NOTE PURCHASE AGREEMENT

Exhibit 10.3

GUARDIAN II ACQUISITION CORPORATION

$20,000,000 12% Senior Secured Note

NOTE PURCHASE AGREEMENT

Dated as of July 21, 2006

TABLE OF CONTENTS

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TABLE OF CONTENTS

Guardian II Acquisition Corporation

1000 Winter Street, Suite 2200

Waltham, MA 02451

Phone: (781) 398-2300

Fax: (781) 398-2530

$20,000,000 12% Senior Secured Note

Dated as of July 21, 2006

Paul Royalty Fund Holdings II

140 East 45th Street, 44th Floor

New York, NY 10017

Ladies and Gentlemen:

Guardian II Acquisition Corporation, a Delaware corporation (the “ Company ”), agrees with you as follows:

The Company has authorized the issue and sale of a $20,000,000 aggregate principal amount Senior Secured Note (the “ Note ,” such term to include any such notes issued in substitution therefor pursuant to Section 13 of this Agreement). The Note shall be substantially in the form set out in Exhibit 1.1, with such changes therefrom, if any, as may be approved by you and the Company.

The initial maturity date of the Note shall be fourth anniversary of the Closing Date. So long as no Default or Event of Default then exists, the Company may, after the Closing, request (on a one time basis) an extension of the maturity date of the Note to the sixth anniversary of the Closing Date by providing at least 30 and not more than 60 days’ prior written notice to the holders of record of the Note of such requested extension, specifying the effective date of such requested extension. Such request, when made, shall be irrevocable. In order for such request to be made effective, (i) no Default or Event of Default shall exist on the effective date for such requested extension as specified by the Company and (ii) on or before the specified effective date, the Company shall have (A) delivered Extension Warrants to the holder of record of the Note which Extension Warrants shall, in respect of such holder, (1) be for the acquisition of a number of shares of common stock, par value $0.10 per share, of the Parent equal to 10% of the outstanding principal amount of the Note (inclusive of any interest accrued by unpaid hereunder) held by such holder divided by the exercise price per share for the warrants (determined per clause (2) immediately following) and (2) have an exercise price per share equal to the exercise

price per share of the Warrants delivered to you on the Closing (as such exercise price may have been adjusted for any subsequent events as provided in such Warrants), and (B) delivered to the holder of record of the Note such other documents, opinions and agreements as are consistent with the deliveries made to you on the Closing with respect to the Warrants received by you on the Closing (all of which shall be in form and substance reasonably satisfactory to such holder); provided that the condition in clause (i) above may be waived by the holder of the Note, in such holder’s sole discretion.

The Note shall, subject to the addition of any default interest, bear interest at a per annum rate of 12%. Interest shall be payable, in cash, semi-annually in arrears on the last day of each March and September; provided that, unless either prohibited by applicable law or the Company otherwise notifies the holders of the Note of its intention to pay all interest due on the next scheduled due date for interest payments in cash (which notice shall be in writing and irrevocable with respect to the interest payable on the scheduled due date) at least ten (10) days prior to then next scheduled due date for interest payments in respect of the Note, so long has no Default or Event of Default has then occurred and is continuing, 50% of the interest payable on such next scheduled date shall be paid in cash and the remaining 50% of such interest payable shall be added to the principal amount owing under the Note (effective from and after such next scheduled interest payment date). In no event shall the interest charged in respect of the Note, including any interest on amounts added to the principal of the Note pursuant to the preceding sentence, exceed the highest maximum amount chargeable as interest under applicable law, provided that if applicable law limits the amount chargeable as interest hereunder and applicable law subsequently permits a rate of interest in respect of the Note which is higher than the stated rate chargeable under the terms of the Note, the Note shall bear interest at such higher rate until the aggregate interest, including any interest on amounts added to the principal of the Note pursuant to the preceding sentence, charged in respect of the Note equals the amount which would have been charged if the rate chargeable in respect of the Note had not been so previously limited and thereafter the rate chargeable in respect of the Note shall reduce from such higher rate to the stated rate of interest hereunder.

