Metal Complexes as Potential Antimicrobial Agent: A Review

Metal ions play many critical functions in humans. Deficiency of some metal ions can lead to disease like pernicious anemia resulting from iron deficiency, growth retardation arising from insufficient dietary zinc, and heart disease in infants owing to copper deficiency. Antibiotic resistance has been growing at an alarming rate and consequently the activity of antibiotics against Gram-negative and Gram-positive bacteria has dropped dramatically day by day. In this sense there is a strong need to synthesis new substances that not only have good spectrum of activity, but having new mechanisms of action. Inorganic compounds particularly metal complexes have played an important role in the development of new metal based drugs. A significantly rising interest in the design of metal complexes as drugs and diagnostic agents is currently observed in the area of scientific inquiry, specifically termed medicinal inorganic chemistry. In this review our main focused on research undertaken over the past few decades which has sought to possess preclinical pharmacological screenings like anti-bacterial, anti-fungal, anti-inflammatory, anti-cancer, DNAinteraction and anti-tumor action of synthetic metal complexes.


Introduction
Metal ions play important roles in biological processes and the field of knowledge concerned with the application of inorganic chemistry to therapy or diagnosis of disease is medicinal inorganic chemistry [1]. The introduction of metal ions or metal ion binding components into a biological system for the treatment of diseases is one of the main subdivisions in the field of bioinorganic chemistry [2][3][4][5]. A characteristic of metals is that they easily lose electrons to form positively charged ions which tend to be soluble in biological fluids. It is in this cationic form that metals play their role in biology. Metal ions are electron deficient, whereas most biological molecules such as proteins and DNA are electron rich. The attraction of these opposing charges leads to a general tendency for metal ions to bind and interact with biological molecules [6][7]. This same principle applies to the affinity of metal ions for many small molecules and ions crucial to life, such as oxygen. Given this wide scope for the interaction of metals in biology, it is not surprising that natural evolution has incorporated many metals into essential biological functions. Metals perform a wide variety of tasks such as carrying oxygen throughout the body and shuttling electrons. Hemoglobin, an iron-containing protein that binds to oxygen by which it carries this vital molecule to body tissues. Similarly, calcium-containing minerals are the basis of bones, the structural framework of the human body. Metals such as copper, zinc, iron and manganese are incorporated into catalytic proteins, the metalloenzymes, which facilitate a multitude of chemical reactions needed for life. Metal complexes are already in clinical use, and encourage further studies for new metallo drugs such as metal mediated antibiotics, antibacterials, antivirals, antiparasitics, anti-HIV [8], anti-diabetes, radio-sensitizing agents and anticancer compounds [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Transition metal complexes offer two distinct advantages as DNA-binding agents [24][25][26][27][28][29]. Hence, herein the attention is focused primarily on the research concerning with a few pharmacological activities of the cheaper and easily available first-row transition metal coordination compounds V(IV), V(II), Cr(III), Mn(II), Fe(II) and Co(II) complexes. Moreover, these metal ions are the essential elements present in the biological intracellular environment of living organisms. They are most abundantly found trace elements present in biological systems together with iron and most of the metalloproteins have these elements [30][31][32][33]. These metal ions are nowadays present in several inorganic pharmaceuticals used as drugs against a variety of diseases, ranging from antibacterial and antifungal to anticancer applications [34][35][36][37][38][39][40][41]. In addition to that, the N-heterocycles, pyrazolones and quinazolinone derivatives as ligands affect the environment of the complex in such a way that their lipophilicity increases which is a major factor in designing a drug [42][43][44][45][46][47][48][49][50][51][52][53]. The literature survey reveals that numerous review articles and books have been published on medicinal inorganic chemistry in the field of metallodrugs [54][55][56][57][58][59][60][61][62][63][64] and especially on anticancer and anti-tumor treatments [65][66][67][68][69][70][71][72]. In this paper we have explored the overview of important application of metal complexes in medicine and drugs.

