The [Tc-99m(N)(PNP)](2+) metal fragment: A technetium-nitrido synthon for use with biologically active molecules the N-(2-methoxyphenyl)piperazyl-cysteine analogues as examples

The incorporation of a bioactive molecule into a nitrido-containing 99mTc-complex has been successfully achieved by using the [TcN(PNP)]2+ metal fragment. In this strategy, the strong electrophilic [TcN- (PNP)]2+ metal fragment efficiently reacts with bifunctional chelating ligands having a ð-donor atom set, such as N-functionalized O,S-cysteine. The 2-methoxyphenylpiperazine (2-MPP) pharmacophore, which displays preferential affinity for 5HT1A receptors, was conjugated to the amino group of cysteine to obtain 2-MPPP-cys-OS, where 2-MPPP is 3-[4-(2-methoxyphenyl)piperazin-1-yl]propionate. The asymmetric Tc(V)-nitrido complexes, [99g/99mTc(N)(PNP)(2-MPPP-cys-OS)] (PNP ) PNP3, PNP4), were obtained in high yield (95%), by simultaneous addition of PNP and 2-MPPP-cys-OS ligand to a solution containing a starting 99g/99mTc-nitrido precursor. A mixture of syn and anti isomers was observed, the latter being the thermodynamically favored species. In vitro challenge experiments using the anti isomers with glutathione and cysteine indicated that no transchelation reaction occurs. Assessment of the in vitro 5HT1A receptor-affinity of the technetium complexes revealed that only the anti-PNP4 complex possesses some affinity for the receptor, but displayed negligible brain uptake in biodistribution studies in rats in vivo.