Acylhydrazones and Their Biological Activity: A Review

Abstract

Due to the structure of acylhydrazones both by the pharmacophore –CO–NH–N= group and by the different substituents present in the molecules of compounds of this class, various pharmacological activities were reported, including antitumor, antimicrobial, antiviral, antiparasitic, anti-inflammatory, immunomodulatory, antiedematous, antiglaucomatous, antidiabetic, antioxidant, and actions on the central nervous system and on the cardiovascular system. This fragment is found in the structure of several drugs used in the therapy of some diseases that are at the top of public health problems, like microbial infections and cardiovascular diseases. Moreover, the acylhydrazone moiety is present in the structure of some compounds with possible applications in the treatment of other different pathologies, such as schizophrenia, Parkinson’s disease, Alzheimer’s disease, and Huntington’s disease. Considering these aspects, we consider that a study of the literature data regarding the structural and biological properties of these compounds is useful.

1. Introduction

The acylhydrazones, through their structure, have significant malleability both chemically and pharmaceutically. Numerous representatives of this class of organic compounds are intermediates in the synthesis of heterocyclic compounds, including pentatomic ones [1,2,3,4,5,6]. They also present a structural variability that offers the possibility to synthesize compounds belonging to this class with various therapeutic indications (like cytotoxic, antibacterial, antifungal, antiviral, antioxidant, antiparasitic, anti-inflammatory, anticonvulsant, and antihypertensive) [7,8]. A number of derivatives containing the acylhydrazone moiety are used in therapy, such as nitrofurazone (antimicrobial), carbazochrome (antihemorrhagic), nifuroxazide (intestinal antibacterial), dantrolene (muscle relaxant), nitrofurantoin (antibacterial), nifuratel (antitrichomonal and antifungal), nifurzide (intestinal anti-infective), nifurtoinol (urinary anti-infective), naftazone (capillary stabilizing), azimilide (anti-arrhythmic), zorubicin (cytotoxic antibiotic) [9,10,11]. The structures of some representative pharmacologically active agents containing the acylhydrazone scaffold are shown in Figure 1.

The objective of this paper is to review the literature describing the acylhydrazone moiety as an important scaffold for medicinal chemistry highlighting its versatility and drug-like character.

2. Structure

The acylhydrazones have in their structure the –CO–NH–N=CH– group in which there are: an electrophilic carbon atom (CH=N), a nucleophilic imine nitrogen atom, by the doublet of non-participating electrons (CH=N:), and an amino nitrogen atom with acidic character (–NH–) [12,13]. Thus, the acylhydrazone molecules are both electrophilic and nucleophilic [14]. The nucleophilic attack is performed at the amine nitrogen atom (NH), and the electrophilic one at the oxygen atom (CO) [15].

The acylhydrazones can also exhibit keto-enol tautomerism and through the electron donor (the oxygen atom of the carbonyl group) [14], together with the azomethine nitrogen atom (–N=), participate in the chelation of metal ions [16].

Due to the fact that the N=CH bond is in the vicinity of the amide nitrogen atom (CO–NH), the acylhydrazones may have an acidic character manifested by the yielding of the hydrogen atom bound to the azomethine carbon atom [17].

The acylhydrazones can form intermolecular hydrogen bonds through the hydrogen atom bound to the amino nitrogen (–NH–) and the oxygen atom [18,19,20], between the hydrogen atom bound to the imine carbon (CH) and the atomic nitrogen atom (–N=) of another molecule [20].

The acylhydrazones exhibit geometric isomerism due to the imine group (–N=CH–). Thus, they are in a mixture of E and Z isomers, where E is predominant, in general, because its stability is superior to the Z isomer [4,21].

Theoretically, the acylhydrazones can have four isomers, two of which are geometric isomers (E/Z) and are due to the C=N double bond, and two are conformal isomers (syn/anti) and are due to the N–N bond [5,22]. The structures of these isomers are shown in Figure 2 [14].

In the case of N-aroylhydrazones 1a–k (Figure 3), the Z isomer is stabilized by intramolecular hydrogen bonds. Thus, it is found in a higher percentage than the E isomer [5].

The NMR spectra indicated that the N-acylhydrazones usually exist as a mixture of two conformers, namely E_(C=N)(N–N) synperiplanar and E_(C=N)(N–N) antiperiplanar, at room temperature in DMSO-d₆. The E_(C=N) configurational isomers rapidly establish synperiplanar/antiperiplanar equilibrium about the –CO–NH– bond, in the DMSO-d₆ solution. The synperiplanar conformer predominates the antiperiplanar isomer due to its ability to develop intermolecular interactions with polar solvents, like DMSO [23].

