AGREEMENT

Exhibit 10.48.1

AGREEMENT

dated: 1 ^st July 2005

between

VERNALIS DEVELOPMENT LIMITED

and

ENDO PHARMACEUTICALS INC

re.

CO-PROMOTION OF FROVATRIPTAN IN U.S.A.

The confidential portions of this exhibit have been filed separately with the Securities and Exchange Commission pursuant to a confidential treatment request in accordance with Rule 24b-2 of the Securities and Exchange Act of 1934, as amended. REDACTED PORTIONS OF THIS EXHIBIT ARE MARKED BY AN ***.

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CONTENTS

CO-PROMOTION AGREEMENT

     This CO-PROMOTION AGREEMENT (this “Agreement”) is entered into and effective as of this 1 ^st day of July, 2005 (the “Effective Date”), by and between ENDO PHARMACEUTICALS INC., a Delaware corporation whose principal place of business is at 100 Endo Boulevard, Chadds Ford, PA 19317, USA (together with its Affiliates, “Endo”), and VERNALIS DEVELOPMENT LTD., a company organized under the laws of England and Wales whose principal place of business is at Oakdene Court, 613 Reading Road, Winnersh, Wokingham, RG41 5UA, England (“Vernalis”).

RECITALS

     WHEREAS, Vernalis granted to Endo a sole and exclusive license to Commercialise its product known as frovatriptan in the Territory by way of a License Agreement dated July 14, 2004 (the “License”) as and to the extent more particularly set forth in the License;

     WHEREAS, Endo is engaged in the business of and has expertise in, among other things, the promotion to physicians of pharmaceutical products;

     WHEREAS, the License provides Vernalis certain rights to Commercialise (and in particular to co-promote) the Product in the USA in accordance with the terms set forth in the License and such additional terms as may be agreed between the parties;

     WHEREAS, Vernalis has exercised its option to co-promote Product in the USA, and Endo and Vernalis desire to work together to promote the Product in the USA upon the terms and conditions set forth herein;

     WHEREAS, the parties wish to set forth in this Agreement the detailed terms and conditions to govern such co-promotion and to amend, restate and supercede Article 9 and Schedule 10 of the License and make the other specific amendments to the License explicitly stated in Section 11, but otherwise leave the License (and that certain Safety Agreement, Loan Agreement and Security Agreement between the Parties in connection with the License) unaffected.

     NOW, THEREFORE, in consideration of the mutual covenants and agreements set forth in this Agreement, and for other good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, the parties hereto agree as follows:

1. Definitions . Capitalized terms used herein shall have the meanings specified in this Section 1 (such definitions to be equally applicable to both the singular and plural forms of the terms defined). Other capitalized terms used in this Agreement, unless otherwise set forth in this Section 1, shall have the meanings set forth in the License.

     “Act” shall mean the United States Federal Food, Drug and Cosmetic Act, as it may be amended from time to time.

     “Agreement” means this Agreement, together with all appendices, exhibits and schedules referenced herein or attached hereto, and as the same may be amended or supplemented from time to time hereafter pursuant to the provisions hereof.

     “Annual Co-Promotion Plan” shall mean a written sales plan relating to the sales and promotion of the Product in the USA by Endo sales representatives and Vernalis Specialty Sales Personnel as annually prepared by the Joint Co-Promotion Team.

     “Audited Party” shall have the meaning set forth in Section 8.2(a).

     “Auditing Party” shall have the meaning set forth in Section 8.2(a).

     “Endo Copyright” means copyright or any other intellectual property analogous to copyright including any rights in designs subsisting or relating to any Documents, designs or other embodiments of the trade dress for the Product, any form of advertisement in whatever media, Marketing Materials, sales training materials, samples or other promotional gifts or any other materials in which such rights are capable of subsisting as a matter of law in all cases which are generated by or upon behalf of Endo or its Affiliates during the period of this Agreement in connection with the advertising, promotion, marketing or sale or other Commercialization of Product.

     “Endo Trademarks” means (i) the name and mark ENDO and the associated Endo logo, and (ii) any other trademarks other than the Vernalis Trademarks used, owned by or licensed to Endo in relation to the Product (including without limitation the MAM Product (if and when it receives Marketing Authorization)). This definition shall not be read to modify in any way Section 13.1 of the License.

     “FDA” means the United States Food and Drug Administration or any successor entity thereto.

