Synthesis and characterization of a platinum(II) complex tethered to a ligand of the peripheral benzodiazepine receptor

A peripheral benzodiazepine receptor (PBR) ligand (TZ6, 5) has been selected as receptor-mediated carrier for antitumor cisplatin-like compounds. Compound 5, containing a thiazole ring in position 2 of the imidazopyridine nucleus, is able to act as a dinitrogen chelate toward platinum. The resulting complex, cis-[PtCl2(5)], that is, compound 8, has been fully characterized by NMR techniques and has been shown to possess affinity and selectivity for the PBR comparable to those of 5 (IC50 of 4.6 and 2.81 nM for 8 and 5, respectively; selectivity indexes for PBR greater than 10 000 for both compounds). Hence, a platinum moiety cross-linking the imidazopyridine and the thiazole aromatic rings does not alter the affinity for PBR. The same cross-linking could be responsible for the tendency of 8 to associate in dimers. The equilibrium between monomer and dimer has been investigated by NMR spectroscopy and the corresponding constant determined.