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                                                                   Exhibit 10.07

             EIGHTH AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT

                                 ACUSPHERE, INC.

                                  June 1, 2001


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                                TABLE OF CONTENTS
<TABLE>
<CAPTION>


<S>      <C>                                                                                              <C>
1.       Termination of Seventh Amended and Restated Investors' Rights Agreement; Waiver and Consent..      2
         1.1      Termination of Seventh Amended Investors' Rights Agreement..........................      2
         1.2      Waiver and Consent..................................................................      2

2.       Registration Rights..........................................................................      2
         2.1      Definitions.........................................................................      2
         2.2      Demand Registration.................................................................      3
         2.3      "Piggy-Back" Registration...........................................................      5
         2.4      Form S-3 Registration...............................................................      6
         2.5      Obligations of Acusphere............................................................      7
         2.6      Furnish Information.................................................................      9
         2.7      Expenses of Demand and S-3 Registrations............................................      9
         2.8      Expenses of "Piggy-Back" Registration...............................................      9
         2.9      Delay of Registration...............................................................     10
         2.10     Indemnification.....................................................................     10
         2.11     Reports Under Securities Exchange Act of 1934.......................................     12
         2.12     Assignment of Registration Rights...................................................     12
         2.13     Limitations on Subsequent Registration Rights.......................................     13
         2.14     "Market Stand-Off" Agreement........................................................     13
         2.15     Termination of Registration Rights..................................................     14

3.       Covenants of Acusphere.......................................................................     14
         3.1      Delivery of Financial Statements....................................................     14
         3.2      Inspection and Observation..........................................................     15
         3.3      Termination of Information, Inspection and Observation
                  Covenants; Assignment...............................................................     15
         3.4      Right of First Refusal..............................................................     16
         3.5      Preparation of Audited Financial Statements.........................................     19
         3.6      Stock Purchase Agreements...........................................................     19
         3.7      Termination of Certain Covenants....................................................     19
         3.8      Covenants Relating to SBA Matters ..................................................     20
         3.9      Regulatory Compliance Cooperation ..................................................     21
         3.10     Consideration of Section 1202(c)....................................................     21
         3.11     Registration Rights.................................................................     21

4.       Miscellaneous................................................................................     21
         4.1      Successors and Assigns..............................................................     21
         4.2      Governing Law.......................................................................     22
         4.3      Counterparts........................................................................     22
         4.4      Titles and Subtitles................................................................     22
         4.5      Notices.............................................................................     22
         4.6      Expenses............................................................................     22
</TABLE>

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<Table>
<Caption>
<S>      <C>                                                                                              <C>
         4.7      Amendments and Waivers..............................................................     22
         4.8      Severability........................................................................     22
         4.9      Aggregation of Stock................................................................     22
         4.10     Entire Agreement; Amendment; Waiver.................................................     22
</TABLE>

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             EIGHTH AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT


         THIS EIGHTH AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT (this
"Agreement") is made as of the 1st day of June, 2001, by and among Acusphere,
Inc., a Delaware corporation ("Acusphere"), and those investors listed on
SCHEDULE A attached hereto (the "Investors"); and, solely for purposes of
Sections 1, 3.4 and 4 of this Agreement, Sherri C. Oberg ("Oberg").

                                    RECITALS

         WHEREAS, certain of the Investors hold shares of Acusphere's Series A
Convertible Preferred Stock, $.01 par value per share (the "Series A Stock"),
Acusphere's Series B Convertible Preferred Stock, $.01 par value per share (the
"Series B Stock"), Acusphere's Series C Convertible Preferred Stock, $.01 par
value per share (the "Series C Stock"), Acusphere's Series D Convertible
Preferred Stock, $.01 par value per share (the "Series D Stock"), Acusphere's
Series E Convertible Preferred Stock, $.01 par value per share (the "Series E
Stock"), Acusphere's Series F Convertible Preferred Stock, $.01 par value per
share (the "Series F Stock"), Acusphere's Series G Non-Voting Convertible
Preferred Stock, $.01 par value per share (the "Series G Stock"), and
Acusphere's Series H Convertible Preferred Stock, $.01 par value per share (the
"Series H Stock", and, together with the Series A Stock, Series B Stock, Series
C Stock, Series D Stock, Series E Stock, Series F Stock and Series G Stock, the
"Preferred Stock"), respectively, and a certain Investor, Alexandria Real Estate
Equities, L.P. ("Alexandria"), holds a warrant to purchase shares of Acusphere's
Series F Non-Voting Convertible Preferred Stock (the "Alexandria Warrant
Shares"), and each such Investor possesses certain registration and other rights
in connection therewith pursuant to a Seventh Amended and Restated Investors'
Rights Agreement, dated as of September 22, 2000, by and among Acusphere and
such Investors, as amended by that certain Amendment and Waiver to the Seventh
Amended and Restated Investors' Rights Agreement, dated as of March 30, 2001
(the "Seventh Amended Investors' Rights Agreement"); and

         WHEREAS, certain Investors (the "Series I Investors") are parties to
the Series I Convertible Preferred Stock Purchase Agreement, dated as of the
date hereof (the "Series I Agreement"), by and among such Series I Investors and
Acusphere providing, INTER ALIA, for the purchase by such Series I Investors of
Series I Convertible Preferred Stock, par value $.01 per share (the "Series I
Stock");

         WHEREAS, among the conditions to the consummation of the transactions
contemplated by the Series I Agreement is the execution and delivery of this
Agreement; and

         WHEREAS, each of the parties hereto desires to set forth in a single
document such registration and certain other rights of the Investors.

         NOW, THEREFORE, in consideration of the mutual promises, covenants and
conditions hereinafter set forth and for other good and valuable consideration
Acusphere, each Investor and Oberg, severally and not jointly, hereby agree as
follows:

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1.       TERMINATION OF SEVENTH AMENDED INVESTORS' RIGHTS AGREEMENT; WAIVER AND
CONSENT.

         1.1 TERMINATION OF SEVENTH AMENDED INVESTORS' RIGHTS AGREEMENT. The
parties hereto hereby acknowledge and agree that the Seventh Amended Investors'
Rights Agreement is hereby amended, restated and superseded in all respects by
this Agreement.

         1.2 WAIVER AND CONSENT. The Investors and all other parties to the
Seventh Amended Investors' Rights Agreement hereby (i) consent to the issuance
of the Series I Stock in accordance with the terms of the Series I Agreement and
(ii) waive all preemptive rights and rights of first refusal, and all related
notice provisions, under all existing agreements with respect to the issuance by
Acusphere of such Series I Stock.

2.       REGISTRATION RIGHTS.  Acusphere covenants and agrees as follows:

         2.1      DEFINITIONS.  For purposes of this Section 2:

                  (a) The term "Act" means the Securities Act of 1933, as
amended.

                  (b) The term "Alexandria Registrable Securities" means
(i) those shares of Common Stock issued or issuable upon conversion of the
Alexandria Warrant Shares issued or issuable upon exercise of that certain
Warrant Agreement by and between Acusphere and Alexandria, dated March 30, 2001
(the "Alexandria Warrant") and (ii) any common stock of Acusphere issued as (or
issuable upon the conversion of exercise of any warrant, right or other security
which is issued as) a dividend or other distribution with respect to, or in
exchange for or in replacement of the shares referenced in (i) above, excluding
in all cases, however, any Registrable Securities sold by a person in a
transaction in which the rights under this Section 2 are not assigned.

                  (c) The term "Common Stock" means shares of Common Stock,
$.01 par value per share, of Acusphere.

                  (d) The term "Elan Registrable Securities" means (i) the
Common Stock issuable or issued upon conversion of the Series G Stock and the
Series H Stock, (ii) the Common Stock issued upon conversion of the Note, and
(iii) any common stock of Acusphere issued as (or issuable upon the conversion
of exercise of any warrant, right or other security which is issued as) a
dividend or other distribution with respect to, or in exchange for or in
replacement of the shares referenced in (i) and (ii) above, excluding in all
cases, however, any Registrable Securities sold by a person in a transaction in
which the rights under this Section 2 are not assigned.

                  (e) The term "Form S-1" means such form under the Act as
in effect on the date hereof, or any registration form under the Act
subsequently adopted by the SEC which permits the registration of securities
under the Act for which no other form is authorized or prescribed.

                  (f) The term "Form S-3" means such form under the Act as
in effect on the date hereof or any registration form under the Act subsequently
adopted by the SEC which

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permits inclusion or incorporation of substantial information by reference to
other documents filed by Acusphere with the SEC.

                  (g) The term "Holder" means any person owning or having
the right to acquire Registrable Securities or any assignee thereof in
accordance with Section 2.12 hereof.

                  (h) The term "1934 Act" shall mean the Securities
Exchange Act of 1934, as amended.

                  (i) The terms "register," "registered," and
"registration" refer to a registration effected by preparing and filing a
registration statement or similar document in compliance with the Act, and the
declaration or ordering of effectiveness of such registration statement or
document.

                  (j) The term "Registrable Securities" means (A) (i) the
Common Stock issuable or issued upon conversion of the Series A Stock, the
Series B Stock, the Series C Stock, the Series D Stock, the Series E Stock, the
Series F Stock and the Series I Stock, and (ii) any Common Stock of Acusphere
issued as (or issuable upon the conversion or exercise of any warrant, right or
other security which is issued as) a dividend or other distribution with respect
to, or in exchange for or in replacement of the shares referenced in (i) above,
excluding in all cases, however, any Registrable Securities sold by a person in
a transaction in which the rights under this Section 2 are not assigned, (B) the
Elan Registrable Securities and (C) solely for the purposes of Section 2.1, 2.3
through 2.15, 3.11, 4.1, 4.7 and 4.9 of this Agreement, the Alexandria
Registrable Securities.

                  (k) The number of shares of "Registrable Securities then
outstanding" shall be determined by the number of shares of Common Stock
outstanding which are, and the number of shares of Common Stock issuable
pursuant to then exercisable or convertible securities which are, Registrable
Securities.

                  (l) The term "SEC" means the Securities and Exchange
Commission.

                  (m) The term "SBIC Investor" means any Investor that is a
Small Business Investment Company, as defined in 13 C.F.R. ss.107.

