<PAGE>

                                                                   Exhibit 10.53

                               FIRST AMENDMENT TO
                            LOAN EXTENSION AGREEMENT

     This First Amendment to Loan Extension Agreement (this "Amendment") is
entered into by and between Polar Molecular Corporation, a Delaware corporation
(the "Company"), and Lockhart Holdings, Inc. (the "Holder") to be effective as
of January 31, 2003.

                                    RECITALS

A.   The Company issued to the Holder a Promissory Note (the "Note") in the
     principal amount of $200,000 dated as of October 19, 2001;

B.   The Holder and the Company have entered into a Loan Extension Agreement
     (the "Loan Extension Agreement"), pursuant to which certain terms of the
     Note have been amended; and

C.   The Holder and the Company now desire to amend the Loan Extension to
     provide for the extension of additional credit under the Note in the
     aggregate principal amount of $25,000 (the "Additional Credit Amount"); and

D.   The Holder and the Company acknowledge that the outstanding principal
     amount under the Note as of January 31, 2003 after giving effect to this
     Amendment is $225,000.

                                    AGREEMENT

     In consideration of the foregoing and the mutual promises contained herein,
the Holder and the Company hereby agree as follows:

1.   Section 1 of the Loan Extension Agreement is hereby amended to read in its
     entirety as follows:

     "1. Section 1 of the Note is hereby amended to read in its entirety as
     follows:

     "1. Conversion. The holder of this Note (the "Holder") is entitled, at its
     option, on the Maturity Date to notify the Company to convert up to fifty
     percent (50%) of the outstanding principal amount of this Note into options
     to purchase fully paid and nonassessable shares of common stock, $0.0001
     par value, of the Company ("Common Stock"), at the initial price of $.20
     per share of Common Stock (the "Conversion Price"). The Conversion Price
     shall be subject to adjustment as specified in Section 5 hereof. Conversion
     of this Note shall be effected by surrender of this Note to the Company at
     its address specified below (or such other address as the Company shall
     have given written notice to the Holder hereof), together with a request
     that this Note be converted to options to purchase Common Stock as
     specified herein.""

2.   A new paragraph shall be added to the Loan Extension Agreement to read in
     its entirety as follows:

<PAGE>


     "7.  Principal Amount. As of January 31, 2003, the principal amount
          outstanding under the Note shall be increased by $25,000 and all
          references to outstanding principal contained in the Note shall hereby
          be changed to "$225,000" (which represents the aggregate principal
          amount outstanding under the Note as of January 31, 2003).
          Notwithstanding the extension of the Additional Credit Amount,
          interest shall not accrue against the Additional Credit Amount prior
          to February 1, 2003; following which date interest shall accrue as
          provided in the Note on the aggregate principal amount of $225,000."

3.   Wire Transfer of Funds. Holder hereby agrees to wire transfer the
     Additional Credit Amount as follows:

     Holme Roberts & Owen LLP - Trust Account
     Held for: Polar Molecular Corporation 46887-00050
     Account No. 1010920111
     Routing No. 102000076
     Wells Fargo Bank West, N.A.
     1740 Broadway, Denver, CO 80274

4.   Consideration. As consideration for entering into this Amendment, the
     Company (i) has agreed to reduce the conversion price of the convertible
     portion of the Note as reflected in paragraph 1 of this Amendment and (ii)
     shall issue the Holder 25,000 shares of the Company's common stock.

5.   No Other Changes. Except as explicitly amended by this Amendment, all of
     the terms and conditions of the Loan Extension Agreement shall remain in
     full force and effect.

                            [Signature Page Follows]

                                       2

<PAGE>

     The undersigned have executed this Amendment to be effective as of the date
first set forth herein.


COMPANY:

POLAR MOLECULAR CORPORATION


By:  /s/ Mark L. Nelson
     ---------------------------------
         Mark L. Nelson, President and
         Chief Executive Officer


HOLDER:

LOCKHART HOLDINGS, INC.


By:  /s/ Thomas J. Gillespie
     ---------------------------------------
         Thomas J. Gillespie, Jr., President

                                       3