Document:

<PAGE>

                                                                   EXHIBIT 10.55

                                 PROMISSORY NOTE

                                   6/25/2004
                                     (DATE)

FOR VALUE RECEIVED, LA JOLLA PHARMACEUTICAL COMPANY a corporation located at the
address stated below ("MAKER") promises, jointly and severally if more than one,
to pay to the order of GENERAL ELECTRIC CAPITAL Corporation or any subsequent
holder hereof (each, a "PAYEE") at its office located at 83 WOOSTER HEIGHTS
ROAD, DANBURY, CT or at such other place as Payee or the holder hereof may
designate, the principal sum of ONE HUNDRED THIRTY ONE THOUSAND SIX HUNDRED
THIRTEEN AND 45/00 ($131,613.45), with interest on the unpaid principal balance,
from the date hereof through and including the dates of payment, at a fixed
interest rate of Eight and Seventy Seven Hundredths percent (8.77%) per annum,
to be paid in lawful money of the United States, in Forty-Two (42) consecutive
monthly installments of principal and interest as follows:

<TABLE>
<CAPTION>
   Periodic
  Installment     Amount
---------------  ---------
<S>              <C>
Thirty-Six (36)  $3,856.77
Five (5)         $1,992.06
</TABLE>

each ("Periodic Installment") and a final installment which shall be in the
amount of the total outstanding principal and interest. The first Periodic
Installment shall be due and payable on _________7/01/04___________ and the
following Periodic Installments and the final installment shall be due and
payable on the same day of each succeeding month (each, a "Payment Date"). Such
installments have been calculated on the basis of a 360 day year of twelve
30-day months. Each payment may, at the option of the Payee, be calculated and
applied on an assumption that such payment would be made on its due date.

The acceptance by Payee of any payment which is less than payment in full of all
amounts due and owing at such time shall not constitute a waiver of Payee's
right to receive payment in full at such time or at any prior or subsequent
time.

The Maker hereby expressly authorizes the Payee to insert the date value is
actually given in the blank space on the face hereof and on all related
documents pertaining hereto.

This Note may be secured by a security agreement, chattel mortgage, pledge
agreement or like instrument (each of which is hereinafter called a "SECURITY
AGREEMENT").

Time is of the essence hereof. If any installment or any other sum due under
this Note or any Security Agreement is not received within ten (10) days after
its due date, the Maker agrees to pay, in addition to the amount of each such
installment or other sum, a late payment charge of five percent (5%) of the
amount of said installment or other sum, but not exceeding any lawful maximum.
If (i) Maker fails to make payment of any amount due hereunder within ten (10)
days after the same becomes due and payable; or (ii) Maker is in default under,
or fails to perform under any term or condition contained in any Security
Agreement, then the entire principal sum remaining unpaid, together with all
accrued interest thereon and any other sum payable under this Note or any
Security Agreement, at the election of Payee, shall immediately become due and
payable, with interest thereon at the lesser of eighteen percent (18%) per annum
or the highest rate not prohibited by applicable law from the date of such
accelerated maturity until paid (both before and after any judgment).

Notwithstanding anything to the contrary contained herein or in the Security
Agreement, Maker may not prepay in full or in part any indebtedness hereunder
without the express written consent of Payee in its sole discretion.

It is the intention of the parties hereto to comply with the applicable usury
laws; accordingly, it is agreed that, notwithstanding any provision to the
contrary in this Note or any Security Agreement, in no event shall this Note or
any Security Agreement require the payment or permit the collection of interest
in excess of the maximum amount permitted by applicable law. If any such excess
interest is contracted for, charged or received under this Note or any Security
Agreement, or if all of the principal balance shall be prepaid, so that under
any of such circumstances the amount of interest contracted for, charged or
received under this Note or any Security Agreement on the principal balance
shall exceed the maximum amount of interest permitted by applicable law, then in
such event (a) the provisions of this paragraph shall govern and control, (b)
neither Maker nor any other person or entity now or hereafter liable for the
payment hereof shall be obligated to pay the amount of such interest to the
extent that it is in excess of the maximum amount of interest permitted by
applicable law, (c) any such excess which may have been collected shall be
either applied as a credit against the then unpaid principal balance or refunded
to Maker, at the option of the Payee, and (d) the effective rate of interest
shall be automatically reduced to the maximum lawful contract rate allowed under
applicable law as now or hereafter construed

