Agreement and Plan of Merger

Exhibit 10.5

LEV PHARMACEUTICALS, INC.

675 Third Avenue

Suite 2200

New York, NY 10017

July 15, 2008

Judson Cooper

c/o Lev Pharmaceuticals, Inc.

675 Third Avenue

Suite 2200

New York, NY 10017

Dear Mr. Cooper:

Reference is made to the Agreement and Plan of Merger by and among ViroPharma Incorporated (“Buyer”), HAE Acquisition Corp (“Sub”), and Lev Pharmaceuticals, Inc. (the “Company”), dated July 15, 2008 (the “ Merger Agreement ”) and the Second Amended and Restated Employment Agreement between you and the Company dated December 20, 2007 (the “ Employment Agreement ”). Unless otherwise specified herein, capitalized terms used herein without definition have the meanings ascribed in the Merger Agreement or the Employment Agreement, as applicable.

This letter agreement (the “ Agreement ”) sets forth our mutual agreement concerning (i) your resignation as an executive officer and employee of the Company effective immediately after the closing of the transactions (the “ Merger ”) contemplated by the Merger Agreement (the “ Closing ”), (ii) the payments that will be made to you and (iii) amends the restrictions on competition set forth in the Employment Agreement.

1. Resignation . Your employment with the Company and its subsidiaries and affiliates will terminate in all capacities immediately after the Closing (the “ Effective Time ”).

2. Payments . The Company will provide you with the following payments and benefits:

(a) An amount equal to $14,365,000, which represents 3.25% of the Enterprise Value ($442,000,001) of the Company without regard to the Contingent Value Rights issued in connection with the Merger (such amount being, the “ Total Transaction Amount ”). The Total Transaction Amount shall be paid as follows:

(i) That portion of the Total Transaction Amount which is attributable to the Transaction Fee, or $6,630,000, shall be paid in a lump sum, subject to receipt by the Company and Buyer of a mutual release in the form attached hereto signed by You, on the Closing Date.

(ii) That portion of the Total Transaction Amount which is attributable to the severance amounts payable to you pursuant to Section 8(b) of the Employment Agreement (the “ Severance Amount ”) shall be paid in a lump sum on the day after the six-month period starting on the date of Closing. To illustrate, if the Closing occurs on September 30, 2008, the Severance Amount would be $6,377,823 and the payment will be made on March 31, 2009.

(iii) The remaining portion of the Total Transaction Amount (less the parties good faith determination of the cost of the benefits provided in Section 2(c) below) which is attributable to the Gross-Up Payment described in Section 8(e) of the Employment Agreement (the “ Gross-Up Amount ”) shall be paid to you when such taxes are remitted but in no event later than the last day of the taxable year following the taxable year in which you are required to pay the Excise Tax.

In accordance with Section 8(c) of the Employment Agreement, subject to receipt by the Company and Buyer of a mutual release in the form attached hereto signed by you, on the Closing Date the Company will place the Severance Amount and the Gross-Up Amount into a “rabbi trust.” Such trust shall be maintained pursuant to a standard rabbi trust arrangement by and among you, the Company, and an independent trustee providing for the timely payment of the amounts held in the trust (the “ Trust Arrangement ”). The Trust Arrangement shall be maintained until all sums held in the trust have been paid.

(b) As soon as practicable following, but in no event more than thirty (30) days following, the date that the First CVR Payment Amount is deemed earned, the Enterprise Value shall be increased to $522,363,637 and the Buyer shall pay you an additional amount equal to $1,205,455 as an additional Transaction Fee payment on such date. As soon as practicable following, but in no event more than thirty (30) days following, the date that the Second CVR Payment Amount is deemed earned, the Enterprise Value will increase to $602,727,274 and the Buyer shall pay you an additional amount equal to $1,205,455 as an additional Transaction Fee payment on such date. Any additional Gross-Up Payments applicable to each additional Transaction Fee payments and to the payment of the CVRs up to the applicable Cap (which in each case increases by $2,611,818) shall be paid by the Company within the time specified in Section 2(a)(iii) above. The first CVR Payment Amount and the Second CVR Payment Amount shall not be subject to the rabbi trust provisions described above and shall be paid in accordance with this Section 2(b).