3. Synthesis

The acylhydrazones 4 can be obtained by the condensation reaction of an aldehyde or ketone 3 with a derivative of the class of hydrazides 2 [24] in the presence of an alcohol [25,26], generally at reflux, and in an acidic medium [12,27,28,29,30,31,32] or in the absence of the acid catalyst [4,6,33,34,35,36,37]. The general synthesis reaction of acylhydrazones is presented in Scheme 1 [37].

4. Spectral Analysis

The vibration-rotation spectra of acylhydrazones show bands specific to the –CO–NH–N= moiety present in the structure of derivatives of this class. The intervals in which these bands are recorded are as follows: 1647–1687 cm⁻¹ for the C=O connections [6,18,19], 3194–3440 cm⁻¹ for the NH connection [6,19,29], with the specification that there is variation between symmetrical (3080 cm⁻¹) and asymmetrical vibrations (3194 cm⁻¹) [17], 980–1000 cm⁻¹ for the N–N connection [38], 1578–1623 cm⁻¹ for the N=C connection [17,18,19], and for the CH connection the value of the wavenumber in the region of 3050–3078 cm⁻¹ was reported [6].

The values of the chemical shifts of the protons specific to the acylhydrazone derivatives in the ¹H-NMR spectra are in the following ranges: 11.0–13.5 ppm for the proton of the –CO–NH– group [6,31], 8.5–12.5 ppm for the proton of the N–H bond [12,19,28], 8–9 ppm for the proton of the –N=CH– group [12,31].

In the ¹³C-NMR spectra, the chemical shift values for the imine carbon atom (–N=CH–) are between 157–168 ppm, and for the amide carbon atom (–CO–NH–) are reported between 159.0–173.5 ppm [6,31]. In some cases, the duplicated signals observed in the NMR spectra of acylhydrazones correspond to the presence of two amide bond-related conformers [23].

5. Biological Properties

Acylhydrazones have significant importance in the pharmaceutical field through numerous biological properties with multiple therapeutic indications. In the research studies performed using compounds from the acylhydrazone class, the following actions were reported: antitumor [25,26,27,28,39,40,41] and [42,43,44,45,46,47,48,49,50,51], cytotoxic [52], antibacterial [4,12,31,33,34,35] [53,54,55,56,57,58,59], antifungal [34,60,61], antiviral [62,63,64,65,66,67,68,69], antiparasitic [6,45,70,71,72,73,74,75,76], anti-inflammatory [32,73,77,78,79,80,81,82,83,84], analgesic [36,77,78,79,80,81,85], immunomodulatory [83,86], enzyme inhibition [29,86,87,88] and [89,90,91,92,93,94,95,96,97,98], antidiabetic [18], anticonvulsant [73], antioxidant [34,39,78,99,100,101], and effects on the cardiovascular system [102,103,104,105,106,107,108,109,110,111].

5.1. Antitumor Action

According to a recent study, 5-bromo-1-methyl-N’-[(E)-(1-methyl-1H-indol-3-yl)methylidene]-1H-indol-3-carbohydrazide 5 (Figure 4) showed antitumor action on breast, cervical and colon cancer cell lines by inducing cellular apoptosis. This action is exerted through cyclic adenosine monophosphate (cAMP)-dependent protein kinase A, p53 protein, and by stimulating the generation of reactive oxygen species (ROS) and nitric oxide (NO) [28].

Aneja et al. demonstrated that (E)-1-(4-methoxybenzyl)-N’-(7-methyl-2-oxoindolin-3-ylidene)-1H-1,2,3-triazole-4-carbohydrazide 6 (Figure 5) has an inhibitory effect on the kinase involved in cell replication, microtubule affinity regulatory kinase (4MARK4), fulfilling an antiproliferative effect simultaneously with increasing the production of ROS, inducing apoptosis in cancer cell lines [25].

The cytotoxic action was evidenced for several compounds of which two derivatives, namely N’-(1-(4,7-dihydroxy-2-oxo-2H-chromen-3-yl)ethylidene)benzohydrazide 7a and N’-(1-(4-hydroxy-2-oxo-2H-chromen-3-yl)ethylidene)benzohydrazide 7b (Figure 6), were identified as having an intensity of this effect comparable to that of doxorubicin and colchicine [27].