         2.2      DEMAND REGISTRATION.

                  (a) If Acusphere shall receive at any time after the
earlier of (i) December 31, 2002 or (ii) six months after the closing of an
initial public offering of securities of Acusphere (other than a registration
statement relating either to the sale of securities to employees of Acusphere
pursuant to a stock option, stock purchase or similar plan or a SEC Rule 145
transaction), a written request from (i) the Holders of at least forty percent
(40%) of the Registrable Securities other than the Elan Registrable Securities
then outstanding, (ii) the Holders of at least a majority of the Elan
Registrable Securities then outstanding, or (iii) the Holders of at least a
majority of the Series F Stock and Series I Stock then outstanding (including,
for such purposes, any shares of Common Stock issued upon conversion thereof),
that

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Acusphere effect a registration statement under the Act with respect to all
or a part of the Registrable Securities, then Acusphere shall:

                           (i)  within ten (10) days of the receipt thereof,
         give written notice of such request to all Holders; and

                           (ii) effect as soon as practicable, and in any event
         within ninety (90) days of the receipt of such request, the
         registration under the Act of all Registrable Securities which the
         Holders request to be registered, subject to the limitations of
         subsection 2.2(b), within thirty (30) days of the mailing of such
         notice by Acusphere in accordance with Section 4.5.

                  (b) If the Holders initiating the registration request
hereunder (the "Initiating Holders") intend to distribute the Registrable
Securities covered by their request by means of an underwriting, they shall so
advise Acusphere as a part of their request made pursuant to subsection 2.2(a)
and Acusphere shall include such information in the written notice referred to
in subsection 2.2(a). The underwriter will be selected by a majority in interest
of the Initiating Holders, subject to the reasonable approval of Acusphere. In
such event, the right of any Holder to include Registrable Securities in such
registration shall be conditioned upon such Holder's participation in such
underwriting and the inclusion of such Holder's Registrable Securities in the
underwriting to the extent provided herein. All Holders proposing to distribute
their securities through such underwriting shall (together with Acusphere as
provided in subsection 2.5(e)) enter into an underwriting agreement in customary
form with the underwriter or underwriters selected for such underwriting.
Notwithstanding any other provision of this Section 2.2, if the underwriter
advises the Initiating Holders in writing that marketing factors require a
limitation of the number of shares to be underwritten, then the Initiating
Holders shall so advise all Holders of Registrable Securities which would
otherwise be underwritten pursuant hereto, and the number of shares of
Registrable Securities that may be included in the underwriting shall be
allocated among all Holders thereof, including the Initiating Holders, in
proportion (as nearly as practicable) to the amount of Registrable Securities of
Acusphere owned by each Holder; PROVIDED, HOWEVER, that the number of shares of
Registrable Securities to be included in such underwriting shall not be reduced
unless all other securities are first entirely excluded from the underwriting.

                  (c) Notwithstanding the foregoing, if Acusphere shall
furnish to the Holders requesting registration pursuant to this Section 2.2, a
certificate signed by the Chief Executive Officer of Acusphere stating that in
the good faith judgment of the Board of Directors of Acusphere, it would be
seriously detrimental to Acusphere and its stockholders for a registration
statement to be filed and it is, therefore, essential to defer the filing of
such registration statement, Acusphere shall have the right to defer taking
action with respect to such filing for a period of not more than one hundred
twenty (120) days after receipt of the request of the Initiating Holders;
PROVIDED, HOWEVER, that Acusphere may not utilize this right more than once in
any twelve-month period.

                  (d) Acusphere shall not be obligated to effect, or to
take any action to effect, any registration pursuant to this Section 2.2 after
Acusphere has effected four (4) registrations on

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Form S-1 or its then equivalent pursuant to this Section 2.2 and such
registration statements have been declared or ordered effective and the sales of
Registrable Securities under such registration statements have closed; PROVIDED,
HOWEVER, that each such obligation shall be deemed to be satisfied only when a
registration statement covering at least sixty percent (60%) or more of the
Registrable Securities held by the Holders initiating such registration and
requested to be included in such registration statement has been declared or
ordered effective and the sale of such Registrable Securities under such
registration statements have closed; and FURTHER PROVIDED, HOWEVER, that in any
event (i) the Holders of at least forty percent (40%) of the Registrable
Securities other than the Elan Registrable Securities shall be entitled to at
least one demand registration pursuant to Section 2.2(a) hereto, (ii) the
Holders of at least a majority of the Elan Registrable Securities shall be
entitled to at least one demand registration pursuant to Section 2.2(a) hereto,
and (iii) the Holders of at least a majority of the Series F Stock and Series I
Stock (including, for such purposes, any shares of Common Stock issued upon
conversion thereof) shall be entitled to at least one demand registration
pursuant to Section 2.2(a) hereto.

                  (e) No incidental right under this Section 2.2 shall be
construed to limit any registration required under Section 2.3 or Section 2.4
herein.

         2.3      "PIGGY-BACK" REGISTRATION.

                  (a) If (but without any obligation to do so) Acusphere
proposes to register (including for this purpose a registration effected by
Acusphere for stockholders other than the Holders) any of its stock or other
securities under the Act in connection with the public offering of such
securities solely for cash (other than a registration relating solely to the
sale of securities to participants in a stock plan, a registration on any form
which does not include substantially the same information as would be required
to be included in a registration statement covering the sale of the Registrable
Securities, a registration in which the only Common Stock being registered is
Common Stock issuable upon conversion of debt securities which are also being
registered, or a registration relating to the sale of securities in connection
with a transaction covered by Rule 145 promulgated under the Act), Acusphere
shall, at such time, promptly give each Holder written notice of such
registration. Upon the written request of each Holder given within twenty (20)
days after mailing of such notice by Acusphere in accordance with Section 4.5,
Acusphere shall, subject to the provisions of subsection 2.3(b), cause to be
registered under the Act all of the Registrable Securities that each such Holder
has requested to be registered.

                  (b) UNDERWRITING REQUIREMENTS. In connection with any
offering involving an underwriting of shares of Acusphere's capital stock,
Acusphere shall not be required under this Section 2.3 to include any of the
Holders' securities in such underwriting unless such Holders accept the terms of
the underwriting as agreed upon between Acusphere and the underwriters selected
by it (or by other persons entitled to select the underwriters pursuant to the
terms of this Agreement), and then only in such quantity as the underwriters
determine in their sole discretion will not jeopardize the success of the
offering by Acusphere. If the total amount of securities, including Registrable
Securities, requested by stockholders to be included in such offering exceeds
the amount of securities to be sold (other than by Acusphere) that the
underwriters determine in their sole discretion is compatible with the success
of the offering, then Acusphere shall be required to include in the offering
only that number of such securities, including


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Registrable Securities, which the underwriters determine in their sole
discretion will not jeopardize the success of the offering; PROVIDED, HOWEVER,
there shall first be excluded from such registration statement all shares of
Common Stock sought to be included therein by (i) any director, consultant,
officer or employee of Acusphere or any subsidiary of Acusphere (except those
joining such registration pursuant to the Stock Repurchase and Registration
Agreement, dated April 30, 1996, among Acusphere, Robert S. Langer, Harry R.
Allcock and Sherri C. Oberg, the "Founders' Registration Agreement"), and (ii)
stockholders exercising any contractual or incidental registration rights
subordinate and junior to the rights of the Holders of Registrable Securities.
Thereafter, the shares of Common Stock sought to be included in such
registration statement pursuant to this Agreement and, if applicable, the
Founders' Registration Agreement, shall be excluded from such registration
statement in proportion (as nearly as practicable) to the amount of such shares
owned by the holders of such shares. If after such shares are excluded, the
underwriters shall determine in their sole discretion that the number of
securities which remain to be included in the offering exceeds the amount of
securities to be sold that the underwriters determine is compatible with the
success of the offering, then the Common Stock to be included, if any, shall be
apportioned pro rata among the holders of Common Stock providing notice of their
desire to participate in the offering pursuant to this Agreement and the
Founders' Registration Agreement (the "Selling Stockholders") according to the
total amount of securities entitled to be included therein owned by each Selling
Stockholder, or in such other proportions as shall mutually be agreed to by such
Selling Stockholders. Notwithstanding the foregoing, in no event shall the
amount of securities of the Selling Stockholders included in the offering be
reduced below thirty percent (30%) of the total amount of securities included in
such offering, unless such offering is the initial public offering of
Acusphere's securities in which case the Selling Stockholders may be excluded if
the underwriters make the determination described above and no other
stockholder's securities are included. For purposes of the preceding sentence
concerning apportionment, for any Selling Stockholder which is a partnership or
corporation, the partners, retired partners and stockholders of such Selling
Stockholder, or the estates and family members of any such partners and retired
partners and any trusts for the benefit of any of the foregoing persons shall be
deemed to be a single "Selling Stockholder", and any pro-rata reduction with
respect to such "selling holder" shall be based upon the aggregate amount of
shares carrying registration rights owned by all entities and individuals
included in such "Selling Stockholder", as defined in this sentence.

                  (c) No incidental right under this Section 2.3 shall be
construed to limit any registration required under Section 2.2 or Section 2.4
herein.

         2.4 FORM S-3 REGISTRATION. In case Acusphere shall receive from any
Holder or Holders of Registrable Securities a written request or requests that
Acusphere effect a registration on Form S-3 and any related qualification or
compliance with respect to all or a part of the Registrable Securities owned by
such Holder or Holders, Acusphere will:

                  (a) promptly give written notice of the proposed registration,
and any related qualification or compliance, to all other Holders; and

                  (b) as soon as practicable, effect such registration and
all such qualifications and compliances as may be so requested and as would
permit or facilitate the sale and


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distribution of all or such portion of such Holder's or Holders' Registrable
Securities as are specified in such request, together with all or such portion
of the Registrable Securities of any other Holder or Holders joining in such
request as are specified in a written request given within fifteen (15) days
after receipt of such written notice from Acusphere; PROVIDED, HOWEVER, that
Acusphere shall not be obligated to effect any such registration, qualification
or compliance, pursuant to this Section 2.4: (i) if Form S-3 is not available
for such offering by the Holders; (ii) if the Holders, together with the holders
of any other securities of Acusphere entitled to inclusion in such registration,
propose to sell Registrable Securities and such other securities (if any) at an
aggregate price to the public of less than $3,000,000; or (iii) if Acusphere
shall furnish to the Holders a certificate signed by the Chief Executive Officer
of Acusphere stating that in the good faith judgment of the Board of Directors
of Acusphere, it would be seriously detrimental to Acusphere and its
stockholders for such Form S-3 registration to be effected at such time, in
which event Acusphere shall have the right to defer the filing of the Form S-3
registration statement for a period of not more than sixty (60) days after
receipt of the request of the Holder or Holders under this Section 2.4;
provided, however, that Acusphere shall not utilize this right more than once in
any twelve (12) month period.

