<PAGE>   1
                                                                   EXHIBIT 10.16


                                FOURTH AMENDMENT
                     TO 7000 BROADWAY BUILDING OFFICE LEASE
                                    between
                    DENVER JACK LIMITED PARTNERSHIP (Lessor)
                                      and
           ALLOS THERAPEUTICS, INC., A Delaware Corporation (Lessee)
                              Dated June 29, 1998

THIS FOURTH AMENDMENT is to that certain office lease (the "Lease") dated
October 30, 1995, First Amendment to 7000 Broadway Building Office lease dated
October 30, 1995, Second Amendment to 7000 Broadway Building Office Lease dated
June 7, 1996, and Third Amendment to 7000 Broadway Building Office Lease dated
March 26, 1998, by and between DENVER JACK LIMITED PARTNERSHIP ("Lessor") and
ALLOS THERAPEUTICS, INC., A DELAWARE CORPORATION ("Lessee") with respect to
approximately 4,022 and 2,308 square feet of space on the 3rd floor, known as
Suite 310 and 320 (the "Premises"), in the building known as 7000 Broadway,
Denver, Colorado (the "Building"). In the event of any conflict between the
terms and provisions of the Lease as amended by the First, Second and Third
Amendments, and the terms and provisions of this Fourth Amendment, the terms and
provisions of this Third Amendment shall control.

     1. AMENDMENT TO ARTICLE 34 OF RIDER NUMBER ONE TO THE LEASE DOCUMENT AND
        ARTICLE 1 OF THE SECOND AMENDMENT. Lessor and Lessee do hereby agree
        that effective July 15, 1998 or when Landlord delivers Premises to
        Tenant (the "Effective Date") the description of the Premises shall be
        deemed to be amended to include approximately 9,208 rentable square feet
        of office space located on the fourth (4th) floor known as Suite 400
        (the "New Premises") as outlined on the diagram of the fourth (4th)
        floor of the building, attached hereto as Exhibit "A".

        As of the Effective Date, Lessee shall be released of its obligation
        from suite 310 excluding the shower area consisting of approximately
        3,882 rentable square feet leaving approximately 2,448 rentable square
        feet in suite 320 and shower area ("suite 320"), attached hereto as
        Exhibit "C".

        Lessee's right to occupy and obligation to pay Base Rent on the "New
        Premises" shall commence on the "Effective Date" and shall terminate
        with the lease term which is redefined in paragraph 2 of this Fourth
        Amendment.

        Except as otherwise specifically provided herein, as of the "Effective
        Date" any reference to the term "Premises" and/or "New Premises" in the
        Lease or any amendments thereto, including this Amendment, shall be
        known as suite 400, approximately 9,208 rentable square feet and suite
        320, approximately 2,448 rentable square feet and Lessee's occupancy
        thereof shall be subject to all of the terms and conditions of the
        Lease.

     2. AMENDMENT TO ARTICLE 1 OF THE LEASE DOCUMENT AND ARTICLE 2 OF THE SECOND
        AMENDMENT. Lessor and lessee do hereby agree that as of the "Effective
        Date", the lease term as described in Article 1 of the Lease and Article
        2 of the Second Amendment shall remain the same for suite 320 and shall
        be extended through July 14, 2002 for suite 400.

     3. AMENDMENT TO ARTICLE 35 OF RIDER NUMBER ONE TO THE LEASE DOCUMENT AND
        ARTICLE 3 OF THE SECOND AMENDMENT. As of the "Effective Date", Lessor
        and Lessee do hereby agree that the Rent obligation in Article 35 of
        Rider Number One and Article 3 of the Second Amendment to the lease
        document be amended as outlined below:


               Suite 400 Approximately 9,208 Rentable Square Feet
               --------------------------------------------------

<TABLE>
<S>                      <C>                      <C>

07/15/98 - 07/31/98      $ 5,680.74 per month     ($13.50/RSF/Yr)
08/01/98 - 07/31/99      $10,359.00 per month     ($13.50/RSF/Yr)
08/01/99 - 07/31/00      $10,550.83 per month     ($13.75/RSF/Yr)
08/01/00 - 07/31/01      $10,934.50 per month     ($14.25/RSF/Yr)
08/01/01 - 06/30/02      $11,126.33 per month     ($14.50/RSF/Yr)
07/01/02 - 07/14/02      $ 5,024.79 per month     ($14.50/RSF/Yr)

</TABLE>


               Suite 320 Approximately 2,448 Rentable Square Feet
               --------------------------------------------------

<TABLE>

<S>                      <C>                      <C>

07/15/98 - 7/31/98       $ 1,393.45               ($12.10/RSF/Yr)

</TABLE>
<PAGE>   2

        08/01/98 - 12/31/98          $2,541.00 per month         ($12.10/RSF/Yr)
        01/01/99 - 06/30/01          $2,608.20 per month         ($12.42/RSF/Yr)

        Such other charges as may be required by the terms of this Lease to be
        paid by Lessee may be referred to in the Lease document as "Additional
        Rent." Lessor shall have the same rights as to the Additional Rent as it
        has to the payment of the rent set forth above.

