<PAGE>

                                                                    EXHIBIT 10.6

                               DATED 6 APRIL 2005
               ---------------------------------------------------

                         BIOMED RESEARCH CONSULTING LTD

                            LEE PLACE HOUSE LEE PLACE
                         PULBOROUGH WEST SUSSEX RH20 1DF
                                 UNITED KINGDOM

                                      AND

                        OXFORD THERAPEUTIC CONSULTING LTD

                     MAGDALEN CENTRE THE OXFORD SCIENCE PARK
                         OXFORD OX4 4GA UNITED KINGDOM

               ---------------------------------------------------
                              CONSULTANCY AGREEMENT
               ---------------------------------------------------

                                                                               1
<PAGE>

THIS AGREEMENT is made the 6th day of April 2005

BETWEEN:

(a)      BioMed Research Consulting Ltd, Lee Place House, Lee Place, Pulborough,
         West Sussex RH20 1DF ("the Consultant")

(b)      Oxford Therapeutics Consulting Ltd, Magdalen, The Oxford Science Park,
         Oxford OX4 4GA ("the Company")

NOW IT IS HEREBY AGREED as follows:

1.       DEFINITIONS

         In this Agreement (unless it is expressly otherwise provided or the
         context otherwise requires):

1.1.     the following words and expressions shall have the following meanings:

1.1.1    "Consultant" shall mean Dr E Christine Siepl on behalf of BioMed
         Research Consulting Ltd retained by the Company to perform the
         Services.

1.1.2    "Business" means any business of the Company

1.1.3    "Confidential Information" includes without limitation:

                  any and all information concerning the business plans,
                  finances, intellectual property, customers, suppliers,
                  associates, consultants or employees of the Company; and

                  any information which is of a confidential nature or is given
                  or received or discovered in circumstances in which the
                  Consultant knows or should reasonably have concluded that the
                  information will be regarded by the Company as confidential.

1.1.4    "Expenses" means the out-of-pocket expenses approved by the Company and
         reasonably incurred by the Consultant wholly and exclusively in
         connection with the Services.

1.1.5    "Fee" means the sum of (pound) 640.- plus VAT per 8 hour day or
         such other rate as the Company and the Consultant shall agree in
         writing from time to time.

1.1.6    "Intellectual property" means all forms of intellectual property
         including, without limitation, patents, know-how, copyrights, designs,
         trade marks.

1.1.7    "Parties" means the parties to this Agreement.

1.1.9    "Services" means the services reasonably required of the Consultant
         from time to time by the Company.

1.2      References to Clauses are to those of this Agreement.

1.3      Reference to the singular number shall include the plural number and
         vice versa and those denoting one gender only shall include the other.

                                                                               2
<PAGE>

1.4      Headings are inserted for convenience only and shall not affect the
         construction or interpretation of this Agreement.

2.       APPOINTMENT AND TERM

2.1      The Company engages the Consultant to provide consultancy services to
         the Company and the Consultant agrees to provide the Services to the
         Company upon the terms and conditions of this Agreement.

2.2      This Agreement shall commence on 6th April 2005 and expire on 5th April
         2007 unless renewed by agreement between the Company and the
         Consultant. It may be terminated earlier by either party giving to the
         other not less than one calendar month's written notice of intention to
         determine the Agreement such notice to expire at any time.

3.       OBLIGATIONS OF THE CONSULTANT

3.1      The Consultant is retained on a "when needed" basis to provide the
         Services and shall perform the Services at such times and such
         locations as the Company and the Consultant shall agree from time to
         time.

3.2      The Consultant shall at all times during the period of this Agreement
         faithfully and diligently perform those duties and exercise such powers
         consistent with them which are from time to time necessary in
         connection with the provision of the Services;

3.3      The Consultant shall not be subject to control by the Company in the
         performance of the Services.

3.4      Any work that the Consultant produces shall be to pre-agreed standards.
         In the event that these standards are not met and the work requires
         correction the Consultant will carry this out in her own time and at
         her own expense.

3.5      Provided prior written agreement by the Company in each case, the
         Consultant may at her own cost engage subcontractors, employees or
         agents of her own to assist in providing the Services. The Company may
         require that such subcontractors, employees or agents first enter into
         covenants direct with the Company in terms similar to the relevant
         terms of this Agreement.

3.6      The consultant shall be responsible for her own costs of office
         facilities, computer equipment and software, technical literature,
         motorcar maintenance and so on.

3.7      The Consultant shall keep detailed timesheets and records of all things
         done in relation to the provision of the Services and at the Company's
         request shall make them available for inspection and provide copies to
         the Company.

