New insides into chimeric and hybrid azines derivatives with antifungal activity

Abstract

During the last decades, five or six member rings azaheterocycles compounds appear to be an extremely valuable source of antifungal agents. Their use seems to be a very attractive solution in antifungal therapy and to overcome antifungal resistance in agriculture. The present review highlights the main results obtained in the field of hybrid and chimeric azine (especially pyridine, quinoline, phenanthroline, bypyridine, naphthyridine and their fused derivatives) derivatives presented in scientific literature from the last 10 years, with emphasis on antifungal activity of the mentioned compounds. A special attention was played to hybrid and chimeric azole–azine class, having in view the high antifungal potential of azoles.

Plain language summary

Article highlights.

• Six-member ring azaheterocycles with one nitrogen atom. Hybrid compounds.

• Hybrid azole–azine derivatives.

• Other hybrid azine derivatives.

• Six-member ring azaheterocycles with one nitrogen atom. Hybrid quinoline.

• Azine heterocycles with one nitrogen atom. Chimeric derivatives.

• Hybrid azole–azine derivatives containing imidazole or triazole moiety have an excellent antifungal activity.

• Hybrid azine derivatives with quinoline skeleton are generally more active comparative with those one with pyridine moiety.

• Chimeric imidazo–pyridine, pyrimidine–pyridine and pyrazolo–quinoline classes manifest a powerful antifungal activity.

• Both classes of hybrid and chimeric derivatives are an attractive way in future antifungal therapy.

1. Introduction

Resistance of microorganisms to antimicrobials represents one of the main challenges we are facing today. This phenomenon is fully acknowledged and is considered by the WHO as one of the biggest medical issues worldwide, with a high risk of pan-resistance development, resulting in many deaths [1,2].

Although the resistance to antimycotics in fungi is not as widespread as it is the resistance of bacteria to antibiotics [3], it represents a major problem for both agriculture (the need to eradicate fungal diseases of crops) and human health (treating fungal infections in humans). Especially in agriculture, antimycotics resistance is largely spread in some agricultural systems and leads to alarming economical costs. In 2015, the spread of Fusarium oxysporum f.sp. cubensis in many countries that are important bananas producers, has strongly affected this crop cultivation. According to the Food and Agriculture Organization (FAO), this disease conducted to a strong decrease of production and an increment of the price [4].

Furthermore, some antifungal agents are no more available for large scale use due to national and international regulations, dictated by the necessity to protect the environment and human health [5]. Some fungicides widely used in crop protection have been banned in many countries for commercialization or use, due to their environmental side effects, such as toxicity to wildlife in their initial form or as degradation residues. Other fungicides are believed to possess carcinogenic, teratogenic, cytotoxic, endocrine-disruptive or other negative properties, affecting human health [6]. Another issue of concern is the rise of new fungal pathogens, more often in the agricultural sector [7,8].

All the mentioned reasons have made the scientific researchers from both academia or related public bodies and the private sector, to investigate new alternatives to counteract fungal pathogens of humans and cultivated plants. The number of chemical compounds known for their antifungal activity is tremendous today [9], however, only a tiny fraction of it is of interest, as many of them fail to comply the current requirements related to applicable legislation or economic feasibility.

A long-time used approach to find new antifungals has been the screening of living organisms [10,11], especially microorganisms. However, natural chemicals are being synthesized by respective organisms based on genes that are present in a wider environmental gene pool and are subjected to vertical or horizontal transfer [12], including to pathogenic fungi. This process results in acquisition of resistance to the respective compounds. Therefore, chemically synthesised compounds with antifungal properties might present an advantage compared with the natural ones.

In the past decades, the interest in discovering newly active compounds has resulted in publication of many scientific studies on the topic, only in the last decade being published thousands of them. A promising group is comprised in natural or synthetic heterocyclic compounds with one or more nitrogen atoms and their fused derivatives, with different, very interesting, biological activities. Among them are antioxidative, antitumoral, antibacterial, antifungal, antituberculosis, etc [9,13–19]. According to the literature [8,10,13,20], there are several classes of antifungals that are frequently used to counteract fungal infections: allylamines azoles, calcium–calcineurin derivatives, echinocandins, polyenes and to some extent antimetabolites (which are pyrimidine analogues). Related to the mechanism of antifungal activity, the literature describe several possible mechanism according with the structure of compounds/drugs [20–29].

Allylamines (e.g., naftifine, terbinafine) are antifungal agents that block ergosterol biosynthesis as they are reversible, non-competitive inhibitors of squalene epoxidase (ERG1) [24].

In the case of calcium–calcineurin derivatives (e.g., efungumab or mycograb) a first mechanism admit the direct binding and inhibition of calmodulin or calmodulin-like proteins or calmodulin-dependent enzymes. These compounds also induces formation of complexes with immunophilins [25].

Echinocandins derivatives (e.g. rezafungin, ibrexafungerp) are antifungal agents that are able to inhibit β-1,3-glucan synthase and damage the cell wall [20,26].

In the case of polyenes (e.g., amphotericin B) it is admitted that these class of fungicide are able to interfere the cell wall with the ion homeostasis of the fungal cell, establishing hydrophobic interactions with ergosterol of the membrane and creating pores. These derivatives also induces an accumulation of reactive oxygen species (ROS) [27].

Azoles and their derivatives are one of the main groups, and it is known that these compounds inhibit lanosterol 14α-demethylase (P450 14DM, CYP51), an enzyme that plays role in ergosterol biosynthetic pathway. Azines derivatives have also attracted the attention for their biological activities, antifungal including [20–23,28–30]. As a matter of fact, many of the existing drugs from the market are derived from azole [28–30]. Especially the drugs containing imidazole and triazole moiety are the most used and ones of the most efficient antifungal drugs. The most representative used drugs in current therapy with azole skeleton [20,28–30] are azanidazole, butoconazole, clotrimazole, ketoconazole, miconazole, metronidazole, tioconazole, etc. (with imidazole moiety) respectively albaconazole, fluconazole, itraconazole, isavuconazole, posaconazole, ravuconazole, terconazole, voriconazole, etc. (with triazole moiety). Substantially less drugs are on the market with azine skeleton. For instance, ibrexafungerp (a hybrid triterpene glycoside containing triazole and pyridine moieties) is used as antifungal for vulvovaginal candidiasis [30]. As antifungal mechanisms ibrexafungerp is a cell wall inhibitor, being a competitive inhibitor of the β-1,3-glucan synthase [20,26,30]. These type of drugs have the advantages of selectivity, damaging the wall of fungus, without affecting the cells of host.

This paper summarizes the literature published in the field of azine derivatives in the last 10 years, with emphasis on their antifungal activity, possible mechanisms of action and promises for their future use. One of the main challenges in addressing this aim is the difficulty of comparing different compounds for their antifungal potential based on the available literature, due to the non-similar protocols used for assessing antifungal activity. In this review the most promising results according to the literature data are being discussed, for different groups of antifungals and their supposed ways of action. A special attention was played to hybrid and chimeric azole–azine class, having in view the high antifungal potential of azoles. The main results achieved in the last decade in the field of antifungal azines are summarized in Table 1 (see also Supporting Information).

Table 1. Antifungal activity of azine compounds against different fungi.

