Radiolabelled Cyclic Bisarylmercury: High Chemical and in vivo Stability for Theranostics

Abstract

We show the synthesis of an in vivo stable mercury compound with functionality suitable for radiopharmaceuticals. The designed cyclic bisarylmercury was based on the water tolerance of organomercurials, higher bond dissociation energy of Hg-Ph to Hg-S, and the experimental evidence that acyclic structures suffer significant cleavage of one of the Hg-R bonds. The bispidine motif was chosen for its in vivo stability, chemical accessibility, and functionalization properties. Radionuclide production results in ^197(m) HgCl₂ (aq), so the desired mercury compound was formed via a water-tolerant organotin transmetallation. The Hg-bispidine compound showed high chemical stability in tests with an excess of sulfur-containing competitors and high in vivo stability, without any observable protein interaction by human serum assay, and good organ clearance demonstrated by biodistribution and SPECT studies in rats. In particular, no retention in the kidneys was observed, typical of unstable mercury compounds. The ^nat Hg analogue allowed full characterization by NMR and HRMS.

Keywords: Bispidine; Mercury; Organomercury; Radiopharmaceuticals; Theranostics.

Scheme 1

Cyclic bisarylmercury bispidine synthesis.

Figure 1

Overlay of radiochromatogram (γ, red) and UV absorption chromatogram (220 nm, black), for co‐injection of 3 a* (retention time 10.2 min) and 3 a (retention time 10.1 min).

Figure 2

Relative abundance of the calculated and measured [M+H]⁺ isotopic distribution pattern of substance 3 a as analysed by mass spectrometry.

Figure 3

Species analysis of ^197(m)Hg. Overlay of four radio‐TLCs: RP−C18 (dotted lines) and trithiol impregnated RP−C18 (solid lines) of ^197(m)HgCl₂ reference and 3 a*. NS3=tris(2‐mercaptoethyl)ammonium oxalate.

Figure 4

Degradation analysis of ^197(m)Hg‐species. Overlay of five radio‐TLCs (iTLC‐SG) of ^197(m)HgCl₂ reference, 3 a* reference, and 3 a* with competitor trials after 2 d at room temperature. ^aIntensity normalized with respect to 3 a*. GSH=l‐glutathione. NS3=tris(2‐mercaptoethyl)ammonium oxalate.

Figure 5

Analysis of ^197(m)Hg‐incorporation into human serum proteins by SDS‐PAGE. 20 % SDS‐polyacrylamide gel Coomassie blue staining (A), showing bands of human serum proteins, and autoradiograph (B), showing ^197(m)Hg‐labelled bands. Lane 1: ^197(m)Hg‐radiolabelled bispidine 3 a*. Lane 2: ^197(m)Hg‐radiolabelled EDTA. M: molecular‐weight size marker.

Figure 6

Biodistribution of 3 a* in healthy male Wistar rats (^197(m)HgCl₂: n=4, 3 a*: n=8). ∼300 kBq per animal. All other organs measured well below 1 %ID/g.

Figure 7

Standardized Uptake Value coloured SPECT/CT images of rats 24 h after injection of ^197(m)HgCl₂ (left, retention in kidneys) and 3 a* (right, rapid renal excretion with minor liver uptake followed by hepatobiliary clearance); maximum intensity projections (top) and coronal slice images (bottom); intestine (int), kidneys (ki), liver (li), rectum (re), stomach (st).