Di-2-pyridylketone-N¹-substituted thiosemicarbazone derivatives of copper(II): Biosafe antimicrobial potential and high anticancer activity against immortalized L6 rat skeletal muscle cells

Highlights

• Copper complexes have distorted square planar geometry.

• Moderate to high antimicrobial activity; High in vitro cell viability 89–91%

• Active against S. aureus and methicillin resistant S. aureus

• Also active against K. pneumoniae 1, S. typhimurium 2 and C. albicans

• High anticancer activity against immortalized L6 rat skeletal muscle cell lines

Abstract

The basic aim of this study pertains to developing antimicrobial or anticancer agents based on N, S-donor organic ligands bonded to metals. In the present investigation, di-2-pyridylketone-N¹-substituted thiosemicarbazone (py₂tscH-N¹HR², Chart 2) thio-ligands were reacted with copper(I) halides in organic solvents yielding copper(II) complexes of stoichiometry, [Cu(N,N,S-py₂tsc-N¹HR²)X] (X = I, R² = H, 1; Me, 2; Et, 3; Ph, 4; X = Br, R² = H, 5; Me, 6; Et, 7; Ph, 8; X = Cl, R² = H, 9; Me, 10; Et, 11; Ph, 12); the formation of Cu^II probably occurs through a proton coupled electron transfer (PCET) process. Electron spin resonance, ultraviolet-visible spectroscopy and X-ray crystallography (2, 3, 5, 7, 11) supported a distorted square planar geometry of these complexes. Moderate to high antimicrobial activities of these complexes against methicillin resistant Staphylococcus aureus, Gram positive bacteria, Staphylococcus aureus and Gram negative bacteria, Klebsiella pneumoniae 1, Salmonella typhimurium 2 and one yeast Candida albicans were recorded. Complexes were found to be biosafe with 88–91% cellular viability. All complexes have shown high anticancer activity against the immortalized L6 rat skeletal muscle cell line with very low IC₅₀ values.

Introduction

Copper is an essential metal in biological processes because of its ability to switch between reduced Cu(I) and oxidized Cu(II) states [1,2]. This redox-switching tendency is important for copper to act as catalytic co-factor in enzymes such as cytochrome C oxidase and superoxide dismutase (SOD) for important biological processes. In cytochrome C oxidase, copper is bonded to four nitrogen atoms of tetra-pyrrole rings and to one sulfur donor atom of cysteine. Most of the enzyme mimics possess at least one redox active metal similar to the natural enzymes/tissues (Mn, Fe, Co, Cu, Zn, Hg, Na, K, Mg, Ca) [3]. Among metal complexes, cis-platin was the first metallo-drug introduced for the treatment of cancer in the seventies. However, owing to the cytotoxicity and drug-resistant problems, new metal based (Cu, Pd, Pt) complexes have been investigated which exhibited activity against human lung carcinoma cell line, cervical carcinoma cell line, breast cancer cell lines, Chinese hamster ovary and metastatic breast cancer [[4], [5], [6]]. Interestingly, copper based complexes, namely, (3-methoxysalicylate)(1,10-phenanthroline) -copper(II) showed significant anti-proliferative activity, even in the cancer cells resistant to cis-platin [7], while a tetra-nuclear copper cluster [Cu₄(2,4-di-tert-butyl-6-(1H-imidazo-[1,10]phenanthrolin-2-yl)phenol)₄] (each ligand is dianion-loss of –OH and –NH protons) was cytotoxic to human liver carcinoma and mouse neurone-like cells [8]. Binucleating hydrazonic ligands and their μ-hydroxodicopper(II) complexes are reported as promising structural starting motifs for the development of a new generation of highly active potential antitumor agents [9]. Interestingly, silver(I) bonded to N,S-donor atoms of heterocyclic-2-thiones has shown high anticancer activity against the MG63 human osteosarcoma cell line [10].

