Exhibit 10.3

COMPLETION, PAYMENT AND PERFORMANCE GUARANTY

THIS COMPLETION, PAYMENT AND PERFORMANCE GUARANTY (this “Guaranty”), dated as of July 11, 2006, is made and entered into by ALEXION PHARMACEUTICALS, INC., a Delaware corporation (“Guarantor”), in favor of iSTAR FINANCIAL INC., a Maryland corporation (collectively, with its successors and/or assigns, “Lender”), with an address for notice hereunder of 1114 Avenue of the Americas, 27 ^th Floor, New York, New York 10036.

Background

A. ALEXION MANUFACTURING LLC, a Delaware limited liability company (the “Borrower”) and Lender have entered into that certain Loan Agreement (as amended, modified, supplemented or restated from time to time, the “Loan Agreement”), dated of even date herewith, wherein, among other things, Lender has agreed to make, and Borrower has agreed to accept, a loan in an original maximum principal amount up to TWENTY SIX MILLION AND 00/100 DOLLARS ($26,000,000.00) (the “Loan”) upon the terms and conditions set forth in the Loan Agreement. Any capitalized term used but not defined in this Guaranty shall have the meaning ascribed to such term in the Loan Agreement.

B. The Loan is evidenced by a Promissory Note, dated of even date herewith, in the original principal amount of up to $26,000,000.00 (together with any notes given in substitution or exchange from time to time, as such Promissory Note and substitute or exchange notes may, from time to time, be amended, modified, supplemented or restated, the “Note” ). The Note is secured by, among other things, a Construction Mortgage Deed, Assignment of Leases and Rents, Security Agreement, and Fixture Filing of even date herewith (as amended, modified, supplemented or restated from time to time, the “Mortgage”).

C. Guarantor is the sole member of Borrower.

D. It is a condition to the making of the Loan that Guarantor execute and deliver this Guaranty.

NOW, THEREFORE, in consideration of the Loan, and for other good and valuable consideration, the receipt whereof is hereby acknowledged, Guarantor and its successors and assigns, hereby covenants and agrees with Lender for the benefit of Lender, its endorsees, participants, successors and assigns, as follows:

1. Guaranty .

(a) Scope of Guaranty . Guarantor as a primary obligor and not merely as a surety, hereby absolutely, unconditionally and irrevocably guarantees to Lender the prompt and complete payment (and performance, in the case of non-pecuniary obligations) of all of the Guaranteed Obligations (as defined below) in full, when and as the same shall become due, whether on any due date or performance date or at stated maturity thereof, or by declaration, acceleration or required payment, or upon demand or otherwise (including amounts and

performance that would become due but for the operation of the automatic stay under Section 362(a) of the United States Bankruptcy Code, Title 11, United States Code, as amended (the “Bankruptcy Code”).

Guarantor hereby agrees, in furtherance of the foregoing and not in limitation of any other right that the Lender may have at law or in equity against Guarantor by virtue hereof, that upon the failure of Borrower to pay or perform any of the Guaranteed Obligations when and as the same shall become due (or, as provided below, would have become due), whether at stated maturity or due date or performance date, as the case may be, by required payment or prepayment, declaration, acceleration, demand or otherwise (including without limitation amounts that would have become due, or could have been accelerated, but for the operation of the automatic stay under Section 362(a) of the Bankruptcy Code, whether or not any Creditor has taken any action to enforce or exercise any right or remedy in respect of the Guaranteed Obligations, and interest and fees which, but for the filing of a petition in bankruptcy with respect to Borrower, would have accrued and become due on, or constituting, such Guaranteed Obligations, whether or not a claim is allowed against Borrower for such amounts in any such bankruptcy proceeding), Guarantor will forthwith pay and perform, or cause to be paid and performed, all Guaranteed Obligations then due (or that would have accrued and/or become due but for the filing of a petition in bankruptcy with respect to Borrower) as aforesaid, and all other Guaranteed Obligations then owed to the Lender as aforesaid.

(b) Obligations Guaranteed . As used in this Guaranty, “Guaranteed Obligations” means, collectively: (1) the prompt and complete payment of the all principal and interest payments and all other amounts, when due, in accordance with the Loan Agreement, including, without limitation, the Loan, whether at stated maturity or otherwise, (2) the payment and performance of all Obligations as defined in the Loan Agreement, as and when due, and (3) all of the obligations, duties and agreements of Borrower under the Loan Agreement and the other Loan Documents relating to the construction, renovation, redevelopment, equipping and furnishing of the Mortgaged Property and the Completion of the Initial Construction Work in accordance with the Project Plans and Specifications, all applicable Legal Requirements and all provisions of the Loan Documents. Without limiting the generality of the foregoing, Guarantor absolutely, irrevocably and unconditionally guarantees to Lender that:

