Formulating for Function: Strategies to Maximize Stability of Emerging Biologics

The increasing diversity of biologic modalities such as bioconjugates, including antibody–drug conjugates (ADCs), antibody-oligo conjugates (AOCs), bispecific antibodies, and fusion proteins, is reshaping modern drug development. These complex molecules enable targeted therapies and novel mechanisms of action across oncology, immunology, and rare diseases; however, their inherent structural complexity introduces unique formulation challenges.

Ensuring stability, manufacturability, and patient-friendly delivery requires an integrated understanding of molecular behavior, excipient interactions, and process compatibility. The ability to predict and manage these factors has become essential as biologic pipelines expand and patient expectations evolve.

Formulation scientists must balance physicochemical stability, manufacturability, and delivery constraints from discovery through to commercialization. Their work increasingly relies on high-throughput analytical methods, computational modeling and a lifecycle-based formulation design.

This critical work, which ensures a molecule's safety, efficacy and accessibility, makes formulation the lynchpin of biologic success. This article explores strategies for formulating complex and emerging biologics, the role of predictive modeling, and evolving trends in stability management, lyophilization and delivery science.

Understanding the core challenges of complex biologics

Biologic molecules are complex and highly sensitive to their surroundings. Factors including temperature fluctuations, pH changes, and mechanical agitation can disrupt their native structures, leading to aggregation, degradation (e.g., deamidation and oxidation), or phase separation.

These physicochemical complexities can cause irreversible damage and a subsequent loss of therapeutic efficacy or induce adverse effects in patients. Consequently, having an in-depth understanding of the core factors that can impact a molecule’s stability is crucial for the successful formulation of biologics.

Modality specific formulation considerations

The unique structural characteristics of emerging biologics demand tailored formulation strategies:

• Bioconjugates: Formulating bioconjugates, such as ADCs, AOCs and RDCs, involves maintaining stability across multiple components: the antibody, the payload, and the linker connecting the two. The payload and distribution of conjugation sites can alter surface characteristics, leading to aggregation or reduced solubility if not carefully controlled. Stabilizing the monoclonal antibody (mAb) must be balanced with ensuring the chemical stability of its highly sensitive linker payload system. For example, the hydrophilic nature of the payload for an AOC will require a completely different approach to that of a hydrophobic payload often used in an ADC.
• Bispecific antibodies: These constructs possess two or more binding domains, often with engineered linkers joining the domains. Their unique architectures can produce new interfaces that foster self-association or conformational instability, requiring conditions that stabilize multiple binding domains simultaneously.
• Fusion proteins: These combine functional domains from different proteins to achieve new mechanisms of action. While individual domains may be stable, the fusion can introduce unanticipated aggregation or solubility issues due to altered surface charge or hydrophobic patches.

From fit-for-purpose to optimized formulations

Early clinical formulations are often designed for rapid development and safety assessment. For first-in-human studies, a simple composition at a lower concentration (e.g., 10-20 mg/mL) can enable quick entry into the clinic as an intravenous (IV) infused product while minimizing risks.

Formulations in this phase typically use well-characterized excipients and frozen storage to ensure stability is maintained. Once early data confirms safety and efficacy, development progresses to an optimized format aligned with the target product profile.

This stage often requires reformulating to extend shelf life, simplify the cold chain or, when necessary, support subcutaneous (SC) delivery. These optimized formulations emphasize manufacturability, stability and, in the case of SC injections, patient convenience.

For example, an mAb formulation that initially performed well in IV administration might need to be concentrated for SC delivery. However, simply increasing concentration can drive viscosity beyond practical limits. Screening pH, ionic strength and excipients such as amino acids or sugars can reduce viscosity while maintaining stability.

Designing and refining screening strategies

To efficiently convert a complex biologic into a stable, manufacturable drug product, formulation scientists rely on a multi-pronged strategy that integrates thorough molecular characterization with advanced, high-throughput experimentation and predictive computational tools.

This systematic approach allows developers to rapidly explore the formulation landscape, minimize trial-and-error and mitigate risks early in development:

• Preformulation studies: Successful formulation development begins with a detailed understanding of molecular properties, including isoelectric point, hydrophobicity and chemical stability. Analytical tools such as size exclusion chromatography (SE-HPLC), dynamic light scattering (DLS), viscosity measurements, differential scanning calorimetry (DSC), and imaged capillary isoelectric focusing (icIEF) provide complementary insights into unfolding, aggregation, viscosity, and charge heterogeneity. These methods allow researchers to monitor protein behavior under stress conditions, giving them the ability to fine tune formulations to meet clinical and regulatory requirements.
• High throughput screening (HTS): Modern workflows increasingly use microplate-based methods to evaluate excipients, pH ranges and ionic strengths across hundreds of conditions. Design of experiments (DoE) approaches identify key variables and their interactions, accelerating discovery of optimal formulations. High throughput systems can identify indicative stable, low viscosity formulations within days rather than weeks.
• Predictive modeling: Machine learning (ML) and molecular modeling are being integrated with experimental data to forecast aggregation and viscosity risks. By analyzing historical datasets and sequence features, predictive algorithms can flag problematic domains or compositions before they are tested, reducing iteration cycles and resource use.

The viscosity hurdle for high concentration

At higher concentrations, such as those needed for SC delivery, viscosity becomes a significant challenge for drug delivery because it makes injection difficult and can also contribute to instability, leading to aggregation, precipitation or even gelation during storage.

As concentration increases, short range attractive interactions between protein molecules form transient networks that raise solution viscosity. Adjusting pH or including viscosity-reducing excipients (e.g., arginine) can mitigate this effect, but requires a careful balance so as not to inhibit the interactions responsible for promoting stability.

