Discovery of New Imidazole Derivatives Containing the 2,4-Dienone Motif with Broad-Spectrum Antifungal and Antibacterial Activity

A compound containing an imidazole moiety and a 2,4-dienone motif with significant activity toward several fungi was discovered in a screen for new antifungal compounds. Then, a total of 26 derivatives of this compound were designed, synthesized and evaluated through in vitro and in vivo antifungal activity assays. Several compounds exhibited improved antifungal activities compared to the lead compound. Of the derivatives, compounds 31 and 42 exhibited strong, broad-spectrum inhibitory effects toward Candida spp. In particular, the two derivatives exhibited potent antifungal activities toward the fluconazole-resistant isolate C. albicans 64110, with both having MIC values of 8 µg/mL. In addition, they had significant inhibitory effects toward two Gram-positive bacteria, Staphylococcus aureus UA1758 (compound 31: MIC = 8 µg/mL; compound 42: MIC = 4 µg/mL) and Staphylococcus epidermidis UF843 (compound 31: MIC = 8 µg/mL; compound 42: MIC = 8 µg/mL). The results of an animal experiment indicated that both compounds could improve the survival rate of model mice infected with ATCC 90028 (fluconazole-susceptible isolate). More importantly, the two compounds exhibited notable in vivo effects toward the fluconazole-resistant C. albicans isolate, which is promising with regard to the clinical problem posed by fluconazole-resistant Candida species.


Introduction
Infections caused by bacteria and fungi lead to diseases and an enormous social burden as millions of people are infected by bacteria and fungi every year worldwide. Therefore, a large number of antimicrobial drugs have been listed, which play an important role in treating infections [1]. As the need for antifungal intervention has increased, so too has the prevalence of resistance [2]. With the irrational use of antibiotics, the resistance of microorganisms has become a very serious clinical problem. Growing antifungal resistance poses the threat that there will be no available drugs for the treatment of common infections in the future [3], so there is an urgent need for the discovery of new compounds with antibacterial and antifungal activities [4], especially those with mechanisms of action that are distinct from the well-known classes of antifungal agents [5][6][7].
The development of derivatives based on heterocyclic scaffolds is a fast emerging subject in medicinal chemistry. Azole compounds in particular play a remarkably important role in the field of medicinal chemistry. A great deal of azole-based antibacterial and antifungal agents have been extensively studied as drug candidates, and some of them have been used in the clinic, for instance itraconazole, fluconazole, posaconazole and voriconazole, which suggests the great development value of azole compounds [8]. However, the reduced susceptibility of Cryptococcus neoformans to fluconazole has been reported in a number of sub-Saharan countries including Kenya, Uganda, Rwanda and South Africa [9]. As a result, there is an unmet need for the development of new azole antifungals.
Imidazole compounds containing two nitrogen atoms in a five-membered aromatic azole ring have received special attention in recent years. As reported, imidazole rings are widely employed as spin-trapping species in the interesting application of designing drugs with neuroprotective activity [10,11]. These imidazole-based derivatives also have several favorable properties such as excellent bioavailability, good tissue penetrability and permeability and a relatively low incidence of adverse and toxic effects, which suggests that they have considerable development potential in chemistry, materials science and medicinal chemistry [12]. Their extensive applications, especially as antifungal agents, are frequently investigated, and this has become one of the most active areas in antifungal drug development. Many imidazole-based derivatives have been marketed as antifungal drugs such as ketoconazole (1), miconazole (2), clotrimazole (3), tioconazole (4), econazole (5), tinidazole (6), enilconazole/imazalil (7), parconazole (8), eberconazole (9), lanoconazole (10), fenticonazole (11), bifonazole (12), sulconazole (13), lombazole (14), and sertaconazole (15) (Figure 1) [13][14][15][16][17][18][19], which indicates their large development value and broad potential as antifungal agents. The imidazole ring has been demonstrated to be a versatile core of many biologically active molecules, especially those with antifungal properties. Thus, we screened the imidazole-based compounds in our inventory. As a result, compound 24 in Figure 2 was discovered in our screening to exhibit antifungal activity. The MIC values of this compound toward Candida albicans ATCC 90028, Candida glabrata 923, Candida glabrata 168 and Candida parapsilosis 27 were 2, 4, 4 and 16 µg/mL, respectively, but this compound was found to be inactive against Candida albicans 205, Candida albicans 64110, Candida krusei ATCC 6528, and Candida tropicalis 657. Considering the biological potency and synthetic accessibility of compound 24, it has the potential to serve as a reasonable lead compound for further development.

