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                                                                   Exhibit 10.23

                         Metaphore Pharmaceuticals, Inc.
                           One Bridge Plaza, Suite 500
                               Fort Lee, NJ 07024

                                                               December 28, 2005

ActivBiotics, Inc.
110 Hartwell Avenue
Lexington, MA 02421

Ladies and Gentlemen:

     Reference is hereby made to the Agreement and Plan of Merger and
Reorganization, dated as of December 7, 2005 ("Merger Agreement") by and among
ActivBiotics, Inc., Metaphore Acquisition Co., Metaphore Pharmaceuticals, Inc.
and the Stockholder Representatives party thereto (collectively, the "Parties").
Capitalized terms used herein but not otherwise defined herein shall have such
meanings as defined in the Merger Agreement.

     Notwithstanding anything to the contrary contained in the Merger Agreement,
the Parties hereby agree as follows:

     1. The current action entitled Daniela Salvemini PhD vs. Metaphore
Pharmaceuticals, Inc. and Alan W. Dunton M.D. (Superior Court of New Jersey,
Bergen County (Docket No. L-2076-05) (the "Salvemini Matter"), set forth on
Schedule 6.17 of the Company Disclosure Schedule shall be considered a D&O Claim
for which ABI and the Surviving Corporation shall indemnify and hold harmless
Alan W. Dunton, M.D., in accordance with Section 8.12 of the Merger Agreement.
The Parties further agree that if at any time ABI or the Surviving Corporation
is required to provide indemnification to Mr. Dunton pursuant to Section 8.12 of
the Merger Agreement in connection with the Salvemini Matter, then ABI or the
Surviving Corporation shall have the right to make a claim for indemnification
therefor pursuant to Section 13.3 of the Merger Agreement.

     2. Except as modified hereby the Merger Agreement remains in full force and
effect.

[SIGNATURE PAGE TO FOLLOW]

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     Please confirm your agreement to the foregoing by signing the enclosed
duplicate of this letter.

ABI:

ACTIVBIOTICS, INC.

By: /s/ STEVEN C. GILMAN
    ---------------------------------
    Name: Steven C. Gilman
    Title: President & CEO

Merger Sub:

METAPHORE ACQUISITION CO.

By: /s/ STEVEN C. GILMAN
    ---------------------------------
    Name: Steven C. Gilman
    Title: President

Company:

METAPHORE PHARMACEUTICALS, INC.

By: /s/ Alan W. Dunton M.D.
    ---------------------------------
    Name: ALAN W. DUNTON M.D.
    Title: President & CEO<Page>

                                                               Exhibit 10.24

                               [ACTIVBIOTICS LOGO]

March 12, 2002

James L. Warren
48 Groton Road
N. Chelmsford, MA 01863

Dear Jim,

I am pleased to offer you the position of VICE PRESIDENT OF FINANCE AND
ADMINISTRATION AND CHIEF FINANCIAL OFFICER with ActivBiotics, Inc., with a start
date of March 18, 2002.

Your annual base salary will be $190,000 payable in bi-weekly installments of
$7916.67. You will also be eligible for the company stock option program, with
an initial grant of 400,000 shares. This award is subject to a vesting schedule
of 25% per year for four years, and will have an exercise price of $.10 per
share. Jim, this award represents an important equity position in ActivBiotics,
and we look forward to working together to continue build the value of this
company. You will also participate in our bonus program, eligible to earn an
additional $25,000 and 25,000 stock options. This bonus payout will be
determined by your individual performance against agreed upon objectives, as
well as company and management team performance against objectives.

Your first 90 days of employment at ActivBiotics are considered an orientation
period, and your performance will be reviewed at the conclusion of this period.
After 12 months in the position, you will receive a complete performance review,
and you will be eligible for a salary review. This position reports to Chalom
Sayada, Chief Executive Officer.

