<PAGE>

                                                                    EXHIBIT 10.1

                            FOURTH AMENDMENT TO LEASE

THIS FOURTH AMENDMENT TO LEASE (the "Amendment") is made and entered into as of
the 18th day of January, 2005, by and between Marina Business Center, LLC, a
California limited liability company ("Landlord"), and CancerVax Corporation, a
Delaware corporation ("Tenant").

                                    RECITALS

      A.    Landlord (as successor in interest to Spieker Properties, LP, a
            California limited partnership) and Tenant are parties to that
            certain lease dated July 22, 1999, which lease has been previously
            amended by First Amendment to Lease dated October 1, 2001, by Second
            Amendment to Lease dated September 4, 2002, and the Third Amendment
            to Lease dated November 14, 2003 (collectively, the "Lease").
            Pursuant to the Lease, Landlord has leased to Tenant space
            containing approximately 50,750 rentable square feet (the
            "Premises") described as Suite Nos. 100, 150 and the Mezzanine
            commonly known as Marina Business Center, 4503 Glencoe Avenue,
            Marina Del Rey, California 90292 (the "Building").

      B.    Tenant has submitted to Landlord written plans for making certain
            tenant improvements to the Premises (the "Tenant Improvements")
            known as the Manufacturing Facility Remodel, dated October 29, 2004
            (the "Plans"). Details of the Plans are incorporated herein by
            reference.

      C.    Tenant has further requested that Landlord approve the Plans and
            Landlord has agreed to approve the same on the following terms and
            conditions.

            NOW, THEREFORE, in consideration of the mutual covenants and
            agreements contained herein and other good and valuable
            consideration, the receipt and sufficiency of which are hereby
            acknowledged, Landlord and Tenant agree as follows:

      I.    Plans and Specifications.

            A.    Tenant shall be responsible for ensuring that the Plans are
                  compatible with the design, construction and equipment of the
                  Building, comply with applicable Regulations and Standards
                  (defined below), people loads, floor loads, power and
                  plumbing, regular and HVAC needs, telephone communications,
                  telephone and electrical outlets, lighting, light fixtures and
                  related power, and electrical and telephone switches, B.T.U.
                  calculations, electrical requirements and special receptacle
                  requirements. The Plans shall also include mechanical,
                  electrical, plumbing, structural and engineering drawings
                  mutually satisfactory to Landlord and Tenant. Notwithstanding
                  Landlord's review and approval of the Plans and any revisions
                  thereto, Landlord shall have no responsibility or liability

                                       1
<PAGE>

                  whatsoever for any errors or omissions contained in the Plans
                  or any revisions thereto, or to verify dimensions or
                  conditions, or for the quality, design or compliance with
                  applicable Regulations of any improvements described therein
                  or constructed in accordance therewith. Tenant hereby waives
                  all claims against Landlord relating to, or arising out of the
                  design or construction of, the Tenant Improvements.

            B.    The plans for the new emergency generator are not approved
                  (note 3 page A1.01 of the Plans). Such Plans are hereby
                  approved, provided that, the generator surround is constructed
                  of the same materials and construction used on the Tenant's
                  existing generator facility with the access as shown on the
                  Plans moved 90 degrees to the back of the enclosure from the
                  side.

      II.   Tenant Improvement Cost.

      The cost of the Tenant Improvements shall be paid for by Tenant,
      including, without limitation, the cost of: Plans and studies;
      architectural and engineering fees; consulting fees in connection with
      obtaining U.S. Food and Drug Administration approval for the operation of
      the Permitted Use from the Premises; permits, approvals and other
      governmental fees; labor, material, equipment and supplies; construction
      fees and other amounts payable to contractors or subcontractors; taxes;
      off-site improvements; remediation and preparation of the Premises for
      construction of the Tenant Improvements; filing and recording fees;
      premiums for insurance and bonds; code compliance and related expenses
      triggered as a result of the construction of the Tenant Improvements;
      attorney's fees; financing costs; and all other costs expended or to be
      expended in the construction of the Tenant Improvements.

