Document:

<PAGE>

                                                                        EX-10.42

                            FIRST AMENDMENT TO LEASE

         This First Amendment to Lease ("FIRST AMENDMENT") is made as of
September 12, 2000, between ARE-10933 NORTH TORREY PINES, LLC, a Delaware
limited liability company ("LANDLORD"), and IDEC PHARMACEUTICALS CORPORATION, a
Delaware corporation ("TENANT"), with reference to the following Recitals.

                                    RECITALS

         A. Landlord and Tenant entered into that certain Lease Agreement dated
as of June 24, 1999 (the "LEASE"), pursuant to which Tenant leases certain
premises (the "PREMISES") located at 3010 Science Park Road, San Diego,
California and more particularly described in the Lease. Initially capitalized
terms used herein without definition shall have the meanings defined for such
terms in the Lease unless the context clearly indicates otherwise.

         B. The rentable square footage of the Premises, the Building and the
Project has been measured as required pursuant to Section 5 of the Lease and
Landlord and Tenant desire to amend the Lease to reflect the results of such
measurement. Landlord and Tenant desire to also amend the Lease as set forth
below.

                                    AGREEMENT

         NOW, THEREFORE, in consideration of the foregoing Recitals, and for
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree that the Lease is amended as
follows:

1.       MEASUREMENT. Landlord and Tenant hereby acknowledge that they have
reviewed the results dated July 17, 2000, of the measurement of the Premises,
the Building and the Project by McGraw/Baldwin Architects. Landlord and Tenant
hereby acknowledge and agree that the Premises, the Building and the Project
have been measured by McGraw/Baldwin Architects pursuant to, and in accordance
with, the requirements set forth in Section 5 of the Lease.

2.       BASIC LEASE TERMS. Based on the results of such measurement of the
Premises, the Building and the Project, Landlord and Tenant hereby agree that,
effective as of the Commencement Date, the following basic Lease terms set forth
on page 1 of the Lease shall be amended and restated as follows:

PREMISES:         That portion of the Building, containing approximately 44,809
                  rentable square feet, as shown on EXHIBITS A-1 AND A-2,
                  together with an exclusive use hazardous materials storage pad
                  outside the Building, as shown on EXHIBIT B-1.

TENANT'S SHARE: 60.10%

RENTABLE AREA OF PREMISES: 44,809 sq. ft.

RENTABLE AREA OF PROJECT: 182,690 sq. ft.

RENTABLE AREA OF BUILDING: 74,557 sq. ft.

BUILDING'S SHARE OF PROJECT: 40.81%

                                           [Science Park - IDEC Pharmaceuticals]

                                       1

<PAGE>

Landlord and Tenant hereby agree that Exhibit A-1, Exhibit A-2 and Exhibit B-1
to the Lease are hereby replaced with Exhibit A-1, Exhibit A-2 and Exhibit B-1
which are attached hereto and incorporated herein by this reference.

3.       TI ALLOWANCE. Landlord and Tenant acknowledge that pursuant to Section
5.2 of the Work Letter, Landlord agreed to provide to Tenant a TI Allowance of
$100 per rentable square foot of the Premises. Based on the results of the
measurement of the Premises, Landlord and Tenant hereby agree that the TI
Allowance is increased from $4,132,800 to $4,480,900 and that, subject to final
reconciliation of the TI Costs, Tenant shall be entitled to a refund of the
Excess TI Deposit in the amount of $280,900 which Tenant deposited with
Landlord.

4.       STORAGE AREA. Notwithstanding anything to the contrary contained in the
Lease, during the term of the Lease, Landlord hereby waives its right to collect
monthly rent from Tenant in connection with Tenant's use of the Storage Area and
the mechanical storage area located in the southwest corner of the basement of
the Building.

5.       MISCELLANEOUS.

         (a)      This First Amendment is the entire agreement between the
parties with respect to the subject matter hereof and supersedes all prior and
contemporaneous oral and written agreements and discussions. This First
Amendment may be amended only by an agreement in writing, signed by the parties
hereto.

         (b)      This First Amendment is binding upon and shall inure to the
benefit of the parties hereto, their respective successors and assigns.

