Exhibit 1.1

ALLOS THERAPEUTICS, INC.

(a Delaware corporation)

10,800,000 Shares of Common Stock

PURCHASE AGREEMENT

Dated: May 22, 2008

ALLOS THERAPEUTICS, INC.

(a Delaware corporation)

10,800,000 Shares of Common Stock

PURCHASE AGREEMENT

May 22, 2008

MERRILL LYNCH & CO.

Merrill Lynch, Pierce, Fenner & Smith

Incorporated

as Representative of the several Underwriters

4 World Financial Center

New York, New York  10080

Ladies and Gentlemen:

Allos Therapeutics, Inc., a Delaware corporation (the “Company”), confirms its agreement with Merrill Lynch & Co., Merrill Lynch, Pierce, Fenner & Smith Incorporated (“Merrill Lynch”) and each of the other Underwriters named in Schedule A hereto (collectively, the “Underwriters,” which term shall also include any underwriter substituted as hereinafter provided in Section 10 hereof), for whom Merrill Lynch is acting as representative (in such capacity, the “Representative”), with respect to (i) the issue and sale by the Company and the purchase by the Underwriters, acting severally and not jointly, of the respective number of shares of the Company’s common stock, par value $.001 per share (the “Common Stock”) set forth in Schedule A and (ii) the grant by the Company to the Underwriters, acting severally and not jointly, of the option described in Section 2(b) hereof to purchase all or any part of 1,620,000 additional shares of Common Stock to cover overallotments, if any.  The aforesaid 10,800,000 shares of Common Stock (the “Initial Securities”) to be purchased by the Underwriters and all or any part of the 1,620,000 shares of Common Stock subject to the option described in Section 2(b) hereof (the “Option Securities”) are hereinafter called, collectively, the “Securities.”

The Company understands that the Underwriters propose to make a public offering of the Securities as soon as the Representative deem(s) advisable after this Agreement has been executed and delivered.

The Company has filed with the Securities and Exchange Commission (the “Commission”) a shelf registration statement on Form S-3 (No. 333-143198), including the related preliminary prospectus or prospectuses, which registration statement became effective on June 5, 2007 under the rules and regulations of the Commission (the “1933 Act Regulations”) under the Securities Act of 1933, as amended (the “1933 Act”).  Such registration statement covers the registration of the Securities under the 1933 Act. Promptly after execution and delivery of this Agreement, the Company will prepare and file a prospectus in accordance with the provisions of Rule 430B (“Rule 430B”) of the 1933 Act Regulations and paragraph (b) of Rule 424 (“Rule 424(b)”) of the 1933 Act Regulations.  Any information included in such prospectus that was omitted from such registration statement at the time it became effective but

1

that is deemed to be part of and included in such registration statement pursuant to Rule 430B is referred to as “Rule 430B Information.”  Each prospectus used in connection with the offering of the Securities that omitted Rule 430B Information is herein called a “preliminary prospectus.”  Such registration statement, at any given time, including the amendments thereto to such time, the exhibits and any schedules thereto at such time, the documents incorporated by reference therein pursuant to Item 12 of Form S-3 under the 1933 Act at such time and the documents otherwise deemed to be a part thereof or included therein by 1933 Act Regulations, is herein called the “Registration Statement.”  The Registration Statement at the time it originally became effective is herein called the “Original Registration Statement.”  The final prospectus in the form first furnished to the Underwriters for use in connection with the offering of the Securities, including the documents incorporated by reference therein pursuant to Item 12 of Form S-3 under the 1933 Act at the time of the execution of this Agreement and any preliminary prospectuses that form a part thereof, is herein called the “Prospectus.”  For purposes of this Agreement, all references to the Registration Statement, any preliminary prospectus, the Prospectus or any Term Sheet or any amendment or supplement to any of the foregoing shall be deemed to include the copy filed with the Commission pursuant to its Electronic Data Gathering, Analysis and Retrieval system (“EDGAR”).

All references in this Agreement to financial statements and schedules and other information which is “contained,” “included” or “stated” in the Registration Statement, any preliminary prospectus or the Prospectus (or other references of like import) shall be deemed to mean and include all such financial statements and schedules and other information which is incorporated by reference in or otherwise deemed by 1933 Act Regulations to be a part of or included in the Registration Statement, any preliminary prospectus or the Prospectus, as the case may be; and all references in this Agreement to amendments or supplements to the Registration Statement, any preliminary prospectus or the Prospectus shall be deemed to mean and include the filing of any document under the Securities Exchange Act of 1934 (the “1934 Act”) which is incorporated by reference in or otherwise deemed by 1933 Act Regulations to be a part of or included in the Registration Statement, such preliminary prospectus or the Prospectus, as the case may be.

SECTION 1.           Representations and Warranties.

(a)           Representations and Warranties by the Company.  The Company represents and warrants to each Underwriter as of the date hereof, the Applicable Time referred to in Section 1(a)(ii) hereof and as of the Closing Time referred to in Section 2(b) hereof, and agrees with each Underwriter, as follows:

(i)              Compliance with Registration Requirements. (A) At the time of filing the Original Registration Statement, (B) at the time of the most recent amendment thereto for the purposes of complying with Section 10(a)(3) of the 1933 Act (whether such amendment was by post-effective amendment, incorporated report filed pursuant to Section 13 or 15(d) of the 1934 Act or form of prospectus), and (C) at the date hereof, the Company met the requirements for use of Form S-3 under the 1933 Act.

2

(ii)             Registration Statement, Prospectus and Disclosure at Time of Sale.  The Original Registration Statement became effective on June 5, 2007, and any post-effective amendment thereto has become effective under the 1933 Act.  No stop order suspending the effectiveness of the Registration Statement has been issued under the 1933 Act and no proceedings for that purpose have been instituted or are pending or, to the knowledge of the Company, are contemplated by the Commission, and any request on the part of the Commission for additional information has been complied with. The Company has paid any and all required registration fees relating to the Securities within the time, and in the manner, required by the 1933 Act and the 1933 Act Regulations.

At the respective times the Original Registration Statement and each amendment thereto became effective, at each deemed effective date with respect to the Underwriters pursuant to Rule 430B(f)(2) of the 1933 Act Regulations and at the Closing Time (and, if any Option Securities are purchased, at the Date of Delivery), the Registration Statement complied and will comply in all material respects with the requirements of the 1933 Act and the 1933 Act Regulations, and did not and will not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading.

Neither the Prospectus nor any amendments or supplements thereto, at the time the Prospectus or any such amendment or supplement was issued and at the Closing Time (and, if any Option Securities are purchased, at the Date of Delivery), included or will include an untrue statement of a material fact or omitted or will omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

Each preliminary prospectus (including the prospectus or prospectuses filed as part of the Original Registration Statement or any amendment thereto) complied when so filed in all material respects with the 1933 Act Regulations and each preliminary prospectus and the Prospectus delivered to the Underwriters for use in connection with this offering was identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T.

As of the Applicable Time, neither (x) the Issuer General Use Free Writing Prospectus(es) (as defined below) issued at or prior to the Applicable Time  (as defined below), the Statutory Prospectus (as defined below) and the information included on Schedule C hereto, all considered together (collectively, the “General Disclosure Package”), nor (y) any individual Issuer Limited Use Free Writing Prospectus (as defined below), when considered together with the General Disclosure Package, included any untrue statement of a material fact or omitted to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

As used in this subsection and elsewhere in this Agreement:

3

“Applicable Time” means 7:00 a.m. (Eastern time) on May 23, 2008 or such other time as agreed by the Company and Merrill Lynch.

“Issuer Free Writing Prospectus” means any “issuer free writing prospectus,” as defined in Rule 433 of the 1933 Act Regulations (“Rule 433”), relating to the Securities that (i) is required to be filed with the Commission by the Company, (ii) is a “road show that is a written communication” within the meaning of Rule 433(d)(8)(i), whether or not required to be filed with the Commission or (iii) is exempt from filing pursuant to Rule 433(d)(5)(i) because it contains a description of the Securities or of the offering that does not reflect the final terms, in each case in the form filed or required to be filed with the Commission or, if not required to be filed, in the form retained in the Company’s records pursuant to Rule 433(g).

“Issuer General Use Free Writing Prospectus” means any Issuer Free Writing Prospectus that is intended for general distribution to prospective investors, as evidenced by its being specified in Schedule D hereto.

“Issuer Limited Use Free Writing Prospectus” means any Issuer Free Writing Prospectus that is not an Issuer General Use Free Writing Prospectus.

“Statutory Prospectus” as of any time means the prospectus relating to the Securities that is included in the Registration Statement immediately prior to that time, including any document incorporated by reference therein and any preliminary or other prospectus deemed to be a part thereof.

Each Issuer Free Writing Prospectus, as of its issue date and at all subsequent times through the completion of the public offer and sale of the Securities or until any earlier date that the issuer notified or notifies Merrill Lynch as described in Section 3(e), did not, does not and will not include any information that conflicted, conflicts or will conflict with the information contained in the Registration Statement or the Prospectus, including any document incorporated by reference therein and any preliminary or other prospectus deemed to be a part thereof that has not been superseded or modified.

The representations and warranties in this subsection shall not apply to statements in or omissions from the Registration Statement, the Prospectus or any Issuer Free Writing Prospectus made in reliance upon and in conformity with written information furnished to the Company by any Underwriter through Merrill Lynch expressly for use therein.

(iii)            Incorporated Documents.  The documents incorporated or deemed to be incorporated by reference in the Registration Statement and the Prospectus, when they became effective or at the time they were or hereafter are filed with the Commission, complied and will comply in all material respects with the requirements of the 1934 Act and the rules and regulations of the Commission thereunder (the “1934 Act Regulations”), as applicable, and, when read together with the other information in the Prospectus, (a) at the time the Original Registration Statement became effective, (b) at the earlier of the time the

4

Prospectus was first used and the date and time of the first contract of sale of Securities in this offering and (c) at the Closing Time, did not and will not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading.

