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                                                                   EXHIBIT 10.20

Predix
Pharmaceuticals
                            PERSONAL AND CONFIDENTIAL

January 31, 2005

Dr. Dale Dhanoa
[ADDRESS]

Dear Dale:

      The purpose of this letter agreement (the "Agreement") is to describe the
terms of your separation from Predix Pharmaceuticals ("Predix Pharmaceuticals"
or the "Company"). Payment of the separation pay and benefits described below is
contingent on your agreement to and compliance with the terms of this Agreement
as set forth below. To accept this Agreement, you must return a signed copy to
the Company by February 21, 2005.

      1.    SEPARATION OF EMPLOYMENT. Your employment with the Company will end
effective January 31, 2005 (the "Separation Date"). Except as consistent with
Section 5 herein, you shall not be required to perform any job duties. You
acknowledge that from and after January 31, 2005, you shall have no authority to
speak on behalf of or bind the company in any way. From and after the Separation
Date, you shall not represent yourself as an employee or agent of the Company.

      2.    SEPARATION PAY AND BENEFITS. In exchange for the mutual promises set
forth in this Agreement, and as soon as practicable following the Rescission
Period described in Section 6, the Company agrees to provide you with the
following Separation Pay and Benefits:

      (i)   Payment of six (6) months base salary, paid out under normal
            payroll, less all applicable federal, state, local and other
            employment-related taxes and deductions;

      (ii)  Payment in the amount of $488.00 (4.53 hours), less all applicable
            federal, state, local and other employment-related taxes and
            deductions, which reflects payments for unused vacation days; and

      (iii) Upon completion of the appropriate COBRA(1) forms, and subject to
            all the requirements of COBRA, continuation of your participation in
            the Company's medical and dental insurance plans at the Company's
            expense from January 31, 2005, through July 31, 2005, or until such
            time as you

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      1 "COBRA" is the Consolidated Omnibus Budget Reconciliation Act of 1985,
as amended. Your right to elect COBRA is not contingent on your acceptance of
this Agreement. However, if you don't accept this Agreement, you will be
responsible for all COBRA payments.

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            become eligible for similar benefits from another source (whichever
            period is shorter). After that time, you will have the right to
            continue your medical and dental insurance, subject to the
            requirements of COBRA, at your own cost. The "qualifying event"
            under COBRA shall be deemed to have occurred on January 31, 2005.

      You acknowledge and agree that except as noted above, the separation pay
and benefits described above are not otherwise due or owing to you under any
Company employment agreement (oral or written) or Company policy or practice.
You also agree that the separation pay and benefits described above are not
intended to and do not constitute a severance plan, nor do they confer a benefit
on anyone other than the parties hereto. Additionally, you acknowledge that
except for the specific financial consideration set forth in this Agreement, you
have been paid all final wages. You further acknowledge that you are not now and
shall not in the future be entitled to any other compensation from the Company
including, without limitation, other wages, commissions, incentive pay, bonuses,
vacation pay, holiday pay, personal time, sick time, paid time off or any other
form of compensation or benefit.

      3.    EQUITY. To the extent applicable, all of the terms, rights and
conditions of the Company Stock Option Plan (the "Plan") and any documents
executed by you pursuant to the issuance of any stock options under the Plan
(your "Stock Option Agreements" ), are hereby incorporated by reference and
shall survive the signing of this Agreement.

      4.    TAXES. You and the Company acknowledge that the Company makes no
guarantees with regard to tax consequences to you for any form of compensation
provided by the Company, including, but not limited to, tax treatment under
Section 409A of the Internal Revenue Code of 1986, as amended.

      5.    YOUR CONFIDENTIALITY NON-DISPARAGEMENT AND FUTURE COOPERATION
OBLIGATIONS. You expressly acknowledge and agree to the following:

      (i)   that you promptly will return to the Company all Company documents
            (and any copies thereof) and property, and that you shall abide by
            the provisions of your Employment Agreement previously signed by
            you, the terms of which are hereby incorporated by reference and
            which shall survive the signing of this Agreement. Further, you
            agree that you will abide by any and all common law and/or statutory
            obligations relating to protection and non-disclosure of the
            Company's trade secrets and/or confidential and proprietary
            documents and information;

