Monoclonal Antibodies - The Challenges in Drug Development

Monoclonal antibodies (mAb) are approved in a wide range of conditions and represent the largest class of biotherapeutic products. mAbs are large molecules with complex structures and functions. Though bestowed with remarkable specificity and flexibility; the unique properties of mAbs remain a challenge to their drug development, with only a few achieving clinical success.

As mAb are proteins, most of the time, there are stability and pharmacokinetics issues, which negatively impair development. Immunogenicity issues and difficulty in translating data points from animals to humans are other challenges.

Unsuitable biophysical properties make large scale manufacture difficult

Protein engineering has rendered mAbs with better functional characteristics; however, given the complexity of these molecules, there are many issues that can create manufacturing problems.

mAbs are large multimeric proteins containing numerous disulfide bonds and post-translational modifications. Hence, the manufacturing of therapeutic antibodies requires large cultures of mammalian cells and extensive purification steps, which leads to high production costs.

Another issue with mAbs is the high concentrations required to achieve clinical efficacy. Many mAb formulations are used at concentrations higher than 100 mg/mL. Highly concentrated formulations can lead to irreversible aggregation, irreversible precipitation, and high viscosity, and these conditions can present unique challenges to product development.

Stability - A matter of huge concern

Maintaining stability within a highly concentrated product is a matter of concern. If proper stability is not achieved, and the product degrades during shipment or storage, patient safety could be at risk.

The efficacy of mAB therapeutics depends on the structural, conformational, and chemical stability; hence preventing physical and chemical degradation is pivotal. A thorough understanding of the mechanisms and the causes of mAB degradation has allowed developers to craft various stabilization strategies, including pH optimization and identifying effective buffer components to stabilize the mAb. 

· Addition of stabilizers 

There are different ways of stabilizing a protein-based drug, and one of them is the addition of stabilizers such as sugars and polyols. Stabilizers serve as antioxidants, metal chelators, or cryoprotectants. 

Stabilizers, like sugars, generally prefer to interact with water molecules and are steered away from the hydration shell of protein molecules. This state increases the partial molar free energy of the denatured protein relative to the native conformation, thus pushing the protein molecules to maintain their native structure and reduce aggregates' formation. Though useful, caution should be exercised while using stabilizers as excessive use can cause challenges such as an increase in tonicity of highly concentrated mAb formulations.

· Improving the molecule structure through protein engineering 

Another widely used strategy to enhance stability is improving the molecule structure through protein engineering. In vitro molecular engineering¹ enables modifications to the structural and the functional properties of mAbs to render them suitable for use. These modifications are often precise and target a small group of amino acids that form the protein of interest.

The main strategy in protein engineering is to locate the key spots for mutation. Studies show that engineering the variable domains can help stabilize the antibody in various denaturation or proteolytic conditions.

Along with molecular biology techniques, structure-based computational design methods can be used in existing or predicted protein structures to identify aggregation-prone regions and create improved versions of original mAbs.

Undesirable Pharmacokinetics

A major contributor to the failure of mAbs is an insufficient or poor understanding of the pharmacokinetics (PK) and pharmacodynamic (PD) properties. Compared to small molecules, mAbs have complex PK/PD properties. A thorough understanding of the mechanisms influencing the PK and disposition of mAbs can help in improving the product design to achieve the intended activity, and accomplish clinical success. 

To avoid late-stage failures, it is essential to include "PK developability"² as one of the lead selection criteria. The use of small-scale, rapid, and predictive tests to evaluate biochemical/biophysical features and functional properties can help identify the best candidate at an early stage of development. Before³ testing the candidate in humans, robust preclinical data should be obtained to get an insight into the full PD pathway. 

Translation from animals to human is difficult 

Interspecies translational issues are a leading cause of attrition within mAb therapeutic development. For most antibody-based products, non-human primates (NHPs) are used for nonclinical safety and toxicology testing. However, due to the distinct differences in primate and human physiology, the perfect translation of results from NHP study to human safety and effectiveness is tough. 

With advancements in technology and growing research, significant progress⁴ has been made to address these developmental challenges. In place of the NHP, it is now possible to use mouse target knockout phenotypes for risk assessment in human beings. Another approach is the use of surrogate molecules in rodent species to demonstrate safety and efficacy. Additionally, the use of novel in vitro and in-silico technologies, systems pharmacology, and modeling can facilitate human safety prediction, improve the development process, and offset interspecies translation challenges.

Immunogenicity issues

Induction of unwanted immunogenicity⁵ is a potential problem with the use of mAbs. The incidence of immunogenicity, its characteristics, and clinical consequences vary considerably and are affected by different extrinsic and intrinsic factors. Besides causing safety issues, immunogenicity also impacts product efficacy; hence, assessing immunogenicity is a crucial aspect of product development. However, predicting whether a product will be immunogenic and how this will affect safety and efficacy is challenging.

Protein engineering on antibody molecules can limit immunogenicity by reduction of the antibody’s non-human sequence content (chimerization and humanization) or by de-immunization, a process of locating and removing T cell epitopes. 

Many companies are now performing in silico optimization of complementarity-determining regions (CDRs) to minimize immunogenicity. Yeast display⁶ is another new technology that can avoid some of the immunogenicity problems of traditional mAb production. Next-generation transgenic mice with a more complete complement of human antibody genes are also being harnessed to develop fully human mAb.

Moving forward despite the challenges

As the speed to clinic is a huge determinant of success, many companies have incorporated platform approaches⁷ for discovery, product, and process development of mAbs. Apart from speeding up the development process, platform approaches help streamline the regulatory and quality record-keeping processes, which are essential to save time and resources. 

There have been many strides forward, as well as challenges, but mAbs have grown from a small part of the drug industry to occupying a big chunk of the overall biotherapeutic market. Despite challenges, there is still significant interest from companies, and there is a massive pipeline of antibodies yet to be commercialized.

Reference：

1、https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3499339/ 

2、https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6226118/

3、http://www.appliedclinicaltrialsonline.com/monoclonal-antibodies-clinical-pharmacology-knowledge-support-fih-and-early-development

4、https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5524158/

5、https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2881252/

6、https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6939334/

7、https://www.pharmamanufacturing.com/articles/2018/the-evolution-of-therapeutic-monoclonal-antibodies/

About the Author:

Neeta Ratanghayra is a freelance medical writer, who creates quality medical content for Pharma and healthcare industries. A Master’s degree in Pharmacy and a strong passion for writing made her venture into the world of medical writing. She believes that effective content forms the media through which innovations and developments in pharma/healthcare can be communicated to the world."

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