Research progress on the synthesis and pharmacology of 1,3,4-oxadiazole and 1,2,4-oxadiazole derivatives: a mini review

Abstract Oxadiazole is a five-membered heterocyclic compound containing two nitrogen atoms and one oxygen atom. The 1,3,4-oxadiazole and 1,2,4-oxadiazole have favourable physical, chemical, and pharmacokinetic properties, which significantly increase their pharmacological activity via hydrogen bond interactions with biomacromolecules. In recent years, oxadiazole has been demonstrated to be the biologically active unit in a number of compounds. Oxadiazole derivatives exhibit antibacterial, anti-inflammatory, anti-tuberculous, anti-fungal, anti-diabetic and anticancer activities. In this paper, we report a series of compounds containing oxadiazole rings that have been published in the last three years only (2020–2022) as there was no report or their activities described in any article in 2019, which will be useful to scientists in research fields of organic synthesis, medicinal chemistry, and pharmacology.


Oxadiazole
Kashid et al. synthesised a series of novel 2,5-disubstituted 1,3,4-oxadiazole derivatives. The preliminary anti-oxidation and anti-inflammatory in vitro screening results indicated that compound 2 ( Figure 1) had a strong antioxidant activity of 2,2diphenyl-1-picrylhydrazyl (DPPH) with an IC 50 value of 23.07 ± 0.27 mM when compared it with synthetic antioxidant Diclofenac sodium (IC 50 ¼ 90.21 ± 0.77 mM), but the nitric oxide free radical scavenging activity was poor (IC 50 ¼ 88.04 ± 0.71 mM). Additionally, molecular docking studies revealed that all compounds have a high affinity for the COX-2 enzyme, providing an important starting point for structure-based drug design, resulting in the development of a series of novel derivatives with strong anti-inflammatory activity compared with synthetic antioxidant Diclofenac sodium 12 . Alam et al. synthesised nine compounds containing 1,2,3-triazole and 1,3,4-oxadiazole and evaluated their anticancer and in vitro thymidylate synthase activities. The results showed that compounds 3a and 3b ( Figure 1) exhibited significant inhibitory effects on MCF-7 and HCT-116 cells. The inhibitory activity of compound 3a on MCF-7 cells was 4-fold than that of the standard drug 5-Fluorouracil (IC 50 ¼ 24.74 mM) which was comparable to that of Tamoxifen (IC 50 ¼ 5.12 mM), while compound 13 was 5-fold than that of Tamoxifen and 24-fold than that of 5-Fluorouracil on MCF-7 cells. Compounds 3a and 3b inhibited thymidylate synthase (TS) with IC 50 values of 2.52 mM and 4.38 mM compared to the standard drug Pemetrexed (IC 50 ¼ 6.75 mM). In conclusion, compounds 3a and 3b can serve as candidate lead compounds 13 .
Alam et al. synthesised a series of novel naproxen based 1,3,4oxadiazole derivatives and screened their cytotoxicity as EGFR inhibitors. Among them, compound 4 ( Figure 1) was the most potent compound against MCF-7 and HepG2 cells with IC 50 values of 2.13 and 1.63 mg/mL, respectively, which was comparable to Doxorubicin (MCF-7: IC 50 ¼ 1.62 mg/mL, HepG2: IC 50 ¼ 1.62 mg/ mL). In addition, compound 4 inhibited EGFR kinase with an IC 50 value of 0.41 mM compared to the standard drug Erlotinib (IC 50 ¼ 0.30 mM). The results showed that these synthetic naproxen hybrids have EGFR inhibition and can serve as leads for cancer therapy 14 .
The antibacterial results showed that some compounds 5a, 5b, 5c, 5d and 6 ( Figure 1) exhibited good inhibitory effects on Escherichia coli and Staphylococcus aureus 15 .
