Document:

<PAGE>   1
                                                                 Exhibit 10.4.43

                                 AMENDMENT NO. 1
                                       TO
                   AMENDED AND RESTATED REVOLVING CREDIT NOTE
                          DATED AS OF DECEMBER 31, 1999

         THIS AMENDMENT NO. 1 ("Amendment") is dated as of March 2, 2000,
between NEOPROBE CORPORATION, a Delaware corporation, whose address is 425 Metro
Place North, Suite 400, Dublin, Ohio 43017 (the "Borrower"), hereby promises to
pay to the order of THE PROVIDENT BANK, an Ohio banking corporation ("Bank").

                                   WITNESSETH:

         WHEREAS, the Borrower issued to the Bank, parties to that certain
Amended and Restated Revolving Credit Note dated as of December 31, 1999 in the
amount of $400,000 (the "Revolving Note") have agreed to amend the Revolving
Note on the terms and conditions hereinafter set forth. Terms not otherwise
defined herein are used as defined in the Agreement or the Revolving Note as
amended hereby.

         NOW, THEREFORE, the Borrower and the Bank hereby agree as follows:

         SECTION 1. AMENDMENT OF THE REVOLVING NOTE. The Revolving Note is,
effective the date hereof, hereby amended as follows:

         Section 1.1. The definition of "Maturity Date" in the introductory
paragraph of the Revolving Note is hereby amended by deleting "February 29,
2000" and substituting in its place "May 31, 2000."

         SECTION 2. GOVERNING LAW. This Amendment shall be governed by and
construed in accordance with the laws of the State of Ohio.

         SECTION 3. COSTS AND EXPENSES. The Borrower hereby agrees to pay on
demand all costs and expenses of the Bank in connection with the preparation,
execution and delivery of this Amendment and the other documents to be delivered
in connection herewith, including, without limitation, the reasonable fees and
out-of-pocket expenses of counsel to the Bank with respect thereto.

         SECTION 4. COUNTERPARTS. This Amendment may be executed in any number
of counterparts and by different parties hereto in separate counterparts, each
of which when so executed shall be deemed to be an original and all of which
when taken together shall constitute one and the same agreement.

         SECTION 5. CONFESSION OF JUDGMENT. The Borrower hereby authorizes any
attorney at law to appear for the Borrower, in an action hereon, at any time
after the same becomes due, as herein provided, in any court of record in or of
the State of Ohio, or elsewhere, to waive the issuing and service of process
against the Borrower and to confess judgment in favor of the holder hereof or
the party entitled to the benefits hereof against the Borrower for the amount
that may be due, with interest at the rate herein mentioned and costs of suit,
and to waive and
<PAGE>   2

release all errors in said proceedings and judgment, and all petitions in error,
and right of appeal from the judgment rendered.

         Borrower has executed this Amendment as of the date first above
written.

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WARNING - BY SIGNING THIS PAPER YOU GIVE UP YOUR RIGHT TO NOTICE AND COURT
TRIAL. IF YOU DO NOT PAY ON TIME, A COURT JUDGMENT MAY BE TAKEN AGAINST YOU
WITHOUT YOUR PRIOR KNOWLEDGE AND THE POWERS OF A COURT CAN BE USED TO COLLECT
FROM YOU REGARDLESS OF ANY CLAIMS YOU MAY HAVE AGAINST THE CREDITOR WHETHER FOR
RETURNED GOODS, FAULTY GOODS, FAILURE ON HIS PART TO COMPLY WITH THE AGREEMENT,
OR ANY OTHER CAUSE.

--------------------------------------------------------------------------------

                                   NEOPROBE CORPORATION

                                   By:  /s/ David C. Bupp
                                        -----------------
                                   Name:  David C. Bupp
                                   Its:  President and Chief Executive Officer<PAGE>   1
                                                                 Exhibit 10.4.44

                                 AMENDMENT NO. 1
                                       TO
                         TENNESSEE REVOLVING CREDIT NOTE
                          DATED AS OF DECEMBER 31, 1999

         THIS AMENDMENT NO. 1 ("Amendment") is dated as of March 2, 2000,
between NEOPROBE CORPORATION, a Delaware corporation, whose address is 425 Metro
Place North, Suite 400, Dublin, Ohio 43017 (the "Borrower"), hereby promises to
pay to the order of THE PROVIDENT BANK, an Ohio banking corporation ("Bank").

