**Exhibit
      10.18**




**AMENDMENT
      TO LOAN AND SECURITY AGREEMENT**




This
      AMENDMENT TO LOAN AND SECURITY AGREEMENT is made as of May 19, 2008 (this
      " **Amendment** "),
      by
      and among Mercator Momentum Fund III, LP, a California limited partnership
      (together with its successors, assigns and designees, the " **Lender** "),
      as
      Lender, and Global Clean Energy Holdings, Inc., a Utah corporation formerly
      known as Medical Discoveries, Inc. (" **Borrower** "),
      as
      borrower. Unless expressly defined in this Amendment, all capitalized terms
      have
      the meanings given to them in that certain Loan and Security Agreement, dated
      September 7, 2007 (the “ **Loan
      Agreement** ”).




**RECITALS**




A. Pursuant
      to the Loan Agreement, Lender agreed to make available to Borrower a secured
      term credit facility (the " **Loan** ")
      in the
      amount of up to One Million Dollars ($1,000,000) (the " **Loan
      Amount** ").




B. Lender
      has, to date, only extended a total of $350,000 under the Loan Agreement, of
      which $150,000 has been repaid and $200,000 of principal remains outstanding
      (the “ **Existing
      Loan Balance** ”).
      The
      Existing Loan Balance is due and payable on June 21, 2008.




C. Each
      of the draw-downs to be made under the Loan Agreement (the, “ **Loans** ”)
      are to
      be evidenced by, among other things, secured promissory notes in the form of Exhibit
      A attached
      to the Loan Agreement (together with all renewals, rearrangements, replacements,
      modifications, substitutions, and extensions thereof, each, a “ **Note** ”
and,
      collectively, the “ **Notes** ”).




D. Borrower
      now desires to draw-down an additional $250,000 under the Loan (the
“ **Additional
      Draw-Down** ”),
      and
      combine the Existing Loan Balance and the Additional Draw-Down (collectively,
      the “ **Total
      Outstanding Loan Balance** ”)
      into
      one Note having a principal balance of $450,000, with an extension of the
      maturity date of the Loan to August 19, 2008 (the “ **Revised
      Note** ”).




E. As
      an inducement to Lender to lend the Additional Draw-Down and to accept the
      Revised Note, Borrower agrees (i) that the Revised Note shall bear interest
      at
      the rate of 8.68% *per
      annum* ,
      and
      (ii) to issue to Lender a warrant to purchase shares of Common Stock as more
      particularly set forth herein.




E. Lender
      and Borrower now desire to amend the Loan Agreement to reflect the (i) loan
      of
      the Additional Draw-Down, (ii) the terms of the Revised Note, and (iii) certain
      other amendments to the Loan Agreement.




**AGREEMENT**




NOW,
      THEREFORE, for good and valuable consideration, the receipt and sufficiency
      of
      which are hereby acknowledged by the parties, the parties agree as
      follows:




Section
      1. Section 1 of the Loan Agreement is hereby amended and restated to read in
      its
      entirety as follows:







“Section
      1. The
      Loan .




1.1 Agreement
      to Lend .
      In
      reliance upon the representations and warranties contained herein and subject
      to
      compliance by Borrower with the terms and conditions of this Amendment, the
      parties hereby agree as follows:




(a) Effective
      as of the date of this Amendment, Lender shall lend Borrower the Additional
      Draw-Down.




(b) The
      Note evidencing the Existing Loan Balance will be superseded by the Revised
      Note, which will evidence the Total Outstanding Loan Balance, with a principal
      balance of $450,000, and will bear interest at the rate of 8.68% *per
      annum* , the
      form of which is attached hereto as Exhibit
      A .




Borrower
      and Lender hereby agree that the Existing Loan Balance shall remain a “Loan” as
      defined under the Loan Agreement, and that the Additional Draw-Down under this
      Amendment likewise shall constitute a “Loan” under the Loan Agreement and shall
      be subject to the terms and conditions of the Loan Agreement, as modified by
      this Amendment and by the Revised Note.




1.2 Delivery
      of Revised Note and Existing Note. Concurrently with the execution and delivery
      of this Amendment and the funding of the Additional Draw-Down, (i) Lender is
      returning the originally executed Existing Note to Borrower, and (ii) Borrower
      is delivering to Lender an originally executed Revised Note.




1.3 Interest
      Payment on Existing Loan. Concurrently with the execution of this Amendment,
      Borrower is paying all unpaid interest that has accrued through the date of
      this
      Amendment on the Existing Loan Balance.




1.4 Interest
      on the Principal Indebtedness .
      Interest on the outstanding principal indebtedness of the Revised Note shall
      accrue at the rate of 8.68% *per
      annum* and be
      payable upon maturity or the prepayment thereof, in the manner and at the times
      set forth in the Revised Note.




1.5 Consideration .
      In
      consideration for Lender’s agreement to loan the Additional Draw-Down and extend
      the maturity of the Loan and for the other agreements made by Lender hereunder,
      Borrower and Lender agree that, concurrently with the execution of this
      Amendment and the Revised Note and the funding of the Additional Draw-Down
      ,
      Borrower will, on the first business day following the date of this Amendment,
      issue to Lender a one-year Common Stock purchase warrant (the “ **New
      Warrant** ”),
      pursuant to which Lender shall have the right to purchase a number of shares
      of
      Common Stock equal to $75,000 divided by 130% of the Closing Price (as defined
      below). The New Warrant shall have an exercise price equal to 130% of the
      Closing Price. For the purposes of this Section 1.5, the term “Closing Price”
shall mean the last price of the Common Stock on the date of this Agreement
      as
      published by the OTC Bulletin Board.





