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                                                                     Exhibit 4.3

NEITHER THIS WARRANT NOR THE SHARES OF COMMON STOCK THAT MAY BE PURCHASED UPON
EXERCISE OF THIS WARRANT HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933,
AS AMENDED, OR ANY STATE SECURITIES ACT, AND NEITHER THIS WARRANT NOR THE SHARES
OF COMMON STOCK THAT MAY BE PURCHASED UPON EXERCISE OF THIS WARRANT CAN BE SOLD
OR TRANSFERRED EXCEPT PURSUANT TO THE REGISTRATION PROVISIONS OF SUCH ACTS OR AN
EXEMPTION THEREFROM.

THIS WARRANT AND THE SHARES OF COMMON STOCK ISSUABLE UPON EXERCISE OF THIS
WARRANT ARE SUBJECT TO CERTAIN RESTRICTIONS ON TRANSFER CONTAINED HEREIN.

MOREOVER, UNDER CERTAIN CIRCUMSTANCES SET FORTH IN THE CORPORATION'S SECOND
AMENDED AND RESTATED CERTIFICATE OF INCORPORATION, AS AMENDED FROM TIME TO TIME,
THE HOLDER OF THIS WARRANT OR THE SHARES OF COMMON STOCK THAT MAY BE PURCHASED
UPON EXERCISE OF THIS WARRANT WILL BE OBLIGATED TO SELL OR TRANSFER SUCH
SECURITIES TO ANOTHER PERSON, IN ACCORDANCE WITH THE TERMS THEREOF, PURSUANT TO
SECTION 202 OF THE GENERAL CORPORATION LAW OF THE STATE OF DELAWARE.

THIS WARRANT CERTIFICATE WAS AMENDED AND RESTATED EFFECTIVE AS OF JUNE 27, 2002
TO REFLECT CHANGES IN THE ANTI-DILUTION PROVISIONS AND THE CORPORATION'S REVERSE
STOCK SPLIT, EACH OF WHICH WERE AGREED TO BY THE OWNER AND THE CORPORATION IN
CONNECTION WITH THE ISSUANCE OF THE CORPORATION'S SERIES C CONVERTIBLE PREFERRED
STOCK.

                                                                        Warrants

                           VOID AFTER AUGUST 30, 2010

                         METAPHORE PHARMACEUTICALS, INC.

                    AMENDED AND RESTATED WARRANT CERTIFICATE

          THIS CERTIFIES THAT for value received _________________, or
registered assigns, is the owner of the number of Warrants set forth above, each
of which entitles the owner thereof to purchase, upon presentation and surrender
of this Warrant Certificate with the Form of Election to Purchase attached
hereto duly executed and, subject to Section 13 hereof, accompanied by the
aggregate Purchase Price (as defined below) in the form of a certified check (or
such other means of cash payment as permitted by the Company) at any time prior
to 4:00 P.M. (St Louis time) on August 30, 2010, at the principal executive
offices of MetaPhore Pharmaceuticals, Inc, a Delaware corporation (the
"Corporation"), one fully paid and nonassessable share of the Common Stock,
$0.0119 par value of the Corporation ("Common

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Stock"), at the purchase price of $3.74 per share ("Purchase Price"). No
fractional shares of Common Stock will be issued upon exercise of the Warrants
evidenced hereby.

     The Warrants are subject to the following provisions, terms and conditions:

     1. The number of Warrants evidenced by this Warrant Certificate (and the
number of shares of Common Stock that may be purchased upon exercise hereof) set
forth above, and the Purchase Price set forth above, are the number and Purchase
Price as of June 27, 2002 (the "Re-Issue Date"), based on the shares of Common
Stock of the Corporation as constituted at such date. In order to prevent
dilution of the exercise rights granted under this Warrant, the Purchase Price
shall be subject to adjustment from time to time as follows:

     (A) ADJUSTMENTS TO PURCHASE PRICE FOR CERTAIN DILUTING ISSUES.

          (i) SPECIAL DEFINITIONS. For purposes of this Section A, the following
definitions apply:

               (a) "OPTIONS" shall mean rights, options and warrants to
subscribe for, purchase or otherwise acquire Common Stock or Convertible
Securities (defined below).

               (b) "ORIGINAL ISSUE DATE" means August 31, 2000.

               (c) "RE-ISSUE DATE" shall have the meaning set forth above.

               (d) "SERIES B WARRANTS" shall mean the Warrants evidenced by
this Warrant Certificate and any other warrants issued on the Original Issue
Date to purchase up to 35,000 shares (as of the Re-Issue Date, 29,906.52
shares), (less the shares that may be purchased upon exercise of the Warrants
evidenced by this Warrant Certificate) of Common Stock of the Corporation at an
exercise price of not less than $3.33 per share (as of the Re-Issue Date, $3.74
per share), subject to adjustment for all subdivisions and combinations, to be
issued only to agents of certain holders of the Series B Preferred Stock in
exchange for the provision Of BONA FIDE research and other services to the
Corporation that are not in connection with the offer or sale of securities in a
capital-raising transaction or the direct or indirect promotion or maintenance
of a market for the Corporation's securities, as interpreted consistently with
Rule 701 under the Securities Act;

               (e) "CONVERTIBLE SECURITIES" shall mean any evidences of
indebtedness, shares (other than Common Stock, Series A Preferred Stock, Series
B Preferred Stock, Series C Preferred Stock or the Amended and Restated
Convertible Debenture by and between Pharmacia Corporation ("Pharmacia") and the
Corporation dated as of the Re-Issue Date, (originally dated January 26, 1998))
or other securities convertible into or exchangeable for Common Stock.

               (f) "ADDITIONAL SHARES OF COMMON STOCK" shall mean all shares of
Common Stock issued (or, pursuant to subsection (ii) of this Section A, deemed
to be issued) by the Corporation after the Re-Issue Date, other than shares of
Common Stock issued or issuable in any of the following transactions:

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                    (1) in connection with the issuance of Options to purchase
or otherwise acquire Common Stock or Convertible Securities to employees,
consultants, officers or directors of the Corporation pursuant to stock purchase
or stock option plans or agreements on terms from time to time approved by the
Board of Directors (including Options granted prior to the Original Issue Date);

                    (2) in connection with acquisition transactions;

                    (3) upon the issuance of securities in connection with
commercial credit arrangements, equipment financings or similar transactions;

                    (4) upon conversion of shares of Series A Preferred Stock,
Series B Preferred Stock or the Convertible Debenture by and between Pharmacia
and the Corporation dated January 26, 1998;

                    (5) upon the issuance or exercise of the Series B Warrants;

                    (6) upon the issuance of securities in a public offering;

                    (7) pursuant to the exercise of Options which are not
Additional Shares of Common Stock hereunder,

                    (8) pursuant to which adjustment of the Purchase Price has
been made pursuant to the terms hereof; and