Chemistry
The synthetic methods used to prepare compounds 21-46 are illustrated in Scheme 1. The non-commercial intermediates (compounds 19a, 19k, 19m, 19n, 19o, 19q, 19r, 19s) were synthesized by irradiating the appropriate acetophenone in the presence of Br2 at a power of 250 W for 5 h (Scheme 1). Then, compound 20, the key intermediate, was obtained through the reaction of intermediate 19 with imidazole. Finally, various aldehydes were reacted with the corresponding intermediate 20 to produce the target compounds 21-44 and 46 via a Knoevenagel reaction. There was an interconversion between the E and Z isomers in the reaction solvent, which complicated the purification of the products. The ultimate solution was to purify the compounds by recrystallization from a mixed solvent of ethyl acetate and petroleum ether (v/v, 1/70) after silica gel column chromatography. The two-dimensional hydrogen spectrum suggested that the obtained compounds had only the Z-configuration.
To investigate the influence of the carbonyl moiety, compound 45 was also synthesized by reducing compound 31 with sodium borohydride, as shown in Scheme 1. All compounds were fully analyzed and characterized by 1 H nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS) before biological evaluation.

Antifungal Susceptibility Assay
All of the synthesized compounds were evaluated using in vitro antifungal activity assays against Candida albicans ATCC 90028, Candida albicans 205, Candida albicans 64110, Candida krusei ATCC 6528, Candida tropicalis 657, Candida glabrata 923, Candida glabrata 168 and Candida parapsilosis 27 (Table 1). The minimum inhibitory concentrations (MICs) of 26 compounds and fluconazole (Pfizer Pharmaceuticals, Shanghai, China) for the tested isolates were determined based on the standard guidelines described in the Clinical and Laboratory Standards Institute document M27-A3 using the microdilution reference method.
The MICs of all antifungal agents against the tested strains of Candida spp. determined by broth microdilution are listed in Table 1. The MICs of most compounds were higher than those of fluconazole, while compounds 31 and 42 exhibited more potent inhibitory effects on the tested Candida spp. In particular, the two derivatives showed potent antifungal activity toward the fluconazole-resistant isolate C. albicans 64110, with a MIC value of 8 µg/mL, which was a relatively excellent result.

Conclusion of the Structure-Activity Relationship
SAR of this class of compounds has been well studied through variation of the styryl, imidazole and benzoyl moieties, as shown in Table 1. First, the substituents on the A-ring were varied to improve the antifungal activity of the compounds. Thus, the first batch of compounds (21-23, 25, 26) was synthesized by replacing the 4-Br (24 in Figure 2

30, 41)
and introducing electron-donating groups (32, 33) or second substituents (35-40) resulted in relatively poor inhibitory activities. The results above indicated that substitution of fluoro in the para position of the phenyl group was essential for maintaining the antifungal activity. Furthermore, the antifungal activities of compounds 45 and 46 were also explored. Both of them exhibited a significant decrease in activity, which suggested that the conjugated double bonds and carbonyl were important for the activity.

