177Lu-, 225Ac- and 111In-labelled nonadentate bispidine ligands synthesis, radiolabelling experiments and stability assays

Objectives
Bispidines (3,7-diazabicyclo[3.3.1]nonanes) are of great interest for the use in radiopharmaceutical applications. Combining the advantages of highly preorganised rigid macrocyclic ligands and the flexibility of open-chain ligands, they are able to form highly stable complexes at mild reaction conditions with a broad range of di- and trivalent metal ions.1,2 Here, we present a nonadentate bispidine ligand (Fig. 1) labelled with [177Lu]LuCl3, [225Ac]AcCl3 and [111In]InCl3 at mild conditions and report their stability and inertness in aqueous solution and human serum.3 The results in terms of radiolabelling conditions and serum stabilities are compared with the “gold standard” DOTA, which requires harsh radiolabelling conditions, and the octadentate bispidine ligand H2bispa.2,4

Methods
The bispidine ligand was synthesised according to literature on a multigram scale, with an overall yield of 5% over 9 steps.2

Radiolabelling experiments of the nonadentate bispidine ligand with [177Lu]Lu3+ and [225Ac]Ac3+ were carried out at 40 °C using 150 mM NH4OAc buffer (pH 6). Radiolabelling with [111In]In3+ was performed in the same buffer at room temperature. For comparison, the DOTA and the octadentate bispidine ligand H2bispa2 were labelled with the trivalent radiometals as well. The radiochemical yields and purities were monitored via radio-TLC and radio-HPLC for different ligand concentrations after 5 min, 30 min and 60 min. Radiostabilities in human serum were studied by radio-TLC and radio-SEC after 1 h, 1 d, 3 d and 7 d.

Results
Radiolabelling experiments gave quantitative yields for the formation of both 177Lu- and 225Ac‑complexes at 40 °C after 5 minutes for ligand concentrations of 10-6 mol/L. With [111In]In3+ a quantitative conversion was obtained even at room temperature after 60 minutes with a ligand concentration of 5·10-6 mol/L. For [177Lu]Lu3+, a molar activity of >100 MBq/nmol, for [225Ac]Ac3+ 0.2 MBq/nmol and for [111In]In3+ >20 MBq/nmol was found at the end of synthesis. Furthermore, the radiolabelled bispidine complexes showed high stability in human serum after 7 d ([225Ac]Ac‑bispidine: 80±2%; n=3, [177Lu]Lu-bispidine: 94%; n=2,), whereas only moderate stability was achieved for [111In]In-bispidine (74%; n=2 ) after 3 d.

Conclusions
The presented 177Lu- and 225Ac-labelled bispidine complexes showed favourable labelling kinetics and radiostabilities in human serum. The 111In-labelled bispidine gave moderate molar activities and stabilities in human serum. In comparison to DOTA, milder conditions for bispidine ligands allow the radiolabelling of heat-sensitive biomolecules.

References

[1] Comba, P.; Kerscher, M.; Rück, K.; Starke, M. Bispidines for Radiopharmaceuticals. Dalton Trans. 2018, 47, 9202–9220.

[2] Abad‐Galán, L.; Cieslik, P.; Comba, P.; Gast, M.; Maury, O.; Neupert, L.; Roux, A.; Wadepohl, H. Excited State Properties of Lanthanide(III) Complexes with a Nonadentate Bispidine Ligand. Chem. – Eur. J. 2021, 27, 10303–10312.

[3] Cieslik, P.; Kubeil, M.; Stephan, H.; Comba, P. Bispidine derivatives and the use thereof. EP20216739, 2020.

[4] Comba, P; Jermilova, U.; Orvig, C.; Patrick, B. O.; Ramogida, C.; Rueck, K.; Schneider, C.; Starke, M. Inorg. Chem. 2017, 23, 15945 – 15956.