Type of compounds	Fungal species	Std.	Inhibition/(IZD)†/[MIC]‡		Ref.
			Compound	Std.
Quinoline derivatives	A. clavatus	gris.	[25–>1000]	[100]	[61]
Pyridine and pyrimidine derivatives	A. flavus	flu.	[9–19]	[31]	[46]
Metal–pyridine derivatives	A. flavus	carb.	[6–30]	[35]	[41]
Metal–pyridine derivatives	A. fumigatus	carb.	[12–25]	[28]	[41]
Quinoline–pyrimidines	A. niger	gris.	[25–>1000]	[100]	[61]
Metal–pyridine derivatives	A. niger	amph.	[19.93–>41.66]	[0.18]	[66]
Metal–pyridine-pyrazine	A. niger	carb.	[13–25]	[30]	[41]
Metal–pyridine derivatives	A. niger	NA	[9.67–46]	–	[45]
Quinoline–pyrimidines	C. albicans	gris.	[500–>100]	[500]	[54]
Triazol–pyridines	C. albicans	flu.	[6.2–>200	[25–>100]	[38]
Pyridine and pyrimidine derivatives	C. albicans	flu.	[10–17]	[32]	[46]
Quinoline derivatives	C. albicans	flu.	[0.0625–0.5]	[0.25–>64]	[68]
Metal–pyridine-pyrazine derivatives	C. albicans	carb.	[13–24]	[30]	[41]
Metal–pyridine derivatives	C. albicans	NA	[7.67–69]	–	[45]
Triazol–pyridines	C. glabrata	flu.	[25–100]	[25–>100]	[38]
Quinoline derivatives	C. glabrata	flu.	[1]	[1]	[68]
Pyrazole–pyridines	C. gloeosporioides	carb.	[5.73–68.51]	[100]	[35]
Triazol–pyridines	C. guilliermondii	flu.	[12,5–100]	[12.5–>100]	[38]
Metal–pyridines derivatives	C. krusei	amph.	[9.96–>41.66]	[0.36]	[66]
Triazol–pyridines	C. krusei	flu.	[12.5–100]	[25–50]	[38]
Triazol–pyridines	C. lusitaniae	flu.	[12.5–100]	[12.5]	[38]
Quinoline derivatives	C. neoformans	flu.	[0.5–2]	[0.5–4]	[68]
Pyrazole–pyridines	C. orbiculare	bosc.	[29.41–92.81]	[83.61]	[36]
Metal–pyridines derivatives	C. parapsilosis	amph.	[19.93–>41.66]	[0.18]	[66]
Triazol–pyridines	C. parapsilosis	flu.	[>200]	[3.1–25]	[38]
Pyrazine–pyridines	C. parapsilosis	NA	[4.9–>1830]	–	[43]
Quinoline derivatives	C. parapsilosis	flu.	[1]	[0.25]	[68]
Triazol–pyridines	C. tropicalis	flu.	[25–100]	[6.2–50]	[38]
Pyrazole–pyridines	F. moniliforme	bosc.	[12.01–43.39]	[31.08]	[36]
Pyrazole–pyridines	F. moniliforme	carb.	[6.67–69.43]	[63.5]	[35]
Pyrazole–pyridines	F. oxysporum	carb.	[6.35–64.17]	[100]	[35]
Triazole–pyridines	F. oxysporum	manc.	[50–100]	[25]	[65]
Triazole–pyridines	F. recini	manc.	[25–100]	[25]	[65]
Triazol–pyridines	G. candidum	flu.	[>200]	[12.5–50]	[38]
Pyrazole–pyridines	P. aphanidermatum	bosc.	[26.15–90.79]	[85.64]	[36]
Pyrazole–pyridines	P. infestans	bosc.	[8.59–56.06]	[36.36]	[36]
Quinoline derivatives	P. oryzae	teb.	[65–90]	[70]	[31]
Metal–pyridine derivatives	Penicillium	NA	[8–41]	–	[45]
Triazol–pyridines	R. mucilaginosa	flu.	[6.2–100]	[>100]	[38]
Pyrazole–pyridines	R. solani	bosc.	[31.91–92.59]	[91.74]	[36]
Metal–pyridine derivatives	Rhizopus sp.	NA	[8.33–42]	–	[45]
Pyrazole–pyridines	S. sapinea	carb.	[7.53–66.67]	[65]	[35]

^†Expressed as mm inhibition area.

^‡Expressed in μg/ml.

amph.: Amphotericin B; bif.: Bifonazole; bosc.: Boscalid; carb.: Carbendazim; clot.: Clotrimazole; eco.: Econazole; flu.: Fluconazole; gris.: Griseofulvin; itr.: Itraconazole; ket.: Ketoconazole; manc.: Mancozeb; mic.: Miconazole; nyst.: Nystatin; pol.: Polyoxin B; pyr.: Pyrimetharil; teb: Tebufloquin; terb.: Terbinafine; trif.: Triflucan; xym.: Xymexazole; Std.: Standard antimycotic used; IZD: Inhibition zone diameter; MIC: Minimum inhibitory concentration) Expressed as % inhibition compared with the control.

2. Results & discussion

An increased number in immunosuppressed patients has led to a rise of invasive fungal infections incidence, including infections made by opportunistic fungi that normally would not affect clinically healthy patients. Some germs, as Candida, Cryptococcus and Aspergillus are more prevalent and can cause severe infections resulting in live losses, especially to immunocompromised patients (such as persons with AIDS).

During the last few years an attractive tool in fungal drug design is to project and develop multiple targeting drugs (MTD) where a single chemical entity interacts with two or more distinct biological targets related to a disease. In general, MTD possess the advantages of expressing better biological activity and specificity, lower drug–drug interactions, lower toxicity and side effects, etc., generally a consistent improvement of pharmacodynamic, pharmacokinetic, toxicologic properties of the potential drug candidates. These types of drugs are usually classified in hybrid drugs (HD) and chimeric drugs (CD) [31,32]. The HD are designed by the molecular hybridization technique (MHT), wherein two or more drug pharmacophores with different biological activities are connected via a flexible linker resulting in a single chemical entity, more flexible and with a higher molecular weight [15,31–34]. The CD are designed by merging or fusing the pharmacophores of two different drugs in a single chemical entity using an appropriate core [16,31–34]. However, both HD and CD have also some disadvantages like drug–drug interactions, toxicity, etc. So far searching for new chemical entities with improved antifungal properties, remains a very challenging and important task in medicinal chemistry.

2.1. Six-member ring azaheterocycles with one nitrogen atom, hybrid compounds

A prime group of hybrids azaheterocycle compounds with antifungal activity consist in pyridine derivatives. According with the literature data [21–23], derivatives with azole moiety are one of the most successful classes in fungal treatment, this is why we decide to accord a special treatment to the hybrid azole–azine class of potential antifungal drugs. Complete information concerning synthesis of the compounds presented bellow are supplied in the Supporting Information.

2.1.1. Hybrid azole–azine derivatives

Liang et al. [35] synthesized two new classes of hybrid pyridine derivatives (namely pyrazole–pyridine 1a–s and 2a–q) and studied their antifungal activities. The synthesis is using as starting material 2-amino-pyridine derivatives which is Boc protected to amino group, than alkylated with bromo–benzyl derivatives and subsequent acylated with pyrazole carbonyl chloride, to produce the desired pyrazole–pyridine hybrid derivatives 1a–s and 2a–q (Figure 1).

Figure 1.

Hybrid azole–azine derivatives with antifungal activity.

The newly hybrid pyrazole–pyridine derivatives 1a–s and 2a–q were evaluated for plant pathogenic fungi inhibition, by measuring mycelial growth rate, on four fungi: Colletotrichum gloeosporioides, Fusarium oxysporum, Sphaeropsis sapinea and Fusarium moniliforme. Some of the tested compounds gave better results in comparison with carbendazim (positive control) against Sphaeropsis sapinea and Fusarium moniliforme. Thus, compounds N-(4-chlorobenzyl)-3-(difluoromethyl)-1-methyl-N-(pyridin-2-yl)-1H-pyrazole-4-carboxamide 1d, N-(3-bromobenzyl)-3-(difluoromethyl)-1-methyl-N-(pyridin-2-yl)-1H-pyrazole-4-carboxamide 1e, N-(3-iodobenzyl)-3-(difluoromethyl)-1-methyl-N-(pyridin-2-yl)-1H-pyrazole-4-carboxamide 1g and N-(4-iodobenzyl)-3-(difluoromethyl)-1-methyl-N-(spyridin-2-yl)-1H-pyrazole-4-carboxamide 1h, were the most potent compounds synthesized (the antifungal inhibition rates were: 1d [68.68%], 1e [71.62%], 1g [66.29%], 1h [69.43%]; [63.50%] for positive control, Carbendazim. The authors established interesting correlation structure–biological activity (SAR), suggesting that introduction of halogen groups onto benzene ring might increase the antifungal activity.

Du et al. [36] synthesized and tested for antifungal activity different classes of hybrid pyrazole derivatives, these including pyrazole–pyridine derivatives type 3a–f. The synthesis is using as starting material 3-amino-2-chloro-pyridine, which was in situ initially protected to the amino group, then undergo a Suzuki cross-coupling reactions generating the intermediary. The deprotections of intermediates followed by a subsequent acylations with pyrazole carbonyl chloride, are leading to the desired pyrazole–pyridine hybrid derivatives 3a–f (Figure 1).

The hybrid pyrazole–pyridine derivatives 3a–f were screened for their in vitro antifungal activity against seven phytopathogenic fungi: Colletotrichum orbiculare, Rhizoctonia solani, Phytophthora infestans, Pythium aphanidermatum; Fusarium moniliforme Sheld, Botryosphaeria berengeriana, Botrytis cinerea. Some of the tested pyrazole–pyridine derivatives 3a–f revealed a very good antifungal activity against the tested fungi, superior to the standard fungicide used, Boscalid. The most promising compounds against Botryosphaeria berengeriana was found to be N-(2-(4-(tert-butyl)phenyl)pyridin-3-yl)-3-(difluoromethyl)-1-methyl-1H-pyrazole-4-carboxamide, 3d, (inhibition rate of 88.92% comparative with Boscalid, 79.55%) and against C. orbiculare was found to be 3b (inhibition rate of 83.66% comparative with Boscalid, 83.61%). The authors claim that the good activity of these hybrid pyrazole–pyridine derivatives could be explained by the combined action of amide group from the fourth position of pyrazole and Y substituent from phenyl moiety.

Felefel et al. [37] obtained and tested the antifungal properties of some new hybrid pyrazole–pyridine 4a–e derivatives. The synthesis of the target compounds started from pyridine derivatives which undergo different chemical reactions, mainly cyclocondensations, leading to hybrid pyrazole–pyridine derivatives 4a–e (Figure 1).

Using the disk diffusion agar technique, the pyrazole–pyridine derivatives 4a–e, were evaluated in vitro against two fungi, Aspergillus flavus and Candida albicans. The tested hybrids manifest a moderate antifungal activity against both fungi, inferior to the standard fungicide used, Ketoconazole.

Szafranski et al. [38] designed and obtained new hybrid N-(5-amino-1H-1,2,4-triazol-3-yl)-4-R-pyridine-3-sulfonamide derivatives 5a–k and assessed their effect over multiple yeasts of clinical importance. The synthesis of the hybrid derivatives was performed in three steps: the starting material pyridine-sulfonamides reacts with dimethyl N-cyanoiminodithiocarbonate, then the obtained intermediary is treated with KOH and subsequent with hydrazine hydrate, leading to the desired products, the hybrid triazol–pyridine–sulfonamide derivatives 5a–k (Figure 1).

The hybrid triazol–pyridine–sulfonamide derivatives 5a–k were tested for their in vitro antifungal activity against 31 yeast strains isolated from patients with candidiasis of the oral cavity and respiratory tract: Candida albicans – 8 strains, Candida glabrata-4 strains, Candida parapsilosis-3 strains, Candida tropicalis-3 strains, Candida guilliermondii-2 strains, Candida krusei-2 strains, Candida lusitaniae-2 strains, Geotrichum candidum-2 strains, Rhodotorula mucilaginosa-2 strains, Candida utilis-1 strain, Saccharomyces cerevisiae-1 strain. Also six standardized strains of Candida albicans were used in the tests. Among the tested strains, Candida albicans proved to be more sensitive, some of the compounds [5a (R = 3-(4-phenylpiperazin-1-yl), 5c (R = 3-(4-(p-floro)piperazin-1-yl), 5e (R = 3-(4-(o-methoxy)piperazin-1-yl), 5f (R = 3-(3,5-dimethyl-1H-pyrazol-1-yl), 5g (R = 3-(3,4,5-trimethyl-1H-pyrazol-1-yl), 5i (R = 3-(3,5-diethyl-1H-pyrazol-1-yl), 5j (R = 3-(benzylthio)), 5k (R = (-3-ylthio)methanamine] exerted an inhibition higher or similar (Minimum inhibitory concentration [MIC] = 25–100 μg/ml) to Fluconazole. Against Rhodotorula mucilaginosa compounds 5a, 5c, 5g, 5j and 5k manifested higher activity than Fluconazole (MIC ≥ 100 μg/ml). Also, several compounds exerted an outstanding inhibitory effect over Candida glabrata, Candida guilliermondii, Candida krusei and Candida tropicalis with MIC values ranging from 12.5 to 25 μg/ml. Interesting SAR correlation has been performed by authors highlighting the influence of substituents R from the 4th position of pyridine ring: the presence of a piperazine or thio-acetamide moiety being favourable for activity.

Sribalan et al. [39] synthesized and tested the antifungal properties of some new hybrid tetrazolo–pyridine derivatives 6a–d. The synthesis of the target compounds took place by a cyclocondensation reaction of amide precursors with sodium azide, when the desired tetrazolo–pyridine 6a–d hybrids are obtained (Figure 1).

The hybrid tetrazolo–pyridine 6a–d derivatives were tested for their in vitro antifungal activity against fungus Candida albicans. The antifungal activity was moderate to low, inferior to the standard fungicide used, Ketoconazole.

Eryılmaz et al. [40] designed and obtained two new classes of hybrid pyridine derivatives, anchored in the 2- and 4-position of the ring with a thiazole moiety. The synthesis took place by a Hantzsch cyclocondensation of pyridine carbothioamide (2- and 4-substituted) with acetophenone derivatives, when the corresponding hybrid thiazole–pyridine 7a–e and thiazole–pyridine 8a–e are obtained (Figure 1).

The obtained hybrid thiazole–pyridine 7a–e and 8a–e derivatives were tested for their in vitro antifungal activity against fungus Candida albicans. Some of the tested compounds revealed a good antifungal activity against the tested Candida albicans, the 4-(4-bromophenyl)-2-(pyridin-2-yl)thiazole 7c and 4-(4-bromophenyl)-4-(pyridin-2-yl)thiazole 8c derivatives were found to be more effective with a MIC value of 0.15 mM (compared with the standard etalon Fluconazole, 8.5 mM).

2.1.2. Other hybrid azine derivatives

Amperayani et al. [41] obtained new classes of piperine–pyridine hybrid derivatives and screened their antifungal activities. The synthesis took place via an acylation reaction of different substituted amino–pyridine, when the corresponding hybrids piperine–pyridine derivatives 9a–h are obtained (Figure 2).

Figure 2.

Other hybrid azine derivatives with antifungal activity.

The hybrid piperine–pyridine derivatives 9a–h was screened for their antifungal properties. By using the disk diffusion agar technique, the following fungal strains was used in screening: Aspergillus niger, Aspergillus flavus, Aspergillus fumigatus and Candida albicans. The obtained results indicate that one compound, the diastereoisomer (2E,4E)-5-(benzo[d][1,3]dioxol-5-yl)-N-(3-hydroxypyridin-2-yl)penta-2,4-dienamide (2E,4E)-5-(benzo[d][1,3]dioxol-5-yl)-N-(3-hydroxypyridin-2-yl)penta-2,4-dienamide 9e manifested a strong activity in inhibiting Aspergillus flavus (MIC = 30 μg/ml), Aspergillus fumigatus (MIC = 25 μg/ml) and Candida albicans (MIC = 23 μg/ml), compared with the standard Carbendazim (MIC = 35 μg/ml, MIC = 28 μg/ml and MIC = 30 μg/ml respectively).

Andrejević et al. [42] synthesized and tested the antifungal properties of some newly hybrid metal–naphthyridine and metal–phenanthroline complexes of zinc (II). Using ZnCl₂ and naphthyridine and phenanthroline heterocycles, in a molar ration 1:2, the corresponding metal–naphthyridine 10 and metal–phenanthroline 1 complexes was obtained (Figure 2).

The obtained hybrid metal–naphthyridine 10 and metal–phenanthroline 11 derivatives were tested for their in vitro antifungal activity against fungi Candida albicans and Candida parapsilosis using disc diffusion assay. The metal–phenanthroline hybrid 11 proved to be very active against fungi Candida albicans and Candida parapsilosis, the activity against Candida albicans being spectacular. This is why authors claim that metal–phenanthroline hybrid 11 prove could be considered as a lead compound for further tests. The significant higher antifungal activity of metal–phenanthroline hybrid comparative with metal-naphthyridine hybrid was explained by authors through the complementary influence of metal and phenanthroline ligand.

In a subsequent work Andrejević et al. [43] synthesized different hybrid metal–pyrazine–pyridine complexes of copper(II) and silver(I) of type 12–15 with antifungal effect. The synthesis of the target compounds is using as starting material the pyrazine–pyridine ligand, which is treated with the corresponding copper(II) and silver(I) salts, generating the desired metal–pyrazine–pyridine complexes 12–15 (Figure 2).

