Organometallic technetium-99m RGD-peptide conjugates with tunable lipophilicity

Introduction:

Radiolabeled RGD-peptides have been studied for in vivo targeting of integrin receptors which are over-expressed in carcinomas [1]. The high pharmacological influence of the radiometal complex and limitations of various labelling concepts are reasons for exploring new approaches. The so-called “4+1” mixed-ligand chelate system, [Tc(NS₃)(CN-R)], is an interesting alternative due to high labelling efficiency and in vivo stability [2]. Here we present the application of this concept to the labelling of the peptide c(RGDyK).

Experimental:

The figure shows the labelled peptides, corroborated upon coinjection with the corresponding rhenium compounds. The RGD-peptide, serving as monodentate ligand, was functionalized with 4-(isocyanomethyl)benzoic acid (L1) and 4-isocyanobutanoic acid (L2). ^99mTc labelling was performed starting from ^99mTc-EDTA followed by ligand exchange reaction with a mixture of the appropriate isocyanide-modified peptide and a tetradentate ligand (NS₃R). The resulting series of ^99mTc peptide conjugates [^99mTc(NS₃R)(L-c(RGDyK))] with R = R1, R2, R3 and L = L1, L2 was evaluated in biodistribution studies using tumour-bearing mice.

Results and Discussion:

The radiochemical yield was about 60 % after labelling 0.05-0.1 mg of isocyanide-modified peptide. Purification by HPLC resulted in radiochemical purity > 95 %. The stability of the 99mTc conjugates in PBS was higher than 90 % after 24 h. Distribution ratios (logD, octanol/PBS, pH 7.4) covered the range from -0.59 ± 0.14 for [⁹⁹mTc(NS₃R1)(L1-c(RGDyK))] to -3.3 ± 0.2 for [^99mTc(NS₃R3)(L2-c(RGDyK))]. The compounds exhibited only low tumour uptake and a fast hepatobiliary elimination. Substitution of the tripodal chelator bearing R1 or R2 by the carboxyl group-bearing ligand NS₃R3 resulted in a predominant urinary excretion.

Conclusion:

The "4+1" mixed-ligand approach enables the ^99mTc-labelling of c(RGDyK). The chelate unit dominates the biodistribution profile which could be influenced by varying the tripodal chelator.

[1] Haubner, R., Wester, H.-J. Curr. Pharm. Design 10 (2004) 1439-1455.
[2] Seifert, S. et al., Bioconjugate Chem. 15 (2004) 856-863.

IAEA, CSIC, FMP-CHLCC, Lab. de vacunas recombinantes, Fac. de Medicina, Uruguay for financial support.