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Issue 42, 2017
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Technetium-99m complexes of L-arginine derivatives for targeting amino acid transporters

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Although relevant from the clinical point of view, radiotracers targeting cationic amino acid transporters are relatively unexplored and, in particular, no metal-based radiotracers are known. The rare examples of complexes recognized by amino acid transporters, namely by the Na+-independent neutral L-type amino acid transporter 1 (LAT1), are 99mTc(I)/Re(I) compounds. Herein, we describe conjugates comprising a pyrazolyl-diamine chelating unit and the cationic amino acid L-arginine (L-Arg) linked by a propyl (L1) or hexyl linker (L2), which allowed the preparation of stable complexes of the type fac-[99mTc(CO)3(k3-L)]+ (Tc1, L = L1; Tc2, L = L2) and of the respective surrogates Re1 and Re2. Interestingly, complex Tc2 exhibited moderate levels of time-dependent internalization in three human tumoural cell lines, with approximately 3% of total applied activity internalized, corresponding to 21% of the cell-associated activity. A putative mechanism of retention in the cytoplasm of cells could be the interaction of the complex with inducible nitric oxide synthase (iNOS), which is the enzyme responsible for the catalytic oxidation of L-Arg to citrulline and nitric oxide. However, the surrogate complex Re2 does not recognize iNOS, as demonstrated by the in vitro assays with purified iNOS and in studies with lipopolysaccharide(LPS)-activated macrophages. Preliminary mechanistic studies suggest that the internalization of Tc2 is linked to the cationic amino acid transporters, namely system y+. This finding might open the way towards the development of novel families of metal-based radiotracers for probing metabolically active cancer cells.

Graphical abstract: Technetium-99m complexes of l-arginine derivatives for targeting amino acid transporters

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Publication details

The article was received on 30 Mar 2017, accepted on 30 May 2017 and first published on 31 May 2017

Article type: Paper
DOI: 10.1039/C7DT01146F
Dalton Trans., 2017,46, 14537-14547

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    Technetium-99m complexes of L-arginine derivatives for targeting amino acid transporters

    M. Morais, V. F. C. Ferreira, F. Figueira, F. Mendes, P. Raposinho, I. Santos, B. L. Oliveira and J. D. G. Correia, Dalton Trans., 2017, 46, 14537
    DOI: 10.1039/C7DT01146F

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