Encapsulation of a hydrophobic drug into a polymer-micelle core explored with synchrotron SAXS

Synchrotron small-angle X-ray scattering (SAXS) at the SPring-8 40B2 and 45XU beamlines was carried out on aqueous solutions of (PEG-P(Asp(Bzl))): partially benzyl-esterified poly(ethylene glycol)-block-poly(aspartic acid) with LE540 loaded up to 8.3 wt %, where LE540 is a very hydrophobic retinoid antagonist drug. The scattering profiles showed characteristic features for core-shell spherical micelles, confirming that P(Asp(Bzl)) forms a hydrophobic core and PEG forms a hydrophilic shell. Before the addition of LE540, a diffraction peak was observed around q = 4 nm(-1), where q is the magnitude of the scattering vector. This peak can be attributed to ordering between alpha-helices made of P(Asp(Bzl)), the so-called nonspecific hexatic arrangement. The P(Asp(Bzl)) helices disappeared as LE540 was added. This result can be interpreted by assuming a uniform distribution of LE540 in the core. By use of a core-shell spherical micelle model, the SAXS data could be well fitted for all of the samples. The analysis indicated that the core radius increases sigmoidally from 5.9 to 6.9 nm upon addition of LE540 whereas the shell radius stayed at 12.5-12.8 nm. The aggregation number that is the average number of PEG-P(Asp(Bzl))'s consisting of one micelle slightly increased from 145 to 182.