Document:

<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

<PAGE>   3
                                 CB COMMERCIAL

                                 [EXHIBIT "A"]

                                ALLOS SPACE PLAN
<PAGE>   4
                                  [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]<PAGE>   1
                                                                   EXHIBIT 10.17

                                Lease Term Sheet

<TABLE>
<CAPTION>
Blank
 No.    Information Needed                              Applicable Information
-----   ------------------                              ----------------------
<S>     <C>                                             <C>
[1]     Date of Lease                                   July 29, 1999

[2]     Name of Tenant and type of organization         A Virginia Corporation
        (e.g., XYZ Corporation, a Virginia
        corporation)

[3]     Suite Number                                    200

[4]     Floor on which Lease Premises located           2nd

[5]     Number of rentable square feet                  1,860

[6]     Lease commencement date                         November 1, 1999

[7]     Lease Year at the end of which                  October 31, 2004
        Lease terminates

[8]     Monthly rent during first Lease Year            $4,330.70

[9]     Annual rent escalation                          Three percent

[10]    Amount of security deposit                      $4,330.70

[11]    Notice address for Tenant                       Allos Therapeutics, Inc.
                                                        7000 North Broadway
                                                        Suite 400
                                                        Denver, CO 80221
</TABLE>
<PAGE>   2

                                LEASE AGREEMENT

     THIS LEASE AGREEMENT (the "Lease"), dated July 28, 1999 between VIRGINIA
BIOTECHNOLOGY RESEARCH PARK AUTHORITY, a political subdivision of the
Commonwealth of Virginia (the "Landlord"), and ALLOS THERAPEUTICS, INC, a
Delaware Corporation (the "Tenant"), recites and provides:

RECITALS:

     The Landlord owns a real estate project located in the City of Richmond,
Virginia, known as the Virginia Biotechnology Research Park (the "Project").

     By lease dated as of August 18, 1994 (as amended, the "Master Lease"), the
Landlord leased to Virginia Commonwealth University ("VCU") the Biotech one
building (the "Building") a portion of phase one of the Project.

     The Landlord desires to lease a portion of the Building to the Tenant in
accordance with the terms of this Lease. VCU joins in this Lease solely to
release (but only during the term of this Lease) all of its right, title and
interest under the Master Lease in and to the premises hereby leased to the
Tenant, during the term of this Lease to the Tenant.

     NOW, therefore, for and in consideration of the terms, conditions,
covenants, promises and agreements herein made, the Landlord hereby leases and
demises unto Tenant the following described real property (the "Leased
Premises"), located on the 2nd floor of the Building, consisting of
approximately 1,860 rentable square feet as shown outlined in red on the floor
plan attached hereto as Exhibit A, together with a right to use in common with
other tenants of the Project the common areas and common facilities as may be
designated from time to time by the Landlord. This Lease shall be subject to all
of the terms and conditions of the Master Lease.

     1. Term. The term of this Lease (the "Term") and the Tenant's obligation to
pay rent hereunder shall commence on the later of November 1,1999 or on the date
when the improvements required to be constructed by the Landlord under Section 3
have been substantially completed (the "Commencement Date"), unless the Tenant
takes possession of the Leased Premises on an earlier date, in which event such
earlier date shall be the Commencement Date. This Lease shall terminate on the
last day of the 5th consecutive full Lease Year. The Lease may be renewed for
two more five-year periods. The term "Lease Year" as used herein shall mean a
period of 12 consecutive full calendar months. The first Lease Year shall begin
on the Commencement Date if it is the first day of a calendar month. If the
Commencement Date is

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<PAGE>   3
not the first day of a calendar month, the first Lease Year shall begin on the
first day of the calendar month immediately following the Commencement Date.
Each succeeding Lease Year shall commence on the anniversary of the first day
of the first Lease Year.

     2.   Rent. (a) Tenant covenants to pay Landlord the sum of $51,968 as base
rent during the first Lease Year in monthly installments of $4,330.70, due and
payable in advance on the first day of each month (the "Monthly Rent"), without
notice or demand and without setoff. If the Monthly Rent is not received in the
Authority's office within five (5) days after the first day of each rental
month, the Authority has the right to apply a service charge of 1.5% of the
monthly rental amount. If the Commencement Date is not the first day of a
calendar month, the Tenant shall pay on the first day of the first calendar
month following the Commencement Date a proportionate amount of the Monthly
Rent for the period of time from the Commencement Date to the date on which
such first payment of Monthly Rent is due. Rent shall be paid to the Landlord
at 800 East Leigh Street, Richmond, Virginia 23219, or to such other party and
such other place as Landlord may from time to time designate in writing as
provided herein.

          (b) The Monthly Rent shall be increased annually on each anniversary
of the first day of the first Lease Year by an amount equal to three percent
(3%) of the Monthly Rent payable during the preceding Lease Year.