3.8      If the Consultant shall be prevented by illness or injury from
         performing the Services she shall report that fact forthwith to the
         Company.

4.       EXPENSES

4.1      The Consultant shall be reimbursed for all reasonable expenses properly
         incurred by her wholly and exclusively in the provision of the
         Consultancy Services. The Consultant shall provide the Company with
         such receipts or other evidence of actual payment of the said expenses
         as shall be requested by the Company.

                                                                               3
<PAGE>

5.       PAYMENT OF CONSULTANCY FEE AND EXPENSES

5.1      The Consultant shall be paid by the Company monthly in arrears the Fee
         and Expenses for the Services hereunder (inclusive of any value added
         tax payable thereon) such sums to be paid by the Company within 30 days
         of receipt of the Consultant's invoice.

5.2      Any delay in the payment of undisputed Consultant's invoices by the
         Company will incur an interest charge of 2% (two per cent) per month
         above the National Westminster Bank base rate prevailing on the date of
         this Agreement on any amounts overdue.

5.3      The Consultant shall be responsible for making appropriate income tax
         and national insurance contributions. In consideration of the Company
         agreeing to engage the Consultant, the Consultant hereby indemnifies
         the Company in respect of any claims that may be made by the relevant
         authorities against the Company in respect of tax demands or national
         insurance or similar contributions relating to the provision of the
         Services hereunder.

6.       NATURE OF THE AGREEMENT

6.1      Nothing in this Agreement is intended by the Parties to create any
         contract of employment and it is hereby agreed that the Consultant is
         an independent contractor.

6.2      The Consultant shall not be entitled to any pension, bonus or other
         benefits from the Company.

7.       CONFIDENTIALITY

7.1      The Consultant shall:

         7.1.1    use the Confidential Information disclosed to her exclusively
                  to provide the Services.

         7.1.2    keep all Confidential Information that she may acquire in any
                  manner confidential and not disclose it to any person.

         7.1.3    keep confidential all reports written on behalf of the Company
                  unless the Company has given written permission to disclose
                  all or part of a report.

7.2      The foregoing restrictions on the Consultant shall not apply to any
         Confidential Information that the Consultant can establish by
         reasonable proof:

         7.2.1    was already in her possession and at her disposal before its
                  disclosure to the Consultant by the Company.

         7.2.2    was hereafter disclosed to her without any obligations of
                  confidence by a third party who insofar as is known by the
                  Consultant after due enquiry has not derived it directly or
                  indirectly from the Company.

         7.2.3    is or becomes generally available to the public through no act
                  or default on the part of the Consultant or its agents.

         7.2.4    was disclosed by the Consultant in accordance with the
                  requirements of the laws of any jurisdiction provided that
                  such disclosure was limited to the

                                                                               4
<PAGE>

                  extent of that requirement and was limited as far as possible
                  to that jurisdiction.

         7.2.5    was disclosed with the knowledge and consent of the Company.

7.3      The confidentiality and non-use obligations under this Agreement shall
         expire on the tenth anniversary of the date first written above.

7.4      Upon the termination of this Agreement (for whatever reason) the
         Consultant shall upon request deliver to the Company any property of
         the Company which may be in its possession or under its control
         including all documents and any copies thereof except that the
         Consultant shall be permitted to retain in its legal files one copy of
         any documents relating to continuing obligations on or rights of the
         Consultant

8.       INTELLECTUAL PROPERTY RIGHTS

8.1      The Consultant undertakes that all copyright, design right, rights to
         apply for patents, patents and other intellectual property in the
         results shall belong to the Company. In consideration of the fees
         payable under this Agreement, the Consultant agrees on demand to assign
         forthwith to the Company all intellectual property in the results at
         any time after their coming into existence. At the Company's request
         and expense the Consultant will use all reasonable endeavours to enable
         the Company at its discretion to make formal application anywhere in
         the world to obtain and maintain intellectual property in the results.

9.       TERMINATION FOR CAUSE

9.1      This Agreement may be terminated forthwith by the Consultant by notice
         in writing to the Company without notice and without compensation if
         the Company shall at any time have:

         9.1.1    failed to make the payments properly due to the Consultant
                  under this Agreement; or

         9.1.2    become insolvent or made any arrangements or composition with
                  its creditors.