Section
      2. Section 2 of the Loan Agreement is hereby amended and restated in its
      entirety to read as follows:




“Section
      2. Representations
      and Warranties of Borrower .
      As an
      inducement to Lender to enter into this Amendment and the Revised Note, Borrower
      represents and warrants to Lender that, as of the date hereof, all
      representations and warranties contained in the Loan Agreement are true and
      correct in all material respects except as supplemented, updated or revised
      as
      follows:




2.1 Amendment
      and Revised Note Authorized .
      The
      execution, delivery and performance of this Amendment and the Revised Note
      by
      Borrower, are duly authorized and do not require the consent or approval of
      any
      governmental body or other regulatory authority; are not in contravention of
      or
      conflict with any law or regulation or any term or provision of its
      organizational documents; and the Amendment and the Revised Note are valid
      and
      binding obligations of Borrower enforceable in accordance with their
      terms.




2.2 Collateral.




(a) Borrower
      represents and warrants that it has no place of business or offices where its
      respective books of account and records are kept (other than temporarily at
      the
      offices of its attorneys or accountants) or places where the Collateral is
      stored or located, other than at its offices at 6033 W. Century Blvd., #1090
      Los
      Angeles, CA 90045.




2.3 Change
      in Name. Borrower has changed its name to “Global Clean Energy Holdings, Inc.”




2.4 Financial
      Information . The
      historical financial data concerning Borrower that has been delivered by
      Borrower to the Lender, consisting of the unaudited, internally prepared balance
      sheet and income statement for the period ending March 31, 2008, is true,
      complete and correct in all material respects and fairly presents the financial
      condition of the persons or entities covered thereby as of the date of such
      reports. Since the delivery of such data, except as otherwise disclosed in
      writing to the Lender, there has been no material adverse change in the assets,
      liabilities or financial position of Borrower or in the results of operations
      of
      Borrower. Borrower has not incurred any obligation or liability, contingent
      or
      otherwise, not reflected in such financial data which could reasonably be
      expected to cause a Material Adverse Effect.




2.5 Eucodis
      Sale. The co-development and licensing agreements between Borrower and
      Eucodis Pharmaceuticals Forschungs - und Entwicklungs GmbH have been terminated.
      In addition, the agreements between Borrower and Eucodis regarding the Eucodis
      Sale have expired and, as a result, there currently is no agreement in effect
      regarding the possible sale by Borrower of its rights to that certain topical
      aromatase inhibitor cream to Eucodis. Borrower is continuing to discuss the
      possible sale of its rights in the topical aromatase inhibitor cream with
      Eucodis as well as with other potential purchasers. For the purposes of this
      Amendment and the Revised Note, the sale of the topical aromatase inhibitor
      cream to Eucodis or any other party is hereafter referred to as the
‘ **SaveCream
      Sale** .’”




Section
      3. Section 4.3 of the Loan Agreement is hereby deleted in its entirety.





Section
      4. Miscellaneous Provisions .




4.1 Termination
      of Emmes Group .
      Lender
      and Borrower hereby agree and acknowledge that the Advisors have resigned.
      Lender and Borrower further agree that the Letter Agreement with the Advisors
      has been terminated and that the parties no longer need to comply with the
      terms
      thereof .




4.2 Notices
      to Parties .
      All
      notices or other communications hereunder or under any other Loan Document by
      any
      party to any other party shall be in writing unless otherwise provided for
      herein and shall be served by hand, certified or registered mail, postage
      prepaid, return receipt requested, or facsimile transmission confirmed by
      certified or  registered
      mail. All such notices or other communications shall be deemed to have been
      sufficiently given for all purposes hereof on the date of receipt or refusal
      to
      accept delivery. Addresses for notices are as listed below. Any party may change
      the address to which notices are to be sent by notice of such change to the
      other parties given as provided herein.




(i)    
       if to Lender:




Mercator
      Momentum Fund III, LP


555
      South
      Flower Street, Suite 4200


Los
      Angeles, CA 90071


Attention:
      David F. Firestone


Telephone:
      (213) 533-8288


Telecopier:
      (213) 533-8285





with
      a
      copy to:





Paula
      Winner Barnett, Esq.


17967
      Boris Drive


Encino,
      CA 91316


Attention:
      Paula Winner Barnett, Esq.


Telephone:
      (818) 776-9881


Telecopier:
      (818) 743-7491




(ii)    
       if to Borrower:




Global
      Clean Energy Holdings, Inc.


6033
      W.
      Century Blvd., #1090


Los
      Angeles, CA 90045


Attention:
      Richard Palmer


Telephone:
      (310) 378-8529


Telecopier:
      (310) 378-7620





with
      a
      copy to:




Troy
      Gould PC


1801
      Century Park East, Suite 1600


Los
      Angeles, CA 90067


Attention:
      Istvan Benko, Esq.


Telephone:
      (310) 789-1226


Telecopier:
      (310) 789-142




4.3 Waiver.
      Lender hereby waives any and all Events of Default that may have occurred prior
      to the date of this Amendment or that exist as of the date of this
      Amendment.




4.4 No
      Other
      Changes to Loan Agreement . Except
      as
      amended hereby, the Loan Agreement shall remain in full force and
      effect.




IN
      WITNESS WHEREOF, the parties hereto have caused this Amendment to be executed
      by
      their duly authorized representatives, as of the date first above
      written.






**LENDER:****MERCATOR
                MOMENTUM FUND III, LP**,a
                California limited partnershipBy:
            /s/
                MERCATOR MOMENTUM FUND III, LPName:Title:**BORROWER:****GLOBAL
                CLEAN ENERGY HOLDINGS, INC.**,a
                Utah corporationBy:
            /s/
                GLOBAL CLEAN ENERGY HOLDINGS, INC.Name:Title: