**4.1 Alkylating agents**

*Current Topics in Chirality - From Chemistry to Biology*

*The structure of gold complex, which is under clinical investigation for treatment cancer.*

The mechanism of action of Au(I) complexes is thought to trigger apoptosis *via* inhibition of selenium- and sulfur-containing enzymes such as thioredoxin reductase (TrxR), glutathione peroxidase, cysteine protease or glutathione-S-

The novel chiral Au(III) complexes containing chiral P stereogenic phosphine ligand: *R,R* and *S,S*- QuinoxP\* (2,3-bis(tert-butylmethylphoshino)quinoxaline) were prepared **Figure 13** [81]. The antiproliferative activities of the two compound were evaluated against panel of cell lines and exhibited cytotoxicity slightly higher than that of cisplatin and auranofin. However, no diffrence was found in the cyto-

**148**

transferase [80].

**Figure 13.**

**Figure 11.**

**Figure 12.**

toxic effect between the two enantiomers.

*Chemical structures of the enantiomeric phosphine-Au(III) complexes.*

*Examples of enantiomeric phosphine-Au(I) complexes.*

Alkylating agents were among the first group of chemicals determined to be useful in cancer chemotherapy, and the largest drug group among conventional cytotoxic chemotherapeutics. They are so named because of their ability to add alkyl groups to negatively charged groups on biological molecules such as DNA and proteins [83]. Classical alkylating agents include nitrogen mustards, nitroso ureas, aziridines, and alkyl sulfonates. Nonclassical alkylating agents include hydrazine, triazene, and altretamines. In addition, the alkylating-like agent group, which similarly to alkylating agents functions by crosslinking with DNA, includes platinum compounds. The high clinical efficacy of one of the main classes of alkylating agents, nitrogen mustards, makes them the current choice for the first-line treatment of different tumor types. However, severe side effects limit the therapeutic value of such compounds, and new effective compounds are required [84].

The group of bifunctional chloropiperidine derivatives has been revealed to possess novel efficient DNA-alkylating properties, leading to direct strand cleavage at guanine nucleotides and indirect effects on the human topoisomerase II enzyme [85, 86].

## *4.1.1 Chiral chloropiperidines*

Carraro et al. [87] investigated series of racemic and enantiomerically pure monofunctional chloropiperidines – **Figure 14**. Derivatives of chloropiperidines demonstrated the ability to alkylate DNA *in vitro*. On a panel of carcinoma cell lines, M-CePs exhibited low nanomolar cytotoxicity indexes, which showed their remarkable activity against pancreatic cancer cells and in all cases performed strikingly better than the chlorambucil control. Interestingly, stereochemistry modulated the activity of the chloropiperidines.

An analysis of the cytotoxicity of enantiomers *N*-butyl derivatives of chloropiperidines D-1 and L-1 in three cancer cell lines revealed that D-1 was the most

**Figure 14.** *Chemical structure of racemic chloropiperidines and enatiomerically pure compounds.* active compound, again with a clear tropism for pancreatic cancer cells, while its enantiomer, L-1 enantiomer was less cytotoxic, with an eudismic ratio of ∼40 in the case of BxPC-3 cells. The direct damage observed to isolated DNA was not sufficient to explain their nanomolar cytotoxicity, especially when considering the enantiomeric couple.

The D-1 enantiomer turned out to be the most cytotoxic compound of the entire series, although it was inactive in the DNA cleavage assay. In contrast, its mirror image, L-1, found to efficiently nick and fragment the plasmid *in vitro*, happened to be much less cytotoxic.

The enantiomers also differed in permeation through an artificial membrane, that simulates passive diffusion. Because D-1 was the most cytotoxic but least permeable enantiomer, the permeation analysis of the chiral compounds suggests the involvement of active mechanisms of uptake into cells.

#### **4.2 Inhibitors of topoisomerases**

#### *4.2.1 Chiral epoxy-substituted chromones*

Human DNA topoisomerases (Top) have been recognized as a good target molecule for the development of anticancer drugs because they play an important role in solving DNA topological problems caused by DNA strand separation during replication and transcription [88].

Jo et al. [89] designed and synthesized novel chiral epoxy-substituted chromone analogues -**Figure 15** that exhibit an anticancer effect by inhibiting the DNA synthesis of cancer cells. Their ability to alkylate DNA and inhibit topo enzymes and cancer cell growth was evaluated.

