(M1530-11-62) Method Development and Validation for Leachable Silicone Oil in Pharmaceutical Drug Products by Graphite Furnace AAS

Purpose: During the process of bringing a new drug product (DP) to the market a lot of test phases need to be conducted and should be successful to ensure patient’s safety. Besides the clinical trials which focus on the efficacy of the medicinal product itself, some studies are designed to study the interaction between the drug product and its container closure system (CCS), the so-called Extractables and Leachables studies. These analytical chemical studies are obligatory from the regulatory point of view and their main objective is to identify the chemical molecules or elemental species which leach into the DP whereafter a toxicological assessment can be done. Due to its chemical characteristics, silicone oil is used in a wide variety of applications, from the use in medical equipment to contact lenses and in the manufacture of elastomers, e.g. rubber plungers and needle caps in disposable or single-use syringes. It also used in shampoo, as a lubricant, with heat-resistant tiles, etc. Silicone oil itself, also referred to as PDMS (PolyDiMethylSiloxane; CAS 63148-62-9), is not considered to be a toxic chemical species within Extractables and Leachables studies, but unfortunately the presence of PDMS may have detrimental effects on the drug products (DP) when it contains proteins as it’s a well-known phenomenon that the latter may denature or degrade in the presence of silicone oil leading to a decreased activity of the medicinal product. When silicone oil is a leachable in orally inhaled nasal drug products (OINDPs) it has an impact on the size of the aerosol droplets leading to a decreased delivery of the active ingredient to the patient. Obviously, the effects in these two application domains are unwanted. The presence of silicone oil, even in trace concentrations, is an issue within the field of Leachables studies for certain applications, e.g. protein containing DP in single-use syringes, and there’s a need for an analytical technique capable of measuring low concentrations of PDMS in these challenging pharmaceutical matrices. Moreover, this analytical method, not readily available in literature, should be able to distinguish between inorganic silicon (e.g. SiO₂ when using a glass syringe) and organosilicon compounds (like PMDS) in aqueous-based drug products. PDMS exhibits a low solubility in water and its viscosity, depending on the chain length, is a key characteristic affecting this.

Methods: A Perkin Elmer AA600 GF-AAS Graphite Furnace Atomic Absorption Spectrometer was used with a L’Vov Platform for the thermal atomization, a wavelength of 251.6 nm for the analysis of the elemental Si by absorption spectroscopy, a background correction by means of the Zeeman-effect (magnetic field), the use of an autosampler, a peak area integration during 5 seconds of atomization, drying and ashing steps preceding the atomization. Chemicals used to conduct the method validation are of high grade quality with CoA’s.

Results: First step in the method development was the selection of the appropriate solvent for the GF-AAS analysis as due to the inorganic Si possibly also present, the drug product needs to be extracted using an organic solvent, targeting the complete dissolution of PDMS therein. A GF-AAS is designed to analyze aqueous samples, some instrumental parameters like the injection speed need to be optimized for this organic solvent injection of typically 40 µL. The absorption spectrum of PDMS as measured by GF-AAS (Zeeman-corrected) is shown in Figure 1 for a series of calibration standards between 60 and 600 µg/L PDMS, including the blank solvent injection. The linearity for silicone oil in the organic solvent was proven on various days and by various operators exhibiting good calibration curves with each time a correlation coefficient ≥ 0.995 indicating good reproducibility (an example shown in Figure 2). The analytical method, i.e. the combination of the extraction procedure and analysis by means of GF-AAS, was fully validated according to the international ICH-guideline including a.o. accuracy and precision at three different concentration levels, both repeatability and intermediate precision, method detection and quantification limits, specificity, linearity. Examples of typical experimentally determined detection and quantification limits in UltraPure water and in some Drug Products are given in Table 1. The QL is determined based on six replicates (n =6) and complies to an average recovery between 75 and 125 % and a RSD < 25 %. The results demonstrate the capability of this GF-AAS method to detect and quantify silicone in aqueous based drug products at trace concentrations.

Conclusion: A fully validated, analytical method, i.e. Graphite Furnace Atomic Absorption Spectrometry (GF-AAS), has been developed, sensitive enough to quantify silicone oil in concentrations at trace concentrations in various aqueous-based drug products. The method involves a solvent extraction procedure prior to the GF-AAS analysis.

References: Kuo, Alex C. M. Polydimethylsiloxane. [book auth.] James E. Mark. Polymer Data Handbook. s.l.: Oxford Unversity Press, Inc., 1999.
Silicone Oil Induced Aggregation of Proteins. Latoya S. Jones, Allyn Kaufmann, C. Russel Middaugh. s.l.: Wiley Interscience, 2005, Journal of Pharmaceutical Sciences, Vol. 94, pp. 918-927.
International Conference on Harmonization ICH Q2(R1): Guidance on Validation of Analytical Procedures: Text and Methodology, November 2005.
‘Guidance for Industry, Container Closure Systems for Packaging Human Drugs and Biologics’ FDA (1999).
‘Guideline on Plastic Immediate Packaging Materials’ EMA (2005).
USP Assessment of Extractables Associated with Pharmaceutical Packaging/Delivery Systems (2020).
USP Assessment of Drug Product Leachables Associated with Pharmaceutical Packaging/Delivery Systems (2020).
USP Orally Inhaled and Nasal Drug Products (2015)
USP: Plastic components and systems used to manufacture pharmaceutical drug products and biopharmaceutical drug substances and products (to be official 2026).

Figure 1: GF-AAS absorption signal for PMDS as a function of increasing concentration at 251.6 nm and Zeeman corrected. (peak area and read time 5 seconds)

Figure 2: Calibration curve for PMDS (Dow Corning 360, 1000 cSt) analysis in an organic solvent by GF-AAS at 251.6 nm and Zeeman corrected. (peak area and read time 5 seconds)

Table 1: Detection and quantification limits experimentally determined in UltraPure Water and three Drug Products by GF-AAS (n = 6 at QL concentration).