Biotherapeutics ADME and PK/PD Principles

1.1

Monoclonal Antibodies

An antibody, also known as an immunoglobulin (Ig), is a large protein produced by the host's immune system in response to a specific antigen. Antibodies are generated to recognize biological substances that are foreign to the host body, such as viruses or bacteria, and can be instrumental to the neutralization and elimination of such substances. The molecular structure of an antibody resembles a distorted Y-shape with two arms conjoined by one region known as the fragment crystallizable (Fc) portion of the antibody. The end of each of these two arms (also referred to as the N -terminus of the antibody) contains the antigen-specific binding sites (Fabs), with two identical sites per antibody molecule. Both arms are joined by a flexible hinge region that enables bivalent binding of the antibody with antigen sites at a range of distances.

Five different antibody classes or isotypes IgA, IgD, IgE, IgG, and IgM can be found in mammals. Ig classes differ in their sequence, number of constant domains, hinge structure, and valency, with the vast majority (70%–85%) of the total antibody pool consisting of the IgG class. IgG antibodies are mostly found in monomeric form and have a molecular weight of approximately 150 kDa. IgGs consist of two identical heavy chains with two identical light chains as shown in Figure 4.1A . The light chain has two domains VL and CL (one variable and one constant among antibodies respectively), both within the Fab region of the antibody. The heavy chain comprises one variable and three constant domains (VH, CH1, CH2, and CH3, respectively), with the first two being in the Fab region and the last two forming the Fc portion of the antibody. There are four IgG subclasses (IgG1, IgG2, IgG3, and IgG4) each comprising a different heavy chain that imparts different functionalities and capabilities to activate the immune system via effector functions.

One of the key roles of antibodies in the immune system is to trigger immune responses by binding via the Fc domain to specific molecules that could drive chemical cytotoxic responses—such as elements in the complement pathway that result in complement-dependent cytotoxicity (CDC)—or receptors in specialized cells that result in cell killing or cell phagocytosis—as in the case of the antibody-dependent cellular cytotoxicity (ADCC) or antibody-dependent cellular phagocytosis (ADCP) ( Figure 4.1B ). Mainly, IgG1 and IgG3 can trigger potent effector responses, while the IgG2 and IgG4 subtypes drive weaker effects. Differences in the observed Ig functionality can be explained by the differences in binding affinity to elements of the complement pathway as well as binding affinity to Fc gamma receptors expressed on immune cells that drive the ADCC/ADCP responses. IgGs owe their unusually long half-life to their ability to bind the neonatal Fc receptor, also known as FcRn, in a pH-dependent manner ( Figure 4.1C ). Serum proteins are continuously taken up by endothelial cells via pinocytosis into the endosome and incorporated into the lysosome for degradation following the fusion of the endosome and lysosome. Binding to FcRn in the slightly acidified environment (pH < 7.0) within the endosome rescues serum IgGs from lysosomal degradation and recycles them back to circulation where they are released at physiological pH (pH > 7.0).

The IgG has been the large molecule format most heavily used in the design and development of therapeutic antibodies because of its extended half-life, ability to modulate the immune system, and relative abundance compared to other Igs. The first generation of therapeutic antibodies were of murine origin. These provided limited therapeutic benefit because the patients' immune systems would recognize these as foreign and generate a human antimouse antibody response that would clear these therapeutics. To overcome this, the following generation of antibody-based therapeutics have focused on making these large molecule therapeutics more “human like.” These efforts resulted in chimeric antibodies (structural chimeras made by fusing variable regions from one species like a mouse with the constant regions from another species such as a human being) as well as humanized antibodies (where the nonhuman portions of the antibody have been modified to increase similarity of antibody molecules endogenously produced by humans). Further developments in antibody production technology have enabled the generation of fully human antibodies either by using display technologies (e.g., phage, ribosomes, or yeast that display human antibody variable regions) or by using transgenic mice (that is, mice that have been engineered to produce fully human antibodies as part of their normal immune response to foreign agents).