Antibacterial Susceptibility Assay
The antibacterial activities of the two compounds (compounds 31 and 42) with excellent antifungal activities were also evaluated by antibacterial activity assays. The minimum inhibitory concentrations (MICs) of the two compounds were determined based on standard guidelines described in the Clinical and Laboratory Standards Institute document M27-A3 using the broth microdilution reference method. The results are shown in Table 2. Among the tested strains, the two compounds were effective toward two Gram-positive bacteria, Staphylococcus aureus UA1758 (compound 31: MIC = 8 µg/mL; compound 42: MIC = 4 µg/mL) and Staphylococcus epidermidis UF843 (compound 31: MIC = 8 µg/mL; compound 42: MIC = 8 µg/mL).  n.t. a n.t. a n.t. a n.t. a n.t. a Erythromycin 2 >256 24 n.t. a n.t. a n.t. a n.t. a n.t. a

Antifungal Activity in Vivo
As compounds 31 and 42 demonstrated excellent antifungal activities in vitro and were more potent than fluconazole, their in vivo activities were investigated in the further studies. In these studies, a mouse model of systemic candidiasis was used. First, the toxicities of these compounds in the mice were explored. As the structures of the two compounds were similar, we only assessed the toxicity of compound 31. Each group of mice was given compound 31 by intragastric administration consecutively with dosages of 32, 16 or 8 mg/kg once a day. After five continuous days of drug administration, nothing abnormal was detected. Therefore, the further studies could be continued under these dosages. As shown in Figure 3, after a prolonged time of injection of ATCC 90028 (fluconazole-susceptible isolate), the mice in the control group died gradually, while the survival rates of mice treated with compound 31 or 42 increased. Compound 42 exhibited particularly excellent antifungal activity in vivo; all mice treated with 32 mg/kg of 42 survived to day 10. However, compared with the reference drug fluconazole, the effects of the two compounds were weak, which was not consistent with the in vitro assay results. This discrepancy may be due to the low hydrophilicity of compounds 31 and 42. In future studies, we will focus on improving the hydrophilicity of these compounds to enhance their in vivo activity.
The effects of the two compounds in model mice treated with C. albicans 64110 (fluconazole-resistant C. albicans isolate) were also investigated, and the results are shown in Figure 4. Both compounds improved the survival rate of model mice, which indicated that they were effective against the fluconazole-resistant C. albicans isolate in vivo. Notably, 32 mg/kg of compound 31 kept most mice alive, displaying outstanding in vivo activity toward the fluconazole-resistant C. albicans isolate, which is promising regarding to solve clinical problems caused by fluconazole-resistant Candida species. The ED50 of the compounds were also calculated and the results are presented in Table 3. Compound 31 was more effective toward fluconazole-resistant strain 64110 than fluconazole-susceptible isolate ATCC 90028, with ED50 values of 2.693 and 21.653, respectively. As for compound 42, the two ED50 values were very similar, indicating similar activity against both strains.

General Information
Reagents were purchased from Alfa Aesar (Shanghai, China), Acros Organics (Shanghai, China), Adamas-beta (Shanghai, China) and Shanghai Chemical Reagent Company (Shanghai, China) and were used without further purification. Analytical thin-layer chromatography (TLC) was performed using HSGF 254 plates (150-200-µm thickness, Yantai Huiyou Company, Yantai, China). Melting points were measured without correction in capillary tubes on a SGW X-4 melting point apparatus. Nuclear magnetic resonance (NMR) spectra were acquired on a Bruker AVANCE 400 NMR instrument, using TMS as an internal standard. Chemical shifts are reported in parts per million (ppm, δ) downfield from tetramethylsilane. Proton coupling patterns were described as singlet (s), doublet (d), triplet (t), quartet (q) or multiplet (m). Low-and high-resolution mass spectra (LRMS and HRMS) were acquired with electric ionization (EI) produced by a Finnigan MAT-95 spectrometer.
3.1.1. General Procedure for the Synthesis of 18o The synthesis of 18o was performed according to a reported method [22].

General Procedure for the Synthesis of 18r
Compound 18r was synthesized according to a reported method [23].

