Often the key to potency is timing. Although the cancer-killing nanomedicine that Leu-Wei Lo and colleagues have devised uses state-of-the-art chemotherapeutic and genetic drugs – doxorubicin (Dox) and DNAzyme, respectively – both have been used before. However, this is the first time they have been delivered with the time lag between the two that makes the combination even more deadly to cancer cells.

"We started working on nanomaterials and nanomedicine around 2004, and were one of the first groups reporting in vivo medical applications using MSN in 2009, so we know MSN very well," says Lo. The key advantage MSN offers as a delivery vehicle is the three distinct topological domains of its structure, which Lo likens to a house with walls, rooms and a roof. Lo’s team had already exploited the distinct topological domains in MSN to deliver a modified two-photon antenna molecule capable of efficient and controllable energy transfer, alongside a photosensitizer to make the energy transferred more deadly to the target breast cancer cells. "So we thought we can also use this topological domain structure for sequential release of the Dox and DNAzyme and have a synergistic effect," says Lo.

Getting the right ingredients

Dox is a widely used chemotherapeutic drug whose anticancer effects have been recognised since the late 1960s, although higher and higher doses may be required to combat the cancer cells' protection mechanism of flushing it out. In contrast the DNAzyme used in the study is a relatively recent discovery. It is a catalytically active DNA strand that targets and cleaves the mRNA of the protein c-Jun, a transcription factor that controls expression of the multidrug resistant protein MRP1.

"MRP1 has one purpose – to pump," explains Lo. "Its function is cell protection, so the baseline levels are very low until it is induced by the presence of the drug." Other studies have used small interfering RNA (siRNA) molecules to inhibit expression of other multidrug resistant proteins, but these siRNA are relatively unstable. The DNAzyme that Lo and his colleagues use is specially modified so that it is particularly stable in the intracellular environments where it is set to work.

Killing a cell twice

Another advantage of the MSN delivery mechanism is that lysozymes – antimicrobial enzymes in animal immune systems – carry the MSNs to the peripheral parts of the cell nucleus. Here the doxorubicin can more readily enter the nucleus to induce cell death. The transcription factors the DNAzyme targets are located here too.

There are also advantages to targeting the transcription factor c-Jun rather than the protein MRP1 itself. “We chose c-Jun because many reports have indicated it is one of the most important factors for cell growth and metastasis,” says Lo.

He and his team tested the effect of the Dox and DNAzyme loaded MSNs on PC-3, a human prostate carcinoma cell line that is not only capable of Dox-resistance by MRP1 efflux pumping, but it also has a particularly high potential for metastasis. They found the efficacy of the chemotherapeutic effects of Dox greatly enhanced, resulting in 80% reduction of the surviving fraction, compared with 60% surviving exposure to nanoparticles carrying Dox alone. The increased efficacy means lower Dox doses can be used, reducing the potential for side effects.

In addition, when studying the effect of the nanoparticles on the cells in a wound-healing assay, they were able to confirm that cell migration and invasiveness – key actions in the first stage of metastasis – were also reduced.

The next stage is to test the Dox and DNAzyme loaded MSNs in vivo using a mouse model.

Full details are reported in Nano Futures.