Transgenic technology in the production of therapeutic proteins

Transcription

1 Transgenic technology in the production of therapeutic proteins Transgenic technology represents a new generation of biopharmaceutical production system to meet the medical needs of the new millennium. Dr Patricia F Dimond, Genzyme Transgenics Corporation Transgenic technology enables the high level expression of recombinant human therapeutic proteins in the milk of transgenic animals. The technology has the potential to transform the way in which biopharmaceuticals are produced by providing a robust, highly efficient process that substantially lowers capital expenditure and production costs. In particular, the technology enables the economical, high-volume production of proteins such as monoclonal antibodies - therapeutics that may be required in relatively large and/or repeated doses for chronic illnesses such as cancer and autoimmune diseases. Specifically, the recent FDA approvals of antibodies as human therapeutics is driving demand for production systems that can cost-effectively deliver these molecules at the 100 kilogram per year scale or greater. Benefits of transgenic production The production of recombinant human proteins in the milk of transgenic animals presents several advantages over mammalian cell culture for proteins required at high levels. Expression levels in the milk of transgenic animals are typically fold higher than those in Chinese Hamster Ovary (CHO) cell culture systems; for example, Genzyme Transgenics Corporation (GTC) has demonstrated expression of recombinant antibodies at levels between 1g/L and 10g/L in transgenic mice and goats, compared with less than 0.1g/L to 1g/L for CHO cell cultures. Transgenically produced recombinant proteins have the same amino acid sequence as native human proteins because they are synthesised by the cells of the mammary gland from a recombinant version of the native gene. Recombinant proteins expressed in milk have mammalian glycosylation patterns. While these patterns may differ from human patterns, the various different, complex proteins that have been expressed in the milk of transgenic animals exhibit the appropriate folding, assembly and biological activity of the native molecule. Transgenic production enables the development of certain scarce and valuable human proteins. For example, therapeutic proteins currently purified from pooled human plasma - a source subject to periodic shortages - are now being produced transgenically (antithrombin III, human recombinant alpha-1-proteinase inhibitor, human serum albumin). Pooled human plasma also carries a theoretical risk of human viral disease transmission, such as hepatitis C. Transgenic production of such proteins may avoid the potential risks associated with transmission of viral diseases through the human blood supply, as well as guarantee an uninterrupted supply for patients suffering from hereditary deficiencies. Additionally, certain proteins cannot easily be produced in cell culture because the cells making them do not secrete them into tissue culture supernatant, thereby complicating the purification process. To overcome this impediment for one potential therapeutic, GTC has transgenically produced glutamic acid decarboxylase, a possible drug for the treatment of diabetes. 92 Innovations in Pharmaceutical Technology

2 Transgenic technology also offers a highly flexible production system that can be applied to a wide range of recombinant protein types, ranging from human plasma proteins to soluble cell surface receptors. To date, GTC has produced 50 diverse proteins in the milk of transgenic animals such as mice, rabbits and goats including, for example, monoclonal antibodies for the treatment of cancer and autoimmune diseases, as well as other proteins including beta interferon, human growth hormone, tissue plasminogen activator, insulin and prolactin. Transgenic production also provides significant capital flexibility in the development process. Given the time and risks associated with the development of new biopharmaceuticals, costly investment in cell culture facilities can restrict product development choices. The capital costs for the transgenic production of 100kg of a monoclonal antibody are $6 million for herd scale-up, farm and dairy costs, but excluding purification facility costs. Typical costs associated with cell-culture based bioreactor plants approach at least $50 million, and more often reach the $200 million level. Furthermore, expanded transgenic production output to supply clinical trial and marketing needs can be met by breeding and milking more animals, rather than by a significant investment in new production facilities. The transgenic production process A transgenic animal is an animal whose cells incorporate sequences of DNA that are not normally part of its genome. To produce a transgenic animal, a segment of DNA, or expression vector, is To date, GTC has produced 50 diverse proteins in the milk of transgenic animals such as mice, rabbits and goats Figure 1. Typical transgenic development plan. Innovations in Pharmaceutical Technology 93

