Document:

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                                                              EXHIBIT 10.20

                            IMARX THERAPEUTICS, INC.

                   AMENDED AND RESTATED RIGHT OF FIRST REFUSAL
                              AND CO-SALE AGREEMENT

     THIS AMENDED AND RESTATED RIGHT OF FIRST REFUSAL AND CO-SALE AGREEMENT (the
"AGREEMENT") is made and entered into as of October 10, 2002, by and among IMARX
THERAPEUTICS, INC., a Delaware corporation (the "COMPANY"), each of the persons
and entities listed on Exhibit A hereto (the "INVESTORS"), and Evan C. Unger,
M.D. ("FOUNDER").

                                    RECITALS

     WHEREAS, certain of the Investors, Founder and the Company have entered
into that certain Right of First Refusal and Co-Sale Agreement dated August 9,
2001 (the "PRIOR CO-SALE AGREEMENT") and, in accordance with Section 6.3 of the
Prior Co-Sale Agreement, the Company and certain of the signatories to the Prior
Co-Sale Agreement whose signatures appear below and who hold, in the aggregate,
the percentage of capital stock of the Company required to amend and restate the
Prior Co-Sale Agreement desire to amend and restate the Prior Co-Sale Agreement
as set forth herein.

     WHEREAS, the Investors have purchased shares of the Company's Series A
Convertible Preferred Stock and/or Series D Convertible Preferred Stock
(collectively, the "PREFERRED STOCK") pursuant to various stock purchase
agreements, most recently pursuant to that certain Series D Preferred Stock
Purchase Agreement, dated as of the date hereof (the "PURCHASE AGREEMENT");

     WHEREAS, the obligations in the Purchase Agreement are conditioned upon the
execution and delivery of this Agreement; and

     WHEREAS, in connection with the consummation of the transactions
contemplated by the Purchase Agreement, the parties desire to enter into this
Agreement to grant first refusal and/or co-sale rights to the Company and to the
Investors prior to certain transfers of Founder Stock, and to subject the shares
of Founder Stock (as defined in Section 1.1) to a market stand-off provision.

     NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties agree hereto agree that the Prior Co-Sale Agreement
shall be amended and restated in its entirety as follows:

1.   DEFINITIONS.

     1.1 "FOUNDER STOCK" shall mean shares of the Company's Common Stock now
owned or subsequently acquired by Founder by gift, purchase, dividend, option
exercise or any other means whether or not such securities are only registered
in Founder's name, including any interest of a spouse in any of the shares of
Founder Stock, whether that interest is asserted pursuant to marital property
laws or otherwise, but shall not include any Common Stock acquired by Founder
upon conversion of the Preferred Stock held by him, or any shares of
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Common Stock held as of the date hereof by irrevocable trusts for the benefit of
his children as to which neither he nor his spouse is trustee. As of the date
hereof, Founder owns 1,421,040 shares of Common Stock that are deemed to be
Founder Stock hereunder.

     1.2 "INVESTOR STOCK" shall mean the shares of the Company's Common Stock
now owned or subsequently acquired by the Investors whether or not such
securities are only registered in an Investor's name or beneficiary or otherwise
legally owned by such Investor.

     1.3 "COMMON STOCK" shall mean the Company's Common Stock and shares of
Common Stock issued or issuable upon conversion of the Company's outstanding
Preferred Stock or exercise of any option, warrant or other security or right of
any kind convertible into or exchangeable for Common Stock.

     1.4 "TRANSFER" shall include any sale, assignment, encumbrance,
hypothecation, pledge, conveyance in trust, gift, transfer by request, devise or
descent, or other transfer or disposition of any kind, including, but not
limited to, transfers to receivers, levying creditors, trustees or receivers in
bankruptcy proceedings or general assignees for the benefit of creditors,
whether voluntary or by operation of law, directly or indirectly, of any of the
shares of Founder Stock.