The obtained metal–pyrazine–pyridine hybrids 12–15 were tested for their in vitro antifungal activity against fungi Candida albicans and Candida parapsilosis using disc diffusion assay. The hybrid complexes with silver (I) 14 and 15 manifested a strong inhibitory effect over Candida albicans and Candida parapsilosis with MIC values of 4.9 and 3.9 μg/ml respectively, superior to standard clinically used AgSD (silver sulfadiazine) complex. Moreover, authors found an anti-biofilm formation activity, which is a very important feature of antifungal drugs, as biofilms tend resist to antimycotics. The higher antifungal activity of silver or copper ligand hybrids was explained by authors through the influence of metal:

• –The silver ion interact with proteins, enzymes and DNA and, also interfere the ROS production.
• –The copper ion interfere the metalloenzymes, catechol oxidase and phenoxazinone synthase.

Cinarli et al. [44] synthesized and tested the antifungal properties of a new hybrid metal–pyridine derivatives of type 16. The hybrid pyridine 16 was synthesized by an initial cyclocondensation of pyridine-2-acyl derivative followed by a complexation with Zn(CH₃COO)₂ of the intermediary (Figure 2).

The hybrid derivatives 16 was screened for its antifungal activity against fungus C. albicans manifesting a good activity, with a MIC of 23.43 μg/ml. The antifungal activity of metal–pyridine hybrid was explained by authors through the complementary influence of Zn metal and pyridine ligand.

Kpomah et al. [45] synthesized a series of hybrid metal–bipyridine complexes 17a–c with antifungal effect. The synthesis was performed by complexation reaction of bypyridine, ligand and the corresponding metal(II) chloride complexes of copper, nickel and zinc, generating the desired metal–bypyridine complexes 17a–c (Figure 2).

The obtained metal–bypyridine hybrids 17a–c were tested for their in vitro antifungal activity against four fungi Aspergillus niger, Penicillium sp, Rhizopus sp. and Candida albicans using disc diffusion assay. The complexes containing Cu(II) and Ni(II) were the most potent related to inhibition of filamentous fungi Aspergillus niger, Penicillium sp. and Rhizopus sp, with a diameter of inhibition zone around 40–46 mm (positive control DMSO, 0 mm). SAR correlation indicate that copper-bypyridine compounds are the most active. Authors suggested that mechanisms of action relate to copper's affinity for binding sites and redox potentials is critical for activity. Also, this metal ion forms the active center of different metalloproteins, increasing its antifungal activity.

Khalifa et al. [46] designed, synthesized and tested the antifungal properties of a newly class of hybrid furan–pyridine derivatives of type 18–22. The hybrids furan–pyridine were synthesized by variously cyclocondensation reactions of furan intermediary with different reagents, when the corresponding furan–pyridine derivatives 18–22 are obtained (Figure 2).

The furan–pyridine hybrids 18–22 were tested for their in vitro antifungal activity against fungi Aspergillus flavatus and Candida albicans using disc diffusion method. Two hybrids, 2-(4-bromophenyl)-7-(5-methylfuran-2-yl)-5-oxo-1,5-dihydro-[1,2,4]triazolo[1,5-a]pyridine-6,8-dicarbonitrile 20a and 2-(d-galactol)-7-(5-methylfuran-2-yl)-5-oxo-1,5-dihydro-[1,2,4]triazolo[1,5-a]pyridine-6,8-dicarbonitrile 20e, manifested the strongest effect over Candida albicans isolated from animal byproducts, producing an 18 and 19 mm inhibition zone diameter compared with 32 mm inhibition zone of 100 μg/ml for Fluconazole. Against Aspergillus flavatus activity is moderate to low.

2.2. Six-member ring azaheterocycles with one nitrogen atom, hybrid quinoline 

Ghorab et al. [47] synthesized and tested the antifungal properties of a large variety of hybrid azole–sulfonamide–quinoline derivatives 23a–f and azine–sulfonamide–quinoline derivatives 24g–o. The hybrids were obtained via an N-alkylation reaction of 4-chloro-quinoline intermediary with different types of sulfonamide derivatives when the corresponding azole–sulfonamide–quinoline derivatives 23a–f and azine–sulfonamide–quinoline derivatives 24g–o are obtained (Figure 3).

Figure 3.

Hybrid six-member ring azaheterocycles with one nitrogen atom – quinoline.

The obtained hybrid azole–sulfonamide–quinoline derivatives 23a–f and azine–sulfonamide–quinoline derivatives 24g–o were tested for their in vitro antifungal activity against two fungi (Cryptococcus neoformans and Candida albicans) using disc diffusion assay. Two hybrids, 24g and 24k, manifested an excellent activity against the tested fungi. Thus, against Candida albicans 4-((7-methoxyquinolin-4-yl)amino)-N-(pyridin-2-yl)benzenesulfonamide 24g produced an inhibition zone diameter of 12 mm (and a MIC of 125 μg/ml) and against Candida neoformans produced an inhibition zone diameter of 18 mm (and a MIC of 31 μg/ml) compared with 11 mm (and a MIC of 125 μg/ml) for control Fluconazole. Against Candida albicans N-(2,6-dimethylpyrimidin-4-yl)-4-((7-methoxyquinolin-4-yl)amino)benzenesulfonamide 24k produced an inhibition zone diameter of 8 mm (and an MIC of 500 μg/ml) and against Candida neoformans produced an inhibition zone diameter of 10 mm (and a MIC of 500 μg/ml) compared with 19 mm (and an MIC of 250 μg/ml) for control Fluconazole. As far for SAR correlations, the authors indicate that the hybrid azine–sulfonamide–quinoline derivatives are more active than the azole–sulfonamide–quinoline derivatives, introducing of an azine moiety onto sulfonamide–quinoline skeleton being favourable for antifungal activity.

Vishal et al. [48] obtained and tested the antifungal properties of some hybrid aryl–sulfonamide–quinoline derivatives 25a–e and thiophene–sulfonamide–quinoline derivative 23f. The hybrids were synthesized via an N-acylation reaction of 6-amino-quinoline intermediary with variously sulfonyl chloride when the corresponding hybrid aryl–sulfonamide–quinoline derivatives 25a–e and thiophene–sulfonamide–quinoline derivative 25f are obtained, Figure 3.

The obtained hybrid derivatives were tested for their in vitro antifungal activity against four fungi (Candida albicans, Aspergillus niger, Aspergillus flavus and Fusarium oxysporum) by using two methods: disc diffusion assay and broth dilution method. The obtained results indicate that the hybrid thiophene–sulfonamide–quinoline derivative 5-hydroxy-N-(4-methyl-2-oxo-1,2-dihydroquinolin-6-yl)thiophene-2-sulfonamide 25f show an excellent nonselective antifungal activity against all fungal strains, being at least two to eight-times more potent (MIC = 0.78 mg/ml against Aspergillus flavus and Aspergillus niger, MIC = 3.12 mg/ml against Fusarium oxisporum and Candida albicans) comparative to the control drug Ketoconazole. Against Aspergillus flavus the hybrid N-(4-methyl-2-oxo-1,2-dihydroquinolin-6-yl)-4-(trifluoromethyl)benzenesulfonamide 25d were two-times more potent (MIC = 0.78 mg/ml) comparative to the reference drug Ketoconazole. Against Aspergillus niger the hybrid 3,4,5-triisopropyl-N-(4-methyl-2-oxo-1,2-dihydroquinolin-6-yl)benzenesulfonamide 25e have (MIC = 6.25 mg/ml) a similar activity with the control drug Ketoconazole. Against Fusarium oxisporum the hybrid 25a were eight-times more potent (MIC = 1.56 mg/ml) comparative to the reference drug Ketoconazole while 25d manifest a similar activity (MIC = 12.5 mg/ml) with the control. Against Candida albicans the hybrids 25e and 25d have an excellent antifungal activity (MIC = 1.56 and MIC = 0.78 mg/ml), which is two- and four-times greater comparative to the reference drug Ketoconazole. The remaining hybrids have a moderate to low activity against all four species of fungi.

Albayrak et al. [49] designed, synthesized and tested the antifungal properties of new classes of hybrid triazolo–quinoline derivatives of type 26a–j and 27a–j. The hybrids were synthesized by an initial alkylation of 8-aminoquinoline followed by a click reaction of variously azide intermediaries with different compounds with triple bond (via copper(I)-catalyzed azide-alkyne 3+2 dipolar cycloaddition reactions), when the corresponding hybrid triazolo–quinoline derivatives of type 26a–j and 26a–j are obtained (Figure 3).

The obtained hybrid triazolo–quinoline derivatives of type 26a–j and 27a–j were tested for their in vitro antifungal activity against two fungi (Candida parapsilosis and Candida albicans) using the MIC method. Two hybrid triazolo–quinoline derivatives N-(3-(4-butyl-1H-1,2,3-triazol-1-yl)propyl)quinolin-8-amine 26i and 3-(1-(3-(quinolin-8-ylamino)propyl)-1H-1,2,3-triazol-4-yl)propan-1-ol 26j manifest a very good activity against both the tested strains Candida parapsilosis and Candida albicans (MIC = 62.5 μg/ml), being two-times more potent comparative to control Fluconazole (MIC = 31.25 μg/ml). The remaining hybrids have a moderate to low activity against the tested strains. The SAR correlation reveal that the introduced of an alkyl substituent on triazole moiety is favourable for antifungal activity.

Vishnuvardhan et al. [50] synthesized and tested the antifungal properties of three new classes of hybrid triazolo–quinoline derivatives of type 28a–d, 29a–d and 30a–d. The hybrids were synthesized by click reactions of the quinoline (with a triple bond substituent) intermediaries with aryl–azide derivatives, when the corresponding hybrid triazolo–quinoline derivatives of type 28a–d, 29a–d and 30a–d are obtained (Figure 3).

The obtained hybrid triazolo–quinoline derivatives of type 28–30 were tested for their in vitro antifungal activity against two fungi (Aspergillus niger and Candida metapsilosis) using the disc diffusion method. The hybrids (E)-1-(2-((1-(4-nitrophenyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(2-(p-tolyloxy)quinolin-3-yl)prop-2-en-1-one 28d, (E)-1-(3-((1-(4-nitrophenyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(2-(p-tolyloxy)quinolin-3-yl)prop-2-en-1-one 29d and (E)-1-(4-((1-(2-nitrophenyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(2-(p-tolyloxy)quinolin-3-yl)prop-2-en-1-one 30c manifest good activity against the two fungal strains while the remaining compounds showed moderate to low antifungal activity.

Irfan et al. [51] obtained and tested the antifungal properties of some hybrid triazole–quinoline derivatives 31a–b. The hybrids were synthesized by click reactions of the quinoline (with a triple bond substituent) intermediaries with phenyl–azide, when the corresponding hybrid triazolo–quinoline derivatives 31a–b are obtained (Figure 3).

The obtained hybrids 31a–b were tested for their in vitro antifungal activity against different fungi [Candida albicans, Candida glabrata, Candida tropicalis and four clinical isolates of Candida albicans namely D27, D31, D39 (FLC-sensitive) and D15.9 (FLC-resistant)], using the broth dilution technique to determine the IC₅₀ values. The hybrids 31a–b showed a very good antifungal activity. Compound 8-((1-phenyl-1H-1,2,3-triazol-4-yl)methoxy)quinoline 31a showed fungicidal activity against Candida albicans (standard) and Candida albicans (fluconazole resistant) strains having a IC₅₀ values of 0.044 μg/ml and 2.3 μg/ml respectively, comparative with the standard Fluconazole (IC₅₀ = 15.62 and IC₅₀ = 7.5 μg/ml respectively). Compound 5-chloro-8-((1-phenyl-1H-1,2,3-triazol-4-yl)methoxy)quinoline 31b showed a fungistatic activity having a IC₅₀ values of 25.4 and 32.8 μg/ml for the same strains.

Hryhoriv et al. [52,53] obtained and tested the antifungal properties of two new classes of hybrid analogous of fluoroquinolones, namely 1,2,3-triazolo-piperidino–quinoline 33a–k and piperidino–quinoline 32a,b. Initially was obtained the hybrid piperidino–quinoline 32a–b via an N-alkylation reaction of piperidino–quinoline intermediary. The piperidino–quinoline 32a–b undergo a click cyclocondensation reaction leading to the second class of hybrids, the 1,2,3-triazolo–piperidino–quinoline 33a–k (Figure 3).

The obtained hybrids 32a–b and 33a–k were tested for their in vitro antifungal activity against fungus Candida albicans standard and clinical isolated, using disc diffusion method. The 1,2,3-triazolo-piperidino–quinoline hybrids 7-(4-(5-amino-1-(p-tolyl)-1H-1,2,3-triazole-4-carbonyl)piperazin-1-yl)-1-ethyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid 33a, 7-(4-(5-amino-1-(o-tolyl)-1H-1,2,3-triazole-4-carbonyl)piperazin-1-yl)-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid 33d, 7-(4-(5-amino-1-(4-(methylthio)phenyl)-1H-1,2,3-triazole-4-carbonyl)piperazin-1-yl)-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid 33g and 7-(4-(5-amino-1-(3-(methylthio)phenyl)-1H-1,2,3-triazole-4-carbonyl)piperazin-1-yl)-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid 33h manifest a very good activity against standard Candida albicans (31 mm, 25 mm, 22 mm, 21 mm), superior to control (DMSO, 20 mm). The remaining hybrids have a moderate to low activity against the tested Candida albicans standard. Against Candida albicans clinical isolated all hybrids manifest a negligible activity.

Awolade et al. [54] synthesized obtained and tested the antifungal properties of four new classes of hybrid 1,2,3-triazolo–quinoline derivatives of type phenyl hybrid 34a–u, anilino hybrids 35a–n, phenoxy hybrids 36a–z and alkyloxy hybrids 37a–b. The hybrids were obtained by click reactions of alkyne–quinoline intermediary with variously azide (Figure 3).

The obtained hybrids 34a–u, 35a–n, 36a–z and 37a–b were tested for their in vitro antifungal activity against fungi Candida albicans and Cryptococcus neoformans. Mostly of the tested hybrids from phenyl hybrid series 34a–u, phenoxy hybrid series 36a–z and 37a–b series, manifest a very good antifungal activity against the two strains. Overall, the most promising activity against Candida neoformans is manifested by the alkyloxy hybrid 2-(4-((quinolin-8-yloxy)methyl)-1H-1,2,3-triazol-1-yl)ethyl 4-methylbenzenesulfonate 37b which have a MIC value of 2.36 μM, being 11-times more potent comparative to control Fluconazole (MIC = 26.12 μM). From the phenyl hybrid series 34a–u, the most active compound against Candida neoformans was 3-(4-((quinolin-8-yloxy)methyl)-1H-1,2,3-triazol-1-yl)phenol 34q (MIC = 12.57 μM), being two-times more potent comparative to control Fluconazole (MIC = 26.12 μM). From the phenoxy hybrid series 36a–z, the most active compounds against Candida neoformans were 8-((1-(2-(3,4-difluorophenoxy)ethyl)-1H-1,2,3-triazol-4-yl)methoxy)quinoline 36d, 5-(2-(4-((quinolin-8-yloxy)methyl)-1H-1,2,3-triazol-1-yl)ethoxy)benzene-1,3-diol 36v, and 8-((1-(2-(naphthalen-2-yloxy)ethyl)-1H-1,2,3-triazol-4-yl)methoxy)quinoline 36y, showing equipotent activity comparative to control Fluconazole. The SAR correlations reveal the importance for antifungal activity of the O or NH-linked alkane spacer, triazole core and phenyl ring. Also, the presence of a phenyl moiety is important for lipophilicity.

Nehra et al. [55] obtained and tested the antifungal properties of new hybrids triazole–benzothiazole–quinoline type 38a–f. The hybrids were synthesized by click reactions of the alkyne–quinoline intermediary with azide–alkyl–benzothiazole, when the corresponding hybrid triazole–benzothiazole–quinoline type 38a–f are obtained (Figure 4).

Figure 4.

Hybrid six-member ring azaheterocycles with one nitrogen atom – quinoline.

The obtained hybrids 38a–f were tested for their in vitro antifungal activity against two fungi Candida tropicalis and Aspergillus terreus, using disc diffusion method. The synthesized triazole–benzothiazole–quinoline hybrids 38a–f showed a very good activity against both fungal strains, and compound 2-(2-(2-(4-((quinolin-8-yloxy)methyl)-1H-1,2,3-triazol-1-yl)ethoxy)phenyl)benzo[d]thiazole 38a manifest an excellent activity (with a diameter of inhibition zone [DIZ] of 34 mm for Candida tropicalis and 31 mm for Aspergillus terreus), superior to reference drug Fluconazole (DIZ of 21mm respectively 19 mm).

Ammar et al. [56] obtained and tested the antifungal properties of several new classes of hybrids thiazole–quinoline hybrids. A first set of hybrids were obtained by condensation reaction between formil–quinoline intermediary with different thiazolone derivatives, when the corresponding hybrids thiazolone–quinoline type 39–42, are obtained (Figure 4).

A second set of hybrids were obtained by cyclization of quinoline–thiosemicarbazone intermediary with the halogenated compounds, when the corresponding hybrid thiazolone–quinoline type 43–48 are obtained (Figure 4).