                  (c) Subject to the foregoing, Acusphere shall file a
registration statement covering the Registrable Securities and other securities
so requested to be registered as soon as practicable after receipt of the
request or requests of the Holders. At the election of a majority in interest of
the Holders of Registrable Securities to be included in such registration
statement, such registration may be firmly underwritten by a managing
underwriter selected by such Holders, and, if so elected, be subject to the
requirements and provisions of Section 2.2(b). Registrations effected pursuant
to this Section 2.4 shall not be counted as demands for registration or
registrations effected pursuant to Sections 2.2 or 2.3, respectively.

         2.5 OBLIGATIONS OF ACUSPHERE. Whenever required under this Section 2 to
effect the registration of any Registrable Securities, Acusphere shall, as
expeditiously as reasonably possible:

                  (a) Prepare and file with the SEC a registration
statement with respect to such Registrable Securities and use its best efforts
to cause such registration statement to become effective, and, upon the request
of the Holders of a majority of the Registrable Securities registered
thereunder, keep such registration statement effective for a period of up to one
hundred twenty (120) days or until the distribution contemplated in the
Registration Statement has been completed; provided, however, that (i) such
120-day period shall be extended for a period of time equal to the period the
Holder refrains from selling any securities included in such registration at the
request of an underwriter of Common Stock (or other securities) of Acusphere;
and (ii) in the case of any registration of Registrable Securities on Form S-3
which are intended to be offered on a continuous or delayed basis, such 120-day
period shall be extended, if necessary, to keep the registration statement
effective until all such Registrable Securities are sold, provided that Rule
415, or any successor rule under the Act, permits an offering on a continuous or
delayed basis, and provided further that applicable rules under the Act
governing the obligation to file a post-effective amendment permit, in lieu of
filing a post-effective amendment which (A) includes any prospectus required by
Section 10(a)(3) of the Act or (B) reflects facts or events representing a
material or fundamental change in the information set forth in the registration
statement, the

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<PAGE>   11

incorporation by reference of information required to be included in (A) and (B)
above to be contained in periodic reports filed pursuant to Section 13 or 15(d)
of the 1934 Act in the registration statement.

                  (b) Prepare and file with the SEC such amendments and
supplements to such registration statement and the prospectus used in connection
with such registration statement as may be necessary to comply with the
provisions of the Act with respect to the disposition of all securities covered
by such registration statement.

                  (c) Furnish to the Holders such numbers of copies of a
prospectus, including a preliminary prospectus, in conformity with the
requirements of the Act, and such other documents as they may reasonably request
in order to facilitate the disposition of Registrable Securities owned by them.

                  (d) Use its best efforts to register and qualify the
securities covered by such registration statement under such other securities or
Blue Sky laws of such jurisdictions as shall be reasonably requested by the
Holders; provided that Acusphere shall not be required in connection therewith
or as a condition thereto to qualify to do business or to file a general consent
to service of process in any such states or jurisdictions, unless Acusphere is
already subject to service in such jurisdiction and except as may be required by
the Act.

                  (e) In the event of any underwritten public offering,
enter into and perform its obligations under an underwriting agreement, in usual
and customary form, with the managing underwriter of such offering. Each Holder
participating in such underwriting shall also enter into and perform its
obligations under such an agreement.

                  (f) Notify each Holder of Registrable Securities covered
by such registration statement at any time when a prospectus relating thereto is
required to be delivered under the Act as a result of the happening of any event
as a result of which the prospectus included in such registration statement, as
then in effect, includes an untrue statement of a material fact or omits to
state a material fact required to be stated therein or necessary to make the
statements therein not misleading in the light of the circumstances then
existing.

                  (g) Cause all such Registrable Securities registered
pursuant hereunder to be listed on each securities exchange on which similar
securities issued by Acusphere are then listed.

                  (h) Provide a transfer agent and registrar for all
Registrable Securities registered pursuant hereunder and a CUSIP number for all
such Registrable Securities, in each case not later than the effective date of
such registration.

                  (i) Furnish, at the request of any Holder requesting
registration of Registrable Securities pursuant to this Section 2, on the date
that such Registrable Securities are delivered to the underwriters for sale in
connection with a registration pursuant to this Section 2, if such securities
are being sold through underwriters, or, if such securities are not being sold
through underwriters, on the date that the registration statement with respect
to such securities becomes

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<PAGE>   12

effective, (i) an opinion, dated such date, of the counsel representing
Acusphere for the purposes of such registration, in form and substance as is
customarily given to underwriters in an underwritten public offering, addressed
to the underwriters, if any, and to the Holders requesting registration of
Registrable Securities and (ii) a letter dated such date, from the independent
certified public accountants of Acusphere, in form and substance as is
customarily given by independent certified public accountants to underwriters in
an underwritten public offering, addressed to the underwriters, if any, and to
the Holders requesting registration of Registrable Securities.

                  (j) Make available for inspection by each seller of
Registrable Securities, any underwriter participating in any distribution
pursuant to such registration statement, and any attorney, accountant or other
agent retained by such seller or underwriter, all financial and other records,
pertinent corporate documents and properties of Acusphere, and cause Acusphere's
officers, directors and employees to participate in "road show" meetings with
potential investors and to supply all information reasonably requested by any
such seller, underwriter, attorney, accountant or agent in connection with such
registration statement.

         2.6 FURNISH INFORMATION. It shall be a condition precedent to the
obligations of Acusphere to take any action pursuant to this Section 2 with
respect to the Registrable Securities of any selling Holder that such Holder
shall furnish to Acusphere such information regarding itself, the Registrable
Securities held by it, and the intended method of disposition of such securities
as shall be required to effect the registration of such Holder's Registrable
Securities.

         2.7 EXPENSES OF DEMAND AND S-3 REGISTRATIONS. All expenses other than
underwriting discounts and commissions incurred in connection with
registrations, filings or qualifications pursuant to Sections 2.2 and 2.4,
including (without limitation) all registration, filing and qualification fees,
printers' and accounting fees, fees and disbursements of counsel for Acusphere
and transfer taxes, if any, and the reasonable fees and disbursements of one
counsel for the selling Holders shall be borne by Acusphere; PROVIDED, HOWEVER,
that Acusphere shall not be required to pay for any expenses of any registration
proceeding begun pursuant to Section 2.2 or 2.4 if the registration request is
subsequently withdrawn at the request of the Holders of a majority of the
Registrable Securities to be registered (in which case all Holders participating
in the registration shall bear such expenses), unless, in the case of a
registration pursuant to Section 2.2, the Holders of a majority of the
Registrable Securities agree to forfeit their right to one demand registration
pursuant to Section 2.2; PROVIDED FURTHER, HOWEVER, that if at the time of such
withdrawal, the Holders have learned of a material adverse change in the
condition, business, or prospects of Acusphere from that known to the Holders at
the time of their request and have withdrawn the request with reasonable
promptness following disclosure by Acusphere of such material adverse change,
then the Holders shall not be required to pay any of such expenses and shall
retain their rights under Sections 2.2 and 2.4.

         2.8 EXPENSES OF "PIGGY-BACK" REGISTRATION. Acusphere shall bear and pay
all expenses incurred in connection with any registration, filing or
qualification of Registrable Securities with respect to the registrations
pursuant to Section 2.3 for each Holder, including (without limitation) all
registration, filing, and qualification fees, printers' and accounting fees
relating or apportionable thereto and transfer taxes, if any, and the fees and
disbursements of one

                                      -9-
<PAGE>   13

counsel for the selling Holders selected by them, but excluding underwriting
discounts and commissions relating to the Registrable Securities.

         2.9 DELAY OF REGISTRATION. No Holder shall have any right to obtain or
seek an injunction restraining or otherwise delaying any such registration as
the result of any controversy that might arise with respect to the
interpretation or implementation of this Section 2.

         2.10     INDEMNIFICATION.  In the event any Registrable Securities are
included in a registration statement under this Section 2:

                  (a) To the extent permitted by law, Acusphere will
indemnify and hold harmless each Holder, any underwriter (as defined in the Act)
for such Holder and each person, if any, who controls such Holder or underwriter
within the meaning of the Act or the 1934 Act, against any losses, claims,
damages, or liabilities (joint or several) to which they may become subject
under the Act, the 1934 Act or other federal or state law, insofar as such
losses, claims, damages, or liabilities (or actions in respect thereof) arise
out of or are based upon any of the following statements, omissions or
violations (collectively a "Violation"): (i) any untrue statement or alleged
untrue statement of a material fact contained in such registration statement,
including any preliminary prospectus or final prospectus contained therein or
any amendments or supplements thereto, (ii) the 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 violation or alleged
violation by Acusphere of the Act, the 1934 Act, any state securities law or any
rule or regulation promulgated under the Act, the 1934 Act or any state
securities law; and Acusphere will pay to each such Holder, underwriter or
controlling person, as incurred, any legal or other expenses reasonably incurred
by them in connection with investigating or defending any such loss, claim,
damage, liability, or action; provided, however, that the indemnity agreement
contained in this subsection 2.10(a) shall not apply to amounts paid in
settlement of any such loss, claim, damage, liability, or action if such
settlement is effected without the consent of Acusphere (which consent shall not
be unreasonably withheld), nor shall Acusphere be liable in any such case for
any such loss, claim, damage, liability, or action to the extent that it arises
out of or is based upon a Violation which occurs in reliance upon and in
conformity with written information furnished expressly for use in connection
with such registration by any such Holder, underwriter or controlling person.

                  (b) To the extent permitted by law, each selling Holder,
severally but not jointly, will indemnify and hold harmless Acusphere, each of
its directors, each of its officers who has signed the registration statement,
each person, if any, who controls Acusphere within the meaning of the Act, any
underwriter, any other Holder selling securities in such registration statement
and any controlling person of any such underwriter or other Holder, against any
losses, claims, damages, or liabilities, to which any of the foregoing persons
may become subject, under the Act, the 1934 Act or other federal or state law,
insofar as such losses, claims, damages, or liabilities (or actions in respect
thereto) arise out of or are based upon any Violation, in each case to the
extent (and only to the extent) that such Violation occurs in reliance upon and
in conformity with written information furnished by such Holder expressly for
use in connection with such registration; and each such Holder will pay, as
incurred, any legal or other expenses reasonably incurred by any person intended
to be indemnified pursuant to this subsection 2.10(b),

                                      -10-
<PAGE>   14

in connection with investigating or defending any such loss, claim, damage,
liability, or action; provided, however, that the indemnity agreement contained
in this subsection 2.10(b) shall not apply to amounts paid in settlement of any
such loss, claim, damage, liability or action if such settlement is effected
without the consent of the Holder, which consent shall not be unreasonably
withheld; provided, that, in no event shall any indemnity under this subsection
2.10(b) exceed the net proceeds from the offering received by such Holder.