2.   RESTRICTIONS ON TRANSFER

     2.1 NOTICE OF TRANSFER. If Founder proposes to Transfer any shares of
Founder Stock, then Founder shall promptly give written notice (the "NOTICE")
simultaneously to the Company and to each of the Investors at least thirty (30)
days prior to the closing of such Transfer. The Notice shall describe in
reasonable detail the proposed Transfer including, without limitation, the
number of shares of Founder Stock to be transferred, the nature of such
Transfer, the consideration to be paid, and the name and address of each
prospective purchaser or transferee. In the event that the Transfer is being
made pursuant to the provisions of Section 3.1, the Notice shall state under
which section the Transfer is being made.

     2.2 COMPANY RIGHT OF FIRST REFUSAL. For a period of ten (10) days following
receipt of any Notice described in Section 2.1, the Company shall have the right
to purchase all or a portion of the shares of Founder Stock subject to such
Notice on the same terms and conditions as set forth therein. The Company's
purchase right shall be exercised by written notice signed by an officer of the
Company (the "COMPANY NOTICE") and delivered to Founder. The Company shall
effect the purchase of the shares of Founder Stock, including payment of the
purchase price, not more than five (5) business days after delivery of the
Company's Notice, and at such time Founder shall deliver to the Company the
certificate(s) representing the shares of Founder Stock to be purchased by the
Company, each certificate to be free and clear of any liens, claims or
encumbrances and properly endorsed for transfer. The shares of Founder Stock so
purchased shall thereupon be canceled and cease to be issued and outstanding
shares of the Company's Common Stock.

     2.3 INVESTOR RIGHT OF FIRST REFUSAL.

          (A) In the event that the Company does not elect to purchase all of
the shares of Founder Stock available pursuant to its rights under Section 2.2
within the period set forth therein, Founder shall promptly give written notice
(the "SECOND NOTICE") to each of the

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Investors, which shall set forth the number of shares of Founder Stock not
purchased by the Company and which shall include the terms of Notice set forth
in Section 2.1. Each Investor shall then have the right, exercisable upon
written notice to Founder (the "INVESTOR NOTICE") within fifteen (15) days after
the receipt of the Second Notice, to purchase its pro rata share of Founder
Stock subject to the Second Notice and on the same terms and conditions as set
forth therein. The Investors who so exercise their rights (the "PARTICIPATING
INVESTORS") shall effect the purchase of the shares of Founder Stock, including
payment of the purchase price, not more than ten (10) days after delivery of the
Investor Notice, and at such time Founder shall deliver to the Participating
Investors the certificate(s) representing the shares of Founder Stock to be
purchased by the Participating Investors, each certificate to be free and clear
of any liens, claims or encumbrances and properly endorsed for transfer.

          (B) Each Investor's pro rata share shall be equal to the product
obtained by multiplying (i) the aggregate number of shares of Founder Stock
covered by the Second Notice and (ii) a fraction, the numerator of which is the
number of shares of Common Stock owned by the Participating Investor at the time
of the Transfer and the denominator of which is the total number of shares of
Common Stock owned by all of the Investors at the time of the Transfer.

          (C) In the event that not all of the Investors elect to purchase their
pro rata share of Founder Stock available pursuant to their rights under Section
2.3(a) within the time period set forth therein, then Founder shall promptly
give written notice to each of the Participating Investors the ("OVERALLOTMENT
NOTICE"), which shall set forth the number of shares of Founder Stock not
purchased by the other Investors, and shall offer such Participating Investors
the right to acquire such unsubscribed shares. The Participating Investors shall
have five (5) days after receipt of the Overallotment Notice to deliver a
written notice to Founder (the "PARTICIPATING INVESTORS OVERALLOTMENT NOTICE")
of its election to purchase its pro rata share of the unsubscribed shares on the
same terms and conditions as set forth in the Second Notice. For purposes of
this Section 2.3(c) the denominator described in subsection 2.3(b)(ii) above
shall be the total number of shares of Common Stock owned by all Participating
Investors at the time of Transfer. The Participating Investors shall then effect
the purchase of the shares of Founder Stock, including payment of the purchase
price, not more than ten (10) days after delivery of the Participating Investors
Overallotment Notice to Founder, and at such time, Founder shall deliver to the
Investors the certificates representing the shares of Founder Stock to be
purchased by the Participating Investors, each certificate to be free and clear
of any liens, claims or encumbrances and properly endorsed for transfer.