(iv)            Independent Accountants.  The accountants who certified the financial statements and supporting schedules, if any, included or incorporated by reference in the Registration Statement, are independent public accountants as required by the 1933 Act and the 1933 Act Regulations.

(v)             Financial Statements.  The financial statements, together with the related schedules and notes, included or incorporated by reference in the Registration Statement, the General Disclosure Package and the Prospectus, together with the related schedules and notes, present fairly the financial position of the Company at the dates indicated and the statement of operations, stockholders’ equity and cash flows of the Company for the periods specified; said financial statements have been prepared in conformity with generally accepted accounting principles (“GAAP”) applied on a consistent basis throughout the periods involved.  The supporting schedules, if any, included or incorporated by reference in the Registration Statement, the General Disclosure Package and the Prospectus, present fairly in accordance with GAAP the information required to be stated therein.  The selected financial data and the summary financial information included in the Prospectus present fairly the information shown therein and have been compiled on a basis consistent with that of the audited financial statements included in the Registration Statement.  Neither the Registration Statement, the General Disclosure Package nor the Prospectus contain any “non-GAAP financial measures” (as such term is defined by the rules and regulations of the Commission).

(vi)            No Material Adverse Change in Business.  Since the respective dates as of which information is given in the Registration Statement, the General Disclosure Package or the Prospectus, except as otherwise stated therein, (A) there has been no material adverse change in the condition, financial or otherwise, or in the earnings, business affairs or business prospects of the Company, whether or not arising in the ordinary course of business (a “Material Adverse Effect”), (B) there have been no transactions entered into by the Company, other than those in the ordinary course of business, which are material with respect to the Company and (C)  there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock.

(vii)           Good Standing of the Company.  The Company has been duly organized and is validly existing as a corporation in good standing under the laws of the state of Delaware and has corporate power and authority to own, lease and operate its properties and to conduct its business as described in the Prospectus and to enter into and perform its obligations under this Agreement; and the Company is duly qualified as a foreign corporation to transact business and is in good standing in each other jurisdiction in which such qualification is required, whether by

5

reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not result in a Material Adverse Effect.

(viii)          Subsidiaries.  The Company has no subsidiaries.

(ix)             Capitalization.  The authorized, issued and outstanding capital stock of the Company is as set forth in the Prospectus in the column entitled “Actual” under the caption “Capitalization” (except for subsequent issuances, if any, pursuant to this Agreement, pursuant to reservations, agreements or employee benefit plans referred to in the Prospectus or pursuant to the exercise of convertible securities or options referred to in the Prospectus.  The shares of issued and outstanding capital stock of the Company have been duly authorized and validly issued and are fully paid and non-assessable; none of the outstanding shares of capital stock of the Company was issued in violation of the preemptive or other similar rights of any securityholder of the Company.

(x)              Authorization of Agreement.  The Company has the requisite corporate power and authority to enter into this Agreement and to perform its obligations hereunder and the transactions contemplated hereby have been duly authorized by the Company.  This Agreement has been duly authorized, executed and delivered by the Company.

(xi)             Authorization of the Securities.  The Securities have been duly authorized for issuance and sale to the Underwriters pursuant to this Agreement and, when issued and delivered by the Company pursuant to this Agreement against payment of the consideration set forth herein, will be duly and validly issued and will be fully paid and nonassessable.

(xii)            Description of the Securities.  The Securities will conform in all material respects to all statements relating thereto in the Prospectus and the General Disclosure Package and no holder of the Securities will be subject to personal liability for any obligation of the Company by reason of being such a holder.  The issuance of the Securities is not subject to any preemptive or similar rights, other than such rights as have been satisfied or duly and validly waived.

(xiii)           Absence of Defaults and Conflicts.  The Company is not in violation of its charter or by-laws or in default in the performance or observance of any obligation, agreement, covenant or condition contained in any contract, indenture, mortgage, deed of trust, loan or credit agreement, note, lease or other agreement or instrument to which the Company is a party or by which it may be bound, or to which any of the property or assets of the Company is subject (collectively, “Agreements and Instruments”) except for such defaults that would not result in a Material Adverse Effect; and the execution, delivery and performance of this Agreement and the consummation of the transactions contemplated herein and in the Registration Statement (including the issuance and sale of the Securities and the use of the proceeds from the sale of the Securities as

6

described in the Prospectus under the caption “Use of Proceeds”) and compliance by the Company with its obligations hereunder have been duly authorized by all necessary corporate action and do not and will not, whether with or without the giving of notice or passage of time or both, conflict with or constitute a breach of, or default under, or result in the creation or imposition of any lien, charge or encumbrance upon any property or assets of the Company pursuant to, the Agreements and Instruments (except for such conflicts, breaches, defaults or liens, charges or encumbrances that would not result in a Material Adverse Effect), nor will such action result in (x) any violation of the provisions of the charter or by-laws of the Company or (y) a violation of any applicable law, statute, rule, regulation, judgment, order, writ or decree of any government, government instrumentality or court, domestic or foreign, having jurisdiction over the Company or any of its material assets, properties or operations.

(xiv)          Absence of Labor Dispute.  No organized labor dispute with the employees of the Company exists or, to the knowledge of the Company, is imminent, and the Company is not aware of any existing or imminent labor disturbance by the employees of any of its principal suppliers, manufacturers, customers or contractors, which, in either case, would result in a Material Adverse Effect.

(xv)           Absence of Proceedings.  There is no action, suit, proceeding, inquiry or investigation before or brought by any court or governmental agency or body, domestic or foreign, now pending, or, to the knowledge of the Company, threatened, against or affecting the Company, which is required to be disclosed in the Registration Statement (other than as disclosed therein), or which could reasonably be expected to result in a Material Adverse Effect, or which might materially and adversely affect the properties or assets thereof or the consummation of the transactions contemplated in this Agreement or the performance by the Company of its obligations hereunder. The aggregate of all pending legal or governmental proceedings to which the Company is a party or of which any of its property or assets is the subject which are not described in the Registration Statement, including ordinary routine litigation incidental to the business, would not reasonably be expected to result in a Material Adverse Effect.

(xvi)          Accuracy of Exhibits.  There are no contracts or documents which are required to be described in the Registration Statement, the Prospectus or the documents incorporated by reference therein or to be filed as exhibits thereto which have not been so described and filed as required.

(xvii)         Possession of Intellectual Property.  The Company owns or possesses, or can acquire on reasonable terms, adequate patents, patent rights, licenses, inventions, copyrights, know-how (including trade secrets and other unpatented and/or unpatentable proprietary or confidential information, systems or procedures), trademarks, service marks, trade names or other intellectual property (collectively, “Intellectual Property”) necessary to carry on the business now operated by it, and the Company has not received any notice and is otherwise not

7

aware of any infringement of or conflict with asserted rights of others with respect to any Intellectual Property or of any facts or circumstances which would render any Intellectual Property invalid or inadequate to protect the interest of the Company therein, and which infringement or conflict (if the subject of any unfavorable decision, ruling or finding) or invalidity or inadequacy, singly or in the aggregate, would result in a Material Adverse Effect.

(xviii)        Absence of Manipulation.  Neither the Company nor any affiliate of the Company has taken, nor will the Company or any affiliate take, directly or indirectly, any action which is designed to or which has constituted or which would be expected to cause or result in stabilization or manipulation of the price of any security of the Company to facilitate the sale or resale of the Securities.

(xix)           Absence of Further Requirements.  No filing with, or authorization, approval, consent, license, order, registration, qualification or decree of, any court or governmental authority or agency is necessary or required for the performance by the Company of its obligations hereunder, in connection with the offering, issuance or sale of the Securities hereunder or the consummation of the transactions contemplated by this Agreement, except such as have been already obtained or as may be required under the 1933 Act or the 1933 Act Regulations or state securities laws.

(xx)            Possession of Licenses and Permits.  The Company possesses such permits, licenses, approvals, consents and other authorizations (collectively, “Governmental Licenses”) issued by the appropriate federal, state, local or foreign regulatory agencies or bodies necessary to conduct the business now operated by it, except where the failure so to possess would not, singly or in the aggregate, result in a Material Adverse Effect; the Company is in compliance with the terms and conditions of all such Governmental Licenses, except where the failure so to comply would not, singly or in the aggregate, result in a Material Adverse Effect; all of the Governmental Licenses are valid and in full force and effect, except where the invalidity of such Governmental Licenses or the failure of such Governmental Licenses to be in full force and effect would not, singly or in the aggregate, result in a Material Adverse Effect; and the Company has not received any notice of proceedings relating to the revocation or modification of any such Governmental Licenses which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would result in a Material Adverse Effect.

(xxi)           Title to Property.  The Company has good and marketable title to all real property owned by the Company and good title to all other properties owned by them, in each case, free and clear of all mortgages, pledges, liens, security interests, claims, restrictions or encumbrances of any kind except such as (a) are described in the Prospectus or (b) do not, singly or in the aggregate, materially affect the value of such property and do not interfere with the use made and proposed to be made of such property by the Company; and all of the leases and subleases material to the business of the Company, and under which the Company holds properties described in the Prospectus, are in full force and effect,

8

and the Company has not received any notice of any material claim of any sort that has been asserted by anyone adverse to the rights of the Company under any of the leases or subleases mentioned above, or affecting or questioning the rights of the Company to the continued possession of the leased or subleased premises under any such lease or sublease.

(xxii)          Investment Company Act.  The Company is not required, and upon the issuance and sale of the Securities as herein contemplated and the application of the net proceeds therefrom as described in the Prospectus will not be required, to register as, an “investment company” under the Investment Company Act of 1940, as amended (the “1940 Act”).

(xxiii)         No Undisclosed Relationships.  No relationship, direct or indirect, exists between or among the Company, on the one hand, and the directors, officers, stockholders, customers or suppliers of the Company, on the other, that is required by the Securities Act to be described in the Registration Statement and the Prospectus and that is not so described in such documents and in the General Disclosure Package.

