the resolutions of the Board of Directors of the Company (the
"Board"), for the period and upon the other terms and conditions set forth in
this Agreement.

      2. Term. The term of Executive's employment hereunder (the "Term") shall
commence upon the date of execution of the Former Agreement and shall continue
until this Agreement is terminated as set forth in Section 5 below.

      3. Position and Duties.

            3.1. Service with the Company. During the Term of this Agreement,
Executive agrees to perform the duties of the President and Chief Executive
Officer of the Company as set forth in the Organizational Documents and such
other duties as the Board may from time to time prescribe that are consistent
with the duties of a Chief Executive Officer of a company of the size and nature
of the Company. In addition, during the Term, Executive shall serve without any
additional compensation on the Company's Board. The Company agrees that
Executive shall remain a member of its Board, and furthermore that should the
Executive be relieved of his position as CEO as a result of the hiring of or
promotion of a person to that position, then Executive shall automatically be
appointed to position as Chairman of the Board and Chief Scientific Officer. The
position of Executive as member of the Board does not depend upon continued
employment by Company.

            3.2. No Conflicting Duties. Executive hereby confirms that he is
under no contractual commitments inconsistent with his obligations set forth in
this Agreement, and that

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during the Term of this Agreement, he will not render or perform services, or
enter into any contract to do so, for any other corporation, firm, entity or
person that are inconsistent with the provisions of this Agreement or
Executive's fiduciary obligations to the Company. Except as otherwise provided
herein, Executive shall not serve as a director of any other corporation (except
nonprofit organizations to the extent such service does not materially affect
Executive's performance of his duties for the Company) without the prior
approval of the Board of Directors. Company acknowledges that Executive is
Professor of Radiology and Bioengineering at the University of Arizona and
furthermore approves Executive to work up to 6 days per month at the University
of Arizona (or other University) in research, administration, teaching and/or
clinical work.

      4. Compensation and Benefits.

            4.1. Base Salary. (a) As compensation for all services to be
rendered by Executive under this Agreement, the Company shall pay to Executive
during the Term an annual salary of $250,000 (the "Base Salary"). Subject to
Section 4.1(b), the Base Salary shall be reviewed at least annually and changed
at the discretion of the Board (or its Compensation Committee), provided,
however, that the Base Salary may not be decreased without the written consent
of the Executive. The Company shall pay the Base Salary to Executive in
accordance with the Company's normal payroll procedures and policies.

      (b) If Executive's Base Salary is increased at any time, it shall not
thereafter be decreased during the Term of this Agreement, unless such decrease
is the result of a general reduction (on the same percentage basis) affecting
the base salaries of all other executive officers of the Company and such
decrease does not result in a Base Salary of less than $200,000.

            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, payable quarterly, as determined by annual
pre-determined milestones, attached as Exhibit A, which shall be mutually
agreeable to Executive and the Board (or its Compensation Committee) in its sole
discretion.

            4.3. Stock Options. (a) As of the commencement of the Term of this
Agreement, the Company shall grant (the "Original Grant") to Executive an
incentive stock option to purchase 600,000 shares of the Company's common stock
(the "Shares"), pursuant to the Company's 2000 Stock Plan (the "Plan"), which
shall have a term of ten years, shall vest and be exercisable as hereinafter set
forth, and shall otherwise be in the form attached hereto as Exhibit C. The
Original Grant shall, to the extent permissible, be for "incentive stock
options" and otherwise shall be for "nonqualified stock options", and the
exercise price per share shall be the fair market value of the Company's common
stock at the time of grant as determined by the Board in its good faith
discretion. Subject to Executive being employed hereunder at the time of each
vesting, as applicable, the Original Grant shall vest and become exercisable as
follows: 45,000 shares shares shall vest annually on each anniversary date
following the Effective Date of the Former Agreement over four years; and
420,000 shares shall vest according to the milestones

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established in Exhibit B. In addition, a portion of the unvested Options shall
vest as described in Sections 6.3 and 7 hereof. If the Company's common stock is
registered under Section 12 of the Securities Exchange Act of 1934 (the
"Exchange Act"), the Company shall use its good faith efforts to register under
the Securities Act of 1933 on Form S-8 (or any successor form) all common stock
issued and/or issuable under the 2000 Stock Plan, including the Shares subject
to the Original Grant and all other stock options or incentives granted or
awarded to Executive under the 2000 Stock Plan, and its good faith efforts to
qualify such common stock for sale under such state securities or `blue sky'
laws as the Company shall determine are required to issue the shares of common
stock under the 2000 Stock Plan and for the resale of such shares by the
recipients thereof, provided that the Company shall have no obligation to
qualify such stock in any particular jurisdiction in which the Company would be
required to execute a general consent to service of process in effecting such
qualification, unless the Company is already subject to service in such
jurisdiction and except as may be required by the Securities Act.

            4.4. Participation in Benefit Plans. Executive shall be included to
the extent eligible thereunder in any and all plans of the Company providing
general benefits for the Company's executive employees, including, without
limitation, medical, dental, vision, short and long term disability insurance,
life insurance, 401(k) plan, sick days, vacation, and holidays. Executive's
participation in any such plan or program shall be subject to the provisions,
rules, and regulations applicable thereto. In addition, during the Term of this
Agreement, Executive shall be eligible to participate in all non-qualified
deferred compensation and similar compensation, incentive, bonus, profit sharing
and stock plans offered, sponsored or established by Company on substantially
the same or a more favorable basis as any other employee of Company.

            4.5. Business Expenses. In accordance with the Company's policies
established from time to time, the Company will pay or reimburse Executive for
all reasonable and necessary out-of-pocket expenses incurred by him in the
performance of his duties under this Agreement, subject to the presentment of
appropriate supporting documentation. During the Term of this Agreement, the
Company shall at its expense provide Executive with reasonable office space and
furnishings (including without limitation desk and lap top computers) at the
Company's principal executive offices, and a cell telephone.

            4.6. Key Man Life Insurance. During the Term of this Agreement, the
Company shall have the option of purchasing and paying the premiums for a "Key
Man" life insurance policy relating to Executive in a coverage amount determined
by the Company, and the Company shall be named as the beneficiary of such
policy. Executive represents and warrants that he currently is insurable for
such policy on an unrated basis and agrees to fully cooperate with the Company
in obtaining the policy.

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      5. Termination.

            5.1. Disability. At the Company's election, Executive's employment
and this Agreement shall terminate upon Executive's becoming totally or
permanently disabled for a period of ninety (90) days or more in any twelve (12)
month period. For purposes of this Agreement, the term "totally or permanently
disabled" or "total or permanent disability" means Executive's inability on
account of sickness or accident, whether or not job-related, to engage in
regularly or to perform adequately his assigned duties under this Agreement and
Executive is qualified and eligible to receive disability benefits under the
disability policies maintained by the Company for Executive.

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

            5.3. Termination for Cause. The Company may terminate Executive's
employment and this Agreement at any time for "Cause" (as hereinafter defined)
immediately upon written notice to Executive. As used herein, the term "Cause"
shall mean that Executive shall have (i) been convicted of a felony; or (ii)
committed a single act of fraud, embezzlement, or breach of trust, or (iii)
committed an act of willful misconduct or gross negligence resulting in a
material loss to the Company, or (iv) 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, or (v) refused to follow the
reasonable written directions given by the Board or its 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.

            5.4. Resignation. Executive's employment and this Agreement shall
terminate on the earlier of the date that is one (1) month following the written
submission of Executive's resignation to the Company or the date such
resignation is accepted by the Company.

            5.5. Termination Without Cause. The Company may terminate
Executive's employment and this Agreement without cause upon written notice to
Executive. Termination "without cause" shall mean termination of employment on
any basis (including no reason or no cause) other than termination of
Executive's employment hereunder pursuant to Sections 5.1, 5.2, 5.3, or 5.4.

            5.6. Surrender of Records and Property. Upon termination of his
employment with the Company, Executive shall deliver promptly to the Company all
credit cards, computer equipment, cellular telephone, records, manuals, books,
blank forms, documents, letters, memoranda, notes, notebooks, reports, data,
tables, calculations or copies thereof, that are the property of the Company and
that relate in any way to the business, strategies, products, practices,
processes, policies or techniques of the Company, and all other property, trade
secrets and confidential information of the Company, including, but not limited
to, all documents that in whole or in part contain any trade secrets or
confidential information of the Company that in any

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of these cases are in his possession or under his control, and Executive shall
also remove all such information from any personal computers that he owns or
controls.