     2.4  RIGHT OF CO-SALE.

          (A) In the event that the Company and/or the Investors fail to
exercise their respective rights to purchase all of the shares of Founder Stock
subject to Sections 2.2 or 2.3 hereof, following the exercise or expiration of
the rights of purchase set forth in Section 2.2 and 2.3, then Founder shall
deliver to the Company and each Investor written notice (the "CO-SALE NOTICE")
that each Investor shall have the right, exercisable upon written notice to
Founder within fifteen (15) days after receipt of the Co-Sale Notice, to
participate in such Transfer of the shares of Founder Stock on the same terms
and conditions. Such notice from the Investor shall indicate the number of
shares of Investor Stock such Investor wishes to sell under his or her right to
participate. To the extent one or more of the Investors exercise such right of
participation in

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accordance with the terms and conditions set forth below, the number of shares
of Founder Stock that Founder may sell shall be correspondingly reduced.

          (B) Each Investor may sell all or any part of that number of shares
equal to the product obtained by multiplying (i) the aggregate number of shares
of Founder Stock covered by the Co-Sale Notice by (ii) a fraction the numerator
of which is the number of shares of Common Stock owned by such Investor at the
time of the Transfer and the denominator of which is the total number of shares
of Common Stock owned by Founder and all Investors at the time of the Transfer.
If not all of the Investors elect to sell their shares of Common Stock proposed
to be transferred within said fifteen (15) day period, then Founder shall
promptly notify in writing the Investors who do so elect and shall offer such
Investors the additional right to participate in the sale of such additional
shares of Co-Sale Stock proposed to be transferred on the same percentage basis
as set forth above in this subsection 2.4(b). The Investors shall have five (5)
days after receipt of such notice to notify Founder of their election to sell
all or a portion thereof of the unsubscribed shares.

          (C) Each Investor who elects to participate in the Transfer pursuant
to this Section 2 (a "CO-SALE PARTICIPANT") shall effect its participation in
the Transfer by promptly delivering to Founder for transfer to the prospective
purchaser one or more certificates, free and clear of any liens, claims or
encumbrances and properly endorsed for transfer, which represent:

               (I) the type and number of shares of Common Stock which such
Co-Sale Participant elects to sell; or

               (II) that number of shares of Preferred Stock which is at such
time convertible into the number of shares of Common Stock which such Co-Sale
Participant elects to sell; provided, however, that if the prospective purchaser
objects to the delivery of Preferred Stock in lieu of Common Stock, such Co-Sale
Participant shall convert such Preferred Stock into Common Stock and deliver
Common Stock as provided in Section 2.4(c)(i) above. The Company agrees to make
any such conversion concurrent with the actual transfer of such shares to the
purchaser.

          (D) The stock certificate or certificates that the Co-Sale Participant
delivers to Founder pursuant to Section 2.4(c) shall be transferred to the
prospective purchaser in consummation of the sale of the Common Stock pursuant
to the terms and conditions specified in the Co-Sale Notice, and Founder shall
concurrently therewith remit to such Co-Sale Participant that portion of the
sale proceeds to which such Co-Sale Participant is entitled by reason of its
participation in such sale. To the extent that any prospective purchaser or
purchasers prohibits such assignment or otherwise refuses to purchase shares or
other securities from a Co-Sale Participant exercising its rights of co-sale
hereunder, Founder shall not sell to such prospective purchaser or purchasers
any shares of Founder Stock unless and until, simultaneously with such sale,
Founder shall purchase such shares or other securities from such Co-Sale
Participant on the same terms and conditions specified in the Co-Sale Notice.

          (E) The exercise or non-exercise of the rights of the Investors
hereunder to participate in one or more Transfers shall not adversely affect
their rights to participate in subsequent Transfers subject to Section 2.

