GTC Biotherapeutics Presents ATryn Safety and Efficacy Data.

Business Editors/Health/Medical Writers

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FRAMINGHAM, Mass. & HAMBURG, Germany--(BUSINESS WIRE)--Feb. 27, 2004

GTC Biotherapeutics, Inc. ("GTC", Nasdaq: GTCB) presented today, at the German Society of Thrombosis and Haemostasis Research meeting in Hamburg, Germany, safety and efficacy data from its recently completed multinational trial of ATryn(R) (recombinant human antithrombin) in the treatment of patients with hereditary antithrombin deficiency (HD) undergoing high risk procedures such as surgery and childbirth. Presentations were made on GTC's behalf by Dr. Mario von Depka of the Medizinische Hochschule Hannover, an investigator in the trial, and Dr. Johan Frieling of Genzyme Europe B.V., the trial's Medical Monitor. In the trial, treatment with ATryn(R) was found to be efficacious in preventing the development of symptomatic deep vein thromboses (DVT) and other thromboembolic complications which may occur in patients with confirmed hereditary antithrombin deficiency who are undergoing high risk surgical procedures or labor and delivery.

"We believe that ATryn(R) will be found to be an effective treatment for the prevention of peri- and post-surgical thromboembolic complications in patients with hereditary antithrombin deficiency," stated Johan Frieling, M.D., Ph.D, Associate Director of Clinical Research at Genzyme Europe B.V. in The Netherlands. Genzyme Europe B.V. collaborates with GTC on the clinical studies and regulatory submission for ATryn(R).

The clinical study was conducted in accordance with scientific advice received from the European Medicines Evaluation Agency (EMEA) and evaluated the safety and efficacy of ATryn(R) in HD patients in clinical high risk situations. Based on this and prior studies, GTC recently submitted a Market Authorization Application (MAA) for ATryn(R) to the EMEA which has been accepted for review. The safety and efficacy study enrolled 14 patients from Europe and the United States. The high risk situations treated included five surgeries and nine childbirths. The surgeries included four hip replacement procedures. The childbirths included natural, induced and Cesarean section procedures. The study used serial Duplex ultrasound scans of the lower extremities to monitor for the development of DVTs.

ATryn(R) was well tolerated and any adverse events that occurred were consistent with the surgical or childbirth procedures performed.

There were no clinical signs or symptoms of DVTs in any of the patients treated with ATryn(R). The central blinded review of the serial Duplex ultrasound examinations performed in all patients, regardless of the presence or absence of symptoms of DVT, indicated a thrombosis in two patients. The first was discovered by the central blinded review of the Duplex ultrasound data that was taken as scheduled seven days after cessation of ATryn(R) treatment, while local review at the time of the ultrasound was assessed negative. Patient management was therefore not changed and the patient remained without symptoms. The second was observed by scheduled Duplex ultrasound on the intended stop date, day 13, of a course of treatment with ATryn(R) as part of a hip replacement procedure. There were no clinical symptoms of the DVT at any time and it was resolved without complication through continuing treatment, including continuing administration of ATryn(R). It is of note that DVTs are found by screening in about 16% to 30% of hip replacement surgeries involving patients with normal coagulation characteristics who have been treated with effective prophylaxis.

Other diagnostic techniques were employed, as clinically appropriate, to monitor for the development of thromboemboli other than DVT. No thromboemboli were observed.

Antibodies formed against ATryn(R) were not detected in the first ten patients in this study. Antibody analysis of the other four patients is pending.

Patients with HD have an increased risk for the development of DVTs or thromboemboli, especially during high-risk medical procedures. These patients are normally treated with an orally available anticoagulant. However, this maintenance therapy often must be discontinued during events such as childbirth or surgery to reduce the risks of excessive bleeding or because these anticoagulants are contraindicated. Currently, human plasma derived antithrombin is often given to cover the period of high risk for the development of DVT.

Commercially available antithrombin is a blood protein with anticoagulant and anti-inflammatory properties that is purified from donors of human source plasma. GTC produces and purifies its recombinant version of this protein from the milk of goats that have the human antithrombin gene linked to a milk-protein promoter. No human blood products are added to ATryn(R).

GTC is hosting a conference call with webcast to review the clinical and regulatory progress of ATryn(R) as well as its 2003 financial results on Wednesday, March 3, at 10:00 a.m., Eastern Time. Dr. Frieling and Richard Scotland, GTC's Vice President of Regulatory Affairs, will participate in this conference call. The dial-in number from inside the United States is 1-800-901-5213. The dial-in number from outside the United States is 1-617-786-2962. The webcast may be found at www.gtc-bio.com.

GTC Biotherapeutics is a leader in the development, production, and commercialization of therapeutic proteins through transgenic animal technology. GTC has three internal proprietary products in its pipeline and a portfolio of external program production opportunities. In addition to the ATryn(R) program, GTC is developing a recombinant human serum albumin and a malaria vaccine. In its external programs, GTC's technology is used to develop transgenic production of its partners' proprietary products, including both large-volume protein therapeutics as well as products that are difficult to produce in significant quantities from conventional bioreactor systems. GTC's external program collaborations are developing transgenic versions of products such as monoclonal antibodies and immunoglobulin fusion proteins for conditions such as rheumatoid arthritis, HIV/AIDS and cancer. One of the external programs recently entered clinical trials with a transgenically produced product. Additional information is available on the GTC web site, http://www.gtc-bio.com.

This press release contains forward-looking statements as defined in the Private Securities Litigation Reform Act of 1995, including without limitation statements regarding the efficacy of ATryn(R) and the expected outcomes of regulatory submissions for the ATryn(R) program. Such forward-looking statements are subject to a number of risks, uncertainties and other factors that could cause actual results to differ materially from future results expressed or implied by such statements. Factors that may cause such differences include, but are not limited to, the risks and uncertainties discussed in GTC's most recent Annual Report on Form 10-K and its other periodic reports as filed with the Securities and Exchange Commission, including the uncertainties associated with conducting clinical studies, and the risks and uncertainties associated with dependence upon the actions of regulatory agencies. GTC 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 document, and GTC undertakes no obligation to update or revise the statements, except as may be required by law.