Chapter 7 Biological studies
7.3 Results and Discussion
7.3.1 Cytotoxic Activity of the Copper(II) Chelates
The IC50 values measured for the six synthesised copper(II) chelates against a representative panel of four human cell lines (three neoplastic and one healthy) are reported in Table 7.3.1.
Table 7.3.1: The IC50 values (µM) of the copper(II) chelates against three human cancer cell lines and one healthy human cell line.
Cell line HEK293 TK-10 U937 MT-4
Human embryo kidney cells
Human renal carcinoma
Human lymphoblast
lung cells
Human leukaemia T
cells
Mean IC50
Value of cancer cells
[Cu(L)(Cl)2] 3.18 - 3.22 1.16 2.19 ± 1.45
[Cu(L1)(Cl)] 5.66 20.00 24.64 11.28 18.64 ± 6.78
[Cu(L2)(Cl)2] 3.43 - 5.06 1.30 3.18 ± 2.66
[Cu(L1)(Bpy)](Cl) 6.95 8.62 5.67 4.73 6.34 ± 2.03
[Cu(L1)(Phen)](Cl) 11.08 3.65 11.18 4.18 6.33 ± 4.20
[Cu(L1)(Phen-NH2)](Cl) 4.65 3.95 17.02 4.00 8.32 ± 7.53
In General, compounds with lower IC50 values are more cytotoxic against the cancer cell lines.
This data is counterintuitive, thus –log IC50 values are calculated. The lower the IC50 value of the copper(II) chelates, the larger its –log IC50 value. This allows for a clearer graphical representation of the data. Figure 7.3.1 shows a bar graph of the –logIC50 values of the metal chelates.
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Figure 7.3.1: The -log IC50 values for the copper(II) chelates against four cell lines and their mean cytotoxicity.
The data in Figure 7.3.1 illustrates that each compound has a unique cytotoxicity profile (i.e.
each cell line responds differently to the drug candidate) and confirms that the compounds are genuine chemotherapeutics with a well-defined mechanism of action.
HEK293 is a normal human cell line and was used a control in this study. The cytotoxic activity of the copper chelates towards the normal healthy cells illustrates the need for targeted drug delivery. The copper(II) chelates are similarly toxic to both healthy and tumour cells. By incorporating biomarkers (such as biotin) into the drug design it will ensure a higher uptake of the drug in neoplastic over healthy tissue. An interesting point highlighted by the data in is that [Cu(L2)Cl2] shows a significantly reduced cytotoxicity towards the healthy cell line while showing good toxicity towards tumour cell lines. This suggests that it may be a more suitable primary ligand for the development of the next generation of heteroleptic complexes. The reason for the reduced cytotoxicity is not clear, though it may be related to the reduced aromaticity and associated reduction in DNA binding affinity.
The copper chelates all show the greatest cytotoxicity towards the leukaemia cell line MT-4.
This suggests that the compounds could be most suitable for the treatment of this particular class of cancer. It would therefore be pertinent to test the compounds against a wider variety of leukaemia cell lines in future work. It is noteworthy that [Cu(L1)(Cl)] is the least cytotoxic of all the chelates. The addition of a co-ligand is required to increase the cytotoxicity. If the same trend holds for the other single-ligand chelates then the addition of co-ligands to [Cu(L)Cl2] could lead to a potent chemotherapeutic. [Cu(L1)(Phen-NH2)](Cl) shows modestly enhanced
4 4.5 5 5.5 6 6.5
HEK293 TK-10 U937 MT-4 Mean
-Log IC50
[Cu(L)(Cl)2] [Cu(L1)(Cl)] [Cu(L2)(Cl)2]
[Cu(L1)(Bpy)](Cl) [Cu(L1)(Phen)](Cl) [Cu(L1)(Phen-NH2)](Cl)
[Cu(L)(Cl)2] [Cu(L2)(Cl)2]
[Cu(L1)(Phen-NH2)](Cl)
Page | 125 activity when compared to [Cu(L1)(Phen)](Cl). This is likely due to the NH2 group’s hydrogen bonding capabilities which stabilises the DNA/Drug conjugate.
As previously discussed, the copper(II) chelates were designed to bind DNA as a key step to inhibit cell replication. A comparison of the DNA binding affinities and cytotoxicities is therefore relevant. Table 7.3.2 shows (rather unexpectedly) little relationship between DNA binding affinity and cytotoxicity. In the case of [Cu(L)(Cl)2], this chelate exhibits the highest cytotoxicity of the screened copper chelates, however, it possesses one of the lowest DNA binding affinities. Chelate [Cu(L1)(Cl)] possessed the lowest cytotoxicity, but had the highest DNA binding affinity. This shows that the mechanism of action is more complex than DNA binding alone. The redox activity of the copper(II) ion is therefore likely to be a key component in the cytotoxicity. Ligands which are more flexible, such as in complex [Cu(L)Cl2], may allow for the geometric changes required for a transition from copper(II) to copper(I) since each of these metal ions have different preferences for coordination geometry.
