Tricarbonyl-Rhenium Complexes of a Thiol-Functionalized Amphoteric Poly(amidoamine).

An amphoteric thiol-functionalized poly(amidoamine) (referred to as ISA23SH10%) has been obtained by Michael-type polyaddition between 2,2-bis(acrylamido)acetic acid and a 9:1 mixture of 2-methylpiperazine and cysteamine. The copolymer is able to tightly bind up to 0.8 equivalents of Re(CO)3+ fragments, with respect to the thiol groups. The polymeric complexes 1 and 2, containing 0.5 or 0.8 equivalents of rhenium, respectively, are easily obtained by reacting ISA23SH10% with [Re(CO)3(H2O)3](CF3SO3), in aqueous solution, at pH 5.5. The reaction is fast: complex 1 is formed in 15 min at 80°C in a 5 mM solution and this reaction time is further reduced to 2 min by microwave heating. No reaction occurs at pH higher than 5.5, due to the fast oligomerization of [Re(CO)3(H2O)3]+. The complexes maintain the water solubility of the parent polymer and are stable in physiological conditions and also in the presence of cysteine. The coordination of the Re(CO)3 fragment involves the cysteamine-deriving moiety. A detailed 1H, 13C and 15N NMR characterization of ISA23SH10% and of complex 1 provided a clear evidence that the binding of rhenium occurs by chelation through the S and N atoms of the β-amino-thiolate fragment. Pulsed Gradient Spin Echo (PGSE) NMR experiments allowed to evaluate the diffusion coefficients and then the hydrodynamic radii of the free copolymer ISA23SH10% and its complex 1, that resulted very similar, but always slightly larger for the rhenium containing polymer. The radii values and the small decrease of the size measured in more diluted solutions suggest a certain extent of aggregation of the polymer coils in solution. A morphological evaluation by TEM analysis showed that both complexes 1 and 2 form nanoparticles with a regular spherical morphology and a narrow size distribution with sizes in good agreement with those evaluated by PGSE. In vivo toxicological tests showed that ISA23SH10% is highly biocompatible, with a maximum tolerated dose (MTD) of 500 mg/kg. Preliminary biological studies in vitro and in vivo have been performed also on complexes 1 and 2. No hemolytic activity was observed, up to a concentration of 5 mg/mL, neither citotoxicity effect was observed on Hela cell after 48 h of incubation. No toxic side effects were observed after the intravenous injection in mice of the two complexes in doses up to 20 mg/kg. The good toxicological properties observed suggest that the tricarbonyl-rhenium complexes of thiol-functionalized amphoteric PAAs may clear the way for a new family of radiodiagnostic and radiopharmaceuticals.