(M1030-06-39) Impact of Polyethylene Glycol Polymer in the Development of Long-Acting Injectable Suspensions

Purpose: Polyethylene glycol (PEG) polymer is commonly used in long-acting injectable (LAI) suspensions to aid in the dispersion of hydrophobic active pharmaceutical ingredients (APIs) in aqueous media [1]. The presence of terminal hydroxyl groups and ether linkages imparts hydrophilicity to PEG, enabling it to enhance wettability, sterically stabilize particles, and inhibit API recrystallization. However, there is limited or no literature available on the impact of PEG source variability on in vitro drug release. Furthermore, several reports [2,3,4] indicate that LAI suspensions may induce local inflammation at the injection site, leading to the formation of a fibrous capsule around the injected depot volume, which could hinder in vivo drug release. The extent of capsule formation may vary depending on depot characteristics. Therefore, the objective of this research was to investigate the influence of PEG source variation on in vitro drug release and to explore the role of PEG in intramuscular depot formation.

Methods: Depo-Provera® (medroxyprogesterone acetate, MPA) was selected as a model LAI product. To evaluate the impact of polymer source variability, two formulations—FA and FC—were prepared using MPA from Spectrum Chemicals® as received. Formulation FA utilized a suspending medium containing PEG3350 from Spectrum Chemicals®, whereas formulation FC incorporated PEG3350 sourced from BASF®. The PEG polymers were characterized using thermogravimetric analysis (TGA) for residual content, gel permeation chromatography for molecular weight distribution, and liquid chromatography–mass spectrometry (LC/MS) for molecular fraction profiling. Using Mastersizer®, formulations FA and FC were further characterized for particle size both with shear (syringe) and without shear (pipette). In vitro drug release studies were conducted to compare release profiles between the two formulations. To further investigate the role of PEG in depot formation, three additional Q1/Q2-equivalent formulations—FB, FD, and FE—were developed. Formulation FB incorporated larger, recrystallized MPA particles, while formulations FD and FE used homogenization and wet milling, respectively, to reduce particle size. All three were prepared using a suspending medium containing PEG3350 from Spectrum Chemicals®. All five Q1/Q2-equivalent formulations were administered via intramuscular injection into the quadriceps muscles of rabbits. Tissue surrounding the injection site was collected for histopathological evaluation to assess local tissue response. The depot was also analyzed for PEG3350 content.

Results: Thermal analysis revealed a broad derivative peak for BASF PEG3350 and a narrower peak for Spectrum Chemicals PEG3350 (Fig.1B), indicating greater heterogeneity in the BASF polymer likely due to variations in chain length or the presence of residual monomers. Gel permeation chromatography (GPC) showed differences in the weight-average molecular weight (Mw) between the two PEG3350 sources (Table.1), while liquid chromatography-mass spectrometry LC-MS analysis revealed that BASF PEG3350 contained a lower proportion of high molecular weight polymeric chains compared to the Spectrum PEG3350 [Fig.1C(I)& C(II)]. These differences in polymer characteristics influenced the formation of particle–polymer agglomerates. When subjected to shear stress (passage through a syringe), formulation FC (prepared with BASF PEG3350) showed ~33% reduction in particle size, whereas formulation FA (containing Spectrum PEG3350) exhibited only ~5% reduction (Fig.1DI). This suggests that the agglomerates in FC were weaker compared to those in FA, resulting in slightly faster drug release from FC in the initial five days (Fig.1DII). The enhanced agglomerate strength in FA may be attributed to the higher fraction of high molecular weight chains in Spectrum PEG3350, which likely facilitate increased polymer–particle entanglement and interaction. In vivo depot analysis in rabbits injected with Depo-Provera® and its Q1/Q2-equivalent formulations revealed distinct depot characteristics. Gross examination (Fig. 2C) showed that the depots formed by FA and FC were gel-like, while those formed by FB, FD, and FE were gritty in texture—most notably in FB. Higher PEG3350 retention in formulations FA and FC (~90% and ~75%, Fig. 2B) likely contributed to their gel-like appearance, potentially through osmotic-driven water absorption into the depot. Histological evaluation indicated that FA and FC, which contained higher PEG concentrations, and FD and FE, which had smaller particle sizes, exhibited inflammatory cell infiltration primarily at the surface of the depot. This may be attributed to water retention in FA and FC, and compact depot structure in FD and FE, both of which potentially limited deeper immune cell penetration. In contrast, FB, which contained larger MPA particles, showed inflammatory cell presence both at the surface and within the core of the depot (Fig.2D).

