GUARANTY

Exhibit 4.5

EXECUTION COPY

ARONEX PHARMACEUTICALS, INC.

GUARANTEE

Dated as of October 30, 2006

Ingalls & Snyder, as Agent

TABLE OF CONTENTS

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ARONEX PHARMACEUTICALS, INC.

GUARANTEE

This Agreement, dated as of October 30, 2006, is among Aronex Pharmaceuticals, Inc., a Delaware corporation (the “Guarantor”), and Ingalls & Snyder LLC, as agent (the “Agent”) for itself and the other Buyers under the Securities Purchase Agreement (as defined below). The parties agree as follows:

1. Reference to Securities Purchase Agreement; Definitions; Certain Rules of Construction . Reference is made to the Securities Purchase Agreement dated as of the date hereof, as from time to time in effect (the “ Securities Purchase Agreement ”), among Antigenics Inc., a Delaware corporation (the “ Borrower ”), the Buyers and the Agent. Capitalized terms defined in the Securities Purchase Agreement and not otherwise defined herein are used herein with the meanings so defined. Certain other capitalized terms are used in this Agreement as specifically defined below in this Section 1. Except as the context otherwise explicitly requires, (a) the capitalized term “Section” refers to sections of this Agreement, (b) the capitalized term “Exhibit” refers to exhibits to this Agreement, (c) references to a particular Section shall include all subsections thereof, (d) the word “including” shall be construed as “including without limitation”, (e) references to a particular statute or regulation include all rules and regulations thereunder and any successor statute, regulation or rules, in each case as from time to time in effect and (f) references to a particular Person include such Person’s successors and assigns to the extent not prohibited by this Agreement and the other Transaction Documents. References to “the date hereof” mean the date first set forth above.

1.1. “ Agreement ” means this Guarantee as from time to time in effect.

1.2. “ Bankruptcy Code ” means Title 11 of the United States Code.

1.3. “ Credit Obligations ” means all present and future liabilities, obligations and Indebtedness of the Company, any of its Subsidiaries or any other Obligor owing to the Agent or any Buyer (or any Affiliate of a Buyer) under or in connection with this Agreement or any other Transaction Document.

1.4. “ Credit Security ” is defined in section 2.1 of the Pledge Agreement.

1.5. “ Default ” means any Event of Default and any event or condition which with the passage of time or giving of notice, or both, would become an Event of Default, including the filing against the Company, any of its Subsidiaries or any other Obligor of a petition commencing an involuntary case under the Bankruptcy Code.

1.6. “ Event of Default ” is defined in section 4(a) of the Note.

1.7. “ Guarantee ” is defined in section 2.1 of this Agreement.

1.8. “ Pledge Agreement ” means the Pledge and Security Agreement, dated as of the date hereof, between the Borrower and the Agent.

1.9. “ Obligors ” means the Borrower, the Guarantor and the Subsidiaries of the Borrower party hereto from time to time.

1.10. “ Subsidiary ” means any person of which the Borrower (or other specified Person) shall at the time, directly or indirectly through one or more of its subsidiaries, (a) own at least 50% of the outstanding capital stock (or other shares of beneficial interest) entitled to vote generally, (b) hold at lest 50% of the partnership, joint venture or similar interests or (c) be a general partner or joint venturer.

2. Guarantee .

2.1. Guarantee of Credit Obligations . The Guarantor unconditionally guarantees (the “Guarantee”) that the Credit Obligations will be performed and paid in full in cash when due and payable, whether at the stated or accelerated maturity thereof or otherwise, this guarantee being a guarantee of payment and not of collectability and being absolute and in no way conditional or contingent. In the event any part of the Credit Obligations shall not have been so paid in full when due and payable, the Guarantor will, immediately upon notice by the Agent pay or cause to be paid to the Agent for the account of each Buyer in accordance with the Buyers’ respective percentage interests therein the amount of such Credit Obligations which are then due and payable and unpaid. The obligations of the Guarantor hereunder shall not be affected by the invalidity, unenforceability or irrecoverability of any of the Credit Obligations as against the Borrower, any other Obligor, any other guarantor thereof or any other Person. For purposes hereof, the Credit Obligations shall be due and payable when and as the same shall be due and payable under the terms of the Securities Purchase Agreement or any other Transaction Document notwithstanding the fact that the collection or enforcement thereof may be stayed or enjoined under Bankruptcy Code or other applicable law.

