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2 Publications
Towards Tc(III)- and Re(III)-labelled peptides with tunable lipophilicity
Gniazdowska, E.; Künstler, J.-U.; Stephan, H.; Pietzsch, H.-J.;
1 Introduction
Organometallic Tc and Re (III) compounds in terms of so called “4+1” complexes enable high specific labelling of biomolecules, e.g., peptides under mild conditions and exhibit high complex stability [1, 2]. However, 99mTc-labelled peptides containing a “4+1” complex as in 99mTc(NS3)(CN-GGY) (Fig. 1) showed a high liver uptake [1]. To improve the biobehavior of such compounds a new hydrophilic “4+1” complex bearing three carboxyl groups was introduced and evaluated coupled to a model peptide.

2 Results
The carboxyl-groups bearing ligand NS3(COOH)3 was synthesized by conjugation (DCC) of the dendritically functionalized amine H2N-C(-CH2-O-CH2-CH2-COO-Me)3 [3] to the S-benzyl protected NS3COOH [1] followed by deprotection.
The isocyanide group was coupled to the model peptide Gly-Gly-Tyr via an aliphatic linker (CN-BFCA). For 99mTc-labelling a two-step procedure [1] was applied using 99mTc-EDTA/mannitol (400 MBq) which was reacted with 20 µg isocyanide-modified peptide and 300 µg NS3(COOH)3. 99mTc-labelling resulted in a radiochemical yield of app. 25 % (HPLC), Fig. 2. The yield can be increased by using a higher peptide amount. 99mTc(NS3(COOH)3)(CN-GGY) showed a high in vitro stability (> 90 % in PBS after 24 h).
To verify the identity of the 99mTc-labelled peptide, the analogous "4+1" Re compound Re(NS3(COOH)3)(CN-GGY) was synthesized (Fig. 1).

For a convenient synthesis of peptides containing the new hydrophilic Re “4+1” complex, the active ester Re(NS3(COOMe)3)(CN-BFCA) as shown in Fig. 1 was prepared starting from NS3(COOH)3, [Re(tu-S)6]Cl3 and PMe2Ph followed by ligand exchange with CN-BFCA. Reaction of Re(NS3(COOMe)3)(CN-BFCA) with the peptide and hydrolysis of the methyl ester gave the desired peptide derivative.
The increased hydrophilicity of the new “4+1” complex was shown by HPLC investigations (Fig. 2) and determination of the partition coefficient (logD, octanol/PBS, pH 7.4).
99mTc(NS3(COOH)3)(CN-GGY) showed a logD value of -2.6 ± 0.3 (n = 3) compared with the logD value of -1.5 ± 0.2, 99mTc(NS3)(CN-GGY).

3 Conclusion
A new hydrophilic “4+1” complex characterized by a dendritically modified tetradentate ligand was synthesized and has been introduced for peptide labelling. Increasing generation of dendritic branches will be grafted on the chelating unit in order to tune the lipophilicity. As a next step peptides bearing the new modified “4+1” compound have to be evaluated in animal experiments.

4 References
[1] Seifert S., Künstler J.-U., Schiller E., Pietzsch H.-J., Pawelke B., Bergmann R. and Spies H. [2004] Bioconjugate Chem. 15: 856-863.
[2] Schiller E., Seifert S., Tisato F., Refosco F., Kraus W., Spies H. and Pietzsch H.-J. [2005] Bioconjugate Chem. 16: 634-643.
[3] Newkome G. R., Lin X. and Young J. K. [1992] Synlett 1: 53-54.

5 Acknowledgements
The work was performed in the framework of the project “Chemical Studies for Design and Production of New Radiopharmaceuticals” (No MTKD-CT-2004-509224 (POL-RAD-PHARM)) supported by the European Community within the 6th Framework Programme Marie Curie: Host Fellowships for Transfer of Knowledge.
  • Poster
    European Symposium on Radiopharmacy and Radiopharmaceuticals - ESRR 06, 30.03.-02.04.2006, Lucca, Italy
  • Abstract in refereed journal
    Quarterly Journal of Nuclear Medicine and Molecular Imaging 51(2007)1, 92

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Publ.-Id: 8387