Synthesis and Anticancer Evaluation of 4-Chloro-2-((5-aryl-1,3,4-oxadiazol-2-yl)amino)phenol Analogues: An Insight into Experimental and Theoretical Studies

We report herein the synthesis, docking studies and biological evaluation of a series of new 4-chloro-2-((5-aryl-1,3,4-oxadiazol-2-yl)amino)phenol analogues (6a-h). The new compounds were designed based on the oxadiazole-linked aryl core of tubulin inhibitors of IMC-038525 and IMC-094332, prepared in five steps and further characterized via spectral analyses. The anticancer activity of the compounds was assessed against several cancer cell lines belonging to nine different panels as per National Cancer Institute (NCI US) protocol. 4-Chloro-2-((5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazol-2-yl)amino)phenol (6h) demonstrated significant anticancer activity against SNB-19 (PGI = 65.12), NCI-H460 (PGI = 55.61), and SNB-75 (PGI = 54.68) at 10 µM. The compounds were subjected to molecular docking studies against the active site of the tubulin–combretastatin A4 complex (PDB ID: 5LYJ); they displayed efficient binding and ligand 4h (with docking score = −8.030 kcal/mol) lay within the hydrophobic cavity surrounded by important residues Leu252, Ala250, Leu248, Leu242, Cys241, Val238, Ile318, Ala317, and Ala316. Furthermore, the antibacterial activity of some of the compounds was found to be promising. 4-Chloro-2-((5-(4-nitrophenyl)-1,3,4-oxadiazol-2-yl)amino)phenol (6c) displayed the most promising antibacterial activity against both Gram-negative as well as Gram-positive bacteria with MICs of 8 µg/mL and a zone of inhibition ranging from 17.0 ± 0.40 to 17.0 ± 0.15 mm at 200 µg/mL; however, the standard drug ciprofloxacin exhibited antibacterial activity with MIC values of 4 µg/mL.

The oxadiazole-linked aryl core (represented as bold blue bond) of IMC-038525 (tubulin inhibitor; IC50 = 0.39 ± 0.06 µM), IMC-094332 (tubulin inhibitor), and FABT (anticancer; ID50 = 6.2 ± 1.4 (HCV29T), 3.9 ± 1.1 (SW707) and 4.2 ± 1.2 (T47D) µg/mL, respectively) was used in the design strategy for the newer oxadiazole analogues shown in Figure 2 [27][28][29]. Similarly, FTAB is another anticancer compound, whose 1,3,4-thiadiazole ring was replaced with an isostere 1,3,4-oxadiazole in the title compounds (6a-h). Cancer, on the other hand, is one of the most pressing health concerns today, with nearly 10 million deaths and 19.3 million new cases reported worldwide in 2020. The number of new cancer cases is expected to exceed 28.4 million by 2040 [25]. Breast (2.26 million), lung (2.21 million), colon and rectum (1.93 million), and prostate (1.41 million) cancers were the most common cancer types reported in 2020 [26]. Chemotherapy is the most widely used and well-known cancer treatment. It is, however, linked to a number of negative consequences, hence the search for new compounds is always admired. Inspired by the miraculous biological activities of 1,3,4-oxadiazoles, we have synthesized eight new oxadiazole analogues and studied their antiproliferative effect against the nearly five dozen cancer cell lines, as well as their antibacterial potentials.
The oxadiazole-linked aryl core (represented as bold blue bond) of IMC-038525 (tubulin inhibitor; IC 50 = 0.39 ± 0.06 µM), IMC-094332 (tubulin inhibitor), and FABT (anticancer; ID 50 = 6.2 ± 1.4 (HCV29T), 3.9 ± 1.1 (SW707) and 4.2 ± 1.2 (T47D) µg/mL, respectively) was used in the design strategy for the newer oxadiazole analogues shown in Figure 2 [27][28][29]. Similarly, FTAB is another anticancer compound, whose 1,3,4-thiadiazole ring was replaced with an isostere 1,3,4-oxadiazole in the title compounds (6a-h). Microtubules are highly dynamic structures made up of αand β-tubulin heterodimers that are indispensable for cell division (mitosis) and growth [30]. The expression of specific tubulin isotypes has been linked to cancer, but the exact mechanism is still unknown. Many tubulin-binding chemotherapeutics target the tubulin family protein to suppress the dynamics of the mitotic spindle to cause mitotic arrest and cell death [31]. Because the synthesized compounds have the same pharmacophoric core as the tubulin inhibitors IMC-038525 and IMC-094332, they can be used as tubulin inhibitors. As a result, a putative mechanism of action of the target compounds (6a-h) as anticancer agents was discovered using a molecular docking approach.
Microtubules are highly dynamic structures made up of α-and β-tubulin heterodimers that are indispensable for cell division (mitosis) and growth [30]. The expression of specific tubulin isotypes has been linked to cancer, but the exact mechanism is still unknown. Many tubulin-binding chemotherapeutics target the tubulin family protein to suppress the dynamics of the mitotic spindle to cause mitotic arrest and cell death [31]. Because the synthesized compounds have the same pharmacophoric core as the tubulin inhibitors IMC-038525 and IMC-094332, they can be used as tubulin inhibitors. As a result, a putative mechanism of action of the target compounds (6a-h) as anticancer agents was discovered using a molecular docking approach. The oxadiazoles were also reported as antibacterial and antifungal agents [16,18,32,33]. Hence, we further explored their antibacterial activity as well as their molecular docking studies against DNA gyrase, a potential target for many antibacterial drugs. Since some of the compounds exhibited promising antibacterial activity, molecular docking against DNA gyrase was studied for the title compounds (6a-h).

