Influence of the denticity of ligand systems on the in vitro and in vivo behavior of Tc-99m(I)-tricarbonyl complexes: A hint for the future functionalization of biomolecules

Functionalization of biologically relevant molecules for the labeling with the novel fac-[Tc-99m(OH2)(3)(CO)(3)](+) precursor has gained considerable attention recently. Therefore, we tested seven different tridentate (histid ine L-1, iminodiacetic acid L-2, N-2-picolylamineacetic acid L-3, N,N-2-pic olylaminediacetic acid L-4) and bidentate (histamine L-5, a-picolinic acid L-6, 2,4-dipicolinic acid L-7) ligand systems, with the potential to be bif unctionalized and attached to a biomolecule. The ligands allowed mild radio labeling conditions with fac-[Tc-99m(OH2)(3)(CO)(3)](+) (30 min, 75 degrees C). The ligand concentrations necessary to obtain yields of >95% of the co rresponding organometallic complexes 1-7 ranged from 10(-6) to 10(-4) M. Co mplexes of the general formula "fac-[(TcL)-Tc-99m(CO)(3)]" (L = tridentate ligand) and "fac-[Tc-99m(OH2)L'(CO)(3)]" (L' = bidentate ligand), respectiv ely, were produced. Challenge studies with cysteine and histidine revealed significant displacement of the ligands in complexes 5-7 but only little ex change with complexes 1-4 after 24 h at 37 degrees C in PBS buffer. However , no decomposition to (TCO4-)-T-99m was observed under these conditions. Al l complexes showed a hydrophilic character (log P-o/w values ranging from - 2.12 to 0.32). Time-dependent FPLC analyses of compounds 1-7 incubated in h uman plasma at 37 degrees C showed again no decomposition to (TCO4-)-T-99m after 24 h at 37 degrees C. However, the complexes with bidentate ligands ( 5-7) became almost completely protein bound after 60 min, whereas the compl exes with tridentate coordinated ligands (1-4) showed no reaction with seru m proteins. The compounds were tested for their in vivo stability and the b iodistribution characteristics in BALB/c mice. The complexes with tridentat e coordinated ligand systems (1-4) revealed generally a good and fast clear ance from all organs and tissues. On the other hand, the complexes with onl y bidentate coordinated ligands (5-7) showed a significantly higher retenti on of activity in the liver, the kidneys, and the blood pool. Detailed radi ometric analyses of murine plasma samples, 30 min p.i. of complex fac-[(TcL 1)-Tc-99m(CO)(3)], 1, revealed almost no reaction of the radioactive comple x with the plasma proteins. By contrast, in plasma samples of mice, which w ere injected with complex fac-[Tc-99m(OH2)L-5(CO)(3)](+), 5, the entire rad ioactivity coeluded with the proteins. On the basis of these in vitro and i n vivo experiments, it appears that functionalization of biomolecules with tridentate-chelating ligand systems is preferable for the labeling with fac -[99mTc(OH2)(3)(CO)(3)](+), since this will presumably result in radioactiv e bioconjugates with better pharmacokinetic profiles.