     “Governmental or Regulatory Authority” shall mean any U.S. court, tribunal, arbitrator, agency, commission, official or other instrumentality of any federal, state, county, city or other political subdivision thereof.

     “Initial Term” shall have the meaning set forth in Section 9.1.

     “Joint Co-Promotion Team” shall have the meaning set forth in Section 7.

     “License” means the License Agreement dated as of July 14, 2004 between Endo and Vernalis.

     “Marketing Materials” shall have the meaning set forth in Section 5.3.

     “Non-Serious Adverse Event” shall mean any adverse drug experience associated with the use of the Product in humans, whether or not considered drug-related, which is not a Serious Adverse Event.

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     “PDMA” shall mean the Prescription Drug Marketing Act, as amended, and the implementing rules and regulations thereunder.

     “Person” shall mean an individual, corporation, partnership, limited liability company, trust, business trust, association, joint stock company, joint venture, pool, syndicate, sole proprietorship, unincorporated organization, governmental authority, or any other form of entity not specifically listed herein.

     “Phase One Training” shall mean a Product training program (including compliance training) designed by Endo and provided to sales representatives, a sample of which is set forth in Exhibit B hereto. Phase One Training may include sales force automation training if so requested by Vernalis and if Vernalis uses the same sales force automation system as Endo.

     “Product” shall have the meaning set forth in the License.

     “Product Details” shall mean face-to-face contacts by a sales representative with a Target Healthcare Professional for the purpose of discussing information about the Product.

     “Product Technical Complaint” or “PTC” shall mean any complaint that questions the purity, identity, potency or quality of the Product, its packaging or labeling or the compliance of any batch of the Product with applicable laws, including the Act, and current Good Manufacturing Practice; any complaint that concerns any incident that causes the Product or its labeling to be mistaken for, or applied to, another article; any bacteriological contamination or significant chemical, physical or other change or deterioration in the Product; any failure of one or more batches of the Product to meet the specifications therefor in the NDA; or any complaint or evidence of tampering with the Product.

     “Product Trademarks” shall mean the Vernalis Trade Marks, including without limitation the trademark Frova® associated with the Product, any other related trademark or service mark containing the word “Frova” and any other trademark or service mark (whether registered or unregistered) currently used on or with the Product or in any Marketing Material in the U.S. and its territories.

     “Safety Agreement” shall mean the Safety Data Exchange Agreement version 12 August 2004 entered into between Endo Pharmaceuticals Inc. and Vernalis Development Ltd.

     “Specialty Sales Personnel” shall have the meaning set forth in the License for Vernalis Specialty Sales Force

     “Target Healthcare Professionals” shall mean medical professionals with prescribing or dispensing authority and who practice in the medical specialties set forth on Exhibit C, as may be amended by agreement of the Parties from time to time.

     “Term” shall have the meaning set forth in Section 9.1.

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     “USA” shall mean the United States of America and its territories.

     “Vernalis Detail Report” shall have the meaning set forth in Section 3.2.

     “Vernalis Detailing Services” shall have the meaning set forth in Section 3.1(a).

2. Grants of Rights .

     2.1 Co-Promotion Rights .

          a. Endo hereby grants to Vernalis, together with Endo, and to the extent, if any, that Vernalis does not already have the right to do so under the License given the rights it has retained thereunder, the right to promote the Product in the USA during the Term subject to the terms and conditions set forth in this Agreement.

          b. The promotion rights granted to Vernalis herein shall not prevent Vernalis from promoting other products in the USA. The Parties agree that during the time in which Vernalis is co-promoting the Product in the USA, the Specialty Sales Personnel shall have the right to detail other products in addition to the Product provided that such other products are not (x) for the treatment of migraine or post herpetic neuralgia or (y) in direct competition in the USA with products in other therapeutic areas that are being Commercialised by Endo or are in Phase III Clinical Trial or have completed clinical proof of principle by Endo.

     2.2 Rights to Trademarks and Copyrights .

          a. Licenses .

               (i) Endo hereby grants to Vernalis a non-exclusive, royalty-free license to use the Endo Trademarks and Endo Copyright solely in connection with performing its obligations and exercising its rights to co-promote the Product pursuant to the terms and conditions of this Agreement.

          b. Required Use and Compliance .

