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Review

Design of Hetero-Dinuclear Metallic Complexes as Potential Metal-Based Drugs With a Zinc Metal Center in a Square-Pyramidal Structure

by
Tanja V. Soldatović
Department of Natural-Mathematical Sciences, State University of Novi Pazar, Vuka Karadžića 9, 36300 Novi Pazar, Serbia
Drugs Drug Candidates 2025, 4(1), 12; https://doi.org/10.3390/ddc4010012
Submission received: 17 February 2025 / Revised: 5 March 2025 / Accepted: 12 March 2025 / Published: 17 March 2025
(This article belongs to the Collection Bioinorganic Chemistry in Drug Discovery)
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Abstract

:
The mini-review highlights the innovative development of hetero-dinuclear metallic complexes, with a specific focus on zinc(II) metal centers arranged in a square-pyramidal configuration. The work presented, stemming from our research group in collaboration with others between the years 2020 and 2024, makes significant contributions to this area, emphasizing their potential applications in bioinorganic chemistry, particularly in the context of drug discovery. These advances not only expand the fundamental understanding of such complexes but also lay the groundwork for the design of novel hetero-dinuclear metallic compounds with therapeutic potential. The interaction of these complexes with biological systems and their implications for drug development are critical for future research in bioinorganic chemistry, offering new pathways for targeted treatments and molecular therapies.

1. Introduction

In recent decades, cancer research and the development of new therapies have made significant strides, particularly in the area of metal-based coordination compounds; platinum-based compounds, such as cisplatin, have been widely used in cancer treatment, but their use is limited due to severe side effects and the development of drug resistance [1,2,3]. This has prompted the recent design of novel metal-based drugs with two metal centers that maintain or enhance the anticancer effect while minimizing adverse effects. Hetero-dinuclear metallic complexes involving metal ions, such as zinc(II), gold(I), platinum(II/IV), ruthenium(II), renium(I), and other metals, have shown promise as alternatives due to their unique mechanisms of action [4,5,6,7].
The combination of advanced chemical designs, metal–ligand interactions, and biological targeting mechanisms holds promise in improving the efficacy, selectivity, and safety of cancer treatments. A special strategy involves the combination of two metal centers that differ in their chemical properties, with zinc emerging as an interesting metal center [8,9,10,11,12]. The following question arises: why zinc?
Zinc is often selected in various chemical and biological contexts due to its unique properties. It is relatively abundant and non-toxic and plays a vital role in many enzymatic processes [13,14]. Zinc also has a flexible coordination chemistry, allowing it to form stable complexes with a variety of ligands [14]. Additionally, zinc’s ability to coordinate different donor atoms of biomolecules enhances their functionality in drug design and other applications. Its properties can also provide a balanced combination of stability and reactivity, which is essential in complex molecular systems [12,14]. The ability to switch zinc(II) complexes between different geometries, such as tetrahedral, octahedral, or square-pyramidal, makes zinc an interesting metal ion in the design of complexes with potential biological applications [15]. These geometric changes are crucial for the reactivity and functionality of zinc-containing complexes. The type of chelating ligand or pincer ligand can influence the stability of the zinc structure as well as its additional properties [15,16]. The most suitable ligands for zinc complexes are those that can provide strong coordination and stabilize the metal center through multiple binding sites. Pincer ligands, a subset of chelating ligands, consist of a rigid framework with multiple donor atoms that bind to the metal from various directions, creating a strong and stable structure. These ligands are particularly effective at stabilizing metal centers like zinc(II), enhancing the stability of the complex by preventing unwanted reactions and maintaining the coordination sphere. Pincer and chelating ligands can also influence the geometry of zinc complexes, ensuring they adopt the most stable configurations (such as tetrahedral, octahedral, or square-pyramidal), while improving reactivity to facilitate or interactions with specific biomolecules [8]. Since square-pyramidal geometry is uncommon for zinc(II), it is an intriguing feature in coordination chemistry. Researchers often emphasize such unusual geometries because they highlight the unique structural and chemical properties of these complexes [9]. These distinct properties can have significant implications for reactivity, stability, and potential applications, particularly in fields such as catalysis, drug design, and materials science.
When considering the square-pyramidal structure of zinc in hetero-dinuclear metallic complexes, notable biological activity can be observed due to this unique coordination geometry [8,9,16]. This structure enables zinc to effectively interact with various ligands and different donor atoms, which enhances its reactivity and targeting specificity. Furthermore, the inclusion of a second metal ion in these complexes can optimize the overall biological activity [8,9]. The cooperative interactions between the metals make hetero-dinuclear metallic complexes especially promising for therapeutic applications. By combining the properties of both metals, these complexes can offer enhanced efficacy, particularly in areas such as anticancer and antimicrobial treatments [8,9,10,11,12].
The mini-review emphasizes the innovative hetero-dinuclear metallic complexes as potential metal-based drugs that feature a zinc(II) metal center in a square-pyramidal configuration, which is connected to another metal center through bridging ligands like pyrazine and bipyridine. The described biological activity of these complexes showcases the role of these ligands in facilitating coordination between the metal centers, thereby enhancing the stability and reactivity of the complexes.