(xxiv)         Environmental Laws.  Except as described in the Registration Statement, the General Disclosure Package or the Prospectus and except for such matters as would not, singly or in the aggregate, result in a Material Adverse Effect, (A) the Company is not in violation of any federal, state, local or foreign statute, law, rule, regulation, ordinance, code, policy or rule of common law or any judicial or administrative interpretation thereof, including any judicial or administrative order, consent, decree or judgment, relating to pollution or protection of human health, the environment (including, without limitation, ambient air, surface water, groundwater, land surface or subsurface strata) or wildlife, including, without limitation, laws and regulations relating to the release or threatened release of chemicals, pollutants, contaminants, wastes, toxic substances, hazardous substances, petroleum or petroleum products, asbestos-containing materials or mold (collectively, “Hazardous Materials”) or to the manufacture, processing, distribution, use, treatment, storage, disposal, transport or handling of Hazardous Materials (collectively, “Environmental Laws”), (B) the Company has all permits, authorizations and approvals required under any applicable Environmental Laws and is in compliance with their requirements, (C) there are no pending or, to the Company’s knowledge, threatened administrative, regulatory or judicial actions, suits, demands, demand letters, claims, liens, notices of noncompliance or violation, investigation or proceedings relating to any Environmental Law against the Company and (D) to the Company’s knowledge, there are no events or circumstances that would reasonably be expected to form the basis of an order for clean-up or remediation, or an action, suit or proceeding by any private party or governmental body or agency, against or affecting the Company relating to Hazardous Materials or Environmental Laws.

(xxv)          Accounting Controls and Disclosure Controls.  The Company maintains a system of internal accounting controls sufficient to provide reasonable

9

assurances that (1) transactions are executed in accordance with management’s general or specific authorization; (2) transactions are recorded as necessary to permit preparation of financial statements in conformity with GAAP and to maintain accountability for assets; (3) access to assets is permitted only in accordance with management’s general or specific authorization; and (4) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences.  Except as described in the Prospectus, since the end of the Company’s most recent audited fiscal year, there has been (I) no material weakness in the Company’s internal control over financial reporting (whether or not remediated) and (II) no change in the Company’s internal control over financial reporting that has materially affected, or is reasonably likely to materially affect, the Company’s internal control over financial reporting.

The Company employs disclosure controls and procedures that are designed to ensure that information required to be disclosed by the Company in the reports that it files or submits under the 1934 Act is recorded, processed, summarized and reported, within the time periods specified in the Commission’s rules and forms, and is accumulated and communicated to the Company’s management, including its principal executive officer or officers and principal financial officer or officers, as appropriate, to allow timely decisions regarding disclosure.

(xxvi)         Compliance with the Sarbanes-Oxley Act.  There is and has been no failure on the part of the Company or any of the Company’s directors or officers, in their capacities as such, to comply in all material respects with any provision of the Sarbanes-Oxley Act of 2002 and the rules and regulations promulgated in connection therewith (the “Sarbanes-Oxley Act”), including Section 402 related to loans and Sections 302 and 906 related to certifications.

(xxvii)        Pending Proceedings and Examinations.  The Registration Statement is not the subject of a pending proceeding or examination under Section 8(d) or 8(e) of the 1933 Act, and the Company is not the subject of a pending proceeding under Section 8A of the 1933 Act in connection with the offering of the Securities.

(xxviii)       Taxes.  The Company has paid all federal and state taxes and filed all federal and state tax returns required to be paid or filed through the date hereof and is not in default in the payment of any taxes which were payable pursuant to said returns or any assessments with respect thereto, other than any which the Company is contesting in good faith.  To the best of its knowledge, the Company has paid all local and foreign taxes and filed all local and foreign tax returns required to be paid or filed through the date hereof and is not in default in the payment of any taxes which were payable pursuant to said returns or any assessments with respect thereto, other than any which the Company is contesting in good faith.  There is no pending dispute with any taxing authority relating to any of such returns and the Company has no knowledge of any proposed liability for any tax to be imposed upon the properties or assets of the Company for which

10

there is not an adequate reserve reflected in the Company’s financial statements included in the Registration Statement.

(xxix)         FDA Compliance.  Except as described in the Registration Statement, the General Disclosure Package and the Prospectus, the Company:  (A) is and at all times has been in material compliance with all statutes, rules, regulations, or guidances applicable to the ownership, testing, development, manufacture, packaging, processing, use, distribution, marketing, labeling, promotion, sale, offer for sale, storage, import, export or disposal of any product under development, manufactured or distributed by the Company (“Applicable Laws”); (B) has not received any FDA Form 483, notice of adverse finding, warning letter, untitled letter or other correspondence or notice from the U.S. Food and Drug Administration (the “FDA”) or any other federal, state, local or foreign governmental or regulatory authority alleging or asserting material noncompliance with any Applicable Laws or any licenses, certificates, approvals, clearances, authorizations, permits and supplements or amendments thereto required by any such Applicable Laws (“Authorizations”); (C) possesses all material Authorizations and such Authorizations are valid and in full force and effect and the Company is not in material violation of any term of any such Authorizations; (D) has not received notice of any claim, action, suit, proceeding, hearing, enforcement, investigation, arbitration or other action from the FDA or any other federal, state, local or foreign governmental or regulatory authority or third party alleging that any product operation or activity is in material violation of any Applicable Laws or Authorizations and has no knowledge that the FDA or any other federal, state, local or foreign governmental or regulatory authority or third party is considering any such claim, litigation, arbitration, action, suit, investigation or proceeding; (E) has not received notice that the FDA or any other federal, state, local or foreign governmental or regulatory authority has taken, is taking or intends to take action to limit, suspend, modify or revoke any material Authorizations and has no knowledge that the FDA or any other federal, state, local or foreign governmental or regulatory authority is considering such action; (F) has filed, obtained, maintained or submitted all material reports, documents, forms, notices, applications, records, claims, submissions and supplements or amendments as required by any Applicable Laws or Authorizations and that all such reports, documents, forms, notices, applications, records, claims, submissions and supplements or amendments were materially complete and correct on the date filed (or were corrected or supplemented by a subsequent submission); and (G) has not, either voluntarily or involuntarily, initiated, conducted, or issued or caused to be initiated, conducted or issued, any recall, market withdrawal or replacement, safety alert, post sale warning, “dear doctor” letter, or other notice or action relating to the alleged lack of safety or efficacy of any product or any alleged product defect or violation and, to the Company’s knowledge, no third party has initiated, conducted or intends to initiate any such notice or action.

(xxx)          Clinical Studies.  The studies, tests and preclinical and clinical trials conducted by or on behalf of the Company were and, if still pending, are being conducted in all material respects in accordance with experimental protocols,

11

procedures and controls pursuant to accepted professional scientific standards and all Applicable Laws and Authorizations, including, without limitation, the Federal Food, Drug and Cosmetic Act and the rules and regulations promulgated thereunder (collectively, “FFDCA”); the descriptions of the results of such studies, tests and trials contained in the Registration Statement, the General Disclosure Package and the Prospectus are accurate and complete in all material respects and fairly present the data derived from such studies, tests and trials; except to the extent disclosed in the Registration Statement, the General Disclosure Package and the Prospectus, the Company is not aware of any studies, tests or trials, the results of which the Company believes reasonably call into question the study, test, or trial results described or referred to in the Registration Statement, the General Disclosure Package and the Prospectus when viewed in the context in which such results are described and the clinical state of development; and, since December 31, 2001, the Company has not received any notices or correspondence from the FDA or any other federal, state, local or foreign governmental or regulatory authority requiring the termination, suspension or material modification of any studies, tests or preclinical or clinical trials conducted by or on behalf of the Company.

(xxxi)         Compliance With ERISA.  (i) Each employee benefit plan, within the meaning of Section 3(3) of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”), for which the Company or any member of its “Controlled Group” (defined as any organization which is a member of a controlled group of corporations within the meaning of Section 414 of the Internal Revenue Code of 1986, as amended (the “Code”)) would have any liability (each, a “Plan”) has been maintained in material compliance with its terms and the requirements of any applicable statutes, orders, rules and regulations, including but not limited to ERISA and the Code;  (ii) no prohibited transaction, within the meaning of Section 406 of ERISA or Section 4975 of the Code, has occurred with respect to any Plan excluding transactions effected pursuant to a statutory or administrative exemption; (iii) for each Plan that is subject to the funding rules of Section 412 of the Code or Section 302 of ERISA, no “accumulated funding deficiency” as defined in Section 412 of the Code, whether or not waived, has occurred or is reasonably expected to occur; (iv) the fair market value of the assets of each Plan exceeds the present value of all benefits accrued under such Plan (determined based on those assumptions used to fund such Plan); (v) no “reportable event” (within the meaning of Section 4043(c) of ERISA) has occurred or is reasonably expected to occur; and (vi) neither the Company nor any member of the Controlled Group has incurred, nor reasonably expects to incur, any material liability under Title IV of ERISA (other than contributions to the Plan or premiums to the PBGC, in the ordinary course and without default) in respect of a Plan (including a “multiemployer plan”, within the meaning of Section 4001(a)(3) of ERISA).

(xxxii)        Insurance.  The Company has insurance covering its properties, operations, personnel and businesses, including business interruption insurance, which insurance is in amounts and insures against such losses and risks as are, to

12

the knowledge of the Company, adequate to protect the Company and its  business; and the Company has not (i) received notice from any insurer or agent of such insurer that capital improvements or other expenditures are required or necessary to be made in order to continue such insurance or (ii) any reason to believe that it will not be able to renew its existing insurance coverage as and when such coverage expires or to obtain similar coverage at reasonable cost from similar insurers as may be necessary to continue its business.  All such insurance is outstanding and duly in force on the date hereof.

(xxxiii)       No Unlawful Payments.  Neither the Company nor, to the knowledge of the Company, any director, officer, agent, employee or other person associated with or acting on behalf of the Company has (i) used any corporate funds for any unlawful contribution, gift, entertainment or other unlawful expense relating to political activity; (ii) made any direct or indirect unlawful payment to any foreign or domestic government official or employee from corporate funds; (iii) violated or is in violation of any provision of the Foreign Corrupt Practices Act of 1977; or (iv) made any bribe, rebate, payoff, influence payment, kickback or other unlawful payment.

