<PAGE>   1
                                                                   EXHIBIT 10.21


              GREATER CAPITAL AREA ASSOCIATION OF REALTORS(R), INC.

                    SHORT FORM COMMERICAL AND APARTMENT HOUSE
                         REAL ESTATE PURCHASE AGREEMENT

                                Date of Offer:  July 27, 1999

     THIS PURCHASE AGREEMENT ("Agreement") is made and entered into by and
between United Therapeutics Corp. ("Purchaser"), and 1106 Spring Street
Associates ("Seller").

     The parties hereto are desirous of entering into an agreement relating to
the sale by Seller to Purchaser of certain property. In consideration of the
foregoing and of the mutual covenants and agreements hereinafter set forth, the
parties hereto hereby agree as follows:

     1. DEPOSIT: DESCRIPTION OF PROPERTY. Hanes Realty, Inc. ("Agent") has
received from Purchaser a deposit ("Deposit") wire transfer (by check, by cash,
by note) of Seventy-Five Thousand Dollars ($75,000.00) to be applied as part
payment of the Purchase Price of the real property legally described as Lot P4,
Square Blk "A" with improvements thereon known by street address as 1106 Spring
Street, Silver Spring, Maryland 20910 a three story and basement office/medical
building in J.C. Wilson estates in Montgomery County together with the
easements, rights, privileges and appurtenances belonging to the same, and
including all of the furniture, fixtures, furnishings, machinery, equipment and
(see addendum) (the "Personal Property") owned by Seller situated on or about
said property (said real and personal property being collectively referred to
herein as the "Property"). Seller agrees to sell and convey to Purchaser, and
Purchaser agrees to buy from Seller, the Property, upon the terms and conditions
herein set forth.

     2. PURCHASE PRICE. The purchase price ("Purchase Price") of the Property is
One Million, Five Hundred and Forty Thousand dollars ($1,540,000.00). If the
Property shall be encumbered by a mortgage or a deed of trust which Purchaser is
to assume or take title subject to and if any regular payments on account of the
principal thereof shall be made by the Seller between the date of this Agreement
and the date of settlement, the aggregate amount of any such principal payments
shall be DNA (paid to Seller by Purchaser at settlement in addition to the
amount set forth in paragraph 3(a) hereof; added to the principal amount of the
DNA Deed of Trust provided for below.)

     3. PAYMENT OF PURCHASE PRICE. [FINANCING CONTINGENCY (IF ANY) AND
ADDITIONAL OR ALTERNATIVE FINANCING TERMS (IF ANY) ARE ADDED BY ADDENDUM
     YES    XX  NO]
-----     -----

The Purchaser agrees:

     To pay One Million, Five Hundred and Forty Thousand Dollars ($1,540,000.00)
cash on the date of settlement, of which the Deposit shall be a part.

     4. TITLE. The Property is sold free of encumbrance except as aforesaid and
except for the rights of tenants in possession; title is to be good of record
and marketable, subject, however, to covenants, rights of way, easements,
conditions and restrictions of record, if any, otherwise, said Deposit is to be
returned and this Agreement declared void at the option of the Purchaser, unless
the defects are of such character that they may be readily remedied by legal
action or the Purchaser fails to object as provided below. The Seller and Agent
are hereby expressly released from all liability for damages by reason of any
defect in unmarketability of the title. In case legal steps are necessary to
perfect the title, such action must be taken by the Seller promptly at his own
expense, whereupon the time herein specified for full settlement by the
Purchaser will thereby be extended for period necessary for such perfecting of
title. Purchaser hereby authorizes Agent to order the examination of title from
_________ ("Title Company") a survey (if required), and the preparation of all
necessary conveyancing papers, at Purchaser's expense; provided, however, that
if upon examination the title should be found defective or unmarketable, and is
not


<PAGE>   2

remedied as aforesaid, the Seller hereby agrees to pay the cost of the
examination of the title and also to pay the Agent herein the commission
hereinafter provide for just as though the sale had actually been consummated
and the parties had complied with all the terms of this Agreement. All claims of
defect in or unmarketability of title by Purchaser shall be delivered by
Purchaser to Seller within fifteen (15 days) of issuance of the title report and
shall be deemed waived if not so delivered. Issuance by a title insurance
company of a title insurance commitment naming Purchaser as the proposed insured
for an owners' policy of title insurance on American Land title Association
standard form of owners' marketability policy (ALTA Form B-1970, as from time to
time amended), conforming to the requirements of this Agreement, shall be
conclusive evidence of marketability of title.