2. Hetero-Dinuclear Metallic Complexes with Zinc(II) in a Square-Pyramidal Structure

As the starting Zn(II) complex for the synthesis of new hetero-dinuclear metallic complexes, Zn(II) complexes with 2,2′:6′,2″-terpyridine, 4′-chloro-2,2′:6′,2″-terpyridine, and 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine ligands were used. The results of the crystal structures analysis showed that these complexes have a square-pyramidal structure [16,17,18]. Specifically, in the [ZnCl2(terpy)] complex, Zn(II) is five-coordinated in a distorted square-pyramidal geometry by two Cl atoms and three N atoms from the 2,2′:6′,2″-terpyridine ligand. The latter is not planar and shows dihedral angles between the least-squares planes of the central pyridine ring and the terminal rings of 3.18 (8)° and 6.36 (9)° [16]. In the [ZnCl2(terpy-Cl)] complex, the Zn(II) atom is also five-coordinated by two chloride ions and three terpyridine N atoms in a slightly distorted square-pyramidal geometry [17], while X-ray structure analysis revealed that the compound [ZnCl2(terpytBu)] crystallizes in the trigonal space group P3221, where the Zn(II) atom is five-coordinated with a highly distorted square-pyramidal geometry, more resembling a trigonal bipyramidal geometry [18].

2.1. Hetero-Dinuclear Metallic Complexes with General Formula Zn(II)-L-Cu(II)

Novel hetero-dinuclear metallic complexes [{ZnCl(terpy)(μ-pyrazine) CuCl(terpy)}](ClO4)2 (Zn-L1-Cu) and [{ZnCl(terpy)(μ-4,4′-bipyridyl)CuCl(terpy)}](ClO4)2 (Zn-L2-Cu) (where terpy = 2,2′:6′,2″-terpyridine, L1 = pyrazine, L2 = 4,4′-bipyridyl) are designed according to the biological nature of the two metal ions, which have identical intermediate Lewis acidity, arranged in a square-pyramidal configuration (Figure 1). The complexes exhibit stability at physiological pH, with pKa values indicating stability, and the distance between the metal centers influencing the pKa values [10].
The square-pyramidal coordination of both metal centers enables them to bond with various donor atoms, like nitrogen from 5′-GMP and 5′-IMP, or oxygen from the carboxylate group in GSH. Zinc(II) and copper(II) ions, which are borderline hard Lewis acids, have a strong attraction for nitrogen and oxygen donor atoms, particularly when the coordination number is 5.
Studies on cytotoxic activity revealed that both complexes decreased the viability of healthy and colorectal cancer cell lines in a time- and dose-dependent manner, with a more significant effect seen after prolonged exposure. After 72 h, both complexes exhibited strong activity against the HCT-116 cell line, showing an IC50 of 0.01 µM (see Figure 2). The selectivity index (SI), calculated by dividing the IC50 of the healthy MRC-5 cell line by the IC50 of the HCT-116 cancer cell line, was 130 for Zn-L1-Cu and 80 for Zn-L2-Cu. Since an SI value greater than 10 indicates a selective compound, these results suggest that both complexes are highly selective for the colorectal cancer cell line [10,19,20,21].
Furthermore, both complexes induced a strong pro-oxidative response in the tested cell lines, leading to significant cytotoxicity and high selectivity. This pronounced effect on the colorectal cancer HCT-116 cells prompted us to explore their potential antitumor activity in other types of tumor cell lines in future.

2.2. Hetero-Dinuclear Metallic Complexes with General Formula Pt(II)-L-Zn(II)

The general design of these types of complexes includes well-known metal-based drug, such as cisplatin and its geometric isomer transplatin, coordinated through bridging ligands like pyrazine and 4,4′-bipyridine with Zn(II)–terpyridine type complexes, which differ by the substituents in the terpyridine moiety. The influence of substituents on their biological activity is significant [8,9,11,12].