<PAGE>

by the courts having jurisdiction thereof. It is further agreed that without
limitation of the foregoing, all calculations of the rate of interest contracted
for, charged or received under this Note or any Security Agreement which are
made for the purpose of determining whether such rate exceeds the maximum lawful
contract rate, shall be made, to the extent permitted by applicable law, by
amortizing, prorating, allocating and spreading in equal parts during the period
of the full stated term of the indebtedness evidenced hereby, all interest at
any time contracted for, charged or received from Maker or otherwise by Payee in
connection with such indebtedness; provided, however, that if any applicable
state law is amended or the law of the United States of America preempts any
applicable state law, so that it becomes lawful for the Payee to receive a
greater interest per annum rate than is presently allowed, the Maker agrees
that, on the effective date of such amendment or preemption, as the case may be,
the lawful maximum hereunder shall be increased to the maximum interest per
annum rate allowed by the amended state law or the law of the United States of
America.

The Maker and all sureties, endorsers, guarantors or any others (each such
person, other than the Maker, an "OBLIGOR") who may at any time become liable
for the payment hereof jointly and severally consent hereby to any and all
extensions of time, renewals, waivers or modifications of, and all substitutions
or releases of, security or of any party primarily or secondarily liable on this
Note or any Security Agreement or any term and provision of either, which may be
made, granted or consented to by Payee, and agree that suit may be brought and
maintained against any one or more of them, at the election of Payee without
joinder of any other as a party thereto, and that Payee shall not be required
first to foreclose, proceed against, or exhaust any security hereof in order to
enforce payment of this Note. The Maker and each Obligor hereby waives
presentment, demand for payment, notice of nonpayment, protest, notice of
protest, notice of dishonor, and all other notices in connection herewith, as
well as filing of suit (if permitted by law) and diligence in collecting this
Note or enforcing any of the security hereof, and agrees to pay (if permitted by
law) all expenses incurred in collection, including Payee's actual attorneys'
fees. Maker and each Obligor agrees that fees not in excess of twenty percent
(20%) of the amount then due shall be deemed reasonable.

THE MAKER HEREBY UNCONDITIONALLY WAIVES ITS RIGHTS TO A JURY TRIAL OF ANY CLAIM
OR CAUSE OF ACTION BASED UPON OR ARISING OUT OF, DIRECTLY OR INDIRECTLY, THIS
NOTE, ANY OF THE RELATED DOCUMENTS, ANY DEALINGS BETWEEN MAKER AND PAYEE
RELATING TO THE SUBJECT MATTER OF THIS TRANSACTION OR ANY RELATED TRANSACTIONS,
AND/OR THE RELATIONSHIP THAT IS BEING ESTABLISHED BETWEEN MAKER AND PAYEE. THE
SCOPE OF THIS WAIVER IS INTENDED TO BE ALL ENCOMPASSING OF ANY AND ALL DISPUTES
THAT MAY BE FILED IN ANY COURT (INCLUDING, WITHOUT LIMITATION, CONTRACT CLAIMS,
TORT CLAIMS, BREACH OF DUTY CLAIMS, AND ALL OTHER COMMON LAW AND STATUTORY
CLAIMS.) THIS WAIVER IS IRREVOCABLE MEANING THAT IT MAY NOT BE MODIFIED EITHER
ORALLY OR IN WRITING, AND THE WAIVER SHALL APPLY TO ANY SUBSEQUENT AMENDMENTS,
RENEWALS, SUPPLEMENTS OR MODIFICATIONS TO THIS NOTE, ANY RELATED DOCUMENTS, OR
TO ANY OTHER DOCUMENTS OR AGREEMENTS RELATING TO THIS TRANSACTION OR ANY RELATED
TRANSACTION. IN THE EVENT OF LITIGATION, THIS NOTE MAY BE FILED AS A WRITTEN
CONSENT TO A TRIAL BY THE COURT.

