Exhibit 10.15  SECURITY AGREEMENT  THIS SECURITY AGREEMENT, dated as of
                    ,
2008, is made by Callisto Pharmaceuticals, Inc.,
a Delaware corporation(the Pledgor)
and Sommer & Schneider, LLP(the
Escrow Agent) for the benefit of the parties set forth on the signature page hereto
(the Lenders).  RECITALS  A.In order to induce the Lenders to make a loan
to Pledgor pursuant to a Securities Purchase Agreement of even date herewith
(the Securities Purchase Agreement) in the aggregate principal amount of
$500,000 made concurrently with the execution of this Security Agreement and
evidenced by a series of 11% Secured Promissory Notes due April 15, 2010
(the Notes) of even date herewith, the Pledgor has agreed to pledge 1,900,000shares of the Common Stock (the Pledged
Securities) of Synergy Pharmaceuticals, Inc., a Florida corporation (SGYP)
as security for the full payment and performance of Pledgors obligation under
the Notes.  B.The Lenders have agreed to accept the Pledged
Securities as security for such obligations.  NOW, THEREFORE, it is agreed as follows:  1.Pledge.  As collateral security for the
payment, performance and observance of the terms of the Notes, the Pledgor
herewith pledges, assigns, transfers, hypothecates, and grants to the Lenders,
a security interest in the Pledged Securities and the certificate(s) evidencing
the same and such additional property at any time and from time to time
receivable by the Pledgor as a dividend or in respect of or in exchange for any
or all such shares, the Pledgor herewith pledges, together with any and all
products and proceeds of any of the foregoing in whatever form (the Pledged
Securities and the products and proceeds thereof may be referred to
collectively as the Pledged Collateral).  2.Delivery to Agent for Lender.  Concurrently with the execution of this
Security Agreement, the Pledgor shall deliver the certificates representing the
Pledged Securities to the Escrow Agent, acting as agent for the Lender solely
for the purposes of perfecting the security interest granted hereby.  Acting in such capacity, the Escrow Agent
shall maintain possession of the Pledged Securities until they are required to
be released to either the Pledgor or the Lender pursuant to the provisions of
this Agreement. The Pledgor has deposited with the Escrow Agent stock powers
duly executed in blank with signatures guaranteed by a Medallion Participant.  3.Title.  The Pledgor agrees to use
reasonable efforts to defend the Lenders right, title, lien and security
interest in and to the Pledged Collateral against the claims and demands of all
persons whomsoever. The Pledgor also represents and warrants to the Lender that
it has good title to all of the Pledged Collateral, free and clear of all
claims, mortgages, pledges, liens, encumbrances and security interests of every
nature whatsoever (Liens) except those


granted
to the Lender herein and that no consent or approval of any governmental or
regulatory authority, or of any securities exchange is required to be obtained
by the Pledgor in connection herewith.  4.Delivery of the Pledged Securities by Escrow
Agent.  Upon the failure of the Pledgor to pay the
interest or principal on the Note when due in accordance with the terms thereof
or occurrence of an Event of Default defined in the Note (an Event of Default)
and upon five business days written notice given by each Lender on its own
behalf to the Escrow Agent and the Pledgor, the Escrow Agent shall deliver the
Pro Rata portion of the Pledged Securities and additional collateral held
pursuant to Section 5 hereof or otherwise to the Lender giving such
notice. For the purposes of this agreement, Pro Rata means that fraction
determined by dividing the amount of the principal amount of the Notes held by the
Lender to the total principal amount of the Notes outstanding as shown on the
books of the Pledgor, and multiplying that fraction by the number of Pledged
Securities or other Pledged Collateral, as the case may be. Upon payment in
full of the Note (and if in default, such penalties and reimbursable expenses
as are set forth in the Note) and upon one business day notice given by the
Pledgor to the Escrow Agent and all the Lenders, the Escrow Agent shall deliver
the certificate(s) for the Pledged Securities and additional collateral
held pursuant to Section 5 hereof or otherwise to the Pledgor and the
Pledged Collateral shall no longer be subject to the security interest granted
hereby.  5.Dividends.  If, upon the dissolution or
liquidation (in whole or in part) of the Lender, any sum shall be paid as a
liquidating dividend or otherwise upon or with respect to any of the Pledged
Securities, and if any other dividends of any kind shall be otherwise paid upon
or with respect to any of the Pledged Securities under any other circumstances,
such sum shall be paid over to the Escrow Agent, to be held as additional
collateral hereunder. In case any stock or other dividend shall be declared on
any of the Pledged Securities, or any shares of stock or fractions thereof
shall be issued pursuant to any stock split involving any of the Pledged
Securities, or any shares shall be distributed upon or with respect to the
Pledged Securities pursuant to a recapitalization or reclassification of the
capital of the Lender, or pursuant to the dissolution, liquidation (in whole or
in part), bankruptcy or reorganization of the Lender, or to the merger or
consolidation of the Lender with or into another Lender, the shares,
obligations or other property so distributed shall be delivered to the Escrow
Agent (together with stock powers executed in blank by the Pledgor, where
appropriate), to be held by it as additional collateral hereunder, and all of
the same shall constitute Pledged Collateral for all purposes hereof.  6.Voting.  (a)  Until there is an Event of Default, the Pledgor shall have
the sole and absolute right to exercise all voting power with respect to the
Pledged Securities.  The voting rights
granted to Pledgor shall survive the retention or purchase of the Pledged
Securities by Lender and are coupled with an interest in the surplus which may
be recouped by Pledgor in the event of the Pledged Securities are sold in
part.  In such event the Lenders hereby
appoint the Pledgor as the Lenders true and lawful proxy to vote such shares
in any manner which the Pledgor deems advisable for or against all matters
which may be submitted to a vote of such stockholders.