In the brief structure–activity relationship analysis, no clear correlation was seen between stereochemistry and topos inhibitory and cytotoxic activity. However, compounds **6**, **10** and **11** were more potent than the others in both Top I and IIα inhibitory activity.

The 5(*R*),7(*S*)-bisepoxy-substituted compound **11** showed the most potent cell antiproliferative activity against all tested cancer cell lines with particularly strong inhibition of K562 myelogenous leukemia cancer cell proliferation.

#### *4.2.2 Chiral hydroxyanthraquinone analogs*

Natural and synthetic analogs of hydroxyanthraquinones (e.g., anthracyclines, mitoxantrone and emodin) are additional prototypes for the design of anticancer drug candidates with the ability to bind double-stranded DNA and inhibit topoisomerases 1 and 2 mediated relaxation of supercoiled DNA [90, 91].

Derivatives of (4,11-dihydroxynaphtho[2,3-*f*]indole-5,10-dione) were identified as a promising scaffold for the search of agents active against resistant tumor cells [92]. Shchekotikhin et al. [93] explored new TopI and TopII antagonists based on a 4,11-dihydroxy naphtho[2,3-*f*]indole-5,10-dione scaffold bearing the cyclic chiral diamine in the side chain - **Figure 16**.

Potent cytotoxicity (at submicromolar to low micromolar concentrations) against a panel of wild type mammalian tumor cells and isogenic drug resistant sublinies was observed for all novel derivatives of naphtho[2,3-*f*]indole-5,10-diones. Only isomer **7** induced the formation of specific DNA cleavage products similar for classical Top1 inhibitors camptothecin and indenoisoquinoline MJ-III-65. Importantly, the derivative of (*R*)- aminopyrrolidine **7** increased the life span of mice bearing P388 leukemia while its enantiomer **6** was inactive.

**151**

their activity.

*Chirality in Anticancer Agents*

**Figure 15.**

**Figure 16.**

**4.3 Chiral thiosemicarbazones**

*The chemical structures of naphthoindolediones with some chiral substituents.*

Another group of potent anticancer agents is made up from chiral thiosemicarbazones derived from homochiral amines **Figure 17** [94]. Their antiproliferative activity was evaluated against several panels of cancel cell lines (A549 (human alveolar adenocarcinoma), MCF-7 (human breast adenocarcinoma), HeLa (human cervical adenocarcinoma), and HGC-27 (human stomach carcinoma) cell lines. Some of the compounds, especialy thiosemicarbazones with substituted hydroxyl group, 4-chlorophenoxy, 4-fluorophenoxy showed inhibitory activities on the growth of cancer cell lines. Compounds with substituted piperidine ring exhibited higher activity against HCG-27 than taxol, which was uses as the standard. In every active thiosemicarbazone derivatives the (*S*) enantiomer was more active than (*R*)-enantiomer. The most active compounds the (*S*)-isomers of the thiosemicarbazone derivative with substituted piperidine group, also showed the best fit for the generated pharmacophore hypothesis. In the pharmacophore model, the (*S*)-enantiomer was better matched than the (*R*) enantiomer. This showed that the configuration of isomers greatly influenced

*DOI: http://dx.doi.org/10.5772/intechopen.98977*

*The structures of prepared chiral chromane analogs.*

*Chirality in Anticancer Agents DOI: http://dx.doi.org/10.5772/intechopen.98977*

**Figure 15.**

*Current Topics in Chirality - From Chemistry to Biology*

the involvement of active mechanisms of uptake into cells.

enantiomeric couple.

be much less cytotoxic.

**4.2 Inhibitors of topoisomerases**

replication and transcription [88].

and cancer cell growth was evaluated.

*4.2.2 Chiral hydroxyanthraquinone analogs*

diamine in the side chain - **Figure 16**.

inhibitory activity.