Conclusion: This study demonstrates that PEG polymers are a critical material attribute in the formulation development of LAI suspensions. The investigation into source variability suggests that manufacturing differences can lead to variations in the molecular composition of PEG, which in turn may influence particle–polymer interactions and result in subtle differences in in vitro drug release. Furthermore, particle characteristics appear to affect PEG retention at the depot site, which may subsequently influence depot morphology and the extent of the local inflammatory response.

References: [1] Li, Dan, Zhou Lan Yin, and Qi Yuan Chen. "Study on influence of polyethylene glycol on dispersion of pyrite powder in aqueous solution." Advanced Materials Research 554 (2012): 349-352.
[2] Bauer, Andrea, Philippe Berben, Sudhir S. Chakravarthi, Sayantan Chattorraj, Ashish Garg, Betty Gourdon, Tycho Heimbach et al. "Current state and opportunities with long-acting injectables: industry perspectives from the innovation and quality consortium “long-acting injectables” working group." Pharmaceutical Research 40, no. 7 (2023): 1601-1631.
[3] Darville, Nicolas, Marjolein Van Heerden, Dirk Mariën, Marc De Meulder, Stefaan Rossenu, An Vermeulen, An Vynckier et al. "The effect of macrophage and angiogenesis inhibition on the drug release and absorption from an intramuscular sustained-release paliperidone palmitate suspension." Journal of Controlled Release 230 (2016): 95-108.
[4] Kinnunen, Hanne M., and Randall J. Mrsny. "Improving the outcomes of biopharmaceutical delivery via the subcutaneous route by understanding the chemical, physical and physiological properties of the subcutaneous injection site." Journal of Controlled Release 182 (2014): 22-32.

Acknowledgements: The project was funded by a contract from the U.S. Food and Drug Administration (contract # 75F40121C00133). Disclaimer: The views expressed here do not reflect official policies of the U.S. FDA or the Department of Health and Human Services, nor does any mention of trade names imply endorsement by the U.S. Government.


(A) Schematic representation of preparation of Q1Q2 equivalents of Depo-Provera®; (B) graphical representation of thermogravimetric derivative curves of Spectrum Chemicals® and BASF PEG3350®; (C) Deconvoluted mass spectrum of PEG3350 samples from (CI) Spectrum Chemicals® and (CII) BASF®; Graphical representation of (DI) mean particle size after introducing suspensions into the particle sizing well using pipette or syringe; and (DII) in vitro drug release profiles of formulation FA and FC performed using 1% w/v SDS at 37 ̊C temperature and a flow rate of 8 ml/min All data are presented as mean ± SD, n = 3


(A) Pictorial representation of tissue extraction for PEG3350® content analysis; (B) Percentage of PEG3350 remaining in depots removed 14-days post intramuscular injection; (C) Gross appearance of depots retrieved from rabbit quadriceps muscles 14 days post-injection; and (D) H&E and CD68+ stain of tissue exposed to Depo Provera 150® and its Q1Q2 equivalents, (*) indicates the area of macrophages recruitment.


Number-average and weight-average molecular weights of PEG 3350 polymers from Spectrum Chemicals® and BASF®. Molecular weights were determined using gel permeation chromatography (GPC).