The obtained thiazole–quinoline hybrids 39–48 were tested for their in vitro antifungal activity against two fungi Candida albicans and Fusarium oxysporum. The synthesized hybrids 43–48 manifested a good activity against both fungal strains, and for the most promising derivatives depending on inhibition zone values, MIC and minimum fungicide concentration (MFC) were determined. The obtained results indicate that four thiazole–quinoline hybrids have a superior antifungal activity against both fungi compared with the control Amphotericin B (MIC = 15.62 μg/ml and MFC = 34.62 μg/ml against Candida albicans and respectively, MIC = 31.25 μg/ml and MFC = 65.62 μg/ml against Fusarium oxysporum). Thus: against Candida albicans the hybrids (Z)-5-((2-chloro-7-ethoxyquinolin-3-yl)methylene)-3-methyl-2-thioxothiazolidin-4-one 39a and (E)-4-(2-(2-((2-chloro-7-ethoxyquinolin-3-yl)methylene)hydrazinyl)thiazol-4-yl)phenol 44b have the MIC values of 7.81 μg/ml and MIC = 9.25 μg/ml and respectively, MFC values of 12.49 μg/ml and MIC = 17.57 μg/ml; against Fusarium oxysporum the hybrids 39a and the 44b have the MIC values of 15.62 μg/ml and MIC = 18.51 μg/ml and respectively, MFC values of 26.55 μg/ml and MFC = 29.61 μg/ml. Two others thiazolo–quinoline hybrids, (Z)-2-amino-5-((2-chloro-7-ethoxyquinolin-3-yl)methylene)thiazol-4(5H)-one 42 and (E)-2-(2-((2-chloro-7-ethoxyquinolin-3-yl)methylene)hydrazinyl)-4-(4-chlorophenyl)thiazole 44c, have a fungicidal activity lower than compounds 39a and 44b, but still higher comparative with Amphotericin B. Having in view the above results, the author conclude that a thiazolone–moiety anchored to quinoline moiety is more favourable for activity comparative with a thiazole-moiety anchored.

Wang et al. [57] obtained and tested the antifungal properties of three new classes of hybrids benzimidazole–quinolone hybrids: unsaturated allyl or propargyl derivatives 49a–b, alkyl substituted derivatives 50a–g and chloro/fluoro–benzyl derivatives 51a–f. The hybrids were obtained by N-alkylation reaction of quinoline intermediary with different benzimidazole derivatives, when the corresponding hybrids 49a–b, 50a–g and 51a–f, are obtained (Figure 4).

The obtained hybrid derivatives were tested for their in vitro antifungal activity against five drug resistant fungal strain isolated from infected patients (Candida albicans, Candida tropicalis, Aspergillus fumigatus, Candida albicans ATCC 90023, Candida parapsilosis ATCC 22019) by using disc diffusion assay. The obtained results indicate that some of the hybrid benzimidazole–quinolone derivative showed an excellent activity toward the clinical drug-resistant isolates. Thus against Candida tropicalis the hybrids have a MIC in the range of 1–256 μg/ml, superior to control Fluconazole (MIC = 256 μg/ml), a remarkable activity having compound ethyl 7-chloro-6-fluoro-1-((1-(2-fluorobenzyl)-1H-benzo[d]imidazol-2-yl)methyl)-4-oxo-1,4-dihydroquinoline-3-carboxylate 51b (MIC = 1 μg/ml), 256-times more active comparative with Fluconazole. Also, the hybrid 51b manifest a good activity against Candida parapsilosis ATCC 22019 (MIC = 1 μg/ml) comparative with Fluconazole (MIC = 2 μg/ml). In the chloro/fluoro–benzyl derivatives 51a–f series, it was found that fluorobenzyl hybrids have an equivalent or even stronger antifungal activities in comparison with the chlorobenzyl derivatives. Alkyl substituted derivatives 50a–g showed moderate antifungal activity against most of the tested fungi, with a special mention for hybrids ethyl 7-chloro-1-((1-ethyl-1H-benzo[d]imidazol-2-yl)methyl)-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylate 50a and ethyl 1-((1-butyl-1H-benzo[d]imidazol-2-yl)methyl)-7-chloro-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylate 50b (with an MIC value of 16 μg/ml), which is superior to control Fluconazole against Candida tropicalis. The unsaturated allyl or propargyl derivatives 49a–b also showed a moderate antifungal activity against the tested fungi.

Lu et al. [58] synthesized and tested the antifungal properties of some newly hybrid metal–bipyridine–phenanthroline and metal–phenanthroline–phenanthroline complexes of ruthenium (II) type 52a–c and iridium (III) type 53a–c. The hybrids were obtained from ruthenium (II) or iridium (III) complex of 1,10-phenanthroline-5,6-dione which were condensed with 5,6-diamino-1,10-phenanthroline to obtain the corresponding ruthenium (II) {[Ru(bpy)₂(tpphz)](PF₆)₂ (52a), [Ru(phen)₂(tpphz)](PF₆)₂ (52b), [Ru(DIP)₂(tpphz)](PF₆)₂ (52c)} and iridium (III) {[Ir(ppy)₂(tpphz)](PF₆) (53a), [Ir(thpy)₂(tpphz)](PF₆) (53b), [Ir(dfppy)₂(tpphz)](PF₆) (53c)} complexes was (Figure 4).

The obtained hybrid metal–azine complexes of ruthenium (II) type 52a–c and iridium (III) respectively type 53a were tested for their in vitro antifungal activity against two fungi (sensitive Candida albicans SC5314 and FLC resistant Candida albicans CA23) using disc diffusion assays. Ruthenium (II) and iridium (III) complexes alone did not inhibit both sensitive and resistant strain of Candida albicans (MIC >100 μg/ml). Instead, the combination of ruthenium (II) type 52a-c with Fluconazole manifested moderate antifungal activity against sensitive Candida albicans SC5314, with a MIC value of 25.32 μg/ml (52a), MIC = 11.02 μg/ml (52b) and MIC = 8.93 μg/ml (52c) (comparative with the standard Fluconazole with MIC >100 μg/ml). The combination of iridium (III) complexes with FLC did not have activity against sensitive Candida albicans SC5314. Against the FLC resistant Candida albicans CA23 the best antifungal activity was manifested by iridium (III) pyridine–phenanthroline hybrid 53b, with a MIC = 2.09 μg/ml, net superior standard Fluconazole with MIC >100 μg/ml.

Naz et al. [59] synthesized and tested the antifungal properties of some newly hybrid metal–bipyridine 54 and metal–phenanthroline complexes type 55, 56. The hybrids were obtained from the corresponding bipyridine or phenanthroline heterocycles with zinc–isophtalate salts (Figure 4).

The obtained hybrid metal–azine complexes 54–56 were tested for their in vitro antifungal activity against four fungi (Aspergillus niger, Aspergillus fumigatus, Fusarium solani and Mucor sp.) using disc diffusion assays. All hybrid metal–azine complexes 54–56 manifest a good antifungal activity against all tested strains (with a zone inhibition higher than 13 mm and a MIC in the range of 0.2–1 μg/ml) (comparative with the standard Clotrimazole with a zone inhibition in the range of 20–30 mm and a MIC in the range of 0.3–1 μg/ml).

2.3. Azine heterocycles with one nitrogen atom – chimeric derivatives

The chimeric azine heterocycles represent a very important class of fused heterocycles possessing a large variety of biologic activities, antifungal including [1,9,15,60]. However, a thoroughly study of literature indicated that the field of fused azine derivatives with antifungal activity it is a relative less studied research area and could be a fertile field of research.

Desai et al. [61] synthesized and tested the antifungal properties of some newly chimeric pyrimidine–pyridine derivatives anchored with pyrazole moiety of type 57a–o. The chimeric compounds 57a–o were obtained in two steps, a condensation of formil–pyrazole with acetophenone derivative followed by a cyclocondensation with 6-amino-uracil (Figure 5).

Figure 5.

Azine heterocycles with one nitrogen atom – chimeric derivatives.