2.1.2. Aryl-1,3,4-oxadiazole schiff bases Ullah et al. synthesised eighteen aryl-1,3,4-oxadiazole derivatives bearing Schiff bases and assessed their inhibitory effect against a-glucosidase. All of the synthesised derivatives exhibited good inhibitory activity against a-glucosidase. They discovered that two of these compounds had a significantly greater inhibitory effect against a-glucosidase than that of the standard drug Acarbose, and compounds 7a (IC 50 ¼ 0.6 ± 0.05 mM) and 7b (IC 50 ¼ 0.30 ± 0.2 mM, Figure 1) had more than hundredfold inhibitory activity against a-glucosidase compared with the standard inhibitor Acarbose (IC 50 ¼ 38.45 ± 0.80 mM). SAR analysis revealed that compounds with ortho-and para-hydroxyl groups were more active than those with nitro and chloro groups. Molecular docking studies confirmed the binding sites and interactions between ligands and enzymes. Therefore, structural modification of active compounds may facilitate the discovery of promising antidiabetic lead compounds 16 .

Quinoxaline-1,3,4-oxadiazole hybrid derivatives
Ono et al. synthesised a series of quinoxaline-1,3,4-oxadiazole hybrid derivatives and evaluated their anticancer activity against human leukaemia HL-60 cells. While these compounds have a significant inhibitory effect on HL-60 cell proliferation, they are highly cytotoxic to normal human cells. Compounds 8a and 8b ( Figure  1) can significantly inhibit cell proliferation when compared to the positive control XK469. Even at a concentration of 10 lM, compounds 8a and 8b exhibited strong anti-proliferative effects, with cell viability less than 10% of that of the control drug. They are, however, toxic to normal human fibroblast WI-38 cells (cell viability 8a: 1.8 ± 0.2%; 8b: 2.9 ± 0.2%). Therefore, the quinoxaline-1,3,4oxadiazole compounds must be further optimised. To better identify the optimised molecules, compounds 9a, 9b, 9c and 9d ( Figure 1) had obtained by linking the quinoxaline frameworks to phenyl, piperazine, piperidine, and morpholine. The optimised compound successfully induced apoptosis in HL-60 cells and had low toxicity on WI-38 cells. Additionally, RT-PCR analysis revealed that these compounds mainly inhibited Bcl-2expression, with Bcl-2 relative mRNA expression levels less than 10% of the untreated level. Compounds 9a, 9b, 9c and 9d had inhibition rates of 98.3%, 97.9%, 98.2% and 98.0% on HL-60 cell, respectively. The results of the study indicated that compounds 9a-d had great potential as proapoptotic anticancer agents 17 .

1,3,4-Oxadiazole conjugated pyrmidinones
Said et al. synthesised a series of new oxadiazole conjugated pyrmidinones compounds and tested their analgesic activity. All compounds showed good analgesic activity when compared to the standard Indomethacin. Among all of them, compound 10 ( Figure  1) had the highest analgesic activity with 100% protection surpassing that of Indomethacin (83.33%protection). Moreover, compound 10 was evaluated for anti-inflammatory activity, ulcerative, and in vitro COX-1 and COX-2 enzyme inhibition tests. The results showed that compound 10 had good anti-inflammatory activity, relatively low ulcer index (ulcer index ¼ 3.8), and strong inhibitory activity on COX-1 and COX-2 (COX-1: IC 50 ¼ 0.140 ± 2.38 lM, COX-2: IC 50 ¼ 0.007 ± 0.11 lM) compared with the standard drug Indomethacin 18 .   and Salmonella typhi and two Gram-positive strains, namely Bacillus subtilis and Bacillus megaterium and antifungal activity against fungal strains such as Aspergillus niger, Rhizopus oryzae, Penicillium chrysogenum and Candida albicans. The synthesised compounds exhibited significant broad-spectrum of antibacterial and antifungal potential. Compared with standard drug Streptomycin, compounds 19a-c ( Figure 2) showed high antibacterial activity with very low MIC values between 3.9 and 31.25 lM. The synthesised compounds were also screened for free radical scavenging activity by OH and DPPH assays, which showed that they are good antioxidants. Furthermore, haemolysis studies showed that the series of 1,3,4-oxadiazole derivatives had little cytotoxicity compared to streptomycin 27 .
Izgi et al. synthesised novel 1,3,4-oxadiazole derivatives and tested the in vitro inhibitory potential on a-glucosidase. The results showed that compound 20 ( Figure 2) has an IC 50 value of 0.46 ± 0.15 mM for GAA inhibition, which is still too low to compete with AC1 28 .