Among thio-ligands, the coordination activity of N,S-donor thiosemicarbazones {Chart 1: R¹R²C² = N³-N²(H)-C¹(=S)N¹R³R⁴; Structure I} towards different metals is versatile due to the easy possibility of a variation of R groups at the C²/N¹ atoms of the thio-ligands. These ligands have generated a wide range of metal complexes with a variety of coordination modes, including several complexes exhibiting cyclometallation or exhibiting biochemical activity [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]]. Chart 1 shows several pyridine based thiosemicarbazones (Structure II, Types A–D) whose metal derivatives (Fe, Cu, Au, Pd, Pt, Ga, Zn, Cd, In, Sn) have been studied for their anticancer activity [[28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]]. Biochemically significant 3d-metal (Fe, Cu, Zn) derivatives of these ligands, are found as anticancer agents against ovarian carcinoma, breast cancer (Fe, Cu, Zn), human liver (Fe) and hepatocellular carcinoma (Fe, Zn) cell lines, colon carcinoma, neuro-epithelioma, and promyelocytic leukemia cell lines (Cu); kidney fibroblast, human bladder, prostate carcinoma and chronic myelogenous leukemia cell lines (Zn) [31,32,[34], [35], [36],[39], [40], [41], [42], [43], [44]].

In view of our background in metal-thiosemicarbazone coordination chemistry [11,12,[18], [19], [20]], we have recently reported that copper(II)/zinc(II) derivatives of salicylaldehyde N-substituted thiosemicarbazones exhibited good antimicrobial efficacy against several Gram positive and Gram negative bacterial strains and a yeast, but were found to be mostly bactericidal [[21], [22], [23], [24], [25]]. Subsequently, pyridine based thiosemicarbazones (Structure III) were investigated for different substituents at the C² carbon: R¹ = H, CH₃ or Ph and N¹ nitrogen: R² = H, Me, Et and Ph, and were found to exhibit high antimicrobial activity against several bacterial strains and a yeast. Interestingly, most of these complexes were observed to be non-toxic [49,50].

In continuation of our broad plan to explore metallo-drugs and keeping in view the high potential of metal derivatives of pyridine based thiosemicarbazones (Structures II-III), both as anticancer [[28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]] and antimicrobial agents [49,50], in this paper we report antimicrobial activity of copper(II) derivatives of di-pyridylketone-N-substituted thiosemicarbazones (Chart 2: Structure IV, R¹ = pyridyl, R² = H, Me, Et and Ph; abbrev. py₂tscH-N¹HR²) against methicillin resistant S. aureus (MRSA)- a clinical isolate bacteria, S. aureus, K. pneumoniae 1, S. typhimurium 2 and one yeast C. albicans. It will be pertinent to state that there are some related octahedral bis-complexes of 3d metals (Mn, Fe, Ni, Cu) with py₂tscH-N¹HR²/py₂tscH-N¹Me₂ (R² = H, Me, Ph) ligands, namely, [(Mn(N,N,S-py₂tsc-N¹HPh)₂], [Fe^III(N,N,S-py₂tsc-N¹H₂)₂]-(ClO₄), [(Ni(N,N,S-py₂tsc-N¹HPh)₂], [(Zn(N,N,S-py₂tsc-N¹HMe)₂] and [Cu^II(N,N,S-py₂tsc-N¹Me₂)₂] which have shown anticancer activity, but no antimicrobial studies are reported [41,[51], [52], [53], [54], [55]]. Further, with the py₂tsc-N¹H₂ ligand, dinuclear complexes of stoichiometry, [Cu₂(μ₄-N,N,N,S-py₂tsc-N¹H₂)Cl₂]∙2dmf and [Cu₂(μ₄-N,N,N,S-py₂tsc-N¹H₂)(CN)₂]∙2MeOH with square pyramid geometry of each metal center are reported with no bioactivity [56]. It is added here that the copper complexes being reported in this paper are new, although the ligands were reported in the literature [54]. Finally, this paper in addition to the antimicrobial studies as mentioned above, provides the first report on anticancer activity of metal complexes against immortalized L6 rat skeletal muscle cells followed by in vitro cell viability of complexes using MTT assay {MTT = 3-[(4,5-dimethylthiazol-2-yl)-2,5-diphenyl] tetrazolium bromide} [57,58].