DX-88 (ecallantide) Demonstrated Relief of Symptoms in Hereditary Angioedema Acute Attacks in All Attack Locations

An analysis demonstrating the ability of DX-88 (ecallantide) to resolve symptoms of acute hereditary angioedema (HAE) attacks in all anatomic locations and to sustain effect will be presented in an oral presentation this week at the American College of Allergy, Asthma and Immunology (ACAAI) 2009 Annual Meeting in Miami Beach. This evaluation, and two others to be presented, highlight efficacy and safety analyses from the integrated analysis of the two Phase 3 placebo-controlled trials, EDEMA3^® and EDEMA4^®, of DX-88, the lead product candidate of Dyax Corp. (NASDAQ:DYAX), for the treatment of acute attacks of HAE.

In the time to response to treatment analysis, a statistically significant effect over placebo was achieved with DX-88 for two key endpoints: time to sustained improvement (improvement that endured for at least 45 minutes within 4 hours of dosing) (p=0.005) and time to significant improvement (defined as the first time within 4 hours of dosing that a patient reported feeling "a little better” or "a lot better or resolved;” indicates complete or near complete resolution) (p=0.015).

The time of response of the clinical effect appeared to vary with location of the angioedema attack. Abdominal attacks, typically the most painful, and laryngeal attacks, which can be potentially life-threatening, improved more rapidly than peripheral attacks, demonstrating a significant benefit of DX-88 over placebo within 4 hours. The therapeutic effect of DX-88 in peripheral attacks was apparent by 24 hours.

"Positive findings from the clinically stringent hurdle of complete or near complete resolution of an HAE attack provide encouraging evidence that treatment with DX-88 can potentially offer patients a quick and durable treatment for what is often a debilitating attack that can last for several days if untreated,” commented Marc Riedl, MD, MS, Assistant Professor of Clinical Immunology and Allergy at the David Geffen School of Medicine at UCLA in Los Angeles. "Of even greater importance is that a rapid response was seen for laryngeal attacks, which can be life-threatening.”

A second oral presentation provides data on the pediatric experience of DX-88 for treating acute attacks of HAE. A total of 40 acute HAE attacks were treated with subcutaneously-administered DX-88 in 17 pediatric patients. For these patients, individual attack data were compared to the outcomes for the population receiving the corresponding DX-88 dose in the same study (called the reference group).

Symptom improvement, as measured by change in two patient-reported outcomes measures – Mean Symptom Complex Severity (MSCS) score and Treatment Outcome Score (TOS) – were better than or equal to that for the reference group in 75% and 78%, respectively, of acute attacks in pediatric patients four hours after treatment with DX-88. Time to onset of improvement (median: 37 minutes) and time to near complete resolution (median: 82.5 minutes) were shorter than or equal to that for the reference group in 58% and 63%, respectively, of acute attacks. There was a low rate of treatment-related adverse events in pediatric patients treated with 30 mg subcutaneous DX-88. Although hypersensitivity was observed in two pediatric patients treated with intravenous DX-88, no hypersensitivity was observed in the pediatric population following subcutaneous dosing. The efficacy and safety profiles for pediatric patients appeared similar to that of the overall treatment group.

Complete List of Presentations on DX-88 at ACAAI 2009

Oral Presentations

• Time to Response with Ecallantide for the Treatment of Acute Attacks of Hereditary Angioedema: Results from the EDEMA Development Program – Marc Riedl, MD, MS, David Geffen School of Medicine at UCLA – Monday, November 9, 2009, 1:00 PM
• Ecallantide for the Treatment of Acute Attacks of Hereditary Angioedema in Pediatric Patients: Experience in the EDEMA Development Program – Andrew J. MacGinnitie, MD, PhD, Children’s Hospital of Pittsburgh of UPMC – Monday, November 9, 2009, 1:45 PM

Poster Presentations

• Time of Intervention with Ecallantide for the Treatment of Acute Attacks of Hereditary Angioedema: Results from the EDEMA Development Program – Erin Banta, MD, Milton S. Hershey Medical Center, Hershey, PA – Saturday, November 7, 2009 and Sunday, November 8, 2009
• Successful Use of Ecallantide After an Anaphylactoid Reaction – Henry Li, MD, PhD, FAAAAI, Institute for Asthma and Allergy at Wheaton and Chevy Chase, MD – Saturday, November 7, 2009 and Sunday, November 8, 2009

About DX-88 for HAE

DX-88, a recombinant, small protein, was discovered utilizing Dyax’s proprietary phage display technology and is being evaluated as a subcutaneous therapy for treating acute HAE attacks. DX-88 is a potent and selective plasma kallikrein inhibitor, a key mediator of inflammation in angioedema, and has demonstrated effectiveness in all attack locations, including life-threatening laryngeal attacks. DX-88 has been evaluated in five clinical trials for HAE, including two Phase 3 placebo-controlled trials (EDEMA3 and EDEMA4), which represent the largest placebo-controlled evaluation for this indication. A continuation trial is ongoing.

