Understanding How Monoclonal Antibodies Work

Function

The five antibody classes (as classified by heavy chain sequence) are IgM, IgD, IgG, IgE, and IgA.[3][4] Each antibody class performs a unique function in human biology. The most abundant IgG is then further divided into four subclasses based on their properties (namely, the location and quantity of disulfide bonds). For mAb therapeutics, IgG is presently the only class of antibodies utilized.[6] This is due to the pharmacokinetics, stability, low immunogenicity (especially newer, humanized/human agents), limited toxicity profiles, and accessible producibility of a large number of mAbs to a variety of antigens with relative simplicity. General properties of antibodies include composition with two light and heavy chains, with both light and heavy chains containing variable and constant domains (one variable, one constant, and one variable, three constant domains in light and heavy chains, respectively).[6] 

Complementary-determining regions (CDRs) contained within each antigen-binding fragment (Fab) of each respective antibody play an essential role in determining the specificity and affinity with which antibodies bind their target antigens. These highly specific regions are why mAbs can be applied to target precise targets whilst limiting the effects on alternate systems. The Fc region is another region of the antibody that contains constant domains and acts to activate the immune system against the target of mAbs. These functions are mediated by the binding and activation of other Fc receptors expressed on endogenous cells and the complement system, leading to the activation of effector function.[1][3][4]

The initial mAbs were limited not only in their utility (secondary to intolerance, as discussed above) and also in their targets. Early options focused on targeting soluble molecules (namely, cytokines), whilst novel agents have expanded to include a broader range of targets, including membrane-bound structures. For example, direct tumor elimination in oncology applications of mAbs can be accomplished through the inhibition of essential functions, such as dimerization, kinase activation, and signal cascade propagation. The dysfunction of these mechanisms can lead to reduced growth and eventual apoptosis of malignant cells. Additionally, mAb binding of enzymes can lead to elimination, and antibody-drug conjugates can be used to effectively deliver therapeutic agents directly to malignant cells. 

Immune-mediated cell destruction is another mechanism by which mAbs have been applied in oncology. This strategy utilized the immune system to target malignant cells and induce destruction through a variety of mechanisms. Examples of such mechanisms include complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated phagocytosis (ADCP), and antibody-dependent cellular toxicity (ADCC). These mechanisms are triggered when Fc receptors expressed on endogenous cells (natural killer cells, macrophages, etc.) are activated through binding with Fc receptors on mAbs. Finally, vascular disruption is also a potential target for mAbs in oncological applications. Several mechanisms are possible, including toxin administration to vasculature or stromal cells, stromal cell inhibition, and vascular receptor antagonism. One example of an anti-vascular agent is bevacizumab, a humanized mAb that binds directly to vascular endothelial growth factor-A (VEGF-A) and thereby defers vascular growth in tumors.

In addition to the oncological application of mAbs, another major area of application is seen in autoimmune conditions including, but not limited to, rheumatoid arthritis (RA), inflammatory bowel disease (IBD), systemic lupus erythematosus (SLE), and spondyloarthropathies. These diseases create an inflammatory environment in the body whereby CD4+ T-cells are exposed to antigens (via antigen-presenting cells (APCs) and B-cells) and activated in an autoimmune fashion.[4] 

Following activation, the T-cells attack the body's tissue to fight the antigens (in this case, autologous antigens) that were inadvertently presented to the immune system, creating an excess of pro-inflammatory molecules. mAbs have come to play an important role in the therapeutics of autoimmune diseases due to their ability to specifically target small molecules or cell surface molecules, as described above. In the native immune system, there are many complex interplays between a variety of cells and molecules in the innate and adaptive immune systems (i.e., T-cells, B-cells, APCs, cytokines) that provide ample opportunity for intervention. A common example of one such agent is Adalimumab, a human mAb that inhibits TNF-alpha (TNF-a). TNF-a is a cytokine produced by macrophages that leads to the production of multiple cytokines (i.e., interleukin (IL)-1, IL-6, IL-8, GM-CSF) and increased inflammation.[5] 