3 Transgenic production also provides significant capital flexibility in the development process Figure 2a. Transgenic purification process. Figure 2b. Key purification technologies. constructed consisting of a promoter DNA sequence and a DNA sequence that codes for the desired recombinant protein. The presence of the promoter segment directs production of the protein to the mammary gland. The recombinant proteins controlled by milk promoters are then generally expressed only in the mammary gland during lactation. Transgenic animals are produced by introducing - usually through microinjection - the expression vector into an early-stage embryo. The embryo is then transferred to a surrogate mother. Following the birth of animals resulting from these embryo transfers, animals carrying the transgene for the protein are identified. The female offspring of these founder animals form the production herd. Production lines established from a single transgenic founder ensure that all animals in the herd exhibit consistency in recombinant protein expression levels and characteristics that are stable from lactation to lactation and generation to generation. While GTC has chosen goats as its production species, transgenic mice, rabbits, sheep, pigs and cows have been developed. GTC routinely achieves about 5-10% transgenic animals among live births subsequent to the microinjection procedure. These first generation transgene carriers, or founder animals, may be male or female. If the founder is female, then the time from transgene introduction to the first natural lactation is 18 months for goats. Lactation may be induced in females at three months of age, however, thereby supplying material for testing and early clinical trial supply. If the founder is male, then he must produce transgenic daughters, which in turn must produce transgenic daughters before full-scale milk production can begin. The time to first lactation is about 2.5 years for transgenic lines derived from male founders. More recently, transgenic female animals have been produced by somatic cell nuclear transfer (cloning). By combining nuclear transfer technology with lactation-induction in prepubertal goats, it may be possible to considerably shorten the time line associated with founder production and subsequent herd development (Figure 1). GTC reported the development of three cloned female goats carrying a transgene targeting the expression of antithrombin III (rh AT III). These goats were produced from a female cell line originating from goats that were transgenic for rh AT III produced by microinjection. Expression of rh AT III in the milk of the cloned animals was consistent with the expression found in the milk of other transgenic females from the same line obtained by natural breeding. Therapeutic protein recovery from milk Milk as a source material of recombinant proteins offers a much higher protein concentration - typically 10-fold to 1,000 fold greater than tissue culture supernatant. Furthermore, milk does not contain active proteases that can break down the protein of interest. Because this high concentration decreases the volume of material input into the purification process, the need for materials and equipment such as buffer salts, chromatography columns and custom materials for chromatographic capture steps is significantly reduced. The concentration of protein in milk also remains consistent over the production cycle, whereas in cell culture bioreactors, the protein concentration varies considerably with the stage of growth and state of the cells that are producing it. Therapeutic proteins are generally more readily purified from milk than from tissue culture 94 Innovations in Pharmaceutical Technology