     5. DEED. Seller agrees to execute and deliver at settlement a good and
sufficient special warranty deed: or, if the Property is located in the
Commonwealth of Virginia, seller agrees to execute and deliver at settlement a
good and sufficient general warranty deed.

     6. ADJUSTMENTS. Rents, taxes, water rent, insurance and interest on
existing encumbrances (if any), cost of fuel in storage tanks, salaries and
accrued benefits of employees (if any), and operating charges (including,
without limitation, utilities charges) are to be adjusted to the date of
transfer. Rental security deposits, if any, plus any interest thereon shall be
transferred to Purchaser at the time of settlement. Taxes, general and special,
are to be adjusted according to the certificate of taxes as issued by the
Collector of Taxes, except that assessments for improvements completed prior to
the date hereof, whether assessment therefore has been levied or not, shall be
paid the Seller or allowance made therefor at the time of transfer. If the
Property is serviced by the Washington Suburban Sanitary Commission, annual
benefit charges of said Commission are to be adjusted to date of transfer and
assumed thereafter by Purchaser.

     7. CLOSING AND RECORDING COSTS. Examination of title (except as otherwise
provided above), tax certificate, conveyancing, notary fees, survey if required,
State revenue stamps, if any, and all recording charges, including those for
purchase money trust, if any, are to be at the cost of the Purchaser. Transfer
and recordation taxes shall be equally divided between Seller and Purchaser.

     8. SETTLEMENT: FORFEITURE OF DEPOSIT. On or before the 27th day of August,
1999, or as soon thereafter as (a) the report on the title can be secured if
promptly ordered by the Purchaser, (b) an appointment for settlement can be
arranged with the Title Company, (c) a survey procured, if one is necessary, and
(d) any and all contingencies herein set forth are satisfied, the Seller and
Purchaser agree to and are required to make full settlement of said purchase in
accordance with the terms hereof. At the settlement, in addition to all other
documents required to be delivered by the provisions hereof (a) Seller shall
deliver to Purchaser a bill of sale for the Personal Property, an assignment of
such insurance policies on the Property as shall be assignable, all books and
records pertaining to the operation of the Property, plans and specifications of
the Property in Seller's possession, assignments of the leases described in
paragraph 11 hereof, such certificates, permits and licenses with respect to the
Property as shall be assignable, and DNA (additional items, if any); and (b)
Purchaser shall deliver to Seller a promissory note an deed of trust, in a form
reasonably satisfactory to Seller, for financing (if any) taken back by Seller
in accordance with the terms hereof, a security agreement and financing
statement secured by the Personal Property, and DNA (additional items if any).
As of the settlement date (a) no part of the Property shall have been acquired,
or shall be about to be acquired, by any governmental authority or agency in the
exercise of its power of eminent domain or by private purchase in lieu thereof,
nor shall there by any threat or imminence of any such acquisition or purchase;
and (b) all representations and warranties as set forth in this Agreement shall
be true and correct.

        If Purchaser shall fail to make full settlement in accordance herewith,
the Deposit herein provided for may be forfeited as liquidated damages at the
option of the Seller, in which event the Purchaser shall be relieved from
further liability hereunder.

        If after breach by Purchaser, the Seller (a) elects to declare the
Purchaser's Deposit forfeited under the provisions of this Agreement, in any
such event, the Seller and Purchaser agree that Agent shall be paid by Seller,
or shall retain from the Deposit (if then in escrow), one-half (1/2) of said
Deposit as compensation for services rendered by Agent, said amount not to
exceed the full commission as hereinafter provided. If Seller elects to declare
the Deposit forfeited, the Purchaser agrees that the Agent may deliver the
remaining one-half (1/2) of said Deposit to the Seller, to be retained as
liquidated damages; and when the foregoing has been accomplished, the Purchaser
and Seller hereby expressly relieve and release said Agent from any further
liability or responsibility.