                                   WITNESSETH:

         WHEREAS, the Borrower issued to the Bank, parties to that certain
Tennessee Revolving Credit Note dated as of December 31, 1999 in the amount of
$100,000 (the "Revolving Note") have agreed to amend the Revolving Note on the
terms and conditions hereinafter set forth. Terms not otherwise defined herein
are used as defined in the Agreement or the Revolving Note as amended hereby.

         NOW, THEREFORE, the Borrower and the Bank hereby agree as follows:

         SECTION 1. AMENDMENT OF THE REVOLVING NOTE. The Revolving Note is,
effective the date hereof, hereby amended as follows:

                  Section 1.1. The definition of "Maturity Date" in the
introductory paragraph of the Revolving Note is hereby amended by deleting
"February 29, 2000" and substituting in its place "May 31, 2000."

         SECTION 2. GOVERNING LAW. This Amendment shall be governed by and
construed in accordance with the laws of the State of Ohio.

         SECTION 3. COSTS AND EXPENSES. The Borrower hereby agrees to pay on
demand all costs and expenses of the Bank in connection with the preparation,
execution and delivery of this Amendment and the other documents to be delivered
in connection herewith, including, without limitation, the reasonable fees and
out-of-pocket expenses of counsel to the Bank with respect thereto.

         SECTION 4. COUNTERPARTS. This Amendment may be executed in any number
of counterparts and by different parties hereto in separate counterparts, each
of which when so executed shall be deemed to be an original and all of which
when taken together shall constitute one and the same agreement.

         SECTION 5. CONFESSION OF JUDGMENT. The Borrower hereby authorizes any
attorney at law to appear for the Borrower, in an action hereon, at any time
after the same becomes due, as herein provided, in any court of record in or of
the State of Ohio, or elsewhere, to waive the issuing and service of process
against the Borrower and to confess judgment in favor of the holder hereof or
the party entitled to the benefits hereof against the Borrower for the amount
that may be due, with interest at the rate herein mentioned and costs of suit,
and to waive and
<PAGE>   2
release all errors in said proceedings and judgment, and all petitions in error,
and right of appeal from the judgment rendered.

         Borrower has executed this Amendment as of the date first above
written.

--------------------------------------------------------------------------------
WARNING - BY SIGNING THIS PAPER YOU GIVE UP YOUR RIGHT TO NOTICE AND COURT
TRIAL. IF YOU DO NOT PAY ON TIME, A COURT JUDGMENT MAY BE TAKEN AGAINST YOU
WITHOUT YOUR PRIOR KNOWLEDGE AND THE POWERS OF A COURT CAN BE USED TO COLLECT
FROM YOU REGARDLESS OF ANY CLAIMS YOU MAY HAVE AGAINST THE CREDITOR WHETHER FOR
RETURNED GOODS, FAULTY GOODS, FAILURE ON HIS PART TO COMPLY WITH THE AGREEMENT,
OR ANY OTHER CAUSE.
--------------------------------------------------------------------------------

                                     NEOPROBE CORPORATION

                                     By:  /s/ David C. Bupp
                                          -----------------
                                     Name:  David C. Bupp
                                     Its:  President and Chief Executive Officer<PAGE>   1
                                                                   Exhibit 10.10

                      [APPLIED INNOVATION INC. LETTERHEAD]

June 14, 1999

Mr. Michael P. Keegan
5208 Whisper Willow Drive
Fairfax, VA 22030

Dear Mike:

We are pleased to confirm our offer for, and your acceptance of, the position of
VP & Chief Financial Officer. We are excited about your joining the Company, and
we are confident you have the ability to make a major contribution to Applied's
future performance. We know that Applied will offer you challenging and
rewarding opportunities professionally and personally.

Following are the terms of your employment offer:

1.       EMPLOYMENT

         You will initially report to Gerard Moersdorf--President, & Chief
         Executive Officer.

         Your targeted start date is July 26, 1999, but this date will stay
         flexible based on your needs.

2.       COMPENSATION

         Your base salary compensation will be at a bi-weekly salary of
         $5,769.24 (26 pay periods per year), payable in accordance with the
         normal payroll practices of the company. With this offer comes our
         liberal benefits package on which you have already received
         information.

         You will also be eligible to participate in the management bonus plan.
         The bonus plan will be based on corporate objectives. Your objectives
         will be the same as those currently in place for the President/CEO and
         the Vice President of Sales and Marketing. The objectives are primarily
         relate to targeted sales and pre-tax profits, as determined by the
         President's Operating Report. The bonus, if any, will be paid after
         completion of the 1999 audit, which is anticipated to be in February,
         2000.

