Introduction
The development of tumour-specific, fluorescent smart probes has the potential to improve the early diagnosis of cancer and metastatic spread. TKTL1 thereby seems an interesting enzyme to target, as it has been described to be overexpressed in a number of malignancies. It plays a central role in the pentose phosphate pathway, and its overexpression might lead to a selection advantage in tumour cells as their metabolism might become widely independent of the presence of oxygen. Therefore, they can survive under the common hypoxic conditions within tumours, while at the same time lactate and CO2 production increase, which in turn promotes matrix degradation and favours metastasis formation.