Several independent laboratories have determined that differences in glycosylation of AT do not affect the intrinsic rate
constant of the uncatalyzed or heparin catalyzed inhibition of thrombin, indicating that the carbohydrate chains solely affect
heparin binding and not heparin activation or proteinase binding functions. Thus, glycosylation does not impact the major
biological activity of AT, which is thrombin inhibition, but explains the differences in affinity for heparin and in pharmacokinetics.
The manufacturing process for rhAT has been validated for its viral and prion removal capacity. The rhAT viral validation
studies demonstrated that a significant virus reduction of >8.5 to >25.3 log10 (roughly 300 million fold to septillion fold) was accomplished across the distinctly different modes of the rhAT process.19 All GTC goats are certified free of scrapie in the United States Department of Agriculture (USDA) Scrapie Flock Certification
Program and various risk minimization measures have been instituted to protect this highly controlled closed donor goat population.
The rhAT purification process has been validated for its ability to reduce scrapie contamination over a 100 billion fold (>11.3
log10 scrapie removal).
Ten human clinical studies have been undertaken with rhAT, and several other studies are ongoing. Two clinical indications
- Heparin resistance in patients undergoing cardiac surgery involving CPB20,21
- Prevention of deep vein thrombosis (DVT) in patients who have a hereditary deficiency of AT and who are in high risk situations
such as delivery or surgery 22-24
In all the human studies completed, rhAT has not generated significant adverse events and has met the primary clinical endpoints.
All studies demonstrated that rhAT was well tolerated in these patient populations.
A European regulatory filing was submitted in January 2004, for the use of rhAT in the prophylaxis of DVT in Hereditary AT
deficient patients in a high-risk situation. On June 2, 2006 the CHMP of the EMEA adopted a positive opinion on the market
authorization application for Atryn.25 The CHMP has recommended that ATryn be granted market authorization for the prophylaxis of venous thromboembolism in surgery
of patients with congenital antithrombin deficiency. ATryn may be given in association with heparin or low molecular weight
heparin in these situations.
Upon approval, expected about three months after the positive opinion, ATryn will be the first antithrombin product approved
for use in all 25 countries of the European Union. ATryn will also be the only available recombinant antithrombin product
that is not derived from the human blood supply. Furthermore, GTC Biotherapeutics anticipates using the results from both
the completed study reviewed by the CHMP and an ongoing pivotal Phase 3 study to prepare a Biologics License Application for
the FDA. The results of the pivotal Phase 3 study will also be submitted for consideration by the CHMP for expansion of the
use of ATryn in Europe to prevent deep vein thromboses and thromboembolisms in women with hereditary antithrombin deficiencies
who are undergoing childbirth.
The development of a recombinant option for antithrombin will provide a safe and reliable supply of this important factor
and will facilitate the resumption of clinical trials aimed at acquired deficiencies of antithrombin, such as cardiovascular
surgery, severe burns, and severe sepsis. For example, LEO Pharma, in partnership with GTC Biotherapeutics, has also begun
development of ATryn in Europe as a potential treatment for disseminated intravascular coagulation, or DIC, associated with
severe sepsis. DIC occurs in an estimated 220,000 severe sepsis cases in the European Union each year. Approximately 50% are
fatal, representing a major unmet medical need of significant interest in critical care. Fulfilling the AT needs associated
with the DIC clinical indication, if these development activities lead to an approval, would require several hundred kilograms
of purified rhAT. This can be easily accomplished by scaling-up the existing herd of rhAT-producing transgenic goats.
Other recombinant proteins expressed in the milk of transgenic animals are currently in development, including human albumin,
human growth hormone, C1-esterase inhibitor, alpha1-antitrypsin, as well as monoclonal antibodies. It is likely that in the
coming years several regulatory filings in various jurisdictions will be submitted with some of these products, realizing
the promise of transgenic technology in offering a safe cost-efficient alternative for the production of complex recombinant