<PAGE>   2

Michael P. Keegan
June 14, 1999
Page 2

3.       VACATION

         You will be entitled to 15 days paid vacation during each of your first
         full six calendar years of your employment. After that you will receive
         vacation in accordance with the Applied Innovation Inc. vacation
         policy. Presently, that policy provides one additional day per year up
         to a maximum of 20 days in year eleven. You will have 6 days of paid
         vacation available for use during 1999.

4.       RELOCATION

         APPROVAL

         The Chief Executive Officer will have the ultimate authority for all
         decisions relating to reimbursement of relocation expenses.

         GUIDELINES

         A.       HOUSE HUNTING TRIPS

                  The company will reimburse expenses for up to two trips for
                  the employee and spouse to select a residence at the new
                  location. The Company will also reimburse expenses for one of
                  the trips to include children of the employee and spouse,
                  subject to review by the Company. Typically these trips should
                  not exceed 3 days/2 nights, but necessary exceptions will be
                  considered on a case by case basis. The Company will reimburse
                  reasonable expenses for flights/mileage, meals and hotel.

         B.       TEMPORARY LIVING EXPENSES

                  The relocating employee is expected to make all reasonable
                  efforts to secure housing at the new location prior to the
                  sale of the current home and moving of the employee and
                  family, thereby eliminating the need for temporary housing and
                  property storage; however, the Company recognizes that there
                  may be circumstances beyond the employee's control relating to
                  the sale of the current home and/or purchase of the new home
                  which may result in the need for temporary housing. In these
                  circumstances the Company will consider providing temporary
                  housing (rent and utilities for furnished apartment or hotel
                  room selected by the Company) and property storage (van line
                  storage or self storage facility selected by the Company) on a
                  case by case basis. Such approved storage would include the
                  additional labor costs associated with the additional loading
                  and unloading of household goods.

                  Temporary housing and property storage should typically not
                  exceed 90 days and are subject to advance approval.

<PAGE>   3

Michael P. Keegan
June 14, 1999
Page 3

              C.  TRANSPORTATION OF HOUSEHOLD GOODS

                  The company will pay for reasonable costs of transporting
                  normal household goods, including packing, loading, hauling
                  and unloading of household goods (excluding cars, boats and
                  campers). The carrier will be selected by the Company.

              D.  TRANSPORTATION OF EMPLOYEE AND FAMILY

                  The company will pay for reasonable costs associated with
                  transporting you and your family to your new location.
                  Reasonable costs include standard mileage reimbursement (10
                  cents per mile) per vehicle driven, plus meals and in-transit
                  lodging.

              E.  SALE OF RESIDENCE

                  Payment of the following expenses incidental to the sale of
                  the employee's home will be considered:

              1.  Realtor commission/Broker fee up to 6% of sale price of home.

              2.  Appraisal, credit report and survey when required by law or
                  mortgage holder.

              3.  Mortgagee's title policy, abstract or title guarantee.

              4.  Recording of mortgage and deed.

              5.  Local mortgage tax.

              6.  Reasonable attorney fees consistent with local requirements
                  limited to $300.00.

              7.  Transfer fees up to $2.00 per thousand dollars of selling
                  price.

              8.  Title insurance up to $3.50 per thousand dollars of selling
                  price.

              9.  Deed preparation up to $35.00.

              10. Gas and termite inspection up to $90.00.

              11. Septic/well inspection up to $100.00.

              12. Home warranty up to $300.00.

              Not reimbursable:

              o   Full or pro-rated hazard insurance

              o   Property taxes/aggregate escrow reserve

              o   Interest charges

              o   Home owner association dues

<PAGE>   4

Michael P. Keegan
June 14, 1999
Page 4

              F.  PURCHASE OF RESIDENCE

                  Payment of the following expenses incidental to the purchase
                  of the employee's home will be considered:

              o   Buyer closing costs: the Company will pay for the buyer
                  closing costs for the purchase of the new home, providing the
                  selling price of the current home and purchase price of the
                  new home have a difference of less than 10%. If the purchase
                  price of your new home is more than 10% higher than the
                  selling price of your current home, the Company will pay for a
                  prorated amount of the closing costs. Reimbursable closing
                  costs will include the costs listed in section "E" above for
                  items 2 through 12.

              o   Mortgage related points: if the interest rate on the existing
                  mortgage is less than the available interest rate on the new
                  mortgage of comparable terms (30 year fixed to 30 year fixed,
                  ARM to ARM, etc.), the Company will pay the necessary points
                  to buy down the interest rate to the same level as the
                  interest rate on your existing mortgage, providing the total
                  length of the new mortgage is not longer than the original
                  length of your existing mortgage. This buy down is subject to
                  Company review and approval.

              o   The cost of standard utility hook-ups at the new residence.