      6. Compensation Upon the Termination of Executive's Employment.

            6.1. In the event that Executive's employment and this Agreement are
terminated pursuant to Section 5.1 (Disability), 5.3 (Cause), or 5.4
(Resignation), then Executive shall be entitled to receive Executive's then
current Base Salary through the date his employment is terminated, and such
other amounts that accrued prior to the termination date and are required to be
paid to him pursuant to any employee benefit plan in accordance with such plan
and/or by law, but no other compensation of any kind or amount.

            6.2. In the event Executive's employment and this Agreement are
terminated pursuant to Section 5.2 (Death), Executive's beneficiary or a
beneficiary designated by Executive in writing to the Company, or in the absence
of such beneficiary, Executive's estate, shall be entitled to receive
Executive's then current Base Salary through the end of the month in which his
death occurs, and such other amounts that accrued prior to the termination date
and are required to be paid to him pursuant to any employee benefit plan in
accordance with such plan and/or by law, but no other compensation of any kind
or amount.

            6.3. Unless Section 7 applies, in the event Executive's employment
and this Agreement are terminated by the Company pursuant to Section 5.5
(Without Cause) or by Executive for "Good Reason" (as defined below), (A) the
Company shall pay to Executive, as a severance allowance, his then current
monthly Base Salary for the six (6) month period following the date of
termination, paid on the Company's regular paydays throughout that 6-month
period, followed by a one-time lump sum payment (the "Lump Sum Payment") on the
next regular payday of an amount equal to six months of his then current monthly
Base Salary (i.e. one-half of his annual Base Salary), for a total severance
allowance equal to one (1) year of his Base Salary, and such other salary that
accrued prior to the termination date and amounts required to be paid to him
pursuant to any employee benefit plan in accordance with such plan and/or by
law; and (B) Executive shall receive accelerated vesting for twelve (12) months
from the date of Executive's termination for all stock options granted by the
Company to Executive before or after the Commencement Date and extension of the
option exercise period for an additional twelve (12) months beyond the period
set forth in Exhibit A attached hereto for such exercise, provided, however,
that such stock options shall not be extended beyond the date on which they
would have terminated had Executive continued to be employed by the Company; but
no other compensation or benefits of any kind. Executive shall be entitled to
receive these benefits and payments only if he complies with his continuing
obligations to the Company as set forth in this Agreement, provided, however,
that any deductions to the benefits described in Section 6.3(A) are subject only
to the offsets set forth in Section 13.4, and provided, further, that with
respect to the benefits described in Section 6.3(B), the Company shall not deny
any such benefits unless it has provided written notice to Executive of any
claimed breach of such continuing obligations and Executive has cured each such
breach within fifteen days after receipt of such written notice of breach and
the exercise period of any such options shall be tolled during such fifteen (15)
day period or any longer period required to resolve any disputed claim of

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breach of such continuing obligations, and provided, further, that the foregoing
shall not limit Executive's ability to exercise any such options that had vested
prior to the date of termination of his employment which he exercises within
ninety (90) days after such termination date.

            6.4. The provisions of Section 6 shall not affect Executive's
participation in or terminating distributions and vested rights under, any
pension, profit sharing, insurance or other employee benefit plan of the Company
to which Executive is entitled pursuant to the terms of such plans.

      7. Change in Control. In the event a Change in Control (as defined below)
occurs and, in the six month periods preceding or following the Change in
Control, Executive's employment and this Agreement are terminated by the Company
or its successor, assigns or transferee pursuant to Section 5.5 (Without Cause)
or by Executive for "Good Reason" (as defined below), then the Company shall,
within thirty (30) days after occurrence of the last of these conditions (the
"Trigger Date"), pay Executive a lump sum amount equal to one hundred percent
(100%) of Executive's then current annual Base Salary (less applicable
withholdings). In addition, one hundred percent (100%) of Executive's unvested
options (whether granted before or after the Commencement Date) as of the
Trigger Date, shall automatically vest as of the Trigger Date and the exercise
period for all such stock options shall be extended an additional twelve (12)
months.

      7.1. Definition of Change in Control. For purposes of this Agreement, a
"Change in Control" shall be deemed to have
occurred if, at any time during the Term, any of the following events occurs:

            7.1.1. if the Company does not have a class of securities registered
under Section 12 of the Securities Exchange Act of 1934, as amended (the
"Exchange Act"):

                  7.1.1.1. any person, as that term is used in Section 13(d) and
            Section 14(d)(2) of the Exchange Act, becomes a beneficial owner (as
            defined in Rule 13d-3 under the Exchange Act or any successor rule
            or regulation), directly or indirectly, of securities of the Company
            representing 50% or more of the combined voting power of the
            Company's then outstanding Voting Stock, as defined below, provided,
            however, that for purposes of this paragraph, the term "person"
            shall exclude (A) a trustee or other fiduciary holding securities
            under an employee benefit plan of the Company or any of its
            subsidiaries, (B) a person who beneficially owns 25% or more of the
            Company's outstanding Voting Stock on the Effective Date, or (C) any
            person who becomes such a beneficial owner in connection with the
            Company's financing.

                  7.1.1.2. the Company is merged, consolidated or reorganized
            into or with another corporation or other legal person (an
            "Acquiring Person") or securities of the Company are exchanged for
            securities of an Acquiring Person, and as a result of such merger,
            consolidation, reorganization or exchange less than a majority of
            the combined voting power of the then outstanding

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            securities of the Acquiring Person immediately after such
            transaction are held, directly or indirectly, in the aggregate by
            the holders of securities entitled to vote generally in the election
            of directors ("Voting Stock") of the Company immediately prior to
            such transaction;

                  7.1.1.3. the Company, in any transaction or series of related
            transactions, sells or otherwise transfers, directly or indirectly,
            all or substantially all of its assets, on a consolidated basis, to
            an Acquiring Person, and less than a majority of the combined voting
            power of the then outstanding securities of the Acquiring Person
            immediately after such sale or transfer are held, directly or
            indirectly, in the aggregate by the holders of Voting Stock of the
            Company immediately prior to such sale or transfer; or

                  7.1.1.4. during any period of two consecutive years,
            commencing upon the closing of the Company's private placement
            financing and the appointment of the Board of Directors in
            connection therewith, individuals who at the beginning of any such
            period constitute the directors of the Company cease for any reason
            to constitute at least a majority thereof, unless the election, or
            the nomination for election by the Company's stockholders, of each
            director of the Company first elected during such period was
            approved by at least a majority vote of the directors of the Company
            then still in office who were directors of the Company at the
            beginning of any such period;

            OR

            7.1.2. if the Company has a class of securities registered under
Section 12 of the Exchange Act:

                  7.1.2.1. any person, as that term is used in Section 13(d) and
            Section 14(d)(2) of the Exchange Act, becomes, is discovered to be,
            or files a report on Schedule 13D or 14D-1 (or any successor
            schedule, form or report) disclosing that such person is, a
            beneficial owner (as defined in Rule 13d-3 under the Exchange Act or
            any successor rule or regulation), directly or indirectly, of
            securities of the Company representing 25% or more of the combined
            voting power of the Company's then outstanding Voting Stock,
            provided, however, that for purposes of this paragraph, the term
            "person" shall exclude (A) a trustee or other fiduciary holding
            securities under an employee benefit plan of the Company or any of
            its subsidiaries, (B) a person who becomes such a beneficial owner
            in connection with the Company's initial public offering of common
            stock or (C) any person who was a shareholder of the Company
            immediately prior to the Company's initial public offering of common
            stock; OR

      7.1.2.2. any of the events described in Sections 7.1.1.2, 7.1.1.3 or
7.1.1.4.

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Notwithstanding the foregoing, a transaction shall not constitute a Change in
Control if its sole purpose is to change the state of the Company's
incorporation or to create a holding company that will be owned in substantially
the same proportions by the persons who held the Company's securities
immediately before such transaction.

            7.2. Definition of Good Reason. As used in this Agreement, "Good
Reason" means any of the following: (i) a material reduction in Executive's
title, status, authority, or responsibility at the Company, including without
limitation the failure of Executive to be elected to the Board of Directors of
the Company during the Term for any reason other than termination of Executive's
employment pursuant to Section 5.3; or (ii) a material reduction in the benefits
in effect for the Executive, and comparable reductions have not been made in the
benefit of the other members of senior management of the Company; or (iii)
except with Executive's prior written consent, relocation of Executive's
principal place of employment to a location more than 25 miles from the
Company's executive offices in Tucson, Arizona; or (iv) ) any breach by the
Company of its material obligations under this Agreement unless such breach is
cured within 30 days after written notice of breach from Executive.

