Excipients Emerge as Key Guardians of Liquid Phage Formulations Against Adsorption, Aggregation and Structural Damage

Bacteriophages have moved decisively from the laboratory bench toward clinical and commercial reality. As antibiotic resistance intensifies, therapeutic phages are being formulated for clinical trials, compassionate-use programs and, increasingly, regulated products. Yet a persistent obstacle stands between a well-characterized phage suspension in a research vial and a stable, shelf-ready liquid medicine: the physical and chemical fragility of the viral particle itself. A new study published in npj Viruses examines, in systematic detail, how pharmaceutical excipients can be deployed to protect liquid phage formulations against three of their most damaging enemies: adsorption to container surfaces, aggregation between particles, and degradation of capsid structure during storage. The work offers formulators a practical framework for keeping phage preparations viable without resorting to freeze-drying.

The problem begins with a paradox inherent to phages themselves. These viruses are essentially nucleic acids wrapped in protein coats, decorated with tail fibers and baseplate structures that must remain precisely folded to recognize and inject their genetic payload into bacterial hosts. Unlike small-molecule drugs, which can often tolerate considerable environmental variation, phage infectivity depends on conformational integrity at the nanometer scale. A tail fiber that unfolds, a capsid that cracks, or a protein subunit that detaches can render a particle noninfectious even though the genetic material remains fully intact. Liquid formulations, which are preferable for many clinical settings because they avoid the stresses and costs of lyophilization, expose particles continuously to the surfaces of their containers, to each other, and to solution conditions that shift subtly over time.

Surface adsorption is the first and often the fastest loss mechanism. Phage particles, like many proteins, carry a mosaic of charged and hydrophobic patches that attract them to interfaces. Glass vials, plastic syringes, rubber stoppers and tubing all present surfaces to which virions can adhere, sometimes irreversibly. For low-concentration preparations, intended for pediatric dosing or for phages that are difficult to propagate to high titers, even a small fraction of adsorbed particles can translate into a clinically significant loss of dose. The study underscores that adsorption is not uniform across container materials: hydrophobic polymers tend to sequester particles more aggressively than borosilicate glass, but glass introduces its own risks through leached metal ions and alkaline surface chemistry. The authors emphasize that adsorption must be evaluated for every combination of phage, container and buffer, because a formulation that works beautifully for one phage type may fail entirely for another.

The second threat, aggregation, arises when particles collide and stick together, forming dimers, larger clusters and eventually visible precipitates. Aggregation is driven by electrostatic attraction between oppositely charged regions of adjacent particles, by hydrophobic interactions between exposed protein patches, and by the removal of stabilizing hydration shells during freezing and thawing or during shifts in ionic strength. Aggregated phages are problematic on multiple levels. Aggregates sediment during storage, creating heterogeneous dosing; they are cleared more rapidly from the bloodstream after injection; and they can trigger immune responses or exceed particulate limits set by pharmacopoeial standards for injectable products. Crucially, aggregation and infectivity loss are not always correlated in simple ways, which means that turbidity measurements or particle counts alone cannot substitute for plaque assays when assessing formulation quality.

The third mechanism, structural degradation, is the slowest but perhaps most insidious. Over weeks and months in liquid storage, capsid proteins can undergo deamidation, oxidation and backbone cleavage, while tail structures can gradually lose the precise geometry required for host recognition. Temperature accelerates these processes dramatically, which is why cold-chain dependence has been such a burden on phage therapy programs. The research highlights that structural degradation is often detected only through functional assays: a formulation may look perfectly clear and show normal particle counts by electron microscopy, yet exhibit a steady decline in plaque-forming units as more and more particles lose their ability to infect. This decoupling of appearance from activity is a central argument for rigorous, infectivity-based stability testing.

