<PAGE>   1
                               AMENDMENT NO. 2 TO
                               PURCHASE AGREEMENT
                                     BETWEEN
                           BIOVENTURE INVESTMENTS KFT
                                       AND
                                 ENTREMED, INC.


         THIS AMENDMENT NO. 2 TO PURCHASE AGREEMENT (the "Amendment") is made
and entered into on this 30th day of July, 2001 by and between EntreMed, Inc., a
Delaware corporation ("Seller") and Bioventure Investments kft, a company
organized under the laws of Hungary and a wholly-owned subsidiary of Royalty
Pharma AG, a Swiss stock corporation ("Buyer").

         WHEREAS, Seller and Buyer are parties to that certain Purchase
Agreement dated as of June 15, 2001, as amended by Amendment No. 1 dated as of
July 13, 2001 (as so amended, the "Purchase Agreement") pursuant to which Seller
agreed, subject to the terms thereof, to sell, transfer, assign and deliver to
Buyer all of Seller's right, title and interest in and to the Net Celgene
Payments;

         WHEREAS, Seller and Buyer wish to amend the Purchase Agreement to
provide that the Purchase Agreement may be terminated at any time prior to the
Closing by either Buyer or Seller if the Closing shall not have been consummated
on or before August 3, 2001;

         NOW, THEREFORE, in consideration of the mutual promises and covenants
contained in this Amendment and in the Purchase Agreement, and pursuant to
Section 9.02(a) of the Purchase Agreement, Seller and Buyer do hereby amend the
Purchase Agreement, as follows:

         1.       Section 8.01(b) of the Purchase Agreement is hereby amended by
deleting the reference to "July 30, 2001" wherever it appears in said Section
8.01(b) and replacing it with "August 3, 2001."

         2.       Definitions. All capitalized terms used, but not defined
herein, shall have the respective meanings ascribed to them in the Purchase
Agreement.

         3.       Governing Law. This Amendment shall be governed by, construed
in accordance with, the laws of the State of New York (without regard to
conflict of laws provisions).

         4.       Entire Agreement. The Purchase Agreement, as amended hereby,
constitutes the full and entire understanding between the parties regarding the
subject matter herein. Except as otherwise expressly provided herein, the
provisions hereof shall be binding upon and inure to the benefit of the parties
hereto and their respective successors and assigns.

         5.       Full Force and Effect. Except as amended hereby, the Purchase
Agreement shall remain in full force and effect.




<PAGE>   2

                                      -2-

         6.       Counterparts. This Amendment may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

         7.       Headings. Headings in this Amendment are included for
reference only and have no effect upon the construction or interpretation of any
part of this Amendment.


                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]



<PAGE>   3




         IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed by their respective authorized officers as of the day and year
first above written.


                                 ENTREMED, INC.


                                 By: /s/ James D. Johnson
                                    -------------------------------------------
                                    Name:  James Dean Johnson
                                    Title: General Counsel



                                BIOVENTURE INVESTMENTS KFT


                                 By: /s/ I Zentai
                                    -------------------------------------------
                                    Name:  Istvan Zentai
                                    Title: Managing Director