*4.2.1 Chiral epoxy-substituted chromones*

active compound, again with a clear tropism for pancreatic cancer cells, while its enantiomer, L-1 enantiomer was less cytotoxic, with an eudismic ratio of ∼40 in the case of BxPC-3 cells. The direct damage observed to isolated DNA was not sufficient to explain their nanomolar cytotoxicity, especially when considering the

The D-1 enantiomer turned out to be the most cytotoxic compound of the entire series, although it was inactive in the DNA cleavage assay. In contrast, its mirror image, L-1, found to efficiently nick and fragment the plasmid *in vitro*, happened to

The enantiomers also differed in permeation through an artificial membrane, that simulates passive diffusion. Because D-1 was the most cytotoxic but least permeable enantiomer, the permeation analysis of the chiral compounds suggests

Human DNA topoisomerases (Top) have been recognized as a good target molecule for the development of anticancer drugs because they play an important role in solving DNA topological problems caused by DNA strand separation during

analogues -**Figure 15** that exhibit an anticancer effect by inhibiting the DNA synthesis of cancer cells. Their ability to alkylate DNA and inhibit topo enzymes

inhibition of K562 myelogenous leukemia cancer cell proliferation.

somerases 1 and 2 mediated relaxation of supercoiled DNA [90, 91].

mice bearing P388 leukemia while its enantiomer **6** was inactive.

Jo et al. [89] designed and synthesized novel chiral epoxy-substituted chromone

In the brief structure–activity relationship analysis, no clear correlation was seen between stereochemistry and topos inhibitory and cytotoxic activity. However, compounds **6**, **10** and **11** were more potent than the others in both Top I and IIα

The 5(*R*),7(*S*)-bisepoxy-substituted compound **11** showed the most potent cell antiproliferative activity against all tested cancer cell lines with particularly strong

Natural and synthetic analogs of hydroxyanthraquinones (e.g., anthracyclines, mitoxantrone and emodin) are additional prototypes for the design of anticancer drug candidates with the ability to bind double-stranded DNA and inhibit topoi-

Derivatives of (4,11-dihydroxynaphtho[2,3-*f*]indole-5,10-dione) were identified as a promising scaffold for the search of agents active against resistant tumor cells [92]. Shchekotikhin et al. [93] explored new TopI and TopII antagonists based on a 4,11-dihydroxy naphtho[2,3-*f*]indole-5,10-dione scaffold bearing the cyclic chiral

Potent cytotoxicity (at submicromolar to low micromolar concentrations) against a panel of wild type mammalian tumor cells and isogenic drug resistant sublinies was observed for all novel derivatives of naphtho[2,3-*f*]indole-5,10-diones. Only isomer **7** induced the formation of specific DNA cleavage products similar for classical Top1 inhibitors camptothecin and indenoisoquinoline MJ-III-65. Importantly, the derivative of (*R*)- aminopyrrolidine **7** increased the life span of

**150**

*The structures of prepared chiral chromane analogs.*

**Figure 16.**

*The chemical structures of naphthoindolediones with some chiral substituents.*

#### **4.3 Chiral thiosemicarbazones**

Another group of potent anticancer agents is made up from chiral thiosemicarbazones derived from homochiral amines **Figure 17** [94]. Their antiproliferative activity was evaluated against several panels of cancel cell lines (A549 (human alveolar adenocarcinoma), MCF-7 (human breast adenocarcinoma), HeLa (human cervical adenocarcinoma), and HGC-27 (human stomach carcinoma) cell lines. Some of the compounds, especialy thiosemicarbazones with substituted hydroxyl group, 4-chlorophenoxy, 4-fluorophenoxy showed inhibitory activities on the growth of cancer cell lines. Compounds with substituted piperidine ring exhibited higher activity against HCG-27 than taxol, which was uses as the standard. In every active thiosemicarbazone derivatives the (*S*) enantiomer was more active than (*R*)-enantiomer. The most active compounds the (*S*)-isomers of the thiosemicarbazone derivative with substituted piperidine group, also showed the best fit for the generated pharmacophore hypothesis. In the pharmacophore model, the (*S*)-enantiomer was better matched than the (*R*) enantiomer. This showed that the configuration of isomers greatly influenced their activity.

**Figure 17.**

*The chemical structures of chiral thiosemicarbazones.*

#### **Figure 18.**

*The structure of stereoisomers* δ*-iodo-*γ*-lactones with p-isopropylphenyl substituent.*

#### **4.4 Chiral** δ**-iodo-**γ**-lactones**

Compounds with both a lactone function and an aromatic ring in their structure are promising as potential anticancer agents [95, 96]. Four stereoisomers of δ-iodo-γlactones with a 4-isopropylphenyl substituent at the β-position - **Figure 18** were tested against a broad panel of canine cancer cell lines representing hematopoietic and mammary gland cancers. The investigated isomers exerted higher activity against canine lymphoma/leukemia cell lines than against mammary tumors, whereas the configuration of stereogenic centres of the examined compounds affected their activity.

