**4. Anticancer activity of flavonoids metal complexes**

The cytotoxic activity of flavonoids involves the inhibition of several molecular targets and pathways: DNA topoisomerases I and II [64], cyclin‐dependent kinases CDK2 and/or CDK1 [65], androgen receptor signaling [66], actin polymerization [67, 68], activation of p53, and inhibition of NFkB pathways [69]. Flavonoids activate the caspase-mediated signal transduction pathways, consecutively stimulating the tumor‐suppressor protein p53, which consequently triggers cell apoptosis [69].

In many cases, the antitumor activity of the flavonoid metal complexes has been reported to be greater than that of the free corresponding flavonoids [47]. This may be mediated through the regulation of important cell‐cycle events, alterations in the DNA structure, prooxidant effects, or interactions with the phospholipid bilayer. Flavonoid metal complexes have been reported to be active against gastric cancer cells, human hepatocellular carcinoma cells [70], human cervical carcinoma cells [71], leukemia cells [61], human colon adenocarcinoma cells, human hepatoma cells, and osteoblast cancer cells [62]. A very important and promising feature displayed by these complexes is that a number of them have proven to be selective toward cancerous cells over normal cells [72]. **Table 3** contains a selection of metal complexes of flavonoids which have shown antitumoral activity.

As it has been shown in **Table 5**, complexes contain diverse structures, derived from the variety of flavonoids and metal ions used in the drug design. Regarding the flavonoid structures in metal complexes, the type of the substituents on either ring in the ligand structure, whether electron‐withdrawing or electron donating, seems to have minor importance. However, their position appears to be crucial for the cytotoxic activity. *Ortho*-substitution of the B-ring appears to be unfavorable, while *meta‐* and *para*-substitution augments the anticancer activity. This may be due to a structural effect, as the B (phenyl)‐ring is more twisted in the *ortho*-substituted compounds than in the *meta*- and *para*‐derivatives, which may increase the interaction with biological targets. The purpose of combining these ligands with metal centers is to obtain anticancer agents with extended mechanistic range, facilitating single‐molecule multi‐target anticancer therapy.


By analyzing the data from **Table 2** and the scientific literature, some observations appear to

• most metal complexes behave as more powerful antioxidants compared to the parent

• the higher antioxidant activity of the metal complexes was generally explained by acquisition of an additional radical‐scavenging metal center by the complexes, probably a super-

• in some cases, the metal (e.g., Sn2+, Cd2+) complexes show lower antioxidant activity than the parent flavonoids; the lower antioxidant activity can be explained by the fact that chelation of these metal ions significantly changes the chemical properties of the flavonoid, or

The cytotoxic activity of flavonoids involves the inhibition of several molecular targets and pathways: DNA topoisomerases I and II [64], cyclin‐dependent kinases CDK2 and/or CDK1 [65], androgen receptor signaling [66], actin polymerization [67, 68], activation of p53, and inhibition of NFkB pathways [69]. Flavonoids activate the caspase-mediated signal transduction pathways, consecutively stimulating the tumor‐suppressor protein p53, which consequently

In many cases, the antitumor activity of the flavonoid metal complexes has been reported to be greater than that of the free corresponding flavonoids [47]. This may be mediated through the regulation of important cell‐cycle events, alterations in the DNA structure, prooxidant effects, or interactions with the phospholipid bilayer. Flavonoid metal complexes have been reported to be active against gastric cancer cells, human hepatocellular carcinoma cells [70], human cervical carcinoma cells [71], leukemia cells [61], human colon adenocarcinoma cells, human hepatoma cells, and osteoblast cancer cells [62]. A very important and promising feature displayed by these complexes is that a number of them have proven to be selective toward cancerous cells over normal cells [72]. **Table 3** contains a selection of metal complexes

