Hydrazide–hydrazones as potential antimicrobial agents: overview of the literature since 2010

Hydrazide–hydrazone derivatives are present in many bioactive molecules and display a wide variety of biological activities, such as antibacterial, antitubercular, antifungal, anticancer, anti-inflammatory, anticonvulsant, antiviral, and antiprotozoal action. Therefore, many medicinal chemists synthesize various hydrazide–hydrazones and evaluate them for biological activities. Among biological properties of this class of compounds, antimicrobial activity is the most frequently encountered in scientific literature. This paper is focused on the overview of the literature findings of the last six years (2010–2016) covering the research on antimicrobial activity of hydrazide–hydrazone derivatives. This review may also serve as a useful guide for the development of new hydrazide–hydrazones as potential antimicrobial agents.


Introduction
Hydrazide-hydrazones constitute a class of organic compounds, which attracts the attention of medicinal chemists due to the fact that they contain azomethine group (-NH-N=CH-) connected with carbonyl group, which is responsible for their different pharmaceutical applications and makes possible the synthesis of different heterocyclic scaffolds (Rollas and Küçükgüzel 2007), like 1,3, 4-oxadiazolines (Doğan et al. 1998), azetidin-2-ones (Kalsi et al. 2006), coumarins (Mohareb et al. 2011), 1,3-thiazolidin-4-ones (Popiołek et al. 2015, and 1,3-benzothiazin-4-ones . The main route to synthesize hydrazide-hydrazone derivatives is the heating of appropriate hydrazides of carboxylic or heterocarboxylic acids with different aldehydes or ketones in various organic solvents like ethanol, methanol or butanol (Bala et al. 2013;Popiołek et al. 2015Popiołek and Biernasiuk 2016a, b). The molecular stucture of synthesized hydrazide-hydrazone derivatives can be easily confirmed by spectral methods. In the IR spectra, three characteristic bands are observed. The peaks around 1550 cm −1 correspond to the presence of C=N group. Carbonyl group (C=O) gives a characteristic band around 1650 cm −1 , whereas the NH group can be found in the area around 3050 cm −1 . In the 1 H NMR spectra of hydrazide-hydrazones, we can observe a characteristic singlet signal in the range of δ 8-9 ppm, and the second singlet signal around δ 10-13 ppm, which correspond to =CH and NH groups, respectively. In the 13 C NMR spectra, the signal for =CH group usually appears around δ 145-160 ppm, whereas in the range of δ 160-170 ppm we can observe the signal for carbonyl group (C=O) (Mohareb et al. 2011, Popiołek andBiernasiuk 2016a, b).
In recent years, a lot of biologically important hydrazide-hydrazone derivatives with a number of functional groups have been synthesized from many different carbonyl compounds. They were found to possess anticancer (Kumar et al. 2012;Yadagiri et al. 2014;Machakanur et al. 2012;Nasr et al. 2014), anti-inflammatory (Kumar et al. 2015), anticonvulsant (Çakır et al. 2001), antiviral (Şenkardes et al. 2016), and antiprotozoal (Siddiqui et al. 2014) activities. Among the biological properties of this class of compounds, the antimicrobial activity is the most frequently encountered one in scientific literature. Additionally, widely used chemotherapeutic agents such as nitrofurazone (McCalla et al. 1970), furazolidone (Chatterjee and Ghosh 1979;Ali 1983), and nitrofurantoin (McOsker and Fitzpatrick 1994;Munoz-Davila 2014) are known to contain typical hydrazide-hydrazone moiety or hydrazidehydrazone moiety, in which the carbonyl group and nitrogen atom are included in the 1,3-oxazolidine-2-one or imidazolidine-2,4-dione ring (Fig. 1).
Encouraged by the above mentioned facts, this study is an attempt to collect the hydrazide-hydrazone derivatives, which can be considered as potential antimicrobial agents, reported in the literature in the years 2010-2016.