      8. Release. As a condition precedent to the Company's obligation to
provide Executive with the amounts set forth in Section 6.3 or Section 7,
Executive must first execute and deliver to the Company a mutual legal release
in the form attached hereto as Exhibit B, with such changes as the Company deems
necessary in order to maintain the breadth of such release in the event of
changes in applicable laws, rules or regulations, it being the intent of the
parties that the release be as broad as possible.

      9. Ventures. If, during the Term of this Agreement, Executive is engaged
in or associated with the planning or implementing of any project, program, or
venture involving the Company and a third party or parties, all rights in the
project, program, or venture shall belong to the Company and shall constitute a
corporate opportunity belonging exclusively to the Company. Except as approved
in writing by the Board, Executive shall not be entitled to any interest in such
project, program, or venture or to any commission, finder's fee, or other
compensation in connection therewith other than the Base Salary to be paid to
Executive as provided in this Agreement.

      10. Restrictions.

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

                  10.1.1. "Trade Secrets" means information that is not
generally known about the Company or its business, including without limitation
about its products, recipes, projects, designs, developmental or experimental
work, computer programs, data bases, know-how, processes, business partners,
manufacturers, customers, suppliers, business plans, marketing plans and
strategies, financial or personnel information, and information obtained from
third parties under confidentiality agreements. "Trade Secrets" also means
formulas,

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patterns, compilations, programs, devices, methods, techniques, or processes
that derive independent economic value, actual or potential, from not being
generally known to the public or to other persons who can obtain economic value
from its disclosure or use, and is the subject of efforts that are reasonable
under the circumstances to maintain its secrecy. In particular, the parties
agree and acknowledge that the following list, which is not exhaustive and is to
be broadly construed, enumerates some of the Company's Trade Secrets, the
disclosure of which would be wrongful and would cause irreparable injury to the
Company: (i) pharmaceutical manufacturing; (ii) formulation technology; (iii)
pricing information; (iv) product development, marketing, sales, customer,
manufacturer and supplier information related to any Company product or service
available commercially or in any stage of development during Executive's
employment with the Company; and (v) Company marketing and business strategies,
ideas, and concepts. Executive acknowledges that the Company's Trade Secrets
were and are designed and developed by the Company at great expense and over
lengthy periods of time, are secret, confidential, and unique, and constitute
the exclusive property of the Company.

                  10.1.2. "Restricted Field" means the business of developing,
manufacturing, licensing and selling (i) perfluorocarbon (excluding
perfluorocarbon used as a propellant) or ultrasound energy-mediated therapy or
drug delivery products, (ii) targeted in vivo ultrasound diagnostics, and/or
(iii) non-energy mediated nanoparticle hydrophobic drug delivery formulations
(excluding liposomal formulations).

                  10.1.3. "Non-Competition Period" means a period of 12 months
after the termination of Executive's employment with the Company unless a court
of competent jurisdiction determines that that Period is unenforceable under
applicable law because it is too long, in which case the Non-Competition Period
shall be for the longest of the following periods that the court determines is
reasonable under the circumstances: 11 months, 10 months, 9 months, 8 months, 7
months, or 6 months after the termination of Executive's employment with the
Company.

                  10.1.4. "Business Territory" means the entire United States,
unless a court of competent jurisdiction determines that that geographic scope
is unenforceable under applicable law because it is too broad, in which case the
Business Territory shall be amended by eliminating geographical areas and states
from the following list until the Business Territory is determined to be
reasonable: Alabama, Alaska, Arizona, Arkansas, California, Colorado,
Connecticut, Delaware, District of Columbia, Florida, Georgia, Hawaii, Idaho,
Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland,
Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska,
Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North
Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina,
South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, Washington,
District of Columbia, West Virginia, Wisconsin, Wyoming, Pima County, Arizona,
Maricopa County, Arizona, Tucson, Arizona, Phoenix, Arizona. The parties
acknowledge and agree that if any of the geographic areas or States listed above
are required by law to be eliminated, it would be fair and appropriate to do so
in the inverse order of the volume of revenue received or projected to be
received by the Company from such area or State at the time of determination.

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                  10.1.5. "Non-Solicitation Period" means a period of 12 months
after the termination of Executive's employment with the Company.

            10.2. Non-Disclosure Obligations. Executive shall not at any time
during the period specified in Section 4.A. of the Invention and Confidential
Information Agreement attached hereto as Exhibit C, without the express written
consent of an officer of the Company, publish, disclose, or divulge to any
person, firm or corporation, or use directly or indirectly for the Executive's
own benefit or for the benefit of any person, firm, corporation or entity other
than the Company, any Trade Secrets of the Company.

            10.3. Non-Competition Obligations. Executive acknowledges the
substantial amount of time, money, and effort that the Company has spent and
will spend in developing its products and other strategically important
information (including Trade Secrets), and agrees that during Executive's
employment with the Company hereunder and during the Non-Competition Period,
Executive will not, alone or with others, directly or indirectly, as an
employee, agent, consultant, advisor, owner, manager, lender, officer, director,
employee, partner, stockholder, or otherwise, engage in any Restricted Field
activities in the Business Territory, nor have any such relationship with any
person or entity that engages in Restricted Field activities in the Business
Territory; provided, however, that nothing in this Agreement will prohibit
Executive from owning a passive investment of less than one percent of the
outstanding equity securities of any company listed on any national securities
exchange or traded actively in any national over-the-counter market so long as
Executive has no other relationship with such company in violation of this
Agreement. The Non-Competition Period set forth in this Section 10.3 shall be
tolled during any period in which the Executive is in breach of the restriction
set forth in this Section 10.3.

            10.4. Agreement Not to Solicit Customers. Executive agrees that
during Executive's employment with the Company hereunder and during the
Non-Solicitation Period, Executive will not, either directly or indirectly, on
Executive's own behalf or in the service or on behalf of others, solicit,
divert, or appropriate, or attempt to solicit, divert, or appropriate, to any
business that engages in Restricted Field activities in the Business Territory
(i) any person or entity whose account with the Company was sold or serviced by
or under the supervision of Executive during the twelve (12) months preceding
the termination of such employment, or (ii) any person or entity whose account
with the Company has been directly solicited at least twice by the Company
within the year preceding the termination of Executive's employment (the
"Customers"). The Non-Solicitation Period set forth in this Section 10.4 shall
be tolled during any period in which the Executive is in breach of the
restriction set forth in this Section 10.4.

            10.5. Agreement Not to Solicit Employees. Executive agrees that
during Executive's employment with the Company hereunder and during the
Non-Solicitation Period, Executive will not, either directly or indirectly, on
Executive's own behalf or in the service or on the behalf of others solicit,
divert, or hire away, or attempt to solicit, divert, or hire away any person
then employed by the Company, nor encourage anyone to leave the Company's
employ. The Non-Solicitation Period set forth in this Section 10.5 shall be
tolled during any period in which the Executive is in breach of the restriction
set forth in this Section 10.5.

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            10.6. Defamatory Statements. Executive agrees that during
Executive's employment with the Company hereunder and thereafter, he will not,
either directly or indirectly, defame the reputation, character, or image of the
Company or its products, services, employees, directors, or officers.

            10.7. Reasonableness. Executive and the Company agree that the
covenants set forth in this Agreement are appropriate and reasonable when
considered in light of the nature and extent of the Company's business.
Executive further acknowledges and agrees that (i) the Company has a legitimate
interest in protecting the Company's business activities and its current,
pending, and potential Trade Secrets; (ii) the covenants set forth herein are
not oppressive to Executive and contain reasonable limitations as to time,
scope, geographical area, and activity; (iii) the covenants do not harm in any
manner whatsoever the public interest; (iv) Executive's chosen profession,
trade, or business is in manufacturing, developing, and marketing pharmaceutical
drugs, products and devices (the "Profession") (v) the Restricted Field is only
a very small or limited part of the Profession, and Executive can work in many
different jobs in Executive's Profession besides those in the Restricted Field;
(vi) the covenants set forth herein do not completely restrain Executive from
working in Executive's Profession, and Executive can earn a livelihood in
Executive's Profession without violating any of the covenants set forth herein;
(vii) Executive has received and will receive substantial consideration for
agreeing to such covenants, including without limitation the consideration to be
received by Executive under this Agreement; (viii) if Executive were to work for
a competing company that engages in activities in the Restricted Field, there
would be a substantial risk that Executive would inevitably disclose Trade
Secrets to that company; (ix) the Company competes with other companies that
engage in Restricted Field Activities in the Business Territory, and if
Executive were to engage in prohibited activities in the Restricted Field within
the Business Territory, it would harm the Company; (x) the Company expends
considerable resources on hiring, training, and retaining its employees and if
Executive were to engage in prohibited activities during the Non-Solicitation
Period, it would harm the Company; and (xi) the Company expends considerable
resources acquiring, servicing, and retaining its Customers and if Executive
were to engage in prohibited activities during the Non-Solicitation Period, it
would harm the Company.

