1 , 2 , 4-and 1 , 3 , 4-Oxadiazoles as Scaffolds in the Development of Antiparasitic Agents

In this review, we present the potential use of the heterocyclic oxadiazole rings in the design and synthesis of new drugs to treat parasitic infections. We intend to compare herein all the four isomeric forms of oxadiazole rings as well as discuss the differences and similarities between them. In addition, we discuss aspects on their reactivity that justify the great importance of both 1,2,4and 1,3,4-oxadiazoles isomers when compared with their other two isomers. Although some oxadiazole isomers satisfy Hückel’s rule, there are differences concerning their aromaticity, which have a great impact on the possible interactions of the oxadiazole ring with biological receptors. The set of works selected from the literature and discussed herein points out the oxadiazole core as an important and versatile scaffold in the development of new chemical entities potentially useful as antiparasitic drugs.


Nitrogen heterocycles and drug discovery
Nitrogen heterocycles play an important role in the drug discovery scenario. 1The nitrogenated cores commonly occur as fragments in the structure of most drugs with varied ring sizes; aromatic and nonaromatic rings; fused and bicyclic rings.Nitrogen heterocyclic drugs are present in all therapeutic areas including cardiovascular and metabolic illnesses, central nervous system (CNS) disorders, antiinflammatory, antineoplastic, anti-infective drugs, among others.In the remarkable review published by Njardarson's group, 2 from the University of Arizona, the authors indicate that 59% of unique small-molecule drugs approved by the FDA (Food and Drug Administration, USA) contain a nitrogen heterocycle.Figure 1 shows the 881 FDAapproved drugs containing a nitrogen heterocycle presented by ring size among other structural profiles. 2igure 2 shows four examples of bestseller drugs containing nitrogen heterocyclic systems in their structures.Atorvastatin (1, Pfizer) is a statin that acts to inhibit HMG-CoA reductase, an enzyme that plays a key role in the production of cholesterol.Sildenafil (2, Pfizer) is a PDE5 inhibitor used clinically in the treatment of erectile dysfunction.Imatinib (3, Novartis) is a 2-phenylaminopyrimidine derivative which functions as a specific inhibitor of several tyrosine kinase enzymes.Imatinib is a drug used to treat chronic myelogenous leukemia and other types of cancers.Losartan (4, Merck & Co) is an antagonist of angiotensin II type 1 receptor (AT 1 ), indicated for the treatment of hypertension (Figure 2). 3 Figure 3 shows the structures of five heterocyclic anti-infective agents.Ketoconazole (5) is an important imidazolic antifungal that acts inhibiting the enzyme responsible for fungal ergosterol biosynthesis, 14α-demethylase. 4Metronidazole (6) and benznidazole (7)  are two antiprotozoal drugs acting through the generation of intracellular radical species which can damage both parasites' DNA and other cellular machinery. 5The antibacterial fluoroquinolone ciprofloxacin (8) acts through the damage of bacterial DNA. 6Raltegravir ( 9) is an HIV-1 integrase inhibitor, useful for the treatment of HIV infections. 7y two atoms of carbon, two atoms of nitrogen and one atom of oxygen.They were firstly discovered in 1884 by Tiemann and Krüger, 8 then named furo[ab]diazoles.Oxadiazoles can be isosterically compared with furan, 9 but the replacement of two methine groups (−CH=) by two sp 2 nitrogen (−N=) reduces their aromaticity so that some of their isomers are electronically comparable to conjugated diene systems.Oxadiazoles can be found in four different isomeric structures (Figure 4). 10 It is possible to find molecules bearing oxadiazole moieties with application in several areas, as luminescent materials, 11 electron-transport materials, 12 polymers, 13 herbicides, 14 and corrosion inhibitors, 15 for example.This review will focus on molecules with importance to medicinal chemistry, more specifically to the development of new chemical entities with antiparasitic properties.The stability of the oxadiazole rings in aqueous medium is one of the most important characteristics that justify the interest in the development of bioactive molecules containing this motif.1,2,4-Oxadiazole rings, for example, are stable even in the presence of concentrated sulfuric acid. 8Another relevant aspect of oxadiazoles is their capability of acting as a hydrogen bond acceptor, due to the nonligand electron pairs from the heteroatoms present    in their structures. 16In drug discovery and development, oxadiazole rings are commonly employed as bioisosteric replacement for carbonyl-containing groups such as esters, amides, carbamates and hydroxamic esters.Such groups are usually unstable in biological media, which poses as an obstacle for their use in the structure of drug candidates. 17t the same time, their spatial geometry is similar to the oxadiazole bioisosters, so the new compounds shall bind similarly at the same bioactive sites. 18or a long time, oxadiazolic rings could only be obtained via synthetic routes.However, in 2011, two naturally occurring 1,2,4-oxadiazoles were found: phidianidines A (10a) and B (10b), whose structures are shown in Figure 5.Such molecules were isolated from the aeolid opisthobranch Phidiana militaris, 19 a shell-less marine mollusk that inhabits Southern India. 20The natural phidianidines displayed a potent and selective cytotoxic profile against tumor and nontumor cell lines (C6, HeLa, CaCo-2, 3T3-L1 and H9c2).In addition, they exhibited selective binding properties with CNS sites, such as the μ-opioid receptor and the dopamine transporter.These features represent an interesting starting point for chemical optimization based on phidianidine's structure targeting cancer, pain treatment and some CNS diseases. 21oactive molecules containing the oxadiazole ring have increasingly been discovered and studied over the last few years (Figure 6). 22It is possible to find the oxadiazole group in various molecules with reported activity as antitumor, 23 antioxidant, 24 antibacterial, 25 antiviral, 26 antiinflammatory, 27 insecticidal 28 and antiparasitic 29 agents, for example.