As it has been shown in **Table 5**, complexes contain diverse structures, derived from the variety of flavonoids and metal ions used in the drug design. Regarding the flavonoid structures in metal complexes, the type of the substituents on either ring in the ligand structure, whether electron‐withdrawing or electron donating, seems to have minor importance. However, their position appears to be crucial for the cytotoxic activity. *Ortho*-substitution of the B-ring appears to be unfavorable, while *meta‐* and *para*-substitution augments the anticancer activity. This may be due to a structural effect, as the B (phenyl)‐ring is more twisted in the *ortho*-substituted compounds than in the *meta*- and *para*‐derivatives, which may increase the interaction with biological targets. The purpose of combining these ligands with metal centers is to obtain anticancer agents with extended mechanistic range, facilitating single‐molecule multi‐target anticancer therapy.

might increase the oxidation potentials relative to those of the free flavonoid;

**4. Anticancer activity of flavonoids metal complexes**

of flavonoids which have shown antitumoral activity.

• some metal complexes, especially those of Fe(III) ions, display prooxidant activity.

be relevant:

flavonoids;

oxide‐dismutase‐mimicking center;

312 Flavonoids - From Biosynthesis to Human Health

triggers cell apoptosis [69].



**Table 3.** Metal complexes of flavonoids with antitumor activity.

**Complex Comments Ref.**

disorders *in vivo*.

of the complexes).

*HepG2*: IC50(µM) values: (que)2

between DNA base pairs.

their mechanistic range.

*A549*: IC50(µM) values: cisplatin: 3.8 ± 0.8; **(1)**: ∼100; **(2)**: 5.3 ± 0.4; **(3)**: 3.8 ± 0.3; *A2780* (ovarian cancer): IC50(µM) values: cisplatin: 0.2 ± 0.1; **(1)**: 39.6 ± 3; **(2)**: 2.5 ± 0.2; **(3)**: 1.8 ± 0.1; *A2780cis* (cisplatin‐resistant subline): IC50(µM) values: cisplatin: 16.8 ± 1.5; (1): 62.5 ± 16; (2): 4.6 ± 0.2; (3): 1.5 ± 0.04; *Toledo* (diffuse large B‐cell lymphoma): IC50(µM) values: cisplatin: 0.5 ± 0.07; **(1)**: 13.9 ± 1.2; **(2)**: 0.5 ± 0.1; **(3)**: 0.6 ± 0.04; *Toledo‐cis* (cisplatin‐ resistant subline): IC50(µM) values: cisplatin: 8.3 ± 0.6; **(1)**: 56.6 ± 2.8; **(2)**: 2.9 ± 0.2; **(3)**: 2.8 ± 0.1. *Lymphocytes*: IC50 (µM) values: cisplatin: 0.2 ± 0.1; **(1)**: 19.4 ± 3; **(2)**: 3.4 ± 0.3; **(3)**: 1.6 ± 0.2. The complexes had minor effects on hemostasis or on the red blood cell lysis *in vitro,* at cytotoxic concentrations and are therefore unlikely to cause hematologic

*CH1 (ovarian carcinoma), SW480 (colon carcinoma, A549 (non‐small‐cell lung carcinoma), 5637 (human urinary bladder), LCLC‐103H (human large‐cell lung), DAN‐G (human pancreatic carcinoma cell lines).* The IC<sup>50</sup> values were in the low micromolar range. The chemosensitive CH1 cell line was very sensitive to the complexes with IC50 values < 7.9 µM; for SW480, the IC50 values: 3.4–26 µM. The lowest cytotoxic potency was found for *A549* (IC50: 8.6–51 µM). Some of the complexes have an influence on the cell cycle distribution; at concentrations around the IC50 values of the complexes, an increase in the cell fraction in G0/G1 phase was observed. Most of the complexes strongly inhibit CDK2, inhibit topoisomerase IIα to a greater extent than the corresponding free ligands (the activity on topo‐IIα correlates with the cytotoxic effect