Antimicrobial Activity of Some Metal Complexes
Antibiotics are substances which, even at low concentrations, inhibit the growth and reproduction of bacteria and fungi. The treatment of infectious diseases would be inconceivable today without antibiotics (Koolman and Roehm, 2005). Hakan Arslan et al. [73] synthesized five thiourea derivatives ligand and their Ni 2+ and Cu 2+ complexes. Those compounds were screened for their in vitro anti-bacterial activity using Gram-positive bacteria ( [78]. Abdullah M. Asiri and Salman A. Khan prepared Pd(II) metal complexes with Steroidal Thiosemicarbazones and investigated their in vitro antibacterial activity against bacterial species. The results showed that steroidal complexes are better inhibitors of both types of the bacteria (Gram-positive and Gram-negative) as compared to steroidal thiosemicarbazones [79]. A. Budakoti, synthesized new Pd(II) complexes with 1-N-substituted thiocarbamoyl-3,5-diphenyl-2-pyrazoline derivatives and investigated their in vitro biological activity [80]. 3-Aminocoumarin has been synthesized and used as a ligand for the formation of Cr(III), Ni(II), and Cu(II) complexes by Abdul Amir H et al. [81]. The ligand and metal complexes (figure 1) were screened against both Gram positive and Gram negative bacteria. Palladium complexes of pyrazoline thiocarbamoyl derivatives were investigated for antiamoebic activity. In general, their effects on Entamoeba histolytica were more pronounced that those of the corresponding ligands. The IC 50 value of complex (figure 2) was significantly less than that of metronidazole. Coordination compounds of other thiocarbamoyl dihydropyrazoles with copper and nickel showed good inhibitory activity against several Candida strains. The activity decreased in the order CuL > NiL > L for the separate compounds. Co-administration of the coordination compounds (figure 3) and amphotericin B or fluconazole gave the reverse order L > NiL > CuL. The Schiff base hydrazone ligand was prepared by the condensation reaction of 7-chloro-4-quinoline with o-hydroxyacetophenone. The ligand behaves either as monobasic bidentate or dibasic tridentate and contain ONN coordination sites. This was accounted for the presence in the ligand of a phenolic azomethine and imine groups. It reacts with Cu(II), Ni(II), Co(II), Mn(II), UO 2 (VI) and Fe(II) to form either mono-or binuclear complexes (figure 4). The ligand HL and metal complexes were tested against a strain of Gram positive bacteria (Staphylococcus aureus), Gram negative bacteria (Escherichia coli), and fungi (Candida albicans). The tested compounds exhibited high antibacterial activities, investigated by Nora H. Al-Shaalan [82]. A series of Schiff bases derived from 2-acetylpyridne and their metal complexes were synthesized by Nura Suleiman Gwaram [83]. The complexes were screened for anti-bacterial activity against Methicillin-resistant Staphylococcus aureus (MRSA), Acinetobacter baumanni (AC), Klebsiella pneumonie (KB) and Pseudomonas aeruginosa (PA) using the disc diffusion and micro broth dilution assays. Based on the overall results, the complexes showed the highest activities against MRSA while a weak antibacterial activity was observed against A. baumanii and P. aeruginosa. The reaction pathway of the coordinated complexes was shown in ( Figure  5).  Two novel organoantimony (V) and two organobismuth (V) complexes were synthesized with benzoic acid derivatives by Arshad Islam and et al. [85]. The metal complexes, their metal salts and ligands ( Figure 7) were evaluated in vitro for their activities against Leishmania infantum and amazonensis promastigotes and Staphylococcus aureus and Pseudomonas aeruginosa bacteria. Both the metal complexes showed antileishmanial and antibacterial activities but the bismuth complexes were the most active. Intriguingly, complexation of organobismuth (V) salt reduced its activity against Leishmania, but increased it against bacteria.  Some new Zn(II) and Cu(II) complexes with novel cyclohexane-1,3-dione ligands were synthesized by Nevin Turan et al. [86] The synthesized ligands and their complexes were tested for antibacterial activity against Escherichia coli ATCC 25922, Enterococcus faecalis ATCC 29212, Staphylococcus aureus ATCC 25923, and Salmonella typhimurium CCM 583. Some of complexes showed medium-level antibacterial activity against the tested bacteria compared with ampicillin. Ikechukwu P. Ejidike and Peter A.  Figure 9) were screened for antibacterial activity against Gram positive and Gram negative bacteria by the agar well diffusion method [87].
