These tungsten residuals have been reported to have a marked effect on drug products stored in prefilled syringes. Lee et al. examined the aggregation of proteins resulting from leached tungsten (11). They report that concentrations of tungsten as low as 1 ppm in the drug product can produce measurable protein aggregation. Analysis of 500 syringes from various manufacturers indicated that, in general, the level of leached tungsten is well below this critical concentration. These authors suggest that tungsten speciation in solution is an important factor in protein aggregation, with large tungstate polyanions (the dominant aqueous species at low pH) having a greater potential impact on aggregation than the smaller tungstate anion that dominates tungsten speciation at higher pH values.

Similarly, Osterberg reports a situation in which tungsten leached from a syringe caused the oxidation of the contained drug product, resulting in drug degradation and aggregation (12). This interaction ultimately led to an unacceptable drug product. Rosenberg and Worobec also report the tungsten-induced aggregation of a protein arising from oxidation mediated by leached tungsten (13), and Markovic documents a case where leached tungsten oxide triggered protein oxidation followed by aggregation (14). In the latter case, the aggregation was mitigated by switching from a tungsten to a platinum filament. Finally, Wen et al. report a case in which protein aggregation results from small particles of tungsten that were present in the drug product as a result of its storage in a prefilled syringe (15).

Silicon oil. Silicon oil is a common lubricant used in many container–closure systems, including prefilled syringes. The potential influence that silicone oil can have on the viability of protein-drug molecules is well documented.

Leaching. One of the most widely documented instances of an unanticipated incompatibility between a container–closure system and a protein-drug product is that of "Eprex" (epoetinum alfa) (Janssen-Ortho, ON, Canada) and its prefilled syringe packaging system (19–21). At some point in its product lifetime (1998), Eprex, a product containing recombinant human erythropoietin, was reformulated with polysorbate 80, which replaced human serum albumin as a formulation stabilizer. Shortly after this change, there were increased incidents of antibody-mediated pure red cell aplasia (PCRA) with Eprex use by chronic renal failure patients. The cause of PCRA was directly linked to the formation of neutralizing antibodies to both recombinant and endogenous erythropoietin in patients adminstered Eprex.

A considerable, crossfunctional technical effort was undertaken to establish the root cause of this phenomenon. One potential root cause involved leached substances. The presence of previously unidentified leachables was suggested as new peaks in the tryptic map of Eprex. Leaching studies determined that the polysorbate 80 extracted low levels of vulcanizing agents (and related substances) from the uncoated rubber components of the prefilled syringe. This leaching issue was addressed by replacing the rubber components with components coated with a fluoropolymer. Because the fluoropolymer is an effective barrier to migration, the leaching of the rubber components was greatly reduced. Since the conversion from the uncoated to the coated components, the incidence of PRCA has returned to the baseline rate seen for all marketed epoetin products. This is strong circumstantial evidence that leaching of the vulcanizing agent was, in fact, the root cause of the observed effect. The circumstantial case against rubber leachables as the root cause was further strengthened when the adjuvant effect of the leached vulcanizing agent (and related substances) was confirmed in animal models. Nevertheless, it is noted that the link between the extracted vulcanizing agent and the adverse patient effect has not been conclusively established and there are alternate proposals as to the root cause of this phenomenon.