The obtained chimeric derivatives of type 57a–o were tested for their in vitro antifungal activity against three fungal strains (Aspergillus flavus, Aspergillus niger, Candida albicans) by MIC method. The investigated compounds manifest a significant activity against the fungus Candida albicans, while against the others two have a modest activity. The most promising chimeric derivatives was 7-(3,4-dichlorophenyl)-5-(1,3-diphenyl-1H-pyrazol-4-yl)pyrido[2,3d]pyrimidine-2,4(1H,3H)-dione 57n (with a MIC = 200 μg/ml) and 5-(1,3-diphenyl-1H-pyrazol-4-yl)-7-(p-tolyl)pyrido[2,3d]pyrimidine-2,4(1H,3H)-dione 57k (with a MIC = 250 μg/ml), comparative with the standard drug Griseofulvin (with a MIC = 500 μg/ml). As far for SAR correlations, the chimeric derivatives having electron-withdrawing substituents Y (bromo, chloro, and nitro) on the fourth position of phenyl ring possessed comparable antifungal activity against Candida albicans. The chimeric derivatives with electron donating substituents Y (methoxy and hydroxy) were also found to have comparable antifungal activity against Candida albicans. The author claim that the activity is closely related with the mechanism of activity, the chimeric pyrimidine–pyridine derivatives producing ergosterol biosynthesis inhibition.

Devi et al. [62] synthesized and tested the antifungal properties of some newly chimeric pyrazole–imidazo–pyridine derivatives of type 58 and 59dA. The chimeric compounds 58 were obtained via one-pot three component strategy, from 4-formyl-pyrazole, pyridin-2-amines and tert-butyl isonitrile; for 58dA a N-methylation reaction is leading to 59dA (Figure 5).

The obtained chimeric derivatives of type 58 and 59dA were tested for their in vitro antifungal activity against two fungal strains (Aspergillus niger, Candida albicans) by MIC method. Some of the investigated compounds (58aA, 58aF, 58bE, 58cE, 58cF, 58eD and 59dA) manifest a very good activity against both fungi, with a MIC ranging from 3.12 to 0.39 μg/ml), comparative with the standard drug Fluconazole (MIC = 0.39 μg/ml, for both fungi). The most promising chimeric derivatives was 58aA (R₁ = -Ph; R₂ = -H; MIC = 0.39 μg/ml) and 59dA (MIC = 0.39 μg/ml), being equipotent with the standard drug Fluconazole against Candida albicans and exhibiting a good antifungal activity against Aspergillus niger (with MIC = 1.56 μg/ml for both compounds). From the SAR studies, the authors conclude that introduction of methyl group, either in R1 or R2 substituents, increase substantially the antifungal activity.

Zhou et al. [63] obtained and tested the antifungal properties of some newly chimeric imidazo–pyridine derivatives of type 60a–j. The chimeric derivatives 60a–j were obtained in two steps, an initial N-alkylation of toluidine derivative (with the corresponding α-bromo-acetophenone) followed by a cyclocondensation of the formed intermediary (Figure 5).

The obtained chimeric imidazo–pyridine derivatives of type 60a–j were tested for their in vitro antifungal activity against fungus Candida albicans. The obtained results indicate that the chimeric derivatives 60a–j are effective Bdf1 inhibitors, with very good binding and site selectivity.

Kuthyala et al. [64] synthesized and tested the antifungal properties of some newly chimeric imidazo–pyridine derivatives of type 61a–j. The synthesis involves a several successive reactions: a first cyclocondensation reaction of amino–pyridine with ethyl–chloro–acetoacetate, followed by a reactions with hydrazine, an finally, another cyclocondensation of hydrazonyl–imidazopyridine with the corresponding benzoic acids, when the imidazo–pyridine 61a–j were obtained (Figure 5).

The obtained chimeric imidazo–pyridine derivatives of type 61a–j were tested for their in vitro antifungal activity against fungi Aspergillus niger and Candida albicans. Some of the obtained chimeric compounds manifest a good activity against the tested fungi, the most active compound against Candida albicans was 2-(2,7-dimethylimidazo[1,2-a]pyridin-3-yl)-5-(3-nitrophenyl)-1,3,4-oxadiazole 61f (MIC = 12.5 μg/ml), superior to control Fluconazole (MIC = 16 μg/ml). Based on the above findings, the authors to conclude that introduction of methyl or nitro group in any position of pyridine moiety increase substantially the antifungal activity.

Marepu et al. [65] obtained and tested the antifungal properties of some newly chimeric triazolo–pyridine derivatives of type 62a–d. The chimeric derivatives 62a–d were obtained by a cyclocondensation reaction of the corresponding diamino–pyridine derivative with sodium nitrite (Figure 5).

The obtained chimeric triazolo–pyridine derivatives of type 62a–d were tested for their in vitro antifungal activity against two species of fungi, Fusarium oxyxporum and Fusarium recini. Three of the tested compounds (3-benzyl-6-chloro-3H-[1,2,3]triazolo[4,5-c]pyridine 62a, 6-chloro-3-(tetrahydro-2H-pyran-4-yl)-3H-[1,2,3]triazolo[4,5-c]pyridine 62b, 6-chloro-3-(4-methoxyphenyl)-3H-[1,2,3]triazolo[4,5-c]pyridine 62d, manifest a promising antifungal activity against Fusarium recini (with a MIC = 25 μg/ml), being equipotent to the standard drug Mancozeb (MIC = 25 μg/ml).

Felefel et al. [37] synthesized and tested the antifungal properties of some newly chimeric triazolo/tetrazolo–pyridine derivatives of type 63–67. The chimeric derivatives 63–67 were obtained using as starting material 6-(3,4-dimethylphenyl)-2-hydrazinyl-4-(thiophen-2-yl)-pyridine-3-carbonitrile, which react with the appropriate formic acid, acetic acid, benzoyl chloride, carbon disulfide, respectively sodium nitrite, to produce the desired hybrid derivatives 63–67 (Figure 5).

The synthesized triazolo/tetrazolo–pyridine 63–67 were tested for their in vitro antifungal activity against two fungi, Aspergillus flavus and Candida albicans. The obtained results indicate that these compounds manifest a week antifungal activity, exception being tetrazolo–pyridine 5-(3,4-dimethylphenyl)-7-(thiophen-2-yl)-3-thioxo-2,3-dihydro-[1,2,4]triazolo[4,3-a]pyridine-8-carbonitrile 66, which have a moderate activity comparative with control drug Ketoconazole.

Kumar et al. [66] synthesized and tested the antifungal properties of some newly chimeric seleno–imidazo–pyridine derivatives type 68a–l (Figure 5).

The synthesized compounds were tested for their in vitro antifungal activity against four fungi: Aspergillus fumigatus, Aspergillus niger, Candida krusei and Candida parapsilosis. The obtained results indicate that only one chimeric compound, 3-nitro-2-(pyridin-2-ylselanyl)imidazo[1,2-a]pyridine 68e, have a good non-selective activity against Aspergillus fumigatus, Candida krusei, Aspergillus niger, and Candida parapsilosis with a MIC values of 9.96, 9.96, 19.93 and 19.93 μg/ml, comparative with the control Amphotericin B (MIC = 0.18 μg/ml for Aspergillus fumigatus, MIC = 0.36 μg/ml for Candida krusei, MIC = 0.18 μg/ml for Aspergillus niger, MIC = 0.18 μg/ml for Candida parapsilosis).

Abdellattif et al. [67] obtained and tested the antifungal properties of some newly chimeric seleno–pyridine derivatives type 69a–d. The synthesis involves some typical organic reactions (substitutions and cyclocondensations) (Figure 5).

The synthesized seleno–pyridine derivatives were in vitro screened against three fungi, Candida albicans, Aspergillus niger and Aspergillus clavatus. One compound (E)-1-(3-amino-5-(argiodiazenyl)-4,6-dimethylselenopheno[2,3-b]pyridin-2-yl)ethan-1-one 69d exhibited a good activity against the tree fungi (with a inhibition zone around 70%) compared with control Griseofulvin (with a inhibition zone around 80%).

Villa et al. [68] obtained and tested the antifungal properties of some newly chimeric benzimidazole–pyrrolo–quinoline derivatives of type 70a–i. The chimeric compounds 70a–i was obtained via a catalysed cyclocondensation reaction (Figure 5).

The synthesized benzimidazole–pyrrolo–quinoline derivatives of type 70a–i were screened against three fungi (Candida parapsilosis, Candida albicans and Candida glabrata) and fungal strains that are resistant to Fluconazole, Candida albicans (15 strains), Cryptococcus neoformatus (8 strains). One compound 12H-benzo[4′,5′]imidazo[1′,2′:1,2]pyrrolo[3,4-b]quinoline 70a exhibited a very good activity against resistant fungi Candida albicans NR-29448, Candida albicans NR-29446, C. albicans NR-29366, Candida albicans NR-29367 and Candida albicans NR-29439, with the MIC ranging from 0.25 to 0.5 μg/ml. In addition, the chimeric compound 70a significantly reduced the metabolic activity of fungal cells in the Candida albicans biofilms.