      (ii)  that all information relating in any way to this Agreement,
            including the terms and amount of financial consideration provided
            for in this Agreement, shall be held confidential by you and shall
            not be publicized or disclosed to any person (other than an
            immediate family member, legal counsel or financial advisor,
            provided that any such individual to whom disclosure is made agrees
            to be bound by these confidentiality

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            obligations), business entity or government agency (except as
            mandated by state or federal law), except that nothing in this
            paragraph shall prohibit you from participating in an investigation
            with a state or federal agency if requested by the agency to do so;

      (iii) that you will not make any statements that are professionally or
            personally disparaging about, or adverse to, the interests of the
            Company (including its officers, directors, employees and
            consultants) including, but not limited to, any statements that
            disparage any person, product, service, finances, financial
            condition, capability or any other aspect of the business of the
            Company, and that you will not engage in any conduct which could
            reasonably be expected to harm professionally or personally the
            reputation of the Company (including its officers, directors,
            employees and consultants);

      (iv)  that until 15 February 2005 you will make yourself available to the
            Company either by telephone or, if the Company believes necessary,
            in person upon reasonable notice, to assist the Company in
            connection with any matter relating to services performed by you on
            behalf of the Company prior to the Separation Date. Such assistance
            may include, but is not limited to, updating Company employees on
            current intellectual property issues, executing waivers related to
            intellectual property as deemed necessary by the Company, and
            executing a power of attorney appointing the Company to sign on your
            behalf for any intellectual property generated during your time with
            the Company;

      (v)   further, you agree that following the Separation Date you shall
            cooperate fully with the Company in the defense or prosecution of
            any claims or actions now in existence or that may be brought or
            threatened in the future against or on behalf of the Company,
            including any claims or actions against its officers, directors and
            employees. Your cooperation in connection with such matters, actions
            and claims shall include, without limitation, being available to
            meet with the Company regarding personnel matters in which you have
            been involved, and any contract matters or audits; to prepare for
            any proceeding (including, without limitation, depositions,
            consultation, discovery or trial); to provide affidavits; to assist
            with any audit, inspection, proceeding or other inquiry; and to act
            as a witness in connection with any litigation or other legal
            proceeding affecting the Company. You further agree that should you
            be contacted (directly or indirectly) by any person or entity (for
            example, by any party representing an individual or entity) adverse
            to the Company, you shall promptly (within 48 hours) notify Dr.
            Michael Kauffman at the Company. You shall be reimbursed for any
            reasonable costs and expenses incurred in connection with providing
            such cooperation under this Section. In addition, for cooperation
            provided after the first 90 days, you will be compensated for your
            time at the rate of two hundred dollars ($200.00) per hour; and

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      (vi)  that a breach of this Section shall constitute a material breach of
            this Agreement and, in addition to any other legal or equitable
            remedy available to the Company, shall entitle the Company to
            recover any sums, and the monetary equivalent of any benefit, paid
            to you under Section 2 of this Agreement, and that the other
            benefits discussed herein will be deemed sufficient consideration to
            enforce this Agreement.

      6.    YOUR RELEASE OF CLAIMS. You hereby agree and acknowledge that by
signing this Agreement and accepting the Separation Pay and Benefits discussed
in Section 2, and for other good and valuable consideration, you are waiving
your right to assert any and all forms of legal claims against the Company(2) of
any kind whatsoever, whether known or unknown, arising from the beginning of
time through the date you execute this Agreement (the "Execution Date"). Except
as set forth below, your waiver and release herein is intended to bar any form
of legal claim, charge, complaint or any other form of action (jointly referred
to as "Claims") against the Company seeking any form of relief including,
without limitation, equitable relief (whether declaratory, injunctive or
otherwise), the recovery of any damages, or any other form of monetary recovery
whatsoever (including, without limitation, back pay, front pay, compensatory
damages, emotional distress damages, punitive damages, attorneys' fees and any
other costs) against the Company, for any alleged action, inaction or
circumstance existing or arising through the Execution Date.