2.2.1. Hetero-Dinuclear Metallic Complexes [{cis-PtCl(NH3)2(μ-4,4′-bipyridyl)ZnCl(terpy)}](ClO4)2 (C1), [{trans-PtCl(NH3)2(μ-4,4′-bipyridyl)ZnCl(terpy)}](ClO4)2 (C2), [{cis-PtCl(NH3)2(μ-pyrazine)ZnCl(terpy)}](ClO4)2 (C3) and [{trans-PtCl(NH3)2(μ-pyrazine) ZnCl(terpy)}](ClO4)2 (C4) and Their Biological Activity

The hetero-dinuclear metallic complexes C1–C4 were synthesized using a method outlined in Scheme 1, where the starting Pt(II) complex is either cisplatin or transplatin. The geometry around the Pt(II) metal center is square-planar, while that around Zn(II) is square-pyramidal with terpy (2,2′:6′,2″-terpyridine) as an inert ligand (Figure 3).
The results obtained from the UV-Vis spectrophotometric method for determining the pKa values of the diaqua complexes indicate the stability of the C1–C4 complexes, particularly under physiological conditions (approximately pH 7.4). However, the pKa2 of the complex with the 4,4′-bipy bridging ligand (C1, C2) is very close to physiological conditions [8,9]. For comparison, the pKa values of the aqua analogs for cisplatin and transplatin are 5.93 and 7.87, and 4.35 and 7.40, respectively [22,23]. Therefore, it can be concluded that both cis- and trans-Pt-L-Zn isomers remain stable at physiological pH and do not undergo hydrolysis (Table 1).
The geometrical structures of zinc(II) and platinum(II) affect their affinity for donor atoms. In square-pyramidal coordination, zinc(II) interacts with N7 or phosphate groups from 5′-GMP and 5′-IMP, as well as O-carboxylate from GSH. The greater reactivity of 5′-GMP and 5′-IMP compared to GSH is attributed to the binding of zinc(II) to the phosphate group. The slower second reaction step results from steric hindrance and charge distribution [8,9]. The first step is driven by Zn(II) reactivity, while the second depends on Pt(II). The inert terpyridine ligand enhances metal center electrophilicity through π-back bonding. Trans-Pt-L-Zn isomers are more reactive than cis-isomers due to the kinetic trans effect along the Cl-Pt-N axis, similar to transplatin. Bridging ligands also affect reactivity. Complexes containing pyrazine (C3, C4) are more reactive than those with 4,4′-bipyridyl because pyrazine’s π-acceptor properties enhance the electrophilicity of the metal centers. In contrast, C1 and C2 are less reactive due to the greater Zn(II)-Pt(II) distance, reducing electronic communication [8,9]. Hetero-dinuclear metallic Zn(II)-Pt(II) complexes, differing in Lewis acidity, geometry, and ligand arrangement, significantly impact reactivity and biomolecular coordination.
The cytotoxic activity of the investigated hetero-dinuclear metallic C1–C4 complexes, in comparison to cisplatin (CDDP), was assessed based on IC50 values (see Table 2) against various cell lines, including lung carcinoma (A549), colon carcinoma (HCT116 and LS-174), breast carcinoma (MDA-MB-231), and human non-tumor fetal lung fibroblast cells (MRC-5). The results revealed distinct activity profiles and selectivity [8,9].
The obtained results indicate that complexes C1 and C3 exhibited strong growth inhibition against HCT116 cells, with IC50 values of 0.42 and 0.51 µM, respectively. The trans-isomers demonstrated significant cytotoxic activity in HCT116, MDA-231, and A549 cells, with notable selectivity for HCT116 (C2 IC50 3.08 µM; C4 IC50 8.83 µM). Among them, C2 displayed strong inhibitory effects across all tested tumor cell lines. Complex C1 demonstrated the highest selectivity for HCT116 cells (SI = 117), whereas the trans-isomer C2 (SI = 6.8) displayed significantly greater selectivity than cisplatin (SI = 0.5) [8,9].
The effects of hetero-dinuclear metallic Pt(II)-L-Zn(II) complexes C1 and trans-C2 on cell cycle progression in HCT116 cells showed significant changes after 24 h of incubation. Complex C2 caused an accumulation of cells in the G0/G1 phase (64.69% vs. control 47.57%) and a reduction in the G2/M phase (13.33% vs. control 27.94%). In contrast, cisplatin (CDDP) treatment also decreased the percentage of cells in the G2/M phase (14.77% vs. control 27.94%) while increasing the Sub-G1 population (15.19% vs. control 5.56%). After 48 h of treatment, C2 resulted in an increase in the Sub-G1 population (7.76% vs. control 3.11%), indicating that cells, after being blocked in the G2/M phase, failed to repair damage and ultimately underwent cell death. The Sub-G1 peak is linked to DNA fragmentation. After 48 h, CDDP treatment arrested cells in the S phase (24.25% vs. control 13.32%), leading to an increase in Sub-G1 cells (22.92% vs. control 3.11%) (Figure 4) [9].
Complex C1 did not induce any changes in the cell cycle progression in HCT116 cells. This implies that C1 is not capable of causing lethal or irreparable DNA damage. Despite showing high cytotoxicity in HCT116 cells (with an IC50 of 0.42 µM), the lack of cell cycle arrest suggests that DNA may not be the primary target of its cytotoxic effect.
Treatment with complex C2 produced effects akin to those of typical DNA-damaging agents such as cisplatin, including a p53-mediated response, cell cycle arrest, and activation of apoptotic genes. Both C1 and C2 complexes were also capable of inducing PARP1 mRNA expression and binding DNA in vitro (Figure 5). However, the interactions of C1 and C2 with DNA appeared to be less harmful, and the upregulation of PARP1 and H2AX phosphorylation may not be closely related in the signaling pathway activated by these complexes [9].
Although PARP inhibitors (PARPi) are used in treating DNA-damaged tumor cells, AZD2461, a PARP1 inhibitor, did not influence the sensitivity of HCT116 cells to C1, C2, or cisplatin [9]. Platinum-based drugs generally cause complex cross-links, which are more lethal than the single-strand breaks (SSBs) induced by PARPi, and have different mechanisms of action and resistance. Further preclinical research is necessary to assess the potential of combining PARPi with hetero-dinuclear metallic complexes for therapeutic use.
Additionally, the cytotoxicity of the cis- or trans-Pt-L-Zn complexes may be attributed to their interference with various intracellular processes involving zinc, such as DNA recognition, transcriptional regulation, and apoptosis. The molecular mechanisms behind the response of HCT116 cells to these complexes seem complex, with significant interactions between various signaling pathways.