(xxxiv)       Compliance with Money Laundering Laws.  The operations of the Company are and have been conducted at all times in compliance with applicable financial recordkeeping and reporting requirements of the Currency and Foreign Transactions Reporting Act of 1970, as amended, the money laundering statutes of all jurisdictions, the rules and regulations thereunder and any related or similar rules, regulations or guidelines, issued, administered or enforced by any governmental agency (collectively, the “Money Laundering Laws”) and no action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Company with respect to the Money Laundering Laws is pending or, to the knowledge of the Company, threatened.

(xxxv)        Compliance with OFAC.  Neither the Company nor, to the knowledge of the Company, any director, officer, agent, employee or Affiliate (as defined in Section 6(a) below) of the Company is currently subject to any U.S. sanctions administered by the Office of Foreign Assets Control of the U.S. Department of the Treasury (“OFAC”); and the Company will not directly or indirectly use the proceeds of the offering of the Securities hereunder, or lend, contribute or otherwise make available such proceeds to any subsidiary, joint venture partner or other person or entity, for the purpose of financing the activities of any person currently subject to any U.S. sanctions administered by OFAC.

(xxxvi)       No Broker’s Fees.  The Company is not a party to any contract, agreement or understanding with any person (other than this Agreement) that would give rise to a valid claim against the Company or any Underwriter for a brokerage commission, finder’s fee or like payment in connection with the offering and sale of the Securities.

13

(xxxvii)      No Registration Rights.  No person has the right to require the Company to register any securities for sale under the Securities Act by reason of the filing of the Registration Statement with the Commission or the issuance and sale of the Securities, other than such rights as have been satisfied or duly and validly waived.

(xxxviii)     Margin Rules.  Neither the issuance, sale and delivery of the Securities nor the application of the proceeds thereof by the Company as described in the Registration Statement, the General Disclosure Package and the Prospectus will violate Regulation T, U or X of the Board of Governors of the Federal Reserve System or any other regulation of such Board of Governors.

(xxxix)        Forward-Looking Statements.  No forward-looking statement (within the meaning of Section 27A of the Securities Act and Section 21E of the Exchange Act) contained in the Registration Statement, the General Disclosure Package and the Prospectus has been made or reaffirmed without a reasonable basis or has been disclosed other than in good faith.

(xl)             Statistical and Market Data.  Nothing has come to the attention of the Company that has caused the Company to believe that the statistical and market-related data included in the Registration Statement, the General Disclosure Package and the Prospectus is not based on or derived from sources that are reliable and accurate in all material respects.

(xli)            Audit Committee/Internal Controls.  The Company’s board of directors has validly appointed an audit committee whose composition satisfies the requirements of Rule 4350(d)(2) of the Rules of the NASD (the “NASD Rules”) and the Company’s board of directors and/or the audit committee has adopted a charter that satisfies the requirements of Rule 4350(d)(1) of the NASD Rules.  Neither the Company’s board of directors nor the audit committee has been informed, nor is any director of the Company aware, of (x) any significant deficiencies in the design or operation of the Company’s internal controls that could adversely affect the Company’s ability to record, process, summarize and report financial data or any material weakness in the Company’s internal controls; or (y) any fraud, whether or not material, that involves management or other employees of the Company who have a significant role in the Company’s internal controls.

(xlii)           Listing on the Nasdaq Global Market.  The common stock of the Company is listed on the Nasdaq Global Market under the ticker symbol “ALTH”. The Company has not received any notice that it is not in compliance with the listing requirements of the Nasdaq Global Market. The Company is, and has no reason to believe that it will not in the foreseeable future continue to be (except as a result of minimum trading price requirements), in compliance with all such listing requirements. There are no affiliations with the Financial Industry Regulatory Authority (the “FINRA”) among the Company’s officers or directors.

14

A registration statement relating to the Common Stock on Form 8-A or other applicable form under the Exchange Act has become effective.

(xliii)          Stock Options.  With respect to the stock options (the “Stock Options”) granted pursuant to the stock-based compensation plans of the Company (the “Company Stock Plans”), in all material respects: (i) each Stock Option intended to qualify as an “incentive stock option” under Section 422 of the Code as of its Grant Date (as defined below) so qualified, (ii) each grant of a Stock Option was duly authorized no later than the date on which the grant of such Stock Option was by its terms to be effective (the “Grant Date”) by all necessary corporate action, including, as applicable, approval by the board of directors of the Company (or a duly constituted and authorized committee or subcommittee thereof) and any required stockholder approval by the necessary number of votes or written consents, (iii) each such grant was made in accordance with the terms of the Company Stock Plans, the Exchange Act and the rules of The Nasdaq Global Market and (iv) each such grant was properly accounted for in accordance with GAAP in the financial statements (including the related notes) of the Company and disclosed in the Company’s filings with the Commission to the extent required in accordance with the Exchange Act and all other applicable laws.  There is no and has been no policy or practice of the Company to coordinate the grant of Stock Options prior to the release or other public announcement of material information regarding the Company or its results of operations or prospects.

(b)           Officer’s Certificates.  Any certificate signed by any officer of the Company delivered to the Representative or to counsel for the Underwriters shall be deemed a representation and warranty by the Company to each Underwriter as to the matters covered thereby.

SECTION 2.           Sale and Delivery to Underwriters; Closing.

(a)           Initial Securities.  On the basis of the representations and warranties herein contained and subject to the terms and conditions herein  set forth, the Company agrees to sell to each Underwriter, severally and not jointly, and each Underwriter, severally and not jointly, agrees to purchase from the Company, at the price per share set forth in Schedule C, that proportion of the number of Initial Securities set forth in Schedule A, which the number of Initial Securities set forth in Schedule A opposite the name of such Underwriter, plus any additional number of Initial Securities which such Underwriter may become obligated to purchase pursuant to the provisions of Section 10 hereof, bears to the total number of Initial Securities, subject in each case to such adjustments among the Underwriters as the Representative in its sole discretion shall make to eliminate any sales or purchases of fractional securities.

(b)           Option Securities.  In addition, on the basis of the representations and warranties herein contained and subject to the terms and conditions herein set forth, the Company hereby grant(s) an option to the Underwriters, severally and not jointly, to purchase up to an additional 1,620,000 shares of Common Stock, as set forth in Schedule B, at the price per share set forth in Schedule C, less an amount per share equal to any dividends or distributions declared by the Company and payable on the Initial Securities but not payable on the Option Securities.  The

15

option hereby granted will expire 30 days after the date hereof and may be exercised in whole or in part from time to time only for the purpose of covering overallotments in connection with the offering and distribution of the Initial Securities upon notice by the Representative to the Company setting forth the number of Option Securities as to which the several Underwriters are then exercising the option and the time and date of payment and delivery for such Option Securities.  Any such time and date of delivery (a “Date of Delivery”) shall be determined by the Representative, but shall not be later than seven full business days after the exercise of said option, nor in any event prior to the Closing Time, as hereinafter defined.  If the option is exercised as to all or any portion of the Option Securities, each of the Underwriters, acting severally and not jointly, will purchase that proportion of the total number of Option Securities then being purchased which the number of Initial Securities set forth in Schedule A opposite the name of such Underwriter bears to the total number of Initial Securities, subject in each case to such adjustments as the Representative in its discretion shall make to eliminate any sales or purchases of fractional shares.

(c)           Payment.  Payment of the purchase price for, and delivery of certificates for, the Securities shall be made at the offices of Latham & Watkins LLP, 650 Town Center Drive, 20^th Floor, Costa Mesa, California, 92626, or at such other place as shall be agreed upon by the Representative and the Company, at 9:00 A.M. (Eastern time) on the third (fourth, if the pricing occurs after 4:30 P.M. (Eastern time) on any given day) business day after the date hereof (unless postponed in accordance with the provisions of Section 10), or such other time not later than ten business days after such date as shall be agreed upon by the Representative and the Company (such time and date of payment and delivery being herein called “Closing Time”).

In addition, in the event that any or all of the Option Securities are purchased by the Underwriters, payment of the purchase price for, and delivery of certificates for, such Option Securities shall be made at the above-mentioned offices, or at such other place as shall be agreed upon by the Representative and the Company on each Date of Delivery as specified in the notice from the Representative to the Company.

Payment shall be made to the Company by wire transfer of immediately available funds to a bank account designated by the Company, against delivery to the Representative for the respective accounts of the Underwriters of certificates for the Initial Securities and the Option Securities, if any, to be purchased by them.  It is understood that each Underwriter has authorized the Representative, for its account, to accept delivery of, receipt for, and make payment of the purchase price for, the Initial Securities and the Option Securities, if any, which it has agreed to purchase.  Merrill Lynch, individually and not as representative of the Underwriters, may (but shall not be obligated to) make payment of the purchase price for the Initial Securities and the Option Securities, if any, to be purchased by any Underwriter whose funds have not been received by the Closing Time, but such payment shall not relieve such Underwriter from its obligations hereunder.

(d)           Denominations; Registration.  Certificates for the Initial Securities and Option Securities, if any, shall be in such denominations and registered in such names as the Representative may request in writing at least one full business day before the Closing Time.  The certificate for the Securities and Option Securities, if any, which may be in temporary form,

16

will be made available for examination and packaging by the Representative in The City of New York not later than 10:00 A.M. (Eastern time) on the business day prior to the Closing Time.

