FIFTH LEASE AMENDMENT between BENAROYA CAPITAL COMPANY, LLC and

Exhibit 10.15(a)

FIFTH LEASE AMENDMENT

between

BENAROYA CAPITAL COMPANY, LLC

and

TARGETED GENETICS CORPORATION

This Fifth Lease Amendment dated January 8, 2004, is attached to and made part of that certain Lease dated October 7, 1996, as amended in that Fourth Lease Amendment dated March 28, 2001, that Third Lease Amendment dated April 19, 2000, that Second Lease Amendment dated February 25, 2000, and that First Lease Amendment, dated May 12, 1997 (the “Lease”), by and between Benaroya Capital Company, LLC, a Washington limited liability company (the “Lessor”), and Targeted Genetics Corporation, a Washington corporation (the “Lessee”), covering Suites 100 and 1200 located in the property commonly known as the Metropolitan Park West Tower, located at 1100 Olive Way, Seattle, Washington (the “Premises”). The Premises are more particularly described in the Lease. The terms used herein shall have the same definitions as set forth in the Lease.

NOW THEREFORE, in consideration of the mutual covenants and promises contained in this Fifth Lease Amendment and the Lease, Lessor and Lessee agree as follows:

“Lessor does hereby lease to Lessee those certain Premises, to wit: approximately 13,556 square feet of office space located on the first floor in Suite 100 (4,990 square feet) and on the twelfth floor in Suite 1200 (8,566 square feet) as outlined on Exhibit A and Exhibit A-1, respectively, attached to this Fifth Lease Amendment (hereinafter called the “Premises”) being situated within the project known as the Metropolitan Park West Tower.” The useable square foot area of suite 100 is 4,825 square feet and the current load factor per 1996 BOMA is 3.42%. The useable area for suite 1200 is 7762 useable square feet and the current load factor per 1996 BOMA is 10.36%.

a) Effective April 1, 2004, Lessee’s prorata share of operating expenses will be 4.022%.

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b) Effective April 1, 2004, the Base Year for operating expenses will be changed to 2004.

“Provided Lessee is not in default of any material term or condition of the Lease (unless the default was cured within the applicable cure period), Lessee shall have two (2) consecutive Options to Extend the lease term, one (1) period of thirty-six (36) months and, if exercised, followed by one (1) additional period of twenty-four (24) months (“Option Terms”), upon the same terms and conditions as are set forth in the Lease, except for the Monthly Minimum Rent which shall be determined as set forth below and except that the Option to Cancel provisions shall not thereafter apply. The first option shall be exercised, if at all, by written notice to Lessor before August 1, 2008, and if applicable, the second option shall be exercised, if at all, by written notice to Lessor before August 1, 2011

The rent shall be the fair market rent as agreed by Lessee and Lessor. Upon receipt of Lessee’s notice to extend, Lessor shall provide Lessee notice stating the rental rate it would be willing to accept for the extended term (the “Lessor’s Notice Rate”). Lessee shall have ten (10) days after receipt of Lessor’s Notice Rate to accept or reject such rate. In the event Lessee rejects the Lessor’s Notice Rate such rejection shall state the rate that Lessee would be willing to pay for the extended term (the “Lessee’s Notice Rate”). If the parties fail to agree on the fair market rental rate, then within two (2) days thereafter each party shall select an appraiser and the two appraisers shall within ten (10) days appoint a third appraiser (the “Determining Appraiser”). The Determining Appraiser shall make an independent determination of the rental rate for the extended term (the “New Rent”) which shall be the average prevailing rental rate for comparable space. The parties agree that for purposes of determining the New Rent, they will attempt to obtain the economic terms of any leases signed within the prior six (6) months for space on the first floors and the upper 35% of the floors of facilities within the perimeter of Denny Road to the North, Pike Street to the South, Interstate 5 to the East and 3^rd Avenue to the West. New Rent will be computed based on facilities within such geographic perimeter of comparable size, condition, age, in-building parking facilities, freeway access, central HVAC systems, views and for comparable floors and square footage. No leases in buildings without central HVAC and modern in-building parking will be considered. The Determining Appraiser will be instructed to contact the owners of those buildings, obtain the most recent lease rates for the applicable space, average the result (if there are more than one) and produce the figure which shall be considered the “New Rent” and which determination will be binding on the parties. The party whose Notice Rate is furthest from the appraiser’s New Rent shall pay the fee of the appraiser. Lessor shall prepare and Lessee shall execute a lease amendment for extension within thirty (30) days after determination of the New Rent.”

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Except as otherwise modified by the terms of this Fifth Lease Amendment, all other terms and conditions of the Lease remain unchanged and in full force and effect.

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I certify that I know or have satisfactory evidence that Larry R. Benaroya is the person who appeared before me, a Notary Public in and for the State of Washington duly commissioned and sworn, and acknowledged that he is the manager of Benaroya Capital Company, LLC, a Washington limited liability company, who executed the within and foregoing instrument, and acknowledged the instrument to be the free and voluntary act and deed of said company for the uses and purposes therein mentioned, and on oath stated that affiant is authorized to execute said instrument on behalf of said company.

IN WITNESS WHEREOF I have hereunto set my hand and affixed my official seal this 9^th day of January, 2004.

I certify that I know or have satisfactory evidence that Todd E. Simpson is the person who appeared before me, a Notary Public in and for the State of Washington duly commissioned and sworn, and acknowledged that he/she is the VP Finance & CFO of Targeted Genetics Corporation, a Washington Corporation, who executed the within and foregoing instrument, and acknowledged the instrument to be the free and voluntary act and deed of said corporation for the uses and purposes therein mentioned, and on oath stated that affiant is authorized to execute said instrument on behalf of said corporation.

IN WITNESS WHEREOF I have hereunto set my hand and affixed my official seal this 8^th day of January, 2004.

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EXHIBIT A

FLOOR PLAN OF PREMISES

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EXHIBIT A-1

FLOOR PLAN OF PREMISES

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