Novel Anticancer Agents and Targets: Recent ...

Mini-Reviews in Medicinal Chemistry, 2013, 13, 000-000

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Novel Anticancer Agents and Targets: Recent Advances and Future Perspectives Kuldipsinh P. Barot1, Stoyanka Nikolova2, Illiyan Ivanov2, Manjunath D. Ghate*,1 1

Department of Pharmaceutical Chemistry, Institute of Pharmacy, Nirma University, Ahmedabad-382481, Gujarat, India; 2Faculty of Chemistry, University of Plovdiv, 24 "Tsar Asen" str., 4000 Plovdiv, Bulgaria Abstract: An urgent need for the discovery of novel anticancer agents is required for the long term therapy of cancer. Many novel bio-active and potential anticancer agents are being used in clinical and pre-clinical trials. Although many heterocyclic compounds are already available commercially as anticancer agents, great efforts have been put to identify novel anticancer targets. This review provides an insight of the novel anticancer targets and molecules of the first and final stage of clinical and pre-clinical trials.

Keywords: Cancer, novel anticancer agents, novel heterocyclic compounds, novel anticancer targets, chemogenomics. 1. INTRODUCTION Cancer is the most widespread and feared disease in the western world. The panic is largely due to the difficulty to cure which is due to uncontrolled multiplication of some modified normal human cells. Chemotherapy is one of the main methods of modern cancer treatment [1]. Medicinal chemistry played a vital role in highly integrated and multidisciplinary process of anticancer drug development [2]. However, the nature of its major contributions has varied over time. About one in four people is caught with some form of cancer during their life time. At the present time, cancer contributes to about one in five of all deaths [3]. Cancer treatment mainly includes radiation therapy, surgery and chemotherapy. Each treatment has significant limitations but chemotherapy offers only approach to treat metastasized cases of cancer. Non-cytotoxic drugs can prevent the multiplication of cancer cells [4]. Over the last fifty years about 500,000 natural and synthetic chemical compounds are tested for anticancer activity, but only 25 of these are used today which indicates the major concern of cancer therapy. Currently available anticancer drugs have significantly reduced the mortality rates for some form of cancers (e.g. Leukemia, Testicular and Ovarian cancer) and given overall longer patient survival times [2, 5]. 1.1. Origins of Cancer Cancer describes a group of 120 different diseases of broad similarities. The common factor among them includes the uncontrollable cell division of a single cell to form a tumor and break down to form new tumors [6]. Normal cell divisions are under tight control by a number of biological mechanisms which are still being explored. Cell division is *Address correspondence to this author at the Department of Pharmaceutical Chemistry, Institute of Pharmacy, Nirma University, Ahmedabad-382481, Gujarat, India; Tel: +91-2717-241900 to 04; Fax: +91-2717-241916; E-mail: [email protected]

1389-5575/13 $58.00+.00

controlled by a relatively small group of enzymes. Some of them form a communication network which relies on growth signals of the cell surface and control the cell division [4, 6]. However, mutation in the DNA of enzyme renders them defective. Such mutated cell will divide uncontrollably and produce same natured daughter cells [6]. A human cell includes approximately 100,000 genes of which 50 are known as proto-oncogenes. Many of them code for the enzymes that makes communication and surveillance systems as described above. If a cell accumulates critical mutations in five or six of these proto-oncogenes, the changes are likely to result in a fully malignant cell which is capable of forming a tumor [5, 7]. 1.2. Background In the United States of America, 562,340 deaths from cancer were reported in 2010 and 1529,560 new cancer cases were expected in 2011. Although overall cancer incident rates have decreased in recent years, the lung cancer is still the leading cause of cancer mortality in men and women [8]. Novel approaches are urgently required for further improvements in current cancer therapies especially the lung cancer to slow down the cancer death rate. More and more research groups have devoted into the cancer area to slow down its death rate. Based on the mechanism of glucose metabolism, inhibition of the glucose uptake might kill cancer cells [9]. 1.3. Warburg Effect In 1924, Warburg and Negelein observed that cancer cells exhibited a higher glycolysis rate than normal cells even in the presence of oxygen. This feature is well known as the "Warburg Effect" and is one of the most important characteristics of cancer cells [9]. Since Warburg effect has been well investigated and revisited. It act as biochemical basis for the development of anticancer therapeutic strategies and novel potential anticancer agents [8, 10].

© 2013 Bentham Science Publishers

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Barot et al.

1.4. Steps for Discovery of Anticancer Agents

2. DESIGNING OF NOVEL ANTICANCER AGENTS

Many heterocyclic molecules are well known to play a critical role in health care and pharmaceutical drug design. Presently, a number of heterocyclic compounds are widely available commercially as anticancer drugs and great efforts have been taken to identify novel anticancer targets for anticancer drug discovery [11].

Anticancer drugs are divided into cytotoxic (cell killing) drugs and cytostatic (cell stabilizing) drugs. Both categories lead to a reduction in the size of the tumor because cancer cells have such a high mortality rate that simply prevents them from dividing. It will lead to a reduction in the population [15].

1.4.1. Step 1 - Initial Discovery

2.1. Cytotoxic Drugs

Synthetic and natural compounds are tested in high capacity screens to discover molecules with useful properties [12].

Cytotoxic drugs work by interfering DNA replication because cancer cells divide and synthesize new DNA rapidly. Three main groups of molecules used to interfere with DNA replication are as follows [15].

1.4.2. Step 2- Molecular Modification of Known Compound

2.1.1. Antimetabolites

A molecule having suitable properties is chemically altered for the best drug-like properties of the most effective anticancer lead compound [12].

They are similar to nucleotides and are incorporated into DNA which lead to non-functional DNA.

1.4.3. Step 3- Development into Useful Pharmaceutical

2.1.2. Alkylating Agents

Above process is very time consuming and expensive, as discovery is usually patented at this time so that the researchers can eventually recover the costs. The most effective route for synthesizing the molecule has been worked out [12]. A process of advanced testing begins and ends up with tests on patients in specialised hospitals. If the results are favorable, the drug is able to be released for use. The process of drug development is very long. Among ten thousand of the molecules originally tested, only one lead compound would be used clinically [13].

They permanently attach to the DNA and distort its shape. Unfortunately, they also attach to many other molecules in the cells.

1.5. Early Rationality of Cancer Treatment

2.2. Cytostatic Drugs

Knowledge of DNA metabolism allowed pioneers such as George Hitchings, Gertrude Elion and Charles Heidelberger to develop nucleotide mimics and antagonists largely by rational design. It lead to the development of 6mercaptopurine I, cytosine arabinoside II, 5-fluorouracil III and methotrexate IV which are widely used in cytotoxic chemotherapy in todays era [13]. Haddow and Ross explored the family of nitrogen mustards which was sparked by the initial observation of depression of white cell count in people exposed to the war gas bis(2-chloroethyl)amine V (mustard gas). It lead to the development of mechlorethamine VI, the less vesicant analog chlorambucil VII, melphalan VIII and the widely used cyclophosphamide IX (Fig. 1) [14].

Cytostatic drugs alter biochemical pathways which enable cancer cells to reproduce quickly. They are designed to deactivate the altered enzymes that are resulted in the changes of the involved oncogenes. They are not designed to kill the cancerous cells but prevent them to reproduce theoretically "cancer cell-specific" [16]. 4-Anilinoquinazoline 4 is a potent inhibitor but modifications at 3rd position of the molecule increase the activity about a million-fold. The related molecule 5 can inhibit the enzyme at a concentration of 0.000025 M. The related tricyclic molecule 6 is more potent inhibitor at a concentration of 0.00008 M. Compound 5 inhibits the EGFR (Epidermal Growth Factor Receptor) about a million-fold more effectively than it inhibits related tyrosine kinase (Fig. 3). That was shown for the first time for

2.1.3. DNA Binding Agents They usually attach to the DNA chain and disengage from the chain. Then they attach to another chain to repeat the process which usually function in conjunction with an enzyme. Tirapazamine 1 and EO-9 3 are in clinical trials of anticancer agents (Fig. 2) [16].

NH2 SH N

H N

OH H 2N

N

N

N

F

NH

OH O

O

HN

N H

O

H2N

N HN

N N

O

H 3C

Cl

Cl

V R = H, VI R = CH3

R

N

Cl

O

VII (CH2)3COOH, VIII CH(NH2)COOH

Fig. (1). Early rationality of cancer treatment with well known anticancer agents.

O P N NH IX

Cl Cl

COOH O

IV Cl

R N

N

III

II

I

COOH

N

HO

Novel Anticancer Agents and Targets: Recent Advances and Future Perspectives

the extremely potent and selective capacity of the EGFR enzymes [17]. Physical properties and biological activities of asulacrine 7 compared to amsacrine resulted in the secondgeneration analogue of amsacrine for clinical trials of cancer. Asulacrine 7 (Fig. 4) was synthesized from 5-methylacridone-4-carboxylic acid. Initial clinical trials of asulacrine were carried out in Auckland and UK in the mid to late 1980s. It has been suggested that the drug has some activity in certain types of lung and breast cancers. Further trials are currently in progress in UK which are controlled by the UK Cancer Research Campaign [15, 18].


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anticancer activity. It was suggested that optimized compounds might potentially constitute a novel class of anticancer agents which is useful for further studies [18, 19]. O R2 HN

R1

N X

H N

S O

8 R1 = -Cl, R2 = 2-methoxyphenyl 9 R1 = -H, R2 = Chloromethyl

O N

O2N

N

N O

NH2

N

N OH

Br

N H

HO

N

NH

HN

Br

H3CO

O

R1

R2

X

10

4-Chlorobenzyl

2-methoxy phenyl

S

11

Benzyl

Chloromethyl

S

12

N-methyl-piperazine-phenyl

Chloromethyl

S

13


2-methoxyphenyl

S

14

4-Chlorobenzyl


S

15


2-methoxy phenyl

N

16


2-methoxy phenyl

O

N Br

N

N NH

H3CO

4

S X

N H

3

Fig. (2). Novel cytotoxic drugs for the treatment of cancer.

N

R2

O

2

N

N

O

HO

1

HN R1

O

H 3C N

N 6

5

Fig. (3). Novel cytostatic drugs for the treatment of cancer. O H N O S CH3

Fig. (5). Benzimidazole, benzothiazole and benzoxazole derivatives as novel anticancer agents.

OCH3 NH

3.2. Phenolic Benzoate Esters as Novel Potential Anticancer Agents N CH3

O

N H

CH3

7

Fig. (4). Asulacrine as novel cytostatic drug for the treatment of cancer.

3. NOVEL ANTICANCER AGENTS IN CLINICAL AND PRE-CLINICAL TRIALS

Phenolic benzoate esters exhibit strong glucose uptake inhibitory activity in the lung cancer cell line H1299 and prove to be cytotoxic for cancer cells [20] and several analogs of 17 were synthesized based on this fact. In order to lower the molecular weight, one galloyl group on the core aromatic ring was replaced by methoxyl group, fluorine, chlorine and hydrogen. Different substituents were introduced into the pendant aromatic rings and the number of hydroxyl groups on the pendant aromatic rings was changed (Fig. 6) [21].

3.1. Benzimidazole, Benzothiazole and Benzoxazole Derivatives as Novel Anticancer Agents In search of novel anticancer agents, lead compounds 8 and 9 were found to be promising (Fig. 5). However, the poor solubility of 8 and 9 was a major drawback for further in vivo studies. In the resolvement of this problem, a lead optimization programme was conducted through introducing N-methyl-piperazine groups at the 2nd and 6th position [18]. To their delight, optimized analog have shown comparable in vitro anticancer activity with better solubility compared with compound 8. Based on these results, the replacement of the benzothiazole scaffold with benzimidazole and benzoxazole moieties has afforded fundamentally high

OH HO OH

O

OH

HO

O O

HO O

O O

R

OH

O O

R OH O

17

O

X

OH

X = OCH3, F, Cl, H R = OCH3, F 18

Fig. (6). Design analogs of phenolic benzoate ester as novel anticancer agent.

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3.2.1. Structure-Activity Relationship (SAR) of Benzoate Ester Derivatives

Aromatic rings on the phenolic ethers can freely rotate with ether linkage between core aromatic ring and pendant aromatic ring which results in different conformations from phenolic benzoate esters (Fig. 8) [23]. Therefore, we can get more information on the SAR of the phenolic ethers by investigating their inhibitory activity. Retrosynthetically, the phenolic ether derivatives can be obtained after coupling different phenols with benzyl chlorides (Fig. 9) [24].

Structure-activity relationship study suggests that one of the ring is not necessary for the activity in both glucose uptake inhibition and cell growth inhibition. The electron withdrawing group on the aromatic ring helps to keep or increase inhibitory activity whereas the electron donating group decreases both inhibitory activities [21]. metahydroxyl group on the pendant aromatic rings play an important role on both inhibitory activities in glucose uptake and cell growth. Inhibitory activity can be decreased when electron donating or withdrawing groups are introduced to the pendant aromatic rings [22] and m-hydroxyl benzoate ester has shown the best inhibitory activity in both glucose uptake and cell growth. Considering the molecular weight and inhibitory activity, compound 19 was selected as the lead compound from the library of benzoate esters (Fig. 7). Overall, the m-hydroxyl group on the pendant aromatic rings is essential for providing good activity in both glucose uptake inhibition and cell growth inhibition. It suggested that H-bond might also contribute to better inhibitory activity in glucose uptake and cell growth [21, 23].

3.4. Novel Anthraquinone Analogs for Treatment of Multi-Drug Resistant Tumor Cells Many acridine and anthracycline derivatives are excellent DNA intercalators which are in market as chemotherapeutic agents. Commercially available acridine and anthracycline derivatives have been widely studied for physico-chemical properties, structural requirements, synthesis and biological activity [25]. However, the clinical application of these and other compounds of the same class has encountered problems such as multi-drug resistance (MDR) and secondary side effects. These short-comings have motivated the search for new compounds to be used either in place of conjunction with the existing compounds [25]. DNA intercalating compounds have been shown little or no biological activity. Research in this area has not shown beneficial results for the synthesis and antitumor properties of compounds tested on diverse tumor cell lines. It analyzes the structural and biological considerations relevant to the use of some anthraquinone analogs (Fig. 10) [26].

3.3. Phenolic Ether Derivatives as Novel Potential Anticancer Agents Phenolic ether derivatives were synthesized to determine the importance of carbonyl group in the ester linkage which act as more stable analog as novel anticancer agent. OH O HO

O

O O F

19

Not necessary for activity

EWG keeps or increase activity EDG decrease activity R

O O

X

O

Ester linkage implies two atoms

O X

O O

meta-OH is the best and necessary for both activities. 3,4-(OH)2 and 3,5-(OH)2 are better than 3,4,5-(OH)3. para-(OH) is similar to 3,4,5-(OH)3, ortho-OH shows poorest activity. X = F, CF3, OCH3 show decreaseed activity.

X

Fig. (7). Structure-activity relationship of benzoate ester derivatives. Y

Y O

X O

O O

X O OH

H

H O

Cl + OH

X

H

OH 20

Y

H

OH OH

21

Fig. (8). Retrosynthetic analysis of phenolic ether derivatives as anticancer agents.

OH

22

OH

23


(OR)n X

24 25 26 27 28

Y

Z

OCH3 H F H H

H Cl H Cl H

H H H H Cl

(OR)n 1,2-(3-OH-C6H4CH2O)2 3,4-(3-OH-C6H4CH2O)2 1,2-(3-OH-C6H4CH2O)2 1,3-(3-OH-C6H4CH2O)2 3,5-(3-OH-C6H4CH2O)2

O Cl H2N Pt Cl NH2

Fig. (9). Phenolic ether derivatives as novel potential anticancer agents.

Cisplatin 38

O O O H2N Pt O NH2

O H2N Pt O NH2

O

OH

OH

O

OH

N H

O

N O

29

OH

OH

OH 33

OH 32

N

Cl H2N Pt Cl CH3

O HN Pt O NH

O

N H

H3COCO Cl H2N Pt Cl NH OCOCH3

N

JM216 44

Oxaliplatin 43

BBR3464 45

HN C R

OH

Fig. (11). Platinum complexes in clinical and pre-clinical trials as potential anticancer agents.

H N N

O

O

HO

30 R = H, 31 R = OH X

O

OH

O

HN

N H O

AMD473 42

ZD0437 41

254-S 40

Carboplatin 39

Cl H2N Pt Cl CH3

Cl H2N Pt Cl CH3 N

O O

5

unexpected monofunctional platinum(II) complexes with one normal and one cyclometalated 2-phenylpyridine ligand 49 were discovered for high antitumor efficacy against cisplatin resistant mouse sarcoma 180 (S-180 cis R) cell lines. Compounds 46-51 are in clinical and pre-clinical studies for anticancer activity (Fig. 12) [26, 28].

Y Z

X


NH

N R

34 35 36 37

R = H, X = NH2 R = H, X = NHCH3 R = H, X = N(CH3)2 R = CH3, X = N(CH3)2

X

Fig. (10). Novel anthraquinone analogues for treatment of multidrug resistant tumor cells.

3.5. Novel Platinum Complexes as Potential Anticancer Agents Progress in platinum based anticancer agents has been achieved for the understanding of DNA-binding and pharmacological effects of cisplatin 38. Many compounds with reduced toxicity and high specificity are developed. Future development of medicinal inorganic chemistry requires an understanding of the physiological processing of metal complexes to provide a rational basis for the designing of novel anticancer drugs [27]. Structure-activity relationships for a class of platinum co-ordinated compounds confirmed the need of cis-geometry to block cancer cell growth. The most active complex cisplatin 38 was found to exhibit antitumor activity, whereas its trans isomer has shown no activity [27]. Carboplatin [Pt(C6H6O4)(NH3)2] 39 has fewer toxic side effects than cisplatin and is more easily used in combination therapy. Carboplatin 39 is used more for the ovarian cancer treatment whereas oxaliplatin 43 is effective in colon cancer treatment. Oxaliplatin 43 has been approved for clinical use in Europe and China for colorectal cancer. Strategies for developing new platinum anticancer agents include the incorporation of carrier groups that can target tumor cells with high specificity. BBR3464 45 is in phase II clinical trial and exhibits activity against pancreatic, lung and melanoma cancers (Fig. 11). It is effective against human tumor mouse xenografts containing mutant p53 gene. ZD0473 41 derivative with improved antitumor activity and

O

O C L1 NH O Pt L2 C NH O O

H N O Pt N O H

Malonatoplatinum (II) (DACH) complex 46

Cl H 3C H CH HN Pt N H3CO Cl

CH3 CH OCH3

(trans-[PtCl2 (E-iminoether)2]) 48

O

O

O O Pt N N

Pt (bpy) (cbdca) 50

OH

O Ethylenediamine-malatoplatinum (II) complex 47

Cl

N Pt N

Pt complex with 2-phenylpyridine ligand 49 HN H3C HN

NH Pt Cl SH

N NH

CH3

N Pt complex with acridinyl thiourea ligand 51

Fig. (12). Novel platinum complexes in clinical and pre-clinical trials as anticancer agents.

3.6. Pyrimidine Derivatives as Novel Targeted Potential Anticancer Agents Many synthetic methods for pyrimidine offer high scope in the field of medicinal chemistry for anticancer drug discovery. Pyrimidines are utilized synthon for various biologically active compounds for anticancer activity. As a result of novel research, many literatures are accumulated over the years and chemistry of pyrimidines continue to be interesting for the development of potentially active anticancer agents [29]. Compound 52 was synthesized and evaluated for anticancer activity. It has shown significant antitumor activity (IC50 = 1.25-6.75 μM) on MCF-7 cell. Compound 53 was synthesized and evaluated for in vivo antitumor activity. It was tested at a dose of 35 mg/kg and 10

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O N

N N

HN

O NH

O

S

NH2

F

CF3

HN

NH

O2N

OCH3

HN

O 52

OCH3

OCH3

53

54 NH2

O

NH2 O

N NH

C 2H 5O O

N

N

H2N

N N

R

N

R2 N R1

R3 R1 = CH3, CH2CH3, CH2CH2OH, CH2COOH, CH2CH2NH2, C4H9, CH2CH2CH2NH2 X X = F, Cl, Br R = COOCH3, OCH3 55

R2 = H, CH2OH R3 = 4-CH3, 4-CH2OH, 2-CH2CH3, 2-OCH2CH3 56

Fig. (13). Pyrimidine derivatives as novel potential anticancer agents.

mg/kg of standard compound three times a week by intraperitonal route against the B16-F10 melanoma implanted in arrthymic male mice [29]. Compound 54 exhibited better in vitro antitumor activity at low concentration (log10 GI50 = -4.7) against the used human tumor cell lines. Pyrido (2,3-d) pyrimidine carboxylate derivative 55 was synthesized and evaluated for cytotoxic activity using HT29 cancer cell line. LC50 of the synthesized pyrimidine derivative was found to be >100 μg/ml for all these cell lines. Derivatives of compound 56 were synthesized as dihydrofolate reductase inhibitor (Fig. 13) [30]. 3.7. Aminonaphthalene Derivative as Starting Material for Synthesis of Anticancer Agents Aminonaphthalene derivative 57 is a key intermediate for the synthesis of the Duocarmycin based prodrug 58 for the selective treatment of cancer. In the development of Duocarmycin based prodrug 58, aminonaphthalene derivate 57 serves as an important building block for the treatment of selective cancer. Due to the need of large amounts of the prodrug 58 in clinical trials, an efficient continuous synthesis of 57 was needed to allow the preparation of 58 in a kilogram scale. Microreactor would open a reasonable way to achieve this aim with good reproducibility of product quality by precise control of reaction conditions in a continuous way. Multi-step synthesis of the aminonaphthalene derivate 57 in a micro-reactor was served as a backbone for the synthesis of novel anticancer agents (Fig. 14) [31]. 3.8. Voreloxin Intercalates Topoisomerase II