SECTION 3.           Covenants of the Company.  The Company covenants with each Underwriter as follows:

(a)           Compliance with Securities Regulations and Commission Requests; Payment of Filing Fees.  The Company, subject to Section 3(b), will comply with the requirements of Rule 430B and will notify the Representative immediately, and confirm the notice in writing, (i) when any post-effective amendment to the Registration Statement or new registration statement relating to the Securities shall become effective, or any supplement to the Prospectus or any amended Prospectus shall have been filed, (ii) of the receipt of any comments from the Commission, (iii) of any request by the Commission for any amendment to the Registration Statement or the filing of a new registration statement or any amendment or supplement to the Prospectus or any document incorporated by reference therein or otherwise deemed to be a part thereof or for additional information, (iv) of the issuance by the Commission of any stop order suspending the effectiveness of the Registration Statement or such new registration statement or of any order preventing or suspending the use of any preliminary prospectus, or of the suspension of the qualification of the Securities for offering or sale in any jurisdiction, or of the initiation or threatening of any proceedings for any of such purposes or of any examination pursuant to Section 8(e) of the 1933 Act concerning the Registration Statement and (v) if the Company becomes the subject of a proceeding under Section 8A of the 1933 Act in connection with the offering of the Securities.  The Company will effect the filings required under Rule 424(b), in the manner and within the time period required by Rule 424(b) (without reliance on Rule 424(b)(8)), and will take such steps as it deems necessary to ascertain promptly whether the form of prospectus transmitted for filing under Rule 424(b) was received for filing by the Commission and, in the event that it was not, it will promptly file such prospectus.  The Company will make every reasonable effort to prevent the issuance of any stop order and, if any stop order is issued, to obtain the lifting thereof at the earliest possible moment.

(b)           Filing of Amendments and Exchange Act Documents.  The Company will give the Representative notice of its intention to file or prepare any amendment to the Registration Statement or new registration statement relating to the Securities or any amendment, supplement or revision to either any preliminary prospectus (including any prospectus included in the Original Registration Statement or amendment thereto at the time it became effective) or to the Prospectus, whether pursuant to the 1933 Act, the 1934 Act or otherwise, and the Company will furnish the Representative with copies of any such documents a reasonable amount of time prior to such proposed filing or use, as the case may be, and will not file or use any such document to which the Representative or counsel for the Underwriters shall object. The Company has given the Representative notice of any filings made pursuant to the 1934 Act or 1934 Act Regulations within 48 hours prior to the Applicable Time; the Company will give the Representative notice of its intention to make any such filing from the Applicable Time to the Closing Time and will furnish the Representative with copies of any such documents a reasonable amount of time prior to such proposed filing and will not file or use any such document to which the Representative or counsel for the Underwriters shall object.

17

(c)           Delivery of Registration Statements.  The Company has furnished or will deliver to the Representative and counsel for the Underwriters, without charge, signed copies of the Original Registration Statement and of each amendment thereto (including exhibits filed therewith or incorporated by reference therein and documents incorporated or deemed to be incorporated by reference therein or otherwise deemed to be a part thereof) and signed copies of all consents and certificates of experts, and will also deliver to the Representative, without charge, a conformed copy of the Original Registration Statement and of each amendment thereto (without exhibits) for each of the Underwriters.  The copies of the Original Registration Statement and each amendment thereto furnished to the Underwriters will be identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T.

(d)           Delivery of Prospectuses.  The Company has delivered to each Underwriter, without charge, as many copies of each preliminary prospectus as such Underwriter reasonably requested, and the Company hereby consents to the use of such copies for purposes permitted by the 1933 Act.  The Company will furnish to each Underwriter, without charge, during the period when the Prospectus is required to be delivered under the 1933 Act, such number of copies of the Prospectus (as amended or supplemented) as such Underwriter may reasonably request.  The Prospectus and any amendments or supplements thereto furnished to the Underwriters will be identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T.

(e)           Continued Compliance with Securities Laws.  The Company will comply with the 1933 Act and the 1933 Act Regulations and the 1934 Act and the 1934 Act Regulations so as to permit the completion of the distribution of the Securities as contemplated in this Agreement and in the Prospectus.  If at any time when a prospectus is required by the 1933 Act to be delivered in connection with sales of the Securities, any event shall occur or condition shall exist as a result of which it is necessary, in the opinion of counsel for the Underwriters or for the Company, to amend the Registration Statement or amend or supplement the Prospectus in order that the Prospectus will not include any untrue statements of a material fact or omit to state a material fact necessary in order to make the statements therein not misleading in the light of the circumstances existing at the time it is delivered to a purchaser, or if it shall be necessary, in the opinion of such counsel, at any such time to amend the Registration Statement or to file a new registration statement or amend or supplement the Prospectus in order to comply with the requirements of the 1933 Act or the 1933 Act Regulations, the Company will promptly prepare and file with the Commission, subject to Section 3(b), such amendment, supplement or new registration statement as may be necessary to correct such statement or omission or to comply with such requirements, the Company will use its best efforts to have such amendment or new registration statement declared effective as soon as practicable (if it is not an automatic shelf registration statement with respect to the Securities) and the Company will furnish to the Underwriters such number of copies of such amendment, supplement or new registration statement as the Underwriters may reasonably request.  If at any time following issuance of an Issuer Free Writing Prospectus there occurred or occurs an event or development as a result of which such Issuer Free Writing Prospectus conflicted or would conflict with the information contained in the Registration Statement (or any other registration statement relating to the Securities) or the Statutory Prospectus or any preliminary prospectus or included or would include an untrue statement of a material fact or omitted or would omit to state a material fact

18

necessary in order to make the statements therein, in the light of the circumstances prevailing at that subsequent time, not misleading, the Company will promptly notify Merrill Lynch and will promptly amend or supplement, at its own expense, such Issuer Free Writing Prospectus to eliminate or correct such conflict, untrue statement or omission.

(f)            Blue Sky Qualifications.  The Company will use its best efforts, in cooperation with the Underwriters, to qualify the Securities for offering and sale under the applicable securities laws of such states and other jurisdictions as the Representative may designate and to maintain such qualifications in effect for a period of not less than one year from the date hereof; provided, however, that the Company shall not be obligated to file any general consent to service of process or to qualify as a foreign corporation or as a dealer in securities in any jurisdiction in which it is not so qualified or so subject itself to taxation in respect of doing business in any jurisdiction in which it is not otherwise so subject. The Company will also supply the Underwriters with such information as is necessary for the determination of the legality of the Securities for investment under the laws of such jurisdictions as the Underwriters may request.

(g)           Rule 158.  The Company will timely file such reports pursuant to the 1934 Act as are necessary in order to make generally available to its securityholders as soon as practicable an earnings statement for the purposes of, and to provide to the Underwriters the benefits contemplated by, the last paragraph of Section 11(a) of the 1933 Act.

(h)           Use of Proceeds.  The Company will use the net proceeds received by it from the sale of the Securities in the manner specified in the Prospectus under “Use of Proceeds.”

(i)            Listing.  The Company will use its reasonable best efforts to effect and maintain the listing of the Securities on the Nasdaq Global Market.

(j)            Restriction on Sale of Securities.  During a period of 90 days from the date of this Agreement, the Company will not, without the prior written consent of Merrill Lynch, (i) directly or indirectly, offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase or otherwise transfer or dispose of any share of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock or file any registration statement under the 1933 Act with respect to any of the foregoing or (ii) enter into any swap or any other agreement or any transaction that transfers, in whole or in part, directly or indirectly, the economic consequence of ownership of the Common Stock, whether any such swap or transaction described in clause (i) or (ii) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise.  The foregoing sentence shall not apply to (A) the Securities to be sold hereunder, (B) any shares of Common Stock issued by the Company upon the exercise of an option or warrant or the conversion of a security outstanding on the date hereof and referred to in the Prospectus, (C) any shares of Common Stock issued or options to purchase Common Stock granted pursuant to existing employee benefit plans of the Company referred to in the Prospectus or (D) any shares of Common Stock issued pursuant to any non-employee director stock plan.  Notwithstanding the foregoing, if (1) during the last 17 days of the 90-day restricted period the Company issues an earnings release or material news or a material event relating to the Company occurs or (2) prior to the expiration of the 90-day restricted period, the Company announces that it will release earnings results or becomes aware that material news or a material event will occur

19

during the 16-day period beginning on the last day of the 90-day restricted period, the restrictions imposed in this clause (j) shall continue to apply until the expiration of the 18-day period beginning on the issuance of the earnings release or the occurrence of the material news or material event, unless Merrill Lynch waives such extension.

(k)           Reporting Requirements.  The Company, during the period when the Prospectus is required to be delivered under the 1933 Act, will file all documents required to be filed with the Commission pursuant to the 1934 Act within the time periods required by the 1934 Act and the 1934 Act Regulations.

(l)            Issuer Free Writing Prospectuses.  The Company represents and agrees that, unless it obtains the prior consent of the Representative, and each Underwriter represents and agrees that, unless it obtains the prior consent of the Company and the Representative, it has not made and will not make any offer relating to the Securities that would constitute an “issuer free writing prospectus,” as defined in Rule 433, or that would otherwise constitute a “free writing prospectus,” as defined in Rule 405, required to be filed with the Commission.  Any such free writing prospectus consented to by the Company and the Representative is hereinafter referred to as a “Permitted Free Writing Prospectus.”  The Company represents that it has treated or agrees that it will treat each Permitted Free Writing Prospectus as an “issuer free writing prospectus,” as defined in Rule 433,  and has complied and will comply with the requirements of Rule 433 applicable to any Permitted Free Writing Prospectus, including timely filing with the Commission where required, legending and record keeping.

(m)          Compliance with Registration Statement. The Company will comply with all the undertakings contained in the Registration Statement.

(n)           Controls and Procedures.  For a period of one year from the date of this Agreement, the Company will use its reasonable best efforts to maintain such controls and other procedures, including without limitation those required by Sections 302 and 906 of the Sarbanes-Oxley Act and the applicable regulations thereunder, that are designed to ensure that information required to be disclosed by the Company in the reports that it files or submits under the Exchange Act is recorded, processed, summarized and reported within the time periods specified in the Commission’s rules and forms, including without limitation, controls and procedures designed to ensure that information required to be disclosed by the Company in the reports that it files or submits under the Exchange Act is accumulated and communicated to the Company’s management, including its principal executive officer and its principal financial officer, or persons performing similar functions, as appropriate to allow timely decisions regarding required disclosure, to ensure that material information relating to Company is made known to them by others within the Company.

