Preparation of Block Copolymer Micelle Formulations

The selection of the appropriate preparative technique for micelle formation is largely based on the solubility of the copolymer and the drug in aqueous media. There are several methods that have been developed for the preparation of drug-loaded block copolymer micelles, including the direct dissolution method, evaporation method, and dialysis method as depicted in Figure 17.1.

The direct dissolution method is employed for block copolymers that are relatively soluble in water (e.g., Pluronic®). In this method, the copolymer and the drug are directly dissolved in water or an aqueous solution such as phosphate-buffered saline (PBS) with subsequent heating and/or stirring to induce micellization. Successful loading of doxorubicin, etoposide, nystatin, and halo-peridol into Pluronic micelles has been achieved using this method. ,

For copolymers and drugs with limited aqueous solubility, the evaporation method can be used to prepare the micelle formulations. This method involves first dissolving the copolymer and the drug in a common solvent or a mixture of two miscible solvents. The mixture is then stirred, and the solvent is allowed to evaporate, yielding a copolymer-drug film that can be reconstituted in warm water or a buffer to form the drug-loaded micelles.35-38

The most commonly used method for the preparation of drug-loaded micelles from poorly water-soluble copolymers is the dialysis method. The copolymer and the drug are first dissolved in a common organic solvent that is miscible with water. Slow addition of water results in the formation of micelles with swollen cores. The solution is then dialyzed against a large volume of water to

3,28,32,39

remove the organic solvent. , , ,9 As the percentage of solvent decreases, the degree of swelling in the micelle core decreases to form intact drug-loaded micelles. Alternatively, the solvent can be removed by evaporation of the solvent from the polymer-drug mixture to form micelles (i.e., emulsification method).38,40,41 The selection of the organic solvent and the ratio of water-to-solvent employed have been shown to greatly affect the physical properties and drug loading of the micelles.39

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