                    (9) upon the issuance of shares of Series C Convertible
Preferred Stock,

          (ii) DEEMED ISSUANCE OF ADDITIONAL SHARES OF COMMON STOCK. In the
event the Corporation at any time or from time to time after the Re-Issue Date
shall issue any Options or Convertible Securities or shall fix a record date for
the determination of holders of any class of securities then entitled to receive
any such Options or Convertible Securities, then the maximum number of shares
(as set forth in the instrument relating thereto without regard to any
provisions contained therein designed to protect against dilution) of Common
Stock issuable upon the exercise of such Options Or, in the case of Convertible
Securities and Options for Convertible Securities, the conversion or exchange of
such Convertible Securities, shall be deemed to be Additional Shares of Common
Stock issued as of the time of such issue or, in case such a record date shall
have been fixed, as of the close of business on such record date, provided that
in any such case in which Additional Shares of Common Stock are deemed to be
issued:

               (a) no further adjustments in the Purchase Price shall be made
upon the subsequent issue of such Convertible Securities or shares of Common
Stock upon the exercise of such Options or conversion or exchange of such
Convertible Securities;

               (b) if such Options or Convertible Securities by their terms
provide, with the passage of time or otherwise, for any increase or decrease in
the consideration payable to

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the Corporation, or decrease or increase in the number of shares of Common Stock
issuable, upon the exercise, conversion or exchange thereof, the Purchase Price
computed upon the original issue thereof (or upon the occurrence of a record
date with respect thereto), and any subsequent adjustments based thereon, shall,
upon any such increase or decrease becoming effective, be recomputed to reflect
such increase or decrease insofar as it affects such Options or the rights of
conversion or exchange under such Convertible Securities (provided, however,
that no such adjustment of the Purchase Price shall affect Common Stock
previously issued upon conversion of the Convertible Securities);

          (c) upon the expiration of any such Options or any rights of
conversion or exchange under such Convertible Securities which shall not have
been exercised, the Purchase Price computed upon the original issuance thereof
(or upon the occurrence of a record date with respect thereto), and any
subsequent adjustments based thereon, shall, upon such expiration, be recomputed
as if:

               (1) in the case of Convertible Securities or Options for Common
Stock, the only Additional Shares of Common Stock issued were the shares of
Common Stock, if any, actually issued upon the exercise of such Options or the
conversion or exchange of such Convertible Securities and the consideration
received therefor was the consideration actually received by the Corporation for
the issue of all such Options, whether or not exercised, plus the consideration
actually received by the Corporation upon such exercise or for the issue of all
such Convertible Securities which were actually converted or exchanged, plus the
additional consideration, if any, actually received by the Corporation upon such
conversion or exchange, and

               (2) in the case of Options for Convertible Securities, only the
Convertible Securities, if any, actually issued upon the exercise thereof were
issued at the time of issue of such Options, and the consideration received by
the Corporation for the Additional Shares of Common Stock deemed to have been
then issued was the consideration actually received by the Corporation for the
issue of all such Options, whether or not exercised, plus the consideration
deemed to have been received by the Corporation (determined pursuant to
subsection (iv) hereof upon the issue of the Convertible Securities with respect
to which such Options were actually exercised;

          (d) no readjustment pursuant to paragraph (b) or (c) above shall have
the effect of increasing the Purchase Price to an amount which exceeds the lower
of: (1) the Purchase Price on the original adjustment date, or (2) the Purchase
Price that would have resulted from any issuance of Additional Shares of Common
Stock between the original adjustment date and such readjustment date;

          (e) in the case of any Options which expire by their terms not more
than 30 days after the date of issue thereof, no adjustment of the Purchase
Price shall be made until the expiration or exercise of all such Options,
whereupon such adjustment shall be made in the same manner provided in paragraph
(c) above; and

          (f) in the event that any adjustment described in this subsection (ii)
is made to any Additional Shares of Common Stock that were originally issued (or
deemed issued)

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for a consideration per share equal to or in excess of the Purchase Price then
in effect that, had such adjustment been made prior to such original issue
date (or deemed original issue date) would have caused such Additional Shares of
Common Stock to be issued for a consideration per share less than the Purchase
Price then in effect, then such Additional Shares of Common Stock shall be
deemed to have been issued as of the date of any such adjustment.

          (iii) ADJUSTMENT OF PURCHASE PRICE UPON ISSUANCE OF ADDITIONAL SHARES
OF COMMON STOCK. In the event this Corporation, at any time after the Re-Issue
Date, shall issue Additional Shares of Common Stock (including Additional Shares
of Common Stock deemed to be issued pursuant to subsection (ii)) without
consideration or for a consideration per share less than the Purchase Price in
effect on the date of and immediately prior to such issue, then and in such
event, the Purchase Price shall be adjusted, concurrently with such issue, to a
price equal to the quotient obtained by dividing:

               (a) the total number of shares of Common Stock outstanding
immediately prior to such issuance multiplied by the Purchase Price in effect
immediately prior to such issuance, plus the consideration received by the
Corporation upon such issuance, by.

               (b) the total number of shares of Common Stock outstanding
immediately prior to such issuance plus the Additional Shares of Common Stock
issued in such issuance.

For the purpose of the above calculation, the number of shares of Common Stock
outstanding immediately prior to such issue shall be calculated on a fully
diluted basis, as if all outstanding shares of Convertible Securities had been
fully converted into shares of Common Stock immediately prior to such issuance
and all outstanding Options had been fully exercised immediately prior to such
issuance (and the resulting securities fully converted into shares of Common
Stock, if so convertible) as of such date, but not including in such calculation
any additional shares of Common Stock issuable with respect to outstanding
shares of Convertible Securities or Options solely as a result of the adjustment
of the respective conversion prices (or other conversion ratios) resulting from
the issuance of the Additional Shares of Common Stock causing the adjustment in
question. All of the above calculations shall be made to the nearest cent.

          (iv) DETERMINATION OF CONSIDERATION. For purposes of subsection (ii)
above, the consideration received by the Corporation for the issue of any
Additional Shares of Common Stock shall be computed as follows:

               (a) CASH AND PROPERTY. Such consideration shall:

                    (1) insofar as it consists of cash, be computed at the
aggregate amount of cash received by the Corporation excluding amounts paid or
payable for accrued interest or accrued dividends;

                    (2) insofar as it consists of property other than cash, be
computed at the fair value thereof at the time of such issue, as determined
in good faith by the Board of Directors; and

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                    (3) in the event Additional Shares of Common Stock are
issued together with other shares or securities or other assets of the
Corporation for consideration which covers both, be the proportion of such
consideration so received, computed as provided in subparagraphs (1) and (2)
above, as determined in good faith by the Board of Directors.