         (c)      This First Amendment may be executed in any number of
counterparts, each of which shall be deemed an original, but all of which when
taken together shall constitute one and the same instrument. The signature page
of any counterpart may be detached therefrom without impairing the legal effect
of the signature(s) thereon provided such signature page is attached to any
other counterpart identical thereto except having additional signature pages
executed by other parties to this First Amendment attached thereto.

         (d)      Except as amended and/or modified by this First Amendment, the
Lease is hereby ratified and confirmed and all other terms of the Lease shall
remain in full force and effect, unaltered and unchanged by this First
Amendment. In the event of any conflict between the provisions of this First
Amendment and the provisions of the Lease, the provisions of this First
Amendment shall prevail. Whether or not specifically amended by this First
Amendment, all of the terms and provisions of the Lease are hereby amended to
the extent necessary to give effect to the purpose and intent of this First
Amendment.

                        [SIGNATURES ARE ON THE NEXT PAGE]

                                           [Science Park - IDEC Pharmaceuticals]

                                       2
<PAGE>

         IN WITNESS WHEREOF, this First Amendment to Lease has been duly
executed and delivered by Landlord and Tenant as of the date first above
written.

                                      TENANT:

                                      IDEC PHARMACEUTICALS CORPORATION,
                                      a Delaware corporation

                                      By: /s/ Phillip Schneider
                                          --------------------------------------
                                      Its: SVP & CFO

                                      LANDLORD:

                                      ARE -10933 NORTH TORREY PINES, LLC,
                                      a Delaware limited liability company

                                      By: ALEXANDRIA REAL ESTATE EQUITIES, INC.,
                                          a Maryland corporation, Managing
                                          Member

                                          By: /s/ [ILLEGIBLE]
                                              ----------------------------------
                                          Its: SENIOR VICE PRESIDENT

                                           [Science Park - IDEC Pharmaceuticals]

                                       3

<PAGE>

                                   EXHIBIT A-1

                               [FIRST FLOOR PLAN]

<PAGE>

                                   EXHIBIT A-2

                               [SECOND FLOOR PLAN]

<PAGE>

                                  EXHIBIT B-1

                                  THE PROJECT

                                  [SITE PLAN]<PAGE>

                                                                        EX-10.43

                            SECOND AMENDMENT TO LEASE

         This Second Amendment (the "SECOND AMENDMENT") to Lease is made as of
November 1, 2000, by and between ARE-10933 NORTH TORREY PINES, LLC, a Delaware
limited liability company, having an address at 135 North Los Robles Avenue,
Suite 250, Pasadena, California 91101 ("LANDLORD"), and IDEC Pharmaceuticals
Corporation, a Delaware corporation, having an address at 11011 Torreyana Road,
San Diego, California 92121 ("TENANT").

                                    RECITALS

         A.       Landlord and Tenant have entered into that certain Lease
Agreement dated as of June 24, 1999, as amended by that certain First Amendment
to Lease dated as of September 12, 2000 (as so amended, the "LEASE"), wherein
Landlord leased to Tenant certain premises (the "PREMISES") located at 3010
Science Park Road, San Diego, California and legally described on EXHIBIT A
attached thereto, and more particularly described in the Lease. Capitalized
terms used herein without definition shall have the meanings defined for such
terms in the Lease.

         B.       Tenant desires to lease the balance of the Building (the
"REMAINING SPACE") and Landlord is willing to lease such Remaining Space on the
terms and conditions set forth herein.

         C.       Landlord and Tenant desire to amend the Lease to, among other
things, add the Remaining Space to the Premises demised under the Lease and to
provide for the improvement of such Remaining Space.

                                    AGREEMENT

         Now, therefore, the parties hereto agree that the Lease is amended as
follows:

         1.       PREMISES. Effective as of November 1, 2000, the Premises
demised under the Lease are hereby expanded to include the Remaining Space,
consisting of three suites (each a "SUITE") containing an aggregate, for all
purposes of the Lease, of 29,748 rentable square feet ("RSF"), as follows:

         Suite A           9,618 RSF located on the 2nd floor of the Building
                           and depicted on EXHIBIT B-2, attached hereto and
                           incorporated herein by this reference

         Suite B           10,214 RSF located on the 1st floor of the Building
                           and depicted on EXHIBIT B-1, attached hereto and
                           incorporated herein by this reference

         Suite C           9,916 RSF located on the 1st floor of the Building
                           and depicted on EXHIBIT B-1, attached hereto and
                           incorporated herein by this reference

Following the addition of all of the Remaining Space, the Rentable Area of the
Project will be 182,690 RSF, the total Premises demised under the Lease will be
74,557 RSF, Tenant's Share of the Building will 100% and the Building's Share of
the Project will be 40.81%. The prorata share of each Suite added to the
Premises will be 12.90%, 13.70% and 13.30% of the Building for Suite A, Suite B
and Suite C, respectively.

