Document:

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                                                                   Exhibit 10.45

                              LINCOLN STREET TRUST
                              POST OFFICE BOX 9198
                       WALTHAM, MASSACHUSETTS 02254-9198

                                    July 30, 1999

Immulogic Pharmaceutical Corporation
610 Lincoln Street
Waltham, Massachusetts 02451

Re:      Letter Agreement regarding Tenant Improvements, Insurance,
         and Reimbursement of Landlord's Expenses
         -------------------------------------------------------------

Gentlemen:

     Reference is hereby made to that certain Lease dated November 29, 1991 and
an amendment thereto dated May 8, 1992 (as amended, the "Lease") between Lincoln
Street Trust as landlord ("Landlord") and Immulogic Pharmaceutical Corporation
as tenant ("Immulogic") whereby Immulogic leased the entire building known and
numbered as 610 Lincoln Street, Waltham, Massachusetts. All terms not defined
herein have the meaning ascribed to them in the Lease.

     In connection with the assignment of Immulogic's interest in the Lease to
Scriptgen Pharmaceuticals, Inc. ("Scriptgen") by virtue of the Second Amendment
to Lease, of even date herewith, among Landlord, Immulogic and Scriptgen,
Landlord and Immulogic agree as follows:

     1.   Commencing August 1, 1999, Landlord shall pay Immulogic a sum equal to
          $54,399.00 monthly during the remaining term of the Lease but only if,
          as and when Landlord has received from Scriptgen (or any successor
          tenant to Scriptgen under the Lease) the monthly installment of Annual
          Fixed Rent, together with all other charges and payments then due and
          payable under the Lease. Landlord shall pay such sum to Immulogic
          within fifteen (15) days of Landlord's receipt thereof.

     2.   Immulogic shall obtain, at its sole cost and expense, an insurance
          policy insuring Landlord, its beneficiary, and their trustees,
          employees, agents and

                                       1
<PAGE>   2

Immulogic Pharmaceutical Corporation
July __, 1999
Page 2

          contractors against any and all loss, cost, damage, expense, claims,
          or liability arising out of any release of hazardous substances as
          defined in Section 5.2 of the Lease caused by Immulogic, its agents,
          employees, invitees, licensees or contractors. The insurance shall be
          for a term of five (5) years in an amount not less than Five Million
          Dollars from an insurance company and in form and substance reasonably
          satisfactory to Landlord. Until such time as Immulogic has procured
          such policy and Landlord has approved same, the sums due and payable
          pursuant to paragraph 1 above shall be retained and held in escrow by
          the Landlord (the "Retained Funds"). If such insurance policy is not
          delivered within ninety (90) days of the date hereof, Immulogic
          forfeits all right, title and interest to the Retained Funds and all
          other amounts due under paragraph 1 above and Landlord shall terminate
          the escrow and shall keep the Retained Funds and all other amounts due
          under paragraph 1 above.

     3.   Immulogic has paid to Landlord as of the date hereof the sum of
          $4,000.00 as reimbursement to Landlord for legal expenses and
          disbursements incurred by Landlord in connection with this
          transaction.

This lease agreement shall be binding upon and for the benefit of the parties'
successors and assigns.

                                             LINCOLN STREET TRUST, Landlord

                                             By: /s/ Michael D. Bank
                                                ------------------------------
                                                   Name: Michael D. Bank
                                                        ----------------------
                                                   Title: Managing Trustee
                                                         ---------------------

Agreed and acknowledged:

IMMULOGIC PHARMACEUTICAL
CORPORATION

By: /s/ J. Richard Crowley
   ------------------------------
      Name: J. Richard Crowley
           ----------------------
      Title: President
            ---------------------

                                       2<PAGE>   1

                                  Exhibit 10.10

                             SUMMIT TECHNOLOGY, INC.

                        1991 EMPLOYEE STOCK PURCHASE PLAN
                       (as amended through July 28, 1999)

ARTICLE 1:  PURPOSE.