Very recently, Vilková et al. investigated the anticancer activity of some acridine acylhydrazone analogs 8a–d (Figure 7), among which 8a and 8c reduced the clonogenic capacity of A549 cells [112].

According to the evaluation of Lis et al., acylhydrazone 9 (Figure 8) induced apoptosis in erlotinib-resistant neoplasms as a result of selective STAT3 inhibition [40].

Recently, Banumathi et al. showed that the azo-hydrazone analog 10 (Figure 9) exerted chemosensitivity specifically against EAC and A549 cells without altering their normal counterpart [113]. It was found that the antiproliferative activity of 10 was due to the induction of apoptosis by inhibiting the STAT3 signal. Furthermore, compound 10 attenuated solid tumor growth without inducing significant toxicological side effects.

The acylhydrazone derivative 11 (Figure 10) exhibited an in vivo antiproliferative effect with a potency similar to that of colchicine both by inducing apoptosis and by inhibiting the polymerization of microtubules [26].

The derivatives 12a–c (Figure 11) presented, besides the antiproliferative action on human erythroleukemia K562 and melanoma Colo-38 cells, an antioxidant action demonstrated based on 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity test, ferric reducing antioxidant power, and oxygen radical absorbance capacity [39].

According to a study by Sun et al., it was found that a derivative of the class of acylhydrazones (13) (Figure 12) showed antitumor action with possible use in gastric cancer as a lysine-specific demethylase 1 (LSD1) inhibitor [41].

Congiu et al. synthesized a series of acylhydrazone derivatives 14a–d (Figure 13) which showed cytotoxic effect and inhibition of tumor development for a relatively large number of neoplasms [42].

A series of acylhydrazone-derived compounds displayed cytotoxic action of variable intensity. Thus, for compound 15, the potency of the effect was higher compared to doxorubicin in promyelocytic leukemia [43]; for the acylhydrazone derivative 16, the intensity of the effect was significant due to the exercise of cytotoxic action on different neoplasms including resistant cell lines [44]. The benzothiazole acylhydrazones 17a–c showed selective inhibition towards cancer cells. Moreover, derivative 17a displayed higher antiproliferative activity than the reference agent cisplatin [114]. The structures of the acylhydrazones 15–17 are presented in Figure 14.

In the case of acylhydrazone derivatives, the cytotoxic mechanism does not involve the generation of ROS leading to apoptosis. The derivative 18 (Figure 15) falls into this category of compounds, influencing the cell cycle, cell division, and ribonucleotide reductase, an enzyme that changes its activity following the chelation of iron ions [46].

In a study by Yu et al., two derivatives of the class of acylhydrazones 19 and 20 (Figure 16) with cytotoxic action superior to the reference substance (5-fluorouracil) were reported [47].

Acylhydrazone 21 (Figure 17) could be used in therapy as an antitumor agent with insignificant effects on normal cell lines due to the fact that it induces apoptosis by depolarizing the mitochondrial membrane and generating ROS in cancer cell lines. In addition to these actions, the compound is involved in the inhibition of tubulin polymerization [48].

Compound 22a showed the strongest cytotoxic action on all cell cultures used, and derivatives 22b and 22c exhibited cytotoxicity only on a certain (ovarian cancer) cell line. In the experimental model of Ehrlich solid carcinoma, the acylhydrazone 22a showed inhibition of tumor development comparable to that of the reference substance, 5-fluorouracil [49]. The structures of acylhydrazones 22a–c are presented in Figure 18.

De Almeida et al. evaluated the cytotoxic action of a series of derivatives from the class of acylhydrazones. The research showed that acylhydrazone 23 (Figure 19) exerted the best action in the series of studied compounds, probably due to the bromine substituent in the para position on the phenyl nucleus [50].

The acylhydrazone derivative 24 (Figure 20) showed antitumor action on the studied cancer cell lines with increased intensity on the lung cancer cell line. The cytotoxic action is due to the generation of ROS and the altering of the cell cycle [51].

Compounds 25a and 25b (Figure 21) showed the inhibitory effect against carbonic anhydrase IX and XII isoforms, respectively, involved in the growth and development of tumors [29].

The derivative 26 (Figure 22) showed antitumor action with a higher potency compared to 5-fluorouracil, due to the inhibitory effect of telomerase [91].

The acylhydrazone 27 (Figure 23) demonstrated the inhibitory activity on phosphatidyl-inositol-3-kinase, which is involved in cell division. Gao et al. assumed that the action was feasible due to the nitrogen atoms and substituents in the compound structure [94].