Anticancer Activity
All the compounds were designed based on the pharmacophore of tubulin inhib and their anticancer activity was accessed using the National Cancer Institute pro (NCI US) reported elsewhere [37][38][39][40]. The title compounds were initially screened vi dose assay at 10 µM drug concentration against 56 NCI cancer cell lines, and their re in the form of growth percent (GP) and percent growth inhibition (PGI) are given in T 1 and Table 2, respectively. The GP and PGI are related as PGI = 100 − GP. Compo 6b, 6e, and 6g showed maximum sensitivity against PC-3 (PGI = 14.13), NCI-H460 (P 19.55), and NCI-H522 (PGI = 11.4), respectively, while compound 6a was found to be sitive against NCI-H522 (PGI = 23.

Anticancer Activity
All the compounds were designed based on the pharmacophore of tubulin inhibitors, and their anticancer activity was accessed using the National Cancer Institute protocol (NCI US) reported elsewhere [37][38][39][40]. The title compounds were initially screened via one dose assay at 10 µM drug concentration against 56 NCI cancer cell lines, and their results in the form of growth percent (GP) and percent growth inhibition (PGI) are given in Tables 1 and 2 Table 2]. The average PGI of individual panels was calculated and compared with the anticancer activity of the reference drug imatinib and their results are given in Table 3. Compound 6h exhibited far better anticancer activity against non-small cancer, colon, CNS, melanoma, ovarian, renal, and breast cancer cell lines. The data for imatinib were retrieved from the NCI website with NSC Code 759854 [37]. The structure-activity relationship was established based on the anticancer activity. The 3,4,5-trimethoxy (6h) substitution of the phenyl ring exhibited promising anticancer activity followed by 2-hydroxy (6f) substitution. The overall structure-activity relationship (SAR) can be represented as 3,4,5-trimethoxy > 2hydroxy > 4-nitro > 4-hydroxy-3-methoxy substitution in the phenyl ring directly attached to the oxadiazole ring.

