4 Best Practices for Preclinical Formulation Optimization of Injectable Macromolecule Drugs

Currently, injectable drugs, particularly intravenous (IV) and subcutaneous (SC) formulations, remain the primary delivery method for macromolecular drugs. This route effectively bypasses gastrointestinal barriers, ensuring the drug reaches its target intact. This blog systematically reviews 4 key strategies used in preclinical formulation development for injectable drugs, including buffer solutions for pH control, stabilizing agents, surfactants, and excipients for viscosity reduction. It also discusses how rational excipient selection and formulation design can improve both in-formulation stability and in vivo delivery performance.

The Rise of Injectable Drugs in the Era of Biologics

Since the beginning of the 21st century, macromolecular drugs, including proteins/peptides, antibodies and their conjugates (ADCs), enzyme replacement therapies, nucleic acid drugs, and vaccines, have become increasingly dominant in disease treatment and the pharmaceutical market. Their success is driven by exceptional target specificity, improved pharmacokinetic/pharmacodynamic properties, and favorable safety profiles ^[1,2]. However, compared to small molecule drugs, these macromolecular therapeutics exhibit far greater molecular complexity ^[3].

For developers focused on injectable formulation development, this complexity presents a unique challenge. Macromolecules are highly susceptible to environmental stressors such as pH shifts, temperature fluctuations, and mechanical stress. These factors can trigger chemical degradation (e.g., deamidation, oxidation) and physical aggregation (e.g., oligomer or visible particle formation). Consequently, ensuring physical and chemical stability is a paramount concern.

Figure 1. Comparison of macromolecular drugs in terms of relative molecular mass, cell permeability, and complexity ^[3]

4 Best Practices of Formulation in Injectable Drugs

Injectable delivery is currently the most mature and widely used approach for macromolecular drugs. A scientifically sound formulation strategy is essential to improve physicochemical stability, preserve bioactivity, and ensure long-term storage performance. Below is a comprehensive overview of the four core areas in macromolecule injectable formulation development.

Strategy 1 – Precise pH Control and Buffer Solutions

The foundation of any biologic formulation is establishing a robust buffer system to resist pH changes. Solution pH profoundly impacts a biomacromolecule’s tertiary and quaternary structure, aggregation propensity, and chemical degradation rates. While traditional systems like phosphate and citrate have historically been used, the industry is currently trending toward multifunctional histidine buffers (typically in the pH 5.5–6.5 range). Histidine offers an optimal pH window near the isoelectric point of many antibodies, effectively reducing charge-based aggregation while doubling as a stabilizer, antioxidant, and viscosity-reducing agent.

Strategy 2 – Building a Protective Molecular Net with Stabilizing Agents

To protect macromolecules against denaturation and aggregation driven by heat, freeze-thaw cycles, and mechanical stress, formulations rely heavily on targeted stabilizing agents. These generally fall into two categories:

• Conventional small-molecule stabilizers: Including sugars (like sucrose and trehalose) that utilize a "preferential hydration" mechanism to create a protective barrier, as well as carefully balanced polyols and inorganic salts.

• Amino-acid-based stabilizers: A rapidly growing trend utilizing amino acids like arginine to reduce protein-protein interactions and methionine to scavenge oxidative threats. Modern biologics are increasingly utilizing multi-amino acid combinations to replace traditional sugars and salts.

Strategy 3 – Interfacial Protection via Surfactants

During manufacturing, transport, and clinical use, macromolecules inevitably encounter air-liquid and container-liquid interfaces. These interfaces generate tension that can unfold and aggregate delicate proteins. Surfactants address this by forming micelles and adsorbing at these interfaces to create a protective molecular shield. The industry predominantly utilizes non-ionic polysorbates (PS-80 and PS-20) due to their compatibility and effectiveness, though alternatives like Poloxamer 188 are utilized when specific degradation or compatibility issues arise.

Strategy 4 – Viscosity Reduction for High-Concentration Injectable Drugs

The shift toward patient‑friendly subcutaneous administration has driven the need for high‑concentration formulations that deliver large doses in small injection volumes. This presents challenges related to viscosity, injectability, and aggregation. Two main strategies have emerged:

• Enzymatic tissue remodeling using recombinant human hyaluronidase PH20 (rHuPH20) temporarily degrades hyaluronic acid in the subcutaneous space, allowing larger volumes (up to 5 mL or more) to be administered and rapidly absorbed ^[9].

• Viscosity‑reducing additives, particularly specific amino acids like arginine and histidine, lower the intrinsic viscosity of the formulation by disrupting hydrophobic interactions, shielding electrostatic attractions, and increasing effective molecular spacing ^[10].

These approaches have enabled the successful conversion of many intravenous biologics to subcutaneous formulations.

Conclusion and Future Trends

By combining precise buffer systems, multifunctional stabilizers, protective surfactants, and strategic use of enzymes and viscosity reducers, developers of injectable drugs can overcome the inherent instability of macromolecules. These strategies not only ensure product quality and shelf life but also improve dosing convenience through subcutaneous administration. Future innovation will focus on longer-acting formulations, non-injectable routes, and continuous manufacturing.

WuXi AppTec DMPK platform possesses extensive experience and a comprehensive suite of in vitro and in vivo capabilities for pharmacokinetic formulation optimization of macromolecular therapeutics. The platform supports early-stage vehicle screening, physicochemical characterization, and relevant in vitro studies, thereby enabling full support for preclinical in vivo studies. Supported by advanced bioanalytical technologies and integrated models, the platform enables efficient identification of developable candidates for macromolecular drugs. As innovation continues, with novel excipients, smart delivery systems, and integrated CRDMO support, the future of injectable formulation development promises even greater efficacy, safety, and accessibility for biologics.

Authors: Xinyue Wang, Lijin Zheng, Quanli Feng, Cheng Tang, Qian Huijuan

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