Against this backdrop, the study evaluates the protective roles of specific classes of excipients, the pharmaceutically accepted, nonactive ingredients that surround the phage in the final product. Sugars and sugar alcohols, including sucrose, trehalose and mannitol, emerge as foundational stabilizers. These molecules act through preferential exclusion, a thermodynamic effect in which the sugar is excluded from the surface of the protein, effectively making the folded state energetically favorable and discouraging both unfolding and aggregation. Trehalose in particular has a long history in the stabilization of biological drugs and vaccines, and the findings reinforce its value for phages, where it buffers particles against osmotic shocks and reduces interfacial stress. Polysorbates and other nonionic surfactants occupy a complementary niche: by adsorbing competitively to container surfaces and air-liquid interfaces, they deny phage particles access to the very interfaces where adsorption and surface-induced denaturation occur. Even low concentrations of surfactant can dramatically reduce the fraction of virions lost to a plastic syringe barrel.

Amino acids and buffering agents form the third pillar of the protection strategy. Arginine, glycine and histidine appear repeatedly in biologics formulations because they disrupt electrostatic attractions that drive aggregation, while histidine buffer offers mild pH control near neutrality, where most phages are comfortable. Plasma proteins and serum albumin, historically used as catch-all stabilizers in early phage work, are discussed in the context of modern pharmaceutical expectations: although effective, animal-derived proteins raise regulatory and supply concerns, pushing formulators toward chemically defined alternatives. Salts deserve careful handling, the authors note, because ionic strength simultaneously affects electrostatic repulsion between particles, adsorption to charged surfaces and the osmotic environment of the capsid; there is no universally optimal salt concentration, and each phage-buffer-container system must be characterized empirically.

Perhaps the most practically important message of the work is that excipient effects are context-dependent and phage-specific. A tailed, contractile Myoviridae-like phage with a large, complex capsid responds differently to stress than a small, robust Podoviridae-like particle. The study therefore advocates a structured development workflow: characterize the phage’s isoelectric point and hydrodynamic behavior, screen a panel of excipient candidates across relevant pH ranges, stress-test the leading candidates with accelerated temperature challenges, and validate the winners under real storage conditions in the intended container closure system. Analytical tools discussed in the article include plaque assays for infectivity, dynamic light scattering and nanoparticle tracking for aggregation, electron microscopy for morphological integrity, and differential scanning fluorimetry or similar methods for probing thermal stability. Combining these orthogonal readouts, rather than relying on any single metric, gives formulators the confidence that a liquid product will remain potent throughout its shelf life.

The implications for the phage therapy field are considerable. Liquid formulations stored at standard refrigeration temperatures, or even at room temperature in optimized compositions, would remove one of the most burdensome logistics constraints on clinical deployment, particularly in regions where deep-freeze distribution is unreliable. Standardized, excipient-protected formulations would also simplify regulatory pathways, since regulators can evaluate excipient safety profiles that are already well established for other biologics. As phage products progress from bespoke compassionate-use preparations toward licensed medicines, the kind of systematic formulation science presented in this study will be indispensable. It transforms the stabilization of phages from a craft of trial and error into a rational engineering discipline, giving the next generation of antiviral antibacterials a fighting chance to arrive intact, potent and ready at the patient’s bedside.

Subject of Research: Use of pharmaceutical excipients to stabilize bacteriophages in liquid formulations by preventing surface adsorption, aggregation and structural degradation.

Article Title: Investigating the role of excipients in mitigating surface adsorption, aggregation and structural degradation in liquid phage formulations

Article References: Siafakas, E., Duong, H. T. T., & Iredell, J. R. (2026). Investigating the role of excipients in mitigating surface adsorption, aggregation and structural degradation in liquid phage formulations. npj Viruses. https://doi.org/10.1038/s44298-026-00233-1

Image Credits: AI Generated

DOI: 10.1038/s44298-026-00233-1

Keywords: bacteriophage, phage therapy, liquid formulation, excipients, trehalose, polysorbate, surface adsorption, aggregation, capsid stability, biologics formulation, antibiotic resistance, npj Viruses