One can observe that the 1,2,4-and 1,3,4-isomers of oxadiazolic rings are significantly more common in medicinal chemistry than both their 1,2,3-and 1,2,5analogues.1,2,3-Oxadiazoles are commonly observed in the diazoketone tautomeric form, due to its instability in the cyclic form.Studies have shown that 3-oxides of 1,2,3-oxadiazoles are more stable structures.However, the complicated synthetic route for their preparation still restricts the exploration of this isomeric form. 30,2,5-Oxadiazoles are more stable structures and more explored in medicinal chemistry research, including as antiparasitic agents. 31In spite of that, this isomeric form orients side chains to different positions compared with 1,2,4-and 1,3,4-oxadiazole counterparts, which display closer chain arrangement comparing to most natural ligands. 18Recently, Andricopulo and co-workers 32 reported the development of a 2D-QSAR-(quantitative structure-activity relationship) and 3D-QSAR-based pharmacophore for cruzain using several differently substituted oxadiazoles as molecular probes.This abundant T. cruzi cysteine protease is a validated biochemical target to the development of antichagasic chemotherapy.The wider application of 1,2,4-and 1,3,4-oxadiazoles on medicinal chemistry is also observed regarding research on antiparasitic agents.Therefore, this review will focus on the physicochemical and biological properties, synthetic methods and antiparasitic applications of such isomers.
These physicochemical properties reflect in the reactivity of each heterocycle.For example, 1,2,4-oxadiazoles can react with strong nucleophiles such as n-butyllithium in THF (tetrahydrofuran) at -78 °C, via a nucleophilic addition, to form the derivatives depicted below (Figure 8). 36However, 1,3,4-oxadiazoles do not react under the same reaction condition due to the fact they are much less electrophilic.It is certainly linked to the aromaticity of the 1,3,4-oxadiazolic core, which favors substitution in place of addition reactions.The 1,3,4-oxadiazole core's properties are similar to an aromatic heterocycle.Being so, it has typical interactions of aromatic systems, such as π-π stacking with hydrophobic amino acids such as tyrosine, phenylalanine and tryptophan.Dhumal et al. 37 synthesized a series of 1,3,4-oxadiazoles aiming to inhibit the mycobacterial enoyl reductase (InhA) of Mycobacterium tuberculosis.Docking studies carried out by Dhumal et al. 37 on InhA showed that the oxadiazole core of 16 interacts with Phe149 and Tyr158 through a π-π stacking, as shown in Figure 9.This kind of interaction also happens in other 1,3,4-oxadiazoles designed by the authors, 37 with relevant changes only at the level of the other structure decorations present, like a pyridine ring.
Another possible interaction involving 1,3,4-oxadiazoles and proteins is the cation-π interaction.This type of noncovalent interaction is rather uncommon and quite different from the complexes of transition metals and π-systems because of the d orbitals present in the metal.The interaction is based on the effects caused by the proximity of a monopole (the cation) and a quadrupole (the electron-rich π-system). 38Taha et al. 39 synthesized a series of α-glucosidase inhibitors with a 5-aryl-2-(6'-nitrobenzofuran-2'-yl)-1,3,4-oxadiazole scaffold.Compound 17 was the second most active (the first one was the reference drug, acarbose).Docking studies show that the 1,3,4-oxadiazole moiety interacted with residue His279 by a cation-π interaction, as shown in Figure 10. 39nsidering the aromaticity of the 1,3,4-oxadiazole core as a 6π electron system, the two nitrogen atoms have each one electron pair fully available to receive a hydrogen bond.These interactions are observed in the binding site   of raltegravir (9) in the HIV-1 integrase.The nitrogens are linked to the cation Mg 2+ by two water molecules, as shown in Figure 11a.Curiously, in another possible binding pose, the oxadiazole moiety can also interact with histidine residue His114 by a cation-π interaction as presented before (Figure 11b). 40n a different manner, the 1,2,4-oxadiazole motif present on derivative 18 forms a hydrogen bond with Thr206 of the binding pocket of the GABA A receptor, even being close to Tyr209.This might be explained by the less aromatic profile of this heterocycle, as seen in Figure 12. 41 Kamal et al. 42 reported a series of 1,2,4-oxadiazolic derivatives, such as 19a-b, which tend to form hydrophobic interactions between the heterocycle and α-Thr179 apart from π-π stacking with β-Asn258 on the colchicine binding site of bovine tubulin (Figure 13).

