Applying Next-Generation In Vitro Antibody Libraries to Drug Discovery 

In the race to develop effective antibody therapies for cancer and other prevalent diseases, researchers have historically relied on two approaches: immunization and in vitro antibody libraries.

Immunization-based approaches are time intensive and ethically challenging due to their heavy use of animals. For more than 30 years, in vitro antibody libraries have provided an alternative, and they align with 2010 European Union and European Medicines Agency guidance recommending non-animal alternatives in drug discovery and development. 

Recently, in vitro libraries have demonstrated the ability to deliver antibodies with affinities comparable to those obtained by immunization.

To be effective drugs, antibodies must have biological activity and usually require high affinity. Also, they must be “developable,” a concept describing the likelihood a candidate can be successfully developed as a drug and comprises desirable features, such as stability and low polyreactivity. Generating therapeutic candidates is often a multistep process involving improvement of affinities and developability of an antibody lead after it has been identified.

Early in the COVID-19 pandemic, when treatment options were limited, the need for rapid drug development became urgently clear. In addition to expediting the slow pace of drug discovery, the pandemic also set the stage for an inadvertent direct comparison of different antibody development methods. This clearly demonstrated the value of next-generation in vitro antibody libraries in lead generation, with antibodies obtained more rapidly, with higher affinities, higher neutralization activities, and fewer developability issues compared to the best immunization approaches. 

Conceptually, an in vitro antibody library comprises billions of different antibodies from which those with a biological activity of interest can be easily isolated. The challenges lie in designing the library such that most antibodies within it are potentially functional, and then devising selection strategies to isolate functional antibodies. One approach is to use diverse, therapeutic antibodies as library scaffolds, into which natural antibody binding loops have been embedded, as detailed below. 

Therapeutic antibodies as scaffolds

Of the hundreds of antibodies approved and under study in clinical trials, some are extremely developable while others fail to gain approval. By choosing genetically diverse, developable therapeutic antibodies as library scaffolds, developability is improved.

Well-behaved, natural immune antibody binding loops as diversity  

Naturally, antibodies have six highly diverse binding loops responsible for target binding. These mutate during an immune response, and by sequencing antibody genes from many donors, the full diversity of the immune system can be understood. Highly effective antibody libraries can be built by eliminating binding loops with problematic sequences and inserting the remainder into genetically matched therapeutic antibody scaffolds. 

Focusing on developability 

Ultimately, the goal of antibody discovery is to develop safe and effective therapies for patients in need. As such, an essential part of antibody discovery is developability. Designing in vitro antibody libraries in which most antibodies are already developable reduces the need for extensive downstream engineering. 

High affinity antibodies 

By creating libraries in which well-behaved binding loops are embedded within well-behaved therapeutic antibody scaffolds, the functionality of the library is significantly increased. A beneficial effect of this higher functionality is that selected antibodies have higher affinities. These antibodies have less need for further affinity maturation and move the molecule to the next phase faster. 

Additional properties

In addition to generating antibodies with excellent developability and high affinities, an advanced antibody discovery platform can also be used to select antibodies that along with recognizing the human target, are cross-reactive and recognize the equivalent murine target. This allows antibodies to be easily tested in murine disease models. 

For solid tumor cancers and autoimmune diseases, in vitro libraries can be particularly advantageous. For example, anti-cancer target antibodies, even if highly specific, may not be usable if the recognized target is also found on normal cells. Since solid tumors are known to be more acidic than normal tissue, in vitro methods for selecting antibodies binding under acidic conditions can deliver antibody candidates that can destroy cancer cells without damaging healthy cells, resulting in safer, targeted therapy.

Improving all, not one or the other

To expedite drug discovery with tangible benefits for patients, in vitro antibody libraries improve diversity, affinity, and developability simultaneously, which can accelerate drug development timelines.