9.2      This Agreement may be terminated forthwith by the Company by notice in
         writing to the Consultant without notice and without compensation if
         the Consultant shall at any time have:

         9.2.1    committed any serious or (after warning) any persistent or
                  continuing breach of this Agreement; or

         9.2.2    been guilty of any serious misconduct or gross neglect in the
                  performance of her duties hereunder; or

         9.2.3    become insolvent or made any arrangement or composition with
                  her creditors; or

         9.2.4    been convicted of any criminal offence other than an offence
                  which in the reasonable opinion of the Company does not affect
                  her position and standing in relation to the Business; or

                                                                               5
<PAGE>

         9.2.5    been guilty of any act or conduct that causes or is likely to
                  cause serious damage or discredit to the Company or its
                  customers.

9.3      Termination of this Agreement shall not affect the provisions of Clause
         7 and 8, which shall remain in full force and effect notwithstanding
         termination howsoever termination shall have occurred.

10.      LIABILITY OF THE CONSULTANT

10.1     Subject to Clause 2, in the event that the Consultant shall
         substantially fail to perform the Services agreed between the Company
         and the Consultant, the Consultant's liability shall be limited to the
         amount of the Fees already paid to the Consultant by the Company in
         respect of any uncompleted stages of the provision of the Services but
         excluding any Expenses paid by the Company.

10.2     While the Consultant will make all reasonable efforts to comply with
         any time schedule requested by the Company for the provision of the
         Services, the Consultant cannot guarantee compliance with such time
         schedule and shall have no liability for any failure to comply with
         such time schedule.

10.3     The Consultant shall have no liability to the Company if they are
         unable to provide the Services agreed between the Company and the
         Consultant as a result of war, strike, lockout, industrial disputes,
         riot, civil commotion, acts of Government, fire, blockade, accident,
         natural catastrophe, disaster, default of suppliers or other
         circumstances beyond the Consultant's control.

11.      ENTIRE AGREEMENT AND SEPARABILITY

11.1     This Agreement represents the entire agreement and understanding
         between the Parties relating to the provision of the services and the
         terms of this Agreement may only be varied by a document in writing
         signed by the Consultant and the Company.

11.2     The various provisions of this Agreement are separable and if any
         provision is held to be invalid or unenforceable by any court of
         competent jurisdiction then such invalidity or unenforceability shall
         not affect the remaining provisions of this Agreement.

12.      NOTICES

12.1     Any notice or other document to be given under this Agreement shall be
         in writing and shall be deemed to have been duly given sent by:

         12.1.1   first class post or express or air mail or other fast postal
                  service; or

         12.1.2   registered or special delivery post; or

         12.1.3   facsimile or other electronic media to a Party at the address
                  or relevant telecommunications number for such Party or such
                  other address as the Party may from time to time designate by
                  written notice to the other.

12.2     Notices to the Consultant shall be addressed to:

         Dr E Christine Siepl
         BioMed Research Consulting Ltd,

                                                                               6
<PAGE>

         Lee Place House, Lee Place
         Pulborough, West Sussex RH20 1DF
         United Kingdom.
         Fax +44 (0)1403 701 832
         siepl@bmrconsulting.com

     and notices to the Company shall be addressed to:

         Dr Jan Steiner
         Oxford Therapeutics Consulting Ltd
         Magdalen Centre
         The Oxford Science Park
         Oxford OX4 4GA
         Fax +00 (0)1865 784 875
         jansteiner@oxrx.com

12.3     All such notices shall be in the English language. Any notice or other
         document shall be deemed to have been received by the addressee two
         working days following the date of dispatch of the notice or other
         document by post or, where the notice or other document is sent by hand
         or is given by facsimile or other electronic media, simultaneously with
         the delivery or transmission. To prove the giving of a notice or other
         document it shall be sufficient to show that it was dispatched or
         validly transmitted.

13.      GENERAL

13.1     This Agreement shall be construed in accordance with and shall be
         subject to English Law and the Parties hereby submit to the
         non-exclusive jurisdiction of the English Courts.

13.2     This Agreement is personal to the Consultant and may not be assigned by
         the Consultant. This Agreement does not give the Consultant any
         authority to act as agent of the Company.

13.3     For the purpose of ensuring compliance with the Consultant's
         obligations under this Agreement the Company shall have access to and
         the right to inspect any work being carried out by the Consultant under
         this Agreement.

SIGNED BY  /s/ Dr E. Christine Siepl            DATE: 7 APRIL 2005
           ------------------------------
           DR E CHRISTINE SIEPL

FOR AND ON BEHALF OF BIOMED RESEARCH CONSULTING LTD

SIGNED BY  /s/ Dr Jan Steiner                  DATE: 6/4/2005
           ------------------------------
           DR JAN STEINER

FOR AND ON BEHALF OF OXFORD THERAPEUTIC CONSULTING LTD

                                                                               7