About HAE

Hereditary angioedema (HAE) is an acute inflammatory condition characterized by episodes of severe, often painful swelling affecting the extremities, the gastrointestinal tract, the genitalia, and in potentially life-threatening cases, the larynx. HAE is caused by low or dysfunctional levels of C1 esterase inhibitor (C1-INH), a naturally occurring molecule that inhibits plasma kallikrein and other serine proteases in the blood.

About Dyax

Dyax is focused on advancing novel biotherapeutics for unmet medical needs, with an emphasis on inflammatory and oncology indications. Dyax utilizes its proprietary drug discovery technology to identify antibody, small protein and peptide compounds for clinical development. Dyax’s lead product candidate is DX-88, a recombinant, small protein that is currently being evaluated for its therapeutic potential in three separate indications. On June 1, 2009, Dyax submitted a response to the FDA’s Complete Response letter regarding the review of Dyax’s Biologics License Application (BLA) of DX-88 for the treatment of hereditary angioedema (HAE). The FDA accepted the submission and assigned Dyax’s BLA a new Prescription Drug User Fee Act (PDUFA) action date of December 1, 2009. DX-88 has orphan drug designation in the U.S. and E.U., as well as Fast Track designation in the U.S., for this indication. Additionally, DX-88 is being evaluated in two Phase 2 trials for the reduction of blood loss during on-pump cardiothoracic surgery (CTS), which are being conducted by Dyax’s partner, Cubist Pharmaceuticals. Dyax licensed to Cubist the intravenous formulation of DX-88 for surgical indications in North America and Europe. DX-88 is also being evaluated in a Phase 1 trial for retinal vein occlusion-induced macular edema by Dyax’s partner, Fovea Pharmaceuticals, which has recently entered into an agreement to be acquired by sanofi-aventis. Dyax licensed to Fovea the ocular formulation of DX-88 for the treatment of retinal diseases in the EU but retains all rights outside of the EU.

DX-88 and other compounds in Dyax’s pipeline were identified using its patented phage display technology, which rapidly selects compounds that bind with high affinity and specificity to therapeutic targets. Dyax leverages this technology broadly with over 70 revenue generating licenses and collaborations for therapeutic discovery, as well as in non-core areas such as affinity separations, diagnostic imaging, and research reagents. Dyax is headquartered in Cambridge, Massachusetts. For online information about Dyax Corp., please visit www.dyax.com.

Dyax Disclaimer

This press release contains forward-looking statements, including statements regarding the prospects for therapeutic benefits and treatment advantages of DX-88 for HAE. Statements that are not historical facts are based on Dyax’s current expectations, beliefs, assumptions, estimates, forecasts and projections about the industry and markets in which Dyax competes. The statements contained in this release are not guarantees of future performance and involve certain risks, uncertainties and assumptions, which are difficult to predict. Therefore, actual outcomes and results may differ materially from what is expressed in such forward-looking statements. Important factors which may affect the prospects for therapeutic benefits and treatment advantages of DX-88 for HAE and include the risks that: DX-88 could take a significantly longer time to gain regulatory approval than Dyax expects or may never gain approval; others may develop technologies or products superior to DX-88 or that are on the market before DX-88; DX-88 may not gain market acceptance; Dyax is dependent on the expertise, effort, priorities and contractual obligations of third parties in the manufacture, marketing, sales and distribution of DX-88 ; and other risk factors described or referred to Item 1A, "Risk Factors” in Dyax’s most recent Annual Report on Form 10-K and other periodic reports filed with the Securities and Exchange Commission. Dyax cautions investors not to place undue reliance on the forward-looking statements contained in this release. These statements speak only as of the date of this release, and Dyax undertakes no obligations to update or revise these statements, except as may be required by law.

Dyax and the Dyax logo are registered trademarks of Dyax Corp. EDEMA3 and EDEMA4 are registered service marks of Dyax.