Platinum(II) Complexes with Bioactive Carrier Ligands Having High Affinity for the Translocator Protein

Peripheral benzodiazepine receptors (PBRs, also named TSPO) are overexpressed in many tumor types, with the grade of TSPO overexpression correlating with the malignancy of the tumor. For this reason, TSPO-binding ligands have been widely explored as carriers for receptor-mediated drug delivery. In this paper we have selected a ligand with nanomolar affinity for TSPO, [2-(4-chlorophenyl)-8-aminoimidazo[1,2-a]pyridin-3-yl]]-N,N-di-n-propylacetamide (3), for preparing platinum adducts that are structural analogues to picoplatin, cis-[PtCl2(NH3)(2-picoline)] (AMD0473, 6), a platinum analogue currently in advanced clinical investigation. In vitro studies assessing receptor binding and cytotoxicity against human and rat glioma cells have shown that the new compounds cis-[PtX2(NH3){[2-(4-chlorophenyl)-8-aminoimidazo[1,2-a]pyridin-3-yl]-N,N-di-n-propylacetamidel}] (X = I,4; X = Cl, 5) keep high affinity and selectivity for TSPO (nanomolar concentration) and are as cytotoxic as cisplatin. Moreover, they appear to be equally active against sensitive and cisplatin-resistant A2780 cells. Similar to cisplatin, these compounds induce apoptosis but show a favorable 10- to 100-fold enhanced accumulation in the glioma cells.