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                                                                   EXHIBIT 10.12

                              [NITROMED LETTERHEAD]

                                  July 14, 1997

Michael D. Loberg, Ph.D.
1770 Dolington Road
New Town, PA 18940

Dear Michael:

We would like to formally offer you the position of Chief Executive Officer of
NitroMed. I know you share with us the enthusiasm for building an excellent and
successful company. I should add that the employees of the Company, the
scientific founder, and the board of directors are very excited about your
joining the Company.

The terms of the offer arc described below:

     Start Date:              On or before September 1, 1997

     Reporting to:            Board of Directors

     Base Salary:             $245,000, to be reviewed annually and adjusted by
                              the Board of Directors to reflect performance.

     Performance Bonus:       Annual bonus based upon performance and the
                              achievement of milestones mutually agreed upon by
                              you and the Board of Directors in an amount up to
                              35% of base salary.

     Stock:                   Options granted under the Company's stock option
                              plan of 350,000 -- 5% of the outstanding shares of
                              the Company on a fully diluted basis. These
                              options would vest at 20% per year over four
                              years, with the initial 20% vesting to occur on
                              your start date and subsequent vesting on each of
                              the next four anniversaries thereof. The exercise
                              price of these options will be the fair market
                              value as set by the Board of Directors on the day
                              you commence employment. The Company commits to
                              directionally correcting this position following
                              the next round of financing. In the event that the
                              Company is sold or is a party to a merger in which
                              the stockholders of the Company do not
                              collectively own a majority of the stock of the
                              surviving entity [ a "sale of the Company"], your
                              non-vested options will become 50% vested.
                              Additionally, if your employment with the Company
                              is terminated for reasons other than cause within
                              12 months after a sale of the Company, the
                              remainder of

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                              your non-vested options will become 100% vested.

     Severance:               Should your employment with the Company be
                              terminated for reasons other than cause, you will
                              receive six months' base salary as termination
                              payment.

     Agreements:              As a condition of employment, you will be required
                              to sign the Company's invention, non-disclosure
                              and non-compete agreements.

     Moving Costs:            The Company will reimburse you up to $100,000 for
                              all reasonable expenses incurred in the sale of
                              your current house, removal of your household
                              goods, and expenses incurred in the purchase of a
                              home in the Boston area.

This offer is open through July 22, 1997 and is subject to a satisfactory
pre-employment physical exam including drug screen and review by your attorneys.

I am personally very much looking forward to working with you and having you as
part of the NitroMed team.

Sincerely

/s/ Mark Leschly
-----------------
Mark Leschly
Director

                                          Agreed and Accepted by:

                                          /s/ Michel D. Loberg
                                          -----------------------------
                                          Michael D. Loberg, Ph.D.<Page>

                                                                   EXHIBIT 10.13

                              [NITROMED LETTERHEAD]

NitroMed, Inc.                                        Maria I Marmarinos
One Kendall Square                                    EXECUTIVE VICE PRESIDENT
Building 100                                          Phone: 617-374-9074
Cambridge, MA 02139                                   Facsimile: 617-374-9173

July 29, 1993

Manuel Worcel, M.D.
46 Pomeroy Road
Madison, New Jersey 07940

Dear Dr. Worcel:

We would like to formally offer you the position of President and C.E.O. of
NitroMed. I know you share with us the enthusiasm of building a company around
the work of Joe Loscalzo and Jonathan Stamler.

As C.E.O. you would be responsible for all aspects of the Company and its
operations. You will report to the Board of Directors.

The terms of the offer are described below:

     Base Salary:             $190,000 per annum paid in 24 equal payments.

     Performance Bonus:       Cash bonus based upon performance of 0-20% of base
                              salary paid at year end. For the first year 10%
                              will be guaranteed. In the event your employment
                              terminates due to a sale of the Company prior to
                              July 1 of any year, you will receive 50% of the
                              bonus you would have received for that year. In
                              the event your employment terminates due to a sale
                              of the Company after July 1 of any year, you will
                              receive the entire bonus that you would have
                              received for that year.

     Stock:                   You will be offered options to purchase 325,000
                              shares of common stock of the Company. The
                              exercise price of these options will be set by the
                              Board of Directors and is expected to be
                              $.01/share. These options will vest over four
                              years in equal installments at the anniversary of
                              your joining the Company. These options must be
                              exercised within 90 days after any termination of
                              your employment unless termination is for cause.
                              Vesting of these options will be accelerated if
                              the Company is sold and as a result you will not
                              be employed after the acquisition.

     Group Benefits:          You will receive comprehensive group health,
                              disability,

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                              accident and life insurance benefits in line with
                              the attached plan of Guardian or such other
                              comparable benefits and protection as the Company
                              is able to reasonably obtain from another plan.
                              You will be entitled to participate in and enjoy
                              the benefits of a retirement and pension plan or
                              plans as and when instituted by the Company for
                              the benefit of its employees generally. You will
                              also be entitled to any other benefits as may be
                              deemed by the Board of Directors to be appropriate
                              to the position held by you as CEO and to the
                              discharge of your duties.

     Leave:                   You will be entitled to illness and vacation days
                              consistent with the standard policies to be
                              developed for the Company's executive employees
                              and any such additional leave benefits which the
                              Board of Directors may deem appropriate.

     Agreements:              As a condition of employment, you will be required
                              to sign customary invention, non-disclosure and
                              non-compete agreements.

     Relocation Costs:        The Company will reimburse you for all reasonable
                              expenses incurred in the sale of your current
                              house; the purchase of a home in the Boston area;
                              the removal, storage, insuring, moving and
                              unpacking of your house-hold goods and
                              automobiles, house hunting expenses, and rental of
                              temporary living quarters.

     Severance Compensation:  If your employment is terminated by the Company
                              without cause, you will be entitled to receive up
                              to six months continued base salary payments.

This offer is open through July 30th, and is subject to a satisfactory
pre-employment physical exam, including drug screen and Bishops Report and to my
final reference checks, which I hope to complete by next week.

We would like to move forward as soon as possible and would like to suggest an
early start date. I personally very much look forward to working with you.

Sincerely

/s/ John W. Littlechild
-------------------------
John W. Littlechild
President                                      Agreed and Accepted by:

                                               /s/ Manuel Worcel, MD
                                               --------------------------
                                               Manuel Worcel, M.D.

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