Benzothiazole derivatives as anticancer agents

Abstract Benzothiazole (BTA) belongs to the heterocyclic class of bicyclic compounds. BTA derivatives possesses broad spectrum biological activities such as anticancer, antioxidant, anti-inflammatory, anti-tumour, antiviral, antibacterial, anti-proliferative, anti-diabetic, anti-convulsant, analgesic, anti-tubercular, antimalarial, anti-leishmanial, anti-histaminic and anti-fungal among others. The BTA scaffolds showed a crucial role in the inhibition of the metalloenzyme carbonic anhydrase (CA). In this review an extensive literature survey over the last decade discloses the role of BTA derivatives mainly as anticancer agents. Such compounds are effective against various types of cancer cell lines through a multitude of mechanisms, some of which are poorly studied or understood. The inhibition of tumour associated CAs by BTA derivatives is on the other hand better investigated and such compounds may serve as anticancer leads for the development of agents effective against hypoxic tumours.


Introduction
Heterocyles are important pharmcophores and have significance to create privileged chemical structures possessing pharmacological activities. Five membered heterocyclic which incorporate oxygen, nitrogen and sulphur are found in broad spectrum therapeutic agents which have an enormous significance in drug discovery and drug development processes 1 . Benzothaizole (BTA) is a fused benzoheterocyle which is present in many naturally occurring products and is responsible for the medicinal, pharmacological and pharmaceutical applications of such natural products 2 . BTA is present in terrestrial as well as marine compounds which exhibit various biological activities 3 . The BTA nucleus is formed by the fusion of the thiazole ring with a benzene ring 4 .
The pharmacological profile of the drug used for the management of amyotrophic lateral sclerosis Riluzole (Figure 1) attracted the attention of medicinal chemists towards biologically active benzothiazole 5 .
Cancer is the most prominent, notably complex and lethal disease which became a serious concern of today's medical science. It poses a great challenge to medical scientific community for development of drugs, medicines and procedures for safer treatment and cure of cancer disease 20 . These neoplasm tumour cells are diversified, heterogeneous cells with rapid proliferative properties. These neoplasm malignant tumours, have potential to invade or spread to other parts of body through blood stream and lymphatic system 21 . The plethora of research mentioned in the present review of last decade on anticancer potential of BTA derivatives will be helpful in future drug discovery and drug development for the treatment of lethal cancer disease. afforded the N-bis-benzothiazole and benzothiazolyl thiocarbamide derivatives and screened for cytotoxic activities against two human cell lines U-937 (human macrophage cell line), THP-1 (human leukaemia monocytic cell line) and B16-F10 (mouse melanoma cell line). The thiourea containing benzothiazole derivative 3 ( Figure 3) demonstrated the best antiproliferative activity against the U-937 cell line as compared to standard drug Etoposide. The IC 50 values of compound 3 were higher (16.23 ± 0.81 mM)), (4847.73 ± 2.39 mM)) and (34.58 ± 1.73 mM)) as compared to standard compound etoposide IC 50 values (17.94 ± 0.89), (18.69 ± 0.94) and (2.16 ± 0.11 mM)) against U-937, B16-F10 and THP-1 cell lines respectively 22 .