Based on the structure of the Twist1 inhibitor harmine, Zhao et al. designed a series of 1,3,4-oxadiazole derivatives. The results showed that compound 25 ( Figure 2) exhibited significant antiproliferative activity against A549 and H2228 cell lines, with IC 50 values of 2.03 lM and 9.80 lM, respectively, which were superior to Harmine (A549: IC 50 ¼ 17.12 lM, H2228: IC 50 ¼ 31.06 lM). Overall, compound 25 was identified as a potential lead drug against advanced non-small cell lung cancer 33 .

Ferulic acid based 1,3,4-oxadiazole hybrids
Tripathi et al. designed and synthesised thirty 1,3,4-oxadiazole derivatives based on ferulic acid, and evaluated them for their multifunctional inhibitory activity against acetylcholinesterase (AChE), butyrylcholinesterase (BChE) and beta-secretase-1 (BACE-1). Compound 26a (Figure 2) was the most effective AChE inhibitor (IC 50 ¼ 0.068 mM). The IC 50 values of equipotent inhibition against BChE and BACE-1 were 0.218 mM and 0.255 mM, respectively. Compound 26b ( Figure 2) showed the strongest inhibitory effect on BChE and BACE-1 with IC 50 values of 0.163 mM and 0.211 mM, respectively. Compounds 26a and 26b can be used in place of propidium iodide in PAS-AChEsince as they had good blood-brain barrier permeability in PAMPA experiment, and their anti-Ab aggregation activity in the self and AChE induction experiments showed neuroprotective activity on neuroblastoma SH-SY5Y cells. In vitro studies had shown that these compounds have inhibitory AChE and antioxidant activity. In addition, the strongest activity of compound 26a may be attributed to the presence of a strong electron-withdrawing group on the terminal phenyl group, which is capable of penetrating the CAS-AChE. Compound 26a exhibited a constant binding affinity for PAS-AChE and aspartate dyads of BACE-1. Furthermore, compound 26a improved the learning and memory behaviour of the Ab-induced Alzheimer's disease-like-phenotypic ICV rat model in the Morris water maze experiment, and pharmacokinetic studies confirmed the good oral absorption characteristics of the compounds. The overall findings suggest that compound 26a may be a promising multifunctional lead drug candidate for the treatment of Alzheimer's disease 34 .

Benzenesulfonamides incorporating 1,3,4-oxadiazole hybrids
Sharma et al. designed and synthesised benzenesulfonamides containing 1,3,4-oxadiazole hybrids as a novel selective inhibitor of carbonic anhydrase (hCA) I, II, IX and XII isoenzymes. Researchers determined the inhibitory activity of these compounds against the two dominant cytosolic isoforms hCA I/II and the tumour-associated isoforms hCA IX/XII. Compared with the positive control drug Acetazolamide (AAZ), the majority of the compounds had relatively weak inhibitory effects, with Ki values ranging from 469.6 nM to 3.89 mM, but compounds 27 (Ki ¼ 70.7 nM, Figure 2) and 28a (Ki ¼ 73.2 nM, Figure 2) showed a stronger inhibitory effect on hCA I than on AAZ (Ki ¼ 250 nM). Compound 28a exhibited strong inhibitory activity against hCA II and hCA IX, and most of the compounds were moderately effective hCA XII inhibitors, with Ki values ranging from 230.6 to 3.62 mM. CA inhibition results indicated that 1,3,4-oxadiazole containing benzenesulfonamide with an amide tail/linker was a potent inhibitor of hCA IX. Therefore, compound 28b (Ki ¼ 29.0 nM, Figure 2) acts as a selective inhibitor of tumour-associated isoforms hCA IX 35 .
2.1.9. S-Alkylated-1,3,4-oxadiazole-sulphonamide hybrids Javid et al. designed and synthesised a series of 10 different oxadiazole-sulphonamide hybrid compounds through a facile method. The synthesised products were evaluated for their potential inhibitory activity against two aldo-keto reductase family enzymes: ALR1 and ALR2. The majority of the compounds showed good activity, particularly 30a, 30b, 30c and 30d (Figure 3). Compound 30a inhibited ALR1 selectively with an IC 50 value of 4.77 ± 0.47 mM, whereas compound 30c inhibited ALR2 selectively with an IC 50 value of 2.21 ± 0.73 mM. The remaining analogs inhibited both enzymes in tandem 37 .
