Hydrophobic oligopeptide-based star-block copolymers as unimolecular nanocarriers for poorly water-soluble drugs

Hydrophobic oligopeptide (HOP)-based star-block copolymers of the form PEI-g-(HOP-b-PEG) were synthesized, characterized and evaluated as nanocarriers for poorly water-soluble drugs. The designed PEI-g-(HOP-b-PEG) polymers were composed of a hyperbranched polyethylenimine (PEI) core, a HOP [i.e., oligo(l-tryptophan), oligo(l-phenylalanine), oligo(l-leucine), oligo(γ-benzyl-l-glutamate) and oligo(ɛ-benzyloxycarbonyl-l-lysine)] inner shell and a hydrophilic poly(ethylene glycol) (PEG) outer shell. The synthesized polymers were characterized using (1)H NMR, gel permeation chromatography (GPC) and transmission electron microscopy (TEM). Their micellization behavior was investigated by the dynamic light scattering (DLS) and fluorescence spectroscopy using pyrene as a probe; the results demonstrated that these star-block copolymers predominantly resembled unimolecular micelles, particularly when shorter HOP blocks and/or elongated PEG chains were incorporated. The encapsulation properties of these unimolecular micelles were evaluated using pyrene, oil-red O (OR) and doxorubicin (DOX) as guest hydrophobic compounds, which revealed that poorly water-soluble guests can be efficiently solubilized in PEI-g-(HOP-b-PEG) with a loading capacity of up to 10%. The encapsulated DOX demonstrated sustained release from PEI-g-(HOP-b-PEG). The synthesized star-block copolymers, given their structural versatility, water solubility and biodegradability, could potentially be used as unimolecular nanocarriers for the delivery of poorly water-soluble drugs.