Purpose: The aim of this work was to study the assembly, drug loading, and stability of poly(ethylene glycol)-block-poly(epsilon-caprolactone) (PEG-b-PCL) micelles.
Methods: Three PEG-b-PCL compositions with PCL number average molecular weights of 1000, 2500, and 4000 g/mol were used. The assembly of PEG-b-PCL micelles, induced by the addition of water to acetonitrile (ACN), was characterized with 1H nuclear magnetic resonance spectroscopy (1H-NMR) and dynamic light scattering (DLS) with and without the presence of fenofibrate, a poorly water-soluble drug. PEG-b-PCL micelles with encapsulated fenofibrate were prepared through the removal of a negative ACN-water azeotrope under reduced pressure. Fenofibrate content was measured using reverse-phase high-performance liquid chromatography (HPLC), whereas the kinetic stability of PEG-b-PCL micelles with and without encapsulated fenofibrate was evaluated using size exclusion chromatography (SEC).
Results: The critical water content (CWC), the water content at which amphiphilic block copolymer (ABC) micelle assembly begins, was determined using DLS and ranged from 10% to 30% water, depending on both PCL molecular weight and PEG-b-PCL concentration. As the water content was increased, the PEG-b-PCL unimers assembled into swollen structures with hydrodynamic diameters ranging from 200 to 800 nm. The 1H-NMR peaks associated with the PCL block exhibited line-broadening, following the addition of D2O, indicating that the PCL blocks reside in the core of the PEG-b-PCL micelle. With further addition of water, the PCL cores collapsed to form fairly monodisperse PEG-b-PCL micelles (20-60 nm). In the presence of fenofibrate, the CWC value was lowered, perhaps due to hydrophobic interactions of fenofibrate and the PCL block. Further addition of water and subsequent evaporation of the negative ACN-water azeotrope resulted in fenofibrate-loaded PEG-b-PCL micelles of under 50 nm. The extent of fenofibrate encapsulation was dependent on PCL block size. At a polymer concentration of 1.0 mg/ml, PEG-b-PCL (5000:4000) and (5000:2500) micelles could encapsulate more than 90% of the initial loading level of fenofibrate, whereas PEG-b-PCL (5000:1000) micelles encapsulate only 28%. SEC experiments revealed that PEG-b-PCL (5000:4000) and (5000:2500) micelles eluted intact, indicating kinetic stability, whereas PEG-b-PCL (5000:1000) micelles eluted primarily as unimers.
Conclusions: PEG-b-PCL in ACN assembles with fenofibrate into drug-loaded polymeric micelles with the addition of water and the subsequent removal of a negative ACN-water azeotrope.