One of the key features that distinguishes antibody-based therapeutics is their high specificity for their intended therapeutic target. Antibodies interact with their intended targets via the complementarity-determining regions located within variable region, area also known as the paratope of the antibody. On the targeted antigen, the specific region that binds to the antibody is known as the target epitope. This interaction can be characterized by the rate of antibody–antigen association (referred to as k _on ), rate of dissociation (referred as k _off ), and the ratio between these two (i.e., k _off /k _on ), known as the equilibrium constant or affinity (typically referred as K _D ) of the antibody–antigen interactions. Affinity is a critical property in the design of antibodies because it quantitates the strength of the antibody–antigen interaction and can be tuned to optimize the therapeutic performance of the antibody. The affinity value of an antibody has units of concentrations and the lower the value of K _D , the stronger the antibody–antigen interaction.

Quantitative PK and PD modeling can leverage our knowledge of relevant target properties, such as turnover rate, antigen concentrations under physiological and pathological conditions, and biodistribution, together with the PK and biodistribution properties of the antibody to optimize affinity design goals for antibody-based therapeutics. Figure 4.2 shows an example of an application of a PK/PD model for optimal affinity determination ( ). Here, it is shown that increases in affinity can result in lower clinical doses because of improved antibody potency. However, beyond a certain point, further improvements in the affinity do not produce significant improvements in potency and clinical dose reductions due to limitations in stoichiometry of the antigen. Where this ceiling value in dose lies depends on the properties of the system, such as antigen concentration, turn over, etc., and it is a critical consideration during the design and selection of antibody candidates.

1.2

Antibody-Based Therapeutics

The antibody structure confers multiple advantages to therapeutic molecules, and not surprisingly, the structure of the antibody has been leveraged to test various therapeutic approaches beyond monoclonal antibodies. For example, antibody drug conjugates (ADCs) are a class of antibody-based therapeutics that combines the specificity and prolonged exposure of antibodies with the potent pharmacological activity of small molecules. Most of the existing ADCs being explored in the clinic or approved for clinical use consist of a cytotoxic payload (the small molecule) conjugated to a full-size antibody for the treatment of solid or hematological malignancies ( ). The antibody acts as a carrier that releases its small molecule cargo selectively upon binding to (and most frequently internalization and catabolism by) target positive cells. Current efforts are extending this concept to other payloads beyond cytotoxic, including antibiotic payload to treat infectious disease ( ; ) and steroid payloads in the immunology field ( ; ), cardiovascular diseases ( ), and other disease states ( ).

Another well-established antibody-based modality is bispecific antibodies, which are a special class of antibodies designed to target two different antigens (one with each of the antibody arms) ( ). These have been most used to tackle mechanistically the disease biology where the combination of the pharmacological activity on different antigens would result in additive or synergistic activity of the constructs in comparison to the monospecific antibody alternative. Moreover, a number of therapeutics in this class have been designed in such a way as to harness the immune system by binding with one arm targeting a cancer cell surface antigen and another arm targeting T-cell receptors to engage the patient's immune system, most commonly through CD3. These bispecific molecules can bridge T-cells to tumor cells through an artificial immunologic synapse, resulting in selective killing of target-expressing tumor cells which, in the case of CD3, occurs independently of the presence of MHC-I or costimulatory molecules. The approach was pioneered by the BiTE molecules ( ) but has expanded into a plethora of formats during the last few decades ( ).

Fc fusion proteins are another class of antibody-based therapeutic and consist of the Ig Fc fragment domain directly fused to a peptide or protein intended to drive a pharmacological effect. In most existing clinical applications, this approach has been used to replace or enhance the pharmacological action of an endogenous molecule and, many times, this has been achieved leveraging the structure (or a modification thereof) of the endogenous molecule itself ( ). Attachment to an Fc domain grants the therapeutic protein many of the advantages of the monoclonal antibody, including improved exposure due to interaction with FcRn. However, the overall resulting PK and biodistribution of Fc-containing constructs varies from molecule to molecule and depends on many factors such as local and global electrostatics charges, hydrophobicity, glycosylation patterns, target-mediated and nonspecific mechanisms, etc.

As mentioned earlier in this chapter, the variable region of antibodies possesses unique properties of high affinity, specificity, and selectivity. The variable region has been explored as a therapeutic alternative where the properties of the Fc backbone are not critical or not desired. The antigen binding fragment (or Fab) is a therapeutic modality comprising only one constant and one variable domain of each light and heavy chain. Due to their smaller size, Fabs are believed to result in better tissue penetration than full-length antibodies. Single-chain variable fragments (commonly referred as scFvs) are even smaller proteins with a peptide linker that connects the variable domain of the heavy and light chain. These smaller proteins can be used as stand-alone therapeutics or as imaging agents; but also their small size has made them amenable to be used as building blocks for multitarget therapy ( ).