      11. Other Agreements. Executive reaffirms Executive's obligations set
forth in the Invention and Confidential Information Agreement attached hereto as
Exhibit C, which Executive has executed and delivered herewith. Executive
further acknowledges and agrees that he will comply with all other Company
policies and procedures.

      12. Assignment. This Agreement shall not be assignable, in whole or in
part, by either party without the written consent of the other party, except
that the Company may, without the consent of Executive, assign its rights and
obligations under this Agreement to any corporation, firm or other business
entity (i) with or into which the Company may merge or consolidate, (ii) to
which the Company may sell or transfer all or substantially all of its assets or
(iii) of which at least a majority of the equity investment and of the voting
control is owned, directly or indirectly, by, or is under common ownership with,
the Company. Upon such assignment by the Company, the Company shall attempt to
obtain the assignees' written agreement enforceable by

                                       11
<PAGE>
Executive to assume and perform, from and after the date of such assignment, the
terms, conditions, and provisions imposed by this Agreement upon the Company.

      13. Other Provisions.

            13.1. Governing Law. This Agreement is made under and shall be
governed by and construed in accordance with the laws of the State of Arizona
without reference to conflicts of law provisions thereof.

            13.2. Injunctive Relief. Executive agrees that it would be difficult
to compensate the Company fully for damages for any violation of the provisions
of this Agreement. Accordingly, Executive specifically agrees that the Company
shall be entitled to temporary and permanent injunctive relief to enforce the
provisions of this Agreement. This provision with respect to injunctive relief
shall not, however, diminish the right of the Company to claim and recover
damages in addition to injunctive relief.

            13.3. Prior Agreements. This Agreement and its exhibits contain the
entire agreement of the parties relating to the subject matter hereof and
supersedes all prior agreements and understanding 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 which are not set
forth herein.

            13.4. Withholding Taxes and Right of Offset. The Company may
withhold from all payments and benefits under this Agreement all federal, state,
city, or other taxes as shall be required pursuant to any law or governmental
regulation or ruling. Executive agrees that the Company may offset any payments
owed to Executive pursuant to this Agreement by any amounts owed by Executive to
the Company pursuant to Section 4.7.1 or for travel advances, provided that
following a termination of Executive's employment pursuant to Section 5.5
(Without Cause) or resignation by Executive for Good Reason, only the "Lump Sum
Payment" payable pursuant to Section 6.3(A) shall be subject to any such offset.

            13.5. Amendments. No amendment or modification of this Agreement
shall be deemed effective unless made in writing signed by Executive and the
Company.

            13.6. No Waiver. No term or condition of this Agreement shall be
deemed to have been waived nor shall there be any estoppel to enforce any
provisions of this Agreement, except by a statement in writing signed by the
party against whom enforcement of the waiver or estoppel is sought. Any written
waiver shall not be deemed a continuing waiver unless specifically stated, shall
operate only as to the specific term or condition waived, and shall not
constitute a waiver of such term or condition for the future or as to any act
other than that specifically waived.

            13.7. Severability. To the extent any provision of this Agreement
shall be invalid or unenforceable, it shall be considered deleted from this
Agreement and the remainder

                                       12
<PAGE>
of such provision and of this Agreement shall be unaffected and shall continue
in full force and effect.

            13.8. Indemnification. Executive shall be entitled as an officer and
director to be indemnified by the Company under the Organizational Documents, as
set forth therein, and to receive the benefits, if any, of any director and
officer liability insurance obtained by the Company in its discretion from time
to time, subject to the terms, provisions and conditions of any such insurance.

            13.9. Headings. The headings in this Agreement are included solely
for reference purposes and shall not be considered in the interpretation or
construction of this Agreement.

            13.10. Notices. All notices, requests, demands and other
communications under this Agreement shall be in writing and shall be delivered
personally, by overnight courier or similar means, by United States certified or
registered mail, return receipt requested, postage prepaid, or sent by facsimile
transmission with written confirmation of receipt, at the addresses specified
below or such other addresses as the parties may designate by like notice. Any
such notice shall be effective upon receipt if delivered personally, by
overnight courier or by United States certified or registered mail, or on the
next business day following transmittal if sent by facsimile transmission.

      IF TO THE COMPANY:   ImaRx Therapeutics, Inc.
                           1635 East 18th Street
                           Tucson, AZ  85719
                           Attn:  Chairman of the Board
                           Telephone:   520-770-1259
                           Facsimile:   520-791-2437

      COPY TO:             Osborn Maledon, P.A.
                           2929 N. Central, Suite 2100
                           Phoenix, Arizona  85012
                           Attn:  Thomas H. Curzon, Esq.
                           Telephone:   (602) 640-9000
                           Facsimile:   (602) 640-6067

      IF TO EXECUTIVE:     Evan C. Unger, MD
                           6227 E. Miramar Dr.
                           Tucson, AZ 85719
                           Telephone (520) 298-9234
                           Facsimile (520) 298-9254

            13.11. No Mitigation Obligation. It will be difficult, and may be
impossible, for Executive to find reasonably comparable employment following the
termination of Executive's

                                       13
<PAGE>
employment with the Company, and the noncompetition covenant contained in
Section 10.1 hereof will further limit the employment opportunities for
Executive. Accordingly, the parties hereto expressly agree that the payments
provided for in Section 6.3 and Section 7 by the Company to Executive will be
liquidated damages, and that Executive shall not be required to seek other
employment, or otherwise, to mitigate any payment provided for hereunder.

            13.12. Remedies Cumulative. Remedies under this Agreement of either
party hereto are in addition to any remedy or remedies to which such party is
entitled or may become entitled at law or in equity.

            13.13. Attorneys' Fees. In the event suit is brought to enforce the
terms of this Agreement or to collect any moneys due hereunder, or to collect
money damages for breach hereof, the prevailing party shall be entitled to
recover, in addition to any other remedy, reimbursement for reasonable
attorneys' fees, court costs, costs of investigation and other related expenses
incurred in connection therewith.

            13.14. Counterparts. This Agreement may be executed in any number of
counterparts, all such counterparts shall be deemed to constitute one and the
same instrument, and each of said counterparts shall be deemed an original
hereof.

            13.15. Survivability. Sections 4.3, 6, 7, 8, 10 and 13 of this
Agreement shall survive the termination of this Agreement and the termination of
Executive's employment with the Company.

      IN WITNESS WHEREOF, the parties have executed this Agreement as of the day
and year first set forth above.

                                 "COMPANY":     IMARX THERAPEUTICS, INC.

                                                By: /S/Greg Cobb
                                                   -----------------------------
                                                Name:  Greg Cobb
                                                Title: Chief Financial Officer

                                 "EXECUTIVE":   /S/ Evan C. Unger
                                                --------------------------------
                                                EVAN C. UNGER

                                       14
<PAGE>
                                    EXHIBIT A

             BONUSABLE MILESTONES FOR THE YEAR ENDING JUNE 30, 2006

<TABLE>
<CAPTION>
        BONUSABLE MILESTONE                   BONUS EXPIRATION      BONUS AS A
                                                    DATE          PERCENTAGE OF
                                                                    BASE SALARY
--------------------------------------------------------------------------------
<S>                                          <C>                  <C>
a) Allowance of introduction of              September 30, 2005       a) 5%,
MRX-815 into clinical trials,                                           or
  or                                                                  b) 10%
b) Allowance of introduction of
MRX-815 into clinical trials with
device indication

Execution and approval by the Board           December 31, 2005        10%
and licensee of a licensing
contract for SonoLysis with
milestone payments of greater than
$5 million, provided however, no
Bonus payment will be made until
the Company has received
unreimbursable cash payments under
the contract of at least $500,000

Completion of an IPO or private                March 28, 2006          10%
placement of $20 million or greater

Completion of Phase II/Feasibility             March 31, 2006          10%
study in ischemic stroke (n=40
patients)

Execution and approval by the Board             June 30, 2006          10%
and licensee of a licensing
contract for ImaRx technology other
than SonoLysis and vulnerable
plaque with milestone payments of
greater than $5 million, provided
however, no Bonus payment will be
made until the Company has received
unreimbursable cash payments under
the contract of at least $500,000
</TABLE>
<PAGE>
                                    EXHIBIT B