2.2.2. Hetero-Dinuclear Metallic Complexes [{cis-PtCl(NH3)2(μ-4,4′-bipyridyl)ZnCl(terpy-Cl)}](ClO4)2 (C1a), [{trans-PtCl(NH3)2(μ-4,4′-bipyridyl)ZnCl(terpy)}](ClO4)2 (C2a), [{cis-PtCl(NH3)2(μ-pyrazine)ZnCl(terpy-Cl)}](ClO4)2 (C3a) and [{trans-PtCl(NH3)2(μ-pyrazine)ZnCl(terpy-Cl)}](ClO4)2 (C4a) and Their Biological Activity

Four new designed complexes contain terpy-Cl (4′-chloro-2,2′:6′,2″-terpyridine) as an inert ligand coordinated to the zinc(II) ion (Figure 6). Chloride has a dual nature in terms of electron behavior: it exhibits the −I inductive effect due to its high electronegativity and the +M mesomeric (or resonance) effect, where the lone pairs on the chlorine atom can donate electron density. As a result, the chloride atom at the 4′ position on the middle pyridine ring of the terpyridine ligand increases the electron density, reducing the electrophilicity of the zinc(II) center. This leads to a higher pKa1 compared to complexes with an unsubstituted terpyridine ligand [9,11].
The electronic communication between the metal centers is enhanced by the conjugated π-electron system of the bridging ligands, which also affects the pKa values. Previous studies revealed variations in reactivity and the formation of both cis- and trans-isomers of the diaqua species. The data obtained showed that the C1a–C4a complexes exhibited moderate stability under physiological conditions (around pH 7.4). The pKa2 values were found to be close to physiological pH, with the -I and +M effects of chloride at the 4′ position of 2,2′:6′,2″-terpyridine playing a significant role in the ability of the complexes. These findings reveal low cytotoxic activity, which was confirmed in additional experiments.
The investigated complexes displayed moderate anticancer activity. After 72 h of exposure, cytotoxicity decreased in the following order: C2a, C1a, and C3a. Previous studies on Pt(II)-Zn(II) complexes, including the C1–C4 series, demonstrated notable cytotoxic effects after 72 h, with IC50 values ranging from 0.42 to 8.83 µM against HCT-116 cells, which is consistent with the findings presented here. The IC50 values for HCT-116 colon cancer cells indicate a higher sensitivity compared to SW-480 cells, with IC50 values exceeding 200 µg/mL suggesting a lack of significant cytotoxicity in SW-480 cells (Table 3).
The introduction of the chloride atom clearly reduced the cytotoxic activity in the tested colon cancer cells, highlighting differences in sensitivity between them. Based on the observed cytotoxicity, these complexes alone do not demonstrate significant antitumor potential. However, they could be explored in future combined treatments with standard chemotherapeutic agents. This strategy may enhance both the cytotoxic activity and selectivity of the complexes, warranting further evaluation in various cancer cell lines [11].
The impact of the chloride substituent at the 4′ position of the terpyridine (terpy) ligand is evident, as the increased electron density on the zinc(II) center reduces the stability of the complex, particularly after the coordination with biomolecules, resulting in low cytotoxic activity.