4 Milk as a source material of recombinant proteins offers a much higher protein concentration Therapeutic proteins are generally more readily purified from milk than from tissue culture supernatants supernatants. Recombinant protein typically represents a minor component of supernatant that must be separated from cells, cellular debris, lipids, DNA, host proteins, enzymes, pyrogens and other potential contaminants or impurities. Serumsupplements to culture supernatants further complicate the purification problem by adding exogenous bovine proteins, including immunoglobulins, and dyes. By contrast, milk provides a relatively clean feedstream, consisting of about 87% water, 4% fat and 9% non-fat solids. The soluble phase contains the normal milk proteins (casein and whey protein), mineral salts and vitamins; the non-soluble phase contains fat globules, protein aggregates and cells. Recombinant proteins usually accumulate in the soluble whey fraction of milk, thereby facilitating their recovery. Standard dairy procedures remove as much as 95% of fat by centrifugation, and casein can be precipitated at low ph or with enzyme treatment. Ultrafiltration can be used to remove fat, casein and any cellular components of the milk, as well as provide a barrier to microbial or viral contaminants. The protein of interest can then be purified by chromatography. Figure 2 depicts the generic process for protein recovery from milk. GTC has developed proprietary procedures for protein purification from milk that typically attain 65% yields at 99.99% final product purity. Transgenic production economics The most significant driver for transgenic production of biopharmaceuticals is the need for recombinant human proteins in quantities that cannot be produced economically from either scarce or expensive alternative sources. Currently, these proteins are produced from pooled human blood plasma. Given the relatively low levels of protein production typical of cell culture bioreactors, transgenic production offers a safe, viable and economic alternative for the recombinant production of blood proteins such as rh AT III, human serum CHO cells albumin and other proteins important in the clotting cascade. This case may be the most compelling for rh AT III and human serum albumin, where yearly market demand exceeds 50 kilograms and 440 metric tons, respectively. Furthermore, the recent regulatory approvals for monoclonal antibodies as therapeutics for chronic diseases - including cancers, rheumatoid arthritis and heart disease - are driving the need for production systems that can produce proteins economically at the hundreds of kilograms scale. Monoclonal antibody production provides a good model for an examination of the relative costs of large-scale recombinant protein production performed in either cell culture or transgenic animals. To date, 14 monoclonal antibodies have been approved for human or diagnostic use in the US, with an additional two dozen therapeutic antibodies in clinical trials and another 200 in preclinical development. Annual production requirements for a given antibody therapeutic approach the 100 kg scale. Table 1 depicts a comparison between producing 100 kg of recombinant monoclonal antibody in Chinese hamster ovary (CHO) cells or in the milk of transgenic goats. For production at this scale, 170,000 litres of CHO cell culture supernatant containing 1 gram of antibody per litre are required, compared with 21,000 litres of milk containing about 9 grams of protein per litre from a transgenic goat. Ultimately, one gram of CHO-produced antibody costs from $300 to $3,000, compared with $105 for transgenicallyproduced protein. Safety considerations Genzyme Transgenics Corporation has taken a leading role in working with regulatory agencies to establish criteria to ensure appropriate control over both the manufacturing process for transgenic biopharmaceuticals and the products themselves. GTC operates its farm and milking procedures Transgenic Production days/year 200 (20 runs) 300 (2 litres/goat/day) Reactor capacity required 8,500-litre vessels 35 goats (plus seed culture) Unit product cost $300-$3,000 $105 Capital expenditures $20-50 million $2-5 million Table 1. The economics of transgenic MAb production (100kg scale). 96 Innovations in Pharmaceutical Technology

5 under Good Agricultural Practices (GAP), which provide the highest animal care standards for reduction of disease transmission. GTC has established methods to prevent, monitor and control viral infections in its herds, and maintains a US Department of Agriculturecertified scrapie-free goat herd for the production of rh AT III, as well as for other products in development. With respect to minimisation of viral contamination risk, rigorous control is exercised at the level of the animal, the milk and the final product. Animal control measures, for example, include use of defined breeding stock, identification and tracking of individual animals, frequent health screening and provision of a defined feed source that does not contain any animal products. Milk controls include monitoring of milk sources, collection and handling. Product safety is assured by specific steps in the protein purification process that remove and/or inactivate bacterial, viral and prion agents. Conclusion The production of recombinant proteins in the milk of transgenic animals offers an enabling technology that supports the production of large quantities of biopharmaceuticals at highly competitive costs. Transgenic technology further enables the production of unique molecules that cannot be produced by other means, and protein recovery at high yield and purity. Transgenicallyproduced proteins show appropriate protein structure, stability, bioactivity and safety. In summary, transgenic technology represents the next generation of biopharmaceutical production systems, providing a unique capability for the supply of biotherapeutics to address unmet medical needs as we enter the new millennium. Patricia F Dimond, PhD, joined Genzyme Transgenics Corporation in 1996 as Director of Corporate Development and Communications; she is also responsible for development and funding of the company s idiotypic cancer vaccine programmes for B-cell lymphoma and myeloma. Dr Dimond received a Masters degree in membrane biophysics from Georgetown University and a PhD in cellular biochemistry from Catholic University in Washington, DC. She completed a National Institutes of Health (NIH) post-doctoral research fellowship in the Department of Biochemistry at Boston University School of Medicine. Dr Dimond has over 15 years experience in the biotechnology industry, including positions in communications and corporate development at Becton-Dickinson, TSI Corporation and PerSeptive Biosystems. Innovations in Pharmaceutical Technology 97