SECURED PROMISSORY NOTE

Exhibit 10.62

SECURED PROMISSORY NOTE

THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR THE SECURITIES LAW OF ANY STATE AND MAY NOT BE SOLD, TRANSFERRED OR OTHERWISE DISPOSED OF EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER SUCH ACT AND APPLICABLE STATE SECURITIES LAWS OR PURSUANT TO AN APPLICABLE EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF SUCH ACT AND SUCH LAWS.

FOR VALUE RECEIVED, the receipt of which is hereby acknowledged, BIOVEST INTERNATIONAL, INC., a Delaware corporation having its corporate office at 8500 Evergreen Blvd., Minneapolis MN 55433 (the “Company”), hereby promises to pay to the order of Peter J. Pappas, Sr. having an address at c/o PJ Mechanical, 135 W. 18^th Street, Second Floor, New York, N.Y. 10011 (“Holder”), or such other address as Holder may notify the Company, the principal sum of $460,000.00 (the “Principal”), together with interest thereon, as follows:

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(b) For purposes of this Note, “Collateral” shall mean all of the Company’s right, title and interest in and to all of its tangible personal property and intangible property (including, without limitation, the Company’s cash on hand, contract rights, securities, accounts receivables, equipment, inventory, trademarks, copyrights and other intellectual property, wherever located; BUT EXCLUDING: (i) the Company’s rights in that certain Cooperative Research and Development Agreement for Non-Hodgkin Lymphoma Therapeutic Cancer Vaccine between the Company and the National Cancer Institute (as hereafter amended and supplemented), (ii) and the Company’s rights arising from and relating to Non-Hodgkin Lymphoma Therapeutic Cancer Vaccine, and (iii) the

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Company’s grant receivable from the National Institutes of Health; in each case, (x) whether tangible or intangible; (y) all and any proceeds from any sale, lease, license or other disposition thereof, and (z) all proceeds and products thereof).

(c) The Company hereby grants Holder a limited power of attorney to prepare and file appropriate UCC-1 financing statements (as secured party) in appropriate jurisdictions to evidence and publish the liens created by this Note.

(d) Except as contemplated by this Note and except as specifically set forth in the immediately following proviso, the Company shall not encumber or grant a security interest in any of the Collateral without the prior written consent of Holder and, other than the grant of the security interest contemplated hereby, the Collateral pledged by the Company hereunder is, and will be, owned by Maker free and clear of all liens and encumbrances; provided that, by Holder’s acceptance of this Note, Holder hereby agrees that the indebtedness evidenced by this Note (and the security interest granted and lien created hereby) shall be subordinated in right of payment to all existing secured debts of the Company outstanding on the date of issuance of this Note, plus payment in full in cash of up to $5,000,000 aggregate principal amount of indebtedness to third parties (other than Accentia, Inc. and/or its affiliates) and accrued interest thereon as may be outstanding from time to time after the date hereof (“Senior Debt”). Such terms of subordination shall be the terms reasonably requested by the lender(s) of such Senior Debt and, in connection therewith, Holder shall execute and deliver such subordination

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IN WITNESS WHEREOF, this Note has been executed as of the date first above written.

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AGREEMENT REGARDING CONVERSION OF PROMISSORY NOTE

THIS AGREEMENT dated as of July 28, 2005, by and between Biovest International, Inc. a Delaware corporation (“Biovest”) and Peter J. Pappas, Sr., who is the Holder of Certain Convertible Promissory Note(s) convertible into Biovest Common Stock (hereinafter “Holder”), is as follows:

WHEREAS Biovest and Holder understand and acknowledge that Biovest Common Stock is in the process of being listed for sale on the NASD Over the Counter-Bulletin Board for public trading; and

WHEREAS it is in the interest of both Biovest and Holder that the number of shares of Common Stock of Biovest that are subject to contingent issuance under Warrants and Convertible Notes (“Overhang”) be consistent with comparable public companies in order to be able to sustain a good market value accurately reflecting the company’s actual value and the potential of its future growth and success; and

WHEREAS both Biovest and Holder acknowledge and agree that the existence of large outstanding Overhang creates uncertainty and a potentially destabilizing influence on the market value of Biovest Common Stock, which tends to prevent the maintenance of a trading price accurately reflecting the value of Biovest;

NOW THEREFORE, for good and valuable consideration, receipt of which is hereby mutually acknowledged, the parties hereby agree as follows:

1. Subject to the terms contained herein, Holder hereby irrevocably covenants and agrees to convert the entire outstanding balance (Principal and all accrued interest) due on Holder’s Promissory Notes as particularly identified on the attached Exhibit A into Common Stock of Biovest at $0.60 per share. The foregoing provision replaces and supersedes all provisions regarding conversion rights into Biovest Common Stock contained in Holder’s Convertible Promissory Note(s).

