Tethered Iridium(III) Cyclopentadienyl Half-Sandwich Complexes for Biological Applications

Resumo

This thesis is focused on the design of half-sandwich iridium(III) complexes of general formula [Ir(η5:κ 1-C5Me4CH2py)(XY)]n+, where C5Me4CH2C5H4N is a hemilabile tethering ligand, XY is a bidentate chelating ligand, and n = 1 or 2, for their development as anticancer agents. We were particular concerned about exploring and understanding the activation of the tether pyridine bond (Ir–N(py)) dynamics in aqueous solution. In Chapter 1, which does not intend to be comprehensive, we introduce the field of metals in medicine and summarize the most significant research breakthroughs on metal complexes based on platinum, ruthenium, osmium, rhodium and iridium as anticancer agents, and in particular on iridium(III) compounds. Chapter 2 introduces the pyridine tethered Cp-iridium(III) half-sandwich core structure in complexes of formula [Ir(η5:κ1-C5Me4CH2py)(XY)]n+ (1–5), where XY is a bidentate chelating ligand with different imino groups (L1–L5). In this chapter we begin to understand the effect of the different building blocks on the overall structure and on the biological potential of this new type of metallodrug candidates. The cyclopentadienyl ligand C5Me4CH2C5H4N, once doubly coordinated to the iridium centre in an η5- and a κ1-manner, forms a 5-membered chelate, the Ir(η5:κ1-C5Me4CH2pyN) tether-ring, which is a chelate additional to that formed by the XY ligand. The derivatised Cp ligand is hemilabile and we propose that the Ir–N(py) bond can be reversibly cleaved by various stimuli. In Chapter 3, we further investigate the tethered Cp-Ir(III) core structure in complexes of general formula [Ir(η5:κ1-C5Me4CH2py)(N,N)](PF6)2, bearing a N,N-chelating ligand (ethylenediamine (en), 6, 1,3-diaminopropane (dap), 7, 2,2’-bipyridine (bipy), 8 and 1,10-phenanthroline (phen), 9). The four complexes are unreactive toward hydrolysis at pH 7. Interestingly, 6 and 7 react with hydrochloric acid and with formate, and speciation between the closed- and open-tether complexes can be followed by 1H NMR spectroscopy. Complex 6 binds to nucleobase guanine (9-EtG), yet no interaction to calf thymus DNA was observed. New X-ray structures of closed tether complexes 6, 7, 8 and 9, and open tether complexes 6·HCl, [Ir(η5-C5Me4CH2pyH)(en)Cl](PF6)2, and 6·hyd, [Ir(η5-C5Me4CH2py)(en)H]PF6, have been determined. Hydride capture is efficient for 6 and 7. The kinetics of Ir–H bond formation and hydride transfer to a model organic molecule is also investigated, revealing a strong dependence on temperature for both. Co-incubation of complex 6 with non-toxic concentrations of sodium formate decreases the IC50 value in MCF7 breast cancer cells, indicating that intracellular activation of the Ir–N(py) tether bond to generate cytotoxic activity via iridium-mediated transfer hydrogenation is possible. These results are included in a recent publication.1 Results obtained in the previous chapter prompted us to further study the catalytic properties of the half-sandwich Ir(III) pyridine tethered complexes [Ir(η5:κ1-C5Me4CH2py)(N,N’)]+ (10–14), where N,N’ is picolinamidate (L6), N-phenylpicolinamidate (L7), N-methylpicolinamidate (L8), 6-methylpicolinamidate (L9), or N,6-dimethylpicolinamidate (L10), which is compiled in Chapter 4. We study such catalytic properties by testing them in the well-established iridium-catalyzed reactions of formic acid (FA) dehydrogenation and water oxidation (WO). Complexes 10–14 resulted inactive towards both reactions, and no cytotoxic activity is observed for any of them. Our results suggest that complexes bearing both the hemilabile ligand C5Me4CH2C5H4N and picolinamidate derivatives are completely sheltered and behave as inert under our experimental conditions, no being capable to activate a reactive site in the first coordination sphere of the Ir(III) organometallic structure. In Chapter 5, which compiles two publications resulting from this work,2, 3 six complexes of formula [Ir(η5:κ1-C5Me4CH2py)(C,N)]PF6, where C,N is 2-phenylpyridine (15), 7,8-benzoquinoline (16), 1-phenylisoquinoline (17), 2-(p-tolyl)pyridine (18), 4-chloro-2-phenylquinoline (19) or 2-(2,4-difluorophenyl)pyridine (20), have been synthesized. Non-tether versions of 15 and 16 were synthesized to aid unambiguous correlation between structure and activity. Whilst non-tether complexes are highly potent towards MCF7 cancer cells (similar to cisplatin), complexes bearing the tether-ring structure, 15–20, are exceptionally more potent (1–2 orders of magnitude). Additionally, 15–20 disrupt mitochondrial membrane potential (MMP) and induce oxidative stress. Internalization studies strongly correlate intracellular accumulation and anticancer activity in tether and non-tether complexes. The iridium half-sandwich complex 15, [Ir(η5:κ1-C5Me4CH2py)(2-phenylpyridine)]PF6, which is highly cytotoxic: ca. 15–250× more potent than cisplatin in several cancer cell lines tested, was selected to further experiments of intracellular localization. We developed a correlative 3D cryo X-ray imaging approach to specifically localize the iridium distribution within the whole hydrated cell at nanometre resolution. By means of cryo soft X-ray tomography (cryo-SXT), which provides the cellular ultrastructure at 50 nm resolution, and cryo hard X-ray fluorescence tomography (cryo-XRF), which provides the elemental sensitivity with a 70 nm step size, we located the iridium exclusively in the mitochondria. Our methodology provided unique information on the metallodrug’s intracellular fate and its quantification, without the need for chemical fixation, labelling, or mechanical manipulation of the cells. The cryo-3D correlative imaging method can be applied to a number of biochemical processes for specific elemental localization within the cellular landscape. In summary, we present a new class of organo-iridium drug candidates bearing a structural feature that results in a leap in anticancer potency within the field.