(b)  If an Event of Default occurs and is continuing, the Lenders
shall have the sole and absolute right, in addition to any other rights granted
herein, to exercise any and all voting power with respect to their Pro Rata
share of the Pledged Securities until such Default has been cured. The voting
rights granted to Lenders are coupled with an interest.  In such event the Pledgor hereby appoints the
Lenders as the Pledgors true and lawful proxy to vote their respective Pro
Rata portion of the Pledged Securities in any manner which each Lender deems
advisable for or against all matters which may be submitted to a vote of such
stockholders.  7.Rights of Secured Party.  The Pledgor agrees that, upon an Event of
Default, the Lenders shall have the rights and remedies provided in the Uniform
Commercial Code in force in the State of New York at the date of this agreement
and in this connection, the Lender may upon five days notice to the Pledgor,
sent by registered mail, and without liability for any diminution in price
which may have occurred, sell all the Pledged Collateral in such manner and for
such price as the Lender may determine or propose to retain the Pledged
Collateral in lieu of repayment of the Loan (including any penalties and
expenses due thereon).  At any bona fide
public sale the Lender shall be free to purchase all or any part of the Pledged
Securities.  Out of the proceeds of any
sale the Lender may retain an amount equal to the principal and interest
(including penalty interest) then due on the Loan, plus the amount of the
expenses of the sale.  Lender agrees
that, upon an Event of Default, it shall look first to the sale or retention of
the Pledged Collateral for satisfaction of any unpaid obligations of the
Pledgor under the terms of the Loan; provided, however, that the foregoing
shall in no way limit the rights and remedies of the Lender provided in the
Uniform Commercial Code in force in the State of New York at the date of this
Agreement.  8.Endorsement.  Upon an Event of Default in
respect of the Note, the Lender shall have the right, for and in the name,
place and stead of the Pledgor, to execute endorsements, assignments or other
instruments of conveyance or transfer with respect to their Pro Rata share of
the Pledged Collateral and each Lender hereby appoints the Escrow Agent their
attorney-in-fact to complete the stock powers and other instruments of transfer
delivered to the Escrow Agent in order to deliver the Pledged Securities in
accordance with Section 4 of this Agreement.  9.Indemnification and Costs.  (a)The Pledgor and the Lenders release the
Escrow Agent from any claims, causes of action and demands at any time arising
out of or with respect to this Agreement, the Pledged Collateral and/or any
actions, taken or omitted to be taken by the Escrow Agent with respect thereto,
and the Pledgor and the Lenders hereby agree to hold the Escrow Agent harmless
from and with respect to any and all such claims, causes of action and demands
other than those resulting from the gross negligence or willful misconduct of
the Escrow Agent.  (b)The Pledgor shall be exclusively responsible
for all costs and expenses (if any) incidental to the performance by the Escrow
Agent of its duties as agent hereunder.


