Antithrombin alfa (AT alfa or recombinant human AT) became the first approved medicine produced in the milk of transgenic goats, when EMEA granted it marketing authorization in 2006 for the prophylaxis of venous thromboembolism in patients with congenital AT deficiency undergoing surgery. AT alfa has been evaluated in 10 clinical studies in support of hereditary and acquired AT deficiency indications. In all studies, patient serum samples were collected prior to treatment with AT alfa and at various times after treatment (depending on the study up to 90 days). Patient samples were also collected to measure baseline and post treatment AT activity levels to assess the development of possible neutralizing antibodies to AT. All assays used to screen for potential patient immune responses were validated prior to their use in these analyses. In all studies, pre- and post-treatment patient samples were assessed for development of IgG antibodies to AT alfa by ELISA and confirmatory radioimmunoprecipitation assay. Additionally, evaluations were performed for IgM antibodies to AT alfa (ELISA) and for antibodies to potential contaminating goat milk proteins including goat antithrombin (validated immunodot blots) in the studies supporting the hereditary deficiency indication. In the clinical studies, 211 individuals have been treated with AT alfa on one (173) or two (38) occasions. Dosing was done by single injection, multiple daily injections or continuous infusion. These individuals were healthy volunteers or subjects with hereditary or acquired AT deficiencies. There were no clinical immunological reactions noted in the clinical trials. None of the treated subjects developed an IgG or IgM antibody response to AT alfa nor showed any evidence on immunodot blots of the induction of antibodies to potential contaminating goat milk proteins, including goat antithrombin. In addition, upon follow-up, all subjects returned to their functional baseline AT levels suggesting that no neutralizing antibodies to AT had developed in treated individuals. These findings suggest that AT alfa, as it was administered in these clinical trials, is immunologically tolerated. Since, except for some rare type II hereditary deficiencies, patients are heterozygous for their AT mutations, typical hereditary AT deficient patients have approximately 50% normal functional AT and AT alfa appears not to be considered foreign. This is a major distinction as compared to for example F-VIII deficiency. Although AT alfa has incomplete glycosylation, the glycosylation intermediates are the same as found in humans. Finally, AT is a molecule that undergoes significant conformational changes (exposing previously buried epitopes) during its lifetime in the circulation, which might also contribute to the lack of immunogenicity found in these clinical studies. However, to obtain more data on more patients, immuno-monitoring continues to be done in ongoing studies with AT alfa, as well as part of a post-marketing surveillance study in Europe.

Author notes

Disclosure:Employment: Ziomek, Masiello, Peters, Bonfiglio, Chen, and Meade are employees from GTC Biotherapeutics, the manufacturer of antithrombin alfa. Consultancy: Theobald, Richards, and Welch are employees from Genzyme Corp, who consulted in the development of antithrombin alfa. Frieling is a consultant for GTC Biotherapeutics. Ownership Interests:; Employees may have ownership interest in their own and/or other company.