This article reviews the reasons why transgenic milk is a cost-effective system. Also reviewed is the earlier research on targeting heterologous proteins to the mammary glands of many different animals. The final section describes the process by which goats express rhAT in their milk at approximately 2 g/L. The human AT purified from milk is structurally indistinguishable from human plasma–derived AT with the exception of carbohydrates. Clinical studies are ongoing on the prevention of deep-vein thrombosis.
LOW- VOLUME, NON-MILK TRANSGENIC PROTEINS
The first microbial bioreactors, in particular Escherichia coli and Saccharomyces cerevisiae, were found to be satisfactory for the production of simple polypeptides such as insulin and human growth hormone. However, microbial bioreactors were found to be unsuitable for proteins with complex post-translational modifications or intricate folding requirements, such as the coagulation factors, or monoclonal antibodies. This led to the development of large-scale mammalian cell culture, for example, the use of Chinese Hamster Ovary (CHO) cell bioreactors.
These technologies permitted the development of numerous monoclonal antibodies, cytokines, and other complex bioactive biomolecules. However, there are proteins that, due to a combination of complex structure and large therapeutic dosing, have until now eluded recombinant production using traditional bacterial and cell culture bioreactors. For example, commercial recombinant production of complex molecules, such as antithrombin and alpha1-antitrypsin, has not yet been achieved in microbial or mammalian cell derived bioreactors. The only source is human plasma because of the high dose needed.
Even human serum albumin, the therapeutic protein used in largest amounts (>400 tons, worldwide), the use of the recombinant form, produced in Saccharomyces cerevisiae, is limited to excipient applications. These are within the practical production capacity of this system but far too small for high-volume therapeutic indication (volume replacement).
Capital investments in production plants represent a significant portion of the development cost of new recombinant drugs. Also, the inherent risk associated with the regulatory approval process is a stimulus for the development of flexible and inexpensive approaches for the manufacture of therapeutic proteins. Milk-specific production offers a way to lessen the bite.