DNA

and

Poisons

As a quinolone derivative, voreloxin 59 is a toposiomerase II targeting agent with a unique mechanism of action. A detailed understanding of molecular mechanism of voreloxin in combination with its evolving clinical profile explains understanding of structure-activity relationships to develop safer and more effective topoisomerase II targeted

therapies for the treatment of cancer [32]. Voreloxin 59 is a topoisomerase II poison and induces site selective DNADSB mediated by human topoisomerase IIa and IIb. It induces G2 arrest which is partially dependent on topoisomerase II (Fig. 15). It is a quinolone derivative currently completing phase II clinical trials for platinumresistant ovarian cancer. Voreloxin can cause DNA damage by interfering topoisomerase II function. It exerts highly potent anticancer activity through a mechanism that parallels the activity of the quinolones in bacterial cells namely interaction with DNA and topoisomerase II poisoning [33]. Voreloxin 59 increases intercalation of double stranded DNA and topoisomerase II poisoning to induce site selective DNA DSB in GC rich regions compared to other antibacterials. Based upon both chemical and mechanistic differences, it provides clinical advantages over other topoisomerase II poisons that are currently in use. Continued interrogation of the molecular and cellular activities of voreloxin and other members of this new family of quinolone derivatives promises a potential anticancer therapeutics in anticancer research [33]. H H

Cl NHBoc O OBn 57

N

O OH OH OH OH

O

O N

N

58

Fig. (14). Aminonaphthalene derivative 57 as starting material for the synthesis of novel anticancer agent 58.

3.9. Substituted Quinoline Antibreast Cancer Agents

Derivatives

as

Novel

Potent antibreast cancer agents are derived from substituted quinolines. These quinoline derivatives are readily synthesized in large scale from a sequence of


reactions starting from 4-acetamidoanisole. Effects of the substituted quinolines on cell viability of T47D breast cancer cells using trypan blue exclusion assay were examined for antibreast cancer activity [34]. Various substituted quinolines were synthesized from a tandem Michael addition followed by electrophilic aromatic substitution reaction of substituted aniline with vinyl methyl ketone, reduction of the nitro function and alkylation of the resulting amine moiety. Several of these compounds possess potent anticancer activities against T47D breast cancer cells in nM ranges [34]. The trypan blue exclusion assay for each compound is conducted in triplicate. The results showed the lowest IC50 value (16 ± 3 nM) of quinoline 66 among others. The weak inhibition of quinoline 65 may be due to the oxidation of sulfur of the thiophene moiety during the two day incubation with the T47D breast cancer cells. IC50 values of 60, 61 and 63 were in the nanomolar activity range which was desirable range for pharmacological testing (Fig. 16). The absence of asymmetric center in these molecules alleviates the chiral synthesis and purification of enantiomers for bio-evaluation. Variation of substituents of these lead compounds along with the mechanism of action of their anticancer activity are being studied [35]. O

O OH

HN H3C

N

N

N S

H3CO

N

Naphthyridine core (Quinolone derivative) 59

Fig. (15). Voreloxin as novel quionolone derivatives for potent anticancer activity.


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were synthesized by the reaction of 2-amino-7-chloro-6fluoro benzothiazole with ethyl-2-cyano-3,3-bismethyl thioacrylate in the presence of dimethyl formamide and anhydrous potassium carbonate. The newly synthesized compounds were evaluated for their in vitro anticancer activity towards human cancer cell lines derived from various cancer types [36]. Compound 71 exhibited remarkable inhibitory effects against all the cell lines studied such as MCF-7, HeLa, A549, B16 and Hepg2. The parent compounds 67 and 70 were also exhibited anticancer activity against Hepg2 (Fig. 17). Fused benzothaizoles demonstrated cytotoxic properties justifying further investigation as the potential anticancer agents and may be used as a basis for the design of novel potential anticancer agents [37]. X

Y

Y Cl

N

S

SCH3

N

F

CN

Cl

N

S N

F

O

X CN

O

67

68 R = COOC2H5

COCH3

69 R = COOC2H5

CN

70 R = COCH3

COCH3

71 R = CN

CN

Fig. (17). Benzothiazole and its 2-substituted derivatives as potential anticancer agents.

Substituted 2-phenyl benzothiazole and 1,3benzothiazole-2-yl-4-carbothiaote derivatives were synthesized. Substituted 2-phenyl-benzothiazoles were synthesized by condensing substituted benzoic acid with 2amino thiophenol in the presence of phosphoric acid. 1,3Benzothiazole-2-yl-4-substituted carbothiaote derivatives were synthesized by condensing 2-mercaptobenzothiazole with substituted acidchlorides which were screened for anticancer activity. It was also found that compounds 72, 73, 76 and 78 showed highly potent anticancer activity whereas other compounds showed mild to moderate anticancer activity (Fig. 18) [37].

RHN N

R1

OCH3 CF3

R3

O

73 R1 = F, R2 = H, R3 = Cl, R4 = H

S

CH3 60 R = CH2CH2CH2NH2

72 R1 = H, R2 = H, R3 = NH2, R4 = H

R2

N

R4

65 R =

H 2C

S

74 R1 = H, R2 = H, R3 = Cl, R4 = H

O R

N

61 R = CH2CH2CN

S

76 R = NH2

S

62 R = CH2CH2C(=NH)OCH2CH3

66 R =

H 2C

O

63 R = CH2CH2C(=NH)NH2 H H2C N 64 R = N

R1

75 R = NO2

R2 77 R1 = H, R2 = H, R4 = H

N N(CH2CH2Cl)2 S R4

78 R1 = H, R2 = NH2, R4 = NH2

Fig. (16). Substituted quinoline derivatives as novel antibreast cancer agents.

Fig. (18). Substituted 2-phenyl benzothiazole and 1,3benzothiazole-2-yl-4-carbothiaote derivatives as novel anticancer agents.