Antibacterial activity of a series of six nitrate silver (I) complexes with pyridine and (benz)imidazole derivatives were investigated by Urszula Kalinowska-Lis et al. against three Gram-negative strains: Pseudomonas aeruginosa ATCC 15442, Escherichia coli ATCC 25922 and Proteus hauseri ATCC 13315. The results were compared with those of silver nitrate, a silver sulfadiazine drug and appropriate ligands. The most significant antibacterial properties were exerted by silver (I) complexes containing benzimidazole derivatives [88]. New Cu(II), Pd(II) and Pt(II) complexes were synthesized from 8-ethyl-2 hydroxytricyclo (7. 3. 1. 02, 7)tridecan-13-one thiosemicarbazone by Elena Pahont et al [89]. The free ligand and the metal complexes ( Figure 10) have been tested for their antimicrobial activity against E. coli, S. enteritidis, S. aureus, E. faecalis, C. albicans and cytotoxicity against the NCI-H1573 lung adenocarcinoma, SKBR-3 human breast, MCF-7 human breast, A375 human melanoma and HL-60 human promyelocytic leukemia cell lines. Copper complex 2 exhibited the best antiproliferative activities against MCF-7 human breast cancer cells.   Four copper(II) complexes with a series of differently substituted fluorine-containing Schiff bases starting from the drug isoniazid (isonicotinylhydrazide) were prepared by Ladislav Habala and et al. The prepared compounds were evaluated for their antimicrobial activity and urease inhibition.
Zahid H. Chohan and et al. were prepared and characterized Co(II) and Ni(II) complexes of N-(2-furanylmethylene)-2aminothaiadiazole and role of SO 4 2-, NO 3 -, C 2 O 4 2and CH 3 COOanions. The complexes were screened against in vitro antibacterial activities [95]. New bidentate or tridentate Schiff bases and their VO(II) and Co(II) complexes formed by the condensation of methyl isobutyl ketone with nicotinamide (mna)/2-amino-4 chlorophenol (map) and 2-hydroxy acetophenone with nicotinamide (han)/isoniazide (hai). Some of the complexes have been screened for their antimicrobial activity by the well diffusion technique using DMSO as solvent on different species of pathogenic bacteria/fungi, that is, E. coli, S. aureus, S. fecalis, A. niger, T. polysporum [96]. Some antifertility inhibitors of 18 to 24-membered tetraazamacrocyclic complexes of iron(II) and manganese(II) have been synthesised by the template condensation using 1,3-phenylenediamine with malonic acid, succinic acid, glutaric acid and adipic acid by Ashu Chaudharyl et al [97].
The complexes have been screened in vitro against a number of fungi and bacteria to assess their growth inhibiting potential. Thirty two new Cu(II), Ni(II) and Zn(II) complexes (1-32) with salicylidene thiosemicarbazones (H 2 L 1 -H 2 L 10 ) were synthesized ( Figure 12). Salicylidene thiosemicarbazones, of general formula (X)N-NH-C(S)-NH(Y), were prepared through the condensation reaction of 2-hydroxybenzaldehyde and its derivatives (X) with thiosemicarbazide or 4-phenylthiosemicarbazide (Y = H, C 6 H 5 ). All the ligand and metal complexes have also been tested for their in vitro antibacterial activity against Staphylococcus aureus (Wood-46  Sulekh Chandra et al. [99] were synthesized transition metal complexes of Co(II), Ni(II) and Cu(II) metal ions with pentadentate Schiff base derivative of 4-Aminoantipyrine. The ligand ( Figure 14) and its complexes have been screened for their antifungal and antibacterial activities against three fungi, i.e. Alternaria brassicae, Aspergillus niger and Fusarium oxysporum and two bacteria, i.e. Xanthomonas compestris and Pseudomonas aeruginosa. Manganese(ll) and iron(ll) macrocyclic complexes of polyamide groups have been synthesized by the template condensation of diamines (2, 6 diaminopyridine, 1, 2 phenylenediamine and 1,3 phenylenediame) and triamine (diethylenetriamine) with phthalic acid in 1:2:2 molar ratios by Ashu Chaudhary [100]. The complexes ( Figure 15) have been screened in vitro against a number of fungi and bacteria to assess their growth inhibiting potential. Six diorganotin(IV) carboxylates prepared by reacting diorganotin(IV) dichlorides with the respective silver carboxylate have been tested for antifungal activity against Aspergillus. niger, Aspergilluus flavus and Pencillium. citrinum in Sabourand dextrose broth. The compounds generally exhibit greater fungitoxicity than the diorganotin(IV) dichlorides and the carboxylic acids from which they were synthesized [101]. Metal complexes of dichloro-tetramorpholino-cyclophosphazatriene containing divalent cations such as Ni(II), Co(II), and Mn(II) have been prepared and characterised by standard physico-chemical procedures. The newly synthesized compounds possessed antifungal activity against Aspergillus and Candida spp., some of them showing effects comparable to ketoconazole (with minimum inhibitory concentrations in the range of 2-30 ktg/mL) but being generally less active as compared to the azole. Best activity was detected against C. albicans, and worst activity against A. niger. The mechanism of action of these compounds probably involves inhibition of ergosterol biosynthesis, and interaction with lanosterol-14-o-demethylase (CYP51A 1), since reduced amounts of ergosterol were evidenced by means of HPLC in cultures of the sensitive strain A. niger treated with some of these inhibitors [102].