Yu et al. designed and synthesised a series of novel 1,3,4-oxadiazole neuraminidase inhibitors, and tested their inhibitory activity against neuraminidase in vitro. The results showed that compound 31 (Figure 3) had the best inhibitory activity (IC 50 ¼ 0.027 mM), significantly 3.04-fold lower than Oseltamivir carboxylate (IC 50 ¼ 0.082 mM). Compound 31 also showed stronger inhibitory potency against H5N1-H274Y mutant relative to Oseltamivir carboxylate 38 .  against human liver cancer cell lines (HepG2 and HCCLM3) to determine their anti-proliferation ability. The OTDs compounds 33a, 33b, 33c and 33d (Figure 3) all showed significant cytotoxicity, with IC 50 values of 19.5, 21.4, 24.5 and 22.3 lM, respectively. Additionally, the toxicity of these compounds was determined in normal liver LO2 cell lines. All of these compounds, particularly 33a and 33b, showed no toxicity in LO2 cells, indicating that compounds 33a and 33b are more selective for cancer cells. In vitro, western blot, flow cytometry and molecular docking analysis and other studies revealed that compound 33a induced apoptosis in HepG2 cells by inhibiting the expression of SIRT2 40 .   respectively, and strong cytotoxicity to MDA-MB-231 cells. Compound 37c (Figure 3) exhibited no cytotoxicity to the four cancer cell lines. Interestingly, none of the compounds was cytotoxic to normal human embryonic kidney cells HEK-293. It was worth noting that compound 37a is a promising lead drug 44 .
Kumar et al. designed and synthesised indole-oxadiazole derivatives 38a-e ( Figure 3) and assessed their anti-inflammatory and analgesic activity in vivo. As demonstrated by the results, the majority of the compounds exhibited COX-2 selectivity. Compounds 38d and 38e were particularly selective for COX-1. With a selectivity index (SI) of 2.19, compound 38c was the most selective COX-2 inhibitor, followed by compound 38a (SI ¼ 1.95). At a concentration of 10 lM, compound 38b inhibited COX-2 at a rate of 63.23% and a SI of 1.49 45 . In vivo anti-inflammatory activity of the compounds was further investigated. Compounds 46a and 46e showed superior anti-inflammatory activity than that of the positive drug celecoxib in vivo. These derivatives were also tested for their ulcerogenic potential, with compound 46e showing a better safety profile compared to Celecoxib, and compound 46a showed minor damage 52 .  IC 50 values for compounds 49a, 49b, 50a and 50b (Figure 4) were 2.32 ± 0.03, 6.48 ± 0.10, 2.01 ± 0.00, 1.96 ± 0.01 lg/mL, respectively, when Donepezile (IC 50 ¼ 0.03 ± 0.00 lg/mL) was used as a control 55 .

Combination of 1,3,4-oxadiazole and thiazole
Rashad et al. synthesised a febuxostat derivative with carboxamide and 1,3,4-oxadiazole functionalities. Compounds 51 and 52 ( Figure 4) were tested for their inhibitory activity against xanthine oxidase (XO) and COX. The results showed that compounds 51 and 52 had a significant inhibitory effect on XO and COX in vitro.
To further evaluate the anti-inflammatory activity (AI%) of compound 52 in vivo, Celecoxib was used as a reference standard. The results showed that after 8 h, compound 52 (AI% ¼ 89.29%) showed higher activity than Celecoxib (AI% ¼ 83.57%) 56 Figure 4) had lower cytotoxicity (CC 50 ¼ 57.34 lg/mL), especially the inhibition on hERG (IR ¼ 11.3 ± 1.7% at 10 lM). It has a better oral PK profile than the anti-TB drug candidate PBTZ169 (currently in Phase II clinical trials) 61
Chen et al. modified compound 67a (SEW2871, Figure 5) to obtain compound 67b (ASR396, Figure 5), allowing the new compound to be loaded into nanoliposomes (nanoASR396). NanoASR396 was capable of greatly inhibiting the permeability increase and gap formation of vascular endothelium and EC monolayer models induced by inflammatory cytokines. NanoASR396 strengthened the integrity of the endothelial cell layer by inhibiting the phosphorylation of MLC, hence inhibiting the metastasis and spread of cancer cells 70 .
