Six new TSPO binding ligands with specific metal binding sites and 15 new anticancer metallodrug candidates were synthesized and characterized by multinuclear NMR spectroscopy, infra-red spectroscopy and mass spectrometry and their purity was confirmed by elemental analysis and high performance liquid chromatography (HPLC). UV/Vis spectroscopic and HPLC studies confirmed the stabilities of the drug candidates under physiological conditions and in cell culture medium. The interactions of the gold complexes with biological sulfur ligands were studied using the amino acid cysteine as a model for sulfur-containing biomolecules. The results showed that phosphorus and N-heterocyclic carbene co-ligands are easily replaced by cysteine, while the TSPO ligands remain coordinated to gold. Three Au complexes were shown to be highly active towards human bladder cancer cell lines (5637, T24, Caki) with IC50 values in the 0.1 to 5.0 μM range. A correlation between cellular uptake and cytotoxicity in bladder cancer cells could be established. Structure–activity relationship studies showed that compounds with a phosphorus co-ligand are the most active compounds. The cytotoxic Au complexes were shown to be effective inhibitors of TrxR. TSPO molecular docking studies confirmed that all ligands and complexes can interact with the TSPO binding pocket. Thus, the newly synthesized Au complexes represent the first example for combining TrxR inhibition and TSPO binding.
In addition to gold complexes, complexes with Pt(IV) and Cu(II), two other metal ions widely studied in anticancer metalldrug research, were developed and investigated. Two Pt(IV) complexes exhibited IC50 values in breast cancer cells which were 3 - 13 fold lower than that of the reference metallodrug cisplatin and showed selectivity for cancer cells over non-cancerous cells. It was shown for the first time that Pt(IV) pro-drugs for cisplatin with a TSPO binder in an axial position exert cytotoxicity in MCF-7 cells via a multimodal mechanism combining the effects of DNA damage, depolarisation of the mitochondrial membrane potential, swelling and breaking down of the mitochondria and induction of oxidative stress due to enhanced ROS production. Two Cu complexes were synthesized that are more active than the reference metallodrug toward cisplatin-sensitive and cisplatin-resistant ovarian cancer cell lines with IC50 values in the low micromolar range (1.4 to 6.0 μM).