          (c) All costs in addition to the Monthly Rent required to be paid by
the Tenant in accordance with the terms of this Lease shall be deemed to be
additional rent (the "Additional Rent"). Additional Rent shall be due and
payable with the next installment of Monthly Rent unless some other due date is
expressly provided under this Lease.

     3.   Tenant Improvements. The Landlord agrees to provide improvements as
shown or described on architectural plans and specifications prepared by
Baskervill & Son dated July 28, 1999 approved by the Landlord and Tenant and
incorporated herein (the "Tenant Improvements").

     4.   Use of Premises. The Tenant shall use the Leased Premises solely for
biomedical and biotechnology research and related uses approved in writing by
the Landlord and VCU.

     5.   Security Deposit.

          (a) Upon execution of this Lease, the Tenant shall deposit with the
Landlord a cash deposit (the "Security Deposit") in the amount of $4,330.70
(one month's rent). The Security

                                       2
<PAGE>   4
Deposit shall be held by the Landlord, without interest due to the Tenant, for
the term of this Lease, and until all of the Tenant's monetary obligations
under this Lease have been paid in full and Tenant has performed all of its
obligations under this Lease.

          (b) If, at any time during the term of this Lease, any Monthly Rent,
Additional Rent or other cost is not paid by Tenant when due, or if the Tenant
fails to perform any of its obligations under this Lease, the Landlord may, at
its option, apply all or any part of the Security Deposit to (1) the payment
of any such Monthly Rent, Additional Rent or other cost, and/or (2) reimburse
the Landlord for any loss or damage sustained as a result of the Tenant's
failure to perform its obligations, and/or (3) reimburse the Landlord for the
expenses incurred in performing obligations which were the responsibility of
the Tenant.

          (c) If the Security Deposit, or any part thereof, is applied by the
Landlord as provided in Section 5(b), upon written demand by the Landlord, the
Tenant shall pay to the Landlord forthwith an amount equal to the amount
necessary to restore the Security Deposit to the original amount set forth in
Section 5(a). At the termination of this Lease, if all the Tenant's monetary
obligations have been paid in full and Tenant has performed all of its
obligations under this Lease, the Landlord shall return to the Tenant the
Security Deposit or any lesser amount remaining after application of all or any
portion of the Security Deposit as provided in Section 5(b). The Landlord shall
have the right to transfer the Security Deposit to VCU or to any purchaser or
mortgagee of the Project, and upon any such transfer the Landlord shall be
discharged from any further liability with respect thereto.

     6.   Common Areas and Common Facilities. All common areas and common
facilities furnished by the Landlord in or about the Project for the general
use, in common, of tenants of the Project and their employees and invitees
shall at all times be subject to the exclusive control and management of the
Landlord. The Landlord, with the approval of VCU, shall have the right from
time to time to establish, modify and enforce reasonable rules and regulations
with respect to all such common areas and facilities. The Landlord shall have
the right to remove common areas from common use with the approval of VCU; to
construct, maintain and operate lighting facilities in or on all such common
areas and facilities; to police such areas and facilities; from time to time to
change the area and arrangement of the common facilities with the approval of
VCU; to close temporarily all or any portion of such areas or facilities for
maintenance purposes; and to do and perform such other acts in and to such
areas and facilities as the Landlord, in its reasonable discretion, shall

                                       3

<PAGE>   5
deem advisable. The Tenant shall have the right to use, in common with other
tenants of the Project, all such common areas and facilities of the Project. In
the event of removal of the right to use a common area or diminution of a
common area, the Landlord shall not be subject to any liability for damages
caused to Tenant by such removal or diminution, nor shall the Tenant be
entitled to any compensation or diminution or abatement of Monthly Rent or
Additional Rent.

     7.   Services and Utilities.

          (a) The Landlord shall provide the Tenant the following services
during the term hereof:

               (i) Hot and cold water and lavatory supplies. Hot and cold water
shall be provided only at those points of supply provided for the general use of
other tenants of the Project.

               (ii) Automatically operated elevator service.

               (iii) Cleaning and janitorial services after normal business
hours Monday through Friday of each week, except holidays recognized by the
Commonwealth of Virginia, which services shall not be performed between 9:00
a.m. and 5:00 p.m.

               (iv) Heating and air conditioning in season Monday through Friday
from 8:00 a.m. to 6:00 p.m. and on Saturday from 9:00 a.m. to 2:00 p.m., except
on the following holidays: Christmas, New Year's Day, Thanksgiving, July 4th,
Memorial Day and Labor Day.

               (v) Maintenance and electric lighting service for all common
areas in the Project.

               (vi) Electricity and proper electrical facilities to furnish
sufficient electricity, comparable to services provided for typical business
office operations, for equipment of the Tenant installed under this Lease. To
the extent the Leased Premises is not then separately metered, then if any
portion or all of the Tenant's equipment shall cause its demand for electricity
to exceed an average of 5 watts per square foot of area in the Leased Premises,
the Tenant shall pay for the cost of such excess electricity at the rate charged
by the public utility for furnishing such excess, assuming continuous operation
of the Tenant's equipment during normal business hours.