1.3

Beyond Antibody-Based Therapeutics

Despite the fact that the large majority of large molecule therapeutics consist of monoclonal antibody or antibody-based therapeutics, other protein modalities are also being pursued. Here, we describe some of the most frequent types and formats of biotherapeutics that could be encountered in clinical applications; however, advances in protein engineering have unlocked countless possibilities in terms of how biotherapeutic molecules can be constructed. An exhaustive review of this topic is beyond the scope of this chapter.

Albumin binding therapeutics . The role of FcRn in antibody PK has been described. Similar to IgG, albumin is also rescued from lysosomal degradation by FcRn, giving albumin an extended half-life compared to like proteins in vivo. Protein therapies have taken advantage of this mechanism by generating molecules that can noncovalently bind to endogenous albumin and have potential to exhibit the prolonged half-life while still maintaining the advantages of small proteins in terms of tissue distribution and penetration. Alternatively, another therapeutic strategy involves leveraging the favorable half-life properties of albumin by generating drug with covalent bonds to albumin molecules (via chemical conjugation or genetic fusion). Either of these approaches has demonstrated to be capable of altering the serum half-life ( ; ) as well as the tissue distribution ( ; ; ; ) of the pharmacological agent.

Pegylated proteins. Pegylation refers to the conjugation of drugs to polyethylene glycol molecules as a strategy to modify their PK properties. Mainly, pegylation reduces the renal clearance of the conjugated molecules, resulting in more sustained and constant circulating concentrations which may lead to improved therapeutic effects. Because pegylation can substantially change the physical and chemical properties of the drug, pegylated drugs can exhibit tissue distribution patterns that are dramatically different from that of the parent drug. Moreover, differences in the steric environment around the drug's active site can impact its ability to interact with its intended target, limit the drug's susceptibility to be a substrate for various enzymes, or even change the drug's immunogenic potential ( ; ; ).

Multimers and other large proteins . Nonantibody–based multimeric structures have also been explored. These structures are most likely sought when the Fc fragment structure would not confer critical advantages and/or when the multimeric structure is believed to be critical for the therapeutic mechanism of action (such as in the case of agonists where the multimeric binding results in superagonistic activity). For local administration, elimination mechanisms in tissue may differ from those after systemic administration and the large size may constitute an exposure advantage. Systemically, these molecules clear fast, so they are typically not utilized for pharmacology that requires sustained engagement of the target.

Gene and Cell Therapy . The concept of gene therapy comprises the use of genes to treat disease. The idea is that genetic material will be delivered into the cells in the target tissues and replace genes that are defective or nonfunctional. The therapeutic gene could aim to replace a mutated gene with a healthy copy, inactivate a gene that is not functioning properly, or introduce new genes which are designed to treat the disease. So far, gene therapy has shown promising success in treating genetic disorders, cancers, and some infectious diseases and is currently being tested for a number of diseases with severe unmet needs. One of the key challenges in gene therapy is how to efficiently deliver the genetic material into target cells and tissues. Currently, genes are typically delivered to cells using a plasmid or a virus. Viruses are specialized agents capable of entering cells and inserting their genetic material; however, there are potential concerns about immune responses and genome manipulation.

Cell therapy entails the use of live cells to treat disease. These cells may be obtained directly from the patient via autologous gene therapy or from a donor via allogenic gene therapy. Different kinds of cells can be used, including hematopoietic stem cells (HSCs), skeletal muscle stem cells, mesenchymal stem cells, lymphocytes, dendritic cells, and pancreatic islet cells depending on the application and disease indication. HSC transplantation is the most frequently used cell therapy and is used to treat a variety of blood cancers and hematologic disorders. Sometimes the cells administered as cell therapy may be genetically altered to treat specific diseases. Such is the case of CAR (chimeric antigen receptor)–T-cells, which is a combination of both gene and cell therapy. Here, T-cells are harvested from a patient and are taken into the laboratory to be genetically modified to become specific to an antigen expressed on a tumor that is not expressed on healthy cells. Then, these modified T-cells are infused back into the patient where they become activated upon binding to the specific antigen, proliferate, and become cytotoxic ( ; ).