BTA derivatives as anticancer agents
Kumbhare et al. reported the synthesis of mannich base arylimidazo derivatives containing benzothiazole moiety and screened for their anticancer activities against HepG2, MCF -7 and HeLa cell lines. All these synthesised mannich bases BTA scaffolds showed cytotoxicity against all tested cell lines but the pyrrolidine based imidazo benzothiazole derivative 4 ( Figure 3) demonstrated specific features of apoptosis as enhancement in the levels of caspase-3. The compound 4 exhibited anti cancer activity and proved to be the best antiproliferative agent as compared to other derivatives against HepG2, MCF-7 and HeLa cell line when screened at 4.0 mM concentrations. The SAR studies revealed that the incorporation of fluorine atom at the 7 th position of derivative 4 enhanced the cytotoxicity. The compound 4 have potential to lead in the treatment of cancer especially against hepatocaricinoma. The anticancer activity potential of BTA scaffold 4 is encourging for the development of new anti-cancer therapeutic agents and this will be good addition in armamentarium that consists of paclitaxel, cisplatin and doxorubicin drugs 23 .
Caputo et al. afforded two types of five derivatives on the basis of an aryl amide and an aryl urea functionalities attached at C-2 of benzothiazole core and these scaffolds were screened against 60 human cancer cell lines. The urea moiety based fluorophenyl containing benzothiazole derivative 4 ( Figure 3) and cyanophenyl containing benzothiazole derivative 5 ( Figure 4) demonstrated remarkable anticancer activities. The BTA derivative 4 exhibited the anticancer activity at 10 À5 M against different cell lines such as leukaemia cell lines (log GI 50       (log GI 50 value À6.0) as compared with reference drug 5-fluorouracil NSC 19893. The scaffolds 4 and 5 showed the best anticancer therapeutic potential due to presence of electron with drawing groups on para position of phenyl ring 23 .
El-Damasy et al. synthesised the novel amide and urea based BTA series of 20 sorafenib analogues in which the pyridylamide privileged functionality was attached with an ether linkage at 6position of the BTA ring. A selected group of 12 potent scaffolds were evaluated and appraised for anti-proliferative activities against sixty human cancer cell lines. These chlorotrifluoromethyl phenyl ureido picolinamide benzothiazoles 7 ( Figure 5), bistrifluoromethyl phenyl ureido based picolinamide benzothiazole derivative 8 ( Figure 5) and dichlorophenyl ureido based picolinamide benzothiazoles 9 ( Figure 5) were more potent in the treatment of renal cell carcinoma than the standard drug sorafenib, used for the treatment of such tumours.

Imidazole based benzothiazole derivatives as anticancer agents
Yurttas et al. obtained 2-(4-aminophenyl)BTA derivatives substituted with different heterocyclic rings and tested their antitumor potential against 60 human tumour cell lines. The BTA derivatives Figure 8) showed remarkable antitumor potential against different   cancer cell lines. The heterocylic substitutions affect the activity and antitumor potential of these BTA derivatives, with derivative 14 having comparable antitumor potential with the standard drugs whereas derivative 13 being less active compared to 14. The order overall antitumor potential of 2-(4-aminophenyl) benzothiazole derivatives with reference to the heterocyclic substitution was benzimidazole ! imidazole > benzothiazole > benzoxazole 28 . Singh et al. reported the synthesis of imidazole based benzothiazoles by treatment of substituted anilines with KSCN which afforded the desired benzothiazole derivatives, and studied their anticancer activities. Compound 15 ( Figure 9) showed excellent anticancer activity possessing IC 50 value 10 mM when compared with the standard drug doxorubicin 29 . Gurdal et al. synthesised BTA derivatives which incorporate piperazine moieties and evaluated their cytotoxicity against different cancer cell lines such as HUH-7 (Heptacellular), MCF-7 (Breast) and HCT-116 (Colorectal). GI 50 values of these derivatives indicated that all compounds exhibited good potential against the aforementioned cell lines but the pyridine containing derivative 18 ( Figure 11) had a remarkable cytotoxic activity with GI 50 value 7.9 mM, 9.2 mM and 3.1 mM for HCT-116, MCF-7 and HUH-7 respectively. Apoptosis caused by this derivative during cell cycle arrest at subG1 phase was confirmed by Hoechst staining and fluorescence activated cell sorting analysis 31 .      Figure  15) and 26 (imidazolinyl-substituted thiophene based BTA; Figure  15) exhibited low cytotoxic effects on normal human fibroblasts and strong antiproliferative effects on MiaPaCa-2 and MCF-7 cancer cell lines. The experimental data indicated that the benzothiazole derivatives having thiophene and imidazole substitions possessed interesting antiproliferative activities 35 .