         The purpose of this 1991 Employee Stock Purchase Plan (the "Plan") is
to advance the interests of Summit Technology, Inc. (the "Company") by
encouraging and enabling the acquisition of a larger personal proprietary
interest in the Company by all eligible employees of the Company and of the
participating subsidiaries of the Company. The Plan is designed to encourage
eligible employees to remain in the employ of the Company and have a personal
interest in the success of the Company. The options issued pursuant to this Plan
are intended to constitute options issued pursuant to an "employee stock
purchase plan" as defined in Section 423(b) of the Internal Revenue Code of
1986, as amended (the "Code").

ARTICLE 2:  ADMINISTRATION OF THE PLAN.

         The Plan shall be administered by the Company's Compensation Committee.
Subject to the provisions of the Plan, the Compensation Committee shall have the
complete authority, in its discretion, to interpret the Plan, to ascribe, amend
and rescind rules and regulations relating to the Plan, and to make all of the
determinations necessary or advisable for the administration of the Plan.

         The Compensation Committee shall consist of at least two members of the
Board, each of whom shall be a disinterested person as defined in Rule 16b-3
under the Securities Exchange Act of 1934, as such Rule may be hereafter
amended. The Compensation Committee shall be appointed by the Board, which may
at any time, and from time to time, remove any member of the Compensation
Committee, with or without cause, appoint additional members to the Compensation
Committee and fill vacancies, however caused, in the Compensation Committee. A
majority of the members of the Compensation Committee shall constitute a quorum.
All determinations of the Compensation Committee shall be made by a majority of
its members. Any decision or determination of the Compensation Committee reduced
to writing and signed by all members of the Compensation Committee shall be
fully effective as if it had been made at a meeting duly called and held.

ARTICLE 3:  ELIGIBLE EMPLOYEES DEFINED.

         As used in the Plan, the phrase "eligible employees" means all common
law employees of the Company and of the participating subsidiaries of the
Company, except the following shall not be deemed "employees" for purposes of
the Plan: (a) employees who have been employed by the Company, or its
subsidiaries, for less than 90 days, (b) employees customarily employed for less
than 20 hours per week, and (c) employees customarily employed for less than 5
months per year. All eligible employees shall have the same rights and
obligations under this Plan. Persons who are eligible employees on the first day
of a Payment Period shall receive their options on such day. Persons who become
eligible employees during the course of a Payment Period shall first receive
their options on the first day of the next succeeding Payment Period. However,
notwithstanding any other provision of this ARTICLE 3, an employee shall not be
deemed an eligible employee for purposes of the Plan if, immediately after the
option is granted, such employee owns stock possessing 5% or more of the total
combined voting power or value of all classes of the stock of the Company, or of
any participating subsidiary of the Company. For purposes of determining this 5%
limitation, the rules of Code Section 424(d) shall apply when determining the
stock ownership of an individual, and stock which the employee may purchase
under outstanding options shall be treated as stock owned by the employee.
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ARTICLE 4:  STOCK SUBJECT TO THE PLAN.

         The Stock subject to the options under the Plan shall be shares of the
Company's authorized but unissued common stock, par value $.01 per share
("Stock"), or shares of Stock reacquired by the Company, including shares
purchased in the open market. The aggregate number of shares which may be issued
pursuant to the Plan is 375,000, subject to adjustment as provided in ARTICLE
12. In the event that any option granted under the Plan shall expire or
terminate for any reason without having been exercised in full or shall cease
for any reason to be exercisable in whole or in part, the unpurchased shares
subject thereto shall again be available under the Plan.

ARTICLE 5:  PAYMENT PERIODS AND STOCK OPTIONS.

         The six-month periods, October 1 to March 31 and April 1 to September
30, are Payment Periods during which payroll deductions will be accumulated
under the Plan. To the extent any of these days is not a business day, the
Payment Period shall begin or end, as the case may be, on the first business day
following such date. The first Payment Period under the Plan will begin on
October 1, 1999.