(c) Continuation of Insurance Coverage . As provided in Section 7(e)(ii)(B) of the Employment Agreement, the Company or the Buyer will continue (or provide comparable substitute coverage) your health, dental, disability and life insurance coverage as in effect on the date of your termination, and pay the applicable premiums, until the earlier of (a) December 31, 2012 or (b) the date on which you are covered under another comparable plan. You agree to notify the Company in writing within 15 (fifteen) days in the event that you obtain coverage under another such plan.

(d) 401(k) Plan . You will be entitled to receive your vested accrued benefits under the Company’s 401(k) plan in accordance with the terms and conditions of such plan.

(e) Options; Restricted Stock. Your options to purchase the Company’s Common stock and your restricted shares of Common Stock will be deemed fully vested at the Closing and will be treated as contemplated in the Merger Agreement.

(f) Accrued Compensation . You shall also be entitled to receive the Accrued Compensation in accordance with the Employment Agreement.

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3. Restrictive Covenants .

(a) Non-competition . Effective as of the Closing, Section 9(d) of the Employment Agreement is amended in its entirety to read as follows:

“Cooper agrees that during the period beginning on the Closing Date and ending three years after the Closing Date (the “Noncompete Restricted Period” ), he shall not, except for a Permitted Activity, directly or indirectly, for his own benefit or for the benefit of any other person or entity (whether as an officer, director, employee, partner, joint venturer, consultant, investor or otherwise):

For purposes hereof, “Permitted Activity” means (i) the ownership of publicly-traded securities of any entity not exceeding 3% of the total amount outstanding of such securities, (ii) following Cooper’s notice to Buyer, the direct employment with a government entity or university (or contracting relationship with such an entity or university solely as an individual contributor) other than work performed for a university that directly benefits a competitor of the Company or is performed in connection with a project the subject matter of which is in direct competition with the products or services offered by the Company as of the Closing Date or (iii) accepting employment (or performing services for) any entity whose business is diversified but which engages in the Business, so long as (A) Cooper does not render any services or assistance to any division or part of such entity that is in any way engaged in the Business, and (B) Buyer shall have received, prior to Cooper rendering any services or assistance, written assurance from such entity that Cooper shall not render any services or assistance to any division or part of such entity that is in any way engaged in the Business; provided that the Permitted Activity does not include (a) any operational management position in any such entity referenced in (i) above or (b) any activity described in Section 9(d)(ii) above.

(b) Non-solicitation of Employees . Effective as of the Closing, Section 9(e) of the Employment Agreement is amended in its entirety to read as follows:

“Cooper agrees that during the period beginning on the Closing Date and ending one year after the Closing Date, he shall not (i) solicit for employment or recruit (whether as an employee, principal, consultant or otherwise) any person who is an employee of or consultant to the Company immediately prior to the Closing Date, or (ii) induce or encourage any such person to sever his/her relationship with the Company or Buyer; provided, that the provisions of this Section 9(e) shall not be violated if such employee or consultant or former employee or consultant (x) responds to a general advertisement for services not specifically directed at such person, or (y) has been terminated by the Company or the Buyer. This provision shall not apply to Joshua Schein.”

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(c) Notwithstanding anything to the contrary, you shall not be required to comply with any provision of Sections 9(d) or 9(e) of the Employment Agreement if the amounts required to be paid pursuant to Section 2 of this Agreement are not timely paid.

4. Communications . You and the Buyer agree that the press release and any related statements regarding your resignation will be in the form to be mutually agreed upon, and that no subsequent comments should be made to the media or through other public statements by either party or by any subsidiary, officer or director of the Buyer regarding your departure that are inconsistent with such statement. From and after the Effective Time, you will refrain from taking actions or making public statements, written or oral, which denigrate, disparage or defame the goodwill or reputation of the Buyer and its subsidiaries and their former and current executive officers and directors. From and af