The role of Dextran in lyophilisation
How can dextran support freeze-drying formulation development? Explore what published studies reveal about drying performance, protein stability and the role of molecular weight—and why promising thermal properties are only part of the picture.
Dextran has been investigated as an excipient for modifying the thermal and physical properties of lyophilised protein formulations. Studies in model systems report shorter primary drying, improved activity retention and changes in dried-matrix structure. However, these outcomes are formulation-dependent: improvements in thermal properties do not necessarily translate into improved protein stability. Evidence from monoclonal antibody formulations also identifies potential chemical stability and reconstitution trade-offs. [1-4]
This article considers the experimental evidence for dextran in lyophilisation, with emphasis on molecular weight, combinations with disaccharides and the distinction between process performance and protein stability
Why excipient selection matters in lyophilisation
Lyophilisation comprises freezing, primary drying, during which ice sublimes, and secondary drying, during which unfrozen water is removed by desorption. Formulation composition and freezing conditions influence the material properties relevant to subsequent drying. Process development therefore needs to consider both the physicochemical characteristics of the formulation and the heat- and mass-transfer conditions of the drying process. [5]
Two glass transition temperatures describe different material states. The glass transition temperature of the maximally freeze-concentrated solution, Tg′, characterises the frozen formulation, whereas the glass transition temperature of the dried product, Tg, is relevant to the dried matrix during storage. Collapse temperature, Tc, describes a separate structural limit relevant to primary drying. These parameters are related but should not be treated as interchangeable measures of formulation performance. [1,4,5]
A higher critical product temperature may allow less restrictive primary-drying conditions. Nevertheless, cycle selection must also account for product quality and engineering constraints. Thermal characterisation provides a basis for process design, rather than evidence that a particular protein will remain stable. [4,5]
Primary drying and protein activity in a model system
Strøm Larsen et al. investigated the thermal properties of dextrans of different molecular weights and evaluated Dextran 1, Dextran 40 and their mixtures using lactate dehydrogenase (LDH) as a model protein. The study compared these formulations with sucrose under the investigated drying and storage conditions. [1]
• Primary drying was completed in approximately 5 hours for the dextran-containing formulations, compared with 16.5 hours for the sucrose formulation.
• Dextran 40 formulations retained approximately 97% LDH activity immediately after freeze-drying and 94% after seven weeks at 40°C.
• The sucrose formulation retained approximately 86% activity immediately after freeze-drying and 27% after seven weeks at 40°C.
The shorter primary-drying time was associated with the higher Tg′ of the dextran formulations, which allowed a higher primary-drying temperature. The activity results provide evidence for protection of LDH under the tested conditions; they do not demonstrate an equivalent benefit for other proteins or a corresponding reduction in total cycle duration.
Combining dextran with sucrose or trehalose
In a study of freeze-dried actin, Allison et al. found that adding dextran to sucrose or trehalose increased the formulation’s Tg while preserving the disaccharide’s ability to limit protein unfolding during lyophilisation. The combinations improved storage stability compared with the individual excipients in the systems investigated. These findings support considering matrix properties and structural protection together when evaluating excipient combinations. [2]
The authors also described dextran as an amorphous bulking agent that supported the formation of mechanically strong cakes during rapid lyophilisation. Cake structure and protein stabilisation represent distinct formulation attributes, even when both are influenced by the excipient system. [2]
Evidence from actin formulations provides a rationale for investigating dextran-disaccharide blends, but does not establish a generally applicable composition. The protein, excipient ratio and drying and storage conditions remain important variables. Studies in monoclonal antibodies further illustrate why the properties of a blend need to be assessed experimentally rather than inferred from Tg alone. [2,4]
Physical properties of the dried sugar mixtures
Imamura et al. investigated freeze-dried sucrose-dextran mixtures containing dextrans of different molecular weights. The effect on Tg depended on composition and relative humidity. Adding a small amount of dextran was also reported to prevent collapse of the amorphous sugar during freeze-drying. [3]
These findings describe the physical behaviour of sugar mixtures and are relevant to matrix design with other components mixed with dextran. But, they should not be interpreted, for example, as evidence of retained biological activity or chemical compatibility with a protein.