          (b)  If any law, regulation or executive or administrative order
requires the Landlord or Tenant to reduce or maintain at a certain level the
consumption of electricity from the Leased Premises or the Project which
affects the heating, air

                                       4
<PAGE>   6
conditioning, lighting or hours of operation of the Leased Premises or the
Project, the Tenant shall adhere to and abide by such laws, regulations or
executive or administrative order without any reduction in rent.

          (c)  Failure by the Landlord to any extent to furnish the services
listed above or any cessation thereof resulting from causes beyond the control
of the Landlord shall not render the Landlord liable for damages to either
person or property, nor be construed as an eviction of the Tenant, nor work an
abatement of rent, nor relieve the Tenant from fulfillment of any of the terms
and conditions of this Lease.  If any equipment or machinery ceases to function
properly, the Tenant shall have no claim for rent abatement or damages due to
any interruptions in service occasioned thereby or resulting therefrom.

     8.   Tenant's Equipment, Fixtures and Alterations. (a)  The Tenant will not
install in the Leased Premises any electrically operated equipment or other
machinery, other than personal desktop computers (other than mainframe
computers), electric typewriters or word processing equipment, adding machines,
radios, televisions, tape recorders, dictaphones, bookkeeping machines, copying
machines, a coffee-maker, a small refrigerator and clocks, without the prior
written consent of the Landlord, which consent may be conditioned upon payment
by the Tenant of Additional Rent as compensation for additional consumption of
electricity or water, to the extent the Leased Premises is not then separately
metered.  The Tenant shall not install any equipment which will necessitate any
changes, replacements or additions to, or changes in the use of, the water
system, heating system, plumbing system, air-conditioning system or electrical
system in the Leased Premises without the prior written consent of the
Landlord.  If Tenant uses equipment that increases the heat levels in the Leased
Premises above those customarily found in an office, Landlord shall have the
option to install supplemental air conditioning equipment.  All costs of such
installation, operation, maintenance and replacement of equipment by
Landlord shall be at Tenant's expense.

          (b)  The Tenant shall not make or cause to be made any interior or
exterior modifications or additions to the Leased Premises without the prior
written consent of the Landlord.  The Tenant shall present to the Landlord plans
and specifications for such work at the time such consent is sought. If the
plans and specifications are approved, the Tenant shall employ a contractor,
selected by the Tenant and approved by the Landlord, to make the improvements.
All improvements shall comply with the requirements of all federal, state and
local governmental authorities.  The Tenant shall be solely responsible for the
prompt payment of all costs related to the improvements and, if any mechanics'
or materialmen's lien is filed, the Tenant shall

                                        5
<PAGE>   7
have such lien released by payment or bond within ten days after the Tenant
receives notice thereof.

          (c) Upon the termination or expiration of the term of this Lease, the
Tenant shall remove all trade fixtures and furnishings installed by the Tenant
and not a part of the real estate or mechanical equipment and shall repair all
damage to the Leased Premises caused by the installation or removal of any
trade fixtures or other furnishings. All trade fixtures and furnishings which
the Tenant has not removed prior to the termination or expiration of the term
hereof shall, at the Landlord's option, become the property of the Landlord.

     9.   Tenant's Insurance; Compliance with Laws.

          (a) Tenant shall maintain at its sole cost and expense the following
insurance with respect to the Leased Premises:

               (i)   Contractual and commercial general liability insurance
against claims for bodily injury, death or property damage occurring on, in or
about the Project, which insurance shall be written on an occurrence basis and
shall provide protection with a combined single limit of Two Million
($2,000,000) Dollars (or in such increased limit to reflect declines in the
purchasing power of the dollar as Landlord may, from time to time, reasonably
request). Tenant shall hold the Landlord and VCU harmless from any liability for
death or bodily injury to any person or damage to property owned by a third
party occurring in the Leased Premises arising from any cause whatsoever.

               (ii)  Insurance for the Tenant's furniture, furnishings,
equipment, improvements and trade fixtures located in the Leased Premises under
a standard fire and extended coverage insurance policy, in amounts to prevent
Landlord, and Tenant from becoming co-insurers, and in any event, in amounts not
less than the actual replacement cost of such property. The Landlord shall not
be responsible for providing insurance of any kind on the Tenant's furniture and
furnishings or its equipment, improvements or trade fixtures and the Tenant
shall be solely liable to repair or replace the same in the event of damage or
destruction to the Leased Premises.

               (iii) Such additional and/or other insurance in such amounts as
is requested by Landlord and customarily carried by prudent tenants with respect
to improvements similar in character, location and use.

          (b)  Each policy of insurance required by this Section shall name the
Landlord, VCU and any person or entity having an interest in the Project and
designated by the Landlord or VCU as

                                       6
<PAGE>   8
insured parties and shall provide that the insurer shall not cancel or change
the terms of such insurance policy without first giving the Landlord, VCU and
other insureds thirty (30) days' prior written notice thereof.  The insurance
policies shall be issued by an insurance company, licensed to do business in
Virginia, and approved by the Landlord.  Copies of the policies or the issuer's
certificates of insurance shall be delivered to the Landlord, VCU and to all
other insured parties on or before the Commencement Date and thereafter promptly
upon request.

