<PAGE>
                                                                 Exhibit 10.26

                              CONSULTING AGREEMENT

      This Agreement dated this 18th day of August 2000, is executed by and
between Advanced Pharma, Inc., a Delaware corporation ("COMPANY"), located at
15200 Shady Grove Road, Suite 350, Rockville, Maryland 20850 and Jenefir
Isbister, Ph.D., ("CONSULTANT"), located at George Mason University, Center for
Bioresource Development. Shared Research Instrumentation Facility, MS 4D7,
Fairfax, Virginia 22030-4444.

      WHEREAS, Company seeks to engage Consultant to accomplish the
objectives described on Exhibit A hereto;

      WHEREAS, Consultant possesses the requisite skills, training and
experience to perform the services called for under this Agreement, and wishes
to perform the services based on the terms and conditions herein; and

      WHEREAS, based on the nature of the relationship that the parties intend
to establish, Company hereby engages Consultant as an independent Consultant.

      NOW, THEREFORE, in consideration of the mutual promises and covenants of
the parties as herein contained, the parties hereto agree and contract as
follows:

1)    Company hereby engages Consultant to accomplish the objectives described
      in Exhibit A hereto, which can be amended from time to time in accordance
      with Paragraph 16 hereof. Consultant shall perform such objectives in a
      thorough, efficient, and workmanlike manner, promptly and with due
      diligence and care, and in accordance with that standard of care and skill
      ordinarily exercised by members of the profession performing similar
      services. In addition, Consultant shall comply with all applicable
      federal, state and local statutes, ordinances and regulations in the
      course of performing such objectives.

2)    The original term of this Agreement shall be the 12 month period
      commencing on the 18th day of August 2000. This Agreement may be renewed
      for additional 12 month periods by mutual agreement of the parties hereto.

3)    Company enters in this Agreement based on Consultant's demonstrated
      ability to accomplish the objectives described in Exhibit A hereto.
      Consequently, Company will not provide Consultant with any training or
      instructions with respect to the services to be provided hereunder.
      Similarly, Consultant is responsible for providing any equipment,
      materials or supplies that Consultant determines is necessary to
      accomplish the objectives.

4)    Consistent with the parties' intent that the relationship created by this
      Agreement be that of service recipient and independent Consultant,
      Consultant shall retain the exclusive right to control and direct all
      details of the services that Consultant performs hereunder, including
      where, when and how the services are to be performed. Consultant's failure
      to accomplish an objective described on Exhibit A by the applicable
      deadline, however, shall constitute a material breach of this Agreement,
      unless Company agrees, in accordance with Paragraph 16 hereof, to an
      extended deadline.
<PAGE>
                                                            Consulting Agreement
                       between Advanced Pharma, Inc. and Jenefir Isbister, Ph.D.
                                                           dated August 18, 2000
                                                                     Page 2 of 7

5)    Consultant (and its employees, if any) shall not be eligible to
      participate in any benefit programs that Company now or hereafter
      maintains for its employees and, in the event Consultant (and its
      employees, if any) for any reason were to become eligible to participate
      in a Company-sponsored benefit program, Consultant hereby waives any such
      right to participate in the program. This waiver of any right to
      participate in Company-sponsored employee benefit programs represents a
      material component of the terms of compensation agreed to by these parties
      and is not in any way conditioned on any representation or assumption
      concerning status of Consultant (and its employees, if any) with respect
      to the Company as an employee or independent Consultant.

6)    For all purposes, including but not limited to the Federal Insurance
      Contributions Act ("FICA"), the Social Security Act, the Federal
      Unemployment Tax Act ("FUTA"), income tax withholding and any and all
      other federal, state and local laws, rules and regulations, Consultant
      (and its employees, if any) shall be treated as an independent Consultant
      and not as an employee with respect to Company.

7)    Consultant acknowledges and agrees that Consultant shall be responsible
      (as a self-employed individual) for filing all tax returns, tax
      declarations and tax schedules, and for the payment of all taxes required,
      when due, with respect to any and all compensation earned by Consultant
      under this Agreement. Company will neither pay nor withhold any employment
      taxes with respect to the compensation it pays Consultant. Rather, Company
      will report the amounts it pays Consultant on IRS Forms 1099, to the
      extent required to do so under applicable Internal Revenue Code
      provisions.

8)    Consultant's fees for services provided under this Agreement shall be
      $1,500.00 per day, unless otherwise specified in Exhibit A hereto. Company
      shall have no obligation to pay a fee prior to receipt of an invoice from
      Consultant requesting payment of the same. Company will not reimburse
      Consultant for any expenses incurred in connection with the performance of
      services hereunder unless otherwise agreed by Company in accordance with
      Paragraph 16 hereof.