                  (c) Promptly after receipt by an indemnified party under
this Section 2.10 of notice of the commencement of any action (including any
governmental action), such indemnified party will, if a claim in respect thereof
is to be made against any indemnifying party under this Section 2.10, deliver to
the indemnifying party a written notice of the commencement thereof and the
indemnifying party shall have the right to participate in, and, to the extent
the indemnifying party so desires, jointly with any other indemnifying party
similarly noticed, to assume the defense thereof with counsel mutually
satisfactory to the parties; PROVIDED, HOWEVER, that an indemnified party
(together with all other indemnified parties which may be represented without
conflict by one counsel) shall have the right to retain one separate counsel,
with the fees and expenses to be paid by the indemnifying party, if the
indemnified party shall have reasonably concluded that there may be reasonable
defenses available to it which are different from or additional to those
available to the indemnifying party or if the interests of the indemnified party
reasonably may be deemed to conflict with the interests of the indemnifying
party. The failure to deliver written notice to the indemnifying party within a
reasonable time of the commencement of any such action, if prejudicial to its
ability to defend such action, shall relieve such indemnifying party of any
liability to the indemnified party under this Section 2.10, but the omission so
to deliver written notice to the indemnifying party will not relieve it of any
liability that it may have to any indemnified party otherwise than under this
Section 2.10.

                  (d) If the indemnification provided for in this Section
2.10 is held by a court of competent jurisdiction to be unavailable to an
indemnified party with respect to any loss, liability, claim, damage, or expense
referred to therein, then the indemnifying party, in lieu of indemnifying such
indemnified party hereunder, shall contribute to the amount paid or payable by
such indemnified party as a result of such loss, liability, claim, damage, or
expense in such proportion as is appropriate to reflect the relative fault of
the indemnifying party on the one hand and of the indemnified party on the other
in connection with the statements or omissions that resulted in such loss,
liability, claim, damage, or expense as well as any other relevant equitable
considerations. The relative fault of the indemnifying party and of the
indemnified party shall be determined by reference to, among other things,
whether the untrue or alleged untrue statement of a material fact or the
omission to state a material fact relates to information supplied by the
indemnifying party or by the indemnified party and the parties' relative intent,
knowledge, access to information, and opportunity to correct or prevent such
statement or omission; provided, that, in the case of contribution by a selling
Holder, in no event shall any contribution under this subsection 2.10(d) exceed
the net proceeds from the offering received by such contributing party.

                  (e) Notwithstanding the foregoing, to the extent that the
provisions on indemnification and contribution contained in the underwriting
agreement entered into in connection with the underwritten public offering are
in conflict with the foregoing provisions, the provisions in the underwriting
agreement shall control.

                                      -11-
<PAGE>   15

                  (f) The obligations of Acusphere and Holders under this
Section 2.10 shall survive the completion of any offering of Registrable
Securities in a registration statement under this Section 2, and otherwise.

         2.11 REPORTS UNDER SECURITIES EXCHANGE ACT OF 1934. With a view to
making available to the Holders the benefits of Rule 144 promulgated under the
Act and any other rule or regulation of the SEC that may at any time permit a
Holder to sell securities of Acusphere to the public without registration or
pursuant to a registration on Form S-3, Acusphere agrees to use its best efforts
to:

                  (a) make and keep public information available, as those
terms are understood and defined in SEC Rule 144, at all times after ninety (90)
days after the effective date of the first registration statement filed by
Acusphere for the offering of its securities to the general public;

                  (b) take such action, including the voluntary
registration of its Common Stock under Section 12 of the 1934 Act, as is
necessary to enable the Holders to utilize Form S-3 for the sale of their
Registrable Securities, such action to be taken as soon as practicable after the
end of the fiscal year in which the first registration statement filed by
Acusphere for the offering of its securities to the general public is declared
effective;

                  (c) file with the SEC in a timely manner all reports and
other documents required of Acusphere under the Act and the 1934 Act; and

                  (d) furnish to any Holder, so long as the Holder owns any
Registrable Securities, forthwith upon request (i) a written statement by
Acusphere that it has complied with the reporting requirements of SEC Rule 144
(at any time after ninety (90) days after the effective date of the first
registration statement filed by Acusphere), the Act and the 1934 Act (at any
time after it has become subject to such reporting requirements), or that it
qualifies as a registrant whose securities may be resold pursuant to Form S-3
(at any time after it so qualifies), (ii) a copy of the most recent annual or
quarterly report of Acusphere and such other reports and documents so filed by
Acusphere, and (iii) such other information as may be reasonably requested in
availing any Holder of any rule or regulation of the SEC which permits the
selling of any such securities without registration or pursuant to such form.

         2.12 ASSIGNMENT OF REGISTRATION RIGHTS. The rights to cause Acusphere
to register Registrable Securities pursuant to this Section 2 may be assigned
(but only with all related obligations) by a Holder only to a transferee or
assignee of such securities who, after such assignment or transfer, holds at
least 75,000 shares of Registrable Securities (subject to appropriate adjustment
for stock splits, stock dividends, combinations and other recapitalizations),
provided: (a) Acusphere is, within a reasonable time after such transfer,
furnished with written notice of the name and address of such transferee or
assignee and the securities with respect to which such registration rights are
being assigned; (b) such transferee or assignee agrees in writing to be bound by
and subject to the terms and conditions of this Agreement, including without
limitation the provisions of Section 2.14 below; and (c) such assignment shall
be effective only if immediately following such transfer the further disposition
of such securities by the transferee or assignee is restricted under the Act.


                                      -12-
<PAGE>   16

         2.13 LIMITATIONS ON SUBSEQUENT REGISTRATION RIGHTS. From and after the
date of this Agreement, Acusphere shall not, without the prior written consent
of the Holders of a majority of the Registrable Securities then outstanding,
enter into any agreement with any holder or prospective holder of any securities
of Acusphere which would allow such holder or prospective holder (a) to include
such securities in any registration filed under Section 2.2 or described in
Section 2.3 hereof, unless under the terms of such agreement, such holder or
prospective holder may include such securities in any such registration only to
the extent that the inclusion of such securities will not reduce the amount of
the Registrable Securities of the Holders which are to be included or (b) to
make a demand registration which could result in such registration statement
being declared effective prior to the earlier of either of the dates set forth
in subsection 2.2(a) or within one hundred twenty (120) days of the effective
date of any registration effected pursuant to Section 2.2 or 2.4.

         2.14 "MARKET STAND-OFF" AGREEMENT. Each Investor hereby agrees that,
during the period of duration (not to exceed one hundred eighty (180) days)
specified by Acusphere and an underwriter of Common Stock or other securities of
Acusphere, following the effective date of a registration statement of Acusphere
filed under the Act, such Investor shall not, to the extent requested by
Acusphere and such underwriter, directly or indirectly sell, offer to sell,
contract to sell (including, without limitation, any short sale), grant any
option to purchase or otherwise transfer or dispose of (other than to donees who
agree to be similarly bound) any securities of Acusphere held by it at any time
during such period except Common Stock included in such registration; provided,
however, that:

                  (a) such agreement shall be applicable only with respect to a
         registration statement covering Acusphere's initial public offering of
         securities; and

                  (b) all officers and directors of Acusphere and all other
         persons with registration rights (whether or not pursuant to this
         Agreement) enter into similar agreements.

                  In order to enforce the foregoing covenant, Acusphere may
impose stop-transfer instructions with respect to the Registrable Securities of
an Investor (and the shares or securities of every other person subject to the
foregoing restriction) until the end of such period.

                  Notwithstanding the foregoing, the obligations described in
this Section 2.14 shall not apply to a registration relating solely to employee
benefit plans on Form S-1 or Form S-8 or similar forms which may be promulgated
in the future, or a registration relating solely to a SEC Rule 145 transaction
on Form S-4 or similar forms which may be promulgated in the future.


                                      -13-
<PAGE>   17

         2.15 TERMINATION OF REGISTRATION RIGHTS. No Holder shall be entitled to
exercise any right provided for in this Section 2 if there is then an active
public trading market for the Common Stock, such Holder may sell all of such
Holder's Registrable Securities under Rule 144 promulgated under the Act in a
three-month period and such Holder's Registrable Securities represent less than
3% of Acusphere's outstanding shares of capital stock. No holder shall be
entitled to exercise any right provided for in Section 2.2 after five (5) years
following the consummation of the sale of securities pursuant to a registration
statement filed by Acusphere under the Act in connection with the initial firm
commitment underwritten offering of its securities to the general public.

3.       COVENANTS OF ACUSPHERE.

         3.1 DELIVERY OF FINANCIAL STATEMENTS. Acusphere shall deliver to each
Significant Investor (as defined in Section 3.2(b) hereof), as long as such
Investor holds shares of Preferred Stock (or any shares of Common Stock issuable
upon conversion thereof):

                  (a) as soon as practicable, but in any event within
ninety (90) days after the end of each fiscal year of Acusphere, an income
statement for such fiscal year, a balance sheet of Acusphere and statement of
stockholders' equity as of the end of such year, and a schedule as to the
sources and applications of funds for such year, such year-end financial reports
to be in reasonable detail, prepared in accordance with generally accepted
accounting principles ("GAAP"), and audited and certified by independent public
accountants of nationally recognized standing selected by Acusphere;

                  (b) as soon as practicable, but in any event within
forty-five (45) days after the end of each of the first three (3) quarters of
each fiscal year of Acusphere, an unaudited profit or loss statement and
schedule as to the sources and application of funds for such fiscal quarter and
an unaudited balance sheet as of the end of such fiscal quarter;

                  (c) within thirty (30) days of the end of each month, an
unaudited income statement and schedule as to the sources and application of
funds and balance sheet for and as of the end of such month, in reasonable
detail;

                  (d) as soon as practicable, but in any event forty-five
(45) days prior to the end of each fiscal year, a budget and business plan for
the next fiscal year, prepared on a monthly basis, including balance sheets and
sources and applications of funds statements for such months and, as soon as
prepared, any other budgets or revised budgets prepared by Acusphere;

                  (e) with respect to the financial statements called for
in subsections (a), (b) and (c) of this Section 3.1, an instrument executed by
the Chief Financial Officer or Chief Executive Officer of Acusphere and
certifying that such financials were prepared in accordance with GAAP
consistently applied with prior practice for earlier periods (with the exception
of footnotes that may be required by GAAP) and fairly present the financial
condition of Acusphere and its results of operation for the period specified,
subject to year-end audit adjustment; and

                                      -14-
<PAGE>   18

                  (f) such other information relating to the financial
condition, business, prospects or corporate affairs of Acusphere as such
Significant Investor or any assignee of such Significant Investor may from time
to time request, provided, however, that Acusphere shall not be obligated under
this subsection (f) or any other subsection of Section 3.1 to provide
information which it deems in good faith to be a trade secret or similar
confidential information.