Inflamed joints in RA patients have demonstrated elevated TNF-a content within the synovium, which effectively correlates with disease activity. Adalimumab has demonstrated a reduction in inflammatory cytokines (IL-1, IL-6, IL-8, granulocyte-macrophage colony-stimulating factor (GM-CSF)) and acute-phase reactants (i.e., erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP)), providing effective treatment for patients with RA who meet criteria to receive mAb therapy [1]

Issues of Concern

Adverse drug reactions (ADRs) are classically classified into five subcategories (A-E) in pharmacology. Type A reactions (augmented) are expected, dose-dependent reactions, and due to the pharmacology of the respective agent. Type B (bizarre) reactions are not related to the pharmacology of the respective medication and, thus, are not predictable. They are not dose-dependent and may range from immune-mediated ADRs to hypersensitivity reactions. Type C (chronic) reactions are dependant on drug metabolism and are continuous. Type D (delayed) reactions are typically challenging to identify from a diagnostic perspective and encompass ADRs that can occur with prolonged (years) use of medications. Type E (end of treatment) reactions occur after withdrawal of medication from a treatment regimen. This classification scheme works well to encompass most drugs that are chemical compounds (xenobiotics) but may prove insufficient to classify mAb associated ADRs.[7]

As outlined above, mAbs are proteins that have been engineered to closely reflect the structure of naturally produced human proteins, and thus, are similarly handled by the body (i.e., not metabolized like drugs). A review article by Pichler et al. (2006) published a classification system to better classify the ADRs associated with the use of mAbs.[7][8] Type alpha reactions refer to high cytokine and cytokine release syndrome reactions. These reactions occur due to the administration or release of high doses of cytokines into the systemic circulation. Type beta reactions are allergy-type reactions that may be mediated by IgE, IgG, or T-cells (immediate vs. delayed). Type gamma reactions are termed cytokine or immune imbalance syndromes that can lead to autoimmunity, allergic/atopic disorders, or impaired immune function. Type delta reactions refer to cross-reactivity reactions that can occur when targeted antigens are also expressed on other cells/ structures. Type epsilon reactions refer to non-immunologic adverse effects.[7][8]

As evidenced by the classification system by Pichler et al. (2006), adverse events are still possible despite the advancements from murine antibodies to human/ humanized mAbs and the associated reduction in immunogenicity. Recombinant mAbs do not harbor the infectious concerns associated with human donor blood-type products, yet they are associated with their own unique risks as biologic products. This unique class of medications is also associated with its own subset of additional considerations during prescribing. For example, the use of TNF-a inhibitors such as Adalimumab warrants the additional step of ensuring that the patient does not have underlying latent tuberculosis (TB). The granulomas formed in TB infections contain the bacteria in a localized area and are kept by TNF-a signaling. Once inhibition of TNF-a occurs (for example, by administration of mAbs), the bacteria’s confinement is compromised, and reactivation of latent infection occurs. By the same token, biologics act by inhibiting the host's immune function and thereby lower the body’s defense mechanisms, potentially predisposing recipients to opportunistic infections, atypical infections (i.e., fungal), and sepsis. With these implications in mind, it is important for prescribers to be aware of and prepared to address adverse events that may occur with these agents.

Infusion-related reactions typically occur during administration or within hours of administration of mAb infusions but may occur later. These reactions are acute and are related to the pharmacologic activity (intended), immunogenicity, or host immune response. Exact mechanisms are poorly understood for these reactions and are thought to involve the release of pro-inflammatory cytokines as well as complement cascade activation. Infusion-related reactions can be classified into Type alpha reactions and can feature localized injection site reactions, cytokine release syndromes (tachycardia, fever, dyspnea, nausea in mild cases), and, in extreme cases, cytokine storm. As with other infusion-related reactions, the severity of reactions will dictate management options. In severe reactions with multiple organ failure, cessation of the offending medication and commencement of supportive measures (i.e., mechanical ventilation, IV fluids, vasopressors, HD, etc.) is necessary.[9] In more mild cases, the infusion can be temporarily interrupted, and supportive medications (for example, diphenhydramine and paracetamol) can be given before restarting the infusion at slower rates.