         2.       RENT. Base Rent for the Remaining Space as of the date hereof
shall be $2.37 per month per RSF, which amount shall be subject to periodic
adjustment when and as provided in the Lease; provided, however, that Tenant's
obligation to pay Base Rent and Additional Rent on the Remaining Space shall be
abated until the earlier of (x) Tenant's use of any Suite for any purpose in the
conduct of Tenant's business ("OCCUPANCY") (provided that the installation and
testing of

                                       1
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Tenant's fixtures shall not constitute the conduct of business), or (y) the
satisfaction of the conditions described in clauses (ii) through (iv) of Section
3 hereof.

         3.       RENT COMMENCEMENT. Tenant may Occupy the three Suites
comprising the Remaining Space in such order as Tenant shall elect, provided
however, that (i) Occupancy of any portion of a Suite shall constitute Occupancy
of all of such Suite for purposes of the Lease and this Second Amendment, (ii)
if no Suite is Occupied on or before June 1, 2001, rent shall commence on Suite
A on June 1, 2001, and shall continue until the earliest of (A) Occupancy of a
Suite on or before August 1, 2001, (B) Occupancy of two Suites on or before
December 1, 2001, or (C) the expiration or earlier termination of the Lease,
(iii) if less than two Suites are Occupied on or before August 1, 2001, rent
shall commence on Suite B (or if Suite B is Occupied, Suite C) on August 1,
2001, and shall continue until the earlier of (D) Occupancy of two Suites on or
before December 1, 2001, or (E) the expiration or earlier termination of the
Lease, and (iv) if less than three Suites are Occupied on or before December 1,
2001, rent shall commence on the remaining Suite on December 1, 2001, and shall
continue until the expiration or earlier termination of the Lease. Upon the
satisfaction of the conditions described in clauses (A), (B) or (D) of the
preceding sentence, rent shall be based on the Suite(s) so Occupied.
Notwithstanding the foregoing, any Landlord Delay, as defined below, shall
extend the dates set forth in clauses (A), (B) and (D) of the preceding sentence
above, as applicable, on a day-for-day basis. "LANDLORD DELAY" shall mean any
delay, beyond the periods permitted under the Work Letter, for Landlord to give
or withhold its approval of any matter requiring Landlord's review and approval.

         4.       IMPROVEMENT OF REMAINING SPACE. Landlord shall provide a
Tenant Improvement Allowance of $100.00 per RSF of the Remaining Space, less the
cost per RSF applied to the central plant (the "CENTRAL PLANT COSTS") (such
amount, less the Central Plant Costs, the "TI ALLOWANCE"). The Central Plant
Costs shall be determined upon the final reconciliation of the tenant
improvement costs for the portion of the Building initially demised under the
Lease. Such TI Allowance shall be used to improve the Remaining Space as
described in the Work Letter attached hereto as EXHIBIT C.

         5.       PERMITTED USE. Tenant may, as a Permitted Use, elect to use a
portion of the Building of Tenant's selection, not to exceed 21,000 square feet,
for clinical manufacturing.

         6.       USE. Section 7 of the Lease is hereby modified with respect to
any Suite used for clinical manufacturing as follows: In the sentence in such
section which states "Tenant shall not place any equipment weighing 500 pounds
or more in or upon the Premises or transport or move such items through the
Common Areas or in the Building elevators without the prior written consent of
Landlord, which shall not be unreasonably withheld or delayed" the figure 500
shall be replaced by the figure 1,000.