         Twice each year, on the first business day of each Payment Period, the
Company will grant to each eligible employee who is then a participant in the
Plan an option to purchase on the last day of such Payment Period, at the Option
Price hereinafter provided for, a maximum of the number of shares of Stock
equaling the number of whole shares obtained by dividing 5% of the eligible
employee's yearly wages (or such lesser percentage as the employee shall
designate pursuant to Article 8) by the average market price on the first day of
the Payment Period (the "Maximum Share Limit"), on condition that such employee
remains eligible to participate in the Plan throughout such Payment Period. The
participant shall be entitled to exercise such option so granted only to the
extent of the participant's accumulated payroll deductions on the last day of
such Payment Period. In the event that the participant's accumulated payroll
deductions on the last day of the Payment Period would enable the participant to
purchase more than the Maximum Share Limit were it not for the Maximum Share
Limit, the excess of the amount of the accumulated payroll deductions over the
aggregate purchase price of the Maximum Share Limit shall be promptly refunded
to the participant by the Company, with such interest as is provided in ARTICLE
8.

         The Option Price for each Payment Period shall be the lesser of (i) 85%
of the closing price of the Stock on the first business day of the Payment
Period, or (ii) 85% of the closing price of the Stock on the last business day
of the Payment Period, in either event rounded up to avoid fractions of a dollar
other than 1/4, 1/2 and 3/4. The foregoing limitation on the number of shares
which may be granted in any Payment Period and the Option Price per share shall
be subject to adjustment as provided in ARTICLE 12.

         For purposes of this Plan, the term "closing price" on any day means
(i) the last reported sale price of the Stock on that day on the principal
national securities exchange on which the Stock is traded, if the Stock is then
traded on a national securities exchange; or (ii) the last reported sale price
(on that date) of the Stock on the Nasdaq National Market List, if the Stock is
not then traded on a national securities exchange; or (iii) the average of the
closing bid and asked prices last quoted (on that date) by an established
quotation service for over-the-counter securities, if the Stock is not reported
on a national securities exchange or on the NASDAQ National Market List. If the
Stock is not publicly traded at the time the option is granted under this Plan,
"closing price" shall mean the fair market value of the Stock as determined by
the Compensation Committee after taking into consideration all the factors which
it deems appropriate, including, without limitation, recent sale and offer
prices of the Stock in private transactions negotiated at arm's length.

         For purposes of this Plan, the term "business day" means a day on which
there is trading on the NASDAQ National Market System or on the aforementioned
national securities exchange, whichever is applicable pursuant to the preceding
paragraph.

         No employee shall be granted an option which permits the employee's
right to purchase Stock under the Plan, and under all other Section 423(b)
employee stock purchase
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plans of the Company or any parent or subsidiary corporations, to accrue at a
rate which exceeds $25,000 of fair market value of Stock (determined at the time
such option is granted) for each calendar year in which such option is
outstanding at any time. The purpose of the limitation in the preceding sentence
is to comply with Code Section 423(b)(8).

ARTICLE 6:  EXERCISE OF OPTION.

         Each eligible employee who continues to be a participant in the Plan on
the last business day of a Payment Period shall be deemed to have exercised
his/her option on such date and shall be deemed to have purchased from the
Company such number of full shares of Stock reserved for the purpose of the Plan
as his/her accumulated payroll deductions plus additional payments on such date
will pay for at the Option Price, subject to the Maximum Share Limit on the
option. If a participant is not an employee on the last business day of a
Payment Period, he/she shall not be entitled to exercise his/her option. Only
full shares of Stock may be purchased under the Plan. Unused payroll deductions
remaining in an employee's account at the end of a Payment Period (other than
amounts refunded to the employee pursuant to ARTICLE 5) will be carried forward
to the succeeding Payment Period.

ARTICLE 7:  AUTHORIZATION FOR ENTERING THE PLAN.

         The Compensation Committee shall develop regulations concerning the
procedure which each employee must follow to become a participant of the Plan.

ARTICLE 8:  AMOUNT OF PAYROLL DEDUCTIONS.

