The therapeutic performance of biodegradable micellar drugs is far from optimal due to existing challenges like poor
tumor cell uptake and intracellular drug release. Here, we report on
ligand-directed reduction-sensitive shell-sheddable biodegradable
micelles based on poly(
ethylene glycol)-poly(ε-
caprolactone) (PEG-PCL) copolymer actively delivering
doxorubicin (DOX) into the nuclei of target
cancer cells, inducing superb in vitro antitumor effects. The
micelles were constructed from
PEG-SS-PCL and
galactose-PEG-PCL (Gal-PEG-PCL) block copolymers, in which Gal-PEG-PCL was designed with a longer PEG than that in
PEG-SS-PCL (6.0 vs 5.0 kDa) to fully expose Gal
ligands onto the surface of
micelles for effective targeting to
hepatocellular carcinoma cells.
PEG-SS-PCL combining with 10 or 20 wt % of Gal-PEG-PCL formed uniform
micelles with average sizes of 56.1 and 58.2 nm (denoted as
PEG-SS-PCL/Gal10 and
PEG-SS-PCL/Gal20, respectively). The in vitro release studies showed that about 81.1 and 75.0% DOX was released in 12 h from
PEG-SS-PCL/Gal10 and
PEG-SS-PCL/Gal20
micelles under a reducing condition containing 10 mM
dithiothreitol (DTT). In contrast, minimal DOX release (<12%) was observed for
PEG-SS-PCL/Gal10 and
PEG-SS-PCL/Gal20
micelles under nonreducing conditions as well as for reduction-insensitive Gal-PEG-PCL and PEG-PCL/Gal20
micelles in the presence of 10 mM DTT. MTT assays in HeLa and HepG2 cells showed that DOX-loaded
PEG-SS-PCL/Gal20
micelles exhibited apparent targetability and significantly enhanced antitumor efficacy toward
asialoglycoprotein receptor (
ASGP-R)-overexpressing HepG2 cells with a particularly low half maximal inhibitory concentration (IC50) of 1.58 μg DOX equiv/mL, which was comparable to free DOX and approximately six times lower than that for nontargeting
PEG-SS-PCL counterparts under otherwise the same conditions. Interestingly, confocal microscopy observations using
FITC-labeled
PEG-SS-PCL/Gal20
micelles showed that DOX was efficiently delivered and released into the nuclei of HepG2 cells in 8 h. Flow cytometry revealed that cellular DOX level in HepG2 cells treated with DOX-loaded
PEG-SS-PCL/Gal20
micelles was much greater than that with reduction-insensitive PEG-PCL/Gal20 and nontargeting
PEG-SS-PCL controls, signifying the importance of combining shell-shedding and active targeting.
Ligand-directed, reduction-sensitive, shell-sheddable, and biodegradable
micelles have emerged as a versatile and potent platform for targeted
cancer chemotherapy.