Antibacterial activity
Searching for effective and non-toxic chemotherapeutic agents is still a very important issue due to the increase of multi-resistant bacterial strains (Moellering 2011). The treatment of bacterial infections is especially challenging in patients with compromised immune systems or with other associated diseases (Coates et al. 2002). Some of currently used antibacterial agents are known to contain hydrazide-hydrazone moiety (McCalla et al. 1970;Chatterjee and Ghosh 1979;Ali 1983;McOsker and Fitzpatrick 1994;Munoz-Davila 2014) (Fig. 1). Due to this fact, it is reasonable to search for novel antibacterial agents among hydrazide-hydrazone derivatives.
The in vitro screening results of newly synthesized benzimidazole derivatives bearing hydrazone moiety revealed that some of the compounds had significant antimicrobial activity (Özkay et al. 2010). Among synthesized derivatives, compounds 1 and 2 had bactericidal effect on the growth of Salmonella typhimurium, two times better (compound 1: MIC = 6.25 μg/ml) or equal (compound 2: MIC = 12.5 μg/ml) to the activity of chloramphenicol (MIC = 12.5 μg/ml), which was used as positive control (Fig. 2). The activity of these compounds against other Gramnegative bacterial strains, like Escherichia coli, Proteus vulgaris, Klebsiella pneumoniae, or Pseudomonas aeruginosa, was good (MIC = 25-100 μg/ml). The activity against Gram-positive bacteria was assessed on four strains: Listeria monocytogenes, Staphylococcus aureus, Enterococcus faecalis, and Bacillus subtilis. The best activity equal to the  (2) activity of chloramphenicol was found against E. faecalis (MIC = 12.5 μg/ml). Against other Gram-positive bacterial strains, the activity of compounds 1 and 2 was good to moderate (MIC = 25-200 μg/ml) (Özkay et al. 2010).
In another research, Rasras et al. (2010) synthesized novel hydrazide-hydrazones of cholic acid and tested them for antibacterial activity against three Gram-negative and three Gram-positive bacterial strains (Fig. 3). The activity of five derivatives (3-7) against E. coli was strong (MIC = 3.91-7.81 µg/ml), but weaker than cefixime, which was used as control. Interestingly, none of the tested compounds had any activity against P. aeruginosa and Enterobacter aerogenes. In turn, the activity against Gram-positive bacterium Enterococcus faecalis for the compounds 3 and 6 (MIC = 1.96 µg/ml), 4 and 7 (MIC = 3.91 µg/ml), and 5 (MIC = 7.82 µg/ml) was almost 16, 8, and 4 times higher, respectively, than the activity of cefaclor (MIC = 31.25 µg/ ml) and cefixime (MIC = 31.25 µg/ml). The MIC values for tested compounds against two other Gram-positive bacteria Staphyloccocus aureus and Bacillus megaterium were also good (MIC = 7.82-62.5 µg/ml) and comparable to chemotherapeutics used as controls (Table 1) (Rasras et al. 2010). Kumar et al. (2011a) evaluated novel hydrazidehydrazones of 4-chlorophenylsulfonyl acid for in vitro antibacterial activity. The tested compounds (8, 9 and 10) showed moderate to mild antibacterial activity on the basis of the measurement of the zone of the inhibition growth (ZOI = 10-21 mm) against two Gram-positive (Bacillus subtillis and S. aureus) and two Gram-negative (E. coli and Salmonella typhi) bacterial strains, when compared to ampicillin sodium used as a control (ZOI = 20-24 mm) (Fig. 4). The best antibacterial activity was displayed by compound 8 (ZOI = 21 mm) against S. aureus (control ZOI = 22 mm) and compound 10 against E. coli, whose zone of inhibition was 21 mm, whereas for ampicillin it was 20 mm (Table 2.) (Kumar et al. 2011a).
In another research, Kodisundaram et al. (2013) obtained a series of heterobicyclic methylthiadiazole hydrazones and investigated their activity in vitro against Gram-positive and Gram-negative bacteria. Two of the synthesized compounds, 26 and 27, showed interesting activity, especially against B. subtilis (Fig. 10). Their activity against this bacterium (MIC = 6.25 μg/ml) was two times higher than the activity of streptomycin (MIC = 12.5 μg/ml), which was used as positive control. The activity of these compounds against S. aureus was good (MIC = 25-50 μg/ml). As for not applicable  the Gram-negative bacteria, the synthesized compounds (26, 27) showed two times better activity (MIC = 6.25 μg/ ml) than streptomycin (MIC = 12.5 μg/ml) against K. peneumoniae, whereas against P. aeruginosa, compound 27 showed the activity equal to streptomycin (MIC = 12.5 μg/ml). The activity of the inhibition of the growth of E. coli was also two times higher (MIC = 12.5 μg/ml) in comparison with the control (MIC = 25 μg/ml) (Kodisundaram et al. 2013  showed the best activity towards Staphylococcus epidermidis ATCC 12228 (MIC = 4 μg/ml) (Fig. 11). The activity of these compounds against this bacterium was two times higher than the activity of nitrofurantoin (MIC = 8 μg/ml). The MICs against the other bacterial strains like, S. aureus ATCC 6538, S. aureus ATCC 29213, and S. aureus ATCC 29213, were also low (MIC = 11-27 μg/ml) (Pieczonka et al. 2013). The infection caused by P. aeruginosa constitutes a severe problem, especially for the patients with weak immune system. In response to this, hydrazide-hydrazones of 2,5-difluorobenzoic acid were synthesized by Narisetty et al. (2013) (Fig. 12). Among a series of new derivatives, compound 30 showed better activity (ZOI = 21 mm) against P. aeruginosa MTCC 424 than ampicillin (ZOI = 20 mm). Two of other synthesized compounds (31, 32) showed also very good activity (ZOI = 21-24 mm) against E. coli MTCC 443, S. aureus MTCC 96 and Streptococcus pyogenes MTCC 442. The activity of these compounds was higher than or comparable to the activity of ampicillin (ZOI = 19-22 mm) (Table 4) (Narisetty et al. 2013). Morjan et al. (2014) synthesized novel hydrazidehydrazones of nicotinic acid by the condensation reaction of nicotinic acid hydrazide with various ketones (Fig. 13). The synthesized derivatives were screened in vitro for antibacterial activity against Gram-positive and Gram-negative bacteria. The results of antimicrobial assay revealed that the synthesized compounds had interesting antibacterial activity against Gram-negative strains. Especially compounds 33 and 34 showed very strong activity against P. aeruginosa (MIC = 0.22 and 0.19 μg/ml, respectively). The activity of these compounds against K. pneumoniae was also high (33: MIC = 3.12 μg/ml, 34: MIC = 14.00 μg/ml). The inhibitory potency against Gram-positive bacteria Streptococcus pneumoniae (33: MIC = 3.12 μg/ml) and against S. aureus (34: MIC = 7.03 μg/ml) was also very significant (Morjan et al. 2014).   Satyanarayana et al. (2014) synthesized novel hydrazide-hydrazone derivatives of 4-(4-chlorophenyl) cyclohexanecarboxylic acid as potential antibacterial agents (Fig. 14). Three of the obtained compounds (35, 36 and 37) showed good antibacterial activity (MIC = 32-64 μg/ml) against a panel of four bacterial strains, including: S. aureus, B. subtilis, E. coli, and P. aeruginosa (Table 5). Unfortunately, the MIC values of the synthesized compounds were higher than the activity of the antibiotic used as control-ciprofloxacin (MIC = 5 μg/ml) (Satyanarayana et al. 2014).
2-(2,3-Dihydrobenzofuran-5-yl)acetic acid was used as a starting material for the synthesis of new hydrazide-hydrazone derivatives by Kaki et al. (2014). Among a series of synthesized derivatives, two compounds (38 and 39) possessed interesting antibacterial activity measured by the diameter of the zone of the inhibition growth against two Gram-negative (E. coli and P. aeruginosa) and two Gram-positive (S. aureus and S. pyogenes) bacterial strains (Fig. 15). Compounds (38 and 39) according to the antimicrobial activity assays, had similar values of inhibition zone (ZOI = 19-22 mm) as ampicillin, which was used as positive control (ZOI = 19-22 mm) (Kaki et al. 2014).

Antitubercular activity
Despite the fact that tuberculosis is a curable and treatable disease, it still remains a major cause of death and illness (WHO 2010), which confirms that seeking and developing new antitubercular agents is needed. A survey of scientific literature reveals that several hydrazide-hydrazones synthesized during last 6 years possess interesting antitubercular activity (Unissa et al. 2016). Additionally, it is worth to mention that according to the most recent article published by John et al. (2016) the 2-hydroxy-1-naphthaldehyde isonicotinoyl hydrazone may act as a novel inhibitor of methionine aminopeptidases from Mycobacterium tuberculosis.
Cihan-Üstündağ and Çapan (2012) evaluated a series of indole derivatives containing hydrazide-hydrazone scaffold for in vitro antitubercular activity (Fig. 24). Antimycobacterial activity was tested against M. tuberculosis H37Rv ATCC 27294 with the use of rifampicine as a control. Unfortunately the synthesized compounds showed weaker activity (MIC > 6.25 μg/ml) than the reference substance (MIC = 0.125 μg/ml) (Cihan-Üstündağ and Çapan 2012). Velezheva et al. (2016) designed and synthesized a series indole-pyridine derived hydrazide-hydrazones and evaluated them against two M. tuberculosis strains (H37Rv and CN-40) (Fig. 25). Based on the obtained results, hydrazide-hydrazone derivative 85 appeared to be the most potent among examined compounds (MIC=0.05 µg/ml) against the M. tuberculosis H37Rv strain. Its activity was equal to that of isoniazid used as positive control in this assay. Besides, this compound 85, unlike isoniazid, showed significant activity against isoniazidresistant M. tuberculosis CN-40 strain. Other synthesized derivatives also displayed high antitubercular activity (Table 9) (Velezheva et al. 2016).