Choi et al. reported a new compound 68 (JY-2, Figure 5). The compound had an inhibitory effect on FoxO1 (IC 50 ¼ 22 lM). JY-2 was shown to protect hepatocytes and pancreatic b-cells from lipotoxicity induced by pa, and improved glucose tolerance in vivo, and associated with the corresponding liver gluconeogenesis and b-cell newborn mRNA expression 71 .
Mohan et al. designed and synthesised 1,2,4-oxadiazole-1,2,3triazole-pyrazole derivatives, and evaluated their anti-cancer activity against PC3, DU-145 (prostate cancer), A549 and MCF-7 cells. Among them, compound 69 ( Figure 5) showed good anticancer Kumar Kushwaha et al. synthesised a series of isoxazole-1,2,4oxadiazole derivatives and tested the highest non-cytotoxic concentration (HNC) of the synthesised derivatives using the MTT cell viability assay in TZM-bl reporter cells. None of the compounds appeared to be cytotoxic at the 100 nM concentration. The antiviral activity of HNC was then tested using a luciferase reporter gene assay in HIV-1 NL4.3 virus-infected TZM-bl cells at a concentration of 100 nM. The results showed that compound 74 (IC 50 ¼ 85.7 ± 1.91 lM, Figure 5) showed significant inhibition on luciferase activity, which indicated that viral gene expression and replication were inhibited, and the HIV-1 inhibitor AUY922 (IC 50 ¼ 66.25 ± 1.38 lM) was used as a positive control for comparison 77  Furthermore, no cytotoxicity was observed in normal human cells 79 .

Coumarin linked 1,2,4-oxadiazoles
Thacker et al. synthesised a series of 1,2,4-oxadiazole derivatives linked to coumarin, and evaluated them against four physiologically and pharmacologically related subtypes, namely the cytosolic subtypes hCA I and II and the transmembrane tumour-related subtypes hCA IX and XII. The results indicated that all compounds selectively inhibited hCA IX and XII, but not hCA I or II. The Ki range for hCA IX was 23.6 $ 315.6 nM, and compound 77b exhibited the best inhibitory activity. The Ki value of hCA XII ranged from 1.0 to 752.6 nM, and the inhibitory effects of compounds 77a-d ( Figure 5) were 1.58, 1.26, 5.7 and 1.29 times than that of standard AAZ, respectively. It is worth noting that compound 77c had a Ki value of 1 nM, making it a promising lead drug candidate for the design of new hCA XII inhibitors. Compound 77b showed the strongest inhibitory effect on hCA IX, with a Ki value of 23.6 nM. Therefore, compound 77b can be used as a lead compound for the design of dual hCA IX and XII inhibitors 80 .

