Dual targeting of the cancer antioxidant network with 1,4-naphthoquinone fused Gold(i) N-heterocyclic carbene complexes.

To achieve a systems-based approach to targeting the antioxidant pathway, 1,4-naphthoquinone annulated N-heterocyclic carbene (NHC) [bis(1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i)] [silver(i) dichloride] (1), [bis(1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i)] chloride (2), and 1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i) chloride (3)) were designed, synthesized, and tested for biological activity in a series of human cancer cell lines. The solution phase of complexes 1-3 were assigned using several spectroscopy techniques, including NMR spectroscopic analysis. Complexes 1 and 3 were further characterized by single crystal X-ray diffraction analysis. Electrochemical and spectroelectrochemical studies revealed that quinone reductions are reversible and that the electrochemically generated semiquinone and quinone dianions are stable under these conditions. Complex 1, containing two NHC-quinone moieties (to accentuate exogenous ROS via redox cycling) centered around a Au(i) center (to inactivate thioredoxin reductase (TrxR) irreversibly), was found to inhibit cancer cell proliferation to a much greater extent than the individual components (i.e., Au(i)-NHC alone or naphthoquinone alone). Treatment of A549 lung cancer cells with 1 produced a 27-fold increase in exogenous reactive oxygen species (ROS) which was found to localize to the mitochondria. The inhibition of TrxR, an essential mediator of ROS homeostasis, was achieved in the same cell line at low administrated concentrations of 1. TrxR inhibition by 1 was similar to that of auranofin, a gold(i) containing complex known to inhibit TrxR irreversibly. Complex 1 was found to induce cell death via an apoptotic mechanism as confirmed by annexin-V staining. Complex 1 was demonstrated to be efficacious in zebrafish bearing A549 xenografts. These results provide support for the suggestion that a dual targeting approach that involves reducing ROS tolerance while concurrently increasing ROS production can perturb antioxidant homeostasis, enhance cancer cell death in vitro, and reduce tumor burden in vivo, as inferred from preliminary zebra fish model studies.

Crystal structure of 4-(2-bromo-prop-ion-yl)-3-phenyl-sydnone.

Sydnones are a class of mesoionic compounds containing a five-membered heterocyclic ring. In general, sydnone com-pounds are synthesized with an aromatic substutuent at the N(3) position. This feature, adds to the stability of the heterocyclic ring. In the title compound {systematic name: 4-(2-bromo-propano-yl)-3-phenyl-1,2,3λ(5)-oxa-diazol-3-ylium-5-olate}, C11H9BrN2O3, the aromatic substitutent is an unsubstituted phenyl ring. The sydnone ring is almost planar, with a maximum deviation from the mean plane of 0.023 (1) Å, but is not coplanar with the phenyl ring, having a dihedral angle of 40.93 (8)°. The carbonyl side chain is twisted relative to the syndone ring by 15.8 (2)°. The mol-ecules are packed in the unit cell as pairs related by an inversion center at (1, 0, 1/2). The pairs inter-act via π-stacking, with the distance separating the centroids being 3.824 (1) Å. The Br atom has two contacts, one to an N atom in a neighboring asymmetric unit with a distance of 3.346 (2) Š(the sum of the van der Waals radii is 3.40 Å) and a second to an H atom with a distance of 3.03 Å. The contact with the H atom is perpendicular (C-Br⋯H = 98.60°) to the C-Br bond, and that to the N atom is linear [C-Br⋯N = 169.10 (5)°] to the C-Br bond. The O atom of the sydnone ring is involved in two hydrogen bonds, one intra-molecular with a donor-acceptor distance of 3.1486 (19) Šand a second that is inter-molecular, with a phenyl H atom as the donor and has a donor-acceptor distance of 3.346 (2) Å.