Production of Therapeutic Proteins in Plants

Abstract

Until recently, pharmaceuticals used for the treatment of diseases have been based largely on the production of relatively small organic molecules synthesized by microbes or by organic chemistry. These include most antibiotics, analgesics, hor-mones, and other pharmaceuticals. Increasingly, attention has focused on larger and more complex protein molecules as therapeutic agents. Proteins are large molecules composed of long chains of subunits called amino acids (see Suslow, Thomas, and Bradford 2002). Just as words are composed of the 26 letters of the alphabet, pro-teins are composed of different combinations of the 20 or so amino acids, except that the length of proteins is often 100 to 1,000 amino acids (" letters ") long. The struc-ture and functionality of a given protein is determined by its sequence of amino acids, which, in turn, determines its three-dimensional conformation, or structure. Internal bonds (sulfur and hydrogen bonds) among the amino acids give the protein its final shape and form. Complex proteins undergo further processing such as the addition of phosphate groups (phosphorylation) or carbohydrate molecules (glycosy-lation), which modify the proteins' functions. Information stored in DNA directs the protein-synthesizing machinery of the cell to produce the specific proteins required for its structure and metabolism. Since proteins play critical roles in cell biology, they have many potential therapeutic uses in preventing and curing diseases and dis-orders. The first protein used to treat disease was insulin, a small peptide that revo-lutionized the treatment of diabetes. In addition, the antigens used in vaccinations to induce immune responses are often proteins. While short peptide chains (containing fewer than 30 amino acids) can be syn-thesized chemically, larger proteins are best produced by living cells. The DNA that encodes the instructions for producing the desired protein is inserted into cells, and as the cells grow they synthesize the protein, which is subsequently harvested and purified. Since 1982, more than 95 therapeutic proteins, or peptides (" biologics "), have been licensed for production using bacterial, fungal, and mammalian cells grown in sterile cultures, and hundreds of additional therapeutic proteins are cur-rently being developed and tested. In fact, many analysts anticipate that in the near future the capacity of cell culture facilities will fall far short of demand, as aug-menting cell culture facilities requires large investments in buildings and equip-ment. Recently, transgenic (i.e., plants engineered to produce specific proteins) plant expression systems (Suslow, Thomas, and Bradford 2002) were developed as alter-native sources for the production of biologics, known as plant-made pharmaceuticals, or PMPs. In general, the use of plants means a lower cost of production and easier expansion for large-volume production than cell culture systems. Instead of a large capital investment in cell culture facilities, plant production systems can be expand-ed simply by growing and harvesting additional plants. However, about 50 percent of the total cost of production is in extraction and purification of the proteins, which is required in both systems. This reduces the potential cost advantage for PMPs.