The acylhydrazone 28 (Figure 24) was reported as an inhibitor with significant action on lactate dehydrogenase A, an isoform that exhibits abnormal activity in tumor cells [96].

5.2. Antimicrobial Action

5.2.1. Antibacterial Action

There are many acylhydrazones described to have antimicrobial effects on various bacterial strains. It is difficult to analyze the structure-activity relationships because of the high chemical diversity of these compounds. As a general rule, the compounds active on Gram-negative bacteria are more hydrophilic than those effective on Gram-positive bacteria because of the differences in their cell wall structure [115,116]. Many of the studies reported here used acylhydrazone scaffold as the rationale for their drug-design process and presented only the phenotypic antibacterial activity without the mechanism of the effect.

A series of acylhydrazone salts 29a,b (Figure 25) were synthesized and their antimicrobial action was studied. It is noteworthy that the investigated derivative 29a exerted antimicrobial action on methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli, Clostridium difficile, and Candida albicans. A high potency action was registered on methicillin-resistant Staphylococcus aureus and Escherichia coli (29b) [31]. Overall, molecular dynamics simulation analysis showed that the effect of structural features, such as pyridinium scaffold, hydrophobic side chains, and –CO–NH–N= linker, in the diffusion of such substances across the cell membrane and that it could be responsible for their antibacterial activity. In order to understand the mechanism of acylhydrazone salts 29a,b as anti-bacterial agents, docking experiments were performed against the microbial target, E. coli glucosamine-6-P synthase. The acylhydrazone salts 29a,b were predicted to form stable hydrogen bonding and hydrophobic interactions. Molecular dynamics simulation highlighted the target interaction behavior of these derivatives at the surface of cell membranes indicating a passive diffusion mechanism at the surface layer.

Among the pathogenic microorganisms, for which the antimicrobial action of acylhydrazone derivatives was demonstrated there is also Mycobacterium tuberculosis. Rohane et al. synthesized an acylhydrazone 30 (Figure 26) with the most intense action among the obtained derivatives due to the substituents on the benzene ring. The reference substance used was isoniazid [33]. Molecular docking studies investigating acylhydrazone analogs using enoyl acyl carrier protein reductase as their potential biological target indicate that the hydroxyl, azide, amino, and phenyl groups of the spacer of the acylhydrazone play an important role in the interactions with the active site [33]. The enoyl acyl carrier protein reductase is an attractive target for drug-design, being essential in the type II fatty acid synthase system found in microorganisms and without homologue in mammals [117].

Siddique et al. obtained a series of new compounds 31a–g (Figure 27), that showed antibacterial and antifungal actions with varying intensities studied on Escherichia coli, Bacillus subtilis, Salmonella typhimurium, Staphylococcus aureus, and Candida albicans [34].

The mechanism of antibacterial action, in the case of acylhydrazones 32a–d (Figure 28), studied by Xia et al., is to modulate the expression of genes responsible for hemolysis and virulence of tested pathogenic microorganisms [35].

The acylhydrazone derivatives 33a,b and 34a–c (Figure 29) showed antibacterial action on Escherichia coli by inhibiting the enzymatic pyruvate dehydrogenase complex (PDHc). Among the compounds studied, the most active was 34b. The acylhydrazones 33a and 33b exhibited selectivity for the enzymatic complex [12].

The antimicrobial action of some acylhydrazone derivatives against Escherichia coli, resulting from the inhibition of the multienzyme PDHc-E1, was also investigated. Among the compounds studied, acylhydrazones 35a–d (Figure 30) exerted the best action with good selectivity [53].

The acylhydrazone derivatives 36a–d (Figure 31) showed intense antibacterial action on Pseudomonas aeruginosa, a resistant microorganism [4].

According to studies by Jin et al., the acylhydrazones 37a,b (Figure 32) exhibited a broad antibacterial spectrum, being active on both Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa) and Gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus) [54,55].

The complex of acylhydrazone 38 (H₂L) with zinc (II) ion as [Zn(HL)₂]∙EtOH showed an intense antimicrobial action on most of the tested bacterial strains. Among the microorganisms on which this property was studied are Bacillus subtilis, methicillin-resistant Staphylococcus aureus, Escherichia coli, and Haemophilus influenzae. The potency of the complex on Haemophilus influenzae was significant [56]. The structure of acylhydrazone 38 is presented in Figure 33.