         7.       MISCELLANEOUS.

                  (a)      This Second Amendment is the entire agreement between
         the parties with respect to the subject matter hereof and supersedes
         all prior and contemporaneous oral and written agreements and
         discussions. This Second Amendment may be amended only by an agreement
         in writing, signed by the parties hereto.

                  (b)      This Second Amendment is binding upon and shall inure
         to the benefit of the parties hereto, their respective agents,
         employees, representatives, officers, directors, divisions,
         subsidiaries, affiliates, assigns, heirs, successors in interest and
         shareholders.

                  (c)      This Second Amendment may be executed in any number
         of counterparts, each of which shall be deemed an original, but all of
         which when taken together shall constitute one and the same instrument.
         The signature page of any counterpart may be detached therefrom

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<PAGE>

         without impairing the legal effect of the signature(s) thereon provided
         such signature page is attached to any other counterpart identical
         thereto except having additional signature pages executed by other
         parties to this Second Amendment attached thereto.

                  (d)      Landlord and Tenant each represent and warrant that
         it has not dealt with any broker, agent or other person (collectively
         "BROKER") in connection with this transaction other than John Burnham &
         Company, and that no Broker other than John Burnham & Company, who
         shall be paid by Landlord pursuant to a separate Agreement, brought
         about this transaction. Landlord and Tenant each hereby agree to
         indemnify and hold the other harmless from and against any claims by
         any Broker other than John Burnham & Company claiming a commission or
         other form of compensation by virtue of having dealt with Tenant or
         Landlord, as applicable, with regard to this leasing transaction.

                  (e)      Except as amended and/or modified by this Second
         Amendment, the Lease is hereby ratified and confirmed and all other
         terms of the Lease shall remain in full force and effect, unaltered and
         unchanged by this Second Amendment. In the event of any conflict
         between the provisions of this Second Amendment and the provisions of
         the Lease, the provisions of this Second Amendment shall prevail.
         Whether or not specifically amended by this Second Amendment, all of
         the terms and provisions of the Lease are hereby amended to the extent
         necessary to give effect to the purpose and intent of this Second
         Amendment.

         IN WITNESS WHEREOF, the parties hereto have executed this First
Amendment as of the day and year First above written.

                                      TENANT:

                                      IDEC PHARMACEUTICALS CORPORATION,
                                      a Delaware corporation

                                      By: /s/ Phillip Schneider
                                          --------------------------------------
                                      Its: SVP

                                      LANDLORD:

                                      ARE-10933 NORTH TORREY PINES, LLC, a
                                      Delaware limited liability company

                                      By: ALEXANDRIA REAL ESTATE EQUITIES, INC.,
                                          a Maryland corporation, managing
                                          member

                                          By: /s/ Vincont R. Ciruzzi
                                              ----------------------------------
                                          Its: Senior Vice President

                                       3
<PAGE>

                                   EXHIBIT A

                          LEGAL DESCRIPTION OF PROJECT

Lot 1 of Torrey Pines Science Park Unit No. 1 in the City of San Diego, County
of San Diego, State of California, according to Map thereof No. 6229, filed in
the Office of the County Recorder of San Diego County on November 21, 1968.

<PAGE>

                                  EXHIBIT B-1

                                  THE PREMISES

                                  [FLOOR PLAN]

<PAGE>

                                  EXHIBIT B-2

                                  THE PREMISES

                                  [FLOOR PLAN]

<PAGE>

                                    EXHIBIT C

                                   WORK LETTER

         THIS WORK LETTER dated as of November 1,2000 (this "WORK LETTER") is
made and entered into by and between ARE-10933 NORTH TORREY PINES, LLC, a
Delaware limited liability company ("LANDLORD"), and IDEC PHARMACEUTICALS
CORPORATION, a Delaware corporation ("TENANT"), and is attached to and made a
part of the Second Amendment to Lease dated November 1,2000 (the "SECOND
AMENDMENT"), by and between Landlord and Tenant. Any initially capitalized terms
used but not defined herein shall have the meanings given them in the Second
Amendment.