Stereoisomers with the 4*S* configuration shown to be were more active, with the cis-(4*S*,5*S*,6*R*) isomer as the most potent. The investigated δ-iodo-γ-lactones act as an anticancer agent by the induction of apoptosis of canine cancer cells via a mitochondrial-mediated, caspase-dependent pathway [97].

#### **5. Conclusions**

Chirality has become a major task for the synthesis and development of drugs. One enantiomer of a chiral drug may be a medicine for particular disease

**153**

**Author details**

Jindra Valentová\* and Lucia Lintnerová

in Bratislava, Bratislava, Slovak Republic

provided the original work is properly cited.

\*Address all correspondence to: valentova@fpharm.uniba.sk

Department Chemical Theory of Drugs, Faculty of Pharmacy, Comenius University

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Chirality in Anticancer Agents*

**Acknowledgements**

**Conflict of interest**

The authors declare no conflict of interest.

*DOI: http://dx.doi.org/10.5772/intechopen.98977*

whereas; another enantiomer of the molecule may be not only inactive but can even be toxic. The pharmacological activity of chiral drugs depends mainly on the drug's interaction with chiral biological molecules such as proteins, nucleic acids and bio membranes. This is valid not only for organic drugs, but also for metalbased drugs which are developed for anticancer application. This review outlines the effect of chirality on the efficiency of anticancer metal-based drugs and interesting potential organic chiral drug molecules. The influence of stereoselectivity on anticancer activity can hardly be generalized, it is manifested specifically for each individual chiral compound and depend on the type of cellular targets. However, knowledge of the stereochemistry of anticancer compounds can influence some critical processes underlying their toxicity towards cancer cells

and provide a rational basis for the design of new antitumor drugs.

This work was supported by VEGA grant No 1/0145/20 (Slovak Republic).

*Chirality in Anticancer Agents DOI: http://dx.doi.org/10.5772/intechopen.98977*

*Current Topics in Chirality - From Chemistry to Biology*

*The chemical structures of chiral thiosemicarbazones.*

**152**

**5. Conclusions**

**4.4 Chiral** δ**-iodo-**γ**-lactones**

**Figure 18.**

**Figure 17.**

Compounds with both a lactone function and an aromatic ring in their structure are promising as potential anticancer agents [95, 96]. Four stereoisomers of δ-iodo-γlactones with a 4-isopropylphenyl substituent at the β-position - **Figure 18** were tested against a broad panel of canine cancer cell lines representing hematopoietic and mammary gland cancers. The investigated isomers exerted higher activity against canine lymphoma/leukemia cell lines than against mammary tumors, whereas the configura-

tion of stereogenic centres of the examined compounds affected their activity. Stereoisomers with the 4*S* configuration shown to be were more active, with the cis-(4*S*,5*S*,6*R*) isomer as the most potent. The investigated δ-iodo-γ-lactones act as an anticancer agent by the induction of apoptosis of canine cancer cells via a

Chirality has become a major task for the synthesis and development of drugs. One enantiomer of a chiral drug may be a medicine for particular disease

mitochondrial-mediated, caspase-dependent pathway [97].

*The structure of stereoisomers* δ*-iodo-*γ*-lactones with p-isopropylphenyl substituent.*

whereas; another enantiomer of the molecule may be not only inactive but can even be toxic. The pharmacological activity of chiral drugs depends mainly on the drug's interaction with chiral biological molecules such as proteins, nucleic acids and bio membranes. This is valid not only for organic drugs, but also for metalbased drugs which are developed for anticancer application. This review outlines the effect of chirality on the efficiency of anticancer metal-based drugs and interesting potential organic chiral drug molecules. The influence of stereoselectivity on anticancer activity can hardly be generalized, it is manifested specifically for each individual chiral compound and depend on the type of cellular targets. However, knowledge of the stereochemistry of anticancer compounds can influence some critical processes underlying their toxicity towards cancer cells and provide a rational basis for the design of new antitumor drugs.