         3.2      INSPECTION AND OBSERVATION.

                  (a) Acusphere shall permit each Significant Investor (as
defined in Section 3.2(b) hereof), at such Significant Investor's expense, to
visit and inspect Acusphere's properties, to examine its books of account and
records and to discuss Acusphere's affairs, finances and accounts with its
officers, all at such reasonable times as may be requested by such Significant
Investor; provided, however, that Acusphere shall not be obligated pursuant to
this Section 3.2 to provide access to any information which its Board of
Directors reasonably considers to be a trade secret or similar confidential
information.

                  (b) Acusphere will permit any Significant Investor (as
hereinafter defined), any authorized representative thereof or any Series E
Investor holding at least five percent (5%) of the Common Stock on a
fully-diluted basis, assuming the conversion or exercise, as the case may be, of
all outstanding securities or rights to acquire shares of Acusphere's Common
Stock (each, a "Series E Five-Percent Holder"), to attend all meetings of the
Board of Directors of Acusphere in a nonvoting observer capacity and shall, upon
the written request of such Significant Investor or Series E Five-Percent
Holder, provide such Significant Investor or Series E Five-Percent Holder with
such notice and other information with respect to such meetings as are delivered
to the directors of Acusphere; provided, however, that it is hereby acknowledged
and agreed that only a single representative of each group consisting of a
Significant Investor and its affiliates will be entitled to attend any such
meeting pursuant to this Section 3.2(b). Upon the written request of any such
Significant Investor or Series E Five-Percent Holder, Acusphere shall notify
such Significant Investor or Series E Five-Percent Holder, within ten (10) days
thereof, of the taking of any action by the Board of Directors of Acusphere in
lieu of a meeting thereof. As used in this Section 3, the term "Significant
Investor" shall mean (i) Elan so long as it, together with its affiliates, holds
5% or more of Acusphere's issued and outstanding shares of Common Stock on a
fully diluted basis (assuming the conversion or exercise, as the case may be, of
all outstanding securities or rights to acquire shares of Acusphere's Common
Stock) and (ii) any other Investor other than Alexandria who, together with its
affiliates, holds at least 200,000 shares of Preferred Stock, including Common
Stock issuable upon the conversion of Preferred Stock (as presently constituted
and subject to subsequent adjustment for stock splits, stock dividends, reverse
stock splits, recapitalizations and the like).

         3.3 TERMINATION OF INFORMATION, INSPECTION AND OBSERVATION COVENANTS,
ASSIGNMENT. The covenants set forth in Section 3.1 and Section 3.2 shall
terminate as to each Investor and be of no further force or effect when the sale
of securities pursuant to a registration statement filed by Acusphere under the
Act in connection with the firm commitment underwritten offering of its
securities to the general public is consummated or when Acusphere first becomes
subject to the periodic reporting requirements of Sections 12(g) or 15(d) of the
1934 Act, whichever event shall first occur; PROVIDED, HOWEVER, that the
covenants set forth in Section 3.2 shall not terminate with

                                      -15-
<PAGE>   19

respect to Elan so long as it, together with its affiliates, holds 5% or more of
Acusphere's issued and outstanding shares of Common Stock on a fully diluted
basis (assuming the conversion or exercise, as the case may be, of all
outstanding securities or rights to acquire shares of Acusphere's Common Stock).
The rights to receive and access information relating to Acusphere pursuant to
Sections 3.1 and 3.2 may be assigned (but only with all related obligations) by
an Investor to a transferee or assignee of such securities who, after such
assignment or transfer, holds at least 75,000 shares of Registrable Securities
(subject to appropriate adjustment for stock splits, stock dividends,
combinations and other recapitalizations), provided: (a) Acusphere is, within a
reasonable time after such transfer, furnished with written notice of the name
and address of such transferee or assignee and the securities with respect to
which such information rights are being assigned; and (b) such transferee or
assignee agrees in writing to be bound by and subject to the terms and
conditions of this Agreement.

         3.4 RIGHT OF FIRST REFUSAL. Subject to the terms and conditions
specified in this Section 3.4, Acusphere hereby grants to each of the Investors
a right of first refusal with respect to future sales by Acusphere of its Shares
and Venture Capital Shares (each, as hereinafter defined). For purposes of this
Section 3.4 and Section 4 only, the term "Investor" shall include Oberg and any
general partners and affiliates of an Investor. For purposes of this Section 3.4
only, the term "Investor" shall not include Elan or any of its affiliates after
June 30, 2004. Each Investor shall be entitled to apportion the right of first
refusal hereby granted it among itself and its partners and affiliates in such
proportions as it deems appropriate. For purposes hereof, an "affiliate" of any
Investor is an entity or a person that directly or indirectly controls, is
controlled by, or is under common control with such Investor. For purposes
hereof, (i) all notice provisions shall be deemed satisfied with respect to any
general partner or affiliate of an Investor if such notice is deemed satisfied
with respect to such Investor, (ii) all waiver and amendment provisions shall be
deemed satisfied with respect to any general partner or affiliate of an Investor
if such waiver or amendment is deemed satisfied with respect to such Investor,
and (iii) in no event shall any Investor, its general partners and its
affiliates, together as a group, have a right pursuant to this Section 3.4 to
purchase in the aggregate more than such Investor's pro rata portion of the
Shares and Venture Capital Shares, if any, offered hereby.

                  Each time Acusphere proposes to offer any shares of, or
securities convertible into or exercisable for any shares of, any class of its
capital stock ("Shares"), Acusphere shall first make an offering of such Shares
to the Investors in accordance with the following provisions:

                  (a) Acusphere shall deliver a notice by certified mail
("Notice") to each Investor other than Alexandria ("Remaining Investors")
stating (i) its bona fide intention to offer such Shares, (ii) the number of
such Shares to be offered, and (iii) the price and terms, if any, upon which it
proposes to offer such Shares. If the Shares being offered by Acusphere are
offered in connection with a Venture Capital Financing (as defined in paragraph
(c) below), the Notice shall be delivered to the Remaining Investors
simultaneously with the Alexandria Notice (as defined below) delivered to
Alexandria.

                  (b) Within thirty (30) calendar days after receipt of the
Notice, each Remaining Investor may elect to purchase or obtain, at the price
and on the terms specified in the

                                      -16-
<PAGE>   20

Notice (provided, that, in the event any such consideration is non-cash
consideration, at the election of such Remaining Investor, such Remaining
Investor may pay cash equal to the fair market value of such non-cash
consideration, and provided, that, the closing of any such purchase may be
extended to any such Remaining Investor for purposes of obtaining necessary
governmental approvals), some or all of such Remaining Investor's pro rata
portion of the Shares offered by Acusphere in the Notice. Each Remaining
Investor's pro rata portion of such Shares shall be equal to a fraction of such
Shares, the numerator of which is the number of shares of Registrable Securities
or Common Stock as the case may be, then held by such Remaining Investor and the
denominator of which shall be equal to the sum of the total number of shares of
Registrable Securities and Common Stock then held by all Remaining Investors;
PROVIDED, HOWEVER, that, solely for purposes of determining such pro rata
portion pursuant to this Section 3.4(c), "Registrable Securities" shall not
include any shares of Series G Stock or any share of Common Stock issued as (or
issuable upon the conversion or exercise of any warrant, right or other security
which is issued as) a dividend or other distribution with respect to, or in
exchange for or in replacement of such shares. If any Remaining Investor does
not elect to exercise its or his right to purchase its or his total pro rata
portion of the Shares, Acusphere shall promptly give notice of such failure to
the other Remaining Investors who did elect to purchase (the "Participants").
Each Remaining Investor shall have a right of over-allotment such that if any
Remaining Investor fails to exercise its or his right to purchase its or his
total pro rata portion of the Shares, the Participants may purchase such
portion, by giving written notice to Acusphere with five (5) days from the date
that Acusphere provides written notice to the other Participants of the number
of Shares with respect to which such non-purchasing Remaining Investor has
failed to exercise its rights hereunder.

                  (c) Notwithstanding the foregoing rights contained in
clause (b) of this Section 3.4, if such Shares are offered in connection with a
private placement pursuant to a Venture Capital Financing (as defined below)
("Venture Capital Shares"), Acusphere shall first deliver a notice by certified
mail ("Alexandria Notice") to Alexandria stating (i) its bona fide intention to
offer such Venture Capital Shares, (ii) the number of Venture Capital Shares to
be offered, and (iii) the price and terms, if any, upon which it proposes to
offer such Venture Capital Shares. Within thirty (30) calendar days after
receipt of the Alexandria Notice, Alexandria may elect to purchase or obtain, at
the price and on the terms specified in the Alexandria Notice (provided, that,
in the event any such consideration is non-cash consideration, at the election
of Alexandria, Alexandria may pay cash equal to the fair market value of such
non-cash consideration, and provided, that, the closing of any such purchase may
be extended to Alexandria for purposes of obtaining necessary governmental
approvals), such number of Venture Capital Shares as is equal the number of
Venture Capital Shares determined by dividing $250,000 by the per share price at
which the Venture Capital Shares are offered by Acusphere. The term "Venture
Capital Financing" as used herein shall refer to the sale by Acusphere of
Venture Capital Shares in a private placement to one or more institutional
investors resulting in aggregate gross proceeds to Acusphere of at least
$5,000,000, excluding any securities issued in connection with an acquisition
transaction. In the foregoing clauses (a) and (b) of this Section 3.4, the term
Shares shall exclude any such Shares which constitute Venture Capital Shares and
which are purchased by Alexandria pursuant to this clause (c).