3-Aryl-1,2,4-oxadiazoles derived from indomethacin
Marzouk et al. designed and synthesised a series of 3-aryl-1,2,4oxadiazole compounds 79a-g ( Figure 5) derived from indomethacin (compound 78, Figure 5). At 5 mM, all the compounds showed no obvious cytotoxicity. The anti-inflammatory effect of compounds 79a-g containing 3-aryl-1,2,4-oxadiazole was found to be weak to moderate. Compounds 79d and 79f demonstrated the highest anti-inflammatory activity (strongest inhibitory effect on NO production), with inhibition rates of 37.2% and 33.7%, respectively. The anti-inflammatory activity of compounds 79b, 79c and 79e were moderate (25.5%, 29.3% and 26.9%, respectively), while compounds 79a and 79g showed the least anti-inflammatory activity (9.6% and 17.3%, respectively). After 3 h, the anti-inflammatory activity of compounds 79a, 79b, 79d and 79g were 60.9%, 69.9%, 53.8% and 57.9%, respectively, which was significantly higher than Celecoxib (46.56%) and Indomethacin (55.63%). Compound 79b exhibited the strongest anti-inflammatory activity of all the compounds. Compound 79c had a moderate anti-inflammatory activity (48.5%), while compounds 79e and 79f exhibited the least anti-inflammatory activity (29.4% and 21.3%, respectively). After 4 h, compound 79g showed prolonged anti-inflammatory activity (42.78%), which was comparable to that of Indomethacin (43.9%). Compound 79b, which had the strongest anti-inflammatory activity, was orally administered to male mice to determine acute toxicity. The study demonstrated that when the compound was administered at a dose of less than 400 mg/ kg, no evidence of acute toxicity or death was observed. The tested compounds in the study were non-toxic and well-tolerated. However, Indomethacin has a higher ulcerogenic potential, with an injury index of 78.7%, whereas the tested compound had a lower ulcerogenic potential, with an injury index of 35 $ 61%. Among these, compound 79b demonstrated potent anti-inflammatory activity in vivo and, when compared to Indomethacin, it showed a lower ulcerogenic, with an injury index of 38%. In contrast, all compounds studied were shown to be more ulcerogenic than Celecoxib 81 . 2.2.6. Oxadiazole-hydroxy small molecule hybrids Fernandes et al. used molecular hybridisation methods to design and synthesise a series of vinyl-1,2,4-oxadiazole and oxadiazolehydroxy hybrid derivatives as antiparasitic agents. The obtained compounds showed in vitro inhibitory activity against intracellular amastigotes of two protozoan parasites, Trypanosoma cruzi and Leishmania infantum. HFF-1 fibroblasts and HepG2 hepatocytes were used to evaluate cytotoxic activity. The majority of the compounds were not cytotoxic to HFF-1 fibroblasts, but moderate cytotoxic to the liver HepG2 cell line. It was worth noting that compounds 81a-d had lower toxicity than that of the positive control drug Doxorubicin. The IC 50 values for compounds 81a, 81b, 82a and 82b ( Figure 6) were 6.20, 2.20, 2.30 and 2.20 mM, respectively, against Trypanosoma cruzi. Compounds 81c, 81d, 82c and 82d ( Figure 6) were highly selective for Leishmania infantum (IC 50 values of 3.89, 2.38, 2.50 and 2.85 mM, respectively) 83 .

1,2,4-Oxadiazole based trans-acrylic acid derivatives
The alpha and gamma nuclear receptors of peroxisome proliferator-activated receptors (PPARs) are considered to be a promising target for diabetes treatment. Activating PPAR-c can result in an increase in insulin resistance, while activating PPAR-a can result in a decrease intriglycerides, thus reducing the complications of diabetes, particularly cardiovascular disease. Therefore, Kaur et al. designed 3,5-substituted 1,2,4-oxadiazole derivatives and screened them using molecular docking. Simultaneously, toxicity analysis of selected compounds revealed that most of the compounds were not carcinogenic or mutagenic. Additionally, in vitro analysis of PPAR-c and PPAR-a demonstrated that compounds 83a-d ( Figure  6) had a more favourable effect on both receptors. Compound 83a and 83c were found to be the most potent on both PPAR-a and PPAR-c receptors with EC 50 of 0.781 ± 0.008 mM, 3.29 ± 0.03 mM and 0.07 ± 0.0006 mM, 0.06 ± 0.0005 mM respectively than the positive control drug Pioglitazone having EC 50 of 32.38 ± 0.2 and 38.03 ± 0.13 mM for both receptors. Compared to the standard drug pioglitazone (5 mg/kg/day), the evaluation of compounds 83a and 83c on a diabetic rat model revealed that two compounds significantly lowered the blood glucose level of diabetic rats. The liver and kidney biochemical indicators TBARS, GSH, CAT were normal following treatment with compounds 83a and 83c when being compared to the negative control group (diabetes group). Furthermore, histological examinations of the kidney and pancreas confirmed that compounds 83a and 83c were effective at reversing tissue regeneration. Therefore, compounds 83a and 83c acted as PPAR-a and PPAR-c agonists 84 . Compound 84c demonstrated higher anti-proliferative activity and was also found to be active against the tumour-associated carbonic anhydrase IX (hCAIX) isoform, with an IC 50 value of 4.23 mM. As a result of further optimisation, compound 84c was transformed into 84d, which doubled its anti-proliferative effect in HCT-116 cells (IC 50 ¼ 6.0 mM) and demonstrated significant hCAIX inhibitory potential (IC 50 ¼ 0.74 mM). Compound 84d treatment decreased CAIX expression, induced apoptosis and reactive oxygen species (ROS) production, and inhibited colon cancer cellcolony formation and migration. Additionally, this result establishes a reasonable preclinical basis for further optimising compound 84d as a potential lead for the treatment of colorectal cancer 85 .