     9. PLACEMENT OF SETTLEMENT AND TITLE COMPANY. Settlement is to be made at
the Title Company or other agency employed to examine the title. Deposit with
the Title Company or such other agency of that part of the Purchase Price to be
paid in case, the deed of conveyance for execution and such other papers as are
required of either the Seller or Purchaser by the terms of this Agreement,
within the time above provided for the


<PAGE>   3

consummation of this Agreement, shall be considered good and sufficient tender
of performance by either of said parties of the terms hereof. IT IS UNDERSTOOD
THAT THE PURCHASER HAS THE RIGHT TO NAME AND EMPLOY A PARTICULAR TITLE INSURANCE
COMPANY, SETTLEMENT, OR ESCROW COMPANY OR TITLE ATTORNEY, WHICH HAS BEEN
DESIGNATED BY PURCHASER AS SET FORTH IN PARAGRAPH 4 ABOVE.

    10. DISPOSITION OF DEPOSIT. The entire Deposit, receipt of which is hereby
acknowledged, shall be held by Agent as escrow agent and deposited in an account
in accordance with applicable law until settlement hereunder is made, or until
disposition thereof is made pursuant to the terms of this Agreement. Agent shall
have the right to disburse the Deposit to Purchaser or Seller and the interest
earned thereon to Purchaser (a) at the settlement, or (b) otherwise as provided
in this Agreement upon ten (10) days' written notice to the parties, provided,
however, that Agent shall not have received any written objections to such
disbursement within ten (10) days after receipt by Purchaser and Seller of said
notice. The parties hereto hereby acknowledge that Agent shall have no liability
to any party on account of Agent's failure to disburse the Deposit or interest
thereon if a dispute shall have arisen with respect to the propriety of such
disbursement; and, in the event of any dispute as to who is entitled to receive
the Deposit or interest earned thereon, Agent shall have the right to retain the
funds and disburse them in accordance with the final order of a court of
competent jurisdiction, or to deposit such funds with said court pending a final
decision of such controversy. The parties further agree that Agent shall not be
liable for failure of any depository in which the Deposit is placed, and that
Seller and Purchaser each will indemnify, defend and save harmless Agent from
and against every claim, liability, cost, damage, loss and expense arising out
of the disbursement of the Deposit or failure to disburse the Deposit by Agent,
except in the case of Agent's gross negligence or willful misconduct.

    11. EXISTING TENANCY. The Property is sold and shall be conveyed subject to
an existing tenancy or tenancies as follows: as listed in addendum. Without the
prior written consent of Purchaser (which shall not be unreasonably withheld),
Seller shall not (a) modify the terms or terminate any such tenancies, except
for nonpayment of rent; or (b) enter into any new leases or tenancies with
respect to the property.

    12. POSSESSION. Seller agrees to give possession of the Property at time of
settlement, subject to the aforesaid existing tenancy or tenancies. If Seller
shall fail so to do and occupies said Property, Seller shall be a DNA (tenant by
sufferance, trespasser), and Seller agrees to pay Purchaser $DNA per week for
use and occupancy, and hereby expressly waives all notice to quit, if any,
provided by law.