               (i) Except for the use of the Endo Trademarks and Endo Copyright in labeling, package inserts, Product monographs, packaging for Products, and Marketing Materials, each party shall promote the Product only under the Product Trademarks. To avoid any doubt, the Vernalis Specialty Sales Personnel are permitted to use business cards indicating their association with Vernalis and bearing any trademark or tradename of Vernalis and are permitted in their Product promotion to refer to their association with Vernalis.

               (ii) To the extent Vernalis makes any use of the Endo Trademarks in writing other than in a writing provided by Endo, Vernalis shall ensure that each use of the Endo Trademarks in writing is accompanied by an acknowledgement that the Endo Trademarks are owned by Endo. Vernalis shall not (A) use the Endo Trademarks in a way that might materially prejudice their distinctiveness or validity or

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the goodwill of Endo therein, or (B) use in its Product-related activities any trademarks or tradenames so resembling any of the Endo Trademarks as to be likely to cause confusion or deception.

          c. Notice of Infringement— Endo Trademarks.

               (i) Vernalis shall give Endo prompt notice of any infringement or threatened infringement of which it becomes aware of any of the Endo Trademarks used in connection with the Product in the USA.

               (ii) Endo shall determine in its sole discretion what action, if any, to take in response to the infringement or threatened infringement of any Endo Trademark. The costs of any such action in response to the infringement or threatened infringement in the USA shall be borne solely by Endo. Endo shall keep Vernalis informed of such action and shall use reasonable efforts to advise Vernalis to the extent such action impacts or affects the Vernalis Specialty Sales Personnel’s Product promotion in the USA.

          d. Notice of Infringement — Product Trademarks. The parties rights and obligations as regards infringement of the Product Trademarks are as set forth in Clauses 13.7 — 13.11 of the License.

3. Responsibilities of Vernalis .

     3.1 Promotion by Vernalis .

          a. Commencing after January 1, 2006 and by January 31, 2006, and continuing throughout the Term, Vernalis shall promote the Product to Target Healthcare Professionals in the USA in accordance with the then-current Annual Co-Promotion Plan (collectively, the “Vernalis Detailing Services”). The targeting and frequency of Product Details to be provided by Vernalis will be determined by the Joint Co-Promotion Team.

          b. By January 31, 2006, Vernalis shall have hired and shall thereafter maintain a sales force of Specialty Sales Personnel detailing the Product in the USA to Target Healthcare Professionals, and such sales force shall complete the number of Product Details specified in the then current Annual Co-Promotion Plan, not to exceed the maximum number of details set forth in Clause 9.1.2 of the License, in each calendar year during the Term. All Product Details made by Vernalis and reimbursed by Endo shall contribute to Endo’s Estimated Detailing Effort as set forth in the License.

          c. Vernalis shall after 31 Jan 2006 and at all times during the remainder of the Term of this Agreement, use Commercially Reasonable Efforts to provide at least the number of Product Details set forth in the Annual Co-Promotion Plan. If for any *** consecutive calendar quarters, Vernalis fails to provide a minimum of *** of the required Product Details set forth for Vernalis to provide in the Annual Co-Promotion Plan, then Endo shall have the right to terminate this Agreement upon *** days prior written notice to Vernalis; provided, however, that if the reason Vernalis has failed to provide such *** is for a reason which meets the definition of Force Majeure,

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then Endo shall not be entitled to terminate this Agreement on that basis and the Joint Co-Promotion Team shall agree in good faith to an appropriate adjustment to the Annual Co-Promotion Plan going forward.

          d. Vernalis shall notify Endo prior to September 30, 2005 and prior to June 30 of each year thereafter during the Term of the proposed number of Product Details to be included in the promotion of the Product for the following calendar year. Vernalis may revise the proposed number of Product Details provided to Endo by up to *** as long as such revisions are made in writing on or before August 31 ^st of subsequent years. For planning purposes only, at the same time as Vernalis notifies Endo of the proposed number of Product Details, Vernalis shall also notify Endo of the estimated number of Specialty Sales Personnel Vernalis intends to employ for Product Details.