                       EXECUTIVE OPTION VESTING MILESTONES

<TABLE>
<CAPTION>
                GRANT MILESTONE               GRANT EXPIRATION      SHARES
                                                    DATE
--------------------------------------------------------------------------------
<S>                                           <C>                   <C>
    Execution and approval by the Board and   December 31, 2005     100,000
    licensee of a licensing contract for
    SonoLysis with milestone payments of
    greater than $5 million, provided
    however, no Bonus payment will be made
    until the Company has received
    unreimbursable cash payments under the
    contract of at least $500,000

    Completion of an IPO or private            March 28, 2006       100,000
    placement of $20 million or greater

    Achieve a liquidity event or market       December 31, 2008     100,000
    capitalization as a public company of
    over $400M for 20 consecutive trading
    days

    FDA approval of SonoLysis for stroke      December 31, 2009     120,000
</TABLE>
<PAGE>
                                    EXHIBIT C

                          [2000 STOCK OPTION AGREEMENT]
<PAGE>
                                    EXHIBIT D

                                [FORM OF RELEASE]
<PAGE>
                                    EXHIBIT E

              [INVENTION AND CONFIDENTIAL INFORMATION AGREEMENT]<PAGE>
                                                                   Exhibit 10.19

                                                                  EXECUTION COPY

                            IMARX THERAPEUTICS, INC.
                          REGISTRATION RIGHTS AGREEMENT

            THIS REGISTRATION RIGHTS AGREEMENT (this "Agreement") is made as of
January 19, 2001 by and among ImaRx Therapeutics, Inc., a Delaware corporation
(the "Company"), ELAN INTERNATIONAL SERVICES, LTD., a Bermuda exempted limited
liability company ("EIS"), and ELAN PHARMA INTERNATIONAL LIMITED, an Irish
private limited liability company ("EPIL").

                                R E C I T A L S:

            A. Pursuant to a Securities Purchase Agreement dated as of the date
hereof by and among the Company, EIS and EPIL (the "Purchase Agreement"), (i)
EIS has acquired, or will acquire in the future, (a) certain shares of Series B
Preferred Stock of the Company (the "Series B Preferred Stock") convertible into
shares of common stock of the Company (the "Common Stock") or Series C Preferred
Stock of the Company (the "Series C Preferred Stock"), as the case may be, and
(b) certain shares of Series C Preferred Stock of the Company convertible into
shares of Common Stock, and (ii) EPIL has acquired a certain note of the Company
(the "Note") convertible into shares of Common Stock or Series C Preferred
Stock, as the case may be. The Series B Preferred Stock, the Series C Preferred
Stock and the Note collectively are referred to herein as the "Securities".

            B. The execution of the Purchase Agreement has occurred on the date
hereof and it is a condition to the closing of the transactions contemplated
thereby that the parties execute and deliver this Agreement.

            C. The parties desire to set forth herein their agreement on the
terms and subject to the conditions set forth herein related to the granting of
certain registration rights to the Holders (as defined below) relating to the
Common Stock, if any, held and the Common Stock issuable upon conversion or
exercise of the Securities by such Holders.

                               A G R E E M E N T:

            The parties hereto agree as follows:

      1.    Certain Definitions.  As used in this Agreement, the following
terms shall have the following respective meanings:

            "Commission" shall mean the U.S. Securities and Exchange Commission.
<PAGE>
                                      -2-

            "Exchange Act" shall mean the U.S. Securities Exchange Act of 1934,
as amended, and the rules and regulations of the Commission thereunder, all as
the same shall be in effect from time to time.

            "Holders" or "Holders of Registrable Securities" shall mean EIS,
EPIL and any Person who shall have acquired Registrable Securities from EIS or
EPIL as permitted herein and in the Purchase Agreement, either individually or
jointly, as the case may be, in a transaction pursuant to which registration
rights are transferred pursuant to Section 10 hereof.

            "Initial Offering" shall mean the Company's first firm commitment
underwritten public offering of its Common Stock registered under the Securities
Act.

            "Person" shall mean an individual, a partnership, a company, an
association, a joint stock company, a trust, a joint venture, an unincorporated
organization or a governmental or quasi-governmental entity, or any department,
agency or political subdivision thereof.

            "Registrable Securities" means (i) any shares of Common Stock issued
or issuable upon conversion or exercise of the Securities; and (ii) any shares
of Common Stock issued or issuable in respect of the securities referred to in
clause (i) above, until, in the case of any such security, it is effectively
registered under the Securities Act and disposed of in accordance with the
registration statement covering it; excluding in all cases, however, any
Registrable Securities that are sold by a Person to the public either pursuant
to a registration statement or under Rule 144 promulgated under the Securities
Act, as such rule may be amended from time to time, or any similar rule or
regulation hereafter adopted by the Commission, and Registrable Securities sold
by a Person in a transaction (including a transaction pursuant to a registration
statement under this Agreement and a transaction pursuant to Rule 144
promulgated under the Securities Act) in which registration rights are not
transferred pursuant to Section 10 hereof. Whenever a number or percentage of
Registrable Securities is to be determined pursuant to this Agreement, each then
outstanding Security that is convertible into or exercisable for shares of
Common Stock will be deemed to be equal to the number of shares of Common Stock
for which such Security is then so convertible or exercisable.

            The terms "register," "registered" and "registration" refer to a
registration effected by preparing and filing a registration statement in
compliance with the Securities Act.

            "Registration Expenses" shall mean all expenses, other than Selling
Expenses, incurred by the Company in complying with Sections 2 or 3 hereof,
including without limitation, all registration, filing fees, exchange listing
fees (if shares of Common Stock are then listed on a securities exchange),
printing expenses, escrow fees, fees and disbursements of counsel for the
Company, blue sky fees and expenses, the expense of any special audits incident
to or required by any such registration.
<PAGE>
                                      -3-

            "Securities Act" shall mean the Securities Act of 1933, as amended.

            "Selling Expenses" shall mean all underwriting discounts, selling
commissions and stock transfer taxes and the reasonable costs, fees and expenses
of any accountants, attorneys or other experts retained by the Holders.

      2.    Demand Registrations.

            (a) Requests for Registration. From and after the Initial Offering,
any Holder or Holders who collectively hold Registrable Securities representing
at least 5% of the Registrable Securities then outstanding shall have the right
(subject to the limitations below) to request registration under the Securities
Act of all or part of their Registrable Securities on Form S-1 or S-3 (or any
successor form to Form S-1 or S-3, or any similar short-form registration
statement) (a "Demand Registration"). The request for the Demand Registration
shall specify the approximate number of Registrable Securities requested to be
registered, which must have a minimum expected aggregate offering price to the
public of at least, (x) within the first 12 months following the consummation of
an Initial Offering, U.S.$5,000,000, net of underwriting discounts and
commissions, and (y) thereafter, U.S.$2,000,000. Within 10 days after receipt of
any such request, the Company will give written notice of such requested
registration to all other Holders of Registrable Securities. The Company shall
include such other Holders' Registrable Securities in such offering if they have
responded affirmatively within 10 days after the receipt of the Company's
notice. The Holders shall be permitted only one Demand Registration hereunder. A
registration will not count as a permitted Demand Registration until it has
become effective (unless such Demand Registration has not become effective due
solely to the fault of the Holders requesting such registration or a request by
such Holders that such registration be withdrawn).

            (b) Priority on Demand Registrations. If a Demand Registration is an
underwritten offering and the managing underwriters advise the Company in
writing that in their opinion the number of Registrable Securities and, if
permitted hereunder, other securities requested to be included in such offering,
exceeds the number of Registrable Securities and other securities, if any, which
can be sold in such offering without adversely affecting the marketability of
the offering, the Company will include in such registration:

            (i) first, the Registrable Securities requested to be included in
      such registration by the Holders (or, if necessary, such Registrable
      Securities pro rata among the Holders thereof based upon the number of
      Registrable Securities owned by each such Holder); and

            (ii) thereafter, other securities requested to be included in such
      registration, as determined by the Company.
<PAGE>
                                      -4-

            (c) Restrictions on Demand Registration. The Company may postpone or
suspend for up to three months in any 12-month period the filing or the
effectiveness of a registration statement for a Demand Registration if the
Company determines in good faith that such Demand Registration (i) would
reasonably be expected to have a material adverse effect on (x) any proposal or
plan by the Company to engage in any financing, acquisition or disposition of
assets (other than in the ordinary course of business) or (y) any merger,
consolidation, tender offer or similar transaction or (ii) would require
disclosure of any information that the board of directors of the Company
determines in good faith the disclosure of which would be detrimental to the
Company; provided, however, that in such event, the Holders initially requesting
such Demand Registration will be entitled to withdraw such request and, if such
request is withdrawn, such Demand Registration will not count as a permitted
Demand Registration hereunder and the Company will pay any Registration Expenses
in connection with such registration.

