Versatile synthetic approach to new bifunctional chelating agents tailor made for labeling with the fac-[M(CO) 3] + core (M = Tc, 99mTc, Re): synthesis, in vitro, and in vivo behavior of the model complex [M(APPA)(CO) 3] (appa = [(5-amino-pentyl)-pyridin-2-yl-methyl-amino]-acetic acid)

Abstract

A general synthetic approach for potent tridentate, bifunctional chelating agent (BFCA) for the [M(CO) 3] + fragment (M = 99gTc, 99mTc, and Re) has been elaborated. The strategy allows the facile preparation of BFCA with a pendent amino or carboxylic acid functionality for coupling to peptides and proteins via formation of an amide bond. [(5-amino-pentyl)-pyridin-2-yl-methyl-amino]-acetic acid (APPA) and [pyridin-2-yl-methyl-amino]-diacetic acid (PADA) were synthesized according to this protocol. The BFCA were labeled with the [M(CO) 3] + fragment, which resulted in formation of uniform products with a ligand to metal ratio of 1:1. The complexes have been fully characterized by means of mass spectrometry, IR, and NMR ( 1H, 13C, 99Tc) spectroscopy. Coordination of the tricarbonyl core with APPA and PADA was exclusively tridentate (via the acid function, the ternary amine, and the pyridine nitrogen). On the n.c.a. level the complexes were almost quantitatively formed (yield >90%) at ligand concentrations of 10±2 μM (PADA) or 50±4 μM (APPA) after 30 min at 70°C. Chromatographic behavior of the 99mTc complexes is similar to that of the corresponding 99Tc/Re complexes suggesting the identical chemical structure. Pharmacokinetic experiments with the 99mTc-APPA complex were performed in BALB/c mice and compared with previously published results of the 99mTc-PADA complex. The 99mTc-APPA complex revealed good clearance from the blood pool (0.29 ± 0.03% ID after 24h p.i.) and a low uptake in the liver (2.41 ± 0.14% ID/g), in the kidneys (2.81 ± 0.12% ID/g) and other tissue and organs.