Thiadiazole based benzothiazole derivatives as anticancer agents
Sekar et al. reported the synthesis and anticancer activities of six noval BTA derivatives. All derivatives exhibited anticancer activities from a high to a moderate activity level with the substituted thiadiazole flourobenzothiazole 27 ( Figure 16) and methoxybenzothiazole 28 (Figure 16) showing the best anti-cancer potential due to the presence of highly electronative and electron denoting pharmacophores (e.g. fluorine and methoxy moieties) 36 .

Pyrazole based benzothiazole derivatives as anticancer agents
Gabr et al. reported the synthesis and evaluation of novel BTA scaffolds against 60 tumour cell lines at a single dose of 10 mM. The best derivatives were 32 ( Figure 19) and 33 (Figure 19), which were further screened at 5 doses. These derivatives demonstrated interesting anticancer activity at micro molar and sub micro molar concentrations, against all sixty tumour cell lines with GI 50 in the low micro molar or submicromolar range. The SAR study revealed that introduction of the pyrazole moiety significantly enhanced the antitumor activity of both derivatives. Furthermore the presence of 2-hydroxy ester and 3-oxopyrazole within the pyrimidine moiety increased the anti-tumour activities of both derivatives. The simple BTA scafolds having pyrazole functionalities were more potent against different cell lines as compared to derivatives having the pyrazole ring within a pyrimidine moiety 39 .

Pyrimidine based benzothiazole derivatives as anticancer agents
Kambhare et al. synthesised the isoxazole pyrimidine based BTAs and evaluated them for anticancer activity by the MTT assay against different cell lines such as A549, Colo205, MCF-7 and U937        derivatives exhibited better anticancer activities as compared to the standard drug cisplatin. The substituted methoxybenzamide benzothiazole 41 and the substituted chloromethylbenzamide benzothiazole 42 (Figure 25) showed good anti-tumour potential in vitro, with IC 50 values ranging from 1.1 mM to 8.8 mM. The introduction of chloromethyl and methoxy functionalities in compounds 41 and 42 increased their anticancer activity compared to other synthesised analogs 46 .
Bolelli et al. synthesised 2-substituted benzothiazoles by the reaction of carboxylic acid with thionyl chloride which afforded acyl chlorides, further treated with substituted benzothiazole to give 2-substituted benzothiazoles, which were studied for their inhibitory activities against human glutathione transferases (hGSTP1-1). The benzamide benzothiazole derivative 43 and benzamide methylbenzothiazole 44 ( Figure 26) showed potent hGSTP1-1 inhibitory activities, useful in cancer chemotherapy. The SAR studies pinpointed that both these scaffold showed effective inhibition due to the presence of para-substitutions on the phenyl ring of the benzamide group 47 .
Corbo et al. reported 19 derivatives of benzamide based BTAs by the reaction of substituted aniline, potassium thiocyante and bromine, which afforded substituted compounds which were tested for anti-proliferative activity against MCF-7 and HepG2 cell lines. The substituted diflourobenzamide containing benzothiazole 45 ( Figure 27) proved to be a potent anti-proliferative compound with the percentual inhibition of 64 ± 2 mM for MCF-7 cell line and of 64 ± 6 mM for HepG2 cell line 48 .        receptors. The A3 receptor is overexpressed in different cancer cell lines. The quinolone based derivative 48 (Figure 29) showed the maximum potency for the hA3 adenosine receptor 50 . The SAR studies investigated that anticancer activities of these compounds were due to the substitutions (methoxy and chloro groups) present in their molecules 51 .