               (b) OPTIONS AND CONVERTIBLE SECURITIES. The consideration per
share received by the Corporation for Additional Shares of Common Stock deemed
to have been issued pursuant to subsection (ii), relating to Options and
Convertible Securities shall be determined by dividing:

                    (1) the total amount, if any, received or receivable by the
Corporation as consideration for the issue of such Options or Convertible
Securities, plus the minimum aggregate amount of additional consideration (as
set forth in the instruments relating thereto, without regard to any provision
contained therein designed to protect against dilution) payable to the
Corporation upon the exercise of such Options or the conversion or exchange of
such Convertible Securities, or in the case of Options for Convertible
Securities, the exercise of such Options for Convertible Securities and the
conversion or exchange of such Convertible Securities, by

                    (2) the maximum number of shares of Common Stock (as set
forth in the instruments relating thereto, without regard to any provision
contained therein designed to protect against dilution) issuable upon the
exercise of such Options or conversion or exchange of such Convertible
Securities

     2. ADJUSTMENTS TO PURCHASE PRICE FOR STOCK DIVIDENDS AND FOR COMBINATIONS
OR SUBDIVISIONS OF COMMON STOCK. In the event that the Corporation at any time
or from time to time after the Re-Issue Date shall declare or pay, without
consideration, any dividend on the Common Stock payable in shares of Common
Stock or in any right to acquire shares of Common Stock for no consideration, or
shall effect a subdivision of the outstanding shares of Common Stock into a
greater number of shares of Common Stock (by stock split, reclassification or
otherwise than by payment of a dividend in shares of Common Stock or in any
right to acquire shares of Common Stock), or in the event the outstanding shares
of Common Stock shall be combined or consolidated, by reclassification or
otherwise, into a lesser number of shares of Common Stock, then the Purchase
Price in effect immediately prior to such event shall, concurrently with the
effectiveness of such event, be proportionately decreased or increased, as
appropriate. In the event that the Corporation shall declare or pay, without
consideration, any dividend on the Common Stock payable in any right to acquire
shares of Common Stock for no consideration, then the Corporation shall be
deemed to have made a dividend payable in shares of Common Stock in an amount
equal to the maximum number of shares issuable upon exercise of such rights to
acquire Common Stock. The owner of this Warrant Certificate and the Corporation
acknowledge and agree that this Warrant Certificate, as currently amended and
restated, reflects the Corporation's reverse stock split which was effected in
connection with the issuance of the Series C Convertible Preferred Stock.

     3. ADJUSTMENTS FOR RECLASSIFICATION AND REORGANIZATION. If the Common Stock
issuable upon exercise of these Warrants shall be changed into the same or a
different number of

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shares of any other class or classes of stock, whether by capital
reorganization, reclassification or otherwise (other than a subdivision or
combination of shares provided for in Section 2 above) the Purchase Price then
in effect shall, concurrently with the effectiveness of such reorganization or
reclassification, be proportionately adjusted so that these Warrants shall
thereafter be convertible into, in lieu of the number of shares of Common Stock
which the holders would otherwise have been entitled to receive, a number of
shares of such other class or classes of stock equivalent to the number of
shares of Common Stock that would have been received by the holders upon
conversion of the Series B Preferred Stock immediately before that change.

     4. CERTIFICATES AS TO ADJUSTMENTS. Upon the occurrence of each adjustment
or readjustment of the Purchase Price pursuant to Sections 1,2 or 3, as
applicable, the Corporation at its expense shall promptly compute such
adjustment or readjustment in accordance with the terms hereof and prepare and
furnish to the holder a certificate executed by the Corporation's President,
Executive Vice President or Chief Financial Officer setting forth such
adjustment or readjustment and showing in detail the facts upon which such
adjustment or readjustment is based.

     5. NOTICES OF RECORD DATE. In the event that the Corporation shall propose
at any time prior to the exercise of the Warrants evidenced by this Warrant
Certificate: (a) to declare any dividend or distribution upon its Common Stock,
whether in cash, property, stock or other securities, whether or not a regular
cash dividend and whether or not out of earnings or earned surplus; (b) to
offer for subscription pro rata to the holders of any class or series of its
stock any additional shares of stock of any class or series or other' rights;
(c) to effect any reclassification or recapitalization of its Common Stock
outstanding involving a change in the Common Stock; or (d) to merge or
consolidate with or into any other corporation, or sell, lease or convey all or
substantially all of its assets, or to liquidate, dissolve or wind up; then, in
connection with each such event, the Corporation shall send to the holders:

          (i) at least twenty (20) days' prior written notice of the date on
which a record shall be taken for such dividend, distribution or subscription
rights (and specifying the date on which the holders of Common Stock shall be
entitled thereto) or for determining rights to vote, if any, in respect of the
matters referred to in Subsections (c) or (d) of this Section 5; and

          (ii) in the case of the matters referred to in Subsections (c) or (d)
of this Section 5, at least twenty (20) days' prior written notice of the date
when the same shall take place (and specifying the date on which the holders of
Common Stock shall be entitled to exchange their Common Stock for securities or
other property deliverable upon the occurrence of such event).

     6. ISSUE TAXES The Corporation shall pay any and all issue and other taxes
that may be payable in respect of any issue or delivery of shares of Common
Stock on exercise of this Warrant pursuant to the terms hereof; provided,
however, that the Corporation shall not be obligated to pay any transfer taxes
resulting from any transfer requested by any holder in connection with any such
conversion.

     7. FRACTIONAL SHARES. No fractional share of Common Stock shall be issued
upon the exercise of the Warrants evidenced by this Warrant Certificate. All
shares of Common Stock

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(including fractions thereof) issuable upon exercise of the Warrants evidenced
by this Warrant Certificate shall be aggregated for purposes of determining
whether the conversion would result in the issuance of any fractional share. If,
after the aforementioned aggregation, the conversion would result in the
issuance of a fraction of a share of Common Stock, the Corporation shall, in
lieu of issuing any fractional share, pay the holder otherwise entitled to such
fraction a sum in cash equal to the fair market value of such fraction on the
date of conversion (as determined in good faith by the Board of Directors).

     8. NOTICES. Any notice required by the provisions of this Warrant
Certificate to be given to its holder shall be deemed given if deposited in the
United States mail, postage prepaid, or if sent by facsimile or delivered
personally by hand or nationally recognized courier and addressed to each holder
of record at such holder's address or facsimile number appearing in the records
of the Corporation.

     9. COVENANTS AND REPRESENTATIONS. The Corporation covenants that it will at
all times reserve and keep available out of its authorized Common Stock, solely
for the purpose of issuance upon exercise of the Warrants evidenced by this
Warrant Certificate, such number of shares of Common Stock as shall from time to
time be issuable upon the exercise of these Warrants; and if at any time the
number of authorized but unissued and issued but not outstanding shares of the
Common Stock, on a fully diluted basis, shall not be sufficient to effect the
exercise of these Warrants at the Purchase Price then in effect, the Corporation
will take such corporate action as may, in the opinion of its counsel, be
necessary to increase its authorized but unissued or issued but not outstanding
shares of the Common Stock to such number of shares as shall be sufficient for
such purpose. The Corporation represents that all shares of Common Stock which
shall be so issuable, when issued upon conversion of these Warrants, shall be
duly and validly issued, fully-paid and non-assessable.