         Subject to the limitations on stock purchases set forth in this Plan,
at the time specified in regulations issued pursuant to ARTICLE 7 an eligible
employee may designate the percentage of his/her wages during each Payment
Period that the employee wishes to have deducted and applied to the Plan;
provided, however, that the Compensation Committee may set such minimum and/or
maximum payroll deductions as the Compensation Committee, in its sole
discretion, deems appropriate. Any minimum and/or maximum payroll deductions
mandated by the Compensation Committee shall apply equally to all eligible
employees.

         The Company will accumulate and hold for the employee's account the
amounts deducted from his/her pay. No interest will be paid on these amounts,
unless the Compensation Committee, in its sole discretion, determines that
interest should be paid on the amounts deducted from eligible employees' wages.
If the Compensation Committee decides that interest should be paid on the
amounts deducted from eligible employees' wages, the Compensation Committee may
set that interest rate at whatever rate the Compensation Committee, in its sole
discretion, deems appropriate, and may change that rate at whatever time, and to
whatever rate, the Compensation Committee, in its sole discretion, deems
appropriate.

ARTICLE 9:  CHANGE IN PAYROLL DEDUCTIONS.

         Deductions may not be increased or decreased during a Payment Period.
However, an employee may withdraw in full from the Plan, to the extent such
withdrawal is permitted by ARTICLE 10 of the Plan.

ARTICLE 10:  WITHDRAWAL FROM THE PLAN.

         An employee may withdraw from the Plan, in whole but not in part, by
delivering a withdrawal notice to the Compensation Committee not less than 10
days prior to the last business day of a Payment Period, in which event the
Compensation Committee shall promptly refund without interest the entire balance
of the employee's payroll deductions not previously used to purchase Stock under
the Plan. An employee who is an officer or director of the Company who withdraws
from the Plan may not participate in the Plan during the next succeeding Payment
Period.
<PAGE>   4
ARTICLE 11:  ISSUANCE OF STOCK.

         Certificates for Stock issued to participants will be delivered as soon
as practicable after each Payment Period by the Company's transfer agent.

         Stock purchased under the Plan will be issued only in the name of the
employee, or if his/her authorization so specifies, in the name of the employee
and another person of legal age as joint tenants with rights of survivorship.

ARTICLE 12:  ADJUSTMENTS.

         Upon the happening of any of the following described events, an
optionee's rights under options granted under the Plan shall be adjusted as
hereinafter provided:

                  A. In the event shares of Stock shall be subdivided or
         combined into a greater or smaller number of shares or if, upon a
         reorganization, split-up, liquidation, recapitalization or the like of
         the Company, the shares of the Stock shall be exchanged for other
         securities of the Company, each optionee shall be entitled, subject to
         the conditions herein stated, to purchase such number of shares of
         stock or amount of other securities of the Company as were exchangeable
         for the number of shares of Stock which such optionee would have been
         entitled to purchase except for such action, and appropriate
         adjustments shall be made in the purchase price per share to reflect
         such subdivision, combination or exchange; and

                  B. In the event the Company shall issue any of its shares as a
         stock dividend upon or with respect to the Stock, each optionee upon
         exercising an option shall be entitled to receive (for the purchase
         price paid upon such exercise) the shares as to which he/she is
         exercising his/her option and, in addition thereto (at no additional
         cost), such number of shares of the class or classes in which such
         stock dividend or dividends were declared or paid, and such amount of
         cash in lieu of fractional shares, as is equal to the number of shares
         thereof and the amount of cash in lieu of fractional shares,
         respectively, which he/she would have received if he/she had been the
         holder of the shares as to which he/she is exercising his/her option at
         all times between the date of the granting of such option and the date
         of its exercise.

         Upon the happening of any of the foregoing events, the class and
aggregate number of shares set forth in ARTICLE 4 hereof which are subject to
options which have been or may be granted under the Plan and the Maximum Share
Limit set forth in ARTICLE 5 shall also be appropriately adjusted to reflect the
events specified in paragraphs A and B above. Notwithstanding the foregoing, any
adjustments made pursuant to paragraphs A and B shall be made only to the extent
that the Compensation Committee, based on advice of counsel for the Company,
determines that such adjustments will not constitute a change requiring
stockholder approval under Code Section 423(b)(2).

