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                                                                   EXHIBIT 10.20

                                AMENDMENT TO THE

                              EMPLOYMENT AGREEMENT

                             OF CHARLES J. CASAMENTO

            This Amendment to the Employment Agreement (the "Agreement"), dated
as of April 4, 1999, by and between Questcor Pharmaceuticals, Inc., a California
corporation (formerly named "Cypros Pharmaceutical Corporation") (the
"Company"), and Charles J. Casamento ("Executive") is made and entered into as
of March 21, 2003.

                                    RECITALS

            A. The Company, Cypros Acquisition Corporation and RiboGene, Inc.
entered into the Agreement and Plan of Reorganization dated as of August 4,
1999, which provided that Cypros Acquisition Corporation would be merged with
and into RiboGene, Inc., on the terms and subject to the conditions set forth
therein (the "Merger").

            B. The Company and Executive entered into the Agreement, which
provided that Executive would become employed by the Company, effective as of
the Merger, on the terms and subject to the conditions set forth in the
Agreement.

            C. The Company and Executive desire to amend the Agreement in
certain respects and continue Executive's employment in accordance with the
Agreement, as amended.

            NOW, THEREFORE, in consideration of the mutual covenants and
agreements contained herein, and with reference to the above recitals, the
parties hereby agree to amend the Agreement as follows:

            1. Subsections 7.1(a), (b) and (c) of the Agreement are hereby
amended to read in their entirety as follows:

                  (a) Subject to Section 7.3 below, if Executive's employment
      with the Company is terminated by the Company (other than for 'Cause' or
      'Disability' or upon Executive's death) at any time within sixty (60) days
      before, or within twelve (12) months after, a Change of Control, or is
      terminated by Executive within six (6) months after a Change of Control,
      Executive shall be entitled to receive a severance benefit, in an amount
      equal to twenty-four (24) months of Base Salary at the annual rate in
      effect immediately prior to the Change of Control (or, if greater, the
      annual rate in effect on the date of termination of employment). In
      addition, in the event of such termination of employment, the Company
      shall pay to Executive a prorated bonus for the fiscal year of the Company
      in which such termination of employment occurs (the 'Termination Fiscal
      Year') in an amount equal to Executive's Bonus Opportunity for the
      Termination Fiscal Year, multiplied by a fraction, the numerator of which
      is the number of days during the Termination Fiscal Year ending prior to
      such termination of employment, and the denominator of which is the number
      of days

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      in the Termination Fiscal Year. Furthermore, in the event of such
      termination of employment: (i) if the Board (or the Compensation Committee
      thereof) has determined the amount of Executive's bonus for the fiscal
      year of the Company immediately preceding the Termination Fiscal Year (the
      'Prior Fiscal Year') prior to the Change of Control, and the Company has
      not paid the bonus for the Prior Fiscal Year (if any) to Executive prior
      to such termination of employment, the Company shall pay Executive's bonus
      (as so determined) for the Prior Fiscal Year to Executive, or (ii) if the
      Board (or the Compensation Committee thereof) has not determined the
      amount of Executive's bonus for the Prior Fiscal Year prior to the Change
      of Control, the Company shall pay to Executive a bonus for the Prior
      Fiscal Year in an amount not less than Executive's Bonus Opportunity for
      the Prior Fiscal Year. For purposes of this subsection, the Executive's
      Bonus Opportunity for any fiscal year shall be not less than 50% of
      Executive's annual rate of Base Salary determined as of the first day of
      such fiscal year, and any reference to the fiscal year of the Company
      shall include the fiscal year of any successor thereto. Such payments
      shall be paid in cash in a lump sum payment not later than ten (10) days
      following such termination of employment.

                  (b) In the event Executive is entitled to a severance benefit
      pursuant to Section 7.1(a), then in addition to such severance benefit,
      Executive shall receive 100% Company-paid health, term life and disability
      insurance coverage and medical expense reimbursement benefits coverage as
      is provided to such Executive (and his dependents, if applicable)
      immediately prior to Executive's termination of employment (the
      'Company-Paid Coverage'). Company-Paid Coverage shall continue for
      twenty-four (24) months following termination of employment or until
      Executive becomes covered under another employer's group insurance plan or
      plans providing health, term life and disability insurance coverage,
      whichever occurs first. In addition, for twenty-four (24) months following
      the termination of Company-Paid Coverage, the Company shall provide
      Executive (and his dependents, if applicable) with the health, term life
      and disability insurance coverage provided immediately prior to
      Executive's termination of employment, at Executive's election and
      expense.

                  (c) In the event Executive is entitled to severance benefits
      pursuant to Section 7.1(a), each of the Prior Stock Options (as assumed
      and converted as provided in Section 3.4), the New Options and each other
      stock option exercisable for shares of Company Common Stock (or any option
      into which such Options or any such options have been converted) held by
      Executive shall become immediately vested on the date of Executive's
      termination of employment and shall be exercisable in full in accordance
      with the provisions of the plan and option agreement pursuant to which
      such Option or option was granted. Executive shall have the right to
      require an extension of the exercise period of each such Option or option
      for a period of two (2) years following the later of: (i) the termination
      of employment, or (ii) the expiration of a lock-up agreement (if any)
      imposed on the Company's optionees at the time of Executive's termination
      of employment; provided, however, that in no event will such extension of
      such Option or option extend beyond the expiration of the

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      original term of the Option or option. In addition, the Prior Restricted
      Stock (as substituted as provided in Section 3.5) and each other share of
      Company's Common Stock held by Executive that is subject to a forfeiture,
      reacquisition or repurchase option held by the Company shall become fully
      vested, nonforfeitable and no longer subject to reacquisition or
      repurchase by the Company or other restrictions on the date of Executive's
      termination of employment.