     10. RIGHTS OF HOLDER. No holder of this Warrant Certificate shall be
entitled to vote or receive dividends or be deemed the holder of Common Stock or
any other securities of the Corporation which may at any time be issuable on the
exercise hereof for any purpose, nor shall anything contained herein be
construed to confer upon the holder hereof, as such, any of the lights of a
stockholder of the Corporation or any right to vote for the election of
directors or upon any matter submitted to stockholders at any meeting thereof,
or to give or withhold consent to any corporate action (whether upon any
recapitalization, issue of stock, reclassification of stock, change of par
value, consolidation, merger, conveyance, or otherwise), or except as provided
above, to receive notice of meetings, or to receive dividends of subscription
rights or otherwise, until the Warrants evidenced by this Warrant Certificate
shall have been exercised.

     11. TRANSFER ABILITY OF WARRANT. Every holder of this Warrant Certificate
by accepting the same consents and agrees with the Corporation that:

          (a) Until the termination of the Investors Rights' Agreement dated as
of the Re-Issue Date (or any successor agreements), the holder shall not
transfer this Warrant Certificate or the Common Stock purchasable upon the
exercise of the Warrants evidenced hereby to a "Competitor" (as defined in the
Investors Rights Agreement) of the Corporation, and any direct or indirect
attempted transfer to such a Competitor shall be null and void AB INITIO;

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          (b) The Warrants evidenced by this Warrant Certificate are
transferable only on the registry books of the Corporation if surrendered at the
principal office of the Corporation, duly endorsed, or accompanied by a proper
instrument of transfer; and

          (c) The Corporation may deem and beat the person in whose name the
Warrant Certificate is registered as the absolute owner thereof and of the
Warrants evidenced thereby (notwithstanding any notations of ownership or
writing on the Warrant Certificates made by anyone other than the Corporation)
for all purposes whatsoever, and the Corporation shall not be affected by any
notice to the contrary.

     12. INCIDENTAL REGISTRATION. In the event that (but without any obligation
to do so) the Corporation proposes to register any shares of Common Stock in
connection with the public offering of such shares solely for cash on any form
of registration statement in which the inclusion of the Common Stock is
appropriate (other than a registration pursuant to a registration statement on
Form S-4 or Form S-8 (or any successor forms) or any form that does not include
substantially the same information, other than information relating to the
selling holders or their plan of distribution, as would be required to be
included in a registration statement under the Securities Act of 1933, as
amended, covering the sale of Common Stock), the Corporation shall promptly give
the holder of this Warrant Certificate written notice of such registration at
least thirty (30) days before the anticipated filing date of any such
registration statement. Upon the written request of any holder within fifteen
(15) days after the receipt by such holder of such notice from the Corporation,
the Corporation shall cause to be registered under the Securities Act all of the
Common Stock that such holder has so requested to be registered; provided,
however that if the underwriter(s) advise the holder in writing that marketing
or other factors require that less than 100% of the shares of Common Stock
requested by the holder be included in the underwriting, then the Corporation
shall so advise the holder. The Corporation shall not be required to proceed
with, or maintain the effectiveness of, any registration of its securities after
giving the notice herein provided, and the right of any holder to have Common
Stock included in such registration statement shall be conditioned upon
participation in any such underwriting to the extent so allowed. The Corporation
shall not be required to include any Common Stock in such underwriting unless
the holder enters into an underwriting agreement with the underwriter(s)
selected by the Corporation in customary form, and upon terms and conditions
agreed upon between the Corporation and such underwriter(s).

     13. CASHLESS EXERCISE. The holder of the Warrants represented by this
Warrant Certificate may elect to exercise such Warrants without any payment by
certified check by surrendering this Warrant Certificate at the offices of the
Corporation, together with the Notice of Exercise indicating an election
pursuant to this Section 13. In such event the Corporation shall issue to such
holder that number of shares of Common Stock as is calculated according to the
following formula:

    Y (A-B)
X = -------
       A

Where: X = the number of shares of Common Stock to be issued to the holder.

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          Y= the number of shares of Common Stock which may be purchased upon
exercise of this Warrant.

          A= the fair market value of one share of Common Stock.

          B= Purchase Price.

     As used herein, the fair market value of the Common Stock shall mean, with
respect to each share of Common Stock, the average of the closing prices of the
Corporation's Common Stock sold on all securities exchanges on which the Common
Stock may at the time be listed (including, for this purpose, the Nasdaq
National Market), or, if at any time the Common Stock is not so listed, the
average of the representative bid and asked prices quoted in the Nasdaq System
as of 4:00 p.m., St. Louis, Missouri, time, or, if at any time the Common Stock
is not quoted in the Nasdaq System, the average of the highest bid and lowest
asked price in the domestic over-the-counter market as reported by the National
Quotation Bureau, Incorporated, or any similar successor organization, in each
such case on the day on which the Notice of Exercise is received or if no sales
of the Common Stock have occurred on such date, on the next preceding date on
which there were such sales. If at any time the Common Stock is not listed on
any securities exchange or quoted in the Nasdaq System or the over-the-counter
market, the fair market value of the Common Stock shall be determined in good
faith by the Board of Directors, unless (i) the Corporation shall become subject
to a merger, acquisition or other consolidation pursuant to which the
Corporation is not the surviving party, in which case the fair market value of
the Common Stock shall be deemed to be the value received by the holders of the
Corporation's Common Stock for each share of Common Stock pursuant to the
Corporation's acquisition; or (ii) the holder shall purchase such shares in
conjunction with the initial underwritten public offering of the Corporation's
Common Stock pursuant to a registration statement filed under the Securities Act
of 1933, in which case, the fair market value of the shares of Common Stock
which may be purchased upon exercise of this Warrant shall be the price at which
all registered shares of Common Stock are sold to the public in such offering.
The Corporation agrees to advise the holder, within thirty days after receiving
such holder's written request therefor, of the fair market value of the Common
Stock for purposes of enabling the holder to determine whether it wishes to
exercise the Warrants evidenced by this Warrant Certificate in accordance with
this Section 13. Notwithstanding anything to the contrary, a holder's sole
remedy in the event of disagreement regarding the fair market value of the
Common Stock shall be to withdraw such holder's request to exercise pursuant to
this Section 13, in which event such holder may, in such holder's discretion,
purchase such shares by certified check or other cash payment, as set forth
above.

          THIS WARRANT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES HEREUNDER
SHALL BE GOVERNED BY, AND CONSTRUED AND ENFORCED IN ACCORDANCE WITH, THE
SUBSTANTIVE LAWS OF THE STATE OF MISSOURI, WITHOUT REGARD TO ITS CONFLICTS OF
LAWS PRINCIPLES.