         If the Company is to be consolidated with or acquired by another entity
in a merger, or undergo a sale of all or substantially all of the Company's
assets or otherwise (all such events collectively referred to as an
"Acquisition"), the Compensation Committee shall, with respect to options then
outstanding under this Plan, (i) make appropriate provision for the continuation
of such options by arranging for the substitution on an equitable basis for the
shares then subject to such options the consideration payable with respect to
the outstanding shares of the Stock in connection with the Acquisition; (ii)
terminate all outstanding options in exchange for a cash payment equal to the
excess of the fair market value of the shares subject to the options (determined
as of the date of the Acquisition) over the Option Price thereof (determined
with reference only to the first business day of the applicable Payment Period)
if such fair market value exceeds such Option Price; or (iii) terminate all
outstanding options and promptly refund without interest the balance of employee
deductions not used to purchase Stock under the Plan in the event that the fair
market value of the shares subject to the options (determined as of the date of
the Acquisition) is less than the Option Price thereof (determined with
reference only to the first business day of the applicable Payment Period).
<PAGE>   5
         The Compensation Committee shall determine the adjustments to be made
under this ARTICLE 12, and its determination shall be conclusive.

ARTICLE 13:  NO TRANSFER OR ASSIGNMENT OF EMPLOYEE'S RIGHTS.

         An employee's option rights under the Plan are the employee's alone and
may not be transferred or assigned to, or availed of by, any other person other
than by will or the laws of descent and distribution. Any option granted under
the Plan to an employee may be exercised, during the employee's lifetime, only
by the employee.

ARTICLE 14:  TERMINATION OF EMPLOYEE'S RIGHTS.

         An employee's rights under the Plan will terminate when he/she ceases
to be an employee of the Company or of a participating subsidiary of the Company
(other than as a result of death or an Approved Leave of Absence, as defined
below). A withdrawal notice will be considered as having been received from the
employee on the day his/her employment so ceases, and all payroll deductions not
used to purchase Stock will be refunded without interest.

         If an employee's payroll deductions are interrupted by any legal
process, a withdrawal notice will be considered as having been received from the
employee on the day the interruption occurs.

ARTICLE 15:  DEATH OF AN ELIGIBLE EMPLOYEE.

         Upon an eligible employee's death, his or her beneficiary may elect to
(i) withdraw the accumulated balance of the employee's payroll deductions during
a Payment Period, or (ii) apply such balance at the end of the Payment Period to
the purchase of Stock. The beneficiary must make this election by written notice
to the Board of Directors within the earlier of 60 days of the participant's
death, or the end of the Payment Period. Failure to make a timely election in
accordance with this Article shall result in the application of the balance of
the employee's payroll deductions to the purchase of Stock. The Company shall
not be obligated to deliver any Stock or cash to a beneficiary until the Company
shall have received proof satisfactory to the Company of his or her identity,
existence and valid designation.

ARTICLE 16:  APPROVED LEAVES OF ABSENCE.

         An eligible employee may continue to participate in the Plan during a
leave of absence from his or her employment with the Company or a participating
subsidiary of the Company for which he or she obtains prior approval (an
"Approved Leave of Absence"). During an Approved Leave of Absence that is paid,
an employee's participation in the Plan will continue in the same manner as
before such leave of absence began. An Approved Leave of Absence that is paid
shall mean one in which regular payroll disbursements to the eligible employee
from the Company or a participating subsidiary of the Company, and payroll
deductions under the Plan, continue to occur as before such leave of absence.
During all other Approved Leaves of Absence, an eligible employee may (in lieu
of payroll deductions) make contributions under the Plan from disability, sick
or vacation pay, or in cash, for the first 90 days of the Approved Leave of
Absence; provided, however, that the aggregate amount of such contributions made
during this period shall not exceed the aggregate amount of payroll deductions
that otherwise would be permitted during this period under the Plan. After this
period, a participant may make no further contributions under the Plan until the
Approved Leave of Absence has ended and he or she has returned to employment
with the Company or a participating subsidiary of the Company.

