MANAGED CARE November 2006. ©MediMedia USA
The use of transgenic goats to produce a recombinant form of human antithrombin is much more efficient than using mammalian cell cultures
Few Americans have missed the white-mustached celebrities on billboards and television with the catchy phrase "Got Milk." Another series of commercials shows "happy cows" being used to produce milk for cheese. The milk producers and processors have certainly marketed the benefits of milk to our society. But these ads fail to mention a much more interesting technology that uses milk as a source of therapy for human ailments such as hereditary antithrombin (AT) deficiency.
Milk-based therapy
In August 2006, the first milk-based therapy for hereditary AT deficiency was approved by the European Commission. ATryn, a recombinant form of human antithrombin (termed rhAT), is manufactured by GTC Biotherapeutics under a collaborative agreement with LEO Pharma A/S.
This form of antithrombin is derived from genetically-modified goats that secrete the drug in their milk, thus avoiding the very complex and expensive bioreactor methods required for biotechnology drug production. The active protein is secreted in 10-fold higher concentrations compared to the concentrations produced in highly optimized cell cultures typical of those used in other biotechnology processes.
ATryn will soon be available in Europe and is in phase 3 clinical trials in the United States. It has an amino acid sequence that is structurally indistinguishable from human plasma AT.
This transgenic product is considered too complex and large to be created in the Chinese hamster ovary cell line — the common mammalian cell line used for other complex biotechnology products currently available in the United States. Less complex biotechnology molecules such as human insulin and growth hormone utilize bacteria or yeast as the preferred biologic factory.
The transgenic goats used for AT production were developed 15 years ago. The AT gene is isolated on chromosome 1, linked to a milk-specific promoter, and introduced into early-stage goat embryos that are transferred to surrogate goat mothers. After the goat is born, the presence of the transgene is verified.
The animals are bred repeatedly to create a herd of transgenic goats specific for rhAT production. Only goats that have passed the following criteria are used for drug production: stable transgene integration, suitable transmission of the transgene to progeny, commercially feasible protein levels in milk, and overall good health. Lactation can occur post-birth or be induced hormonally in immature female or male goats. This process takes years to build a viable herd, but once established, it is scalable in a much less expensive way than building large bioreactor plants that are the norm for biotechnology products.
According to GTC, using transgenic drug production using live animals is much more efficient than using cell cultures. Goats generate 2 to 10 grams of recombinant protein per liter of milk, are capable of producing up to 800 gallons of milk per year, and can easily be bred to increase the herd size and subsequent drug yield. They are also theoretically capable of manufacturing much more complex proteins than the typical cell culture process.
Hereditary antithrombin deficiency
Hereditary antithrombin deficiency, historically called hereditary antithrombin III deficiency, was first described by Olav Egeberg, a Norwegian hematologist, in 1965 after he observed several members in a family who had high rates of venous thrombosis. He discovered antithrombin levels 40 percent–50 percent lower than those of nonaffected family members. Since then, genetic and traditional medical research has led us to an understanding of two different classifications of AT deficiency — type I and type II.
Patients with type I hereditary antithrombin deficiency have less AT because of a mutation of the portion of the AT gene that controls production of AT. Patients with type II deficiency have mutations in the gene that codes for the protein chain. This leads to a dysfunctional AT molecule. There are several known mutations that can cause dysfunctional AT.
Hereditary AT deficiency has a prevalence of about 1 per 2,000 to 5,000 live births and an incidence of about 0.2 percent–0.4 percent. The deficiency is autosomal dominant (occurs on a non-sex-determining chromosome) and affects men and women equally.
Hereditary AT deficiency is associated with spontaneous episodes of thrombosis and pulmonary embolism, usually appearing after the age of 20. Its incidence increases with age, surgery, pregnancy, and other conditions. By the age of 50, more than 85 percent of individuals with hereditary AT deficiency have had a significant thrombotic event. Complete deficiency of AT activity is thought to be incompatible with life.
There are also acquired forms of AT deficiency secondary to extensive burns, sepsis, malignancies, DIC (disseminated intravascular coagulation), preterm birth, acute respiratory distress syndrome, pregnancy, liver disease, and nephritic syndrome.
Heparin
Antithrombin is a single-chain glycoprotein composed of 432 amino acids. AT basically balances the clotting cascade: In its absence, clotting goes on unchecked. AT inactivates a number of the clotting factors after they have been activated by the clotting cascade. AT binds to these factors on a one-to-one basis in an accelerated process in the presence of heparin.
Treatment options for patients with hereditary AT deficiency must take into account the number and types of venous thrombotic events, age, and plasma AT activity. Besides traditional anticoagulants such as heparin, LMWH, and warfarin, current therapies in the United States include replacement of purified AT derived from pooled human plasma (termed hpAT and available as Thrombate).
Research and development over several decades continue to yield therapies for rare and not-so-rare diseases. We are now seeing that the traditional bioreactor is not the only way to create viable human therapies.
Live-animal drug manufacturing processes, previously limited to hormonal production from pregnant mare urine and a few forms of antitoxin (tetanus, botulism, and diphtheria, for example) may soon be supplemented by customized genetically-modified herds providing an ever-increasing array of complex "human" proteins for human ailments. If this promise is fulfilled with FDA approval, the phrase "Got Milk" may take on a whole new meaning in the world of Tomorrow's Medicine!