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 betahomo-Glu-betaAla-betaAla-[Cha(13),Nle(14)]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).