This Note and any Security Agreement constitute the entire agreement of the
Maker and Payee with respect to the subject matter hereof and supercedes all
prior understandings, agreements and representations, express or implied.

No variation or modification of this Note, or any waiver of any of its
provisions or conditions, shall be valid unless in writing and signed by an
authorized representative of Maker and Payee. Any such waiver, consent,
modification or change shall be effective only in the specific instance and for
the specific purpose given.

<PAGE>

Any provision in this Note or any Security Agreement which is in conflict with
any statute, law or applicable rule shall be deemed omitted, modified or altered
to conform thereto.

                                       LA JOLLA PHARMACEUTICAL COMPANY

/s/ Jeri Lynn Holm                    By:  /s/ Gail A. Sloan
--------------------------------           -------------------------------------
(Witness)

Jeri Lynn Holm                        Name: Gail A. Sloan
--------------------------------            ------------------------------------
(Print name)

6455 Nancy  Ridge Drive               Title: V.P. of Finance and Controller
--------------------------------             -----------------------------------
San Diego, CA 92121
-------------------
(Address)

                                       Federal Tax ID #: 330361285

                                       Address: 6455 Nancy Ridge Drive,
                                                San Diego, San Diego County,
                                                CA 92121<PAGE>

                                                                   EXHIBIT 10.56

                                FOURTH AMENDMENT

THIS FOURTH AMENDMENT (the "AMENDMENT") is made and entered into as of July 8,
2004, by and between EOP-INDUSTRIAL PORTFOLIO, L.L.C., A DELAWARE LIMITED
LIABILITY COMPANY ("LANDLORD") and LA JOLLA PHARMACEUTICAL COMPANY, A DELAWARE
CORPORATION ("TENANT").

                                    RECITALS

A.    Landlord (as successor in interest pursuant to merger with Spieker
      Properties, L.P., a California limited partnership, as successor in
      interest to BRE Properties, Inc., a Delaware corporation) and Tenant are
      parties to that certain lease dated July 27, 1992, which lease has been
      previously amended by the Addendum to Lease dated July 27, 1992, First
      Amendment to Lease dated March 15, 1993, Second Amendment to Lease dated
      July 18, 1994 and Third Amendment to Lease dated January 26, 1995
      (collectively, the "LEASE"). Pursuant to the Lease, Landlord has leased to
      Tenant space currently containing approximately 36,526 rentable square
      feet (the "PREMISES") in the building commonly known as Westridge I
      located at 6455 Nancy Ridge Drive, San Diego, California (the "BUILDING").

B.    Landlord and Tenant hereby acknowledge and agree that due to a scrivener
      error in paragraph A of the Recitals of that certain Third Amendment to
      Lease dated January 26, 1995, incorrectly stated that the Premises
      contained approximately "36,256" rentable square feet of space. Landlord
      and Tenant retroactively agree that from and after January 26, 1995, the
      Premises contains approximately "36,526" rentable square feet.

C.    The Lease by its terms shall expire on JULY 31, 2004 ("PRIOR EXTENDED
      TERMINATION DATE"), and the parties desire to extend the Lease Term, all
      on the following terms and conditions.

      NOW, THEREFORE, in consideration of the above recitals which by this
reference are incorporated herein, the mutual covenants and conditions contained
herein and other valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, Landlord and Tenant agree as follows:

1.    EXTENSION. The Lease Term is hereby extended for a period of sixty (60)
      months and shall expire on JULY 31, 2009 ("SECOND EXTENDED TERMINATION
      DATE"), unless sooner terminated in accordance with the terms of the
      Lease. That portion of the Lease Term commencing the day immediately
      following the Prior Extended Termination Date ("SECOND EXTENSION DATE")
      and ending on the Second Extended Termination Date shall be referred to
      herein as the "SECOND EXTENDED TERM".