Miscellaneous benzothiazole derivatives as anticancer agents
Noolvi et al. reported the synthesis of chloro substituted benzothiazole amines in order to produce isothiocyanates and thioureases. These BTA derivatives were tested for their anticancer activities. The dichlorophenyl containing chlorobenzothiazole 51 ( Figure 31) showed good anticancer activity against 9 different cancer cell lines having GI 50 values in the range of 1.60 mM-71.8 nM. The derivative 51 exhibited GI 50 ¼ 7.18 Â 10 À8 M against non-small cell lung cancer (HOP-92). The SAR studies showed that the highest activity of compound 51 was due to the presence of three chlorine atoms in the compound as compared to other derivatives 52 .
Havrylyuk et al. screened novel 4-thiazolidinone benzothiazole derivatives against ovarian, renal, prostate, leukaemia, melanoma, lung, colon, CNS and breast cancer cell lines. The thioxothiazolidine acetamide benzothiazole 52 (Figure 32) showed the most         The SAR studies showed that the presence of a phenyl group on the thiazolidine part of the structure increased the selectivity while substitution with an electron withdrawing or donating groups decreased the selectivity 66 .
Benzothiazole derivatives with carbonic anhydrase inhibitory and antitumor action Carbonic anhydrases (CAs, EC 4.2.1.1) are zinc enzymes which catalyse the reversible interconversion between CO 2 and bicarbonate [67][68][69][70][71] . CO 2 is efficiently hydrated through a zinc hydroxide intermediate from the enzyme active site with generation of the weak base bicarbonate and the strong acid H þ . As a consequence, CAs are involved in pH regulation, electrolyte secretion and metabolism, in normal and tumour tissues [70][71][72] . Fifteen a-CA isoforms are present in humans, with at least two of tyhem overexporessed in hypoxic tumours (CA IX and XII) 67,71,72 , as a consequence of the hypoxia inducible factor (HIF-1a) transcription factor cascade activation 67,70 . CAs are efficiently inhibited by a range of compounds, such as the sulphonamides and their isosteres [73][74][75][76] , and inorganic anions 77 , which constitute the main zinc-binding CA inhibitor (CAI) classes 67,76,77 . Some of the CAIs belonging to the sulphonamide 78 , sulfocoumarin 79 , saccharin 80 or other structurally related chemotypes 81,82 , were shown to possess significant antitumor effects, with one such derivative (SLC-0111) in Phase I/IIb clinical trials for the management of hypoxic metastatic tumours 78,83 . Indeed, by inhibiting the tumour-associated isoforms CA IX and XII, such CAIs interfere with the pH regulation and metabolism of    tumours, leading to the inhibition of growth of the primary tumour, metastases and reducing the population of cancer stem cells 78 . As a consequence, many CAIs belonging to various classes are nowadays investigated for their antitumor/antimetastatic effects, including many BTA derivatives ( Figure 43) 84-91 . .Indeed, using the ethoxzolamide 72, a CAI in clinical use for decades as lead molecule, a multitude of primary sulphonamides (e.g. compounds 73-75) as well as the secondary siulfonamide 76 were reported to act as highly efficient, frequently low nanomolar inhibitors against the tumour-associated isoforms CA IX and XII [84][85][86][87][88][89][90][91] . No ex vivo or in vivo studies are available so far with these potent CA IX/XII inhibitors, but compounds belonging to other classes of sulphonamides were proved to possess significant antitumor effects in vivo when they act as potent inhibitors of these two CA isoforms 78,[92][93][94][95][96][97][98] . Thus future studes may address this issue,      considering the fact that the BTA scaffold present in these compounds may induce interesting phisico-chemical and pharmacologic properties to the CA IX/XII inhibitors, of which many chemical families are alredy reported 99-103 .

Conclusions
Benzothiazole is a pharmacophore widely used in medicinal chemistry. This review points out to a growing interest in the development of lead or hybrid structures bearing the BTA moiety as antiproliferative and anticancer agents. The present work describes the potential of BTA scaffolds in the management of various types of cancers such as ovarian, prostate, central nervous system, renal, gastric, pancreatic, liver, breast and colon cancers. SAR studies reaveled that the anticancer activity of BTA scaffolds depends upon the nature of substituents present in these molecules, being multifactorial and not always easy to rationalise. The plethora of research on the anticancer profile of BTA derivatives mentioned in this review and their rationalisation based on the drug targets of these derivatives, when this was possible, may be useful for the development of novel such agents.

Disclosure statement
No potential conflict of interest was reported by the authors.