DNA Binding and Cleavage Properties of Metal Complexes
Mixed ligand copper(II) complexes containing derivatives of salicylic acid and heterocyclic ligands with nitrogen donor atoms have been prepared by Lenka Kuckova and et al. The derivatives of salicylic acids in the coordination environment of copper(II) complexes are responsible for radicals scavenging activity (predominantly towards superoxide radical anion), incorporation of chelating ligand 2,9-dimethyl-1,10-phenantroline into the copper(II) complexes significantly enhances their capability of binding to DNA via intercalation [103]. The metal complexes investigated so far, Cu(II) complexes of 1,10-phenanthroline (phen) and their derivatives have attracted much attention as they function as chemical nucleases. Sigman [104] by catalyzing the formation of reactive oxygen species (ROS), which involves Cu(II)/Cu(I) redox cycle [105]. The coordination geometry of a Cu(II) complex bound to DNA affects the Cu(II)/Cu(I) redox behavior and a change in the coordination geometry has been found to primarily determine the properties of Cu(II) rather than Cu(I) complex species [106]. The Cu 2+ , Ni 2+ , and Zn 2+ complexes were prepared from two bidentate NS ligands. The ligands ( Figure 16) were synthesized by the condensation reaction of S-2-methylbenzyldithiocarbazate (S2MBDTC) with 2 methoxybenzaldehyde (2MB) and 3-methoxybenzaldehyde (3MB). The Schiff bases and their metal complexes were evaluated for their biological activities against estrogen receptor-positive (MCF-7) and estrogen receptor-negative (MDA-MB-231) breast cancer cell lines. Only the Cu(II) complexes showed marked cytotoxicity against the cancer cell lines. Both Schiff bases and other metal complexes were found to be inactive. In concordance with the cytotoxicity studies, the DNA binding studies indicated that Cu(II) complexes have a strong DNA binding affinity [107]. The interaction of [Co(H 2 O) 4 (p-NO 2 C 6 H 4 COO) 2 ]. 2H 2 O with sheep genomic DNA has been investigated by spectroscopic studies and electrophoresis measurements. The interaction between cobalt(II) p-nitrobenzoate and DNA has been followed by gel electrophoresis while the concentration of the complex. The spectroscopic study and electrophoretic experiments support the fact that the complex binds to DNA by intercalation via p-nitrobenzoate into the base pairs of DNA. The mobility of the bands decreased as the concentration of complex was increased, indicating that there was increase in interaction between the metal ion and DNA [108]. Interaction between cobalt(II) p-nitrobenzoate and genomic DNA was shown in (Figure 17) Two novel cobalt(III) pyridine complexes (1) [Co(en) 2 (py) 2 ] 3+ and (2) [Co(en) 2 (mepy) 2 ] 3+ (en=ethylenediamine, py=pyridine, and mepy = methylpyridine) have been synthesized and characterized. The interaction of these complexes with calf thymus DNA was investigated by absorption, emission spectroscopy, viscosity measurements, DNA melting, and DNA photocleavage. Results suggest that the two complexes bind to DNA via groove mode and complex 2 binds more strongly to CT DNA than complex 1. Moreover, these Co(III) complexes have been found to promote the photo cleavage of plasmid DNA pBR322 under irradiation at 365 nm, cytotoxicity results of complexes are also showing anticancer activity [109]. A novel transition metal coordination polymer [Co 3 (C 14 H 8 NO 4 Cl) 4 (CH 3 OH) 4 4CH 3 OH]n with Schiff base (C 14 H 8 NO 4 Cl: 4-chloroanthranilic acid-2,4 dihydroxybenzaldehyde) was synthesized using 4-chloroanthranilic acid, 2, 4dihydroxybenzaldehyde and cobalt(II) acetate as source, and its structure was characterized by IR