In this work, a pair of gold(III)
complexes derived from the analogous
tetrapyridyl ligands H
2
biqbpy1 and H
2
biqbpy2
was prepared: the rollover, bis-cyclometalated [Au(biqbpy1)Cl ([1]Cl)
and its isomer [Au(biqbpy2)Cl ([2]Cl). In [1]
+
, two pyridyl
rings coordinate to the metal via a Au–C bond (C
∧
N
∧
N
∧
C coordination) and the two
noncoordinated amine bridges of the ligand remain protonated, while
in [2]
+
all four pyridyl rings of the ligand coordinate
to the metal via a Au–N bond (N
∧
N
∧
N
∧
N coordination), but both amine bridges are deprotonated.
As a result, both complexes are monocationic, which allowed comparison
of the sole effect of cyclometalation on the chemistry, protein interaction,
and anticancer properties of the gold(III) compounds. Due to their
identical monocationic charge and similar molecular shape, both complexes
[1]Cl and [2]Cl displaced reference radioligand [
3
H]dofetilide
equally well from cell membranes expressing the K
v
11.1
(hERG) potassium channel, and more so than the tetrapyridyl ligands
H
2
biqbpy1 and H
2
biqbpy2. By contrast, cyclometalation
rendered [1]Cl coordinatively stable in the presence of biological
thiols, while [2]Cl was reduced by a millimolar concentration of glutathione
into metastable Au(I) species releasing the free ligand H
2
biqbpy2 and TrxR-inhibiting Au
+
ions. The redox stability
of [1]Cl dramatically decreased its thioredoxin reductase (TrxR) inhibition
properties, compared to [2]Cl. On the other hand, unlike [2]Cl, [1]Cl
aggregated into nanoparticles in FCS-containing medium, which resulted
in much more efficient gold cellular uptake. [1]Cl had much more selective
anticancer properties than [2]Cl and cisplatin, as it was almost 10
times more cytotoxic to human cancer cells (A549, A431, A375, and
MCF7) than to noncancerous cells (MRC5). Mechanistic studies highlight
the strikingly different mode of action of the two compounds: while
for [1]Cl high gold cellular uptake, nuclear DNA damage, and interaction
with hERG may contribute to cell killing, for [2]Cl extracellular
reduction released TrxR-inhibiting Au
+
ions that were taken
up in minute amounts in the cytosol, and a toxic tetrapyridyl ligand
also capable of binding to hERG. These results demonstrate that bis-cyclometalation
is an appealing method to improve the redox stability of Au(III) compounds
and to develop gold-based cytotoxic compounds that do not rely on
TrxR inhibition to kill cancer cells.