2.3. Comparative Cytotoxic Studies of Newly Synthesized Mononuclear zinc(II) and Hetero-Dinuclear Platinum(II)/Zinc(II) Complexes toward Colorectal Cancer Cells

A series of mono- and hetero-dinuclear platinum(II) and zinc(II) complexes with 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine ligand were synthesized and characterized (Figure 7). The cytotoxic studies of [ZnCl2(terpytBu)] (C1), [{cis-PtCl(NH3)2(μ-pyrazine)ZnCl(terpytBu)}](ClO4)2 (C2), [{trans-PtCl(NH3)2 (μ-pyrazine)ZnCl(terpytBu)}](ClO4)2 (C3)[{cis-PtCl(NH3)2(μ-4,4′-bipyridyl), ZnCl(terpytBu)}](CIO4)2 (C4) and trans-PtCl(NH3)2(μ-4,4′-bipyridyl) [{ZnCl(terpytBu)}](CIO4)2 (C5) (where terpytBu = 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine) also indicate the influence of the substituent of terpy ligand on their activity [12].
The in vitro cytotoxicity of newly synthesized mono- and hetero-dinuclear platinum(II) and zinc(II) complexes, coordinated to a 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine ligand (denoted as C1–C5), was systematically evaluated using MTT assay methodology. This study targeted a range of cell lines, including murine colorectal carcinoma (CT26), human colorectal carcinoma (HCT116 and SW480), and non-cancerous murine mesenchymal stem cells (mMSCs). The cells were exposed to varying concentrations of the synthesized complexes (1.17–150 μM) for a period of 48 h (Table 4). The findings revealed that the mono- and hetero-dinuclear platinum(II) and zinc(II) complexes (C1–C5) exhibited pronounced cytotoxic effects against both human (HCT116, SW480) and murine (CT26) colorectal carcinoma cells [12].
Notably, the complexes demonstrated a dose-dependent cytotoxic profile, with significant reductions in cell viability in colorectal cancer cell lines. In contrast, the complexes displayed relatively low cytotoxicity toward non-cancerous mMSCs, indicating selective tumor cell targeting. Moreover, similar cytotoxic patterns were observed with the zinc(II)-terpyridine complexes, which significantly impaired cell viability across several cancer cell lines, including human lung adenocarcinoma (A549), hepatocellular carcinoma (Bel-7402), breast adenocarcinoma (MCF-7), and esophageal squamous carcinoma (Eca-109) [24]. Similarly, platinum(II)–terpyridine complexes exhibited notable antiproliferative activity across a wide array of cancer cell lines, including human squamous cell carcinoma (A431), cervical carcinoma (HeLa), breast carcinoma (MCF-7), non-small-cell lung carcinoma (A549), and the cisplatin-resistant A549 subline (A549/DDP) [25], further corroborating the therapeutic potential of these metal-based complexes.
Furthermore, the therapeutic selectivity of the synthesized complexes was quantitatively assessed through the calculation of the selectivity index, as presented in Table 5. As mentioned previously, this index serves as a crucial parameter for evaluating the differential cytotoxicity of a compound against cancerous versus non-cancerous cell populations. The selectivity index is determined by calculating the ratio of the IC50 value in mouse mesenchymal stem cells (mMSCs) to the IC50 value in tumor cells. A higher selectivity index reflects a compound’s preferential cytotoxic effect on tumor cells while minimizing toxicity to normal cells, which is crucial for therapeutic efficacy in oncology. A key finding from this study was the exceptionally high selectivity index of the C1 complex. This suggests that the C1 complex has a significantly higher cytotoxic effect on colorectal cancer cell lines (CT26, HCT116, and SW480) compared to non-cancerous mMSC. The strong selectivity demonstrated by the C1 complex highlights its potential as a candidate for targeted cancer therapy, offering the possibility of reduced off-target effects and minimal damage to healthy tissues.
In light of these promising results, the C1 complex was chosen for further detailed investigation to assess its therapeutic potential. The capacity of the C1 complex to induce apoptotic cell death in cancer cells was evaluated through flow cytometric analysis. CT26 cells were treated with the C1 complex and subsequently stained with Annexin V FITC and Propidium Iodide. As shown in Figure 8, a significant proportion of CT26 cells treated with the C1 complex for 24 h underwent both early and late apoptosis stages. Additionally, a substantial fraction of the CT26 cells exhibited necrosis compared to the untreated controls. These findings suggest that the C1 complex induces cell death through both apoptotic and necrotic pathways, demonstrating its potential as an effective anti-carcinoma agent. In a related study, NCI-H460 human large-cell lung carcinoma cells were treated with dinuclear platinum(II) complexes containing4′-substituted-2,2′:6′,2′′-terpyridine ligands for 24 h. The results indicated a significant increase in the proportion of apoptotic cells, which strongly correlated with exposure to these platinum(II) complexes, further supporting their potential for promoting cell death in cancer therapy [26].
The C1 complex induced a significant apoptotic response in CT26 cells, as evidenced by the upregulation of the pro-apoptotic protein Bax and the activation of Caspase-3, alongside a notable decrease in the expression of the anti-apoptotic protein Bcl-2 and the proliferative marker Ki67 [27,28,29]. These changes strongly suggest that the C1 complex can promote apoptosis and inhibit cellular proliferation. Additionally, the C1 complex effectively caused cell cycle arrest at the G0/G1 phase, which was linked to a selective downregulation of Cyclin D (Figure 9).
Further analysis revealed changes in the expression of the cyclin-dependent kinase inhibitor p21 and phosphorylated AKT (p-AKT), suggesting that the C1 complex disrupts key cell cycle checkpoints and survival signaling pathways (Figure 10). Collectively, these findings suggest that the C1 complex exerts a multifaceted mechanism of action, modulating both apoptotic signaling and cell cycle progression, thereby highlighting its potential as a targeted therapeutic agent in cancer treatment.
In conclusion, the presence of substituents, particularly tert-butyl groups, on the tridentate terpyridine ligands plays a crucial role in modulating the antiproliferative activity and DNA-binding interactions of both mononuclear and hetero-dinuclear platinum(II)/zinc(II) complexes. The steric bulk introduced by the tert-butyl groups increases the overall size of the hetero-dinuclear complexes, which results in reduced flexibility and reactivity. This steric effect is likely a significant contributor to the observed differences in cytotoxicity, as evidenced by the higher selectivity index of the mononuclear C1 complex relative to the hetero-dinuclear complexes (C2–C5). The enhanced selectivity index of the C1 complex indicates its superior ability to target tumor cells with minimal toxicity to normal cells, suggesting its potential as a promising candidate for targeted anticancer therapy.