          e. In performing its duties hereunder, Vernalis shall, and shall cause its employees and designees to, comply with all regulatory, professional and legal requirements, including, without limitation, the FDA’s regulations and guidelines concerning the advertising of prescription drug products, state laws and regulations relating to promotion of pharmaceutical products, the American Medical Association’s Guidelines on Gifts to Physicians, the OIG Compliance Program Guidelines for Pharmaceutical Manufacturers, the PhRMA Guidelines for Marketing Practices, and the ACCME Standards for Commercial Support of Continuing Medical Education, which may be applicable to the co-promotion of the Product by Vernalis. No employee or designee of Vernalis shall (i) make any representation, statement, warranty or guaranty with respect to the Product that is not consistent with then-current labeling of the Product or Marketing Materials approved by Endo, that is deceptive or misleading or that disparages the Products or the good name, goodwill and reputation of Endo or (ii) make any arrangements with, make payments to or provide gifts or other incentives to any healthcare professionals in violation of applicable laws, regulations or guidelines relating thereto. Vernalis covenants that its services hereunder will be provided in a professional, ethical and competent manner in accordance with the foregoing standards. Notwithstanding Section 9.2 hereof, upon written notice to Vernalis by Endo, Vernalis shall take prompt action to address any breach of this Section 3.1(e) and, within *** days shall submit to Endo a written plan to correct such breach. If the breach is not cured within *** days after Endo’s receipt of such corrective plan, or if Vernalis fails to timely submit to Endo such corrective plan, Endo may, where the breach has been caused by one or more identifiable Vernalis Specialty Sales Personnel, require that such Specialty Sales Personnel cease to promote the Product and, in all other cases, may terminate this Agreement immediately upon written notice to Vernalis.

          f. Vernalis shall ensure that its Specialty Sales Personnel are familiar with the procedures, obligations, rights, and responsibilities imposed by the terms of this Agreement as applicable to the performance of promotional activities hereunder. Vernalis shall at all times ensure that its Specialty Sales Personnel are providing Vernalis Detailing Services that are consistent with the marketing messages developed and communicated to Vernalis by Endo and shall use only Marketing Materials provided by Endo.

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          g. To avoid any doubt, nothing express or implied in this Section 3.1 (including without limitation Sections 3.1(a) and 3.1(b)) shall be read to limit in any way Vernalis’ rights under Section 6.2 of the License.

          h. To avoid any doubt, because Endo determines and provides the Marketing Materials for Product promotion in the USA, nothing express or implied in this Section 3.1 or elsewhere in this Agreement shall be read to make Vernalis liable for its Product promotion based on the content of the Marketing Materials provided by Endo; provided that Vernalis Specialty Sales Personnel are properly using and promoting such content in accordance with applicable laws and instructions from Endo.

     3.2 Vernalis Detail Reports . Vernalis shall provide Endo with a report (each a “Vernalis Detail Report”), within *** calendar days after the end of each calendar quarter during the Term (and within such period after the end of the Term as may be necessary to finalize this Agreement), setting forth the following information regarding the efforts of Vernalis Specialty Sales Personnel in promoting the Product during the preceding quarter (or part thereof): (i) the number of Product Details made and recorded by Vernalis standard record keeping procedures based on data recorded by the Specialty Sales Personnel and (ii) such other information as may be required in the then-current Annual Co-Promotion Plan or deemed appropriate by the Joint Co-Promotion Team (provided that Vernalis has in its discretion agreed to include such information required in such plan, but in any case data provided must be sufficient to allow Endo to meet its obligations to report detailing activity to Vernalis consistent with the format set forth in Schedule 5 of the License). Each such Vernalis Detail Report shall be in an electronic format and in hard copy form. An example of a Vernalis Detail Report is attached to this Agreement as Exhibit B. Each Vernalis Detail Report shall be treated as Confidential Information of Vernalis pursuant to Section 10 of this Agreement and shall not be disclosed to third parties without Vernalis’ prior written approval or direction. Vernalis shall use Commercially Reasonable Efforts to use sales force tracking and reporting systems that are complimentary with Endo’s sales force tracking and reporting systems.

     3.3 Vernalis Specialty Sales Personnel .

          a. Hiring . Vernalis shall be solely responsible for the recruitment and hiring of the Specialty Sales Personnel; provided that:

               (i) In recruiting its Specialty Sales Personnel Vernalis shall hire representatives who, at a minimum, have the qualifications of *** . Vernalis shall take into consideration Endo’s hiring profile for specialty sales representatives, which is attached hereto as Exhibit D, as may be subsequently amended,

               (ii) Incentive compensation for Product promotion for Vernalis Specialty Sales Personnel shall be based partly upon similar performance metrics as those which apply to Endo sales representatives for promotion of the Product, and

               (iii) Upon *** days written request from Vernalis, Endo shall implement a referral program to provide an incentive to its sales representatives to refer

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qualified candidates to Vernalis for the Specialty Sales Personnel positions and Vernalis shall reimburse Endo for all amounts paid to Endo sales representatives under such referral program.

          b. Costs .