Among the compounds investigated are acylhydrazones 39 with action on Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus subtilis [57] and 40 which was only active on Mycobacterium tuberculosis among the microorganisms included in the study [58]. The structures of acylhydrazones 39 and 40 are shown in Figure 34.

Shah et al. synthesized a series of isonicotinic hydrazid-based acylhydrazone analogs 41a–f (Figure 35), which were evaluated for their antibacterial activity against two Gram-positive strains, namely Staphylococcus aureus, Bacillus subtilis, and a Gram-negative bacterium, i.e., Escherichia coli [118]. The results showed that the studied compounds 41a–f had appreciable antibacterial activity against the tested strains, among which the derivatives 41c and 41e proved to be the most active, being promising agents in the treatment of bacterial infections. The acylhydrazones 41a–f were also screened for their cytotoxic effect, the maximum activity being noted for analogs 41e and 41f.

The acylhydrazone 42 (Figure 36) showed good antimicrobial activity on Escherichia coli by inhibiting the PDHc-E1 due to the para-NO₂ group grafted on the benzene ring [93].

Yao et al. designed and synthesized a series of aminoguanidine derivatives containing an acylhydrazone moiety 43a–h (Figure 37), which were evaluated for their antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus, Enterococcus faecalis, Bacillus subtilis, and Streptococcus mutans) and Gram-negative strains (Escherichia coli; Pseudomonas aeruginosa) [119]. Penicillin, oxacillin, and norfloxacin were used as positive controls. The derivative 43d displayed a wide spectrum of antibacterial effects, being active on both Gram-positive and Gram-negative bacterial strains.

5.2.2. Antifungal Action

The acylhydrazones 31a, 31b, 31c, and 31e (Figure 27) studied on Candida albicans exerted a moderate antifungal effect [34]. Additionally, the derivatives 44a–e (Figure 38) showed modest antifungal activity against different fungal strains (Candida albicans, Candida tropicalis, Candida krusei, Candida glabrata, and Candida parapsilosis) [60]. In the case of compounds 44a–d, the association of the carbohydrate unit with the acylhydrazone moiety determined the increase of the fungicidal effect on Candida parapsilosis. The acylhydrazone derivatives 45a,b (Figure 38), from the series synthesized by Reis et al., had selectivity for Candida glabrata and a potency comparable to that of the nystatin [61].

All the compounds 46a–g (Figure 39), obtained by Kumar et al., showed excellent antifungal activity against Aspergillus niger compared to the reference drug (clotrimazole), good antimalarial effect against Plasmodium falciparum compared to the standard drug chloroquine, and moderate to good antibacterial activity against Gram-positive bacterium strain Bacillus cereus compared to clotrimazole [120].

5.3. Antiviral Action

In the case of some derivatives from the acylhydrazone class, it is reported in the literature that they exhibit antiviral action. This effect was identified for acylhydrazones 47a,b and 48 (Figure 40), which were studied as inhibitors targeting Human immunodeficiency virus type 1 (HIV-1) capsid protein [62] and on Tobacco mosaic virus, respectively [63].

Additionally, the acylhydrazone derivatives 49–55 (Figure 41) were studied for their antiviral action. Through the research undertaken, the following results were obtained, namely, compound 49 displayed antiviral action on HIV-1 by blocking the activity of the viral envelope glycoprotein [64], analog 50 showed intense action on HIV-1 [65], and derivatives 51 and 52 had antiviral action on the Epstein–Barr virus [66]. Compound 53, with possible application in the treatment of the Influenza virus, had neuraminidase inhibitory action more potent than oseltamivir [67]. The derivatives 54 and 55, containing in their structure a monosaccharide moiety (D-mannose, D-ribose), displayed the highest potency in the series of studied compounds on Hepatitis A virus (54) and Herpes simplex 1 (55), using as reference substance amantadine, respectively, acyclovir [68].

In the case of acylhydrazone derivatives, the antiviral action against HIV and Influenza A virus subtype H1N1 was shown to be determined by the enzymatic inhibition resulting from the chelation of metal ions in the viral structure and endonucleases [69].

The acylhydrazone class derivative 56 (Figure 42) was found to be an influenza virus endonuclease inhibitor due to the ability of complexation of metal ions (through –OH groups) in the enzyme structure and forming hydrogen bonds [98].