9)    Consultant shall have the right to engage others to assist in the
      accomplishment of the objectives described in Exhibit A hereto. Consultant
      shall be solely responsible for (a) complying with all applicable
      employment and labor laws with respect to any assistants it engages, (b)
      paying all compensation owed to such assistants, and (b) paying, and/or
      withholding and remitting to the appropriate government agency, any
      applicable employment taxes that might be owed with respect to such
      compensation. Consultant also shall indemnify and hold Company harmless
      against any and all liabilities attributable to the obligations imposed on
      Consultant under this Paragraph 9. This Paragraph 9 shall survive the
      termination of this Agreement.

10)   Consultant reserves the right to, and intends to, perform services for
      others, so long as the performance of such services does not interfere
      with the performance of services hereunder.

11)   Consultant acknowledges and agrees that in the course of the performance
      of services pursuant to this Agreement, Consultant will be given access
      to, or come into possession of,

                                      - 2 -
<PAGE>
                                                            Consulting Agreement
                       between Advanced Pharma, Inc. and Jenefir Isbister, Ph.D.
                                                           dated August 18, 2000
                                                                     Page 3 of 7



      confidential information of the Company, which information includes trade
      secrets, proprietary data or other confidential information. Consultant
      acknowledges and agrees that he will not use, duplicate or divulge to
      others any such confidential information of the Company except in
      connection with the performance of services under this Agreement.
      Consultant agrees and covenants that Consultant shall not remove or copy
      any data, research, memoranda, reports, records, documents, publications,
      journals, diaries, computer programs, files, information contained in
      files, or other information or material pertaining to the business,
      research, or technology of Company without the express written consent of
      Company, which in all events shall be considered to be the owner and
      possessor of all such data, research, memoranda, reports, records,
      documents, publications, journals, diaries, computer programs, files,
      information contained in files, or other information or material.
      Consultant covenants and agrees that Consultant shall in no way utilize
      any such information in Consultant's possession for the gain or advantage
      of Consultant (other than in Consultant's performances of services
      hereunder) and/or to the detriment of Company. Upon a termination of this
      Agreement, or at such earlier date as Company may request, Consultant
      shall deliver forthwith to Company all such data, research, memoranda,
      reports, records, documents, publications, journals, diaries, computer
      programs, files, information contained in files, or other information or
      material (including all extracts, abstracts, copies, or portions thereof)
      which are then in Consultant's possession or control. The obligations of
      this Paragraph 11 shall survive the termination of this Agreement.

12)   (a) Consultant acknowledges and agrees that, as part of his engagement
      with the Company, he is expected to make new contributions of value to the
      Company and agrees to promptly disclose to the Company any and all ideas,
      discoveries, works of authorship, writings, computer software programs,
      know-how, processes, formulas, improvements or revisions (collectively,
      "Works"), whether subject to copyright, patent, registration or other
      protection as intellectual property or not, which he may make, devise,
      conceive, create, design, invent, develop or discover, either solely or
      jointly with another or others, during his engagement by the Company,
      whether at the request or upon the suggestion of the Company or otherwise,
      which (i) stem from his work for the Company; (ii) were created using
      Company facilities, equipment, or resources, Company personnel, or during
      any time he is performing services for the Company; (iii) come about as a
      result of Consultant's access to confidential, proprietary, or trade
      secret information belonging to the Company; or (iv) relate to, or are
      capable of use in connection with any business of the Company, or any
      services, programs or products offered, used, sold or being developed by
      the Company at the time he creates or develops such Works, of which
      Consultant became aware through his consulting relationship with the
      Company or otherwise through Consultant's provision of services hereunder.
      Any and all of the foregoing shall belong exclusively to the Company and,
      to the extent it is copyrightable material, shall be deemed to be "Works
      Made for Hire," and the Company shall be deemed the author or creator
      thereof.

                                      - 3 -
<PAGE>
                                                            Consulting Agreement
                       between Advanced Pharma, Inc. and Jenefir Isbister, Ph.D.
                                                           dated August 18, 2000
                                                                     Page 4 of 7

      (b) Consultant shall assign to the Company, and hereby does so assign, all
      Works disclosed, or required to be disclosed, in accordance with this
      Section and assigns the right to obtain patents or copyright registrations
      on any and all such Works in any or all countries in his name or
      otherwise.

13)   Upon the request of Company, whether or not made during the period of his
      engagement with Company, Consultant shall assist Company in any way
      necessary, including, but not limited to executing documents, to
      accomplish the following, in any or all countries, with respect to any and
      all Works disclosed, or required to be disclosed, in accordance with
      Paragraph 12: (a) to perfect in Company all right, title and interest in
      and to the Work; (b) to file for and/or obtain a patent or patents,
      copyright registration or copyright registrations, trademark, trade name,
      domain name or similar registration, or other means established for the
      protection of intellectual property on the Work, in the United States or
      any other country; and (c) to protect and enforce Company's rights in the
      Work. The provisions of Paragraphs 11, 12 and 13 shall survive the
      termination of this Agreement.