2.    BASE RENT. As of the Second Extension Date, the schedule of Base Rent
      payable with respect to the Premises during the Second Extended Term is
      the following:

<PAGE>

<TABLE>
<CAPTION>
                PERIOD                   ANNUAL RATE PER SQUARE FOOT   MONTHLY BASE RENT
<S>                                      <C>                           <C>
August 1, 2004 through July 31, 2005               $13.80                  $42,004.90
August 1, 2005 through July 31, 2006               $14.35                  $43,679.01
August 1, 2006 through July 31, 2007               $14.93                  $45,444.43
August 1, 2007 through July 31, 2008               $15.52                  $47,240.29
August 1, 2008 through July 31, 2009               $16.14                  $49,127.47
</TABLE>

      All such Base Rent shall be payable by Tenant in accordance with the terms
of the Lease.

3.    REAL PROPERTY TAXES. For the period commencing on the Second Extension
      Date and ending on the Second Extended Termination Date, Tenant shall pay
      for Tenant's pro rata share of the real property taxes and Tenant's pro
      rata share of all costs incurred by Landlord for the operation and
      maintenance of the Common Areas in accordance with the terms of the Lease.

4. IMPROVEMENTS TO PREMISES.

      4.01. CONDITION OF PREMISES. Tenant is in possession of the Premises and
            accepts the same "as is" without any agreements, representations,
            understandings or obligations on the part of Landlord to perform any
            alterations, repairs or improvements, except as may be expressly
            provided otherwise in the Lease or this Amendment.

      4.02. RESPONSIBILITY FOR IMPROVEMENTS TO PREMISES. Any construction,
            alterations or improvements to the Premises shall be performed by
            Tenant at its sole cost and expense using contractors selected by
            Tenant and reasonably approved by Landlord and shall be governed in
            all respects by the provisions of the Lease.

5.    OTHER PERTINENT PROVISIONS. Landlord and Tenant agree that, effective as
      of the date of this Amendment (unless different effective date(s) is/are
      specifically referenced in this Section), the Lease shall be amended in
      the following additional respects:

      5.01. PARKING. For the period commencing on the Second Extension Date and
            ending on the Second Extended Termination Date, Tenant shall retain
            its existing vehicle parking rights as set forth in the Lease.

      5.02. RIGHT OF FIRST REFUSAL TO LEASE 6465 NANCY RIDGE DRIVE AS SET FORTH
            IN THE ADDENDUM TO LEASE shall be deleted in its entirety and is of
            no further force and effect.

                                       2

<PAGE>

      5.03. LIMITATION ON LANDLORD'S LIABILITY. The obligations of Landlord
            under the Lease (including any actual or alleged breach or default
            by Landlord) do not constitute personal obligations of the
            individual partners, directors, officers or shareholders of Landlord
            or Landlord's partners, and Tenant shall not seek recourse against
            the individual partners, directors, officers or shareholders of
            Landlord or Landlord's partners, or any of their personal assets for
            satisfaction of any liability with respect to the Lease. In
            addition, the liability of Landlord for its obligations under the
            Lease shall be limited solely to, and Tenant's sole and exclusive
            remedy shall be against, Landlord's interest in the Industrial
            Center, and no other assets of Landlord.

6. MISCELLANEOUS.

      6.01. This Amendment and the attached exhibits, which are hereby
            incorporated into and made a part of this Amendment, set forth the
            entire agreement between the parties with respect to the matters set
            forth herein. There have been no additional oral or written
            representations or agreements. Under no circumstances shall Tenant
            be entitled to any Rent abatement, improvement allowance, leasehold
            improvements, or other work to the Premises, or any similar economic
            incentives that may have been provided Tenant in connection with
            entering into the Lease, unless specifically set forth in this
            Amendment.

      6.02. Except as herein modified or amended, the provisions, conditions and
            terms of the Lease shall remain unchanged and in full force and
            effect.

      6.03. In the case of any inconsistency between the provisions of the Lease
            and this Amendment, the provisions of this Amendment shall govern
            and control.

      6.04. Submission of this Amendment by Landlord is not an offer to enter
            into this Amendment. Landlord shall not be bound by this Amendment
            until Landlord has executed and delivered the same to Tenant.