3.10. Benzothiazole and 2-Substituted Derivatives as Potential Anticancer Agents

3.11. Amide and Urea Derivatives of Benzothiazole as Raf-1 Inhibitor for Treatment of Cancer

9-Chloro-3-cyano-8-fluoro-2-methylthio-4-oxo-4H-pyrimido [2,1-b] [1,3] benzothiazole and its 2-substituted derivatives

Amide and urea derivatives of benzothiazole have been synthesized and evaluated for their antiproliferative profile

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Barot et al.

in human SK-Hep-1 (liver), MDA-MB-231 (breast) and NUGC-3 (gastric) cell lines. Compounds 79 and 80 shown highly potent to moderate inhibitory activities among them (Fig. 19). These compounds were also investigated for their ability to inhibit Raf-1 activity. Raf was the first identified and most characterized downstream effector kinase of Ras [37]. There are three members of the Raf family of kinases, Raf-1 (C-raf), A-raf and B-raf. The structure comprises benzothiazole part, linker and phenyl ring. Depending upon the nature of linker group between benzothiazole and phenyl ring, compounds bear amide linkages and urea linkage. The role of various substitutions on benzothiazole and phenyl ring has been investigated for potent anticancer activity [38].

presence of bromine and chloroform yield thiourea and their 2-aminobenzothiazole derivatives. These compounds were evaluated for in vitro cytotoxicity against Mouse Ehrlich Ascites Carcinoma (MEAC) and two human cancer cell lines (MCF-7 and HeLa). In preliminary MTT [3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] cytotoxicity studies, optically active thiourea derivatives 83, 84 and 85 were found most effective [39]. In EAC cells IC50 values for 83, 84, 85 and 86 (Fig. 21.) were found in the range of 10-24 M, whereas in MCF-7 and HeLa cells the IC50 values were observed in the range of 15-30 M and 3348 M respectively [40].

OCH3 O H3C

N

S H C HN R C NH R'

O

N NH

NH S

S CH3 OCH3

79

NO2

80

3.12. Preliminary Anticancer Activity of Prop-2eneamido, Thiazole and 1-Acetylpyrazolin Derivatives of Aminobenzothiazoles 2-Aminobenzothiazole fused with prop-2-eneamido, 1acetyl-pyrazolin and thiazole moieties were evaluated for their anticancer activity against 60 different human tumor cell lines which were derived from nine neoplastic cancer types. Compound 81 (Fig. 20) was found to be active with selective influence on renal cancer cell lines especially on RXF 393 with a growth percentage of -71.40. It was also active with non-small cancer cell lung cancer cell lines. The obtained results prove the necessity for further investigations to clarify the features underlying the antitumor potential of tested compounds [39]. O NH

H N

N

S N

Cl N N

S

CH3 81

O

Cl Cl

F

Cl

O F

85 F

83 R =

Cl R' = -CH3

84 R =

R' = -CH3

S

R'

H

C R NH

N 86 R = -CH2CH2CH2CH2CH3 R' = -CH3 Cl

Fig. (19). Amide and urea derivatives of benzothiazole as Raf-1 inhibitor for cancer treatment.

H3CO

S HN C HN

Cl

82

Fig. (20). Prop-2-eneamido, thiazole and 1-acetylpyrazolin derivatives of aminobenzothiazoles as novel potential anticancer agents.

3.13. Thiourea and 2-Aminobenzothiazole Derivatives as Novel Anticancer Agents 2-Aminobenzothiazole derivatives were synthesized by reaction of optically active amine with thiophosgene to obtained optically active isothiocyanates which on condensation with 4-fluoro-3-chloro aniline yield various optically active thioureas. Further oxidative cyclisation in the

Fig. (21). Thiourea and their 2-aminobenzothiazole derivatives as anticancer agents.

4. ANTICANCER ACTIVITY OF HETEROCYCLIC COMPOUNDS 4.1. Acridine NSC 601316/DACA 87 is in clinical trials as an potent anticancer agent which is a DNA intercalating agent and dual topoisomerase I/II inhibitor (Fig. 22). Substitutions in the acridine ring of DACA have significant effects on biological activity. 7-Substituted DACA analogs have cytotoxicities similar to DACA whereas 5-substituted derivatives are more cytotoxic but relatively less effective against JLA and JLD cell lines than the wild type JLC. 5,7-Disubstituted analogs of DACA retain both the broad spectrum effectiveness of the 7-substituted derivatives and the higher cytotoxic potency of the 5-substituted derivatives [41]. 4.2. Benzimidazole A number of 5-substituted terbenzimidazoles were synthesized and evaluated as mammalian topoisomerase-I poisons and for cytotoxicity against a human lymphoblastoma cell line such as RPMI-8402 in which 5-chloro derivative 88 exhibited potent anticancer activity (Fig. 22) [42]. 4.3. Benzothiazole Cytotoxic activity of 2-[(substituted-1,3-benzothiazole-2yl)aminomethyl]-5,8-dimethoxy-1,4-naphthoquinones 89 was evaluated against SNU-1 cancer cells (Fig. 22). It was observed that the compounds with highly hydrophobic 6substituents will be more active for anticancer activity [43]. 4.4. Camptothecin Cytotoxic activity of camptothecin was discovered with a novel mechanism of action involving the nuclear enzyme

Novel Anticancer Agents and Targets: Recent Advances and Future Perspectives N

O N

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N N H

N HN HN

N

O

NH2

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99

9

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O


100

O

N 101

Fig. (22). Acridine, benzimidazole, benzothiazole, camptothecin, indole, isatin, isoquinoline and phenanthredine derivatives as novel anticancer agent.

topoisomerase-I. This discovery of unique mechanism involves camptothecin and its analogs as anticancer agents. Camptothecin derivative 90 was found to be highly active against SKOV-3 human ovarian cancer cells (Fig. 22) [44]. 4.5. Indole Indolylpyrimidine 91 and indolylpyrazine 92 were synthesized and evaluated for their cytotoxicities against a panel of 60 human tumor cell lines. QSAR results suggested that the most important determinant for the cytotoxic activities of these compounds against different cancer cell lines is the hydrophobic parameters of the whole molecules and ClogP models suggested that the highly hydrophobic molecules will be more active for potent anticancer activity (Fig. 22) [45]. 4.6. Isatin Isatin (1H-indole-2,3-dione) class of compounds exhibited many biological activities. A number of isatins were synthesized. Out of which compound 93 exhibited potent in vitro cytotoxic activities against the human monocyte like histiocytic lymphoma (U937) cell line (Fig. 22) [46]. 4.7. Isoquinoline A series of (4, 8, 9, 10 or 11)-substituted-2[2-(dimethylamino)ethyl]-1,2-dihydro-3H-dibenz[de,h] isoquinoline-1,3-diones 94 and 3-arylisoquinoline 95 exhibited excellent cytotoxic activities against SK-MEL-2 melanoma cancer cell lines (Fig. 22) [47]. 4.8. Phenanthredine Esters and amides of 2,3-dimethoxy-8,9-methylenedioxybenzo [i] phenanthridine-12- carboxylic acid 96 (Fig. 22) were synthesized as potent cytotoxic and DNA topoisomerase-I targeting agent for highly potent anticancer activity [48]. 4.9. Pyrrolo[2,3-d] pyrimidines Development issue was observed for IGF-1R inhibitor 97 with IC50 of 2.0 nM whereas an acid mediated cyclization of the pyrimidine moiety onto the pendant carboxamide lead to facile hydrolysis in vitro and in vivo. Improvements in both inhibitors were realized via substitution at C(4) with carboxamide containing 5-membered heteroaryl amines 98 (Fig. 22) with IC50 of 1.6 nM and constrained lactam 99 with IC50 of 0.8 nM or indolines 100 with IC50 of 0.5 nM (Fig. 23) [49].