3. Conclusions

In conclusion, the studies on hetero-dinuclear metallic complexes with zinc(II) in square-pyramidal coordination structures, particularly those containing 2,2′:6′,2″-terpyridine (terpy) as a ligand, reveal important insights into their potential as anticancer agents. These complexes, exemplified by both Zn(II)-Cu(II) and Pt(II)-Zn(II) systems, demonstrate a variety of promising features, such as significant cytotoxicity and selective targeting of cancer cells, while sparing healthy cells. The square-pyramidal geometry around zinc facilitates coordination with various donor atoms, contributing to the reactivity and stability of these complexes, which are stable under physiological pH.
The presence of substituents on the terpyridine ligands, such as tert-butyl groups or chlorine atoms (terpy-Cl), plays a crucial role in modulating the electronic properties of the zinc center. These changes impact the stability and cytotoxicity of the complexes. For instance, the introduction of a chloro group on the terpyridine ligand was found to decrease the overall reactivity and cytotoxic activity, especially in the case of the Pt(II)-Zn(II) complexes, which exhibited lower stability and anticancer efficacy in comparison to their unsubstituted counterparts.
Further studies revealed that the square-pyramidal Zn(II) centers, in combination with other metals like copper or platinum, exhibit unique coordination properties that influence the reactivity of the complexes toward biomolecules. This contributes to their anticancer potential, which is evident in their ability to induce apoptosis and cell cycle arrest in various cancer cell lines. The cis- and trans-isomers of these hetero-dinuclear metallic complexes show varying degrees of cytotoxicity, with trans-isomers often exhibiting stronger activity due to enhanced electronic effects.
Overall, the incorporation of both zinc and other metals in a hetero-dinuclear metallic arrangement with terpyridine ligands, particularly when influenced by substituents like chloro or tert-butyl groups, can significantly affect the biological activity, stability, and selectivity of the complexes. These findings underscore the potential of such hetero-dinuclear metallic systems as candidates for targeted cancer therapies, with further studies needed to optimize their performance and understand their underlying molecular mechanisms.

Funding

Financial support for this study was provided by the Ministry of Science, Technological Development, and Innovation of the Republic of Serbia, under the agreement referenced as contract number 451-03-137/2025-03/200252.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

The author would like to express his gratitude to members of the State University of Novi Pazar Republic of Serbia for their support.

Conflicts of Interest

The author declare no conflicts of interest.