            2. Section 10.11 of the Agreement is hereby amended to read in its
entirety as follows:

                  10.11 ATTORNEYS' FEES.

                  (a) If any legal action or other proceeding is brought for the
      enforcement of the Agreement, or because of an alleged dispute, breach or
      default in connection with any of the provisions of the Agreement, the
      successful or prevailing party shall be entitled to recover attorneys'
      fees and other expenses and costs incurred in that action or proceeding,
      in addition to any other relief that may be granted.

                  (b) Notwithstanding subsection (a), in the case of any legal
      action or other proceeding by Executive to enforce Section 7.1 of the
      Agreement, or an alleged dispute, breach or default in connection with
      Section 7.1 of the Agreement, regardless of whether Executive is
      successful or prevails: (i) the Company shall bear its attorneys' fees and
      other expenses and costs, and (ii) the Company shall reimburse Executive
      for Executive's reasonable attorneys' fees and other reasonable expenses
      and costs, to the extent incurred in connection with such enforcement or
      attempted enforcement or alleged dispute, breach or default as part of the
      Initial Adjudication (as defined below), in addition to any other relief
      that may be granted. The Company shall not reimburse Executive for any
      attorneys' fees and other expenses and costs incurred in connection with
      such enforcement or attempted enforcement or alleged dispute, breach or
      default incurred by Executive following the Initial Adjudication. Such
      reimbursements of Executive's attorneys' fees and other expenses and costs
      shall be made monthly not later than 30 days after the Company has
      received a copy of the written invoice evidencing such fees, expenses or
      costs. In the event that a court of competent jurisdiction or arbitrator
      determines that Executive has acted in connection with such enforcement or
      attempted enforcement, or alleged dispute, breach or default, in bad
      faith, or that Executive's positions, claims or assertions were frivolous
      or without substantial basis, Executive shall repay to the Company any
      attorneys' fees and other expenses and costs paid by the Company to
      Executive pursuant to this subsection (b). For the purposes of this
      Agreement, the "Initial Adjudication" shall mean the final order, decree
      or other adjudication of Executive's claims by a court of competent
      jurisdiction or arbitrator with regard to such enforcement or attempted
      enforcement or such alleged dispute, breach or default.

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            3. This Amendment shall be effective as of the date hereof. The
Agreement, as amended by this Amendment, shall remain in full force and effect.

            IN WITNESS WHEREOF, the parties have executed this Amendment to the
Agreement as of the date first written above.

                                        "COMPANY"

                                        QUESTCOR PHARMACEUTICALS, INC.,
                                        a California corporation

                                        By:    /s/ Timothy E. Morris
                                               ---------------------------------

                                        Title: Vice President, Finance &
                                               Administration and
                                               Chief Financial Officer
                                               ---------------------------------

                                        Date:  March 21, 2003
                                               ---------------------------------

                                        "EXECUTIVE"

                                        By: /s/ Charles J. Casamento
                                            ------------------------------------

                                        Date:  March 21, 2003
                                               ---------------------------------

                                       4<PAGE>
                                                                   Exhibit 10.23

                                                           [QUESTCOR LETTERHEAD]

                                                            Charles J. Casamento
                                                       Chairman, President & CEO

May 2, 2000

Questcor Pharmaceuticals, Inc.
3260 Whipple Road
Union City, CA 94587

Dear Ken:

I am pleased to offer you the position of Vice President, Marketing and Sales
for Questcor Pharmaceuticals, Inc. The terms of your offer are as follows:

-    A base salary of $184,000 annually.

-    An annual performance review and corresponding merit increase based on
     performance, each January 1, beginning in 2001.

-    Stock options in the amount of 150,000 shares. The price will be determined
     as of the closing price on your first day of employment. Your options will
     vest monthly over a forty-eight month period with vesting beginning in
     month seven at which point you will be vested in 6/48ths of your options
     with another 1/8th being vested each month thereafter.

-    Participation in our management bonus plan. Historically, our corporate
     officers have been eligible for a bonus equal to 33% ($61,000 in your case)
     of base salary. The exact amount to be paid will be determined based on
     achievement of agreed upon objectives.

-    Participation in sales commission plan that will provide up to an
     additional $40,000 income based on over-achievement of the sales budget.

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5/2/00 letter from C. Casamento
Page 2
-------------------------

-    You will participate in the Company's benefits, which are appropriate for
     individuals on a Vice President level. This includes medical, dental, life
     insurance, disability, 401-K plan and a company stock purchase plan.

-    You currently have commission due from your current employer. They
     will not pay commission if you leave the company prematurely. To
     compensate for this we will pay a one-time bonus of $20,000 when you
     become employed by Questcor.

-    We will provide you with six months severance should your employment
     be terminated. If the Company ever merges or is acquired and a severance
     plan is put in place providing for severance for a period of time different
     than six months, you will receive severance for the greater period of time.

Ken, I believe you can make significant contributions to the growth and
development of Questcor. I look forward to a mutually beneficial working
relationship.

Sincerely,                                  Accepted by,

/s/ Charles J. Casamento                     /s/ Kenneth R. Greathouse
---------------------------                  ----------------------------
Charles J. Casamento                         Kenneth R. Greathouse
Chairman, President and CEO
Questcor Pharmaceuticals, Inc.

Date: May 2, 2000                            Date: 5/3/00
     ----------------------                  -------------------------------

CJC:ls

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