Design of Protein-Targeted Organometallic Complexes as Anticancer Agents.

Abstract

DNA is considered as the ultimate target of platinum based anticancer drugs which are widely used in clinics but the toxicity and resistance induced by these compounds have halted their success. In recent past, proteins or enzymes have been explored as alternate targets for metal-based anticancer agents. These enzymes or proteins are involved in metabolic pathways associated with cancer development. These include transferrin, albumin, kinase, cathepsin B, thioredoxin reductase, plectin, carbonic anhydrase and histone deacetylase etc. Many compound classes of metal complexes have been investigated against such targets. The ruthenium and osmium complexes of pyridine-2-carbothioamides (PCAs) stabilized by η6-arene ring were introduced as orally administrable anticancer agents with potential to bind with the histone proteins to interrupt the chromatin activity (Chemical Science., 2013, 4, 1837–1846). Recently, in vivo examination of these compounds revealed selective binding to plectin and they termed as plecstatin (Angewandte Chemie International Edition., 2017, 56, 8267-8271). In this doctoral thesis, PCA ligands were functionalized with groups which can bind to specific enzymes or proteins such as carbonic anhydrase and histone deacetylase. The new PCA ligands were then converted to their respective organometallic compounds of Ru(II), Os(II), Rh(III) and Ir(III). All novel PCAs and their corresponding complexes were evaluated for their cytotoxic potential against different cancer cell. The organometallic compounds were studied for their hydrolytic stability as well as their interactions with biomolecules such as amino acids and proteins by using a range of biophysical methods. For structure activity relationships study, a series of N-phenyl substituted pyridine-2- carbothiamides (PCAs) were obtained by systematically varying the substituents at the phenyl ring. The PCAs were then converted to their corresponding RuII(η6-pcymene) complexes. In preliminary examination, these metal based compounds were studied for their acidic and hydrolytic stability. In cytotoxic assay, the lipophilic PCAs 1–4 showed cytotoxicity in the low micromolar range and 6 was the most potent compound of the series with an IC50 value of 1.1 μM against HCT116 colon cancer cells. These observations were correlated with calculated octanol/water partition coefficient (clogP) data and quantitative estimated druglikeness. A similar 17 trend as for the PCAs was found in their Ru complexes, where the complexes with more lipophilic ligands proved to be more cytotoxic in all tested cell lines. In general, the PCAs and their organoruthenium derivatives demonstrated excellent drug-likeness and cytotoxicity with IC50 values in the low micromolar range, making them interesting candidates for further development as orally active anticancer agents. In order to investigate the impact of metal centres on anticancer activity, Rh and Ir analogues of the most promising and orally active compound plecstatin (9) were prepared. Within the same group, the lighter metal fragments ruthenium and rhodium complexes showed increased cytotoxicity as compared to their respective heavier congener i.e. osmium and iridium. However, changing the halido leaving group resulted in slight decrease in activity with exception of ruthenium-bromido 17 and osmium-iodido 20 complexes in H460 cancer cell line. To further explore the carbonic anhydrase as another potential target for these compounds, PCA was functionalized with sulfonamide group and convert into RuII and OsII(η6-p-cymene) complexes. The presence of the sulfonamide motif in many organic drugs and metal complexes endowed these agents with interesting biological properties and may result in the latter case in multitargeting agents. The compounds were characterized with standard methods and the in vitro anticancer activity data was compared with studies on the hydrolytic stability of the complexes and their reactivity to small biomolecules. A molecular modelling study against carbonic anhydrase II revealed plausible binding modes of the complexes in the catalytic pocket. In a multitargeting approach, by incorporating several bioactive components – a metal centre, a pyridinecarbothioamide and a hydroxamic acid – in a novel pharmacophore, highly cytotoxic functionalized PCAs and their organometallic compounds were obtained. The PCA ligand 31 bearing the vorinostat (SAHA) pharmacophore and their respective organoruthenium, osmium, rhodium and iridium complexes 38–41 displayed potent cytotoxicity but these results showed slight correlation towards HDACi studies. In HDAC inhibition assay against HDAC1, HDAC6 and HDAC8, the PCA-SAHA derivative 31 and its organometallic compounds 38–41 showed inhibitory activity in nanomolar range and some derivatives were more potent inhibitors than the approved drug SAHA. The HDACi mechanism further confirmed by dynamic simulation where compound 31 and its enantiomeric complexes 39 and 18 40 chelated with Zn2+ ion of HDAC8 and HDAC6 and formed several interactions within their binding pocket. Overall, this doctoral thesis comprises of seven new ligands (6, 7, 23, 28–31) and twenty six novel organometallic complexes(10–18, 20–22, 24–27, 32–41), while single crystals of four ligands (3, 6, 23, 28) and seven complexes (12, 13, 17, 18, 20, 27neutral, 33) are reported.