Subject to the terms and conditions of this Agreement, the Company will issue and sell to you and you will purchase from the Company, at the Closing provided for in Section 3, the Note in the aggregate principal amount specified in Section 1.1 at the purchase price of 100% of the principal amount thereof.

The sale and purchase of the Note to be purchased by you shall occur at the offices of Ropes & Gray LLP, Boston, Massachusetts at 9:00 a.m., New York time, at a closing (the “ Closing ”) on such Business Day on or prior to October 30, 2006 as may be agreed upon by you and the Company (the “ Closing Date ”). At the Closing, the Company will deliver to you the Note to be purchased by you in the form of a single Note dated the date of the Closing and registered in your name (or in the name of your nominee), against delivery by you to the

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Company or its order of immediately available funds in the amount of the purchase price therefor by wire transfer of immediately available funds for the account of the Company to an account provided in writing to the Company. If at the Closing the Company fails to tender the Note to you as provided above in this Section 3, or any of the conditions specified in Section 4 shall not have been fulfilled to your satisfaction, you shall, at your election, be relieved of all further obligations under this Agreement, without thereby waiving any rights you may have by reason of such failure or such nonfulfillment. At the Closing, the dates left blank in this Agreement (i.e., the issuance date of the Note and maturity date of the same) shall be completed based on the date of the Closing.

Your obligation to purchase and pay for the Note to be sold to you at the Closing is subject to the fulfillment to your satisfaction, prior to or at the Closing, of the following conditions:

The representations and warranties of the Company set forth in this Agreement shall be true, correct and complete in all material respects as of the date of Closing.

The Company shall have performed and complied with all agreements and conditions contained in this Agreement required to be performed or complied with by it prior to or at the Closing and after giving effect to the issue and sale of the Note (and the application of the proceeds thereof as contemplated by Section 5.5) no Default or Event of Default shall have occurred and be continuing.

No action, suit, litigation, proceeding or investigation shall have been instituted, be pending or threatened (i) challenging or seeking to make illegal, to delay or otherwise directly or indirectly to restrain or prohibit the consummation of the transactions contemplated by this Agreement, or seeking to obtain damages in connection with the transactions contemplated by this Agreement, or (ii) seeking to restrain or prohibit your rights or performance under the Revenue Interests Assignment Agreement.

All notices to, consents, approvals, authorizations and waivers from third parties and Government Authorities that are required for the consummation of the transactions contemplated by this Agreement shall have been obtained or provided for and shall remain in effect.

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The Company shall have delivered to you a certificate certifying as to the resolutions attached thereto and any other corporate proceedings relating to the authorization, execution and delivery of the Note and this Agreement.

You shall have received opinions in form and substance satisfactory to you, dated the date of the Closing from Ropes & Gray LLP, covering the matters set forth in Exhibit 4.6.

On the date of the Closing your purchase of Note shall (i) not violate any applicable law or regulation (including, without limitation, Regulation T, U or X of the Board of Governors of the Federal Reserve System) and (ii) not subject you to any tax, penalty or liability under or pursuant to any applicable law or regulation, which law or regulation was not in effect on the date hereof. If requested by you at least three Business Days prior to the Closing, you shall have received an Officer’s certificate certifying as to such matters of fact as you may reasonably specify to enable you to determine whether such purchase is so permitted.

Your due diligence review of the Company shall have been completed to your reasonable satisfaction.

All corporate and other proceedings in connection with the transactions contemplated by this Agreement and in connection with the Acquisition and all documents and instruments incident to such transactions shall be reasonably satisfactory to you and your special counsel, and you and your special counsel shall have received all such counterpart originals or certified or other copies of such documents as you or they may reasonably request, including without limitation, the Security Agreement.

The Company shall have consummated the transactions contemplated by the Acquisition and the Revenue Interests Assignment Agreement and the Stock Purchase Agreement shall contemporaneously be consummated, all on the terms and conditions described therein. The Company shall have issued the Warrant to Paul Royalty Fund Holdings II.