General Procedure for the Synthesis of Bromoacetophenone Derivatives 19
A three-neck flask was charged with glacial acetic acid (35 mL) and the appropriate ketone (18, 0.02 mol). To this solution, bromine (1.1 eq) was added dropwise over 30 min at room temperature. The reaction mixture was irradiated at a power of 250 W for 5 h. The mixture was cooled, poured into 100 mL of H2O, and extracted three times with CH2Cl2 (60 mL). The combined organic extracts were washed with brine, dried over anhydrous sodium sulfate, filtered and evaporated under reduced pressure. The crude residue was purified by column chromatography (EtOAc/petroleum ether) to obtain the products. The data of the synthesized intermediates 19 is given in Table 4.

General Procedure for the Synthesis of 2-(1H-Imidazol-1-yl)-1-phenylethanone Derivatives 20
Bromoacetophenone derivatives 19 (1.0 mmol) and imidazole (2.0 mmol) were dissolved in dry tetrahydrofuran (10 mL) under a nitrogen atmosphere. The reaction mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and the residue was purified by silica gel column chromatography with dichloromethane/methanol (40:1) as the eluent to obtain the resulting intermediates. The data of the synthesized intermediates 20 is given in Table 5.  To a 25-mL round-bottom flask charged with the appropriate 2-(1H-imidazol-1-yl)-1-phenyl-ethanone derivative 20 (1.6 mmol), (E)-3-(4-nitrophenyl) acrylaldehyde (1.2 eq) and toluene (4 mL) was added piperidine (55 µL) and glacial acetic acid (12 µL). The flask was evacuated, filled with nitrogen and heated to 75 °C for 4 h. The reaction mixture was cooled to room temperature and the solvent was removed. The reaction mixture was purified by silica gel column chromatography with triethylamine/petroleum ether/ethyl acetate (0.15:2:1, v/v/v) as the eluent to obtain the crude product. The crude product was washed with petroleum ether/ethyl acetate (70:1, v/v) to obtain the final product.

Synthesis of 45
A mixture of 31 (300 mg, 0.76 mmol) and sodium borohydride (165 mg) in absolute methanol (15 mL) was stirred at room temperature for 2 h. After removing the solvent, the residue was purified by flash column chromatography on silica gel, eluted with triethylamine/ petroleum ether/ethyl acetate (0.15:2:1, v/v/v), to afford 45 (200 mg, 66%) as yellow oil. The data of target compounds 21-46 are given in Table 6.

Antifungal Susceptibility Tests
The prepared compounds and fuconazole (Pfizer Pharmaceuticals, Shanghai, China) were dissolved in DMSO to prepare primary stocks.
The stock was then gradually diluted to prepare secondary stocks with different concentrations. Finally, the working concentrations of the derivatives were obtained by adding the appropriate amount of the secondary DMSO stocks to RPMI 1640 medium.The amount of DMSO in working solutions did not exceeded 1%. Antifungal susceptibility tests were performed according to the standard guidelines described in the Clinical and Laboratory Standards Institute document M27-A3, and the microdilution reference method was used. Next, 100 μL of RPMI 1640 medium containing the desired concentrations (128 mg/L to 0.5 mg/L) of the appropriate compound was added to each well of a 96-well plate.   Eight strains of Candida spp. were used in the assay, including the quality control Candida albicans ATCC 90028 and Candida krusei ATCC 6528 isolates. They were cultured in solid Yeast Extract Peptone Dextrose (YPD) medium at 37 °C in a humidified atmosphere of 5% CO2 in air. The cells were dissolved in normal saline at a density of 5 × 10 6 CFU/mL. Then, the solution was diluted 1000 times with RPMI 1640 medium, and 100 μL of diluted solution was added to the 96-well plate containing the compounds. After incubation for 48 h at 37 °C, the MIC was read as the lowest concentration that produced a prominent decrease in growth (inhibition ≥ 80%) compared to the control cells (without compound).