Antifungal activity
Fungal infections still remain a serious problem, even though there are many available medicines on the market  (Lewis 2011). In this section, I will present several examples of hydrazide-hydrazones which possess significant antifungal activity. Benzimidazole derivatives bearing hydrazone moiety (Özkay et al. 2010) were also screened for antifungal activity against three species of yeasts: Candida albicans, Candida glabrata, and Candida tropicalis. The activity of compounds 1 and 2 against these fungi was good (MIC = 50-100 μg/ml). In the case of C. tropicalis, the MICs of the synthesized compounds (MIC = 50 μg/ml) were equal to the MIC of ketoconazole used as control (MIC = 50 μg/ml) (Özkay et al. 2010).
Similar research was performed by Kumar et al. (2011a). New hydrazide-hydrazones of 4-chlorophenylsulfonyl acid were synthesized and tested for antifungal activity on the basis of the measurement of the zone of inhibition growth against C. albicans and Aspergillus niger. Three of the synthesized compounds (8, 9 and 10) showed promising antifungal activity compared with the clotrimazole, which was used as positive control. In the case of compound 8, the antifungal activity (ZOI = 31 mm) was greater than the activity of clotrimazole (ZOI = 30 mm) against C. albicans (Kumar et al. 2011a).
New anacardic acid hydrazone derivatives were subjected to in vitro antifungal assays against C. albicans and A. niger (Rambabu et al. 2015). Three of the synthesized compounds (40, 41 and 42) showed good to moderate antifungal activity (ZOI = 11-20 mm) based on the measurement of the zone of inhibition growth in comparison with the gryseofulvin used as control (ZOI = 24-28 mm) (Rambabu et al. 2015).
In the case of compounds 88 and 89 the antifungal activity against C. tenuis and A. niger was much better (Fig. 27). Compounds 88 and 89 against C. tenuis showed the MIC values of 1.9 and 0.9 µg/ml, respectively. Compound 88 displayed strong activity (MIC = 3.9 µg/ml) against A. niger and compound 89 showed moderate activity (MIC = 125 µg/ml) (Rutkauskas et al. 2013).
In the case of hydrazide-hydrazones of 4-nitrobenzoic acid, the antifungal activity against the above mentioned fungi strains was even better (Fig. 29). The MIC 80 parameters for compounds 94 and 95 were 0.5 µg/ml against C. albicans, whereas for compound 95 it was even 0.125 µg/ml against C. glabrata. The MIC 80 value for compound 94 against C. glabrata was also very low (MIC 80 = 0.5 µg/ml) (Backes et al. 2014).

Conclusions
In conclusion, this paper gives an overview of the antibacterial, antitubercular and antifungal properties of hydrazide-hydrazone derivatives. As presented in this study hydrazide-hydrazone moiety may be found and   incorporated in various bioactive molecules. Thus this paper appears to be important for further development of hydrazide-hydrazones as potential antimicrobial agents.

Compliance with ethical standards
Conflict of interest The author declares that he has no competing interests.
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