the mitochondria, higher activity in our micellar system is very
promising for the potential development of this polymer-gold
system into a chemotherapy treatment.
The increased activity can be explained via the different
route of cell entry. Whilst low molecular weight drugs such as
2 diffuse through the cell membrane, nanoparticles are taken
up by endocytosis. This route not only provides a more
efficient entry pathway, but also offers the possibility of
by-passing the drug resistance mechanism induced by small
molecules.20
It has been demonstrated that glycopolymers bearing pro-
tected thiosugar units can be efficiently converted to polymeric
Au(I) complexes. In micellar form, the block copolymer system
displayed higher activity against OVCAR-3 cells than its small
molecule analogue, offering a promising alternative delivery
mechanism which may overcome the stability and toxicity
issues faced by discrete Au(I) based chemotherapeutics.
We are currently investigating the protection offered by the
micellar system in preventing ligand displacement with serum
proteins, and the controlled synthesis of related polymer-gold
systems.
Fig. 3 Proliferation of OVCAR-3 human ovarian carcinoma cells in
the presence of 2 (closed squares) and poly(HEA)-b-poly(4-AuPEt3)
(open triangles) as a function of Au(I) content.
macroRAFT agent, but after purification by dialysis and
cleavage of the pyridyl disulfide groups using DTT, the
molecular weight distribution narrowed and shifted to higher
molecular weight in a similar fashion to the homopolymer
system (Fig. S8, ESIw). Complexation of AuPEt3Cl to the
purified block copolymer using the same approach generated
an amphiphilic block copolymer with the AuPEt3 complexed
to the thiol units of the pendant sugars. Based on TGA
analysis (Fig. S9, ESIw), the block copolymer contained 20%
gold by mass which corresponds to a loading efficiency of
approximately 74%. It was hoped that the introduction of
the AuPEt3 group would render the Au(I)-containing block
hydrophobic relative to the readily water soluble poly(HEA)
block and thereby encourage self-assembly into a core-shell
structure in an aqueous environment. Indeed, controlled addition
of water to a DMSO solution of the pure block copolymer
followed by dialysis against water produced micelles with a mean
diameter of approximately 75 nm by dynamic light scattering
(DLS, Fig. S10, ESIw). Transmission electron microscopy (TEM)
confirmed the presence of spherical structures containing distinct
gold-rich domains which strongly scattered the electron beam
and generated excellent contrast in the images (Inset, Scheme 1).
The activity of the micellar poly(HEA)-b-poly(4-AuPEt3)
system against tumour cells was evaluated by measuring the
proliferation OVCAR-3 human ovarian carcinoma cells in the
presence of either the discrete drug 2 or its micellar analogue at
increasing concentrations (Fig. 3). It was not deemed appropriate
to use either poly(HEA)-b-poly(4) or poly(HEA)-b-poly(4-SH)
precursors as negative controls, because neither the pyridyl
disulfide groups nor the free thiols in each would constitute a
valid reference in the same way that an unloaded micelle serves as
the negative control for a micelle system physically loaded with a
drug. The dose-response curves of cell proliferation as a percentage
of the control (cells incubated with medium only) demonstrate
similar profiles for 2 and poly(HEA)-b-poly(4-AuPEt3), with
poly(HEA)-b-poly(4-AuPEt3) actually exhibiting slightly higher
activity compared to 2. Indeed, regression analysis of the two
curves indicates that the IC50 inhibitory concentration for
poly(HEA)-b-poly(4-AuPEt3) is 0.94 mM (based on the
concentration of Au(I) in the system) and the value for 2 is
1.51 mM. Considering that 2 is one of the most potent known
compounds for inducing cell death via direct interaction with
MS thanks the Australian Research Council for financial
support.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4695–4697 4697