Glycopolymer Self-Assemblies with Gold(I) Complexed
to the Core as a Delivery System for Auranofin
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Abstract
A new glycomonomer <b>1</b> containing a thioacetate group
in the anomeric position and mimicking the thiosugar ligand of the
gold-based drug auranofin was designed and synthesized in four steps
from d-glucose. Both CPADB-mediated homopolymerization and
chain extension of a hydrophilic poly(OEGMEMA) macroRAFT agent were
well-controlled with dispersities (<i><i><i>Đ</i></i></i>) below 1.2, highlighting the suitability of thioacetate
as a thiol protecting group in RAFT polymerization. Using the homopolymer
as a test system, the thioacetate protective groups were selectively
removed using hydrazine acetate, and AuPEt<sub>3</sub>Cl was subsequently
complexed to the exposed thiols to generate a polymeric auranofin
analogue with 52% complexation efficiency. Extension of this successful
procedure to three block copolymers with differing hydrophobic block
lengths, poly(OEGMEMA)<sub>34</sub>-<i>b</i>-poly(<b>1</b>)<sub>47</sub>, poly(F-OEGMEMA)<sub>32</sub>-<i>b</i>-poly(<b>1</b>)<sub>27</sub>, and poly(F-OEGMEMA)<sub>32</sub>-<i>b</i>-poly(<b>1</b>)<sub>7</sub> (where “F”
in the last two indicates the incorporation of 2 wt % fluorescein
methacrylate into the hydrophilic block), produced well-defined complexed
block copolymers with complexation efficiencies comparable to that
of the homopolymer. Self-assembly of the longest complexed polymer
poly(OEGMEMA)<sub>34</sub>-<i>b</i>-poly(<b>1</b>-AuPEt<sub>3</sub>)<sub>47</sub> generated spherical micelles with a hydrodynamic
diameter <i>D</i><sub>h</sub> of 28 nm when prepared by
slow water addition to a dilute DMF solution. The IC<sub>50</sub> value
against OVCAR-3 cells in a serum-free media was 44 μM on a gold
concentration basis, compared to 0.3 μM for auranofin itself.
The two shorter fluorescent complexed block copolymers formed spherical
micelles with <i>D</i><sub>h</sub> 23 and 9 nm, respectively,
and proved more cytotoxic than their longer counterpart, both displaying
IC<sub>50</sub> values of 13.5 μM. The addition of serum to
the cell growth medium reduced the cytotoxicity of auranofin by a
factor of 3.6 but had a less marked effect on the fluorescent micellar
systems, reducing their toxicities by between 2.4 and 2.8 times. These
micellar systems therefore show less susceptibility to deactivation
by serum proteins (which is the primary limitation to auranofin’s <i>in vivo</i> effectiveness) than the free auranofin, suggesting
some protective benefit offered by the hydrophilic shell. Fluorescence
microscopy of the two fluorescent systems revealed an accumulation
in the lysosomes of the OVCAR-3 cells. The cytotoxicity mechanism
may therefore differ from that of auranofin, which is known to interact
with mitochondrial proteins