(i) Borrower shall timely construct, renovate, redevelop, equip, furnish and substantially complete and achieve Substantial Completion of Initial Construction Work on or before Required Completion Date in compliance with the Project Plans and Specifications, applicable Legal Requirements and the Loan Documents;

(ii) The Borrower shall keep the Loan “in balance” (as more particularly described in Section 3.2(d) of the Loan Agreement);

(iii) The Borrower shall fully and punctually deposit amounts required to be paid pursuant to the loan balancing provisions of Section 3.2(d) of the Loan Agreement;

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(iv) Borrower shall fully and punctually pay and discharge any and all costs and expenses and liabilities incurred for or in connection with the construction, renovation, redevelopment, equipping, furnishing and Completion of Initial Construction Work, when and as the same may become due and payable, and also pay and discharge any and all claims and demands for labor and materials used and services rendered for or in connection with the construction, renovation, redevelopment and Completion Initial Construction Work and/or installation of all items of fixtures, furnishings and equipment and other personalty in connection therewith; and

(v) The Mortgaged Property shall be and remain free and clear of any and all Liens, claims, and demands from any and all Persons furnishing materials, labor or services for or in connection with the construction, equipping, furnishing or completion of the Initial Construction Work and/or the installation of all items of fixtures, furnishings and equipment and other personalty in connection therewith.

(c) Indemnity, Reimbursement and Performance . In the event that Borrower does not fully perform the Guaranteed Obligations, then:

(i) Guarantor shall, within ten (10) days of demand by Lender, perform the Guaranteed Obligations;

(ii) In the event that Guarantor fails to commence performance under the immediately preceding Section l(c)(i) (i) within ten (10) Business Days after Lender’s written demand and to diligently prosecute such performance to completion thereof, and if Lender shall (a) cause any construction, renovation, redevelopment, equipping and furnishing of the Initial Construction Work and the Project Improvements, or takes any action whatsoever toward Completion of the Project, (b) pay any costs in connection with the construction, renovation, redevelopment, equipping and furnishing of the Initial Construction Work or the completion of the Project Improvements, or (c) cause any lien, claim or demand to be released or paid, then Guarantor shall promptly reimburse Lender within ten (10) Business Days after written demand, for all sums paid and all costs and expenses incurred by Lender in connection therewith; and

(iii) Guarantor will fully indemnify, defend and save Lender harmless from all actual out of pocket third party costs and damages (including reasonable attorney’s fees, including any diminution in value) that Lender may suffer by reason of Guarantor’s failure to promptly and fully perform under the immediately preceding Section l(c) (i) and (ii) above. In no event shall the indemnification contained herein include consequential or punitive damages.

(d) Balancing . If, at any time and for any reason, Borrower shall fail to deposit amounts required to be deposited by Section 3.2 of the Loan Agreement within the ten (10) Business Day period provided therein for such payment, then Guarantor shall, within five (5) Business Days after a written request by Lender, deposit with Lender cash in an amount sufficient to cover the deficiency not paid by Borrower. Lender shall hold and apply such deposited cash as provided in the Loan Agreement.

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(e) Enforcement Costs . In addition to the foregoing payment obligations, Guarantor further agrees to pay any and all actual out of pocket third party costs and other expenses (the “Enforcement Costs”) that may be paid or incurred directly or indirectly by each of the Lender in, or allocable to, collecting any or all of the Guaranteed Obligations and/or preserving and/or enforcing any rights and remedies under this Guaranty and/or in respect of the Guaranteed Obligations (including, without limitation, all reasonable fees and expenses incurred by the Lender and its respective agents and representatives in connection with any default or event of default, beyond any applicable notice and cure period, relating to, or other breach or violation of, this Guaranty and/or Guaranteed Obligation and, to the extent the Lender from time to time deem it necessary to employ counsel and/or consultants for any purpose relating to this Guaranty and/or any Guaranteed Obligation, the reasonable fees and expenses of such counsel and/or consultants). For purposes of this paragraph, the term “counsel” includes attorneys who are employees of Lender acting as counsel for Lender, and the terms “costs and expenses” and “fees and expenses” shall include, without limitation, the fees charged by Lender for its in-house counsel provided such fees are within the range of fees charged by attorneys of similar experience at medium to large sized law firms located in the City of Chicago, Illinois.

(f) Maximum Amount of Guaranty . Guarantor shall be liable under this Guaranty for the maximum amount of such liability that can be hereby incurred without rendering this Guaranty, as it relates to Guarantor, voidable under applicable Legal Requirements relating to fraudulent conveyance or fraudulent transfer, and not for any greater amount.

(g) Continuing Guaranty . This is an irrevocable, absolute, continuing guaranty of payment and performance. This Guaranty may not be revoked by Guarantor and shall continue to be effective with respect to the Guaranteed Obligations arising or created after any attempted revocation by Guarantor and after Guarantor’s dissolution (in which event this Guaranty shall be binding upon Guarantor’s successors and assigns). It is the intent