2. Holder hereby covenants and agrees to convert the entire balance, including accrued interest on each of the Convertible Promissory Notes set forth on Exhibit A within 30 days of the date of this Agreement contingent upon and subject only to the following:

A. Biovest’s commencement of trading on the Over the Counter-Bulletin Board ;

B. Holder’s previously having converted these Notes into Accentia Common Stock in accordance with the terms of the Convertible Note(s).

3. Biovest agrees to issue to Holder and Holder agrees to accept from Biovest in full and complete satisfaction of all rights and obligations pursuant to the Convertible Promissory Note(s) identified in Exhibit A hereto the number of shares of Biovest Common Stock set forth on Schedule B hereto, which shares shall be fully-paid and non-assessable.

4. Biovest and Holder acknowledge and agree that either the Biovest Common Stock (or, in the event that Holder elects to convert into Accentia Common Stock as set forth in paragraph 2(b),the Accentia Common Stock) issued pursuant to this Agreement shall bear a restrictive legend stating that the shares are not registered securities under the Securities Act of 1933, as amended, or any state securities laws.

5. Representations:

A. Holder acknowledges and represents that Holder has had access to all reports filed with the SEC by Biovest;

B. Holder is an “Accredited Investor” as defined in Regulation D promulgated under the Securities Act of 1933, as amended, (ii) Holder has the financial ability to bear the economic risk of Holder’s investment as contemplated by this Agreement, and (iii) Holder was given the opportunity to ask questions to (and received satisfactory answers from) Biovest regarding the terms and conditions of this Agreement;

C. Other than as specifically set forth herein, there have been no representations or warranties made by either party to the other.

6. Holder and Biovest hereby agree to submit any dispute that may arise pursuant to this agreement to arbitration under the auspices of the American Arbitration Association Rules of Commercial Arbitration, to be venued in Tampa, FL. This agreement is subject to final approval by the Board of Directors of Biovest.

EXHIBIT A

1. Promissory Note dated August 31, 2001 in the Principal Amount of $1,000,000;

2. Promissory Note dated February 26, 2003 in the Principal Amount of $160,000;

3. Promissory Note dated June 16, 2003 in the Principal Amount of $300,000.

Schedule B

Biovest shall issue a total of 3,010,596 shares of Common Stock, fully-paid and non-assessable, in full conversion and satisfaction of all sums due and obligations owed pursuant to the Notes set forth above.

CLARIFICATION AND MODIFICATION OF

AGREEMENT REGARDING CONVERSION OF PROMISSORY NOTE

THIS CLARIFICATION AND MODIFICATION OF PROMISSORY NOTE hereby amends and supplements that certain AGREEMENT REGARDING CONVERSION OF PROMISSORY NOTE (the “Agreement”) dated as of July 29, 2005, by and between Biovest International, Inc., a Delaware corporation (“Biovest”), and the undersigned (“Holder”) as follows:

Paragraph 1 of the Agreement is hereby modified by adding the following sentence to the end of said paragraph: “The conversion described in this paragraph will occur at the time and upon the conditions set forth in paragraph 2 below, and such conversion will occur automatically and without any further action on the part of the Holder or Biovest.”

Paragraph 2 of the Agreement is hereby deleted and replaced with the following: “The automatic conversion of the principal and all accrued interest under the Convertible Promissory Notes pursuant to Paragraph 1 above shall occur on the date which is the earlier of (a) thirty (30) days after the closing of the initial underwritten public offering of Accentia Biopharmaceuticals, Inc. or (b) December 1, 2005. Notwithstanding the foregoing, such conversion shall be conditioned on the following: (i) Biovest’s common stock must be quoted on the OTC Bulletin Board as of the date of conversion, and (ii) the Holder shall not have previously converted such Convertible Promissory Notes into common stock of Accentia Biopharmaceuticals, Inc pursuant to the terms of such notes. Upon such conversion, all principal and accrued but unpaid interest through the date of conversion shall be deemed to have been automatically and irrevocably paid in full.”

All other terms of the Agreement shall continue in full force and effect. This Clarification and Modification shall be deemed to be effective as of September 27, 2005.