4.10. Pyrrole Pyrrole moiety is incorporated into several nonnucleoside reverse transcriptase inhibitors and antiproliferative agents as well as the DNA minor groove binders Distamycin A and Tallimustine and in vitro anticancer activity of diazopyrroles and triazenopyrroles was reported. A number of agents of this class are under development by research groups throughout the world. Nphenyl-3-pyrrolecarbothioamide 101 (Fig. 23) was synthesized and exhibited potent cytotoxic activities against melanoma cell lines [50]. 5. NOVEL ANTICANCER TARGETS 5.1. Matrix Metalloproteinases (MMPs) Mammal MMPs gene family consists of at least 26 structurally related members. Among which MMP-2 and MMP-9 are proved to be highly correlated with cancer [51]. Compounds currently under clinical trials as Matrix metalloproteinase inhibitors (MMPIs) include marimastat 102, tanomastat (Bay 129566) 103, prinomastat (AG3340) 104 and CGS27023A 105. All these compounds are applied to treat different types of cancer such as ovarian cancer, breast cancer, malignant glioma, pancreatic cancer, NSCLC, advanced bladder carcinoma etc. Positive results are observed in gastric cancer with marimastat 102. The structures of pyrrolidine peptidomimetic inhibitors 106 and 107 have shown high inhibitory activity against MMP-2 and MMP-9 with IC50 of 11.5 nM and 7.7 nsM respectively (Fig. 24). In vivo, all these derivatives display favorable inhibitory potency to metastasis of tumor cells with the metastasis inhibition rate of higher than 92% in H22 mouse liver carcinoma model. It indicates the strategy to design MMPIs of praline analogs is a remarkable success for development of novel anticancer agents [52]. 5.2. Tyrosine Kinase (TK) Tyrosine kinase (TK) is activated and it phosphorylates the C terminal tyrosine residues. The process is named as autophosphorylation. After that a subsequent phosphorylating activation process occurs, called a "kinases cascade" which results in the amplification of the signal. During this process, a number of proteins are phosphorylated which lead to a subsequent cellular event such as proliferation, division, adhesion, morphogenesis, angiogenesis, metastasis and antiapoptosis of cells [53]. TK plays a crucial role in the action of Epidermal Growth Factor Receptor (EGFR). EGFR is a member of the ErB family of receptors with intrinsic protein tyrosine kinase activity. Over expression

10 Mini-Reviews in Medicinal Chemistry, 2013, Vol. 13, No. 9

Barot et al. O O

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Fig. (23). Pyrrolo [2, 3-d] pyrimidines and pyrrole derivatives as anticancer agent. Cl O HO

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104 IC50 - 0.04 nM

103 IC50 - 11 nM

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102 IC50 - 2 nM

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105 IC50 - 1.9 nM

106 IC50 - 11.5 nM

107 IC50 - 7.7 nM

OH

Fig. (24). Matrix Metalloproteinases (MMPs) as novel target for anticancer agents.

of EGFR could result in the transformation of cultured cells and be associated with a number of cancer onset and progression. EGFR is the cell-surface receptor for members of the epidermal growth factor family (EGF-family) of extracellular protein ligands. EGFR is a member of the ErbB

family of receptors, a subfamily of four closely related receptor tyrosine kinases EGFR (ErbB-1), HER2/c-neu (ErbB-2), Her 3 (ErbB-3) and Her 4 (ErbB-4). Mutations affecting EGFR expression or activity could result in cancer. EGFR and its receptor were discovered by Stanley Cohen of


Vanderbilt University. Cohen shared the 1986 Nobel Prize in Medicine with Rita Levi-Montalcini for their discovery of growth factors. Mutation of the ATP-binding site of EGFR is closely associated with TK activity disrupting the formation of tumorigenic signals. Inhibition of TK will result in the suppression of cell activity related to EGFR which provides a new strategy for the treatment of cancer. The enzyme inhibitors 108 and 109 are in clinical trial to tyrosine kinase for potent anticancer activity (Fig. 25) [54].

O

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angiogenesis of tumor tissues and degrading bio-active peptides such as interleukin, cytokines and immunoactive substance [56, 57]. Inhibitors targeting to APN are bestatins 112 have been marketed for a long time. Probestin 113 is designed on the basis of proline which exists in collagen. AHPA (3-amino-2-hydroxy-4-phenyl butyric acid) scaffold derivatives 114 and 115 have been synthesized and found to have highly selective activity (IC50) of 16-200 nM against APN. Bestatin 112 can inhibit the invasion of human metastatic tumor cells and induce apoptosis in non small lung cancer cell lines. Phase-III clinical trial of bestatin in patients with completely resected stage-I squamous cell lung carcinoma has been completed in 2003 (Fig. 27) [58].

N

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N

OH

NH2 O

108

109

Fig. (25). Tyrosine kinase (TK) as novel target for anticancer agents.

OH

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112

5.3. Farnesyltransferase (FTase) A phase II clinical trial is conducted by the M. D. Anderson Cancer Center evaluating the efficiency of SCH66336 110 with a combination of temozolomide for recurrent glioblastoma multiform (GBM). Three human GBM xenografts demonstrated substantial growth inhibition in response to SCH66336 110 with up to 69% growth inhibition after 21 days of treatment [55]. Zarnestra 111 is also known as tipifarnib or R115777, a nonpeptidomimetic methyl quinolone derivative, can inhibit Ras FTase selectively. Recently, two clinical trials are underway: one is under phase II for breast cancer and renal carcinoma and other is under phase II/III for pancreatic carcinoma. The main dose limiting toxicities have been reported are myelosuppression, fatigue and neurotoxicity with R115777 111. Two other phase III trials of R115777 111 in colorectal and pancreatic cancers have failed to show a survival benefit (Fig. 26) [56].

N H

N

O O

OH NH

O

H N

S

HN O

N

NH

O

O HN OH

114

115

Fig. (27). Aminopeptidase N (APN) as novel target for anticancer agents.

5.5. Histone Deacetylase (HDACs) A class of compounds that suppresses the activity of HDACs to gene expression provides a novel target for the treatment of cancer identified as HDAC inhibitors. It includes electrophilic ketone derivative SAHA 116 (Fig. 28). Phase-I clinical trial was performed on 116 to evaluate the safety, pharmacokinetics and biological activity. It has been concluded that SAHA 116 is well tolerated, inhibits the biological target in vivo and has antitumor activity in solid and hematological tumors [59].

Cl Cl Br

NH2

N

H Rb O N

N

O

H N O Cl

N NH2

OH

116

Fig. (28). Histone Deacetylase (HDACs) as novel target for anticancer agents.

N

O

N H

O 110

111

Fig. (26). Farnesyltransferase (FTase) as novel target for anticancer agents.

5.4. Aminopeptidase N (APN) Aminopeptidase N plays a critical role in tumorigenesis by degrading extra cellular matrix, promoting growth and metastasis of primary tumor. It also participates in the

5.6. Cycloxygenase-2 (COX-2) In cancer cells, the gene expressing CUGBP-2 is closed and results in the enhancement of COX-2 activity by promoting the gene expression associated with angiogenesis. As a result, COX-2 becomes a potential target for the prevention of tumor [58, 60]. Number of COX-2 inhibitors have been evaluated in all kinds of clinical trials for the treatment of cancer including celecoxib 117, rofecoxib 118 and NS-398 119 (Fig. 29). Among which 117 is combined


with isotreinoin for the treatment of recurrent and deteriorated malignant neuroglioma in a phase-II clinical trial. Rofecoxib induces growth arrest and apoptosis of HCA-7 cells only at concentrations significantly higher than the IC50 for COX-2 inhibition. Rofecoxib 118 may negatively regulate angiogenesis in human CRC liver metastases [61]. NH2 O S O O2N

F

F

O

O S O

N N F

O HN S O

O 119

118

117

Fig. (29). Cycloxygenase-2 (COX-2) as novel target for anticancer agents.