Synthesis of 1,2,4-oxadiazoles
The classic synthetic routes for 1,2,4-oxadiazoles rely in the use of nitriles as precursors, as shown in Scheme 1: (a) one approach is based on the 1,3-dipolar cycloaddition of nitriles to nitrile N-oxides; (b) the second approach is through obtaining an amidoxime intermediate from a nitrile followed by O-acylation of the amidoxime and cyclization of the O-acylamidoxime as the final step. 9  The formation of the 1,2,4-oxadiazolic ring via amidoxime route needs an acylating agent, which carries the substituent group that will be positioned at C 5 .Mohammadi-Khanaposhtani et al. 41 reacted differently substituted benzamidoximes (20) with 2-chloro acetyl chloride (21) as acylant agent.Because of their high reactivity, acyl chlorides enable reactions of O-acylation at room temperature, yielding the acylamidoximes (22).The reaction to form 1,2,4-oxadiazole intermediates (23)  was then carried out in toluene under reflux, followed by an S N 2 reaction with differently substituted acridones (24), yielding compounds 18 with anticonvulsant profiles (Scheme 2). 41utirite et al. 43 presented a faster and easier methodology to prepare 1,2,4-oxadiazoles as part of a program focused on antibacterial agents and corrosion inhibitors.They used aromatic nitriles (25) in the presence of hydroxylamine hydrochloride and sodium carbonate in ethylene glycol/ water 3:1 as solvent.The reaction was carried out under microwave irradiation until reflux for 2 h in a one-pot reaction (Scheme 3).Yields ranging from 71 to 92%, simpler experimental procedure, lower reaction times and, consequently, lower energy consumption than previous methods are interesting features of this experimental procedure.On the other hand, it only allows the synthesis of symmetrically substituted 1,2,4-oxadiazoles (26). 43t is also possible to find protocols employing ultrasound irradiation to synthesize 1,2,4-oxadiazoles ( 29), as reported by Bretanha et al. 44 Short reaction times (15 min) and high yields (84-98%) were reported for the reaction between trichloroacetoamidoxime ( 27) and the properly substituted benzoyl chlorides (28) in ethyl acetate as solvent (Scheme 4). 44n a different approach, Yoshimura et al. 45 reacted nitrile oxides (34) and nitriles (31) to obtain 1,2,4-oxadiazoles (32).Hydroxy(aryl)iodonium (IBA-OTf, 33) was employed allowing the formation in situ of nitrile oxides (34)  from the respective nitriles (31).IBA-OTf (33) can be generated in situ by addition of 2-iodo-benzoic acid (35), trifluoromethanesulfonic acid (TfOH) and Scheme 3. Synthetic methodology for obtaining symmetrically substituted 1,2,4-oxadiazoles (26) as potential antibacterial agents and corrosion inhibitors.