         1. GENERAL REQUIREMENTS

                  (a) TENANT'S AUTHORIZED REPRESENTATIVE. Tenant designates Paul
         Draper and Robert F. Dilworth collectively, "TENANT'S REPRESENTATIVE")
         as the only persons authorized to act for Tenant pursuant to this Work
         Letter. Landlord may rely and act on, without further inquiry of any
         kind, any written request, approval, inquiry or other communication
         ('COMMUNICATION') from or on behalf of Tenant in connection with this
         Work Letter given by either Tenant's Representative. Tenant's
         Representative may be contacted at the voice, facsimile and pager
         telephone numbers and e-mail addresses set forth on Schedule C-1,
         attached hereto and incorporated herein by this reference. When
         Landlord has attempted to contact Tenant's Representative by each of
         such means, Landlord shall have no further obligation of any kind to
         attempt to locate either Tenant's Representative or Tenant with respect
         to any Communication under this Work Letter. Tenant may change Tenant's
         Representatives or any telephone number or e-mail address set forth on
         Schedule C-1 at any time upon not less than 5 business days advance
         written notice to Landlord. No period set forth herein for any approval
         of any matter by Tenant's Representative shall be extended by reason of
         any change in Tenant's Representative. Neither Tenant nor Tenant's
         Representative shall be authorized to direct Landlord's contractors in
         the performance of Landlord's Work (as hereinafter defined).

                  (b) LANDLORD'S AUTHORIZED REPRESENTATIVE. Landlord designates
         Vincent Ciruzzi and Gregory Margritz collectively, ("LANDLORD'S
         REPRESENTATIVE") as the only person authorized to act for Landlord
         pursuant to this Work Letter. Tenant shall not be obligated to respond
         to or act upon any request, approval, inquiry or other Communication
         from or on behalf of Landlord in connection with this Work Letter
         unless such Communication is in writing from Landlord's Representative.
         Landlord's Representative may be contacted at the voice, facsimile and
         pager telephone numbers and e-mail addresses set forth on Schedule C-2,
         attached hereto and incorporated herein by this reference. When Tenant
         has attempted to contact Landlord's Representative by each of such
         means, Tenant shall have no further obligation of any kind to attempt
         to locate Landlord's Representative or Landlord with respect to any
         Communication under this Work Letter. Landlord may change Landlord's
         Representatives or any telephone number or e-mail address set forth on
         Schedule C-2 at any time upon not less than 5 business days advance
         written notice to Tenant. No period set forth herein for any approval
         of any matter by Landlord's Representative shall be extended by reason
         of any change in Landlord's Representative.

                  (c) ARCHITECTS, CONSULTANTS AND CONTRACTORS. Landlord and
         Tenant hereby acknowledge and agree that the general contractor for the
         Tenant Improvements shall be DPR Construction. Tenant, shall select the
         architect (the "TI ARCHITECT") and the subconsultants for the Tenant
         Improvements, subject to Landlord's approval, which approval shall not
         be unreasonably withheld, conditioned or delayed.

                                       1
<PAGE>

         2. TENANT IMPROVEMENTS.

                  (a) TENANT IMPROVEMENTS DEFINED. As used herein, "TENANT
         IMPROVEMENTS" shall mean all improvements to each of the three Suites
         (as defined in the Second Amendment) desired by Tenant of a fixed and
         permanent nature. Other than funding the TI Allowance (as defined
         below) as provided herein, Landlord shall not have any obligation
         whatsoever with respect to the finishing of the Suites for Tenant's use
         and occupancy.

                  (b) TENANT'S SPACE PLANS. Tenant shall deliver to Landlord
         schematic drawings and outline specifications (the "TI DESIGN
         DRAWINGS") detailing Tenant's requirements for the Tenant Improvements
         for any Suite not less than within 60 business days before the date
         Tenant proposes to commence construction of such Tenant Improvements.
         Not more than 5 business days thereafter, Landlord shall deliver to
         Tenant Landlord's reasonable written objections, questions or comments
         of Landlord with regard to such TI Design Drawings. Tenant shall cause
         the TI Design Drawings to be revised to address such written comments
         and shall resubmit said drawings to Landlord for approval within 10
         business days thereafter. Such process shall continue until Landlord
         has approved the TI Design Drawings. In no event shall Landlord have
         the right to direct upgrades in the quality (above the generic
         Laboratory standard of Tenant's improvements in its facility at 11011
         Torreyana Road, San Diego, California) or quantity (other than to
         comply with any Legal Requirement) of any of the materials or equipment
         to be installed in connection with the Tenant Improvements, nor shall
         any such changes adversely affect the safety or quality of the Tenant
         Improvements.