spectroscopy, elemental analysis, 1H NMR and single crystal X-ray diffraction. The interaction of the Co(II) complex with calf-thymus DNA (CT-DNA) was investigated by UV absorption spectroscopy, fluorescence emission spectroscopy, viscosity and cyclic voltammetry. All measurements revealed that the Co(II) complex binds to DNA via a intercalative mode [110]. DNA binding properties of the ligand and its metal complexes have been investigated by electronic absorption spectroscopy, fluorescence spectra, ethidium bromide displacement experiments, iodide quenching experiments, salt effect and viscosity measurements. Results suggest that all the compounds bind to DNA via an intercalation binding mode. J. Joseph & K. Nagashri were synthesized copper(II) complexes of hydroxyflavone Schiff bases and investigated their antimicrobial and DNA interaction properties [111]. Abijit Pal and et al. [112]    The neutral mononuclear Ln(III) complexes (Ln = La, Sm) with 7-methoxychrom-one-3-carbaldehyde-isonicotinoyl hydrazone ligand (L) have been synthesized, characterized and investigated their interactions with calf-thymus DNA by Qian Wang E et al [113]. Novel copper(II) and zinc(II) complexes (Figure 19) of the type [ML(dppz)]Cl 2 , [L = Schiff base derived from the condensation of 3-(3-phenyl-allylidene) -pentane-2,4-dione and para-substituted aniline; X = -NO 2 (L 1 ), -H (L 2 ), -OH (L 3 ) and -OCH 3 (L4); dppz = dipyrido phenazine] were synthesized and characterized by various analytical and spectral techniques by N. Ramana and et al. [114]. The hydrolytic cleavage of DNA by the zinc complexes was supported by the evidence from free radical quenching and T4 ligase ligation. The copper(II) complexes of N, O-donor ligand and ciprofloxacin have been synthesized and were characterized by physicochemical parameters like elemental analysis, electronic, FT-IR and LC-MS spectra. The complexes were screened for their antimicrobial activity against Gram positive i.e. Staphylococcus aureus, Bacillus subtilis, and Gram negative, i.e. Serratia marcescens, Pseudomonas aeruginosa and Escherichia coli, microorganisms in terms of minimum inhibitory concentration and colony-forming unit. To determine the binding mode of complexes with Herring Sperm DNA, absorption titration and viscosity measurement were employed. DNA cleavage activity was carried out by gel electrophoresis experiment using supercoiled form of pUC19 DNA [115]. The DNA interaction efficacies of mixedligand complexes [ML(phen) 2 ]Cl [L = a Schiff base derived from p-substituted benzaldehyde (L1 = NO 2 / L2 = -OCH 3 ) and methionine; phen = 1,10-phenanthroline, the co-ligand] have been explored by absorption, electrochemistry, viscosity measurements, and agarose gel electrophoresis. The results indicate that these complexes can bind to calf thymus DNA via an intercalative mode and show efficient cleavage activity in the presence of H 2 O 2 [116]. Ahmed M. Abu-Dief and et al. [117] synthesized five novel Cu(II) complexes derived from the condensation between 5-bromosalicylaldehyde (bs) and α-amino acids (L-alanine, l-phenylalanine, L-aspartic acid, L-histidine and L-arginine) and characterized by their elemental analyses, thermogravimetric analysis, IR, mass and electronic spectra, conductance and magnetic measurements. The interaction of the base amino acid Cu(II) complexes with calf thymus DNA has been investigated by absorption spectra, viscosity and gel electrophoresis measurement and the mode of CTDNA binding to the complexes has been explored. The investigated complexes exhibit cooperative binding and presumably intercalate into DNA and follow the trend: bsarc>bshc>bsalc>bsphalc>bsasc (Table-1).