Synthesis of 1,3,4-oxadiazoles
There are several multistep and one-step protocols in the literature to the synthesis of 1,3,4-oxadiazole derivatives.Taha et al. 39 reacted methyl 6-nitrobenzofuran-2-ate (44)  with diazene yielding the hydrazide (45).Next, it was reacted with properly substituted aromatic acids in the presence of phosphoryl chloride, furnishing 1,3,4-oxadiazoles (17), as shown in Scheme 8.The products were evaluated for their inhibitory activity over the enzyme α-glucosidase. 39hao et al. 48produced nonsymmetrical disulfides bearing a 1,3,4-oxadiazolic ring with antiproliferative activity.To build the 1,3,4-oxadiazole moiety the authors reacted differently substituted benzohydrazides (46) in the presence of carbon disulfide and potassium hydroxide in ethanol, yielding intermediates (47).Then, the oxadiazoles were reacted with the previously prepared S-alkyl-thioisothiourea intermediates (48), furnishing the disulfides (49) (Scheme 9). 48n their investigation for antitubercular agents, Desai et al. 49 obtained a 1,3,4-oxadiazole derivative through the reaction of indole-3-carbaldehyde (50)  with isoniazid (51) under microwave irradiation.The intermediate 52 reacted with acetic anhydride under microwave irradiation yielding the 1,3,4-oxadiazole (53) (Scheme 10). 49iu et al. 50used iodine to mediate the formation of C-O bond in a one-pot procedure.Aldehydes with different substitution patterns (54) were reacted with semicarbazide hydrochloride in the presence of sodium acetate in methanol/water.After removing the solvents, the residue was dissolved in 1,4-dioxane followed by addition of iodine and potassium carbonate.The 1,3,4-oxadiazole derivatives (55) were obtained after 1-4.5 h at 80 °C, as shown in Scheme 11. 50t is also possible to find protocols employing heterogeneous catalyst systems to obtain 1,3,4-oxadiazoles in one step.Sanghshetti et al. 51 described the reaction of hydrazide (56) and several aroyl aldehydes (57) in the presence of nanoparticles of ZnO-TiO 2 under microwave irradiation, affording the respective 1,3,4-oxadiazoles (58).Reduced reaction times (12-15 min) and high yields (91-96%) are interesting features of this protocol (Scheme 12). 51uresh et al. 52 described the use of Al 3+ -K10 clay catalyst in the reaction of hydrazides (59) and trimethyl orthoesters (60).The reaction was performed solvent-free under microwave irradiation (T = 55 °C) giving 1,3,4-oxadiazole derivatives (61).One advantage of this protocol is the possibility of synthesizing 1,2,4-oxadiazoles (63) just by using amidoximes (62) instead of hydrazides, keeping the same experimental procedure (Scheme 13). 52ouhani et al. 53 utilized ultrasound irradiation to react aromatic carboxylic acids (64) and acenaphthoquinone (65), obtaining 1,3,4-oxadiazoles (67).The reaction was carried out in the presence of (N-isocyanimino)triphenylphosphorane (66) and acetonitrile as solvent.Overall yields ranged from 67 to 85% (Scheme 14).The comparison between the procedures with and without sonication showed a significant reduction in time: 15 min with sonication versus 24 h without sonication.In addition, an average increase of 20% in the yields was observed when sonication was employed, corroborating the relevance of this method. 53 Antiparasitic Activity of Selected 1,2,4-and 1,3,4-Oxadiazoles 1,2,4-and 1,3,4-oxadiazole rings play significant roles in medicinal chemistry and drug design.They can be found in the structure of molecules with reported bioactivity to several targets and related to a wide range of diseases.Parasitic diseases, such as trypanosomiasis, leishmaniasis, helminthiasis and ectoparasitosis, for example, form an important niche of study in this context.Such illnesses are commonly associated with poor countryside populations living in tropical countries.They get few research efforts from big pharmaceutical companies, which do not see the drug development for these diseases as a profitable investment.That is the reason the World Health Organization (WHO) has labeled this set of infections as neglected tropical diseases (NTDs).54 There are several reviews in the literature of 1,2,4and 1,3,4-oxadiazoles focusing on their synthesis and applications.55 In spite of that, the present work is the first review highlighting the importance of these heterocycles specifically in the development of new drugs with antiparasitic properties.