Jitender et al. [69] synthesized and tested the antifungal properties of some newly chimeric pyrazolo–quinoline derivatives of type 71–75. The chimeric compounds 71–75 were obtained in several steps: an initial cyclocondensation reaction with hydrazine, followed by N-alkyaltion or N-acylation reactions (Figure 5).

The synthesized pyrazolo–quinoline derivatives of type 73a–f, 74a–r and 75a–h, were screened against several fungal strains such as Candida albicans MTCC 183, Candida albicans MTCC 227, Candida albicans MTCC 854, Candida albicans MTCC 1637, Candida albicans MTCC 3017, Candida albicans MTCC 3018, Candida albicans MTCC 3958, Candida albicans MTCC 4748, Candida albicans MTCC 7315, Candida parapsilosis MTCC 1744, Candida anseri MTCC 1962, Candida glabrata MTCC 3019, Candida krusei MTCC 3020 and Issatchenkia hanoiensis MTCC 4755. Some of the chimeric pyrazolo–quinoline derivatives (namely N-(4-methoxyphenyl)-2-(4-phenyl-3-(trifluoromethyl)-1H-pyrazolo[3,4-b]quinolin-1-yl)acetamide 73e, 2-(4-phenyl-3-(trifluoromethyl)-1H-pyrazolo[3,4-b]quinolin-1-yl)-N-(3-(piperazin-1-yl)propyl)acetamide 74r and 2-(4-phenyl-3-(trifluoromethyl)-1H-pyrazolo[3,4-b]quinolin-1-yl)-1-(piperidin-1-yl)ethan-1-one 75f) exhibited a very good nonselective antifungal activity, the most active compound being 74r with a MIC on the same range comparative with the control drug Miconazole. These findings make the authors to conclude that the presence of pyrazolo–quinoline moiety with specific substituents in the pyrazole ring (such as acetamidoethyl piperazine and trifluoromethyl groups) increase substantially the antifungal activity.

Al-Matarneh et al. [70] obtained and tested the antifungal properties two new series of chimeric pyrrolo–quinoline and pyrrolo–isoquinoline derivatives of type 76a–c and 77a–c. The chimeric compounds were obtained via a 3+2 dipolar cycloaddition reaction of the quinolinium and isoquinolinium yieldes (generated in situ from the corresponding salts) with N-ethyl- or N-phenyl-maleimide (Figure 5).

The synthesized pyrrolo–quinoline and pyrrolo–isoquinoline derivatives of type 76a–c and 77a–c were screened against fungus Candida albicans, compounds being inactive.

Antoci et al. [71] obtained and tested the antifungal properties two new series of chimeric pyrrolo–benzo–quinoline derivatives of type 78a–d. The chimeric compounds were obtained via a 3+2 dipolar cycloaddition reaction of the quinolinium ylides (generated in situ from the corresponding salts) with different dipholarophiles with triple bound (Figure 5).

The synthesized pyrrolo–benzo–quinoline derivatives of type 78a–d were screened against fungus Candida albicans, one compound, methyl 3-propionylbenzo[f]pyrrolo[1,2-a]quinoline-1-carboxylate 78b, having a moderate activity.

Banoth et al. [72] synthesized and tested the antifungal properties two new series of chimeric imidazo–naphthyridine derivatives of type 79a–g and 80a–g. The chimeric compounds were obtained by cyclocondensation reactions of 2-amino-naphthyridines with the corresponding α-bromo derivatives (Figure 5).

The synthesized chimeric derivatives were screened against two fungi, Aspergillus niger and Candida metapsilosis, by using the disc diffusion method. The obtained results indicate that these compounds manifest a good antifungal activity against the tested fungi, the activity of some derivatives (6-(4-bromophenyl)-9-(4-chlorophenyl)imidazo[1,2-a][1,8]naphthyridine 79c, 9-(4-chlorophenyl)-6-(3-nitrophenyl)imidazo[1,2-a][1,8]naphthyridine 79d, 3-(6-(4-bromophenyl)imidazo[1,2-a][1,8]naphthyridin-9-yl)-2H-chromen-2-one 80c, 3-(6-(3-nitrophenyl)imidazo[1,2-a][1,8]naphthyridin-9-yl)-2H-chromen-2-one 80d, 3-(6-(3-hydroxyphenyl)imidazo[1,2-a][1,8]naphthyridin-9-yl)-2H-chromen-2-one 80e and 3-(6-(4-nitrophenyl)imidazo[1,2-a][1,8]naphthyridin-9-yl)-2H-chromen-2-one 80f) being excellent, superior to drug control Griseofulvin.

3. Conclusion

In conclusion, chimeric and hybrid six member rings azine are very promising antifungal agents, with numerous reports of their activity against a very broad spectrum of fungi, from clinically important species that cause serious infections in humans to phytopathogenic fungi that lead to hard economic losses. In the hybrid azine series we notice that there are some certain classes of compounds that manifest a powerful antifungal activity, the most active being the hybrid azole–azine and azole–sulfonamide–azine derivatives containing an imidazole or triazole moiety. Also, we notice that the hybrid thiazole– or thiazolone–quinoline derivatives manifest a strong antifungal activity. As a matter of fact, the findings from this review make us to believe that this class of azole–azine antifungals have a certain future for a possible drug candidate. The metal–azine hybrids seems to be also a good approach in antifungal therapy, these compounds having a significant activity. We also notice that the hybrid azine derivatives with quinoline skeleton are generally more active comparative with those one with pyridine moiety; generally, the fused azine proved to be more active than the non fused one. In the class of chimeric azine derivatives we notice that there are some certain classes of compounds that manifest a powerful antifungal activity, the most active being the imidazo–pyridine, pyrimidine–pyridine and pyrazolo–quinoline classes.

All these findings suggest that both classes of hybrid and chimeric derivatives are an attractive way in future antifungal therapy.

4. Future perspective

It appear very clear that there is an urgent need to seek new alternatives at existing methods to counteract pathogenic fungi and to respond to the high demand from pharmaceutic sector for new drugs. Antifungal activity varies widely with the structure and pharmacophore group, but also with the fungal species tested, as there are significant differences in sensitivity of fungi to antimycotics. When the pharmacophore group is kept unaltered, different substitutions might increase the potency of the drug, especially when two different active compounds are combined in a chimeric or hybrid structure. In many cases, tested heterocycles proved an even stronger antifungal potential compared with conventional fungicides that are commercially available, which is an outstanding achievement. The available literature reveals that most studies are conducted on the common pathogenic yeasts and filamentous fungi, like Aspergillus species, while a larger group of species should be screened. As some researchers demonstrated, various compounds exert a strong inhibitory effect against unexpected fungi. There are multiple mechanisms involved in antifungal activity, many of them incompletely understood, but it is supposed that a membranar mechanisms is frequent. Alteration of ergosterol synthesis pathway is demonstrated by several authors for species of Candida. Some compounds showed an inhibition of biofilm formation for species of pathogenic yeast, an important feature for human antifungal therapy. A particular needed property of a new antifungal drug relates to the lack of cytotoxicity toward mammalian cells, and there are multiple reports of such studies, making this large group of compounds suitable candidates for formulation of such drugs.

Finally, there are very promising results in the field of antifungal hybrid and chimeric azine derivatives, but there is also space to further explore. Understanding the mechanisms of action together with a rational drug design should be assessed with priority to make future drugs more efficient. Furthermore, extensive screening over a wide spectrum of fungi is required for finding activity at derivatives less expected. No doubts, the chimeric and hybrid azine group of compounds are a remarkable source for antifungal agents in the next future!

Supplemental material

Supplemental data for this article can be accessed at https://doi.org/10.1080/17568919.2024.2351288

Author contributions

Data curation, all authors.; formal analysis, T Balaes, CG Marandis, V Mangalagiu, G Mihai, II Mangalagiu; methodology, II Mangalagiu; investigation and resources, all authors.; supervision, T Balaes, G Mihai, II Mangalagiu; writing-original draft, T Balaes, CG Marandis, V Mangalagiu, G Mihai, II Mangalagiu; validation, II Mangalagiu; writing-review & editing, T Balaes, CG Marandis, V Mangalagiu, G Mihai, II Mangalagiu. All authors have read and agreed to the published version of the manuscript.

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The authors have no financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Competing interests disclosure

The authors have no competing interests or relevant affiliations with any organization or entity with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

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