                                      -17-
<PAGE>   21

                  (d) If the Investors do not elect to purchase all of the
Shares and Venture Capital Shares, if applicable, referred to in the Notice and
Alexandria Notice, if applicable, Acusphere may, during the 90-day period
following the expiration of the periods provided in subsections 3.4(b) and (c)
hereof (including the additional five day period provided for over-allotments),
offer the remaining unsubscribed portion of such Shares and Venture Capital
Shares, if applicable, to any person or persons at a price not less than, and
upon terms no more favorable to the offeree than those specified in the Notice
and Alexandria Notice, if applicable. If Acusphere does not enter into an
agreement for the sale of the unsubscribed portion of such Shares and Venture
Capital Shares, within such period, or if such agreement is not consummated
within thirty (30) days of the execution thereof, the right provided hereunder
shall be deemed to be revived and such Shares and Venture Capital Shares, shall
not be offered unless first reoffered to the Investors in accordance herewith;
PROVIDED, HOWEVER, that if during such 90-day period, Acusphere offers such
Shares and Venture Capital Shares, if applicable, to any person or persons at a
price and/or upon terms more favorable to the offeree than those specified in
the Notice and Alexandria Notice, if applicable, then Acusphere must first
reoffer the Shares and Venture Capital Shares, if applicable, to the Investors
on such favorable terms and the procedure set forth above shall be followed,
with the exception that the 30-day periods set forth in subsections 3.4(a) and
(c) shall be a 15-day period.

                  (e) The rights of first refusal in this Section 3.4 shall
not be applicable (i) to the issuance or sale on or after March 30, 1994, of up
to 3,985,458 shares of Common Stock (or options therefor) to Acusphere's
employees, consultants, or directors (adjusted to reflect subsequent stock
dividends, stock splits or recapitalizations affecting the number of outstanding
shares of the Common Stock) for the primary purpose of soliciting or retaining
their services, provided each such employee, consultant or director executes an
agreement providing for (A) vesting of shares (or the option to purchase shares)
over at least a four-year period, (B) market stand-off agreements similar to
that set forth in Section 2.14 of this Agreement, and (C) rights of first
refusal on transfers of such shares which shall first be a right of Acusphere,
then of Acusphere's assignees; PROVIDED, HOWEVER, that any such requirements may
be waived by action of Acusphere's Board of Directors, provided that all
representatives of the Investors on the Board of Directors of Acusphere consent
to such waiver, (ii) to the issuance of securities in or after consummation of a
bona fide, firmly underwritten initial public offering of shares of Common
Stock, registered under the Act pursuant to a registration statement on Form S-1
or other available form, (iii) to the issuance of securities pursuant to the
conversion of convertible securities, (iv) to the issuance of 130,169 and 22,216
shares of Common Stock to the Massachusetts Institute of Technology ("MIT") on
or about March 30, 1995 and June 4, 1996, respectively, (v) to the issuance of
securities solely to prevent dilution as a result of any stock splits, stock
dividends, combinations, recapitalizations or issuances of securities below the
then applicable conversion price for the Preferred Stock, (vi) to the issuance
of securities (including options, warrants or convertible securities and the
securities issuable upon conversion or exercise of such options, warrants or
convertible securities) to lending or leasing institutions pursuant to debt or
lease financing arrangements approved by the Board of Directors, including all
of the directors then serving on the Board of Directors which were designated by
the holders of the Company's Preferred Stock in accordance with the Sixth
Amended and Restated Voting Agreement dated as of the date hereof, as amended
and in effect from time to time (the "Sixth

                                      -18-
<PAGE>   22

Amended and Restated Voting Agreement"), not to exceed, in the aggregate, five
percent (5.0%) of the capital stock of Acusphere on a fully-diluted basis
(assuming the conversion or exercise of all outstanding option, warrants or
convertible securities), (vii) to the issuance of shares of Common Stock
pursuant to the redemption by the Company of the Series G Stock pursuant to
Acusphere's Amended and Restated Certificate of Incorporation, as in effect from
time to time after the date hereof, (viii) to the issuance of shares of Common
Stock pursuant to the conversion feature of the Note, (ix) to the issuance of
Series G Paid-in-Kind Dividends pursuant to Acusphere's Amended and Restated
Certificate of Incorporation, as in effect from time to time after the date
hereof, or (x) to the issuance of the Alexandria Warrant, the Alexandria Warrant
Shares issuable upon exercise thereof and the shares of Common Stock issuable
upon conversion thereof.

                  (f) The right of first refusal set forth in this Section
3.4 may be assigned (but only with all related obligations) by a Holder only to
a transferee or assignee of such securities who, after such assignment or
transfer, holds at least 75,000 shares of Registrable Securities (subject to
appropriate adjustment for stock splits, stock dividends, combinations and other
recapitalizations), provided: (a) Acusphere is, within a reasonable time after
such transfer, furnished with written notice of the name and address of such
transferee or assignee and the securities with respect to which such
registration rights are being assigned; (b) such transferee or assignee agrees
in writing to be bound by and subject to the terms and conditions of this
Agreement; and (c) such assignment shall be effective only if immediately
following such transfer the further disposition of such securities by the
transferee or assignee is restricted under the Act.

         3.5 PREPARATION OF AUDITED FINANCIAL STATEMENTS. Acusphere shall
undertake to prepare and shall deliver to each Investor other than Alexandria,
as soon as practicable, its audited financial statements (balance sheet, profit
and loss statement, statement of stockholders' equity and statement of cash
flows, including notes thereon) as of December 31, 2000, and for each fiscal
year thereafter.

         3.6 STOCK PURCHASE AGREEMENTS. All officers and employees of and
consultants to Acusphere who purchase shares of Acusphere's capital stock shall
be required to execute a stockholder agreement providing for (i) vesting of
shares (or option to purchase shares) over a four-year period, (ii) market
stand-off agreements similar to that set forth in Section 2.14 of this
Agreement, and (iii) rights of first refusal on transfers of such shares which
shall first be a right of Acusphere, then of Acusphere's assignees; provided
however, that any such requirements may be waived by action of Acusphere's Board
of Directors, provided that any representative of the Investors on the Board of
Directors of Acusphere consents to such waiver.

         3.7 TERMINATION OF CERTAIN COVENANTS. The provisions set forth in
Section 3.6 shall terminate and be of no further force or effect upon the
consummation of the sale of securities pursuant to a registration statement
filed by Acusphere under the Act in connection with the firm commitment
underwritten offering of its securities to the general public.

                                      -19-
<PAGE>   23

         3.8 COVENANTS RELATING TO SBA MATTERS. At such times as any SBIC
Investor holds any securities of Acusphere, Acusphere shall comply with the
covenants set forth in this Section 3.8.

                  (a) Use of Proceeds. The proceeds from the prior
issuances and sales of preferred stock which the Company still retains (the
"Proceeds") shall be used by Acusphere for its growth, modernization or
expansion. Acusphere shall provide each SBIC Investor and the Small Business
Administration (the "SBA") reasonable access to Acusphere's books and records
for the purpose of confirming the use of Proceeds.

                  (b) Business Activity. During the period ending on April
25, 2001, Acusphere shall not change the nature of its business activity if such
change would render Acusphere ineligible as provided in 13 C.F.R. Section
107.720.

                  (c) Compliance. So long as any SBIC Investor holds any
securities of Acusphere, Acusphere will at all times comply with the
non-discrimination requirements of 13 C.F.R. Parts 112, 113 and 117.

                  (d) Information. Within 45 days after the end of each
fiscal year and at such other times as an SBIC Investor may reasonably request,
Acusphere shall deliver to such SBIC Investor a written assessment, in form and
substance satisfactory to such SBIC Investor, of the economic impact of such
SBIC Investor's investment specifying the full-time equivalent jobs created or
retained in connection with such investment, and the impact of the investment on
Acusphere's business in terms of profits and on taxes paid by Acusphere and its
employees. Upon request, Acusphere promptly (and in any event within 20 days of
such request) will furnish to each SBIC Investor all information reasonably
requested by such SBIC Investor in order for such SBIC Investor to comply with
the requirements of 13 C.F.R. Section 107.620 or to prepare and file SBA Form
468 and any other information requested or required by any governmental agency
asserting jurisdiction over such SBIC Investor. Acusphere shall afford to
representatives of the SBA reasonable access to the books, records and
properties of Acusphere and its subsidiaries. Any submission of any financial
information under Section 3.1, above, or this Section 3.8(d) shall include a
certificate of Acusphere's president, chief executive officer, treasurer or
chief financial officer. If a SBIC Investor requests confidential information
from Acusphere for the purpose of providing such information to the SBA, then,
if requested in writing by Acusphere at the time Acusphere provides such
information to such SBIC Investor, the SBIC Investor shall, to the extent
permitted by law, request that such confidential information be treated
confidentially by the SBA.

                  (e) Number of Holders of Voting Securities. Acusphere
shall notify each SBIC Investor (i) on or prior to the taking of any action
which increases the number of record holders of Acusphere's voting securities
from fewer than 50 to 50 or more, and (ii) of any other action or occurrence
which increases the number of record holders of Acusphere's voting securities
from fewer than 50 to 50 or more, as soon as practicable after Acusphere becomes
aware that such other action or occurrence has occurred or is proposed to occur.

                                      -20-
<PAGE>   24

         3.9 REGULATORY COMPLIANCE COOPERATION. In the event that Bank of
America Ventures determines that it has a Regulatory Problem (as defined below),
it shall have the right to transfer its Registrable Securities without regard to
any restrictions on transfer set forth in this Agreement, the Series I
Agreement, the Sixth Amended and Restated Voting Agreement, or the Stock
Restriction Agreement dated as of the date hereof and in effect from time to
time (the "Stock Restriction Agreement") and Acusphere shall take all such
actions as are reasonably requested by Bank of America Ventures in order to (i)
effectuate and facilitate any transfer by it of any securities of Acusphere then
held by it to any person designated by Bank of America Ventures provided that
such person is reasonably acceptable to Acusphere and agrees in writing to be
bound by this Agreement, the Series I Agreement, the Seventh Amended and
Restated Voting Agreement, and the Amended and Restated Stock Restriction
Agreement, (ii) permit Bank of America Ventures (or any of its affiliates) to
exchange all or any portion of any voting security of Acusphere then held by it
on a share-for-share basis for shares of a nonvoting security of Acusphere,
which nonvoting security shall be identical in all respects to the voting
security exchanged for it, except that it shall be nonvoting and shall be
convertible into a voting security on such terms as are requested by it in light
of regulatory considerations then prevailing and (iii) amend this Agreement, as
amended from time to time, to effectuate and reflect the foregoing. The parties
to this Agreement agree to vote all of Acusphere's securities held by them in
favor of such amendments and actions. For purposes of this Agreement, a
"Regulatory Problem" means any set of facts or circumstances wherein it has been
asserted by any governmental regulatory agency that Bank of America Ventures is
not entitled to hold, or exercise any significant right with respect to
securities of Acusphere.

