Synthesis, characterisation and evaluation of a novel copper-64 complex with selective uptake in EMT-6 cells under hypoxic conditions

Abstract

The radiometal ⁶⁴Cu is now widely used in the development of diagnostic imaging agents for positron emission tomography (PET). The present study has led to the development and evaluation of a novel chelating agent for ⁶⁴Cu: the new monothiourea tripodal ligand 1-benzoyl-3-{6-[(bis-pyridin-2-ylmethyl-amino)-methyl]-pyridin-2-yl}-thiourea (MTUBo). X-ray crystallographic analysis has shown this ligand forms a mononuclear complex with copper(II) and co-ordinates via a trigonal bipyramidal N₄S array of donor atoms. Promisingly, cell uptake studies revealed that ⁶⁴Cu-MTUBo selectively accumulates in EMT-6 cells incubated under hypoxic conditions which may result from its relatively high Cu^II/I redox potential. Small-animal PET imaging and ex vivo biodistribution studies in EMT-6 tumor bearing BALB/c mice revealed significant tumor uptake after 1 h p.i., yielding tumor-to-muscle (T/M) and tumor-to-blood (T/B) ratios of 8.1 and 1.1, respectively. However, injection of ⁶⁴Cu-acetate resulted in similar uptake indicating that the observed uptake was most likely non-specific. Despite showing high in vitro stability, it is likely that in vivo the complex undergoes transchelation to proteins within the blood in a relatively short timeframe. For comparison, the hypoxia imaging agent ⁶⁴Cu-ATSM was also evaluated in the same murine tumor model and showed about 60% higher tumor uptake than ⁶⁴Cu-MTUBo.