(o)           Sarbanes-Oxley Act.  For a period of one year from the date of this Agreement, the Company will use its reasonable best efforts to comply in all material respects with all effective applicable provisions of the Sarbanes-Oxley Act.

(p)           No Stabilization.  The Company will not take, directly or indirectly, any action designed to or that could reasonably be expected to cause or result in any stabilization or

20

manipulation of the price of the Securities and will not take any action prohibited by Regulation M under the Exchange Act in connection with the distribution of the Securities contemplated hereby.

SECTION 4.           Payment of Expenses.

(a)           Expenses.  The Company will pay all expenses incident to the performance of its obligations under this Agreement, including (i) the preparation, printing and filing of the Registration Statement (including financial statements and exhibits) as originally filed and of each amendment thereto, (ii) the preparation, printing and delivery to the Underwriters of this Agreement and such other documents as may be required in connection with the offering, purchase, sale, issuance or delivery of the Securities, (iii) the preparation, issuance and delivery of the certificates for the Securities to the Underwriters, (iv) the fees and disbursements of the Company’s counsel, accountants and other advisors, (v) the qualification of the Securities under securities laws in accordance with the provisions of Section 3(f) hereof, including filing fees and the reasonable fees and disbursements of counsel for the Underwriters in connection therewith and in connection with the preparation of the Blue Sky Survey and any supplement thereto, (vi) the printing and delivery to the Underwriters of copies of each preliminary prospectus, any Permitted Free Writing Prospectus and of the Prospectus and any amendments or supplements thereto and any costs associated with electronic delivery of any of the foregoing by the Underwriters to investors, (vii) the preparation, printing and delivery to the Underwriters of copies of the Blue Sky Survey and any supplement thereto, (viii) the fees and expenses of any transfer agent or registrar for the Securities, (ix)  the costs and expenses of the Company relating to investor presentations on any “road show” undertaken in connection with the marketing of the Securities, including without limitation, expenses associated with the production of road show slides and graphics, fees and expenses of any consultants engaged with the Company’s consent in connection with the road show presentations, travel and lodging expenses of the representatives and officers of the Company and any such consultants, and the cost of aircraft and other transportation chartered in connection with the road show, (x) the filing fees and the reasonable fees and disbursements of counsel for the Underwriters (such fees and disbursements of counsel for the Underwriters not to exceed, when combined with the fees and disbursements of such counsel referred to in (v) above, an aggregate of $20,000) incident to the review by the FINRA of the terms of the sale of the Securities and (xi) the fees and expenses incurred in connection with the listing of the Securities on the Nasdaq Global Market.

(b)           Termination of Agreement.  If this Agreement is terminated by the Representative in accordance with the provisions of Section 5 or Section 9(a)(i) hereof, the Company shall reimburse the Underwriters for all of their out-of-pocket expenses actually incurred by them, including the reasonable fees and disbursements of counsel for the Underwriters. If this Agreement is so terminated, the Company shall not in any event be liable to any of the Underwriters for loss of anticipated profits from the transaction covered by this Agreement.

SECTION 5.           Conditions of Underwriters’ Obligations.  The obligations of the several Underwriters hereunder are subject to the accuracy of the representations and warranties of the Company contained in Section 1 hereof or in certificates of any officer of the Company delivered pursuant to the provisions hereof, to the performance by the Company of its covenants and other obligations hereunder, and to the following further conditions:

21

(a)           Effectiveness of Registration Statement; Filing of Prospectus; Payment of Filing Fee.  The Registration Statement has become effective, and at the Closing Time no stop order suspending the effectiveness of the Registration Statement shall have been issued under the 1933 Act or proceedings therefor initiated or threatened by the Commission, and any request on the part of the Commission for additional information shall have been complied with to the reasonable satisfaction of counsel to the Underwriters.  A prospectus containing  the Rule 430B Information shall have been filed with the Commission in the manner and within the time period required by Rule 424(b) without reliance on Rule 424(b)(8) (or a post-effective amendment providing such information shall have been filed and become effective in accordance with the requirements of Rule 430B).  The Company has paid any and all required registration fees relating to the Securities within the time, and in the manner, required by the 1933 Act and the 1933 Act Regulations.

(b)           Opinion of Counsel for Company.  At the Closing Time, the Representative shall have received an opinion, dated as of Closing Time, of Cooley Godward Kronish LLP, counsel for the Company, in form and substance satisfactory to counsel for the Underwriters, to the effect set forth in Exhibit A hereto, together with signed or reproduced copies of such letter for each of the other Underwriters.

(c)           Opinion of Special Regulatory Counsel for Company.  At the Closing Time, the Representative shall have received an opinion, dated as of Closing Time, of Hyman, Phelps & McNamara PC, special counsel for the Company for regulatory matters, in form and substance satisfactory to counsel for the Underwriters, to the effect set forth in Exhibit B hereto, together with signed or reproduced copies of such letter for each of the other Underwriters.

(d)           Opinion of Special Intellectual Property Counsel for Company.  At the Closing Time, the Representative shall have received an opinion, dated as of Closing Time, of Swanson & Bratschun, LLC, special counsel for the Company for intellectual property matters, in form and substance satisfactory to counsel for the Underwriters, to the effect set forth in Exhibit C hereto, together with signed or reproduced copies of such letter for each of the other Underwriters.

(e)           Opinion of Counsel for Underwriters.  At the Closing Time, the Representative shall have received an opinion, dated as of Closing Time, of Latham & Watkins LLP, counsel for the Underwriters, together with signed or reproduced copies of such letter for each of the other Underwriters with respect to such matters as the Representative may reasonably request.  In giving such opinion such counsel may rely, as to all matters governed by the laws of jurisdictions other than the law of the State of New York and the federal law of the United States, upon the opinions of counsel satisfactory to the Representative.  Such counsel may also state that, insofar as such opinion involves factual matters, they have relied, to the extent they deem proper, upon certificates of officers of the Company and certificates of public officials.

(f)            Officers’ Certificate.  At Closing Time, there shall not have been, since the date hereof or since the respective dates as of which information is given in the Prospectus or the General Disclosure Package, any Material Adverse Effect, and the Representative shall have received a certificate of the President or a Vice President of the Company and of the chief financial or chief accounting officer of the Company, in their respective capacities as officers,

22

dated as of Closing Time, to the effect that (i) there has been no such Material Adverse Effect, (ii) the representations and warranties in Section 1(a) hereof are true and correct with the same force and effect as though expressly made at and as of Closing Time, (iii) the Company has complied with all agreements and satisfied all conditions on its part to be performed or satisfied at or prior to Closing Time, and (iv) no stop order suspending the effectiveness of the Registration Statement has been issued and no proceedings for that purpose have been instituted or are pending or, to their knowledge, contemplated by the Commission.

(g)           Accountant’s Comfort Letter.  At the time of the execution of this Agreement, the Representative shall have received from PricewaterhouseCoopers LLP a letter dated such date, in form and substance satisfactory to the Representative, together with signed or reproduced copies of such letter for each of the other Underwriters containing statements and information of the type ordinarily included in accountants’ “comfort letters” to underwriters with respect to the financial statements and certain financial information contained in the Registration Statement and the Prospectus.

(h)           Bring-down Comfort Letter.  At Closing Time, the Representative shall have received from PricewaterhouseCoopers LLP a letter, dated as of Closing Time, to the effect that they reaffirm the statements made in the letter furnished pursuant to subsection (g) of this Section, except that the specified date referred to shall be a date not more than three business days prior to Closing Time.

(i)            Approval of Listing.  At Closing Time, the Securities shall have been approved for listing on the Nasdaq Global Market.

(j)            No Objection.  The FINRA shall have confirmed that it has not raised any objection with respect to the fairness and reasonableness of the underwriting terms and arrangements.

(k)           Lock-up Agreements.  At the date of this Agreement, the Representative shall have received an Agreement substantially in the form of Exhibit D hereto signed by the persons listed on Schedule E hereto.

(l)            Conditions to Purchase of Option Securities.  In the event that the Underwriters exercise their option provided in Section 2(b) hereof to purchase all or any portion of the Option Securities, the representations and warranties of the Company contained herein and the statements in any certificates furnished by the Company hereunder shall be true and correct as of each Date of Delivery and, at the relevant Date of Delivery, the Representative shall have received:

(i)            Officers’ Certificate.  A certificate, dated such Date of Delivery, of the President or a Vice President of the Company and of the chief financial or chief accounting officer of the Company, in their respective capacities as officers, confirming that the certificate delivered at the Closing Time pursuant to Section 5(f) hereof remains true and correct as of such Date of Delivery.

(ii)           Opinion of Counsel for Company.  An opinion of Cooley Godward Kronish LLP, counsel for the Company, together with the opinion of Hyman, Phelps & McNamara PC,

23

special regulatory counsel for the Company and Swanson & Bratschun, LLC, special counsel for the Company, in form and substance satisfactory to counsel for the Underwriters, dated such Date of Delivery, relating to the Option Securities to be purchased on such Date of Delivery and otherwise to the same effect as the opinion required by Sections 5(b), 5(c) and 5(d) hereof, respectively.

(iii)          Opinion of Counsel for the Underwriters.  The favorable opinion of Latham & Watkins LLP, counsel for the Underwriters, dated such Date of Delivery, relating to the Option Securities to be purchased on such Date of Delivery and otherwise to the same effect as the opinion required by Section 5(f) hereof.

(iv)          Bring-down Comfort Letter.  A letter from PricewaterhouseCoopers LLP, in form and substance reasonably satisfactory to the Representatives and dated such Date of Delivery, substantially in the same form and substance as the letter furnished to the Representatives pursuant to Section 5(g) hereof, except that the “specified date” in the letter furnished pursuant to this paragraph shall be a date not more than five days prior to such Date of Delivery.