          Any legal action or proceeding with respect to this Warrant may be
brought in the courts of the City of St. Louis, State of Missouri, or of the
United States of America for the Eastern District of Missouri in St. Louis,
Missouri, and, by execution and issuance or acceptance of this Warrant, the
Corporation and the holder of this Warrant hereby accepts for itself and in

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respect of its property, generally and unconditionally, the jurisdiction of the
aforesaid courts. The Corporation and the holder of this Warrant further
irrevocably consents to the service of process out of any of the aforementioned
courts in any action or proceeding by the mailing of copies thereof by
registered or certified mail, postage prepaid, to the Corporation at its
principal executive offices and to the holder of this Warrant at its address as
it appears on the Corporation's records (or such other address as may be
indicated by the holder of this Warrant upon written notice to the Corporation),
such service to become effective seven days after such mailing. Nothing herein
shall affect the right of the Corporation or the holder of this Warrant to serve
process in any other manner permitted by law or to commence legal proceedings or
otherwise proceed in any other jurisdiction. The Corporation and the holder of
this Warrant further hereby irrevocably waives any objection which it may now or
hereafter have to the laying of venue of any of the aforesaid actions or
proceedings arising out of or in connection with this Warrant brought in the
courts referred to above and hereby further irrevocably waives and agrees not to
plead or claim in any such court that any such action or proceeding brought in
any such court has been brought in an inconvenient forum.

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          IN WITNESS WHEREOF, the Corporation has caused this Warrant to be
executed and issued by its duly authorized officer as of June 27, 2002.

                                       METAPHORE PHARMACEUTICALS, INC.

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

ATTEST:

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

ACCEPTED AND ACKNOWLEDGED

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

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                          FORM OF ELECTION TO PURCHASE

     (TO BE EXECUTED IF HOLDER DESIRES TO EXERCISE THE WARRANT CERTIFICATE.)

To MetaPhore Pharmaceuticals, Inc.

          The undersigned hereby irrevocably elects to exercise the
_________________ Warrants represented by this Warrant Certificate to purchase
the shares of Common Stock purchasable upon the exercise of such Warrants and
requests that certificates for such shares be issued in the name of:

Please insert social security
or other identifying number:

________________________________________________________________________________
                         (Please print name and address)

_________ I have accompanied this Notice of Election with a certified check or
other means of cash payment acceptable to the Corporation in the amount equal to
the aggregate Purchase Price, representing payment in full for such exercise.

_________ I elect to exercise the ______________ Warrants represented by this
Warrant Certificate pursuant to Section 13 hereof.

Dated: _____________, 20__

                                        ----------------------------------------
                                        Signature

                                        (SIGNATURE MUST CONFORM IN ALL RESPECTS
                                        TO NAME OF HOLDER AS SPECIFIED ON THE
                                        FACE OF THIS WARRANT CERTIFICATE)

<Page>

                               FORM OF ASSIGNMENT

         (TO BE EXECUTED BY THE REGISTERED HOLDER IF SUCH HOLDER DESIRES
                      TO TRANSFER THE WARRANT CERTIFICATE.)

FOR VALUE RECEIVED _____________________________________________________________
hereby sells, assigns and transfers unto _______________________________________
______________________________________________________ this Warrant Certificate,
together with all right, title and interest therein, and does hereby irrevocably
constitute and appoint________________________________________ Attorney, to
transfer the within Warrant Certificate on the books of the within-named
Corporation, with full power of substitution.

Dated: __________, ____

                                        ----------------------------------------
                                        Signature

                                        (SIGNATURE MUST CORRESPOND IN ALL
                                        RESPECTS TO THE NAME OF HOLDER AS
                                        SPECIFIED ON THE FACE OF THIS WARRANT
                                        CERTIFICATE).<Page>

                                                                     Exhibit 4.4

                                                               EXECUTION VERSION

THIS WARRANT AND THE SHARES ISSUABLE HEREUNDER HAVE NOT BEEN REGISTERED UNDER
THE SECURITIES ACT OF 1933, AS AMENDED, OR ANY STATE SECURITIES LAWS, AND MAY
NOT BE SOLD, PLEDGED, OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE
REGISTRATION THEREOF UNDER SUCH ACT AND LAWS OR, SUBJECT TO SECTION 5.3 HEREOF,
AN OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE CORPORATION AND ITS
COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED

                            WARRANT TO PURCHASE STOCK

Issuer: ActivBiotics, Inc.
Number of Shares: 30,000, subject to adjustment
Class of Stock: Series B Convertible Preferred Stock, $O.01 par value per share
Exercise Price: $1.00 per share, subject to adjustment
Issue Date: November 12, 2003
Expiration Date: November 12, 2013

     FOR THE AGREED UPON VALUE of $1.00, and for other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged,
this Warrant is issued to SILICON VALLEY BANK (together with its successors and
permitted assigns, "Holder") by ActivBiotics, Inc., a Delaware corporation (the
"Company").

     Subject to the terms and conditions hereinafter set forth, the Holder is
entitled upon surrender of this Warrant and a duly executed Notice of Exercise
in substantially the form attached hereto as Appendix 1 (the "Notice of
Exercise"), at the principal office of the Company, 128 Spring Street,
Lexington, Massachusetts 02421 or such other office as the Company shall notify
the Holder of in writing, to purchase from the Company up to thirty thousand
(30,000) fully paid and non-assessable shares (the "Shares") of the Company's
Series B Convertible Preferred Stock, $O.01 par value per share (the "Class"),
at a purchase price per Share of One Dollar ($1.00) (the "Exercise Price") This
Warrant may be exercised in whole or in part at any time and from time to time
until 5:00 PM, Eastern time, on the Expiration Date set forth above, and shall
be void thereafter. Until such time as this Warrant is exercised in full or
expires, the Exercise Price and the number of Shares are subject to adjustment
from time to time as hereinafter provided

     Notwithstanding the foregoing definition of Class, upon and after the
automatic or voluntary conversion, redemption or retirement of all (but not
less than all) of the outstanding shares of such Class, including without
limitation the Company's initial registered underwritten public offering and
sale of its securities ("IPO"), then from and after the date upon which all such
outstanding shares have been so converted, redeemed or retired, "Class" shall
mean the Company's common stock, $0.01 par value per share ("Common Stock"), and
this Warrant shall be exercisable for such number of shares of Common Stock as
shall equal the number of shares of Common Stock into which the Shares would
have been converted pursuant to the Company's Certificate of Incorporation, as

<Page>

amended, including without limitation the Certificate of Designation, if any,
applicable to the same class or series of preferred stock as the Shares (the
"Certificate") had the Shares been issued and outstanding immediately prior to
such conversion, redemption or retirement, and the Exercise Price shall be the
Common Stock conversion price as determined pursuant to the Certificate
immediately prior to such conversion, redemption or retirement (all subject to
further adjustment as provided herein).