Molecular weight as a formulation parameter
Molecular weight is a relevant formulation variable, rather than a criterion that can be optimised independently. In the LDH study, glass transition temperatures generally increased with dextran molecular weight before approaching a plateau around 40 kDa. This observation relates to the thermal properties measured in that system and does not establish 40 kDa as a universal optimum. [1]
Haeuser et al. extended this assessment to dextrans of 1, 40, 150 and 500 kDa and their mixtures with sucrose. Using bovine serum albumin to examine formulation properties, they observed higher collapse temperatures with medium- to high-molecular-weight dextrans, particularly at higher dextran-to-sucrose ratios. Dextran-containing formulations also showed higher dried-product Tg and lower residual moisture, but slower reconstitution. These findings illustrate a balance between thermal properties and other product attributes. [4]
Chemical stability: evidence from monoclonal antibodies
In the same study, Haeuser et al. evaluated two monoclonal antibodies in sucrose-dextran formulations after storage at 40°C for 14 days. Although relative potency was retained, size-exclusion chromatography and isoelectric focusing revealed increases in high-molecular-weight and acidic species that depended on dextran molecular weight. Retained potency therefore did not exclude changes in other stability-indicating measurements. [4]
Further analysis of selected formulations containing 1 kDa dextran linked high-molecular-weight species to glycation involving dextran’s terminal glucose group. Adding sucrose substantially reduced this modification in the formulations examined. The findings support explicit assessment of chemical compatibility and show why physical matrix properties alone are insufficient to establish protein stability. [4]
Together, the LDH, actin and antibody studies describe different aspects of formulation performance. Their results are not interchangeable: a dextran grade or blend that performs well for one protein cannot be assumed to provide the same protection for another. [1,2,4]
Implications for formulation and process development
The process-design framework described by Tchessalov et al. considers freezing, primary drying and secondary drying as linked stages governed by formulation properties and engineering conditions. Their guidance combines thermal characterisation with heat- and mass-transfer modelling to support selection of drying conditions. It provides general process-development context, rather than dextran-specific evidence of stabilisation. [5]
For dextran-containing formulations, the combined evidence supports comparing candidate molecular weights and dextran–disaccharide ratios using complementary measurements: thermal behaviour, protein activity or potency, aggregation, chemical modification, residual moisture, cake structure and reconstitution. Drying performance and storage stability should be evaluated separately, under conditions relevant to the intended product. [1-5]
Considering dextran in formulation development
Dextran is a candidate excipient for modifying the physical properties of lyophilised formulations. Its suitability depends on the protein, molecular weight, excipient combination and process. The available studies provide a basis for experimental evaluation, not a general claim of superiority over sucrose or trehalose. [1-5]
Information on the Pharmacosmos dextran portfolio is available at dextran.com. For enquiries about molecular weights, pharmaceutical quality and formulation requirements, contact dextran@pharmacosmos.com.
Frequently asked questions
What is the role of dextran in lyophilisation?
Dextran has been investigated as a cryo- and lyoprotective excipient and as an amorphous bulking agent. Its effects on protein protection, thermal behaviour and cake structure depend on the formulation. These functions should be evaluated separately. [1-4]
Can dextran reduce freeze-drying time?
Shorter primary drying was demonstrated in the LDH model study. The result supports investigating dextran during process development, but does not establish an equivalent reduction in total cycle duration or the same outcome for other products. [1,5]
Can dextran be combined with sucrose or trehalose?
In short, yes. Dextran-sucrose and dextran-trehalose combinations improved storage stability in the actin study. In antibody formulations, adding sucrose reduced dextran-associated glycation in the systems examined. These findings support screening blends, while establishing the appropriate composition for each protein. [2,4]
Why was Dextran 40 of interest in the LDH study?
Dextran 40 combined favorable thermal properties with retained LDH activity in the investigated model. This makes it a relevant candidate for comparison, but the study does not establish it as the preferred molecular weight for every protein formulation. [1,4]
Does a higher glass transition temperature guarantee protein stability?
No. A higher Tg describes a property of the dried matrix, not a complete measure of protein stability. Antibody formulations showed stability-indicating changes despite favourable thermal properties and retained relative potency. Activity or potency therefore needs to be considered alongside physical and chemical stability measurements. [4]
Can Dextran 40 replace sucrose or trehalose in every formulation?
No universal replacement claim is supported. The LDH results are specific to the protein and conditions studied, while the actin and antibody studies highlight the importance of excipient combinations and protein-specific compatibility. [1,2,4]
What should be considered when selecting dextran for a lyophilised formulation?
Selection should consider molecular weight and excipient ratio alongside thermal behaviour, retained protein activity or potency, aggregation, chemical compatibility, residual moisture, cake structure and reconstitution. The antibody study specifically highlights glycation as a potential risk that should not be overlooked when evaluating dextran-containing formulations. [1-5]
References
Additional related references
Toni Vitasovic is a Development Chemist at Pharmacosmos, working within CMC pharmaceutical development. His experience spans formulation and process development, analytical characterization, and CMC activities supporting regulatory submissions, with a focus on iron-carbohydrate drug substances and products. He holds a PhD in Nanoscience from the Interdisciplinary Nanoscience Center (iNANO) at Aarhus University, Denmark.