Exhibit 10.19  SIRTRIS PHARMACEUTICALS, INC.  April 15, 2005  David Sinclair
Harvard Medical School
Department of Pathology
NRB, 9th Floor
77 Avenue Louis Pasteur
Boston, MA 02115  Dear David:  Thank
you for agreeing to become a consultant to Sirtris Pharmaceuticals, Inc.
(the Company). This letter is to confirm our understanding with respect to
the services the Company will ask of you, the compensation you will receive in
return and certain related matters. We have agreed as follows.  1.    Services.    You
agree to serve as a consultant to the Company under the terms of this
Agreement. You will report directly to the Chief Executive Officer of the
Company and perform such tasks as the Chief Executive Officer may request (the Services).
It is anticipated that the Services will include presenting data to SAB and
BOD, fundraising, grant writing, assisting the SVP Research and Director of
Biology in the design of experiments and clinical trials, evaluation and
recruitment of employees, business development, strategic planning, providing
specific advice and intellectual property unless it is confidential or
conflicts with the policy of Harvard Medical School regarding information
dissemination. You agree to make yourself available to render the Services on
an on-call basis, as requested by the Company at such times and locations as
may be mutually agreed subject to the other terms of this Agreement. You will
use commercially reasonable efforts in rendering services to and promoting the
best interests of the Company. It is understood that pursuant to your
commitments to Harvard, your activities with the Company cannot exceed 20% of
your total working time.  2.    Approvals.    You
represent that you have received the requisite approvals that will allow you to
perform the Services contemplated by this Agreement.  3.    Term.    The
term of this Agreement shall commence as of March 1, 2005 and shall
continue until February 28, 2006. The term of this Agreement may be
extended by mutual agreement between you and the Company. You may terminate
this Agreement at any time, with or without cause, on 60 days prior notice
to the Company. The Company may terminate this Agreement at any time, with or
without cause, upon 60 days written notice to the Consultant.  4.    Expenses.    We
will reimburse you for reasonable and necessary expenses which are incurred in
connection with your performance of the Services hereunder and with respect to
which you promptly provide to us a detailed expense account and receipts,
provided that any expenses (other than travel expenses incurred in connection
with attendance at a meeting of the Scientific Advisory Board) in excess of
$500 per month have been approved by the Company in advance and in writing.  5.    Compensation.    As
compensation for you providing the Services to the Company, you will be paid
one hundred and fifty thousand dollars ($150,000) per annum, payable in twelve
equal monthly installments in arrears by the last day of each month.  6.    Confidentiality, Etc.    As
a condition of your engagement as a consultant by the Company, you will execute
the Companys Consultant Non-Solicitation, Non-Disclosure and Proprietary
Information Assignment Agreement in the form attached to this Agreement as Exhibit A (the Non-Disclosure
Agreement).


7.    Independent Contractor.    It
is agreed that you will perform your Services to the Company as an independent
contractor and that nothing in this agreement shall in any way be construed to
give rise to an employment relationship between you and the Company. You will
not be eligible for any employee benefits.  8.    Representations; Consent.    You
represent and warrant that you have no commitments or obligations inconsistent
with this Agreement or the Non-Disclosure Agreement or that would in any way
restrict your activities as a consultant to the Company. You consent to being
named as a consultant to the Company in press releases, offering documents,
reports or other documents in printed or electronic form, and any documents
filed with the Securities and Exchange Commission.  9.    Miscellaneous.    This
agreement constitutes the entire understanding of the parties with respect to
the subject matter hereof and supersedes all prior or current understandings
and agreements, whether written or oral, with respect to such subject matter.
The invalidity or unenforceability of any provision hereof shall not affect the
validity and enforceability of any other term or provision hereof. You may not
assign your rights or delegate your responsibilities under this agreement
without our prior written consent. The provision of the Non-Disclosure
Agreement. This agreement shall be governed by and construed in accordance with
the laws (other than the conflict of laws principles) of the Commonwealth of
Massachusetts. The parties agree that any disputes between the parties shall be
resolved pursuant to final and binding arbitration before a single arbitrator
selected from and administered by the American Arbitration Association in
accordance with its then existing arbitration rules or procedures
regarding business disputes.  Again,
thank you for agreeing to become a consultant to the Company. We look forward
to working with you and believe that your service will be significant to the
long-term success and viability of the Company. If this letter accurately sets
forth our agreement, please so indicate by signing below on both copies of this
letter and returning both to us. We will forward to you a fully executed
agreement for your files.  Very truly yours,SIRTRIS
  PHARMACEUTICALS, INC.By:
  /s/  CHRISTOPH WESTPHALName:
  Christoph WestphalTitle:
  President and Chief
  Executive OfficerDate:
  4/15/05Agreed and Accepted/s/  DAVID SINCLAIRName:
  David SinclairDate:
  4/15/05


Sirtris
Pharmaceuticals, Inc.
790 Memorial Drive, Suite 104
Cambridge, MA 02139  March 5,
2006   David A. Sinclair
8 Preston Road
West Roxbury, MA 02132  Re:Relationship with
Sirtris Pharmaceuticals, Inc.  Dear David:  This
amends the consulting agreement between you and Sirtris dated April 15,
2005. The Term of the agreement is hereby amended to extend until Feb 28, 2007.  This
amendment will be effective as of March 1, 2006. Except as described in
this amendment, the letter agreement will continue in accordance with its
original terms.  If
this is your understanding, please sign this letter in duplicate in the space
provided below and return an executed copy to me. The other copy if for your
records.  Very truly yours,/s/  CHRISTOPH WESTPHALChristoph Westphal,
  M.D., Ph.D.Vice Chair and CEO  Acknowledged
and Agreed  /s/  DAVID A. SINCLAIRDavid A. Sinclair


GRAPHIC  February 26, 2007  David A. Sinclair
8 Preston Road
West Roxbury, MA 02132  Re:  Relationship
with Sirtris Pharmaceuticals, Inc.  Dear David:  This
letter amends the consulting agreement between you and Sirtris Pharmaceuticals, Inc.
dated April 5, 2005 and the letter agreement amendment to that agreement
dated March 5, 2006 as follows:  ·The term of the agreement is hereby
amended to extend to February 29, 2008  Except
for the change in duration of the agreement all other terms of the consulting
agreement and the amendment thereto remain unchanged.  Sincerely,/s/ CHRISTOPH WESTPHALChristoph WestphalVice Chairman and Chief
  Executive OfficerAcknowledged and
  agreed:/s/ DAVID A. SINCLAIRDavid A. Sinclair  Sirtris
Pharmaceuticals, Inc.
790 Memorial Drive, Suite 104
Cambridge, MA 02139


February 8, 2008   David A. Sinclair 43 Newbrook Circle Chestnut Hill, MA 02467   Re:
Relationship with Sirtris Pharmaceuticals, Inc.   Dear David:  This letter amends the
consulting agreement between you and Sirtris Pharmaceuticals, Inc. dated April 5,
2005 and the letter agreement amendments to that agreement date March 5,
2006 and February 26, 2007 as follows:         The term of the agreement is hereby
amended to extend to February 28, 2009  Except for the change in
duration of the agreement, all other terms of the consulting agreement and the
amendments thereto remain unchanged.   Sincerely,  /s/ Christoph WestphalChristoph WestphalVice Chairman and Chief
  Executive OfficerAcknowledged and agreed:/s/ David A. SinclairDavid A. Sinclair   Sirtris
Pharmaceuticals, Inc.
200 Technology Square
Cambridge, MA 02139 617-252-6920