        All Payments are due on or before the first day of the first full
        calendar month of the term hereof and a like sum on or before the first
        day of each and every successive calendar month thereafter during the
        term hereof. Rent for any period during the term hereof, which is less
        than one month, shall be prorated based on a thirty (30) day calendar
        month. All rents payable hereunder shall be paid to Lessor in lawful
        money of the United States of America constituting legal tender at the
        time of payment, at the office of the building or to such other person
        at such other place as Lessor may, from time to time, designate in
        writing. The current payments or notice to Lessor shall be c/o CB
        Commercial Real Estate Group, Inc., 1050 Seventeenth Street, Suite 800,
        Denver, Colorado 80265.

     4. LESSEE IMPROVEMENTS. Lessor shall provide "turnkey" Lessee improvements
        with building standard materials per the attached Exhibit "A" prepared
        by McDermott Planning dated March 26, 1998. Any increase in cost of the
        Lessee improvements due to Lessee changing, modifying or altering the
        plan shall be the responsibility of Lessee.

     5. AMENDMENT TO ARTICLE 39 OF RIDER NUMBER ONE OF THE LEASE DOCUMENT AND
        ARTICLE 5 OF THE SECOND AMENDMENT. As of the Effective Date, Lessee
        shall have the right to use on an unassigned, unreserved basis, one (1)
        parking space for every 200 rentable square feet leased in the 7000
        Broadway parking area free of charge for the term of the Lease.

     6. AMENDMENT TO ARTICLE 38 OF RIDER NUMBER ONE OF THE LEASE DOCUMENT AND
        ARTICLE 6 OF THE SECOND AMENDMENT. Lessee's percentage of Operating
        Expenses as of the "Effective Date" shall be 5.39% for suite 320 and
        20.27% for suite 400.

     7. MOVING ALLOWANCE. Lessor shall provide Lessee with up to $18,416.00 to
        offset costs associated with moving of furniture, equipment,
        telecommunications and data communications lines, and changing of
        letterhead. Said allowance shall be paid to Lessee upon Lessee's
        furnishing original receipts of each expense to Lessor.

     8. CONFLICTS. If there is any conflict between the terms and provisions of
        this Fourth Amendment and the terms and provisions of the Lease as
        amended by the First, Second and Third Amendments, the terms and
        provisions of this Third Amendment shall govern. Except as herein
        specifically set forth, all other provisions of the Lease shall remain
        in full force and effect and be binding upon the parties in accordance
        with their terms.

LESSOR:                                      LESSEE:

DENVER JACK LIMITED PARTNERSHIP BY           ALLOS THERAPEUTICS
CB RICHARD ELLIS, INC. PROPERTY              A DELAWARE CORPORATION
MANAGER

By:  /s/  [ILLEGIBLE]                        By:  /s/  STEPHEN J. HOFFMAN
     ---------------------------------            -----------------------------

Its:   Vice President                        Its:   President
     ---------------------------------            -----------------------------

By:  /s/  [ILLEGIBLE]
     ---------------------------------

Its:   Senior Vice President
     ---------------------------------


STATE OF COLORADO

COUNTY OF ADAMS

     The foregoing instrument was acknowledge before me this 1st day of July,
1998 by Stephen J. Hoffman as President.

Witness my hand and official seal.

My commission expires: May 2, 1999                 /s/ LISA D. LANGE
(SEAL)                                             ---------------------------
                                                   Notary Public




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                                 CB COMMERCIAL








                                 [EXHIBIT "A"]









                                ALLOS SPACE PLAN
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                                  [FLOOR PLAN]
















                                 [EXHIBIT "B"]


<PAGE>   5
                                 7000 BROADWAY
                                   MANAGED BY
                                 CB COMMERCIAL







                                 [EXHIBIT "C"]









                               ALLOS THIRD FLOOR
                                  2448 R.S.F.


NOT TO SCALE                                                     [NORTH LOGO]