Antibody-drug conjugates (ADCs) have great potential as targeted cancer therapeutics. In 2011, the ADC brentuximab vedotin was approved for treating anaplastic large cell lymphoma and Hodgkin’s lymphoma. And in early 2012 it was reported that trastuzumab emtansine increased median survival time among patients with a type of breast cancer that overexpresses the HER2 protein. Brentuximab vedotin and trastuzumab emtansine are both full monoclonal antibodies, and in the wake of their success, new and established companies are attempting to make ADCs with a variety of antibody backbones, including antibody fragments.
While monoclonal antibody expression must be done in mammalian cells, several types of antibody fragments can be produced in microbial cells (mainly bacteria or yeast).
Recombinant protein production in microbial systems tends to be faster and cheaper than in mammalian systems. Because bacteria and yeast are unicellular, they have a robust cellular structure that makes them amenable to culturing. In contrast, mammalian cells, which are derived from multicellular organisms, are not adapted to survive outside the body and are thus sensitive to external conditions like shear stress and osmotic shock. The doubling time of bacteria and yeast is every 20 minutes to 2 hours versus every 24 hours to 2 days for mammalian cells. The cost for bacterial cell media is 90% lower than for mammalian Chinese hamster ovary (CHO) media. Producing a stable recombinant protein production system takes approximately 4 to 6 months in mammalian cells versus 1 month in the bacteria Escherichia coli.
Expression of antibody fragments does not always work well in mammalian cells. For example, the product can get stuck in the endoplasmic reticulum, which precludes secretion and isolation. In these cases, expression may be more effective in microbial systems, such as the following.