ARTICLE 17:  TERMINATION AND AMENDMENTS TO PLAN.

         The Plan may be terminated at any time by the Company's Board of
Directors but such termination shall not affect options then outstanding under
the Plan. The Plan will terminate in any case when all or substantially all of
the unissued (or issued but not outstanding) shares of Stock reserved for the
purposes of the Plan have been purchased. If at any time shares of Stock
reserved for the purpose of the Plan remain available for purchase but not in
sufficient number to satisfy all then unfilled purchase requirements,
<PAGE>   6
the available shares shall be apportioned among participants in proportion to
their options and the Plan shall terminate. Upon such termination or any other
termination of the Plan, all payroll deductions not used to purchase Stock will
be refunded without interest.

         The Board of Directors may from time to time adopt amendments to the
Plan provided that, without the approval of the shareholders of the Company, no
amendment may materially increase the benefits accruing to participants under
the Plan, materially increase the number of shares that may be issued under the
Plan, or materially modify the requirements for eligibility for participation in
the Plan.

ARTICLE 18:  LIMITS ON SALE OF STOCK PURCHASED UNDER THE PLAN.

         The Plan is intended to provide shares of Stock for investment and not
for resale. SHARES OF STOCK PURCHASED UNDER THE PLAN MAY NOT BE SOLD,
TRANSFERRED OR ASSIGNED FOR 90 DAYS FROM THE DATE OF PURCHASE. Except for such
limitation, the Company does not intend to restrict or influence any employee in
the conduct of his/her own affairs. An employee may, therefore, sell Stock
purchased under the Plan at any time the employee chooses after 90 days from the
date of purchase, subject to compliance with any applicable Federal or state
securities laws; provided, however, that because of certain Federal tax
requirements, each employee agrees by entering the Plan promptly to give the
Company notice of any such stock disposed of within two years after the date of
grant of the applicable option showing the number of such shares disposed of.
THE EMPLOYEE ASSUMES THE RISK OF ANY MARKET FLUCTUATIONS IN THE PRICE OF THE
STOCK.

ARTICLE 19:  PARTICIPATING SUBSIDIARIES.

         The term "participating subsidiary" shall mean any subsidiary of the
Company, as that term is defined in Section 425(f) of the Code, which is
designated from time to time, by the Board of Directors to participate in the
Plan. The Board of Directors shall have the power to make such designation
before or after the Plan is approved by the shareholders.

ARTICLE 20:  OPTIONEES NOT SHAREHOLDERS.

         Neither the granting of an option to an employee nor the deductions
from his/her pay shall constitute such employee a shareholder of the shares
covered by an option until such shares have been actually purchased by the
employee.

ARTICLE 21:  APPLICATION OF FUNDS.

         The proceeds received by the Company from the sale of Stock pursuant to
options granted under the Plan will be used for general corporate purposes.

ARTICLE 22:  GOVERNMENTAL REGULATIONS.

         The Company's obligation to sell and deliver shares of the Stock under
this Plan is subject to the approval of any governmental authority required in
connection with the authorization, issuance or sale of such shares, including
the Securities and Exchange Commission and the Internal Revenue Service.

ARTICLE 23:  APPROVAL OF SHAREHOLDERS.

         The Plan shall be subject to approval by the holders of a majority of
the shares of the Company present or represented by proxy at a duly called
meeting of shareholders, which approval must occur within twelve months before
or after the date on which this Plan was adopted by the Board of Directors.
Prior to such approval no shares shall be issued to participants under the Plan.
In the event that the approval of the shareholders is not received within twelve
months before or after the date of the adoption of this Plan, any and all
options previously granted shall be rescinded, and the Company will promptly
refund the entire balance of each participating employee's deductions without
interest.

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