          (c)  The Tenant shall not do or allow to be done in or about the
Leased Premises anything which is prohibited under any policy of insurance
carried by the Landlord or VCU. If Tenant's use or occupancy of the Leased
Premises causes the premium for fire or other insurance carried by the Landlord
or VCU to be higher than the premium otherwise payable for such insurance, the
Tenant shall pay the difference as Additional Rent.

          (d)  The Tenant shall at all times and at its expense, promptly
comply with, or promptly cause to be complied with, all applicable laws, rules,
regulations and other governmental requirements, now existing or hereafter
enacted, to which the Tenant or the Leased Premises is subject.

     10.  Landlord's Access to Leased Premises.    The Landlord shall have the
right at all reasonable times to enter the Leased Premises to perform routine
maintenance, repairs and alterations, to inspect the Leased Premises and to
show such property to prospective tenants, mortgages or purchasers.  If the
Tenant is not present to open and permit entry to the Leased Premises at any
time when for any reason entry there is necessary or permissible hereunder, the
Landlord may use a master key to enter the Leased Premises.  The Landlord may
enter the Leased Premises at any time to make emergency repairs and may
forcibly enter the Leased Premises in an emergency situation.  Such entry shall
not be deemed to constitute an eviction of the Tenant or to give the Tenant any
right to abatement of rent for loss or interruption of the business of the
Tenant.  The Landlord shall have the right to place upon the Leased Premises
"For Rent" signs at any time during the final Lease Year.

     11.  Surrender of Possession.      The Tenant shall, at the expiration or
termination of the term of this Lease, surrender possession of the Leased
Premises peacefully and promptly in as good condition as at the commencement of
its occupancy, ordinary wear and tear excepted.  The Tenant shall surrender all
keys used in connection with the Leased Premises.

                                        7
<PAGE>   9
     12.  Tenant's Personal Property.

          (a) All property of the Tenant kept or stored in the Leased Premises
shall be kept or stored at the sole risk of the Tenant, and the Tenant shall
hold the Landlord and VCU harmless from any liability for loss of or damage to
such property.

          (b) The Tenant agrees to notify the Landlord immediately of any fire
or other casualty or known defect in the Leased Premises, the Building and in
the fixtures or equipment located therein.

          (c) The Tenant shall be responsible for and shall pay when due all
municipal, city or state taxes assessed during the term of this Lease against
any leasehold interest or personal property of any kind, owned by or placed in
the Leased Premises by the Tenant.

     13.  Rules and Regulations. The rules and regulations attached to this
Lease as Exhibit C, as amended and supplemented from time to time as provided
below, are hereby made a part of this Lease (the "Rules and Regulations"), and
Tenant shall be subject to all provisions thereof. The Landlord, with the
approval of VCU, reserves the right to make reasonable amendments and
supplements to the Rules and Regulations. The Landlord shall give the Tenant at
least ten (10) days written notice prior to the implementation of any
amendments or supplements.

     The Tenant shall be liable for the payment of all costs and expenses,
including reasonable attorneys' fees incurred by the Landlord in enforcing the
Rules and Regulations and other provisions of this Lease.

     14.  Indemnification. (a) The Tenant agrees to indemnify and hold the
Landlord and VCU harmless from all claims, actions, damages, liability and
expense in connection with loss of life or personal injury to persons and
damage to property arising in or about the Leased Premises, or the occupancy or
use of the Leased Premises, or any part thereof, by the Tenant, or occasioned
wholly or in part by any act or failure to act by the Tenant, its agents,
contractors, employees, or subtenants.

          (b) The Tenant waives all claims, demands or rights of indemnity
against the Landlord or VCU due to loss of or damage to the Leased Premises or
to any personal property located therein resulting from fire or other casualty
regardless of the cause of the damage or destruction.

     15.  Estoppel Certificate. Upon request by the Landlord, the Tenant shall
deliver to the Landlord, or to any party designated by the Landlord, a written
statement in recordable

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<PAGE>   10
form certifying: the commencement and termination dates of this Lease; that
this Lease is or is not in full force and effect; that the Tenant has or has
not accepted the Leased Premises and is or is not in complete possession
thereof; that this Lease has not been modified or amended, or if it has,
stating the specific modifications or amendments thereto; that all improvements
to the Leased Premises to be made by the Landlord have been fully completed in
accordance with the applicable plans and specifications or stating specifically
any failure to complete such improvements; that, as of the date of
certification, the Tenant has not paid Monthly Rent more than 30 days in
advance (or stating the amount of rent paid more than 30 days in advance);
that no proceedings are pending or threatened against Tenant before or by any
court or administrative agency which, if adversely decided, would materially
and adversely affect this Lease or the Tenant's ability to perform its
obligations hereunder or, if any such proceedings are pending or threatened,
specifying and describing such proceedings; and that there are no defaults
under this Lease, nor defenses or offsets, or stating the specific defaults,
defenses or offsets claimed by the Tenant. Such statement shall include any
other certification reasonably requested.

