<PAGE>   1
                                                                   EXHIBIT 10.52


                                 PROMISSORY NOTE



$250,000                                                          April 14, 1999
                                                          Gaithersburg, Maryland


        FOR VALUE RECEIVED, MARK D. GESSLER ("BORROWER"), hereby unconditionally
promises to pay to the order of GENE LOGIC INC., a Delaware corporation
("LENDER"), at 708 Quince Orchard Road, Gaithersburg, Maryland 20878, or at such
other place as the holder hereof may designate in writing, in lawful money of
the United States of America and in immediately available funds, the principal
amount of Two Hundred Fifty Thousand Dollars ($250,000.00) together with
interest accrued on the unpaid principal.

        1.     PRINCIPAL REPAYMENT. The outstanding principal amount of this
Note shall be due and payable on April 13, 2004; provided, however, that in the
event that the undersigned's employment by or association with the Company is
terminated for any reason prior to the payment in full of this Note, this Note
shall be accelerated and all unpaid principal and interest shall become due and
payable on the three-month anniversary of the date of such termination.
Notwithstanding the foregoing, until the payment in full of this Note has been
made, Borrower agrees to apply any proceeds from the sale of common stock of
Lender by the Borrower to payment of this Note. If the undersigned fails to pay
any of the principal and accrued interest when due, the Company, at its sole
option, shall have the right to accelerate this Note, in which event the entire
principal balance and all accrued interest shall become immediately due and
payable, and immediately collectible by the Company pursuant to applicable law.

        2.     INTEREST RATE. Borrower further promises to pay interest on the
outstanding principal amount hereof from the date hereof until payment in full,
which interest shall be payable at the rate of 5.25% per annum or the maximum
rate permissible by law, whichever is less. Interest shall be payable annually
in arrears not later than the first day of each calendar year for the preceding
year and shall be calculated on the basis of a 360-day year for the actual
number of days elapsed.

        3.     APPLICATION OF PAYMENTS. Payment on this Note shall be applied
first to accrued interest, and thereafter to the outstanding principal balance
hereof.

        4.     COSTS. The Lender shall be entitled to recover, and the
undersigned agrees to pay when incurred, all costs and expenses of collection of
this Note, including, without limitation, reasonable attorneys' fees.

        5.     WAIVER. Borrower waives presentment and demand for payment,
notice of dishonor, protest and notice of protest of this Note, and shall pay
all costs of collection when incurred, including, without limitation, reasonable
attorneys' fees, costs and other expenses. The right to plead any and all
statutes of limitations as a defense to any demands hereunder is hereby waived
to the full extent permitted by law.


<PAGE>   2

        6.     GOVERNING LAW. This Note shall be governed by, and construed and
enforced in accordance with, the laws of the State of Delaware, excluding
conflict of laws principles that would cause the application of laws of any
other jurisdiction.

        7.     PREPAYMENT. Borrower may prepay the unpaid principal in whole or
in part, without penalty, at any time, upon the payment of all unpaid interest
accrued to the date of such prepayment.

        8.     NON-TRANSFERABLE. The right of Borrower to request and receive
the amounts under this Note, as well as the other benefits under this Note,
shall not be assignable or otherwise transferable by Borrower.

        9.     SUCCESSORS AND ASSIGNS. The provisions of this Note shall be
binding on any successor in interest to Borrower and shall inure to the benefit
of any such successor in interest.



                                       /s/ Mark D. Gessler
                                       ----------------------------------------
                                       MARK D. GESSLER


ACKNOWLEDGED AND AGREED TO:

GENE LOGIC INC.



/S/ MICHAEL J. BRENNAN, M.D., PH.D.
-----------------------------------
MICHAEL J. BRENNAN, M.D., PH.D.
Chief Executive Officer