Hexadentate Bispidine Derivatives as Versatile Bifunctional Chelate Agents for Copper(II) Radioisotopes

The preparation and use of bispidine derivatives (3,7-diazabicyclo[3.3.1]nonane) as chelate ligands for radioactive copper isotopes for diagnosis (⁶⁴Cu) or therapy (⁶⁷Cu) are reported. Starting from the hexadentate bispidine-based bis(amine)tetrakis(pyridine) ligand 1 with a keto and two ester substituents, the corresponding mono-ol 2 and two dicarboxylic acid derivatives 3 and 5 have been synthesized. A range of techniques, including single-crystal X-ray structure analysis, UV/vis spectroscopy, cyclic voltammetry, thin-layer- (TLC), and high-performance liquid chromatography (HPLC), have been used to characterize the structure and stability of the copper(II)-bispidine complexes. A rapid formation (within 1 min) of stable copper(II)-bispidine complexes under mild conditions (ambient temperature, aqueous solution) has been observed. Challenge experiments of these complexes in the presence of a high excess of competing ligands, such as glutathione, cyclam, or superoxide dismutase (SOD), as well as in rat plasma, gave no evidence of demetalation or transchelation. The bifunctional bispidine derivative 5 can be readily functionalized with biologically active molecules at the pendant carboxylate groups. The coupling of a bombesin analogue βhomo-Glu-βAla-βAla-[Cha¹³,Nle¹⁴]BBN(7−14), by condensation of a carboxylate of the bispidine backbone with the N-terminus of the peptide produced the bifunctional ligand 6. The radiocopper(II) complex of this bombesin−bispidine conjugate has a considerable hydrophilicity (log D_o/w < −2.4), and this leads to a very fast blood clearance (blood: 0.28 ± 0.02 SUV, 1 h p.i.), low liver tissue accumulation (liver: 1.20 ± 0.27 SUV, 1 h p.i.), and rapid renal-urinary excretion (kidneys: 6.06 ± 2.96 SUV, 1 h p.i.) as shown by biodistribution studies of ⁶⁴Cu-6 in Wistar rats. Preliminary in vivo studies of ⁶⁴Cu-6 in NMRI nu/nu mice, bearing the human prostate tumor PC-3 showed an accumulation of the conjugate in the tumor (2.25 ± 0.13 SUV, 12.5 min p.i.; 0.94 ± 0.05 SUV, 55 min p.i.) and allowed a clear visualization of the gastrin-releasing peptide receptor distribution by positron emission tomography (PET).