      III.  Construction of Tenant Improvements.

            A.    Tenant shall be responsible for obtaining all governmental
                  approvals to the full extent necessary for the construction
                  and installation of the Tenant Improvements and for Tenant's
                  occupancy of the Premises, in compliance with all applicable
                  Regulations. Tenant shall employ Lusardi Construction, or
                  another Contractor that is reasonably acceptable to Landlord,
                  to construct the Tenant Improvements in conformance with the
                  approved Plans. The contractor shall be duly licensed.

            B.    Without limiting the provisions of Paragraph 3.5 of the Lease,
                  Landlord shall not be liable for any direct or indirect
                  damages suffered by Tenant as a result of delays in
                  construction beyond Landlord's reasonable control, including,
                  but not limited to, delays due to strikes or unavailability of
                  materials or labor, or delays caused by Tenant (including
                  delays by the Space Planner, the contractor or anyone else
                  performing services on behalf of Landlord or Tenant).

                                       2
<PAGE>

            C.    All Tenant Improvement work to be performed on the Premises by
                  Tenant or Tenant's contractor or agents shall be subject to
                  the following conditions:

                  i.    All work shall be done in conformity with a valid
                        building permit when required, a copy of which shall be
                        furnished to Landlord before such work is commenced, and
                        in any case, all such work shall be performed in a good
                        workmanlike manner, and in accordance with all
                        applicable Regulations and the requirements and
                        standards of any insurance underwriting board,
                        inspection bureau or insurance carrier insuring the
                        Premises pursuant to the Lease. Notwithstanding any
                        failure by Landlord to object to any such work, Landlord
                        shall have no responsibility for Tenant's failure to
                        comply with all applicable Regulations. Tenant shall be
                        responsible for ensuring that construction and
                        installation of the Tenant Improvements will not
                        materially affect the structural integrity of the
                        Building.

                  ii.   At reasonable times and upon reasonable notice, Landlord
                        or Landlord's agents shall have the right to inspect the
                        construction of the Tenant Improvements by Tenant during
                        the progress thereof. If Landlord shall give notice of
                        faulty construction or any other material deviation from
                        the approved Plans, Tenant shall inform the contractor
                        of such deviation and shall require the contractor to
                        make corrections promptly. However, neither the
                        privilege herein granted to Landlord to make such
                        inspections, nor the making of such inspections by
                        Landlord, shall operate as a waiver of any right of
                        Landlord to require good and workmanlike construction
                        and improvements erected in accordance with the approved
                        Plans.

                  iii.  Unless otherwise agreed in writing by Landlord and
                        Tenant, Tenant's construction of the Tenant Improvements
                        shall comply with the following: (a) the Tenant
                        Improvements shall be constructed in substantial
                        accordance with the approved Plans, as may be amended
                        from time to time; and (b) Tenant shall abide by all
                        reasonable rules made by Landlord with respect to the
                        use of freight, loading dock, and service elevators,
                        storage of materials, coordination of work with the
                        contractors of other tenants, and any other matter in
                        connection with this Amendment, including, without
                        limitation, the construction of the Tenant Improvements.

                  iv.   Tenant shall promptly reimburse Landlord upon demand for
                        any reasonable expense actually incurred by the Landlord
                        by reason of faulty work done by Tenant or its
                        contractors or by reason of any delays caused by such
                        work, or by reason of inadequate clean-up.

                                       3
<PAGE>

            D.    Notwithstanding Section 12 of the Lease, Landlord may at
                  Landlord's option and upon written notice to Tenant no later
                  than 30 days after receipt of the Improvement Removal
                  Notification Letter, require that Tenant, at tenant's expense,
                  remove any and all Tenant Improvements made by Tenant as
                  specified by Landlord (specifically excluding any Tenant
                  Improvements that are not part of the Plans referenced in this
                  Amendment) and restore the Premises by the expiration or
                  earlier termination of this lease to the condition existing
                  prior to the construction of such Alterations. No sooner than
                  the 180th day prior to termination of the lease or any
                  extension thereof, Tenant shall send a written Notification
                  ("Improvement Removal Notification Letter") to Landlord asking
                  the specific Tenant Improvements which will be required to be
                  removed and restored.

