Avidin–biotin system: a small library of cysteine biotinylated derivativesdesigned for the [99mTc(N)(PNP)]2+ metal fragment

Using the avidin–biotin system as model, we investigate here the effective application of [Tc(N)L(PNP)]+/0 technology (L=Nfunctionalized cysteine [O-,S-]; PNP=aminodiphosphine) to the preparation of target-specific radiopharmaceuticals. A series of 99mTc-nitrido complexes containing functionalized biotin ligands was prepared and their biological profile was determined. To minimize the steric and the electronic influences of the Tc-carrying complex on the biotin–avidin receptor interaction, the following N-functionalized cysteine–biotin derivatives were synthesized: (1) Biot-CysOSH; (2) Biot-Abu-CysOSH; (3) Biot-Abz-CysOSH; (4) Biot-L- (Ac)Lys-CysOSH; (5) Biot-D-(Ac)Lys-CysOSH; (6) Biot-Glu-CysOSH. The asymmetrical nitrido-Tc(V) 99g/99mTc(N)(Biot-X-CysOS)(PNP3) (X=spacer) complexes, where PNP3 was N,N-bis-[(dimethoxypropyl) phosphinoethyl] methoxy-ethylamine, were obtained by simultaneous addition of PNP3 and the relevant biotinylated ligand to a solution containing a 99mTc-nitrido precursor (yields N95%). In all cases, a mixture of syn- and anti isomers was observed. In vitro challenge experiments with glutathione and cysteine indicated that no transchelation reactions occurred. Assessment of the in vitro binding to avidin of the complexes revealed that only the complexes containing Biot-Abu-CysOS and Biot-Glu-CysOS ligand maintained a good affinity for the concentrator. Stability studies carried out in human and mouse plasma as well as in rat and mouse liver homogenate evidenced a rapid enzymatic degradation for the 99mTc(N)(Biot-Abu-CysOS)(PNP3) complex, whereas the 99mTc(N)(Biot-Glu-CysOS)(PNP3) one was stable in all conditions. Tissue biodistribution in normal Balb/C mice of the most stable candidate showed a rapid clearance both from the blood and the other tissues. The activity was eliminated both through the hepatobiliary system and the urinary tract