nH2O, where L = [(pyridine-2-carboxaldhyde)-3-isatin]-bishydrazone (cpish), [(2-acetyl pyridine)-3-isatin]-bishydrazone (apish) and [(2-benzoyl pyridine)-3-isatin]-bishydrazone (bpish) have been synthesized and characterized by Mostafa K. Rabia and et al. [119]. Investigation of their interaction with CTDNA under physiological conditions, using spectroscopic (UV-visible) and hydrodynamic techniques (viscosity measurements). Binding constant 'Kb' obtained from spectroscopic methods revealed significant binding of compounds with DNA via intercalation, Furthermore, free energies of compounds-DNA interactions indicated spontaneity of their binding.
Mixed-ligand Cu(II), Ni(II), Co(II), and Zn(II) complexes using a tryptophan-derived Schiff base (obtained by the condensation of tryptophan and benzaldehyde) as the primary ligand and 1,10-phenanthroline as the co-ligand were synthesized and characterized by D. Shiva leela et al [118]. The binding properties of metal complexes with DNA ( Figure  20) were investigated by electronic absorption spectroscopy, cyclic voltammetry, and by performing viscosity measurements, and the results showed that these complexes have the ability to interact with DNA via an intercalative mode. The DNA cleavage efficiencies of these complexes with pUC19 DNA were investigated by gel electrophoresis. The complexes were found to promote the cleavage of pUC19 DNA from the supercoiled form I to the open circular form II and the linear form III in the presence of ascorbic acid.

Antitumor Activity Figure 21. Structure of the experimental compounds: [Au(dppe)2]Cl [A], [Au(d4pype)2]Cl [B], [Au(dpmaaSnMe2)(dpmaaH2)]Cl [C], and [Au(dpmaaSnMe2)2]Cl [D]. (50 µM).
The in vitro cytotoxicities of a number of gold(I), silver(I) and copper(I) complexes containing chiral tertiary phosphine ligands have been examined by Mark J. McKeage et al. [120] against the mouse tumour cell lines P815 mastocytoma, B16 melanoma [gold(I) and silver(I) compounds] and P388 leukaemia [gold(I) complexes only] with many of the complexes having IC50 values comparable to that of the reference compounds cis-diamminedichloroplatinum(ll), cisplatin, and bis[1,2-bis(diphenylphosphino) ethane]gold(I) iodide. M. P. Sathisha et al. [121] synthesized Co(II), Ni(II), Cu(II), and Zn(II) complexes of thiocarbohydrazone ligand. The ligand was synthesized starting from thiocarbohydrazide and isatin. The compounds showed considerable cytotoxic activity in the trypan blue exclusion method. In the in vivo cancer model (Ehrlich ascitic carcinoma model), the compounds significantly (P < .05) reversed the tumor-induced changes in the parameters monitored viz, percentage increase in body weight, percentage increase in lifespan, tumor-viable count, and hematological parameters (total and DLC of WBC, total RBC, and Hemoglobin count). These effects were almost comparable to cisplatin-the standard drug used in the study. The compounds, however, were found to have good effect in prolonging the lifespan (ILS) as compared to standard drug cisplatin. These findings imply that the compounds might be having some anticancer principles. Murine L1210 cytotoxicity assays demonstrated good activity for compound 2 (EDs0 2.68 tg/ml) which was more effective than the parent 1 (EDs0 3.34 lag/ml) 35. Rat osteogenic sarcoma UMR-106 activity was noted for only compound 3 (EDs0 3.24 lag/ml). Against the growth of Tmolt3 only the parent 1 was active (EDs0 1.31 g/ml). Compounds 2 (EDs0 2.37 lag/ml) and 3 (EDs0 1.91 lag/ml) were active against human HeLa-S suspended cervical carcinoma growth while the parent compound was inactive. In contrast compounds 1 (EDs0 3.38 lag/ml) and 2 (EDs0-1.82 lag/ml) were active against solid HeLa cervical carcinoma growth while compound 3 was inactive [122]. Sherika Mahepal and et al. [123]  Metal complexes of 5-carboxy-2-thiouracil with Mn(ll), Co(ll), Ni(ll), Cu(ll), Zn(ll) and Cd(ll) ions were synthesized, characterized, and subjected to a screening system for evaluation of antitumour activity against Sarcoma-180 (S-180) tumour cells (Udai P and et al. [124].

