Rhenium and technetium tricarbonyl complexes anchored by pyrazole-based tripods: novel lead structures for the design of myocardial imaging agents

This report describes the synthesis and biological evaluation of cationic ^99mTc–tricarbonyl complexes anchored by ether-containing tris(pyrazolyl)methane or bis(pyrazolyl)ethanamine ligands to be applied in the design of radiopharmaceuticals for myocardial imaging: fac-[^99mTc(CO)₃{RC(pz)₃}]⁺ (R = H (1a), MeOCH₂ (2a), EtOCH₂ (3a), ⁿPrOCH₂ (4a)) and fac-[^99mTc(CO)₃{RNHCH₂CH(pz)₂}]⁺ (R = H (5a), MeO(CH₂)₂ (6a)) (pz = pyrazolyl). At the no carrier added level, complexes 1a–6a were obtained in high radiochemical yield (> 98%) by reaction of fac-[^99mTc(CO)₃(H₂O)₃]⁺ with the corresponding tripod chelator in aqueous medium. All these complexes display a high in vitro and in vivo stability, except 6a which metabolizes invivo yielding fac-[^99mTc(CO)₃{HO(CH₂)₂NHCH₂CH(pz)₂}]⁺ (7a). Biological studies in mice have shown that among the radiotracers evaluated in this work, 3a, anchored by a tris(pyrazolyl)methane chelator bearing an ethyl methyl ether substituent, has the highest heart uptake (3.6 ± 0.5%ID g⁻¹ at 60 min p.i.). Complex 3a presents also the best heart : blood, heart : liver and heart : lung ratios, appearing as the most promising as a potential myocardial imaging agent. The chemical identity of 1a–7a was ascertained by HPLC comparison with the previously reported fac-[Re(CO)₃{HC(pz)₃}]Br (1) and with the novel fac-[Re(CO)₃{RC(pz)₃}]Br (R = MeOCH₂ (2), EtOCH₂ (3), ⁿPrOCH₂(4)) and fac-[Re(CO)₃{RNHCH₂CH(pz)₂}]Br (R = H (5), MeO(CH₂)₂ (6) HO(CH₂)₂ (7)). The novel Re(I) tricarbonyl complexes, 2–7, were characterized by the common analytical techniques, including single crystal X-ray diffraction analysis. The solid state structure confirmed the presence of facial and tridentate (κ³-N₃) anchor ligands. Solution NMR studies have also shown that this κ³-N₃ coordination mode is retained in solution for all complexes (2–7).