      6.05. The capitalized terms used in this Amendment shall have the same
            definitions as set forth in the Lease to the extent that such
            capitalized terms are defined therein and not redefined in this
            Amendment.

      6.06. Tenant hereby represents to Landlord that Tenant has dealt with no
            broker other than Irving Hughes in connection with this Amendment.
            Tenant agrees to indemnify and hold Landlord, its members,
            principals, beneficiaries, partners, officers, directors, employees,
            mortgagee(s) and agents, and the respective principals and members
            of any such agents (collectively, the "LANDLORD RELATED PARTIES")
            harmless from all claims of any brokers other than Irving Hughes
            claiming to have represented Tenant in connection with this
            Amendment. Landlord hereby represents to Tenant that Landlord has
            dealt with no broker in connection with this Amendment. Landlord
            agrees to indemnify and hold Tenant, its members, principals,
            beneficiaries, partners, officers, directors, employees, and agents,
            and the respective principals and members of any such agents

                                       3

<PAGE>

            (collectively, the "TENANT RELATED Parties") harmless from all
            claims of any brokers claiming to have represented Landlord in
            connection with this Amendment.

      6.07. Landlord and Tenant each represent that each signatory of this
            Amendment has the authority to execute and deliver the same on
            behalf of the party hereto for which such signatory is acting.

      6.08. Equity Office Properties Management Corp. ("EOPMC") is an affiliate
            of Landlord and represents only the Landlord in this transaction.
            Any assistance rendered by any agent or employee of EOPMC in
            connection with this Lease or any subsequent amendment or
            modification hereto has been or will be made as an accommodation to
            Tenant solely in furtherance of consummating the transaction on
            behalf of Landlord, and not as agent for Tenant.

      6.09. CALIFORNIA WAIVERS. Notwithstanding anything to the contrary
            contained in the Lease, Tenant hereby waives any and all rights
            under and benefits of subsection 1 of Section 1932, Sections 1941
            and 1942 (Tenant's Repairs and Alterations), 1932(2) (Casualty
            Damage), and Section 1265.130 (Condemnation) of the California Code
            of Civil Procedure, or any similar or successor laws now or
            hereinafter in effect.

      6.10. NO EXTENSION OPTIONS DURING THE SECOND EXTENDED TERM. The parties
            hereto acknowledge and agree that during the Second Extended Term
            Tenant shall have no rights to extend the term of the Lease. The
            parties agree that any rights existing in the Lease to extend the
            term of the Lease shall be deleted in their entirety and are of no
            further force and effect.

      6.11. LEASE CANCELLATION RIGHT AND RIGHT OF FIRST OFFER TO PURCHASE THE
            PROJECT. The parties hereto acknowledge and agree that during the
            Second Extended Term, the Lease Cancellation Right as set forth in
            the Third Amendment to Lease and the Right of First Offer to
            Purchase the Project as set forth in the Addendum to Lease shall
            remain in full force and effect.

                       [SIGNATURES ARE ON FOLLOWING PAGE]

                                       4

<PAGE>

      IN WITNESS WHEREOF, Landlord and Tenant have duly executed this Amendment
as of the day and year first above written.

                             LANDLORD:

                             EOP-INDUSTRIAL PORTFOLIO, L.L.C.,
\                            A DELAWARE LIMITED LIABILITY COMPANY

                             By: EOP Operating Limited Partnership, a
                                 Delaware limited partnership, its sole member

                                   By: Equity Office Properties Trust, a
                                       Maryland real estate investment trust,
                                       its general partner

                                       By: /s/ Robert E. Dezzutti
                                           ----------------------------
                                       Name:  Robert E. Dezzutti
                                       Title: Senior Vice President

                             TENANT:

                             LA JOLLA PHARMACEUTICAL COMPANY,
                             A DELAWARE CORPORATION

                             By: /s/ Steven B. Engle
                                 --------------------------------------
                             Name:  Steven B. Engle
                             Title: Chairman and CEO
                             Tenant's FEIN: 33-0361285

                              5

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00069-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00069-of-00352.parquet"}]]