5.4. Antiparasitic Action

Some acylhydrazone derivatives were studied for their antiparasitic activity. For example, compounds 57a,b had antiparasitic action against Entamoeba histolytica which was superior to that of metronidazole with lower toxicity [6]. Compound 58a showed antimalarial activity as an inhibitor of β-hematin synthesis and derivative 58b displayed antiamoebic effect [70]. Compounds 59 [71] and 60a–c exhibited antiparasitic action against the Plasmodium falciparum, 60b being the most potent compound in the series [72]. The structures of acylhydrazone compounds 57–60 are presented in Figure 43.

The derivatives 61a,b [74], 62 [45] with antiparasitic action on Trypanosoma cruzi, and analog 63 [75] active against Leishmania amazonensis were also reported (Figure 44).

The mechanism of antiparasitic action of the acylhydrazone derivative 64 (Figure 45) is based on membrane depolarization, production of ROS, and alteration of cell membrane integrity in the case of the parasite L. amazonensis [121].

A compound with an inhibitory effect on the development of Plasmodium falciparum was obtained by complexing the acylhydrazone 65 (Figure 46) with iron ions [73].

The acylhydrazones 66a,b (Figure 47) showed antiparasitic action via inhibition of cruzain, the major cysteine protease of Trypanosoma cruzi. The effect was comparable to that of the reference substance nifurtimox [76,122].

5.5. Anti-Inflammatory Action

Acylhydrazone class compounds 67–72 (Figure 48) exerted anti-inflammatory activity. Thus, compound 67 inhibited the cascade of arachidonic acid based on the naphthyl group which facilitates hydrophobic interactions with IKK-β [32]. The derivatives 68a,b showed anti-inflammatory and analgesic activities, the effects exerted by 68a being of lower intensity [77]. In the case of compound 69a, the anti-inflammatory action was determined by the presence of the –NO₂ group [78]. The derivative 69b had an anti-inflammatory action comparable to that of nimesulide [79]. Compounds 69c and 70a,b demonstrated the anti-inflammatory effect by inhibiting the NF-kB pathway and the release of IL-8 [80]. Analog 71 also exerted analgesic action in addition to the anti-inflammatory one [81]. The derivative 72 had an anti-inflammatory effect by reducing the eosinophilia due to low IL-4, IL-5, and IL-13 cytokine levels [82]. This suggests its therapeutic potential for treating allergic diseases. Additionally, 72 demonstrated the anti-inflammatory action by modulating IL-1β secretion and PGE2 synthesis in macrophages and by inhibiting calcineurin phosphatase activity in lymphocytes [83].

The anti-inflammatory effect of acylhydrazone 73 (Figure 49) was due to the selective inhibition of cyclooxygenase-2 (COX-2) and decreasing lymphocyte proliferation [84]. Moreover, the in silico analysis and experimental results suggested that 73 exhibits a well-balanced pharmacodynamic and pharmacokinetic profile.

Compound 74 (Figure 50), synthesized by Ünsal-Tan et al., was reported as a non-selective COX inhibitor with the highest potency among the studied derivatives [97].

The acylhydrazones 75a–c (Figure 51) exhibited an anti-inflammatory effect comparable to that of indomethacin, but do not affect the gastric mucosa [73].

In addition to the anti-inflammatory activity, compounds of the acylhydrazone class 68a,b [77], 69a [78], 69b [79], 70a [80], and 71 [81] (Figure 48) demonstrated analgesic action. This effect, in association with the anti-inflammatory activity, may have possible therapeutic applications in various pathologies.

It was found that the analgesic action mediated by acylhydrazones 76a,b was exerted via the opioidergic system [36]. Cordeiro et al. showed that amino-pyridinyl-N-acylhydrazone 77 exhibited anti-inflammatory activity by inhibiting p38α, reducing inflammatory pain, cell migration, and inflammatory mediators participating in the MAPK pathway, such as IL-1β and NF-α [123]. The structures of acylhydrazones 76a,b and 77 are presented in Figure 52.

5.6. Immunomodulatory Action

The action of acylhydrazone derivatives on the immune system was also reported in the literature. The acylhydrazone class derivative 72 (Figure 48) showed immunomodulatory effect by inhibiting cytokine production and lymphocyte proliferation [83].

According to a study conducted by Guimarães et al., acylhydrazone 78 (Figure 53) exhibited immunosuppressive activity due to the inhibitory action of phosphodiesterase-4 (PDE-4), inhibiting phosphorylation of IkB protein which interferes with the NF-kB pathway [86].

