Exploiting Tumour Hypoxia to Deliver Mononuclear Bioreductive Cobalt-based Anticancer Chemotherapeutic Agents with Novel Coordination Geometry

Abstract

To circumvent the wide range of adverse effects of many traditional anticancer chemotherapeutics, there has been a shift towards targeting methodologies which exploit tumour microenvironmental differences between cancer and healthy cells. One biologically relevant tumour-specific microenvironmental difference is the presence of hypoxic pockets which results in a change in protein expression, cellular energetics, and an overall reducing environment. The reduction potential in these areas is significantly lower than in healthy cells enabling the selective activation-by-reduction of organic and metal-based compounds. CoIII compounds have been identified as potential agents to deliver cytotoxic ligands to tumours due to the significantly faster aqua ligand exchange kinetics for the CoII ion compared to CoIII upon reduction. The electro- and physicochemical properties of the CoIII species can be controlled by the specific ancillary and cytotoxic ligands coordinated to the Co centre. CoIII compounds containing tetraazamacrocycle ancillary ligands, cyclen and cyclam, were synthesised containing three classes of cytotoxic ligands coordinating through different coordination motifs, namely the O,O’-coordinating hydroxamate (e.g. vorinostat), the O,O’- and S,O-coordinating (thio)maltolate, and S,N-coordinating pyridinecarbothioimide ligands. As the cytotoxic and ancillary ligands have a distinct impact on the overall properties of the final CoIII compound, clear patterns of reduction potential, reversibility, and ligand release. All the Co(cyclen) compounds with O,O’-coordinating ligands were irreversibly reduced while the Co(cyclam) O,O’- and all S,O-derivatives were quasireversible. For these compounds only a portion of the ligands were released by reduction with sodium dithionite. However, for the S,N-coordinating PCA compounds, only the para-fluoro derivative was quasireversible with all other species being irreversibly reduced and complete ligand release observed in 1H NMR spectra. In normoxic conditions, the bioactivity of the cytotoxic ligand is masked by coordination to CoIII and is an important metric for determining hypoxia-dependent antiproliferative activity. Antiproliferative assays revealed a range of anticancer activities for the ligands with selected compounds showing high potency, especially for vorinostat and those featuring thiomaltol-derived ligands. In virtually all cases, coordination to CoIII resulted in reduced antiproliferative activity demonstrating the potential to improve the selectivity of the native ligands by CoIII coordination. Unfortunately, no significant improvement in antiproliferative activity was observed in vitro when cells were treated with selected CoIII compounds in hypoxic conditions either due to poor reduction of the CoIII centre or poor uptake into the tumour cells. Nevertheless, when the impacts of CoIII–vorinostat species were determined after incubation with AGS cells in vitro under hypoxic conditions, the same spectrum of changes to gene expression and histone acetylation as native vorinostat were observed, thereby demonstrating a degree of intracellular uptake, reduction of CoIII, and ligand release. Despite the lack of hypoxia-dependent antiproliferative activity, this work lays the foundation to further investigate the coordination of cytotoxic ligands with alternative coordination motifs to CoIII centres.