            (d) Selection of Underwriters. The Holders will have the right to
select the investment banker(s) and manager(s) to administer an offering
pursuant to the Demand Registration, subject to the Company's prior written
approval, which will not be unreasonably withheld, delayed or conditioned.

            (e) Other Registration Rights. Except as provided in this Agreement,
so long as any Holder owns any Registrable Securities, the Company will not
grant to any Persons the right to require the Company to register any equity
securities of the Company, or any securities convertible or exchangeable into or
exercisable for such securities, which conflicts with the rights granted to the
Holders hereunder, without the prior written consent of the Holders of at least
50% of the Registrable Securities.

      3.    Piggyback Registrations.

            (a) Right to Piggyback. If at any time the Company shall propose to
register shares of Common Stock under the Securities Act (other than in a
registration statement on Form S-3 relating to sales of securities to
participants in a Company dividend reinvestment plan, or Form S-4 or S-8 or any
successor form or in connection with an acquisition or exchange offer or an
offering of securities solely to the existing shareholders or employees of the
Company or a registration statement on Form S-1 relating to the Initial
Offering) (each, a "Piggyback Registration"), the Company (i) will give prompt
written notice to all Holders of Registrable Securities of its intention to
effect such a registration at least 30 days prior to the filing of such
registration statement and (ii) subject to Section 3(b) and the other terms of
this Agreement, will include in such registration all Registrable Securities
which are permitted under applicable securities laws to be included in the form
of registration statement selected by the Company and with respect to which the
Company has received written requests for inclusion therein within 30 days after
the receipt of the Company's notice. The Holders will be permitted to withdraw
all or any part of the Registrable Securities from a Piggyback Registration at
any time prior to the effective date of such Piggyback Registration.
<PAGE>
                                      -5-

            (b) Priority on Piggyback Registrations. If a Piggyback Registration
is to be an underwritten offering, and the managing underwriters advise the
Company in writing that in their opinion the number of securities requested to
be included in such registration exceeds the number which can be sold in such
offering without adversely affecting the marketability of the offering, the
Company will include in such registration:

            (i)   first, the securities the Company proposes to sell;

            (ii) second, the Registrable Securities requested to be included in
      such registration by the Holders and any securities requested to be
      included in such registration by any other Person other than Persons
      having a lower priority of registration than the Holders, pro rata among
      the Holders of such Registrable Securities and such other Persons, on the
      basis of the number of securities requested to be included in such
      registration by each of such Holders and such other Persons; and

            (iii) thereafter, other securities requested to be included in such
      registration, as determined by the Company.

            No such reduction shall reduce the securities being offered by the
Company for its own account to be included in such registration statement. The
Holders of any Registrable Securities included in such an underwritten offering
will execute an underwriting agreement in customary form and in form and
substance satisfactory to the managing underwriters.

            (c) Right to Terminate Registration. If at any time after giving
written notice of its intention to register any of its securities as set forth
in Section 3(a) and prior to the effective date of the registration statement
filed in connection with such registration, the Company shall determine for any
reason not to register such securities, the Company may, at its election, give
written notice of such determination to each Holder of Registrable Securities
and thereupon be relieved of its obligation to register any Registrable
Securities in connection with such registration (but not from its obligation to
pay the Registration Expenses in connection therewith as provided herein).

            (d) Selection of Underwriters. The Company will have the right to
select the investment banker(s) and manager(s) to administer an offering
pursuant to a Piggyback Registration.

      4.    Expenses of Registration.

            Except as otherwise provided herein or as may otherwise be
prohibited by applicable law, all Registration Expenses incurred in connection
with all registrations pursuant to Sections 2 and 3 hereof shall be borne by the
Company. All Selling Expenses relating to securities registered on behalf of the
Holders of Registrable Securities shall be borne by such Holders.
<PAGE>
                                      -6-

      5. Holdback Agreements. Each Holder agrees, if requested by the managing
underwriters in an underwritten offering of Common Stock or securities
convertible for Common Stock of the Company (including without limitation the
Company's Initial Offering of Common Stock), not to exercise their rights
pursuant to Section 2 hereof or to effect any offer, sale, distribution or
transfer, including a sale pursuant to Rule 144 (or any similar provision then
effect) under the Securities Act (except as part of such underwritten
registration), during the 10-day period prior to, and during the 180-day period
in the case of the Company's Initial Offering or a 90-day period in the case of
any other offering of Common Stock (or, in each case, such shorter period as may
be agreed to in writing by the Company and the Holders of at least 50% of the
Registrable Securities) following, the effective date of such Registration
Statement; provided, however, that no Holder shall be required to enter into
more than one such agreement in any 12-month period.

      6.    Registration Procedures.

            Whenever the Holders of Registrable Securities have requested that
any Registrable Securities be registered pursuant to this Agreement, the Company
will use all reasonable efforts to effect the registration and the sale of such
Registrable Securities in accordance with the intended method or methods of
distribution thereof, and pursuant thereto the Company will as expeditiously as
possible:

            (a) subject to Section 2(c) hereof, prepare and file with the
      Commission a registration statement on any appropriate form for which the
      Company qualifies with respect to such Registrable Securities and use all
      reasonable efforts to cause such registration statement to become
      effective (provided that before filing a registration statement or
      prospectus or any amendments or supplements thereto, the Company will (i)
      furnish to the counsel selected by the Holders copies of all such
      documents proposed to be filed, which documents will be subject to the
      reasonable review of such counsel, and (ii) notify each Holder of
      Registrable Securities covered by such registration of any stop order
      issued or threatened by the Commission);

            (b) subject to Section 2(c) hereof, prepare and file with the
      Commission such amendments and supplements to such registration statement
      and the prospectus used in connection therewith as may be reasonably
      necessary to keep such registration statement effective for a period equal
      to the shorter of (i) six months and (ii) the time by which all securities
      covered by such registration statement have been sold, and comply with the
      provisions of the Securities Act with respect to the disposition of all
      securities covered by such registration statement during such period in
      accordance with the intended methods of disposition by the sellers thereof
      set forth in such registration statement;

            (c) furnish to each seller of Registrable Securities such number of
      copies of such registration statement, each amendment and supplement
      thereto, the prospectus included in such registration statement (including
      each preliminary prospectus) and such
<PAGE>
                                      -7-

other documents as such seller may reasonably request in order to facilitate the
disposition of the Registrable Securities owned by such seller;

            (d) use its reasonable efforts to register or qualify such
      Registrable Securities under the securities or blue sky laws of such
      jurisdictions as any seller of Registrable Securities reasonably requests
      and do any and all other acts and things which may be reasonably necessary
      or advisable to enable such seller to consummate the disposition in such
      jurisdictions of the Registrable Securities owned by such seller (provided
      that the Company will not be required to (i) qualify generally to do
      business in any jurisdiction where it would not otherwise be required to
      qualify but for this Section 6(d), (ii) subject itself to taxation in any
      jurisdiction or (iii) take any action that would subject it to general
      service of process in any such jurisdiction);

            (e) notify each seller of such Registrable Securities, at any time
      when a prospectus relating thereto is required to be delivered under the
      Securities Act, of the happening of any event as a result of which the
      prospectus included in such registration statement contains an untrue
      statement of a material fact or omits any fact necessary to make the
      statements therein not misleading, and, at the request of any such seller,
      the Company will prepare a supplement or amendment to such prospectus so
      that, as thereafter delivered to the purchasers of such Registrable
      Securities, such prospectus will not contain an untrue statement of a
      material fact or omit to state any fact necessary to make the statements
      therein not misleading; provided, however, that the Company shall not be
      required to amend the registration statement or supplement the Prospectus
      for a period of up to six months if the board of directors determines in
      good faith that to do so would reasonably be expected to have a material
      adverse effect on any proposal or plan by the Company to engage in any
      financing, acquisition or disposition of assets (other than in the
      ordinary course of business) or any merger, consolidation, tender offer or
      similar transaction or would require the disclosure of any information
      that the board of directors determines in good faith the disclosure of
      which would be detrimental to the Company, it being understood that the
      period for which the Company is obligated to keep the Registration
      Statement effective shall be extended for a number of days equal to the
      number of days the Company delays amendments or supplements pursuant to
      this provision. Upon receipt of any notice pursuant to this Section 6(e),
      the Holders shall suspend all offers and sales of securities of the
      Company and all use of any prospectus until advised by the Company that
      offers and sales may resume, and shall keep confidential the fact and
      content of any notice given by the Company pursuant to this Section 6(e);

            (f) cause all such Registrable Securities to be listed on each
      securities exchange, if any, on which similar securities issued by the
      Company are then listed;

