were developed in the 1970s and as the name suggests, would lead to the harvesting of only one (‐mono) type of antibody, which leads to more specificity for antigen binding. mAbs are typically produced by injecting a mouse with antigens, resulting in the mouse developing an immune response mediated by B cells. At this point, the B cells are removed and fused with myeloma cells, resulting in what is called a “hybridoma.” Myeloma cells are used for their long lifespan and ability to replicate, like cancer cells. This allows mAbs to be produced continuously resulting in an efficient manufacturing machinery. Since mAbs are still produced from animal, the potential for allergic reaction or neutralization by the human body exists.2
Advances in science allowed for mAbs to become more humanized over time; first with chimeric antibodies that contained mouse and human proteins (an example is the drug, Reopro®), then with humanized antibodies that further minimized the components made from mice (e.g. Herceptin®), and finally, fully human antibodies (e.g. Humira®, the first fully human antibody approved by the FDA). Antibodies can also be conjugated with other products such as small molecules or radiopharmaceuticals that use the specificity of the antibody‐to‐antigen to target a site and then release a secondary pharmaceutical agent for therapeutic purposes.2
The future of mAbs development may include bi‐specific antibody development, meaning antibodies that possess two binding specificities. This could be advantageous as targeting multiple targets simultaneously could inhibit various receptor‐ligand signaling pathways more effectively and limit development of disease‐cell resistance. In 30 years since the first therapeutic mAb was approved in 1986, there are more than 294 mAbs being used clinically, with almost 90% of them being humanized mAbs with the remainder being chimeric mAbs.13
3.2.4 Enzymes
Enzymes have been a long‐standing target for drug development, including small molecules as well as biologics. The biologic enzyme replacement therapies (ERTs) for the treatment of lysosomal storage diseases (LSDs) such as Fabry disease, Gaucher disease, and Pompe disease offer a vantage point into some of the current successes and future potential in this area of biotechnology. There are 50–60 different rare, genetically inherited disorders resulting in deficient lysosomal enzymes. When lysosomal enzymes do not work properly, fats and other enzyme substrates build up throughout the body resulting in widespread cellular complications including death. Enzyme replacement involves the production of enzyme proteins through rDNA technology, then these enzymes are infused into the patient on a recurrent basis as life‐long replacement therapy. ERTs have resulted in significant clinical benefit to patients including improved quality of life, walking ability, and respiratory function improvements. However, challenges exist such as neutralizing antibodies and other immune reactions. ERTs are not always able to reach the desired target cells such as those in the central nervous system. Potentially 75% of patients with neurologic dysfunctions may not be adequately treated with ERTs, often due to challenges with drug design and inability to penetrate the blood–brain barrier. Drugs marketed for ERT are some of the most expensive drugs in the world today. Gene therapy is one of many scientific advances being explored to treat these conditions in patients.14
3.2.5 Cytokines
Cytokines are proteins that are involved in cell communication and mediating immune system processes. They represent a diverse group of molecules but mainly growth factors and hormones and falling into one of two broad categories: Type I cytokines include interleukins and colony‐stimulating factors, and Type II cytokines that are typically interferons. Erythropoietin, thrombopoietin, growth hormone, and prolactin have similar structures and signaling mechanisms as Type I cytokines.15
Improperly regulated cytokines can result in a variety of diseases such as autoimmunity and cancer. These characteristics make them attractive targets for therapeutic purposes. For their function, cytokines need to bind to specific receptors. As such, drugs that target the cytokine‐mediated immune system can include antibodies that neutralize the cytokine or cytokine receptors, recombinant proteins that are receptor agonists or antagonists, or false receptors that will bind the cytokine itself and neutralize it.16
Cytokines are well studied in animal models that are beneficial for developing therapeutic targets; however, there are limitations due to their natural (native) properties. Cytokines have overlapping activity meaning that, when one cytokine is blocked, another may make up for the lost activity. They are also multifunctional in that they affect several processes, potentially in multiple organs in parallel, so affecting or disrupting a specific cytokine may result in unwanted side effects. Inhibiting the natural activity of cytokines can also result in a severely blunted immune system. Some key characteristics of cytokine type of drugs are provided in Table 3.5.
3.2.6 Cytokine‐Interferons
Interferon alpha was the first cytokine to be produced using rDNA technology.18 Interferon is a regulator of growth and differentiation and has clinical efficacy in malignant, viral, immunologic, angiogenic, inflammatory, and fibrotic diseases. Interferon‐beta was approved by the FDA in 1993 and is the oldest and most frequently used medication for treating multiple sclerosis to date.18 Interferon (IFN) works in multiple sclerosis in a variety of ways, decreasing proinflammatory cytokines while also leading to the production of anti‐inflammatory cytokines by increasing the activity of suppressor T‐cells. The activity of cytokines also makes for a strong candidate in oncology. IFN‐alpha was found to have tumor suppressing activity in a rare B‐cell neoplasm, which led to expansive research throughout the 1990s on clinical utilization of the molecule for cancer treatment.19 Intron A® as found in Table 3.5 is approved for five unique cancer types.
3.2.7 Cytokine‐Interleukins
Another group of cytokines with established clinical use are interleukins. There have been over three hundred and fifty thousand scientific articles published on interleukin since it was first discovered in 1977. More than sixty cytokines have been designated as interleukins since the initial discovery of monocyte interleukin (IL‐1) and lymphocyte interleukin (IL‐2). The numbering convention (i.e. IL‐___) is based on functional properties and biological structure.20
Like other cytokines, the activity of interleukins in the immune system makes them an excellent target to treat immune system‐mediated disease such as allergy, asthma, autoimmunity, and chronic infections. For example, describing the interleukin products from Table 3.5, Actemra® binds to IL‐6 receptors inhibiting IL‐6 mediated signaling. IL‐6 is a pro‐inflammatory cytokine produced by T‐cells, B‐cells, lymphocytes, monocytes, and fibroblasts. IL‐6 is also produced in synovial cells, which leads to local joint inflammation in rheumatoid arthritis. Cosentyx® is a recombinant human monoclonal antibody that binds to interleukin‐17A (IL‐17A) cytokine and inhibits its interaction with the IL‐17 receptor. Through this interaction, IL‐17A inhibits the release of proinflammatory cytokines and chemokines. Dupixent® is a human monoclonal antibody that inhibits IL‐4 and IL‐13 by binding to a receptor subunit shared by both complexes. Nucala® is an IL‐5 antagonist impacting eosinophil activity. Kineret® is an IL‐1R antagonist that impacts cartilage degradation and bone resorption, and Zinbryta® binds to IL‐2 that is presumed to impact lymphocytes resulting in therapeutic effects in multiple sclerosis.17
3.2.8 Tumor Necrosis Factor
Two of the top five best‐selling drugs of all time are tumor necrosis factor