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As of the date hereof (it being understood that the representations and warranties contained herein shall be deemed to be made both before and after giving effect to the Acquisition and the consummation of the transactions contemplated under the Revenue Interests Assignment Agreement and the Stock Purchase Agreement), the Company represents and warrants to you that:

The Company has all necessary power and authority to enter into, execute and deliver the this Agreement and the Note and to perform all of the obligations to be performed by it hereunder and thereunder and to consummate the transactions contemplated hereunder and thereunder. This Agreement and the Note each have been duly authorized, executed and delivered by the Company and each of this Agreement and the Note constitutes the valid and binding obligation of the Company, enforceable against it in accordance with their respective terms, subject, as to enforcement of remedies, to bankruptcy, insolvency, reorganization, moratorium or similar laws affecting creditors’ rights generally or general equitable principles.

The execution and delivery by the Company of this Agreement and the Note, and the performance by it of its obligations hereunder and thereunder, does not require any notice to, action or consent by, or in respect of, or filing with, any Governmental Authority, except for the filing of financing statements under the UCC.

The Company has no Subsidiaries and owns no equity interests or debt interests of any other Person. Immediately following the Closing, the sole assets and liabilities of the Company will consist of the assets and liabilities as set forth in the Antara Purchase Agreement, an equity investment in the Company by Parent, and amounts payable by the Company to Parent. The Company is a wholly-owned Subsidiary of Parent and there are no options, warrants, convertible instruments or other rights held by any Person other than Parent to acquire any equity interest (or interest convertible or exchangeable for any equity interest) in the Company.

The Company shall use all proceeds from the issuance of the Note to fund the purchase price of and related transaction expenses in connection with the Acquisition. The Company shall not use any such proceeds for any other purpose.

Other than with respect to its obligations in respect of the Note, the Revenue Interests Assignment Agreement, the Antara Purchase Agreement and subordinated intercompany indebtedness owing to Parent or its Affiliates, the Company has no Debt.

(a) Neither the sale of the Note by the Company hereunder nor its use of the proceeds thereof will violate the Trading with the Enemy Act, as amended, or any of the foreign assets control regulations of the United States Treasury Department (31 CFR, Subtitle B, Chapter V, as amended) or any enabling legislation or executive order relating thereto.

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(b) The Company (i) is not a Person described or designated in the Specially Designated Nationals and Blocked Persons List of the Office of Foreign Assets Control or in Section 1 of the Anti-Terrorism Order or (ii) does not engage in any dealings or transactions with any such Person. The Company is in compliance, in all material respects, with the USA Patriot Act.

(c) No part of the proceeds from the sale of the Note hereunder will be used, directly or indirectly, for any payments to any governmental official or employee, political party, official of a political party, candidate for political office, or anyone else acting in an official capacity, in order to obtain, retain or direct business or obtain any improper advantage, in violation of the United States Foreign Corrupt Practices Act of 1977, as amended, assuming in all cases that such Act applies to the Company.

The Company is not subject to regulation under the Investment Company Act of 1940, as amended, the Public Utility Holding Company Act of 2006, as amended, the Interstate Commerce Act, as amended by the ICC Termination Act of 1995, as amended, or the Federal Power Act, as amended.

The representations and warranties of the Company contained in Sections 3.06 through 3.15 , inclusive, of the Revenue Interests Assignment Agreement are hereby incorporated by reference as if such sections were set forth in full herein, mutatis mutandis.

You represent that (i) the Note is being acquired for the your own account and without a view to the resale or distribution of the Note or any interest therein other than in a transaction exempt from registration under the Securities Act; (ii) you are an “ accredited investor ” as such term is defined in Rule 501(a) of Regulation D under the Securities Act; (iii) you understand that the Note being sold hereby has not been registered under the Securities Act, or applicable state securities laws, and is being issued in reliance on exemptions for private offerings contained in Section 4(2) of the Securities Act and in reliance on exemptions from the registration requirements of certain state securities laws. Because the Note has not been registered under the Securities Act or applicable state securities laws, the Note may not be re-offered or resold except through a valid and effective registration statement or pursuant to a valid exemption from the registration requirements under the Securities Act and applicable state securities laws; (iv) you have sufficient knowledge and experience in financial and business matters so as to be capable of evaluating the merits and risks of its investment in the Note and are capable of bearing the economic risks of such investment, including a complete loss of its investment in the Note; and (v) you understand that your investment in the Note involves a high degree of risk.