Molecular Docking
Some of the oxadiazole analogues displayed good anticancer activity against some of the cancer cell lines and the compounds were also designed based on the pharmacophore of tubulin inhibitors, hence a molecular docking study of tubulin was explored against the active site of tubulin-combretastatin A4 (PDB ID: 5LYJ). The molecular docking scores and interactions are given in Table 4. The molecular docking score of the ligands (6a-h) ranged from −7.617 to −8.389 kcal/mol and they displayed two types of interactions, including H-bonding and halogen bonding. Ligand 6d displayed maximum binding affinity with a docking score of −8.389 kcal/mol and exhibited a halogen bond interaction with the residue Ala317, while ligand 6e displayed a docking score of −7.617 kcal/mol with no significant interaction. Ligands 6d and 6f displayed a similar type of interaction with the residue Ala317 through a halogen bond interaction. The 2D and 3D interactions of ligands 6d and 6f are shown in Figures 3 and 4, respectively. Ligands 6a and 6b displayed a H-bond interaction with the residue Asn258, while they showed a halogen bond interaction with the residues Ala317 and Asn349, respectively. Ligand 6h displayed no significant electrostatic interaction; however, the trimethoxy phenyl ring lies with the hydrophobic cavity of containing residues Leu252, Ala250, Leu248, Leu242, Cys241, Val238, Ile318, Ala317, and Ala316, as shown in Figure 5. of tubulin inhibitors, hence a molecular docking study of tubulin was explored against the active site of tubulin-combretastatin A4 (PDB ID: 5LYJ). The molecular docking scores and interactions are given in Table 4. The molecular docking score of the ligands (6a-h) ranged from −7.617 to −8.389 kcal/mol and they displayed two types of interactions, including Hbonding and halogen bonding. Ligand 6d displayed maximum binding affinity with a docking score of −8.389 kcal/mol and exhibited a halogen bond interaction with the residue Ala317, while ligand 6e displayed a docking score of −7.617 kcal/mol with no significant interaction. Ligands 6d and 6f displayed a similar type of interaction with the residue Ala317 through a halogen bond interaction. The 2D and 3D interactions of ligands 6d and 6f are shown in Figure 3 and Figure 4, respectively. Ligands 6a and 6b displayed a H-bond interaction with the residue Asn258, while they showed a halogen bond interaction with the residues Ala317 and Asn349, respectively. Ligand 6h displayed no significant electrostatic interaction; however, the trimethoxy phenyl ring lies with the hydrophobic cavity of containing residues Leu252, Ala250, Leu248, Leu242, Cys241, Val238, Ile318, Ala317, and Ala316, as shown in Figure 5.

Antibacterial Activity
The antibacterial activity of compounds 6a-h against Gram-negative (E. coli, NCIM 5051 and P. aeruginosa, ATCC 10145) as well as Gram-positive (S. aureus, NCIM 2079 and B. subtilis, NCIM 2439) bacterial strains was assessed via a disc diffusion method and broth dilution method as per the reported protocol [41][42][43] . Compounds 6d, 6f, 6g, and 6h displayed less antibacterial activity with zones of inhibition ranging from 4.8 ± 0.75 to 6.7 ± 0.20 mm at 200 µg/mL drug concentrations and exhibited MICs ranging from 128 to 512 µg/mL against various bacterial strains. Compounds 6a and 6d displayed moderate antibacterial activity with MICs ranging between 16 and 64 µg/mL and zones of inhibition ranging between 13.0 ± 0. 21 Table 5. The 4-nitro substitution exhibited promising antibacterial activity followed by 2-chloro substitution in the phenyl ring attached to the oxadiazole ring. The SAR of the antibacterial activity can be represented as 4-nitro > 2-chloro > 4-fluoro > 4chloro substitution in the phenyl ring. Molecular docking studies of the molecular target DNA gyrase were carried out to explore the binding mode of compounds against the 2-oxo-1,2-dihydroquinoline derivative binding active site of DNA gyrase (PDB ID; 6KZV). The results of the molecular docking studies are given in Table S1 and the 2D binding interactions of ligands 6c and 6e are given in Figure S1 (refer to Supplementary Materials). The antibacterial activity was supported by in silico studies, and the compounds (6c and 6e) that had the maximum docking score (of −6.200 and −6.424 kcal/mol, respectively) demonstrated promising antibacterial activity.