14)   Neither party hereto may terminate this Agreement unless the party first
      provides the other party with 30 days advance written notice. In the case
      of a material breach of this Agreement by one party, however, the other
      party shall have the right to terminate this Agreement immediately by
      providing the breaching party with a notice of termination. The
      termination shall be effective at the time the notice is received by the
      breaching party.

15)   This Agreement shall be governed by and construed and enforced in
      accordance with the laws of the State of Delaware. Any and all disputes
      arising out of, relating to the performance of services contracted for
      under, this Agreement, shall be settled by arbitration in accordance with
      the Commercial Arbitration Rules of the American Arbitration Association,
      or any successor thereto then prevailing. Such arbitration shall be final
      and binding upon the parties, and shall be the sole and exclusive remedy
      of the parties with respect to any dispute arising out of, relating to, or
      resulting from the interpretation of the terms of this Agreement, or any
      breach thereof. The costs of such arbitration shall be borne equally by
      the parties. Notwithstanding the foregoing provisions of this Paragraph 15
      to the contrary, matters in which an equitable remedy or injunctive relief
      is sought by a party, shall not be required to be submitted to
      arbitration, if the party seeking such remedy or relief objects thereto,
      but instead shall be submitted to a court of law having appropriate
      jurisdiction. This Paragraph 15 shall survive the termination of this
      Agreement.

16)   This Agreement shall constitute the entire written agreement between the
      parties, and shall supersede any and all agreements or understandings in
      effect between the parties hereto. Neither this Agreement nor Exhibit A
      hereto may be modified except by written agreement executed by the parties
      hereto.

17)   Each provision of this Agreement shall be treated as a separate and
      independent clause, and the unenforceability of any one clause shall in no
      way impair the enforceability of any of the other clauses herein.
      Moreover, if one or more of the provisions contained in this Agreement
      shall for any reason be held to be excessively broad as to scope,
      activity, subject or otherwise


                                     - 4 -
<PAGE>
                                                            Consulting Agreement
                       between Advanced Pharma, Inc. and Jenefir Isbister, Ph.D.
                                                           dated August 18, 2000
                                                                     Page 5 of 7

      so as to be unenforceable at law, such provision or provisions shall be
      construed by the appropriate judicial body by limiting or reducing such
      provision or provisions, so as to be enforceable to the maximum extent
      comparable with the applicable law as such law shall then be.

18)   No breach of any provision hereof can be waived unless in writing. Waiver
      of any breach of any provision hereof shall not be deemed to be a waiver
      of any other breach of the same, or any other provision.

19)   Any notice required or permitted under this Agreement shall be in writing
      and shall be deemed given upon the earlier of (a) when it is personally
      delivered, (b) three (3) days after having been mailed by certified mail,
      postage prepaid, return receipt requested or (c) two (2) days after having
      been sent by recognized overnight delivery service, in all cases sent to
      the address first set forth above, or to such other address as the person
      to whom notice is to be given may have previously furnished to the other
      in writing in the manner set forth herein.

                         [signatures on following page]


                                     - 5 -
<PAGE>
                                                            Consulting Agreement
                       between Advanced Pharma, Inc. and Jenefir Isbister, Ph.D.
                                                           dated August 18, 2000
                                                                     Page 6 of 7


   IN WITNESS WHEREOF, the parties hereto have executed this Agreement on the
day and year first above written.

ADVANCED PHARMA, INC.                           CONSULTANT



By: /s/ Edward M. Rudnic                        /s/ Jenefir Isbister
    ------------------------------------        --------------------------------
    Edward M. Rudnic, Ph.D.,                    Name:  Jenefir Isbister, Ph.D.
    President and Chief Executive Officer              George Mason University


                                                TIN:
                                                    ----------------------------


                                     - 6 -
<PAGE>
                                                            Consulting Agreement
                       between Advanced Pharma, Inc. and Jenefir Isbister, Ph.D.
                                                           dated August 18, 2000
                                                                     Page 7 of 7


                                    EXHIBIT A

            Description of Services to be Performed by Consultant
    Under an Independent-Consultant Agreement Entered into by the Parties


Dated:  August 18, 2000


<TABLE>
<CAPTION>
Objective                                      Fee (if applicable)
---------                                      -------------------
<S>                                            <C>
Work with Advanced Pharma to                   $1,500 per day
identify and develop genomic and
genetic strategies and/or insight into
pulsatile antibiotic delivery.
</TABLE>


      IN WITNESS WHEREOF, the parties hereto have executed this Attachment A to
that certain Agreement entered into by these parties dated August 18, 2000.