                  (c) WORKING DRAWINGS. Not less than 30 business days prior to
         Tenant's commencement of construction of the Tenant Improvements for
         such Suite, Tenant shall cause the TI Architect to prepare and deliver
         to Landlord for review and comment construction plans, specifications
         and drawings for the Tenant Improvements for such Suite ("TI
         CONSTRUCTION DRAWINGS"), which TI Construction Drawings shall be
         prepared substantially in accordance with the TI Design Drawings for
         such Suite. Tenant shall be solely responsible for ensuring that the TI
         Construction Drawings reflect Tenant's requirements for the Tenant
         Improvements for such Suite. Landlord shall deliver its written
         comments on such TI Construction Drawings to Tenant not later than 7
         business days after Landlord's receipt of the same; provided, however,
         that Landlord may not disapprove any matter that is consistent with the
         TI Design Drawings for such Suite. Tenant and the TI Architect shall
         consider all such comments in good faith and shall, within 10 business
         days after receipt, notify Landlord how Tenant proposes to respond to
         such comments. Any disputes in connection with such comments shall be
         resolved in accordance with Section 3(d) hereof. Provided that the
         design reflected in such TI Construction Drawings is consistent with
         the TI Design Drawings for such Suite, Landlord shall approve the TI
         Construction Drawings submitted by Tenant. Once approved by Landlord,
         subject to the provisions of Section 3(d) below, Tenant shall not
         materially modify the TI Construction Drawings except as may be
         reasonably required in connection with the issuance of the TI Permit
         (as defined in Section 4(b) below).

                  (d) DISPUTE RESOLUTION. Upon any dispute regarding the design
         of the Tenant Improvements for any Suite which is not settled within 10
         business days after notice of such dispute is delivered by one party to
         the other, Tenant shall make the final decision regarding the design of
         such Tenant Improvements, provided Tenant acts reasonably and such
         final decision is either consistent with or a compromise between
         Landlord's and Tenant's positions with respect to such dispute,
         provided further that all costs and expenses resulting from any such
         decision by Tenant shall be payable as a TI Cost. Any changes to the TI
         Construction Drawings for any Suite following Landlord's and Tenant's
         approval of same requested by Tenant shall be processed as provided in
         Section 5 hereof.

                                       2
<PAGE>

         3. PERFORMANCE OF TENANT'S WORK

                  (a) DEFINITION OF TENANT'S WORK. As used herein, "TENANT'S
         WORK" shall mean the work of constructing the Tenant Improvements for
         each Suite.

                  (b) COMMENCEMENT AND PERMITTING OF TENANT'S WORK. Tenant shall
         not commence construction of the Tenant Improvements for each Suite
         until it has obtained a building permit (the "TI PERMIT") authorizing
         the construction of such Tenant Improvements consistent with the TI
         Construction Drawings approved by Landlord for such Suite. The cost of
         obtaining the TI Permit shall be payable as a TI Cost of such Suite.
         Landlord shall, upon Tenant's request, assist Tenant in obtaining the
         TI Permit.

                  (c) SELECTION OF MATERIALS, ETC. Where more than one type of
         material or structure is indicated on the TI Construction Drawings
         approved by Tenant and Landlord, the option will be within Tenant's
         reasonable discretion.

         4. CHANGES. Any changes requested by Tenant to the Tenant Improvements
for a Suite after the delivery and approval by Landlord of the TI Design
Drawings for such Suite, shall be requested and instituted in accordance with
the provisions of this Section and shall be subject to the written approval of
Landlord, such approval not to be unreasonably withheld, conditioned or delayed.

                  (a) TENANT'S RIGHT TO REQUEST CHANGES. If Tenant shall request
         changes ("CHANGES") to the Tenant Improvements for any Suite, Tenant
         shall request such Changes by notifying Landlord in writing in
         substantially the same form as the AIA standard change order form (a
         "CHANGE REQUEST"), which Change Request shall detail the nature and
         extent of any such Change. Tenant's Representative must sign such
         Change Request. Landlord shall review and approve or disapprove such
         Change Request within 5 business days thereafter, provided that
         Landlord's approval shall not be unreasonably withheld, conditioned or
         delayed.