 You wil1 also receive the following additional employee benefits when they
 become generally available:

-    Health and dental insurance

-    Long and short term disability insurance

-    8 fixed and 4 floating holidays

-    401K retirement plan

-    Direct deposit of payroll (OPTIONAL)

-    Section 125 plan, allowing pre-tax deduction of health and dental insurance
     contributions

-    4 weeks (20 days) paid vacation time per year.

-    An option to purchase shares of the Company's common stock in accordance
     with the Company's stock option plan. This award, including the strike
     price, is subject to the approval of the Company's Board of Directors.

Your employment will be at will, with a four-week notification period requested
for resignation of employment. In the event that your employment with
ActivBiotics is terminated not "For Cause" as defined in the attached separation
agreement, you will be entitled to receive your base salary through the date
which is three (3) months following termination. As a condition to the receipt
of the payments described in the separation agreement, you will be required to
execute a release of all claims arising from employment. As a condition of
employment, you will be required to sign the ActivBiotics' Confidentiality
agreement, and abide by company policies as they are established and introduced.

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Jim, we are confident that you will make an important contribution to the
success of ActivBiotics, and look forward to working with you. This offer will
be valid for 5 working days, please do not hesitate to contact me if you have
any questions. In order to accept the position as offered, please sign this
letter and the attached application for employment for our records, and return
to me by March 15, 2002.

                                        I accept the offer of employment as
Sincerely,                              outlined above,

/s/ Cathy Carew                         /s/ James L. Warren
-------------------------------------   ----------------------------------------
Cathy Carew,                            James L. Warren
Vice President, Human Resources         Vice President, Finance and
                                        Administration
SEPARATION AGREEMENT:

     (a) In the event that your employment with the Corporation is terminated
     not For Cause, you will be entitled to receive your base salary through the
     date which is three (3) months following termination (such payments being
     referred to herein as the "Severance Payments" and such period being
     referred to as the "Severance Period"); provided that the Severance
     Payments shall be reduced by any amounts you earn as a result of your
     employment by any business (whether as a director, officer, employee,
     manager, owner, consultant, independent contractor or otherwise) during the
     Severance Period.

     (b) In the event that your employment with the Corporation is terminated
     (i) For Cause (as hereinafter defined), (ii) you resign, you will not be
     entitled to any severance payment. For purposes of this Agreement, the term
     "For Cause" shall mean (A) your willful or reckless failure to perform, or
     willful or reckless violation of, your duties as (other than a failure
     resulting from your death or Disability), which failure or violation shall
     not have been cured within a reasonable cure period, upon your receipt of
     notice from the Board of said failure or violation; (B) your refusal or
     failure to follow any reasonable and lawful direction of the Company's
     President or Board of Directors; (C) your commission of an act of fraud,
     dishonesty, embezzlement or theft against the Corporation or any of its
     subsidiaries; or (D) your conviction of (or your plea of nolo contendere
     to) any felony: (E) serious misconduct, negligence, dishonesty directly
     related to the performance of your duties for the company. For purposes of
     this Agreement, you shall be deemed "Disabled" if an independent medical
     doctor (selected by the Corporation's health or disability insurer)
     certifies that you have for six (6) consecutive months or an aggregate of
     one hundred eighty (180) days within a one-year period, been disabled in a
     manner which seriously interferes with your ability to perform your
     responsibilities. Any refusal by you to submit to a medical examination for
     the purposes of certifying disability under this Agreement shall be deemed
     to constitute conclusive evidence of your disability.<Page>

                                                               Exhibit 10.25

[ACTIVBIOTICS LOGO]

June 13, 2006

James L. Warren
48 Groton Road
N. Chelmsford, MA 01863

Dear Jim,

     Following our recent discussions about your termination from the Company,
this letter contains the complete terms and conditions under which the Company
would provide certain benefits as described below.

     1. RESIGNATION. Your last day of work will be Wednesday, June 14, 2006 (the
"Last Work Day"). As of the Last Work Day, you resign any positions of the
Company that you may hold or have held and the Company accepts such resignations
and relieves you of any responsibilities, obligations or duties associated with
such positions after the Last Work Day.

