Why Patients Want Prefilled Syringes
Ask a patient which they would prefer for a monthly self-injection: a glass vial, a needle, and a syringe they have to assemble themselves, or a prefilled autoinjector they click and hold against their arm. The answer is obvious.
Prefilled syringes and autoinjectors have become the standard delivery format for biologics intended for self-administration. They reduce administration errors, improve patient adherence, and eliminate the multi-step preparation that creates both errors and anxiety. For many biologic drug products, the prefilled syringe is not a convenience upgrade; it is a commercial requirement.
The formulation challenge is that the syringe barrel is not a passive container. It is a component that interacts with the drug product. Getting that transition right takes careful development work.
The Key Compatibility Challenges
Silicone Oil
Glass syringes are siliconised to allow the plunger to slide smoothly during injection. The silicone oil droplets that migrate into the drug solution are a known source of protein aggregation for biologic drug products. High shear at the needle tip during injection can further fragment silicone droplets and increase the surface area available for protein adsorption.
The risk is managed by selecting syringe formats with optimised siliconisation (cross-linked silicone; baked-on silicone coatings) and by demonstrating in formulation development that the drug product is tolerant to the silicone levels expected in the chosen syringe. For highly aggregation-prone proteins, silicone-free syringe options including cyclic olefin polymer (COP) or cyclic olefin copolymer (COC) barrels may be required.
Tungsten Residues
Glass syringes are formed using tungsten pins to create the needle tip geometry. Residual tungsten at low microgram levels can remain in the syringe barrel after manufacture and can interact with some proteins to induce aggregation or precipitation. Tungsten compatibility must be assessed during development by incubating the drug product in the intended syringe format and monitoring for aggregation by DLS or SEC.
Extractables and Leachables
Every component that contacts the drug product, the syringe barrel, the rubber plunger stopper, the tip cap, and the needle, can contribute extractable compounds that migrate into the solution over the shelf life of the product. For prefilled syringes, the longer contact time and the absence of a separate packaging step mean the extractables profile is more complex than for a vial-stopper system.
The PQRI leachables guidance for parenteral and ophthalmic drug products and the ICH Q3E guideline on extractables and leachables provide the framework for extractables and leachables assessment. A complete E&L programme for a prefilled syringe system is a significant analytical undertaking that must be planned well in advance of the registration-stage stability studies.
Formulation Parameters That Affect PFS Performance
| Parameter | Effect in PFS Format | Development Consideration |
| Viscosity | High viscosity formulations require greater injection force; affects autoinjector spring design and patient experience | Target viscosity below 20 cP for standard autoinjectors; reformulate or dilute if above 50 cP |
| pH and buffer | Plunger stopper rubber contains components that can affect pH; buffer capacity must maintain pH over shelf life | Confirm pH stability in the syringe over proposed shelf life; select low-extractables stoppers |
| Surfactant type and concentration | Surfactants protect against agitation-induced aggregation; must be compatible with syringe components | Polysorbate 20 or 80 standard; confirm compatibility with silicone and rubber components |
| Protein concentration | Higher concentrations increase aggregation risk from silicone and tungsten interactions | Assess aggregation at target concentration in the syringe format during compatibility studies |
| Preservatives | Multi-dose PFS may require preservative; single-dose typically preservative-free | Benzyl alcohol and phenol are common; confirm antimicrobial efficacy and compatibility with drug |
Device Development: The Formulation-Device Interface
The autoinjector or pen injector selected for a self-injectable drug product must be compatible with the formulation parameters, particularly viscosity, fill volume, and injection speed. These are not independent design choices. A formulation with a viscosity of 40 cP will require a different spring force than one at 5 cP, which changes the injection duration and the force felt by the patient.
Device selection should happen in parallel with formulation development, not after it. The target product profile for the device and the formulation should be established together, with viscosity, fill volume, and container closure system selected as a package.
Ardena’s Injectable Formulation Capabilities
Ardena’s formulation and analytical teams at Ghent support injectable drug product development for both vial and prefilled syringe formats, including compatibility assessment, extractables and leachables planning, and GMP fill-finish of clinical batches. The team has experience with complex biologic formulations and the specific challenges that arise in the transition from vial to PFS.