1,2,4-Oxadiazoles as antiparasitic agents
The first report on 1,2,4-oxadiazoles with antiparasitic activity was published in 1966 by Ainsworth et al. 56 The group described the synthesis of a series of 3-substituted 1,2,4-oxadiazoles and their anthelmintic activity in different in vivo infected rodent models.The most potent derivative was 3-p-chlorophenyl-1,2,4-oxadiazole, 68 (Table 1).It reduced the Nematospiroides dubius worm count by 94% when administered orally (100 mg kg -1 , single dose) and by 100% when injected subcutaneously (500 mg kg -1 , single dose).Compound 68 was also assayed for anthelmintic activity against Nippostrongylus muris (250 mg kg -1 , oral dose, 100% efficacy), Syphacia obvelata (1000 mg kg -1 , oral dose, 90% efficacy), Strongyloides ratti (1000 mg kg -1 , oral dose, active), and Trichostrongylus axei (250 mg kg -1 , oral dose, active). 56augwitz et al. 57 reported the anthelmintic activity of a series of novel isothiocyanatophenyl-1,2,4-oxadiazoles against N. dubius and Hymenolepis nana in infected mice.The results showed the most potent compounds of the series against H. nana were 69 and 70a-c (Table 2).The drugs displayed 100% efficacy after a 100 mg kg -1 single-dose treatment.Compounds 69 and 70a were the most potent against N. dubius under the same drug regimen, with efficacies of 100% and 90%, respectively.Anthelmintic efficacy was also determined in sheep naturally infected with a mixed gastrointestinal nematode population.A 100% efficacy, determined by fecal egg count, was observed after oral administration of 69 (100 mg kg -1 , a single dose).Compounds 70a and 70b, at the same drug regimen, displayed efficacies of 99 and 73%, respectively.Compound 70c was inactive to mixed gastrointestinal nematodes in sheep.Compound 69 also showed 100% efficacy against Ancylostoma caninum in infected dogs treated with a single oral dose at 200 mg kg -1 .The authors pointed out the presence of the isothiocyanatophenyl moiety at the 3-position of 1,2,4-oxadiazolic ring as an important feature to anthelmintic activity. 57ottrell et al. 58 designed a series of analogues based on the structure of WR85915 (71). 58The prototype had been previously described as a tubulin inhibitor tested in vitro against Leishmania donovani (IC 50 = 5.6 μmol L -1 , promastigotes; IC 50 = 4.3 μmol L -1 , axenic amastigotes). 59he new molecules were assayed for antiprotozoal activity against L. donovani and Trypanosoma brucei.Compound 72a (Table 3) displayed the best antiparasitic profile to both L. donovani (IC 50 = 2.3 μmol L -1 , axenic amastigotes) and T. brucei (IC 50 = 5.2 μmol L -1 , variant 221).On the other hand, it displayed variable cytotoxicity values against mammalian cell lines (J774, PC3 and Vero cells).It influenced the selectivity indexes (SIs), which is the ratio between IC 50 values against mammalian cells and protozoan.The authors observed a positive correlation between the lipophilicity of the 3-aryl ring and the antileishmanial activity.It was also observed that derivatives with an alkyl chain in the C5 of 1,3,4-oxadiazolic ring (72b-c) were ineffective against the different protozoan species assayed.It suggests that the 5-thioisocyanatomethyl group is an important feature for antikinetoplastid activity of this series (Table 3). 58he prototype compound WR85915 (71), for its part, confirmed its activity against protozoan (IC 50 = 4.5 μmol L -1 , axenic amastigotes of L. donovani; IC 50 = 5.2 μmol L -1 , variant 221 of T. brucei) and displayed lower cytotoxic profile against mammalian cell lines.These results justified the antiprotozoal studies of 71 in vivo in a rodent model (BALB/c mice) of visceral leishmaniasis.In comparison with reference drugs pentostam and pentamidine, compound 71 displayed good inhibition values of liver parasitemia, as shown in Table 4. 58 Santos-Filho et al. 60 described the synthesis, biological activity and docking results of a series of 3-(4-substitutedaryl)-1,2,4-oxadiazoles-N-acylhydrazones as anti-Trypanosoma cruzi agents.The in vitro assessment for antiprotozoal activity was conducted against trypomastigotes of T. cruzi (Y strain).Compounds 73a and 73b displayed higher potency than benznidazole, the reference drug for the treatment of Chagas disease (IC 50 values of 3.6 and 3.9 μmol L -1 for 73a and 73b, respectively; IC 50 = 5.0 μmol L -1 for benznidazole).In addition, 73a and 73b were also assayed in vitro for cytotoxicity against BALB/c mouse splenocytes.The results showed moderate to high selectivity in comparison with a mammalian model (SI = 26.4 and 7.7 for 73a and 73b, respectively).A docking study was realized with an in silico model for cruzain (PDB code: 1U9Q). 