Novel 99mTc ‘4 + 1’ peptide conjugates: Tuning the biodistribution by variation of coligands

Abstract

A sophisticated coligand strategy is presented for peptide-derived radioconjugates based on 99mTc ‘4 + 1’ mixed-ligand complexes. The new pharmacologically active coligands are assessed for 99mTc-labeling of the RGD-peptide cyclo(Arg-Gly-Asp- d-Tyr-Lys) which is an established vehicle to target α vβ 3 integrins playing a crucial part in tumor pathogenesis. Complexes of the general formula [ 99mTc(NS 3R)X] were synthesized and evaluated, in which Tc(III) is coordinated by NS 3R, a derivative of the tetradentate chelator 2,2′,2″-nitrilotriethanethiol (NS 3), and by X, a monodentate binding isocyanide bearing the biomolecule. The novel tetradentate chelators (NS 3R = NS 3crown, NS 3en, NS 3(COOH) 3) constitute NS 3 with a crown ether, an amine or a tricarboxylic acid as pharmacological modifiers. The isocyanides (X = L2-RGD, L2-Lys) contained the linker isocyanobutanoic acid (L2) coupled to N 6-Lys of the RGD-peptide and additionally to a single Lys. The lipophilicity (distribution coefficient log D O / W , pH = 7.4) of the RGD-containing radiotracers decreased in the order of the coligands NS 3crown (−1.7 ± 0.1), NS 3en (−2.7 ± 0.1) and NS 3(COOH) 3 (−3.3 ± 0.1). In the same order of the coligands, the biodistribution of the series [ 99mTc(NS 3R)(L2-RGD)] in normal rats showed a decrease of hepatobiliary and an increase of urinary excretion. The ratio of specifically to unspecifically uptaken activity (sum of surface bound and internalized activity) in α vβ 3 integrin-expressing M21 cells was in the range of approximately 4–5 and comparable for all [ 99mTc(NS 3R)(L2-RGD)] tracers. The biodistribution of [ 99mTc(NS 3en)(L2-RGD)] in ν/ν mice bearing M21 and M21L (control) tumor xenografts exhibited a specific tumor uptake with a low target-background ratio. The metabolic stability of the [ 99mTc(NS 3R)(L2-RGD)] tracers in normal rats was high, since 75–87% of the radioactivity in the plasma extract was assigned to the injected radiotracers 60 min after intravenous application in a rat. The hypothetical metabolites [ 99mTc(NS 3R)(L2-Lys)] were not found. These results demonstrate a considerable improvement of in vivo properties of 99mTc ‘4 + 1’ peptide conjugates and open up the possibility of applying the labeling approach for further radiodiagnostic peptides.