An Integrated Prefilled Syringe Platform Approach for Vaccine Development - The authors describe a holistic and integrated approach to focus on the linkage of the prefilled syringe. - BioPharm


An Integrated Prefilled Syringe Platform Approach for Vaccine Development
The authors describe a holistic and integrated approach to focus on the linkage of the prefilled syringe.

BioPharm International
Volume 23, Issue 6, pp. 22-29


The prefilled syringe is a complex system where the component characteristics often depend on the material of construction and processing conditions, which results in variability of the component attributes such as silicone oil level and distribution, as well as residual tungsten. Specifically, silicone oil and tungsten has been implicated in the induction of protein aggregation (6). It is also important to understand whether aggregation is a prerequisite for a cascade phenomenon leading to the visible particulate formation, which is not acceptable for a parenteral product. Not only does this create stability issues, proteinaceous aggregates presented in a highly arrayed structure of sub-visible size range, such as might be found in large non-denatured aggregate species, have been known to potentially interact with the immune system to enhance immune responses (7, 8). This interaction causes great concern and has been critically scrutinized by the regulatory bodies for therapeutic proteins. However, this should not be falsely perceived as a "hand waving favorable" immunogenicity risk assessment for vaccine products. It is categorically essential to ensure that product quality attributes including aggregate species, must be under strict control to demonstrate the intended immune response of the vaccine for the target disease and patient population. As stated earlier, the prophylactic nature of vaccine products impose a higher and more rigorous regulatory standard.

Figure 1: Silicone interaction and formation of visible particulates. (ALL FIGURES COURTESY OF AUTHORS)
In particular, the glass syringe barrel contains silicone oil for functionality purposes, which could migrate into the drug product solution during shelf-life storage. When protein interacts with silicone oil, there are potentially two phenomena that could lead to visible particulates formation. First, hydrophobic interaction could enhance protein aggregation as a prerequisite for visible particulates formation (6). On the other hand, the dispersed silicone oil droplets, although at low level, may be charged dependent on the formulation condition. The charge effect could lead to colloidal instability and flocculation of the silicone oil emulsion together with the protein, which has been reported in literature (9). The protein and silicone oil droplets are quite different in size microscopically, which can be characterized by a recently developed new technique of suspended microchannel resonator (SMR) (10). Silicone oil droplets are charged particles where surface properties have a large effect on emulsion stability. Both the concentration (i.e., quantity) and total surface area (i.e., number and size of oil droplets) are critical attributes to understand the protein silicone interaction phenomenon. Depending on the isoelectric point of the protein and pH of formulation, protein adsorption can change the surface charge properties, and therefore, kinetic instability of the silicone oil emulsion. As a result, the flocculation and coalescence of protein adsorbed silicone oil droplets can lead to the formation of visible particulates. This potential mechanism of action is illustrated in Figure 1.

These concepts are also highly relevant in the case of a complex vaccine product, for example, glycoconjugates. Effective immunization against encapsulated bacteria can be achieved by a type of vaccine based on oligosaccharide conjugated to a carrier protein, for example CRM197, a non-toxic mutant of diphtheria toxin (11). For a particular target disease, different serotypes with different polysaccharides would be needed to achieve majority coverage of the population. For example, serogroups A, B, C, W135, Y, and recently X account for the majority of meningococcal disease in humans (12). Bacterial polysaccharides are high molecular weight and mostly negative-charged carbohydrate molecules. Semi-synthetic glycoconjugates are complex molecules composed of protein and carbohydrate moieties linked together by chemical reactions. The conjugation reactions involve different chemistry and, therefore, different linking sites on the surface of the protein and along the saccharide chain. Moreover, the polysaccharides are high molecular weight carbohydrate moieties with some degree of polydispersion, which increases the heterogeneity of the resulting glycoconjugates.

Figure 2: Charge heterogeneity of three glycoconjugates A, B, C and carrier protein as control measured by isoelectric focusing gel.
The polysaccharides are mostly negatively charged, and there is also significant charge heterogeneity of these glycoconjugate vaccines. Figure 2 shows an example of the charge heterogeneity of three glycoconjugate vaccines as compared to the carrier protein measured by pI. The pI is a well-defined single band for the carrier protein but there is a significant dispersion of isoelectic point (pI) representing charge heterogeneity for the three different glycoconjugates. Each of these highly charge heterogeneous molecules are expected to interact with silicone in a different manner both individually in a monovalent and collectively in a trivalent formulation, which could lead to decrease colloidal stability of the low concentration silicone oil emulsion in a prefilled syringe. It is likely that visible particulates form when the silicone oil droplets associated with these glycoconjugates coalescence.

At present, there is little reported in the literature on systematic and comprehensive characterization of silicone glycoconjugate interactions under a variety of formulation conditions to support the successful development of these types of important vaccine products. On the other hand, it is also important to consider the silicone variability associated with the prefilled syringe components, such as the intra- and inter-batch variability plus the migration during storage before product is filled. After the product is filled, product storage, handling, shipment, agitation, and excursion all contribute to potential significant intra- and inter-batch variability.

In summary, a vaccine product such as glycoconjugate has additional complexity and challenges for prefilled syringe development when compared to therapeutic proteins. It is logical and reasonable to recognize that a large part of development effort should focus on understanding the components, and a standardized toolbox can help build such understanding and a better strategy to control the variability.

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