            (g) provide a transfer agent and registrar for all such Registrable
      Securities not later than the effective date of such registration
      statement;
<PAGE>
                                      -8-

            (h) enter into such customary agreements (including underwriting
      agreements in customary form) and take all such other actions as the
      Holders of a majority of the Registrable Securities being sold or the
      underwriters, if any, reasonably request in order to expedite or
      facilitate the disposition of such Registrable Securities;

            (i) make reasonably available for inspection by a representative of
      the Holders of Registrable Securities included in the registration
      statement, any underwriter participating in any disposition pursuant to
      such registration statement and any attorney, accountant or other agent
      retained by any such seller or underwriter, all pertinent financial and
      other records, pertinent corporate documents and properties of the
      Company, and cause the Company's officers, directors, employees and
      independent accountants to supply all information reasonably requested by
      any such seller, underwriter, attorney, accountant or agent in connection
      with such registration statement;

            (j) otherwise use its reasonable efforts to comply with all
      applicable rules and regulations of the Commission, and make available to
      its security holders, as soon as reasonably practicable, an earnings
      statement covering the period of at least 12 months beginning with the
      first day of the Company's first full calendar quarter after the effective
      date of the registration statement, which earnings statement shall satisfy
      the provisions of Section 11(a) of the Securities Act and Rule 158
      thereunder;

            (k) in the event of the issuance of any stop order suspending the
      effectiveness of a registration statement, or of any order suspending or
      preventing the use of any related prospectus or suspending the
      qualification of any shares of Common Stock included in such registration
      statement for sale in any jurisdiction, use all reasonable efforts
      promptly to obtain the withdrawal of such order; and

            (l) if the registration is an underwritten offering, use all
      reasonable efforts to obtain a so-called "cold comfort" letter from the
      Company's independent public accountants in customary form and covering
      such matters of the type customarily covered by cold comfort letters.

      7.    Obligations of Holders.

            Whenever the Holders of Registrable Securities sell any Registrable
Securities pursuant to a Demand Registration or a Piggyback Registration, such
Holders shall be obligated to comply with the applicable provisions of the
Securities Act, including the prospectus delivery requirements thereunder, and
any applicable state securities or blue sky laws. In addition, each Holder of
Registrable Securities will be deemed to have agreed by virtue of its
acquisition of such Registrable Securities that, upon receipt of any notice
described in Section 6(e), such holder will forthwith discontinue disposition of
such Registrable Securities covered by such registration statement or prospectus
until such holder's receipt of the copies of the supplemented or amended
<PAGE>
                                      -9-

prospectus contemplated by Section 6(e), or until it is advised in writing by
the Company that the use of the applicable prospectus may be resumed, and has
received copies of any additional or supplemental filings that are incorporated
or deemed to be incorporated by reference in such prospectus.

      8.    Indemnification.

            (a) The Company agrees to indemnify, to the fullest extent permitted
by applicable law, each Holder of Registrable Securities, its officers and
directors and each Person who controls such Holder (within the meaning of the
Securities Act) against all losses, claims, damages, liabilities, expenses or
any amounts paid in settlement of any litigation, investigation or proceeding
commenced or threatened (collectively, "Claims") to which each such indemnified
party may become subject under the Securities Act insofar as such Claim arose
out of (i) any untrue or alleged untrue statement of material fact contained, on
the effective date thereof, in any registration statement, prospectus or
preliminary prospectus or any amendment thereof or supplement thereto or (ii)
any omission or alleged omission to state therein a material fact required to be
stated therein or necessary to make the statements therein not misleading,
except insofar as such Claims are caused by or contained in any information
furnished in writing to the Company by such Holder or a partner, officer,
director or controlling person of such Holder, expressly for use therein, by
such Holder's failure to deliver a copy of the registration statement or
prospectus or any amendments or supplements thereto after the Company has
furnished such Holder with a sufficient number of copies of the same or by such
Holder's failure to comply with applicable securities laws. In connection with
an underwritten offering, the Company will indemnify the underwriters, their
officers and directors and each Person who controls the underwriters (within the
meaning of the Securities Act) to the same extent as provided above with respect
to the indemnification of the Holders of Registrable Securities.

            (b) In connection with any registration statements in which a Holder
of Registrable Securities is participating, each such Holder will, to the
fullest extent permitted by applicable law, indemnify the Company, its directors
and officers and each Person who controls the Company (within the meaning of the
Securities Act) against any and all Claims to which each such indemnified party
may become subject under the Securities Act insofar as such Claim arose out of
(i) any untrue or alleged untrue statement of material fact contained, on the
effective date thereof, in any registration statement, prospectus or preliminary
prospectus or any amendment thereof or supplement thereto, (ii) any omission or
alleged omission to state therein a material fact required to be stated therein
or necessary to make the statements therein not misleading or (iii) any failure
to comply with applicable securities laws; provided that with respect to a Claim
arising pursuant to clause (i) or (ii) above, the material misstatement or
omission is contained in the information such Holder provided to the Company
pursuant to Section 11 hereof; provided, further, that the obligation to
indemnify will be individual to each Holder and will be limited to
<PAGE>
                                      -10-

the amount of proceeds received by such Holder from the sale of Registrable
Securities pursuant to such registration statement.

            (c) Any Person entitled to indemnification hereunder will (i) give
prompt written notice to the indemnifying party of any claim with respect to
which it seeks indemnification (but the failure to provide such notice shall not
release the indemnifying party of its obligation under paragraphs (a) and (b),
unless and then only to the extent that, the indemnifying party has been
prejudiced by such failure to provide such notice) and (ii) unless in such
indemnified party's reasonable judgment, based on written advice of counsel, a
conflict of interest between such indemnified and indemnifying parties may exist
with respect to such claim, permit such indemnifying party to assume the defense
of such claim with counsel reasonably satisfactory to the indemnified party. An
indemnifying party who is not entitled to, or elects not to, assume the defense
of a claim will not be obligated to pay the fees and expenses of more than one
counsel for all parties indemnified by such indemnifying party with respect to
such claim, unless in the reasonable judgment of any indemnified party, based on
written advice of counsel, a conflict of interest may exist between such
indemnified party and any other of such indemnified parties with respect to such
claim.

            (d) The indemnifying party shall not be liable to indemnify an
indemnified party for any settlement, or consent to judgment of any such action
effected without the indemnifying party's written consent (but such consent will
not be unreasonably withheld, delayed or conditioned). Furthermore, the
indemnifying party shall not, except with the prior written approval of each
indemnified party, consent to entry of any judgment or enter into any settlement
which does not include as an unconditional term thereof the giving by the
claimant or plaintiff to each indemnified party of a release from all liability
in respect of such claim or litigation without any payment or consideration
provided by each such indemnified party.

            (e) If the indemnification provided for in this Section 8 is
unavailable to an indemnified party under clauses (a) and (b) above in respect
of any losses, claims, damages or liabilities referred to therein, then each
indemnifying party, in lieu of indemnifying such indemnified party, shall
contribute to the amount paid or payable by such indemnified party as a result
of such losses, claims, damages or liabilities in such proportion as is
appropriate to reflect not only the relative benefits received by the Company,
the underwriters, the sellers of Registrable Securities and any other sellers
participating in the registration statement from the sale of shares pursuant to
the registered offering of securities for which indemnity is sought but also the
relative fault of the Company, the underwriters, the sellers of Registrable
Securities and any other sellers participating in the registration statement in
connection with the misstatement or omission which resulted in such losses,
claims, damages or liabilities, as well as any other relevant equitable
considerations. The relative benefits received by the Company, the underwriters,
the sellers of Registrable Securities and any other sellers participating in the
registration statement shall be deemed to be based on the relative relationship
of the total net proceeds from the offering (before deducting
<PAGE>
                                      -11-

expenses) to the Company, the total underwriting commissions and fees from the
offering (before deducting expenses) to the underwriters and the total net
proceeds from the offering (before deducting expenses) to the sellers of
Registrable Securities and any other sellers participating in the registration
statement. The relative fault of the Company, the underwriters, the sellers of
Registrable Securities and any other sellers participating in the registration
statement shall be determined by reference to, among other things, whether the
untrue or alleged untrue statement of a material fact or the omission or alleged
omission to state a material fact relates to information supplied by the Company
or by the sellers of Registrable Securities and the parties' relative intent,
knowledge, access to information and opportunity to correct or prevent such
statement or omission; provided that in no event shall the liability of any
selling Holder hereunder be greater in amount than the dollar amount of the
proceeds received by such Holder upon the sale of the Registrable Securities
giving rise to such indemnification obligation.