                   16. Attornment. In the event of the exercise of any power of
sale under the provisions of any mortgage or deed of trust now or hereafter
encumbering the Project, or a transfer of the Project in lieu thereof, the
Tenant agrees that, if requested by the purchaser at such foreclosure sale or by
the transferee, it shall attorn to the purchaser or transferee and it shall
recognize such purchaser or transferee as the Landlord under the terms of this
Lease and shall continue this Lease in full force and effect regardless of
whether such mortgage or deed of trust was superior or subordinate to this
Lease.

         17. Subordination. This Lease is subject and subordinate to the lien
and the provisions of any and all mortgages, deeds of trust and other security
instruments which may now or hereafter encumber or otherwise affect the
Project, and to all renewals, replacements, extensions and modifications
thereof. The foregoing subordination shall be self-operative and effected by
execution of this Lease. Upon request by the Landlord, the Tenant agrees to
confirm the subordination of its rights hereunder to any mortgage or deed of
trust or any other security instrument resulting from any method of financing
now or hereafter affecting the Project and to all advances made or hereafter to
be made thereunder. Notwithstanding the foregoing, the beneficiary of any deed
of trust on the Project may, at its option, subordinate at any time its deed of
trust to this Lease by executing and recording a unilateral declaration to such
effect.

                                       9

<PAGE>   11
     18.  Assignment and Subletting.

          (a) The Tenant shall not mortgage or assign this Lease, in whole or
in part, nor sublet all or any part of the Leased Premises without the prior
written consent of the Landlord and VCU, which consent may be withheld, delayed
or conditioned in the sole and absolute discretion of Landlord and VCU. This
prohibition on assignment and subletting shall include a prohibition against
any assignment or subletting by operation of law. Any consent given by the
Landlord and VCU to any mortgage, assignment or subletting by the Tenant shall
not constitute a waiver of the requirement for such consent to any subsequent
mortgage, assignment or subletting, nor shall the Tenant be released from
performing any of the terms, covenants and conditions of this Lease. If this
Lease is assigned or the Leased Premises sublet, the Landlord may collect
Monthly Rent and Additional Rent and other costs from the assignee or sublessee
and apply the amount collected to Tenant's obligation for Monthly Rent,
Additional Rent and other costs due in accordance with the provisions of this
Lease.

     19.  Damage or Destruction. (a) If the Leased Premises are damaged by fire
or other casualty, but no portion thereof is thereby rendered untenantable for
the purpose or use for which Tenant has leased the Leased Premises in
Landlord's reasonable opinion, upon being so notified by Tenant by certified or
registered mail, return receipt requested, the Landlord shall [except as
provided in paragraph (ii) of Section 10(a)], at its expense, cause such damage
to be repaired and the rent shall not be abated. If by reason of fire or other
casualty, the Leased Premises are rendered untenantable in part for the purpose
or use for which Tenant has leased the Leased Premises in Landlord's reasonable
opinion, the Landlord shall [except as provided in paragraph (ii) of Section
10(a)], at its expense, subject to availability of insurance proceeds, cause
such damage to be repaired, and the Monthly Rent shall be adjusted on a
pro-rata basis for the period of such repair and restoration for that portion
of the Leased Premises rendered untenantable.  If the leased Premises are
damaged by fire or other casualty so as to render same untenantable for the
purpose or use for which Tenant has leased same in Landlord's reasonable
opinion, the Landlord shall [except as provided in paragraph (ii) of Section
10(a)], at its expense, subject to availability of insurance proceeds, cause
such damage to be repaired, and the Monthly Rent shall be abated in full for
the period of such repair and restoration, provided, however, that if, within
60 days after the fire or other casualty, the Landlord gives the Tenant written
notice that it has elected not to cause the Leased Premises to be repaired and
restored, then this Lease shall terminate as of the date of the fire or other
casualty.

                                       10
<PAGE>   12
          (b)  If the Project is damaged by fire or other casualty with the
result that its general use is impaired, then, notwithstanding that the Leased
Premises may be undamaged, the Landlord shall have the right for ninety (90)
days following the date of the fire or other casualty, to terminate this Lease
by giving the Tenant 30 days' prior written notice of the termination.

     20.  Eminent Domain.   If all or part of the Leased Premises is taken or
condemned by any authority or in the event of any purchase in lieu of any such
taking or condemnation, this Lease shall terminate as of the date on which the
Tenant is deprived of possession of the Leased Premises.  Any award for the land
and buildings of which the Leased Premises are a part and for damages to the
residue, or any negotiated payment by private sale in lieu thereof, shall be
the property of the Landlord, and the Tenant hereby assigns to the Landlord all
its right, title and interest in and to any such award or payment.  The Tenant
shall bear the cost of removing its property from the Leased Premises.