Discussion
The 1,3,4-oxadiazoles were designed using the pharmacophore found in some tubulin inhibitors (IMC-094322 and IMC-038525) and the anticancer agent FTAB. The current research aimed to create scaffolds with the basic pharmacophore of tubulin inhibitors (IMC-094322 and IMC-038525) and a low-molecular-weight small compound. Molecular docking studies were performed for the tubulin-combretastatin A4 binding site of the D-chain (PDB ID: 5LYJ). We discovered that the benzene ring substitution produced higher docking scores than IMC-094322 and IMC-038525; hence, we performed modifications in the benzene ring rather than the dioxane ring of IMC-094322 and IMC-038525. The oxadiazole analogues were prepared in satisfactory yields and characterized using spectroscopic techniques followed by their anticancer evaluation at 10 µM against nine panel of 56 cancer cell lines as per the NCI US protocol. Some of the compounds displayed promising anticancer activity against a few cancer cell lines. Compound 6h demonstrated significant anticancer activity against SNB-19 (CNS cancer), NCI-H460 (non-small-cell lung cancer), and SNB-75 (CNS cancer) with PGIs of 65.12, 55.61, and 54.68 percent, respectively at 10 µM. Compound 6h also displayed efficient binding against the tubulin-combretastatin A4 binding site of the D-chain with a docking score of −8.030 kcal/mol and a trimethoxy phenyl ring lying within the hydrophobic cavity surrounded by important residues Leu252, Ala250, Leu248, Leu242, Cys241, Val238, Ile318, Ala317, and Ala316. The antibacterial activities of the oxadiazoles were also assessed against Gram-negative (E. coli and P. aeruginosa) as well as Gram-positive (S. aureus and B. subtilis) bacterial strains via a disc diffusion method and broth dilution method as per the reported protocol. Compounds 6c and 6e demonstrated good bacterial inhibition against both the Gram-positive as well as Gram-negative bacterial strains. A molecular docking study was conducted for the title compounds (6a-h) against DNA gyrase, one of the attractive molecular targets for many antibacterial drugs. All the compounds exhibited docking scores ranging between −5.883 and −6.424 kcal/mol. The binding affinities of compounds 6c (docking score = −6.200 kcal/mol) and 6e (docking score = −6.424 kcal/mol) were found to be the highest among the series, and these compounds also possessed promising antibacterial activity.

Method for the Synthesis of 4-Chloro-2-nitrophenol (2)
An amount of 21 mL of concentrated H 2 SO 4 and 15 mL of concentrated HNO 3 was mixed in a 250 mL round-bottom flask with a few smaller pieces of porcelain. 4-Chlorophenol (100 mmol; 12.86 g) was added in portions of 2-3 g to the acidic mixture, shaking the reaction mixture after each addition. The temperature was controlled to no more than 55 • C by immersing the flask into the cold-water bath. The reaction mixture was then heated to 45 • C for 45 min with continuous shaking on magnetic stirrer [44]. The content of the flask was poured into the crushed ice with continuous stirring with a glass rod to obtain the precipitate. The precipitate was filtered, washed with water, and re-crystallized from methanol to obtain 4-chloro-2-nitrophenol (2). Colour, light yellow; yield, 82%; Mp 86-88 • C. (3) 4-Chloro-2-nitrophenol (2) (75 mmol; 13.02 g) was dissolved in concentrated HCl (50 mL) and 45 g granulated tin was added, and the reaction mixture was left to heat up in a boiling water bath (100 • C) for 60 min [44]. The reaction mixture was poured into the crushed ice to obtain solid precipitate, which was filtered, dried, and re-crystallized with absolute ethanol to obtain 2-amino-4-chlorophenol (3). Colour, brown; yield, 78%; Mp 142-144 • C.

Anticancer Activity
The anticancer activity of the compounds was assessed using the standard NCI US protocol at 10 µM drug concentration [37][38][39][40]. In our earlier published work, we provided a thorough explanation of the experimental process. [45].

Molecular Docking Studies
The tubulin-combretastatin A4 complex's X-ray crystallographic structure, with a resolution of 2.40 Å and an R-value of 0.192 (observed), was retrieved from protein data bank (PDB) [46]. The combretastatin A4 binding site of D-chain was used to prepare grid generation and molecular docking as per reported protocol [47].

Antibacterial Activity
The antibacterial activities of the compounds were tested against B. subtilis, E. coli, S. aureus, and P. aeruginosa bacterial strains via disc diffusion method and broth dilution method [41][42][43] (please refer to the Supplementary Materials).