    13. RESIDENTIAL TENANCIES. (This clause is inapplicable and of no further
effect if the Property is not subject to residential tenancy.) Within DNA days
following the Date of Agreement, Seller shall provide to any residential tenant,
subtenant, lessee, sublessee, or other person entitled to the possession,
occupancy or benefits of a rental unit in the Property or any group thereof (the
"Tenants") and to any governmental entity such notice of the intended sale of
the Property and of their rights, as shall be required by applicable law. It is
expressly agreed that the obligations of Seller and Purchaser to make settlement
hereunder are conditioned upon the right of the Tenants and/or such governmental
entity to an opportunity to purchase the Property, as and if provided by the
applicable law. The Seller hereby expressly reserves the right to sell the
Property to the Tenants and/or such governmental entity on terms which comply
with the applicable law. Upon settlement of the sale of the Property to the
Tenants, and/or such governmental entity, this Agreement shall be void and the
Deposit shall be returned to the Purchaser within 15 days of the settlement
thereunder; provided, however, that in the event of any such settlement, Seller
shall pay to Agent the commission provided for herein. The Seller shall keep
purchaser and Agent apprised of all negotiation, correspondence, contracts and
other developments with respect to the negotiations with the Tenants and/or such
governmental entity under the applicable law.

    14. RISK OF LOSS. Seller assumes the risk of loss or damage to the Property
by fire or other casualty until the executed deed of conveyance is delivered to
Purchaser or is recorded for him by the title company making the settlement.

    15. LOCAL VIOLATIONS; CONDITION AND OPERATION OF PROPERTY. All written
notices of violations of statutes, municipal order or regulations issued by any
department of the jurisdiction in which the Property is situated, or
prosecutions in any of the courts of the jurisdiction in which the Property is
situated on account thereof, against or affecting the Property that have been
received by Seller as of the Date of Agreement shall be complied with by the
Seller and the Property conveyed free thereof, Seller sates that, tot he best of
his knowledge, all such notices and prosecutions are as set forth in the Addenda
hereto. Purchaser agrees that, except as expressly provided herein, Seller has
made no representation, warranty or other statement as to the physical
condition, operation or any other matter or thing affecting the Property.
Purchaser has inspected the Property, is


<PAGE>   4

fully familiar with the condition thereof, and Purchaser agrees to take the
Property AS IS as of the Date of Agreement. Seller shall: (a) deliver the
Property in substantially the same physical condition as of the Date of
Agreement; (b) not defer normal maintenance of the Property during the period
from the Date of Agreement to the date of settlement; and (c) not enter into,
modify or terminate any maintenance or service contracts relating to the
Property prior to the date of settlement without the prior written consent of
Purchaser.

    16. CHOICE OF LAW; REFERENCES TO DISTRICT OF COLUMBIA. This Agreement, the
rights and obligations of the parties hereto, and any claims or disputes
relating thereto shall be governed by and construed in accordance with the laws
of the jurisdiction in which the Property is located (but excluding the choice
of law rules thereof) (the "applicable law"). If the Property involved in this
Agreement is located in a jurisdiction other than the District of Columbia,
wherever any reference is made to the District or any official thereof, the name
of the jurisdiction in which the Property is located and the proper official
thereof is substituted automatically.

    17. AGENT. Seller recognizes Hanes Realty, Inc. (the "Agent) as agent
negotiating this Agreement and agrees to pay to agent a commission for services
rendered amounting to 6% of the gross Purchase Price set forth herein. The
amount of commission, as aforesaid, is hereby assigned to the Agent by Seller
out of the proceeds of sale and the Deposit. The party through or by whom
settlement hereof is made is hereby irrevocably authorized and directed to make
deduction of the aforesaid commission from the proceeds of sale and to make
payment thereof to said Agent after the proper credit for the Deposit, if any,
retained by Agent, under and by virtue of the terms hereof. However, should
settlement fail to occur within the time herein set forth, Agent shall
nonetheless be entitled to receive the commission herein provided.

    18. GENERAL MASTER PLANS; SEWAGE DISPOSAL; SUBDIVISION PLAT. IF THE
PROPERTY IS LOCATED IN MONTGOMERY COUNTY, MARYLAND, EXECUTE AND APPEND ADDENDUM
ENTITLED "GENERAL MASTER PLANS; SEWAGE DISPOSAL; SUBDIVISION PLAT."

    19. D.C. SOIL DISCLOSURE REQUIREMENTS. (This clause is inapplicable and of
no further effect if Property is not located in the District of Columbia.) The
characteristics of the soil on the Property as described by the Soil
Conservation Service of the United States of Agriculture in the Soil Survey of
the District of Columbia published in 1976 and as shown on the Soil Maps of the
District of Columbia is DNA. For further information, the Purchaser can contact
a soil testing laboratory, the District of Columbia Department of Environmental
Services, or the Soil Conservation Service of the Department of Agriculture.