ARTICLE 1. EXERCISE

          1.1 Method of Exercise. Holder may exercise this Warrant by delivering
this Warrant together with a duly executed Notice of Exercise to the principal
office of the Company. Unless Holder is exercising the conversion right set
forth in Section 1.2, Holder shall also deliver to the Company a bank or
certified check or wire transfer to an account specified by the Company for the
aggregate Exercise Price for the Shares being purchased.

          1.2 Conversion Right. In lieu of exercising this Warrant as specified
in Section 1.1, Holder may from time to time convert this Warrant, in whole
or in part, into a number of Shares determined as follows:

                X = Y (A-B)/A
     where:
                X = the number of Shares to be issued to the Holder

                Y = the number of Shares with respect to which this Warrant is
                being exercised

                A = the Fair Market Value (as determined pursuant to Section 1.3
                below) of one Share

                B = the Exercise Price

          1.3 Fair Market Value.

               1.3.1 If shares of the Class (or shares of the Company's stock
into which shares of the Class are convertible or exchangeable) are traded on a
nationally recognized securities exchange or over the counter market, the fair
market value of a Share shall be the closing price of a share of the Class (or
the closing price of a share of the Company's stock for which shares of the
Class are convertible or exchangeable, multiplied by the number of shares of
such stock into which one share of the Class is convertible or exchangeable)
reported for the business day immediately preceding the date of Holder's Notice
of Exercise to the Company.

               1.3.2 If shares of the Class (or shares of the Company's stock
into which shares of the Class are convertible or exchangeable) are not traded
on a nationally recognized

                                        2

<Page>

securities exchange or over the counter market, the Board of Directors of the
Company shall determine fail market value in its reasonable good faith judgment.

          1.4 Delivery of Certificate and New Warrant. Promptly after Holder
exercises or converts this Warrant, the Company at its sole expense shall
promptly deliver to Holder (i) certificates for the Shares acquired upon such
exercise, and (ii) if this Warrant has not been fully exercised or converted and
has not expired, a new warrant of like tenor representing the Shares for which
this Warrant is still exercisable.

          1.5 Replacement of Warrants. On receipt of evidence reasonably
satisfactory to the Company of the loss, theft, destruction or mutilation of
this Warrant and, in the case of loss, theft or destruction, on delivery of an
indemnity agreement reasonably satisfactory in form and amount to the Company
or, in the case of mutilation, on surrender and cancellation of this Warrant,
the Company at its expense shall execute and deliver, in lieu of this Warrant, a
new warrant of like tenor

          1.6 Assumption on Sale, Merger, or Consolidation of the Company

               16.1. "Acquisition". For the purpose of this Warrant,
"Acquisition" means any sale, assignment, transfer, exclusive license, or other
disposition of all or substantially all of the assets of the Company, or any
acquisition, reorganization, consolidation, or merger of the Company where the
holders of the Company's outstanding voting equity securities immediately prior
to the transaction beneficially own less than a majority of the outstanding
voting equity securities of the surviving or successor entity immediately
following the transaction

               1.6.2. Assumption of Warrant. Upon the closing of any Acquisition
(other than an Acquisition in which the consideration received by the Company's
stockholders consists solely of cash), and as a condition precedent thereto, the
successor or surviving entity shall assume the obligations of this Warrant, and
this Warrant shall be exercisable for the same securities and property as would
be payable for the Shares issuable upon exercise of the unexercised portion of
this Warrant as if such Shares were outstanding on the record date for the
Acquisition and subsequent closing. The Exercise Price shall be adjusted
accordingly, and the Exercise Price and number and class of Shares shall
continue to be subject to adjustment from time to time in accordance with the
provisions hereof.

ARTICLE 2. ADJUSTMENTS TO THE SHARES.

          2.1 Stock Dividends, Splits, Etc. If the Company declares or pays a
dividend on the outstanding shares of the Class, payable in shares of the Class,
Common Stock or other securities, or subdivides the outstanding shares of the
Class into a greater number of shares of the Class, or subdivides the shares of
the Class in a transaction that increases the amount of Common Stock into which
such shares are convertible, then upon exercise of this Warrant, for each Share
acquired, Holder shall receive, without additional cost to Holder, the total
number and kind of

                                        3

<Page>

securities to which Holder would have been entitled had Holder owned the Shares
of record as of the date the dividend or subdivision occurred.

          2.2 Reclassification, Exchange or Substitution. Upon any
reclassification, exchange, substitution, reorganization or other event that
results in a change of the number and/or class of the securities issuable upon
exercise or conversion of this Warrant, Holder shall be entitled to receive,
upon exercise or conversion of this Warrant, the number and kind of securities
and property that Holder would have received for the Shares if this Warrant had
been exercised immediately before such reclassification, exchange, substitution,
reorganization or other event. The Company or its successor shall promptly issue
to Holder a new warrant of like tenor for such new securities or other property.
The new warrant shall provide for adjustments which shall be as nearly
equivalent as may be practicable to the adjustments provided for in this Article
2 including, without limitation, adjustments to the Exercise Price and to the
number of securities or property issuable upon exercise of the new warrant. The
provisions of this Section 2.2 shall similarly apply to successive
reclassifications, exchanges, substitutions, reorganizations or other events.

          2.3 Adjustments for Combinations, Etc. If the outstanding shares of
the Class are combined or consolidated, by reclassification or otherwise, into a
lesser number of shares, the Exercise Price shall be proportionately increased
and the number of Shares issuable upon exercise or conversion of this Warrant
shall be proportionately decreased.

          2.4 No Impairment. The Company shall not, by amendment of the
Certificate or its by-laws or through a reorganization, transfer of assets,
consolidation, merger, dissolution, issue, or sale of securities or any other
voluntary action, avoid or seek to avoid the observance or performance of any of
the terms to be observed or performed under this Warrant by the Company, but
shall at all times in good faith assist in carrying out of all the provisions of
this Article 2 and in taking all such action as may be necessary or appropriate
to protect Holder's rights under this Article against impairment.

          2.5 Adjustments for Dilutive Issuances. The number of shares of Common
Stock for which the Shares are convertible shall be adjusted from time to time
in accordance with Section A.7 of Article 111 of the Certificate as if the
Shares were issued and outstanding on and as of the date of any such required
adjustment.

          2.6 Fractional Shares. No fractional Shares shall be issuable upon
exercise or conversion of the Warrant and the number of Shares to be issued
shall be rounded down to the nearest whole Share. If a fractional Share interest
arises upon any exercise or conversion of this Warrant, the Company shall
eliminate such fractional Share interest by paying Holder an amount computed by
multiplying such fractional interest by the Fair Market Value (determined in
accordance with Section 1.3 above) of one Share.

