CXCR4 as a target for preclinical Positron Emission Tomography of cancer

Abstract

The chemoattractive interactions of the chemokine stromal derived factor-1 (SDF1) and its receptor CXCR4 play a key role in tumour metastasis, with CXCR4 expression found to be significantly higher in cancers with aggressive phenotypes. There is therefore an impetus for non-invasive means to sensitively identify tumours prone to progression. As such, the aim of this work was to develop a number of novel CXCR4 antagonists to assess their suitability as imaging radiotracers for positron emission tomography (PET).

In this study, novel 14-mer peptide, pentapeptide and small molecular antagonists were designed, assessed for in vitro potency using radioligand binding and migration assays, and validated in vivo through imaging and biodistribution experiments.

Several novel pentapeptide antagonists (CCIC7, CCIC15 and CCIC30) were found to inhibit CXCR4 function with moderate potency, although the radiolabelled tracers unfortunately showed poor localisation in vivo, due to rapid clearance or metabolism. Additionally, fluorophenylated derivatives of the small molecular antagonists It1t and AMD3465 were found to antagonise SDF1 binding and function at low concentrations, suggesting potential as CXCR4-targeting molecules. Finally, it was shown that a cation-chelated NO2A-conjugate derivative of the 14mer peptide TN14003 could antagonise CXCR4 at low concentrations, and furthermore that CXCR4-expressing tumours could be identified from non-expressing tumours by PET imaging with the use of a 68Ga-labelled derivative in CXCR4-dependent manner. The tracer accumulated in CXCR4-expressing tumours with a favourable signal-to-muscle ratio of 2.9, and was also found to show metabolic stability in plasma appropriate for PET imaging at 60 minutes.

In conclusion, this study documents the successful validation of [68Ga]-CCIC16 for the identification of CXCR4-expressing tumours in vivo, and highlights several potential small molecular antagonist derivatives suitable for further investigation as PET tracers. While functionalisation of pentapeptide antagonists was generally unsuccessful, the report proposes further analysis and modifications that may potentially be used in future developments.