High concentrations can also exacerbate issues such as opalescence and the formation of visible and sub-visible particles, both of which must be addressed to meet stringent regulatory requirements. The challenge is not only in identifying these issues but also in resolving them without compromising the biologic’s safety, efficacy or manufacturability.

Managing manufacturability and scale up

It is critical to balance excipient concentration against potential impacts on process compatibility. During scale up, factors such as mixing, filtration, and fill-finish operations can stress formulations and promote aggregation.

Early stress mapping studies, including high/low pH stress, agitation, and freeze-thaw testing, help identify vulnerable points before full manufacturing runs. Close collaboration between formulation scientists and process engineers ensures that selected compositions remain robust across scales, from R&D to commercial manufacturing.

Case studies: Real world viscosity and stability solutions

Applying these strategies can resolve critical formulation roadblocks, as illustrated by two common challenges in biologic development:

Managing high viscosity for subcutaneous injection

A product sponsor’s mAb formulation performed well in early-stage trials but required a transition from IV to SC administration for late-stage studies. The target concentration of 150 mg/mL was stable but exhibited a viscosity of >30 cP significantly exceeding the acceptable limit of 20 cP for standard SC injection devices.

The formulation team undertook a systematic screening process to identify viscosity-reducing excipients. This involved identifying and testing a selected panel of buffers and excipients to modulate protein–protein interactions without compromising stability.

The process successfully identified a formulation that reduced the viscosity to within the acceptable range for SC injection. Furthermore, stress testing confirmed that the selected composition did not introduce instability; in fact, it marginally improved the protein’s resistance to aggregation under accelerated conditions, allowing the product to progress successfully to later clinical studies.

Improving solubility and stability

A second product sponsor’s novel mAb exhibited phase separation and gelation during refrigerated storage, a condition that posed significant regulatory and usability concerns. The issue stemmed from a lack of fundamental preformulation assessment of the mAb’s properties.

The formulation team evaluated critical factors such as pH, ionic strength and excipient compatibility. Through this analysis, they identified that the original formulation’s high ionic strength was unsuitable for the mAb’s physicochemical properties.

Adjustments to the combination of tonicity modifiers successfully improved solubility and eliminated the phase separation issue. The optimized formulation demonstrated long term stability and allowed the biologic’s concentration to be increased from 50 mg/mL to 100 mg/mL, meeting the customer’s high dose clinical requirements.

Lyophilization as a stability solution

Freeze drying (lyophilization) is often used when liquid formulations cannot meet stability or cold chain requirements. By removing water under vacuum, lyophilization arrests molecular motion and minimizes degradation.

The process depends on selecting appropriate cryo and lyoprotectants, typically sugars such as sucrose or trehalose, which form an amorphous phase composition to stabilize proteins. Cycle design relies on determining the collapse temperature and glass transition temperature to define the appropriate primary drying conditions.

Analytical tools such as DSC and freeze-drying microscopy are essential for developing efficient cycles that preserve protein integrity. Lyophilized products can also improve global supply chains by reducing dependence on refrigerated storage.

Reconstitution systems, including dual chamber syringes or on body infusors, make these formats more user friendly for high dose SC delivery.

Evolving delivery and sustainability trends

SC administration has become a key driver of biologic formulation innovation. SC delivery enables home administration and reduces healthcare resource burden, but requires small injection volumes, low viscosity, and high stability.

Device co-design with formulation development is essential to ensure both in-use compatibility as well as user comfort and compliance. New delivery systems, such as lipid nanoparticles (LNPs) and polymer–lipid hybrids, offer additional possibilities for encapsulating sensitive payloads and achieving controlled release, particularly for emerging modalities such as mRNA and gene therapies.

These systems must balance stability, biocompatibility, and release kinetics while remaining manufacturable at scale. Sustainability is a growing consideration.

Developing formulations with extended temperature tolerance can reduce the environmental and logistical footprint of biologic distribution by mitigating dependence on energy intensive cold chains. Advances in excipient systems, lyophilization, and the slow adoption of continuous manufacturing are helping to create more resilient and environmentally conscious supply chains.

Integrating analytics and digital tools

Data integration and ML are rapidly enhancing formulation insight. AI and ML are especially valuable in predicting the complex, non-intuitive interactions between biologics and excipients, which are difficult to determine experimentally.

By analyzing historical data and modeling molecular behavior, AI can increasingly identify potential aggregation, viscosity risks or other stability concerns before they arise in the lab. Incorporating these digital tools alongside traditional wet lab work can significantly reduce the number of experimental iterations required, accelerating the development cycle and supporting the move toward knowledge-driven formulation science.

Formulation as the lynchpin of biologic success

Formulation is central to the success of emerging biologics, underpinning efficacy, safety, manufacturability, and patient experience. As bioconjugates, bispecifics and fusion proteins expand therapeutic frontiers, formulation scientists face increasingly complex challenges.

The combination of mechanistic understanding, high throughput experimentation, predictive modeling, and lifecycle management is allowing more efficient development of stable, patient-centric products. Future progress will depend on ever closer integration between analytical science, process engineering, and digital tools.

Biologic formulation requires deep, specialized scientific and technical expertise, and this is where integrated contract development and manufacturing organizations (CDMOs) are integral to success. By partnering with CDMOs, drug developers can efficiently navigate the complexities of biologics from overcoming stability challenges to ensuring manufacturability at scale with the ultimate goal of moving transformative therapies for patients forward.

About the Author

Gary Watts, Head of Formulation Development at Abzena.