Artificial cells as a novel approach to gene therapy - Artificial Cells, Cell Engineering and Therapy - page 250

Review of enzyme replacement therapy/proof of principle
Enzyme replacement therapy (ERT) is simply a subset of protein therapy where
the protein has enzymatic activity. The general principle is to over-express a
desired protein in a cell line, and then to isolate and purify the protein from the
cell culture. When this approach is scaled up for a commercial purpose, large-
scale cell culture using large bioreactor tanks are used. The entire process must
meet stringent manufacturing and purification guidelines, which drives up the
cost of the final product. An alternative approach is to construct a transgenic
animal that over-expresses the protein. If the protein is secreted in the animal's
milk, this provides a non-invasive method of collecting the raw material for
purification.
Protein (or peptide) therapy has been used in a number of disorders for
several decades. The longest-standing and probably most familiar application is
the use of insulin to treat insulin-dependent diabetes. This was first tried on
humans over 80 years ago by Banting and Best shortly after their discovery of
insulin (Banting and Best, 1922), a discovery that would earn the 1923 Nobel
prize in physiology and medicine. However, the first specific administration of a
protein with enzymatic activity was in the treatment of hemophilia ± made
possible by the discovery of cryoprecipitate in the 1960s by Judith Pool (Pool et
al., 1964). Cryoprecipitate was found to be very rich in factor VIII, the blood
clotting factor deficient in hemophilia A, and its administration revolutionized
treatment of hemophilia in the same way the discovery of insulin revolutionized
diabetes care.
When the new molecular biology tools that enabled expression of a
recombinant protein were first developed, one of the very first clinical
applications that were studied in the early 1980s was again insulin therapy of
diabetes. The intervening 60 years of insulin therapy had seen numerous
advances in the production processes and purity of animal-derived insulin.
Indeed, the porcine insulin available at the time that recombinant human
insulin was first produced was so highly purified that there was `no
compelling reason to change patients . . . to human insulin' (Brogden and
Heel, 1987). Eventually, the decreased immune response to human insulin and,
probably more importantly, its lower cost led to the use of recombinant human
insulin over porcine-derived insulin.
Shortly after the cloning of factor IX (Choo et al., 1982, Kurachi and Davie,
1982), the recombinant protein was expressed in cell lines (Anson et al., 1985;
Busby et al., 1985; de la Salle et al., 1985). This opened the way for a
commercial product, but the introduction of recombinant FIX as an available
treatment for hemophilia B patients was much slower than the rollout of a
recombinant product for diabetes. One of the differentiating factors between
these disorders that probably influenced this delay is the prevalence of the
disease, and hence the size of the market. Type 1 diabetes is 250 times more