Table 7.3.2: Relationship between the cytotoxicity and competitive EB-DNA binding affinities of the copper(II) chelates.
Most effective Least effective Cytotoxicity: in vitro cell screening
[Cu(L)(Cl)2] [Cu(L2)(Cl)2] [Cu(L1)(Bpy)](Cl) [Cu(L1)(Phen-NH2)](Cl) [Cu(L1)(Phen)](Cl) [Cu(L1)(Cl)]
EB-ctDNA competitive binding affinities
[Cu(L1)(Cl)] [Cu(L1)(Phen)(Cl)] [Cu(L1)(Phen-NH2)(Cl)] [Cu(L1)(Bpy)(Cl)] [Cu(L)(Cl2)] [Cu(L2)(Cl2)]
The data in Figure 7.3.1 and Table 7.3.1 shows that the cytotoxicity of the metal chelates varies significantly. The cytotoxicity of each chelate also varies against each cell line. The lack of correlation between the cytotoxicity profile and the DNA binding affinities suggests that the mechanism of action of the chelates is more complicated than anticipated. The single-ligand copper(II) chelates are also more cytotoxic than the heteroleptic copper(II) chelates. The opposite is observed with the DNA binding studies which showed the heteroleptic chelates exhibited stronger binding affinities compared to the single-ligand chelates. A stated above the heteroleptic chelates may more effectively stabilise the metal ion, reducing the production of ROS and ultimately reducing the cytotoxicity.
Another interesting trend is noted in both the cytotoxicity and DNA binding affinities of the single-ligand copper(II) chelates. [Cu(L)(Cl)2] and [Cu(L2)(Cl)2] both contained N,N’,N’’ donor sets. Both these chelates were the most active in the cell screening but were the weakest DNA
Page | 126 binders. [Cu(L1)(Cl)] contained N,N’,O donor atoms and displayed contrasting results.
[Cu(L1)(Cl)] was the least active in the cell screening, but had the highest DNA binding affinity.
[Cu(L2)(Cl)2] and [Cu(L1)(Cl)] are structurally comparable with the only difference between the two chelates being the donor atoms to the copper(II) metal centre. This observation suggests that the choice of donor atoms on the primary ligand system is significant and may have an impact on the cytotoxicity and DNA binding affinities.
The –log IC50 values of the copper(II) chelates for cell lines TK-10 and U937 as well as those of cisplatin, carboplatin and bleomycin (as acquired from the National Cancer Institute database and Su et. a.l.)96 are summarised in Table 7.3.3. Figure 7.3.2 illustrates the cytotoxicity profile of the copper(II) chelates with three commercially available chemotherapeutic agents.
Table 7.3.3: The -log IC50 values of the copper(II) chelates as well as cisplatin, carboplatin and bleomycin.5
Compound TK-10 U937
[Cu(L1)(Cl)] 4.70 ± 2.37 4.61 ± 5.56
[Cu(L1)(Bpy)](Cl) 5.06 ± 2.39 5.25 ± 0.14
[Cu(L1)(Phen)](Cl) 5.44 ± 0.92 4.95 ± 2.41
[Cu(L1)(Phen-NH2)](Cl) 5.41 ± 0.54 4.77 ± 1.46
Cisplatin 4.91 5.70
Carboplatin 3.60 5.26
Bleomycin 4.88 -
Cisplatin and its analogue carboplatin are commonly used as industry standards against which anticancer agents are compared due to its high anticancer activity.97 Bleomycins are large water soluble glycoproteins which are derived from the bacteria Streptomyces verticillus.
Bleomycins are known to chelate metal ions, which enhances their cytotoxicity towards cancerous cells.98
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Figure 7.3.2: Comparison of the cytotoxicity of the copper(II) chelates with cisplatin, carboplatin and bleomycin.
From the graph in Figure 7.3.2 it can be concluded that the copper chelates are more active against the TK-10 cell line when compared to the commercially available agents. The three heteroleptic copper(II) chelates showed higher activity than cisplatin and bleomycin, while the single-ligand copper(II) chelate were less active. Carboplatin is the least active of all the compounds listed. These data show that in terms of cytotoxicity the copper(II) chelates synthesised in this work have potential as chemotherapeutics.