(c)The Lenders acknowledge that the Escrow Agent
is acting in solely a ministerial capacity with respect to the release of
Pledged Collateral and that such Pledged Collateral may be commingled with
additional collateral pledged by the Pledgor to secure additional obligations,
including secured promissory notes of like tenor to the Note in the aggregate
principal amount of $500,000.  The
Lenders will not seek to hold the Escrow Agent for following directions of
other parties with respect to the Pledged Collateral, in good faith in order
received, by batch or any other administratively convenient order.  10.Rights Duties and Immunities.  (a)The duties and obligations of the Escrow
Agent shall be determined solely by the express provisions of this
Agreement.  The Escrow Agent shall not be
liable except for the performance of such duties and obligations as are specifically
set out in this Agreement and the Escrow Agent shall not be deemed to have any
knowledge of, or responsibility for, the terms of any other agreement,
instrument or document.  (b)The Escrow Agent shall not be responsible in
any manner whatsoever for any failure or inability of any party hereto, or of
any one else, to deliver documents to the Escrow Agent or otherwise to honor
any of the provisions of this Agreement.  (c)The Escrow Agent shall be entitled to rely
upon any judgment, certification, demand, notice, instrument or other writing
delivered to it hereunder without being required to determine the authenticity
of the correctness of any fact stated therein or the propriety or validity or
the service thereof.  The Escrow Agent
shall be fully protected in acting on and relying upon any written notice,
direction, request, waiver, consent, receipt or other paper or document which
the Escrow Agent believes to be genuine. 
The Escrow Agent may act in reliance upon any instrument or signature it
reasonably believes to be genuine and the Escrow Agent may assume that any
person purporting to give any advice or make any statement in connection with
the provisions hereof has been duly authorized to do so.  (d)The Escrow Agent shall not be liable for any
error of judgment, or for any act done or step taken or omitted by it in good
faith, or for any mistake of fact or law, or for anything which it may do or
refrain from doing in connection herewith, except its own bad faith, willful
misconduct or gross negligence.  (e)The Escrow Agent may seek the advice of legal
counsel as to any question arising from or relating to the construction of any
of the provisions of this Agreement or its duties or obligations hereunder or
otherwise, and it shall incur no liability and shall be fully protected in
respect of any action taken, omitted or suffered by it in good faith in
accordance with the advice of such counsel.  (f)The Escrow Agent does not make and will not
be required or deemed to make any representation as to the validity or
genuineness of any agreement, document or other instrument held by or delivered
to it.  (g)If a dispute arises between one or more of
the parties hereto, or between


any
of the parties hereto and any person not a party hereto, as to whether or not
or to whom the Escrow Agent shall deliver any of the Pledged Securities or as
to any other matter arising from or relating to this Agreement or any related
agreement, the Escrow Agent shall not be required to determine such dispute and
need not make any delivery of any of the Pledged Securities, but will retain
the same until the rights of the parties to the dispute shall have finally been
determined by written agreement among the parties to the dispute or by final
non-appealable order of a court of competent jurisdiction.  In the event that the Escrow Agent has
received notice of such order or any such agreement, the Escrow Agent shall
cause the Pledged Securities to be released and delivered in accordance with
such agreement or order.  (h)The Escrow Agent shall be entitled to assume
that no dispute of the type referred to in Section 10(g) has arisen
unless it has received a written notice that such a dispute has arisen, which
written notice refers specifically to this Agreement and identifies by name and
address the adverse claimants in such dispute. 
Any party delivering written notice of a dispute pursuant to this Section 10(h) shall
simultaneously therewith deliver a copy of such notice to all parties.  For purposes of this Section 10(h), the
Escrow Agent shall not be deemed to have received a written notice until all
parties to this Agreement have received such written notice.  If a dispute of the type referred to in Section 10(g) arises,
the Escrow Agent may, in its sole discretion (but shall not be obligated to),
commence interpleader or similar actions or proceedings for determination of
such dispute.  11.Successor Escrow Agent.  (a)The Escrow Agent may, at any time, resign as
such with or without the prior written consent of all the parties hereto, in
which case the Escrow Agent (and any successor escrow agent) shall deliver the
Pledged Securities, a copy of this Agreement and any other documents delivered
to it hereunder to any successor escrow agent jointly designated by the Lenders
holding not less than 70% of the outstanding principal of the Notes (the Requisite
Amount) and Pledgor in writing, or to any court of competent jurisdiction,
whereupon the Escrow Agent shall be discharged of and from any and all further
duties and obligations arising in connection with this Agreement.  The resignation of the Escrow Agent shall
take effect on the earlier of (i) the appointment of a successor escrow
agent, or (ii) the day which is two business days after the date of the
delivery of the Pledged Securities, a copy of this Agreement and any other
documents delivered to the Escrow Agent hereunder to any court of competent
jurisdiction.  In the event that a
successor escrow agent has not been appointed at the expiration of such two-day
period, the Escrow Agents sole responsibilities hereunder shall be: (i) to
maintain the safekeeping of the Pledged Securities and any other documents
delivered to it hereunder, if any, and (ii) to release and deliver the
Pledged Securities and any such documents in accordance with Section 4 of
this Agreement.  (b)If the Escrow Agent receives a written notice
signed by the Lenders which hold the Requisite Amount of the Notes and Pledgor
stating that they have selected a successor escrow agent, the Escrow Agent
shall deliver the Pledged Securities (and any other documents then held by it
hereunder, if any) to the successor escrow agent named in the aforesaid notice
within 15 days after receipt of such written notice.