ADVANCED PHARMA, INC.                           CONSULTANT


By:  /s/ Edward M. Rudnic                          /s/ Jenefir Isbister
   ------------------------                     -------------------------------
   Edward M. Rudnic, Ph.D.,                      Name:  Jenefir Isbister, Ph.D.
   President and Chief Executive Officer                George Mason University


                                     - 7 -
<PAGE>
August 20, 2001

Jenefir Isbister, Ph.D.
George Mason University
Center for Bioresource Development
Shared Research Instrumentation Facility
MS 4D7
Fairfax, Virginia  22030

Dear Dr. Isbister:

Subject to the following, this letter hereby confirms our understanding that the
Consulting Agreement (the "Original Agreement") by and between you and Advancis
Pharmaceutical Corporation, formerly known as Advanced Pharma, Inc., dated
August 18, 2000, is hereby extended for an additional 12-month period, effective
beginning August 18, 2001.

The Original Agreement, as extended and amended by this letter, contains the
entire understanding of the parties with respect to the subject matter thereof
and hereof and supersedes all prior oral and written agreements and
understandings between the parties with respect to the subject matter.

Please indicate your agreement with the foregoing by signing a copy of this
letter and returning it to my attention. This letter may be signed in
counterparts, and all capitalized terms used in this side letter but not
otherwise defined herein shall have the meanings set forth in the Original
Agreement.

Very truly yours,

ADVANCIS PHARMACEUTICAL CORPORATION
formerly Advanced Pharma, Inc.

By:  /s/ Edward M. Rudnic
     ______________________________
     Edward M. Rudnic, Ph.D.
     President & CEO

ACKNOWLEDGED AND AGREED:

Jenefir Isbister, Ph.D.


By: /s/ Jenefir Isbister                          Date: 8/21/01
______________________________________                  ____________________
<PAGE>
January 28, 2002

Jenefir Isbister, Ph.D.
George Mason University
Center for Bioresource Development
Shared Research Instrumentation Facility
MS 4D7
Fairfax, Virginia  22030

Dear Dr. Isbister:

Subject to the following, this letter hereby confirms our understanding that the
Consulting Agreement (the "Original Agreement") by and between you and Advancis
Pharmaceutical Corporation, formerly known as Advanced Pharma, Inc., dated
August 18, 2000, and extended January 1, 2001, is hereby extended for an
additional 12-month period, effective beginning January 1, 2002.

The Original Agreement, as extended and amended by this letter, contains the
entire understanding of the parties with respect to the subject matter thereof
and hereof and supersedes all prior oral and written agreements and
understandings between the parties with respect to the subject matter.

Please indicate your agreement with the foregoing by signing a copy of this
letter and returning it to my attention. This letter may be signed in
counterparts, and all capitalized terms used in this side letter but not
otherwise defined herein shall have the meanings set forth in the Original
Agreement.

Very truly yours,

ADVANCIS PHARMACEUTICAL CORPORATION
formerly Advanced Pharma, Inc.

By:    /s/ Edward M. Rudnic
     ------------------------
     Edward M. Rudnic, Ph.D.
     President & CEO

ACKNOWLEDGED AND AGREED:

Jenefir Isbister, Ph.D.


By: /s/ Jenefir Isbister        Date:   2/12/02
    ----------------------              ---------
<PAGE>
December 1, 2002

Jenefir D. Isbister, Ph.D.
George Mason University
Center for Bioresearch Development
Shared Research Instrumentation Facility
MSN 4D7 Room 306
10900 University Blvd
Manassas, VA  22030-4444

Dear Jenefir:

Subject to the following, this letter hereby confirms our understanding that the
Consulting Agreement (the "Original Agreement") by and between you and Advancis
Pharmaceutical Corporation, formerly known as Advanced Pharma, Inc., dated
August 18, 2000, and extended January 1, 2001 and January 1, 2002, is hereby
extended for an additional 12-month period, effective beginning January 1, 2003.

The Original Agreement, as extended and amended by this letter, contains the
entire understanding of the parties with respect to the subject matter thereof
and hereof and supersedes all prior oral and written agreements and
understandings between the parties with respect to the subject matter.

Please indicate your agreement with the foregoing by signing a copy of this
letter and returning it to my attention. This letter may be signed in
counterparts, and all capitalized terms used in this side letter but not
otherwise defined herein shall have the meanings set forth in the Original
Agreement.

Very truly yours,

ADVANCIS PHARMACEUTICAL CORPORATION
formerly Advanced Pharma, Inc.

By:    /s/ Edward M. Rudnic
     -------------------------
     Edward M. Rudnic, Ph.D.
     President & CEO

ACKNOWLEDGED AND AGREED:

Jenefir D. Isbister, Ph.D.


By:  /s/ Jenefir Isbister               Date:  12/3/02
     ---------------------                     ---------