(M1130-10-58) Mechanistic Investigation of Gamma Irradiation Effects on Physicochemical Stability of Excipients Used in Colloidal Injectable Suspensions

Purpose: Sterility is a critical quality attribute of injectable dosage forms and needs to be assured using a suitably designed, validated, and controlled manufacturing process to ensure patient safety (1,2,3). However, ready-to-use long-acting injectable suspensions (LAIS) are not amenable to sterile filtration, and also present unique challenges for conventional terminal sterilization due to their colloidal nature and the risk of compromising physical and chemical stability. Consequently, LAIS are often sterilized using gamma irradiation, which assures sterility, but can also potentially inhibit the functionality of surfactants and polymers (used for colloidal stabilization) due to its ionizing effects. These excipients play a critical role in imparting colloidal stability and ease of resuspension throughout the shelf-life. This research aims to understand gamma irradiation effects on excipients widely used in colloidal suspensions, with a particular focus on surfactants and polymer stabilizers, noting that there is limited mechanistic understanding on gamma irradiation effects on stability of these excipients in LAIS formulations.

Methods: Mechanistic studies were conducted to investigate the effect of radiation dose and conditions on physicochemical stability, as well as functional properties of four excipients widely used in injectable products: polysorbate 20 & poloxamer 338 (non-ionic surfactants), and polyethylene glycol 3350 & sodium carboxy methyl cellulose (polymer stabilizers). Preliminary work involved gamma irradiation of neat aqueous solutions of these excipients at concentrations typical for injectable formulations [1%, 3%, and 5% w/v for all but sodium carboxy methyl cellulose (CMC) at 0.5% and 1% w/v].  Samples were prepared in 2R ISO Type 1 glass vials in two different presentations: 2.4 mL fill with a 1 mL headspace air (Group 1), and brimful fill with no headspace air (Group 2). A third group (Group 3) had the bulk solutions sparged with nitrogen gas and filled (2.4 mL) into vials with a 1 mL headspace also purged with nitrogen. These presentations were designed to investigate the influence of headspace gas (headspace air – Group 1, no gas headspace – Group 2 and nitrogen sparged & purged – Group 3) on polymer stability during irradiation. The vials were irradiated at three nominal doses (15 kGy, 25 kGy, and 40 kGy) at ambient temperature. Non-irradiated solutions served as control group. pH and viscosity were determined, as these tests are measurable indicators of physicochemical stability. Analytical methods for high-performance liquid chromatography and size exclusion chromatography with charged aerosol detection (HPLC-CAD and SEC-CAD) were developed for each excipient to quantify residual levels post-irradiation and irradiation-induced molecular weight shifts, which are indicators of chemical stability. Matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectra was collected to understand the molecular weight distribution shifts in irradiated samples and compared with that of non-irradiated controls.

Results: A drop in pH was observed for irradiated samples (versus non-irradiated controls). Visual observation revealed gelation for poloxamer 338 (all concentrations and radiation doses) and PEG3350 (1% w/v at all radiation doses) in nitrogen sparged & purged Group 3. HPLC-CAD chromatogram obtained for above gel samples (digested in mobile phase) did not show any peaks corresponding to the poloxamer 338 and PEG3350 main peaks seen in the neat and non-irradiated samples. This observation suggests that complete removal of oxygen in bulk solutions and headspace gas led to extensive crosslinking of these polymers upon irradiation in solution state. Interestingly, polysorbate 20 and CMC solutions in Group 3 did not show gelation. SEC-CAD chromatogram obtained for irradiated samples showed a broad second peak at early retention time with decrease in corresponding main peak intensity, which corroborates higher molecular weight specie generation (Figures 1, 2 and 3). MALDI-TOF analysis of irradiated samples confirmed molecular weight shift. This finding was consistent for all excipients (at all concentrations and radiation doses). Further, from HPLC-CAD data, all excipients were found to show a loss in potency, i.e., loss in main peak intensity at all concentration levels when exposed to different radiation doses. Percent degradation increases with radiation dose and decreases with higher concentrations for all excipients, except CMC, which showed only slight increase in % degradation for the higher concentration (0.5% versus 1% w/v). This finding for CMC is consistent for all irradiated Groups (1, 2 and 3).

Conclusion: Gamma irradiation plausibly affects native polymer structure and physicochemical properties of polymers in solution state. Radiation dose progressively affects chemical stability or potency of polymers in solution state. Modulating the headspace gas in vial has protective or controlling effect on preserving physicochemical stability of polymers. Radiation dose and headspace gas are critical considerations for physical and chemical stability of surfactant and polymer stabilizers during gamma irradiation in solution state.

References: 1.Lambert WJ. Considerations in developing a target product profile for parenteral pharmaceutical products. AAPS Pharm SciTech. 2010;11(3):1476-81.
2.Kolluru LP, Atre P, Rizvi SA. Characterization and applications of colloidal systems as versatile drug delivery carriers for parenteral formulations. Pharmaceuticals. 2021;14(2):108.
3.Lambert BJ, Hansen JM, Bryans TD, Lam S. Sterility Assurance Across-Sectors—New Paradigms and Tools. Frontiers in Medical Technology. 2021;3:622710.

Figure 1: Representative SEC-CAD chromatogram of gamma irradiated and non-irradiated (control) PEG3350 aqueous solution.

Figure 2: Representative SEC-CAD chromatogram of gamma irradiated and non-irradiated (control) polysorbate 20 aqueous solution

Figure 3: Representative SEC-CAD chromatogram of gamma irradiated and non-irradiated (control) sodium CMC aqueous solution.