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          (F) If none of the Investors elect to participate in the sale of the
shares of Founder Stock subject to the Co-Sale Notice, Founder may, not later
than sixty (60) days following delivery to the Company of the Co-Sale Notice,
enter into an agreement providing for the closing of the Transfer of the shares
of Founder Stock covered by the Co-Sale Notice within thirty (30) days of such
agreement on terms and conditions not more materially favorable to the
transferor than those described in the Co-Sale Notice. Any proposed Transfer of
shares of Founder Stock on terms and conditions materially more favorable than
those described in the Co-Sale Notice, as well as any subsequent proposed
Transfer of any shares of Founder Stock by Founder, shall again be subject to
the first refusal and co-sale rights of the Company and/or Investors and shall
require compliance by Founder with the procedures described in this Section 2.

     2.5 Founder agrees not to Transfer any shares of Founder Stock to
competitors of the Company. For purposes of this Section 2.5, a "COMPETITOR" of
the Company shall mean any person or entity which develops or provides services
or products that are competitive with those services or products developed or
provided by the Company at the time of Transfer, as reasonably determined by the
Company's Board of Directors in good faith.

3.   EXEMPT TRANSFERS.

     3.1 Notwithstanding the foregoing, the provisions of Section 2 shall not
apply to (a) any transfer to the ancestors, descendants or spouse or to trusts,
family limited partnerships or family limited liability companies, for the
benefit of such persons or Founder, or (b) any pledge of shares of Founder Stock
made pursuant to a bona fide loan transaction that creates a mere security
interest; provided that in the event of any such transfer, (i) Founder shall
inform the Investors of such pledge, transfer or gift prior to effecting it and
(ii) the pledgee, transferee or donee shall furnish the Investors with a written
agreement to be bound by and comply with all provisions of this Agreement. Such
transferred shares of Founder Stock shall remain "FOUNDER STOCK" hereunder, and
such pledgee, transferee or donee shall be treated as "FOUNDER" for purposes of
this Agreement.

     3.2 Notwithstanding the foregoing, the provisions of Section 2 shall not
apply to the sale of any shares of Founder Stock to the public pursuant to a
registration statement filed with, and declared effective by, the Securities and
Exchange Commission under the Securities Act of 1933, as amended (the
"SECURITIES ACT").

4.   PROHIBITED TRANSFERS.

     4.1 In the event that a Founder should Transfer any shares of Founder Stock
in contravention of the co-sale rights of each Investor under this Agreement (a
"PROHIBITED TRANSFER"), each Investor, in addition to such other remedies as may
be available at law, in equity or hereunder, shall have the put option provided
below, and Founder shall be bound by the applicable provisions of such option.

     4.2 In the event of a Prohibited Transfer, each Investor shall have the
right to sell to Founder the type and number of shares of Common Stock equal to
the number of shares each Investor would have been entitled to transfer to the
purchaser under Section 2.4 hereof had the

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Prohibited Transfer been effected pursuant to and in compliance with the terms
hereof. Such sale shall be made on the following terms and conditions:

          (A) The price per share at which the shares are to be sold to Founder
shall be equal to the price per share paid by the purchaser to Founder in such
Prohibited Transfer. Founder shall also reimburse each Investor for any and all
fees and expenses, including legal fees and expenses, incurred pursuant to the
exercise or the attempted exercise of the Investor's rights under Section 2.4.

          (B) Within ninety (90) days after the date on which an Investor
received notice of the Prohibited Transfer or otherwise became aware of the
Prohibited Transfer, such Investor shall, if exercising the option created
hereby, deliver to Founder the certificate or certificates representing shares
to be sold, each certificate to be free and clear of any liens, claims or
encumbrances and properly endorsed for transfer.

          (C) Founder shall, upon receipt of the certificate or certificates for
the shares to be sold by an Investor, pursuant to this Section 4.2, pay the
aggregate purchase price therefor and the amount of reimbursable fees and
expenses, as specified in Section 4.2(a), in cash or by other means acceptable
to the Investor.

          (D) Notwithstanding the foregoing, any attempt by a Founder to
transfer shares of Founder Stock in violation of Section 2.4 hereof shall be
voidable at the option of a majority in interest of the Investors if a majority
in interest of the Investors do not elect to exercise the put option set forth
in this Section 4, and the Company agrees it will not effect such a transfer nor
will it treat any alleged transferee as the holder of such shares without the
written consent of a majority in interest of the Investors.

