<PAGE>   1

EXHIBIT 10.15


                    NON-NEGOTIABLE PROMISSORY NOTE - AMENDED



$191,858.73                                                      January 1, 2001


     FOR VALUE RECEIVED, the undersigned, Inhibitex, Inc., a Delaware
corporation, with its principal offices at 8995 Westside Parkway, Alpharetta,
Georgia 30004 (the "Maker"), promises to pay to the order of AtheroGenics, Inc.,
a Georgia corporation (the "Holder"), the principal sum of One Hundred
Ninety-One Thousand Eight Hundred Fifty Eight and 73/100 ($191,858.73), together
with interest on so much thereof as is from time to time outstanding and unpaid,
from the date hereof until the principal sum is paid in full, at the rate of
seven percent (7%) per annum, with principal and accrued interest being due and
payable as follows.

1.   PAYMENTS. All payments due hereunder shall be made in lawful money of the
     United States of America at 8995 Westside Parkway, Alpharetta, Georgia
     30004, or at such other place as Holder may designate in writing to the
     undersigned. Commencing on the date of this Note, interest shall accrue and
     be computed on the daily outstanding balance on a 360-day year simple
     interest basis. Principal and interest shall be due and payable monthly, in
     advance, in accordance with the amortization schedule, attached hereto as
     Attachment A and incorporated herein by this reference, commencing on
     January 1, 2001 and continuing on the first day of each month thereafter
     and including December 1, 2005, at which time all outstanding principal and
     accrued but unpaid interest will be due and payable in full.

2.   EVENT OF DEFAULT. An event of default shall be deemed to occur if Maker
     fails to make or cause to be made any payments required to be made under
     this Note when the same shall be due and payable and such default is not
     cured within three (3) days after Maker receives written notice thereof
     from Holder.

3.   REMEDIES. Upon the occurrence of an event of default, at the option of the
     Holder, the entire principal amount due hereunder, together with all
     accrued interest and any other amounts due hereunder, shall immediately
     become due and payable.

4.   PREPAYMENT. This Note may be prepaid in full or in part prior to maturity,
     without penalty or fee.


                                      -1-

<PAGE>   2

5.   WAIVER. Maker hereby waives diligence, presentment, protest and demand,
     notice of protest, dishonor, notice of dishonor and nonpayment of this
     Note, and expressly agrees that, without in any way affecting the liability
     of Maker, Holder may extend any date or the time for payment of sums due
     hereunder.

6.   CHOICE OF LAW. This Note shall be governed by, and construed and enforced
     in accordance with the laws of the State of Georgia (without regard to its
     rules of conflicts of laws).

7.   COSTS OF COLLECTION. The Maker shall reimburse the Holder for all
     reasonable costs and expenses (including reasonable attorney's fees)
     actually incurred by the Holder in enforcing or collecting, or attempting
     to enforce or collect, the obligations evidenced by this Note, whether or
     not suit is instituted.

8.   SEVERABILITY. Wherever possible, each provision of this Note shall be
     interpreted in such manner as to be effective and valid, but if any
     provision hereof shall be prohibited by law or be otherwise invalid, such
     provision shall be ineffective to the extend of such prohibition or
     invalidity, without invalidating the remainder of such provision or the
     remaining provisions of this Note.

9.   ASSIGNMENT. This Note may not be transferred or assigned by Holder to any
     third party without the prior written permission of Maker.

10.  HEADINGS. The headings and titles used in this Note are intended solely for
     identification and not for any substantive purpose, and no heading or title
     shall in any way limit or extend the provisions hereof.

11.  LIMITATION OF INTEREST. Notwithstanding any provision herein to the
     contrary, the interest rate provided by this Note shall in no case exceed
     the rate allowable under any statute or law applicable to this transaction
     when appropriate consideration is given to borrowers or lenders of like
     character or classification. In the event the rate is determined to exceed
     the rate allowable under any statute or law applicable to this transaction,
     the interest rate shall be the maximum allowed by any such statute or law,
     and any amounts actually paid in excess thereof, shall be credited to
     principal.

     This Amended Non-Negotiable Promissory Note supersedes the note between
Inhibitex, Inc. and AtheroGenics, Inc. executed April 1, 1999. Upon execution of
this Note, there will be no further amounts due on the April 1, 1999 Note.


                                      -2-

<PAGE>   3



     IN WITNESS WHEREOF, this Note has been duly sealed, executed and delivered
     the day and year first above written.



                                       "MAKER"

                                       INHIBITEX, INC.,
                                       a Delaware corporation





                                       By:        /s/ RUSS PLUMB
                                           -------------------------------------
                                           Name:  Russ Plumb
                                           Title: Vice President & CFO



                                      -3-