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Diffusion of active cytotoxic agents throughout an entire solid tumour is a particular challenge to successful drug delivery. Here we show the simple and robust generation of non-toxic, 10-15 nm superparamagnetic iron oxide nanoparticles (SPIONs) that have been sterically stabilized by either 100% anionic or 100% cationic or 100% neutral end-functionalized steric stabilizers or by novel combinations of cationic and neutral end-functionalized polymer. When these nanoparticles were co-administered with various anti-cancer drugs, a significant increase in the diffusion and effectiveness of the cytotoxin in a 3-dimensional model of a solid tumour was shown for specific combinations of surface functionality and cytotoxin. The critical determinant of enhanced cytotoxin diffusion and effectiveness was the end functionality of the steric stabilizers and not the core composition (either iron oxide, silica or gold). We provide evidence that SPIONs stabilized with heterogeneous steric stabilizers enhance nuclear uptake of doxorubicin across multiple cell layers.

We present a bioreductively activated cobalt(III) carrier system for the delivery of curcumin with enhanced drug stability, tumour penetration and efficacy in hypoxic tumour regions. Curcumin is a natural product with potent anticancer activity but low bioavailability and serum stability. With the aim of overcoming these limitations, we prepared a cobalt(III) prodrug of curcumin and compared the stability, cytotoxicity and cellular uptake with those of the free drug. Using a combination of fluorescence lifetime imaging and X-ray absorption spectroscopy, we demonstrated that curcumin is released from the cobalt carrier complex in tumour cells, with strong evidence to suggest that the process occurs via reduction of the cobalt centre. Furthermore, fluorescence lifetime imaging in solid tumour models showed that the cobalt complex delivered curcumin uniformly throughout the tumour model, while free curcumin only accumulated on the outer edges. For comparison, we also investigated the isoelectronic ruthenium(II) complex and found its properties and biological activity to be very different to those of the cobalt analogue.

Hypoxic cancer cells have a more aggressive phenotype than cells in a normoxic environment and are often refractory to current chemotherapy protocols due to an altered proliferation rate. They also represent a means of targeting cancer chemotherapy. We have developed a 3-dimensional cell culture model that readily identifies live hypoxic cells by expression of the Eos photo-convertible green fluorescent protein. Using this model, we have examined the diffusion and hypoxia selectivity of two model Co(III) based compounds. The Co(III) compounds have been designed to target hypoxic cells within a tumour and have fluorescent axial ligands. The fluorescent ligands function as model cytotoxins and their fluorescence is quenched on binding to Co(III). The ligands are released upon the reduction of Co(III) to Co(II) and the recovery of fluorescence enables the detection of the released ligands by confocal microscopy. Despite similarities in electrochemical potential and cytotoxicity, the penetration of the compounds into spheroids and uptake into hypoxic cells was found to differ depending on the properties of the released ligand.

The penetration of anthraquinones and their platinum complexes into cancer cell spheroids reveals that they model well the distribution of such compounds in solid tumours and that the proportion of the compound that accumulates deep in the spheroid is inversely related to the rate of cellular uptake which is affected by the charge of the compound.