Antibacterial Susceptibility Tests
The microdilution reference method was performed according to the standard guidelines described in the Clinical and Laboratory Standards Institute document M07-A9 for the antibacterial susceptibility assays. The desired working concentrations of the derivatives 31, 42, and four positive drugs (amikacin, cefoperazone, vancomycin, and erythromycin) were obtained by adding the secondary DMSO stocks to broth culture, as described for the antifungal susceptibility assays. Then, 100 μL of broth culture containing the appropriate concentrations (128 mg/L to 0.5 mg/L) of each compound was added to each well of a 96-well plate.
Eight strains of bacteria were used: Gram-positive bacterial isolates Staphylococcus aureus UA1758, Staphylococcus epidermidis UF843, and Enterococcus faecalis UA257, and Gram-negative bacterial isolates Klebsiella pneumonia UF222, Escherichia coli UA45, ESBL-producing Escherichia coli, Acinetobacter baumannii UA1037, and Pseudomonas aeruginosa UA1024. After culture in a blood plate at 37 °C in a humidified atmosphere of 5% CO2 in air, cells were suspended in normal saline at a density of 2 × 10 8 CFU/mL. Then, the solution was diluted 1000 times with culture broth, and 100 μL of diluted solution was added to the 96-well plate containing compounds. After incubation for 24 h at 37 °C, the MIC was read as the lowest concentration that produced a prominent decrease in growth (100% inhibition) compared with the control cells (without compound).

Mice Toxicity Assays
Sixteen mice (equal numbers of males and females) were selected to test the toxicity of compound 31 in mice. They were randomly grouped into four groups (four mice in each group): 32 mg/kg group, 16 mg/kg group, 8 mg/kg group and control group. Each group of mice was given compound 31 by intragastric administration consecutively for 5 days with dosages of 32, 16 and 8 mg/kg once a day. The mice in the control group were given 0.5% sodium carboxymethyl cellulose solution at 20 mL/kg. The survival of the mice was recorded

In Vivo Antifungal Activity
To establish a systemic fungal infection model, 160 Kunming mice, half males and half females, were selected, with weights ranging from 18 to 22 g and that had passed the quarantine inspection. They were divided into four groups according to their weight: compound 31 strain No. 1 group (I), compound 31 strain No. 2 group (II), compound 42 strain No. 1 group (III) and compound 42 strain No. 2 group (IV). Each group contained equal numbers of males and females. The 40 mice in group I containing half males and half females were divided into four dose groups and a control group, with 8 mice in each group, containing four of each sex. The mice in each dose group were given a different dosage (0.5, 2, 8 or 32 mg/kg) of compound 31 by intragastric administration, and the mice in the control group were given 0.5% sodium carboxymethyl cellulose solution at 20 mL/kg. A concentration of 2 × 10 6 CFU/mL of strain No. 1 was given by tail vein injection approximately 0.5 h after intragastric administration of the compound. Drugs were administered to mice once a day for 10 consecutive days. The survival rate and ED50 were calculated for each group. The remaining groups (II, III and IV) were given the relevant drugs and strains according to the group designations presented above. The test method was similar to what is described above. Testing of fluconazole (positive control) was performed according to the method similar to that described above.
Result processing: the ED50 and 95% confidence limit were calculated for each group using the regular Bliss method, and a graph of the survival curve was plotted based on the relationship between the survival rates of the mice over time.

Conclusions
In conclusion, we observed inhibitory activity of compound 24 containing an imidazole moiety and a 2,4-dienone motif oward several fungi. Based on this, a total of 26 derivatives were designed and synthesized in three steps. The prepared compounds were tested using in vitro antifungal activity assays, and several compounds exhibited improved antifungal activities compared to the lead compound. Among these compounds, compounds 31 and 42 exhibited strong inhibitory effects toward Candida species. In particular, the two derivatives exhibited potent antifungal activities toward the fluconazole-resistant isolate C. albicans 64110, with both having MIC values of 8 µg/mL. In addition, they displayed obvious effects against two Gram-positive bacteria, Staphylococcus epidermidis UF843 and Staphylococcus aureus UA1758. The results of animal experiments indicated that both compounds could improve the survival rate of model mice treated with ATCC 90028 (fluconazole-susceptible isolate). More importantly, the two compounds exhibited outstanding effects toward a fluconazole-resistant C. albicans isolate in vivo, which is promising with regard to the clinical problem posed by fluconazole-resistant Candida species.