GlaxoSmithKline and Myogen Announce Pulmonary Arterial Hypertension Partnership; GlaxoSmithKline to Commercialize Ambrisentan in All Territories Outside of the U.S.

GlaxoSmithKline (LSE: GSK) (NYSE: GSK) and Myogen, Inc. (Nasdaq:MYOG) today announced a two-part collaboration in Pulmonary ArterialHypertension (PAH). Myogen licensed commercialization rights forambrisentan, Myogen's selective endothelin receptor antagonist (ERA)currently in Phase 3 development, to GlaxoSmithKline in allterritories outside of the United States. Filing for marketingapproval in the U.S. and Europe is expected later this year.Simultaneously, GlaxoSmithKline and Myogen entered into a distributionagreement whereby Myogen will be responsible for the marketing anddistribution of GSK's Flolan (epoprostenol sodium) a life-savingmedicine for many patients, used in the treatment of PAH, in theUnited States.

"We are pleased to be in this alliance with Myogen and view thiscollaboration as a continuation of our corporate pipeline expansion,"said Andrew Witty, President, Pharma Europe GlaxoSmithKline. "Myogen'sinnovative approach to collaboration on ambrisentan has provided bothcompanies with a potentially rewarding opportunity by givingGlaxoSmithKline access to a product candidate in an indication we knowvery well. At the same time, Myogen will be able to establish acommercial presence in the PAH market in the United States."

"I believe that GlaxoSmithKline, one of the premier pharmaceuticalcompanies in the world, is the ideal ex-U.S. partner for ambrisentan,"said J. William Freytag, President and Chief Executive Officer ofMyogen. "They have been a pioneer in the treatment of PAH and, throughtheir decade-long experience with Flolan, have a deep understanding ofthe PAH regulatory and market environments. Meanwhile, the Flolandistribution agreement is expected to underwrite the development ofour own commercial organization and marketing and field sellingexpertise in PAH, well in advance of the potential launch ofambrisentan. We believe this strategic development will accelerate ourunderstanding of customer needs, reimbursement opportunities andmarket dynamics in general."

Under the terms of the ambrisentan license agreement, Myogen willreceive an upfront payment of $20 million and, subject to theachievement of specific milestones, will be eligible to receive up toan additional $80 million in milestone payments. In addition, Myogenwill also receive stepped royalties on product sales with an estimatedaverage royalty in the mid-20% range, which reflect the late stagedevelopment status and market potential of ambrisentan. Myogen will beresponsible for the continued clinical development of ambrisentan.GlaxoSmithKline will be responsible for all regulatory and commercialexpenses in its licensed territories. The companies will share thecosts of certain additional clinical development activities forambrisentan.

Under the terms of the Flolan distribution agreement, Myogen willbuild a commercial support team and field sales organization beginningin the second quarter of 2006 dedicated to the marketing anddistribution of Flolan in the United States. The distributionagreement is a three-year agreement with an option to renew uponmutual consent.

About Pulmonary Arterial Hypertension

PAH is a highly debilitating disease characterized by constrictionof the blood vessels in the lungs, thereby increasing resistance toblood flow in the pulmonary tissues. The increased resistance makes itdifficult for the heart to pump blood through the lungs to beoxygenated. Under this additional strain the heart responds by pumpingharder and the pressure in the pulmonary artery rises, sometimes tovery high levels.

Patients with PAH suffer from increasing shortness of breath asthe heart has to work harder to pump against the elevated resistancefrom the blood vessels in the lungs. Left untreated, patients undergoan inexorable and rapid decline and ultimately may die of heartfailure. PAH may occur without an apparent underlying cause, or it canoccur secondary to diseases such as connective tissue disease,congenital heart defects, cirrhosis of the liver and HIV infection.PAH afflicts approximately 200,000 patients worldwide.

About Ambrisentan

Ambrisentan is an investigational drug being developed as aonce-daily oral therapy for patients with PAH and has been grantedorphan drug designation for the treatment of PAH in both the UnitedStates and European Union. In December, Myogen announced positivetop-line results from the ARIES-2 trial, the first pivotal trialevaluating ambrisentan in PAH. Top-line results of a second importanttrial (ARIES-1) are expected to be announced in April 2006, with FDAand EMEA filing targeted for Q4 this year.