(m)          Additional Documents.  At Closing Time, counsel for the Underwriters shall have been furnished with such documents and opinions as they may require for the purpose of enabling them to pass upon the issuance and sale of the Securities as herein contemplated, or in order to evidence the accuracy of any of the representations or warranties, or the fulfillment of any of the conditions, herein contained; and all proceedings taken by the Company in connection with the issuance and sale of the Securities as herein contemplated shall be reasonably satisfactory in form and substance to the Representative and counsel for the Underwriters.

(n)           Termination of Agreement.  If any condition specified in this Section shall not have been fulfilled when and as required to be fulfilled, this Agreement, or, in the case of any condition to the purchase of Option Securities on a Date of Delivery which is after the Closing Time, the obligations of the several Underwriters to purchase the relevant Option Securities may be terminated by the Representative by notice to the Company at any time at or prior to Closing Time or such Date of Delivery, as the case may be, and such  termination shall be without liability of any party to any other party except as provided in Section 4 and except that Sections 1, 6, 7 and 8 shall survive any such termination and remain in full force and effect.

SECTION 6.           Indemnification.

(a)           Indemnification of Underwriters. The Company agrees to indemnify and hold harmless each Underwriter, its affiliates, as such term is defined in Rule 501(b) under the 1933 Act (each, an “Affiliate”), its selling agents and each person, if any, who controls any Underwriter within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act as follows:

(i)            against any and all loss, liability, claim, damage and expense whatsoever, as incurred, arising out of any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement (or any amendment thereto), including the Rule 430B Information, or the omission or alleged omission therefrom of a material fact

24

required to be stated therein or necessary to make the statements therein not misleading or arising out of any untrue statement or alleged untrue statement of a material fact contained in any preliminary prospectus, any Issuer Free Writing Prospectus or the Prospectus (or any amendment or supplement thereto), or the omission or alleged omission therefrom of a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading;

(ii)           against any and all loss, liability, claim, damage and expense whatsoever, as incurred, to the extent of the aggregate amount paid in settlement of any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or of any claim whatsoever based upon any such untrue statement or omission, or any such alleged untrue statement or omission; provided that (subject to Section 6(d) below) any such settlement is effected with the written consent of the Company;

(iii)          against any and all expense whatsoever, as incurred (including the fees and disbursements of counsel chosen by Merrill Lynch), reasonably incurred in investigating, preparing or defending against any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever based upon any such untrue statement or omission, or any such alleged untrue statement or omission, to the extent that any such expense is not paid under (i) or (ii) above;

provided, however, that this indemnity agreement shall not apply to any loss, liability, claim, damage or expense to the extent arising out of any untrue statement or omission or alleged untrue statement or omission made in reliance upon and in conformity with written information furnished to the Company by any Underwriter through Merrill Lynch expressly for use in the Registration Statement (or any amendment thereto), including the Rule 430B Information or any preliminary prospectus, any Issuer Free Writing Prospectus or the Prospectus (or any amendment or supplement thereto).

(b)           Indemnification of Company, Directors and Officers.  Each Underwriter severally agrees to indemnify and hold harmless the Company, its directors, each of its officers who signed the Registration Statement, and each person, if any, who controls the Company within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act against any and all loss, liability, claim, damage and expense described in the indemnity contained in subsection (a) of this Section, as incurred, but only with respect to untrue statements or omissions, or alleged untrue statements or omissions, made in the Registration Statement (or any amendment thereto), including the Rule 430B Information or any preliminary prospectus, any Issuer Free Writing Prospectus or the Prospectus (or any amendment or supplement thereto) in reliance upon and in conformity with written information furnished to the Company by such Underwriter through Merrill Lynch expressly for use therein.

(c)           Actions against Parties; Notification.  Each indemnified party shall give notice as promptly as reasonably practicable to each indemnifying party of any action commenced against it in respect of which indemnity may be sought hereunder, but failure to so notify an indemnifying party shall not relieve such indemnifying party from any liability hereunder to the

25

extent it is not materially prejudiced as a result thereof and in any event shall not relieve it from any liability which it may have otherwise than on account of this indemnity agreement.  In the case of parties indemnified pursuant to Section 6(a) above, counsel to the indemnified parties shall be selected by Merrill Lynch, and, in the case of parties indemnified pursuant to Section 6(b) above, counsel to the indemnified parties shall be selected by the Company.  An indemnifying party may participate at its own expense in the defense of any such action; provided, however, that counsel to the indemnifying party shall not (except with the consent of the indemnified party) also be counsel to the indemnified party.  In no event shall the indemnifying parties be liable for fees and expenses of more than one counsel (in addition to any local counsel) separate from their own counsel for all indemnified parties in connection with any one action or separate but similar or related actions in the same jurisdiction arising out of the same general allegations or circumstances. No indemnifying party shall, without the prior written consent of the indemnified parties, settle or compromise or consent to the entry of any judgment with respect to any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever in respect of which indemnification or contribution could be sought under this Section 6 or Section 7 hereof (whether or not the indemnified parties are actual or potential parties thereto), unless such settlement, compromise or consent (i) includes an unconditional release of each indemnified party from all liability arising out of such litigation, investigation, proceeding or claim and (ii) does not include a statement as to or an admission of fault, culpability or a failure to act by or on behalf of any indemnified party.

(d)           Settlement without Consent if Failure to Reimburse.  If at any time an indemnified party shall have requested an indemnifying party to reimburse the indemnified party for fees and expenses of counsel, such indemnifying party agrees that it shall be liable for any settlement of the nature contemplated by Section 6(a) (ii) effected without its written consent if (i) such settlement is entered into more than 45 days after receipt by such indemnifying party of the aforesaid request, (ii) such indemnifying party shall have received notice of the terms of such settlement at least 30 days prior to such settlement being entered into and (iii) such indemnifying party shall not have reimbursed such indemnified party in accordance with such request prior to the date of such settlement.

SECTION 7.           Contribution.  If the indemnification provided for in Section 6 hereof is for any reason unavailable to or insufficient to hold harmless an indemnified party in respect of any losses, liabilities, claims, damages or expenses referred to therein, then each indemnifying party shall contribute to the aggregate amount of such losses, liabilities, claims, damages and expenses incurred by such indemnified party, as incurred, (i) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriters on the other hand from the offering of the Securities pursuant to this Agreement or (ii) if the allocation provided by clause (i) is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause (i) above but also the relative fault of the Company on the one hand and of the Underwriters on the other hand in connection with the statements or omissions which resulted in such losses, liabilities, claims, damages or expenses, as well as any other relevant equitable considerations.

The relative benefits received by the Company on the one hand and the Underwriters on the other hand in connection with the offering of the Securities pursuant to this Agreement shall

26

be deemed to be in the same respective proportions as the total net proceeds from the offering of the Securities pursuant to this Agreement (before deducting expenses) received by the Company and the total underwriting discount received by the Underwriters, in each case as set forth on the cover of the Prospectus bear to the aggregate initial public offering price of the Securities as set forth on the cover of the Prospectus.

The relative fault of the Company on the one hand and the Underwriters on the other hand shall be determined by reference to, among other things, whether any such untrue or alleged untrue statement of a material fact or omission or alleged omission to state a material fact relates to information supplied by the Company or by the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission.

The Company and the Underwriters agree that it would not be just and equitable if contribution pursuant to this Section 7 were determined by pro rata allocation (even if the Underwriters were treated as one entity for such purpose) or by any other method of allocation which does not take account of the equitable considerations referred to above in this Section 7.  The aggregate amount of losses, liabilities, claims, damages and expenses incurred by an indemnified party and referred to above in this Section 7 shall be deemed to include any legal or other expenses reasonably incurred by such indemnified party in investigating, preparing or defending against any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever based upon any such untrue or alleged untrue statement or omission or alleged omission.

Notwithstanding the provisions of this Section 7, no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Securities underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages which such Underwriter has otherwise been required to pay by reason of any such untrue or alleged untrue statement or omission or alleged omission.

No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the 1933 Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation.

For purposes of this Section 7, each person, if any, who controls an Underwriter within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act and each Underwriter’s Affiliates and selling agents shall have the same rights to contribution as such Underwriter, and each director of the Company, each officer of the Company who signed the Registration Statement, and each person, if any, who controls the Company within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act shall have the same rights to contribution as the Company.  The Underwriters’ respective obligations to contribute pursuant to this Section 7 are several in proportion to the principal amount of Securities set forth opposite their respective names in Schedule A hereto and not joint.

SECTION 8.           Representations, Warranties and Agreements to Survive.  All representations, warranties and agreements contained in this Agreement or in certificates of officers of the Company submitted pursuant hereto, shall remain operative and in full force and

27

effect regardless of (i) any investigation made by or on behalf of any Underwriter or its Affiliates or selling agents, any person controlling any Underwriter, its officers or directors or any person controlling the Company, and (ii) delivery of and payment for the Securities.

SECTION 9.           Termination of Agreement.

(a)           Termination; General.  The Representative may terminate this Agreement, by notice to the Company, at any time at or prior to Closing Time (i) if there has been, since the time of execution of this Agreement or since the respective dates as of which information is given in the Prospectus (exclusive of any supplement thereto) or the General Disclosure Package, any Material Adverse Effect, or (ii) if there has occurred any material adverse change in the financial markets in the United States or the international financial markets, any outbreak of hostilities or escalation thereof or other calamity or crisis or any change or development involving a prospective change in national or international political, financial or economic conditions, in each case the effect of which is such as to make it, in the judgment of the Representative, impracticable or inadvisable to market the Securities or to enforce contracts for the sale of the Securities, or (iii) if trading in any securities of the Company has been suspended or materially limited by the Commission or the Nasdaq Global Market, or if trading generally on the American Stock Exchange or the New York Stock Exchange or in the Nasdaq Global Market has been suspended or materially limited, or minimum or maximum prices for trading have been fixed, or maximum ranges for prices have been required, by any of said exchanges or by such system or by order of the Commission, the National Association of Securities Dealers, Inc. or any other governmental authority, or (iv)  a material disruption has occurred in commercial banking or securities settlement or clearance services in the United States, or (v) if a banking moratorium has been declared by either Federal or New York authorities.

