Since their discovery over thirty years ago, monoclonal antibodies (mAbs) based on Immunoglobulin G (IgG) have transformed the diagnosis and treatment of many diseases.

In particular, IgGs targeting the programmed cell death protein 1 (PD-1)/programmed cell death ligand 1 (PD-L1) pathway have made the promises of cancer immunotherapy a reality. 

However, despite there now being over 100 FDA-approved mAbs on the market, these well-characterised drugs are yet to live up to their full potential.  

The quest for better biologics 

IgG-based immunotherapies and targeted therapies have made a profound impact on the survival rates for some cancers over the past couple of decades.  

However, many cancers do not respond effectively to these treatments. This is due to a range of factors, from the ​limited number of ​well-characterised tumour-specific antigens to the impact of alternating suppressive pathways on cancer cell survival.  

At the heart of these issues lies the inherent nature of monospecific, monoclonal antibodies.  

An antibody that targets just one protein is much easier for a tumour to evolve resistance to therapy. If a small number of mutated cancer cells lose expression of the protein target, these cells are no longer responsive to the treatment and can proliferate unchecked.  

With these significant drawbacks in mind, researchers have increasingly turned to an innovative solution in the form of bi- or multi-specific antibodies.  

The beauty of bi-specificity 

Bi-specific (bsAb) or multi-specific antibodies are designed to bind to two or more antigens, opening the door to a whole new world of therapeutic potential. 

For example, in a 2020 study by Kotanides et al., researchers at Eli Lilly investigated the possibility of co-targeting PD-1 and PD-L1 with a bi-specific IgG1 antibody, LY3434172.  

They found that LY3434172 exhibited strong, dose-dependent anti-tumour activity and enhanced T cell activation compared to mono- or combination therapy. Overall, the drug showed enhanced immunomodulatory properties and better anti-tumour activity compared with monospecific approaches.  

VHHs: Fantastic flexibility for IgG enhancement 

VHHs – small format single-domain antibody fragments – have a range of unique properties that make them ideal for enhancing the therapeutic potential of conventional IgG antibodies. 

Adding a VHH domain to an IgG antibody can expand the antigen binding repertoire while retaining its Fc effector function and the benefits of prolonged half-life from FcRn recycling.  

VHHs are also more stable and soluble compared to other antibody fragments such as scFvs, creating a simpler molecule. 

With a decade of synthetic VHH discovery campaigns under our belt, it’s only natural we’d want to see how these single-domain antibodies can be used to upskill an IgG.  

Using a similar approach to the Eli Lilly team, we constructed a bi-specific PD-1 x PD-L1 antibody by fusing the blockbuster anti-PD-1 mAb pembrolizumab (Keytruda) to our own anti-PD-L1 VHH.  

We tested the impact of adding our VHH at different positions on pembrolizumab (Figure 1) and saw that each form retained binding to both PD-1 and PD-L1 targets (Figure 2).

Overall, the best activity was retained by adding the VHH to the C-terminal of the heavy chain, leaving the most physical space between the PD-1 and PD-L1 binding regions. Based on our decades of experience in antibody engineering, we know this is likely to be highly dependent on the target antigen and further testing is needed in cells to confirm our findings across different applications.  

Supercharge your IgG 

Although this is only a preliminary study, the benefits of using VHHs for IgG enhancement are clear.  

VHHs are highly soluble, modular, and can be easily reformatted. They show huge promise across the biotherapeutics field, not only in adding additional functions to an existing drug, but also as standalone therapeutic agents.