     21.  Environmental Hazards.  Tenant will not on, about or under the Leased
Premises, handle, store, make, treat or dispose of any "Hazardous Materials"
(defined below), except in accordance with all applicable federal, state and
municipal laws and regulations (collectively, the "Environmental Laws")
governing Hazardous Materials.  "Hazardous Materials" as used herein shall
include, without limitation, "hazardous substances" as defined in the
Comprehensive Environmental Response, Compensation and Liability Act, 42
U.S.C. Section 9601 et seq. ("CERCLA"); all chemicals, petroleum, crude oil or
any fraction thereof, hydrocarbons, polychlorinated biphenyls (PCBs), asbestos,
asbestos-containing materials and/or products, urea formaldehyde, or any
substances which are classified as "hazardous" or "toxic" under CERCLA;
hazardous waste as defined under the Solid Waste Disposal Act, as amended, 42
U.S.C. Section 6901; air pollutants regulated under the Clean Air Act, as
amended, 42 U.S.C. Section 7401, et seq.; pollutants as defined under the Clean
Water Act, as amended, 33 U.S.C. Section 125, et seq., any pesticide as defined
by Federal Insecticide, Fungicide, and Rodenticide Act, as amended, 7 U.S.C.
Section 136, et seq., any hazardous chemical substance or mixture or imminently
hazardous substance or mixture regulated by the Toxic Substances Control Act, as
amended, 15 U.S.C. Section 2601, et seq.; any substance listed in the United
States Department of Transportation Table at 45 CFR 172.101; any chemicals
included in regulations promulgated under the above listed statutes; any
explosives, radioactive material, and any chemical regulated by state statutes
similar to the federal statutes listed above and regulations promulgated under
such state statutes.

          During the term of this Lease, to the extent at any

                                        11
<PAGE>   13
time required by the Environmental Laws, Tenant shall remove any Hazardous
Materials placed, generated or stored by it on or about the Leased Premises,
and Tenant shall defend, indemnify and hold the Landlord and VCU, their
officers, directors, shareholders, partners and employees harmless from and
against any and all causes of action, suits, demand or judgments of any nature
whatsoever, losses, damages, penalties, expenses, fees, claims, costs
(including, but not limited to, attorneys' fees and costs of litigation,
arising out of or in any manner connected with the violation of any applicable
federal, state or local environmental law with respect to the Leased Premises;
the handling, storage, "release" or "threatened release" of or failure to
remove, as required by this Section, Hazardous Materials from the Leased
Premises.

         Tenant represents and warrants that it will not install any
underground storage tank without specific, prior written approval from the
Landlord and VCU. Tenant will not store combustible or flammable materials on
the Leased Premises in violation of the Environmental Laws.

     22. Default; Remedies upon Default.

         (a) Events of Default: Any one or more of the following events shall
constitute a default (a "Default") under this Lease:

             (i)    Failure by the Tenant to pay any Monthly Rent, Additional
Rent or other amount due under this Lease within five days after the date when
due.

             (ii)   Failure by the Tenant to observe or perform any covenant,
condition or agreement on its part to be observed or performed hereunder or
under the Rules and Regulations, other than as referred to in clause (i) above,
which failure shall continue for a period of thirty (30) days after written
notice of such failure is given by the Landlord to the Tenant.

             (iii)  The commencement by Tenant, or the commencement against
Tenant, of a case under federal bankruptcy law, as now or hereafter
constituted, or under any other applicable federal or state bankruptcy,
insolvency, moratorium or similar law.

         (b) Remedies upon Default.

             (i)    Whenever any Default shall have occurred, the Landlord may
take any one or more of the following actions:

                    (a)  The Landlord may immediately re-

                                       12
<PAGE>   14
enter and take possession of all or any part of the Leased Premises without
terminating this Lease, exclude the Tenant from possession of all or any part
of the Leased Premises, and, for the account of the Tenant, keep in force and
effect any assignments or subleases entered into with the consent of Landlord
pursuant to Section 18 (or which Landlord decides to acknowledge even if not
consented to) which then are outstanding and not in default and lease all or
any part of the Leased Premises which is not then so subleased or assigned to
one or more parties, at a rental rate and upon such terms and conditions as
Landlord deems acceptable in its sole discretion, in each case holding the
Tenant liable for the difference between the proceeds of the reletting (less
costs of reletting including without limitation brokerage fees, architect's
fees, attorneys' fees and costs of alterations and redecorating) and all
Monthly Rent, Additional Rent and other amounts due from Tenant under this
Lease.

                    (b) The Landlord may immediately terminate this Lease as to
all or any part of the Leased Premises and exclude the Tenant from possession of
all or any part of the Leased Premises.

                    (c) The Landlord may immediately terminate this Lease and
declare all payments of Monthly Rent due from the date of termination through
the date that would otherwise have been the normal expiration of this Lease to
be immediately due and payable, discounted to present value as of the date of
payment at an annual rate equal to six percent per year.