          2.7 Certificate as to Adjustments. Upon each adjustment of the
Exercise Price, number or class of Shares or number of shares of Common Stock or
other securities for which the

                                       -4-

<Page>

Shares are convertible or exchangeable, the Company at its expense shall
promptly compute such adjustment, and furnish Holder with a certificate of its
chief financial officer setting forth such adjustment and the facts upon which
such adjustment is based. The Company shall at any time and from time to time,
upon written request, furnish Holder with a certificate setting forth the
Exercise Price, number and class of Shares and conversion ratio in effect upon
the date thereof and the series of adjustments leading to such Exercise Price,
number and class of Shares and conversion ratio.

ARTICLE 3. REPRESENTATIONS AND COVENANTS OF THE COMPANY.

          3.1 Representations and Warranties. The Company hereby represents and
warrants to the Holder as follows:

               (a) All Shares which may be issued upon the due exercise of this
Warrant, and all Common Stock or other securities, if any, issuable upon due
conversion of the Shares, shall, upon issuance, be duly authorized, validly
issued, fully paid and non-assessable, and free of any liens and encumbrances
except for restrictions on transfer provided for herein or under applicable
federal and state securities laws.

               (b) The Company covenants that it shall at all times cause to be
reserved and kept available out of its authorized and unissued shares such
number of shares of its Series B Convertible Preferred Stock and shares of its
Common Stock and other securities as will be sufficient to permit the exercise
in full of this Warrant and the conversion of the Shares into shares of Common
Stock or such other securities.

               (c) On and as of the date hereof, (i) $1.00 is the lowest price
per share for which shares of the Class have been sold or issued by the Company,
and the lowest exercise or conversion price per share for which shares of the
Class may be purchased or acquired upon the exercise or conversion of
outstanding securities exercisable or convertible by their terms for shares of
the Class, and (ii) the Common Stock conversion price in effect for shares of
the Class as determined pursuant to the Certificate is $1.00.

               (d) The execution and delivery by the Company of this Warrant and
the performance of all obligations of the Company hereunder, including the
issuance to Holder of the right to acquire the Shares, have been duly authorized
by all necessary corporate action on the part of the Company, and this Warrant
is not inconsistent with the Certificate and/or the Company's by-laws, does not
contravene any law or governmental rule, regulation or order applicable to it,
does not and will not contravene any provision of, or constitute a default
under, any material indenture, mortgage, contract or other instrument to which
it is a party or by which it is bound, and constitutes a legal, valid and
binding agreement of the Company, enforceable in accordance with its terms.

          3.2 Notice of Certain Events. If the Company proposes at any time (a)
to declare any dividend or distribution upon any of its capital stock, whether
in cash, property, stock, or other securities and whether or not a regular cash
dividend; (b) to offer for subscription pro rata to the

                                       -5-

<Page>

holders of any class or series of its stock any additional shares of stock of
any class or series or other rights; (c) to effect any reclassification or
recapitalization of any of its securities; or (d) to merge or consolidate with
or into any other corporation, or sell, lease, license, or convey all or
substantially all of its assets, or to liquidate, dissolve or wind up; then, in
connection with each such event, the Company shall give Holder (1) at least 20
days prior written notice of the date on which a record will be taken for such
dividend, distribution, or subscription rights (and specifying the date on which
the holders of securities of the Company shall be entitled to receive such
dividend, distribution or rights) or for determining rights to vote, if any, in
respect of the matters referred to in (c) and (d) above; and (2) in the case of
the matters referred to in (c) and (d) above at least 20 days prior written
notice of the date when the same will take place (and specifying the date on
which the holders of securities of the Company will be entitled to exchange
their securities of the Company for securities or other property deliverable
upon the occurrence of such event).

          3.3 Registration Under Securities Act of 1933, as amended. The shares
of Common Stock issuable upon conversion of the Shares (and the Shares, at all
times when the Class is Common Stock) shall have certain incidental or
"piggyback" registration rights pursuant to, and as set forth in, that certain
Rights Agreement of even date herewith between the Company and Silicon Valley
Bank. The Company represents and warrants to Holder that the Company's foregoing
grant of registration rights and its execution, delivery and performance of the
aforementioned Rights Agreement (a) have been duly authorized by all necessary
corporate action of the Company's Board of Directors and shareholders, (b) will
not violate the Certificate or the Company's by-laws, each as amended, (c) will
not violate or cause a breach or default (or an event which with the passage of
time or the giving of notice or both, would constitute a breach or default)
under any agreement, instrument, mortgage, deed of trust or other arrangement to
which the Company is a party or by which it or any of its assets is subject or
bound, and (d) do not require the approval, consent or waiver of or by any
shareholder, registration rights holder or other third party which approval,
consent or waiver has not been obtained as of the date of issuance of this
Warrant.

ARTICLE 4. REPRESENTATIONS AND WARRANTIES OF THE HOLDER. Holder represents and
warrants to the Company as follows:

          4.1 Purchase for Own Account. Subject to Silicon Valley Bank's right
to transfer this Warrant and the Shares (and/or the securities, if any, issued
and issuable upon conversion of the Shares) to its parent corporation Silicon
Valley Bancshares and/or any other affiliate, this Warrant and the Shares to be
acquired upon exercise hereof will be acquired for investment for Holder's
account, not as nominee or agent, and not with a view to sale or distribution in
violation of applicable federal and state securities laws.

          4.2 Investment Experience. Holder understands that the purchase of
this Warrant and the Shares covered hereby involves substantial risk Holder (a)
has experience as an investor in unregistered securities, (b) has sufficient
knowledge and experience in financial and business affairs that it can evaluate
the risks and merits of its investment in this Warrant and the Shares, and (c)
can bear the economic risk of such Holder's investment in this Warrant and the
Shares

                                       -6-

<Page>

          4.3 Knowledge of Company Business. The Holder is aware of the
Company's business affairs and financial condition, and has acquired information
about the Company sufficient to reach an informed and knowledgeable decision to
acquire this Warrant.

          4.4 No Public Market. The Holder understands that this Warrant, the
Shares, and the Common Stock issuable upon conversion of the Shares must be held
indefinitely unless subsequently registered under the Securities Act of 1933, as
amended or unless exemptions from registration and qualification are otherwise
available.

          4.5 Accredited Investor. Holder is an "accredited investor" as such
term is defined in Regulation D under the Securities Act of 1933, as amended.

ARTICLE 5: MISCELLANEOUS.

          5.1 Automatic Conversion upon Expiration. In the event that, upon the
Expiration Date, the Fair Market Value of one Share (or other security issuable
upon the exercise hereof) as determined in accordance with Section 1.3 above is
greater than the Exercise Price in effect on such date, then this Warrant shall
automatically be deemed on and as of such date to be converted pursuant to
Section 1.2 above as to all Shares (or such other securities) for which it shall
not previously have been exercised or converted, and the Company shall promptly
deliver a certificate representing the Shares (or such other securities) issued
upon such conversion to the Holder.

