(T1330-01-03) Increasing Oxidative Stability with High Purity Excipients Used in Ophthalmic Formulations

Purpose: Excipients such as flaxseed oil, castor oil, polysorbate 80 and PEG 400 employed in ophthalmic formulations are prone to oxidation due to the high level of unsaturation present in their fatty acid profile and in ether linkages.^1-3 Reducing the impact of the oxidative impurities like acids, peroxide and aldehydes is critical for improved formulation stability and safety.^{4, 5} Therefore, using high purity excipients in ophthalmic formulations would be beneficial. In this study, we aim to reduce these oxidative impurities in these ophthalmic excipients and compare different purity materials for their stability over long term.

Methods: All four excipients were purified using proprietary purification processes to generate high purity (HP) materials. The HP and standard excipients were packed under nitrogen and air, respectively and stored in an incubator at 37 °C for a period of six months. The samples were pulled at regular timepoints to assess their stability by measuring their peroxide value, acid value, carbonyl value, and aldehyde content. Peroxide value and acid values for all excipients were expressed as mg of KOH required to neutralize a gram of the test material (mg KOH/g). The amount of formic acid and acetic acid generated over weeks for PEG 400 was determined using HPLC. The total aldehydes for flaxseed oil and castor oil were assessed using the CDR Foodlab anisidine kit and carbonyl value, respectively whereas formaldehyde content for the Polysorbate 80 and PEG 400 were assessed using HS-GC. The differences between the different material grades were also assessed by measuring color, viscosity, and bacterial endotoxins. Preliminary cellular irritation studies were also conducted using different purities of polysorbate 80 to emphasize the importance of removing oxidative impurities.⁶

Results: The HP excipients packed under nitrogen were observed to have better stability and displayed lower peroxide build up and acid values when compared to standard excipients. Castor oil was observed to be less susceptible to oxidation and possessed lower acid and peroxide values over flaxseed oil. This may be due to the presence of α-linolenic acid as the major fatty acid in the latter making it more prone to oxidation. For flaxseed oil, HP materials packed under nitrogen showed low anisidine values indicating less amount of total aldehydes generated over time when compared to the standard materials packed under air. The carbonyl value for HP castor oil was also observed to be lower than standard material indicating higher stability. The headspace GC analysis on Polysorbate 80 and PEG 400 revealed lower amount of formaldehyde generated for the HP excipients. The HP materials display better color compared to standard materials. The HP flaxseed oil was observed to possess lower viscosity than the standard flaxseed oil. No differences of viscosity were observed between the HP and standard castor oil. The viscosity for HP Polysorbate 80 and PEG 400 was found to be within range of the USP monograph specifications. The endotoxins for all excipients were found to be within range of the USP monograph. Preliminary studies with polysorbate 80 revealed that the HP excipients reduces cellular irritation potential by 40%.

Conclusion: Experiments conducted to assess the long-term stability of ophthalmic excipients revealed several general trends with HP excipients displaying superiority over standard excipients. These HP excipients possess fewer oxidative impurities in comparison to the standard material indicating higher stability for the former. The superior quality of the HP excipients would be advantageous towards the stability and safety of ophthalmic formulations, making them highly recommended for these drug applications.

References: (1) Musakhanian, J.; Rodier, J.-D.; Dave, M. Oxidative Stability in Lipid Formulations: a Review of the Mechanisms, Drivers, and Inhibitors of Oxidation. AAPS PharmSciTech 2022, 23 (5), 151. 10.1208/s12249-022-02282-0
(2) Kerwin, B. A. Polysorbates 20 and 80 used in the formulation of protein biotherapeutics: structure and degradation pathways. J Pharm Sci 2008, 97 (8), 2924-2935. 10.1002/jps.21190
(3) Han, S.; Kim, C.; Kwon, D. Thermal/oxidative degradation and stabilization of polyethylene glycol. Polymer 1997, 38 (2), 317-323. https://doi.org/10.1016/S0032-3861(97)88175-X
(4) Khanum, R.; Thevanayagam, H. Lipid peroxidation: Its effects on the formulation and use of pharmaceutical emulsions. Asian Journal of Pharmaceutical Sciences 2017, 12 (5), 401-411. https://doi.org/10.1016/j.ajps.2017.05.003
(5) Devon Langbein, M. D. Key Considerations for Stabilizing Oxidation-Prone Lipid-Based Drug Delivery Systems. American Pharmaceutical Review, 2021. https://www.americanpharmaceuticalreview.com/Featured-Articles/577086-Key-Considerations-for-Stabilizing-Oxidation-Prone-Lipid-Based-Drug-Delivery-Systems/.
(6) Cottin, M.; Zanvit, A. Fluorescein leakage test: a useful tool in ocular safety assessment. Toxicol In Vitro 1997, 11 (4), 399-405. 10.1016/s0887-2333(97)00020-9

Acknowledgements:The authors claim no conflict of interest.

Pictorial representation of different purities of ophthalmic excipients.

Peroxide value trends in HP and standard flaxseed and castor oil.

Formaldehyde concentration trends in HP and standard Polysorbate 80 and PEG 400.