         3.10 CONSIDERATION OF SECTION 1202(c). At such time or times, if any,
that Acusphere considers repurchasing any shares of its Preferred Stock,
Acusphere agrees to consider the effect that such repurchase would have on such
stock's qualification as "Qualified Small Business Stock," as defined in Section
1202(c) of the Internal Revenue Code of 1986, as amended (the "Code").
Notwithstanding the foregoing, Acusphere shall have no obligation to take, or
refrain from taking any action that would affect such stock's status as
"Qualified Small Business Stock." Acusphere will use reasonable efforts to
comply with the reporting and recordkeeping requirements of Section 1202 of the
Code and the regulations promulgated thereunder.

         3.11 REGISTRATION RIGHTS. Acusphere shall not grant to any person or
entity registration rights which are on par with or senior to the registration
rights granted to the holders of the Registrable Securities, unless holders of a
majority of the then outstanding shares of Registrable Securities consent in
writing.

4.       MISCELLANEOUS.

         4.1 SUCCESSORS AND ASSIGNS. Except as otherwise provided herein, the
terms and conditions of this Agreement shall inure to the benefit of and be
binding upon the respective successors and assigns of the parties (including
transferees of any shares of Registrable Securities). Nothing in this Agreement,
express or implied, is intended to confer upon any party other than the parties
hereto or their respective successors and assigns any rights, remedies,
obligations, or liabilities under or by reason of this Agreement, except as
expressly provided in this Agreement.


                                      -21-
<PAGE>   25

         4.2 GOVERNING LAW.  This Agreement shall be governed by and construed
under the laws of the Commonwealth of Massachusetts as applied to agreements
among Massachusetts residents entered into and to be performed entirely within
Massachusetts.

         4.3 COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument. Any person who may, after
the date hereof, purchase shares of Series I Stock pursuant to the Series I
Agreement shall become a party to this Agreement as an "Investor" and a holder
of "Preferred Stock" and "Registrable Securities" for all purposes hereunder,
all upon execution of a counterpart to this Agreement signed by such person and
Acusphere.

         4.4 TITLES AND SUBTITLES. The titles and subtitles used in this
Agreement are used for convenience only and are not to be considered in
construing or interpreting this Agreement.

         4.5 NOTICES. Unless otherwise provided, any notice required or
permitted under this Agreement shall be given in writing and shall be deemed
effectively given upon personal delivery to the party to be notified or upon
deposit with the United States Post Office, by registered or certified mail,
postage prepaid and addressed to the party to be notified at the address
indicated for such party on the signature page hereof, or at such other address
as such party may designate by ten (10) days' advance written notice to the
other parties.

         4.6 EXPENSES. If any action at law or in equity is necessary to enforce
or interpret the terms of this Agreement, the prevailing party shall be entitled
to reasonable attorneys' fees, costs and necessary disbursements in addition to
any other relief to which such party may be entitled.

         4.7 AMENDMENTS AND WAIVERS. Any term of this Agreement may be amended
and the observance of any term of this Agreement may be waived (either generally
or in a particular instance and either retroactively or prospectively), only
with the written consent of Acusphere and the holders of a majority of the
Registrable Securities then outstanding (provided that if an amendment or waiver
affects any Investor in a manner that is different from the effect of such
waiver or amendment on all other Investors, then the consent of such Investor
shall be required for such amendment or waiver, and further provided that no
such amendment or waiver will be effective to increase any obligations of Oberg
without her written consent). Any amendment or waiver effected in accordance
with this paragraph shall be binding upon each holder of any Registrable
Securities then outstanding, each future holder of all such Registrable
Securities, and Acusphere.

         4.8 SEVERABILITY. If one or more provisions of this Agreement are held
to be unenforceable under applicable law, such provision shall be excluded from
this Agreement and the balance of the Agreement shall be interpreted as if such
provision were so excluded and shall be enforceable in accordance with its
terms.

         4.9 AGGREGATION OF STOCK. All shares of Registrable Securities held or
acquired by affiliated entities or persons shall be aggregated together for the
purpose of determining the availability of any rights under this Agreement.

         4.10 ENTIRE AGREEMENT.  This Agreement (including the Exhibits and
Schedules hereto, if any) constitutes the full and entire understanding and
agreement between the parties with regard to the subjects hereof and thereof.

                                      -22-
<PAGE>   26

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]


                                      -23-

<PAGE>   27

         IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first above written.

                                     ACUSPHERE, INC.
                                     38 Sidney Street
                                     Cambridge, MA 02139


                                     By: /s/ Sherri C. Oberg
                                         ---------------------------------------
                                          Sherri C. Oberg
                                          President and Chief Executive Officer


                                     MVI MEDICAL VENTURE
                                     INVESTMENTS LIMITED


                                     By: /s/ C.A. Roulandson
                                         ---------------------------------------
                                         Name: C.A. Roulandson
                                         Title: For and on behalf of
                                                Healthcare Advisers
                                                Limited Director

                                     BENEFIT CAPITAL MANAGEMENT CORPORATION
                                     (as investment manager for the Prudential
                                     Insurance Company of America separate
                                     account # VCA-GA-5298)
                                     39 Old Ridgebury Road E2-278
                                     Danbury CT 06817
                                     Attn:  Sue DeCarlo


                                     By: /s/ Sue DeCarlo
                                        ----------------------------------------
                                        Name: Sue DeCarlo
                                        Title: Sr. VP and CFO




<PAGE>   28


                                     PRISM VENTURE PARTNERS I, L.P.
                                     100 Lowder Brook Drive
                                     Suite 2500
                                     Westwood, MA  02090

                                     By:  Prism Investment Partners, L.P.

                                     By:  Prism Venture Partners, L.L.C.


                                     By: /s/ John L. Brooks
                                         ---------------------------------------
                                         John L. Brooks
                                         Managing Director

                                     THE CIT GROUP/EQUITY INVESTMENTS, INC.

                                     44 Whippany Road
                                     Morristown, NJ 07960


                                     By: /s/ James Glasheen
                                         ---------------------------------------
                                         Name: James Glasheen
                                         Title: Director


                                     ABS EMPLOYEES' VENTURE FUND L.P.
                                     c/o BT Alex Brown
                                     1 South Street
                                     Mail Stop 1-17-6
                                     Baltimore, MD 21202-3220
                                     Attn:  Dan Gunter / Rick O'Connell


                                     By:
                                         ---------------------------------------
                                         Name:
                                         Title:



<PAGE>   29




                                     BANK OF AMERICA VENTURES 950
                                     Tower Lane, Suite 700 Foster
                                     City, CA 94404

                                     By: /s/ Kate D. Mitchell
                                         ---------------------------------------
                                         Name: Kate D. Mitchell
                                         Title: Managing Director


                                     BA VENTURE PARTNERS II c/o
                                     BankAmerica Ventures 950 Tower
                                     Lane, Suite 700 Foster City,
                                     CA 94404

                                     By: /s/ Rory O'Driscoll
                                         ---------------------------------------
                                         Name: Rory O'Driscoll
                                         Title: General Partner




<PAGE>   30


                                     ALTA V LIMITED PARTNERSHIP
                                     200 Clarendon St.
                                     Floor 51
                                     Boston, MA 02116

                                     By:  Alta V Management Partners, L.P.

                                     By: /s/ Eileen McCarthy
                                         ---------------------------------------
                                         Name: Eileen McCarthy
                                         Title: General Partner

                                     CUSTOMS HOUSE PARTNERS
                                     200 Clarendon St.
                                     Floor 51
                                     Boston, MA 02116


                                     By: /s/ Eileen McCarthy
                                         ---------------------------------------
                                         Name: Eileen McCarthy
                                         Title: under Power of Attorney

                                     POLARIS VENTURE PARTNERS, L.P.
                                     1000 Winter Street
                                     Suite 3350
                                     Waltham, MA 02154

                                     By:  Polaris Venture Management Co., LLC,
                                          Its General Partner


                                     By: /s/ William E. Bilodeau
                                         ---------------------------------------
                                             Name: William E. Bilodeau
                                             Title: Attorney-in-Fact

                                     POLARIS VENTURE PARTNERS FOUNDERS'
                                     FUND, L.P.
                                     1000 Winter Street
                                     Suite 3350
                                     Waltham, MA 02154

                                     By:  Polaris Venture Management Co., LLC
                                          Its General Partner


                                     By:  /s/ William E. Bilodeau
                                          --------------------------------------
                                          Name: William E. Bilodeau
                                          Title: Attorney-in-Fact


<PAGE>   31


                                     THE VENTURE CAPITAL FUND OF NEW
                                     ENGLAND III, L.P.
                                     160 Federal Street, 23rd Floor
                                     Boston, MA 02110

                                     By:  FH & Co. III, L.P., General Partner

                                     By:
                                         ---------------------------------------
                                         Name:
                                         Title:


                                     TRUSTEES OF BOSTON UNIVERSITY
                                     Attn:   Assistant Treasurer

                                     108 Bay State Road
                                     Boston, MA 02215

                                     By: /s/ Matthew J. Burns
                                         ---------------------------------------
                                         Name: Matthew J. Burns
                                         Title: Assistant Treasurer
                                                Boston University


                                     BANCBOSTON VENTURES, INC.
                                     175 Federal Street
                                     Boston, MA 02110


                                     By: /s/ Marcia T. Bates
                                         ---------------------------------------
                                         Name: Marcia T. Bates
                                         Title: Managing Director


                                     PRIVATE EQUITY PORTFOLIO FUND
                                     II, LLC c/o BancBoston Capital
                                     175 Federal Street Boston, MA
                                     02110

                                     By:  Fleet National Bank NA,
                                          Its Manager

                                     By:  /s/ Glen Holland
                                          --------------------------------------
                                          Name: Glen M. Holland
                                          Title: Director


<PAGE>   32



                                      TECHNOLOGY FUNDING PARTNERS III, L.P.,
                                      A Delaware Limited Partnership
                                      2000 Alameda de las Pulgas
                                      San Mateo, CA 94403

                                      By:  Technology Funding, Inc.
                                      Managing General Partner

                                      By: /s/ Michele Lockwood
                                          --------------------------------------
                                          Vice President


                                     TECHNOLOGY FUNDING VENTURE
                                     PARTNERS V, AN AGGRESSIVE GROWTH
                                     FUND L.P.
                                     2000 Alameda de las Pulgas
                                     San Mateo, CA 94403

                                     By:  Technology Funding, Inc.
                                          Managing General Partner

                                     By: /s/ Michele Lockwood
                                         ---------------------------------------
                                         Vice President


                                    TECHNOLOGY FUNDING MEDICAL
                                    PARTNERS I, L.P.
                                    2000 Alameda de las Pulgas
                                    San Mateo, CA 94403

                                    By:  Technology Funding, Inc.
                                         Managing General Partner

                                    By: /s/ Michele Lockwood
                                        ----------------------------------------
                                        Vice President



<PAGE>   33


                                     ELAN INTERNATIONAL SERVICES, LTD.