                  (b) IMPLEMENTATION OF CHANGES. If Landlord approves such
         Change and Tenant deposits with Landlord any Excess TI Costs (as
         defined below) required in connection with such Change, Tenant may
         cause the approved Change to be instituted.

         5. COSTS

                  (a) BUDGET FOR TENANT IMPROVEMENTS. Before the commencement of
         construction of the Tenant Improvements for any Suite, Tenant shall
         obtain a detailed breakdown, by trade, of the costs incurred or which
         will be incurred, in connection with the design and construction of
         such Tenant Improvements (the "BUDGET"). The Budget shall be based upon
         the TI Construction Drawings approved by Landlord for such Suite and
         shall include a payment to Landlord of administrative rent
         ("ADMINISTRATIVE RENT") equal to 1.5% of the TI Costs (as hereinafter
         defined), for monitoring and inspecting the construction of such Tenant
         Improvements, which sum shall be payable as a TI Cost for such Suite;
         provided that such Administrative Rent shall not exceed $15,000 in the
         aggregate for any Suite used for clinical manufacturing, as permitted
         pursuant to Section 5 of the Second Amendment. Such Administrative Rent
         shall include, without limitation, all out-of-pocket costs, expenses
         and fees incurred by or on behalf of Landlord, including the cost of
         any overhead, arising from, out of, or in connection with, such
         monitoring of the construction of the Tenant Improvements, regardless
         whether Landlord's actual costs and expenses exceed such Administrative
         Rent. If the Budget is greater than the TI Allowance for such Suite,
         Tenant shall make the deposit

                                       3
<PAGE>

         described in Section 5(d) hereof, for disbursement by Landlord as
         described in such Section 5(d).

                  (b) TI ALLOWANCE. Landlord shall provide to Tenant a tenant
         improvement allowance for each Suite ("TI ALLOWANCE") as provided in
         the Second Amendment. Such TI Allowances shall be disbursed in
         accordance with this Work Letter.

                  (c) COSTS PAYABLE WITH TI ALLOWANCE. The TI Allowance shall be
         used solely for the payment of design and construction costs in
         connection with the construction of the Tenant Improvements, including,
         without limitation, the cost of preparing the TI Design Drawings and
         the TI Construction Drawings, all costs set forth in the Budget
         (including Landlord's Administrative Rent) and the cost of Changes
         (collectively, "TI COSTS"). Notwithstanding anything to the contrary
         contained herein, the TI Allowance shall not be used to purchase any
         furniture, personal property or other non-building system materials or
         equipment not incorporated into the Improvements, including, without
         limitation, biological safety cabinets and other scientific equipment
         not incorporated into the Improvements.

                  (d) EXCESS TI COSTS. It is understood and agreed that Landlord
         is under no obligation to bear any portion of the cost of any of the
         Tenant Improvements for any Suite, except to the extent of the TI
         Allowance for such Suite. Tenant shall pay all TI Costs, subject to
         Landlord's reimbursement for Landlord's Share of such TI Costs pursuant
         to Section 5(d). "LANDLORD'S SHARE" of any payment by Tenant of TI
         Costs for any Suite shall be equal to the amount of such payment,
         multiplied by a fraction, the numerator of which is the TI Allowance
         for such Suite and the denominator of which is the TI Costs for such
         Suite.

                  (e) PAYMENT FOR TI COSTS. Landlord shall reimburse Tenant for
         Landlord's Share of any TI Costs paid by Tenant once a month on a Suite
         by Suite basis against a draw request in Landlord's standard form,
         containing such certifications, lien waivers, inspection reports and
         other matters as Landlord reasonably requires, to the extent of
         Landlord's approval thereof for payment, no later than 30 days
         following receipt of such draw request. Upon completion of the Tenant
         Improvements for any Suite, Tenant shall deliver to Landlord: (i) sworn
         statements setting forth the names of all contractors and
         subcontractors who did the work and final lien waivers from all such
         contractors and subcontractors; and (ii) "as built" plans for the
         Tenant Improvements for such Suite.

