(M1030-07-40) Fabrication of Drug-Loaded Polymeric Micelles Using Homo-PEG as Both the Solvent and the Lyoprotectant for Freeze-Drying

Purpose: The aim of this study was to develop a method for the fabrication of drug-loaded polymeric micelles which improves safety, scalability and long-term storage of the final product. To do this, polyethylene glycol (PEG) was used both as the solvent and lyoprotectant in the micelle fabrication procedure. The method was named “PEG-assist”.

Methods: Polyethylene glycol-b-poly(D, L-lactide) (PEG-b-PLA, PEG block Mw: 4 kDa, PDLLA block Mw: 2.2 kDa) diblock copolymer together with paclitaxel (PTX) was dissolved in PEGs of different molecular weights (i.e. 200, 400, 600 and 1000 Da) at 60 ˚C and incubated at 60, 40 or 25 ˚C for 2 h prior to addition of water and micelle formation. The effects of homo-PEG molecular weight and hydration temperature on PTX encapsulation efficiency and particle size of the obtained micelles were investigated using High performance liquid chromatography (HPLC, Prominence, Shimadzu, Japan) and Dynamic light scattering (DLS, nano ZS, Malvern), respectively. The optimal sample was studied for the feasibility of 25X scaling and freeze-drying without removing the PEG or adding any other excipient.

Results: Both PEG molecular weight and the incubation/hydration temperature influenced the PTX encapsulation efficiency. Two-way ANOVA suggested an extremely significant interaction between the two variables i.e. PEG molecular weight and temperature (5.2% of the total variance, p < 0.0001). The effect of both variables on encapsulation efficiency was extremely significant (p < 0.0001). Tukey’s multiple comparisons test implied a significantly higher encapsulation efficiency when the mixture was hydrated at 40 ˚C (p < 0.0001) compared to both 25 and 60 ˚C for all the molecular weights of PEG. Hydration at 25 ˚C led to a significantly higher encapsulation efficiency compared to 60 ˚C only for PEG600 (p < 0.0001). PEG_4kDa-b-PLA_2.2kDa used in this study is expected to produce 25-50 nm PTX encapsulated micelles depending on the preparation process [1, 2]. PEG molecular weight and hydration temperature both significantly (p < 0.0001) influenced the average particle size as well. According to two-way ANOVA, the effect of PEG molecular weight was more pronounced – accounting for 68.9% of the total variance (vs. 16.6% for temperature). The average particle size increased with increasing the PEG molecular weight. The maximum encapsulation efficiency for PEG_4kDa-b-PLA_2.2kDa was achieved using PEG1000, with addition of water performed at 40 ˚C. The effects of 25X scaling up and freeze-drying on this sample was investigated. According to our results, 25X scale-up using PEG1000 as the solvent and 40 ˚C as the processing temperature led to no significant change in encapsulation efficiency, average particle size and polydispersity (p >0.05). Freeze-drying of this formulation was performed at -35 ˚C shelf temperature to ensure the sample temperature remains well below the eutectic melting onset [3]. Freeze-drying yielded a high-quality cake that when hydrated, produced drug-loaded micelles that were statistically equivalent (p>0.05) in terms of encapsulation efficiency, average particle size and polydispersity index to the initial sample before freeze-drying.

Conclusion: PEG-assist method for fabrication of drug-loaded micelles offers high efficiency in drug loading, potential scalability and long-term storage. The optimal procedure for PTX loading into PEG_4kDa-b-PLA_2kDa was established as the hydration of PEG_1kDa/PTX/ PEG_4kDa-b-PLA_2kDa at 40 ˚C. PEG_1kDa plays a dual role in the formulation, both as the solvent for drug and copolymer, and as the lyoprotectant for freeze-drying. PEG-assist needs no organic solvent, is rapid, simple and cost-effective, and gives high encapsulation efficiency for the desired formulation.

References: 1. Tomoda, K., et al., Triolimus: A multi‐drug loaded polymeric micelle containing paclitaxel, 17‐AAG, and rapamycin as a novel radiosensitizer. Macromolecular bioscience, 2017. 17(1): p. 1600194.
2. Tam, Y.T., et al., Poly (ethylene glycol)-block-poly (d, l-lactic acid) micelles containing oligo (lactic acid) 8-paclitaxel prodrug: In Vivo conversion and antitumor efficacy. Journal of Controlled Release, 2019. 298: p. 186-193.
3. Kasper, J.C. and W. Friess, The freezing step in lyophilization: physico-chemical fundamentals, freezing methods and consequences on process performance and quality attributes of biopharmaceuticals. European journal of pharmaceutics and biopharmaceutics, 2011. 78(2): p. 248-263.

Acknowledgements: This research was supported by National Cancer Institute of the NIH under award number R01-CA257837. The authors disclose no conflict of interest.


The effect of PEG molecular weight and incubation/hydration temperature on encapsulation efficiency and particle size of PTX-loaded PEG-b-PLA micelles.


The effects of scaling and freeze-drying on drug encapsulation, average particle size and polydispersity index of PTX-loaded PEG-b-PLA micelles prepared using PEG-assist method.