With 85 molecules under development and 3 approvals to date, it seems that bispecific antibodies may be the novel therapeutic booming class on the market.
When the classical monoclonal antibody binds to a specific target with both paratopes recognizing the same antigen, a bispecific antibody or bsAb has the unique ability to bind two different targets and so triggers a different set of reactions :
- delivering a payload to tumor cells, (the less represented mechanism amongst bsAb pipelines - 8%)
- blocking signaling
- bridging two cell types (in-trans binding) or engaging two receptors on the membrane of one cell (in-cis binding); so far the most important mechanisms amongst bsAb in development (66% of all therapeutic bsAb).
Beyond the mechanistic advantages, bsAbs offer the possibility to combine in one single molecule the potential of two different antibodies without the related cost of two parallel development programs.
But with the victory, comes the flip side of the coin. In fact, a pretty ancient coin.
When did bispecific antibody concept come up?
The concept of bsAb is not new. In 1960, researchers described the experiment of agglutination of two different rabbit Fab formats with a wild re-oxidation protocol. In 1975, the set-up of hybridoma technology by Köhler and Milstein allowed the generation of bsAb through the fusion of two hybridoma cell lines or quadroma. And if you remember our past article on anti-IgG4 generation, it has been proved in 1999 that natural human IgG4 is a bispecific isotype antibody.
So the real question is more « why the first bispecific – Catumaxomab - has been approved in 2009 only »?
- First, the toxicity. Despite their promise, CD3-based bispecific antibodies have demonstrated toxicity, particularly for targets with broad tissue expression, such as EGFR and EpCAM.
- Secondly, the « chain-association issue ». In fact, heavy chain-light chain pairing is one of the initial challenges in bsAb development, through co-expression of two different H and two different L chains. The objective is to obtain the functional bsAb from the mixture of different possible recombinations. Thanks to the progress of recombinant DNA, it’s now possible to investigate different leads.
o One strategy to tackle the issue is the development of fragment-base formats, combining two antibody fragments Fab or ScFv without the Fc region. Despite the fact that these compounds are cheap and easy to produce, the lack of Fc prevents the realization of cytotoxicity functions and drastically diminishes the half-life fo the therapeutic in the bloodstream. We can name for instance DART, Tandem, Diabody amongst the 18 fragment-based format bsAbs in development.
o Another alternative that has emerged is the design of symmetric bsAb, incorporating both specificities in a single polypeptide chain. Usually, these therapeutics are tetravalent. You’ll find in this category, DVD-Ig, CODV-Ig, Two-in-one and F(ab)4CrossMab amongst the 12 symmetric bsAb in development.
o The last approach is the asymmetric format, the closest design from native antibodies, which should trigger less immunogenicity. It’s the case for HLE-BiTE, LP-DART, cH IgG, Hetero H CrossMab or CODV-Fab-TL, constituting the major part of the current pipeline with 53 entities.
- Thirdly, their validation. Since bsAb offers specificity for multiple targets and their unique formats may interfere with affinity and function of one arm, a substantial characterization is needed.
What can we expect for the future of bispecific antibodies?
More and more players will enter the field to join Adimab, Merus, Alligator, Xencor, Zymeworks, Regeneron or Crescendo, and looking ahead, promising innovations represent exciting possibilities, including:
- Not only T Cell Engagers, but also NK Engagers and other immune cells engagers,
- therapeutic IgM,
- mRNA-encoded or DNA-encoded formats,
- combination approach, like the recent combination of an oncolytic adenovirus armed with a bsTCE transgene and a CAR-T,
- disease coverage extension to another field that oncology (86% today) including HIV, inflammatory disease, infectious disease, and allergic disorders,
- more and more bsAbs in the diagnostic field: bsAbs function as excellent bifunctional immunoprobes in diagnostic assays. They offer the advantage of one-step addition of reagents compared to traditional multistep procedures. Obviating the requirement to label a reagent directly, such as a secondary antibody, reduces the deleterious effects of chemical modification of either the enzyme or the antibody.