60This cysteine protease is known to participate in several biological processes of the parasite, such as proteolysis, cellular invasion and host immune system evasion. 61The docking scores observed for the most potent molecules of the series indicate their main mechanism of action could be related to cruzain inhibition.Chemical structures, IC 50 values against trypomastigote of T. cruzi (Y strain), cytotoxicity against mouse splenocytes (maximum noncytotoxic concentration), SIs and docking scores (GOLD) for compounds 73a-b and benznidazole are shown in Table 5. 60 A work published by Dürüst et al. 62 in 2012 described the synthesis of a series of novel compounds containing both 1,2,4-oxadiazole and 1,2,3-triazole heterocyclic rings.The molecules were assayed in vitro for antiprotozoal activity against T. brucei rhodesiense (bloodstream forms, STIB 900 strain), T. cruzi (intracellular amastigotes in L6 cells, Tulahuen strain C2C4), L. donovani (axenic amastigotes, MHOM/ET/67/L82 strain) and Plasmodium falciparum (erythrocytic stages, K1 strain).Cytotoxicity was measured against mammalian cells (L6 rat myoblasts cell line) and SI's were calculated for this review.Results showed that compound 74a was the most active against both T. b. rhodesiense (IC 50 = 7.0 μg mL -1 , SI = 6.8) and L. donovani (IC 50 = 1.6 μg mL -1 , SI = 29.6).The most active compound against P. falciparum was 74b (IC 50 = 13.2 μg mL -1 , SI = 1.2), which also showed moderate activity towards T. b. rhodesiense (IC 50 = 8.4 μg mL -1 , SI = 1.8) and L. donovani (IC 50 = 2.2 μg mL -1 , SI = 6.9).However, its toxicity to the mammalian cell line L6 (IC 50 = 15.2 μg mL -1 ), and consequent low selectivity, indicates it can be harmful to both parasite and host.Compound 74c figured as one most active of the series in addition to display the highest SI values when assayed against T. b. rhodesiense (IC 50 = 8.0 μg mL -1 , SI > 12.5), L. donovani (IC 50 = 2.0 μg mL -1 , SI > 50) and P. falciparum (IC 50 = 14.7 μg mL -1 , SI > 6.8).None of the molecules displayed IC 50 values lower than 30 μg mL -1 against T. cruzi.The halogen substitution at para position (74e-f) of phenyl group attached to oxadiazole ring have little impact on antiprotozoal activity.On the other hand, it was observed that fluorination (74d) affects selectivity, once the fluorinated derivative showed no toxicity to L6 cells at 100 μg mL -1 .Compounds 74a-f chemical structures and the respective in vitro results are shown in Table 6. 62 later work by Santos-Filho et al., 63 in 2016, described the synthesis and antimalarial activity of a series of N-acylhydrazone-1,2,4-oxadiazoles derivatives (75).The compounds were assessed in vitro against chloroquineresistant W2 strain of blood stage P. falciparum and for cytotoxicity against human cell line HepG2.The standard drug mefloquine was used as a positive control and SI values calculated in all cases based on the IC 50 values a Pentamidine produced irritation on administration of full dose on day 1.The dose was reduced by 50% for all mice for the following observed, as shown in Table 7. Compound 75b displayed the lowest IC 50 value (0.07 μmol L -1 ), similar to mefloquine (0.04 μmol L -1 ), followed by 75a (IC 50 = 0.12 μmol L -1 ) and 75c (IC 50 = 0.20 μmol L -1 ).Compounds 75d-e presented IC 50 values 37.5 to 47.5-fold lower than the standard drug, but their low cytotoxicity to human HepG2 cells is an interesting result.In addition, in vivo studies were conducted on Swiss mice (groups of n = 5) infected with Plasmodium berghei (NK65 strain) to determine the influence of the treatment on blood parasitemia and survival rates.The drug candidates with lowest IC 50 values (75a-c) on the in vitro tests were selected to this evaluation and compared with the standard drug chloroquine.At 50 mg kg -1 day -1 for 4 days, administered orally, none of the test compounds (75a-c) could either reduce parasitemia (after 8 days) or improve survival rates (after 30 days).Under the same drug regimen, but administered intraperitoneally, only 75c reduced parasitemia (0%) compared with the vehicle (10% DMSO (dimethyl sulfoxide) on saline, 50% parasitemia).
On the other hand, the survival rate for animals treated with 75c was only 50%, which indicates the drug could have toxic effects towards the host.Treatment with chloroquine both orally and intraperitoneally led to 0% parasitemia and 100% survival rate.The in vivo results presented on Table 7 refer to 50 mg kg -1 day -1 drug regimen (4 days) administered intraperitoneally. 63

