New maleimide functionalized NOTA-derivatives: Synthesis and ⁶⁴Cu/⁶⁸Ga labelling of thiol modified L-oligonucleotides

Introduction:

The macrocyclic chelator 1,4,7–triazacyclononane triacetic acid (NOTA) is very suitable to label molecules with ⁶⁴Cu or ⁶⁸Ga due to its fast complex formation resulting in thermodynamically and kinetically stable complexes. Here we describe a convenient synthesis procedure for maleimide-functionalized NOTA derivatives for stable binding thiol groups of carrier molecules such as peptides, oligonucleotides or antibodies. This principle is examplified by coupling of thiol-modified L-oligonucleotides which are of great interest for pre-targeting approaches in radio-immunotherapy 1.
Results and discussion:
NOTA-monomaleimide 3 and NOTA-bismaleimide 4 were synthesized by nucleophilic addition of aminoethyl maleimide 1 or bismaleimide 2 to p–SCN-Bn-NOTA. At the next reaction step the NOTA-maleimides were conjugated
with a 50-mercaptohexyl-modified L-oligonucleotide.
⁶⁴Cu or ⁶⁸Ga labelling of conjugates 5 and 6 occurred very fast and with good radiochemical yields in NH₄OAc buffer. The specific activities resulted in 13.0 GBq/mmol for [⁶⁴Cu]Cu5, 6.7 GBq/mmol for [68Ga]Ga5, 5.8 GBq/mmol for [64Cu]Cu6 and 2.6 GBq/mmol for [⁶⁸Ga]Ga6. Small animal PET imaging in mice with [⁶⁴Cu]Cu5 showed that after 2 hours most of the injected activity was located in the bladder.
Conclusion:
The new ligand systems show advantages which make them interesting for further biological studies, namely the easy synthesis procedure, the fast labelling with ⁶⁴Cu or ⁶⁸Ga and the transferability of the conjugation principle to other carrier molecules such as peptides or antibodies.
Reference
[1] J. Schlesinger et al., Bioconjugate Chem. 2008, 19, 928–939.