5.   MARKET STAND-OFF; LEGEND.

     5.1 Founder hereby agrees that he shall not sell, transfer, make any short
sale of, grant any option for the purchase of, or enter into any hedging or
similar transaction with the same economic effect as a sale, any Common Stock
(or other securities) of the Company held by Founder (other than those included
in a registration) for a period specified by the representative of the
underwriters of Common Stock (or other securities) of the Company not to exceed
one hundred eighty (180) days following the effective date of a registration
statement of the Company filed under the Securities Act.

     5.2 Each certificate representing shares of Founder Stock now or hereafter
owned by Founder or issued to any person in connection with a transfer pursuant
to Section 3.1 or Section 3.2 hereof shall be endorsed with the following
legend:

          "THE SALE, PLEDGE, HYPOTHECATION OR TRANSFER OF THE SECURITIES
          REPRESENTED BY THIS CERTIFICATE IS SUBJECT TO THE TERMS AND CONDITIONS
          OF A CERTAIN RIGHT OF FIRST REFUSAL AND CO-SALE AGREEMENT BY AND AMONG
          THE STOCKHOLDER, THE COMPANY AND CERTAIN HOLDERS OF STOCK OF THE
          COMPANY. COPIES OF SUCH AGREEMENT MAY BE OBTAINED UPON WRITTEN REQUEST
          TO THE SECRETARY OF THE COMPANY."

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     5.3 Each of Founder and the Investors agree that the Company may instruct
its transfer agent to impose transfer restrictions on the shares represented by
certificates bearing the legend referred to in Section 5.2 above to enforce the
provisions of this Agreement and the Company agrees to promptly do so. The
legend shall be removed upon termination of this Agreement.

6.   MISCELLANEOUS.

     6.1 CONDITIONS TO EXERCISE OF RIGHTS. Exercise of the Investors' rights
under this Agreement shall be subject to and conditioned upon, and Founder and
the Company shall use their reasonable efforts to assist each Investor in,
compliance with applicable laws.

     6.2 GOVERNING LAW. This Agreement shall be governed by and construed under
the laws of the State of Delaware as applied to agreements among Delaware
residents entered into and to be performed entirely in Delaware.

     6.3 AMENDMENT. Except as otherwise provided herein, any provision of this
Agreement may be amended and the observance thereof may be waived (either
generally or in a particular instance and either retroactively or
prospectively), only by the written consent signed by (a) the Company, (b)
holders of at least seventy-five (75%) in interest of the Preferred Stock
(including any shares of Common Stock issued upon conversion thereof), acting as
a single class on an as-if-converted to Common Stock basis, and (c) holders of a
majority in interest of the shares of Founder Stock; provided, that no consent
of any holder of shares of Founder Stock shall be necessary for any amendment
and/or restatement which includes additional holders of Preferred Stock of the
Company as "INVESTORS" and parties hereto. Any amendment or waiver effected in
accordance with this Section 6.3 shall be binding upon the Company, each
Investor, Founder, and each of their successors and assigns.

     6.4 BINDING EFFECT. This Agreement and the rights and obligations of the
parties hereunder shall inure to the benefit of, and be binding upon, their
respective successors, assigns and legal representatives.

     6.5 TERM. This Agreement shall continue in full force and effect from the
date hereof through the earliest of the following dates, on which date it shall
terminate in its entirety:

          (A) the date of the closing of a firmly underwritten public offering
of the Common Stock pursuant to a registration statement filed with the
Securities and Exchange Commission, and declared effective under the Securities
Act of 1933, as amended, which results in the Preferred Stock being converted
into Common Stock;

          (B) the date of the liquidation, dissolution or winding up of the
Company; or

          (C) the date as of which the parties hereto terminate this Agreement
by written consent of holders of seventy-five percent (75%) in interest of the
Preferred Stock (including any shares of Common Stock issued upon conversion
thereof), acting as a single class on an as-if-converted to Common Stock basis,
and holders of a majority in interest of the shares of Founder Stock.

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     6.6 OWNERSHIP. Founder represents and warrants that the Unger Family Trust
dated 10/24/95 (the "TRUST") is the sole legal and beneficial owner of those
shares of Founder Stock subject to the Agreement and that no other person other
than beneficiaries of the Trust has any interest (other than a community
property interest) in such shares.