    20. TIME OF ESSENCE. The Purchaser and Seller agree that time is of the
essence of this Agreement.

    21. ACCEPTANCE DATE OF AGREEMENT. This Agreement must be ratified and
accepted by the Seller and spouse (if Seller is married), within five days from
the date set forth under the title hereof (Date of Offer), in order to be
effectual and binding; otherwise the obligation of the Purchaser hereunder shall
cease and terminate and the Deposit shall be refunded. The Date of this
Agreement ("Date of Agreement") shall be the date on which this Agreement is
ratified and accepted as aforesaid.

    22. BINDING EFFECT; ENTIRE AGREEMENT. Purchaser and Seller mutually agree
that this Agreement shall be binding upon them, and their respective heirs,
executors, administrators, successors and assigns; that the provisions hereof
shall survive the execution and delivery of the deed aforesaid and shall not be
merged therein; that this Agreement contains the final and entire Agreement
between the parties hereto, and that they shall not be bound by any terms,
conditions, statements, warranties, or representations, oral or written, express
or implied, not herein contained.

    23. ASSIGNMENT. This Agreement may be assigned by either party hereto
without the consent of the other party hereto, unless paragraph 3(b) or (c)
(Deeds of Trust) is applicable, in which event, this Agreement shall not be
assigned without the prior written consent of Seller.

    24. GRANTEE. The Property is to be conveyed in the names(s) to be
designated in writing by Purchaser prior to settlement.

    25. NO REPRESENTATIONS. THE AGENT ASSUMES NO RESPONSIBILITY FOR THE
CONDITION OF THE PROPERTY NOR FOR THE PERFORMANCE OF THIS AGREEMENT BY ANY OR
ALL PARTIES HERETO PURCHASER, HEREBY WARRANTS AND REPRESENTS UNTO SELLER AND THE
REAL ESTATE BROKERS HEREIN (INCLUDING AGENT) THAT NO AGENT, SERVANT OR EMPLOYEE
OR SAID REAL ESTATE BROKERS HAS MADE ANY STATEMENT, REPRESENTATION OR WARRANTY,
ORAL OR WRITTEN, EXPRESS OR IMPLIED, TO PURCHASER REGARDING THE CONDITION OF THE
PROPERTY OF ANY PART THEREOF UPON WHICH PURCHASE HAS RELIED AND WHICH IS NOT
EXPRESSLY SET FORTH IN THIS AGREEMENT.


<PAGE>   5

    26. ADDITIONAL PROVISION. Special provisions in the attached Addendum,
bearing the signatures of all parties hereto, and all plats and surveys attached
hereto, are incorporated herein and made a part hereof. Addendum attached:
 XX  yes     no; Plat attached     yes  XX  no; Survey attached:    yes
----     ----                  ----    ----                     ----
 XX  no.
----

    27. PRONOUNS. The words "Seller," "Purchaser," all pronouns and any
variations thereof shall be deemed to refer to the masculine, feminine, neuter,
singular or plural as the identity of the person or entity and the context may
require.

    28. NOTICES. All notices required or permitted herein shall be in writing
and effective as of the date on which such notice is mailed in any United States
Post Office, by certified or registered mail, postage prepaid, or hand-delivered
to Seller, the Agent or Purchaser (as shall be applicable) at the addresses
designated herein, or to such other addresses as the parties may designate in
writing from time to time.

    We, the undersigned, hereby ratify, accept and agree to the above and
acknowledge it to be our Agreement.

THIS IS A LEGALLY BINDING AGREEMENT , PURCHASER AND SELLER SHOULD SEEK COMPETENT
LEGAL ADVICE BEFORE SIGNING.