1,3,4-Oxadiazoles as antiparasitic agents
An early work by Hutt et al. 64 described the synthesis of a series of differently substituted 2-trichloromethyl-5-phenyl-1,3,4-oxadiazoles.The compounds were inspired by the chemical structure of 1,4-bis-(trichloromethyl) benzene (76, hetol), based on previous reports of its antimalarial activity in infected monkeys. 65Test molecules were assayed for antimalarial activity in BALB/c mice infected with P. berghei (n = 5 groups).The animals were treated with a single subcutaneous dose of test compounds (640, 320, 160 and 80 mg kg -1 ) and compared with hetol at the same drug regimen.Compounds 77a-b displayed the highest cure rates (60-80%) and extension of mean survival times (ΔMST, days) in comparison with nontreated controls.It was observed 100% cure rate in animals treated with hetol.The series of 1,3,4-oxadiazoles was also tested for antimalarial activity against Plasmodium gallinaceum in chicks.Test compounds were administered subcutaneously in a single dose.Only compound 77c led to cure (60% at 240 mg kg -1 ).Chemical structures and biological evaluation results for compounds 77a-c and hetol are shown in Table 8.
The events of cure are represented by C (n), while the deaths due to drug toxicity are represented by T (n), where n = frequency. 64ore recently, Balaji et al. 66 reported the synthesis and antimalarial evaluation of 20 novel 1,3,4-oxadiazole derivatives.The compounds were assayed against chloroquine-sensitive (NF54) and chloroquine-resistant (Dd2) strains of P. falciparum.Submicromolar IC 50 values were observed for both strains.We highlight here the results for five derivatives (78a-e, Table 9).The compounds displayed comparable (NF54) or higher (Dd2) toxicity to parasites compared with reference drug chloroquine.Based on the full series of compounds tested, the authors indicated the importance of the imidazole ring and the bromine atom to the antimalarial activity. 66as et al. 67 reported in 1980 the synthesis and biological assessment of 1,3,4-oxadiazoles and other 5-membered heterocycles designed similarly to 3,4-dimethyl-2,5-bis(4-guanylphenyl)-furan (79), previously reported as a potent antitrypanosomal molecule. 68Antiparasitic activity of test compounds was determined against T. brucei rhodesiense in infected BALB/c mice (n = 5 groups).Among the active molecules, 1,3,4-oxadiazole 80 (Table 10) displayed a 100% cure rate at 26.5 mg kg -1 .
However, it showed toxicity to the mammalian host at higher doses. 67atel et al. 69 described the synthesis, characterization and anthelmintic activity of a series of 1,3,4-oxadiazoles.Biological assessments were performed against earthworms (Perituma posthuma) in saline solution, having albendazole as reference drug.Paralysis and death times were measured respectively as the necessary times for earthworms to become motionless and to be inert to external stimuli.Compounds 81b, d, e and 82a-d displayed significant paralytic times, while compounds 81a-e showed better lethal times than that observed for the standard drug.Compound 81e displayed better paralysis and death times than albendazole.The chemical structures of compounds 81a-e and 82a-d are shown in Figure 14 and the biological activities against P. posthuma are given in Table 11. 69106 mg kg -1a 80 mg kg -1a 26.5 mg kg -1a 20 mg kg -1a 13.3 mg kg -1a 10 mg kg -1a 6.6 mg kg -1a 5 mg kg -1a 2.5 mg kg -1a 1.   Nitroheterocyclic molecules bearing the 1,3,4-oxadiazole group were reported as active against T. cruzi by Ishii et al. 70 Nitroheterocycles, such benznidazole and nifurtimox, are drugs known for their inhibitory activity against T. cruzi.However, they are also toxic to the mammalian host, being responsible for severe side effects such as anorexia, weight loss, nausea and vomit, nervous excitation, insomnia, psychic depression, convulsion, vertigo, headaches, sleepiness, balance disorder, memory loss and hepatic intoleration. 71The compounds 83a-o were planned based on the structure of nitroheterocyclic drugs.The antitrypanosomal activity was measured against epimastigote forms of T. cruzi (Y strain) along with reference drug benznidazole.IC 50 values revealed all molecules are potent inhibitors.In fact, only compound 83j was less potent than benznidazole, but still comparable to the reference drug.The other derivatives in the series displayed IC 50 values ranging from 1.4-to 2.6-fold lower than benznidazole.Derivative 83o was the most potent of the series showing an IC 50 = 7.91 μmol L -1 against T. cruzi epimastigotes.The analysis of Table 12 shows a slightly higher activity for derivatives substituted with resonance electron-withdrawing groups (83m-o). 70aha et al. 72 discussed the antileishmanial potential of an extensive series of 30 1,3,4-oxadiazole-phenylhydrazones hybrids.In vitro antileishmanial activity was determined on THP-1 cells infected with Leishmania major.Cytotoxicity to mammalian cells was measured on THP-1 and U-937 cell lines.Pentamidine was used as the reference drug.All tested molecules displayed antileishmanial activity, with IC 50 values ranging from 0.95 ± 0.01 μmol L -1 to 78.6 ± 1.78 μmol L -1 .Thirteen of them exhibited IC 50 values lower or comparable to pentamidine's (IC 50 = 7.02 μmol L -1 ).We selected for this review the seven most promising compounds (84a-g) based on their activity against L. major and calculated SI values.With the exception of 84g, all these compounds presented low toxicity against U-937 and THP-1 cells (IC 50 ≥ 100 μmol L -1 ).Chemical structures of selected compounds with respective IC 50 values are shown in Table 13. 72olecular docking studies were carried out on pteridine reductase (PTR1, PDB code: 1E7W).This enzyme, present in Leishmania species, is responsible for reduction of biopterin, a vital condition for in vitro parasite growth. 73Compounds 84d-g were selected to investigate whether the mode of action of the 1,3,4-oxadiazoles described by Taha et al. 72 could be related to PTR1 inhibition.The main interactions observed were hydrogen bonds between the differently oriented -OH groups and hydrogen bond acceptors on the binding site.In addition, well-formed π-π stacking, cation-π, hydrophobic and hydrogen bond interactions are features that help the stabilization of test compounds on PTR1. 72haves et al. (2017) 74 studied the antileishmanial activity of gold(I) complexes with 1,3,4-oxadiazole-2-thione derivatives.All of the complexes prepared for this work displayed IC 50 values below 11 μmol L -1 for promastigotes (Leishmania infantum and Leishmania braziliensis) and 5 μmol L -1 for amastigotes (L.infantum).In addition, cytotoxicity to human macrophages THP-1 was determined for each compound, as well as the respective SI values.The biological assessment also included the evaluation of ligands and precursors involved on the synthesis of the complexes.Amphotericin B (AmB) was used as a positive control.Among this series, 85a-b were the most active candidates against intracellular amastigotes (IC 50 = 0.97 and 1.41 μmol L -1 , respectively).The chemical structures of complexes 85a-b and the respective oxadiazolic ligands 86a-b along with IC 50 values for complexes, ligands, the precursor Au(PEt 3 )Cl and AmB are displayed on Table 14. 74n addition, authors concluded that novel gold(I) complexes can be useful for understanding and overcoming drug resistance in Leishmania.Sb V -based drugs such as pentostam and glucantime are prodrugs.They are reduced in biological media assuming their Sb III active form.These drugs act mostly through inhibition of trypanothione reductase (TryR), disrupting the parasite's redox homeostasis.Gold(I) complexes can act through similar pathways with no cross resistance for Sb-resistant Leishmania parasites.Several biochemical and biophysical changes were observed in parasite's cells, including increased membrane fluidity.However, the mechanism of action related to gold(I) effects on Sb-resistant Leishmania parasites needs to be further investigated. 74