Anaphylaxis is an immediate hypersensitivity reaction (type 1) that can also occur when using mAbs. Anaphylaxis occurs due to the development of IgE antibodies against mAbs and is generally not expected to occur on the first exposure of the recipient to any given mAb. This is because sensitization to the mAb cannot occur without prior exposure, but cases of host IgE cross-reactivity have been documented. Anaphylaxis is classified as a type beta reaction per the classification scheme proposed by Pichler et al. (2006).[7][8] The management of mAb-associated anaphylaxis does not differ from the management of anaphylaxis due to other agents.

As with all other medications, there may be unintended adverse effects related directly to the intended mechanism of action of any given mAb (type A reactions). An example applicable to mAbs would be the use of Abciximab (a mAb against the glycoprotein (GP) IIb/IIIa) in patients undergoing percutaneous coronary intervention (PCI) causing major bleeding events.[10] Antiplatelet medications are used to decrease the risks of associated ischemic events, periprocedural complications, and stent thrombosis in patients with coronary artery disease. Abciximab binds and inhibits the GP IIb/IIIa receptor and prevents platelet aggregation. It has been shown to be effective in preventing platelet aggregation and, by the same mechanism, may lead to major bleeding events in patients receiving this mAb.[10]

Unfortunately, the pathophysiology for many of the conditions in which mAbs are applied tends to be complex and not mediated by a single mechanism or molecule.[11][12] A potential solution to this dilemma is the advent of bispecific mAbs (bsmAbs), whereby the concept of targeting multiple pathophysiological mechanisms may improve the efficacy of therapy.[11][12][13] 

The main areas described in the literature for the development of bsmAbs include inhibition of multiple cell surface receptors, multiple ligand blockade, receptor cross-linkage, and recruitment of Fc receptor absent T-cells (which would normally not be active with antibody stimulation due to the absence of Fc receptors. The design and implementation of bsmAbs have proven to be challenging despite the conceptually sound nature of the propositions. For example, catumaxomab was one such agent that was initially approved for malignant ascites (2009) but withdrawn from the market in 2017 despite its success due to adverse events. The mechanism of action has been described as trifunctional. There are two antigen-binding regions and an Fc region present on the molecule. The first antigen-binding region targets a trans-membrane glycoprotein named EpCAM, which can be found on malignant cells. The second antigen-binding region binds CD3 antigens on T-cells of the immune system. The Fc region functions to activate and recruit immune cells, including macrophages and natural killer cells, to act against malignant cells. Overall, catumaxomab activates multiple pathways for immune-mediated cell destruction (i.e., ADCC, ADCP, etc.) and cytokine-mediated cytotoxicity.[11][12][13][14]

Clinical Significance

MAbs have evolved drastically since their inception in the 1970s by Drs Kohler and Milstein. The initial agents were murine in origin and presented several issues with their continued use, including immunogenicity and the potential for anaphylaxis with repeated exposure. As the research in immunology, molecular biology, and biochemistry continued to advance, it became possible to create chimeric, humanized, and, eventually, fully humanized mAbs.

The mechanisms of action associated with mAbs include direct cell toxicity, immune-mediated cell toxicity, vascular disruption, and modulation of the immune system. Their evolution has placed mAbs at the forefront of highly individualized therapeutics for a variety of cancers and autoimmune diseases whilst limiting the adverse systemic effects associated with traditional treatment options.

Yet, despite these advancements and the wide variety of mAbs now available on the market, there is still significant work to be done to identify novel mAb targets and optimize the pharmacology-related properties of such molecules. Future directions include the continued development and refinement of bispecific antibodies, which have the potential to target multiple therapeutic targets, activate multiple pathways simultaneously, and enhance the potency of therapy.

Enhancing Healthcare Team Outcomes

Patients with complex medical issues as cancers require a multidisciplinary approach which involves a coordinated effort from physicians from different specialties as medical and radiation oncologists, radiation technologists, nursing, infusion therapy. Monoclonal antibodies are a beneficial treatment modality that greatly enhances the overall outcome and preservation of quality of life when provided in the correct patient. [Level 5] [15]