How VHHs Can Redefine the Next Generation of ADCs
Introduction
Antibody–drug conjugates (ADCs) have transformed oncology by combining the specificity of antibodies with the potency of cytotoxic payloads. Over time, ADCs have improved greatly, evolving from first-generation mouse antibodies with low-potency payloads that caused a lot of toxicity through off-target action. After decades of progress, second- and third-generation ADCs have more potent payloads and very homogeneous drug-antibody ratios. As a result, especially with linker-payload chemistry advances, there has been less off-target activity and therefore less toxicity.
However, significant limitations remain — from poor tumour penetration and heterogeneous payload delivery to narrow therapeutic windows and manufacturing complexity.
At the same time, single-domain antibodies (also known as VHHs or nanobodies) have matured into a proven therapeutic format, offering high affinity and stability in a compact, engineerable scaffold. Bringing these two platforms together offers a compelling opportunity: ADCs built on smaller, more versatile antibody frameworks that could help address long-standing design bottlenecks.
In this article, we explore how VHHs can help tackle some of today’s toughest ADC challenges, and what that means for the future of targeted therapies.
The ADC Format and Its Current Challenges: Where Challenges Remain
A traditional antibody–drug conjugate (ADC) consists of three components:
- Monoclonal antibody (mAb) – provides target selectivity and enables internalisation
- Linker – keeps the payload stable in circulation, releasing it at the target site
- Payload – a highly potent cytotoxic compound that drives cell killing
Over successive generations, improvements in linker chemistry, payload potency, and antibody specificity have reduced off-target toxicity. Yet, even with these refinements, ADC developers continue to face several key challenges, particularly:
- Low tumour penetration
- Optimising drug–antibody ratio (DAR)
- Balancing half-life and systemic exposure
- Complex manufacturing
1. Low Tissue Penetration
Conventional IgG-based ADCs are large (~150 kDa) molecules. Their size limits diffusion into dense or poorly vascularised tumour tissue, leaving many cells untouched. The result is uneven payload distribution, patchy efficacy, and, in some cases, treatment resistance. One of the most persistent limitations is poor tumour penetration.
How VHHs Can Help:
VHHs are roughly one-tenth the size of a full antibody (~15 kDa), thus diffuse more rapidly and uniformly through tumour tissue. Their compact structure enables deeper penetration, ensuring more consistent engagement of target cells across the tumour mass. By replacing an IgG scaffold with VHHs, ADC developers can increase therapeutic reach without increasing payload dose, which can potentially translate into improved efficacy and reduced off-target exposure.
Beyond tumour penetration, ADC developers must also consider how efficiently each antibody carries and delivers its drug payload.
2. Optimising Drug–Antibody Ratio (DAR)
Membrane proteins Establishing the optimal drug-to-antibody ratio is a delicate balance. High DAR can increase potency but also promote aggregation, accelerate clearance, and raise toxicity risk. While high DAR ADCs – typically with a DAR of 8 or more – have been researched extensively, low-DAR ADCs with modern, ultrapotent payloads remain relatively underexplored.
How VHHs Can Help:
The modular, single-domain structure of VHHs offers high specificity in conjugation design. With precise engineering options (such as engineered cysteine residues or click-chemistry handles), developers can achieve highly controlled and homogeneous conjugation profiles. However, by using a single site, this can be restricted to DAR of 1, or branched linkers for increased DAR. While data is limited due to the chemistry challenges of creating asymmetrical mAb-based ADCs, initial studies suggest that DAR1 antibodies can exhibit improved tissue penetration and efficacy (Bioconjugate Chem. 2023 34(3): 538-548).
Importantly, the combination of lower DAR and increased tissue penetration means that, for a fixed payload dose, more ADC molecules can be administered, accessing many more individual target cells and driving up efficacy.
In other words, VHHs shift the DAR equation from “more is better” to “smaller, smarter, and more effective.”
Even with an optimal DAR, balancing circulation time remains critical to limit systemic exposure.
3. Balancing Half-Life and Systemic Exposure
The long serum half-life of IgG antibodies — often days to weeks — supports sustained exposure but can also prolong systemic toxicity, particularly when payloads detach prematurely in circulation.
How VHHs Can Help:
VHHs enable developers to fine-tune pharmacokinetics. Left unmodified, they clear rapidly through renal filtration, which can be advantageous for reducing systemic exposure and toxicity. When extended circulation is desirable, half-life can be readily tuned through fusion to albumin-binding domains, Fc regions, or PEGylation.
In addition to biological performance, developers must also consider the manufacturing and consistency challenges that affect ADC scalability.
4. Simplifying ADC Manufacturing
ADCs are among the most complex biologics to manufacture. The antibody component alone can introduce heterogeneity, while conjugation steps add further variability. Maintaining product consistency across batches is a persistent industrial challenge.
How VHHs Can Help:
VHHs consist of a single, stable domain that is easy to express in microbial systems and highly tolerant of chemical modification. Their smaller size and structural simplicity streamline conjugation, resulting in more homogeneous products and consistent DAR profiles.
Moreover, their high thermal and chemical stability expands process options for linker–payload chemistry, enabling site-specific conjugation strategies that can enhance product stability and safety.
This simplicity not only reduces process variability but also supports scalable, cost-effective production.
VHHs: A New Design Space for ADC Innovation
Perhaps the most exciting opportunity lies in the design flexibility that VHHs introduce. Their small size, modularity, and ability to be linked into multivalent or bi-specific constructs open up entirely new ADC architectures. For instance, multi-VHH conjugates could simultaneously engage multiple tumour antigens, enhancing selectivity or overcoming heterogeneity in antigen expression.
By decoupling the huge potential of ADC from the limitations of the IgG scaffold, VHHs expand the design space for payload choice, linker chemistry, and overall pharmacokinetic behaviour.
Together, these properties make VHH-based ADCs a versatile platform for the next generation of targeted therapies.
Redefining ADCs for the Next Generation
ADCs have already shown great promise, but many challenges remain to be addressed, and the next generation demands smarter, more adaptable scaffolds. Single-domain antibodies bring precisely that: compactness, control, and creative freedom.
By aligning the long-standing challenges of ADC design with the inherent advantages of VHHs, researchers can move toward ADCs that are smaller, more stable, and more effective for improved clinical outcomes.
Watch Marion’s webinar on “VHH Single Domain Antibodies for ADC Applications”:
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