Concluding Remarks and Perspectives
Regarding their occurrence in the structure of biologically active molecules, 1,2,4-and 1,3,4-oxadiazoles notably stand out from their two other isomers.These heterocycles can mimic amide and esters in the biological environment, behaving as bioisosters of these functions, with the advantage of being resistant to hydrolysis reactions.The structural and electronic differences regarding the aromaticity of 1,2,4-and 1,3,4-oxadiazoles have an important influence on the possible interactions that these nuclei carry out with several Their relevance is also due to the several synthetic alternatives for their preparation, in addition to the possibility of these heterocycles presenting different substitution patterns, pointing them as scaffolds on which new chemical entities possessing high structural diversity can be planned and easily prepared.Then, 1,2,4-and 1,3,4-oxadiazoles emerge as structural subunits of great importance and versatility for the development of new drug candidates applicable to the treatment of parasitic infections, especially in the field of NTDs.Certainly, in the coming years, we will see an increasing number of works using oxadiazolic nuclei as scaffolds in the design and synthesis of new antiparasitic drug candidates, in addition to other therapeutic applications.

Figure 1 .
Figure 1.Nitrogen containing heterocycles and their relative distribution among the FDA approved drugs (adapted from reference 2).

Figure 2 .
Figure 2. Structures of some bestselling drugs containing nitrogen heterocyclic rings.

Figure 6 .
Figure 6.Annual publications related to oxadiazoles on medicinal chemistry over the period from 1997 to 2017.22

Figure 9 .
Figure 9. Schematic model of the π-π stacking interaction between the oxadiazole derivative 16 and the aromatic aminoacids of the binding site of Mycobacterium tuberculosis mycobacterial enoyl reductase.

Figure 11 .
Figure 11.2D representation of the best binding poses of raltegravir (9, up) in HIV-1 integrase: (a) interaction between the 1,3,4-oxadiazole moiety and the cation Mg 2+ linked by two water molecules in a hydrogen bond; (b) oxadiazole core interacting with His114 via a cation-π interaction.

Figure 12 .
Figure 12.Schematic representation of the 18 derivative forming a hydrogen bond between the 1,2,4-oxadiazole ring and threonine (A) and a π-π stacking with tyrosine via the methoxyphenyl ring (B).

Table 4 .
Activity of 71, pentostam and pentamidine against L. donovani in BALB/c mice

Table 7 .
Chemical structure and biological assessment results for antimalarial activity of 1,2,4-oxadiazoles (75a-e) selectivity index; c i.p.: intraperitoneal; d calculated 8 days after the infection in comparison with the nontreated group; e calculated 30 days after the infection in comparison with the nontreated group.Single s.c.dose in mg kg -1 , milligrams of drug administered subcutaneously per kilogram of animal's body weight; five animals used per dosage level; b calculated by subtraction of mean survival times (MST) of untreated control from treated situations; c cure defined by survival 60 or 30 days after infection for mice and chicks, respectively, n stands for the frequency of the event; d deaths occurred within 48 h after drug administration; n stands for the frequency of the event.
a Mefloquine (in vitro) or chloroquine (in vivo); b a

Table 10 .
Comparison between the antitrypanosomal activity (T.brucei rhodesiense) of prototype compound 79 and 1,3,4-oxadiazole 80 Single s.c.dose in mg kg-1, milligrams of drug administered subcutaneously per kilogram of animal's body weight, five animals used per dosage level; b calculated by subtraction of mean survival times (MST) of untreated control from treated situations; c cure defined by survival 60 days after infection; n stands for the frequency of the event; d deaths occurred within 48 h after drug administration; n stands for the frequency of the event. a

Table 11 .
Anthelmintic activity of compounds 81a-e and 82a-d

Table 13 .
Chemical structures of 1,3,4-oxadiazoles (84a-g) with the respective IC 50 values against L. major (JISH118), THP-1 and U-937 cell lines and SI values are shown Ratio between IC 50 values against THP-1 and L. major; b not active; c standard drug. a