      IV    Insurance Requirements.

            A.    All of Tenant's contractors shall carry worker's compensation
                  insurance covering all of their respective employees, and
                  shall also carry public liability insurance, including
                  property damage, all with limits, in form and with companies
                  as are required to be carried by Tenant as set forth in
                  Paragraph 8 of the Lease.

            B.    Tenant shall carry "Builder's All Risk" insurance in an amount
                  approved by Landlord covering the construction of the Tenant
                  Improvements, and such other insurance as Landlord may
                  require, it being understood and agreed that the Tenant
                  Improvements shall be insured by Tenant pursuant to Paragraph
                  8 of the Lease immediately upon completion thereof. Such
                  insurance shall be in amounts and shall include such extended
                  coverage endorsements as may be reasonably required by
                  Landlord including, but not limited to, the requirement that
                  all of Tenant's contractors shall carry excess liability and
                  Products and Completed Operation coverage insurance, each in
                  amounts not less than $500,000 per incident, $1,000,000 in
                  aggregate, and in form and with companies as are required to
                  be carried by Tenant as set forth in Paragraph 8 of the Lease.

            C.    Certificates for all insurance carried pursuant to this
                  Amendment must comply with the requirements of Paragraph 8 of
                  the Lease and shall be delivered to Landlord before the
                  commencement of construction of the Tenant Improvements and
                  before the contractor's equipment is moved onto the site. In
                  the event the Tenant Improvements are damaged by any cause
                  during the course of the construction thereof, Tenant shall
                  immediately repair the same at Tenant's sole cost and expense.
                  Tenant's contractors shall

                                       4
<PAGE>

                  maintain the foregoing insurance coverage in force until the
                  Tenant Improvements are fully completed and accepted by
                  Landlord. All policies carried under this Paragraph IV shall
                  insure Landlord and Tenant, as their interests may appear, as
                  well as the contractors. All insurance maintained by Tenant's
                  contractors shall preclude subrogation claims by the insurer
                  against anyone insured thereunder. Such insurance shall
                  provide that it is primary insurance as respects the owner and
                  that any other insurance maintained by owner is excess and
                  noncontributing with the insurance required hereunder. If the
                  Tenant Improvements are not completed by December 31, 2005,
                  and if, at that time, Landlord has a reasonable basis for
                  concern regarding Tenant's ability to pay for the cost of the
                  Tenant Improvements or Tenant's actual payment of said costs,
                  Landlord may require Tenant to obtain a lien and completion
                  bond or some alternate form of security satisfactory to
                  Landlord in an amount sufficient to ensure the lien-free
                  completion of the Tenant Improvements and naming Landlord as a
                  co-obligee.

      V.    Completion of Construction.

            A.    Within thirty (30) days after completion of construction of
                  the Tenant Improvements, Tenant shall cause a Notice of
                  Completion to be recorded in the office of the Recorder of the
                  county in which the Building is located in accordance with
                  Section 3093 of the Civil Code of the State of California or
                  any successor statute, and shall furnish a copy thereof to
                  Landlord upon such recordation. If Tenant fails to do so,
                  Landlord may execute and file the same on behalf of Tenant as
                  Tenant's agent for such purpose, at Tenant' sole cost and
                  expense. At the conclusion of construction, (i) Tenant shall
                  cause the contractor (a) to update the approved working
                  drawings as necessary to reflect all changes made to the
                  approved working drawings during the course of construction,
                  (b) to certify to the best of their knowledge that the
                  "record-set" of as-built drawings are true and correct, which
                  certification shall survive the expiration or termination of
                  the Lease, (c) to deliver to Landlord two (2) sets of copies
                  of such record set drawings within ninety (90) days following
                  issuance of a certificate of occupancy for the Premises,
                  together with any and all electronic CAD drawings, and (d) to
                  deliver to Landlord a copy of all warranties, guarantees, and
                  operating manuals relating to all non-removable fixtures
                  included as part of the Tenant Improvements.