            (f) The indemnification provided for under this Agreement will
remain in full force and effect regardless of any investigation made by or on
behalf of the indemnified party or any officer, director or controlling person
of such indemnified party and will survive the transfer of the Registrable
Securities.

      9.    Participation in Underwritten Registrations.

            No Holder may participate in any registration hereunder which is
underwritten unless such Holder (a) agrees to sell such Holder's securities on
the basis provided in any underwriting arrangements approved by the Holder or
Holders entitled hereunder to approve such arrangements, (b) as expeditiously as
possible notifies the Company of the occurrence of any event as a result of
which any prospectus contains an untrue statement of material fact or omits to
state a material fact required to be stated therein or necessary to make the
statements therein not misleading and (c) completes and executes all
questionnaires, powers of attorney, indemnities, underwriting agreements and
other documents reasonably required under the terms of such underwriting
arrangements.

      10.   Transfer of Registration Rights.

            The rights granted to any Holder under this Agreement may be
assigned to any permitted transferee of Registrable Securities, in connection
with any transfer or assignment of Registrable Securities by a Holder; provided,
however, that: (a) such transfer is otherwise effected in accordance with
applicable securities laws, (b) such transfer is permitted by the Purchase
Agreement, (c) if not already a party hereto, the assignee or transferee agrees
in writing prior to such transfer to be bound by the provisions of this
Agreement applicable to the transferor and (d) EIS shall act as agent and
representative for such Holder for the giving and receiving of notices
hereunder.
<PAGE>
                                      -12-

      11.   Information by Holder.

            Each Holder shall furnish to the Company such written information
regarding such Holder and any distribution proposed by such Holder as the
Company may reasonably request in writing and as shall be reasonably required in
connection with any registration, qualification or compliance referred to in
this Agreement and shall promptly notify the Company of any changes in such
information.

      12.   Exchange Act Compliance.

            The Company shall comply with all of the reporting requirements of
the Exchange Act then applicable to it, if any, and shall comply with all other
public information reporting requirements of the Commission that are conditions
to the availability of Rule 144 for the sale of the Registrable Securities. The
Company shall cooperate with each Holder in supplying such information as may be
necessary for such Holder to complete and file any information reporting forms
presently or hereafter required by the Commission as a condition to the
availability of Rule 144.

      13.   Termination of Registration Rights.

            All registration rights and obligations (including, without
limitation, under Section 5) under this Agreement shall terminate and be of no
further force and effect, as to any particular Holder, at such time as all
Registrable Securities held by such Holder are eligible to be sold without
compliance with the registration requirements of the Securities Act pursuant to
Rule 144(k) promulgated thereunder or have been resold pursuant to a
registration statement hereunder.

      14.   Miscellaneous.

            (a) No Inconsistent Agreements. The Company will not hereafter enter
into any agreement with respect to its securities that is inconsistent with or
violates the rights granted to the Holders of Registrable Securities in this
Agreement without the prior written consent of a majority in interest of such
Registrable Securities.

            (b) Remedies. Any Person having rights under any provision of this
Agreement will be entitled to enforce such rights specifically to recover
damages caused by reason of any breach of any provision of this Agreement and to
exercise all other rights granted by law. The parties hereto agree and
acknowledge that money damages may not be an adequate remedy for any breach of
the provisions of this Agreement and that any party may in its sole discretion
apply to any court of law or equity of competent jurisdiction (without posting
any bond or other security) for specific performance and for other injunctive
relief in order to enforce or prevent
<PAGE>
                                      -13-

violation of the provisions of this Agreement; provided, however, that in no
event shall any Holder have the right to enjoin, delay or interfere with any
offering of securities by the Company.

            (c) Amendments and Waivers. Except as otherwise provided herein, the
provisions of this Agreement may be amended or waived only with the prior
written consent of the Company and Holders of at least 50% of the Registrable
Securities; provided, however, that without the prior written consent of all the
Holders, no such amendment or waiver shall reduce the foregoing percentage
required to amend or waive any provision of this Agreement.

            (d) Successors and Assigns. This Agreement and all of the provisions
hereof shall be binding upon and inure to the benefit of the parties and their
respective successors and permitted assigns. This Agreement may be transferred
along with such Holder's Registrable Securities by EIS, EPIL and their permitted
assigns and transferees to their respective affiliates and subsidiaries, as well
as any special purpose financing or similar vehicle established by EIS or EPIL.
Other than as set forth above, no party shall transfer or assign this Agreement
without the prior written consent of the other party, which will not be
unreasonably withheld, delayed or conditioned. In addition, whether or not any
express assignment has been made, the provisions of this Agreement which are for
the benefit of Holders of Registrable Securities are also for the benefit of,
and enforceable by, any permitted transferee of Registrable Securities, in
accordance with Section 10 hereof.

            (e) Severability. In case any provision of this Agreement shall be
invalid, illegal or unenforceable, the validity, legality and enforceability of
the remaining provisions shall not be in any way affected or impaired thereby.

            (f) Counterparts and Facsimile. This Agreement may be executed in
any number of counterparts, and each such counterpart hereof shall be deemed to
be an original instrument, but all such counterparts together shall constitute
one agreement. This Agreement may be signed and delivered to the other party by
facsimile transmission; such transmission shall be deemed a valid signature.

            (g)   Descriptive Headings.  The section and paragraph headings
contained in this Agreement are for reference purposes only and shall not
affect in any way the meaning or interpretation of this Agreement.

            (h) Governing Law; Disputes. This Agreement shall be governed by and
construed in accordance with the internal laws of the State of New York, without
giving effect to principles of conflicts of laws. Any dispute under this
Agreement that is not settled by mutual consent shall be finally adjudicated by
any federal or state court sitting in the City, County and State of New York,
and each party consents to the exclusive jurisdiction of such courts (or any
appellate court therefrom) over any such dispute.
<PAGE>
                                      -14-

            (i) Notices. All notices, demands and requests of any kind to be
delivered to any party in connection with this Agreement shall be in writing and
shall be deemed to have been duly given if personally or hand delivered or if
sent by internationally-recognized overnight courier or by registered or
certified mail, return receipt requested and postage prepaid, or by facsimile
transmission, addressed as follows:

            (i)     if to the Company, to:

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

                    with a copy to:

                    Fennemore Craig
                    3003 North Central Avenue

                    Suite 2600
                    Phoenix, AZ 85012
                    Attention:  Sarah A. Strunk, Esq.
                    Facsimile:  (602) 916-5527

            (ii)(a) if to EIS, to:

                    Elan International Services, Ltd.
                    102 St. James Court
                    Flatts, Smiths Parish

                    Bermuda FL 04
                    Attention:  Chief Executive Officer
                    Facsimile:  (441) 292-2224

                (b) if to EPIL, to:

                    Elan Pharma International Limited
                    Will House
                    Shannon Business Park
                    Shannon, Co. Clare
                    Ireland
                    Attention:  Secretary
                    Facsimile:  011-353-61-362097
<PAGE>
                                      -15-

            with a copy, in the case of (a) or (b) above, to:

                  Cahill Gordon & Reindel
                  80 Pine Street
                  New York, New York  10005
                  Attention:  William M. Hartnett, Esq.
                  Facsimile:  (212) 269-5420

or to such other address as the party to whom notice is to be given may have
furnished to the other party hereto in writing in accordance with provisions of
this Section 14(i). Any such notice or communication shall be deemed to have
been effectively given (i) in the case of personal or hand delivery, on the date
of such delivery, (ii) in the case of an internationally-recognized overnight
delivery courier, on the second business day after the date when sent, (iii) in
the case of mailing, on the fifth business day following that day on which the
piece of mail containing such communication is posted and (iv) in the case of
facsimile transmission, on the date of telephone confirmation of receipt.

            (j) Entire Agreement. This Agreement constitutes the full and entire
understanding and agreement of the parties with regard to the subject matter
hereof and supersedes all prior agreements and understandings among the parties
with respect thereto.

                            [Signature page follows]
<PAGE>
            IN WITNESS WHEREOF, the parties have executed this Registration
Rights Agreement as of the date first written above.

                                 IMARX THERAPEUTICS, INC.

                                 By:  \s\ R. Michael Rodgers
                                     _________________________
                                     Name:  R. Michael Rodgers
                                      Title:  Vice President and Secretary

                                  ELAN INTERNATIONAL SERVICES, LTD.

                                  By:  \s\ Kevin Insley
                                       _________________________
                                       Name:  Kevin Insley
                                       Title:  President and Chief Financial
                                       Officer

                                  ELAN PHARMA INTERNATIONAL LIMITED

                                  By:  \s\ Kevin Insley
                                       _________________________
                                       Name:  Kevin Insley
                                       Title:  Authorized Signatory

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