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Figure 1. Structures of hetero-dinuclear metallic zinc(II)–copper(II) complexes.
Figure 1. Structures of hetero-dinuclear metallic zinc(II)–copper(II) complexes.
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Figure 2. Cytotoxic effects (IC50 values) of Zn-L1-Cu and Zn-L2-Cu complexes following 24 h and 72 h exposure periods [10].
Figure 2. Cytotoxic effects (IC50 values) of Zn-L1-Cu and Zn-L2-Cu complexes following 24 h and 72 h exposure periods [10].
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Scheme 1. Synthetic pathway of Pt-L-Zn complexes [8,9].
Scheme 1. Synthetic pathway of Pt-L-Zn complexes [8,9].
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Figure 3. Structures of the hetero-dinuclear cis- or trans-platinum(II)-zinc(II) complexes [9].
Figure 3. Structures of the hetero-dinuclear cis- or trans-platinum(II)-zinc(II) complexes [9].
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Figure 4. Cell cycle phase distribution of HCT116 cells, treated with complexes C1 and C2 or CDDP after (A) 24 h and (B) 48 h [9].
Figure 4. Cell cycle phase distribution of HCT116 cells, treated with complexes C1 and C2 or CDDP after (A) 24 h and (B) 48 h [9].
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Figure 5. Analysis of PARP1, TP53, ERCC1, XRCC2, Bax, Bcl2 expression, and Bax/Bcl2 ratio 24 h after treatment with investigated complexes C1 and C2 and CDDP (2 × IC50) in HCT116 cells [9].
Figure 5. Analysis of PARP1, TP53, ERCC1, XRCC2, Bax, Bcl2 expression, and Bax/Bcl2 ratio 24 h after treatment with investigated complexes C1 and C2 and CDDP (2 × IC50) in HCT116 cells [9].
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Figure 6. Newly synthesized hetero-dinuclear cis- or trans-platinum(II)–zinc(II) complexes [11].
Figure 6. Newly synthesized hetero-dinuclear cis- or trans-platinum(II)–zinc(II) complexes [11].
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Figure 7. Structures of the investigated zinc(II) and hetero-dinuclear cis- and trans-platinum(II)-zinc(II) complexes [12].
Figure 7. Structures of the investigated zinc(II) and hetero-dinuclear cis- and trans-platinum(II)-zinc(II) complexes [12].
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Figure 8. Apoptotic effects of the C1 complex on CT26 colorectal cancer cells.Statistical significance is denoted by * p < 0.05 and *** p < 0.001, highlighting the differences between C1 complex-treated and untreated CT26 cells [12].
Figure 8. Apoptotic effects of the C1 complex on CT26 colorectal cancer cells.Statistical significance is denoted by * p < 0.05 and *** p < 0.001, highlighting the differences between C1 complex-treated and untreated CT26 cells [12].
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Figure 9. Modulation of Ki67 expression and cell cycle distribution in CT26 cells by C1 complex. Panel (A) showcases representative FACS plots depicting the expression levels of Ki67 in CT26 cells following a 24 h exposure to the C1 complex. Panel (B) focuses on the impact of the C1 complex on the cell cycle distribution of untreated versus treated mouse CT26 colorectal carcinoma cells. Statistical significance is denoted by *** p < 0.001, highlighting the differences between the cells treated with the C1 complex and the untreated control cells [12].
Figure 9. Modulation of Ki67 expression and cell cycle distribution in CT26 cells by C1 complex. Panel (A) showcases representative FACS plots depicting the expression levels of Ki67 in CT26 cells following a 24 h exposure to the C1 complex. Panel (B) focuses on the impact of the C1 complex on the cell cycle distribution of untreated versus treated mouse CT26 colorectal carcinoma cells. Statistical significance is denoted by *** p < 0.001, highlighting the differences between the cells treated with the C1 complex and the untreated control cells [12].
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Figure 10. Expression of cell cycle and signaling proteins in CT26 cells treated with C1 complex. Panel (A) illustrates the expression of Cyclin D, while panel (B) shows the levels of Cyclin E. Panel (C) focuses on the expression of p21, and panel (D) depicts the levels of phospho-AKT (p-AKT) in these cells. Statistical significance is denoted by * p < 0.05, ** p < 0.01 and *** p < 0.001, highlighting the differences between the cells treated with the C1 complex and the untreated control cells [12].