      Without limiting the foregoing general waiver and release, you
specifically waive and release the Company from any Claim arising from or
related to your prior employment relationship with the Company or the
termination thereof, including, without limitation:

      **    Claims under any local, state or federal discrimination, fair
            employment practices or other employment related statute, regulation
            or executive order (as they may have been amended through the
            Execution Date) prohibiting discrimination or harassment based upon
            any protected status including, without limitation, race, national
            origin, age, gender, marital status, disability, veteran status or
            sexual orientation. Without limitation, specifically included in
            this paragraph are any Claims arising under the federal Age
            Discrimination in Employment Act, the Older Workers Benefit
            Protection Act, the Civil Rights Acts of 1866 and 1871, Title VII of
            the Civil Rights Act of 1964, the Civil Rights Act of 1991, the
            Equal Pay Act, the Americans With Disabilities Act, the Worker
            Adjustment and Retraining Notification Act, MASS. GEN. L. c. 151B
            (the Commonwealth's anti-discrimination statute), and any similar
            Massachusetts or other state statute or federal statute.

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(2) For purposes of this Section 6, the term "Company" includes
Predix Pharmaceuticals and any of its divisions, affiliates (which means all
persons and entities directly or indirectly controlling, controlled by or under
common control with the Company), parents, subsidiaries and all other related
entities, and its and their directors, officers, employees, trustees, agents,
lawyers, successors and assigns.

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      **    Claims under any other local, state or federal employment related
            statute, regulation or executive order (as they may have been
            amended through the Execution Date) relating to wages, hours or any
            other terms and conditions of employment. Without limitation,
            specifically included in this paragraph are any Claims arising under
            the Fair Labor Standards Act, the Family and Medical Leave Act of
            1993, the National Labor Relations Act, the Employee Retirement
            Income Security Act of 1974, the Consolidated Omnibus Budget
            Reconciliation Act of 1985 ("COBRA"), and any similar Massachusetts
            or other state statute or federal statute.

      **    Claims under any local, state or federal common law theory
            including, without limitation, wrongful discharge, breach of express
            or implied contract, promissory estoppel, unjust enrichment, breach
            of a covenant of good faith and fair dealing, violation of public
            policy, defamation, interference with contractual relations,
            intentional or negligent infliction of emotional distress, invasion
            of privacy, misrepresentation, deceit, fraud or negligence.

      **    Claims under any local, state or federal securities law, including,
            without limitation, the Securities Act of 1933, as amended, the
            Securities Exchange Act of 1934, as amended, and any other state or
            local securities statutes and regulations; and

      **    Any other claim arising under local, state, or federal law, or the
            laws of any country or governmental entity other than the United
            States.

      Notwithstanding the foregoing, this Section does not release the Company
from any obligation expressly set forth in this Agreement. You acknowledge and
agree that, but for providing this waiver and release, you would not be
receiving the separation pay and benefits being provided to you under the terms
of this Agreement.

      BECAUSE YOU ARE MORE THAN FORTY (40) YEARS OF AGE, YOU HAVE SPECIFIC
RIGHTS UNDER THE OLDER WORKERS BENEFITS PROTECTION ACT ("OWBPA"), WHICH
PROHIBITS DISCRIMINATION ON THE BASIS OF AGE, AND THAT THE RELEASES SET FORTH IN
THIS SECTION 6 ARE INTENDED TO RELEASE ANY RIGHT THAT YOU MAY HAVE TO FILE A
CLAIM AGAINST THE COMPANY ALLEGING DISCRIMINATION ON THE BASIS OF AGE.

      IT IS THE COMPANY'S DESIRE AND INTENT TO MAKE CERTAIN THAT YOU FULLY
UNDERSTAND THE PROVISIONS AND EFFECTS OF THIS AGREEMENT. TO THAT END, YOU HAVE
BEEN ENCOURAGED AND GIVEN THE OPPORTUNITY TO CONSULT WITH LEGAL COUNSEL FOR THE
PURPOSE OF REVIEWING THE TERMS OF THIS AGREEMENT. THE COMPANY ALSO IS PROVIDING
YOU WITH TWENTY-ONE (21) DAYS FROM THE COMPANY'S FIRST OFFER OF SEVERANCE IN
WHICH TO CONSIDER AND ACCEPT THE TERMS OF THIS AGREEMENT BY SIGNING BELOW AND
RETURNING IT TO DR. MICHAEL KAUFFMAN AT PREDIX PHARMACEUTICALS, 10K GILL ST,
WOBURN, MA, 01801. YOU AGREE THAT THE MATERIAL CHANGES REFLECTED IN THIS
AGREEMENT DO NOT RESTART THE 21-DAY CONSIDERATION PERIOD. IN ADDITION, YOU MAY
RESCIND YOUR ASSENT TO THIS AGREEMENT IF, WITHIN SEVEN (7) DAYS AFTER YOU SIGN
THIS AGREEMENT (THE

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"RESCISSION PERIOD"), YOU DELIVER BY HAND OR SEND BY MAIL (CERTIFIED, RETURN
RECEIPT AND POSTMARKED WITHIN SUCH SEVEN (7) DAY PERIOD) A NOTICE OF RESCISSION
TO DR. MICHAEL KAUFFINAN AT THE ABOVE-REFERENCED ADDRESS.