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Following the termination of the Revenue Interests Assignment Agreement, the Company agrees to deliver to you and to perform the following:

(a) Promptly after receipt by the Company or Parent of notice of any action, claim, investigation, proceeding (commenced or threatened), certificate, offer, proposal, material correspondence or other material written communication relating to the transactions contemplated by this Agreement, the Revenue Interests Assignment Agreement, the Stock Purchase Agreement or documents related thereto or transactions contemplated thereby, then, the Company shall inform you of the receipt of such notice and the substance of such action, claim, investigation, proceeding, certificate, offer, proposal, correspondence or other written communication and, if in writing shall furnish you with a copy of such notice and any related materials with respect to such action, claim, investigation, proceeding, certificate, offer, proposal, correspondence or other written communication.

(b) The Company and Parent shall keep and maintain, or cause to be kept and maintained, at all times accurate and complete books and records.

(c) The Company and its Parent shall maintain a system of accounting established and administered in accordance with sound business practices to permit preparation of financial statements in conformity with GAAP.

(d) The Company shall deliver to you the following financial statements:

(i) Within forty-five (45) calendar days after the end of each Fiscal Quarter, copies of the unaudited consolidated financial statements of Parent and its Subsidiaries for such Fiscal Quarter; and

(ii) Within ninety (90) calendar days after the end of each Fiscal Year, copies of the audited consolidated financial statements of Parent and its Subsidiaries for such Fiscal Year.

(e) You and any of your representatives shall have the right, once a year (and at any other time a Default or an Event of Default shall have occurred or be continuing), upon five (5) Business Day’s written notice given by you to the Company (provided one (1) Business Day’s notice shall be required if a Default or Event of Default shall have occurred and be continuing), to visit the Company and Parent’s offices and properties where the Company and Parent keep and maintain books and records relating or pertaining to this Agreement for purposes of conducting an audit of such books and records, and to inspect, copy and audit such books and records, during normal business hours, and the Company will provide you and any of your representatives reasonable access to such books and records, and shall permit you and any of your representatives to discuss the business, operations, properties and financial and other condition of the Company or any of its Affiliates including, but not limited to, matters relating or pertaining to this Agreement with officers of such parties, and with their independent certified public accountants (to the extent such independent certified accountants agree to discuss such matters with you) whose reasonable fees and expenses shall be paid by the Company.

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No regularly scheduled prepayments are due on the Note prior to their stated maturity.

The Company may, at its option, upon notice as provided below, prepay at any time after (a) the occurrence of Change of Control (as defined in the Revenue Interests Assignment Agreement) or (b) on or after the second anniversary of the Closing Date, all or any part of the Note in an amount not less than $1,000,000 in the aggregate in the case of a partial prepayment, at (i) 101% of the principal amount so prepaid if the prepayment is made, in each case, before the third anniversary of the Closing Date or (ii) 100% of the principal amount so prepaid if the prepayment is made on or after the third anniversary of the Closing Date. The Company will give the holder of the Note written notice of each optional prepayment under this Section 8.2 not less than 30 days and not more than 60 days prior to the date fixed for such prepayment. Each such notice shall specify such date (which shall be a Business Day), the principal amount of the Note to be prepaid on such date, and the interest to be paid on the prepayment date with respect to such principal amount being prepaid.

In the case of each prepayment of the Note pursuant to this Section 8, the principal amount of the Note to be prepaid shall mature and become due and payable on the date fixed for such prepayment (which shall be a Business Day), together with interest on such principal amount accrued to such date and prepayment premium, if any. From and after such date, unless the Company shall fail to pay such principal amount when so due and payable, together with the interest and prepayment premium, if any as aforesaid, interest on such principal amount shall cease to accrue. If the Note is paid or prepaid in full, it shall be surrendered to the Company and canceled and shall not be reissued.