Wang et al. designed and synthesised a series of new 1,2,4oxadiazole derivatives according to the strategy of multi-target directed ligands. All compounds were evaluated for glycogen synthase kinase 3b (GSK-3b) inhibition, anti-neurin-inflammatory, neuroprotective activity, and effects on glucose consumption in HepG2 cells. The results showed that compound 85 ( Figure 6) had GSK-3b inhibition (IC 50 ¼ 0.19 mM) and anti-neuroinflammatory potency (IC 50 ¼ 6.94 ± 2.33 mM). The effect of compound 85 on glucose consumption was higher than that of the positive drug Metformin. Compound 85 significantly reduced Ab-induced Tau hyperphosphorylation, thereby inhibiting GSK-3b at the cellular level. Notably, compound 85 exhibited a good inhibitory effect on the formation of intracellular ROS. Furthermore, these compounds cross the blood-brain barrier and are not neurotoxic at a concentration of 50 mM. Finally, in vivo experiments showed that compound 85 ameliorated cognitive impairment in a scopolamineinduced mouse model. The results indicated that compound 85 deserves further study as a multifunctional lead compound 86 .
2.2.9. 1,2,4-Oxadiazole-amide based compounds Qiu et al. found a potent and selective reversible BTK inhibitor. Compound 86 ( Figure 6) exhibited 58 nM BTK inhibitory potency and high kinome selectivity in human whole blood. It inhibited only two off-target kinases, FGR (IC 50 ¼ 5.36 mM) and Src (IC 50 ¼ 27.12 mM), and its inhibitory activity was closest to that of BTK. In addition, the compound exhibited favourable pharmacokinetics and showed potent dose-dependent efficacy in a rat CIA model 87 .
Frejat et al. synthesised a series of 1,2,4-oxadiazole derivatives to evaluate the activity of the compounds against DNA gyrase and topoisomerase IV as well as many Gram-negative and Grampositive strains. The results showed that compound 87 ( Figure 6) had an IC 50 of 0.21 lM against Staphylococcus aureus DNA gyrase and an IC 50 value of 120 nM against Escherichia coli DNA gyrase, which was more active than novobiocin (170 nM). Furthermore, it showed activity against Staphylococcus aureus topoisomerase IV (IC 50 ¼ 5.2 lM) and Escherichia coli topoisomerase IV (IC 50 ¼ 3.07 lM). Compared to Ciprofloxacin (30 and 60 ng/mL, respectively), compound 87 exhibited excellent antibacterial activity against Gram-positive bacteria, with MIC values of 62 and 24 ng/ mL against Escherichia coli and Staphylococcus aureus, respectively. Moreover, the cytotoxicity of compound 87 was very low 88 .
The 1,3,4-oxadiazole and 1,2,4-oxadiazole show bioisosteric equivalency with ester and amide moieties. The presence of 1,3,4oxadiazole or 1,2,4-oxadiazole moiety in medicinal agents can modify their polarity and flexibility, hence the biological activities are significantly improved due to various bonded and nonbonded interactions, such as hydrogen bond, steric, electrostatic, and hydrophobic with target sites. 1,3,4-oxadiazole and 1,2,4-oxadiazole can therefore be deemed as potential framework for the novel drug development. In summary, we believe that the search for promising new modifications of 1,3,4-oxadiazole and 1,2,4-oxadiazole will contribute to the development of specific, low toxic and high potent drugs, still worthy of long-term efforts by chemical researchers.

Disclosure statement
The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this article.