0.3; *SMMC‐7221* (hepatocarcinom): IC50(µM) values: (que)2

1.0; **(1)**: 7.66 ± 0.30; **(2)**: 8.0 ± 0.3; *A549*: IC50(µM) values: (que)2

1.0; (**1)**: 10.0 ± 0.45; **(2)**: 8.0 ± 0.3. The antitumor activity of **(2)** may be partially attributed to the interaction with the GC‐rich DNA sequences and to DNA protein‐binding interactions. These processes lead to downregulation of survivin gene expression, caspase activation, and induction of apoptosis. HepG2 cells had undergone morphological changes typical of apoptosis, characterized by nuclear shrinkage, chromatin condensation, and fragmentation subsequent to exposure to complex **(1)**. The cytotoxic effect is partially attributed to intercalation

*518A2* (melanoma): IC50(µM) values: **(1)**: > 50; **(2)**: > 50; **(3)** > 50; **(4)**: 10.9 ± 1.6; **(5)**: 13.2 ± 1.1; **(6)**: 10.9 ± 1.4; *HCT‐116* (colon carcinoma): IC50(µM) values: **(1)**: > 50; **(2)**: > 50; **(3)** > 50; **(4)**: 14.8 ± 2.7; **(5)**: 15.8 ± 2.5; **(6)**: 10.4 ± 1.6; *KB‐V1/Vbl* (cervix carcinoma): IC50(µM) values: **(1)**: 12.5 ± 2.1; **(2)**: 20.6 ± 1.7; **(3)** 28.8 ± 3.0; **(4)**: 4.6 ± 0.5; **(5)**: 8.0 ± 1.2; **(6)**: 6.7 ± 0.5; *MCF‐7/ Topo* (breast carcinoma): IC50(µM) values: **(1)**: 28.2 ± 6.1; **(2)**: 31.2 ± 0.9; **(3)** 37.0 ± 3.0; **(4)**: 7.8 ± 1.5; **(5)**: 11.8 ± 0.7; **(6)**: 8.5 ± 1.4; The cytotoxic activity of the complexes was correlated with an arrest of the cell cycle of *518A2* cells at the G2/M transition. The complexes gave better results than the free flavonoids in decreasing the migration propensity of *518A2* cells in wound‐healing assays. The antimetastatic effects of complex **(6)** derive from the remodeling of the actin cytoskeleton and the increase in cadherin‐catenin complex formation, factors that favor cell‐cell adhesion. Complex (**6**) decreased the expression and secretion of the metastasis‐related matrix metalloproteinases MMP‐2 and MMP‐9. Thus, the coordination of apigenin and genistein to Cu(II) enhances the antitumoral properties of the free flavonoids and expands

: 13.3 ± 1.0; **(1)**: 5.46 ± 0.36; **(2)**: 8.0 ±

: 13.3 ±

:13.3 ±

[80]

[81]

[82, 83]

[84]

Ru(II) *cis*‐dichlorobis(3‐ imino‐2‐methoxy‐flavanone) **(1)** Ru(II) *cis*‐dichlorobis(3‐ imino‐2‐ethoxy‐flavanone) **(2)** Ru(II) *cis*‐dichloro(3‐ nitrozoflavone)(3‐ hydroxyiminoflavanone)**(3)**

314 Flavonoids - From Biosynthesis to Human Health

Ru(II) complex with


Quercetin complexes with Zn(II), Mn(II) [Zn(que)2

(H2 O)2 ] **(1)**

(H2 O)2 ] **(2)**

Chrysin **(1)**, apigenin **(2)**, genistein **(3)** homoleptic Cu(II) Complexes **(4)**–**(6)**,

[Mn(que)2

respectively

η6

It should be kept in mind that with the failure of ASA404/vadimezan (Antisoma/Novartis) in a phase III trial for advanced non‐small‐cell line cancer when given in combination with carboplatin and taxol [86], there are no more benzopyran‐4‐one derivatives strong candidates for anticancer drugs. The research regarding the antitumor properties of flavonoid metal complexes offers promising results and these should be further improved in order for the complexes to enter clinical trials.
