Exhibit 10.28

February 7, 2008

Curtis Keith

115 Pembroke St., Apt. #3

Boston, MA 02118

Dear Curtis:

This letter agreement is regarding your outstanding options to purchase shares
of common stock of CombinatoRx, Incorporated (the “Company”) and changes to the
Stock Option Agreements (“Agreements”) previously entered into from time to time
between you and the Company.

 

  1. As of your last day of employment with the Company, you have vested and
exercisable options to purchase 272,135 (two hundred seventy-two thousand one
hundred thirty-five) shares of the Company’s common stock (the “Vested
Options”). No further vesting shall take place for these previously granted
options and the maximum number of shares which may be purchased by you pursuant
to the Agreements is 272,135 (two hundred seventy-two thousand one hundred
thirty-five) shares of common stock.

 

  2. The Agreements are all hereby modified such that the requirements of
continuous employment set forth in the Agreements (solely for purposes of
exercising the Vested Options) are hereby amended to substitute your continuous
involvement as a member of the CombinatoRx Scientific Advisory Board or
engagement as a consultant to the Company as the service requirement. As a
result, whenever your continuous involvement with the Scientific Advisory Board
or engagement as a consultant to the Company ends, you will have 90 days to
exercise the Vested Options. If the Vested Options are unexercised within 90
days thereafter, the vested and unexercised options will be irrevocably
cancelled.

 

  3. Notwithstanding the provisions of any section of the Agreements, the
Agreements shall now be deemed not to be an option qualifying as an incentive
stock options within the meaning of Section 422 of the Internal Revenue Code of
1986, as amended or replaced from time to time.

 

  4. As modified hereby, the Agreements shall continue in force in accordance
with their terms.

If you agree to the terms of this letter, please countersign one copy of this
letter and return it in the enclosed stamped envelope.

Best regards,

 

/s/ Alexis Borisy

Alexis Borisy President and Chief Executive Officer

 

Agreed by:  

/s/ Curtis Keith

  Curtis Keith