     6.7 NOTICES. All notices required or permitted hereunder shall be in
writing and shall be deemed effectively given: (a) upon personal delivery to the
party to be notified, (b) when sent by confirmed electronic mail or facsimile if
sent during normal business hours of the recipient; if not, then on the next
business day, (c) five (5) days after having been sent by registered or
certified mail, return receipt requested, postage prepaid, or (d) one (1) day
after deposit with a nationally recognized overnight courier, specifying next
day delivery, with written verification of receipt. All communications shall be
sent to the party to be notified at the address as set forth on the signature
pages or Exhibit A hereto or at such other address as such party may designate
by ten (10) days advance written notice to the other parties hereto.

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

     6.9 ATTORNEYS' FEES. In the event that any suit or action is instituted to
enforce any provision in this Agreement, the prevailing party in such dispute
shall be entitled to recover from the losing party all fees, costs and expenses
of enforcing any right of such prevailing party under or with respect to this
Agreement, including without limitation, such reasonable fees and expenses of
attorneys and accountants, which shall include, without limitation, all fees,
costs and expenses of appeals.

     6.10 ENTIRE AGREEMENT. This Agreement and the exhibits hereto, along with
the Purchase Agreement and each of the exhibits thereto, constitute the full and
entire understanding and agreement among the parties with regard to the subjects
hereof and thereof and no party shall be liable or bound to any other in any
manner by any representations, warranties, covenants and agreements except as
specifically set forth herein and therein.

     6.11 ADDITIONAL INVESTORS. Notwithstanding anything to the contrary
contained herein, if the Company shall issue additional shares of its Preferred
Stock pursuant to the Purchase Agreement, any purchaser of such shares of
Preferred Stock may become a party to this Agreement without the consent of any
other party hereto except the Company by executing and delivering an additional
counterpart signature page to this Agreement and shall be deemed an "INVESTOR"
hereunder.

     6.12 COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

                      [THIS SPACE INTENTIONALLY LEFT BLANK]

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     The foregoing AMENDED AND RESTATED RIGHT OF FIRST REFUSAL AND CO-SALE
AGREEMENT is hereby executed as of the date first above written.

                                        COMPANY:

                                        IMARX THERAPEUTICS, INC.

                                        By: /s/ Evan Unger
                                            ------------------------------------
                                            Evan C. Unger, M.D.
                                        Its: Chief Executive Officer

                                        1635 East 18th Street
                                        Tucson, Arizona 85719
                                        Telephone: (520) 770-1259
                                        Fax: (520) 791-2437
                                        Email: jcarlyle@imarx.com
                                        Attn: Jean Carlyle, CFO

FOUNDER:

/s/ EVAN UNGER
-------------------------------------
EVAN C. UNGER, M.D.
6227 East Miramar
Tucson, AZ 85715

INVESTOR:

-------------------------------------
Name of Entity (if applicable):

By:
    ---------------------------------
Print Individual Name:
                       --------------
Print Title (if applicable):
                             --------
Address:
         ----------------------------

-------------------------------------

Telephone:
           --------------------------
Fax:
     --------------------------------
Email:
       ------------------------------
Attn:
      -------------------------------

  [Amended and Restated Right Of First Refusal And Co-Sale Agreement Signature
                                     Page]<PAGE>
                                                                   EXHIBIT 10.21

                                    AMENDMENT
                                       OF
                            IMARX THERAPEUTICS, INC.
                              AMENDED AND RESTATED
                  RIGHT OF FIRST REFUSAL AND CO-SALE AGREEMENT

     This AMENDMENT is made and entered into as of August __, 2004 among ImaRx
Therapeutics, Inc., a Delaware corporation (the "Company"), Evan C. Unger, M.D.
(the "Founder") and each of the persons and entities listed as investors (the
"Investors") on Exhibit A attached to that certain Amended and Restated Right of
First Refusal and Co-Sale Agreement dated October 10, 2002 among the Company,
the Founder and the Investors (the "Restated Co-Sale Agreement").