     2. CONSULTING AGREEMENT. To assist in the transition of your
responsibilities to other employees, the Company agrees to enter into a
consulting relationship with you. Under the terms of a separately negotiated
Consulting Agreement, the company will compensate you for services provided
under the contract You agree to continue to adhere to the terms and conditions
of your Invention and Non-Disclosure Agreement with the Company, Company work
policies, this letter Agreement, and any Consulting Agreement.

     3. STOCK OPTIONS. With regard to all of your Stock Option Agreements with
the Company, and only for purposes of those Stock Option Agreements, the Company
agrees that the date of termination of your association with the Company shall
be deemed to be the last date (the "Last Consulting Day") as defined under a
separate Consulting agreement You agree to waive any right your might otherwise
have to continued vesting of stock options after the Last Consulting Day, and
you acknowledge that your vested stock options will remain exercisable for
twelve (12) months following Last Consulting Day You acknowledge the sufficiency
of the this extension of exercise time as consideration for your continued
adherence to the terms and conditions of your Invention and Non-Disclosure
Agreement and for your agreements in this letter Agreement to release certain
claims against the Company.

     4. WAIVERS AND RELEASES. (a) You agree that the compensation and benefits
paid to you through the Termination Date, together with vacation pay for unused
vacation accrued as of the Termination Date (which the Company will pay to you
after the Termination Date in accordance with applicable law), constitute full
and complete satisfaction of any and all sums which are now or might hereafter
have become owing to you for services rendered by you during your employment
with the Company or in connection with the termination of your employment with
the Company.

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(b) This letter agreement is not intended to and does not constitute an
admission of liability by either you or the Company, and shall not be used as
evidence of liability or wrongdoing on the part of either you or the Company.

(c) You forever give up, waive and release any and all claims, charges,
complaints, grievances or promises of any and every kind you may have up to the
date of this letter agreement or that may arise after the date of this letter
agreement against the Company, its shareholders, directors, officers and
employees, and related or affiliated persons, including, without limitation--

          (i)  your rights under Title VII of the Civil Rights Act of 1964, as
               amended by the Civil Rights Act of 1991, M.G.L. c 151B, the
               Employee Retirement Income Security Act ("ERISA"), the Equal Pay
               Act, the Americans with Disabilities Act ("ADA"), the Age
               Discrimination in Employment Act ("ADEA") and other federal and
               state statutes prohibiting discrimination on the basis of age,
               gender, sexual orientation, race, religion, national origin or
               handicap;

          (ii) any claims for breach of employment contract (whether express or
               implied); and

          (iii) any common law claims, including without limitation, claims for
               wrongful termination, defamation, invasion of privacy,
               intentional or negligent infliction of emotional distress,
               interference with contractual or advantageous business relations,
               personal injury, breach of contract, unpaid wages or
               compensation, or claims for unreimbursed expenses

     However, for clarity, this waiver and release shall not apply to any claims
that you may have for breach of this letter agreement

     5. CONFIDENTIALITY. You agree that this letter agreement is confidential
and that you will not disclose the fact that it exists or its terms and
conditions with anyone else except your immediate family members, attorney, tax
accountant, or as required by law, and that the individuals to whom disclosure
is made under this paragraph agree to similarly maintain the confidentiality of
this letter agreement

     6. NON-DISPARAGEMENT. You agree that you will not make any disparaging
comments, statements or other communications about the Company, its
shareholders, directors, officers, employees, related or affiliated persons, or
business practices, and that you will not otherwise do or say anything that
could disrupt the good morale of the employees of the Company or otherwise harm
its business or reputation. The Company agrees that its employees who managed
you during your employment with the Company, its officers and directors, and its
other employees and agents to the extent ActivBiotics can control the actions of
such employees and agents, will not make any disparaging comments, statements or
other communications about you or your employment with the Company. The Company
agrees that, in accordance with Company policy, it will refer all inquiries
regarding your employment to its Human Resources department, and the Human
Resources department will only confirm dates of employment and positions held.
For clarity, this Paragraph 6 shall not apply to any truthful statement that any
person is required to make in any legal proceeding, or pursuant to any
governmental or regulatory investigation

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                                        3

     7. RETURN OF PROPERTY. You understand and agree that you must return all
Company property in your possession or control to the Company on or before the
Termination Date, including but not limited to, all building access cards, keys,
parking permits, laptop computers, dial-in access cards, documents, materials
and information relating to the business of the Company, and any other Company
property.