                    (d) The Landlord may exercise rights of self-help and/or
take whatever action at law or in equity may appear necessary or desirable to
collect the Monthly Rent, Additional Rent or other amounts then due and
thereafter to become due, or to enforce performance and observance of any
obligation, agreement or covenant of the Tenant under this Lease. No termination
of this Lease shall relieve Tenant from liability for payment of Monthly Rent,
Additional Rent and other amounts accrued to the date of termination.

          (ii) No re-entry or taking possession of the Leased Premises by the
Landlord shall be construed as an election by the Landlord to terminate this
Lease unless (i) written notice of such termination is given by the Landlord to
the Tenant, or (ii) this Lease is terminated by an order or a decree of a court
of competent jurisdiction. Notwithstanding any re-entry and taking possession
by the Landlord of all or any part of the Leased Premises without terminating
this Lease, the Landlord may at any time thereafter elect to terminate this
Lease for any previous Default by the Tenant. Upon and re-entry and taking

                                       13
<PAGE>   15
possession of the Leased Premises by Landlord, or upon any termination of this
Lease, any property of the Tenant may be removed by Landlord, without liability
for loss of or damage to said property, and stored elsewhere at the risk of and
at the expense of Tenant.

          (iii)  Upon a Default, the Tenant shall on demand pay Landlord as
Additional Rent all reasonable attorneys' fees, court costs and other
reasonable expenses incurred by Landlord in the collection of Monthly Rent,
Additional Rent and other amounts due under this lease and/or the enforcement
of any other obligations of Tenant. Upon a Default, the Tenant shall on demand
pay Landlord as Additional Rent all reasonable expenses incurred by the
Landlord in attempting to cure the Default by the exercise of self-help.

          (iv)   No delay or omission by the Landlord in exercising any right
accruing upon a Default shall be construed to be a waiver of any such right,
but any such right may be exercised from time to time as often as deemed
expedient.

     23. Force Majeure.

     If by reason of acts of God, strikes, lockouts or other industrial
disturbances; acts of public enemies; orders of any kind of the government of
the United States or the Commonwealth of Virginia, or any civil or military
authority; insurrections; riots; epidemics; landslides; lightning; earthquakes;
fires; hurricanes; tornadoes; blizzards, or other storms; floods, washouts;
droughts; arrests; restraint of government and people; civil disturbances;
explosions; breakage or accident to machinery; partial or entire failure of
utilities; or any cause or event not reasonably within the control of the
Landlord, the Landlord is unable in whole or in part to carry out its
agreements contained in this Lease, the Landlord shall not be deemed in Default
during the continuance of such inability.

     24. Successors. Subject to the provisions of this Lease prohibiting or
restricting the right of the Tenant to encumber, sublease or assign its
interests under this Lease and the provisions of the Section headed
"Subordination", this Lease shall be binding upon and inure to the benefit of
the Landlord, VCU and the Tenant and their respective successors and assigns.

     25. Non-Waiver. The failure of the Landlord, to enforce strict compliance
with any of the terms and conditions of this Lease shall not constitute or be
construed as a waiver or relinquishment of the right to enforce strict
compliance thereafter with the terms and conditions of this Lease. The payment
of rent by the Tenant or the acceptance of rent by the Landlord, with knowledge
of the breach of any term or condition

                                       14
<PAGE>   16
of this Lease, shall not be deemed a waiver of such breach.

     26. Notices. All notices required or permitted by this Lease shall be in
writing and delivered or sent by registered or certified mail and addressed as
follows:

     If To the Landlord:

     Virginia Biotechnology Research Park Authority
     800 East Leigh Street
     Richmond, Virginia 23219
     Attn: Executive Vice President

     If to the Tenant:

     Dr. Stephen J. Hoffman
     President & Chief Executive Officer
     Allos Therapeutics, Inc.
     7000 North Broadway
     Suite 400
     Denver, CO 80221

Either party may, at any time, designate in writing a substitute address for
the address set forth above, and thereafter notices shall be directed to such
substitute address.

     27. Exculpation. In the event of a breach by the Landlord of any of its
obligations under this Lease, any monetary judgment against the Landlord, shall
be satisfied solely from the equity of the Landlord in the Project.

     28. Titles. The titles and paragraph headings used herein are for purposes
of convenience only and shall not be construed to limit or extend the meaning
of any part of this Lease.

     29. Applicable Law. This Lease shall be governed by and construed in
accordance with the laws of the Commonwealth of Virginia.

     30. Entire Agreement. This Lease constitutes the entire full and complete
understanding and agreement of the parties and all representations, conditions,
statements, warranties, covenants, promises or agreements previously made or
given by either party to the other are expressly merged into this written Lease
and shall be null, void and without legal effect; provided, however, that this
Lease does not modify or amend the provisions of the Master Lease. Except for
amendments and supplements to the Rules and Regulations as expressly permitted
under the terms of this Lease, this Lease shall not be altered, amended or
modified except by written agreement of the parties hereto with the same
formality as this Lease.