            B.    A default under this Amendment shall constitute a default
                  under the Lease, and the parties shall be entitled to all
                  rights and remedies under the Lease in the event of a default
                  hereunder by the other party.

                                       5
<PAGE>

            C.    Without limiting the "as-is" provisions of the Lease, Tenant
                  hereby affirms and accepts the Premises, including, without
                  limitation, the concrete slab, the utility installations and
                  any and all other Building systems, in their "as-is" condition
                  and configuration and acknowledges that it has had an
                  opportunity to inspect the Premises prior to signing this
                  Amendment.

      VI.   Indemnification of Landlord.

            A.    Landlord shall not be obligated to incur any costs in
                  connection with the construction of the Tenant Improvements.
                  Tenant's indemnity of Landlord, as set forth in Article 8 of
                  the Lease, shall also apply with respect to any and all costs,
                  losses, damages, injuries and liabilities related in any way
                  to any act or omission of Tenant or Tenant's employees,
                  agents, customers, visitors, invitees, licensees, contractors,
                  assignees and subtenants ("Tenant's Agents") or anyone
                  directly or indirectly employed by any of them, or in
                  connection with Tenant's non-payment of any amount arising out
                  of the Tenant Improvements, and/or Landlord's disapproval of
                  all or any portion of any request for payment by any of
                  Tenant's Agents or anyone directly or indirectly employed by
                  any of them with respect to the construction of the Tenant
                  Improvements. Such indemnity by Tenant, as set forth in the
                  Lease, shall also apply with respect to any and all costs,
                  losses, damages, injuries and liabilities related in any way
                  to Landlord's performance of any ministerial acts reasonably
                  necessary (i) to permit Tenant to complete the Tenant
                  Improvements, and (ii) to enable Tenant to obtain any building
                  permit or certificate of occupancy for the Premises.

            B.    Neither Landlord nor any of Landlord's agents or consultants
                  shall be responsible for obtaining any building permit or
                  certificate of occupancy for the Premises and the obtaining of
                  the same shall be the sole responsibility of the Tenant.

      VII   Miscellaneous.

            A.    This Amendment sets 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.

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

                                       6
<PAGE>

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

            D.    Submission of this Amendment by Landlord is not an offer to
                  enter into this Amendment but rather is a solicitation for
                  such an offer by Tenant. Landlord shall not be bound by this
                  Amendment until Landlord has executed and delivered the same
                  to Tenant.

            E.    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.

            F.    Tenant hereby represents to Landlord that Tenant has dealt
                  with no broker in connection with this Amendment. Tenant
                  agrees to indemnify and hold Landlord, its trustees, 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 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
                  trustees, members, principals, beneficiaries, partners,
                  officers, directors, employees, and agents, and the respective
                  principals and members of any such agents (collectively, the
                  "Tenant Related Parties") harmless from all claims of any
                  brokers claiming to have represented Landlord in connection
                  with this Amendment.

            G.    Each signatory of this Amendment represents hereby that he or
                  she has the authority to execute and deliver the same on
                  behalf of the party hereto for which such signatory is acting.

                           [SIGNATURE PAGE TO FOLLOW]

                                       7
<PAGE>

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

LANDLORD:

MARINA BUSINESS CENTER, LLC,
a California limited liability company

By: /s/ W. Scott Dobbins
    --------------------

Print Name: W. Scott Dobbins

Title: Its Authorized Agent

TENANT:

CANCERVAX CORPORATION,
a Delaware corporation

By: /s/ David F. Hale
    -----------------

Print Name: David F. Hale

Title: President and CEO

                                       8