12.Power of Attorney.  Upon
an Event of Default which is continuing, the Pledgor hereby appoints the Lender
or Lenders designees, successors or assigns as the Pledgors attorney-in-fact
for the purpose of carrying out the provisions of this Agreement and taking any
action and executing any instrument which the Lender may deem necessary or
advisable to accomplish the purposes hereof. Without limiting the generality of
the foregoing, the Lender shall have the right and power to receive, endorse
and collect all checks and other orders for the payment of money made payable
to the Pledgor representing any interest or dividend or other distribution
payable in respect of the Pledged Collateral which the Lender is entitled to
receive hereunder or any part thereof and to give full discharge for the same.  13.Notices.  Any notice or demand upon the
Pledgor shall be deemed to have been sufficiently given for all purposes
thereof if mailed, postage prepaid, by registered or certified mail, return
receipt requested, or if delivered, to the addresses set out below or at such
other address as the parties hereto may heretofore have designated in writing:  If to the Pledgor:  Callisto Pharmaceutical, Inc. 420 Lexington Avenue, Suite 1609 New York, New York 10710 Facsimile: 212 297-0020  with a copy to:  Jeffrey Fessler, Esq. Sichenzia Ross Friedman
Ference LLP 61 Broadway New York, NY 10006 Facsimile: 212 930-9725  If to the Lender at the address set forth on the signature page hereto.  If to the Escrow Agent:  Herbert H. Sommer, Esq. Sommer & Schneider LLP 595 Stewart Avenue, Suite 710 Garden City, NY 11530 Facsimile No.: (516) 908-4000  14.Choice of Law.  THIS
AGREEMENT AND THE NOTES SHALL BE CONSTRUED AND ENFORCED IN ACCORDANCE WITH, AND
THE RIGHTS OF THE PARTIES SHALL BE GOVERNED BY, THE LAW OF THE STATE OF NEW
YORK EXCLUDING CHOICE-OF-LAW PRINCIPLES OF THE LAW OF SUCH STATE THAT WOULD
REQUIRE THE APPLICATION OF THE LAWS OF A JURISDICTION OTHER THAN SUCH STATE.


15.Jurisdiction.  Each of the parties hereto hereby irrevocably
and unconditionally consents to submit to the exclusive jurisdiction of the
courts of the State of New York and of the United States of America, in each
case located in New York County, for any action, proceeding or investigation in
any court or before any governmental authority (Litigation) arising out of or
relating to the Transaction Documents and the transactions contemplated thereby
(and agrees not to commence any Litigation relating thereto except in such
courts), and further agrees that service of any process, summons, notice or
document by U.S. registered mail to its respective address set forth in this
Agreement shall be effective service of process of any Litigation brought
against it in any such court.  Each of
the parties hereto hereby irrevocably and unconditionally waives any objection
to the laying of venue of any Litigation arising out of this Agreement or the
transactions contemplated hereby in the courts of the State of New York or the
Untied States of America, in each case located in New York County, and hereby
further irrevocably and unconditionally waives and agrees not to plead or claim
in any court that any such Litigation brought in any such court has been
brought in an inconvenient forum.  16.Counterparts; Signatures By Facsimile. This Agreement may be executed in one or
more counterparts, all of which shall be considered one and the same agreement
and shall become effective when counterparts have been signed by each party and
delivered to the other party. This Agreement, once executed by a party, may be
delivered to the other party hereto by facsimile transmission of a copy of this
Agreement bearing the signature of the party so delivering this Agreement.  17.Headings. The headings of
this Agreement are for convenience of reference and shall not form part of, or
affect the interpretation of, this Agreement.  18.Severability. If any provision
of this Agreement shall be invalid or unenforceable in any jurisdiction, such
invalidity or unenforceability shall not affect the validity or enforceability
of the remainder of this Agreement or the validity or enforceability of this
Agreement in any other jurisdiction.  19.Entire Agreement; Amendments. This Agreement and the instruments
referenced herein contain the entire understanding of the parties with respect
to the matters covered herein and therein and supersede all previous
communication, representation, or Agreements whether oral or written, between
the parties with respect to the matters covered herein. Except as specifically
set forth herein or therein, neither the Company nor the Buyer makes any
representation, warranty, covenant or undertaking with respect to such
matters.  The Agreement may not be orally
modified. Only a modification in writing, signed authorized representatives of
the Pledgor, the Escrow Agent and the Lenders then holding the Requisite Amount
of the Notes.  20.Successors And Assigns. This Agreement shall be binding upon and
inure to the benefit of the parties and their successors and assigns. Neither
the Pledgor nor any Lender shall assign this Agreement or any rights or
obligations hereunder without the prior written consent of the other.
Notwithstanding the foregoing, each Lender may assign its rights hereunder