     WHEREAS, on March 30, 2004, the Company closed a private placement (the
"Private Placement") involving the issue and sale of 2,500,000 shares of its
Common Stock at a purchase price of $2.00 per share through First Montauk
Securities Corp. (the "Placement Agent");

     WHEREAS, pursuant to the Placement Agent Agreement dated February 18, 2004
between the Company and Placement Agent (the "Placement Agent Agreement"),
during the 12-month period immediately subsequent to the closing of the Private
Placement, the Placement Agent has a right of first refusal with respect to the
placement of sales of securities by the Company and any holder of more than 5%
of the Company's capital stock (the "Placement Agent Right of First Refusal");
and

     WHEREAS, pursuant to that certain Consent and Waiver of Preferred
Stockholders dated February 23, 2004 (the "Preferred Consent and Waiver"),
executed by the holder of all of the Company's outstanding Series B Preferred
Stock and Series C Preferred Stock (collectively, the "Series B/C Stock") and
the holders of at least 75% of the Company's Common Stock issued or issuable
upon conversion of the Company's Series A Preferred Stock and Series D Preferred
Stock (collectively, the "Series A/D Stock"), acting on behalf of all holders of
Series A/D Stock (collectively, the "Preferred Stockholders"), the Preferred
Stockholders irrevocably consented to the Private Placement and an amendment to
the Restated Co-Sale Agreement to document: (a) the Founder's agreement not to
transfer any shares of the Company's capital stock in contravention of the
Placement Agent Right of First Refusal and (b) the waiver by the Preferred
Stockholders of any rights of first refusal or co-sale with respect to the
Founder's transfer of up to 250,000 shares of Common Stock to family members or
charities as long as the Founder retains voting control over such transferred
shares, and the transferee agrees that such shares will continue to be subject
to the restrictions set forth in the Co-Sale Agreement (collectively, the
"Amendment");

     NOW, THEREFORE, in consideration of the above and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

     1. AMENDMENT. Effective as of February 23, 2004, the Co-Sale Agreement
shall be amended to document the Amendment by adding new Sections 3.3 and 3.4,
as follows:

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<PAGE>

     "3.3 Notwithstanding the foregoing, the provisions of Section 2 shall not
apply to any transfer of up to 250,000 shares of Founder Stock to family members
or charities; provided that in the event of each such transfer, (a) Founder
retains voting control over such transferred shares during the term of this
Agreement and (b) the transferee shall furnish the Company with a written
agreement to be bound by and comply with all provisions of this Agreement. Such
transferred shares of Founder Stock shall remain "FOUNDER STOCK" hereunder, and
such transferee shall be treated as "FOUNDER" for purposes of this Agreement.

     3.4 Founder hereby agrees that, during the 12-month period ending March 30,
2005, Founder shall not transfer any shares of Founder Stock in contravention of
First Montauk Securities Corp.'s right of first refusal set forth in Section 14
of that certain Placement Agent Agreement dated February 18, 2004 between First
Montauk Securities Corp. and the Company."

     2. REFERENCES TO THE CO-SALE AGREEMENT. From and after the execution of
this Amendment, all references in the Co-Sale Agreement to "this Agreement,"
"hereof," "herein" and similar terms shall mean and refer to the Co-Sale
Agreement as amended by this Amendment, and all references in other documents to
the Co-Sale Agreement shall mean the Co-Sale Agreement as amended by this
Amendment. This Amendment shall not be modified, supplemented or terminated in
any manner whatsoever except in a writing signed by the Company, the Founder and
Preferred Stockholders holding the requisite percentage of shares necessary to
amend the Co-Sale Agreement under Section 6.3 of the Co-Sale Agreement.

     IN WITNESS WHEREOF, the parties hereto have duly executed this Amendment on
the day and year first above written.

THE COMPANY:                            FOUNDER:

IMARX THERAPEUTICS, INC.

By:
    ---------------------------------   ----------------------------------------
    Evan Unger, President               Evan Unger, M.D.

PREFERRED STOCKHOLDERS:

Irrevocable written consent to this Amendment obtained in the Preferred Consent
and Waiver referred to above.

                                       2

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