Antimony (Sb) and its homologue arsenic (As) share similar thiophilicity and affinity for the same charge states. Antimony-119 (t_½ = 38.2 h, ~24 Auger electrons per decay) is a radionuclide for therapeutic applications, whereas positron-emitting ⁷¹As (t_½ = 65.3 h, 28% β⁺) and ⁷²As (t_½ = 26.0 h, 87% β⁺) can be used for diagnostic imaging. This work investigates whether a theranostic relationship between As and Sb radionuclides can be realized due to their chemical homology. Comparative labeling studies were conducted using a trithiol-based chelating ligand, TT (Figure 1), for tridentate coordination of the 3+ pnictogens. Prior studies have shown a 2-step As reaction is required, wherein As is first reacted with a monothiol ligand, SR (Figure 1), to make As(SR)₃, followed by transchelation with TT to form the desired product (^natAs-TT, 80% yield). Conversely, high ^natSb-TT yield (85%) was obtained via direct labeling with TT, while the 2-step reaction yield was low (10%) due to poor transchelation from Sb(SR)₃. Good TT radiolabeling yields (by TLC/HPLC) were obtained with reactor-produced ⁷⁷As (2-step method, >95%) and cyclotron-produced ^120m,122Sb (direct method, 63%) surrogate radiotracers. The logD_7.4 value of ⁷⁷As-TT was +1.4 ± 0.1. The shorter HPLC retention time for Sb-TT (24 min) vs. As-TT (31 min) with the same gradient (50%-90% acetonitrile in water, both with 0.1% trifluoroacetic acid, over 40 min) suggests a lower logD_7.4 value for Sb-TT. This measurement and additional comparative As/Sb studies are ongoing.

Aeli Olson

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