                                       15
<PAGE>   17
     31. EDA Grant Requirements. In the event the Leased Premises are located
in whole or in part on the second floor of the Project, then the following
provisions shall apply:

          (i) Use. The Tenant (i) acknowledges Leased Premises may be improved,
in part, with funding from the United States Economic Development
Administration (the "EDA"), Award Number 01-01-03469 (the "EDA Grant"), and
(ii) agrees to use the Leased Premises in a manner consistent with the
authorized general and special purpose of the EDA Grant, if obtained, which is
biomedical or biotechnology related businesses.

          (ii) Nonrelocation. The Tenant agrees to comply with EDA's
Nonrelocation Regulation as set forth in 13 CFR 309.3, which is incorporated
herein by this reference. Under this restriction, EDA financial assistance may
not be used directly or indirectly to assist employers who transfer one or more
jobs from one commuting area to another. A commuting area is that locality of
the project receiving EDA financial assistance. Upon request by the Landlord,
the Tenant shall execute the EDA form Employer's Certificate of Nonrelocation.

          (iii) Nondiscrimination. The Tenant agrees to provide service without
discrimination to all persons without regard to their age, race, color,
religion, sex, handicap or national origin. Upon request by the Landlord, Tenant
shall execute the EDA form Assurance of Compliance with Civil Rights and Other
Legal Requirements for Other Parties.

          WITNESS the following signatures:

                          [SIGNATURE PAGES TO FOLLOW.]

                                       16
<PAGE>   18
                                 SIGNATURE PAGE

                                LEASE AGREEMENT

                             VIRGINIA BIOTECHNOLOGY RESEARCH
                             PARK AUTHORITY, a political
                             subdivision of the Commonwealth of
                             Virginia

                             By: /s/ ROBERT T. SKUNDA       (SEAL)
                                ----------------------------
                             Name: Robert T. Skunda
                             Title: President and CEO

                                       17
<PAGE>   19
                                 SIGNATURE PAGE

                                LEASE AGREEMENT

                                        ALLOS THERAPEUTICS, INC.
                                        A Virginia Corporation

                                        By: /s/ STEPHEN J. HOFFMAN    (SEAL)
                                           ---------------------------
                                        Name: Dr. Stephen J. Hoffman
                                        Title: President and CEO

                                       18
<PAGE>   20
                                 SIGNATURE PAGE

                                LEASE AGREEMENT

The undersigned joins in the execution of this Lease for the sole purpose of
releasing (but only during the term of this Lease) all of its right, title and
interest under the Master Lease in and to the premises hereby leased to the
Tenant.

                                           COMMONWEALTH OF VIRGINIA, VIRGINIA
                                           COMMONWEALTH UNIVERSITY

                                           By: /s/ EUGENE P. TRANI        (SEAL)
                                              ----------------------------
                                           Name:  Eugene P. Trani
                                           Title: Chair, Virginia
                                                  Biotechnology
                                                  Research Park Authority

                                       19
<PAGE>   21
                                LIST OF EXHIBITS

Exhibit A - Floor Plan

Exhibit B - Tenant Improvements

Exhibit C - Rules and Regulations

Exhibit D - In Premises Support/Shared Access/Shared Use of Common Facilities
            and Equipment in the Virginia Biotechnology Center

                                       20
<PAGE>   22
                                  [FLOOR PLAN]

                                  EXHIBIT "A"

NOTE:

ALL BIOTECH ONE CALCULATIONS ARE TO BE CONSIDERED USABLE SQUARE FEET

BIOTECH ONE - SECOND FLOOR
--------------------------------------------------------------------------------

BASKERVILL & SON

<PAGE>   23
                                   EXHIBIT B

                              TENANT IMPROVEMENTS

Architectural Plans developed by Baskervill & Son dated July 28, 1999.

                                       22
<PAGE>   24
                                   EXHIBIT C

                             RULES AND REGULATIONS

     The following rules and regulations shall be a part of the foregoing Lease
and shall be observed by the Tenant:

     1.  The sidewalks, entrances, elevators, stairways, corridors and other
common areas in and about the Project shall not be obstructed or encumbered by
the Tenant or used for any purpose other than ingress and egress to and from
the Leased Premises. The Tenant shall not interfere with the use and enjoyment
by other tenants of the common areas of Biotech One.

     2.  The Tenant shall not place or maintain any sign, awning, canopy,
advertising matter or other thing on any exterior part of the Leased Premises,
on the doors or on the windows in the Leased Premises, or in any other location
in Biotech One without the prior written consent of the Landlord. No drapes,
blinds, shades or screens shall be attached to or hung in, or used in
connection with any window or door of the Leased Premises without the prior
written consent of the Landlord.

     3.  The Tenant, at its expense, shall make any repairs to the Leased
Premises made necessary as a result of any failure of the Tenant to perform its
obligations hereunder or by the negligence or willful misconduct of the Tenant,
its agents,

                                       23

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