Anticancer Activity
The diverse anticancer utility of cisplatin has stimulated significant interest in the development of additional platinum-based therapies, resulting in several analogues receiving clinical approval worldwide. However, due to structural and mechanistic similarities, the effectiveness of platinum-based therapies is countered by severe side-effects, narrow spectrum of activity and the development of resistance. Nonetheless, metal complexes offer unique characteristics and exceptional versatility, with the ability to alter their pharmacology through facile modifications of geometry and coordination number. This has prompted the search for metal-based complexes with distinctly different structural motifs and non-covalent modes of binding with a primary aim of circumventing current clinical limitations. This review discusses recent advances in platinum and other transition metal-based complexes with mechanisms of action involving intercalation. Cisplatin [cis-diamminedichloroPt(II)] ( Figure 22) is an important chemotherapeutic drug used in the therapy of a broad spectrum of human malignancies such as ovarian, testicular, head and neck, and lung cancers, and in combination with a wide range of other drugs for the treatment of other malignancies. For this reason, it is one of the most widely utilized antitumor drugs in the world. Unfortunately, its use is greatly limited by severe dose-limiting side effects (nephrotoxicity, ototoxicity, and peripheral neurotoxicity) and intrinsic or acquired drug resistance. Thus, numerous Pt derivatives have been further developed with more or less success to minimize toxic effects. Over the last 30 years, 23 other Pt-based drugs have entered clinical trials with only two of these (carboplatin and oxaliplatin), gaining international marketing approval, and another three (nedaplatin, lobaplatin and heptaplatin) approved in individual nations [125]. Currently, there are only four Pt drugs in the various phases of clinical trial (satraplatin, picoplatin, LipoplatinTM and ProLindacTM).  Over the years, research on innovative anticancer metallodrugs has produced several ruthenium-based compounds as alternatives to Pt compounds. The Ru(III) complex trans-[tetrachloro (DMSO) (imidazole) ruthenate(III)] (NAMI-A) was demonstrated to have cisplatin-resistant human colon carcinoma cell lines and in vivo in various tumor types [126]. Due to its higher water solubility, NKP-1339 has now been selected as a lead candidate for further clinical development. Besides these coordination compounds, several classes of other metal complexes and organometallic compounds, based on different metals such as Au, Fe, Ag, Ga, Rh, and Ti, exhibit promising anticancer activity at least in preclinical studies [127][128][129][130][131]. Notably, Jaouen and co-workers have developed organometallic ferrocene-modified tamoxifens (named ferrocifens, Figure 22) as estrogen-targeting molecules effective in hormone-independent breast cancer cells, where hydroxytamoxifen and ferrocene are inactive. In this compound class the β-phenyl ring of tamoxifen has been substituted by a ferrocenyl moiety using classical organic/organometallic synthetic methods. Such structural modifications lead to more lipophilic compounds able to easily cross cell membranes, and, therefore, provide stronger cytotoxic effects. high selectivity for solid tumor metastases and low toxicity at pharmacologically active doses [132]. This is the first ruthenium complex to enter clinical trials. A related Ru(III) compound, indazolium trans-[tetrachlorobis (1H-indazole) ruthenate(III)] (KP1019) and its sodium salt analogue NKP-1339, also entered clinical trials after they were found to exhibit cytotoxic activity in vitro in cisplatin-resistant human colon carcinoma cell lines and in vivo in various tumor types [126]. Due to its higher water solubility, NKP-1339 has now been selected as a lead candidate for further clinical development. The Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) ion complexes with novel Schiff base ligand 2,20-((1Z,10Z)-(1,3-phenylenebis (azanylylidene)) bis (phenylmethanylylidene)) dibenzoic acid (H 2 L) was obtained by the condensation of m-phenylenediamine with o-benzoylbenzoic acid by Walaa H. Mahmoud [133]. The newly synthesized ligand and its metal complexes were screened against a number of bacteria organisms as Bacillus subtilis, Staphylococcus aureus, Escherichia coli, and Neisseria gonorrhoeae and against one fungus, Candida albicans, to assess their inhibiting potential by using the disc diffusion method. The results showed that in some cases the antimicrobial activity of