     8. ADHERENCE TO INVENTION AND NON-DISCLOSURE AGREEMENT: You agree to abide
by any and all employment agreements entered into between you and the Company
prior to or during your employment with the Company, including without
limitation the Invention and Non-Disclosure Agreement.

     9. CONSEQUENCES OF BREACH. In the event that you breach this letter
agreement, any other agreement with the Company (including without limitation
the Invention and Non-Disclosure Agreement with the Company) or Company work
policies, or if you otherwise behave in any way unprofessionally in relationship
to the Company, in addition to whatever legal or equitable remedy may be
available in compensation of that injury, the Company may halt payments of
compensation and provision of benefits under this letter agreement and demand
the return from you of the consideration paid pursuant to this letter agreement,
and you agree to immediately repay all such amounts. If the Company seeks an
equitable remedy, you agree that you will not oppose the granting of such relief
on the grounds that an award of damages would provide an adequate remedy.

     10. GENERAL PROVISIONS. If any term of this letter agreement, or the
application of any such term to any person or circumstance, is or would be, to
any extent, invalid or unenforceable, the remainder of this letter agreement, or
the application of such term to persons or circumstances other than those to
which it is or would be invalid or unenforceable, shall not be affected, and
each term of this letter agreement shall be valid and be enforced to the fullest
extent permitted by law.

     This letter agreement has been made in Massachusetts and Massachusetts law
(other than choice of law provisions) applies to it. You consent to the
exclusive jurisdiction of Massachusetts courts for any dispute or proceeding
concerning this letter agreement.

     Together with the Invention and Non-Disclosure Agreement and any stock
option agreements between you and the Company, this letter agreement sets forth
the entire agreement between you and the Company concerning its subject matter.
You acknowledge that no one has promised you anything that is different from
what is set forth in this letter agreement. No other promises or agreements
shall be binding upon the Company with respect to the subject matter of this
letter agreement unless separately agreed to in writing.

     11. CONSULTATION WITH AN ATTORNEY; REVOCATION. As required by the Older
Workers Benefit Protection Act of 1990, if you are over the age of 40, you
acknowledge and agree that--

          (i)  you have been advised and given the opportunity to consult with
               your own counsel prior to signing this letter agreement;

          (ii) you have been given up to forty-five (45) days from the receipt
               of this letter agreement to consider whether to sign it and that,
               if you execute this letter agreement prior to the end of this
               period, you acknowledge and agree that the prior execution was a
               knowing and voluntary waiver of

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           your right to consider this letter agreement for a full forty-five
           (45) days, and was due to your conclusion that you had ample time
           within which to consider and understand this letter agreement, and
           within which to review this letter agreement with your counsel;

     (iii) you has been advised that even after you sign this letter agreement,
           you may revoke it within seven (7) days of the date of this signing,
           by delivering a signed revocation notice by certified mail to: Cathy
           Carew, ActivBiotics, Inc, 110 Hartwell Ave, Lexington, MA 02421; and

     (iv)  this letter agreement shall not become effective and in force until
           eight (8) days after you sign it (provided then that you have not
           revoked it).

ACTIVBIOTICS, INC.

By: /s/ Steven C. Gilman                06-13-2006
    ---------------------------------   Date
    Steven C. Gilman, Ph D.
    Chief Executive Officer

AGREED:

/s/ James L. Warren                      06-13-2006
-------------------------------------   Date
James L. Warren

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