<TABLE>
<S>                                                           <C>
/s/ M. Rothblatt for                                          /s/ Judith Danish, Managing Partner
--------------------------------------------                  -------------------------------------
Purchaser                                                     Seller

United Therapeutics Corp.                                     for 1106 Spring Street Associates
--------------------------------------------                  -------------------------------------
Purchaser                                                     Seller



                                                              DATE OF RATIFICATION AND ACCEPTANCE
                                                              (DATE OF AGREEMENT)

         7/30/                      , 1999                    9:40 P.M.      8/4/ 1999
------------------------------------------                    ------------------------------------------------

Address of Purchaser:                                         Address of Seller:

United Therapeutics                                           4932 Cloister Drive
------------------------------------------                    ------------------------------------------------
1110 Spring Street                                            Rockville, MD  20852
------------------------------------------                    ------------------------------------------------
Silver Spring, MD  20910
------------------------------------------

Telephone:                                                    Telephone:
     Residence    301-254-2462                                          Residence       301-530-9559
                  ----------------                                                ------------------------
     Office       301-608-9292                                          Office          NA
                  ----------------                                                ------------------------

Agent hereby acknowledges receipt of the Deposit and agrees to hold same pursuant to the terms of this Agreement

AGENT:            Hanes Realty, Inc.
     By
                  -------------------------------
     Title        President/Broker                 (Officer must sign)
                  -------------------------------
     Address:     519 Kennedy Street N.W.
                  -------------------------------
                  Washington, DC  20011
                  -------------------------------
     Telephone:   202-882-6800
                  -------------------------------
</TABLE>


<PAGE>   6


                                    ADDENDUM

                              Additional Provisions

Addendum to contract dated July 27, 1999 between UNITED THERAPEUTICS CORP.,
Purchaser and 1106 SPRING STREET ASSOCIATES, Seller

1.    Re: personal property -

     a.       all cleaning equipment (i.e. carts, supplies, electric buffers,
              etc.) are the property of the cleaning company

     b.       all equipment, both medical and office, in the Suite 301,
              currently occupied by Dr. Louis Butler, is the property of 1106
              Spring Street Associates.

2.   All existing service contracts (i.e. trash, elevator maintenance, cleaning)
     can be assigned or cancelled at the direction of the Purchaser.

3.   The management company, Hanes Realty, Inc., will work with the Purchaser in
     the orderly transfer of all documents and information pertaining to the
     building.

4.    Existing Tenancy:

<TABLE>
<CAPTION>
Suite #           Tenant                             Lease Expires     Monthly Rent
---------------------------------------------------------------------------------------
<S>               <C>                                <C>               <C>
#1                Community Radiology                6/3/00            $ 800.00
#2                Dr. Bernadette Soong               2/28/00             749.00
#3                Dr. Mofikpara Wright               4/30/02             641.50*
#4/5              Dr. Howard Martin                  4/30/02           1,316.00
#10               Levels of Elevation                9/30/03             845.00
#20               Glick Opticians                    5/31/00           1,800.00
#101              Community Radiology                6/30/00           3,200.00
#200              Dr. Crystal McGinty                2/28/02           2,811.62
#205              Affinity Physical Ther.            12/14/01          2,123.00
#301              Dr. Louis Butler                   m/t/m             1,216.00
#303              Dr. Stuart Dulin                   4/30/02           1,316.00
#305              Dr. Jonathan Lyons                 5/31/01           3,200.00
</TABLE>

5.   The Sellers are to pay the following settlement charges:
     State Recordation Tax..................$ 6,776.00
     Loan Origination fee 2 pts.............$23,100.00
     State Transfer Tax.....................$ 7,700.00
     County Transfer Tax....................$15,400.00
                                            ----------
                                            $52,976.00

     Broker's Commission and Seller's legal fees

<TABLE>
<CAPTION>
Purchaser:                                                    Seller:

                                                              1106 Spring Street Associates
<S>                                         <C>
By:      /s/ M. Rothblatt                   By:      /s/ Judith Danish, Managing Partner
         ---------------------------                 -----------------------------------

Date:    7/30/99                            Date:    8/4/99     9:40 pm
         ---------------------------                 ------------------
</TABLE>