Conclusions
We discussed the design, synthesis, and anticancer activity of new 4-chloro-2-((5aryl-1,3,4-oxadiazol-2-yl)amino)phenol analogues (6a-h) in the current investigation. The compounds were designed based on the pharmacophoric features of tubulin inhibitors (IMC-094322 and IMC-038525). Therefore, the compounds were further subjected to molecular docking studies against the active site of the tubulin-combretastatin A4 complex (PDB ID: 5LYJ); they displayed efficient binding with docking scores ranging between −7.617 and −8.389 kcal/mol and demonstrated two types of binding interactions: H-bond and halogen bond interactions. The compounds' binding affinities against the active site of the tubulin-combretastatin A4 complex were found to be higher than those of the tubulin inhibitors IMC-094322 and IMC-038525 in the molecular docking studies, with the exception of compound 6e. The compounds were prepared efficiently in five steps in good yields. The anticancer activity of the compounds was evaluated at 10 µM drug concentration as per the standard protocol. 4-Chloro-2-((5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazol-2yl)amino)phenol (6h) demonstrated significant anticancer activity against some of the cancer cell lines, including SNB-19, NCI-H460, and SNB-75 at 10 µM. Compound 6h was discovered to be highly sensitive to the CNS cancer cell line SNB-19 (PGI = 65.12%), but less sensitive to the renal cancer cell line A498 (PGI = −30.65%). The average anticancer activity of compound 6h against nine panels of cancer cell lines was compared with that of the reference drug imatinib at 10 µM. It was discovered that compound 6h displayed significantly greater anticancer activity against non-small cancer, colon, CNS, melanoma, ovarian, renal, and breast cancer cell lines than imatinib. Based on anticancer activity, the SAR was concluded. It was discovered that the electron-donating group, such as 1,3,4-trimethoxy (compound 6h; mean GP = 81.49%) and 2-hydroxy (compound 6f; mean GP = 96.45%) substitution, as well as the electron-withdrawing 4-nitro (compound 6c; mean GP = 96.97%) substitution on the phenyl ring demonstrated significant anticancer activity compared to other substitutions. The 4-chloro (compound 6b; mean GP = 98.2%) substitution showed slightly more anticancer activity than 2-chloro (compound 6e; mean GP = 100.79%) and 4-fluoro (compound 6a; mean GP = 100.91%) substitution on the phenyl ring. Additionally, the 1,3,4-oxadiazoles' antibacterial activities are well documented in the literature. The antibacterial activity of the compounds was studied against Gram-negative (E. coli and P. aeruginosa) as well as Gram-positive (S. aureus and B. subtilis) bacterial strains via the disc diffusion method and broth dilution method. The compounds 4-chloro-2-((5-(4nitrophenyl)-1,3,4-oxadiazol-2-yl)amino)phenol (6c) and 4-chloro-2-((5-(2-chlorophenyl)-1,3,4-oxadiazol-2-yl)amino)phenol (6e) displayed the most promising antibacterial activities against both the Gram-negative as well as Gram-positive bacteria. Compound 6c displayed the most significant antibacterial activities with MICs of 8 µg/mL and a zone of inhibition ranging from 17.0 ± 0.40 to 17.0 ± 0.15 mm at 200 µg/mL; however, the standard drug ciprofloxacin exhibited MICs of 4 µg/mL. The electron-withdrawing group, such as 4-nitro and 2-chloro substitutions on the phenyl ring, demonstrated the most promising antibacterial activity among the series; 4-chloro and 4-fluoro substitutions exhibited moderate antibacterial activity, while the electron-releasing group, such as 4-methoxy, 2-hydroxy, 4-hydroxy-3-methoxy, and 1,3,4-trimethoxy substitutions on the phenyl ring, displayed less antibacterial activity. A molecular docking study was performed for the title compounds (6a-h) against DNA gyrase (PDB ID: 6KZV), one of the most appealing targets for many antibacterial agents, and it was discovered that both compounds 6c and 6e had efficient binding affinity with docking scores of −6.200 and −6.424 kcal/mol, respectively. The results of the molecular docking studies revealed a correlation between docking scores and antibacterial activity. We have reported the anticancer and antibacterial activities of 1,3,4-oxadiazoles in the present investigation. The bioactivities of the compounds were found to be significant and some of the in vitro experimental results were found to be in good agreement with in silico studies. The current discovery could pave the way for future breakthroughs in cancer and bacterial disease treatment. The biological activities of these oxadiazoles can be increased through structural modification. In our laboratory, we are currently conducting additional research into the anticancer and antibacterial potential of modified oxadiazoles.