title: Influence of the denticity of ligand systems on the in vitro and in vivo behavior of (99m)Tc(I)-tricarbonyl complexes: a hint for the future functionalization of biomolecules.
abstract: Functionalization of biologically relevant molecules for the labeling with the novel fac-[(99m)Tc(OH(2))(3)(CO)(3)](+) precursor has gained considerable attention recently. Therefore, we tested seven different tridentate (histidine L(1)(), iminodiacetic acid L(2)(), N-2-picolylamineacetic acid L(3)(), N, N-2-picolylaminediacetic acid L(4)()) and bidentate (histamine L(5)(), 2-picolinic acid L(6)(), 2,4-dipicolinic acid L(7)()) ligand systems, with the potential to be bifunctionalized and attached to a biomolecule. The ligands allowed mild radiolabeling conditions with fac-[(99m)Tc(OH(2))(3)(CO)(3)](+) (30 min, 75 degrees C). The ligand concentrations necessary to obtain yields of >95% of the corresponding organometallic complexes 1-7 ranged from 10(-)(6) to 10(-)(4) M. Complexes of the general formula "fac-[(99m)TcL(CO)(3)]" (L = tridentate ligand) and "fac-[(99m)Tc(OH(2))L'(CO)(3)]" (L' = bidentate ligand), respectively, were produced. Challenge studies with cysteine and histidine revealed significant displacement of the ligands in complexes 5-7 but only little exchange with complexes 1-4 after 24 h at 37 degrees C in PBS buffer. However, no decomposition to (99m)TcO(4)(-) was observed under these conditions. All 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 human plasma at 37 degrees C showed again no decomposition to (99m)TcO(4)(-) 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 complexes with tridentate coordinated ligands (1-4) showed no reaction with serum proteins. The compounds were tested for their in vivo stability and the biodistribution characteristics in BALB/c mice. The complexes with tridentate coordinated ligand systems (1-4) revealed generally a good and fast clearance from all organs and tissues. On the other hand, the complexes with only bidentate coordinated ligands (5-7) showed a significantly higher retention of activity in the liver, the kidneys, and the blood pool. Detailed radiometric analyses of murine plasma samples, 30 min p.i. of complex fac-[(99m)TcL(1)(CO)(3)], 1, revealed almost no reaction of the radioactive complex with the plasma proteins. By contrast, in plasma samples of mice, which were injected with complex fac-[(99m)Tc(OH(2))L(5)(CO)(3)](+), 5, the entire radioactivity coeluded with the proteins. On the basis of these in vitro and in vivo experiments, it appears that functionalization of biomolecules with tridentate-chelating ligand systems is preferable for the labeling with fac-[(99m)Tc(OH(2))(3)(CO)(3)](+), since this will presumably result in radioactive bioconjugates with better pharmacokinetic profiles.
authors.affiliation: Center for Radiopharmaceutical Science of the ETH Zürich, Paul Scherrer Institute, CH-5232 Villigen, Switzerland.
authors.forename: R
authors.initials: R
authors.lastname: Schibli
authors.forename: R
authors.initials: R
authors.lastname: La Bella
authors.forename: R
authors.initials: R
authors.lastname: Alberto
authors.forename: E
authors.initials: E
authors.lastname: Garcia-Garayoa
authors.forename: K
authors.initials: K
authors.lastname: Ortner
authors.forename: U
authors.initials: U
authors.lastname: Abram
authors.forename: P A
authors.initials: PA
authors.lastname: Schubiger
mesh_terms: D000818:Animals; D001798:Blood Proteins / Q000378:metabolism; D003545:Cysteine / Q000737:chemistry; D004355:Drug Stability; D006639:Histidine / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry / Q000493:pharmacokinetics; D006801:Humans; D007553:Isotope Labeling; D007668:Kidney / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D008807:Mice, Inbred BALB C; D015394:Molecular Structure; D009928:Organ Specificity; D015609:Organotechnetium Compounds / Q000138:chemical synthesis* / Q000493:pharmacokinetics; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000493:pharmacokinetics; D013055:Spectrophotometry, Infrared; D013329:Structure-Activity Relationship; D013667:Technetium / Q000737:chemistry*
chemical_list: D001798:Blood Proteins; D015609:Organotechnetium Compounds; D019275:Radiopharmaceuticals; C411324:tricarbonylhistidinetechnetium(0); D006639:Histidine; D013667:Technetium; D003545:Cysteine