         6. MISCELLANEOUS

                  (a) CONSENTS. Whenever consent or approval of either party is
         required under this Work Letter, that party shall not unreasonably
         withhold, condition or delay such consent or approval, except as may be
         expressly set forth herein to the contrary.

                  (b) MODIFICATION. No modification, waiver or amendment of this
         Work Letter or of any of its conditions or provisions shall be binding
         upon Landlord or Tenant unless in writing signed by Landlord and
         Tenant.

                  (c) COUNTERPARTS. This Work Letter may be executed in any
         number of counterparts but all counterparts taken together shall
         constitute a single document.

                  (d) GOVERNING LAW. This Work Letter shall be governed by,
         construed and enforced in accordance with the internal laws of the
         state in which the Premises are located, without regard to choice of
         law principles of such State.

                                       4

<PAGE>

                  (e) TIME OF THE ESSENCE. Time is of the essence of this Work
         Letter and of each and all provisions thereof.

                  (f) DEFAULT. Notwithstanding anything set forth herein or in
         the Lease to the contrary, Landlord shall not have any obligation to
         perform any work hereunder or to fund any portion of the TI Allowance
         during any period Tenant is in Default under the Lease.

                  (g) SEVERABILITY. If any term or provision of this Work Letter
         is declared invalid or unenforceable, the remainder of this Work Letter
         shall not be affected by such determination and shall continue to be
         valid and enforceable.

                  (h) MERGER. All understandings and agreements, oral or
         written, heretofore made between the parties hereto and relating to
         Tenant's Work are merged in this Work Letter, which alone (but
         inclusive of provisions of the Lease incorporated herein and the final
         approved constructions drawings and specifications prepared pursuant
         hereto) fully and completely expresses the agreement between Landlord
         and Tenant with regard to the matters set forth in this Work Letter.

                  (i) ENTIRE AGREEMENT. This Work Letter is made as a part of
         and pursuant to the Second Amendment and the Lease and, together with
         the Second Amendment and the Lease, constitutes the entire agreement of
         the parties with respect to the subject matter hereof. This Work Letter
         is subject to all of the terms and limitation set forth in the Lease,
         and neither party shall have any rights or remedies under this Work
         Letter separate and apart from their respective remedies pursuant to
         the Lease.

         IN WITNESS WHEREOF, Landlord and Tenant have executed this Work Letter
to be effective on the date first above written.

                                      TENANT:

                                      IDEC PHARMACEUTICALS CORPORATION,
                                      a Delaware corporation

                                      By: Phillip Schneider
                                          -----------------------------
                                      Its: SVP

                                      LANDLORD:

                                      ARE-10933 NORTH TORREY PINES, LLC, a
                                      Delaware limited liability company

                                      By: ALEXANDRIA REAL ESTATE EQUITIES, INC.,
                                          a Maryland corporation, managing
                                          member

                                          By: /s/ Vincent R. Ciruzzi
                                              ----------------------------------
                                          Its: Senior Vice President

                                       5
<PAGE>

                           SCHEDULE C-1 TO WORK LETTER

                      TENANT'S AUTHORIZED REPRESENTATIVES

Paul Draper
IDEC Pharmaceuticals Corporation
3030 Callan Road
San Diego, CA 92121
Phone:    (858) 431-8764
Fax:      (858) 431-8892
E-Mail:   pdraper@idecpharm.com

Robert F. Dilworth
Coronado Assets, Inc.
34 Blue Anchor Cays Road
Coronado, CA 92118
Phone:    (619)575-4570
Fax:      (619)575-7352
E-Mail:   bobdilworth@earthlink.net

<PAGE>

                           SCHEDULE C-2 TO WORK LETTER

                     LANDLORD'S AUTHORIZED REPRESENTATIVES

Vincent R. Ciruzzi
c/o Alexandria Real Estate Equities, Inc.
9820 Willow Creek Road, Suite 440
San Diego, CA 92131
Phone:    (858) 530-8190
Fax:      (858) 530-8191
E-Mail:   vciruzzi@labspace.com

Gregory Margritz
c/o Alexandria Real Estate Equities, Inc.
9820 Willow Creek Road, Suite 440
San Diego, CA 92131
Phone:    (858) 530-8190
Fax:      (858) 530-8191
E-Mail:   gmarqritz@labspace.com

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