      Also, consistent with the provisions of the ADEA and other federal
discrimination laws, nothing in this release shall be deemed to prohibit you
from challenging the validity of this release under the federal age or other
discrimination laws (the "Federal Discrimination Laws") or from filing a charge
or complaint of age or other employment related discrimination with the Equal
Employment Opportunity Commission ("EEOC"), or from participating in any
investigation or proceeding conducted by the EEOC. Further, nothing in this
release or Agreement shall be deemed to limit the Company's right to seek
immediate dismissal of such charge or complaint on the basis that your signing
of this Agreement constitutes a full release of any individual rights under the
Federal Discrimination Laws, or to seek restitution to the extent permitted by
law of the economic benefits provided to you under this Agreement in the event
that you successfully challenge the validity of this release and prevail in any
claim under the Federal Discrimination Laws.

      7. ENTIRE AGREEMENT/MODIFICATION/CHOICE OF LAW/ENFORCEABILITY. You
acknowledge and agree that, except for your Stock Option Agreements and
Employment Agreement, specifically Sections 5, 6, 7, 8, 9, 10, 11, and 14, this
Agreement supersedes any and all prior or contemporaneous oral and/or written
agreements between you and the Company with respect to your employment with
Predix Pharmaceuticals, and sets forth the entire agreement between you and
Predix Pharmaceuticals. No variations or modifications hereof shall be deemed
valid unless reduced to writing and signed by the parties hereto. The failure of
the Company to seek enforcement of any provision of this Agreement in any
instance or for any period of time shall not be construed as a waiver of such
provision or of the Company's right to seek enforcement of such provision in the
future. This Agreement shall be deemed to take effect as an instrument under
seal and shall be governed by and construed in accordance with the laws of the
state of Massachusetts, without giving effect to conflict of law principles. You
agree that any action, demand, claim or counterclaim relating to the terms and
provisions of this Agreement, or to its breach, shall be commenced in
Massachusetts Superior Court, Middlesex County, and that the parties agree that
specific venue in the Business Litigation Session (located in Suffolk County)
will be sought, and you further acknowledge that venue for such actions shall
lie exclusively in Massachusetts and that material witnesses and documents would
be located in Massachusetts. The provisions of this Agreement are severable and
if for any reason any part the Agreement shall be found to be unenforceable, the
remaining provisions shall be enforced in full except that if your release of
claims against the Company contained in Section 6 is found to be unenforceable,
then the entire agreement is null and void.

      By executing this Agreement, you are acknowledging that you have been
afforded sufficient time to understand the terms and effects of this Agreement,
that your agreements and obligations hereunder are made voluntarily, knowingly
and without duress, and that neither the Company nor its agents or
representatives have made any representations inconsistent with the provisions
of this Agreement.

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      The parties agree that the Agreement may be signed on one or more copies,
each of which when signed will be deemed to be an original, and all of which
together will constitute one and the same Agreement.

              [The rest of this page is intentionally left blank.]

      Predix (CEO, BOD, Sr. Management and Staff) will cooperate and allow the
publication of papers under preparation by Dale Dhanoa particularly on the
discovery of PRX-00023, Series, PRX-03140 and Series and PRX-08066, for 5-HT1A
Agonist, 5-HT4 Agonist and 5-HT2B Antagonists. Including any contributions to
1Kv4.3 ion channel program and others. Laptop is waived from returning to
Predix.

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      Please sign, date and return the enclosed copy of this Agreement to Dr.
Michael Kauffman at the address noted above by February 21, 2005.

                                   Sincerely,

                                   Predix Pharmaceuticals

                                   By:  /s/ MICHAEL KAUFFMAN, MD
                                        ----------------------------------------
                                   Dr. Michael Kauffman, MD, PhD

                                   Dated: January 31, 2005

      Confirmed, Agreed and Acknowledged:

      /s/ DALE DHANOA
      ------------------------
      Dr. Dale Dhanoa

      Dated: February 5, 2005

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