Ambrisentan is a non-sulfonamide, propanoic acid-class, type-Aselective endothelin receptor antagonist. Endothelin is a smallpeptide hormone that plays a critical role in the control of bloodflow and cell growth. Elevated endothelin blood levels are associatedwith several cardiovascular disease conditions, including pulmonaryarterial hypertension, chronic renal disease, coronary artery disease,hypertension and chronic heart failure. Agents that block thedetrimental effects of endothelin may provide benefits in thetreatment of these conditions.

Myogen and GSK believe ambrisentan, which is being developed forless severely ill PAH patients, would be complementary to Flolan whichis usually used in later stages of PAH disease.

About Flolan (epoprostenol sodium)

Flolan was approved by the FDA in 1995 and is indicated for thelong-term intravenous treatment of primary pulmonary hypertension andpulmonary hypertension associated with the scleroderma spectrum ofdisease in NYHA Class III and Class IV patients who do not respondadequately to conventional therapy. Use of Flolan is contraindicatedin patients with congestive heart failure due to severe leftventricular systolic dysfunction. Flolan should not be used inpatients who develop pulmonary edema during dose initiation. Flolan isalso contraindicated in patients with known hypersensitivity to thedrug or structurally related compounds. Flolan should be used only byclinicians experienced in the diagnosis and treatment of pulmonaryhypertension. The diagnosis of PPH or PH/SSD should be carefullyestablished. Please consult complete prescribing information forFlolan at www.gsk.com.

About GlaxoSmithKline

GlaxoSmithKline is one of the world's leading research-basedpharmaceutical and healthcare companies. GlaxoSmithKline is committedto improving the quality of human life by enabling people to do more,feel better and live longer. For company information, visitwww.gsk.com.

About Myogen

Myogen has two product candidates in late-stage clinicaldevelopment: ambrisentan for the treatment of patients with pulmonaryarterial hypertension (PAH) and darusentan for the treatment ofpatients with resistant hypertension. The Company, in collaborationwith Novartis, also conducts a target and drug discovery researchprogram focused on the development of disease-modifying drugs for thetreatment of chronic heart failure and related cardiovasculardisorders. Please visit the company's website at www.myogen.com.

Safe Harbor Statement

This press release contains forward-looking statements thatinvolve significant risks and uncertainties, including the statementsrelating to the future potential of the partnership, ambrisentandevelopment and regulatory approval and projections regarding expectedaverage royalties payable by GSK to Myogen, the potential benefits toMyogen of the Flolan distribution agreement and the expected timing ofcommencement of commercial operations by Myogen. Actual results andevents could differ materially from those projected and investorsshould not place undue reliance on the forward-looking statementscontained in this release.

Among other things, the potential approval and launch ofambrisentan in the United States and abroad, the average royaltiespayable by GSK to Myogen under the ambrisentan partnership agreement,the transition of Flolan distribution from GSK to Myogen and theprojected commencement of Myogen commercial operations, may beaffected by difficulties or delays, including difficulties or delayscaused by regulatory issues, manufacturing issues, hiring and employeeretention issues, competition from other biotechnology andpharmaceutical companies and future clinical trial results, includingMyogen's ARIES-1 trial of ambrisentan in patients with PAH. IfMyogen's product candidates do not meet safety or efficacy endpointsin clinical evaluations, they will not receive regulatory approval andMyogen and its sublicenses will not be able to market them. Even ifthe Myogen's product candidates meet safety and efficacy endpoints,regulatory authorities may not approve them, or Myogen or itssublicenses may face post-approval problems that require thewithdrawal of its product from the market. If Myogen is unable toraise additional capital when required or on acceptable terms, it mayhave to significantly delay, scale back or discontinue one or more ofits drug development or discovery research programs. Myogen is at anearly stage of development and may not ever have any products thatgenerate significant revenue.

Additional risks and uncertainties relating to Myogen and itsbusiness can be found in the "Risk Factors" section of Myogen's Form10-K for the year ended December 31, 2004, in Myogen's reports on Form10-Q and Form 8-K and in other documents filed by Myogen with theSecurities and Exchange Commission (SEC). It is Myogen's policy toonly update or confirm its public guidance by issuing a press releaseor filing a periodic or current report with the SEC. Myogen generallyplans to provide guidance as part of its annual and quarterly earningsreleases but reserves the right to provide guidance at differentintervals or to revise its practice in future periods. Myogenundertakes no duty or obligation to update any forward-lookingstatements contained in this release as a result of new information,future events or changes in expectations.