Multidomain Biologics: Bioanalytical Strategies and Insights

In the past year alone, the FDA has approved a new wave of multidomain biologics—therapies designed not just to bind, but to orchestrate complex biological responses across multiple targets. Molecules such as Lynozyfic (invoseltamab-gcpt), Bizengri (zenocutuzumab-zbco), Imdelltra (tarlatamab-dlle), and Ziihera (zanidatamab-hrii) exemplify this shift toward multispecific design, offering the potential to reshape oncology and autoimmune treatment paradigms. But with this precision comes increased analytical challenges, as traditional approaches for measuring exposure and immune response can fall short in characterizing these complex therapeutics.  

As a leading global bioanalytical laboratory that has supported approximately 30% of the biologics approved by the FDA from 2018-2023, BioAgilytix has specialized expertise in addressing the unique analytical challenges these complex molecules present throughout drug development. In this insider session, BioAgilytix scientists explore the bioanalytical challenges of developing these biologics, providing real-world examples and best practices for creating robust pharmacokinetic and immunogenicity assays for multidomain biologics. 

In this article: 

• Pharmacokinetic (PK) Assays 

• Anti-Drug Antibody (ADA) Assays 

• Neutralizing Antibody (NAb) Assays 

Bioanalytical Challenges 

Traditional bioanalytical approaches developed for monoclonal antibodies are often inadequate for multidomain biologics, necessitating specialized analytical strategies that account for the complex structural and functional properties of these molecules. 

Key challenges include: 

• Requirement for custom critical reagents such as anti-idiotypic antibodies specific to each domain 

• Need for comprehensive immunogenicity assessment that can differentiate domain-specific responses 

• Potential matrix interference from circulating targets 

• Need to confirm that assays capture the pharmacologically active form of the intact multidomain molecule  

In addition, the engineered nature of these molecules can potentially lead to stability concerns in solution, requiring careful attention to formulation conditions, storage requirements, and sample handling procedures during assay development and validation.  

Assay Development and Validation Strategies 

Pharmacokinetic Assays 

Preclinical pharmacokinetic (PK) assays typically utilize commercially available anti-human immunoglobulin G (IgG) antibodies to both bind and detect the drug product in a sandwich immunoassay format. If higher sensitivity is needed, combinations of target antigen capture with anti-human IgG detection can be employed to improve signal-to-noise ratios and lower detection limits. Due to higher drug exposure levels and reduced matrix interference in animal studies, these straightforward methods often provide adequate sensitivity and specificity for preclinical applications. 

However, clinical PK assays require more sophisticated strategies due to high circulating IgG levels in humans and the need for enhanced sensitivity during first-in-human (FIH) studies where drug concentrations may be very low. Clinical PK assays often require custom critical reagents such as anti-idiotypic antibodies that bind specifically to individual domains of the therapeutic.  

This development involves systematic checkerboard-style experiments comparing different capture and detection reagent combinations to maximize sensitivity, robustness, and selectivity. While this approach can be labor-intensive, it leads to identification of optimal reagent pairs that provide the best analytical performance characteristics. Additionally, the use of commercially available reagents can become prohibitively expensive for clinical studies over extended timelines, so investing in custom reagents is often cost-effective for long-term development programs. 

Anti-Drug Antibody Assays 

The traditional gold standard for immunogenicity assessment follows a three-tiered approach designed to balance sensitivity and specificity:  

1. Screening, which targets a 5% false positive rate to minimize false negatives 

2. Confirmation, which targets a 1% false positive rate to establish specificity 

3. Characterization, which provides a qualitative assessment of confirmed positive responses 

For multidomain biologics, standard bridging anti-drug antibody (ADA) assays are developed using the entire molecule as both the capture and detection reagent, with polyclonal positive control antibodies generated against the complete therapeutic.  

However, to determine which specific regions of the molecule are driving immunogenic responses, multidomain biologics require additional domain characterization capabilities that provide insight into both safety and future molecule design optimization. Domain characterization typically leverages competition-based approaches where individual recombinant domains are added to confirmatory assays. The standard approach involves: 

• Developing a screening ADA assay using the entire molecule 

• Implementing a confirmatory assay that adds excess unlabelled whole molecule to compete for ADA binding 

• Adding individual domains in separate confirmatory formats to determine domain-specific ADA responses 

When individual domains compete for ADA binding with resultant signal reduction, this indicates the presence of domain-specific antibodies. Conversely, lack of competition suggests ADA responses are directed against other regions of the molecule or conformational epitopes not present in the isolated domain. While this approach is straightforward to implement since it builds upon standard immunogenicity assay formats, its limitations include potential lack of sensitivity for detecting low levels of domain-specific antibodies, particularly when high prevalence ADA responses exist against other domains. 

Successful implementation of domain characterization requires availability of purified recombinant derivates of each individual domain and, ideally, domain-specific positive control (PC) antibodies. While domain reagents are readily available for some molecule formats, they can be extremely challenging to produce for others, as is the case when domains require specific folding or post-translational modifications for proper antigenicity.  

Neutralizing Antibody Assays 

Neutralizing antibody (NAb) assays for multidomain biologics typically utilize cell-based functional formats rather than competitive ligand-binding approaches. Common cell-based approaches include proliferation assays and apoptosis assays, and the optimal format depends on the specific mechanism of action and target biology of the therapeutic.  

Multidomain biologics present challenges for NAb assay development, including: 

• SPC development. Ideally, separate positive controls specific to each arm of a bispecific molecule are developed to demonstrate the capability of the assay to detect NAbs against either domain. However, generating positive controls for all domains can be technically challenging for certain targets.  

• Drug interference. The extended half-life typical of antibody-based multidomain biologics results in high circulating drug levels in patient samples, which can interfere with NAb detection. Sample pretreatment methods are frequently necessary for removing excess drug while preserving ADAs for subsequent neutralization testing. 

• Matrix interference. Circulating targets, soluble receptors, or other endogenous factors can interfere with assay  

Key Takeaway 

Multidomain biologics require integrated bioanalytical strategies that account for the unique structural and functional properties of these complex modalities. Successful development of these molecules requires early strategic planning to ensure availability of critical reagents and seamless implementation across preclinical and clinical studies. 

To learn more about bioanalytical strategies for multidomain biologics, talk to a BioAgilytix scientist.