First results in ⁶⁴Cu-production and copper complexation by new ligands derived from bispidine and 1,4,7-triazacyclononane

Introduction:

⁶⁴Cu (t_1/2 = 12.7 h; ß⁺ 0.653 MeV, 17.4%; ß^-: 0.578 MeV, 39%) [1,2] is the most used copper isotope in nuclear medicine due to its favorable decay characteristics and its ability to be produced with high specific activity. Bispidines and azamacrocycles containing pyridine pendant arms form thermodynamically stable complexes with copper(II) [1,2]. These chelating agents – possessing an additional linker group – provide an attractive lead for labelling biomolecules with copper radionuclides.

Experimental:

First experiments to adapt the described ⁶⁴Cu-production process [3,4] at a biomedical cyclotron CYCLONE 18/9 were performed. A bombarding energy of 10 MeV and a proton beam current of 10 µA for 15 to 60 min were used. The yields of the ⁶⁴Ni(p,n)⁶⁴Cu reaction were comparable to reference values [179-331 MBq/µA/h versus 185-248 MBq/µA/h [3,4]].
Two novel ligands with carboxylic groups – a hexadentate bispidine 1 and a derivative of 1,4,7-triazacyclononane (TACN) 2 [5] were prepared. Labelling experiments of these ligands indicated the rapid formation of stable ⁶⁴Cu-complexes under mild conditions. The bioconjugates 3 and 4 were obtained by coupling the bombesin (BBN) derivate βAla-βAla-[Cha¹³, Nle¹⁴]BBN(7-14) [6] to the ligands 1 and 2 (HBTU, DIPEA, DMF, r.t.).

Results and Discussion:
The bioconjugation of 1 and 2 to a bombesin derivative was successfully achieved via amide coupling to give 3 and 4. The free ligands and the bioconjugates were labelled with ⁶⁴Cu and the resulting complexes were found to be stable in the presence of a large excess of competing ligands such as cyclam and even in rat plasma.

Conclusion:

The bispidine ligand 1, the TACN derivative 2 and their bombesin bioconjugates 3 and 4 are able to form complexes with high stability. These bifunctional chelating agents may be attractive candidates for the development of new copper radiopharmaceuticals for diagnosis (⁶⁴Cu) and therapy (⁶⁷Cu). Preliminary studies on the bio-distribution of ⁶⁴Cu-complexes of 3 and 4 are currently underway [5].

[1] C. Bleiholder et al., Inorg. Chem. 2005, 44, 8145-8155.
[2] G. A. McLachlan et al., Inorg. Chem. 1994, 33, 4663-4668.
[3] F. Szelecsényi et al., Appl. Radiat. Isotopes 1993, 44, 575-580.
[4] D. W. McCarthy et al., Nucl. Med. Biol. 1997, 24, 35–43.
[5] G. Gasser et al., Bioconjugate Chemistry, 2007, in prep.
[6] E. Garcia Garayoa et al., Nucl. Med. Biol. 2007, 34, 17-28.