EJNMMI Research (May 2019)
Instant kit preparation of 68Ga-radiopharmaceuticals via the hybrid chelator DATA: clinical translation of [68Ga]Ga-DATA-TOC
Abstract
Abstract Purpose The widespread use of 68Ga for positron emission tomography (PET) relies on the development of radiopharmaceutical precursors that can be radiolabelled and dispensed in a simple, quick, and convenient manner. The DATA (6-amino-1,4-diazapine-triacetate) scaffold represents a novel hybrid chelator architecture possessing both cyclic and acyclic character that may allow for facile access to 68Ga-labelled tracers in the clinic. We report the first bifunctional DATA chelator conjugated to [Tyr3]octreotide (TOC), a somatostatin subtype 2 receptor (SST2)-targeting vector for imaging and functional characterisation of SSTR2 expressing tumours. Methods The radiopharmaceutical precursor, DATA-TOC, was synthesised as previously described and used to complex natGa(III) and 68Ga(III). Competition binding assays of [natGa]Ga-DATA-TOC or [natGa]Ga-DOTA-TOC against [125I-Tyr25]LTT-SS28 were conducted in membranes of HEK293 cells transfected to stably express one of the hSST2,3,5 receptor subtypes (HEK293-hSST2/3/5 cells). First in vivo studies were performed in female NMRI-nude mice bearing SST2-positive mouse phaeochromocytoma mCherry (MPC-mCherry) tumours to compare the in vivo SST2-specific tumour-targeting of [68Ga]Ga-DATA-TOC and its overall pharmacokinetics versus the [68Ga]Ga-DOTA-TOC reference. A direct comparison of [68Ga]Ga-DATA-TOC with the well-established PET radiotracer [68Ga]Ga-DOTA-TOC was additionally performed in a 46-year-old male patient with a well-differentiated NET (neuroendocrine tumour), representing the first in human administration of [68Ga]Ga-DATA-TOC. Results DATA-TOC was labelled with 68Ga with a radiolabelling efficiency of > 95% in less than 10 min at ambient temperature. A molar activity up to 35 MBq/nmol was achieved. The hSST2-affinities of [natGa]Ga-DATA-TOC and [natGa]Ga-DOTA-TOC were found similar with only sub-nanomolar differences in the respective IC50 values. In mice, [68Ga]Ga-DATA-TOC was able to visualise the tumour lesions, showing standardised uptake values (SUVs) similar to [68Ga]Ga-DOTA-TOC. Direct comparison of the two PET tracers in a NET patient revealed very similar tumour uptake for the two 68Ga-radiotracers, but with a higher tumour-to-liver contrast for [68Ga]Ga-DATA-TOC. Conclusion [68Ga]Ga-DATA-TOC was prepared, to a quality appropriate for in vivo use, following a highly efficient kit type process. Furthermore, the novel radiopharmaceutical was comparable or better than [68Ga]Ga-DOTA-TOC in all preclinical tests, achieving a higher tumour-to-liver contrast in a NET-patient. The results illustrate the potential of the DATA-chelator to facilitate the access to and preparation of 68Ga-radiotracers in a routine clinical radiopharmacy setting.
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