Why Nanoparticle Fill-Finish Is Different
Aseptic fill-finish for a conventional small molecule injectable involves filling a solution into vials under Grade A conditions and sealing them. The process is technically demanding from a contamination control perspective, but the drug product itself is chemically robust enough to withstand the shear forces, surface contacts, and process-related stresses involved without significant changes to its physicochemical properties.
Nanoparticle drug products, including LNPs, liposomes, and polymeric nanoparticles, are far more sensitive to process-related stress. The particles are nano-scale assemblies held together by relatively weak non-covalent interactions, and they can be disrupted by shear forces during pumping and filling, by adsorption to surfaces in the filling equipment and vial, by temperature fluctuations, and by changes in the ionic strength or pH of the vehicle. Any of these events can lead to aggregation, particle size increase, loss of encapsulation efficiency, or active pharmaceutical ingredient loss to surfaces, all of which compromise the quality and clinical performance of the product.
Particle Loss to Surfaces: The Adsorption Problem
One of the most insidious sources of nanoparticle loss during fill-finish is adsorption of particles or their payload to the surfaces of the filling equipment and the vial itself. Lipid nanoparticles, in particular, are prone to adsorbing to hydrophobic surfaces including the tubing used in peristaltic filling pumps, the silicone gaskets in filling needles, and the inner surface of glass vials. mRNA LNPs have been shown to lose a significant fraction of their payload to glass and silicone surfaces under some conditions, reducing the delivered dose below the intended level.
Mitigation strategies include the use of siliconised or coated vials to reduce surface adsorption, the selection of filling equipment materials that are compatible with the specific nanoparticle formulation, and the use of carrier proteins or surfactants in the formulation that preferentially adsorb to surfaces and reduce nanoparticle-surface contact. The extent of surface adsorption must be assessed during process development by measuring drug content in the first and last vials filled in a batch and comparing to the bulk solution.
Shear-Induced Aggregation During Pumping and Filling
Peristaltic pumps and other filling mechanisms subject the drug product to shear forces as it passes through the tubing, pump head, and filling needle. Conventional small molecule solutions are unaffected by these shear forces, but nanoparticle dispersions can aggregate when exposed to sufficiently high shear, particularly at the outlet of the filling needle where flow velocities are highest. Aggregation during filling increases particle size, broadens the PDI, and can eventually produce visible particles that would cause a batch failure.
The shear sensitivity of a specific nanoparticle formulation must be evaluated during process development by measuring particle size and PDI before and after exposure to the filling conditions. Where shear sensitivity is identified, peristaltic pump settings (tube diameter, pump speed, fill volume) must be optimised to keep shear forces below the threshold for aggregation, and the filling process parameters must be included in the validated process description.
Fill-Finish Process Parameters Critical for Nanomedicines
| Process Parameter | Risk to Nanoparticle Product | Control Strategy |
| Pump type and speed | Shear-induced aggregation; particle size increase | Evaluate peristaltic, piston, and time-pressure pumps; optimise speed to minimise shear without sacrificing fill accuracy |
| Tubing and wetted surface materials | API adsorption; particle disruption at interfaces | Qualify tubing materials for compatibility with specific formulation; use low-adsorption tubing where available |
| Fill temperature | Particle instability if temperature deviates from formulation optimum | Maintain product temperature during filling; temperature log in filling suite |
| Fill volume accuracy | Incorrect dose delivered; overfill required to compensate for surface losses | Account for surface adsorption losses in fill volume calculation; validate overfill requirement |
| Headspace gas | Oxidation of lipid components if air contact occurs | Nitrogen purge of vial headspace before stoppering for lipid-sensitive products |
| Vial treatment | Enhanced adsorption to untreated glass | Evaluate siliconised, PTFE-lined, or alternative-coated vials for high-adsorption formulations |
Analytical Monitoring During Nanoparticle Fill-Finish
In-process monitoring during nanomedicine fill-finish goes beyond the standard fill weight checks used for conventional injectables. Particle size and PDI should be measured on samples taken at the beginning, middle, and end of the fill to detect any process-related changes. Encapsulation efficiency should be confirmed on released product. For mRNA LNP products, mRNA integrity by gel electrophoresis and potency by in vitro translation assay are additional release tests that confirm the product has not been damaged during the fill-finish process.
Ardena’s Nanomedicine Fill-Finish Expertise at Ghent
Ardena’s sterile manufacturing team in Ghent has specific experience with the fill-finish requirements of nanomedicine products, including LNPs and liposomal formulations. Process development for nanoparticle fill-finish at Ardena includes surface adsorption assessment, shear sensitivity testing, and in-process particle size monitoring to ensure that the filled product meets its CQA specifications.