complexes was more biologically active than the Schiff base ligand. Anticancer activity of the ligand and its metal complexes were evaluated in human cancer (MCF-7 cells viability). It was found that [Cd(H 2 L )(H 2 O) 2 Cl 2 ]2H 2 O complex showed lowest IC 50 than the others, and hence was the more active. A series of gold(I) complexes of the general composition [Au(naza)(PPh 3 )] (1-8) was prepared and thoroughly characterized (e.g., electrospray ionization (ESI) mass spectrometry and multinuclear nuclear magnetic resonance (NMR) spectroscopy) by Pavel Starha et al. [134]. The N1-deprotonated anions of 7-azaindole or its derivatives (naza) are coordinated to the metal centre through the N1 atom of their pyrrole ring, as proved by a single crystal X-ray analysis of the complexes [Au(3I5Braza)(PPh 3 [135]. The new Pt(II) complexes exhibited high in vitro cytotoxicity on cisplatin sensitive and resistant cell lines and showed negligible reactivity with nucleobases (Guo and 5-GMP) but selective substitution of DMSO/DMS with soft biological nucleophiles, such as L-methionine. In order to assess the ability of the new complexes with respect to cisplatin to induce apoptosis by interaction with nongenomic targets, the Ames' test, a standard reverse mutation assay, was carried out on two Salmonella typhimurium strains (TA98 and TA100). Sahar I. Mostafa1 and Farid A. Badria [136] were synthesized two new water-soluble mixed ligand [Pd(bpy)(dahmp)]Cl and [Ag(bpy)(Hdahmp)]NO 3 complexes (dahmp and Hdahmp are the deprotonated monoanion and the protonated neutral 4,6-diamino-5-hydroxy-2-mercaptopyrimidine, resp.) is reported. The composition of the reported complexes was discussed on the bases of IR, 1H NMR, and mass spectra, as well as conductivity and thermal measurements. The reported complexes display a significant anticancer activity against Ehrlich ascites tumor cells (EACs). The higher activity of these complexes with their higher conductivity values corresponds to their complete ionization in aqueous solution. The structure of (Hdahmp) ( Figure 23) and their Ag(I), Pd(II) complex (Figure 24, 25).   (Figure 26) caused significant elevation in ROS production following perturbation of mitochondrial membrane potential and cytochrome c release, as assessed by the measurement of fluorescence intensity in stained cells. Furthermore, the activation of caspases 3/7 and 9 was part of the Cu (II) complex-induced apoptosis, which confirmed the involvement of mitochondrial-mediated apoptosis. Meanwhile, there was no significant activation of caspase-8. Taken together, these results imply that the Cu(BrHAP) 2 compound is a potential candidate for further in vivo and clinical colon cancer studies to develop novel chemotherapeutic agents derived from metal-based agents. Dilip Kumar Saha et al. [138] synthesized a square planar copper complex of derivatized NSAID drug (Ketoprofen thiosemicarbazone [3-benzoyl-ct-methyl benzene acetic acid thiosemicarbazone] and characterized by elememal analysis, speeoscopy, electrochemistry and magnetic susceptibility studies which exhibits dose-dependent and enhanced antiproliferative effects on human breast cancer cell line T47D rich in progesterone receptors. The phthalocyanine analogue containing nonperipheral long alkyl-substituted benzenoid rings and pyridine rings, zinc bis (1,4-didecylbenzo)-bis (2,3-pyrido) porphyrazine, was synthesized by Keiichi Sakamoto and et al. [139]. The synthesized Zn(II) complexes was tested against IU-002 cells. Ahmad et al. [140] have synthesized a Co(II) complex, [Co(mpca) 2 ]. H 2 O, which is capable of recognizing a specific sequence in DNA minor groove to inhibit the expression of Topo-I ( Figure 27) and thereby controlling the multiplication of tumor affected cells. This has been made possible by joining an element of specific recognition, 2, 9-dimethyl-1,10-phenanthroline with Co(II) metallic center under a hydrothermal condition in the presence of Na 2 MoO 4 catalyst.

Conclusion
In this work, the pharmacological effects of a few metal complexes have been reviewed. The application of bioinorganic chemistry to medicine is a rapidly developing field. Novel therapeutic and diagnostic metal complexes are now having an impact on medical practice. Advances in bioinorganic chemistry are important for improving the design of compounds to reduce toxic side-effects and understand their mechanisms of action. This review reveals that the pharmacologically interesting metals such as chromium, vanadium, copper, cobalt, nickel, zinc, cadmium, palladium and platinum could be a suitable strategy to develop novel therapeutic tools for the medical treatment.