by ecancer reporter Clare Sansom

It is self-evident that the successful diagnosis and treatment of solid tumours depends on the ability to distinguish tumour tissue from adjacent normal tissue.

Many different tumour imaging techniques have been developed for this purpose including small molecule tracers and novel nano-materials; some nano-materials have been shown to have clear advantages in sensitivity and specificity over tracer molecules.

However, most of these methods depend on gene expression patterns or phenotypic features that differ between tumours.

It is still challenging to find imaging methods that can reliably distinguish tumour from normal tissue in a wide variety of tumour types.

Some physiological differences exist between the tumour micro-environment and surrounding normal tissue that can be exploited in imaging, but these signals are very subtle and cannot be detected without amplification.

Jinming Gao and his colleagues at the University of Texas Southwestern Medical Center, Dallas, Texas, USA have now developed a novel strategy for the amplification of signals from the tumour micro-environment that can be used to image many types of solid tumour.

These methods utilise two ubiquitous “hallmarks of cancer” that are present in all such tumours, regardless of type: the tumour vasculature and aerobic glycolysis.

The latter process, which is also known as the Warburg effect, leads to a small but distinct drop in pH in the tumour micro-environment when compared to adjacent normal tissue.

Gao and his colleagues have developed a series of nanoprobes that are sensitive to pH to detect the acidic, angoigenic micro-environment of tumours.

These probes have three components: an ultra-sensitive core that can respond to changes in pH of less than 0.25 pH unit; a series of fluorophores that fluoresce in colours ranging from green to near infra-red; and a targeting unit that binds to cell surface receptors, enabling the probes to enter cells.

The probe composition was optimised for rapid and precise pH detection in the range required to distinguish the tumour micro-environment (pH 6.5-6.8) from normal blood (pH 7.4) and for a large signal ON/OFF ratio, high reproducibility, and relatively small particle size to aid cell penetration.

The resulting ultra-pH sensitive (UPS) nano-particles were able to produce tumour-specific images of even tiny tumours in mouse models an hour after intravenous injection.

Two types of probe were produced, one with properties optimised for delivery to cells (UPSi nanoprobes) and the other designed to be activated by pH transitions in the extra-cellular medium (UPSe nanoprobes).

UPSe nanoprobes were found to fluoresce 102 times more strongly at a typical tumour pH of 6.7 compared to blood pH; this can be compared to the 2-3 fold difference that has been calculated for small molecule pH sensors.

To investigate probe sensitivity and specificity, Gao and his co-workers first showed that small molecule inhibitors of tumour glycolysis increased tumour pH, and then that pre-treatment of mouse lung tumours with these inhibitors decreased the fluorescence signal obtained.

A higher fluorescence signal was obtained from hypoxic regions of tumours, correlating with previous observations that these regions are more acidic.

Intracelluar (UPSi) nanoprobes were constructed that could bind to αvβ3 integrins, which are biomarkers for angiogenesis; fluorescence was observed once integrin-bound probes had been taken up into acidic endocytic organelles, and this was seen to be localised in the tumour vasculature.

The researchers showed that co-injection of nanoprobes designed to fluoresce at different wavelengths could be used to visualise the pH and the vasculature of a tumour at the same time.

No significant toxicity was observed in mice either one or seven days after injection with any of the probes.

Probe function was tested in a total of ten different mouse models of human cancer, including breast, lung, head and neck, prostate, brain and pancreatic tumours, and tumour-specific fluorescence was observed to a similar extent in all models.

Taken together, these results suggest that the UPS probes may provide a universal, sensitive and specific method for imaging a diverse range of solid tumour types, regardless of tumour histology or gene expression.

This method also has potential applications in visualising the response of solid tumours to drug treatment.

Reference

Wang, Y., Zhou, K., Huang, G. and 7 others (2013). A nanoparticle-based strategy for the imaging of a broad range of tumours by nonlinear amplification of microenvironment signals. Nature Materials, published online ahead of print 8 December 2013. doi: 0.1038/nmat3819