5.7. Antiedematous Action

Compounds 25b (R₁ = C₆H₅, R₂ = 2-pyridyl) and 25c (R₁ = CH₃, R₂ = 4-BrC₆H₄) (Figure 21), having in their structures the acylhydrazone moiety closed in a heterocycle, showed antiedematous effect by inhibiting carbonic anhydrase I isoform [29].

5.8. Antiglaucomatous Action

The acylhydrazone derivatives 25c (R₁ = CH₃ and R₂ = 4-BrC₆H₄), 25d (R₁ = CH₃ and R₂ = C₆H₅), 25e (R₁ = CH₃ and R₂ = 4-CH₃C₆H₄) (Figure 21), and 79 (Figure 54) exhibited antiglaucomatous activity by inhibiting the carbonic anhydrase II isoform [29].

5.9. Activity on the Central Nervous System (CNS)

The acylhydrazones 80a–f showed the inhibitory action on acetylcholinesterase and good antiaggregation activity on plates of β-amyloid. The enzyme inhibition was noted as the effect depending on the conformation of the enzyme-substrate complex, with relatively better results than the other compounds in the case of 80d and 80f [88]. The derivative 81 is one of the substances synthesized by Viegas et al. with possible beneficial effects in Alzheimer’s disease by inhibiting acetylcholinesterase, COX-1, and COX-2 [89]. Compound 82 could be a potential candidate for use in neurodegenerative diseases due to passive diffusion through the blood–brain barrier and controlling neuronal synapses [90]. The structures of compounds 80–82 are presented in Figure 55.

Compounds 83 and 84 (Figure 56), which showed the chelating affinity of iron (II) ions, presented inhibitory action on ten-eleven translocation methylcytosine dioxygenase 1 (TET 1). This protein catalyzes the chemical reaction of transforming 5-methylcytosine into 5-hydroxymethylcytosine, a substance that in abnormal concentrations is associated with diverse pathologies, like leukemia, Parkinson’s disease, and Alzheimer’s disease [87].

An acylhydrazone derivative 85 (Figure 57) was reported as a potent phosphodiesterase 10A (PDE10A) inhibitor probably due to the presence in their structure of the substituted 4-quinoline nucleus [92]. The PDE10A is implicated in diverse central nervous system pathologies, such as Parkinson’s disease and Huntington’s disease, and mental disorders, like schizophrenia [92].

The derivative of acylhydrazone 86 (Figure 58) exerted inhibitory action of an isoform of lipooxygenase (15-LOX-1) with good intensity, due to the ortho-chlorine atom on the benzene nucleus. This isoform is involved in pathologies, such as Alzheimer’s disease and Parkinson’s disease [95].

The piperidinehydrazide-hydrazones 87a–c and 88a–c (Figure 59) showed potential anti-Alzheimer activity by inhibiting the β-amyloid plaque formation [99]. Furthermore, the acylhydrazones 87a,b and 88a,b displayed strong antioxidant activity due to the presence in their molecules of the dimethylamino (87a, 88a), respectively, diethylamino moiety (87b, 88b).

5.10. Antidiabetic Activity

Compounds 89a–e (Figure 60) showed an antidiabetic effect due to the inhibition of α-glucosidase, an enzyme that catalyzes the cleavage of oligosaccharides into monosaccharides. The derivative with an electronegative group in the para position (89c) exhibited the most intense action [18].

5.11. Antioxidant Action

Among the various pharmacological studies performed in the case of some acylhydrazone derivatives are those that showed their antioxidant effects [34,39,78,101].

In addition to the above-mentioned properties identified in the case of acylhydrazones 31a, 31c, 31g (Figure 27) [34], and 69a (Figure 48) [78], the antioxidant action of the specified derivatives was also reported. This activity was tested using DPPH [34,78], ferric-reducing antioxidant power (FRAP) [34], hydroxyl-mediated deoxyribose degradation, and superoxide radical scavenging assays [78].

The antioxidant action was reported for the compounds 90a–i (Figure 61) using the oxidative stress induced by tert-butyl hydroperoxide. Moreover, the cytoprotective effect was investigated, which indicated that derivatives 90a–c and 90g–i showed effects comparable to those of the reference substance (quercetin), and compounds 90d–f exhibited weaker effects compared to this one [100].