          5.2 Legends. This Warrant and the Shares (and the securities issuable,
directly or indirectly, upon conversion of the Shares, if any) shall be
imprinted with a legend in substantially the following form:

     THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
     AMENDED, OR ANY STATE SECURITIES LAWS, AND MAY NOT BE SOLD, PLEDGED OR
     OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH
     ACT AND LAWS OR, SUBJECT TO SECTION 5.3 OF THAT CERTAIN WARRANT TO PURCHASE
     STOCK ISSUED BY THE CORPORATION TO SILICON VALLEY BANK DATED AS OF
     ________________, AN OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE
     CORPORATION AND ITS COUNSEL THAT SUCH REGISTRATION IS NOT REQUIRED.
     ADDITIONALLY, THIS SECURITY IS SUBJECT TO THE PROVISIONS OF THE RIGHTS
     AGREEMENT DATED NOVEMBER __, 2003 BETWEEN ACTIVBIOTICS, INC. AND SILICON
     VALLEY BANK COPIES OF SUCH AGREEMENT MAY BE OBTAINED AT NO COST BY WRITTEN
     REQUEST MADE BY THE HOLDER OF RECORD OF THIS CERTIFICATE TO THE SECRETARY
     OF ACTIVBIOTICS, INC.

                                        7

<Page>

          5.3 Compliance with Securities Laws on Transfer. This Warrant and the
Shares (and the securities, if any, issued and issuable upon conversion of the
Shares) may not be transferred or assigned in whole or in part without
compliance with applicable federal and state securities laws by the transferor
and the transferee (including, without limitation, the delivery of investment
representation letters and legal opinions reasonably satisfactory to the
Company, as reasonably requested by the Company). The Company shall not require
Holder to provide an opinion of counsel if the transfer is to Silicon Valley
Bancshares or other affiliate of Holder.

          5.4 Transfer Procedure. Following its receipt of this executed
Warrant, Silicon Valley Bank will transfer same in whole or in part to its
parent corporation Silicon Valley Bancshares, and thereafter Holder and/or
Silicon Valley Bancshares may, subject to Section 5.3 above, transfer all or
part of this Warrant and/or the Shares (or the securities, if any, issued and
issuable upon conversion of the Shares) at any time and from time to time by
giving the Company notice of the portion of the Warrant and/or Shares (or the
securities, if any, issued and issuable upon conversion of the Shares) being
transferred setting forth the name, address and taxpayer identification number
of the transferee and surrendering this Warrant to the Company for reissuance to
the transferee(s) (and Holder if applicable); provided, that at all times prior
to the Company's IPO, Holder shall not, without the prior written consent of the
Company, transfer this Warrant (or any part hereof), any Shares, or any
securities issued or issuable upon conversion of the Shares, to any person who
directly competes with the Company, unless such transfer is in connection with
an Acquisition of the Company by any such person.

          5.5 Notices. All notices and other communications from the Company to
the Holder, or vice versa, shall be deemed delivered and effective when given
personally, or mailed by first-class registered or certified mail, postage
prepaid, or sent via reputable overnight courier service, fee prepaid, at such
address as may have been furnished to the Company or the Holder, as the case may
be, in writing by the Company or such holder from time to time, but in all
cases, unless instructed in writing otherwise, the Company shall deliver a copy
of all notices to Holder to Silicon Valley Bank, Treasury Department, 3003
Tasman Drive, HA 200, Santa Clara, California 95054.

          5.6 Waiver. This Warrant and any term hereof may be changed, waived,
discharged or terminated only by an instrument in writing signed by the party
against which enforcement of such change, waiver, discharge or termination is
sought.

          5.7 Attorneys Fees. In the event of any dispute between the parties
concerning the terms and provisions of this Warrant, the party prevailing in
such dispute shall be entitled to collect from the other party all costs
incurred in such dispute, including reasonable attorneys' fees.

          5.8 Governing Law. This Warrant shall be governed by and construed in
accordance with the laws of The Commonwealth of Massachusetts, without giving
effect to its principles regarding conflicts of law.

                                        8

<Page>

          5.9 No Rights as a Shareholder. Except as specifically provided in
this Warrant, Holder shall have no rights as a shareholder of the Company in
respect of the Shares issuable hereunder unless and until Holder exercises this
Warrant as to all or any of such Shares.

                  [REMAINDER OF PAGE LEFT BLANK INTENTIONALLY]

                                        9

<Page>

     IN WITNESS WHEREOF, the Company has caused this Warrant to Purchase Stock
to be executed as an instrument under seal by its duly authorized representative
as of the date first above written.

ATTEST:                                   "COMPANY"

                                          ACTIVBIOTICS, INC.

By: /s/ [ILLEGIBLE]                       By: /s/ J. L. Warren
    -----------------------------------       ----------------------------------
Name: [ILLEGIBLE]                         Name: J. L. WARREN
Title: WITNESS                            Title: C.E.O

"HOLDER"

SILICON VALLEY BANK

By: /s/ [ILLEGIBLE]
    -----------------------------------
Name: [ILLEGIBLE]
Title: Vice President

                                       10

<Page>

                                   APPENDIX 1

                               NOTICE OF EXERCISE

     1. The undersigned hereby elects to purchase ______ shares of the
__________ stock of ____________________ pursuant to Section 1.1 of the attached
Warrant, and tenders herewith payment of the Exercise Price of such shares in
full

     1. The undersigned hereby elects to convert the attached Warrant into
Shares in the manner specified in Section 1.2 of the attached Warrant. This
conversion is exercised with respect to ___________ of shares of the
________________________ Stock of ________________________

     [Strike paragraph that does not apply.]

     2. Please issue a certificate or certificates representing said shares in
the name of the undersigned or in such other name as is specified below:

                       ------------------------------------
                           (Name)

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

                       ------------------------------------
                           (Address)

     3. The undersigned represents it is acquiring the shares solely for its own
account and not as a nominee for any other party and not with a view toward the
resale or distribution thereof except in compliance with applicable securities
laws

                                          --------------------------------------
                                             (Signature)

-------------------
   (Date)

<Page>

                                   ASSIGNMENT

For value received, Silicon Valley Bank hereby sells, assigns and transfers
unto:

           NAME: SILICON VALLEY BANCSHARES
           ADDRESS: 3003 TASMAN DRIVE (HA-200)
           SANTA CLARA, CA 95054

           Taxid: 91-1962278

that certain Warrant to Purchase Stock issued by ActivBiotics, Inc.
(the "Company"), on November 12, 2003 (the "Warrant") together with all rights,
title and interest therein.

                                        SILICON VALLEY BANK

                                        By: /s/ R. Bryan Jadol
                                            ------------------------------------
                                        Name: R. Bryan Jadol
                                        Title :Vice President

Date November 12, 2003

By its execution below, and for the benefit of the Company, Silicon Valley
Bancshares makes each of the representations and warranties set forth in Article
4 of the Warrant as of the date hereof.

                                        SILICON VALLEY BANCSHARES

                                        By: /s/ Paulette Mehas
                                            ------------------------------------
                                        Name: Paulette Mehas
                                        Title: Treasurer

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