2.2. Continuing Obligation . The Guarantor acknowledges that the Buyers have entered into the Securities Purchase Agreement (and, to the extent that the Buyers or the Agent may enter into any future Transaction Document, will have entered into such agreement) in reliance on this Section 2 being a continuing irrevocable agreement, and the Guarantor agrees that its guarantee may not be revoked in whole or in part. The obligations of the Guarantor hereunder shall terminate when all of the Credit Obligations have been indefeasibly paid in full in cash and discharged; provided , however , that:

(a) if a claim is made upon the Buyers at any time for repayment or recovery of any amounts or any property received by the Buyers from any source on account of any of the Credit Obligations and the Buyers repay or return any amounts or property so received (including interest thereon to the extent required to be paid by the Buyers) or

(b) if the Buyers become liable for any part of such claim by reason of (i) any judgment or order of any court or administrative authority having competent jurisdiction, or (ii) any settlement or compromise of any such claim,

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then the Guarantor shall remain liable under this Agreement for the amounts so repaid or property so returned or the amounts for which the Buyers become liable (such amounts being deemed part of the Credit Obligations) to the same extent as if such amounts or property had never been received by the Buyers, notwithstanding any termination hereof or the cancellation of any instrument or agreement evidencing any of the Credit Obligations. Not later than five days after receipt of notice from the Agent, the Guarantor shall pay to the Agent an amount equal to the amount of such repayment or return for which the Buyers have so become liable. Payments hereunder by the Guarantor may be required by the Agent on any number of occasions.

2.3. Waivers with Respect to Credit Obligations . Except to the extent expressly required by the Securities Purchase Agreement or any other Transaction Document, the Guarantor waives, to the fullest extent permitted by the provisions of applicable law, all of the following (including all defenses, counterclaims and other rights of any nature based upon any of the following):

(a) presentment, demand for payment and protest of nonpayment of any of the Credit Obligations, and notice of protest, dishonor or nonperformance;

(b) notice of acceptance of this guarantee and notice that credit has been extended in reliance on the Guarantor’s guarantee of the Credit Obligations;

(c) notice of any Default or of any inability to enforce performance of the obligations of the Borrower or any other Person with respect to any Transaction Document or notice of any acceleration of maturity of any Credit Obligations;

(d) demand for performance or observance of, and any enforcement of any provision of the Securities Purchase Agreement, the Credit Obligations or any other Transaction Document or any pursuit or exhaustion of rights or remedies with respect to any Credit Security or against the Borrower or any other Person in respect of the Credit Obligations or any requirement of diligence or promptness on the part of the Agent or the Buyers in connection with any of the foregoing;

(e) any act or omission on the part of the Agent or the Buyers which may impair or prejudice the rights of the Guarantor, including rights to obtain subrogation, exoneration, contribution, indemnification or any other reimbursement from the Borrower or any other Person, or otherwise operate as a deemed release or discharge;

(f) failure or delay to perfect or continue the perfection of any security interest in any Credit Security or any other action which harms or impairs the value of, or any failure to preserve or protect the value of, any Credit Security;

(g) any statute of limitations or any statute or rule of law which provides that the obligation of a surety must be neither larger in amount nor in other respects more burdensome than the obligation of the principal;

(h) any “single action” or “anti deficiency” law which would otherwise prevent the Buyers from bringing any action, including any claim for a deficiency, against the Guarantor before or after the Agent’s or the Buyers’ commencement or

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completion of any foreclosure action, whether judicially, by exercise of power of sale or otherwise, or any other law which would otherwise require any election of remedies by the Agent or the Buyers;

(i) all demands and notices of every kind with respect to the foregoing; and

(j) to the extent not referred to above, all defenses (other than payment) which the Borrower may now or hereafter have to the