Undesirable protein aggregations can thus arise in the filled syringes during storage, which impair the effectiveness of the medication. Syringe system manufacturers like Gerresheimer therefore strive for a reduction or avoidance of syringe components like silicone oil or tungsten. This article concerns itself with the reduction and avoidance of tungsten in prefillable glass syringes.

Gerresheimer in the meantime offers a broad spectrum of tungsten-reduced and tungsten-free prefillable syringes:

• Washing process directly following syringe forming for the significant reduction of the tungsten load
• Replacement of tungsten pins with other metals: Gx RTF^® tungsten-free syringe
• Metal-free GX RTF^® glass syringe, ceramic pin, tungsten-free
• ClearJect^® and Gx RTF^® ClearJect^® COP plastic syringe, tungsten-free

Use of the tungsten pin
Tungsten is a contact material ideally suited to and often used for the production of glass syringes. When the glass tubes are heated to approx. 800 – 1,200°C, form rollers shape these into a cone shape at the later front end of the syringe. In order to keep the bore open and define its interior diameter, a tungsten pin is inserted into every syringe to this purpose.

The advantages of tungsten are:

• Heat-resistance, expansion coefficient similar to that of borosilicate glass
• Self-lubricating
• Flexible, easy to handle

Disadvantages can occur when traces of tungsten remain following the forming process in the cone area:

• Abrasion
• Tungsten oxide
• Possible later interaction with proteins after filling of liquid drugs

Influence of tungsten on biotechnology medications
Tungsten is in fact a heavy metal, but the potentially remaining tungsten quantities are extremely minor and have no toxic effect on patients. Problematic on the other hand is the fact that some biotechnology medications react sensitively to traces of tungsten or their oxides and can induce protein aggregations. This takes place as the result of electrostatic molecular interactions or the formation of chelate complexes with oxygen, nitrogen, or sulfur atoms, which are a component of the protein molecules. As a consequence of the cone-shaping process, invisible or visible deposits can form in the cone area, which are no longer acceptable to the pharmaceuticals customer.

Tungsten oxides are initially not detected during the production of (empty) glass syringes. Potential incompatibility only becomes conspicuous during the stability studies carried out by the pharmaceuticals companies. Pharmaceuticals companies are called upon to test the compatibility of their medications with the primary packaging material. The use of tungsten-reduced or ideally metal-free produced syringes thereby reduces the risk of protein-packaging material interactions.

Regulatory positions can be found in, for example:

• the FDA Guidance for Industry – Immunogenicity Assessment for Therapeutic Protein Products and
• the PDA Technical Report No. 73 – Pre-filled Syringe User Requirements for Biotechnology Applications.

These documents recommend special leachables and extractables analyses, incl. reference to tungsten. Exposition studies (“spiking studies”) are recommended for the tungsten analysis in order to evaluate the risk of interactions. Particularly the chemical milieu (pH value) is decisive for the occurrence of tungsten-induced protein aggregations:

• Tungsten polyanions are primarily created in the acidic milieu
• Proteins can form complexes with polytungstates; depending upon the pH value, these are partially reversible
• Sodium tungstate (Na2WO4) is the most potent molecule.

Only a few protein formulations are to date demonstrably known to form visible deposits in pre-filled syringes, meaning that tungsten compounds only generate protein aggregations in a few cases. Examples include Epoetin Alfa (Seidl et al. 2012) and several monoclonal antibodies at low pH values (Bee et al. 2009).

Influence of the filling technology
The filling process of the prefillable syringe can also have an influence on the formation of protein aggregations due to tungsten. In the case of the vacuum stoppering process, the liquid comes into direct contact with the inner cone, as, in contrast with the stoppering tube process, no small air bubbles remain between the liquid and inner cone. This direct contact can bring tungsten traces into contact with the sensitive proteins during the storage period.

Tungsten values in the production process
Gerresheimer has carried out a variety of studies on the measurement of tungsten in the syringe cone. The cone diameter has a strong influence on these measuring values, meaning that values of 50-500 ppb/syringe can be observed with RTF syringes (RTF needle syringes, washed, siliconized, nested, and sterilized). Individual outliers caused by fluctuations in the process temperature or abrasion of the tungsten pins are also possible.
Another effect is of relevance for needle syringes: Because the needles are glued in, the glue seals the inner bore, which contains tungsten, to a great extent, meaning that fewer tungsten oxides are later found on the surface.

Washing process for tungsten reduction
At Gerresheimer, defined, very low tungsten values can already be achieved by a special washing process directly following syringe body production. This washing step is integrated into the glass forming line and should not be confused with the WFI washing step of RTF production. On average, the tungsten load can in this way be reduced by approx. 90%. Visible particles are also significantly reduced in this process step.

This washing process can be carried out with all syringe formats (0.5 ml, 1 ml, 1-3 ml) and all cone shapes (Luer cone, Luer lock, staked-needle). Another “final rinse” with WFI later takes place in the RTF process.
The washing process takes place directly following the shaping of the syringe body. The measurements were carried out in accordance with the specifications in the USP (NF <730> Plasma Spectrochemistry) and the Ph. Eur. (2.2.58 Inductively Plasma Coupled-Mass Spectroscopy).

Tungsten-free and metal-free glass syringes
With a few decisive technical adaptations of the Gerresheimer glass shaping line, production of syringes with other metal pins or with ceramic pins can also be carried out. When replacing other metals, one speaks of tungsten-free syringe production; with ceramic pins of metal-free production. Metal-free Luer cone and Luer lock syringes in all syringe sizes (0.5 ml, 1 ml long, 1-3 ml standard) are available. Metal-free in this case does not refer to components of the glass or impurities (elemental impurities), but instead to the material of the pin used.

Metal-free syringe – choice of the ceramic pin
The material selection for the pin is an important element in process development. Besides the stability of the process, product quality is also decisive. The ceramic material may thus not generate any particles (abrasion) and the pin material should fulfil biocompatibility requirements in accordance with ISO 10993. Experiments with scanning electron microscopy (REM) in combination with X-ray microanalysis (EDX) were also carried out for the ceramic material used. They show no traces of the ceramic material in the bore. The pin material is hard, durable, and abrasion-resistant. The biocompatibility tests also show no conspicuous features in contact with living cells. The material is not cytotoxic, is suitable for syringe production and fulfils the requirements of the ISO 10993-5 and the USP <87> (Elution test).

COP syringes
As an alternative to glass syringes, plastic syringes (COP) can also be used as a primary packaging material when sensitivity of the liquid formulation exists with regard to tungsten. In the case of ClearJect^® and Gx RTF^® ClearJect^® COP syringes, no tungsten pin is used in production. The production process in injection molding takes place with an in any case tungsten-free mold.

Conclusion
The reduction of tungsten and the metal-free production make a big contribution to making pre-filled syringes safer. The prefillable syringe can thus be used even more broadly and is even better suited for sensitive biotech drugs. In past years, Gerresheimer has already made a decisive contribution to the improvement of sterile, pre-fillable syringe systems with other innovations.

• The baked-on RTF^® syringe reduces the load with silicone oil to less than 10% in comparison with spray siliconization
• Optimized siliconization processes for spray siliconization improve the product properties like breakaway and slide forces
• Compatibility with automatic injection systems has been ensured. The company’s own G3 camera system hereby makes a great contribution to product quality and increases customer benefits
• Extensive functionality studies help find the best syringe system in connection with dedicated plunger stoppers
• Own closure systems like the TELC^® Luer lock seal add additional security with the integrated temper evident feature
• Reduction of glass to glass contact and particle reduction in the production process thanks to the most modern RTF technology.