for their potential utility in healthcare. In 1975 Lebowitz et al. discussed the possible application of thallium-201 for myocardial imaging due to the similarities in size between Tl(I) and K(I). Thallium-201 ( 201 Tl) has a half-life of 73 hours and emits -photons at 72 keV (X-ray, 94% abundance), 135 keV (3% abundance) and 167 keV (10% abundance) making it suitable for application in single photo emission computed tomography (SPECT). As predicted, Tl initially found widespread use as a myocardial imaging agent until it was replaced by emerging technetium-99m tracers. In a few instances [ 201 Tl]TlCl has also been employed for tumour detection and monitoring. Additionally, Tl decays through electron capture (EC) to Hg, emitting an average of 36.9 Meitner-Auger electrons per decay event. Meitner-Auger electronemitting agents have shown significant potential for application in systemic, targeted radionuclide therapy of cancer. With its high decay yield of Meitner-Auger electrons, Tl has promise as a radiotherapeutic isotope and possibly for theranostic applications. The clinical application of Tl is currently limited to its use as the "free" or "unchelated" metal ion. Many other radioactive metallic isotopes have demonstrated clinical utility but are largely used in a complexed or chelated form. For example, [ 68 Ga]Ga 3+ and [ 177 Lu]Lu 3+ can be complexed to chelators which are appended to peptides that target receptors expressed in cancer tissue. The ensuant 68 Ga and 177 Lu radiopharmaceuticals are used for PET imaging and systemic radiotherapy respectively, and have had clinical impact in management and treatment of neuroendrocrine cancer and prostate cancer. Advancements in the development of new Tl-based radiopharmaceuticals are currently prevented by an almost complete lack of studies into the formation of stable compounds between thallium and organic chelators that could feature biological targeting functions. Thus far, only two studies have investigated the coordination of thallium to popular radioimaging chelators. In 2011, Hijnen et al. described the evaluation of [ 201 Tl]Tl(III)-DOTA (1,4,7,10-tetraazacyclododecane-N,N′,N″,N′″ tetraacetic acid) complexes. The authors oxidized the readily available 201 Tl(I) by treatment with hydrochloric acid and ozone and found good radiochemical yields and purity for the formation of DOTA and DTPA (diethylenetriaminepentaacetic acid) [ 201 Tl]Tl(III)-labelled complexes. Unfortunately, the DTPA compound was found to rapidly decay in human blood serum. Although the DOTA complex was found to be stable in serum, the biodistribution of the compound mirrored that of non-chelated or "free" [ 201 Tl]Tl(I), suggesting limited stability of [ 201 Tl]Tl(III)-DOTA in vivo. Interestingly, Fodor et al. described the same complex as an "extraordinarily robust macrocyclic complex" in a 2015 report. However no in vivo experiments were reported, suggesting that while [ nat Tl]Tl(III)-DOTA may possess favorable stability in vitro, the Tl(III) core is readily reduced in vivo, leading to rapid decomplexation.