Re and Tc Tricarbonyl Complexes: From the Suppression of NO Biosynthesis in Macrophages to in Vivo Targeting of Inducible Nitric Oxide Synthase


The in vivo molecular imaging of nitric oxide synthase (NOS), the enzyme responsible for the catalytic oxidation of L-arginine to citrulline and nitric oxide (NO), by noninvasive modalities could provide valuable insights into NO/NOS-related diseases. Aiming at the design of innovative (99m)Tc(I) complexes for targeting inducible NOS (iNOS) in vivo by SPECT imaging, herein we describe a set of novel (99m)Tc(CO)(3) complexes (2-5) and the corresponding rhenium surrogates (2a-5a) containing the NOS inhibitor N(omega)-nitro-L-arginine. The latter is linked through its alpha-NH(2) or alpha-COOH group and an alkyl spacer of variable length to the metal center. The complexes 2a (propyl spacer) and 3a (hexyl spacer), in which the alpha-NH(2) group of the inhibitor is involved in the conjugation to the metal center, presented remarkable affinity for purified iNOS, being similar to that of the free nonconjugated inhibitor (K(i) = 3-8 mu M) in the case of 3a (K(i) = 6 mu M). 2a and 3a are the first examples of organometallic complexes that permeate through RAW 264.7 macrophage cell membranes, interacting specifically with the target enzyme, as confirmed by the suppression of NO biosynthesis in LPS-treated macrophages (2a, ca. 30% inhibition; 3a, ca. 50% inhibition). The (99m)Tc(I)-complexes 2 and 3, stable both in vitro and in vivo, also presented the ability to cross cell membranes, as demonstrated by internalization studies in the same cell model. The biodistribution studies in LPS-pretreated mature female C57BL6 mice have shown that 2 presented an overall higher uptake in most tissues of the LPS-treated mice compared to the control group (30 min postinjection). This increase is significant in lung (3.98 +/- 0.63 vs to 0.99 +/- 0.13%ID/g), which is known to be the organ with the highest iNOS expression after LPS treatment. These results suggest that the higher uptake in that organ may be related to iNOS upregulation.



subject category

Biochemistry & Molecular Biology; Chemistry


Oliveira, BL; Raposinho, PD; Mendes, F; Figueira, F; Santos, I; Ferreira, A; Cordeiro, C; Freire, AP; Correia, JDG

our authors


This work has been partially supported by the Fundacao para a Ciencia e Tecnologia (FCT), Portugal (POCI/SAU-FCF/58855/2004). B. L. Oliveira thanks FCT for a PhD grant (SFRH/BD/38753/2007). Dr. J. Marcalo is acknowledged for performing the ESI-MS analyses. The QITMS instrument was acquired with the support of Contract REDE/1503/REM/2005 - ITN of FCT and is part of RNEM.

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