Figure 10. Expression of cell cycle and signaling proteins in CT26 cells treated with C1 complex. Panel (A) illustrates the expression of Cyclin D, while panel (B) shows the levels of Cyclin E. Panel (C) focuses on the expression of p21, and panel (D) depicts the levels of phospho-AKT (p-AKT) in these cells. Statistical significance is denoted by * p < 0.05, ** p < 0.01 and *** p < 0.001, highlighting the differences between the cells treated with the C1 complex and the untreated control cells [12].
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Table 1. pKa values investigated of diaqua C1–C4 complexes [8,9].
Table 1. pKa values investigated of diaqua C1–C4 complexes [8,9].
ComplexpKa1 pKa2
C13.99 ± 0.066.97 ± 0.01
C23.91 ± 0.027.12 ± 0.05
C33.47 ± 0.035.19 ± 0.02
C42.82 ± 0.016.20 ± 0.05
Table 2. Cytotoxic activity of the investigated hetero-dinuclear Pt(II)-L-Zn(II) complexes C1–C4 vs. CDDP, in terms of IC50 values after 72 h [8,9].
Table 2. Cytotoxic activity of the investigated hetero-dinuclear Pt(II)-L-Zn(II) complexes C1–C4 vs. CDDP, in terms of IC50 values after 72 h [8,9].
AgentsHalf-Maximal Inhibitory Concentration IC50 (µM)
HCT116LS-174MDA-MB-231A549MRC-5
C10.42 ± 0.1545.74 ± 3.0021.93 ± 4.242.88 ± 2.3949.19 ± 4.18
C23.08 ± 0.19>508.11 ± 1.9710.27 ± 4.5620.97 ± 5.51
C30.51 ± 0.0211.3 ± 2.179.52 ± 1.3411.07 ± 3.679.29 ± 1.49
C48.83 ± 0.6440.14 ± 6.1639.41 ± 7.8233.96 ± 4.75>50
CDDP28.7 ± 0.1923.59 ± 4.368.97 ± 0.4728.46 ± 0.3714.60 ± 1.12
Table 3. The cytotoxic effect of investigated C1a–C4a complexes versus cisplatin expressed as IC50 ± SD values after 24 and 72 h [11].
Table 3. The cytotoxic effect of investigated C1a–C4a complexes versus cisplatin expressed as IC50 ± SD values after 24 and 72 h [11].
Half-Maximal Inhibitory Concentration IC50 (µM)
AgentsHCT-116 SW-480
24 h 72 h24 h72 h
C1a>20043.20 ± 0.51>200>200
C2a>20019.52 ± 0.78>200>200
C3a>20073.31 ± 1.95>200>200
C4a>200>200>200>200
cisplatin11.11 ± 0.135.33 ± 0.449.07 ± 0.418.13 ± 0.14
Table 4. Values of IC50 for C1–C5 complexes and cisplatin (CDDP) on CT26, HCT116, and SW480 cell lines as determined by the MTT assay [12].
Table 4. Values of IC50 for C1–C5 complexes and cisplatin (CDDP) on CT26, HCT116, and SW480 cell lines as determined by the MTT assay [12].
AgentsIC50 ± SEM (μM)
CT26HCT 116SW480mMSCs
C144.7 ± 58.7388.23 ± 48.52106.26 ± 126.0856.72 ± 6.93
C235.07 ± 39.6526.57 ± 25.8725.71 ± 2.9310.1 ± 0.98
C3210.25 ± 86.98181.35 ± 104.5168.14 ± 5.2176.24 ± 11.95
C4183.16 ± 72.3789.43 ± 6.794.06 ± 9.8960.34 ± 24.66
C5107.58 ± 79.89151.95 ± 26.8275.44 ± 21.1840.17 ± 3.96
CDDP9.03 ± 3.5514.7 ± 2.2110.1 ± 3.987.32 ± 2.51
Table 5. Selectivity index (SI) of C1-C5 complexes and cisplatin on CT26, HCT116, and SW480 cell lines [12].
Table 5. Selectivity index (SI) of C1-C5 complexes and cisplatin on CT26, HCT116, and SW480 cell lines [12].
AgentsSelectivity Index (IC50 mMSC/IC50)
CT26HCT 116SW480
C11.30.60.5
C20.30.40.4
C30.40.41.1
C40.30.70.6
C50.40.30.5
CDDP0.80.50.7
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Soldatović, T.V. Design of Hetero-Dinuclear Metallic Complexes as Potential Metal-Based Drugs With a Zinc Metal Center in a Square-Pyramidal Structure. Drugs Drug Candidates 2025, 4, 12. https://doi.org/10.3390/ddc4010012

AMA Style

Soldatović TV. Design of Hetero-Dinuclear Metallic Complexes as Potential Metal-Based Drugs With a Zinc Metal Center in a Square-Pyramidal Structure. Drugs and Drug Candidates. 2025; 4(1):12. https://doi.org/10.3390/ddc4010012

Chicago/Turabian Style

Soldatović, Tanja V. 2025. "Design of Hetero-Dinuclear Metallic Complexes as Potential Metal-Based Drugs With a Zinc Metal Center in a Square-Pyramidal Structure" Drugs and Drug Candidates 4, no. 1: 12. https://doi.org/10.3390/ddc4010012

APA Style

Soldatović, T. V. (2025). Design of Hetero-Dinuclear Metallic Complexes as Potential Metal-Based Drugs With a Zinc Metal Center in a Square-Pyramidal Structure. Drugs and Drug Candidates, 4(1), 12. https://doi.org/10.3390/ddc4010012

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