Development and Evaluation of a Chelator for Gallium-67/68

Abstract

The novel chelator, NOTET, a tetrazole analogue of NOTA, was successfully synthesised via the tris-alkylation of TACN with chloroacetonitrile followed by [3+2] cycloaddition using sodium azide in the presence of zinc bromide under aqueous conditions to give NOTET in an overall yield of 52%. NOTET rapidly formed complexes with non-radioactive gallium(III), iron(III), and cobalt(II) to give products sufficiently stable for characterisation by HRMS, NMR (where the metal permitted), elemental analysis, and X-ray crystallography ([Fe(NOTET)], [Co(NOTET)]). Radiolabelling of NOTET with gallium-67/68 was readily, and reproducibly, achieved at pH 3.5 in 20 min to give >99% yield. The radioactive complexes were comprehensively evaluated, in terms of physio chemical characteristics, which included partition coefficient, stability in dPBS, transferrin and HSA, and in vivo PET imaging and biodistribution in healthy mice. Partition coefficient studies in the physiologically relevant range of pH 5.5 – 8 were conducted using [67Ga][Ga(NOTET)] and [67Ga][Ga(NOTA)] and showed both complexes to be hydrophilic, but only [67Ga][Ga(NOTET)] showed an interesting pH dependence over that range, consistent with a notable increase in lipophilicity as the pH was reduced. The complexes [67/68Ga][Ga(NOTET)] and [67/68Ga][Ga(NOTA)] were administered to healthy animals for biodistribution and PET imaging studies. In spite of the fact that HPLC and partition coefficient studies showed [67/68Ga][Ga(NOTET)] to be highly polar, the in vivo assessment of the radioactive complexes showed a statistically significant increase in liver uptake and reduced renal retention compared to [67/68Ga][Ga(NOTA)]. While the mechanism of this improvement is not immediately evident, the results are encouraging and indicate that the use of tetrazoles may play a role in mitigating the troublesome renal retention observed when using existing chelators of gallium.