(b)           Liabilities.  If this Agreement is terminated pursuant to this Section, such termination shall be without liability of any party to any other party except as provided in Section 4 hereof, and provided further that Sections 1, 6, 7 and 8 shall survive such termination and remain in full force and effect.

SECTION 10.         Default by One or More of the Underwriters.  If one or more of the Underwriters shall fail at Closing Time or a Date of Delivery to purchase the Securities which it or they are obligated to purchase under this Agreement (the “Defaulted Securities”), the Representative shall have the right, within 24 hours thereafter, to make arrangements for one or more of the non-defaulting Underwriters, or any other underwriters, to purchase all, but not less than all, of the Defaulted Securities in such amounts as may be agreed upon and upon the terms herein set forth; if, however, the Representative shall not have completed such arrangements within such 24-hour period, then:

(a)           if the number of Defaulted Securities does not exceed 10% of the aggregate number of the Securities to be purchased on such date, each of the non-defaulting Underwriters shall be obligated, severally and not jointly, to purchase the full amount thereof in the proportions that their respective underwriting obligations hereunder bear to the underwriting obligations of all non-defaulting Underwriters, or

28

(b)           if the number of Defaulted Securities exceeds 10% of the aggregate number of the Securities to be purchased on such date, this Agreement or, with respect to any Date of Delivery which occurs after the Closing Time, the obligation of the Underwriters to purchase and of the Company to sell the Option Securities to be purchased and sold on such Date of Delivery, this Agreement shall terminate without liability on the part of any non-defaulting Underwriter.

No action taken pursuant to this Section shall relieve any defaulting Underwriter from liability in respect of its default.

In the event of any such default which does not result in a termination of this Agreement or, in the case of a Date of Delivery which is after the Closing Time, which does not result in a termination of the obligation of the Underwriters to purchase and the Company to sell the relevant Option Securities, as the case may be, either the Representatives or the Company shall have the right to postpone the Closing Time or the relevant Date of Delivery, as the case may be, for a period not exceeding seven days in order to effect any required changes in the Registration Statement or Prospectus or in any other documents or arrangements.  As used herein, the term “Underwriter” includes any person substituted for an Underwriter under this Section 10.

SECTION 11.         Notices.  All notices and other communications hereunder shall be in writing and shall be deemed to have been duly given if mailed or transmitted by any standard form of telecommunication.  Notices to the Underwriters shall be directed to the Representative at 4 World Financial Center, New York, New York 10080, attention of Bryan Giraudo, with a copy to Latham & Watkins LLP, 650 Town Center Driver, 20^th Floor, Costa Mesa, California 92626 attention David B. Allen; and notices to the Company shall be directed to it at 11080 CirclePoint Road, Suite 200, Westminster, Colorado 80020 with a copy to Cooley Godward Kronish LLP, 380 Interlocken Crescent, Suite 900, Broomfield, Colorado 80021, attention of James C. T. Linfield, Esq.

SECTION 12.         No Advisory or Fiduciary Relationship.  The Company acknowledges and agrees that (a) the purchase and sale of the Securities pursuant to this Agreement, including the determination of the public offering price of the Securities and any related discounts and commissions, is an arm’s-length commercial transaction between the Company, on the one hand, and the several Underwriters, on the other hand, (b) in connection with the offering contemplated hereby and the process leading to such transaction each Underwriter is and has been acting solely as a principal and is not the agent or fiduciary of the Company, or its stockholders, creditors, employees or any other party, (c) no Underwriter has assumed or will assume an advisory or fiduciary responsibility in favor of the Company with respect to the offering contemplated hereby or the process leading thereto (irrespective of whether such Underwriter has advised or is currently advising the Company on other matters) and no Underwriter has any obligation to the Company with respect to the offering contemplated hereby except the obligations expressly set forth in this Agreement, (d) the Underwriters and their respective affiliates may be engaged in a broad range of transactions that involve interests that differ from those of the Company, and (e) the Underwriters have not provided any legal, accounting, regulatory or tax advice with respect to the offering contemplated hereby and the Company has consulted its own legal, accounting, regulatory and tax advisors to the extent it deemed appropriate.

29

SECTION 13.         Integration.  This Agreement supersedes all prior agreements and understandings (whether written or oral) between the Company and the Underwriters, or any of them, with respect to the subject matter hereof.

SECTION 14.         Parties.  This Agreement shall each inure to the benefit of and be binding upon the Underwriters and the Company and their respective successors.  Nothing expressed or mentioned in this Agreement is intended or shall be construed to give any person, firm or corporation, other than the Underwriters and the Company and their respective successors and the controlling persons and officers and directors referred to in Sections 6 and 7 and their heirs and legal representatives, any legal or equitable right, remedy or claim under or in respect of this Agreement or any provision herein contained.  This Agreement and all conditions and provisions hereof are intended to be for the sole and exclusive benefit of the Underwriters and the Company and their respective successors, and said controlling persons and officers and directors and their heirs and legal representatives, and for the benefit of no other person, firm or corporation.  No purchaser of Securities from any Underwriter shall be deemed to be a successor by reason merely of such purchase.

SECTION 15.         GOVERNING LAW.  THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

SECTION 16.         TIME.  TIME SHALL BE OF THE ESSENCE OF THIS AGREEMENT.  EXCEPT AS OTHERWISE SET FORTH HEREIN, SPECIFIED TIMES OF DAY REFER TO NEW YORK CITY TIME.

SECTION 17.         Counterparts.  This Agreement may be executed in any number of counterparts, each of which shall be deemed to be an original, but all such counterparts shall together constitute one and the same Agreement.

SECTION 18.         Effect of Headings.  The Section headings herein are for convenience only and shall not affect the construction hereof.

30

If the foregoing is in accordance with your understanding of our agreement, please sign and return to the Company a counterpart hereof, whereupon this instrument, along with all counterparts, will become a binding agreement between the Underwriters and the Company in accordance with its terms.

CONFIRMED AND ACCEPTED,

as of the date first above written:

MERRILL LYNCH & CO.

MERRILL LYNCH, PIERCE, FENNER & SMITH

INCORPORATED

For itself and as Representative of the other Underwriters named in Schedule A hereto.

31

SCHEDULE A

1

SCHEDULE B

2

SCHEDULE C

ALLOS THERAPEUTICS, INC.

10,800,000 Shares of Common Stock

(Par Value $.001 Per Share)

The initial public offering price per share for the Securities, determined as provided in said Section 2, shall be $5.64

The purchase price per share to be paid by the Underwriters for the Securities shall be $5.3016 being an amount equal to the initial public offering price set forth above less $0.3384 per share.

1

SCHEDULE D

Issuer General Free Writing Prospectus, filed with the SEC on May 23, 2008.

2

SCHEDULE E

Persons Entering Lock-Ups

EXHIBIT A

Opinion of Cooley Godward Kronish LLP

We are of the opinion that:

1.             The Company has been duly incorporated and is validly existing as a corporation in good standing under the laws of the State of Delaware with requisite corporate power to own or lease, as the case may be, and to operate its properties and conduct its business as described in the Prospectus.

2.             The Company is duly qualified to do business as a foreign corporation and is in good standing under the laws of the State of Colorado.

3.             The authorized, issued and outstanding capital stock of the Company was as set forth in the Prospectus under the caption “Capitalization” as of the date stated therein; the issued and outstanding shares of capital stock set forth in the Prospectus under the caption “Capitalization” as of the date stated therein have been duly and validly authorized and issued and are fully paid and nonassessable.

4.             The Securities have been duly authorized and, when issued and paid for by the Underwriters pursuant to the Agreement, will be validly issued, fully paid and nonassessable.

5.             The holders of outstanding shares of capital stock of the Company are not entitled to preemptive rights under the Company’s certificate of incorporation, bylaws or the DGCL or, to our knowledge, rights of first refusal or other similar rights to subscribe for the Securities, except for those subscription rights granted to Warburg Pincus Private Equity VIII, L.P. pursuant to that certain Securities Purchase Agreement, dated as of March 2, 2005, which have been waived with respect to the issuance of the Securities.

6.             To our knowledge, there is (i) no action, suit or proceeding by or before any court or other governmental agency, authority or body or any arbitrator pending or overtly threatened against the Company or its properties by a third party of a character required to be disclosed in the Prospectus that is not disclosed in the Prospectus as required by the 1933 Act and the rules thereunder, and (ii) no indenture, contract, lease, mortgage, deed of trust, note agreement, loan or other agreement or instrument of a character required to be filed as an exhibit to the Registration Statement or required to be incorporated by reference into the Prospectus, which is not filed or incorporated by reference as required by the 1933 Act and the rules thereunder.

7.             The statements in the Prospectus under the headings “Description of Capital Stock” and in the Registration Statement in Item 15, insofar as such statements purport to summarize legal matters, agreements or documents discussed therein, fairly present, to the extent required by the 1933 Act and the rules thereunder, in all material respects, such legal matters, agreements or documents.  The statements in Part II, Item 1 of the Company’s Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2008, filed with the Commission on May 7, 2008, insofar as such statements constitute summaries of the proceedings described therein, fairly present the matters referred to therein in all material respects.  The statements in the

Prospectus under the heading “Material United States Federal Income and Estate Tax Consequences for Non-U.S. Holders,” to the extent that they constitute matters of U.S. federal income tax laws and legal conclusions with respect thereto, are accurate in all material respects.

8.             The Registration Statement has become effective under the 1933 Act; any required filing of each Issuer Free Writing Prospectus listed on Schedule I hereto pursuant to Rule 433 has been made in the manner and within the time period specified by Rule 433(d); and, no stop order suspending the effectiveness of the Registration Statement has been issued and no proceedings for that purpose have been instituted or overtly threatened.  Any required filing of the Prospectus, and any supplement thereto, pursuant to Rule 424(b) under the Act, has been made in the manner and within the time period required by Rule 424(b).