VHH Antibodies for Difficult Targets

Isogenica enables discovery of functional VHH antibodies against difficult therapeutic targets relevant to targeted payload delivery, T-cell engagers and next-generation biologics. Successful discovery begins with the right target strategy. We work closely with clients to select the most biologically relevant target materials and the most suitable selection strategy. For particularly challenging targets we may recommend incorporating multiple presentations in parallel. This educated and tailored approach allows us to generate critical insights that help steer the campaign and increase the likelihood of identifying functional binders while reducing downstream risk.

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Scientific & Therapeutic Context

About Difficult Targets

Many of the most biologically relevant drug targets are also the most technically challenging. These include GPCRs, ion channels, multipass transmembrane proteins, carbohydrates, and structurally complex or unstable antigens. Such targets often suffer from limited expression, poor stability outside native membranes, conformational heterogeneity, or sensitivity to assay conditions. As a result, traditional antibody discovery campaigns can fail early, not because the target lacks therapeutic relevance, but because the discovery approach is poorly matched to the biological requirements.

These challenges are increasingly important for targeted delivery platforms such as antibody-oligonucleotide conjugates (AOCs), lipid nanoparticle (LNP) targeting strategies and blood-brain barrier (BBB) delivery systems. In these applications, binder specificity, receptor trafficking and biologically relevant target engagement directly influence therapeutic performance. A critical success factor is the ability to test multiple target materials and presentations in parallel and identifying which formats support productive antibody discovery before committing significant time and resources. Screening across multiple target presentations can help identify which formats support productive binder discovery before committing to large-scale campaigns.

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Target landscape

While some targets, such as CD5, CD7, CD8 or EpCAM are considered more straightforward from a binder discovery perspective, others present significant biological complexity based on biology, membrane complexity and translational predictability. These targets carry a higher discovery risk. Understanding the underlying biological constraints allows selection campaigns to be tailored to each target, improving the likelihood of identifying binders with meaningful translational potential.

Key biological constraints affecting discovery for difficult targets

Difficult targets often share common biological characteristics that reduce the predictive value of conventional recombinant protein screening and increase the need for context-aware discovery strategies.

Epitope sensitivity

Example: CD28, PD-L1

Small differences in epitope location can result in markedly different biological outcomes, making it essential to identify binders that recognise functionally relevant regions rather than simply exhibiting high affinity. This makes biologically relevant selection and screening strategies particularly important early in discovery.

Membrane context and receptor accessibility

Example: CD20

Receptor orientation, membrane organisation, and local protein interactions can influence epitope accessibility, meaning binders identified against soluble proteins may not recognise the target effectively on the cell surface. As a four-pass transmembrane protein, CD20 adopts its native conformation within the cell membrane, where oligomerisation influences the presentation of clinically relevant epitopes. Screening against native cell-surface target presentation can therefore improve identification of functionally relevant binders.

Internalisation and trafficking biology

Example: CD71

For targets used in delivery applications, binding affinity alone is insufficient. Successful binders must also retain the ability to undergo receptor-mediated internalisation and intracellular trafficking in physiologically relevant systems. Incorporating internalisation assays early during triage is important for downstream success.

Post-translation modification

Examples: PD-L1, CD20

Glycosylation and other post-translational modifications can alter protein conformation and epitope accessibility, reducing the translational relevance of binders selected against non-glycosylated recombinant proteins.

Heterogeneous expression and membrane biology

Example: ROR1

Some targets, such as ROR1, can have variable tumour expression and native membrane presentation can influence target accessibility and binder performance. Screening across multiple cell-based models with differing expression levels helps identify binders with robust activity in biologically relevant settings.

Clinical durability challenges

Example: BCMA

Some targets that are relatively straight forward targets for initial discovery present challenges during therapeutic development. BCMA, for example, is a stable, single-pass transmembrane glycoprotein that expresses well and supports early discovery. However, soluble BCMA, generated through receptor shedding, can act as a decoy by binding therapeutic molecules before they reach tumour cells. In addition, low target density and antigen downregulation or loss have been reported as mechanisms of therapeutic resistance. Functional cell-based screening and the development of multi-specific therapeutics targeting BCMA alongside complementary tumour markers may help improve long-term clinical durability and reduce the risk of antigen escape. Together, these examples illustrate that difficult targets arise from diverse biological constraints, each requiring tailored discovery strategies that preserve native biology and prioritise functional relevance alongside affinity.

Implications for discovery

Recombinant protein screening alone is insufficient for most difficult targets.
Binding does not always = function, especially for immune receptors. In addition to CD28 other examples include checkpoint inhibitors such as PD-1 and CTLA-4 and cytokine receptors such as IL-2R, IL-6R.
Cell-based and functional context screening must be considered early in selection campaigns.

Example selection campaign workflow

Figure 1. Workflow diagram of a classic strategy vs parallel selection approaches to discovery

Table 1. Different ways to present targets during selections

Applications enabled by difficult target discovery

AOCs

Bi-specifics

T-cell engagers

BBB delivery

Cell-selective delivery

Immune modulation

LNP targeting

Functional binders against complex membrane-associated targets are increasingly important for precision delivery and next-generation therapeutic platforms.

Why VHH Antibodies?

VHH single-domain antibodies are particularly well suited to binding difficult targets relevant to specific payload delivery, BBB transport and multi-specific therapeutic design:

Their small size and single-domain binding enables access to recessed or conformational epitopes on membrane proteins;
High stability allows screening against partially purified, detergent-solubilised, or non-native target preparations;
Compatibility with in vitro discovery enables rapid iteration across target formats.

These properties make VHHs an ideal format for discovery against challenging antigens where epitope accessibility and target quality are uncertain.

How Isogenica De-Risks Difficult Targets?

Isogenica is structured to support parallel target exploration rather than single-shot discovery.

Large, diverse synthetic VHH libraries allow broad epitope sampling;
In vitro display technologies enable rapid selection without animal immunisation;
Multiple target materials — including peptides, domains, extracellular loops, stabilized constructs, lipoparticles, live cells or membrane preparations — can be evaluated side by side.

This approach allows programmes to identify viable discovery paths early, eliminate unproductive formats quickly, and focus investment where the biology and assay performance align.

Key Advantages

Parallel screening against multiple target materials
Reduced risk when working with unstable or complex antigens
Access to membrane proteins in biologically relevant conformations
Faster go/no-go decisions early in discovery
Improved probability of identifying functional binders against hard targets

Isogenica’s

VHH in bi-specifics

RESOURCES

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Explore how target conformation, environment and presentation influence binder quality in challenging membrane targets, including GPCRs, ion channels, multi-pass proteins and CD3–TCR complexes.

Unlocking Hard-to-Drug Targets: How VHH Antibodies Overcome Drug Discovery Barriers

Today, the term ‘undruggable’ is being redefined. Advances in membrane protein solubilization, allosteric modulation, and therapeutic viral vectors have led to proof that these targets can be tackled. Read more

Our Partnerships

Through our connections with companies like Cube, Kactus and G. Clips specialists in complex membrane protein production and presentation, Isogenica can access and screen against a wide range of high-quality target formats, including GPCRs, ion channels, and multipass transmembrane proteins.

De-Risk Antibody Discovery Against Difficult Targets

Whether you are working on targeted delivery receptors, immune checkpoints or complex membrane proteins, Isogenica can help you explore multiple discovery routes in parallel — and identify the most viable path faster.

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