5.7. Endothelin Receptor (ETR) It is described that endothelin receptors (ETR) are expressed on cancer cells. The synthesis of DNA and active pro-factor of angiogenesis are induced when ET and ETR combines with each other [62]. Atrasentan/ABT-627 120 is a pyridoline-3-carbonyl derivative which can inhibit the ET-A receptor selectively (Fig. 30). A phase-I clinical trial for the treatment of gland carcinoma includes the primary adverse effects of rhinitis, headache, fatigue and edema. Several other clinical trials from phase-I to phase-III have been reported in recent years for the treatment of refractory prostate cancer [63]. O

OH

N

N O

O

O 120

O

Fig. (30). Endothelin Receptor (ETR) as novel target for anticancer agents.

5.8. Other Anticancer Targets Integrins are the family of heterodimeric cell surface receptors which mediate the cell adhesion to the extracellular matrix or other cells and play important role in cell signaling. Integrins as the target for anticancer therapy has obtained much attention recently. A comparison of normal and neoplastic human prostate tissues showed a downregulation of a specific variant of the 1 integrin subunit, and strong evidence shows that reduced expression of 6 and 4 may contribute to the higher tumorigenicity of androgenindependent prostate tumor cells. In a study of metastatic

Barot et al.

melanoma, longer disease free survival and overall survival correlated with 1 expression, while neuroblastoma aggressiveness was correlated with expression of integrin v3 and v5 by the microvascular endothelium [63]. Studies of acute lymphoblastic leukemia showed that 2 expression was significantly associated with splenomegaly, and expression of 51 was associated with positive response to chemotherapy in patients with rectal cancer. Other novel targets and inhibitors for anticancer agents are catching more attention for anticancer drugs discovery and development. It includes (a) Antagonist targeting VEGFR (Vascular Endothelial Growth Factor Receptor). (b) Proteasome antagonist targeting ubiquitin and proteasome systems. (c) Inhibiting substance targeting nuclear transcript factor NF-kB [64]. 6. PLANTS AS SOURCE OF NOVEL ANTICANCER AGENTS Plant derived natural agents are used for the treatment of cancer. Several anticancer agents including taxol, vinblastine, vincristine, camptothecin derivatives, topotecan, irinotecan and etoposide derived from epipodophyllotoxin are in clinical use worldwide. A number of promising agents such as flavopiridol, roscovitine, combretastatin A, betulinic acid and silvestrol are in clinical as well as pre-clinical trials [65]. Natural products or natural product derivatives were comprised within 14 of the top 35 drugs as per worldwide sales in year 2000 [66]. Paclitaxel and camptothecin were estimated to account for nearly one-third of the global anticancer market or about $3 billion of $9 billion in total annually in 2002. It is anticipated that plants can provide potential bio-active compounds for the development of novel potential anticancer agents [65, 67]. Several natural agents are currently in clinical and preclinical trials or undergoing further investigation as anticancer agents. Flavopiridol 121 is a synthetic flavone derived from the plant alkaloid rohitukine which was isolated from Dysoxylum binectariferum Hook. (Meliaceae). It is currently in phase I and phase II clinical trials against a broad range of tumors including leukemia, lymphomas and solid tumors [68]. Roscovitine 122 is derived from natural product olomucine which is originally isolated from Raphanus sativus L. (Brassicaceae) which is in phase II clinical trials in Europe [69]. Combretastatins were isolated from the bark of the South African tree Combretum caffrum (Combretaceae). Combretastatin A 123 is active against colon, lung and leukemia cancers and it is expected that this molecule is the most cytotoxic phytomolecule isolated so far [70]. Betulinic acid 124 is a pentacyclic triterpene which was isolated from Zizyphus species, e.g. Mauritiana, Rugosa and Oenoplia. It has displayed selective cytotoxicity against human melanoma cell lines. The development of systemic and topical formulations of betulinic acid for potential clinical trials by the NCI is under process [71]. Pervilleine A 125 was isolated from the roots of Erythroxylum pervillei Baill (Erythroxylaceae). It is selectively cytotoxic against a multi-drug resistant oral epidermoid cancer cell line (KBV1) in the presence of the anticancer agent vinblastine which is currently in pre-clinical development [72]. Silvestrol 126 was first isolated from the fruits of Aglaila sylvestre (Meliaceae). It has exhibited cytotoxicity against lung and



CH3 N

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127

OH

128

Fig. (31). Plant derived agents for future development of potential anticancer agents.

breast cancer cell lines. Biological studies have been carried out to determine the mechanism of action for silvestrol [73]. Schischkinnin 127 and montamine 128 were isolated from the seeds of Centaurea schischkinii and Centaurea montana. Both of the alkaloids have exhibited significant cytotoxicity against human colon cancer cell lines. The unique structural features of 127 and 128 can be exploited as a template for generating compounds with enhanced anticancer activity (Fig. 31) [71, 74]. 6.1. Novel Strategies for Plant Derived Anticancer Agents Drug discovery from the medicinal plants has played an important role in the treatment of cancer. It has also some new clinical applications of plant secondary metabolites and their derivatives over the last half century cancer research [75]. Betulinic acid 129 is a pentacyclic triterpene which is a common secondary metabolite of plants. It was primarily isolated from Betula species (Betulaceae) (Fig. 32). The ethyl acetate soluble extract displayed selective cytotoxicity against human melanoma cells (MEL-2). It was found to be selectively cytotoxic against several human melanoma cancer cell lines (MEL-1 half maximal effective dose (ED50)= 1.1 g/ml, MEL-2 ED50=2.0 g/ml, and MEL-4 ED50=4.8 g/ ml). It was found to be active in vivo using athymic mice carrying human melanomas with little toxicity. Further biological studies indicated that betulinic acid works by induction of apoptosis [74, 76]. Pervilleine A 130 was isolated from the roots of Erythroxylum pervillei Baill. (Erythroxylaceae) (Fig. 32). The chloroform soluble extract was found to be selectively

cytotoxic against a multi-drug resistant (MDR) oral epidermoid cancer cell line (KB-V1) in the presence of vinblastine [76]. The pervilleines were isolated using bioassay guided fractionation including silica gel chromatography and aluminum oxide chromatography. Pervilleine A was found to be selectively cytotoxic against KB-V1 in the presence of vinblastine (ED50=0.3 g/ml). Pervilleine A may be more effective than verapamil for reversing MDR. Further in vivo evaluation including testing in a xenograft mouse model for MDR is under process [77]. Silvestrol 131 was first isolated from the fruits of Aglaia sylvestris (Meliaceae). The chloroform soluble extract was found to be cytotoxic to several human cancer cell lines and extract was active in P-388 in vivo test system (Fig. 32). It was cytotoxic against lung (Lu1, ED50=1.2 nM), prostate (LNCaP, ED50=1.5 nM) and breast (MCF-7, ED50=1.5 nM) cancer cells as well as against umbilical vein endothelial cells (HUVEC, ED50=4.6 nM). It was tested as in vivo hollow fiber bioassay and exhibited dose-dependent cytotoxicity with no significant weight loss. It was also shown activity at a maximum tolerated dose of 2.5 mg/kg/injection when administered intraperitoneally twice daily for 5 days in the P-388 murine leukemia model. Biological studies are ongoing to determine the mechanism of action for silvestrol. It is subjected to further studies and hopefully pre-clinical development [78]. 6.2. Drug Discovery for Cancer Chemoprevention Cancer chemoprevention is defined as ‘‘a strategy of cancer control by administration of synthetic or natural compounds to reverse or suppress the process of


