Increasing the Cytotoxicity of Ru(II) Polypyridyl Complexes by Tuning the Electronic Structure of Dioxo Ligands

Abstract

<div>Due to the great potential expressed by an anticancer drug candidate previously reported by our group, namely Ru-sq ([Ru(DIP)2(sq)](PF6) (DIP: 4,7-diphenyl-1,10-phenanthroline, sq: semiquinonate ligand), we describe in this work a structure-activity relationship (SAR) that involves a broader range of derivatives resulting from the coordination of different catecholate-like dioxoligands to the same Ru(DIP)2 core. More in detail, we chose catechols carrying either electron-donating or electronwithdrawing groups EDG or EWG and investigated the physico-chemical and biological properties of their complexes. Several pieces of experimental evidences demonstrated that the coordination of catechols bearing EDGs led to deep red positively charged complexes 1–4 in which the preferred oxidation state of the dioxoligand is the uninegatively charged semiquinonate. Complexes 5 and 6, on the other hand, are blue/violet neutral complexes which carry an EWG substituted dinegatively charged catecholate ligand. The biological investigation of complexes 1–6 led to the conclusion that the difference in their physico-chemical properties has a strong impact on their biological activity. Thus, complexes 1–4 expressed much higher cytotoxicities than complexes 5 and 6. Complex 1 constitutes the most promising compound of the series and was selected for a more in-depth biological investigation. Apart from its remarkably high cytotoxicity (IC50 = 0.07–0.7 μM in different cancerous cell lines) complex 1 was taken up by HeLa cells very efficiently by a passive transportation mechanism. Moreover, its moderate accumulation in several cellular compartments (i.e. nucleus, lysosomes, mitochondria and cytoplasm) is extremely advantageous in the search of a potential drug with multiple modes of action. Further DNA metalation and metabolic studies pointed to the direct interaction of complex 1 with DNA and to the severe impairment of the mitochondrial function. Multiple targets, together with its outstanding cytotoxicity, make complex 1 a valuable candidate in the field of chemotherapy research.</div>

Note of Caution for the Aqueous Behaviour of Metal‐Based Drug Candidates

“…[17,18] [Ru(DIP) 2 (sq)](PF 6 ) and [Ru(DIP) 2 (3-methoxysq)](PF 6 ) have been recently reported by our group for their remarkable potential as chemotherapeutic agents against cancer. [7,8] During follow-up experiments, we observed an interesting behaviour of these compounds upon aqueous dilution from DMSO stock solutions. As already reported by our group and others, although widely used, DMSO can be found problematic for the biological assessment of metal complexes due to its coordinative properties.…”

“…[5] Herein, we describe the water-DMSO mixture (1 % v/v) behaviour of two ruthenium polypyridyl complexes previously reported by our group as potential chemotherapeutic agents against cancer, namely [Ru(DIP) 2 (sq)] (PF 6 ) and [Ru(DIP) 2 (3-methoxysq)](PF 6 ), where DIP is 4,7-diphenyl-1,10-phenanthroline, sq = semiquinonate and 3-methoxysq = 3-methoxysemiquinonate (Scheme 1). [7,8] The formation of ruthenium polypyridyl nanoaggregates is a known phenomenon. [9][10][11][12][13][14][15][16] For example, the formation of nanoaggregates of [Ru(DIP) 3 ] 2 + and [Ru(bpy) 3 ] 2 + in water was previously reported in the literature.…”

“…[19][20][21] However, in this case, the stability of [Ru (DIP) 2 (sq)](PF 6 ) and [Ru(DIP) 2 (3-methoxysq)](PF 6 ) in DMSO was found to be very high, the nature of both compounds remaining unaltered up to at least 4 days. [7,8] In this communication, we describe the behaviour of [Ru(DIP) 2 (sq)](PF 6 ) and [Ru (DIP) 2 (3-methoxysq)](PF 6 ) in water-DMSO (1 % v/v) mixtures that we believe will be of interest for the scientists working in the medicinal inorganic chemistry field of research. More specifically, [Ru(DIP) 2 (sq)](PF 6 ) and [Ru(DIP) 2 (3-methoxysq)](PF 6 ) were found to form colloids that are completely transparent and consequently, unnoticeable to the naked eye.…”

“…Those aggregates were not found to be stable after being incubated at 28°C for 24 h, as two main population distributions of~65 nm and~95 nm were observed ( Figure S4). It is noteworthy that despite their extremely similar structure, as reflected by their very close physical properties, [7,8] the NTA analysis of [Ru(DIP) 2 (sq)](PF 6 ) and [Ru(DIP) 2 (3-methoxysq)](PF 6 ) gave significantly different results. This interesting finding prompted us to investigate these systems more deeply using transmission electron microscopy (TEM).…”

“…[Ru(DIP) 2 (sq)](PF 6 ) and [Ru(DIP) 2 (3-methoxysq)](PF 6 ) were synthesised in our laboratories. [7,8] All solvents were purchased (analytical or HPLC grade) and when necessary, degassed by purging with dry, oxygen-free nitrogen for at least 30 min before use. 6 ) in DMSO.…”