              G.  VEHICLE LICENSING COSTS

                  The Company will reimburse the cost of the first 12 months of
                  car license plates/tags for up to two vehicles.

              H.  EMPLOYEE HOME VISITS WHILE IN TEMPORARY HOUSING

                  The Company will determine whether reimbursement for employee
                  home visit expenses will be allowed during the relocation
                  period. When authorized, all reasonable attempts should be
                  made to combine necessary work/business trips with such home
                  visits.

                  All of the above costs must be reasonable in amount and
                  subject to AI review and approval. If special conditions exist
                  and additional assistance is required, appropriate advance
                  approval is required.

              I.  MISCELLANEOUS ACCOUNTABLE COSTS

                  The Company will provide a $2000.00 accountable allowance for
                  other miscellaneous move related expenses. The costs must be
                  approved by the Company and be directly related to the
                  relocation event. As with all other expenses under this policy
                  a full accounting, included written receipts with proper
                  explanation, is to be provided before reimbursement occurs.

<PAGE>   5

Michael P. Keegan
June 14, 1999
Page 5

              J.  WHAT IS TAXABLE AND NON-TAXABLE AS INCOME

                  For clarification, the following items are NOT taxable income:
                  transportation of your household goods, 10 cents per mile for
                  driving personal vehicles to the new location, flights and
                  lodging during transport of immediate family members to the
                  new location. All other relocation costs listed above should
                  be assumed to be taxable income. Federal Internal Revenue
                  Service regulations require that all relocation expenses for
                  which an employee is reimbursed or which are being paid
                  directly by the Company be reported as gross income. These
                  expenses will be included in the gross earnings figure on the
                  employee's W-2 form.

                  The Company will gross up payments/reimbursements to cover all
                  payroll taxes.

                  All expenses reimbursable under these guidelines which are not
                  paid directly by the Company must be submitted on an expense
                  form with appropriate receipts. When submitting relocation
                  expenses for reimbursement, please use an expense form that
                  contains only relocation expenses do not mix relocation
                  expenses and business expenses on a single expense form.

              K.  REPAYMENT SCHEDULE

                  If the employee voluntarily resigns from employment with the
                  Company, or the employee's employment is terminated for any
                  form of gross misconduct, the employee will reimburse the
                  Company as follows:

                  0-30 days      100.00%
                  31-60 days      91.63%
                  61-90days       83.30%
                  91-120 days     74.97%
                  121-150 days    66.64%
                  151-180 days    58.31%
                  181-210 days    49.98%
                  211-240 days    41.65%
                  241-270 days    33.32%
                  271-300 days    24.99%
                  301-330 days    16.66%
                  331-365 days     8.33%

<PAGE>   6

Michael P. Keegan
June 14, 1999
Page 6

5.       STOCK OPTIONS

         Under the 1996 Stock Option Plan, options may be granted by the Board
         of Directors at exercise prices not less than fair market value. In
         accordance with that plan, we offer the following:

         a)   The compensation committee of the Board of Directors, effective
              upon the start of your employment, will grant you options to
              purchase 50,000 shares of common stock of the Company.

         b)   All of the options will be exercisable at a price equal to the
              closing price (as quoted by NASDAQ) of the Company's stock on the
              day prior to the start of your employment.

         c)   The options will be subject to the normal requirements of the Plan
              (copy enclosed), and 20% of the options will vest and become
              exercisable upon each annual anniversary of your employment by the
              Company.

         d)   The options will terminate on the eighth anniversary of the date
              of grant.

Your employment is "at-will employment" (may be terminated by either the Company
or yourself at any time and for any reason). If, in the future, you elect to
terminate your employment with the Company, we request that you provide a
minimum notice of fourteen days.

By accepting this offer, you represent and warrant that you are not a party to
any agreement that would prevent you from performing your duties for the Company
or which would expose the Company to a risk of suit by reason of your employment
by the Company.

In consideration of your employment, you will enter into an Employment Agreement
with the Company. A copy of this agreement is enclosed for your careful review.
Please sign the Employment Agreement and one copy of this letter, and return to
affirm acceptance of this offer. We look forward to working with you and we are
confident that your employment at Applied Innovation will be challenging and
successful.

Sincerely,

Larenda Johnson
Employment Manager

Enclosure:  Employment Agreement

 /s/ Michael P. Keegan                           June 18, 1999
--------------------------                     --------------------
     Michael P. Keegan                          Date

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