5.12. Action on the Cardiovascular System

Among the different biological properties and possible therapeutic indications of the acylhydrazone class compounds, their actions on the cardiovascular system were identified. Thus, acylhydrazone 91a (Figure 62) may be a potential candidate for use in the treatment scheme of cardiac remodeling, respectively in combating diastolic disorders after myocardial infarction. This derivative has the potential to reduce cardiac remodeling after myocardial infarction by regulating inflammatory mediators, leading to reduced inflammation and cardiac fibrosis. The positive inotropic effect of compound 91a was observed by stimulating the activity of the sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 2a (SERCA2a) protein, causing both the uptake of Ca²⁺ ions into the sarcoplasmic reticulum and intracellular Ca²⁺ utilization. This effect was also observed in healthy cardiomyocytes by increasing the intracellular Ca²⁺ concentration. Compound 91a regulates the phosphorylation and dephosphorylation of troponin I, troponin T, and protein C, respectively, but the Ca²⁺ sensitivity of contractile proteins was not noted in this study. Thus, this analog is considered a promising agent for use in the treatment of heart failure after myocardial infarction [102]. The above-mentioned acylhydrazone was also found to prevent exercise intolerance after myocardial infarction, probably by producing NO with vasodilating action by increasing the level of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle cells and by activating adenosine A_2A receptors leading to the decreased inflammatory response. Compound 91a could thus increase the blood flow to muscles, prevent the oxidation of proteins, and reduce the pro-inflammatory cytokines, which could lead to improved skeletal muscle contractile response after myocardial infarction [103].

The acylhydrazone derivative 91b had a vasodilating effect, by increasing the concentrations of NO and cGMP, more potent than its isomer with possible use in the treatment scheme of hypertension. Additionally, compound 91b is an M₃ muscarinic receptor agonist proved by the antagonist effect of a selective antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodide. The acylhydrazone 91b had a reduced number of adverse reactions compared to other parasympathomimetic compounds. This derivative was reported for its hypotensive effect with no modification in heart rate observed for both intravenous and longer-term oral administration with possible use in the treatment of hypertension [104]. The structure of compound 91b [124] is shown in Figure 62.

Sathler et al. obtained an acylhydrazone 92 that demonstrated antithrombotic properties when collagen was used as an agonist and lower toxicity compared to other derivatives. The proposed mechanism is based on the interaction of the compound with TXA₂ synthase, acting as an inhibitor [105]. Additionally, Lima et al. synthesized the arylsulfonate–acylhydrazone derivatives 93–95 with antiplatelet activity [106]. The structures of the acylhydrazones 92–95 are presented in Figure 63.

Other derivates with antiplatelet effect are acylhydrazones containing the 1,2,3-triazole scaffold 96a–e (Figure 64), which exhibited a comparable or even higher potency than acetylsalicylic acid. The inhibitory activity observed in the arachidonic acid test was different in the case of studied compounds due to the various structural fragments, as follows: 96a—adenosine diphosphate (ADP) pathway antagonist, 96a,c,d,e—adrenaline pathway antagonists, and 96b,c,e—arachidonic acid pathway antagonists [107].

According to a research study conducted by Alencar et al., an acylhydrazone derivative 97 (Figure 65) was analyzed pharmacologically. It lowered the pressure on the pulmonary arteries by interacting with adenosine A_2A receptors, which have an important role in the pathophysiological mechanism of pulmonary arterial hypertension. Thus, the acylhydrazone 97 had an effect on ventricular remodeling (right ventricular hypertrophy) by decreasing it, lowering the right ventricular systolic pressure, stimulating SERCA2a protein and endothelial nitric oxide synthase, reducing the levels of phospholamban [108].

Silva et al. stated that the derivatives of acylhydrazones 98a and 98b (Figure 66) displayed vasodilatory action. Compound 98b, containing an allyl moiety linked to the amide nitrogen atom, showed a potency equivalent to that of compound 91a (Figure 62) and of acylhydrazone 98a, with a methyl group substituting the amide hydrogen atom [109]. The same biological property was exerted by compounds 98c and 98d (Figure 66), the vasodilatory action being more intense than that of acylhydrazone 91a [110].

The acylhydrazone 99 (Figure 67) was reported by Feng et al. as a substance that can protect cells from oxygen-glucose deprivation, oxidative stress stimulated by H₂O₂ and glutamate, stimulated apoptosis by oxygen-glucose deprivation, increased intracellular ROS, and increased ATP levels in neuronal cells. Compound 99 also increased the phosphorylation based on extracellular signal-regulated kinase and protein kinase B, based on antagonistic action with selective antagonists, and had favorable effects on stroke, inducing neuroprotection. Therefore, acylhydrazone 99 could be used in ischemic strokes after further research [111].