Barot et al. HO H 3C N O

H COOH

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129

O

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131

Fig. (32). Novel plant derived anticancer agents currently undergoing further investigation.

carcinogenesis’’. Several of these compounds are resveratrol 132, ixocarpalactone A 133, isoliquiritigenin 134 and four other flavonoids from Broussonetia papyrifera (Urticaceae) 135-138 (Fig. 33) [79]. Resveratrol 132 was isolated from Cassia quinquangulata (Caesalpiniaceae). The ethyl acetate soluble extract was found to inhibit the cyclooxygenase-1 (COX-1) enzyme (88% inhibition at 69 g/ml) and was subjected to bio-assay-guided fractionation. It was found to inhibit COX-1 with an ED50 of 15 M and had no activity on COX-2 which indicates the selectivity of the compound. It was found to inhibit the development of DMBA-induced preneoplastic lesions in a MMOC model of carcinogenesis. It was tested in a two stage full term mouse model and was found to inhibit tumorigenesis. Phase-I clinical trials are currently under process to determine the ability of resveratrol to prevent cancer in healthy volunteers [77, 80].

metabolism of chemical carcinogens. Bioassay guided fractionation lead to the isolation of ixocarpalactone A as well as numerous other isolates. Ixocarpalactone A induced QR with a concentration required to double activity (CD) of 0.32 M which can inhibit 50% cell growth (IC50) of 7.54 M and a chemopreventive index (CI=IC50/CD) of 24 [81]. Isoliquiritigenin 134 was isolated from the seeds of Dipteryx odorata. (Fabaceae) which is a botanical dietary supplement commonly known as tonka bean. The ethyl acetate-soluble extract was active in the QR bio-assay and was subjected to bio-assay guided fractionation. It was isolated as an active component with a value of 3.8 M, IC50 of 27.3 M and a CI of 7.2. It was then tested at 10 g/ml in the MMOC in vivo bioassay. It was found to be active in inhibiting the induction of 76% of lesions. It was also found to be worthy of further biological testing [82].

Ixocarpalactone A 133 was isolated from the edible plant Physalis philadelphica (Solanaceae) and commonly known as tomatillo. An ethyl acetate soluble extract of the leaves and stems was found to induce the quinine reductase (QR) enzyme which is a phase II enzyme responsible for

Four potent aromatase inhibitors (2S)-abyssinone II 135, 3'-[-hydroxymethyl-(E)- methylallyl] 2,4,2',4'tetrahydroxychalcone 11'-O-coumarate 136, (2S)-2',4'dihydroxy-2''-(1- hydroxy-1-methylethyl)dihydrofuro[2,3h]flavanone 137 and isolicoflavonol 138 were isolated from O

HO H

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133 OH

O

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132

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OH

HO O

O

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OH OH

HO 136

OH

HO

OH

O 137

Fig. (33). Highly active chemopreventive compounds from medicinal plants.

HO 138

OH


the edible plant Broussonetia papyrifera Vent. (Urticaceae) [83]. The ethyl acetate soluble extract of this plant inhibits the enzyme aromatase which is the rate limiting enzyme in the production of estrogen. The inhibition of aromatase in post-menopausal women has been found to reduce the recurrence of breast cancer [84]. Compounds 135-138 were isolated using bioassay-guided fractionation and found to inhibit aromatase (135: IC50=0.4 M, 136: IC50=0.5 M, 137: IC50=0.1 M, 138: IC50=0.1 M). These four compounds are currently being developed under the Rapid Access to Preventive Intervention Development (RAPID) program through NCI [84]. 7. CHEMOGENOMICS Chemogenomics represents a new approach to target the identification and drug development with the potential for accelerating the process of new drug discovery. Chemogenomics combines the latest tools of genomics and chemistry. Chemogenomics together with chemoinformatics that generate knowledge through data integration will concurrently identify and validate therapeutic targets and detect drug candidates to rapidly and effectively generate new treatments for many human diseases [85]. Chemogenomics strategies are increasingly being harnessed by various fields of medical research, especially those related to cancer or immune, inflammatory and hormone disease, in attempts to develop new targeted therapies as rapidly as possible such as HDACs, Methionine Aminopeptidase Type-2 (MetAP2), HSP90 and Ras-Raf [86]. Chemogenomics is used to identify new drug targets and might allow their biological functions to be understood. The application of chemogenomics to cancer and angiogenesis research has identified HSP90 molecular chaperone as targets of radicicol as an antifungal antibiotic that has antitumor and antineoplastic activity [87]. Secondly, chemogenomics approaches are applied to discover in a highthroughput fashion, new chemical candidates for molecular targets and phenotypes of interest [88]. Predictive chemogenomics strategies have been extensively used in cancer research to determine the relationship between genes, compounds and phenotypes. Using this approach, it has been possible to generate integrated databases of compound–gene interactions by experimental profiling. Metalloproteinases represented by MMP and APN have the HEXXH conserved motif that can interact with the critical zinc ion in the active site. As a result, anticancer drugs design strategy can resort to design small molecules containing zinc-binding group (ZBG), thereby targeting all metalloproteinases associated with various diseases [88, 89]. Furthermore, families of related proteins can be considered together as potential drug targets rather than any single member with the aid of chemogenomics multiple functions and drugs targeting to a gene family. In which each protein having different biological and potential therapeutic function will be designed for the treatment of cancer [90].


Recent developments lead to the invention of drugs with novel mechanism of action that are highly specific to cancer cells. Even the initial stage of drug development in a multidisciplinary efforts by chemists, biologists and biochemists can be used for discovery of novel anticancer agents. We have provided the evidence that specific chemical modifications of conventional drugs can render them active in circumventing drug resistant phenotypes in human cancer cells. Although partial resolvement of the entire problem gives an idea for further efforts to increase the therapeutic potential of anticancer agents. CONFLICT OF INTEREST The authors confirm that this article content has no conflicts of interest. ACKNOWLEDGEMENTS Authors would like to thank Prof. Manjunath Ghate (Director, Institute of Pharmacy, Nirma University, Ahmedabad, Gujarat, India) for continuous support and critical review of the manuscript. We are also thankful to Department of Science & Technology (DST), Govt. of India for providing INSPIRE Fellowship as financial support. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

[16] [17] [18] [19] [20]

8. CONCLUSION The design of novel anticancer drugs active against cancer is one of the most difficult problems which can be achieved by the specific application of chemical principles.

15

[21] [22] [23]

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Revised: March 18, 2013

Accepted: March 28, 2013