to
any person that purchases the Notes in a private transaction from such Lender
or an assignment by Lender to any of its affiliates, as that term is defined
under the 1934 Act, without the consent of the Pledgor.  21.Third Party Beneficiaries. This Agreement is intended for the benefit
of the parties hereto and their respective permitted successors and assigns,
and is not for the benefit of, nor may any provision hereof be enforced by, any
other person.  22.Further Assurances. Each party shall do and perform, or cause
to be done and performed, all such further acts and things, and shall execute
and deliver all such other agreements, certificates, instruments and documents,
as other parties may reasonably request in order to carry out the intent and
accomplish the purposes of this Agreement and the consummation of the
transactions contemplated hereby.  23.No Strict Construction. The language used in this Agreement will be
deemed to be the language chosen by the parties to express their mutual intent,
and no rules of strict construction will be applied against any party.  [SIGNATURE PAGE FOLLOWS]


IN WITNESS
WHEREOF, the Pledgor has executed this Agreement as of the day and year first
above.  PLEDGORCALLISTO
  PHARMACEUTICALS, INC.By:Gary S.
  Jacob, President and CEO  Accepted and Agreed this day of                   ,
2008.  ESCROW AGENT  SOMMER & SCHNEIDER LLP   By:Name:
  Herbert H.
  SommerTitle:
  Partner  LENDERS SIGNATURE PAGES FOLLOW


CALLISTO PHARMACEUTICALS, INC.  Lenders Execution of Security Agreement  Accepted and Agreed this day of                     ,
2008.   Your
  Signature
  
  PRINT
  EXACT NAME OF LENDERDELIVERY INSTRUCTIONS:Name: Please Print
  
  Please type or print
  address where your securities are to be deliveredATTN.:Title/Representative
  Capacity (if applicable)Name of Company You
  Represent (if applicable)
  
  Street AddressPlace of Execution of this
  Agreement
  
  City, State or Province,
  Country, Offshore Postal CodePhone Number (For Federal
  Express)Fax Number (re: Notice)


Exhibit A  CERTIFICATE  Reference is
made to a Security Agreement between the undersigned lender (Lender) and
Callisto Pharmaceuticals, Inc. (Pledgor).  The undersigned, either as Lender or as the
duly appointed agent of Lender, certifies and instructs the Escrow Agent as
follows:  1.The Pledgor has defaulted in the payment
or other obligation of Pledgor, giving the Lender the rights set forth in the
Security Agreement (Pledge).  2.Lender hereby demands delivery of                 
shares of the Pledged Collateral as follows:   Account No.  3.The Lender certifies:  (a)The sum of (1) the
number of shares of common stock beneficially owned by Lender and Lenders
affiliates (other than shares of common stock which may be deemed beneficially
owned through the Pledge) and (ii) the number of shares of common stock to
which the determination in this paragraph is being made, would result in
beneficial ownership by Lender of more than 4.99% of the outstanding shares of
common stock of the issuer of the Pledged Collateral.  For purposes of the immediately preceding
sentence, beneficial ownership shall be determined in accordance with Section 13(d) of
the Securities Exchange Act of 1934, as amended, and Regulation 13D-G
thereunder, except as otherwise provided in the parenthetical set forth in
clause (i) of the immediately preceding sentence.  (b)The Lender is not, and has not for at least
the 90 days preceding the date of this certificate, been an affiliate of the
issuer of the Pledged Collateral.  Dated:Lender or
  AgentPrint Name