                                     102 St. James Court
                                     Flatts, Smiths Parish
                                     Bermuda FL 04


                                     By:
                                        ----------------------------------------
                                        Name:
                                        Title:


<PAGE>   34



                                     DEBAR INVESTMENT PARTNERSHIP LLP
                                     c/o David Barry
                                     2 Puritan Road
                                     Rye, New York 10580


                                     By: /s/ David Barry
                                         ---------------------------------------
                                         Name: David Barry
                                         Title: Managing Member



<PAGE>   35


                                     AUDAX PRIVATE EQUITY FUND, L.P.
                                     c/o Audax Group
                                     101 Huntington Ave.
                                     Floor 24
                                     Boston, MA 02199

                                     By:  Audax Private Equity Business, LLC
                                          Its General Partner

                                     By:  Audax Private Equity Business, L.P.,
                                          Its Managing Member


                                     By: /s/ Marc Wolpow
                                         ---------------------------------------
                                         Name: Marc B. Wolpow
                                         Title: Authorized Member


                                     AUDAX CO-INVEST, L.P.
                                     c/o Audax Group
                                     101 Huntington Ave.
                                     Floor 24
                                     Boston, MA 02199

                                     By:  101 Huntington Holdings, LLC
                                          Title: General Partner


                                     By: /s/ Geoffrey S. Rehnert
                                         ---------------------------------------
                                         Name: Geoffrey S. Rehnert
                                         Title: Authorized Member


                                     By: /s/ Marc Wolpow
                                         ---------------------------------------
                                         Name: Marc B. Wolpow
                                         Title:  Authorized Member



<PAGE>   36



                                     AUDAX TRUST CO-INVEST, L.P.
                                     c/o Audax Group
                                     101 Huntington Ave.
                                     Floor 24
                                     Boston, MA 02199

                                     By: 101 Huntington Holdings, LLC
                                         Title: General Partner


                                     By: /s/ Geoffrey Rehnert
                                         ---------------------------------------
                                         Name: Geoffrey S. Rehnert
                                         Title: Authorized Member


                                     By: /s/ Marc Wolpow
                                         ---------------------------------------
                                         Name: Marc B. Wolpow
                                         Title:  Authorized Member


                                     AFF CO-INVEST, L.P.
                                     c/o Audax Group
                                     101 Huntington Ave.
                                     Floor 24
                                     Boston, MA 02199

                                     By: 101 Huntington Holdings, LLC
                                         Title: General Partner


                                     By: /s/ Geoffrey Rehnert
                                         ---------------------------------------
                                         Name: Geoffrey S. Rehnert
                                         Title: Authorized Member


                                     By: /s/ Marc Wolpow
                                         ---------------------------------------
                                         Name: Marc B. Wolpow
                                         Title:  Authorized Member


<PAGE>   37



                                     AUDAX SPECIAL PURPOSE CO-INVEST, L.P.
                                     c/o Audax Group
                                     101 Huntington Ave.
                                     Floor 24
                                     Boston, MA 02199

                                     By: 101 Huntington Holdings, LLC
                                         Title: General Partner


                                     By: /s/ Geoffrey Rehnert
                                         ---------------------------------------
                                         Name: Geoffrey S. Rehnert
                                         Title: Authorized Member


                                     By: /s/ Marc Wolpow
                                         ---------------------------------------
                                         Name: Marc B. Wolpow
                                         Title:  Authorized Member




<PAGE>   38


                                     ALEXANDRIA REAL ESTATE EQUITIES, L.P.
                                     c/o 135 N. Los Robles Avenue, Suite 250
                                     Pasadena, CA 91101

                                     By:ARE-QRS Corp,
                                        Its General Partner


                                     By: /s/ Laurie Allen
                                         ---------------------------------------
                                         Name: Laurie A. Allen
                                         Title:  Senior Vice President, Business
                                         Development & Legal Affairs




<PAGE>   39


                                     AEGIS MANAGEMENT CORPORATION
                                     PROFIT SHARING TRUST FBO TED R. DINTERSMITH
                                     c/o Charles River Ventures
                                     Bay Colony Corporate Center
                                     1000 Winter Street
                                     Suite 3300
                                     Waltham, MA 02154


                                     By:
                                         ---------------------------------------
                                         Name:
                                         Title:

                                     ROBERT S. LANGER
                                     98 Montvale Rd
                                     Newton, MA 02459

                                     /s/ Robert Langer
                                     -------------------------------------------


                                     DAVID NORMAN
                                     26 Turtle Rock Court
                                     Tiburon, CA 94920


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

                                     FOR THE PURPOSES OF SECTIONS 1, 3.4
                                     AND 4 ONLY

                                     SHERRI C. OBERG
                                     157 Bristol Road
                                     Wellesley, MA 02181

                                     /s/ Sherri Oberg
                                     -------------------------------------------




<PAGE>   40



                                     THOMAS WEISEL CAPITAL PARTNERS, L.P.
                                     c/o One Montgomery Street, Suite 3700
                                     San Francisco, California 94104

                                     By:  Thomas Weisel Capital Partners LLC,
                                          its general partner

                                     By:  Thomas Weisel Partners Group LLC,
                                          its managing member

                                     By:  /s/ Daniel S. Dross
                                          --------------------------------------
                                          Daniel Dross
                                          Attorney-in-fact


                                     TWP CEO FOUNDERS CIRCLE (AI), L.P.
                                     c/o One Montgomery Street, Suite 3700
                                     San Francisco, California 94104

                                     By:  Thomas Weisel Capital Partners LLC,
                                          its general partner

                                     By:  Thomas Weisel Partners Group LLC,
                                          its managing member


                                     By:  /s/ Daniel S. Dross
                                          --------------------------------------
                                          Daniel Dross
                                          Attorney-in-fact


                                     TWP CEO FOUNDERS CIRCLE (QP), L.P.
                                     c/o One Montgomery Street, Suite 3700
                                     San Francisco, California 94104

                                     By:  Thomas Weisel Capital Partners LLC,
                                          its general partner

                                     By:  Thomas Weisel Partners Group LLC,
                                          its managing member

                                     By:  /s/ Daniel S. Dross
                                          --------------------------------------
                                          Daniel Dross
                                          Attorney-in-fact

<PAGE>   41


                                     THOMAS WEISEL CAPITAL PARTNERS
                                     (DUTCH), L.P.
                                     c/o One Montgomery Street, Suite 3700
                                     San Francisco, California 94104

                                     By:  Thomas Weisel Capital Partners LLC,
                                          its general partner

                                     By:  Thomas Weisel Partners Group LLC,
                                          its managing member

                                     By:  Thomas Weisel Capital Partners Group
                                          LLC, its managing member


                                     By: /s/ Daniel S. Dross
                                         ---------------------------------------
                                          Daniel Dross
                                          Attorney-in-fact


                                     THOMAS WEISEL CAPITAL PARTNERS
                                     (DUTCH II), L.P.
                                     c/o One Montgomery Street, Suite 3700
                                     San Francisco, California 94104

                                     By:  Thomas Weisel Capital Partners
                                          (Dutch), LLC, its general partner

                                     By:  Thomas Weisel Capital Partners LLC,
                                          its managing member

                                     By:  Thomas Weisel Capital Partners
                                          Group LLC, its managing member

                                     By: /s/ Daniel S. Dross
                                         ---------------------------------------
                                          Daniel Dross
                                          Attorney-in-fact



<PAGE>   42


                                     THOMAS WEISEL CAPITAL PARTNERS EMPLOYEE
                                     FUND, L.P.
                                     c/o One Montgomery Street, Suite 3700
                                     San Francisco, California 94104

                                     By:  Thomas Weisel Capital Partners LLC,
                                          its general partner

                                     By:  Thomas Weisel Partners Group LLC,
                                          its managing member


                                     By:  /s/ Daniel S. Dross
                                          --------------------------------------
                                          Daniel Dross
                                          Attorney-in-fact


                                     TWP 2000 CO-INVESTMENT FUND, L.P.
                                     c/o One Montgomery Street, Suite 3700
                                     San Francisco, California 94104

                                     By:  Thomas Weisel Capital Partners LLC,
                                          its general partners

                                     By:  Thomas Weisel Partners Group LLC,
                                          its managing member


                                     By:  /s/ Daniel S. Dross
                                          --------------------------------------
                                          Daniel Dross
                                          Attorney-in-fact



<PAGE>   43
                                                                      SCHEDULE A

                                    INVESTORS

         Thomas Weisel Capital Partners, L.P.
         TWP CEO Founders Circle (AI), L.P.
         TWP CEO Founders Circle (QP), L.P.
         Thomas Weisel Capital Partners (Dutch), L.P.
         Thomas Weisel Capital Partners (Dutch II), L.P.
         Thomas Weisel Capital Partners Employee Fund, L.P.
         TWP 2000 Co-Investment Fund, L.P.
         Elan International Services, Ltd.
         MVI Medical Venture Investments Limited
         Benefit  Capital Management Corporation (as investment manager for the
                  Prudential Insurance Company of America separate account #
                  VCA-GA-5298)
         Audax Private Equity Fund, L.P.
         Audax Co-Invest, L.P.
         Audax Trust Co-Invest, L.P.
         AFF Co-Invest, L.P.
         Audax Special Purpose Co-Invest, L.P.

         DeBar Investment Partnership LLP
         Prism Venture Partners I, L.P.
         The CIT Group/Equity Investments, Inc.
         ABS Employees' Venture Fund L.P.
         Bank of America Ventures
         BA Venture Partners II
         Alta V Limited Partnership
         Customs House Partners
         Polaris Venture Partners, L.P.
         Polaris Venture Partners Founders' Fund, L.P.
         The Venture Capital Fund of New England III, L.P.
         Trustees of Boston University
         BancBoston Ventures, Inc.
         Private Equity Portfolio Fund II, LLC
         Technology Funding Venture Partners V, An Aggressive Growth Fund, L.P.
         Technology Medical Partners I, L.P.
         Technology Funding Partners III, L.P.
         Aegis Management Corporation Profit Sharing FBO Ted R. Dintersmith
         Robert S. Langer
         David Norman
         Walter Levison Estate
         Alexandria Real Estate Equities, L.P.