               (i) Endo shall reimburse Vernalis quarterly the Cost Per Detail up to *** over the proposed number of Product Details provided by Vernalis under the terms of Section 3.1(d) above, but not to exceed the maximum number of Product Details set forth in the following table. Such reimbursement shall include reimbursement for all Product Details made in any year by Vernalis Specialty Sales Personnel to Target Healthcare Professionals not included in the allocated lists agreed pursuant to Section 3.3(c) (‘non-allocated Target Healthcare Professionals’); provided that at least *** of all Product Details made in any year by Vernalis Specialty Sales Personnel are made to Target Healthcare Professionals included in the allocated lists agreed pursuant to Section 3.3 (c) (‘allocated Target Healthcare Professionals’). If the proportion of Product Details made in any year by Vernalis Specialty Sales Personnel to allocated Target Health Care Professionals is less than *** of total Product Details, the number of reimbursed Product Details to non-allocated Target Health Care Professionals shall be adjusted such that the proportion of such Product Details is not in excess of *** of all Product Details to be reimbursed.

               (ii) For example, if Vernalis, according to Section 3.1 (d), has provided a proposed number of Product Details of *** and, subsequently, Vernalis Specialty Sales Personnel complete *** Product Details, *** of which are to allocated Target Healthcare Professionals, the following reimbursement calculation applies. First, the criterion that the proportion of Product Details to allocated Target Healthcare Professionals is at least *** is applied. In this example, this is calculated as follows: the number of Product Details to allocated Target Healthcare Professionals *** is divided by *** . As this number of Product Details is less than the maximum number of Product Details for which Endo is required to pay *** , Endo is therefore required to pay for *** Product Details. To the extent necessary, reimbursement payments will be reconciled on an annual basis.

               (iii) Endo shall make such reimbursement payment within *** days after receipt by Endo of the complete Vernalis Detail Report (which shall serve as an invoice, so long as it is so labeled). Endo shall pay such costs to Vernalis in US Dollars by wire transfer of immediately available funds to an account designated by Vernalis to Endo in writing. Endo shall have no obligation to reimburse the Cost Per Detail after *** . Vernalis may in its discretion elect to continue to Detail the Product in the USA at no cost to Endo after *** . All payments by Endo shall be made in accordance with Section 11.15 of the License.

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               (iv) Unless otherwise set forth in this Agreement, Vernalis shall be responsible for all other costs and expenses associated with its Specialty Sales Personnel including without limitation, salary, bonus, benefits, pension, insurance, social security, travel, entertainment, budgets and any other related obligations such as income tax withholding and all applicable reporting requirements. Sales representatives of either party shall not be eligible for awards, prizes, contests or other incentives offered by the other party to its sales representatives, unless otherwise agreed between the parties, in writing.

               (v) Vernalis shall be solely responsible for providing and financing fleet services to its Specialty Sales Personnel.

               (vi) In the instances where Endo allocates a designated promotional budget to individual sales representatives for the purposes of completing specific marketing-directed promotional activities, Endo shall provide such promotional budget to Vernalis Specialty Sales Personnel at the same time as provided to Endo sales representatives and as agreed by the Joint Co-Promotion Team.

          c. Allocation . Vernalis and Endo shall agree on the call planning and targeting of Specialty Sales Personnel; which shall be allocated per physician specialty as set forth in Exhibit C. Unless the parties otherwise agree, such allocation shall

               (i) allocate calls equitably between the parties’ respective sales representatives, including without limitation in relation to decile, geographic location and key opinion leaders; and

               (ii) specify a list of *** Target Healthcare Professionals (identifying from within that list *** Target Healthcare Professionals to be treated as priority for Product Details) available for each of the Vernalis Specialty Sales Personnel to undertake Product Details within their respective geographic locations.

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The parties recognize that some Target Healthcare Professionals may receive Product Details from both Endo sales representatives and Vernalis Specialty Sales Personnel; however the parties agree that the Vernalis Speciality Sales Personnel and Endo sales representatives shall not make Product Details to the same Target Healthcare Professionals in the Primary Care and *** fields.

          d. CSO . Vernalis may enlist the services of a contract sales organization (“CSO”) for the purpose of providing Product Details to Target Healthcare Professionals in the *** field only, in accordance with t